Biofuels Refining and Performance Ahindra Nag, Ph.D. Editor

Biofuels Refining and Performance Ahindra Nag, Ph.D. Editor
Biofuels Refining
and Performance
ABOUT THE EDITOR
Ahindra Nag, Ph.D., is a Senior Assistant Professor in
the Department of Chemistry at the Indian Institute of
Technology, Kharagpur. He has 21 years of teaching
experience and has published 60 research papers in major
national and international journals. He is the author of
three other books: Analytical Techniques in Agriculture,
Biotechnology, and Environmental Engineering;
Environmental Education and Solid Waste Management;
and Foundry Natural Product Materials and Pollution.
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
Biofuels Refining
and Performance
Ahindra Nag, Ph.D.
New York
Editor
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DOI: 10.1036/0071489703
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Contents
Contributors
Preface
xiii
xi
Chapter 1. Energy and Its Biological Resources
1
K. B. De
1.1 Energy (Yesterday, Today, and Tomorrow)
1.2 Energy
1.2.1 Thermodynamics
1.3 Energy-Dependent Ecosystems
1.3.1 Photosynthetic factors
1.4 Bioenergy
1.5 Biological Energetics
1.6 Chemical Cell
1.7 Models of Bioenergy Cells
1.7.1 Oxidative phosphorylation path
1.7.2 Photosynthetic path
1.8 A Living Cell Is an Ideal Cell
1.9 Plant Cells Are Unique
1.9.1 Photosynthetic bacteria
1.10 Biofuels
1.10.1 Heterocystous blue-green algae (example,
Anabaena cylindrica)
1.10.2 Photofermentation by photosynthetic bacteria
(example, Rhodospirillium rubrum)
1.10.3 Methane production
1.11 Plant Hydrocarbons
1.12 Biogas
1.13 Gobargas
1.14 Biomass, Gasification, and Pyrolysis
1.14.1 Biomass
1.14.2 Gasification and pyrolysis
1.15 Bioluminescence
1.16 Hydrogen
1.16.1 Microbial conversion
References
1
5
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Contents
Chapter 2. Photosynthetic Plants as Renewable Energy Sources
45
Ahindra Nag and P. Manchikanti
2.1 Introduction
2.2 Mechanism and Efficiency of Photosynthesis in Plants
2.3 Photosynthetic Process
2.3.1 Hill reaction (light reaction)
2.3.2 Blackman’s reaction (dark reaction)
2.3.3 Efficiency of photosynthesis
2.4 Plant Types and Growing Cycles
2.5 Harvesting Plants for Bioenergy
2.6 Products
2.6.1 Gaseous products
2.6.2 Liquid products
2.6.3 Solid products
References
Chapter 3. Bioethanol: Market and Production Processes
45
46
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69
Mohammad J. Taherzadeh and Keikhosro Karimi
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
Introduction
Global Market of Bioethanol and Future Prospects
Overall Process of Bioethanol Production
Production of Sugars from Raw Materials
3.4.1 Sugar solution from starchy materials
3.4.2 Acid hydrolysis of starch
3.4.3 Enzymatic hydrolysis of starch
Characterization of Lignocellulosic Materials
3.5.1 Cellulose
3.5.2 Hemicellulose
3.5.3 Lignin
Sugar Solution from Lignocellulosic Materials
3.6.1 Chemical hydrolysis of lignocellulosic materials
3.6.2 Pretreatment prior to enzymatic hydrolysis
of lignocellulosic materials
3.6.3 Enzymatic hydrolysis of lignocellulosic materials
Basic Concepts of Fermentation
Conversion of Simple Sugars to Ethanol
Biochemical Basis of Ethanol Production from Hexoses
Chemical Basis of Ethanol Production from Pentoses
Microorganisms Related to Ethanol Fermentation
3.11.1 Yeasts
3.11.2 Bacteria
3.11.3 Filamentous fungi
Fermentation Process
3.12.1 Batch processes
3.12.2 Fed-batch processes
3.12.3 Continuous processes
3.12.4 Series-arranged continuous flow fermentation
3.12.5 Strategies for fermentation of enzymatic
lignocellulosic hydrolyzates
3.12.6 Separate enzymatic hydrolysis and fermentation (SHF)
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Contents
3.12.7 Simultaneous saccharification and fermentation (SSF)
3.12.8 Comparison between enzymatic and acid hydrolysis
for lignocellulosic materials
3.13 Ethanol Recovery
3.14 Distillation
3.15 Alternative Processes for Ethanol Recovery and Purification
3.16 Ethanol Dehydration
3.16.1 Molecular sieve adsorption
3.16.2 Membrane technology
3.17 Concluding Remarks and Future Prospects
References
Chapter 4. Raw Materials to Produce Low-Cost Biodiesel
vii
96
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M. P. Dorado
4.1 Introduction
4.2 Nonedible Oils
4.2.1 Bahapilu oil
4.2.2 Castor oil
4.2.3 Cottonseed oil
4.2.4 Cuphea oil
4.2.5 Jatropha curcas oil
4.2.6 Karanja seed oil
4.2.7 Linseed oil
4.2.8 Mahua oil
4.2.9 Nagchampa oil
4.2.10 Neem oil
4.2.11 Rubber seed oil
4.2.12 Tonka bean oil
4.3 Low-Cost Edible Oils
4.3.1 Cardoon oil
4.3.2 Ethiopian mustard oil
4.3.3 Gold-of-pleasure oil
4.3.4 Tigernut oil
4.4 Used Frying Oils
4.5 Animal Fats
4.6 Future Lines
4.6.1 Allanblackia oil
4.6.2 Bitter almond oil
4.6.3 Chaulmoogra oil
4.6.4 Papaya oil
4.6.5 Sal oil
4.6.6 Tung oil
4.6.7 Ucuuba oil
Acknowledgments
References
Chapter 5. Fuel and Physical Properties of Biodiesel Components
107
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Gerhard Knothe
5.1 Introduction
5.2 Cetane Number and Exhaust Emissions
149
152
viii
Contents
5.3 Cold-Flow Properties
5.4 Oxidative Stability
5.4.1 Iodine value
5.5 Viscosity
5.6 Lubricity
5.7 Outlook
References
Chapter 6. Processing of Vegetable Oils as Biodiesel
and Engine Performance
154
156
157
158
159
159
160
165
Ahindra Nag
6.1 Introduction
6.2 Processing of Vegetable Oils to Biodiesel
6.2.1 Degumming of vegetable oils
6.2.2 Transesterification of vegetable oils by acid or alkali
6.2.3 Enzymatic transesterification of vegetable oils
6.2.4 Engine performance with esters of vegetable oil
6.3 Engine Performance with Esters of Tallow and Frying Oil
References
Chapter 7. Ethanol and Methanol as Fuels
in Internal Combustion Engines
165
169
169
177
181
183
186
187
191
B. B. Ghosh and Ahindra Nag
7.1 Introduction
7.2 Alcohols as Substitute Fuels for IC Engines
7.2.1 Ethanol as an alternative fuel
7.2.2 Production of ethanol
7.3 Distillation of Alcohol
7.4 Properties of Ethanol and Methanol
7.5 Use of Blends
7.6 Performance of Engine Using Ethanol
7.7 Alcohols in CI Engine
7.7.1 Alcohol–diesel fuel solution
7.7.2 Alcohol–diesel fuel emulsions
7.7.3 Spark ignition
7.7.4 Ignition improvers
7.8 Methanol as an Alternate Fuel
7.8.1 Production of methanol
7.8.2 Emission
7.8.3 Fuel system and cold starting
7.8.4 Corrosion
7.8.5 Toxicity of methanol
7.8.6 Formaldehyde emission
7.9 Comparison of Ethanol and Methanol
7.10 Ecosystem Impacts Using Alcohol Fuels
7.10.1 Aquatic system impacts
7.10.2 Terrestrial system impacts
7.10.3 Occupational health impacts
7.10.4 Occupational safety impacts
7.10.5 Socioeconomic impacts
7.10.6 Transportation and infrastructure impacts
References
191
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Contents
Chapter 8. Cracking of Lipids for Fuels and Chemicals
ix
221
Ernst A. Stadlbauer and Sebastian Bojanowski
8.1 Introduction
8.2 Thermal Degradation Process
8.2.1 Catalytic cracking (CC)
8.3 Vegetable Oil Fuels/Hydrocarbon Blends
8.3.1 Refitting engines
8.3.2 Tailored conversion products
8.3.3 Feed component in FCC
8.4 Other Metal Oxide Catalysts
8.5 Cracking by In Situ Catalysts
8.6 Conclusion
References
Chapter 9. Fuel Cells
221
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241
246
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251
A. K. Sinha
9.1 Introduction
9.2 Fuel Cell Basics
9.3 Types of Fuel Cells
9.3.1 Polymer electrolyte membrane fuel cells (PEMFCs)
9.3.2 Direct methanol fuel cells (DMFCs)
9.3.3 Alkaline-electrolyte fuel cells (AFCs)
9.3.4 Phosphoric acid fuel cells (PAFCs)
9.3.5 Molten carbonate fuel cells (MCFCs)
9.3.6 Solid oxide fuel cells (SOFCs)
9.3.7 Biofuel cells
9.4 Fuel Cell System
9.4.1 Fuel processor
9.4.2 Air management
9.4.3 Water management
9.4.4 Thermal management
9.4.5 Power-conditioning system
9.5 Fuel Cell Applications
9.6 Conclusion
References
Appendix
295
Index 297
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Contributors
Sebastian Bojanowski Department of Math, Natural Science, and
Information Technology, Laboratory for Waste Treatment Processes,
University of Applied Sciences Giessen-Friedberg, Giessen, Germany
(CHAP. 8)
K. B. De Department of Chemistry, Indian Institute of Technology,
Kharagpur, India (CHAP. 1)
M. P. Dorado Department of Physical Chemistry and Applied
Thermodynamics, EPS, University of Cordoba, Cordoba, Spain (CHAP. 4)
B. B. Ghosh Department of Mechanical Engineering, Indian Institute
of Technology, Kharagpur, India (CHAP. 7)
Keikhosro Karimi Department of Chemical Engineering, University
of Technology, Isfahan, Iran (CHAP. 3)
Gerhard Knothe National Center for Agricultural Utilization
Research, Agricultural Research Service, U.S. Department of Agriculture,
Peoria, Illinois (CHAP. 5)
P. Manchikanti Agriculture Engineering Department, Indian Institute
of Technology, Kharagpur, India (CHAP. 2)
Ahindra Nag Department of Chemistry, Indian Institute of
Technology, Kharagpur, India (CHAPS. 2, 6, 7)
A. K. Sinha Department of Electrical Engineering, Indian Institute of
Technology, Kharagpur, India (CHAP. 9)
Ernst A. Stadlbauer Department of Math, Natural Science, and
Information Technology, Laboratory for Waste Treatment Processes,
University of Applied Sciences Giessen-Friedberg, Giessen, Germany
(CHAP. 8)
Mohammad J. Taherzadeh School of Engineering, University of
Borås, Borås, Sweden (CHAP. 3)
xi
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Preface
The continuous use of the world’s crude oil reserve and a corresponding
escalation in its price together with the limited coal reserves have stimulated the hunt for renewable sources of energy. The main sources of renewable energy are biomass, biogas, methanol, ethanol, and biodiesel; solar
active (photovoltaic), solar passive (preheating of water), wind, mini hydel,
and mini tidal are important sources which produce less pollution and protect the environment.
Much attention has been given to biomass and its modifications as a
substitute for fossil fuels in the Western world. Among the modifications
are biogas, alcohol, biodiesel, and manure. Presently, electrical power
is attractive in many respects and the search is on for renewable and
nonfinite resources to produce and supplement electrical energy.
The first chapter discusses energy and its biological sources. If biofuel is one of the expected solutions, we must know where is the beginning of the crisis and its solution. This chapter reviews the background
story along with an optimistic outlook for a safe energy resource on our
green earth. The second chapter discusses energy from photosynthetic
plants and their inherent recycling nature, as well as the environmental benefits involved. These sources of energy are the solution for energy
management. The third chapter discusses bioethanol, which is now one
of the main actors in the fuel market. Its market grew from less than a
billion liters in 1975 to more than 39 billion liters in 2006, and is expected
to reach 100 billion liters in 2015. The chapter discusses the variety of
raw materials, such as sugars, starch, and lignocellulosic substances,
that produces bioethanol and also covers some of the market issues. To
extend the use of biodiesel, the main concern is the economic viability of
producing biodiesel. Edible oils are too valuable for human feeding to run
automobiles. So, the emphasis must be on low-cost oils, i.e., nonedible
oils, animal fats, and used frying oils. There are many nonedible feedstock crops growing in underdeveloped and developing countries;
biodiesel programs here would give multiple social and economic benefits.
xiii
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xiv
Preface
The fourth chapter discusses different plant sources used for production
of biodiesel, properties of biodiesel, and processing of vegetable oils as
biodiesel, and compares engine performance with different biodiesels.
Biodiesel is the methyl or other alkyl esters of vegetable oils, animal
fats, or used cooking oils. Biodiesel also contains minor components
such as free fatty acids and acylglycerols. Important fuel properties of
biodiesel that are determined by the nature of its major and minor components include ignition quality and exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity. The fifth chapter discusses how
the major and minor components of biodiesel influence the mentioned
properties.
Different techniques of biodiesel preparation and resulting engine
performance are discussed in detail in Chap. 6. The seventh chapter discusses ethanol and methanol as fuel in the internal combustion engine
and emphasizes their advantages (such as a higher octane number)
over gasoline. Cracking of lipids turns polar esters into nonpolar hydrocarbons. This is accompanied by a fundamental change in physical and
chemical properties. Products formed give rise to new applications in the
fuel sector and for chemical commodities, e.g., detergents. The eighth
chapter explores routes to provide these alternative hydrocarbons from
lipids. It concentrates on substrates (seeds, vegetable oils, animal fat) and
conversion pathways as well as analytical tools.
The ninth chapter discusses the fuel cell, an electrochemical device and
nonpolluting alternative energy source that converts the chemical energy
of a fuel (hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant
(air or oxygen) into electricity with water and heat as by-products.
The book is organized in a manner to cater to the needs of students,
researchers, managerial organizations, and readers at large. We welcome
the reader’s opinions, suggestions, and added information, which will
improve future editions and help readers in the future. Readers’ benefits will be the best reward for the authors.
AHINDRA NAG, PH.D.
Biofuels Refining
and Performance
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Chapter
1
Energy and Its
Biological Resources
K. B. De
1.1 Energy (Yesterday, Today,
and Tomorrow)
Today’s energy concept needs to be modified and should be presented as
an integrated management-oriented approach. For example, the problem
of nutrition of human population and livestock is also an important item
in the energy inventory. So the per capita energy requirement will include
2000 kcal of the basal requirement in the form of nutrients; amount of
energy required to produce that amount of food; energy required to preserve
the food; energy required to collect the daily requirement of 600–800 L of
water; energy required for washing, cleaning, and bathing; and energy
required for lights, fans, air conditioners, and transport.
Today’s energy concept should also include the awareness that heat
is a wasteful form of energy, always downhill, and hence efficiency is at
the most 30–40% and that of an automobile is as low as 15–20%. Even
if we go modern, a solar photovoltaic panel has an efficiency of 8%, a
solar thermal power plant has 15%, and from sunshine to electricity
through biomass is only 1%.
In order to establish innovative technologies for highly effective utilization of solar light energy, fundamental research is being conducted
in the following areas:
1. Dye-sensitized solar cells: New types of dye-sensitized solar cells
mimicking the active sites of the natural photosynthesis system.
1
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2
Chapter One
2. Artificial photosynthesis: Hydrogen production from water, using
metal oxide semiconductor photocatalyst systems and effective fixation of CO2 by metal or metal complex catalysts.
Estimated contribution of renewable energy resources in the United
States by AD 2000, excluding hydro- and geothermal energy, amounts to
approximately 5% of the estimated total consumption of 100 quads.
Tropical countries, namely, India, receive 1648–2108 kWh/m2 of solar
energy in different parts with 250–300 days of sunshine, most of which
is unutilized.
While shifting our attention from today into the future, we should look
at some discussions that took place in the 12th Congress of World
Energy, a conference held in New Delhi, during September 18–23, 1983,
the main themes of which were management, policy, development, and
quality of life. There were four divisions, and each of these divisions had
four sections containing 157 technical papers. In the concluding session,
Dr. J. S. Foster, chairman of the program committee, on behalf of the
International Executive Council, gave a summary.
1. Innovation: Commenting on innovation, a report from Israel narrated absorption refrigeration, and Austria reported on a thermal
power plant, investigating a treble Rankine cycle using three separate working fluid loops. Brazil reported methane from urban refuge
and collaborative international efforts on controlled nuclear fusion
were highlighted.
2. Self-Reliance: Self-reliance has been well emphasized.
3. Diversification: Diversification in national or regional supply
ensures a robust energy structure, reducing vulnerability to vagaries
of nature, resource, or market fluctuations.
4. Dependence: Dependence on fossil fuels can be reduced with proper
substitution by biogas, solar, wind, and nuclear powers.
5. Efficiency and conservation: Waste heat recovery, cogeneration,
and recycling of energy were in the technological aspect. Public and
social consciousness through education is the other aspect.
6. Development: International cooperation and development assistance should involve mainly (a) financial resources, (b) technology
transfers, and (c) transfer of managerial and engineering skills.
7. Care of the environment: Pollutions from fossil fuels, nuclear reactors, and effects on forests and vegetation from dams are to be studied
along with future expansion schemes.
8. Quality of life: Indiscriminate and unplanned use of energy may
lead to negative and harmful impacts. Need for energy education
Energy and Its Biological Resources
3
and man power training with an integrated approach has been recommended.
9. Urgency:
a. World population will reach 10 billion in 2020.
b. Half of the population will have only 20 GJ/yr.
c. The other half in the industrialized countries will use 15 times as
much, i.e., 300 GJ/yr.
d. “Firewood crisis” has changed to “firewood catastrophe” and the
2
forest cover is diminishing globally at the rate of 250,000 km /yr.
Energy administration in developing countries depends mainly on
three denominators:
(1) Growth rate of population
(2) Energy self-reliant populations growing in size but lowest rate
(3) Rural population largest in size, but lowest rate of energy consumption
in most countries
Recommendations of new and alternative energy resources are
available. The emphasis is on nonfossil and renewable resources,
namely, biogas and biomass, solar active (photovoltaic), solar passive
(preheating of water), wind, minihydroelectric, and minitidal resources.
The major attempts for conservation include: conservation side legislation, education, awareness, management, and forecasting.
In Europe and South America, biomass and its modification have
been given a lot of attention as a substitute to fossil fuels. The primary
material, of course, is the waste of different plant and vegetable origins.
The conversions are to biogas, alcohol, and manure. Proper selection of
waste material may lead to optimal production of the right transform.
Advanced countries, point out that electrical power is attractive in many
respects and that the search for renewable and infinite resources to
produce and supplement electrical energy should continue. Hydropower,
solar energy, wind, solid waste, biomass, geothermal energy, ocean tidal
power, and ocean thermal gradients are a few resources that need attention. In fact, many institutions and organizations have created demonstration models for these.
In the United Kingdom, the emphasis seems to be more on proper
selection of local conditions and availability of the resources. Biomass and
biogas need collection, transport, and processing to be properly useful
for energy generation. Setting up aerogenerators, wind pumps, and
solar heating will depend on available and favorable conditions and the
proper location. If successfully implemented, they can reduce the local
demand or share the load of a national power grid. The other resources
that remain to be developed and commercialized are listed in the following discussion.
4
Chapter One
1. Fusion of thermonuclear devices (an application of plasma
physics):
2
1D
21D → 32He n 3.2 Mev
2
1D
2
1D
21D → 31T 11H 4.0 Mev
6
3Li
n → 42He 31T 4.8 Mev
2
1D
31T → 42He n 17.6 Mev
2
1D
63Li → 242He 22.4 Mev
2
32He3 → 42He4 11H 18.3 Mev
1D
31T → 42He n 17.6 Mev
2
4
1
6 1D → 2 2He 2 1H 2n 43.1 Mev
Several designs and modifications are suggested:
2P → e D
DP→T
2T → He 2P
The fusion reaction, omnipresent in the sun, needs to be tried out:
2H1 → e H2
where two protons fuse, and deuterium, positron, and neutrino are
evolved; energy is evolved in two steps; four protons are annihilated for each helium formed. Much of the reaction mechanism is
yet unknown, but the model shows great promise.
2. Geothermal source: Other than volcanic or geyser origin at an
8000-ft depth of the earth’s crust, it is possible to obtain geothermal steam at 2000C, which can be used for producing electricity.
Hot dry rock (HDR) remains out of reach at present capability of
drilling. But “heat mining,” as estimated by Los Alamos Scientific
Laboratory, promises 1.2 cents/MJ compared to 2 cents/MJ from an
oil-fired thermal plant ($34/bbl).
3. Aerodynamic generations: Several models are available. Lowvelocity windmills are also being used. Wind is stronger at upper
atmosphere; array of floating windmills are also designed.
4. Hydrodynamics: High hopes are created by some hydroelectric
firms, who proclaim that power can be effectively generated by
ocean waves and ocean currents.
5. Magnetohydrodynamic generators: High-temperature combustion gas expands through a nozzle where ionized sodium is introduced and directed to a magnetic field and a moving conductor cuts
the field, and an electromagnetic field (EMF) is produced.
6. Oil shale and oil sand: Though of limited supply, these have not
been fully explored.
Energy and Its Biological Resources
5
7. Coal conversion: Many models for fluidization and gasification of
coal are available.
8. Black box or hydrogen fuel cell: Usually, these use hydrogen as
input fuel based on reverse hydrolysis (see last part of Sec. 1.6):
At anode: H2 → 2H 2e
At cathode: O2 4H 4e → 2H2O
9. Hydrogen as fuel: Hydrogen as fuel is gaining popularity. The
most common sources are from (a) excess of nuclear energy, (b)
windmills, (c) hydroelectric power, (d) biological sources to some
extent, (e) fuel cell (see Sec. 1.6), and (f ) microbial hydrogen production (see Sec. 1.16).
10. Biological energy: A number of biological energy transformation
principles, very attractive, remain at the conceptual state.
1.2
Energy
A body can do work, or work can be done upon a body; a body of water
can turn a turbine, or one may pedal a bike to move it. If work is done
on a body, it will possess energy. When energy is possessed by a body,
the body can do work.
An agent may do work when it possesses energy, i.e., the amount of
work that an agent can do is the amount of energy it possesses. So a body
may gain kinetic and potential energy or lose the gained energy by producing heat or converting it to other forms of work.
Kinetic energy is due to the motion of a body.
Potential energy is due to the position or status of a body.
Frictional or colligative motion energy is produced in a waterfall; heat evolves to overcome a frictional resistance or checks the
motion of a body but sets useless motion to others (e.g., rolling of pebbles in a stream or dust behind a vehicle). Mechanical friction causes
a matchstick to ignite.
Units of energy are the same as those of work and are assigned equivalent quantities. Some important definitions and units are given in the
appendix. Energy content of some common substances are provided in
Table 1.1.
1.2.1
Thermodynamics
All three principles of thermodynamics are very much applicable in the
area of biological energy and chemical changes related to it. It is worthwhile to review a few fundamental points. Chemical reaction can take
6
Chapter One
TABLE 1.1
Energy Content of Some Common Substances
Food value or fuel value
Food value
Fuel value, kcal/g
Carbohydrates
Proteins
Fats (lipids)
Plant biomass (wet)
Plant biomass (ash free, dry)
Animal biomass (wet)
Animal biomass (ash free, dry)
Coal
Gasoline
4
5
9
2
4.5
2.5
5.5
7.0 [3200 kcal/lb]
11.5 [42,000 kcal/gal]
Average need for an adult human as consumer
Air
Water
Water (nonreturn)
Energy (food)
Land (vegetarian food)
Land (nonvegetarian food)
Plant body other than food
Personal or
survival need
Total social and
establishment
need
300 cuft/d
0.66 gal/d
0.3 gal/d
1 106 kcal/yr
0.3 acre/yr
0.3 acre/yr
5000 cft/d
2000 gal/d
750 gal/d
87 106 kcal/yr
0.6 acre/yr
4 acre/yr
Ratio
(total:personal)
17:1
3030:1
2500:1
87:1
1 ton dry weight per year 1-acre forest (and/or 3 tall trees of 12-in. diameter or 15 small
trees of 6-in. diameter)
place only if the energy status changes, i.e., A will be converted to B only
if B has a free energy content less than that of a change in free energy
F that is easy and spontaneous; reactions may be written as
A B (F )
or
A B F
or
F FA FB
The reaction is called exergonic, or energy is evolved or given out. If
F has a positive expression, the reaction is driven by the input of energy
and called endergonic; such reactions are difficult to complete. At equilibrium, F 0 (±), a point which may be arrived at by the end of the
reaction, or a reaction may be typically of that type (practically sluggish,
the progress of the reaction will depend on the change in concentration
of reactants, the change of temperature or pressure, etc.).
F F0 RT ln B/A, where B/A is the ratio at equilibrium or equilibrium constant, i.e., Keq. Then, 0 F0 RT ln B/A or F0 RT ln B/A Energy and Its Biological Resources
7
1363 log10 Keq at 25C. Here, R 1.987 cal/mol/K, T (273 25) K 298 K, and ln B/A 2.303 log10 Keq. This expression can be very useful:
log10 Keq
F0 1363 log10 Keq
1 10
0
0
0
1 10
±1
1
1363
1 10
2
2
2726
1 10
3
3
4086
Keq
When A and B exist equimolar, then the expression F F0 RT ln 1
means F F0, and the state is called a standard state.
Chemical conversions and change of state need some other consideration in the light of the third law of chemical thermodynamics:
F H TS
H is the change in heat content, T is the absolute temperature at
which the reaction occurs, and S is the change in entropy (change, GR),
or degree of disorder in the system, understood as the heat gained
isothermally and reversibly per unit rise of temperature at which it
happens (unit being calories per kelvin). The absolute value of H and S
of a system cannot be directly determined. “Heat content” is also known
as “heat content at constant pressure” or “enthalpy.” The third law suggests chemical pathway of finding entropy values in absolute terms.
The first law of thermodynamics deals with conservation of energy and
the second law with the relation between heat and work.
1. Energy cannot be destroyed or created, i.e., the sum of all energies
in an isolated system remains constant.
2. All systems tend to approach a state of equilibrium. This means that
the entropy change of a system depends only on the initial and final
stages of the system, expressed by R. Clausius.
a. The total amount of energy in nature is constant.
b. The total amount of entropy in nature is increasing.
1.3
Energy-Dependent Ecosystems
All forms of life are dependent on availability of energy at all levels, the
creation, growth, and maintenance (defense, offense, and survival). The
requirement and utilization of energy are mainly in two forms; the most
important are nutrient and environmental energy in the form of heat
and light.
8
Chapter One
It is easy to observe that extremely cold or hot regions are not favorable for the growth of living things. Likewise, the absence of light limits
the propagation and proliferation of photosynthetic biotic species.
The sun, of course, radiates energy into space of which only an insignificant part is shared by this planet of ours called Earth. Because of its spin
and its orbital rotation, a seasonal variation occurs in the total insolation
2
on the earth’s surface, which averages approximately 20 kcal/(m yr).
The incident radiation comprises 2000–8000 Å, 50% of which is in the
visible range (3700–7700 Å); only a small part of the incident energy is
utilized by living systems.
Solar constants are given as 1.968 cal/(cm2 min) 3.86 1033 erg/s 1.373 kW/m2. There are variations in the figures, depending on the
source of information. However, the energy received on the earth’s surface is mostly thermal and wasted. Biological fixation is restricted to photophosphorylation.
Let us look at the components of ecosystems that are capable of utilizing incident energy and some interrelationships between them.
Autotrophs (meaning self-surviving), also known as producers,
mainly the photosynthetic systems, are the largest users of sunlight.
Theoretically, anywhere there is light they should grow, provided
other inputs are favorable. In arid land, the lack of nutrients; in deserts,
the lack of water; and at higher-altitude, low temperatures, low CO2
tension and other adverse conditions will prevent the proliferation
of autotrophs, leaving otherwise sufficient insolation unutilized
(energy fixation by photosynthetic pathway is treated elsewhere).
Producers growing on detritus (dead organic materials) are not well
described in the literature, but these could be autotrophs.
Heterotrophs (mixed surviving or unlike surviving), on the other
hand, survive partly depending on the nutrient sources made available by other living systems. Most animals are heterotrophic.
Therefore, animals are also called consumers.
If animals survive mainly on autotrophic materials, they are called
primary consumers, commonly known as herbivores. If animals
largely survive on other animals as their source of food, they are
called secondary consumers, popularly known as carnivores. Predators
are animals that hunt their animate food, known as prey. The
prey–predator relationship plays an important role in nature and contributes to the ecologic balance.
1.3.1
Photosynthetic factors
Assuming that the wavelength of light remains constant, the intensity
influences the rate of photosynthesis, which is why the earlier part of the
Energy and Its Biological Resources
9
forenoon is the most productive, and higher intensity of light energy and
higher temperature slow down the photosynthetic rate. Likewise, a
cloudy day does not slow down the normal photosynthetic rate of particular species to any observable extent.
Metabolically speaking, reports are insufficient to conclude anything
based on this observation, even though the above information itself is
very useful and valuable. At the onset of daybreak, the photosynthetic
machinery gets into action after a dark rest period and the rate is at its
peak; the carbon dioxide tension (partial pressure) at the immediate
microenvironment is also higher (it is yet to be established that higher
carbon dioxide tension facilitates photosynthesis, though the reverse is
true). As the reaction proceeds with time, all other conditions remaining the same, the anabolic machineries including the enzymes and coenzymes (particularly NADP/Co II system) are fully occupied and ATP
systems are also fully utilized. ATP production is, in turn, dependent on
respiration (oxidative process), which to some extent is competitive with
carbon fixation. Geological and geographical factors contribute greatly
to ATP productivity.
Let us turn again to the consideration of biogeological and biogeographical distribution on energy. For an energy-based ecosystem, the
biosphere may be classified into two major types: terrestrial, and aquatic.
These can also be subdivided into eight intraterrestrial types: terrestrial,
subterrestrial, epilimnon, mesolimnon, hypolimnon, estuarine, epimarine, and submarine. What do these have to do with our objective?
Natural distribution of flora and fauna largely depend upon the types of
microenvironments mentioned above.
At this point, it need not be assumed that the arctic belt, being very
cold, is biologically unproductive. The author was surprised to see the
existence of almost a minitropical pocket, 66 north latitude and 20 east
longitude (Jockmock, Sweden) due to uninterrupted insolation for almost
90 days and prolonged daylight for 60 more days. The flora and fauna
have adapted to survival techniques for the cruelty of adverse nature
during the long, dark winter months.
1.4
Bioenergy
Energy can be derived from living systems in restricted forms only.
Lignocelluloses are burned to get heat, and vegetable oils are often used
for illumination. These may also serve as nutrients for different biotic
species in various forms, i.e., cellulose, starch, and sugars. In other
words, chemically stored energy may be reused in the form of fuel (firewood) or nutrients (food, feed, fodder, etc.). Animals can be employed to
do different mechanical work. Animals directly (fish, meat) or indirectly
(egg, milk) may provide nutrients for others. Use of dried cow dung as
10
Chapter One
cooking fuel in rural areas is also a well-known example of animal products indirectly contributing to this field. But examples of direct energy
flow from living systems are still in the conceptual state. Scientists
dream that, one day, light emitted by fireflies or high voltage generated
by electric eels may be of great use in the near future.
The production of alcohol or methane by microbial fermentation of
common plant wastes are well-known phenomena. Recently however,
scientists have started looking into these phenomena with greater interest, so that, in either gas or liquid form, their production and use can
be optimized and made efficient. Plant bodies have been used as antennas, and plant leaves have been demonstrated to work as batteries. The
survival of all biotic species depends directly or indirectly on solar
energy. Studying the energy-based ecosystem raises awareness of this
fact. Obviously, the most common question becomes: If the sun happens
to be the source of all energy, why then is the solar energy not harnessed by different devices? There are inherent limitations of most of
the physical devices by (a) way of efficiency, (b) critical cost, (c) maintenance, (d) reliability, and (e) other factors.
In photosynthetic systems operating in green vegetations of the above
points, (b), (c), and (d) are enormously better. Its characteristic limitations are [for point (a)] the incident insolation, the ability to use only a
narrow spectrum [for point (e)], and requiring the proportionate amount
of soil surface area for insolation, optimal nutrients, temperature, and
moisture in the microenvironment. Here nature provides several
mutants from which we can take, pick, screen, or select the most tolerant variety. We may resort to genetic engineering for tissue cultures or
selective hybridization.
What is our objective? Along with the effort to harness the solar energy
by different physical methods, parallel efforts of optimal use of solar
energy through biotic fixation should be attempted. This involves understanding the following:
1. The living world in its entirety, i.e., ecology.
2. The photosynthetic systems in different species: terrestrial, aquatic,
or mixed.
3. Application of the above to develop science and technology for:
a. Better management of the biotic systems useful for our purpose
b. Conversion of biological raw materials into energy rich products
4. Coordination for quality of life, pollution abatement, and sparing of
nonrenewable resources for future generations.
A few examples that may not be out of place include potato, tomato,
eucalyptus, and so forth. Though of wild origin, they have been appreciated and have been cultivated for this use after studying and admiring
Energy and Its Biological Resources
11
their productivity and receptivity. Later by scientific manipulation,
new strains have been developed for cultivation.
It is justified to discuss certain established facts for making sufficient conceptual clarity for special topics. Some aspects of energy relations in living systems will be discussed in detail. Some other aspects
will not be discussed in detail because existing “know-how” is rather
limited.
1.5
Biological Energetics
The study of bioenergetics leads us into a world of novelty and greater
significance and has found new encouragement in industry. The biogas
generation by anaerobic fermentation has also led to new interest in
research in the light of bioenergetics.
The study of energy relations for each chemical step in the living
system may be an item of bioenergetics. The energy change can be
calculated in terms of calories or joules per mole. This is applicable for
catabolic processes, for example, the anaerobic or glycolytic paths or
oxidative phosphorylation. The anabolic paths are equally fitting, e.g.,
the carbon fixation or the photosynthesis and nitrogen fixation by the
symbiotic organisms [1].
The accounting and balancing of free energy change of certain reactions may lead to some fruitful conclusions. When glucose is oxidized in
a bomb calorimeter (an almost one-step reaction),
C6 H12O6 6O2 → 6CO2 6H2O 686,000 cal (pH 7.0)
but when equivalent CO2 is produced in a biological system (through a
multistep reaction),
C6H12O6 6O2 38ADP 38H3PO4 → 6CO2 38ATP 44H2O
382,000 cal (pH 7.0)
A noteworthy departure is the conservation of –304,000 cal/mol of glucose and gain of 38 moles of ATP, energy-rich (bond) compounds, i.e., 800
cal/mol of ATP. It also means 50,666 cal of energy are wasted if, on average, 1 mole of carbon dioxide produced chemically is wasted in the form
of heat, an inferior quality of energy.
A simple calculation will reveal that each nutrient has some specified
energy or calorific values. This can be compared to the different energy
12
Chapter One
TABLE 1.2 Comparison of Some Common Fuels
Fuel
Gobar gas, m3
Kerosene, L
Firewood, kg
Dry cow dung, kg
Charcoal, kg
Soft coke, kg
LPG (butane), kg
3
Coal gas, m
Electricity, kWh (hot plate)
Kcal
Thermal
efficiency (%)
Effective heat
(kcal)
4713
9122
4708
2092
6930
6292
10,882
4004
860
60
50
17
11
28
28
60
60
70
2828
4561
800
230
1940
1762
6529
2402
602
SOURCE: Permission from KVTC, Mumbai.
values of different fuels, i.e., coal, kerosene, firewood, and so forth (see
Table 1.2). Taking glucose as a model carbohydrate,
C6H12O6 6O2 → 6CO2 6H2O 686,000 cal
(molecular weight, MW 180 g).
686,000 cal
3800 cal/g
180 g
and taking palmitic acid as model fatty acid,
C16H32O2 23O2 → 16CO2 16H2O 2,338,000 cal (MW 256 g)
2,338,000 cal
9133 cal/g
256 g
Similarly, in amino acids, peptides show roughly the same value as that
of carbohydrates. In biological systems (measurement through metabolic
cage), it has been found that the biological energy values are slightly
higher than those shown theoretically. This is more so by “specific
dynamic action.” When mixed foods particularly protein are taken, the
total calorific value is enhanced. The exact reasons are not yet clear. Let
us concentrate on a few examples in the following:
In ethanol fermentation (pH 7.0),
C6H12O6 → 2[C2H5OH CO2] 56,000 cal
In lactic fermentation,
C6H12O6 → 2[CH3CHOHCOOH] 47,000 cal
Energy and Its Biological Resources
13
But in lactic fermentation from polysaccharide,
(Glucosyl)n → 2[CH3CHOHCOOH] (Glucosyl)n1 52,000 cal
CH3CHOHCOOH 3O2 → 3CO2 3H2O 319,500 cal
If glucose is the starting point (as is the case of ethanol fermentation),
then 2 moles of ATP are invested and finally 2 2 moles of ATP are
regenerated and the net gain of ATP remains 2 (see Fig. 1.1). But if glycogen is the starting point, then only 1 mole is invested in the formation
of fructose 1,6-diphosphate.
Hence, net gain in ATP is 4 1 3. Twice a mole of reduced Co I is
produced by the conversion of 3 phosphoglyceraldehyde to 1,3 diphosphoglycerate.
ATP H2O → ADP H3PO4 8000 cal
But F of formation of ATP 12,000 cal.
The energy conservation or efficiency factor can be calculated in two
different ways:
1. How much potential energy-rich chemical compounds are now
gained?
a. Ethanol fermentation: 16,000/56,000, about 29%
b. Lactic fermentation: 24, 000/52,000, about 46%
2. How much energy of reaction has been utilized as heat of formation
of the energy-rich compounds?
a. Ethanol fermentation: 24,000/56,000, about 43%
b. Lactic fermentation: 36,000/52,000, about 69%
+ATP
Glucose
Fructose-6–P
Glycogen
or
starch
3 (p) Glyceric
acid
+ATP
Glucose–l–P
–2ATP
Glucose-6–P
F-1– 6-diphosphate
1, 3-diphosphoglyceric acid
3-phosphoglyceraldehydes and
Dihydroxy acetone p
2 (p) Glyceric acid
( p) Enolpyruvic acid
Figure 1.1 Anaerobic part of biological oxidation.
−2ATP
Pyruvic acid
14
Chapter One
The percentage efficiency figures raise doubt about the interpretation. Such efficiency is never achieved by a man-made machine but
biological systems can. If we accept the lower figures with a margin,
we are conserving no less than 25% of our expenditure in the form
of provident fund energy, even under sudden stress, i.e., anaerobic
conditions.
Let us look at the situation when a reduced coenzyme is regenerated
or oxidized (brief and simplified):
ATP
ATP
ATP
1O
2 2
NADH (H) ⎯→ FAD ⎯→ Cytochrome ⎯→ Cytochrome ⎯→ H2O
Stoichiometrically,
CoIH(H) 1
O 3ADP 3H2PO4 → CoI 3ATP 4H2O
2 2
Similarly in the oxidative part, through the tricarboxylic acid cycle, the
major aspects may be represented as in Fig. 1.2.
From alpha ketogluterate to succinate, 1 mole of energy-rich phosphate in the form of guanosine triphosphate (GTP) is gained. Succinate
to fumarate mediated by FAD coenzymes generates two equivalents of
ATP. In the rest of the events, 4 sets of reduced Co I, when regenerated,
give rise to 4 3 12 equivalents of ATP. In the entire sequence of
events, from pyruvate plus oxaloacetate into citrate/isocitrate and finally
back to oxaloacetate, a total of 15 equivalents of energy-rich phosphate
bonds (ATP) are gained.
In combining the anaerobic part, 2 additional moles of reduced Co I will
be reoxidized and 6 ATP equivalents will be regenerated. Starting from
glucose-6-P all the way to CO2 and H2O, we see that 2 6 (2 15) 38
equivalents of ATP are gained. The balance of the equation has been
Pyruvate
−CO2, Co I
Oxaloactate
Acetyl CoA
Citrate/Isocitrate
Co I
−CO2, Co I
Malate
TCA
α-ketoglutarate
−CO2 , Co I
Fumarate
Succinate GTP
FAD (=2ATP)
Figure 1.2 Tricarboxylic acid cycle (oxidative pathway).
Energy and Its Biological Resources
15
cited earlier. An oxidative pathway is considered to be more effective
from a biochemical energetic viewpoint.
One anabolic example of photosynthesis is briefly discussed.
Theoretically, reversal of this known reaction should fit well for photosynthesis:
C6H12O6 6O2 → 6(CO2 H2O) 686,000 cal
But in fact, we find a slightly different figure. The entire reaction may
be symbolically represented as
2H2O 2NADP ⎯⎯⎯→
2NADPH (H) O2
Chloroplast
hv
3CO2 9ATP 5H2O
Triosephosphate 9ADP
⎯⎯→
6NADPH (H)
8H3PO4 6NADP
But the actual stoichiometric presentation shows
n(CO2 H2O) → ( CH2O)n nO2 n(113,000 cal)
almost 22,000 cal higher than expected; fortunately, however, the endergonic reaction derives its energy from light energy. These figures are justified because the part of the reaction occurring in the absence of light
needs a large excess of energy-rich compounds (ATP). The deficiency of
ATP is, however, taken care of by two linked reactions:
Cyclic photophosphorylation:
hv
nADP nH3PO4 ⎯⎯→ nATP nH2O
Noncyclic photophosphorylation:
4Feox 2ADP 2H3PO4 4H2O ⎯→ 4Fered 2ATP O2 2H2O 4H
hv
or 2Co IIred 2ATP O2
2H2O 2H
The deficiency of 1 mole of ATP per mole of CO2 fixed is provided by cyclic
photophosphorylation. The other anabolic process is the nitrogen fixation, which is also highly energy consuming.
The heat of formation of NH3 by a chemical pathway can only be
determined indirectly. By the Haber process, high pressure and temperature is needed and the yield remains very low. So the input in
energy in the technological process remains in large excess than the theoretical heat of formation of NH3.
Nitrogen fixation can take place in nature in two major ways.
Molecular nitrogen is converted to oxides of nitrogen in the atmosphere
16
Chapter One
by electrical discharge and gets into soil by rainwater in the form of
nitrites and nitrates. These are reduced to ammonia by the biological
nitrogen fixation of symbiotic organisms or by blue-green algae.
In Escherichia coli and Bacillus subtilis, NO3 is reduced to NH3
[NO3 → NO 21 → N2O 22 → NH2OH → NH3]
and an oxidation reduction potential of 0.96 V (pH 7.0) is utilized by
these systems to convert other materials to a more oxidized state.
3
NH3 O2 → NO2 H2O H 36,500 cal
2
1
NO 2 O2 → NO 3 17,500 cal
2
2e
2e
2e
N ≡ N ⎯⎯→ HN NH ⎯⎯→ H2NNH2 ⎯⎯→ 2NH3
Via Mo-protein complex
Hydrogen is made available from reduced coenzymes, and the energy
is available from ATP produced by the oxidation of general metabolites.
In some systems, H2 becomes the by-product, and this could be an ideal
fuel or it can be used in a suitable chemical cell for the production of energy.
1.6
Chemical Cell
Two different metals in contact with a polar or ionic fluid generate the
flow of electrons. When touched simultaneously by two different metallic rods, muscles contract, a pioneering observation that gave birth to
the study of galvanic, voltaic, and Daniel cells.
The potential generated depends on the energy of sublimation, the ionization potential, the electronic work function, and the energy of solvation of ions. The nature of the solvent influences the last factor. The
electronic work function also includes several other conditions of ionic
activity. As a result, a potential difference will arise out of a simple concentration gradient, provided that anionic and cationic stoichiometry is
maintained. A review of the existing knowledge is worthwhile here.
If two small baths, each having either Zn or Cu metal and corresponding dilute solutions of Zn2 and Cu2salts, are in electrical continuity—
say through a capillary of a U tube or a Pt wire—then current will flow
in the two metals when connected outside, with Cu behaving as a cathode
and Zn as an anode (see Fig. 1.3). The setup can also be designed by separating the two systems by a semipermeable membrane.
A similar experience is the cylindrical design of the commonly available dry cells, where a graphite rod at the center serves as a reference
Energy and Its Biological Resources
Zn
Cu
Zn
Cu
Zn2+
Cu2+
Zn2+
Cu2+
Cu
Zn+2
17
Zn
Cu+2
Figure 1.3 Chemical cell.
cathode surrounded by a paste of chemicals, usually NH4Cl, totally
housed in a small cylindrical cup of metallic Zn as an anode.
In each case, Zn gets oxidized and changes to Zn2, and Cu2 is
reduced and is deposited as Cu; in the graphite (carbon) electrode, the
chemical change is not noticeable. (Theoretically, CH4 should be formed,
but slow escape of NH3 takes place.)
The field of electrochemistry has progressed considerably. Standard
electrode potentials and electrochemical charts with a fair degree of
accuracy and reliability are available. Taking Pt (inert) electrodes,
hydrogen gas at 1-atm pressure, immersed in a solution of hydrogen ion
of unit activity is usually a reference or standard hydrogen electrode
(usually referred as zero or standard scale). If an element goes into a
solution, producing cation (Zn → Zn2 0.761 V ), the half cell will give
an oxidation potential with a sign opposite to the potential when the
cation of the same species is deposited as the element, giving rise to a
2
reduction (Zn → Zn 0.761 V); the numerical values are expected
to remain in the same order.
One may observe, on the other hand, that alkali metals have a tendency to become hydrated oxides in water, so they exhibit a tendency to
offer oxidation potential with a sign. When the element approaches
nobility, then converts to the halogen (2X → X2 2e), the situation
is reversed. A representative partial list of the standard electrode potentials is reproduced (see Table 1.3). So one may expect that in a chemical cell with Zn/ZnCl2-CuCl2/Cu, the EMF will be 0.761 (0.340) 1.101 V.
If the electrode pair is made of the same material in a system, and the
concentration difference of electrolyte is maintained between the two
electrodes, a standard potential difference is expected, at the rate of
0.054 V per each tenfold rise in ionic concentration (referred to as concentration cells).
18
Chapter One
TABLE 1.3 Standard Electrode Potentials at 25ⴗC
Electrode
Li → Li
Na → Na
Mg → Mg2
AL → AL3
Zn → Zn2
Fe → Fe2
Sn → Sn2
Pb → Pb2
Cu → Cu2
Ag → Ag
Hg → Hg2
3
Au → Ag
1
Pt, H2 → H
2
1
Pt, Cl2 → Cl
2
Potential, V
3.024
2.715
2.34
1.67
0.761
0.441
0.140
0.126
0.340
0.799
0.799
1.300
0.00
1.358
Electrode
Potential, V
1
Br → Br
2 2
1
Pt, Br2(l) → Br
2
1
Pt, I2(s) → I
2
1
Pt, O2 → OH
2
Pt,
2
3
Pt, Fe → Fe
Pt, Pb2 → Pb4
Pt, Sn2 → Sn4
Pt, Cu → Cu2
1
Pt, Hg2 → Hg2
2
1.087
1.065
0.535
0.401
0.771
1.75
1.75
0.16
0.91
If Zn is used as a common electrode, or better inert-metal electrodes
are used (e.g., Pt) and immersed into NH4Cl or HCl solutions, say 0.1
and 1.0 N, a potential difference of 0.054 V will be experienced. The effect
of temperature and other factors which affect ionic activity will definitely
alter the values of EMF. The strength of the current will depend, expectedly, on the total surface area or participation of the total number of ions
and their charge-carrying capacities.
Electrochemical behavior of certain elements, e.g., carbon and silicon,
must be determined indirectly. Only graphite exhibits direct application
in a chemical cell, but other forms of carbon or silicon do not play any
significant role at this state of knowledge (see Fig. 1.4).
1.7
Models of Bioenergy Cells
One attractive suggestion is based on harvesting the potential produced
in different steps of metabolism in living systems [2]. Basic principles
remain the same in all such models. One of them is to tap the oxidative
phosphorylation path, and the other one is to use the photosynthetic
mechanism. There are a few more novel systems suggested by other
schools: (a) calcium pumps in biological systems by Ernesto Carafoli of
Swiss Federal Institute of Technology, Zurich, (b) constructing cells from
bacteriorhodopsin of the purple membranes of certain bacteria by Lester
Packer of the University of California at Berkeley, United States, and
Energy and Its Biological Resources
−
19
+
Power
N2, H2O, O2
H2
H2 → 2H+ + 2e−
H+
1
O + 2H+ + 2e−
2 2
−
+
H+3 O
3H2O
2H+
H2(2H2O)
O2, N2, H2O
V=
∆F
ηF
dil H3PO4
at 25°C, 1.23 V
1.18 V (water vapor)
Figure 1.4 Gaseous battery (hydrogen fuel cell).
(c) isolated energy-rich compounds, i.e., iron-sulfur proteins, ATP, and
so forth, suggested by many other authors [3].
1.7.1
Oxidative phosphorylation path
In the electron transfer chain, the conversion takes place at lower potentials, i.e., NAD/NAD to NADH/NADH between 0.6 V, favorable at a
pH higher than 7.0. But the process develops other energy-rich compounds,
and thus, very little free energy in the form of heat is directly available.
1.7.2
Photosynthetic path
Cytosolic to mitochondrial compartments, the interconversions of pyruvate to aspartate and to glutamate; malate to -ketoglutarate; the energy
produced is utilized to synthesize higher carbon compounds, ultimately
to glucose or even polysaccharide and polynucleotide (genetic material)
(see Fig. 1.5). Artificial culture of thylakoid or chloroplast, (only remains
a possibility for academic purposes at present); cannot be commercially
achieved as yet.
The most important achievement is the photolysis of water (see Fig. 1.6),
i.e., production of proton to hydrogen, reduction of carbon dioxide, reduction of nitrogenous material, and increase in nitrogenous and carbonaceous biomass. Attempts have been made to utilize the energy-trapping
process of the photosynthetic pigments of the plastoquinones at two
stages: (1) Pigment II utilizes 680–700 nm, converts water to a more
20
Chapter One
O2
Thylakoid membrane
Chloroplast
Heterocysts
H2O
ATP, (NADPH), CO2
Carbohydrate
Biomass
Figure 1.5 Electron flow in biophotolysis.
hn
Chloroplast
NADP
O2
::::::::::::::::::::::::::::::::::::::::::::::::::;;::::
NADPH
OX
Fd
Red
NADPH reductase
N ADP
NADPH
OX
Fd
Red
2H2
Figure 1.6 Separated photolytic chamber design.
Energy and Its Biological Resources
21
energetic intermediate, and undergoes a change of 0.8 to 1.1 V.
(2) Pigment I utilizes 700–730 nm, undergoing 0.5 to almost 1.4 V,
production of hydrogen, oxidation of coenzyme, making electrons
available.
Models can be created where direct tapping from the thylakoid membrane may be made possible. Electrochemical cells have been designed
where living thylakoids are used and exposed to sunlight from which,
through proper instrumentation, the energy can be tapped.
1.8
A Living Cell Is an Ideal Cell
Quite a few prototype experiments have been done, and a large number
of postulations are yet to be worked out, based on the potential difference maintained within and outside the living cell. Two well-known
phenomena are the membrane potential and the injury potential.
If the inside and outside walls of a cell membrane are brought to electrical continuity, current will flow. Usually the inside is anodic, mainly
due to the dominating fixed charges on the membrane protein. When
injury is caused, the excess mobile cations from the outer surface infiltrate the inner layer and a local flow of current takes place. A healthy
(uninjured) cell maintains an intact membrane, spends some metabolic
energy to pump in nutrients and K, and retains them within the cell
against a concentration gradient. Likewise, some of the metabolic products, including Na are pumped out (exceptions, namely, Halobacterium—
are few).
Most of these functions are chemically mediated (by ATPase, ATP –
Mg2, etc.) and amount to mechanical work. Maintenance of the potential difference on the membrane inside out is an indirect electrical manifestation of the chemical activity. The membrane components, particularly
protein, uphold its configuration with desired functional groups projected within. Retention of selective ions with the cell, in addition to
offering electrical neutrality, offers colloid osmotic steady state (through
Donnan equilibrium).
Another interesting phenomenon associated with chemical activity of
cells is the pH specificity of specialized cells. Normally, the mammalian
body fluid behaves as an alkaline buffer, pH 7.4, with only about 0.1 M,
contributed by metal ions, but has high osmolarity due to colloid osmotic
components. In spite of the pH 7.4 of the circulating fluid, the stomach,
part of the kidney, and the respiratory system maintain distinct acid pH.
This mechanism of upholding higher H concentration is by metabolic
expenses. In plants, the tissue fluid is usually acidic, say pH 6.5, and
certain specialized tissues, namely fruits, exhibit strong acidity. In very
rare cases (marine flora), plant tissue fluids show alkaline pH.
22
Chapter One
These examples are sufficient to indicate that if gastric mucosa is
connected to the intravenous system, a potential difference or an EMF
will be experienced. Likewise, if the root tissue and the fruit of a tree
are short-circuited, current (however feeble) will be experienced. This
information is not worth much at this present state of the art because
the magnitude of instrumentation will appear prohibitive. But in space
research, there was no alternative left but to develop solar cells, and silicon cells have found their place despite their cost. Because roughly
4 kcal of energy is available per gram of coal or hydrocarbon, this technique is of limited value at present. However, with enhanced improvement, the renewable resources of flora and fauna may be sources of
direct energy when we run out of oil and coal and will also appear inexpensive under those circumstances.
1.9
Plant Cells Are Unique
Whether they are green algae (chlorella) or the higher plants, autotrophs
in general are gifted in nature to fix carbon dioxide and produce biomass.
In ecologic terms, these are producers. The dominant autotroph is phototrophic. Photosynthesis has two distinct aspects: the light dependent
step, where photolysis of water takes place:
680 nm (52 kcal)
1
NADP H2O ⎯⎯⎯⎯⎯⎯→ H NADPH O2
2
8 kcal
ADP Pi ⎯⎯→ ATP
During this reaction, oxygen is set free, Co II is reduced and phosphorylation of ATP takes place. The photoenergy is chemically utilized twofold.
In the next step, through a very complex enzymic sequence, CO2 is
incorporated into the existing metabolite pool and higher carbohydrates
are biosynthesized. This step of the reaction finds variation in different
species; carbohydrates, proteins, and lipids are biosynthesized. Then, the
first part of the reaction makes the autotrophs unique. Light falling on
chloroplasts develops an electrical field across the membrane.
In the presence of the pigments in chloroplasts, the light energy is
trapped and activates water and lyses it. Ideally, water, if converted into
its elemental components, requires (at 25C) 68.3 kcal/mol (from liquid)
or 57.8 kcal/mol (from vapor). Thermal energy is not sufficient to bring this
change. During photolysis, the plant pigment augments electron flow, and
the electron flow system culminates in the two energy-rich chemical products (reduced Co II, ATP, and O2), as already mentioned (see Fig. 1.7).
Lester Packer’s group at the University of California at Berkeley has
shown the steps of the pathway with chloroplasts from spinach leaves,
Energy and Its Biological Resources
O2
Light
T
C
H
Y
L
A
K
O
I
D
M
E
H
L
O
R
O
P
L
A
S
T
S
H
E
T
E
R
O
C
Y
S
T
S
M
B
E
R
A
N
E
H2O
Ferredoxin
(Reduced)
ATP (NADP)
CO2
SO4 =
N2
2H+
−
NO3
Carbohydrates
H2S
NH3
23
H2
NH3
B
I
O
M
A
S
S
Figure 1.7 Biophotolysis and electron flow system.
ferredoxin from Spirulina, and hydrogenase of Clostridium pasteurianum [3].
2H2O → 4H 4e O2
4 Ferredoxin 4e → 4 Ferredoxin
4H 4 Ferredoxin → 2H2 4 Ferredoxin
O2 Glucose → Gluconate H2O2
H2O2 Ethanol → 2H2O Acetaldehyde
The overall reaction is
Glucose Ethanol → Gluconate Acetaldehyde H2
Two H2 are produced for each O2 produced (if not consumed by an
oxidase-type reaction as shown previously). Dr John Benemann of the
same university has also suggested that hydrogen and methane production is possible by designing a two-stage system separated from each
other (see Fig. 1.8).
24
Chapter One
2H2
O2
Fd
CHLOROPLATS
Fd
Light
OX
Red
OX
Red
NADPH
NADPH
Reductase
Reductase
NADP
NADP
Figure 1.8 Concept of a two-stage separated system for photolytic chamber.
Dibromothymoquinone blocks the natural electron flow system at
plastocyanin level (see Fig. 1.9). Thus, in the presence of an artificial
donor or acceptor, the photo systems I and II can be separated at preand post-blocking points.
H+
Q
FeS
H+
Fd
NADPH
PQ (Cyt 559)
I
II
O2
H2O
H
Mn
+
(a)
(b)
Pcy
+
H
(a) Phenazin methesulphate
(b) Diaminodurol; I and II photosynthesis systems I and II
Figure 1.9 Electron flow system at the plastocyanin level.
Ascorbate
Energy and Its Biological Resources
1.9.1
25
Photosynthetic bacteria
Small vesicles, called chromatophores, can be isolated from the membranes
of photosynthetic bacteria, which exhibit two types of electron transfer
chains resembling mitochondria and chloroplasts. Chroma-tophores
supported on artificial membranes permit the generation of 200 mV on
illumination. The salt-bacteria (Halobacterium halobium) contain a
simple protein–vitamin A aldehyde, known as bacteriorhodopsin, when
supported on artificial membranes that generate 250 mV on illumination. This system is simpler than its counterpart. There is a probability that the entire system may be successfully synthesized or assembled.
Solar photocells made of bacteriorhodopsin show great promise.
1.10
Biofuels
Prospects of ethanol and biodiesel as substitutes for conventional fuels
will not be discussed here; these two aspects are presented in sufficient
detail in Chaps. 3, 4, 5, and 6. One of the promising approaches for
future fuel is, perhaps, hydrogen and methane, both of which could be
obtained from living, particularly microbial resources.
Photosynthesis is the main route through which oxidized carbon is
reduced and again oxidized back to carbon dioxide for the generation
of energy. Based on this principle, we can utilize a few steps from
this life chain. This topic could be called biophotolysis—alternatively,
photobiolysis.
In the system, direct electron transport from water to hydrogen has
not been demonstrated as a technically feasible reaction. For this, continued research is required to elucidate the basic nature of FeS (PEA,
ferredoxin, and hydrogenase). This may lead ultimately to the practical
feasibility of production of hydrogen (ideally 20 L/h). Section 1.16 discusses hydrogen in detail. One inherent problem is the stability of the
hydrogenase system because of its sensitivity to molecular oxygen produced during photosynthesis.
However, one may design a two-step or two-compartment system.
Reduced Co II is the oxygen-stable electron carrier between photosynthesis and hydrogenase. A higher ratio of reduced Co II or Co II
helps the evolution of hydrogen, in spite of the unfavorable redox
potential of the coenzyme. Only Co II (reduced) can be pumped or
transported from one stage (compartment) to the other. Photosynthesis
and hydrogenase systems have to be encapsulated or immobilized separately in order to retain their respective activity; the two stages or
compartments may be connected through fiber filters. An example
could be to use appropriate algae to produce reduced organic compounds which can be pumped into bath of photosynthetic bacteria of
hydrogen fermentation.
26
Chapter One
One partial modification will be to collect oxygen during the day and
hydrogen at night, at the expense of accumulated reduced coenzymes,
made operative by anaerobically adapted microalgae or nonheterocystous
nitrogen- fixing blue-green algae. For product separation, the enzyme
technology or immobilization is inapplicable for biophotolysis. However,
there are potential practical applications of immobilized hydrogenase in
biochemical hydrogen–oxygen fuel cells. If such enzymes can be immobilized on an electrode surface, an inexpensive fuel cell might be developed,
which would increase the energy recoverable for hydrogen to save fuels.
Awareness of the limitations due to efficiency, engineering, and the
economy of these principles will save disappointment and encourage continued research. Geographical location and frequency of weather change
limits the insolation. The best photosynthetic efficiency is only 6% of the
total incident solar radiation, i.e., 5 kg/(m2 yr) of H2 by biophotolysis.
Half of this could be a very satisfactory achievement.
1.10.1 Heterocystous blue-green algae
(example, Anabaena cylindrica)
The heterocyst, regularly spread among more numerous vegetative cells
(ratio 1:15), receives carbon compounds fixed by the neighboring vegetative cells in exchange of the nitrogenous compounds fixed by them.
Nitrogenase, like hydrogenase, needs an anaerobic environment to function and can produce hydrogen only under certain conditions (absence of
molecular nitrogen). The ratio of evolution of hydrogen and oxygen roughly
corresponds to the ratio of the heterocysts and vegetative cells and also
with the ratio of nitrogen and carbon for nutritive requirements.
If the algal culture is exposed to argon for about 24 hours, due to nitrogen starvation, differentiation of the heterocysts increases from 6% up to
20%. In addition, a yellowish color appears due to the loss of the lighttrapping pigment phytocyanin, resulting in less carbon dioxide fixation,
i.e., oxygen evolution and an increase in light conversion efficiency by
almost 0.5%. Induction of reversible hydrogenase in the heterocysts, as its
theoretically higher turnover principle, is less affected by N2 and O2, and
independent of ATP, it becomes more desirable and needs heterocysts to
be genetically improved.
1.10.2 Photofermentation by photosynthetic
bacteria (example, Rhodospirillium rubrum)
Hydrogen production by photoheterotrophic bacteria is principally similar to that of blue-green algae, capable of fixing nitrogen and producing hydrogen. The microbes are capable of converting large varieties of
organic compounds to carbon dioxide and hydrogen up to 50 kg/(m2 yr).
Practical applications of these bacteria are more of an engineering
problem than one of scientific “know-how.” The scope of newer research
Energy and Its Biological Resources
27
exists on the noncyclic hydrogen production by these microbes uninhibited by nitrogen. Dilute wastes can be utilized by the photosynthetic
bacteria, which is an added advantage over those of the methane fermentors. The conventional fermentation of organic substrates to
methane or hydrogen is theoretically limited to 80% and 20%, and practically to 65% and 15%. The difference is accounted for by the synthesis
of ATP and cell biomass. ATP is produced in presence of light and reactions are driven at its expense, if hydrogen is produced by nitrogenase.
1.10.3
Methane production
The biology of an “oxidation pond” is not well understood. The algae-versusbacterial growth needs to be controlled, and anaerobicity and temperature
need to be maintained properly. The carbonaceous matter tends to ferment,
and methane is produced instead of carbon dioxide. The end product,
methane, can be used either as a direct fuel or through a suitably designed
fuel cell. Microbial methane and hydrogen production are discussed later.
1.11
Plant Hydrocarbons
While a significant number of scientists are assessing the future of
renewable and nonrenewable sources of energy, and their potential usefulness and costs, a few of them are busy exploring existing storehouses
of nature and modifying the renewable resources into direct conventional
fuels. Prof. Melvin Calvin and his group at the University of California at
Berkeley emphasize the importance of a group of plants which, in addition to producing polysaccharide, also produce polyisoprenes (rubber)
and similar associated products [4]. While the Hevea produces rubber,
different euphorbiacea produce polyhydrocarbons that have molecular
weights lower than 10% of that of average natural rubber. It is likely
that chemical manipulation may yield liquid fuels similar to that of
conventional gasoline or diesel out of these products.
The interesting aspect of these plants is that rubber plants demand
good insolation and high moisture content in soil as well as in the atmosphere. But many subspecies of Euphorbia can grow comfortably in sunny
semiarid lands, where standard cultivations are not economically viable
[5]. This leads us to two major considerations: (1) soil conservation, ecologic improvement, and increase in P/R (productivity/respiratory) ratio; (2)
production of hydrocarbon and biomass, both of which have energy value.
Avalois is the North Brazil variety, and Euphorbia tirucalli is the
Southern Californian equivalent of the plant. Both of them usually contain 30% hydrocarbon in their latex. Similar or parallel plants in the
Indian Subcontinent are not yet well known. But like rubber plantation,
which successfully migrated from Brazil to Malaysia, one may try a few
28
Chapter One
TABLE 1.4
Yield of Some Important Crops and Their Biomass Utilization
Approximate composition (%)
Latex
Moisture
Water sol.
Organic matter
high MW
Organic matter
Low MW
Heve
Euphorbia
65
63
3
9
31
27
1
1
Ideal yield of some crops
Crop
MT/ Ha/ Yr
Sugarcane
Sugar beet
Algae
Sorghum
Corn
Eucalyptus
Rubber (Malaysia)
30
33
87
36
13
54
2
Example of chemical diversification of biomass
Sugar cane ⎯⎯⎯⎯→
↓
Bagasse
(Cellulose Lignin)
Fodder
Energy
Cane Juice ⎯⎯⎯→ Ethanol
↓
↓
Butanol
Ethylene
Glycerol
Chemicals
Citric Acid
Ethyl chloride, etc.
Aconitic Acid
species of Euphorbia—particularly on the rocky, arid, or laterite belts,
which are rather unproductive for forestry or cultivation. It is worthwhile to take a glance at some information already available on these
products [6].
1.12
Biogas
Age-old phenomena of spontaneous combustion of natural gas, continuously
or intermittently, were called “will-o-wisp” or “fool’s fire.” Later, these phenomena were assigned to “marsh gas” and mainly methane by H. Tappeiner
(1882) [7]. Almost a century passed, through which different postulates had
to be verified in order to unveil the mechanism behind this natural
methanogenesis or biogas formation. First, one-step microbial degradation
of cellulose to methane was proposed. This was replaced by a two-step concept, where lower-molecular-weight organic acids are produced as intermediates, which further undergo conversion to methane. Finally, the three-step
concept has been prevailing (the entire process is anoxic):
Hydrolytic
fermentive stage
S
Acetogenic stage
sMesophilicd
S
Methane, organic
sThermophilicd
Energy and Its Biological Resources
Organic matter →
Organic matter
Methane
CO2
Acetic acid
H2,CO2
(45C, pH 4–6)
←⎯⎯⎯⎯⎯
→
Alcohols, H2 , CO2
(35C, pH 5–6)
⎯⎯⎯⎯→
29
An oversimplified mass balance may be written as
C6H12O6 → 3CH4 3CO2
The technical values of yield coefficient, biological efficiency, chemical/
biological oxygen demand (COD/BOD), biological efficiency in productivity/
ecologic efficiency rate (BEP/EER) ratios, and so forth are yet to be established for each setup or system. Mostly obligate anaerobes and a few facultative microbes contributing to these conversions belong to different
genera. A few may be mentioned: Actinomyces, Aerobacter, Aeromonas,
Arthrobacter, Bacillus, Bacteroides, Cellulomonas, Citrobacter, Clostridium,
Corynebacterium, Enterobacter, Escherichia, Klebsiella, Lactobacillus,
Laptospira, Micrococcus, Nocardia, Peptococeus, Proteus, Pseudomonas,
Ruminococcus, Sarcina, Staphylococcus, Streptococcus, Streptomyces, and
many others. A few methanogenic species are also known: Methanobacterium bryantii, Methanococcus vanniellii, Methano-genum aggregans, Methanomicro-bium mobile, Methanosarcina barkeri, Methanothrix concillii, usually eukaryotic organisms, and blue-green algae are
incapable of performing such bioconversions [8].
Morphologically, the organisms belong to wide groups: coccus, sarcina
(flower-like), rod, filamentous, and other shapes. G C (guanine-cytosine)
values of DNA of these organisms also suggest that they all have varied
origin and hence are likely to have different metabolic patterns. Khan
(1980) found that Acetivibrio cellulolyticus producing acetic acid and
hydrogen from cellulose are readily utilized by M. Barkeri to produce
methane and carbon dioxide. It has been established beyond doubt that
the process is chemolithotrophic metabolism, favored by strict anaerobic condition, and facilitated by the absence of sulfates, abundance of moisture, approximate temperature range of 25–40C (37C), and pH 6.2–8.0
(pH 6.8). The organic materials on which these organisms survive and
grow are usually cellulose in nature. Crop residues, agricultural residues,
animal excreta, municipal sewage, and other organic materials derived
from terrestrial and aquatic origin are also considered as good substrates. Plant materials with high lignin content are an inferior type of
feed for such reactions.
A pretreatment or partial putrefaction or degradation makes the
process easy. In this respect, animal excreta appear to be a ready-made
substrate. The art of producing gaseous fuel out of cattle excreta is well
30
Chapter One
known in the Indian Subcontinent as the gobargas plant, and will be discussed subsequently.
Sargassum tenerrimum, an abundant variety of marine algae found
on the Indian coast of the Arabian Sea, shows promising results in laboratory experiments by anaerobic digestion. A mixed culture of marine
bacteria and methanogens happens to be a better choice. In a prototype
experiment, the partially treated marine algal biomass mixed with
cattle dung could be the initial feed for a digester. In a mixed culture,
the entire process is a complex one. The organisms which are very efficient in cellulolytic activities degrade higher-carbohydrate materials
into simpler products as lower organic acids, including CO2 and less frequently H2, along with other products, but very seldom show a significant
amount of reduction reactions. In absence of methanogens, they usually
produce H2, CO2 (even CO), formate, acetate, and less favorably other
fatty acids and alcohols. It has been established that many methanogens
utilize NH 4 as their nitrogen source, either H2S or cysteine for their
sulfur requirement, and other growth-stimulating amino acids, vitamins, and some trace minerals.
Uncommon in many other anaerobic organisms, methanogens have
shown presence of a cofactor (coenzyme) named CoM, identified as
HSCH2CH2SO3 (2-mercapto-ethanesulfonic acid), and also another lowmolecular-weight factor called F420, as of yet unidentified. This F420 in
an oxidized state fluoresces at 420 nm but loses all optical activity when
reduced. This compound is neither a ferredoxin nor can it be substituted
by ferredoxin. Another interesting part is its dependence on Co II
(NADP) and it cannot be substituted by Co I (NAD system). Occurrence
of oxidative or substrate-level phosphorylation in methanogens could not
be established, and the presence of quinines or cytochrome b/c systems
could not be observed. The involvement of methylcobalamin also could
not be substantiated. So, a large part of the information is yet to be
derived by the next-generation scientists. It will be useful to summarize
some of the metabolic steps, so far understood (see Fig. 1.10).
The ecologic role of biogas is manifold. Chemical anoxic transformation reduces the BOD value of the organic residues, which in turn are
enriched, proportionately in its C, N, P, and mineral ratios. In lignocellulosics, after the anoxic process, enrichment of lignin occurs and may
lead to peat formation. This may be the origin of coal; natural gas and
coal deposits are likely to be found within a reasonable stretch. This is
a built-in machinery of nature for BOD and pollution control.
1.13
Gobargas
As already mentioned in the preceeding section of biogas, gobargas is
an extended version of the biogas. Usually, when cattle excreta (gobar)
Energy and Its Biological Resources
H2, Mg2+, ATP
CH3 − S − CoM
Methyl reductase
+2H/2e−
CO2 + MH
MCOOH
CH4 + HS − CoM
+2H/2e−
MCHO
–H2O
31
MCH2OH
+2H – H2O
CH3 OH + MH
–H2O
Barker’s pathway
MCH3
+2H
MH + CH4
−2H
−CO2
CH3COOH + MH
CO + MH
OH
|
2e−
O=C–M
H2 O
H
H
|
|
2e−
2e−
CH3−M
O=C–M
HO−C−M
H2 O
|
H
CH4 + MH
Gunsalus pathway
MH (reduced metabolite/reduced coenzyme/reduced enzyme complex)
Figure 1.10 Methanation.
is the starting material for anoxic fermentation to flammable gas, it is
called gobargas. Before a scientific and technical approach was given to
this promising field, the technique was developed in the southern part
of India in a very crude way. Partly dehydrated animal excreta, when
ignited, produces fumes and burn for a short duration with a partially
sooty flame a little above the solid fuel. Slurried excreta, when stored
in closed earthen vessels for a while, produced flammable gas. Based on
these observations, villagers developed techniques of producing gas similar to illicit brewing.
Perhaps the greatest benefits of gobargas projects are secondary in
nature. It takes out the pollution and ecologic problems and yields better
biomass as compost and manure. The primary product, the biogas, has
of course become very important in the present energy perspective. The
fuel value of the gas, though not very high, is relatively safe and pollution free. Out of the many reports available so far, the positive and
encouraging points leading to successful implementation of gobargas
projects are very restricted. The negative points or factors which make
the progress slow down are many, and a few are difficult to overcome.
It may be useful to mention a few of them. These points are by no means
32
Chapter One
insurmountable, but may help us to orient our future course of action,
research, and development.
1. Dehydrated cow dung is a popular fuel and does not need special or
expensive containers for keeping throughout the year.
2. Untended herds make the collection of dung laborious and cost
intensive.
3. Installation of community biogas plants is not easy. Due to the fragmentized small households, individual plants are also difficult to erect.
Most families cannot provide the minimum 50-kg average dung input
to the plant. About 50 L of water should also go with it. Fifty percent
of the settlements are located in drought-prone areas. The remaining
50% face water shortage during the 5 months of dry season.
4. Temperature fluctuations throughout the year are significant and
affect the rate of biogas production.
Disfunctioning and malfunctioning of some of the plants, due to the
lack of proper maintenance and servicing, create poor examples to neighbors. This reduces the fresh installation potentialities and leads to an
unwillingness to invest funds. The increasing cost of installation is
another reason for the negative attitude.
The Chinese use mostly underground designs, and their outlays have
been more successful because they have already undergone a generation of restructured social order. As per Neelakantan’s (1974–1975)
report, the wet-dung yield of a cow is on an average 11.3 kg ( 3.6 to 18.6 kg)
and of a buffalo is 11.6 kg ( 5.0 to 19.4 kg). The daily output of dung from
an average of five cattle (a minimum of four) may suffice for a household with a miniature gobargas plant. When underground ambient conditions (30C), are favorable, at least 2.7 m3 of gas (50 m3/ton of wet dung)
per day is expected out of the plant. This gas has a minimum of 9500
kcal (3500 kcal/m3 ) of heat value (equivalent to 1.5 L of kerosene), which
may serve the daily need of a five-member family. It is estimated that
the average daily requirements of the gas per adult per day are 0.3 m3
for cooking and 0.2 m3 for lighting purposes.
Installation of a 3 m3 digester (gobargas plant), partly embedded in the
earth, or preferably constructed underground, as per improved versions
of several designs, suffices for one standard household (see Fig. 1.11). At
the present cost, it comes to about Rs. 10,000 (approximately US $200),
depending on the remoteness of the house or the community. Attractive
cost figures have been developed by competent engineers and social
workers who have estimated an annual savings to the tune of Rs. 1000
(approximately US $20) per family, and the initial investment is likely to
be paid off within 3 years. The estimated average lifetime of a gobargas
Energy and Its Biological Resources
Inlet
1
3
2
GL
c
33
Gas delivery
Outlet and
vapors
(Above)
4
4
Ground level
(Below)
1. Inlet tank
2. Outlet tank
3. Gas outlet
4. 100 M.M. A.C.
pipe
- Improved design for existing Janata model
5. Partition wall
of biogas plant
Gobar gas plant developed and designed
by Khadi & Village Industries Commission
Figure 1.11 Gobargas plant.
plant is supposed to be 20 years. It is perhaps very important that a semiskilled person or a trained “know-how” person tend to the plant.
Once installed, a 3 m3 digester plant will require about 50–60 kg
(4 buckets) of raw wet cattle dung and an equal amount of water. If the
dung is slurried prior to feeding the digester plant, stirring may not
be needed. Initially, a 15-day incubation is necessary and combustible
gas starts coming out after about 3 weeks, when stabilized, and will
continue to produce a gas mixture which is satisfactorily flammable.
The average retention time of the materials in the digester is 3–7 weeks
(average 5 weeks). The optimal temperature, of course, is 40C
(15–65C) with a pH 6.8 (pH 6.5–7.5). In a small digester (family
unit), control of temperature and pH remains out of bounds for ordinary villagers.
The omnipresent microbial flora in the ruminants will start the reaction, initially at a slow rate. No additional microbial culture is usually
required. The gas is composed mainly of CO2 and methane, and traces
of other gases. Objectionable or harmful gases are very rare. Since a mixture of carbon dioxide is present, the gas is less flammable and hazardous than LPG, but needs sufficient precaution to be handled in the
household. Most of the precautions to be observed in handling and using
bottled gas will also apply in this case. The pipeline from the plant to
the burner needs to be checked occasionally for leaks.
34
Chapter One
Human excreta and other animal excreta are equally useful for the
same purpose. In fact, all such domestic excreta and pulped organic
refuge may be mixed together to enrich the feed to the gobargas plant.
Social practices and inhibitions prevent people from combining the feedstock materials. The common septic tank system can also be modified
in design and be made to deliver biogas. The quantity of human excreta
per family is relatively small, and hence, the gas evolved will hardly
meet even the partial requirement of the family, if the biogas plant is
fed exclusively with night soil.
The disappearing forests and forage have a cyclic relation in the
ecosystem. Rising cost of animal feed of all kinds adds to the crisis.
Keeping of cattle in small village households may not be an attractive
proposal very soon. A major part of the animal dung is not collected by
the owner of the cattle while animals graze. The space required to keep
cattle and have a biogas plant will be considered a poor investment, due
to soaring price of land, even in remote villages. Considering these and
a few more unforeseen factors, better prospects of gobargas plants in a
distant future may not be a correct speculation.
1.14
1.14.1
Biomass, Gasification, and Pyrolysis
Biomass
Imitating the coal-based process, biomass conversion has also been tried
and looks promising. Main sources of biomass are agricultural, horticultural, and forest wastes. Municipal organic solid wastes (which are
also plenty) are potential resources as well. Considering biomass as a
renewable resource, the bioconversion may be pyrolytic, where biogas
and bio-oil are the main products and yet the residue contains some calorie value which can be further utilized (as adsorbents, filter beds, chars,
etc.). Supercritical conversion and superheated steam reformation of biomass are recent techniques. During 1990–1997, quite a few reports
appeared in the literature showing success and promise of catalytic or
uncatalytic reformation of biomass to hydrogen (almost to 18% v/v)
without any char or residues.
Temperature ranges of 340–650C, with pressures of 22–35 MPa, are
cited with as low as 30-s residence time, through supercritical flow reactors. The raw materials are widely varying: water hyacinth, algae,
bagasse, whole biomass, sewage sludge, sawdust, and other effluents rich
in organic matters. In some efficient carbon bed–catalyzed reactors, other
products (i.e., carbon mono- and dioxides and methane) were also detected.
1.14.2
Gasification and pyrolysis
Gasification, an exothermic reaction, yields mostly producer gas, a
mixture of carbon monoxide, hydrogen, and methane at temperatures
Energy and Its Biological Resources
Dumping
ground
for
municipal
refuse
35
Cold fuel gas
Refuse
Ribbon conveyor
Motor
Hot fuel gas
Fuel gas and oil
Dry refusel
Drier
Motor
Vibrating
separator
or
Pyrolysis
Furnace
Big stones
and fine ash
Steam Hot air
Kiln
Quencher
Water
and fuel gas oil
Blowers—hot air
Figure 1.12 Flowchart of refuse processing plant.
above 1000C, mostly in the absence of air. The starting materials may
be any kind of organic matter, preferably waste materials like cotton and
jute sticks, corn cobs, bagasse, and many other plant and vegetation
products. In India, annually 16 million tons of rice husk, 160 million tons
of paddy straw, 2 million tons of jute sticks, and 2.2 million tons of
groundnut shells are available as agricultural by-products.
The gas can be directly used as fuel or used to drive irrigation pump
sets. Several designs are available.
Pyrolysis, a thermochemical conversion, also performed in absence
of air at a temperature of 500–600C, yields gaseous components, hydrocarbons, carbon monoxide, hydrogen, methane, butane, some liquids,
tars, and a little coke, all of which have very high energy content.
Starting materials are similar to those mentioned under gasification.
The vegetable matter in the municipal refuse (as much as 50%) is also
good feed for pyrolysis. Very optimistic economic analysis for the
pyrolytic process has been put forward by investigators, and a properly
designed plant, say capable of handling 250 tons of organic refuse per
day, will be fully paid off at the end of 5 years. There are 20 domestic or
family-size models suggested by organizations. As per the available
information, large-scale use of either gasifier or pyrolyser has not been
noticed so far. But for the municipalities, the responsibility of quick disposal of the refuse and the environmental issues will prompt installation of such plants in the near future. One such flowchart of a model
plant is given in Fig. 1.12.
1.15
Bioluminescence
A typical natural phenomenon, probably a unique mating signal by the
“firefly,” also exists in other living species, namely, bacteria, protozoa,
fungi, and worms, in the forms that emit visible light. In most cases, the
36
Chapter One
nature of the luminescent light varies in color and intensity; but chemical pathways are, to a great extent, common. The chemical products
responsible for giving out different colors are different and are not yet
fully known.
A heat-labile simple protein enzyme luciferase (MW 105) makes a
complex (luciferyl adenylate E) with reduced luciferin, in the presence
2
of ATP (Mg ), which subsequently breaks down into different products
in the presence of molecular oxygen. This results in the excitation of
luciferin to a high-energy state. On return of the same to the ground
state, emission of visible light produces bioluminescence (see Fig. 1.13).
LH2 ATP (Mg2) E → LH2 AMP E PPi
LH2 AMP E O2 → Products Light
The phenomenon appears to be insignificant but a substantial supply
of luciferin, ATP (Mg2), and a little enzyme can deliver an appreciable
luminescence of practical use. Whether luciferin, luciferase, and ATP
may also be harvested from animal resources, or the chemical components may be synthesized economically and the enzyme can be procured from flies, remains a matter of investigation and development.
Like bee-keeping, culture of “fireflies” is very likely to become a profitable art. The dream of producing high voltage by animal tissues, imitating the electric eel, may come true in the near future; the
fundamentals are known, but economic viability is not assured, hence
not discussed here.
N
S
S
N
H
H
S
Y–
HO
Y
C–OH
||
O
H
N
H
C–OH
||
O
LH2 (reduced luciferin)
Luciferin (dehydro)
S
Y–
N
OH
|
C—O — P— O — Ribose-Adenine
||
||
O
O
LH2-Adenylate
Figure 1.13 Firefly bioluminescence.
Energy and Its Biological Resources
1.16
37
Hydrogen
The simplest of the elements, containing a single proton and electron
each, of mass almost unity, is the first member of the periodic table.
Data may vary from different sources; solid at 4.2 K (d 0.089), H has
the atomic number (AN) 1, atomic weight (AW) 1.008 g, melting point
(mp) 259.14C, and boiling point (bp) 252.87C (d 0.071 at 20.4 K).
He has a AN 2, AW 4.0026 g, mp 272.2C (20 atm), and bp 268.93C
(specific gravity 0.124).
Commercial consumption at present is mostly in synthetic fuels, say
from coal, mineral oils, petroleum reformation (refineries), and iron and
copper ore reductions. Hydrogen is very important because of the versatility of its physical, chemical, and biological properties. More importantly for our purposes, is its potential as a source of energy. Hydrogen
liquefies at 33.2 K, 12.8 atm, and 0.03 g/mL and occupies a negligible
volume (22.4 times less), compared to its gaseous state. Solid hydrogen
and helium are academic ideas. When hydrogen combines with oxygen
in a volume ratio of 2:1, heat is generated and the product is water in
a vapor state. The reaction in a vapor state occurs with a reduction of
volume to 1/3 and water vapor to water 1/22, which means the reaction is
favored at a higher pressure; alternatively, the change in volume is
compensated by utilizing some of the heat that evolves. The calculations
are already there. Hydrogen as a combustion fuel or as a material for a
fuel cell is less attractive than the fusion reaction such as that which
occurs in the sun. Taking it as a model, we may be able to harness huge
amounts of thermal and traditional energies, but we should also learn
how to manage and handle such enormous outbursts of energy. Two
protons fuse to yield a deuterium, a positron, and a neutrino; the last
one is the clue to the release of energy that is not yet fully understood
by science;
2H1 → e H2
H2 H1 → H3 2H3 → He4 2H1
Solar constant 1.968 cal/(cm2 min) 3.86 1033 erg/s 1.373 kW/m2;
even at such a long distance, we are unable to use all the energies.
Hydrogen in absence of air or oxygen, or in vacuum, will not burn, but
may have a kind of combustion to produce ammonia in air or nitrogen.
Combustion of hydrogen in our atmosphere does not produce simple
water vapor, but mixture of others, i.e., ammonia and NOxS (nitrogen and
oxygen combine at the vicinity of high temperature generated).
Cryogenic and space research have taught us many more lessons.
Liquid hydrogen can be stored in special containers (cylinders), or transported through pipes, and is almost an ideal fuel for rockets and spaceships, perhaps next to azides. But at higher altitudes or in space, in the
absence of atmosphere, optimal liquid oxygen is also needed to perform
38
Chapter One
the dynamism or thrust. Water vapor is transformed into ice particles
instantly due to the very low temperature in space. Liquid hydrogen for
such research or experiment is generated at a very high cost, i.e., electrolytic splitting of water. The alternate resource of hydrogen is a byproduct in the caustic soda plant. A similar minor and indirect source
of hydrogen is water gas (C H2O → CO H2), almost obsolete for any
large-scale production. None of these examples are renewable in nature,
continue to be energy and labor intensive, and cannot stand as competitors as fuel or energy resources. Other commercial sources of hydrogen are dependent on the existing limited supply of natural resources,
i.e., coal, naphtha, and natural gas, which are not renewable. The materials are mainly based on fluidization or gasification of coal, and reformation by superheated steam or from steam–iron process (3Fe 4H2O
→ F3O4 4H2); these processes can be broadly classified into (a) thermochemical or solar gasification and (b) fast pyrolysis or other novel
gasification. These processes may be totally or partly catalytic. The
basic chemical principles are mostly similar to those of classical water gas:
C H2O → CO H2; CO H2O → CO2 H2. Major sources of hydrogen at present are directly or indirectly natural gas; electrolysis;
pyrolytic, thermal, and superheated steam; or geothermal, solar, ocean
current, ocean thermal gradient, and nuclear reactors. Biomass as a
source of hydrogen as well as energy has been discussed in Sec. 1.2.
1.16.1
Microbial conversion
Many or most organic cellulosic matter, after proper mechanical treatment (homogenizing), can be put to microbial conversion for (a) biomethanation and/or (b) hydrogen production.
1. Biomethanation can utilize human or animal excreta as well as mixed
green/organic wastes. This part has been discussed earlier in Secs. 1.12
and 1.13.
2. Hydrogen production is discussed hereafter.
Biohydrogen.
Major routes are
1. Enzymatic (partly microbial) through microbial routes
2. Klebsiella and Clostridium groups of microbes
3. Different cyanobacteria (blue-green algae)
4. Various photosynthetic bacteria
5. Many aerobes, i.e., bacilli and alkaligenes
6. Facultative groups, i.e., enterobacters, and coli forms
7. Various anaerobes, i.e., rumens, methanogenic, methylotropes, and
clostridia
Energy and Its Biological Resources
39
Enzymatic. Glucose dehydrogenase oxidizes glucose into gluconic acid
and NADPH, which helps the reduction of H by hydrogenase. Glucose
dehydrogenase and hydrogenase are purified from Thermoplasma acidophylium and Pyrococcus furiosus (optimal growth at 59C and 100C,
respectively) (Woodward).
Based on metabolic patterns, the microbial systems may be of four types:
1. Photosynthetic microbes evolving H2 mediated through NADPH
(Nicotine Adenine Dinucleotide Phosphate [Coenzyme II-reduced]) by
photoenergy.
2. Cytochrome systems operating in facultative anaerobes that convert
mainly formates to H2.
3. Cytochrome containing strict anaerobe, Desulfovibrio desulfuricans.
4. Clostridia, micrococci, methanobacteria, and others, without
cytochrome, anaerobic heterotrophs.
Klebsiella oxytocae. ATCC (American Type Culture Collection) 13182
can convert formates to H2 (100%), but only 2 moles of H2 for each mole
of glucose (5%). C. butyricum can convert glycerol to 1,3-propanediol,
butyric acid, 2,3-butanediol, formic acid, and CO2 and H2. Klebsiella
pneumoniae can convert glycerol into 1,3-propanediol, acetic acid, formic
acid, and CO2 and H2. The presence of acetate enhances the production
of butyrates and H2, and less propanediol.
Before discussing cyanobacteria and photosynthetic bacteria, we
should review the basic reactions involved in photosynthesis, i.e., steps
in so-called photophosporylation:
hv
H2O NADP PO4 ADP ⎯→ O2 NADPH H ATP
hv
CO2 NADPH H ATP ⎯→ (HCOH)n NADP ADP PO4
hv
Aerobic: 6CO2 6H2O ⎯→ C6H12O6 6O2
hv
Anaerobic: Isopropanol or H2S CO2 ⎯→ Acetone or S (CH2O)n H2O
Cyanobacteria. Popularly known as blue-green algae, and justifiably
so (they consume CO2 and evolve O2), they are bacteria (absence of
nuclei, mitochondria, chloroplasts, etc.) as well as algae.
Cyanobacteria are oxygenic photoautotrophs, possessing photo I and
II systems. Cyanobacteria have been well studied, and the details of
their physiology and biochemistry are available in reviews and books.
They are held by many scientists as potential sources of chemicals, biochemicals, food, feed, and fuel. Most of them are molecular nitrogen
fixers and possess a nitrogenase system for H2 production. They are
40
Chapter One
found to be symbiotic to cycads, lichens, and so forth. Some are heterocystous, lacking photolysis of water, and produce H2 through the nitrogenase step (when N2 is low). The nonheterocystous species produce H2
at higher efficiency at low N2 and O2 concentrations. Some of the species
favor anoxic and dark conditions, but with the presence of organic substrates. They may even use sulfides as a source of electrons under an
anaerobic environment. They are highly adaptable to a changing environment and are widely found in salty or sweet water, deserts, hot
springs (up to 75C), as well as Antarctica. Some heterocystous
Anabaena exhibit H2 production in an atmosphere of argon and absence
of molecular nitrogen. This was the clue to the knowledge that the
enzyme nitrogenase, the main biocatalyst for molecular nitrogen fixation, is present in cyanobacteria and is the key route of H2 production:
N2 8H 8e 12ATP → 2NH3 H2 12ADP 12Pi
A “reversible hydrogenase” (in photolysis of water, 2H2O → 2H2 O2),
is present in both heterocyst and vegetative cells and produces H2 at a
lower rate than a nitrogenase. An “uptake hydrogenase” also operates
(minor) connected to cytochrome chain, providing both H and electrons. H2 evolution is common, but the photolytic O2 is inhibitory to
nitrogenases, which is protected by other biochemical and structural
alternatives existing in heterocysts.
Large amounts of ATP, which is required for the reaction are generated in the event of photosynthesis and respiration. The electron (reductant) supply in the nitrogenase equation comes from metabolites, i.e.,
amino acids, mainly from carbohydrates (maltose, glucose, fructose,
other pentoses, tetroses, etc.), produced and stored in the vegetative
cells through photo I and II systems.
Nitrogenase Co II, i.e., NADPH (gained through the pentose phosphate
route) happens to be an electron donor through NADP oxidoreductase/
ferredoxin or flovodoxin. Other electron-supplying batteries are also
envisaged.
1. Through uptake hydrogenase–ferredoxin (photoactivated)
2. Through pyruvate–ferredoxin oxidoreductase
3. Reduced ferredoxin from isocitrate dehydrogenase
4. NADH generated in the glycolytic route
Under anaerobic or low aerobic conditions, nitrogenase activity may
exist in vegetative cells, but H2 generation is of poor order.
Photosynthetic bacteria. Hydrogen production is guided by the surplus
of ATP and reductant organic metabolites (carbon sources from the
Energy and Its Biological Resources
41
Krebs cycle) and reduced nitrogen sources (glutamate/aspartate).
Interactions of hydrogenase and nitrogenase may be complementary or
competitive in different species or mutants. Nitrogenase (Mo, Ni, or Fe)
also with mixed isozymes are reported. Some mutants liberate H2 more
efficiently, utilizing DL-malate, D-malate, and L-lactate. Photoautotrophic
growth is found to be less efficient in producing H2 than photoheterotrophic
growth with limited nitrogen in nutrients. Normally, in photosynthetic
bacteria, hydrogenase utilizes the hydrogen as a reductant for CO2 fixation and also for fixing molecular nitrogen. Nitrogenase reduces molecular nitrogen, along with the production of molecular hydrogen at the
expense of almost six stoichiometric equivalents of ATP. This means that
concurrent nitrogenase activity during photosynthesis competitively consumes the ATP that is produced and lowers the CO2-fixing efficiency.
Rhodospirillum and Rhodopseudomonas grow aerobically in the dark.
But Rhodospirillum rubrum growing on glutamate (a nitrogen source)
exhibit good hydrogen release during photosynthesis. Quantitative production of hydrogen has also been observed, growing on acetate, succinate, fumarate, and malate, by photosynthesis, initially in the presence
of limited ammonium salts.
In Rhodopsuedomonas acidophilla, hydrogenase and nitrogenase are
genetically linked. Several species of Rhodospirillaceae can perform
nonnitrogenase-mediated hydrogen production in the absence of light,
using glucose and organic acids including formates. Different strains of
Rhodopseudomonas gelatinus and Rhodobacter sphaerolides exhibit
highly efficient production of hydrogen [90 µL/(h mg) cell] grown in a
glutamate–malate medium.
In some cultures of Rhodopseudomonas capsulata, R. rubrum, and
Rhodomicrobium vannielli, replacement of glutamate by N2 gas
improved productivity of H2 (760 mL/d, 10 days) decreasing a little on
aging. The model of a nozzle loop bioreactor, with immobilized R.
rubrum KS–301 in calcium alginate, initial glucose concentration of
5.4 g/L, 70 h at 30C, showed production of hydrogen 91 mL/h (dilution rate of 0.4 mL/h). Improvement was suggested by using an agar
gel for immobilization.
Aerobes.
1. Bacillus licheniformis isolated from cattle dung showed production
of H2 in mixed culture media. Immobilized on brick dust, the aerobe
maintained H2 production for about 2 months in a continuous system,
with an average bioconversion ratio of 1.5 mole of H2 per mol of
glucose.
2. Alcaligenes eutrophus, when grown on gluconates or fructose anaerobically, produces H2. Hydrogenase directly reduces the coenzyme
42
Chapter One
using hydrogen, and the excess hydrogen is spilled out. Higher concentration of formate reduced hydrogen production.
HCOOH ↔ CO2 H2
Facultative anaerobes.
1. Enterobacter: Enterobacter aerogenes, as an example, can use varied
and mixed nutrients, i.e., glucose, fructose, galactose, mannose, peptones, and salts (pH 4.0, 40C); and may show activity for about a
month in a continuous culture; evolution of hydrogen was about 120
mL/h/L of medium; 0.8 mol/mol of glucose. Accumulation of acetic,
lactic, or succinic acids is likely to cause antimetabolic suppression
in older cultures.
2. Escherichia coli: Anaerobically, it can use formate to produce CO2 and
H2. Carbohydrates as nutrient sources usually end up with mixed
products, i.e., ethanol, acetate, hydrogen, formate, carbon dioxide and
succinate.
Various anaerobes.
1. Ruminococcus albus mostly converts cellulose to CO2, H2, HCOOH,
C2H5OH, CH3, and COOH. Pyruvatelyase may be functional in the
production of H2 (237 mol/mol of glucose). Further details are not
available.
2. P. furiosus (thermophilic archeon) possesses nickel-containing hydrogenase and produces hydrogen using carbohydrate and peptone, at
100C. The metabolic system seems to be uncommon to those of nonthermophiles.
3. Methanobacterium (Methanotrix) soehngenii (methanogens) can grow
on acetate and salts media, but can split formate into hydrogen and
carbon dioxide. M. barkeri, in the presence of bromoethane sulphonate,
has suppressed methane production; instead, hydrogen, carbon dioxide,
carbon monoxide, and water were produced.
4. Methylomonas albus BG8 and Methylosinus trichosporium OB3b
(methylotrophs) used various substrates, i.e., methane, methanol,
formaldehyde, formate, pyruvate, and so forth. But formate was
found to be most useful for production of hydrogen under anaerobic
conditions.
5. C. butyricum, C. welchii, C. pasturianum, C. beljerinscki, and so forth
are very efficient in utilizing different carbohydrate sources and even
effluents to produce hydrogen (see Fig. 1.14). Immobilization of these
cells has also been successful.
Energy and Its Biological Resources
Glucose
Pyruvate
Acetyl CoA
C6H12O6 + 2H2O
a
b
K=
Fd
Products
H2
Formate
NADH
43
H2
(NAD)
2CH3COO + 2CO2 + 4H2
c
d
e
c2d 2e2
ab2
Figure 1.14 Hydrogen production.
Removal of or reducing concentrations of either CO2 or H2 or the combination of both is likely to favor a forward reaction, i.e., to improve production of H2. Attempts to remove CO2 by collecting the evolved gases
through 25% (w) NaOH solution, using E. aerogenes (E 82005), showed
better production of H2, which improved further by enriching nitrogenous nutrients in the culture media—from 0.52 moles of H2 per mole of
glucose, increased to 1.58 moles [9].
Similar attempts are made using E. cloacae and reducing the partial
pressure of H2 during the production of gases, by reducing the operating pressure of the reactor and simultaneous removal of CO2 [through
30% (w/v) KOH], maintaining an anoxic condition by flushing Ar at the
onset [10]; by reducing the operating pressure to 0.5 atm, the molar ratio
of H2 yield per mole of substrate doubled (1.9–3.9). Other technical and
economic benefits were also cited. There are other similar claims of
improved biohydrogen production [11], using altered nutrients (20 g of glucose, 5 g of yeast extract, and 5 g/L of tryptone) and different mutants of
E. aerogenes HU-101. HU-101 and mutants A1, HZ3, and AAY, respectively yielded 52.5, 78, 80, and 101.5 mmol of hydrogen per liter of media.
References
1. R. BuveI, M. J. Allen, and J. P. Massue. Living Systems as Energy Converters,
Amsterdam: North Holland, 1977.
2. H. Baltscheffsky. Origin and Evolution of Biological Energy, Amazon Company: United
Kingdom, 1996.
3. L. A. Kristofferson and V. Bokalders. Renewable Energy Technologies: Their
Applications in Developing Countries. A Study of the Beijer Institute and the Royal
Swedish Academy of Sciences, Oxford, Pergamon: Sweden, 1986.
4. M. Calvin. Photosynthesis as a resource for energy and materials, American Scientist
64, 270–278, 1976.
5. A. Nag and K. Vizaykumar. Environmental Education and Solid Waste Management,
New Age Publisher: New Delhi, India, 2006.
6. Proceedings of Bio-Energy Society (Department of Nonconventional Energy), CGO,
New Delhi, India, October 14–16, 1985.
44
Chapter One
7. J. G. Zeikus, Biology of methanogenic bacteria, Bacteriological Reviews 41, 514–541,
June 1977.
8. R. C. Kuhad and A. Singh. Lignocellulose biotechnology: Current and future prospects,
Critical Reviews in Biotechnology 13, 151–172, 1993.
9. S. Tanisho, M. Kuromoto, and N. Kadokura. Effect of CO2 removal on hydrogen production by fermentation, International Journal of Hydrogen Energy, 23(7), 559–563,
1998.
10. D. Das, B. Mandal, and K. Nath. Improvement of biohydrogen production under
decreased partial pressure of H2 by Enterobacter cloacae; Biotechnology Letters 28,
831–835, 2006.
11. M. Abdul Rahaman, Y. Furutani, Y. Nakashimada, T. Kakizono, and N. Nishio.
Enhanced hydrogen production in altered mixed acid fermentation of glucose by
Enterobacter aerogenes, Journal of Farm and Bioengineering 41(4), 356–363, 1997.
Chapter
2
Photosynthetic Plants as
Renewable Energy Sources
Ahindra Nag and P. Manchikanti
2.1
Introduction
Renewable energy is an energy resource naturally regenerated over a
short time scale derived from the sun (such as thermal, photochemical,
and photoelectric) or from other natural environment effects (geothermal
and tidal energy). It is forecasted that approximately half of the total
resources in the world will be exhausted by 2025. This survey has also
revealed that global warming and climate change are serious issues that
need immediate action. The use of fossil fuels (coal, oil, gas, etc.) contributes significantly to global warming and climate change [1].
Worldwide there is strong support for renewable energy, as proven by a
number of surveys [1, 2]. In 2003, a European Commission survey across
the 15 European Union (EU) countries showed that 69% of the citizens
supported more renewable energy-related research, compared to 13% for
gas, 10% for nuclear fission, 6% for oil, and 5% for coal. Understandably,
due to the inherent recycling nature as well as environmental benefits
involved, renewable sources of energy are the solution for energy management. There is an increased investment globally in such technologies
for not only enhancing the preservation of biological resources but also
for increasing energy efficiency and pollution control [1].
Biomass is one such renewable source of energy. Out of the 1.1 1020
kW heat generated every second by the sun, only 47% (~7 1017 kWh)
reaches the earth’s surface. Solar energy is utilized by conversion to different energy forms such as biomass, wind, or hydropower. Green plants
are only able to effectively use visible light of wavelength falling between
45
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
46
Chapter Two
400 and 700 nm. This photosynthetically active radiation constitutes
about 43% of the total incident solar radiation to produce biomass.
Biomass energy generally involves the utilization of energy contents of
such items as agricultural residues (pulp derived from sugarcane, corn
fiber, rice straw and hulls, and paper trash) and energy crops. So, biomass is a comprehensive term that includes essential forms of matter
derived from photosynthesis or ultimately available as animal waste [2].
The production of energy from plants is not a new idea; wood burning
has been in common use since ancient times. About one-seventh of the
energy used around the world is derived from firewood. Biomass supplies 14% of the world’s primary energy consumption and is considered
to be one of the important renewable resources of the future. With the
increase in population and the demand for resources, demand for biomass is expected to increase rapidly. On average, 38% of the primary
energy resources in developing countries is biomass. In the United
States alone, biomass sources provide about 3% of all the energy consumed. In terms of energy efficiency measures and stabilization of
energy consumption between 2010 and 2020, the European Renewable
Energy Council (EREC) survey estimates that among the various types
of renewable energy resources, biomass-derived energy will be a significant portion of energy used [1]. The survey also reveales that biomass
and biofuels are the top two in terms of employment that they generate. Burning new biomass does not contribute to new CO2 into the
atmosphere as replanting harvested biomass ensures that CO2 is
absorbed and returned for a cycle of new growth [2].
2.2 Mechanism and Efficiency
of Photosynthesis in Plants
In photosynthesis, CO2 from the atmosphere and water from the earth
combine to produce carbohydrates, which are the components of biomass and solar energy that drive this process. When biomass is efficiently utilized, the oxygen from the atmosphere combines with the
carbon in plants to produce CO2 and water (see Fig. 2.1). Typically, photosynthesis converts less than 1% of the available sunlight to be stored
as chemical energy.
The advantages of using plants for renewable energies (fuels and
chemicals) are listed follows:
䊏
Advances in agriculture and forestry technologies have resulted in
increased utilization of land resources for cultivation of energy crops.
䊏
By increasing harvesting of solar energy, there is effective usage of
biomass-based resources.
Photosynthetic Plants as Renewable Energy Sources
Sun
47
Carbon reintroduced
in atmosphere
Atmospheric CO2
Refineries
Trees
Trees
Sequestered carbon petroleum, natural gas
Figure 2.1 Simplified carbon cycle.
䊏
Multiple economic benefits can be derived—for example, sugar can be
used as such for fermentation to alcohol—depending on the market.
䊏
Biomass combustion, unlike fossil fuels, does not contribute to
increased CO2 levels in the atmosphere [2].
䊏
Increased employment opportunities resulting from the above.
While the advantages of using biomass-based energies are apparent, it
is important to note that biomass cannot by itself provide complete replacement of fossil fuels. Hence, it is one of the solutions toward achieving energy
efficiency. Further factors, such as competition for biomass between energy
production and human nutritional needs, as well as the possible environmental effects, must be kept in mind. There are several factors that should
be considered in using plants for the generation of energy; efficiency of
solar energy absorption and conversion, quality of biomass produced, plant
growth, growth under marginal conditions, soil characteristics, and costeffectiveness of production of energy and conversion. We will focus on the
utilization of terrestrial plants for production of renewable energies.
2.3
Photosynthetic Process
There are essentially two types of reactions in photosynthesis: a series of
light-dependent reactions that are temperature independent (or light
reaction) and a series of temperature-dependent reactions that are light
48
Chapter Two
Sunlight
Enzymes
RuDP
Chlorophyll
ADP
Water
O2
ATP
NADPH
Glucose
CO2
NADP
Light reaction
Dark reaction
Figure 2.2 General process of photosynthesis.
independent (or dark reactions). The rate of the light reaction can be
increased by increasing light intensity, and the rate of the dark reaction
can be increased by increasing temperature to a certain extent (see Fig. 2.2).
2.3.1
Hill reaction (light reaction)
The process of formation of CO2 and O2 during photosynthesis is called
the Hill reaction or photolysis of water. This primary photochemical
reaction takes place in the presence of sunlight. The reaction is associated with chlorophyll, and after receiving light energy, the chlorophyll
becomes activated. The steps in the Hill reaction can be summed up in
the following manner:
1. Absorption of light and activation of chlorophyll Radiant
light contains very tiny energized particles called photons or quanta,
which are absorbed by the chlorophyll and it becomes activated.
2. Photolysis Is the dissociation of water molecules by light energy
that have been absorbed by the chlorophyll. The reaction can be represented as
Light energy
4H2O ⎯⎯⎯⎯→ 4H 4OH
Chlorophyll
4H ⎯⎯⎯⎯→ 2H2
H2 NADP → NADPH2 (Hydrogen acceptor)
Recombination
4OH ⎯⎯⎯⎯⎯→ 2H2O O2
Photosynthetic Plants as Renewable Energy Sources
49
3. Photophosphorylation This is the stage of formation of ATP from
ADP
ADP Pi → ATP
2.3.2
Blackman’s reaction (dark reaction)
The dark reaction is independent of light. This reaction is purely enzymatic and is carried out in the stoma portion of the chloroplast. Ribulose-1,
5-diphosphate (RuDP), a pentose phosphate present in plant cells, acts
as the initial acceptor of CO2 and changes thereby into a very unstable
C6. The latter is converted into 3-phosphoglyceric acid (3-PGA), which
is transferred to 3-phosphoglyceraldehyde. For this reaction, ATP and
NADPH2 (produced in the light reaction) are necessary as cofactors.
Three molecules of RuDP combine with three molecules of CO2 to give
rise to six molecules of PGA. Three molecules of RuDP utilized initially
as CO2 acceptors are regenerated by five molecules of phosphoglyceraldehyde through different intermediates like xylulose-5-phosphate and
ribulose-5-phosphate. The only molecule of phosphoglyceraldehyde is
converted into fructose-1,6-diphosphate, which may be transformed into
sucrose and starch through other reactions.
2.3.3
Efficiency of photosynthesis
While there are several factors that affect photosynthetic rate, the three
main factors are light intensity, carbon dioxide level, and temperature.
The net efficiency of photosynthesis is estimated by the net growth of
biosynthesis and the amount used for respiration. The requirements for
achieving high energy conversion are optimal temperature, light, nutrition, leaf canopy, absence of photorespiration, and so forth. Many plant
species can be distinguished by the type of photosynthetic pathway they
utilize. Most plants utilize the C3 photosynthesis route. C3 determines
the mass of carbon present in the plant material. Poplar, willow, wheat,
and most cereals are C3 plant species. Plants such as perennial grass,
Miscanthus, sweet sorghum, maize, and artichoke all use the C4 route
of photosynthesis and accumulate significantly greater dry mass of
carbon than the C3 plants. Advances in crop production, agricultural
techniques, and so forth have led to potential applications in low-cost biomass production with high conversion efficiencies. Further, introduction
of alternative nonfood crops on surplus land and the use of biomass as a
sustainable and environmentally safe alternative make biomass an
attractive renewable energy resource. The potential of biomass energy
derived from forest and agricultural residues worldwide is estimated at
about 30 EJ/yr. For the adoption of biomass as a renewable energy
50
Chapter Two
source, the cultivation of energy crops using fallow and marginal land
and efficient processing methods are vital [3].
In C3 plants,
the pathway for reduction of carbon dioxide to sugar involves the reductive pentose phosphate cycle. This involves addition of CO2 to the pentose
bisphosphate, ribulose-1,5-bisphosphate (RuBP). The enzyme-bound
carboxylation product is hydrolytically split, through an internal oxidationreduction process, into two identical molecules of 3-PGA. An acyl phosphate of this acid is formed by reaction with ATP. This is further reduced
with NADPH. Five molecules of the resulting triose phosphate are converted into three molecules of the pentose phosphate, ribulose 5phosphate. Three molecules of ribulose 5-phosphate are converted with
ATP to give the carbon dioxide acceptor, RuBP, thereby completing the
cycle. When these three RuBP molecules are carboxylated and split into
six PGA molecules and these are reduced to triose phosphate, there is
a net gain of one triose phosphate molecule over the five needed to
regenerate the carbon dioxide acceptor. Triose phosphate is formed in
this cycle and can either be converted into starch for storage of energy
inside the chloroplast, or it can serve its primary function by being
transported out of the chloroplast for subsequent biosynthetic reactions.
In a mature leaf, sucrose is synthesized and exported to the rest of the
plant, thus providing energy and reduced carbon for growth [4]. Wheat,
potato, rice, and barley are examples of C3 plants. A representative C3
cycle is shown in Fig. 2.3.
C3 metabolism in plants and the pentose phosphate pathway.
Light stage
site; thylakoid
membranes in
chlorplast
Dark stage
site: stroma of chloroplast
Starch
ATP
O2
ADP
(+H2O)
Chlorophyll
OH -
H2O
Splitting of
water
Electron
carrier
system
6C
Chlorophyll
Noncyclic
photophosphorylation
Calvin
Cycle
ATP
ADP
H+
Sugar
3C sugar
phosphoglyceraldehyde
(PGAL)
NADPH2
5C ribulose
diphosphate
(RuDP)
2
molecules
3C of
CO2
Phosphoglyceric acid-(PGA)
Figure 2.3 Representation pathways of C3 plant photosynthesis. (With permission from
Oxford University Press.)
Photosynthetic Plants as Renewable Energy Sources
51
In air that contains low carbon dioxide in relation to oxygen, oxygen competes for the carbon dioxide binding site of
the ribulose bisphosphate carboxylase. This is known to set off a process
of photorespiration in plants, and it is believed that the C4 plants have
evolved from such a mechanism. Such plants possess a specialized leaf
morphology called “Krantz anatomy” and a special additional CO2 transport mechanism. This typically overcomes the problem of photorespiration. Such avoidance of photorespiration is known to result in higher
growth rates. The Krantz anatomy is characterized by the fact that the
vascular system of the leaves is surrounded by a vascular bundle, or
bundle-sheath cells, which contain enzymes of the reductive pentose
phosphate cycle. The reduction of CO2 is similar to that of C3 plants,
except that the CO2 for carboxylation of CO2 is derived not from the
stomata but is released in bundle-sheath cells by decarboxylation of a
four-carbon acid (C4 acid). This C4 acid is supplied by the mesophyll cells
that surround the bundle sheath cells. The C4 pathway for the transport
of CO2 starts in a mesophyll cell with the condensation of CO2 and
phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by
phosphoenolpyruvate carboxylase (PEPCase), and the reduction of
oxaloacetate to malate [5]. Figure 2.4 shows the C4 cycle of CO2 fixation
in photosynthesis.
Due to the elimination of the photorespiration process, C4 plants are
proposed to be ideal for increased biomass production especially in marginal conditions. Grasses are suitable for this purpose as they can be
C4 metabolism in plants.
Triose-P
Triose-P
NADP
Triose-P
7b
NADPH
7a
3-PGA
3-PGA
NADP
NADPH
3
3-PGA
Malate
Malate
Malate
Triose-P
NADP
4
OAA
5
OAA
NADPH
2P
10
P
2 ADP
11
ATP
6
PEP
PP1 + AMP
3-PGA
2
Calvin
cycle
P
PEP
HCO2
12
9
CO2
RuBP
1
CO2
ATP + P1
Pyruvate
Pyruvate
Pyruvate
8
Chloroplast
Mesophyll cell
Chloroplast
5
Bundle-sheath cell
Figure 2.4 The C4 cycle of CO2 fixation in photosynthesis. (Source: Häusler et al. [5])
52
Chapter Two
TABLE 2.1 Differences between C3 and C4 Plants
Plant
characteristics
Leaf anatomy
C3 cycle type
Chloroplasts
Carboxylase type
Mesophyll (palisade and
spongy type), no chloroplasts
in bundle-sheath cell
Single-type
Primary (Rubisco)
Primary CO2 acceptor
Primary stable product
Ratio of CO2:ATP:NADPH
Productivity (ton/ha yr)
RuBP
3-phosphoglyceric acid (3-PGA)
1:3:2
~20
C4 cycle type
Krantz anatomy, bundlesheath cell with
chloroplasts
Dimorphic
Primary PEPCase in
mesophyll, Secondary
(Rubisco in bundlesheath cell)
PEP
Oxalocetate (OAA)
1:5:2
~30
grown on a repetitive cropping mode for continuous and maximum
production of biomass. Grasses such as Bermuda grass, Sudan grass,
sugarcane, and sorghum are good candidates for energy generation from
biomass. A comparison of the characteristics of C3 and C4 plants, in
terms of leaf anatomy, is shown in Table 2.1.
2.4
Plant Types and Growing Cycles
Several plants have been proposed to be good sources of energy. These
include woody crops and grasses/herbaceous plants, starch and sugar
crops and oilseeds, fast growing trees such as hybrid poplars, shrubs
such as willows, and so forth. Energy crops can be grown on agricultural
lands not utilized for food, feed, and fiber. Farmers could plant these
crops along the riverbanks, along lakeshores, between farms and natural forests, or on wetlands. These crops could be a good source of alternate income, reducing the risk of fluctuating markets and stabilizing
farm income. Woody plants, herbaceous plants/grasses, and aquatic
plants are different sources for biomass production. The type of biomass
selected determines the form of energy conversion process. For instance,
sugarcane has high moisture content, and therefore, a “wet/aqueous”
bioconversion process, such as fermentation, is the predominant method
of use. For a low-moisture content type such as wood, gasification, pyrolysis, or combustion are the more cost-effective ways of conversion.
Characteristics of an ideal energy crop are mentioned below:
䊏
Low energy input to produce
䊏
Low nutrient requirements
䊏
Tolerance to abiotic and biotic stresses
Photosynthetic Plants as Renewable Energy Sources
䊏
High yield/high conversion efficiency
䊏
Low level of contaminants
53
Energy plantations and cropping are means of growing selected
species of trees or crops that can be harvested in a shorter time for fuel,
energy, and other resources. Each type of popular plant species is discussed in brief, with respect to renewable resources.
It is a fast growing plant for firewood (see Fig. 2.5).
Different species such as Eucalyptus nitens, E. fastigata, and E. globulus are used in many countries such as Australia and Brazil. Eucalyptus,
an exotic species from Australia, is a versatile tree which adopts itself
to a variety of edaphic and climatic conditions. It comes up in different
types of soils and climates varying from tropical to warm temperatures
and with annual rainfall ranging from 400 to 4000 mm. It grows well
in deep, fertile, and well-drained loamy soils with adequate moisture.
A large eucalyptus plantation program has been successfully launched
in Brazil to serve as the feedstock for its methanol plant. Amatayakul
et al. suggest that if eucalyptus wood is used for electricity generation,
the cost of electricity generation would be 6.2 US cents/kWh, and consequently, the cost of substituting a wood-fired plant for a coal-fired
plant and a gas-fired plant would be US $107 and $196 per ton of C,
respectively [6]. Eucalyptus plantations could offer economically attractive options for electricity generation and CO2 abatement.
Euclayptus.
Casuarina. Casuarina is a genus of shrubs and trees of the Casurinacea
family, native to Australia and islands of the Pacific. The species involve
Casuarina equisetifolia Linn. It is a big evergreen tree with a trunk
diameter of 30 cm and height 15 m, and is harvested after 5–7 years (see
Fig. 2.6). The plant fixes nitrogen through symbiotic bacteria and thus
adds fertility to the soil. It is very useful for afforesting sandy beaches
and sand dunes. The wood is used for fuel purposes.
Figure 2.5 Eucalyptus plantation.
54
Chapter Two
Figure 2.6 Casuarina plantation.
Mimosa. Mimosa leucocephala or kubabul is a fast-growing species
known for energy plantation (see Fig. 2.7). It has a very high potential
for nitrogen fixation and can be well adapted to poor soils, drought, and
windstorms. It can fix up to 500 kg of nitrogen per hectare per annum.
It coppices readily, and the sprouts, after harvesting, can grow up to 18 ft
in just 1 year. It is also called the wonder tree. Under irrigated conditions, it can give fodder yields up to 80–100 ton/(ha yr). Three different
Figure 2.7 Mimosa plantation. (Source: Creative Commons.)
Photosynthetic Plants as Renewable Energy Sources
55
varieties of this species (Hawaiian, Cunningham, and Brazilian) are
commonly used for plantations in Hawaiian, Salvador, and Peru. The
Hawaiian and the Cunningham varieties are used for energy plantation
in India and Australia, respectively. A Hawaiian plantation of 1.27
hectares can support a 1-MW power plant. In Brazil and the Philippines,
it is converted into charcoal that has 70% of the heating value of oil.
Charcoal can be used to produce calcium carbide, acetylene, vinyl plastics, pig iron, and ferroalloys. The low silica, ash, and lignin contents and
high cellulose content make this plant good for paper and pulp materials, and also for rayons and cellophanes. It not only gives a prolific fuelwood yield but is also a nutrient-rich fodder for livestock.
Sugarcane (Saccharum officinale) is a hardy plant that can
tolerate poor drainage, can be cultivated as a rotation crop, and can be
maintained for years. It is grown in fertile areas with more than 1000 mm
of rain and an abundant supply of water. The ethanol yields from this
are in the range of 3.8–12 kL/(ha yr) [7].
Sugarcane.
Cassava. Cassava (Manihot esculenta), like sugarcane, is grown in tropical climates with an average rainfall of 1000 mm. As it is relatively
drought resistant, it can withstand lower annual rainfall. It needs to be
grown annually and is difficult to mechanize, and compared to sugarcane, it is less energy efficient. Ethanol yields are estimated in the
range of 0.5–4.0 kL/(ha yr).
Sorghum. Sorghum embraces a wide variety of plant types and, unlike
sugarcane and cassava, is found in the tropical summer rainfall zones.
While it can grow in as little as 200–250 mm annual rainfall, maximum
yields are obtained in a minimum of 500–600 mm rainfall. Compared
to other cereals, it can tolerate high temperatures. Due to its deep root
system and low rate of transpiration, it is exceptionally resistant to
drought. Ethanol yields of stems and grains of sorghum are in the range
of 1.0–5.0 and 2.0–5.0 kL/(ha yr), respectively.
Babassu. Babassu (Orbignya sp.) is a palm popular in Brazil for the
ethanol derived from it. The mesocarp of coconut is the raw material for
ethanol production, with an estimate of 0.24 kL/(ha yr).
Oil-bearing crops. Vegetable oils are the most promising alternatives to
diesel fuel. About 97% of all oil-bearing plants are grown in tropical
and subtropical climates. There has been some research into the use of
plant oils from sunflower, peanut, rapeseed, soybean, and coconut oils
as biofuels in unmodified/slightly modified engines. Seed-based oils are
shown to lead to slightly higher fuel consumption, probably due to their
56
Chapter Two
calorific value [8]. About 14% of the oil supplied in the world market is
palm oil, yielding an average 3.4 ton/(ha yr) of oil [9]. Individual palm
seeds, however, are capable of producing much higher yields. The extraction of palm kernel oil increases fuel oil yields by 10%. Current cultivation is mostly in lowland humid tropics such as Malaysia, West Africa,
and Indonesia. While the conditions to grow coconut palms are similar to
oil palms, the yield potential of coconut palms has not yet been developed to that potential. Soybeans and peanuts are annual leguminous
crops that are used as sources of both oil and protein. Soybeans thrive
best in subtropical climates. The individual varieties differ greatly in
terms of their reaction to the length of a day and normally can be grown
in a limited geographical area. Peanut cultivation requires an ambient
temperature for growth, as less than optimal temperatures are known
to result in poor yields. Due to its deep root system, it is relatively resistant to drought. It is also a suitable crop for mixed cultivation along
with oil palms and corn. In terms of calorific value of seed, oil plants such
as Simmondsia chinensis, Pittosporium resinifreum, Ricinus communis,
Jatropha curcas, and Cucurbita foetidissima are found to be ideal.
Buffalo gourd (Cucurbita foetidissima), a desert-adapted plant, produces high-quality oil and fermented starch. The oil has a high ratio of
unsaturated to saturated fatty acids. Crude protein and fat content in
the whole seeds is 32.9% and 33%, respectively [8]. With a seed yield of
3000 kg/ha and estimated 16% hydrocarbon, about 35 barrels of crude
oil could be produced per hectare, in addition to carbohydrate from
roots, forage from vines, and protein-rich oil cakes. Jojoba (Simmondsia
chinensis) is a shrub that grows naturally in the United States and
Mexico. Its seeds contain about 50% of oil by weight and does not
decrease with long-term storage. The oil is remarkably resistant to
degradation by bacteria, probably because it cannot cleave and metabolize the long-chain esters it contains (mostly hydrocarbons containing
38–44 carbon atoms). Jojoba oil has potential uses as a fuel and chemical feedstock, and can also be used as a replacement for vegetable oils
in foods, hair oils, and cosmetics since it does not become rancid.
Additionally, it can be used as a source of long-chain alcohols for
antifoaming agents and lubricants. The hydrogenated oil is a white,
hard crystalline wax and has potential uses in preparation of floor and
automobile waxes, waxing fruit, impregnating paper containers, and
manufacturing of carbon paper and candles. Physic nut (Jatropha
curcas), a tropical American species, is a large shrub, or a small tree.
The seeds yield 46–58% oil of kernel weight and 30–40% of seed weight.
In trade, this oil is called curcas oil. All parts of the plant exude sticky,
opalescent, acidic, and astringent latex, containing resinous substances.
The bark of this plant is a rich source of tannin (31%) and also yields a
dark-blue dye. Now Jatropha oil, a semidrying oil, is in high demand
Photosynthetic Plants as Renewable Energy Sources
57
for use as biodiesel in Asian countries. It is employed in preparation of
soaps and candles and used as an illuminant and lubricant. In China,
a varnish is prepared by boiling the oil with iron oxide, and in England,
it is used in wool spinning. The oil is used for medicinal purposes for skin
diseases, for rheumatism, as an abortifacient, and it is also effective in
dropsy, sciatica, and paralysis.
Miscanthus. Miscanthus, a thin-stemmed grass, has been identified as
an ideal fuel crop as it gives a high dry-matter yield (see Fig. 2.8). Under
adequate rainfall conditions, light-arable soils give good yield. It has been
found that dark-colored soils produce better yield than light-colored
soils. It has been evaluated as a bioenergy crop in Europe for over 10 years
and is grown in several European countries. Annual harvesting ability,
low mineral content, and good energy yield per hectare are desirable
characteristics. It is propagated as rhizomes planted in double rows
about 75 cm apart, with 175-cm gaps between the rows. While disease
control is not a significant issue, weed control measures are important.
In Germany and Denmark, yields are 13–30 ton/ha for 3- to 10-year-old
plantation [10].
Panicum. Panicum virgatum or switchgrass (see Fig. 2.9) is another
thin-stemmed herb that has been used as a model plant [10]. It is a C4
species, and though it has lower moisture content than wood, it has
similar calorific value. It has been found suitable for the development of
Miscanthus. (Source: www.bluestem.ca/
miscanthusgracillimus.htm. Used with permission.)
Figure 2.8
58
Chapter Two
Figure 2.9 Panicum. (Source: www.biology.missouristate.edu/
Herbarium/Plants. Used with permission.)
ethanol for petrol replacement. The low ash and alkali content makes
it a suitable fuel for combustion.
Switchgrass has been identified to be a good model bioenergy species,
due to its high yield, high nutrient-use efficiency, and broad geographical distribution. Further, it also has good attributes in terms of soil
quality and stability, cover value for wildlife, and low inputs of energy,
water, and agrochemicals. Evaluation of the use of switchgrass with
coal in existing coal-fired boilers and the handling, operation, combustion, and emission characteristics of the co-firing process have been
studied. Switchgrass has supplied up to 10% of the fuel energy input.
In comparison to the use of corn for the source of bioethanol, switchgrass
has been found to generate 15 times more efficiency of energy production, and it is predicted that switchgrass may entail more profits than
conventional crops for a specific area [10].
Hemp. Hemp is a member of the mulberry family that includes mulberry, paper mulberry, and the hop plant (see Fig. 2.10). It has a cellulose content of about 80% and has been grown for the production of
medicinal, nutritional, and chemical production. Hemp is the earliest
recorded plant cultivated for production of textile fiber. It has a low-moisture
content for biomass feedstock [11].
Artocarpus hirsute and Ficus elastica. Stem and leaf samples of A. hirsute
and F. elastica have been evaluated for their potential as a renewable
energy source. Stem and leaf samples of F. elastica and A. hirsute were
evaluated for polyphenol, oil, and hydrocarbon contents. F. elastica
Photosynthetic Plants as Renewable Energy Sources
59
Hemp. (Source: www.greenspirit.com.
Used with permission.)
Figure 2.10
shows the maximum accumulation of protein (24.5%), polyphenol (4.2%),
oil (6.1%), and hydrocarbon (2.0%) contents. The leaf of F. elastica has
been identified to be a good renewable energy source [12].
Latex obtained from C. procera could be hydrocracked to obtain hydrocarbons under severe thermochemical conditions. Instead, biodegradation is a less energy-intensive technique for
latex degradation. Enhancements in the heptane level have been found
in C. procera latex that was subjected to different fungal and bacterial
treatments, compared to those of untreated ones. Nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FTIR)
analyses reveals that the latex has undergone demethylation, dehydrogenation, carboxylation, and aromatization during microbial treatment. Petroleum obtained by hydrotreatment of the biotransformed
latex is proposed to be used as fuel [13]. Some of the important latexbearing plants are Hevea brasiliensis, Euphorbia sp., Parthenium agentatum, Pedilanthus macrocarpus, F. elastica, and Manihot glaziorii.
Several resin-rich plants such as Cappaifera multijuga (diesel tree),
Copaifera langsdorffi, Pinus, Dipterocarpus, Shorea sp., and Pithosporum resiniferum produce prolific terpene and oleoresins, and are as
such very desirable fuel crops. Woody and herbaceous plants have specific growth conditions, depending on the soil type, soil moisture, nutrient content, and sunlight. These factors determine their suitability and
growth rates for specific geographical locations. Cereals such as wheat
and maize, and perennial grasses such as sugarcane have varied yields
Calotropis procera.
60
Chapter Two
with respect to the climatic conditions. Depending on the habitat, plants
differ in their characteristic makeup. Their cell walls have varying
amounts of cellulose, hemicellulose, lignin, and other minor components.
The relative proportion of cellulose and lignin is one of the selection criteria in identifying the suitability of a given plant species as an energy
crop. Herbaceous plants are usually perennial, having a lower proportion
of lignin that binds together with cellulose fibers. Woody plants characterized by slow growth are composed of tightly bound fibers resulting in
their hard external surface. Generally, cellulose is the largest component,
representing about 40–50% of the biomass by weight; the hemicellulose
portion represents 20–40% of the material by weight. Cellulose is a
straight-chain polysaccharide composed of D-glucose units. These units are
joined by -glycosidic linkage between C-1 of one glucose unit and C-4 of
the next glucose unit. The number of D-glucose units in cellulose ranges
from 300–2500. Hemicellulose is a mixture of polysaccharides, composed
almost entirely of sugars—such as glucose, mannose, xylose, and arabinose—and methylglucuronic and galacturonic acids, with an average
molecular weight of <30,000 g. Cellulose is crystalline, strong, and resistant to hydrolysis, whereas hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base.
A complete structure of lignin is not well defined because the lignin
structure itself differs between plant species. Generally, lignin consists
of a group of amorphous, high-molecular-weight, chemically related
compounds. Phenylpropanes, three carbon chains attached to rings of
six carbon atoms, are the building blocks of lignin. These might have one
or two methoxyl groups attached to the rings. Sugar/starch feedstocks,
such as cereals, have been traditionally used in biochemical conversion
of biomass to liquids such as ethanol. High-cellulose content of biomass
is generally more efficient and therefore preferred over the lignin-rich
biomass for conversion of glucose to ethanol. Depending on the end use
and type of bioconversion preferred, the choice of the plant species
varies. In northern Europe, the C3 woody species especially grown on
short rotation coppice, such as willow and poplar, and forestry residues,
are used [14]. In Europe, there is wide interest in the use of oilseed rape
for producing biofuel [15]. Brazil was one of the first countries to begin
large-scale fuel alcohol production from sugarcane.
2.5
Harvesting Plants for Bioenergy
Biomass can be converted into different types of products, including:
1. Electrical/heat energy
2. Transport fuel
3. Chemical feedstock
Photosynthetic Plants as Renewable Energy Sources
61
Woody and herbaceous species are the ones used most often by biomass
researchers and industry. Several parameters are important in the biomass
conversion process. The principal considerations in terms of the material
type are moisture content, calorific value, fixed carbon and volatile proportion, ash/residue content, alkali metal content, and cellulose–lignin
ratio. In a wet-biomass conversion process, the moisture content and
cellulose–lignin ratio is of prime concern, while in a dry-biomass conversion process, it is the alkali metal content and cellulose–lignin ratio. The
Laticiferous plant species of Apocyanaceae, Asclepiadaceae, Convolvulaceae,
and Euphorbiaceae have been analyzed for use as renewable energy
sources. Analysis of oil and hydrocarbon contents of 15 different plant
species tested has revealed that Carissa carandas L., Ceropegia juncea
Roxb., Hemidesmus indicus R. Br., and Sarcostemma brunourianum W. A.
are the most suitable species [16]. In another study, five different plant
species Plumeria alba, C. procera, Euphorbia nerifolia, Nerium indicum,
and Mimusops elengi have been evaluated as potential renewable energy
sources. Whole plants and plant parts (leaf, stem, and bark) have been analyzed for oil, polyphenol, hydrocarbons, crude protein, -cellulose, lignin,
ash, and mineral content. The barks of these plants were identified to have
greater hydrocarbon content than the leaves. Based on the dry-biomass
yields, hydrocarbon content, and other properties, these plant species most
suitable for renewable energy sources have been identified [17]. In a study
conducted on 51 plant species in Tennessee, in the United States, an examination of the oil, polyphenol, hydrocarbon, protein, and ash content reveals
that Lapsana communis yields the maximum oil (6.1% dry, ash-free plant
sample basis). Chrysopsis graminifolia, Solidago erecta, and Verbesina
alternifolia have been identified as rubber-producing species with 0.4–0.7%
hydrocarbon [18].
2.6
Products
Several processes similar to petroleum refining are involved in the conversion of biomass into different products. Biorefineries convert biomass
into different products in different stages. The different stages involved
in the conversion of biomass to products are depicted in Fig. 2.11.
Agricultural
residues and
waste
Enzymatic or
acid
hydrolysis
Biological
Figure 2.11 Different products from biomass.
Solid,
liquid, and
gaseous
products
62
Chapter Two
There has been a tremendous increase in biobased products such as
ethanol, high-fructose syrups, citric acid, monosodium glutamate,
lysine, enzymes, and specialty chemicals worldwide. It is estimated
that in 2000–2006 in United States alone, there will be an increase
in the use of liquid fuels, organic chemicals, and biopolymers from the
current level of ~2%, 10%, and 90% each to 10%, 25%, and 95%, respectively [19].
2.6.1
Gaseous products
In Chap. 1, gasification (pyrolysis) of biomass, biogas, gobargas, hydrogen, and biohydrogen were discussed in detail.
2.6.2
Liquid products
An important renewable energy resource for transportation purposes is
liquid fuel based on plant oils. However, pure plant oils are generally
not suitable for use in modern diesel engines. This can be overcome by
the process of transesterification. The resultant fatty-acid methyl esters
have properties similar to those of diesel and are commonly called
biodiesel. Biodiesel presents several advantages, such as better CO2
balance than diesel, low soot content, reduced hydrocarbon emissions,
and low carcinogenic potential [20]. The specification standards for the
European Union (EU) and the United States are EN14214 and ASTM
D6751, respectively. The EU directive established a minimum content
of 2% and 5.75% biodiesel for all petrol and diesel used in transport by
December 31, 2005, and December 31, 2010, respectively. Biodiesel
refers to the pure oil before blending with diesel fuel. Biodiesel blends
are represented as “BXX,” with “XX” representing the percentage of
biodiesel component in the blend (National Biodiesel Board, 2005) [21].
In the biomass-to-liquid conversion processes, biomass is broken down
into a gaseous constituent and a solid constituent by low-temperature
gasification. The next step involves production of synthetic gas, which
is converted into fuel (termed SunFuel) by the Fischer-Tropsch synthesis process, with downstream fuel optimization by hydrogen after treatment [22]. Ethanol has already been introduced in countries such as
Brazil, the United States, and some European countries. In Brazil, it is
currently produced from sugar and, in the United States, from starch
at competitive prices. Ethanol is currently produced from sugarcane
and starch-containing materials, where the conversion of starch to
ethanol includes a liquefaction step (to make the starch soluble) and a
hydrolysis step (to produce glucose). There are generally two types of
processes for production of bioethanol: the lignocellulosic process and the
starch process. Unlike the starch-based process, the lignocellulosic
process has not been as widely adopted due to techno-economic reasons.
Photosynthetic Plants as Renewable Energy Sources
63
High ethanol yield requires complete hydrolysis of both cellulosic and
hemicellulose with a minimum of sugar dehydration, followed by efficient fermentation of all sugars in the biomass. Certain advantages of
using lignocellulose-based liquid biofuels are that they are evenly distributed across the globe and hence are readily available, less expensive
compared to agricultural feedstock, produced at a lower cost, and have
low net greenhouse gas emissions. Enzymatic processes (essentially
using bacteria, yeasts, or filamentous fungi) have been considered for
lignocellulosic processes. The enzymatic process when coupled with the
fermentation process is known as simultaneous saccharification and
fermentation. This has proved to be efficient in the fermentation of
hexose and pentose sugars [23]. Genencor International (www.genencor.com/) and Novozymes, Inc., (www.novozymes.com) have been awarded
$17 million each by the U.S. Department of Energy with a goal to reduce
the enzyme cost tenfold (www.eere.energy.gov/). The Iogen Corp.
(www.iogen.ca/) demo-plant is the only one that produces bioethanol
from lignocellulose, using the enzymatic hydrolysis process. This plant is
known to handle about 40 ton/day of wheat, oat, barley, and straw and is
designed to produce up to 3 ML/yr of cellulose ethanol. Refer to Chap. 3
for bioethanol preparation, Chap. 6 for boidiesel processing, and Chap. 7
for ethanol and methanol used in engines.
2.6.3
Solid products
Refer to Sec. 1.14, Chap. 1, for more details on biomass. Solid products
fall under the following categories:
1. Direct outcome of photosynthesis: Products from forest, shrubs, agricultures, and aquacultures.
2. Nonphotosynthesis: Mushrooms, animal biomass, indirect from
photofixation.
3. Wastes: Forests and agricultural products.
4. Municipal solid wastes: Not all solid biomass may be suitable for different end uses, i.e., energy production or energy recovery. For example, mushrooms are notably useful as food, feed, or fodder, not
otherwise. Biomass properties are guidelines to further and more
fruitful end uses. The properties depend on the following:
a. Water or moisture content (aqueous/dry)
b. Calorific or combustion value
c. Dry residues/ash content/silicates, and so forth
d. Alkali metal/oxides in the ash
e. Ratio of cellulose/liquid/oils/fats/of other carbonaceous matters
f. Ratio of solid/liquid/volatiles
64
Chapter Two
Direct combustion of biomass for heat generation is the most inefficient
technique in energy economy, heat being the most inefficient of all forms
of energy. The best way to utilize biomass is to recycle biomass for production of other or further biomass, namely, agriculture, horticulture,
aquaculture, poultry, animal farming, and so forth. Randomness is
reduced (low entropy change), and environmental chaos is lessened.
Properties (a), (c), and (d) are significant for farming; (b) and (f) are
important for hydrolytic processes; and (e) is important for biofuels and
biodiesel. All the points are important for fermentations and in biorefineries. Biorefinery has become a new science and technology harmony
for a promising future, which takes care of different aspects of biosafety,
minimizes waste, and maximizes energy efficiency. It is a field of engineering and technology for the future. Biorefinery is a system similar to
that of petroleum in its requirements for producing fuels and chemicals
from biomass. A biorefinery is a capital-intensive project and is based on
a conversion technology process of biomass. Hence, several technologies—
thermochemical, chemical, biochemical, and so forth—are combined to
reduce the overall cost. Fernando et al. suggest an integrated biorefinery
process from bio-oil produced from pyrolysis of biomas (see Fig. 2.12),
Feedstock
handling
Crude oil
refinery
Hydrocracking
Hydrogen
products
Lignin
intermediates
Lignin
products
Pretreatment
Gasification/
pyrolysis
Gas
conditioning
& separation
Enzymatic
hydrolysis of
cellulose
Sugar
intermediates
Sugar
intermediates
Multisugar
fermentation
Fuel
products
Synthesis
Steam and
power
generation
Electricity
coproduct
Ethanol
recovery
Fermentation
for
bioproducts
Lignin
residue
Fuel ethanol
Bioproducts
Figure 2.12 An integrated biorefinery process. (Permission from S. Fernando, Associate
Editor, FPEI—American Society of Agricultural and Biological Engineers (ASABE),
Mississippi State University, USA.)
Photosynthetic Plants as Renewable Energy Sources
65
which will not only produce sugar but also different by-products and
electricity [24]. The process can produce its own power.
Fermentation is equally important. Anaerobic and restricted aerobic
digestion with selected algae species allow us to harvest hydrogen and
clean fuels, without much loss of biomass and with the least amount of
waste products. In an aerobic process, the process is carried out by oxidizing the volatile matter into biodegradable organic fractions of solid
waste. Air acts as a source of oxygen, and aerobic bacteria act as a catalyst. The change occurring during the process may be represented as
Biomass 1 O2 sAerobic bacteriad S CO2 1 H2O 1 Organic manure
Anaerobic digestion is carried out by segregating the nonbiodegradables and the biodegradables at the same time. This may be done manually or mechanically. The smaller pieces of inorganic materials like clay
and sand may be removed by washing the biomass with water. The
washed material is then shredded into a size that will not interfere
with mixing and may be more amenable to bacterial action. The shredded biomass is then mixed with sufficient quantity of water, and slurry
is fed into a digester system. If necessary, nutrients like nitrogen, phosphorus, and potassium have to be added to the digester. The process
involves four groups of bacteria in the digested slurry as follows:
1. Hydrolytic bacteria catabolize carbohydrates, proteins, lipids, and
so forth contained in the biomass to fatty acids, H2, and CO2.
2. Hydrogen-producing acetogenic bacteria catabolize certain fatty acids
and some neutral end products to acetate, CO2, and H2.
3. Homoacetogenic bacteria synthesize acetate, using H2, CO2, and
formate.
4. In the final phase, called the methanogenic phase, methanogenic
bacteria cleave acetate to methane and CO2.
Water acts as a catalytic agent in methane formation. Thus water is
acted upon by enzymes, itself breaking down to hydrogen and oxygen.
Hydrogen is used by microorganisms to reduce CO2 to CH4, while oxygen
oxidizes carbon dioxide, i.e., makes it acidic (H2CO3). In simple terms,
acetate (in presence of CoI) is simultaneously oxidized to CO2 and
reduced to CH4. For details, refer to Chap. 1, methanation, and Baker’s
and Ganzalus pathway. Thus, methane-forming bacteria play an important role in the circulation of substances and energy turnover in nature.
They absorb CO, CO2, and H2 to give hydrocarbon and methane and help
synthesis of their own cell substances. During anaerobic digestion, gas
containing mainly CH4 and CO2 is produced. The gas is known as biogas,
which is used for the generation of electricity or fuel. The residual biomass
66
Chapter Two
comes out of the digester in the form of a slurry, which is separated into
a sludge, which is used as fertilizer and a stream of waste water.
Research is ongoing to produce renewable energies from different plant
sources, which will necessarily dominate the world’s energy supply in
the long-term. Using renewable-energy system technologies will create
employment at much higher rates than any other technologies would [1].
There are economic opportunities for industries and craft jobs through
production, installation, and maintenance of renewable energy systems.
References
1. European Renewable Energy Council (EREC). Integration of Renewable Energy
Sources: Targets and Benefits of Large-Scale Deployment of Renewable Energy Sources,
Workshop—Renewable Energy Market Development, Riga, Latvia, May 2004.
2. J. A. Bassham. Increasing crop production through more controlled photosynthesis,
Science 197, 630–638, 1977.
3. P. McKendry. Energy production from biomass (Part I): Overview of biomass,
Bioresource Technology 83, 37, 2002.
4. A. Nag. Analytical Techniques in Agriculture, Biotechnology and Environmental
Engineering, New Delhi: Prentice-Hall, 2006.
5. R. E Häusler, H.-J. Hirsch, F. Kreuzaler, and C. Peterhänsel. Overexpression of C4-cycle
enzymes, Journal of Experimental Botany 53(369), 591–607, 2002.
6. W. Amatayakul and C. Azaul. Eucalyptus for fossil fuel substitution and carbon
sequestration: The costs of carbon dioxide abatement in Thailand, International
Journal of Sustainable Development 6(3), 359–377, 2003.
7. C. L. Schulze, E. Schnepf, and K. Motbes. Uber die Localisation der Kautschukpartikel
in verschiedenen Typen von Milchróhren. Flora, Abstracts 158, 458–460, 1967.
8. Energy Information Administration. Forecast and analysis of energy data, International
Energy Outlook 2005, Report #: DOE/EIA-0484 (2005): www.eia.doe.gov/oiaf/ieo/oil.html.
9. E. Chlorent and R. P. Overend. Liquid fuels from lignocellulosics, In: Biomass
Regenerable Energy, Hall, D. O. and Overend, R. P. (Eds.), Rochester, UK: John Wiley
& Sons, pp. 257–269, 1987.
10. S. B. McLaughlin, R. Samson, D. Bransby, and A. Wiselogel. Evaluating physical,
chemical, and energetic properties of perennial grasses as biofuels, In: Proceedings
of the Seventh National Bioenergy Conference—Bioenergy ’96, Nashville, TN,
September 15–20, 1996.
11. L. Dewey. Hemp Hurds as Papermaking Material, USDA Bulletin No. 404, US
Government Printing Office, Washington, DC, October 14, 1916.
12. R. Palaniraj and S. C. Sati. Evaluation of of Artocarpus hirsute and Ficus elastica as
renewable source of energy, Indian Journal of Agricultural Chemistry 36(1), 23, 2003.
13. B. K. Behera, M. Arora, and D. K. Sharma. Studies on biotransformation of Calotropis
procera latex–A renewable source of petroleum, value-added chemicals, and products,
Energy Sources 22(9), 781, 2000.
14. Ove Arup and Partners. Monitoring of a Commercial Demonstration of Harvesting and
Combustion of Forestry Wastes, ETSU B/1171-P1, London, UK, 1989.
15. F. Culshaw and C. Butler. A Review of the Potential of Bio-Diesel as a Transport Fuel,
ETSU-R-71, The Stationary Office, London, UK, 1992.
16. T. Sekar and K. Francis. Some plant species screened for energy, hydrocarbons and
phytochemicals, Bioresource Technology 65(3), 257–259, 1998.
17. D. Kalita and C. N. Saikia. Chemical constituents and energy content of some latex
bearing plants, Bioresource Technology 92(3), 219–227, 2004.
18. M. E. Carr and M. O. Bagby. Tennessee plant species screened for renewable energy
sources, Economic Botany 41(1), 78–85, 1987.
19. S. Fernando, C. Hall, and S. Jha. NOx reduction from biodiesel fuels, Energy Fuels 20,
376–382, 2006.
Photosynthetic Plants as Renewable Energy Sources
67
20. G. Vicente, M. Martinez, and J. Aracil. Kinetics of Brassica carinata oil methanolysis,
Energy Fuels 20, 1722–1726, 2006.
21. W. Steiger. Biomass-to-liquid fuels: Energy for future, In: Proceedings of World
Renewable Energy Congress VIII: Linking the World with Renewable Energy, Denver,
co, August 29–September, 2004, 32–33.
22. B. Hahn-Hagerdal, S. Galbe, G. Gorwa-Grauslund, and Z. G. Liden. The fuel of tomorrow from the residues of today, Trends in Biotechnology 24(12), 549–556, 2006.
23. National Biodiesel Board. Biodiesel 101: www.biodiesel.org/resources/biodiesel_basics/
default.shtm.
24. S. Fernando, S. Adhikari, C. Chandrapal, and N. Murali. Biorefineries: Current
status, challenges and future direction, Energy and Fuels 20, 1727–1737, 2006.
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Chapter
3
Bioethanol: Market and
Production Processes
Mohammad J. Taherzadeh and Keikhosro Karimi
3.1
Introduction
Ethanol (C2H5OH) is a clear, colorless, flammable chemical. It has been
produced and used as an alcoholic beverage for several thousand years.
Ethanol also has several industrial applications (e.g., in detergents, toiletries, coatings, and pharmaceuticals) and has been used as transportation fuel for more than a century. Nicholas Otto used ethanol in the
internal combustion engine invented in 1897 [1]. However, ethanol did
not have a major impact in the fuel market until the 1970s, when two
oil crises occurred in 1973 and 1979. Since the 1980s, ethanol has been
a major actor in the fuel market as an alternative fuel as well as an oxygenated compound for gasoline. Ethanol can be produced synthetically
from oil and natural gas, or biologically from sugar, starch, and lignocellulosic materials. The biologically produced ethanol is sometimes
called fermentative ethanol or bioethanol. Application of bioethanol as
fuel has no or very limited net emission of CO2 [2] and is able to fulfill
the Kyoto Climate Change Protocol (1997) to decrease the net emission
of CO2 [3]. In this chapter, the global market and the production of
bioethanol are briefly reviewed.
3.2 Global Market of Bioethanol and
Future Prospects
Ethanol is produced from a variety of feedstocks. Fermentative ethanol
is produced from grains, molasses, sugarcane juice, fruits, surplus wine,
69
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
70
Chapter Three
whey, and some other similar sources, which contain simple sugars and
their polymers. On the other hand, synthetic ethanol is produced from
oil, e.g., through hydration of ethylene:
H2O
Oil → CH2 CH2 (ethylene) ⎯→ CH3CH2OH (ethanol)
(3.1)
Several companies, such as Sasol, SADAF, British Petroleum, and Equistar,
produce synthetic ethanol, with capacities of 100–400 kilotons/yr.
However, the share of synthetic ethanol in world ethanol production was
less than 4% in 2006, down from 7% in the 1990s [4]. Furthermore,
increasing oil price or declining ethanol price can harm the economic
competition of synthetic ethanol production, compared to the fermentative one. Ethylene prices in 2005 rose to US $1000 per ton, while
ethanol values were around US $500 per ton. If we consider the theoretical yield of ethanol from ethylene based on Eq. (3.1) as 1.64 kg/kg,
the price of raw materials was higher than that of the product. In this
case, it is economically feasible to produce biobased plastics through
“bioethylene”:
Fermentation
H2 O
Biomass/crops ⎯⎯⎯⎯→ CH3CH2OH ⎯→ CH2 CH2 → Plastics (3.2)
The global demand for ethylene is around 120 megatons [4]. It can be
considered a new market for ethanol in the future.
The total world ethanol production in 2006 was 49.8 GL (gigaliter)
(39 megatons), where 77% of this production was used as fuel, 8% as
beverage, and 15% in industrial applications [4]. Since 1975, potable
ethanol production has not experienced a major growth, while industrial
ethanol production has experienced growth by about 75%. However,
fuel ethanol production has increased aggressively from less than 1 GL
in 1975 to more than 38 GL in 2006 (see Fig. 3.1).
60
Industrial
Beverage
Fuel
Billion liters
50
40
30
20
10
Year
Figure 3.1 World ethanol production since 1976 [4].
2005
2003
2001
1999
1997
1995
1993
1991
1989
1987
1985
1983
1981
1979
1977
1975
0
Bioethanol: Market and Production Processes
71
There is competition between Brazil and the United States to be the
dominant ethanol producer in the world. So far, Brazil has been the largest
ethanol producer, but the statistics from 2006 imply that the United
States is the largest ethanol producer with 19.1 GL, followed by Brazil
with 16.7 GL. Both countries produced almost identical amounts of
ethanol in 2005 (16.2 and 16.0 GL, respectively). The American continents produced 72% of the world ethanol production (see Fig. 3.2), followed by Asia, Europe, Oceania, and the African continents.
There is tough competition between sugar crops (particularly sugarcane juice and molasses) and starch crops (particularly maize) as feedstock for fuel ethanol production. While sugar crops were the feedstock
for more than 60% of fuel ethanol production at the beginning of the
2000s, its share decreased to 47% in 2006 and starch crops were used
for 53% of fuel ethanol production in the same year.
The world fuel ethanol production is predicted to keep the latest
trend, at least until 2015. In comparison to 2006, ethanol production
by Brazil and the United States is expected to increase by 102% and
93%, respectively. However, total production of the rest of the world is
expected to increase by 585% [4]. Therefore, the world fuel ethanol production is expected to increase to around 100 GL. The main reasons for
this sharp increase in ethanol production and demand in the future
might be [2, 5, 6]:
䊏
Possible increase in oil prices
䊏
Higher demand for liquid fuels in the future
䊏
Decline of the crude oil supply in the future
䊏
Environmental legislation in different countries to encourage using
biofuels
䊏
Production of bioplastic materials from ethanol
Europe 9%
Africa 1%
Oceania 4%
Asia 14%
Americas
72%
Figure 3.2 World ethanol production in 2006 divided by continents [4].
72
Chapter Three
3.3 Overall Process of Bioethanol
Production
The process of ethanol production depends on the raw materials used.
A general simplified representation of these processes is shown in Fig. 3.3,
and a brief description of different units of the process is presented in
the rest of this chapter. It should be noted that if sugar substances,
such as molasses and sugarcane juice, are used as raw materials, then
milling, pretreatment, hydrolysis, and detoxification are not necessary.
Milling, liquefaction, and saccharification processes are usually necessary for production of fermentable sugar from starchy materials, while
milling, pretreatment, and hydrolysis are typically used for ethanol production from lignocellulosic materials. Furthermore, a detoxification
unit is not always considered, unless a toxic substrate is fed to the bioreactors.
Raw materials
Milling &
size reduction
Large polymer structures
Pretreatment or
liquefaction
Small polymers
Hydrolysis or
Solid residuals
saccharification
Sugar solution
Detoxification
Fermentation
Yeast
Fermented solution
Distillation
Ethanol (~90–95%)
Soild separation,
evaporation, drying
Dehydration
Dried
by-products
Wastewater
Ethanol (>99%)
Figure 3.3 A general process scheme for ethanol production
from different raw materials.
Bioethanol: Market and Production Processes
73
3.4 Production of Sugars
from Raw Materials
Sugar substances (such as sugarcane juice and molasses), starchy materials (such as wheat, corn, barley, potato, and cassava), and lignocellulosic materials (such as forest residuals, straws, and other agricultural
by-products) are being considered as the raw materials for ethanol production. The dominating sugars available or produced from these popular raw materials are
䊏
Glucose, fructose, and sucrose in sugar substances
䊏
Glucose in starchy materials
䊏
Glucose from cellulose and either mannose or xylose from hemicellulose of lignocellulosic materials
Most ethanol-producing microorganisms can utilize a variety of hexoses such as glucose, fructose, galactose, and mannose, and a limited
number of disaccharides such as sucrose, lactose, cellobiose, and maltose,
and rarely their polymers. Therefore, it is necessary to convert the complex polysaccharides, such as cellulose and starch, to simple sugars or
disaccharides. Different types of substrates that need treatment are
presented in Table 3.1, prior to fermentation.
In this section, sugar production from starchy materials is discussed;
lignocellulosic materials are discussed in Sec. 3.5.
3.4.1 Sugar solution from starchy
materials
There are various raw materials that contain starch and are suitable
for ethanol production. Corn is the most widely used on an industrial
scale for this purpose. However, there are several other cereals, such as
wheat, rye, barley, and sorghum, and crop roots such as potato and cassava, which are used as raw materials for ethanol production. The cereals
contain about 60–70% starch, 8–12% proteins, 10–15% water, and small
TABLE 3.1
Treatment for Different Types of Substrates
Substrate
Potential sugar
substrates
Starchy materials
Lignocellulose
materials
Pretreatment
or liquefaction
Hydrolysis or
saccharification
Detoxification
No
No
Typically no
Yes
Yes
Yes
Yes
No
Depends on the
hydrolysis method
74
Chapter Three
amounts of fats and fibers. The compositions of the crop roots are almost
identical to those of the cereals on a dry basis, but the water content of
the roots is usually 70–80%. The exact composition of each raw material depends on the type and variety of materials used and can be found
in literature (e.g., [7]). Starch from these materials is used as a carbon
and energy source, and part of the proteins as a nitrogen source, by the
microorganisms.
Starch contains two fractions: amylose and amylopectin. Amylose,
which typically constitutes about 20% of starch, is a straight-chain
polymer of ␣-glucose subunits with a molecular weight that may vary
from several thousands to half a million. Amylose is a water-insoluble
polymer. The bulk of starch is amylopectin, which is also a polymer of
glucose. Amylopectin contains a substantial number of branches in
the molecular chains. Branches occur from the ends of amylose segments, averaging 25 glucose units in length. Amylopectin molecules are
typically larger than amylose, with molecular weights ranging up to
1–2 Mg. Amylopectin is soluble in water and can form a gel by absorbing water.
For ethanol production, hydrolysis is necessary for converting starch
into fermentable sugar available to microorganisms. Traditional conversion of starch into sugar monomers requires a two-stage hydrolysis
process: liquefaction of large starch molecules to oligomers, and saccharification of the oligomers to sugar monomers. This hydrolysis may
be catalyzed by acid or amylolytic enzymes.
3.4.2
Acid hydrolysis of starch
Acid hydrolysis is an old process still applied in some ethanol industries.
Sulfuric acid is the most commonly applied acid in this process, where
starch is converted to low-molecular-weight dextrins and glucose [8].
Main advantages of this process are rapid hydrolysis and less cost for
catalyst, compared to the enzymatic hydrolysis. However, the acid
processes possess drawbacks including (a) high capital cost for an acidresistant hydrolysis reactor, (b) destruction of sensitive nutrients such
as vitamins present in raw materials, and (c) further degradation of
sugar to hydroxymethylfurfural (HMF), levulinic acid, and formic acid,
which lowers the ethanol yield and inhibits the fermentation process [9].
The acid hydrolysis process can be performed either in batch or in continuous systems. Dilute-acid hydrolysis can also be used as a pretreatment for enzymatic hydrolysis. It is common to soak the starch or starchy
materials in the dilute acid prior to enzymatic hydrolysis, then to continuously pass it through a steam-jet heater into a cooking tube (called
a jet cooker or mash cooker) with a plug flow residence time for a couple
of minutes, and then subject it to enzymatic hydrolysis.
Bioethanol: Market and Production Processes
3.4.3
75
Enzymatic hydrolysis of starch
Enzymatic hydrolysis has several advantages compared to acid hydrolysis. First, the specificity of enzymes allows the production of sugar
syrups with well-defined physical and chemical properties. Second,
milder enzymatic hydrolysis results in few side reactions and less
“browning” [8]. Different types of enzymes involved in the enzymatic
hydrolysis of starch are -amylase, ␤-amylase, glucoamylase, pulluanases, and isoamylases. The mechanism of action of these enzymes is
presented schematically in Fig. 3.4.
There are two popular industrial processes from starch materials,
dry milling and wet milling. In the dry-milling process, grain is first
ground into flour and then processed without separation of the starch
from germ and fiber components. In this method, the mixture of starch
and other components is processed. Starch is converted to sugar in two
stages: liquefaction and saccharification, by adding water, enzymes,
and heat (enzymatic hydrolysis). Dry-milling processes produce a coproduct, distillers’ dried grains with solubles (DDGS), which is used as an
animal-feed supplement. Without the revenues from that coproduct,
ethanol from dry-milled corn processing would not be economically
favorable [2]. A dry-milling process for alcohol production processes
the whole grain, or components derived from the whole grain. Saccharification and fermentation of dry-milled corn result in ethanol and distillers’ dried grains (DDG). When DDG are combined with fermentation
liquids and dried, they result in DDGS as the major feed by-product [10].
Reducing
end
a-amylase
b-amylase
Glucoamylase
Pulluanases and isoamylases
Figure 3.4 Mechanism of action of amylase on starch.
76
Chapter Three
In the wet-milling process, grain is steeped and separated into starch,
germ, and fiber components. Wet milling is capital intensive, but it generates numerous coproducts that help to improve the overall production
economics [2]. Wet mills produce corn gluten feed, corn gluten meal, corn
germ, and other related coproducts. In this method, after the grain is
cleaned, it is steeped and then ground to remove the germ. Further grinding, washing, and filtering steps separate the fiber and gluten. The starch
that remains after these separation steps is then broken down into fermentable sugars by the addition of enzymes in the liquefaction and saccharification stages. The fermentable sugars produced are then subjected
to fermentation for ethanol production, like the other fermentable sugars.
3.5 Characterization of Lignocellulosic
Materials
Lignocellulosic materials predominantly contain a mixture of carbohydrate polymers (cellulose and hemicellulose) and lignin. The carbohydrate
polymers are tightly bound to lignin mainly through hydrogen bonding,
but also through some covalent bonding. The contents of cellulose, hemicellulose, and lignin in common lignocellulosic materials are listed in
Table 3.2. Different types of carbohydrates (glucan, xylan, galactan,
arabinan, and mannan), lignin, extractive, and ash content of many lignocellulosic materials have been analyzed and are available in the literature [2, 11–14] (see Table 3.2).
3.5.1
Cellulose
Cellulose is the main component of most lignocellulosic materials.
Cellulose is a linear polymer of up to 27,000 glucosyl residues linked by
␤-1,4 bonds. However, each glucose residue is rotated 180 relative to
Contents of Cellulose, Hemicellulose, and Lignin in Common
Lignocellulosic Materials
TABLE 3.2
Lignocellulosic
materials
Cellulose
(%)
Hemicellulose
(%)
Lignin
(%)
Hardwood stems
Softwood stems
Corn cobs
Wheat straw
Rice straw
Sugarcane bagasse
Leaves
Paper
Newspaper
Waste paper from
chemical pulps
Grasses
40–75
30–50
45
30
32–47
40
15–20
85–99
40–55
60–70
10–40
25–40
35
50
19–27
24
80–85
0
25–40
10–20
15–25
25–35
15
15
5–24
25
0
0–15
18–30
5–10
25–40
25–50
10–30
Bioethanol: Market and Production Processes
77
its neighbors so that the basic repeating unit is in fact cellobiose, a
dimer of a two-glucose unit. As glucose units are linked together into
polymer chains, a molecule of water is lost, which makes the chemical
formula C6H10O5 for each monomer unit of “glucan.” The parallel
polyglucan chains form numerous intra- and intermolecular hydrogen
bonds, which result in a highly ordered crystalline structure of native
cellulose, interspersed with less-ordered amorphous regions [15, 16].
3.5.2
Hemicellulose
Hemicelluloses are heterogeneous polymers of pentoses (e.g., xylose and
arabinose), hexoses (e.g., mannose, glucose, and galactose), and sugar
acids. Unlike cellulose, hemicelluloses are not chemically homogeneous.
Hemicelluloses are relatively easily hydrolyzed by acids to their monomer
components consisting of glucose, mannose, galactose, xylose, arabinose,
and small amounts of rhamnose, glucuronic acid, methylglucuronic acid,
and galacturonic acid. Hardwood hemicelluloses contain mostly xylans,
whereas softwood hemicelluloses contain mostly glucomannans. Xylans
are the most abundant hemicelluloses. Xylans of many plant materials
are heteropolysaccharides with homopolymeric backbone chains of 1,4linked ␤-D-xylopyranose units. Xylans from different sources, such as
grasses, cereals, softwood, and hardwood, differ in composition. Besides
xylose, xylans may contain arabinose, glucuronic acid or its 4-O-methyl
ether, and acetic, ferulic, and p-coumaric acids. The degree of polymerization of hardwood xylans (150–200) is higher than that of softwoods
(70–130) [14, 15].
3.5.3
Lignin
Lignin is a very complex molecule. It is an aromatic polymer constructed
of phenylpropane units linked in a three-dimensional structure.
Generally, softwoods contain more lignin than hardwoods. Lignins are
divided into two classes, namely, “guaiacyl lignins” and “guaiacylsyringyl lignins.” Although the principal structural elements in lignin
have been largely clarified, many aspects of their chemistry remain
unclear. Chemical bonds have been reported between lignin and hemicellulose, and even cellulose. Lignins are extremely resistant to chemical and enzymatic degradation. Biological degradation can be achieved
mainly by fungi, but also by certain actinomycetes [15, 17].
3.6 Sugar Solution from Lignocellulosic
Materials
There are several possible ways to hydrolyze lignocellulose (see Fig. 3.5).
The most commonly applied methods can be classified into two groups:
chemical hydrolysis and enzymatic hydrolysis. In addition, there are
78
Chapter Three
Cellulose
Lignocellulosic
materials
Hemicellulose
Hydrolysis
Hydrolysis
Glucose
Xylose,
mannose, etc.
Ethanol
Hexose
fermentation
Pentose
and hexose
fermentation
Ethanol
Lignin
Figure 3.5 Overall view of ethanol production from lignocellulosic materials.
some other hydrolysis methods in which no chemicals or enzymes are
applied. For instance, lignocellulose may be hydrolyzed by ␥-ray or
electron-beam irradiation, or microwave irradiation. However, those
processes are commercially unimportant [15].
3.6.1 Chemical hydrolysis
of lignocellulosic materials
Chemical hydrolysis involves exposure of lignocellulosic materials to a
chemical for a period of time, at a specific temperature, and results in
sugar monomers from cellulose and hemicellulose polymers. Acids are
predominantly applied in chemical hydrolyses. Sulfuric acid is the most
investigated acid, although other acids such as hydrochloric acid (HCl)
have also been used. Acid hydrolyses can be divided into two groups:
concentrated-acid hydrolysis and dilute-acid hydrolysis [18].
Concentrated-acid hydrolysis. Hydrolysis of lignocellulose by concentrated sulfuric or hydrochloric acids is a relatively old process.
Concentrated-acid processes are generally reported to give higher sugar
and ethanol yield, compared to dilute-acid processes. Furthermore, they
do not need a very high pressure and temperature. Although this is a
successful method for cellulose hydrolysis, concentrated acids are toxic,
corrosive, and hazardous, and these acids require reactors that are
highly resistant to corrosion. High investment and maintenance costs
have greatly reduced the commercial potential for this process. In addition, the concentrated acid must be recovered after hydrolysis to make
the process economically feasible. Furthermore, the environmental
impact strongly limits the application of hydrochloric acid [12, 15].
Dilute-sulfuric acid hydrolysis is a favorable
method for either the pretreatment before enzymatic hydrolysis or the
conversion of lignocellulose to sugars. This pretreatment method gives
high reaction rates and significantly improves enzymatic hydrolysis.
Dilute-acid hydrolysis.
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79
Depending on the substrate used and the conditions applied, up to 95%
of the hemicellulosic sugars can be recovered by dilute-acid hydrolysis
from the lignocellulosic feedstock [2, 13]. Of all dilute-acid processes, the
processes using sulfuric acid have been the most extensively studied.
Sulfuric acid is typically used in 0.5–1.0% concentration. However, the
time and temperature of the process can be varied. It is common to use
one of the following conditions in dilute-acid hydrolysis:
䊏
Mild conditions, i.e., low pressure and long retention time
䊏
Severe conditions, i.e., high pressure and short retention time
In dilute-acid hydrolysis, the hemicellulose fraction is depolymerized at
temperatures lower than the cellulose fraction. If higher temperature or
longer retention times are applied, the monosaccharides formed will be further hydrolyzed to other compounds. It is therefore suggested that the
hydrolysis process be carried out in at least two stages. The first stage is
carried out at relatively milder conditions during which the hemicellulose
fraction is hydrolyzed, and a second stage can be carried out by enzymatic
hydrolysis or dilute-acid hydrolysis, at higher temperatures, during which
the cellulose is hydrolyzed [13]. These first and second stages are sometimes called “pretreatment” and “hydrolysis,” respectively.
Hydrolyzates of first-stage dilute-acid hydrolysis usually consist of
hemicellulosic carbohydrates. The dominant sugar in the first-stage
hydrolyzate of hardwoods (such as alder, aspen, and birch) and most agricultural residues such as straw is xylose, whereas first-stage hydrolyzates
of softwoods (e.g., pine and spruce) predominantly contain mannose.
However, the dominant sugar in the second-stage hydrolyzate of all lignocellulosic materials, either by enzymatic or dilute-acid hydrolysis, is glucose, which originates from cellulose.
Detoxification of acid hydrolyzates. In addition to sugars, several by-products
are formed or released in the acid hydrolysis process. The most important by-products are carboxylic acids, furans, and phenolic compounds
(see Fig. 3.6).
Mannan
Mannose
HMF
Acids
Xylan
Xylose
Furfural
Acids
Glucan
Glucose
HMF
Acids
Hemicellulose
Cellulose
Lignin
Acetyl groups
Phenolic Compounds
Acetic acid
Figure 3.6 Formation of inhibitory compounds from ligno-
cellulosic materials during acid hydrolysis.
80
Chapter Three
Acetic acid, formic acid, and levulinic acid are the most common carboxylic acids found in hydrolyzates. Acetic acid is mainly formed from
acetylated sugars in the hemicellulose, which are cleaved off already at
mild hydrolysis conditions. Since the acid is not further hydrolyzed, formation of acetic acid is dependent on the temperature and pressure of
dilute-acid hydrolysis, until the acetyl groups are fully hydrolyzed.
Therefore, the acetic acid yield in the hydrolysis does not significantly
depend on the severity of the hydrolysis process [13, 19].
Furfural and HMF are the only furans usually found in hydrolyzates in
significant amounts. They are hydrolysis products of pentoses and hexoses,
respectively [13]. Formation of these by-products is affected by the type
and size of lignocellulose, as well as hydrolysis variables such as acid type
and concentration, pressure and temperature, and the retention time.
A large number of phenolic compounds have been found in hydrolyzates.
However, reported concentrations are normally a few milligrams per liter.
This could be due to the low water solubility of many of the phenolic compounds, or a limited degradation of lignin during the hydrolysis process.
Vanillin, syringaldehyde, hydroxybenzaldehyde, phenol, vanillic acid, and
4-hydroxybenzoic acid are among the phenolic compounds found in diluteacid hydrolyzates [18].
Biological (e.g., using enzymes peroxidase and laccase), physical (e.g.,
evaporation of volatile fraction and extraction of nonvolatile fraction by
diethyl ether), and chemical (e.g., alkali treatment) methods have been
employed for detoxification of lignocellulosic hydrolyzates [20, 21].
Detoxification of lignocellulosic hydrolyzates by overliming is a
common method used to improve fermentability [22–25]. In this method,
Ca(OH)2 is added to hydrolyzates to increase the pH (up to 9–12) and
keep this condition for a period of time (from 15 min up to several days),
followed by decreasing the pH to 5 or 5.5. Recently, it has been found
that time, pH, and temperature of overliming are the effective parameters in detoxification [26]. However, the drawback of this treatment is
that part of the sugar is also degraded during the overliming process.
Therefore, it is necessary to optimize the process to achieve a fermentable hydrolyzate without any loss of the sugar [21, 26].
3.6.2 Pretreatment prior to enzymatic
hydrolysis of lignocellulosic materials
Native (indigenous) cellulose fractions of cellulosic materials are recalcitrant to enzymatic breakdown, so a pretreatment step is required to
render them amenable to enzymatic hydrolysis to glucose. A number of
pretreatment processes have been developed in laboratories, including:
䊏
Physical pretreatment—mechanical comminution, irradiation, and
pyrolysis
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81
䊏
Physicochemical pretreatment—steam explosion or autohydrolysis,
ammonia fiber explosion (AFEX), SO2 explosion, and CO2 explosion
䊏
Chemical pretreatment—ozonolysis, dilute-acid hydrolysis, alkaline
hydrolysis, organosolvent process, and oxidative delignification
䊏
Biological pretreatment
However, not all of these methods may be technically or economically
feasible for large-scale processes. In some cases, a method is used to
increase the efficiency of another method. For instance, milling could be
applied to achieve better steam explosion by reducing the chip size.
Furthermore, it should be noticed that the selection of pretreatment
method should be compatible with the selection of hydrolysis. For example, if acid hydrolysis is to be applied, a pretreatment with alkali may
not be beneficial [18]. Pretreatment methods have been reviewed by
Wyman [2] and Sun and Cheng [12].
Among the different types of pretreatment methods, dilute-acid, SO2,
and steam explosion methods have been successfully developed for pretreatment of lignocellulosic materials. The methods show promising
results for industrial application. Dilute-sulfuric acid hydrolysis is a
favorable method for either pretreatment before enzymatic hydrolysis
or conversion of lignocellulose to sugars.
3.6.3 Enzymatic hydrolysis
of lignocellulosic materials
Enzymatic hydrolysis of cellulose and hemicellulose can be carried out
by highly specific cellulase and hemicellulase enzymes (glycosyl hydrolases). This group includes at least 15 protein families and some subfamilies [15, 27]. Enzymatic degradation of cellulose to glucose is
generally accomplished by synergistic action of three distinct classes of
enzymes [2]:
䊏
1,4-␤-D-glucan-4-glucanohydrolases or Endo-1,4-␤-glucanases, which
are commonly measured by detecting the reducing groups released
from carboxymethylcellulose (CMC).
䊏
Exo-1,4-␤-D-glucanases, including both 1,4-␤-D-glucan hydrolases and
1,4-␤-D-glucan cellobiohydrolases. 1,4-␤-D-glucan hydrolases liberate
D-glucose and 1,4-␤-D-glucan cellobiohydrolases liberate D-cellobiose.
䊏
␤-D-glucoside glucohydrolases or ␤-D-glucosidases, which release Dglucose from cellobiose and soluble cellodextrins, as well as an array
of glycosides.
There is a synergy between exo–exo, exo–endo, and endo–endo enzymes,
which has been demonstrated several times.
82
Chapter Three
Substrate properties, cellulase activity, and hydrolysis conditions (e.g.,
temperature and pH) are the factors that affect the enzymatic hydrolysis of cellulose. To improve the yield and rate of enzymatic hydrolysis,
there has been some research focused on optimizing the hydrolysis
process and enhancing cellulase activity. Substrate concentration is one
of the main factors that affect the yield and initial rate of enzymatic
hydrolysis of cellulose. At low substrate levels, an increase of substrate
concentration normally results in an increase of the yield and reaction
rate of the hydrolysis. However, high substrate concentration can cause
substrate inhibition, which substantially lowers the rate of hydrolysis,
and the extent of substrate inhibition depends on the ratio of total substrate to total enzyme [12].
Increasing the dosage of cellulases in the process to a certain extent
can enhance the yield and rate of hydrolysis, but would significantly
increase the cost of the process. Cellulase loading of 10 FPU/g (filter
paper units per gram) of cellulose is often used in laboratory studies
because it provides a hydrolysis profile with high levels of glucose yield
in a reasonable time (48–72 h) at a reasonable enzyme cost. Cellulase
enzyme loadings in hydrolysis vary from 5 to 33 FPU/g substrate, depending on the type and concentration of substrates. ␤-glucosidase acts as a
limiting agent in enzymatic hydrolysis of cellulose. Adding supplemental
␤-glucosidase can enhance the saccharification yield [28, 29].
Enzymatic hydrolysis of cellulose consists of three steps [12]: (1) adsorption of cellulase enzymes onto the surface of cellulose, (2) biodegradation of cellulose to simple sugars, and (c) desorption of cellulase.
Cellulase activity decreases during hydrolysis. Irreversible adsorption
of cellulase on cellulose is partially responsible for this deactivation.
Addition of surfactants during hydrolysis is capable of modifying the cellulose surface property and minimizing the irreversible binding of cellulase on cellulose. Tween-20 and Tween-80 are the most efficient
nonionic surfactants in this regard. Addition of Tween-20 as an additive
in simultaneous saccharification and fermentation (SSF) at 2.5 g/L has
several positive effects in the process. It increases the ethanol yield,
increases the enzyme activity in the liquid fraction at the end of the
process, reduces the amount of enzyme loading, and reduces the required
time to attain maximum ethanol concentration [30].
3.7
Basic Concepts of Fermentation
The general reaction for ethanol production during fermentation is
Microorganisms
Sugar(s) ⎯⎯⎯⎯⎯→ Ethanol By-products
In this reaction, the microorganisms work as a catalyst.
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83
3.8 Conversion of Simple Sugars
to Ethanol
Conversion of simple hexose sugars, such as glucose and mannose, in
fermentation into ethanol can take place anaerobically as follows:
Microorganisms
C6H12O6 (Hexoses) ⎯⎯⎯⎯⎯→ 2C2H5OH (ethanol) 2CO2
If the entire sugar is converted into ethanol according to the above reaction, the yield of ethanol will be 0.51 g/g of the consumed sugars, meaning that from 1.0 g of glucose, 0.51 g of ethanol can be produced. This
is the theoretical yield of ethanol from hexoses. However, the ethanol
yield obtained in fermentation does not usually exceed 90–95% of the
theoretical yield, since part of the carbon source in sugars is converted
to biomass of the microorganisms and other by-products such as glycerol
and acetic acid [9, 31].
A similar reaction for anaerobic conversion of pentoses, such as xylose
to ethanol, might be considered. Xylose is generally converted first to
xylulose by a one-step reaction catalyzed by xylose isomerase (XI) in
many bacteria, or by a two-step reaction through xylitol in yeasts and
fungi. It can then be converted to ethanol anaerobically through a pentose
phosphate pathway (PPP) and glycolysis. The general reaction can be
written as
Microorganisms
3C5H10O5 (Pentoses) ⎯⎯⎯⎯⎯→ 5C2H5OH (Ethanol) 5CO2
In this case, we can expect a theoretical ethanol yield of 0.51 g/g from
xylose, as we had from glucose. However, the redox imbalance and slow
rate of ATP formation are two major factors that make anaerobic ethanol
production from xylose very difficult [32, 33]. A few anaerobic ethanolproducing strains have been developed from xylose in research laboratories, but no strain is so far available for industrial-scale processes.
Attempts have been made to overcome this problem of xylose assimilation by cometabolization or working with microaerobic conditions, where
oxygen is available at low concentrations. A number of microorganisms
can produce ethanol aerobically from xylose, where the practical yield
of ethanol from xylose and other pentoses is usually lower than its theoretical yield. The challenges in ethanol production from xylose have
been reviewed by van Maris et al. [34].
3.9 Biochemical Basis of Ethanol
Production from Hexoses
A simplified central metabolic pathway for ethanol production in yeast and
bacteria under anaerobic conditions is presented in Fig. 3.7 [15, 35–37].
84
Chapter Three
Glucose (1 mole)
Media
Cytosol
Glucose
2 ADP
2 ATP
Glycerol
Glycerol
1 NADH
2 NADH
Pyruvate
(2 moles)
Acetaldehyde
Ethanol
Ethanol (2 moles)
1 NADH
CO2
Figure 3.7 Central metabolic pathway in yeast under anaerobic conditions.
Three major interrelated pathways that control catabolism of carbohydrate in most ethanol-producing organisms are
䊏
Embden-Meyerhof pathway (EMP) or glycolysis
䊏
Pentose phosphate pathway (PPP)
䊏
Krebs or tricarboxylic acid cycle (TCA)
In glycolysis, glucose is anaerobically converted to pyruvic acid and
then to ethanol through acetaldehyde. This pathway provides energy in
the form of ATP to the cells. The net yield in glycolysis is 2 moles of pyruvate (or ethanol) and 2 moles of ATP from each mole of glucose. This
pathway is also the entrance of other hexoses such as fructose, mannose,
and galactose to metabolic pathways. With only 2 moles of ATP formed
per glucose catabolized, large amounts of ethanol (at least 3.7 g of
ethanol per gram of biomass) must be formed [15, 38].
The PPP handles pentoses and is important for nucleotide (ribose5-phosphate) and fatty acid biosynthesis. The PPP is mainly used to
Bioethanol: Market and Production Processes
85
reduce NADP. In Saccharomyces cerevisiae, 6–8% of glucose passes
through the PPP under anaerobic conditions [8, 15].
The TCA cycle functions to convert pyruvic and lactic acids and
ethanol aerobically to the end products CO2 and H2O. It is also a common
channel for the ultimate oxidation of fatty acids and the carbon skeletons of many amino acids. In cells containing the additional aerobic
pathways, the NADH that forms during glycolysis results in ATP generation in the TCA cycle [8].
Ethanol production from hexoses is redox-neutral, i.e., no net formation of NADH or NADPH occurs. However, biosynthesis of the cells
results in net formation of NADH and consumption of NADPH. The
PPP is mainly used to reduce NADP to NADPH. Oxidation of surplus
NADH under anaerobic conditions in S. cerevisiae is carried out through
the glycerol pathway. Furthermore, there are other by-products—mainly
carboxylic acids: acetic acid, pyruvic acid, and succinic acid—that add
to the surplus NADH. Consequently, glycerol is also formed to compensate the NADH formation coupled with these carboxylic acids. Thus,
formation of glycerol is coupled with biomass and carboxylic acid formation in anaerobic growth of S. cerevisiae [15, 39].
We should keep in mind that growth of the cells and increasing their
biomass is the ultimate goal of the cells. They produce ethanol under
anaerobic conditions in order to provide energy through catabolic reactions. Glycerol is formed to keep the redox balance of the cells, and carboxylic acids may leak from the cells to the medium. Therefore, the
ethanol-producing microorganisms produce ethanol as the major product
under anaerobic conditions, while biomass, glycerol, and some carboxylic
acids are the by-products.
3.10 Chemical Basis of Ethanol Production
from Pentoses
In general, yeast and filamentous fungi metabolize xylose through a twostep reaction before they enter the central metabolism (glycolysis)
through the PPP. The first step is conversion of xylose to xylitol using
xylose reductase (XR), and the second step is conversion of xylitol to
xylulose using another enzyme, xylitol dehydrogenase (XDH) [40–42].
Wild strains of S. cerevisiae possess the enzymes XR and XDH, but
their activities are too low to allow growth on xylose. Although S. cerevisiae cannot utilize xylose, it can utilize its isomer, xylulose. Thus, if
S. cerevisiae is to be used for xylose fermentation, it requires a genetic
modification to encode XR/XDH or XI [40, 43].
Bacteria have a slightly different metabolic pathway for xylose utilization. They convert xylose to xylulose in one reaction using XI [10,
44–46].
86
Chapter Three
3.11 Microorganisms Related
to Ethanol Fermentation
The criteria for an ideal ethanol-producing microorganism are to have
(a) high growth and fermentation rate, (b) high ethanol yield, (c) high
ethanol and glucose tolerance, (d) osmotolerance, (e) low optimum fermentation pH, (f ) high optimum temperature, (g) general hardiness
under physiological stress, and (h) tolerance to potential inhibitors present in the substrate [31, 47]. Ethanol and sugar tolerance allows the conversion of concentrated feeds to concentrated products, reducing energy
requirements for distillation and stillage handling. Osmotolerance
allows handling of relatively dirty raw materials with their high salt content. Low-pH fermentation combats contamination by competing organisms. High temperature tolerance simplifies fermentation cooling.
General hardiness allows microorganisms to survive stress such as that
of handling (e.g., centrifugation) [47]. The microorganisms should also
tolerate the inhibitors present in the medium.
3.11.1
Yeasts
Historically, yeasts have been the most commonly used microorganisms
for ethanol production. Yeast strains are generally chosen among S. cerevisiae, S. ellypsoideuse, S. fragilis, S. carlsbergensis, Schizosaccharomyces
pombe, Torula cremoris, and Candida pseudotropicalis. Yeast species
which can produce ethanol as the main fermentation product are
reviewed, e.g., by Lin and Tanaka [8].
Among the ethanol-producing yeasts, the “industrial working horse”
S. cerevisiae is by far the most well-known and most widely used yeast
in industry and research for ethanol fermentation. This yeast can grow
both on simple hexose sugars, such as glucose, and on the disaccharide
sucrose. S. cerevisiae is also generally recognized to be safe as a food
additive for human consumption and is therefore ideal for producing
alcoholic beverages and for leavening bread. However, it cannot ferment pentoses such as xylose and arabinose to ethanol [14, 31]. There
have been several research efforts to genetically modify S. cerevisiae to
be able to consume xylose [33, 48–50]. Several attempts have been made
to clone and express various bacterial genes, which is necessary for fermentation of xylose in S. cerevisiae [51, 52]. It resulted in great success,
but probably not enough yet to efficiently ferment xylose with high yield
and productivity [32].
Alternatively, xylose is converted to ethanol by some other naturally
occurring recombinant. Among the wild-type xylose-fermenting yeast
strains for ethanol production, Pichia stipitis and C. shehatae have
reportedly shown promising results for industrial applications in terms of
complete sugar utilization, minimal by-product formation, low sensitivity
Bioethanol: Market and Production Processes
87
to temperature, and substrate concentration. Furthermore, P. stipitis is
able to ferment a wide variety of sugars to ethanol and has no vitamin
requirement for xylose fermentation [2].
Olsson and Hahn-Hägerdal [20] have presented a list of bacteria, yeasts,
and filamentous fungi that produce ethanol from xylose. Certain species
of the yeasts Candida, Pichia, Kluyveromyces, Schizosaccharomyces,
and Pachysolen are among the naturally occurring organisms. Jeffries and
Kurtzman [53] have reviewed the strain selection, taxonomy, and genetics
of xylose-fermenting yeasts.
Utilization of cellobiose is important in ethanol production from lignocellulosic materials by SSF. However, a few ethanol-producing
microorganisms are cellobiose-utilizing organisms. The requirement for
addition of ␤-glucosidase has been eliminated by cellobiose utilization
during fermentation, since presentation of cellobiose reduces the activity
of cellulase. Cellobiose utilization eliminates the need for one class of
cellulase enzymes [2]. Brettanomyces custersii is one of the yeasts identified as a promising glucose- and cellobiose-fermenting microorganism
for SSF of cellulose for ethanol production [54].
High temperature tolerance could be a good characterization for
ethanol production, since it simplifies fermentation cooling. On the other
hand, one of the problems associated with SSF is the different optimum
temperatures for saccharification and fermentation. Many attempts
have been made to find thermotolerant yeasts for SSF. Szczodrak and
Targonski [55] tested 58 yeast strains belonging to 12 different genera
and capable of growing and fermenting sugars at temperatures of
40–46C. They selected several strains belonging to the genera
Saccharomyces, Kluyveromyces, and Fabospora, in view of their capacity
to ferment glucose, galactose, and mannose at 40C, 43C, and 46C,
respectively. Kluyveromyces marxianus has been found to be a suitable
strain for SSF [56].
3.11.2
Bacteria
A great number of bacteria are able to produce ethanol, although many
of them generate multiple end products in addition to ethanol.
Zymomonas mobilis is an unusual Gram-negative bacterium that has
several appealing properties as a fermenting microorganism for ethanol
production. It has a homoethanol fermentation pathway and tolerates
up to 120 g/L ethanol. Its ethanol yield is comparable with S. cerevisiae,
while it has much higher specific ethanol productivity (2.5) than the
yeast. However, the tolerance of Z. mobilis to ethanol is lower than that
of S. cerevisiae, since some strains of S. cerevisiae can produce ethanol
to give concentrations as high as 18% of the fermentation broth. The tolerance of Z. mobilis to inhibitors and low pH is also low. Similarly,
88
Chapter Three
S. cerevisiae and Z. mobilis cannot utilize pentoses [14, 57]. Several
genetic modifications have been performed for utilization of arabinose
and xylose by Z. mobilis. However, S. cerevisiae has been more welcomed
for industrial application, probably because of the industrial problems
that may arise in working with bacteria. Separation of S. cerevisiae from
fermentation media is much easier than separation of Z. mobilis, which
is an important characteristic for reuse of the microorganisms in ethanol
production processes.
Using genetically engineered bacteria for ethanol production is also
applied in many studies. Ingram et al. [58] have reviewed metabolic
engineering of bacteria for ethanol production. Recombinant Escherichia
coli is a valuable bacterial resource for ethanol production. Construction
of E. coli strains to selectively produce ethanol was one of the first successful applications of metabolic engineering. E. coli has several advantages as a biocatalyst for ethanol production, including the ability to
ferment a wide spectrum of sugars, no requirements for complex growth
factors, and prior industrial use (e.g., for production of recombinant
protein). The major disadvantages associated with using E. coli cultures
are a narrow and neutral pH growth range (6.0–8.0), less hardy cultures
compared to yeast, and public perceptions regarding the danger of E. coli
strains. Lack of data on the use of residual E. coli cell mass as an ingredient in animal feed is also an obstacle to its application [8].
Recently, the Japanese Research Institute of Innovative Technology
for the Earth (RITE) developed a microorganism for ethanol production.
The RITE strain is an engineered strain of Corynebacterium glutamicum
that converts both pentose and hexose sugars into alcohol. The central
metabolic pathway of C. glutamicum was engineered to produce ethanol.
A recombinant strain that expressed the Z. mobilis gene coding for pyruvate decarboxylase and alcohol dehydrogenase was constructed [59].
RITE and Honda jointly developed a technology for production of ethanol
production from lignocellulosic materials using the strain. It is claimed
that application of this strain by using engineering technology from
Honda enables a significant increase in alcohol conversion efficiency, in
comparison to conventional cellulosic–bioethanol production processes.
3.11.3
Filamentous fungi
A great number of molds are able to produce ethanol. The filamentous
fungi Fusarium, Mucor, Monilia, Rhizopus, Ryzypose, and Paecilomyces are
among the fungi that can ferment pentoses to ethanol [33]. Zygomycetes
are saprophytic filamentous fungi, which are able to produce several
metabolites including ethanol. Among the three genera Mucor, Rhizopus,
and Rhizomucor, Mucor indicus (formerly M. rouxii) and Rhizopus oryzae
have shown good performances on ethanol productivity from glucose,
Bioethanol: Market and Production Processes
89
xylose, and wood hydrolyzate [60]. M. indicus has several industrial
advantages compared to baker’s yeast for ethanol production, such as
(a) capability of utilizing xylose, (b) having a valuable biomass, e.g., for
production of chitosan, and (c) high optimum temperature of 37C [61].
Skory et al. [62] examined 19 Aspergilli and 10 Rhizopus strains for their
ability to ferment simple sugars (glucose, xylose, and arabinose) as well
as complex substrates. An appreciable level of ethanol has been produced
by Aspergillus oryzae, R. oryzae, and R. javanicus.
The dimorphic organism M. circinelloides is also used for production
of ethanol from pentose and hexose sugars. Large amounts of ethanol have
been produced during aerobic growth on glucose under nonoxygenlimiting conditions by this mold. However, ethanol production on galactose or xylose has been less significant [63]. Yields as high as 0.48 g/g
ethanol from glucose by M. indicus, under anaerobic conditions, have
been reported [64]. However, the yield and productivity of ethanol from
xylose is lower than that of P. stipitis [65].
Although filamentous fungi have been industrially used for a long time
for several purposes, a number of process-engineering problems are
associated with these organisms due to their filamentous growth.
Problems can appear in mixing, mass transfer, and heat transfer.
Furthermore, attachment and growth on bioreactor walls, agitators,
probes, and baffles cause heterogeneity within the bioreactor and problems in measurement of controlling parameters and cleaning of the
bioreactor [66, 67]. Such potential problems might hinder industrial
application of M. indicus for ethanol production. However, this fungus
is dimorphic, and its morphology can be controlled to be yeast-like or
pellet-like through fermentation [65].
3.12
Fermentation Process
In this section, we will discuss different fermentation processes applicable for ethanol production. Fermentation processes, as well as other
biological processes, can be classified into batch, fed-batch, and continuous operation. All these methods are applicable in industrial fermentation of sugar substances and starch materials. These processes are
well established, the fed-batch and continuous modes of operation being
dominant in the ethanol market. When configuring the fermentation
process, several parameters must be considered, including (a) high
ethanol yield and productivity, (b) high conversion of sugars, and (c) low
equipment cost. The need for detoxification and choice of the microorganism must be evaluated in relation to the fermentation configuration.
Presentation of a variety of inhibitors and their interaction effects,
e.g., in lignocellulosic hydrolyzates, makes the fermentation process
more complex than with other substrates for ethanol production [17, 21].
90
Chapter Three
In fermentation of this hydrolyzate, the pentoses should be utilized in
order to increase the overall yield of the process and to avoid problems
in wastewater treatment. Therefore, it is still a challenge to use a
hexose-fermenting organism such as S. cerevisiae for fermentation of
the hydrolyzate.
When a mixture of hexoses and pentoses is present in the medium,
microorganisms usually take up hexoses first and produce ethanol. As
the hexose concentration decreases, they start to take up the pentose.
Fermentation of hexoses can be successfully performed under anaerobic
or microaerobic conditions, with high ethanol yield and productivity.
However, fermentation of pentoses is generally a slow and aerobic
process. If one adds air to ferment pentoses, the microorganisms will
start utilizing the ethanol produced as well. It makes the entire process
complicated and demands a well-designed and controlled process.
3.12.1
Batch processes
In batch processes, all nutrients required for fermentation are present in
the medium prior to cultivation. Batch technology had been preferred
in the past due to the ease of operation, low cost of controlling and monitoring system, low requirements for complete sterilization, use of
unskilled labor, low risk of financial loss, and easy management of feedstocks. However, overall productivity of the process is very low, because
of long turnaround times and an initial lag phase [9].
In order to improve traditional batch processes, cell recycling and
application of several fermentors have been used. Reuse of produced
cells can increase productivity of the process. Application of several
fermentors operated at staggered intervals can provide a continuous
feed to the distillation system. One of the successful batch methods
applied for industrial production of ethanol is Melle-Boinot fermentation. This process achieves a reduced fermentation time and
increased yield by recycling yeast and applying several fermentors
operated at staggered intervals. In this method, yeast cells from previous fermentation are separated from the media by centrifugation
and up to 80% are recycled [9, 68]. Instead of centrifugation, the cells
can be filtered, followed by the separation of yeast from the filter aid
using hydrocyclones and then recycled [69].
In well-detoxified or completely noninhibiting acid hydrolyzates of
lignocellulosic materials, exponential growth will be obtained after inoculation of the bioreactor. If the hydrolyzate is slightly inhibiting, there
will be a relatively long lag phase during which part of the inhibitors
are converted. However, if the hydrolyzate is severely inhibiting, no
conversion of the inhibitors will occur, and neither cell growth nor fermentation will occur. A slightly inhibiting hydrolyzate can thus be detoxified
Bioethanol: Market and Production Processes
91
during batch fermentation. However, very high concentration of the
inhibitors will cause complete inactivation of the metabolism [18].
Several strategies may be considered for fermentation of hydrolyzate
to improve the in situ detoxification in batch fermentation and obtain
higher yield and productivity of ethanol. Having high initial cell density, increasing the tolerance of microorganisms against the inhibitors
by either adaptation of cells to the medium or genetic modification of the
microorganism, and choosing optimal reactor conditions to minimize
the effects of inhibitors are among these strategies.
Volumetric ethanol productivity is low in lignocellulosic hydrolyzates
when low cell-mass inocula are used due to poor cell growth. Usually,
high cell concentration, e.g., 10 g/L dry cells, have been used in order
to find a high yield and productivity of ethanol in different studies. In
addition, a high initial cell density helps the process for in situ detoxification by the microorganisms, and therefore, the demand for a detoxification unit decreases. In situ detoxification of the inhibitors may
even lead to increased ethanol yield and productivity, due to uncoupling
by the presence of weak acids, or due to decreased glycerol production
in the presence of furfural [21]. Adaptation of the cells to hydrolyzate
or genetic modification of the microorganism can significantly improve
the yield and productivity of ethanol. Optimization of reactor conditions can be used to minimize the effects of inhibitors. Among the different parameters, cell growth is found to be strongly dependent on pH
[18, 21].
3.12.2
Fed-batch processes
In fed-batch processes (or semi-continuous processes), the substrate
and required nutrients are added continuously or intermittently to the
initial medium after the start of cultivation or from the point halfway
through the batch process. Fed-batch processes have been utilized to
avoid utilizing substrates that inhibit growth rate if present at high concentration, to overcome catabolic repression, to demand less initial biomass, to overcome the problem of contamination, and to avoid mutation
and plasmid instability found in continuous culture. Furthermore, fedbatch processes do not face the problem of washout, which can occur in
continuous fermentation. A major disadvantage of a fed-batch process
is the need for additional control instruments that require a substantial amount of operator skill. In addition, for systems without feedback
control, where the feed is added on a predetermined fixed schedule,
there can be difficulty in dealing with any deviation (i.e., time courses
may not always follow the expected profiles) [70]. The fed-batch processes
without feedback control can be classified as intermittent fed-batch,
constant-rate fed-batch, exponential fed-batch, and optimized fed-batch.
92
Chapter Three
The fed-batch processes with feedback control have been classified as
indirect-control and direct-control fed-batch processes [70, 71].
The fed-batch technique is one of the promising methods for fermentation of dilute-acid hydrolyzates of lignocellulosic materials. The basic
concept behind the success of this technique is the capability of in situ
detoxification of hydrolyzates by the fermenting microorganisms. Since
the yeast has a limited capacity for conversion of the inhibitors, the
achievement of a successful fermentation strongly depends on the feed
rate of the hydrolyzate. By adding the substrate at a low rate in fedbatch fermentation, the concentrations of bioconvertible inhibitors such
as furfural and HMF in the fermentor remain low, and the inhibiting
effect therefore decreases. At a very high feed rate, using an inhibiting
hydrolyzate, both ethanol production and cell growth can stop, whereas
at a very low feed rate, the hydrolyzate may still be converted, but at a
very low productivity rate, which has been experimentally confirmed.
Consequently, there should exist an optimum feed rate [15, 18, 21].
Similar to batch operations, higher optimum dilution rate in fedbatch cultivation can be obtained by (a) high initial cell concentration,
(b) increasing the tolerance of microorganisms against the inhibitors,
and (c) choosing optimal reactor conditions to minimize the effects of
inhibitors. Productivity in fed-batch fermentation is generally limited
by the feed rate which, in turn, is limited by the cell-mass concentration [21].
3.12.3
Continuous processes
Process design studies of molasses fermentation have shown that the
investment cost was considerably reduced when continuous rather than
batch fermentation was employed, and that the productivity of ethanol
could be increased by more than 200%. Continuous operations can be
classified into continuous fermentation with or without feedback control.
In continuous fermentation without feedback control, called a chemostat,
the feed medium containing all the nutrients is continuously fed at a constant rate (dilution rate D) and the cultured broth is simultaneously
removed from the fermentor at the same rate. The chemostat is quite
useful in the optimization of media formulation and to investigate the
physiological state of the microorganism [71]. Continuous fermentations with feedback control are turbidostat, phauxostat, and nutristat.
A turbidostat with feedback control is a continuous process to maintain
the cell concentration at a constant level by controlling the medium
feeding rate. A phauxostat is an extended nutristat, which maintains the
pH value of the medium in the fermentor at a preset value. A nutristat
with feedback control is a cultivation technique to maintain nutrient concentration at a constant level [71].
Bioethanol: Market and Production Processes
93
When lignocellulosic hydrolyzates are added at a low feed rate in continuous fermentation, low concentration of bioconvertible inhibitors in
the fermentor is assured. In spite of a number of potential advantages
in terms of productivity, this method has not developed much yet
in fermentation of the acid hydrolyzates. One should consider the
following points in continuous cultivation of acid hydrolyzates of lignocelluloses:
䊏
Cell growth is necessary at a rate equal to the dilution rate in order
to avoid washout of the cells in continuous cultivation.
䊏
Growth rate is low in fermentation of hydrolyzates because of the
presence of inhibitors.
䊏
The cells should keep their viability and vitality for a long time.
The major drawback of the continuous fermentation is that, in contrast
to the situation in fed-batch fermentation, cell growth is necessary at a
rate equal to the dilution rate, in order to avoid washout of the cells in
continuous cultivation [21]. The productivity is a function of the dilution rate, and since the growth rate is decreased by the inhibitors, the
productivity in continuous fermentation of lignocellulosic hydrolyzates
is low. Furthermore, at a very low dilution rate, the conversion rate of
the inhibitors can be expected to decrease due to the decreased specific
growth rate of the biomass. Thus, washout may occur even at very low
dilution rates [18]. On the other hand, one of the major advantages of
continuous cultivation is the possibility to run the process for a long time
(e.g., several months), whereas the microorganisms usually lose their
activity after facing the inhibitory conditions of the hydrolyzate. By
employing cell-retention systems, the cell-mass concentration in the
fermentor, the maximum dilution rate, and thus the maximum ethanol
productivity increase. Different cell-retention systems have been investigated by cell immobilization and encapsulation, and cell recirculation
by filtration, settling, and centrifugation. A relatively old study [72]
shows that the investment cost for a continuous process with cell recirculation has been found to be less than that for continuous fermentation
without cell recirculation.
Biostil® is the trade name of a continuous industrial process for
ethanol production with partial recirculation of both yeast and wastewater. The fermentor works continuously; the cells are separated by
using a centrifuge, and a part of the separated cells is returned to the
fermentor. Most of the ethanol-depleted beer including residual sugars
is then recycled to the fermentor. In this process, besides providing
enough cell concentration in the fermentor, less water is consumed and
a more concentrated stillage is produced. Therefore, the process has a
lower wastewater problem. However, the process needs a special type
94
Chapter Three
of centrifuge (which is expensive) in order to avoid deactivation of the
cells [47, 73].
Application of an encapsulated cell system in continuous cultivation
has several advantages, compared to either a free-cell or traditionally
entrapped cell system, e.g., in alginate matrix. Encapsulation provides
higher cell concentrations than free-cell systems in the medium, which
leads to higher productivity per volume of the bioreactor in continuous
cultivation. Furthermore, the biomass can easily be separated from the
medium without centrifugation or filtration. The advantages of encapsulation, compared to cell entrapment, are less resistance to diffusion
through beads/capsules, some degree of freedom in movement of the
encapsulated cells, no cell leakage from the capsules, and higher cell concentration [74].
3.12.4 Series-arranged continuous
flow fermentation
Ethanol can be produced by using continuous flow fermentors arranged
in a series with complete sugar utilization or high ethanol concentration. With two fermentors arranged in a series, the retention time can
be chosen so that the sugar is only partially utilized in the first, with
fermentation completed in the second. Ethanol inhibition is reduced in
the first fermentor, allowing a faster throughput. The second, lowerproductivity fermentor can now convert less sugar than if operated
alone. For high product concentration, productivity of a two-stage system
has been 2.3 times higher than that of a single stage [47, 75].
A two-stage continuous ethanol fermentation process with yeast recirculation is used industrially by Danish Distilleries Ltd., Grena, for
molasses fermentation (see Fig. 3.8). Two fermentors with 170,000-L
volume produce 66 g/L ethanol in 21-h retention time [76].
Yeast
settler Mash
Feed
Fermentor I
Fermentor II
Figure 3.8 Two-stage continuous ethanol fermentation process with yeast recirculation [76, 77]. (A seven-fermentor-series system (70,000-L volume each fermentor)
was also used in the Netherlands to produce 86 g/L ethanol in 8-h retention time
[78]. A Japanese company used a six-fermentor-series system (total volume 100,000 L)
with 8.5-h retention time to produce 95 g/L ethanol [79].)
Bioethanol: Market and Production Processes
95
3.12.5 Strategies for fermentation of
enzymatic lignocellulosic hydrolyzates
The cellulose fraction of lignocelluloses can be converted to ethanol by
either simultaneous saccharification and fermentation (SSF) or separate
enzymatic hydrolysis and fermentation (SHF) processes. A schematic of
these processes is shown in Fig. 3.9. It is also possible to combine the
cellulase production, enzymatic hydrolysis, and fermentation in one
step, called direct microbial conversion (DMC). There are cost savings
because of the reduced number of required vessels. However, there is less
attention to DMC for industrial purposes because of the low ethanol
yield in DMC, formation of several by-products, and low ethanol tolerance of the microorganisms used [2].
3.12.6 Separate enzymatic hydrolysis
and fermentation (SHF)
In SHF, enzymatic hydrolysis for conversion of pretreated cellulose to
glucose is the first step. Produced glucose is then converted to ethanol
in the second step. Enzymatic hydrolysis can be performed in the optimum conditions of the cellulase. The optimum temperature for hydrolysis by cellulase is usually between 45C and 50C, depending on the
microorganism that produces the cellulase. The major disadvantage of
SHF is that the released sugars severely inhibit cellulase activity. The
activity of cellulose is reduced by 60% at a cellobiose concentration as
Cellulolytic enzymes
Pretreated
lignocellulosic
materials
Enzymatic
hydrolysis
Microorganisms
Enzymatic
hydrolyzate
Fermentation
Liquid
Distillation
Ethanol
Solid residue
A: Separate enzymatic hydrolysis
Cellulolytic
enzymes
Pretreated
lignocellulosic
materials
Microorganisms
Simultaneous
saccharification and
fermentation
Liquid
Solid residue
Distillation
Ethanol
B: Simultaneous saccharification and fermentation
Figure 3.9 Main steps in SSF or SHF for ethanol production.
96
Chapter Three
low as 6 g/L. Although glucose also decreases the cellulase activity, the
inhibitory effect of glucose is lower than that of cellobiose. On the other
hand, glucose is a strong inhibitor for ␤-glucosidase. At a level of 3 g/L
of glucose, the activity of ␤-glucosidase reduces by 75% [27, 80]. Another
possible problem in SHF is contamination. Hydrolysis is a lengthy
process (one or possibly several days), and a dilute solution of sugar
always has a risk of contamination, even at rather high temperatures
such as 45–50C.
3.12.7 Simultaneous saccharification
and fermentation (SSF)
SSF combines enzymatic hydrolysis of cellulose and fermentation in
one step. As cellulose converts to glucose, a fermenting microorganism
is presented in the medium and it immediately consumes the glucose
produced. As mentioned, cellobiose and glucose significantly decrease the
activity of cellulase. SSF gives higher reported ethanol yields and
requires lower amounts of enzyme, because end-product inhibition from
cellobiose and glucose formed during enzymatic hydrolysis is relieved
by the yeast fermentation. SSF has the following advantages compared
to SHF:
䊏
Fewer vessels are required for SSF, in comparison to SHF, resulting
in capital cost savings.
䊏
Less contamination during enzymatic hydrolysis, since the presence
of ethanol reduces the possibility of contamination.
䊏
Higher yield of ethanol.
䊏
Lower enzyme-loading requirement.
On the other hand, SSF has the following drawbacks compared to SHF:
䊏
SSF requires that enzyme and culture conditions be compatible with
respect to pH and temperature. In particular, the difference between
optimum temperatures of the hydrolyzing enzymes and fermenting
microorganisms is usually problematic. Trichoderma reesei cellulases,
which constitute the most active preparations, have optimal activity
between 45C and 50C, whereas S. cerevisiae has an optimum temperature between 30C and 35C. The optimal temperature for SSF
is around 38C, which is a compromise between the optimal temperatures for hydrolysis and fermentation. Hydrolysis is usually the
rate-limiting process in SSF [27]. Several thermotolerant yeasts (e.g.,
C. acidothermophilum and K. marxianus) and bacteria have been
used in SSF to raise the temperature close to the optimal hydrolysis
temperature.
Bioethanol: Market and Production Processes
97
䊏
Cellulase is inhibited by ethanol. For instance, at 30 g/L ethanol, the
enzyme activity was reduced by 25% [2]. Ethanol inhibition may be
a limiting factor in production of high ethanol concentration. However,
there has been less attention to ethanol inhibition of cellulase, since
practically it is not possible to work with very high substrate concentration in SSF, because of the problem with mechanical mixing.
䊏
Another problem arises from the fact that most microorganisms used
for converting cellulosic feedstock cannot utilize xylose, a hemicellulose hydrolysis product [8].
3.12.8 Comparison between enzymatic and
acid hydrolysis for lignocellulosic materials
The two most promising processes for industrial production of ethanol
from cellulosic materials are two-stage dilute-acid hydrolysis (a chemical process) and SSF (an enzymatic process). Advantages and disadvantages of dilute-acid and enzymatic hydrolyses are summarized in
Table 3.3. Enzymatic hydrolysis is carried out under mild conditions,
whereas high temperature and low pH result in corrosive conditions for
acid hydrolysis. While it is possible to obtain a cellulose hydrolysis of
close to 100% by enzymatic hydrolysis after a pretreatment, it is difficult to achieve such a high yield with acid hydrolysis. The yield of conversion of cellulose to sugar with dilute-acid hydrolysis is usually less
than 60%. Furthermore, the previously mentioned inhibitory compounds
are formed during acid hydrolysis, whereas this problem is not so severe
for enzymatic hydrolysis. Acid hydrolysis conditions may destroy nutrients sensitive to acid and high temperature such as vitamins, which may
introduce the process together with the lignocellulosic materials.
Advantages and Disadvantages of Dilute-Acid and Enzymatic
Hydrolyses
TABLE 3.3
Parameters
Dilute-acid
hydrolysis
Rate of hydrolysis
Overall yield of sugars
Very high
Low
Catalyst costs
Conditions
Low
Harsh reaction conditions
(e.g., high pressure and
temperature)
Highly inhibitory
hydrolyzate
High
Inhibitors formation
Degradation of sensitive
nutrients such as vitamins
Enzymatic
hydrolysis
Low
High and depend upon
pretreatment
High
Mild conditions (e.g.,
50C, atmospheric
pressure, pH 4.8)
Noninhibitory
hydrolyzate
Low
98
Chapter Three
On the other hand, enzymatic hydrolysis has its own problems in
comparison to dilute-acid hydrolysis. Hydrolysis for several days is necessary for enzymatic hydrolysis, whereas a few minutes are enough for
acid hydrolysis. The prices of the enzymes are still very high, although a
new development has claimed a 30-fold decrease in the price of cellulase.
3.13
Ethanol Recovery
Fermented broth or “mash” typically contains 2–12% ethanol. Furthermore, it contains a number of other materials that can be classified into
microbial biomass, fusel oil, volatile components, and stillage. Fusel oil
is a mixture of primary methylbutanols and methylpropanols formed
from ␣-ketoacids and derived from or leading to amino acids. Depending
on the resources used, important components of fusel oil can be isoamylalcohol, n-propylalcohol, sec-butylalcohol, isobutylalcohol, n-butlyalcohol,
active amylalcohol, and n-amylalcohol. The amount of fusel oil in mash
depends on the pH of the fermentor. Fusel oil is used in solvents for
paints, polymers, varnishes, and essential oils. Acetaldehyde and trace
amounts of other aldehydes and volatile esters are usually produced
from grains and molasses. Typically, 1 L of acetaldehyde and 1–5 L of
fusel oil are produced per 1000 L of ethanol [9, 47].
Stillage consists of the nonvolatile fraction of materials remaining
after alcohol distillation. Its composition depends greatly on the type of
feedstock used for fermentation. Stillage generally contains solids, residual sugars, residual ethanol, waxes, fats, fibers, and mineral salts. The
solids may be originated from feedstock proteins and spent microbial
cells [9].
3.14
Distillation
Mash is usually centrifuged or settled in order to separate the microbial biomass from the liquid and then sent to the ethanol recovery
system. Distillation is typically used for the separation of ethanol, aldehydes, fusel oil, and stillage [9]. Ethanol is readily concentrated from
mash by distillation, since the volatility of ethanol in a dilute solution
is much higher than the volatility of water. Therefore, ethanol is separated from the rest of the materials and water by distillation. However,
ethanol and water form an azeotrope at 95.57 wt% ethanol (89 mol%
ethanol) with a minimum boiling point of 78.15C. This mixture
behaves as a single component in a simple distillation, and no further
enrichment than 95.57 wt% of ethanol can be achieved by simple distillation [9, 47, 81]. Various industrial distillation systems for ethanol purification are (a) simple two-column systems, (b) three- or four-column barbet
systems, (c) three-column Othmer system, (d) vacuum rectification,
Bioethanol: Market and Production Processes
99
(e) vapor recompression, (f) multieffect distillation, and (g) six-column
reagent alcohol system [9, 47]. These methods are reviewed by Kosaric
[9]. The following parameters should be considered for selection of the
industrial distillation systems:
䊏
Energy consumption (e.g., steam consumption or cooling water consumption per kilogram of ethanol produced).
䊏
Quality of ethanol (complete separation of fusel oil and light components).
䊏
How to deal with the problem associated with clogging of the first distillation column and its reboiler because of precipitation or formation
of solids. Special design and use of a vacuum may be applied for overcoming the problem in the column. Using open steam instead of application of a reboiler can prevent clogging of the reboiler, in spite of the
increase in amount of wastewater.
䊏
Simplicity in controlling the system.
䊏
Simplicity in opening column parts and cleaning the columns.
Of course, lower capital investment is also one of the main parameters
in the selection of distillation systems.
Ethanol is present in the market with different degrees of purity. The
majority of ethanol is 190 proof (95% or 92.4%, minimum) used for solvent, pharmaceutical, cosmetic, and chemical applications. Technicalgrade ethanol, containing up to 5% volatile organic aldehyde, ester, and
sometimes methanol, is used for industrial solvents and chemical syntheses. High-purity 200 proof (99.85%) anhydrous ethanol is produced
for special chemical applications. For fuel use in mixture with gasoline
(gasohol), nearly anhydrous (99.2%) ethanol, but with higher available
levels of organic impurities, is used [47].
A simple two-column system is described here, while other systems
are presented in the literature (e.g., [9, 47]). Simple one- or twocolumn systems with only a stripping and rectification section are
usually used to produce lower-quality industrial alcohol and azeotrope
alcohol for further dehydration to fuel grade. The simplest continuous ethanol distillation system consists of stripping and rectification
sections, either together in one column or separated into two columns
(see Fig. 3.10).
The mash produced is pumped into a continuous distillation process,
where steam is used to heat the mash to its boiling point in the stripper
column. The ethanol-enriched vapors pass through a rectifying column
and are condensed and removed from the top of the rectifier at around
95% ethanol. The ethanol-stripped stillage falls to the bottom of the
stripper column and is pumped to a stillage tank. Aldehydes are drawn
100
Chapter Three
Aldehydes
Product alcohol
Feed from fermentor
(Mash)
Stripper
column
Rectifying
column
Fusel oil
Stillage
Steam
Wastewater
Steam
Figure 3.10 Two-column system for distillation of ethanol.
from the head vapor, condensed, and partly used as reflux. Fusel oil is
taken out from several plates of the rectifying section [9, 47, 82].
With efficient distillation, the stillage should contain less than 0.1%
ethanol since the presence of ethanol significantly increases the chemical oxygen demand (COD) of wastewater. For each 1% ethanol left in
the stillage, the COD of the stillage is incremented by more than 20 g/L.
Due to the potential impact of residual ethanol content, therefore, proper
control over distillation can greatly affect the COD of stillage [82].
3.15 Alternative Processes for Ethanol
Recovery and Purification
Since distillation is a highly energy-consuming process, several
processes have been developed for purification of ethanol from fermentation broth: for example, solvent extraction, CO2 extraction, vapor
recompression systems, and low-temperature blending with gasoline
[9]. However, these processes are not established in the industrial production of ethanol.
Bioethanol: Market and Production Processes
3.16
101
Ethanol Dehydration
In order to allow blending of alcohol with gasoline, the water content of
ethanol must be reduced to less than 1% by volume, which is not possible by distillation. Higher water levels can result in phase separation
of an alcohol–water mixture from the gasoline phase, which may cause
engine malfunction. Removal of water beyond the last 5% is called dehydration or drying of ethanol. Azeotropic distillation was previously
employed to produce higher-purity ethanol by adding a third component,
such as benzene, cyclohexane, or ether, to break the azeotrope and produce dry ethanol [82]. To avoid illegal transfer of ethanol from the industrial market into the potable alcohol market, where it is highly regulated
and taxed, dry alcohol usually requires the addition of denaturing agents
that render it toxic for human consumption; the azeotropic reagents
conveniently meet this requirement [82]. Except in the high-purity
reagent-grade ethanol market, azeotropic drying has been supplanted
by molecular sieve drying technology.
3.16.1
Molecular sieve adsorption
The molecular sieve is a more energy-efficient method than azeotropic
distillation. Furthermore, this method avoids the occupational hazards associated with azeotropic chemical admixtures. In molecular
sieve drying, 95% ethanol is passed through a bed of synthetic zeolite
with uniform pore sizes that preferentially adsorb water molecules.
Approximately three-fourths of adsorbed material is water and onefourth is ethanol. The bed becomes saturated after a few minutes and
must be regenerated by heating or evacuation to drive out the adsorbed
water. During the regeneration phase, a side stream of ethanol/water
(often around 50%) is produced, which must be redistilled before returning to the drying process [82].
3.16.2
Membrane technology
Membranes can also be used for ethanol purification. Reverse osmosis
(RO), which employs membranes impermeable to ethanol and permeable to water, can be used for purification of ethanol from water. Using a
membrane permeable to ethanol but not to water is another approach [9].
Pervaporation, a promising membrane technique for separation of
organic liquid mixtures such as azeotropic mixtures or near-boiling-point
mixtures, can also be used for separation of these azeotropes [81, 83].
It involves the separation of ethanol–water azeotrope or near-azeotropic
ethanol–water composition (from about 95 to 99.5 wt% ethanol) through
water-permeable (or water-selective) membranes to remove the rest of
the water from the concentrated ethanol solution [84].
102
Chapter Three
3.17 Concluding Remarks and
Future Prospects
Ethanol has been well established in the fuel market, where its share
from less than 1 GL in 1975 is expected to increase to 100 GL in 2015.
Grains, sugarcane juice, and molasses are the dominant raw materials
for the time being, while lignocellulosic materials are expected to have
a significant share in this market in the future. There have been great
achievements in the development of ethanol production from lignocellulosic materials, and large-scale facilities are expected to be built within
a few years. However, several challenges will still persist in this process
in the future, until the process is fully established.
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Chapter
4
Raw Materials to Produce
Low-Cost Biodiesel
M. P. Dorado
4.1
Introduction
The present energy scenario is undergoing a period of transition, as
more and more energy consumers understand the inevitability of
exhaustion of fossil fuel. The era of fossil fuel of nonrenewable resources
is gradually coming to an end, where oil and natural gas will be depleted
first, followed eventually by depletion of coal. In developing countries,
the energy problem is rather critical. The price paid for petrol, diesel,
and petroleum products now dominates over all other expenditures
and forms a major part of a country’s import bill. In view of the problems stated, there is a need for developing alternative energy sources.
Alternative fuel options are mainly biogas, producer gas, methanol,
ethanol, and vegetable oils. But biogas and producer gas have low
energy contents per unit mass and can substitute for diesel fuel only
up to 80%. Moreover, there are problems of storage because of their
gaseous nature. Methanol and ethanol have very poor calorific value
per unit mass, apart from having a low Cetane number. Therefore,
these are rather unsuitable as substitutes for high-compression diesel
engines. Experimental evidence indicates that methanol and ethanol
can be substituted up to only 20–40%. There exists a number of plant
species that produce oils and hydrocarbon substances as a part of their
metabolism. These products can be used with other fuels in diesel
engines with various degrees of processing. The development of vegetable oils as liquid fuels have several advantages over other alternative fuel options, such as
107
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108
Chapter Four
1. The technologies for extraction and processing are very easy and
simple, as conventional equipment with low energy inputs are needed.
2. Fuel properties are close to diesel fuel.
3. Vegetable oils are renewable in nature.
4. Being liquid, these oils offer ease of portability and also possess stability and no handling hazards.
5. The by-product leftover after extraction of oil is rich in protein and
can be used as animal feed or solid fuel.
6. Cultivation of these oilseeds is adaptable to a wide range of geographic locations and climatic conditions.
7. Biodiesel can be used directly in compression ignition engines with
no substantial modifications to the engine.
8. Biodiesel contains no sulfur, and there is no production of oxides of
sulfur.
Hence, biodiesel is considered an alternative fuel for internal combustion engines derived from oils and fats from renewable biological sources;
it emits far less regulated pollutants than the standard diesel fuel
[1–10]. It entails minimal reduced engine performance as a result of a
slight power loss and increase in specific fuel consumption [8, 11–29].
However, one main concern in further usage of biodiesel is the economic
viability of producing biodiesel.
The main economic criteria are manufacturing cost and the price of
raw feedstock. Manufacturing costs include direct costs for oil extraction, reagents, and operating supplies, as well as indirect costs related
to insurance and storage. Fixed capital costs involved in the construction of processing plants and auxiliary facilities, distribution, and retailing must also be taken into consideration [30].
Several authors have found that biodiesel is currently not economically feasible unless tax credits are applied [23, 31]. Peterson [23] has
found that diesel fuel costs less than biodiesel, and an emergency or
diesel shortage would be required to provide a practical reason for using
biodiesel. Some authors have stated that biodiesel could compete with
diesel fuel if produced in cooperatives [31, 32].
To promote biodiesel consumption, several countries have exempted
biodiesel from their fuel excise tax. Among them, the European Union
(EU) approved the biodiesel tax exemption program in May 2002. The
financial law funded biofuels through excise exemption over a period of
3 years (Art. 21, Finance Law 2001). The U.S. Senate Finance Committee
also approved an excise tax exemption for biodiesel in 2003. Moreover,
the legislation provides a 1% reduction in the diesel fuel excise tax for each
percentage of biodiesel blended with petroleum diesel up to 20%. Also,
Raw Materials to Produce Low-Cost Biodiesel
109
among some other countries, the Australian Senate approved an excise
exemption on biofuels in 2004. However, the tax exemption will one day
come to an end; in order to continue to promote the social inclusion and
economic attraction of biodiesel, other steps will be needed. This could be
facilitated by the selection of low-cost raw materials, such as nonedible
oils, used frying oil, or animal fat, and the use of a lower-cost transesterification process.
A lower-cost biodiesel production can be achieved by the optimization
of the process. Because the chemical properties of the esters determine
their feasibility as a fuel, the intent of the optimization is to investigate
and optimize the involved parameters maximizing the yield of ester,
to develop a low-cost chemical process, and to ensure appropriate oil
chemical properties for both the transesterification and the engine
performance.
Although it is a well-known process since, in 1864, Rochleder described
glycerol preparation through the ethanolysis of castor oil [33], the proportion of reagents affects the process, in terms of conversion efficiency
[34]; this factor differs according to the vegetable oil. Several researchers
have identified the most important variables that influence the transesterification reaction, namely, reaction temperature, type and amount
of catalyst, ratio of alcohol to vegetable oil, stirring rate, and reaction
time [20, 35–42]. In this sense, it is important to characterize the oil (i.e.,
fatty acid composition, water content, and peroxide value) to determine
the correlation between them and the feasibility to convert the oil into
biodiesel [39, 43].
However, several studies have identified that the price of feedstock
oils is by far one of the most significant factors affecting the economic
viability of biodiesel manufacture [30, 44–46]. Approximately 70–95%
of the total biodiesel production cost arises from the cost of the raw
material [44, 45]. To produce a competitive biodiesel, the feedstock price
is a factor that needs to be taken into consideration. Edible oils are too
valuable for human feeding to run automobiles. So, the accent must be
on nonedible oils and used frying oils.
4.2
Nonedible Oils
Among nonedible feedstock, there are many crops and tree-borne oilseed
plants, such as karanja, neem, and jatropha, which have been underutilized due to the presence of toxic components in their oils. Most of
them grow in underdeveloped and developing countries, where a
biodiesel program would give multiple benefits in terms of generation
of employment for rural people (farmers), leverage of starting many
types of industries using by-products from biofuels, and so forth [47].
However, nonedible crops are very much ignored in most cases. They
110
Chapter Four
grow on, regardless, waiting for their energetic potential to be discovered. The key is to find crops or trees that need very little care, have high
oil content, and are resistant to plagues and drainage. The foliage could
be used as manure, giving an added value to the crop. In fact, most of
the trees and crops mentioned in the following (karanja, neem, etc.)
grow well on wasteland and can tolerate long periods of drought and dry
conditions.
4.2.1
Bahapilu oil
Salvadora oleoides Decne, S. persica L., and S. indica—
commonly known as bahapilu, chootapilu, jhal, jaal, pilu, kabbar,
khakan, and mitijar—belong to the family Salvadoraceae and are found
in arid regions of western India and Pakistan (see Figs. 4.1 and 4.2). The
crop is typical of the tropical thorn forest. It is highly salt tolerant and
grows in coastal regions and on inland saline soils [48, 49]. S. oleoides
is a shrub or small tree up to 9 m in height. Seeds contain 40–50% of
a greenish-yellow fat containing large amounts of lauric and myristic
acids [50].
Crop description.
Salvadora persica. (Photo courtesy of Abdulrahman Alsirhan
[www.alsirhan.com/Plants_s/Salvadora_persica.htm].)
Figure 4.1
Raw Materials to Produce Low-Cost Biodiesel
111
Figure 4.2 Salvadora angustifolia. (Photo courtesy of Dr. Kazuo Yamasaki
[http://pharm1.pharmazie.uni-greifswald.de/gallery/gal-salv.htm].)
The fruits are sweet and edible. The seed cake contains
12% protein and is suitable for livestock fodder. The wood is used for
building purposes. It is also an important source of fuelwood. The fat in
seeds can be used for making soap and candles. The leaves and fruits
are used in medicine to relieve cough, rheumatism, and fever. The tree
contributes to erosion control in fragile areas [50]. Some authors have
carried out systematic studies on the lubrication properties of biodiesel
from S. oleoides and its blends. Biodiesel was prepared by base-catalyzed
transesterification using methanol. Results indicate that addition of
biodiesel improves the lubricity and reduces wear scar diameter even
at a 5% blend [51].
Main uses.
4.2.2
Castor oil
Crop description. Ricinus communis L., commonly known as the castoroil plant, belongs to the family Euphorbiaceae (see Fig. 4.3). This perennial tree or shrub can reach up to 12 m high in tropical or subtropical
climates, but it remains 3 m tall in temperate places. Native to Central
Africa, it is being cultivated in many hot climates. The oil contains up
to 90% ricinoleic acid, which is not suitable for nutritional purposes
due to its laxative effect [52]. This hydroxycarboxylic acid is responsible for the extremely high viscosity of castor oil, amounting to almost a
hundred times the value observed for other fatty materials [53].
112
Chapter Four
Figure 4.3 Ricinus communis L. (Photo courtesy of Eric Winder
[www.bio.mtu.edu/~jclewin/bahama_pics/Eric/].)
Castor bean is cultivated for its seeds, which yield a fastdrying oil used mainly in industry and medicine. Coating fabrics, highgrade lubricants, printing inks, and production of a polyamide nylon-type
fiber are among its uses. Dehydrated oil is an excellent drying agent and
is used in paints and varnishes. Hydrogenated oil is utilized in the manufacture of waxes, polishes, carbon paper, candles, and crayons. The
pomace or residue after crushing is used as a nitrogen-rich fertilizer.
Although it is highly toxic due to the ricin, a method of detoxicating the
meal has been developed, so that it can safely be fed to livestock [54].
Several authors have found that castor-oil biodiesel can be considered as
a promising alternative to diesel fuel. Transesterification reactions have
been carried out mainly by using both ethanol and NaOH, and through
enzymatic methanolysis [55–57]. Several authors have studied the influence of the nature of the catalyst on the yields of biodiesel from castor oil.
They found that the most efficient transesterification of castor oil
could be achieved in the presence of methoxide and acid catalysts [58].
The influence of alcohol has also been studied. Comparing the use of
ethanol versus methanol, Meneghetti et al. have found that similar
yields of fatty acid esters may be obtained; however, the reaction with
methanolysis is much more rapid [59]. Cvengros et al. produced both
ethyl and methyl esters, using NaOH in the presence of ethanol and
methanol, respectively. Despite the high viscosity and density values,
they concluded that both methyl and ethyl esters can be successfully
used as fuel. A positive solution to meet the standard values for both
viscosity and density parameters can be a dilution with esters based
Main uses.
Raw Materials to Produce Low-Cost Biodiesel
113
on oils/fats without an OH group, or a blending with conventional diesel
fuel [60].
4.2.3
Cottonseed oil
Gossypium spp., commonly known as cotton, belongs
to the family Malvaceae and is native to the tropical and subtropical
regions (see Fig. 4.4). Four separate domesticated species of cotton
grown in various parts of the world are G. arboreum L., G. herbaceum
L., G. hirsutum L., and G. barbadense L. Cotton shrubs are annual and
found in the United States, Australia, Asia, and Egypt. Some have been
grown for many years in southern Europe, mainly the Balkans and
Spain. It can grow up to 3 m high [61–64].
Crop description.
Cotton is a major world fiber crop. Its fiber grows around
the seeds of the cotton plant and is used to make textile, which is the
most widely used natural-fiber cloth. The seeds yield a valuable oil used
for the production of cooking oil or margarine. The fatty acid composition includes mainly palmitic acid (21%), stearic acid (2.4%), oleic acid
(19.5%), linoleic acid (54.3%), and myristic acid (0.9%). Cottonseed oil,
cake, meal, and hulls for feeding are other uses of the by-products.
Whole cottonseed may be used as a feed for mature cattle. Cottonseed
meal is an excellent protein supplement for cattle. The limitations on
effective utilization of this product in rations for swine and poultry are
of minor significance to ruminant animals. Cottonseed meal has a relatively low rumen degradability and is therefore a good source of by-pass
protein and is especially useful in rations for milking cows [61–64].
Main uses.
Gossypium spp. (Photo
courtesy of Prof. Jack Bacheler
[http://ipm.ncsu.edu/
cotton/InsectCorner/photos/
images/Open_cotton_plant.jpg].)
Figure 4.4
114
Chapter Four
Köse et al. investigated the transesterification of refined cottonseed
oil, using primary and secondary alcohols (oil–alcohol molar ratio 1:4)
in the presence of an immobilized enzyme from Candida antarctica (30%
enzyme, based on oil weight). The reaction was carried out at 50C for 7
h, showing that conversion using secondary alcohols was more effective
[65]. Some authors have also proposed the use of lipase with methanol
[66]. Royon et al. used the same catalyst in a t-butanol solvent. Maximum
yield was observed after 24 h at 50C with a reaction mixture containing 32.5% t-butanol, 13.5% methanol, 54% oil, and 0.017 g of enzyme per
g of oil [67]. Recent tendencies propose the use of ultrasonically assisted
extraction transesterification to increase ester yield [68].
4.2.4
Cuphea oil
Crop description. Cuphea spp., C. carthagenensis, C. painter, C. ignea,
and C. viscosissima—commonly known as cuphea—belong to the family
Lythraceae and grow in temperate and subtropical climates (see Fig. 4.5).
They can be found in Central and South America, and have been grown
in trials in Germany and the United States. The seeds of Cuphea contain about 30–36% oil [69]. Major fatty acid composition of the oil
includes caprylic acid (73% in C. painter, 3% in C. ignea), capric acid
(18% in C. carthagenensis, 24% in C. painteri, 87% in C. ignea, and
83–86% in C. llavea), and lauric acid (57% in C. carthagenensis) [70].
It contains high levels of short-chain fatty acids, very interesting for industrial applications. Previous studies have suggested that
oil composition and chemical properties of C. viscosissima VS-320 are
not appropriate for use as a substitute for diesel fuel without chemical
Main uses.
Cuphea sp. (Photo courtesy of Dr. Alvin R. Diamond
[http://spectrum.troy.edu/~diamond/PIKEFLORA.htm].)
Figure 4.5
Raw Materials to Produce Low-Cost Biodiesel
115
conversion of triglycerides to methyl esters. Further genetic modifications must be made [71, 72]. Later studies have revealed that genetically
modified oils present relatively low viscosity that is predicted to enhance
their performance as alternative diesel fuels [73]. Also, atomization properties suggest better fuel performance, because this oil has short-chain
triglycerides, while traditional vegetable oils contain predominantly
long-chain triglycerides [74].
4.2.5 Jatropha curcas oil
J. curcas—commonly known as pourghere, ratanjyot,
Barbados nut, physic nut, parvaranda, taua taua, tartago, saboo dam,
jarak butte, or awla—belongs to the family Euphorbiaceae and grows
in hot, dry, tropical climates (see Fig. 4.6). It originated from South
America and is now found worldwide in tropical countries. It grows wild
especially in West Africa, and is grown commercially in the Cape Verde
Islands and Malagasy Republic. The tree reaches a height of 8 m and
is a tough, drought-resistant plant that bears oil-rich seeds prolifically
under optimum growing conditions [75]. The seeds contain about 55%
oil [76]. The oil contains a toxic substance, curcasin, which has a strong
purging effect. Major fatty acid composition consists of myristic acid
(0–0.5%), palmitic acid (12–17%), stearic acid (5–6%), oleic acid
(37–63%), and linoleic acid (19–40%) [77].
Crop description.
It has been cultivated as a drought-resistant plant in marginal areas to prevent soil erosion. The oil has been commercially used
Main uses.
Jatropha curcas. (Photo
courtesy of Piet Van Wyk and
EcoPort [www.ecoport.org].)
Figure 4.6
116
Chapter Four
for lighting purposes, as lacquer, in soap manufacture, and as a textile
lubricant. It is also used for medicinal purposes for its strong purging
effect. The leaves are used in the treatment of malaria. Products useful
as plasticizer, hide softeners, and hydraulic fluid have been obtained
after halogenation [75]. The wood is used for fuel. The cake, after oil
extraction, cannot be used for animal feed due to its toxicity, but is a good
organic fertilizer. The wood is very flexible and is used for basket
making. A water extract of the whole plant has molluscicide effects
against various types of snail, as well as insecticidal properties [77].
Recently, there has been considerable interest in the use of the oil in
small diesel engines. This oil has great potential for biodiesel production
[78–80]. Foidl et al. transesterified J. curcas oil, using a solution of KOH
(0.53 mol) in methanol (10.34 mol) and stirring at 30C for 30 min [81].
The ester fuel has high quality and meets the existing standards for
vegetable-oil-derived fuels. Some researchers have proposed the use of
immobilized enzymes such as Chromobacterium viscosum, Candida rugosa,
and Porcine pancreas as a catalyst [82, 83]. Modi et al. have proposed the use
of propan-2-ol as an acyl acceptor for immobilized Candida antarctica lipase
B. Best results have been obtained by means of 10% Novozym-435 based
on oil weight, with a alcohol–oil molar ratio of 4:1 at 50C for 8 h [84].
Zhu et al. have proposed the use of a heterogeneous solid superbase catalyst (catalyst dosage of 1.5%) and calcium oxide, at 70C for 2.5 h, with
a methanol–oil molar ratio of 9:1 to produce biodiesel [85]. The lubrication
properties of this biodiesel have also been taken into consideration [51].
4.2.6
Karanja seed oil
Crop description. Pongamia pinnata (L.) Pierre, P. glabra Vent., Cytisus
pinnatus L., Derris indica (Lam.) Bennett, and Galedupa indica Lam.—
commonly known as karanja, pongam, coqueluche, Vesi Ne Wai, vesivesi,
hongay, and honge—belong to the Leguminaceae family and are widely
distributed in tropical Asia (see Fig. 4.7). The tree is drought-resistant,
tolerant to salinity, and is commonly found in East Indies, Philippines,
and India. The karanja tree grows to a height of about 1 m and bears pods
that contain one or two kernels. The kernel oil content varies from 27%
to 39% and contains toxic flavonoids, including 1.25% karanjin and 0.85%
pongamol [86–88]. The fatty acid composition consists of oleic acid
(44.5–71.3%), linoleic acid (10.8–18.3%), palmitic acid (3.7–7.9%), stearic
acid (2.4–8.9%), and lignoceric acid (1.1–3.5%) [86, 89].
The oil is used mainly in agriculture, pharmacy (particularly
in the treatment of skin diseases), and the manufacture of soaps. It has
insecticidal, antiseptic, antiparasitic, and cleansing properties, like
neem oil [86–88]. The cake after oil extraction may be used as manure.
Main uses.
Raw Materials to Produce Low-Cost Biodiesel
117
Figure 4.7 Pongamia pinnata (L.)
Pierre. (Photo courtesy of the Food
and Agricultural Organization of
the United Nations[www.fao.org].)
All parts of the plant have also been analyzed for its reported medical
importance. Several scientists have investigated and guaranteed
karanja oil as a potential source of biodiesel [78]. Most researchers have
conducted the transesterification of P. pinnata oil by using methanol and
potassium hydroxide catalysts [90–92]. Meher et al. [90] found that
using a methanol–oil molar ratio of 12:1 produced maximum yield of
biodiesel (97%), while Vivek and Gupta [91] stated the optimum ratio
was 8–10:1. In both cases, the optimal temperature was around 65C,
with a reaction time of 180 min [90] and 30–40 min [91]. Vivek and Gupta
used 1.5% w/w of catalyst (KOH), while Meher et al. used 2% w/w solid
basic Li/CaO catalyst [93]. Due to the high FFA (free fatty acid) content,
some researchers have proposed esterification with H2SO4 prior to transesterification with NaOH [94, 95]. In all cases, karanja oil has shown a
feasibility to be used as a raw material to produce biodiesel, saving
large quantities of edible vegetable oils. Diesel engine performance tests
were carried out with karanja methyl ester (KME) and its blend with
diesel fuel from 20% to 80% by volume [92]. Results have revealed a
reduction in exhaust emissions together with an increase in torque,
brake power, thermal efficiency, and reduction in brake-specific fuel
consumption, while using the blends of karanja-esterified oil (20–40%),
compared to straight diesel fuel.
4.2.7
Linseed oil
Linum usitatissimum L.—commonly known as linseed, flaxseed, lint bells, or winterlien—belongs to the family Linaceae
(see Fig. 4.8). This annual herb can grow up to 60 cm in height in most
temperate and tropical regions. This plant is native to West Asia and the
Mediterranean [96]. The seeds contain 30–40% oil, including palmitic
Crop description.
118
Chapter Four
Figure 4.8
Linum usitatissimum.
acid (4.5%), stearic acid (4.4%), oleic acid (17.0%), linoleic acid (15.5%),
and linolenic acid (58.6%).
Medicinal properties of the seeds have been known since
ancient Greece. It is used in pharmacology (antitussive, gentle bulk laxative, relaxing expectorant, antiseptic, antiinflammatory, etc.) [97]. As
the source of linen fiber, it was used by the Egyptians to make cloth in
which to wrap their mummies. However, today it is mainly grown for
its oil [98, 99], which is used in the manufacture of paints, varnishes,
and linoleum. Linseed oil is used as a purgative for sheep and horses.
It is also used in cooking. There is a market for flaxseed meal as both
animal feeding and human nutrition [96].
Lang et al. transesterified linseed oil by using different alcohols
(methanol, ethanol, 2-propanol, and butanol) and catalysts (KOH and
sodium alkoxides). Butyl esters showed reduced cloud points and pour
points [100]. Some authors have found that biodiesel from linseed oil
presents a lower cold filter plugging point (CFPP) than biodiesel from
rapeseed oil, due to large amounts of linolenic acid methyl ester and their
iodine value [101]. Long-term endurance tests have been carried out with
methyl esters of linseed oil, showing low emission characteristics. Wear
assessment has shown lower wear for a biodiesel-operated engine [102].
Experimental investigations on the effect of 20% biodiesel blended with
diesel fuel on lubricating oil have shown a lubricating oil life longer
while operating the engine on biodiesel [103]. Oxidation stability have
shown better results compared with methyl esters of animal origin [104].
Lebedevas et al. have suggested the use of three-component mixtures
Main uses.
Raw Materials to Produce Low-Cost Biodiesel
119
(rapeseed-oil methyl esters, animal methyl esters, and linseed oil
methyl esters) to fuel the engine. These three-component mixtures
reduced exhaust emissions significantly, with the exception of NOx that
increased them up to 13% [105].
4.2.8
Mahua oil
Madhuca indica—commonly known as madhuka, yappa,
mahuda, mahua, mauwa, mohwa, hippe, butter tree, mahwa, mahula, or
elupa—belongs to the family Sapotaceae and grows up to 21 m high. This
deciduous tree is distributed mainly in India (see Fig. 4.9). The kernels are
70% of seed by weight. Seeds content includes 35% oil and 16% protein. Main
fatty acids are palmitic acid (16–28.2%), stearic acid (20–25.1%), arachidic
acid (3.3%), oleic acid (41–51%), and linoleic acid (8.9–13.7%) [106].
Crop description.
Traditionally, it has been used as a source of natural hard
fat in soap manufacture. The seed oil is used as an ointment in rheumatism and to prevent dry, cracked skin in winter. It is used in foods, cosmetics, and lighting. The cake presents toxic and bitter saponins that
preclude its use as animal feed. However, mahua cake can be used as
organic manure [106]. Several approaches to produce biodiesel can be
found. Ghadge and Raheman have proposed a two-step pretreatment to
reduce high FFA levels. Transesterification was carried out adding 0.25 v/v
methanol and 0.7% KOH. Fuel properties were found comparable to those
of diesel fuel [107]. Some authors have proposed different successful
Main uses.
Figure 4.9 Madhuca indica. (Photo courtesy of Antonie van den Bos
[www.botanypictures.com/plantimages/].)
120
Chapter Four
alternatives to produce biodiesel: ethanol and sulfuric acid, and
methanol and NaOH [108–110]. Puhan et al. have found better diesel
engine performance for methyl esters compared to ethyl and butyl esters,
while ethyl esters show lower NOx emissions compared to the rest [111].
Systematic studies on the lubrication properties of biodiesel have shown
that the preferred range of blending with diesel fuel is 5–20% [51].
4.2.9
Nagchampa oil
Crop description. Calophyllum inophyllum—commonly known as
nagchampa, ballnut, ati tree, kamani, ndamanu, fetau, Alexandrian
laurel, nambagura, Indian laurel, and tamanu oil—belongs to the family
Guttiferae and is native to the Indo–Pacific region, particularly Malaysia
[112]. This evergreen tree is commonly found in the coastal regions of
South India and Madagascar (see Fig. 4.10). It usually reaches up to 25 m
high [113]. It tolerates varied kinds of soil, coastal sand, clay, and
degraded soil. The average kernel oil content is about 60.1% [114]. The
fatty acids present in crude oils are stearic (14.3%), palmitic (13.7%),
oleic (39.1%), and linoleic (31.1%) acids [115].
Main uses. It is known best as an ornamental plant. Besides this, its
wood is hard and strong and has been used in construction. The seeds
yield oil for medicinal use and cosmetics. A number of medicinal and
therapeutic properties of various parts of Calophyllum have been
described, including the treatment of rheumatism, varicose veins, hemorrhoids, and chronic ulcers [116]. Fatty acid methyl esters of C. inophyllum oil have been found suitable for use as biodiesel that meets
biodiesel standards of the United States and European Standards
Organization [78].
Figure 4.10 Calophyllum inophyllum. (Photo by Forest Starr
and Kim Starr, courtesy of the U.S.
Geological Society [www.hear.org/
starr/hiplants/images/hires/
h t m l / s t a r r _ 0 4 0 7 11 _ 0 2 3 2 _
calophyllum_inophyllum.htm].)
Raw Materials to Produce Low-Cost Biodiesel
4.2.10
121
Neem oil
Crop description. Azadirachta indica—commonly known as the neem
tree, nim, margosa, veppam, cho do, or nilayati nimb—belongs to the
family Meliaceae and can be found in dry tropical forests (see Fig. 4.11).
The major producing countries are India, Sri Lanka, Burma, Pakistan,
tropical Australia, and Africa. The evergreen neem tree grows up to
18 m high. The fat content of the kernels ranges from 33 to 45% [77].
The fatty acid content includes 42% oleic acid, 20% palmitic acid, 20%
stearic acid, 15% linoleic acid, and 1.4% arachidic acid. Good quality kernels yield 40–50% oil. The cakes, which contain 7–12% oil are sold for
solvent extraction. Neem oil is unusual in that it contains nonlipid associates often loosely termed as bitters and organic sulfur compounds
that impart a pungent, disagreeable odor [88].
The products of the neem tree are known to be antibacterial, antifungal, and antiparasitic. The main uses are in soaps, teas,
medicinal preparations, cosmetics, skin care, insecticides, and repellents. Neem twigs are used as tooth brushes and ward against gum
disease. Neem oil, which is extracted from the seed kernel, has excellent healing properties and is used in creams, lotions, and soaps. It
is also an effective fungicide. The bitter cake after the extraction of oil
Main uses.
Figure 4.11 Azadirachta indica.
(Photo courtesy of Food and
Agricultural Organization of the
United Nations [www.fao.org].)
122
Chapter Four
has no value for animal feeding but is recognized as both a fertilizer
and nematicide [88]. Besides medical use, esters of neem oils have some
important fuel properties that can be exploited for alternative fuels for
diesel engines [78]. Nabi et al. have produced biodiesel from neem oil
by using 20% methyl alcohol and 0.6% anhydrous lye catalyst (NaOH).
The temperature of the materials was maintained at 55–60C.
Compared with conventional diesel fuel, exhaust emissions including
smoke and CO were reduced, while NOx emission was increased with
diesel–biodiesel blends. However, NOx emission with diesel–biodiesel
blends was slightly reduced when exhaust gas recirculation (EGR) was
applied. According to the results, Nabi et al. have recommended the
use of the ester of this oil as an environment-friendly alternative fuel
for diesel engines [117].
4.2.11
Rubber seed oil
Crop description. Hevea brasiliensis—commonly known as Pará rubber
tree, rubber tree, jebe, cauchotero de para, seringueira, or siringa—
belongs to the family Euphorbiaceae (see Fig. 4.12). The rubber tree
originates from the Amazon rain forest (Brazil). Today, most rubber
tree plantations are located in Southeast Asia and some are also in
tropical Africa. The tree can reach up to 30 m high. Oil can be extracted
from the seeds. Although there are variations in the oil content of the
seed from different countries, the average oil yield has been 40% [118].
Figure 4.12
Hevea brasiliensis.
Raw Materials to Produce Low-Cost Biodiesel
123
Its fatty acid composition includes palmitic acid (10.2%), stearic acid
(8.7%), oleic acid (24.6%), linoleic acid (39.6%), and linolenic acid
(16.3%) [118].
The crop is of major economical importance because it produces latex. The wood from this tree is used in the manufacture of highend furniture. In Cambodia and other rubber-manufacturing countries,
rubber seeds are used to feed livestock. Rubber seed contains cyanogenic
glycosides that will release prussic acid in the presence of enzymes or
in slightly acidic conditions. Press cake or extracted meal can be cautiously used as fertilizer or feed for stock [119].
Several studies to check its feasibility as a source of biodiesel have
been undertaken. Ikwuagwu et al. have prepared methyl esters of
rubber seed oil using excess of methanol (6 M) containing 1% NaOH
as a catalyst. Petroleum ether was added to the reaction to produce
two phases. Analysis of the properties have shown a good potential for
use as an alternative diesel fuel, with the exception of the oxidative
stability [120]. Ramadhas et al. have performed a previously acidcatalyzed esterification to reduce the high FFA content, followed by an
alkaline esterification. Sulfuric acid 0.5% by volume and a methanoloil molar ratio of 6:1 was used in the pretreatment. A molar ratio of
9:1 and 0.5% by weight of sodium hydroxide was used during the
second step. The authors found a reduction in exhaust gas emissions.
The lower blends of biodiesel increased brake thermal efficiency and
reduced fuel consumption [121].
Main uses.
4.2.12
Tonka bean oil
Crop description. Dipteryx odorata—commonly known as sarapia, tonka
bean, amburana, aumana, yape, charapilla, and cumaru—belongs to the
family Leguminacea and grows in tropical areas (see Fig. 4.13). Major
producing countries are Guianas and Venezuela. The tonka bean is the
seed of a large tree. The kernel contains up to 46% oil on a dry basis.
Major fatty acid composition of oil includes palmitic acid (6.1%), stearic
acid (5.7%), oleic acid (59.6%), and linoleic acid (51.4%) [77].
The oil is used in perfumery and as a flavoring material.
Tonka extracts are used in the tobacco industry to impart a particular
aroma. Few attempts have been made to use it as a raw material to produce biodiesel. Abreu et al. conducted methanolysis of cumaru oil using
different homogeneous metal (Sn, Pb, and Zn) complexes as catalysts.
They found that pyrone complexes of different metals are active for
cumaru-oil transesterification reaction [122].
Main uses.
124
Chapter Four
Dipteryx odorata. (Photo courtesy of Dr. Davison
Shillingford [www.da-academy.org/dagardens_tonkabean1.html].)
Figure 4.13
4.3
Low-Cost Edible Oils
Besides nonedible oils, there are some edible oils from plants that yield
a relatively lower-cost source to produce biodiesel compared to biodiesel
from rapeseed oil or soybean oil.
4.3.1
Cardoon oil
Crop description. Cynara cardunculus L.—commonly known as cardoon, Spanish artichoke, artichoke thistle, cardone, dardoni, or cardo—
belongs to the family Asteracea (see Fig. 4.14). Artichokes originated in
the Mediterranean region and climates, becoming an important weed
of the Pampas in Argentina, and in Australia, and California because of
its adaptation to dry climate. Its fatty acid composition mainly includes
palmitic acid (19.3%), stearic acid (6.1%), oleic acid (39%), and linoleic acid
(30%) [123].
Main uses. The leaf stalks are eaten as a vegetable. The leaves contain
cynarin, which improves gall bladder and liver functions, increases bile
flow, and lowers cholesterol. The down from the seed heads is used as
rennet.
Encinar et al. transesterified C. cardunculus oil using methanol and
several catalysts (sodium hydroxide, potassium hydroxide, and sodium
methoxide) to produce biodiesel. Best properties were achieved by using 15%
methanol and 1% sodium methoxide as catalyst, at 60C temperature [124].
Raw Materials to Produce Low-Cost Biodiesel
Figure 4.14
125
Cynara cardunculus.
The reaction can also be accomplished by using an ethanol–oil molar
ratio of 12:1 and 1% sodium hydroxide, at 75C [125]. C. cardunculus
methyl esters also provide a significant reduction in particulate emissions, mainly due to reduced soot and sulfate formation [126].
4.3.2
Ethiopian mustard oil
Brassica carinata, commonly known as Ethiopian
mustard, is an adequate oil-bearing crop that is well adapted to marginal regions (see Fig. 4.15). This crop, which is originally from Ethiopia,
is drought-resistant and grown in arid regions [127, 128]. Ethiopian
mustard presents up to 6% saturated hydrocarbon chains. It is native
to the Ethiopian highlands, is widely used as food by the Ethiopians, and
pre-sents better agronomic performances in areas such as Spain,
California, and Italy. This makes B. carinata a promising oil feedstock
for cultivation in coastal Mediterranean areas, which could offer the possibility of exploiting the Mediterranean marginal areas for energetic purposes [129]. Its fatty acid composition includes palmitic acid (3.6%),
stearic acid (1.3%), oleic acid (14.8%), linoleic acid (12.2%), gadoleic acid
(10.3%), and erucic acid (45.4%) [123].
Crop description.
Main uses. It is widely used as food in Ethiopia. Oil from wild species
is high in erucic acid, which is toxic, although some cultivars contain
little erucic acid and can be used as food. The seed can also be crushed
and used as a condiment [127]. There is a genetic relationship among
126
Chapter Four
Figure 4.15
Brassica carinata.
B. carinata genotypes based on oil content and fatty acid composition.
Genet et al. have generated information to plan crosses and maximize
the use of genetic diversity and expression of heterosis [130]. Dorado et al.
found negative effects of singular fatty acids, such as erucic acid, over
alkali-catalyzed transesterification reaction [39]. These researchers
described a low-cost transesterification process of B. carinata oil. An
oil–methanol molar ratio of 1:4.6, addition of 1.4% of KOH, a reaction temperature in the range of 20–45C, and 30 min of stirring are considered
to be the best conditions to develop a low-cost method to produce biodiesel
from B. carinata oil [39, 131]. Biodiesel from Ethiopian mustard oil could
become of interest if a fuel tax exemption is granted [30]. When compared with petroleum diesel fuel, Cardone et al. have found that engine
test bench analysis did not show any appreciable variation of output
engine torque values, while there was a significant difference in specific
fuel consumption data at the lowest loads. Biodiesel produced higher
levels of NOx concentrations and lower levels of particulate matter (PM),
with respect to diesel fuel. Biodiesel emissions contain less soot [132].
4.3.3
Gold-of-pleasure oil
Camelina sativa L. Crantz—commonly known as
gold-of-pleasure and camelina—belongs to the family Cruciferae and
grows well in temperate climates (see Fig. 4.16). It is an annual oilseed
plant and is cultivated in small amounts in France, and to a lesser
Crop description.
Raw Materials to Produce Low-Cost Biodiesel
127
Figure 4.16 Camelina sativa L.
Crantz. (Photo courtesy of Prof.
Arne Anderberg [http://linnaeus.
nrm.se/flora/di/brassica/camel/
camemic.html].)
extent in Holland, Belgium, and Russia. The oil content of camelina
seeds ranges from 29.9% to 38.3%. However, it is an underexploited
oilseed crop at present. Its fatty acid profile includes oleic acid
(14–19.5%), linoleic acid (18.8–24%), linolenic acid (27–34.7%), eicosenoic
acid (12–15%), and erucic acid (less than 4%) [133]. Budin et al. have
concluded that camelina is a low-input crop possessing a potential for
food and nonfood exploitation [133].
This crop has recently been rediscovered as an oil crop. At the
moment, the feasibility of utilizing oil from this plant is being investigated [53, 134]. Oil is used as a luminant and emollient for softening the
skin. Fiber is obtained from the stems. Fröhlich and Rice have investigated production of methyl ester from camelina oil. Biodiesel was prepared by means of a single-stage esterification using methanol and
KOH [135]. Steinke et al. have developed both alkali-catalyzed and
lipase-catalyzed alcoholyses of camelina oil [136, 137].
Main uses.
4.3.4
Tigernut oil
Crop description. Cyperus esculentus L.—commonly known as tigernut,
chufa sedge, yellow nutsedge, and earth almond—belongs to the family
Cyperaceae and grows in warm temperate to subtropical regions of the
Northern Hemisphere (see Figs. 4.17 and 4.18). It can be found in Africa,
South America, Europe, and Asia. It is a perennial herb, growing up to
128
Chapter Four
Figure 4.17 Cyperus esculentus L. (Photo courtesy of Rolv
Hjelmstad [www.rolv.no/bilder/galleri/medplant/
cype_esc.htm].)
90 cm high [138]. Tubers contain 20–36% oil. The oil from the tuber contains 18% saturated (palmitic acid and stearic acid) and 82% unsaturated (oleic acid and linoleic acid) fatty acids [138].
The tubers are edible and have high nutritive value. They
contain 3–15% protein, 15–20% sugar, 20–25% starch, 4–14% cellulose,
and trace amounts of natural resin. They are used in Spain to make
a beverage named horchata, and also consumed fresh after soaking.
In other countries, the tubers are used in sweetmeats or uncooked as a
side dish. New products obtained can enhance the interest in this crop
Main uses.
Figure 4.18 Cyperus esculentus
L. (Photo courtesy of Peter Chen
[www.cod.edu/people/faculty/ch
enpe/PRAIRIE/2005_09_20/
Cyperus_esculentus.jpg].)
Raw Materials to Produce Low-Cost Biodiesel
129
as a source of dietary fiber in food technology, as a high-quality cooking/
salad oil, as a source of starch, as an antioxidant-containing food, and
so forth [139]. The oil extracted from yellow nutsedge can be used as food
oil as well as for industrial purposes. Since the tubers contain 20–36%
oil, the crop has been suggested as a potential oil crop for the production of biodiesel [138]. Preliminary tests using pure nutsedge oil as fuel
in a diesel engine have indicated that the engine operated near its rated
power [140]. Currently, it is being studied as an oil source for fuel production in Africa [53].
4.4
Used Frying Oils
Currently, world oil crop production is about 139,000,000 ton [141]. In
particular, developing countries (97,370,185 ton) and developed countries (41,193,308 ton) are the largest producers, while least developed
countries contribute 4,141,535 ton. Most of this oil is used for deepfrying processes, after which it becomes a disposal problem. Disposal
methods often contaminate environmental water and contribute to world
pollution. Due to high oxidative thermal stress, such waste frying fats
should not be used for human food [142]. Also, since 2002, the EU has
enforced a ban on feeding these mixtures to animals, because during
frying many harmful compounds are formed, which could result in the
return of harmful compounds back into the food chain through the
animal meat [143].
Used oils can be recycled through conversion into soap by saponification and reused as lubricating oil or hydraulic fluid. Nevertheless, biofuel production seems to be the most attractive alternative for waste oil
treatment. Certainly, it will not solve the energy problem, because only
a small percentage of diesel demand can be supplied by this source [20],
but it will decrease the dependence on fossil oil while reducing an environmental problem.
For economic reasons, used frying oil is an interesting feedstock for
biodiesel production. In this sense, Nye et al. were the first to describe
the transesterification of used frying oil using excess of alcohol under
both acidic and basic conditions. The best result was obtained using
methanol with catalysis by KOH [144]. The tests were carried out using
frying margarine and partially hydrogenated soybean oil. The reaction
was carried out at 50C for 24 h, using methanol in a methanol–triglyceride
molar ratio of 3.6:1 and 0.4% KOH. At the same time, Mittelbach et al.
investigated the use of waste oils to produce biodiesel and found that
the increase in the amount of polymers during heating of the oil is a
good indicator for the suitability for biodiesel production [42]. They
proposed a low-temperature process (40C) under alkaline catalysis
and excess of methanol [145]. Considering used olive oil, better results
130
Chapter Four
were also obtained using KOH and methanol instead of NaOH and
ethanol, which decreases transesterification rates. The reaction was
optimized at an ambient temperature, using 1.26% KOH and 12%
methanol, and stirring for 1 min [40]. Some authors have optimized the
reaction by using methanol (alcohol–waste oils molar ratios between 3.6
and 5.4) and 0.2–1% NaOH [146], or methanol (molar ratios in the range
of 1:74 to 1:245) and acid catalyst (sulfuric acid) [147]. Al-Widyan and
Al-Shyoukh have performed waste palm oil transesterification under
various conditions. The best process combination was 2.25 M H2SO4
with 100% excess ethanol in about 3 h of reaction time [148].
Several parameters (e.g., heating conditions, FFA composition, and
water content) can influence conversion from waste oils into biodiesel.
Mittelbach et al. have found that heating over a long period led to a significantly higher FFA content, which can reach values up to 10% and
have detrimental effects during the transesterification process.
Nevertheless, in most cases, simple heating and filtering of solid impurities is sufficient for further transesterification [20]. The methyl and
ethyl esters of fatty acids obtained by alcoholysis of triglycerides seem
to be excellent fuels [5]. Anggraini found that it was also important to
keep the water content of used cooking oils as low as possible [149].
Dorado et al. have compared biofuels from waste vegetable oils from several countries (different FFA composition) including Brazil, Spain, and
Germany. The transesterification process was carried out in two steps,
using a stoichiometric amount of methanol and the necessary amount
of KOH, supplemented with the exact amount of KOH to neutralized
acidity. Both reactions were completed in 30 min [41]. Results revealed
that to carry the reaction to completion, an FFA value lower than 3% is
needed. The two-step transesterification process (without any costly
purification step) was found to be an economic method for biofuel production using waste vegetable oils. To reduce FFA content, a two-step
transesterification using 0.2% ferric sulfate and 1% KOH with methanol
(mole ratio 10:1) was also developed [150]. Acid-catalyzed pretreatment
to esterifiy the FFA before transesterification with an alkaline catalyst
was also proposed [151]. This procedure can reduce the acid levels to less
than 1%. Some authors have proposed a three-step process in a fixedbed bioreactor with immobilized Candida antarctica lipase [152].
Brenneis et al. also developed a process involving C. antarctica through
alcoholysis of waste fats, with excess of water. The optimum amount of
water was found to be 80–10% of the amount of fat [153]. Chen et al.
preferred the use of immobilized lipase Novozym-435 in transesterification of both waste oil and methyl acetate. However, they found that
the reaction rate decreased with increasing water content [154].
Engine tests have been performed with biodiesel from different
kinds of waste oils. Al-Widyan et al. tested several ester–diesel blends in
Raw Materials to Produce Low-Cost Biodiesel
131
a direct-injection diesel engine. Results indicated that the biodiesel burned
more efficiently with less specific fuel consumption. Furthermore,
50% of the blends produced less CO and fewer unburned hydrocarbons
than diesel [155]. Also, Mittelbach and Junek stated that it improves
exhaust gas emissions, as compared to esters made from fresh oil [156].
However, despite the exhaust emission reduction, there are some discrepancies in terms of NOx emission related to the process and raw
material [1, 105, 157]. In general terms, most studies show a slight
decrease in brake power output, besides an increase in specific fuel
consumption [158, 159]. To solve this problem, Kegl and Hribernik
have proposed to modify injection characteristics at different fuel
temperatures [160].
Several authors have worked on related topics. Kato et al. have used
ozone treatment to reduce the flash point of biodiesel from fish waste
oil, resulting in easy combustibility [161]. The immiscibility of canola oil
in methanol provides a mass-transfer challenge in the early stages of
transesterification. To exploit this situation, Dubé et al. developed a
two-phase membrane reactor. The reactor was particularly useful in
removing unreacted oil [162].
4.5
Animal Fats
Bovine spongiform encephalopathy (BSE), commonly known as mad
cow disease, is a fatal neurodegenerative disease of cattle. BSE has
attracted wide attention because it can be transmitted to humans.
Pathogenic prions are responsible for transmissible spongiform
encephalopathies (TSE), and especially for the occurrence of a new
variant of Creutzfeldt-Jakob disease (nvCJD), a human brain-wasting
disease. Due to this problem, the specified risk material is burned under
high temperatures to avoid any hazards for humans and animals.
However, another possibility could be to consider this material as a
source for producing biodiesel by transesterification. In fact, production
of biodiesel from the risk material could represent a more economic
usage than its combustion. Siedel et al. have found that almost every
single step of the process leads to a significant reduction in the concentration of the pathogenic prion protein (PrPSc ) in the main product and
by-products. They concluded that biodiesel from materials with a high
concentration of pathogenic prions can be considered safe [163]. Animal
fats, such as tallow or lard, have been widely investigated as a source
of biodiesel [164–169]. Muniyappa et al. have found that transesterification of beef tallow produced a mixture of esters with a high concentration in saturated fatty acids, but with physical properties similar to
esters of soybean oil [37]. Ma et al. found that 0.3% NaOH completed
132
Chapter Four
methanolysis of beef tallow in 15 min [170]. Some authors have found
that absolute ethanol produced higher conversion and less viscosity
than absolute methanol at 50C, after 2 h [171]. Nebel and Mittelbach
have found n-hexane was the most suitable solvent for extraction of fat
from meat and bone meal. The extracted material was converted into
fatty acid methyl esters through a two-step process [172]. Lee et al.
have performed a three-step transesterification to produce biodiesel
from lard and restaurant grease. They found that a porous substance,
such as silica gel, improved the conversion when more than 1 M methanol
was used as reaction substrate [173]. Mbaraka et al. also synthesized
propylsulfonic acid-functionalized mesoporous silica materials for
methanol esterification of the FFA in beef tallow, as a pretreatment step
for alkyl ester production [174].
Engine tests also showed a reduction in emission, except oxides of
nitrogen that increased up to 11% for the yellow grease methyl ester
[157]. Cold-flow properties of the fat-based fuels were found to be less
desirable than those of soy-based biodiesel, with comparable lubricity and
oxidative stability [175]. To solve this problem, Kazancev et al. blended
up to 25% of pork lard methyl esters with other oil methyl esters and
fossil diesel fuels. In this case, the CFPP showed a value of 5C. In
winter, only up to 5% of esters can be added to the fuel. Depressant
Viscoplex 10-35 with an optimal dose of 5000 mg/kg was found to be the
most effective additive to improve the cold properties [101].
4.6
Future Lines
Research in most of the nonedible oil crops previously mentioned has
been insufficient. To determine the viability of their use as a source of
biodiesel and to optimize the transesterification as well as engine performance, more research is needed. But, there are also other nonedible
and low-cost edible oily crops and trees that could be exploited for
biodiesel production. Amongst them, allanblackia, bitter almond, chaulmoogra, papaya, sal, tung, and ucuuba produce oils that hold immense
potential to be used as a raw material for producing biodiesel. Most of
them grow in underdeveloped and developing countries, where governments may consider providing support to the activities related to collection of seeds, production of oil, production of biodiesel, and its
utilization for cleaner environment. Hence, to facilitate its integration,
a legal framework should be legislated to enforce regulations on
biodiesel. Biodiesel should be seriously considered as a potential source
of energy, particularly in underdeveloped and developing countries with
very tight foreign exchange positions and insufficient availability of
traditional fuels.
Raw Materials to Produce Low-Cost Biodiesel
4.6.1
133
Allanblackia oil
Crop description. Allanblackia stuhlmannii and A. floribunda—commonly
known as allanblackia, mkanyi fat, bouandjo, and kagne butter (see
Fig. 4.19)—belong to the family Guttiferae and grow in tropical areas,
mainly in East Africa, Congo, and Cameroons. A high content of hard
white fat (60–80%) can be extracted from the seed kernels of the trees.
Allanblackia fats consist almost entirely of stearic acid (52–58%), oleic
acid (39–45%), and palmitic acid (2–3%) [87]. Allanblackia has received
considerable attention, based on its fat composition rather than its commercial importance [77].
The use of the fat in soap manufacture has been suggested
[176]. The timber is suitable for use under damp conditions. The pounded
bark is used for medicinal purposes [177]. No references about its use
as a biodiesel source have been found so far.
Main uses.
4.6.2
Bitter almond oil
Crop description. Prunus communis, P. americana, and P. amygdalus—
commonly known as almond, amandier, mandelbaum, almendro, and
mandorlo (see Fig. 4.20)—belong to the family Rosaceae and grow in
temperate Mediterranean areas. Major producing countries are Italy,
Spain, Morocco, France, Greece, and Iran. The almond tree grows to a
height of 3–8 m. Many varieties of almonds are grown, but they can be
Allanblackia stuhlmannii. (Photo courtesy of Josina
Kimottho
(ICRAF)
[www.
worldagroforestry.org/Sites/
TreeDBS/aft/imageSearch.asp].)
Figure 4.19
134
Chapter Four
Figure 4.20 Prunus communis. (Photo courtesy of Gernot
Katzer [www.uni-graz.at/~katzer/pictures/prun_09.jpg].)
broadly divided into two types: bitter and sweet. Bitter almonds contain
amygdalin and an enzyme that causes its hydrolysis to glucose, benzaldehyde, and hydrocyanic acid, making the fruit nonedible. The bitter
almond oil yield is around 40–45%, and sometimes as low as 20% [77,
178]. Major fatty acid composition of oil includes palmitic acid (7.5%),
stearic acid (1.8%), oleic acid (66.4%), and linoleic acid (23.5%) [178].
Bitter almond press cake cannot be used for feed due to its
toxic components [179]. They are pressed at low temperatures, generally at about 30C, to prevent destruction of the hydrolytic enzyme. The
press cake is then used for production of bitter almond oil [77]. Despite
the oil content and fatty acid composition, no references about the use
of bitter almond oil as a raw material to produce biodiesel have been
found so far.
Main uses.
4.6.3
Chaulmoogra oil
Crop description. Taraktogenos kurzii, Hydnocarpus wightiana, Oncoba
echinata (West Africa), and Carpotroche brasiliensis (Brazil)—commonly
known as chaulmoogra, chaulmugra, maroti, hydnocarpus, and gorli
seed—belong to the family Flacourtiaceae and grow in India, Sri Lanka,
Burma, Bangladesh, Nigeria, and Uganda (see Fig. 4.21). The trees
Raw Materials to Produce Low-Cost Biodiesel
135
Figure 4.21 Chaulmoogra leaves.
(Photo courtesy of Prof. Gerald D.
Carr [www.botany.hawaii.edu/
faculty/carr/flacourti.htm].)
grow to a height of 12–15 m. The kernels make up 60–70% of the seed
weight and contain 63% of pale-yellow oil. The oil is unusual in that it
is not made up of straight-chain fatty acids but acids with a cyclic group
at the end of the chain [77].
Chaulmoogra oil has been used for thousands of years in the
treatment of leprosy. However, it has now been replaced by modern
drugs. The expeller cake is a useful manure and is reported to ward off
ants and other insect pests. It cannot be used for animal feed due to its
toxicity. The oil has been highly active against fungal plant pathogens
including Aspergillus niger and Rhizopus nigricans. There may be a
wide scope of integrating the pharmaceutical industries based on chaulmoogra, with the fuel and energy industries dealing with production of
petroleum hydrocarbons, such as biodiesel [180].
Main uses.
4.6.4.
Papaya oil
Crop description. Carica papaya L. (see Fig. 4.22)—commonly known
as papaya, pawpaw, melon tree, papayier, lechosa, or mamon—belongs
to the family Caricaceae and grows in tropical to subtropical areas.
Native to South America, now the crop is widely distributed throughout the tropics. Papaya is a short-lived rapidly growing plant (not a
true tree) having no lignified tissues. The seeds contain 25–29% oil
[77, 179]. The oil contains mainly unsaturated fatty acids, around 70.7%,
and may contain toxic components that make it unusable in human
foods [75]. Fatty acid composition of the oil includes oleic acid (79.1%)
and palmitic acid (16.6%) [179].
Papaya is mainly used as fresh fruit, and for the production
of drinks, jams, and so forth. In some places, the seeds are used for treatment against worms [181]. The green fruit is also a commercial source
Main uses.
136
Chapter Four
Figure 4.22 Carica papaya L. (Photo
courtesy of Barbara Simonsohn
[www.barbara-simonsohn.de/
papaya.htm].)
of the proteolytic enzymes papain and chymopapain—the former finding use in a wide range of industries, particularly brewing for haze
removal, and the latter in medicine. Oil extraction from the seeds could
improve the viability of the industry in countries where papaya is cultivated for papain production and processing. The seeds constitute
around 22% of the waste from papaya puree plants [182]. No references
about its use as a biodiesel source have been found so far.
4.6.5
Sal oil
Shorea robusta Gaertn. f.—commonly known as sal,
shal, saragi, sakhu, sakher, shaal, ral, gugal, mara, sagua, salwa, sakwa,
kandar, and kung—is a large tree belonging to the family Dipterocarpaceae (see Fig. 4.23). The tree is native to southern Asia, ranging south
of the Himalayas, from Myanmar in the east to India, Bangladesh, and
Nepal. It grows in dry tropical forests, in a well-drained, moist, sandy
loam soil. This tree can attain heights up to 35 m. The seeds of sal are
an important source of edible oil. The seed contains around 20% of oil
[183, 184].
Crop description.
Although sal is a highly valued timber species, it is also
used for house construction, and as poles, agriculture implements,
fuelwood, fencing, leaves for cups and plates, and compost [185]. The
oil is used for lighting and cooking purposes, and as a substitute for
Main uses.
Raw Materials to Produce Low-Cost Biodiesel
137
Figure 4.23 Shorea robusta Gaertn.
f. (Photo courtesy of Dr. Mike Kuhns
[http://extension.usu.edu/forestry/
UtahForests/TreeID/Assets/
Images/sal-1.3.jpg].)
cocoa butter in the manufacture of chocolates. It is suitable for soap
making after blending with other softer oils. The oil cakes that remain
after oil extraction contain 10–12% protein and about 50% starch, and
are used as cattle and poultry feed. However, the oil cake contains
5–14% tannin; consequently, not more than 20% is concentrated for
cattle without detrimental effects. As the protein remains completely
undigested, the oil cake yields energy only. Sal resin is burned as
incense in Hindu ceremonies. It is also used for varnishes, for hardening softer waxes for use in the manufacture of shoe polishes, and as
cementing material for plywood, asbestos sheets, and so forth. The
resin is used in an indigenous system of medicine as an astringent and
detergent [184]. No references about its use as a biodiesel source have
been found so far.
4.6.6
Tung oil
Aleurites fordii (Vernicia fordii) and A. montana—
commonly known as the tung tree, Chinese wood, Abrasin, and Mu (see
Fig. 4.24)—belong to the family Euphorbiaceae and grow well in cold climates, but will survive in subtropical conditions (A. fordii). A. montana
prefers a tropical climate. Major producers are China, Argentina,
Paraguay, Brazil, and the United States. The nut of this deciduous tree
contains an oil-rich kernel. The oil content of the air-dried fruit lies
between 15% and 20% [77]. Major fatty acid composition of oil includes
Crop description.
138
Chapter Four
Figure 4.24 Vernicia fordii. (Photo courtesy of Dr. Alvin R. Diamond [http://spectrum.
troy.edu/~diamond/PIKEFLORA.htm].)
palmitic acid (5.5%), oleic acid (4.0%), linoleic acid (8.5%), and eleostearic
acid (82%) [77].
Tung oil is used in paints, varnishes, and so forth. It is also
used in the production of linoleum, resins, and chemical coatings. It
has been used in motor fuels in China [77]. The seed cake after oil
extraction is used as a fertilizer and cannot be used for animal feed as
it contains a toxic protein [75]. No references about its use as a raw material to produce biodiesel have been found to date.
Main uses.
4.6.7
Ucuuba oil
Crop description. Virola surinamensis and V. sebifera (see Fig. 4.25)—
commonly known as ucuhuba, ucuiba, ucuba, muscadier porte-suif, and
yayamadou—belong to the family Myristicaceae and grow in tropical
swampy forests. Major producing countries are Brazil, Costa Rica,
Ecuador, French Guiana, and Guyana. A typical tree is of medium height
and can produce 60–90 L of oil each year. The seeds contain 65–76% oil.
The yellow-brown aromatic oils from both varieties are very similar.
Other related species, such as V. otoba, which grows in Colombia and
Peru, yield a fat similar to ucuuba, which is known as otoba butter or
American nutmeg butter. Major fatty acids present in the oil are lauric
Raw Materials to Produce Low-Cost Biodiesel
139
Ucuuba tree. (Photo courtesy of Eugênio
Arantes de Melo [www.arvores.brasil.nom.br/].)
Figure 4.25
acid (15–17.6%), myristic acid (72.9–73.3%), palmitic acid (4.4–5%), and
oleic acid (5.1–6.3%) [77, 87].
Main uses. This fat has been used traditionally in candle manufacture.
The fat and pulverized kernels find use in traditional medicines. The tree
has been proposed as a potential source of isopropyl myristate, which
is used in cosmetic manufacture [186]. However, no references related
to its use as a raw material to produce biodiesel have been found to date.
Acknowledgments
My sincere thanks to the following people and organizations for their
generosity in letting me use their photos: Dr. Kazuo Yamasaki (Teikyo
Heisei University, Japan), Abdulrahman Alsirhan (www.alsirhan.com),
Eric Winder (Biological Sciences, Michigan Technological University),
Jack Bacheler (Department of Entomology, North Carolina State
University), Dr. Alvin R. Diamond (Department of Biological and
Environmental Sciences, Troy University), Piet Van Wyk and EcoPort,
Food and Agricultural Organization of the United Nations, Antoine van
den Bos (Botanypictures), Forest and Kim Starr (USGS), Dr. Davison
Shillingford (Dominica Academy of Arts and Sciences), Prof. Arne
Anderberg (Swedish Museum of Natural History), Rolv Hjelmstad
(Urtekilden), Peter Chen (College of DuPage), Josina Kimottho (ICRAF),
Gernot Katzer (University of Graz), Prof. Gerald D. Carr (University of
Hawaii, Botany Department), Barbara Simonsohn, Dr. Mike Kuhns
(Utah State University), and Eugênio Arantes de Melo (Árvores do
Brasil).
140
Chapter Four
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Chapter
5
Fuel and Physical Properties
of Biodiesel Components
Gerhard Knothe
5.1
Introduction
Biodiesel is an alternative diesel fuel (DF) derived from vegetable oils
or animal fats [1, 2]. Transesterification of an oil or fat with a monohydric alcohol, in most cases methanol, yields the corresponding monoalkyl esters, which are defined as biodiesel. The successful introduction
and commercialization of biodiesel in many countries around the world
has been accompanied by the development of standards to ensure high
product quality and user confidence. Some biodiesel standards are ASTM
D6751 (ASTM stands for American Society for Testing and Materials)
and the European standard EN 14214, which was developed from previously existing standards in individual European countries.
The suitability of any material as fuel, including biodiesel, is influenced
by the nature of its major as well as minor components arising from production or other sources. The nature of these components ultimately determines the fuel and physical properties. Some of the properties included in
standards can be traced to the structure of the fatty esters in the biodiesel.
Since biodiesel consists of fatty acid esters, not only the structure of the
fatty acids but also that of the ester moiety can influence the fuel properties of biodiesel. The transesterification reaction of an oil or fat leads to a
biodiesel fuel corresponding in its fatty acid profile with that of the parent
oil or fat. Therefore, biodiesel is largely a mixture of fatty esters with each
ester component contributing to the properties of the fuel.
Properties of biodiesel that are determined by the structure of its
component fatty esters and the nature of its minor components include
149
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150
Chapter Five
ignition quality, cold flow, oxidative stability, viscosity, and lubricity.
This chapter discusses the influence of the structure of fatty esters on
these properties. Not all of these properties have been included in
biodiesel standards, although all of them are essential to proper functioning of the fuel in a diesel engine.
Generally, as the least expensive alcohol, methanol has been used to
produce biodiesel. Biodiesel, in most cases, can therefore be termed the
fatty acid methyl esters (FAME) of a vegetable oil or animal fat.
However, as mentioned above, both the fatty acid chain and the alcohol
functionality contribute to the overall properties of a fatty ester. It is
worthwhile to consider the properties imparted by other alcohols yielding fatty acid alkyl esters (FAAE) that could be used for producing
biodiesel. Therefore, both structural moieties will be discussed in this
chapter. Table 5.1 lists fuel properties of neat alkyl esters of fatty acids.
Besides the fuel properties discussed here, the heat of combustion (HG)
of some fatty compounds [3] is included in Table 5. 1, for the sake of underscoring the suitability of fatty esters as fuel with regard to this property.
TABLE 5.1
Properties of Fatty Acids and Estersa
Trivial (systematic)
name; acronymb
Caprylic
(Octanoic); 8:0
Methyl ester
Ethyl ester
Capric
(Decanoic); 10:0
Methyl ester
Ethyl ester
Lauric
(Dodecanoic); 12:0
Methyl ester
Ethyl ester
Myristic
(Tetradecanoic); 14:0
Methyl ester
Ethyl ester
Palmitic
(Hexadecanoic); 16:0
Methyl ester
Ethyl ester
Propyl ester
Isopropyl ester
Butyl ester
2-Butyl ester
Isobutyl ester
Stearic
(Octadecanoic); 18:0
MPc
(C)
BPc,d
(C)
Cetane no.
Viscositye
HGf,
(kcal/mol)
16.5
239.3
43.1
31.5
193
208.5
270
20
44
224
243–5
1311
47.2 (98.1)g
g
51.2 (99.4)
1.40 j; 1.72k 1625
1.99 (25) j 1780
1763.25 (25)
5
1.8fr
58
266766
16325
250.5100
61.4 (99.1)g
1.95 j; 2.43k 1940
2.88 j
2098
2073.91 (25)
18.5
12.3
63
295751
295
350
66.2 (96.5)g
66.9 (99.3)g
2.69 j
30.5
415–8747
19.3/24
20.4
13–4
16.9
19110
19012
1602
74.5 (93.6)g; 3.60 j; 4.38k 2550
85.9i
93.1i
2717
85.0i
82.6i
91.9i
84.8i
83.6i
61.7h
2696.12 (25)
22.5, 28.9 1995
71
360d
33.6 (98.6)g
47.6 (98.0)g
0.99 j; 1.19k 1313
1.37 (25) j 1465
1453.07 (25)
2254
2406
2384.76 (25)
Fuel and Physical Properties of Biodiesel Components
TABLE 5.1
a
Properties of Fatty Acids and Esters (Continued)
MPc
(C)
Trivial (systematic)
name; acronymb
BPc,d
(C)
Methyl ester
39
442–3747
Ethyl ester
Propyl ester
Isopropyl ester
Butyl ester
2-Butyl ester
Isobutyl ester
Palmitoleic (9(Z)Hexadecanoic); 16:1
Methyl ester
Oleic (9(Z)Octadecanoic); 18:1
Methyl ester
Ethyl ester
Propyl ester
Isopropyl ester
Butyl ester
2-Butyl ester
Isobutyl ester
Linoleic (9Z,12ZOctadecadienoic); 18:2
Methyl ester
Ethyl ester
Propyl ester
Butyl ester
Linolenic (9Z,12Z,15ZOctadecatrienoic); 18:3
Methyl ester
Ethyl ester
Propyl ester
Butyl ester
Ricinoleic (12-Hydroxy9Z-octadecenoic);
18:1, 12-OH
Methyl ester
Erucic (13ZDocosenoic); 22:1
Methyl ester
Ethyl ester
31–33.4
19910
27.5
343
16
286100
20
218.520
216–7151
5
229–3016
35
21520
270–5180
a
151
Cetane no.
86.9 (92.1)g;
101i
76.8h; 97.7i
69.9h; 90.9i
96.5i
80.1h; 92.5i
97.5i
i
99.3
Viscositye
4.74 j
2859
3012
i
11
HGf,
(kcal/mol)
2521
2657.4 (25)
51.0
46.1h
55h; 59.3i
53.9h; 67.8i
55.7h; 58.8i
86.6i
59.8h; 61.6i
71.9i
59.6i
31.4h
3.73 j; 4.51k 2828
5.50 (25) j
42.2h; 38.2i
37.1h; 39.6i
40.6h; 44.0i
41.6h; 53.5i
230–217
3.05 j; 3.65k 2794
20.6g; 22.7i
26.7h
26.8h
2.65 j; 3.14k 2750
20.4h
57/–52
1090.018
1742.5
5.5
24510
15.44k
33–4
225–715
26515
221–25
229–305
5.91j
3454
Adapted from Ref. [4].
The numbers denote the number of carbons and double bonds. For example, in oleic acid,
18:1 stands for 18 carbons and 1 double bond.
c
Melting point and boiling point data are from Refs. [5] and [6].
d
Superscripts denote pressure (mm Hg) at which the boiling point was determined.
e
Viscosity values determined at 40C, unless indicated otherwise.
f
HG values are from Refs. [3] and [5].
g
Number in parentheses indicates purity (%) of the material used for CN determination as
given in Ref. [7].
h
Ref. [8].
i
Ref. [9].
j
Dynamic viscosity (mPa s cP), Ref. [10].
k
Kinematic viscosity (mm2/s cSt), Ref. [11].
b
152
5.2
Chapter Five
Cetane Number and Exhaust Emissions
The cetane number (CN), which is related to the ignition properties, is
a prime indicator of fuel quality in the realm of diesel engines. It is conceptually similar to the octane number used for gasoline. Generally, a
compound that has a high octane number tends to have a low CN and
vice versa. The CN of a DF is related to the ignition delay (ID) time, i.e.,
the time between injection of the fuel into the cylinder and onset of
ignition. The shorter the ID time, the higher the CN, and vice versa.
Standards have been established worldwide for CN determination,
e.g., ASTM D613 in the United States, and internationally the
International Organization for Standardization (ISO) standard ISO
5165. A long straight-chain hydrocarbon, hexadecane (C16H34; trivial
name cetane, giving the cetane scale its name) is the high-quality standard on the cetane scale with an assigned CN of 100. A highly branched
isomer of hexadecane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), a compound with poor ignition quality, is the low-quality standard with an
assigned CN of 15. The two reference compounds on the cetane scale
show that CN decreases with decreasing chain length and increasing
branching. Aromatic compounds that are present in significant amounts
in petrodiesel have low CNs but their CNs increase with increasing size
of n-alkyl side chains [12, 13]. The cetane scale is arbitrary, and compounds with CN 100 or CN 15 have been identified. The American
standard for petrodiesel (ASTM D975) prescribes a minimum CN of 40,
while the standards for biodiesel prescribe a minimum of 47 (ASTM
D6751) or 51 (European standard EN 14214). Due to the high CNs of
many fatty compounds, which can exceed the cetane scale, the lipid
combustion quality number for these compounds has been suggested [14].
The use of biodiesel reduces most regulated exhaust emissions from
a diesel engine. The species reduced include carbon monoxide, hydrocarbons, and particulate matter (PM). Nitrogen oxide (NOx) emissions
are slightly increased, however. When blending biodiesel with
petrodiesel, the effect of biodiesel is approximately linear to the blend
level. A report summarizing exhaust emissions tests with biodiesel is
available [15], and other summaries are given in Refs. [16, 17].
The structure of the fatty esters in biodiesel affects the levels of exhaust
emissions. When using a 1991-model, 6-cylinder, 345-bhp (257-kW),
direct-injection, turbocharged, and intercooled diesel engine, NOx exhaust
emission increased with increasing number of double bonds and decreasing chain length for saturated chains [18]. Although often a trade-off is
observed between NOx and PM exhaust emissions, no trade-off has been
observed in this work when varying the chain length [18]. The CN and
density were correlated with emission levels [18]. However, emissions are
likely affected by the technology level of the engine. When conducting
tests on a 2003-model, 6-cylinder, 14–L, direct-injection, turbocharged,
Fuel and Physical Properties of Biodiesel Components
153
intercooled diesel engine with exhaust gas recirculation (EGR), no chain
length effect has been observed for NOx exhaust emissions, although
the level of saturation still played a significant role [19]. PM exhaust
emissions were reduced to levels close to the US 2007 regulations
required for ultra-low-sulfur petrodiesel fuel. Also, PM levels were lower
than those for neat hydrocarbons which would be enriched in “clean”
petrodiesel fuel [19]. In both studies [18, 19], NOx emissions of the saturated esters were slightly below those of the reference petrodiesel fuel.
For petrodiesel fuel, higher CNs have been correlated with reduced
NOx exhaust emissions [20]. This correlation has led to efforts to improve
the CN of biodiesel fuels by using additives known as cetane improvers
[8]. Despite the inherent relatively high CNs of fatty compounds, NOx
exhaust emissions usually increase slightly when operating a diesel
engine on biodiesel, as mentioned above. The relationship between the
CN and engine emissions is complicated by many factors, including the
technology level of the engine. Older, lower-injection pressure engines
are generally very sensitive to CN, with increased CN causing significant reductions in NOx emissions, due to shorter ID times and the resulting lower average combustion temperatures. More modern engines that
are equipped with injection systems that control the rate of injection are
not very sensitive to CN [21–23].
Historically, the first CN tests were carried out on palm oil ethyl
esters [24, 25], which have a high CN, a result confirmed by later studies on many other vegetable oil-based DFs and individual fatty compounds. The influence of the compound structure on CNs of fatty
compounds has been discussed in more recent literature [26], with the
predictions made in that paper being confirmed by practical cetane tests
[7–9, 13]. CNs of neat fatty compounds are given in Table 5.1. In summary, the results show that CNs decrease with increasing unsaturation
and increase with increasing chain length, i.e., uninterrupted CH2 moieties. However, branched esters derived from alcohols such as isopropanol have CNs competitive with methyl or other straight-chain
alkyl esters [9, 27]. Thus, one long, straight chain suffices to impart a
high CN, even if the other moiety is branched. Branched esters are of
interest because they exhibit improved low-temperature properties.
Recently, cetane studies on fatty compounds have been conducted
using the Ignition Quality Tester™ (IQT ) [9]. The IQT is a further,
automated development of a constant volume combustion apparatus
(CVCA) [28, 29]. The CVCA was originally developed for determining
CNs more rapidly with greater experimental ease, better reproducibility, reduced use of fuel, and therefore less cost than the ASTM D613
method utilizing a cetane engine. The IQT method, which is the basis
of ASTM D6890, was shown to be reproducible and the results competitive with those derived from ASTM D613. Some results from the IQT
154
Chapter Five
are included in Table 5.1. For the IQT, ID and CN are related by the following equation [9]:
CNIQT 83.99 (ID 1.512)
0.658
3.547
(5.1)
In the recently approved method ASTM D6890, which is based on this
technology, only ID times of 3.6–5.5 ms [corresponding to 55.3–40.5
DCN (derived CN)] are covered as the precision may be affected outside
that range. However, for fatty compounds, the results obtained by using
the IQT are comparable to those obtained by other methods [9].
Generally, the results of cetane testing for compounds with lower CNs,
such as more unsaturated fatty compounds, show better agreement over
various related literature references than the results for compounds
with higher CNs, because of the nonlinear relationship [see Eq. (5.1)]
between the ID time and the CN, which was observed previously [30].
Thus, small changes at shorter ID times result in greater changes in CN
than at longer ID times. This would indicate a leveling-off effect on
emissions such as NOx, as discussed above, once a certain ID time with
corresponding CN has been reached as the formation of certain species
depend on the ID time. However, for newer engines, this aspect must
be modified as discussed above.
5.3
Cold-Flow Properties
One of the major problems associated with the use of biodiesel is poor
low-temperature flow properties, documented by relatively high cloud
points (CPs) and pour points (PPs) [1, 2]. The CP, which usually occurs
at a higher temperature than the PP, is the temperature at which a fatty
material becomes cloudy due to the formation of crystals and solidification of saturates. Solids and crystals rapidly grow and agglomerate,
clogging fuel lines and filters and causing major operability problems.
With decreasing temperature, more solids form and the material
approaches the PP, the lowest temperature at which the material will
still flow. Saturated fatty compounds have significantly higher melting
points than unsaturated fatty compounds (Table 5.1), and in a mixture,
they crystallize at higher temperatures than the unsaturates. Thus,
biodiesel fuels derived from fats or oils with significant amounts of saturated fatty compounds will display higher CPs and PPs.
Besides the CP (ASTM D2500) and PP (ASTM D97) tests, two test
methods for the low-temperature flow properties of petrodiesel exist,
namely, the low-temperature flow test (LTFT) (used in North America;
e.g., ASTM D4539) and cold filter plugging point (CFPP) (used outside
North America; e.g., the European standard EN 116). These methods
have also been used to evaluate biodiesel and its blends with No. 1 and
Fuel and Physical Properties of Biodiesel Components
155
2 petrodiesel. Low-temperature filterability tests were stated to be necessary because of better correlation with operability tests than the CP
or PP test [31]. However, for fuel formulations containing at least 10
vol% methyl esters, both LTFT and CFPP are linear functions of the CP
[32]. Additional statistical analysis have shown a strong 1:1 correlation
between LTFT and CP [32].
Several approaches to low-temperature problems of esters have been
investigated, including blending with petrodiesel, winterization, additives,
branched-chain esters, and bulky substituents in the chain. The latter
approach may be considered a variation of the additive approach, as the
corresponding compounds have been investigated in biodiesel at additive
levels. Blending of esters with petrodiesel will not be discussed here.
Numerous, usually polymeric, additives were synthesized and
reported mainly in the patent literature to have the effect of lowering
the PP or sometimes even the CP. A brief overview of such additives has
been presented [33]. Similarly, the use of fatty compound-derived materials with bulky moieties in the chain [34] at additive levels has been
investigated. The idea associated with these materials is that the bulky
moieties in these additives would destroy the harmony of the crystallizing solids. The effect of some additives appears to be limited because
they more strongly affect the PP than the CP or they have only a slight
influence on the CP. The CP, however, is more important than the PP
for improving low-temperature flow properties [35].
The use of branched esters such as isopropyl, isobutyl, and 2-butyl
esters instead of methyl esters [36, 37] is another approach for improving the low-temperature properties of biodiesel. Branched esters have
lower melting points in the neat form (Table 5.1). These esters showed
a lower TCO (crystallization onset temperature), as determined by differential scanning calorimetry (DSC) for the isopropyl esters of soybean
oil (SBO) by 7–11C and for the 2-butyl esters of SBO by 12–14C [36].
The CPs and PPs were also lowered by the branched-chain esters. For
example, the CP of isopropyl soyate was given as 9C [7] and that of
2-butyl soyate as 12C [36]. In comparison, the CP of methyl soyate is
around 0C [32]. However, in terms of economics, only isopropyl esters
appear attractive as branched-chain esters, although even they are
more expensive than methyl esters. Branching in the ester chain does
not have any negative effect on the CN of these compounds, as discussed above.
Winterization [35, 38, 39] is based on the lower melting points of
unsaturated fatty compounds than saturated compounds (Table 5.1).
This method removes by filtration the solids formed during the cooling
of the vegetable oil esters, leaving a mixture with a higher content of
unsaturated fatty esters and thus with lower CP and PP. This procedure
can be repeated to further reduce the CPs and PPs. Saturated fatty
156
Chapter Five
compounds, which have higher CNs (Table 5.1) than unsaturated fatty
compounds, are among the major compounds removed by winterization.
Thus the CN of biodiesel decreases during winterization. Loss of material was reduced when winterization was carried out in presence of coldflow improvers or solvents such as hexane and isopropanol [39].
In other work [40], tertiary fatty amines and amides have been
reported to be effective in enhancing the ignition quality of biodiesel
without negatively affecting the low-temperature properties. Also, saturated fatty alcohols of chain lengths C12 increased the PP substantially. Ethyl laurate weakly decreased the PP.
5.4
Oxidative Stability
Oxidative stability of biodiesel has been the subject of considerable
research [41–62]. This issue affects biodiesel primarily during extended
storage. The influence of parameters such as presence of air, heat, traces
of metal, antioxidants, and peroxides as well as nature of the storage
container was investigated in the aforementioned studies. Generally, factors such as the presence of air, elevated temperatures, or the presence
of metals facilitate oxidation. Studies performed with the automated oil
stability index (OSI) method have confirmed the catalyzing effect of
metals on oxidation; however, the influence of the compound structure
of the fatty esters, especially unsaturation as discussed below, was even
greater [52]. Numerous other methods, including not only wet-chemical
ones such as the acid value and peroxide value, but also pressurized differential scanning calorimetry, nuclear magnetic resonance (NMR), and
so forth, have been applied in oxidation studies of biodiesel.
Two simple methods for assessing the quality of stored biodiesel are
the acid value and viscosity since both increase continuously with
increasing fuel degradation, i.e., deteriorating fuel quality. The peroxide
value is less suitable because it reaches a maximum and then can
decrease again due to the formation of secondary oxidation products [48].
Autoxidation occurs due to the presence of double bonds in the chains
of many fatty compounds. Autoxidation of unsaturated fatty compounds
proceeds with different rates, depending on the number and position of
double bonds [63]. Especially the positions allylic to double bonds are
susceptible to oxidation. The bis-allylic positions in common polyunsaturated fatty acids, such as linoleic acid (double bonds at .C-9 and
.C-12, giving one bis-allylic position at C-11) and linolenic acid (double
bonds at .C-9, .C-12, and C-15, giving two bis-allylic positions at C-11
and C-14), are even more prone to autoxidation than the allylic positions.
The relative rates of oxidation given in the literature [63] are 1 for
oleates (methyl, ethyl esters), 41 for linoleates, and 98 for linolenates.
This is essential because most biodiesel fuels contain significant amounts
Fuel and Physical Properties of Biodiesel Components
157
of esters of oleic, linoleic, or linolenic acids, which influence the oxidative stability of the fuels. The species formed during the oxidation
process cause the fuel to eventually deteriorate.
A European standard (EN 14112; Rancimat method) for oxidative
stability has been included in the American and European biodiesel
standards (ASTM D6751 and EN 14214). Both biodiesel standards call
for determining oxidative stability at 110C; however, EN 14214 prescribes a minimum induction time of 6 h by the Rancimat method while
ASTM D6751 prescribes 3 h. The Rancimat method is nearly identical
to the OSI method, which is an AOCS (American Oil Chemists’ Society)
method.
Besides preventing exposure of the fatty material to air, adding antioxidants is a common method to address the issue of oxidative stability.
Common antioxidants are synthetic materials such as tert-butylhydroquinone (TBHQ), butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and propyl gallate (PG) as well as natural materials
such as tocopherols. Antioxidants delay oxidation but do not prevent it,
as oxidation will commence once the antioxidant in a material has been
consumed.
5.4.1
Iodine value
The iodine value (IV) has been included in the European biodiesel standards to purportedly address the issue of oxidative stability and the
propensity of the oil or fat to polymerize and form engine deposits. The
IV is a measure of the total unsaturation of a fatty material measured
in grams of iodine per 100 g of sample when formally adding iodine to
the double bonds. An IV of 120 has been specified in EN 14214 and 130
in EN 14213, which would largely exclude vegetable oils such as soybean
and sunflower oils as biodiesel feedstock. Thus the IV has not been
included in biodiesel standards in the United States and Australia, and
is limited to 140 in the South African standard (which would permit sunflower and soybean oils); the provisional Brazilian standard requires
that it only be noted.
The IV of a vegetable oil or animal fat is almost identical to that of the
corresponding methyl esters; however, the IV of alkyl esters decreases
with higher alcohols used in their production since the IV is molecular
weight dependent. For example, the IV of methyl, ethyl, propyl, and
butyl linoleate is 172.4, 164.5, 157.4, and 150.8, respectively [64].
The use of the IV of a mixture for such purposes does not take into
consideration that an infinite number of fatty acid profiles can yield the
same IV and that different fatty acid structures can give the same IV,
although the propensity for oxidation can differ significantly [64]. Other,
158
Chapter Five
new structure indices termed allylic and bis-allylic position equivalents
(APE and BAPE), which are based on the number of such positions in
a fatty acid chain and are independent of molecular weight, are likely
more suitable than the IV [64]. The BAPE index distinguishes mixtures
having nearly identical IV correctly by their OSI times. Note that the
BAPE index is the decisive index compared to the APE because it relates
to the more reactive bis-allylic positions. Engine performance tests with
a mixture of vegetable oils of different IVs did not yield results that
would have justified a low IV [65, 66]. No relationship between the IV
and oxidative stability has been observed in another investigation on
biodiesel with a wide range of IV [52].
5.5
Viscosity
Viscosity affects the atomization of a fuel upon injection into the combustion chamber and, thereby, ultimately the formation of engine
deposits. The higher the viscosity, the greater the tendency of the fuel to
cause such problems. The viscosity of a transesterified oil, i.e., biodiesel,
is about an order of magnitude lower than that of the parent oil [1, 2].
High viscosity is the major fuel property why neat vegetable oils have
been largely abandoned as alternative DF. Kinematic viscosity has been
included in most biodiesel standards. It can be determined by standards
such as ASTM D445 or ISO 3104. The difference in viscosity between the
parent oil and the alkyl ester derivatives can be used in monitoring
biodiesel production [67]. The effect on viscosity of blending biodiesel and
petrodiesel has also been investigated [68], and an equation has been
derived, which allows calculating the viscosity of such blends.
The prediction of viscosity of fatty materials has received considerable
attention in the literature. Viscosity values of biodiesel/mixtures of fatty
esters have been predicted from the viscosities of the individual components
by a logarithmic equation for dynamic viscosity [10]. Viscosity increases
with chain length (number of carbon atoms) and with increasing degree
of saturation. This holds also for the alcohol moiety as the viscosity of
ethyl esters is slightly higher than that of methyl esters [11]. Factors such
as double bond configuration influence viscosity (cis double bond configuration giving a lower viscosity than the trans configuration), while
the double bond position affects viscosity less [11]. Thus, a feedstock
such as used frying oils, which is more saturated and contains some
amounts of trans fatty acid chains, has a higher viscosity than its parent
oil. Branching in the ester moiety, however, has little or no influence on
viscosity, again showing that this is a technically promising approach for
improving low-temperature properties without significantly affecting
other fuel properties. Values for dynamic viscosity and kinematic viscosity
of neat fatty acid alkyl esters are included in Table 5.1.
Fuel and Physical Properties of Biodiesel Components
5.6
159
Lubricity
With the advent of low-sulfur petroleum-based DFs, the issue of DF lubricity is becoming increasingly important. Desulfurization of petrodiesel
reduces or eliminates the inherent lubricity of this fuel, which is essential for proper functioning of vital engine components such as fuel pumps
and injectors. Several studies [10, 11, 67–82] on the lubricity of biodiesel
or fatty compounds have shown a beneficial effect of these materials on
the lubricity of petrodiesel, particularly low-sulfur petrodiesel fuel.
Adding biodiesel at low levels (1–2%) restores the lubricity to low-sulfur
petroleum-derived DFs. However, the lubricity-enhancing effect of
biodiesel at low blend levels is mainly caused by minor components of
biodiesel such as free fatty acids and monoacylglycerols [83], which
have free COOH and OH groups. Other studies [84, 85] also point out
the beneficial effect of minor components on biodiesel lubricity, but these
studies do not fully agree on the responsible species [83–85]. Thus,
biodiesel is required at 1–2% levels in low-lubricity petrodiesel, in order
for the minor components to be effective lubricity enhancers [83]. At
higher blend levels, such as 5%, the esters are sufficiently effective
without the presence of minor components.
While the length of a fatty acid chain does not significantly affect
lubricity, unsaturation enhances lubricity slightly; thus an ester such
as methyl linoleate or methyl linolenate improves lubricity more than
methyl stearate [80, 83]. In accordance with the above observation on
the effect of free OH groups on lubricity, castor oil displayed better
lubricity than other vegetable oil esters [75, 80, 81]. Ethyl esters have
improved lubricity compared to methyl esters [75].
Standards for testing DF lubricity use the scuffing load ball-on-cylinder
lubricity evaluator (SLBOCLE) (ASTM D6078) or the high-frequency
reciprocating rig (HFRR) (ASTM D6079; ISO 12156). Lubricity has not
been included in biodiesel standards despite the definite advantage of
biodiesel over petrodiesel with respect to this fuel property. However, the
HFRR method has been included in the petrodiesel standards ASTM
D975 and EN 590.
5.7
Outlook
The fuel properties of biodiesel are strongly influenced by the properties of the individual fatty esters as well as those of some minor components. Both moieties, the fatty acid and alcohol, have considerable
influence on fuel properties such as CN, with relation to combustion and
exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity.
It therefore appears reasonable to enrich (a) certain fatty ester(s) with
desirable properties in the fuel, in order to improve the properties of the
whole fuel. For example, from the presently available data, it appears
160
Chapter Five
that isopropyl esters have better fuel properties than methyl esters.
The major disadvantage is the higher price of isopropanol in comparison to methanol, besides modifications needed for the transesterification reaction. Similar observations likely hold for the fatty acid moiety.
It may be possible in the future to improve the properties of biodiesel
by means of genetic engineering of the parent oils, which could eventually lead to a fuel enriched with (a) certain fatty acid(s), possibly oleic
acid, that exhibits a combination of improved fuel properties.
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Chapter
6
Processing of Vegetable Oils
as Biodiesel and Engine
Performance
Ahindra Nag
6.1
Introduction
Processing of vegetable oils as biodiesel [1, 2] and its engine performance is very challenging. From an environmental point of view, diesel
engines are a major source of air pollution. Exhaust gases from diesel
engines contain oxides of nitrogen, carbon monoxide, organic compounds
consisting of unburned or partially burned hydrocarbons and particulate matter (consisting primarily of soot).
Interest in clean burning fuels is growing worldwide, and reduction
in exhaust emissions from diesel engines is of utmost importance. It is
widely recognized that alternative diesel fuels produced from vegetable
oils and animal fats can reduce exhaust emissions from compression
ignition (CI) engines, without significantly affecting engine performance. But reducing pollutant emissions from diesel engines requires a
detailed knowledge of the combustion process. However, the complex
nature of the combustion process in an engine makes it difficult to
understand the events occurring in the combustion chamber that determine the emission of exhaust gases.
Dr. Rudolf Diesel [3], the inventor of the CI engine, used peanut oil
in one of his engines for a demonstration at the Paris exhibition in 1900.
Then there was considerable interest in the use of vegetable oils as fuel
in diesel engines.
165
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166
Chapter Six
Several studies have reported the effects of fuel and engine parameters on diesel exhaust emissions. Chowdhury [4] claims to have successfully used raw vegetable oils in diesel engines. He observed that no
major changes were necessary in the engine, but the engine could not
be run for more than 4 h. The performance and economic aspects of
vegetable oil were also discussed.
Barve and Amurthe [5] cite an example of using groundnut oil as
fuel in a diesel engine generator set (103 kW) of a local water pump
house. They claimed that the power output and fuel consumption were
very much comparable with certified diesel fuel. Weibe and Nowakowska
[6] have reported the use of palm oil as a motor fuel. The performance
was found satisfactory with higher fuel consumption. Fang [7] has
reported that soybean and castor oil blended with diesel fuel burns
adequately in a small diesel engine. Engelman et al. [8] has presented
data on the performance of soybean diesel oil blends compared with
diesel fuel. Results from a short-duration test showed that the use of
blends was feasible in the diesel engine; but in fact, in the long-term,
test problems associated with lubrication, sticking piston rings, and
injector atomization patterns contributed to mechanical difficulties in
the engine. Cruze et al. [9] have found that atomization of the fuel by
the injector, in some cases, has caused delayed ignition characteristics
and reduced efficiency of mechanical power production, compared to
diesel fuel. Pryde [10] has stated that raw vegetable oil has had no great
promise for engine tests and that modified oil esters were required for
further engine tests. Bruwer [11] has reported that even without modification, nine diesel engines started and operated almost normally on
sunflower oil and delivered power equal to that of diesel fuel. Brake
thermal efficiency and maximum engine power were 3% lower, while
the specific fuel consumption was 10% higher than that of diesel fuel.
The bench test, however, showed that atomization of 100% sunflower
oil was much poorer than diesel but could be improved by reducing
the viscosity of oil. Energy wise, sunflower oil was favorable for running diesel engines for a shorter duration.
Baranescu et al. [12] have conducted tests on a turbocharged engine,
using mixtures of sunflower oil in 25%, 50%, and 75% with diesel fuel.
They have concluded that the use of sunflower oil blended with diesel
brought modification in the fuel injection process that mainly included
an increase in injection pressure and a longer ignition duration. These
effects led to longer combustion duration. Cold-temperature operation
was very critical due to high viscosity that caused fuel system problems
such as starting failure, unacceptable emission levels, and injection
pump failure. Engine shutdown for a long duration accelerated gum
formation, where the fuel contacted the bare metal. This might further
impair the engine or injection system.
Processing of Vegetable Oils as Biodiesel and Engine Performance
167
Wagner et al. [13] have conducted tests on a number of diesel engines
with different blends of winter rape and safflower oil with diesel fuel.
The following specific conclusions were drawn from the results obtained:
䊏
High viscosity and tendency to polymerize within the cylinder were
major physical and chemical problems.
䊏
Attempt to reduce the viscosity of the oil by preheating the fuel by
increasing the temperature of the fuel at the injector to the required
value was not successful.
䊏
Short-term engine performance showed power output and fuel consumption equivalent to diesel fuel.
䊏
Severe engine damage occurred within a very short duration when the
test was conducted for maximum power with varying engine rpm
(revolutions per minute).
䊏
A blend of 70% winter rape with 30% diesel was successfully used for
50 h. No adverse effect was noted.
䊏
A diesel injector pump when run for 154 h with safflower oil had no
abnormal wear, gumming, or corrosion.
Borgelt et al. [14] have conducted tests on three diesel engines containing 25–75% and 50–50% soybean oil and diesel. The engines were
operated under 50% load for 1000 horsepower (HP); the output ranged
from 2.55 to 2.8 kW. Thermal efficiency ranged from 19.3 to 20%. Engine
performances were not significantly different. Carbon deposit increased
with increased percentage of soybean oil. Thus, Borgelt et al. concluded
that use of 25% or less soybean oil caused negligible changes in engine
performance.
Barsic and Humke [15] performed a study in which blends of unrefined peanut and sunflower with diesel fuel (50–50%) were used in a
single-cylinder engine. The engine produced equivalent power or a minor
increase (6%) with vegetable oils and blends, with a 20% increase in specific fuel consumption. Performance tests at equal energy showed that
the power level remained constant or decreased slightly, thermal efficiency decreased slightly, and the exhaust temperature increased with
an increase in the percentage of vegetable oil in the fuel. Exhaust emission at equal energy input showed slightly higher NOx for vegetable oils
and their blends. Unburned hydrocarbon emission was about 50% higher
than pure diesel fuel because the injection system was not optimized for
more viscous fuels. Ziejewski et al. [16] reported the results of an
endurance test using a 25–75% blend of alkali-refined sunflower oil with
diesel and 25–75% blend of safflower oil with diesel on a volume basis. The
major problems experienced were premature injection, determination of
nozzle performance, and heavier carbon deposits in the piston ring grooves.
168
Chapter Six
There was no significant problem with engine operation when the blend
of safflower oil was used. That investigation revealed that chemical differences between vegetable oil and diesel had a very important influence
on long-term engine performance. Bhattacharya et al. [17] have reported
that a blend of 50% rice bran oil with diesel could be a supplementary
fuel for their 10-bhp CI engine. No significant difference in the brake
thermal efficiency was reported.
Samson et al. [18] have reported the use of tallow and stillingia oil in
25–75% and 50–50% blends by mass with diesel. The fuel properties of the
blends were found to be within the limits proposed for diesel. The heat of
combustion appeared to decrease. Specific gravity and kinematic viscosity increased with the increase in concentration of oil. Dunn et al. [19] conducted the test on rubber seed oil blended with diesel in 25%, 50%, 75%,
and 100% in an air-cooled engine with 4.9 kW at 3600 rpm. Higher specific fuel consumption and slightly higher thermal efficiency were observed.
But, carbon deposits were heavier than those for pure diesel fuel.
Samga [20] conducted a test on a water-cooled single-cylinder diesel
engine, using hone oil (ken seed oil). He concluded that hone oil gave
acceptable performance, smooth running, and ease in starting without
preheating. The exhaust temperature and specific fuel consumption
were higher than those for diesel. The partial-load efficiency was lower,
but full-load efficiency was better than with diesel fuel.
Auld et al. [21] evaluated the potential yield and fuel qualities of
winter rape, safflower, and sunflower as sources of fuel for diesel engines.
Vegetable oils contained 94–95% heat value of diesel fuel, but were
11.1–17.6 times more viscous and also 7–9% heavier than diesel fuel.
Viscosities of vegetable oils were closely related to fatty acid chain length
and number of unsaturated bonds. During short-term engine tests, all
vegetable oils produced power comparable to that of diesel, and the
thermal efficiency was 1.8–2.8% higher than that of diesel. Based on the
results, they concluded that vegetable oil as fuel should be selected on
identification of the crop species that produced the most optimum yield
of fuel quality vegetable oils.
Ryan et al. [22] have tested four different types of vegetable oils (soybean,
sunflower, cottonseed, and peanut) in at least three different stages of
processing. All the oils were characterized according to their physical and
chemical properties. The spray characteristics of oils were determined
at different fuel temperatures, using a high-pressure, high-temperature
injection bomb, and high-speed motion picture camera. The injection
study pointed out that vegetable oil behaved differently from diesel fuel.
Normally, as the viscosity decreased, the penetration rate decreased
and the spray cone angle increased. Using vegetable oils, however,
increased the penetration rate, and increasing the temperature of the
oil from 45C to 145C reduced the cone angle and decreased the viscosity.
Processing of Vegetable Oils as Biodiesel and Engine Performance
169
Engine test results, based on the specific energy, showed that degummed
soybean performed as well as the base fuel, but performance of the
deodorized sunflower was the worst of those tested with an energy consumption 10% higher than the base fuel. Vegetable oils had a much
smaller premixed combustion stage, with the diffused stage of combustion being flatter for sunflower and soybean oil than for the diesel fuel.
Engine inspection showed that heating of the oil reduced the carbon
deposit problem. It was concluded that deposits and overall durability
were related to viscosity differences and the chemical structures of the
other oil as compared to diesel fuel.
Mathur and Das [23] have conducted tests on diesel engines, using
blends of mahua and neem oil with diesel. Results showed that neem
oil could be substituted for up to 35% with marginal reduction in efficiency and power output. Mahua oil with diesel had exhaust characteristics similar to those of diesel. Further, savings in the diesel fuel
through the use of both these nonedible oils outweighed the demerits of
a marginal drop in efficiency and a slight loss in power output.
Goering et al. [24] conducted tests on a diesel engine using a hybrid
fuel formed by micro-emulsion of aqueous ethanol in soybean oil. The
test data were compared with the data from a baseline test on diesel fuel.
The nonionic emulsion produced the same power as diesel fuel, with 19%
lower heating value. Brake specific fuel consumption (BSFC) was 16%
higher, and the brake thermal efficiency was 6% higher, with diesel at
full power. Diesel knock for the hybrid fuel was not worse than for diesel
fuel; thus the low cetane number of the hybrid fuel was not reflected in
engine performance. Hybrid fuels were less volatile than ethanol and
thus safer. The effect of hybrid fuel on the engine durability was
unknown.
6.2 Processing of Vegetable Oils
to Biodiesel
Different techniques adopted for converting vegetable oils to biodiesel are
(a) degumming of vegetable oils, (b) transesterification by acid or alkali,
and (c) enzymatic transesterification.
6.2.1
Degumming of vegetable oils
Degumming is an economical chemical process involving acid treatment
to improve the viscosity and cetane number up to a certain limit so that
the blends of nonedible oils with diesel can be used satisfactorily in a
diesel engine. It is a very simple process by which the gum of the vegetable oil is removed to decrease the viscosity of oil by using an appropriate acid that can be optimized for reduction in viscosity. The quantity
of acid and the duration of the process are very important to obtain
170
Chapter Six
optimum results. Compared to transesterification, the process of degumming is simple, very easy, and less costly, and the reduction in viscosity
of vegetable oil is very small.
Nag et al. [25] degummed karanja, putranjiva, and jatropha oils by
phosphoric acid treatment. Before degumming the oils, the fuel properties
of three oils have been measured and compared with diesel (Table 6.1).
Acid concentrations of 1%, 2%, 3%, 4%, and 5% were used at 40C with
vigorous stirring. The stirring was continued for 10 min after adding the
acid. After stirring, the mixtures were held for 1 week to complete the
reactions and to settle the gum materials. Then the mixtures were filtered
through a packed bed filled with charred sawdust. Viscosities of the filtrate were then measured.
After studying the properties of
the jatropha, karanja, and putranjiva oils, they were degummed. In this
context, the Ricardo variable-compression engine (Ricardo & Co. Engineers
Ltd., England, single cylinder, 3-in bore, 35/8 in stroke) was run with
10%, 20%, 30%, and 40% blends of degummed karanja, jatropha, and
putranjiva oils with diesel at different loads (0–2.7 kW) and different
timings (45, 40, 35, and 30 bTDC [before top dead center]). To measure emissions, an automotive exhaust monitor (model PEA205) and smoke
meter (model OMS103, Indus Scientific Pvt. Ltd., India) were used.
Degumming by acid treatment lowers the viscosity. Viscosities of
karanja, jatropha, and putranjiva oils degummed at 40C and at various
acid concentrations are shown in Fig. 6.1. Karanja oil with 4% acid
treatment had the lowest viscosity, whereas jatropha and putranjiva oils
both had the lowest viscosities with 1% acid treatment.
Performance and emission measurement.
Effect of timing. By observing the performance data at various timings
(45, 40, and 35 bTDC) in Fig. 6.2, it was concluded that at 45 bTDC
timing, the nonedible karanja, jatropha, and putranjiva oils gave the
highest yields, whereas at 40 bTDC timing, diesel gave the highest
yield. That may have been due to the different ignition temperatures of
the nonedible oils from diesel.
TABLE 6.1 Fuel Properties of Three Nonedible Oils and Diesel
Properties
Karanja
Jatropha
Putranjiva
Diesel
Viscosity in cSt (at 40C)
Cetane number
Calorific value (kJ/kg)
Pour point (C)
Specific gravity at 25C
Flash point (C)
Fire point (C)
Carbon residue (%)
43.67
29.9
36,258
5
0.932
215
235
1.4
35.38
33.7
38,833
2
0.916
280
291
0.2
37.62
31.3
39,582
3
0.918
48
53
0.9
5.032
46.3
42,707
12
0.834
78
85
0.1
Processing of Vegetable Oils as Biodiesel and Engine Performance
Jatropha
Karanja
Putranjiva
45
Viscosity (cSt)
40
35
30
25
0
1
2
3
Acid concentration (%)
4
5
Viscosity versus acid concentration of jatropha, karanja,
and putranjiva oils at 40C.
Figure 6.1
22
45° bTDC
40° bTDC
35° bTDC
20
Efficiency (%)
18
16
14
12
10
Diesel
Jatropha
Karanja
Putranjiva
Oils
Figure 6.2 Brake thermal efficiency at various timings of diesel and 20%
vegetable oil blends at 1-kW brake power, 1200 rpm, and 20 compression
ratio.
171
172
Chapter Six
Performance of various blends. Performances of blends of degummed
vegetable oil with diesel are shown in Figs. 6.3 and 6.4. The 20% blends
of jatropha, karanja, and putranjiva oils with diesel gave quite satisfactory performance related to BSFC and brake thermal efficiency (␩bt).
Beyond the 20% blends, the cetane numbers and viscosities of the blends
were not so effective.
As per Figs. 6.5 and 6.6, engine
performance using jatropha and karanja oils was better than diesel but
the use of putranjiva oil gave reverse results at all loads, although the
results were more or less the same. Degummed karanja oil blends gave
better performance, but at high loads, the performance of jatropha oil
blends was better in comparison to the performance of karanja oil blends.
The performance data showed that all three vegetable oils could be used
as alternative fuels for diesel engines.
Comparison of the performance of blends.
As per
Figs. 6.7 and 6.8, it is interesting to note that for the karanja, jatropha,
and putranjiva oils, in every case, smoke and particulates decreased,
which was very favorable in terms of their environmental impact on
human beings. The rate of increase in smoke and particulate generation
with the load of jatropha oil, in comparison to karanja and putranjiva
Effect of loads on emissions of vegetable oil blends and comparison.
500
Diesel
10% blend
20% blend
30% blend
40% blend
BSFC (kg/kWh)
450
400
350
300
250
200
Karanja
Jatropha
Putranjiva
Oils
Figure 6.3 Brake specific fuel consumption versus vegetable oils and diesel
blends at 1200 rpm, 45 bTDC, 20 compression ratio, and 1.4-kW brake power.
Processing of Vegetable Oils as Biodiesel and Engine Performance
28
Diesel
10% blend
26
20% blend
30% blend
40% blend
24
Efficiency (%)
22
20
18
16
14
12
10
Karanja
Jatropha
Putranjiva
Oils
Figure 6.4 Brake thermal efficiency versus brake horsepower of vegetable oil and diesel blends at 1200 rpm, 45 bTDC, 20 compression
ratio, and 1.4-kW brake power.
700
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
650
BSFC (gm/kWh)
600
550
500
450
400
350
300
250
0.0
0.5
1.0
1.5
2.0
Brake power (kW)
2.5
3.0
Brake specific fuel consumption versus brake power of diesel,
20% karanja oil, jatropha oil, and putranjiva oil blends at 1200 rpm,
45 bTDC, and 20 compression ratios.
Figure 6.5
173
174
Chapter Six
35
30
Efficiency (%)
25
20
Diesel
20% K blend
20% J blend
15
10
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
5
0
0.0
0.5
1.0
1.5
2.0
Brake power (kW)
2.5
3.0
Figure 6.6 Brake thermal efficiency versus brake power of diesel, 20%
karanja oil, 20% jatropha oil, and 20% putranjiva oil blends at 1200 rpm,
45 bTDC, and 20 compression ratio.
40
35
Smoke (Hu)
30
25
20
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
15
10
5
0
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Brake power (kW)
Figure 6.7 Smoke versus brake power of diesel, 20% karanja oil, 20%
jatropha oil, and 20% putranjiva oil blends at 1200 rpm, 45 bTDC, and
20 compression ratio.
Processing of Vegetable Oils as Biodiesel and Engine Performance
175
110
100
Particulates (mg/m3)
90
80
70
60
50
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
40
30
20
10
0
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Brake power (kW)
Figure 6.8 Particulates versus brake power of diesel, 20% karanja oil, 20%
jatropha oil and 20% putranjiva oil blends at 1200 rpm, 45 bTDC, and 20
compression ratio.
oils, was very low. It is very interesting to observe that although the particulates and smoke for all the oils decreased, jatropha oil blends gave
the highest reduction.
In Figs. 6.9 and 6.10, the CO, CO2, NOx, and HC (hydrocarbon) emissions for the three nonedible oils were less in comparison to diesel at high
loads. However, at low loads, emissions from the nonedible oils are
almost parallel to diesel. Because of the higher ignition temperature of
nonedible oils than diesel, the better combustion of these oils gave less
exhaust emissions.
Thus, degumming is an economic chemical process for a 20% blend of
karanja, jatropha, and putranjiva oils with diesel to have very satisfactory
results. The degumming method, therefore, offers a potential low-cost
method with simple technology for producing an alternative fuel for CI
engines. Out of the three nonedible oils, jatropha oil was the most promising to yield good performance and emissions at high loads in all
respects. Comparing CO, CO2, NOx, HC, smoke, and particulate emissions from using the three nonedible oils, jatropha oil was very encouraging (see Fig. 6.11). Considering the above-mentioned points, it can be
concluded that the diesel engine can be run very satisfactorily using a
20% blend of vegetable oil with diesel at 45 bTDC, 1200 rpm, and 20
compression ratios. Any diesel engine can be operated with a 20% blend
176
Chapter Six
100
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
NOx (ppm)
80
60
40
20
0
0.0
0.4
0.8
1.2
Brake power
1.6
2.0
2.4
Figure 6.9 Nitrogen oxide versus brake power of diesel, 20% karanja oil,
20% jatropha oil, and 20% putranjiva oil blends at 1200 rpm, 45 bTDC,
and 20 compression ratio.
100
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
HC (ppm)
80
60
40
20
0
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Brake power (kW)
Unburnt hydrocarbon versus brake power of diesel, 20%
karanja oil, 20% jatropha oil, and 20% putranjiva oil blends at 1200 rpm,
45 bTDC, and 20 compression ratio.
Figure 6.10
Processing of Vegetable Oils as Biodiesel and Engine Performance
177
0.05
Diesel
20% K blend
20% J blend
20% P blend
K-Karanja
J-Jatropha
P-Putranjiva
CO (%)
0.04
0.03
0.02
0.01
0.00
0.0
0.4
0.8
1.2
1.6
2.0
2.4
Brake power (kW)
Figure 6.11 Carbon monoxide versus brake power of diesel, 20%
karanja oil, 20% jatropha oil, and 20% putranjiva oil blends at 1200 rpm,
45 bTDC, and 20 compression ratio.
of degummed vegetable oils as a prime mover for agriculture purposes
without any modification of the engine.
6.2.2 Transesterification of vegetable oils
by acid or alkali
Goering et al. [24] have suggested that vegetable oils are too viscous for
prolonged use in direct-injected diesel engines, which has led to poor fuel
atomization and inefficient mixing with air, contributing to incomplete
combustion. These chemical and physical properties caused vegetable
oils to accumulate and remain as charred deposits when they contacted
engine cylinder walls. The problem of charring and deposits of oils on
the injector and cylinder wall can be overcome by better esterification
of the oil to reduce the viscosity and remove glycerol.
Acid-catalyzed alcoholysis of triglycerides (TG) can be used to produce
alkyl esters for a variety of traditional applications and for potentially
large markets in the biodiesel fuel industry [26]. It can overcome some
of the shortcomings of traditional base catalysis for producing alkyl
esters. A significant disadvantage of base catalysts is their inability to
esterify free fatty acids (FFA). These FFA are present at about 0.3 wt%
in refined soybean oil and at significantly higher concentrations in waste
greases, due to hydrolysis of the oil with water to produce FFA. The FFA
react with soluble bases to form soaps through the saponification reaction
178
Chapter Six
mechanism. The soap forms emulsions and makes recovery of methyl
esters (ME) difficult. Saponification consumes the base catalyst and
reduces product yields. The use of alkaline catalysts requires that the
oil reagent be dry and contain less than about 0.3 wt% FFA [27, 28].
Acid catalysts can handle large amounts of FFA and are commonly
used to esterify FFA in fat or oil feedstock prior to base-catalyzed FFA
alcoholysis to ME [29]. Though it solves FFA problems, it adds additional
reaction and cleanup steps that increase batch times, catalyst cost, and
waste generation.
Generally, acid-catalyzed methanolysis of TG is carried out at temperatures at or below that of methanol reflux (65C). Using sulfuric acid
catalysis under reflux conditions, Harrington and D’Arcy-Evans [30]
first explored the feasibility of in situ transesterification, using homogenized whole sunflower seeds as a substrate. Using reflux conditions, a
560-fold molar excess of methanol and a 12-fold molar excess of sulfuric acid relative to the number of moles of triacylglycerol (TAG) were
used. They observed ester production, with yields up to 20% greater than
in the transesterification of preextracted oil, and suggested that this was
an effect of the water content of the seeds, an increased extractability
of some seed lipids under acidic conditions, and also the transesterification of seed-hull lipids.
Stern et al. [31] have developed a process to prepare ethyl esters for
use as a diesel fuel substitute from various vegetable oils using hydrated
ethyl alcohol and crude vegetable oil, with sulfuric acid as a catalyst.
Ethyl ester of 98% purity with a very low acidity has been reported.
Schwab et al. [32] have compared acid and base catalysts and confirmed that, although base catalysts performed well at lower temperatures, acid catalysis requires higher temperatures. Liu [33] has
compared the influence of acid and base catalysts on yield and purity
of the product, and suggested that an acid catalyst is more effective for
alcoholysis if the vegetable oil contains more than 1% FFA.
Goff et al. [34] have conducted acid-catalyzed alcoholysis of soybean
oil using sulfuric, hydrochloric, formic, acetic, and nitric acids, which were
evaluated at 0.1 and 1 wt% loadings at temperatures of 100C and 120C
in sealed ampoules, and observed sulfuric acid was effective. Kinetic
studies at 100C with 0.5 wt% sulfuric acid catalyst and 9 times methanol
stoichiometry provided more than 99 wt% conversion of TG in 8 h, and
with less than 0.8 wt% FFA concentration in less than 4 h (see Fig. 6.12).
Base catalysts are generally preferred to acid catalysts because they
lead to faster reactions [35]. Base catalysts generally used in transesterification reactions are NaOH, KOH, and their alkoxides. KOH is preferred to other bases because the end reaction mixture can be neutralized
with phosphoric acid, which produces potassium phosphate, a well-known
fertilizer [36].
Processing of Vegetable Oils as Biodiesel and Engine Performance
179
0.16
1
0.12
Ester
Triglyceride
Free fatty acid
Diglyceride
0.6
0.1
0.08
0.4
0.06
0.04
0.2
Mass fraction (FFA, DG)
Mass fraction (ME, IG)
0.14
0.8
0.02
0
0
0
5
10
15
20
25
Time (h)
Figure 6.12 Kinetics of 0.5 wt% sulfuric acid catalyst at 100C and 9:1 methanolTG molar ratio. (Used with permission from Goff et al. [34] .)
Darnoko et al. [37] explained transesterification of palm oil with
methanol and KOH as a catalyst by the following three-step reaction
sequence:
Knothe et al. [38] have reported optimal conditions of a 1 wt% KOH
catalyst at 69C and 7:1 alcohol—vegetable oil molar ratio gave 97.7%
conversions in 18 min, when KOH was used with high-purity feedstocks.
Freedman et al. [39] have studied transesterification of sunflower oil
and soybean oil with the reaction variables (a) molar ratio of alcohol to
vegetable oil, (b) type of alcohol (methanol, ethanol, and t-butanol), (c) type
of catalyst (acidic and alkali), and (d) reaction temperature (60C, 45C,
and 32C). They have suggested that esterification was 90–98% completed at the respective molar ratio of methanol to sunflower oil 4:1 and
6:1. All three alcohols produced high yields of esters. Alkaline catalysts were
180
Chapter Six
generally much more effective than acid catalysts. The reaction was
performed successfully at both 45C and 60C in 4 h, with the production
of 97% of ME.
Kruclen et al. [40] have presented a process for conversion of a highmelting point palm oil fraction into ethyl esters, which could be used as a
diesel fuel substitute. The amount of catalyst used (KOH) was 0.1–1%, and
the reaction was completed rapidly at 80C with yields of 80–94%, depending on the concentration of catalysts. The specific gravity of ethyl ester
varied from 0.847 to 0.864 with kinematic viscosity of 4.4–4.6 cSt at 40C.
Gelbard et al. [41] have determined the yield of transesterification of
1
rapeseed oil with methanol and base by H-NMR (nuclear magnetic
resonance) spectroscopy. The relevant signals chosen for integration
are those of methoxy groups in ME at 3.7 ppm (parts per million) (singlet) and of the ␣-carbonyl methylene groups present in all fatty ester
derivatives at 2.3 ppm. The latter appears as a triplet, so accurate measurements require good separation of this multiple at 2.1 ppm, which is
related to allylic protons.
Chadha et al. [42] have studied base-catalyzed transesterification of
monoglycerides from pongamia oil. They separated monoglyceride fractions (MG) by column chromatography and then characterized the fractions by 1H-NMR spectroscopy in deuterated chloroform (CDCl3) and
tetramethylsilane (TMS) (see Fig. 6.13). They explain that 1- or 2-MG
are positional isomers. Consequently, in 1-MG, the methylene protons at
O
O
HO
H
OH
(2H)
O
O
HO
O
OH
(1H)
OH
R O
R
OH
(1H)
5.6
5.2
4.25
1
4.15
δ (ppm)
4.05
3.95
Figure 6.13 Characteristic H-NMR signals of 1- and 2-MG. (Used with per-
mission from Chadha [42].)
Processing of Vegetable Oils as Biodiesel and Engine Performance
181
C-1 and C-3 are magnetically nonequivalent, due to four double doublets,
which are observed in the spectra. But 2-MG, on the other hand, are
symmetrical, and C-1 and C-3 methylene protons are magnetically equivalent and appear as a multiplate.
6.2.3 Enzymatic transesterification
of vegetable oils
Enzymatic transesterification of TG by lipases (3.1.1.3) is a good alternative over a chemical process due to its eco-friendly, selective nature
and low temperature requirement. Lipases break down the TAG into
FFA and glycerol that exhibits maximum activity at the oil–water interface. Under low-water conditions, the hydrolysis reaction is reversible,
i.e., the ester bond is synthesized rather than hydrolyzed. Scientists
are interested in the development of lipase applications to the interesterification reactions of vegetable oils for production of biodiesel.
Nag has reported [43] celite-immobilized commercial Candida rugosa
lipase and its isoenzyme lipase 4 efficiently catalyzed alcoholysis (dry
ethanol) of various TG and soybean oil (see Fig. 6.14). This process has
many advantages over chemical processes such as (a) low reaction temperature, (b) no restriction on organic solvents, (c) substrate specificity
on enzymatic reactions, (d) efficient reactivity requiring only the mixing
of the reactants, and (e) easy separation of the product.
Kaieda et al. have developed [44] a solvent-free method for methanolysis of soybean oil using Rhizopus oryzae lipase in the presence of 4–30 wt%
100
Conversion (%)
80
60
40
20
0
0
2
4
6
8
16
24
Time (h)
Figure 6.14 Conversion versus reaction for ethanolysis of soybean oil catalyzed by immobilized lipase 4 at 40C and 250 rpm. Ethyl oleate (); ethyl
palmitate (♦); ethyl stearate (䊊); ethyl linoleate (•).
182
Chapter Six
water in the starting materials. Oda et al. [45] have reported methanolysis of the same oil using whole-cell biocatalyst, where R. oryzae cells
were immobilized within porous biomass support particles (BSP). Köse
et al. [46] have reported the lipase-catalyzed synthesis of alkyl esters
of fatty acids from refined cottonseed oil using primary and secondary
alcohols in the presence of an immobilized enzyme from C. antarctica,
commercially called Novozym-435 in a solvent-free medium. Under the
same conditions, with short-chain primary and secondary alcohols, cottonseed oil was converted into its corresponding esters.
Alcoholysis of soybean oil with methanol and ethanol using several
lipases has been investigated. The immobilized lipase from Pseudomonas
cepacia was the most efficient for synthesis of alkyl esters, where 67 and
65 mol% of methyl and ethyl esters, respectively, were obtained by
Noureddini et al. [47]. Shimada et al. [48] have reported transesterification of waste oil with stepwise addition of methanol using immobilized C. antarctica lipase, where they have successfully converted
more than 90% of the oil to fatty acid ME. They have also implemented
the same technique for ethanolysis of tuna oil.
Dossat et al. [49] have found that hexane was not a good solvent as
the glycerol formed after the reaction was insoluble in n-hexane and
adsorbed onto the enzyme, leading to a drastic decrease in enzymatic
activity. Enzymatic transesterification of cottonseed oil has been studied using immobilized C. antarctica lipase as catalyst in t-butanol solvent by Royon et al. [50].
Sometimes, gums present in the oils used inhibit alcoholysis reactions
due to interference in the interaction of the lipase molecule with substrates by the phospholipids present in the oil gum. Crude soybean oil
cannot be transesterified by immobilized C. antarctica lipase. So,
Watanabe et al. [51] have used degummed oil as a substrate for a transesterification reaction, in order to minimize this problem, and have effectively achieved conversion of 93.8% oil to biodiesel.
Methanol is insoluble in the oil, so it inhibits the lipases, thereby
decreasing its catalytic activity toward the transesterification reaction.
Du et al. [52] transesterified soybean oil using methyl acetate in the
presence of Novozym-435 (see Fig. 6.15). Further, glycerol was also insoluble in the oil and adsorbed easily onto the surface of the immobilized
lipase, leading to a negative effect on lipase activity. They have suggested
that methyl acetate was a novel acceptor for biodiesel production and
no glycerol was produced in that process, as shown below:
Processing of Vegetable Oils as Biodiesel and Engine Performance
183
Methyl ester yield (%)
100
80
60
40
20
0
0
2
4
6
8
Time (h)
10
12
14
Effect of the substrate ratio of methyl acetate to oil as biodiesel
production. Reaction conditions: 40C; 150 ppm; 30% Novozym-435, based on
oil–methyl acetate molar ratio 6:1 (䊐), 8:1 (•), 10:1 (䉱), 12:1 (♦), and 24:1 (䊊).
(Used with permission from Du et al. [52] .)
Figure 6.15
They found 92% yield with a methyl acetate–oil molar ratio of 12:1,
and methyl acetate showed no negative effect on enzyme activity.
The comparison of biodiesel production by acid, alkali, and enzyme is
given in Table 6.2.
6.2.4 Engine performance with esters
of vegetable oil
Hawkins et al. [53] have conducted combustion studies on methyl and
ethyl esters of degummed sunflower oil, maize oil, cottonseed oil, peanut
oil, soybean oil, and castor oil. Fuel properties of the esters were very similar to each other, except the esters of castor oil which were much more viscous. The heating values of ethyl esters were also considerably lower.
Engine results indicated that the power output for esters varied from 44.4
to 45.5 kW, with diesel delivering 45.1 kW. The brake thermal efficiencies
were also slightly higher than diesel. High esterification yields (around 90%)
must be obtained to avoid choking of injector tips. Further, sticking of injector needles after a shutdown time of 48 h has been reported.
Fort and Blumberg [54] have tested a diesel engine with a mixture of
cottonseed oil and ME of this oil. Results indicate that viscosity and density increased whereas the heating value and the cetane number
decreased, when the percentage of the cottonseed oil was increased in
the blend. The durability test with 50–50% cottonseed oil and ME was
terminated after 183 h of running the engine, because the engine was
noisy. After disassembly, the engine indicated severe wear scouring and
184
Chapter Six
TABLE 6.2 Comparison of Biodiesel Production by Acid, Alkali, and Enzyme
Acid-catalyzed
transesterification
Base-catalyzed
transesterification
The glycerides and
alcohol need not be
anhydrous
Does not make a soaplike product
Easy to water wash
for product separation
The glycerides and alcohol
must be substantially
anhydrous
Soap formation taking
place during the reaction
Water washing is difficult
due to soap emulsifier
Recommended for any
free-fatty acid content
of vegetable oil
Converts free fatty acid
to ester
Product yield is high
Recommended for low
free-fatty acid content of
vegetable oil
Converts free fatty acid to
soap
Product yield is comparatively low
Is slower than alkalicatalyzed transesterification
Percentage of conversion
is low
Under low water content
in reactants (oil or
alcohol),
transesterification
reaction is not hindered
Is faster comparatively
Need high temperature
Percentage of conversion
is high
Water content in the
reactant inhibits the
reaction rate
Reacts even at room
temperature
Enzymatic
Transesterification
The alcohol needs to be
anhydrous
Does not make a soap-like
product
Separation of the product is
very easy; product is obtained
only by filtration
Recommended for any freefatty acid content of
vegetable oil
Converts free fatty acid to
ester
Product yield depends on
different types of enzymes
used; reaction is selective
Ezymatic reaction is slower
than acid and alkalicatalyzed reaction
Percentage of conversion is
high
Under low water conditions,
the hydrolysis reaction is
reversible, i.e., the ester bond
is synthesized rather than
hydrolyzed. Lipases break
down the triacylglycerols into
free fatty acids and glycerol
that exhibit maximum activity
at the oil–water interface
Conversion takes place at low
temperature
heavy carbon deposits. But specific emissions and visible smoke characteristics of diesel fuel and esterified cottonseed oil were comparable.
Ziejewski and Kaufman [55] conducted a long-term test using a 25–75%
blend of alkali-refined sunflower oil and diesel fuel in a diesel engine, and
compared the results with that of a baseline test on diesel fuel. Engine
power output over the tested speed range was slightly higher for this blend.
At 2300 rpm, the difference was 25%. At 1800 rpm, the gain in power was
6%. The smoke level increased at a higher engine speed from 1 to 2.2 and
decreased at a lower engine speed. Greater exhaust temperature was caused
by a higher intake air temperature. The major problems experienced were:
1. Abnormal carbon buildup in the injection nozzle tips.
2. Injector needle sticking.
Processing of Vegetable Oils as Biodiesel and Engine Performance
185
3. Secondary injection.
4. Carbon buildup in the intake port and exhaust-valve stems.
5. Carbon filling of the compression ring grooves.
6. Abnormal lacquer and varnish buildup.
Tahir [56] has determined the fuel properties of sunflower oil and its
ME. The properties were favorable for diesel engine operation, but the
problem of high viscosity (14 times higher than diesel at 37C) of sunflower
oil might cause blockage of fuel filters, higher valve-opening pressure, and
poor atomization in the combustion chamber. Transesterification of sunflower oil to its ME has been suggested to reduce viscosity of the fuel. The
viscosity of ME at 0C was closer to that of No. 2 diesel fuel, but below
0C, it was not possible because of the pour point of 4C.
Pryor et al. [57] have conducted a short-term performance test on a
small, test diesel engine using crude soybean oil, crude degummed soybean oil, and soybean ethyl ester. The engine developed about 3% more
power output with crude legume soybean oil, but the development was
insignificant with soybean ethyl ester. The fuel flow of soybean oil was
13–30% higher and for the ethyl ester it was 11–15% higher, depending
upon the load on the engine. The exhaust temperature throughout the
test was 2–5% higher for soybean oil and 2–3% lower for ethyl ester than
the diesel fuel.
Clark et al. [58] have tested methyl and ethyl esters of soybean oil as
a fuel in CI engine. Esters of soybean oil with commercial diesel fuel
additives revealed fuel properties comparable to diesel fuel, with the
exception of gum formation which manifested itself in problems with
the plugging of fuel filters. Engine performance with esters differed
little from the diesel fuel performance. Emissions of nitrous oxides for
the esters were similar, or slightly higher than diesel fuel. Measurement
of engine wear and the fuel injection test showed no abnormal characteristics for any of the fuels after 200 h of testing.
Laforgia et al. [59] has prepared biodiesel from degummed vegetable
oil with 99.5% methanol and an alkaline catalyst (KOH). On engine
performance, pure biodiesel and blends of biodiesel combined with 10%
methanol had a remarkable reduction in smoke emissions. When the
injection timing was advanced, better results were obtained.
Pischinger et al. [60] have conducted engine and vehicle tests with ME
of soybean oil (MESO) 75–25% gas oil—MESO blend and 68–23–9%
gas oil—MESO—ethanol blend. The fuel properties of the blend indicated a 6% lower volumetric calorific value of the ester, a drastic reduction in kinematics viscosity, and a greater ethane number than that of
gas oil. The engine results indicated about 7% higher BSFC with a marginal difference in power and torque in comparison with gas oil. The
smoke emission was much lower with ME.
186
Chapter Six
Ali et al. [61] have observed that engine performance with diesel
fuel—methyl soyate blends did not differ to a great extent up to a
70–30% (v/v) from that of diesel-fueled engine performance. There was
a slight increase in NOx emissions with increasing methyl soyate content in the blends at higher speeds but at lower speeds there was a
quadratic trend with diesel fuel content.
Carbon monoxide emissions were very similar for blends up to 70–30%
(v/v) diesel fuel—methyl soyate blends at any speed. Visible smoke
decreased with increasing speed and methyl soyate content. More smoke
was produced with neat diesel fuel at full load.
6.3 Engine Performance with Esters
of Tallow and Frying Oil
The estimated amount of good quality and nutritive-value oils and fats
used for frying around the world is around 20 million metric tons (MT).
In frying, the hot oil serves as a heat exchange medium by which heat
is transferred to the material being fried. As a result of frying, the oil
darkens from the formation of polar materials such as minor phenolic
components; elevated FFA; high total polar materials; compounds
having high foaming property, low smoke point, low iodine value, and
increased viscosity; and color compounds.
Sims [62] reported has conversion of tallow, a by-product of the meat
industry, into esters. The fuel properties of methyl, ethyl, and butyl
esters of tallow were similar to diesel fuel, particularly ME, which were
remarkably similar except for the higher liquidification temperature of
tallow esters. Short-term engine performance tests with methyl, ethyl,
and butyl esters gave comparable results as diesel fuel, but at higher
BSFC. Blends with diesel in 50–50% proportion by volume gave intermediate results between esters and neat diesel fuel.
Richardson et al. [63] have tested an engine with ME of tallow.
Preliminary engine tests indicated that the use of 10% and 20% blends
(volume basis) performed similar to diesel fuel. However, lubricant quality aspects were not studied and an endurance test was not conducted.
The ignition quality of the blend was significantly better than that of
diesel. Overall, it was concluded that tallow ME on 10% (volume basis)
can be successfully used as diesel fuel where large amounts of tallow are
produced and temperatures below 10C are not encountered. The fuel
consumption of ME of used frying oil has been measured by Mittelbach
and Tritthard [64]. The ester fuel showed slightly lower hydrocarbon and
carbon monoxide emissions but increased oxides of nitrogen, compared
with that of diesel fuel. The particulate emissions, however, were significantly lower for used frying oil. But, they suggest long-term engine
testing to prove the quality of this fuel.
Processing of Vegetable Oils as Biodiesel and Engine Performance
187
The results discussed contribute to a better understanding of the
structure—physical property relationships in different fatty acid esters
from different vegetable oils which give the desired biodiesel quality and
optimal performance of engines.
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Chapter
7
Ethanol and Methanol as Fuels
in Internal Combustion Engines
B. B. Ghosh and Ahindra Nag
7.1
Introduction
The increasing industrialization and motorization of the world has led to
a steep rise in the demand of petroleum products. Petroleum-based fuels
are stored fuels in the earth. There are limited reserves of these stored
fuels, and they are irreplaceable. Figure 7.1 shows the difference in
demand and supply of petroleum products, and how this depletion will
create a problem before the world within a decade or two.
Geologists throughout the world have been searching for further
deposits. Although the present reserves seem vast, the accelerating consumption is challenging the world to create new types of fuels to replace
the conventional ones. New oil reserves appear to grow arithmetically
while consumption is growing geometrically. Under this situation, when
consumption overtakes discovery, the world will be heading toward an
industrial disaster.
Apart from the problems of fast-vanishing reserves and the irreplaceable nature of petroleum fuels, another important aspect of their
use is the extent and nature of environmental pollution caused by combustion in vehicular engines. Petroleum-fueled vehicles discharge significant amounts of pollutants like CO, HC, NOx, soot, lead compounds,
and aldehydes.
A light-vehicular engine (car engine) discharges 1–2 kg of pollutants
a day, and a heavy automobile discharges 660 kg of CO a year. CO is
highly toxic, and exposure for a couple of hours to concentrations of 30 ppm
can cause measurable impairments to physiological functions.
191
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
192
Chapter Seven
100
Desired
demand
80
60
Gap
Potential
supply
40
20
Difference in demand
and supply of petroleum products.
Figure 7.1
0
1970
1985
2000
2015
Oxides of nitrogen and unburned hydrocarbons from exhausts cause
environmental fouling by forming photochemical smog. Their interaction involves the formation of certain formaldehydes, peroxides, and
peroxyacylnitrate, which cause eye and skin irritation, plant damage,
and reduced visibility. Present day leaded gasoline contains lead compounds. Lead coming out with the exhaust finds its way into the human
body, and causes brain damage in infants and children.
Vehicular exhaust fouling of the environment has already become a
serious problem in Western countries and is a growing menace in developing countries like India [1]. They exhaust huge quantities of harmful
pollutants in urban areas. Everyday, vehicles running in Delhi discharge about 240 tons of CO, 30 tons of HC, 20 tons of NOx, and 2 tons
of SO2. The disastrous effect of these pollutants on human health, animal
and plant life, and property are well known.
In view of these problems, attempts must be made to develop technology to produce alternative, clean-burning synthetic fuels. These fuels
should be renewable, should perform well in the engine, and their potential for environmental pollution should be quite low.
Various fuels have been considered as substitutes for petroleum fuels
used in automobiles. The most prominent of these include ethanol,
methanol, NH3, H2, and natural gases [2]. The suitability of each of these
fuels for internal combustion (IC) engines used in automobiles has been
under investigation throughout the world. A few of them are already in use
in different countries. This chapter introduces different types of unconventional fuel such as ethanol and methanol, their burning properties
when used in IC engines, their performance characteristics compared
Ethanol and Methanol as Fuels in Internal Combustion Engines
193
with conventional engines, the modifications required in the engine if
used in practice, and their environmental pollution characteristics.
7.2 Alcohols as Substitute Fuels
for IC Engines
Due to the global energy crisis and continuous increase in petroleum
prices, scientists have been in search of new fuels to replace conventional
fuels that are used in IC engines. Among all the fuels, alcohols, which can
be produced from sugarcane waste and many other agricultural products,
are considered the most promising fuels for the future. There are two types
of alcohols: ethanol (C2H5OH) and methanol (CH3OH). Many other agricultural products (renewable sources) also have a vast potential for alcohol production, and it is necessary to tap this source to the maximum level
in national interest. The use of alcohol as a motor fuel is itself not a new
idea. Nicolas Otto, the pioneering German engine designer, suggested it
as early as 1895. But, as long as crude oil was plentiful and inexpensive,
petroleum gasoline was the most economical fuel for the IC engine.
Due to the global energy crisis, many countries that used to export
molasses to be used as cattle feed are now setting up distilleries to manufacture ethanol.
7.2.1
Ethanol as an alternative fuel
Ethanol (ethyl alcohol) as a transport fuel has attracted a lot of attention because it is seen as a relatively cheap nonpetroleum-based fuel.
It is produced to a large extent from biomass, which aids agricultural
economies by creating a stable market. Ethanol, being a pure compound,
has a fixed set of physical as well as chemical properties. This is in contrast to petrol and diesel, which are mixtures of hydrocarbons [3].
The use of alcohol in spark ignition (SI) engines began in 1954 in
countries like the United States, Germany, and France. During World
Wars I and II, gasoline shortages occurred in France and Germany, and
alcohol was used in all types of vehicles, including military planes.
Nowadays, it is used with gasoline (a mixture) in the United States and
has become a major fuel in Brazil.
Ethyl alcohol can be produced by fermentation of vegetables and plant
materials. But in countries like India, ethanol is a strong candidate
since they possess the agricultural resources for the production of ethyl
alcohol. It is a more attractive fuel for India because the productive
capacity from sugarcane crops is high, of the order 1345 L/ha. Earlier,
this fuel was not used in automobiles due to low energy density, high production cost, and corrosion. The current shortage of gasoline has made
it necessary to substitute ethanol as fuel in SI engines.
194
Chapter Seven
TABLE 7.1 Comparative Properties of Ethanol with Petrol and Diesel
Sr no.
Property
Petrol
Diesel
Ethanol
1.
2.
3.
4.
5.
6.
Specific gravity (at 15C)
Boiling point (C)
Specific heat (MJ/kg)
Heat of vaporization (kJ/kg)
Octane number (Research)
Cetane number
0.73
30–225
43.5
400
91–100
Below 15
0.82
190–280
43.0
600
NA
40–60
0.79
78.3
27.0
900
NA
Below 15
Any new fuel that is going to be introduced should be evaluated from
the aspect of availability, renewability, safety, and cost adaptability to
the existing engines’ performance, economy, and finally emission. A massive research effort has been put into the study and analysis of all these
aspects for ethanol, which is now an established, viable alternative fuel
for IC engines. The comparative properties of ethanol with petrol and
diesel are shown in Table 7.1.
7.2.2
Production of ethanol
Ethanol is the most appropriate fuel for India to replace petrol, and the
utmost of efforts have been made to increase alcohol production in the
country. India is in an extremely happy position in this regard as it is
the world’s largest producer of sugarcane, a major source of alcohol.
India topped the world in sugar production with 181 Mton (in 1978),
followed by Brazil (130 Mton) and Cuba (67 Mton).
Alcohol is derived not directly from sugarcane but molasses–sugarcane by-products. All starch-rich plants like maize, tapioca, and potato
can be used to produce alcohol; cellulosic waste materials can also be
used. Production of ethanol from biomass involves fermentation and
distillation of crops. India has a vast potential to produce ethanol, and
only 2.5% of the country’s irrigated land is used to produce sugarcane.
This can be raised to a much higher level without adversely affecting
the production of food-bearing crops.
At present, Brazil is the only country that produces fuel alcohol on a
large scale from agricultural products (mainly sugarcane). Other countries, especially those with an substantial agricultural surpluses, such
as the United States and Canada, are also bound to enter into this field
of so-called energy forming. The area of land required is substantial. A
medium-sized car with an annual run of 15,000 km needs 2000 L of
ethanol. To produce this amount, the crop areas required are given in
Table 7.2. To provide enough sugar beet alcohol to fuel 20 million cars
in Germany requires half the area of the entire country.
Ethanol and Methanol as Fuels in Internal Combustion Engines
195
TABLE 7.2 Crops Area Required for Growth
Crop
Area (ha)
Sugarcane
Sweet
sorghum
Sugar
beet
Cassava
Potatoes
Wheat
0.49
0.38
0.5
1.43
1.2
2.52
The present method adopted to obtain alcohol for energy
purposes requires three stages: (1) extracting the juice from sugarcane,
(2) fermentation of the juice, and (3) distillation into 90–95% alcohol.
Sugarcane.
Molasses. The black residue remaining after the sugar is extracted
from sugarcane is called molasses. It contains mostly invert sugars and
some sucrose. This sucrose also undergoes hydrolysis to produce invert
sugar by a catalytic action of acids in molasses.
C12H22O11 H2O → C6H12O11 (D-Glucose) C6H12O6 (D-Fructose)
This mixture product is not crystallizable. Yeast organisms in the presence of oxygen oxidize sugars into CO2 and H2O and convert sucrose
mostly into ethyl alcohol.
C6H12O6 → 2C2H5OH 2CO2
Process adopted. Molasses is mixed with water so that the concentration of sugar in it is 10–18% (optimum is 12%). If the concentration is
high, more alcohol may be produced and may kill the yeast. Then, a
selected strain of yeast is added (it should not contain any wild yeast).
For some nutrient substances like ammonium and phosphates, the pH
value is kept between 4 and 5, which favors the growth of yeast organisms. H2SO4 is used for lowering pH. The temperature of the mixture
is kept at 15–25C. The fermentation takes place as follows:
1. First, the yeast cells multiply at an optimum temperature (30C).
2. Rapid fermentation takes place at the boiling temperature, and
oxygen is given off. The optimum temperature (50C) is maintained,
and the process is continued for 20–30 h.
3. The fermentation rate is reduced, and alcohol is produced slowly.
Total time for fermentation is 36–48 h, depending upon the temperature and sugar content. Last, the formed ethanol is distilled.
In this process, starchy materials are first converted into fermentable sugars. This is done by enzymatic conversion (by means of
malt process) or by acid hydrolysis.
Starch.
Starch → C12H22O11 (Maltose) C6H12O6 (Dextrose)
196
Chapter Seven
Malt process. Malt is prepared by germination of barley grains to produce required enzymes. The grain is ground and steam cooked at
100–150C to break the cell wall of starch. For every 25 kg of grain, 100 L
of water is added. Then the formed mass is cooled to 60–70C and taken
to large vessels where malt is added within 2 h and 60–70% of the stock
is converted into maltose. Converted mash is cooled to a fermenting
temperature of 20–25C. pH is adjusted and fermentation is affected,
producing ethanol.
Acid hydrolysis. This process involves treatment with concentrated
sulfuric or hydrochloric acid at pH 2–3 and 10–20 kg pressure in an autoclave to make sugar and then conversion of sugar to alcohol by yeast.
Cellulose material.
Wood. Cellulose from wood is hydrolyzed into simple sugars by using
diluted acid at a high temperature or concentrated acid at a low temperature. Similarly, cellulosic agricultural waste and straws can be used
in place of wood.
Sulfite waste liquor from paper manufacture. Waste liquor contains 2–3.5%
of sugar, out of which 65% is fermentable into alcohol. Before fermentation, all acids in the liquor are removed by adding calcium. Then fermentation is carried out by special yeasts. Generally, 1% of liquor is
converted into alcohol.
Hydrocarbon gases.
Hydration of ethylene. Conversion of ethylene to ethyl alcohol can be
carried out with high yield by first treating ethylene with H2SO4, forming
ethyl hydrogen sulphate and diethyl sulfate, as given by the following
reactions:
C2H5HSO4 → (C2H5)2SO4
2C2H4 H2SO4 → (C2H5)2SO4
These products, ethyl sulfuric acid and diethyl sulfate, when treated
with water give ethanol as per the following reactions:
C2H5HSO4 H2O → C2H5OH H2SO4
(C2H5)2SO4 2H2O → 2C2H5OH H2SO4
Direct hydration. Ethanol is also formed as per the following chemical
reaction:
C2H4 H2O → C2H5OH
This type of conversion is very small as the reaction is exothermic; it is
not a suitable method for mass production. The corn is first ground, then
Ethanol and Methanol as Fuels in Internal Combustion Engines
197
mixed with water and enzymes, and cooked at 150C to convert starch
to sugar. The mixture is then cooled and sent to fermentation tanks,
where yeast is added and the sugar is allowed to ferment into ethanol.
After 60 h in the tanks, the mixture is sent to distillation columns, where
ethanol is evaporated out, condensed, and mixed with unleaded gasoline to form gasohol, which contains 90% gasoline and 10% ethanol.
Tapioca materials. Tapioca is available in plenty in Asia, the United
States, central Europe, and Africa. Its production can be increased
through modern cultivation techniques. The process consists of converting the tapioca flour into fermentation sugars with enzymes prior to
fermentation with yeast. Modern technology uses ␣-amyl glycosidase,
one of two enzymes required in the process, and then saccharification
of the material into alcohol by using yeast.
Anhydrous alcohol from vegetable wastes. The Philippines has embarked
on an “alcogas program” to produce its own anhydrous alcohol from
local vegetable wastes for blending with petrol. The program is currently based on sugarcane juice and molasses, but it plans to diversify
by using other raw materials. In the basic process, cellulose conversion
begins with the pretreatment of the raw materials, which may include
coffee hulls, rice straw, grass—even sawmill wastes. Enzymes then take
over by converting the feedstock into a sugary liquid that is fermented
and finally distilled into anhydrous alcohol. After distillation, waste
residues can be evaporated into syrup to feed animals, while unconverted cellulose is used as the primary fuel for the plant. If the
Philippines could engineer a breakthrough in this area, its agricultural
and forestry wastes could supply energy equivalent to 9720 mL of oil
annually. In the years to come, this new energy source could make a significant economic impact on a country that depends on imports of crude
oil for 95% of its energy.
As oil prices continue to rise, more and more work is being done
on alternatives. Manioc is one such staple crop in many tropical lands.
Brazil has planned to use manioc in its ethanol production plants, aiming
to make 35,000 bbl a day from 400 103 ha of manioc plantation.
Conversion of manioc to ethanol is somewhat more complex than is the
case with sugarcane. The raw material has to be turned into sugar by fermentation. This first step requires the use of enzymes. Danish Co. has
developed the necessary heat-resistant enzymes in a pilot plant in Brazil.
Manioc does not grow in higher temperature zones; so scientists have
turned to other plants, and there is work being done in Sweden that is
in an advanced stage. They have developed fast-growing poplars and willows. Their yield is 30 ton/ha, which is equal to 12 tons of fuel oil.
Manioc.
198
Chapter Seven
It is estimated that 1000 103 ha planted with such trees can provide 10% of Sweden’s electricity. Also in Sweden, work has been carried
out on the common reed, and the estimated yield is 10 ton/(yr ha),
which is equal to 4.5 tons of oil. Sweden has plans to have 100 103
ha of reeds. Brazil’s program of ethanol from sugarcane and manioc may
employ 200 103 people and save $1600 million each year in foreign
exchange.
7.3
Distillation of Alcohol
If a mixture of water and alcohol is boiled, the percentage of alcohol to
water is greater in vapor than in liquid. Therefore, by repeated distillation and condensation, the alcoholic strength of the distillate can be
increased until it contains 97.6% alcohol. There are different methods
of distillation, but they are not discussed here, as ethanol production is
our prime concern.
7.4
Properties of Ethanol and Methanol
Both ethanol and methanol, as listed in Table 7.3, have high knock
resistance (as the octane numbers are 89 and 92, against 85 for gasoline), wide ignition limit, high latent heat of vaporization, and nearly
TABLE 7.3 Important Alcohol Properties
Sr no.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Property
Molecular weight (g)
Boiling point at 1 bar (C)
Freezing point (C)
Specific gravity (150C)
Latent heat (kJ/kg)
Viscosity (centipoise)
Stoichiometric A:F (ratio)
Mixture heating value (kJ/kg)
(for stoicmixture)
Ignition limits (A/F)
Self-ignition temperature
Octane number
a. Research
b. Motor
Cetane number
Lower CV (kJ/kg)
Vapor pressure at 38G (bar)
Flame speed (m/sec)
Autoignition temperature (C)
Gasoline
C8H18 isooctane
Ethyl
alcohol
Methyl
alcohol
114.2
43–170
107.4
0.72–0.75
400
0.503
14.6
2930
46
78
117.2
0.79
900
0.60
9
2970
32
66
161.8
0.79
1110
0.596
6.45
3070
8–19
335
3.5–17
557
2.15–2.8
574
80–90
85
15
44,100
0.48–1
0.43
222
111
92
8
26,880
0.17
––
––
112
91
3
19,740
0.313
0.76
467
Ethanol and Methanol as Fuels in Internal Combustion Engines
199
the same specific gravity. All those properties are of great advantage if
used in SI engines. Some important advantages of alcohol-fueled engines
compared with gasoline engines are listed below:
1. The alcohols (both) have higher heat of vaporization. As the liquid
fuel evaporates into the air stream being charged to the engine, a
higher heat of vaporization cools the air, allowing more mass to be
drawn into the cylinder. This increases the power produced from the
given engine size. High latent heat of vaporization leads to higher
volumetric efficiency and provides good internal cooling.
2. The high octane number of alcohols compared to petrol means higher
compression ratios can be used, which results in higher engine efficiency and higher power from the engine.
3. Ethanol burns faster than petrol, allowing more uniform and
efficient torque development. Both alcohols have wider flammability limits, which results into a rich air–fuel (A:F) ratio being
used when needed to maximize power by injecting more fuel per
cycle.
4. Alcohols also have lower exhaust emissions than gasoline engines
except for aldehydes. Both alcohols have lower carbon–hydrogen
ratio than petrol and diesel, and produce less CO2. For the same
power output, CO2 produced by an ethanol-fired engine is about
80% of the petrol engine. Because of high heat of vaporization, the
fuels burn at lower flame temperatures than petrol, forming less
NOx. The CO percentage in both cases (alcohol and petrol) remains
more or less the same.
5. Contamination of water in alcohols is less dangerous than petrol or
diesel because alcohols are less toxic to humans and have a recognizable taste.
6. The alcohols can also be blended with gasoline to form the so-called
gasohol (80% petrol and 20% alcohol), which is widely used in the
United States.
7. Ethyl alcohol as a fuel offers great safety due to its low degree of
volatility and higher flash point (17C).
8. The heating value of alcohol is 60% of that of petrol (60% only), and
it shows equally good thermal efficiency and lower fuel consumption,
because the air required for petrol and alcohol is in the ratio of 15:9
by weight, which is the same as their calorific value, i.e., the same
heat is developed per cylinder charge in petrol and alcohol engines.
The power per unit volume of cylinder for petrol, ethanol, and
methanol are closely similar.
200
Chapter Seven
9. In many hot-climate countries, more precautions are often taken for
the use of more volatile spirit-based fuels, while alcohol is perfectly
safe in the hottest climate.
10. The major problem faced with ethanol is corrosion; special metals
should be used for the engine parts to avoid corrosion.
Alcohols are clean-burning, renewable alternative fuels that can come
to our rescue to meet the duel challenge of vehicular fuel oil scarcity and
fouling of the environment by exhaust emissions.
Alcohols inherently make very poor diesel engine fuels as their cetane
number is considerably lower. They can be used in dual-fuel engines or
with assisted ignition in diesel engine. In the dual-fuel mode, alcohol is
inducted along with air, compressed, and then ignited by a pilot spray
of diesel oil.
7.5
Use of Blends
Alcohol can be used as a blend with gasoline as this has the advantage
that the existing engines need not be modified and tetra-ethyl lead
(TEL) can be eliminated from gasoline, due to the octane-enhancing
quality of alcohol. If the engine is to be operated using only pure alcohol, then some major modifications are required in the engine and fuel
system, as listed below:
1. Both alcohols and blends with gasoline are corrosive to many of the
engine materials. These materials have to be changed.
2. Adjustment of the carburetor and fuel injection need to be made to
compensate for the leaning effect.
3. Change in the fuel pump and circulation system need to be made to
avoid vapor lock, as the methanol vaporization rate is very high.
4. Introduction of high energy ignition system with lean mixture.
5. Increase in compression ratio to make better antiknock properties of
the fuel.
6. Addition of detergent and volatile primers to reduce engine deposits
and assist in cold starting.
7. Use of cooler-running spark plugs to avoid preignition.
General properties of the blends are listed in Table 7.4. The volatility shown by the American Standard Testing Method (ASTM) distillation
characteristics of petrol is a compromise between opposing factors to
ensure good performance in petrol engines. This requires petrol to have
a sufficiently lighter reaction and a 10% distillation temperature in
order to start the engine as well as warm up, but the temperature should
Ethanol and Methanol as Fuels in Internal Combustion Engines
201
TABLE 7.4 Evaluation of Ethanol and Gasohol against Petrol
Characteristics
ASTM distillation
Initial boiling
point, C
10% volume
50% volume
90% volume
Final boiling, C
Gum residue,
mg/100
Aniline point, C
Specific gravity
Petrol and Ethyl Alcohol
(Gasohols)
BIS
specification
for petrol
Petrol
Ethyl
alcohol
95%
5%
90%
10%
85%
15%
80%
20%
....
70 (max)
125 (max)
180 (max)
215 (max)
4
55
64
92
128
143
22
78
....
....
....
....
55
55
59
95
145
....
51
50
56
73
127
147
91
48
57
70
130
156
131
46
57
70
125
156
180
....
....
44
0.720
30
0.7966
40
0.7230
35
0.722
32
0.7289
30
0.733
not be so low that vapor-locking takes place and stops the engine due
to the nonsupply of fuel. As far as volatility is concerned, ethanol–petrol
blends are as good as petrol, if not better. Also gum resistance is greater
than that of petrol. Aniline points for blends are lower, which indicates
more aromatic content than petrol, due to the adding of ethanol to petrol,
which helps to improve the octane number marginally. If a small quantity of water is introduced into a gasoline–alcohol blend, phase separation
takes place, with gasoline–content in the upper phase and alcohol in the
lower. This separation produces some undesirable effects. The alcohol–water mixture tends to pick up sediment and stall the engine on
reaching the carburetor [4]. To improve the water tolerance of the blend,
benzene and heptanes are added.
Since 1979, gasohol has been sold at 500 filling stations in the midwestern United States, where the corn from which alcohol is commonly
made is abundant. This blend yields about the same mileage as unleaded
gasoline and even offers an ever renewable source of energy. Moreover,
if this blend were to replace gasoline, it could cut as much as 10% of the
nation’s oil imports, which totalled $40 billion in 1979. This fuel has a
good future in wealthy countries. The blends have some important
advantages over pure ethanol, as listed below:
1. The starting difficulty can be removed.
2. There is no abnormal corrosion compared with pure ethanol.
3. Lubrication in a petrol–alcohol blend is more or less the same.
4. Some benzene is added to prevent separation of the layers of petrol
and alcohol.
202
Chapter Seven
If blends are used, some minor modifications in the engine are required,
as listed below:
1. The carburetor jet should be increased to increase the flow 1.56 times
that of petrol.
2. The float has to be weighted down to correct levels due to higher specific gravity.
3. The air inlet should be modified to get less air as blends require less
air for complete combustion than petrol.
4. Specific arrangement of heating the carburetor and intake manifold
should be provided as lower vapor pressure of alcohol makes the
starting difficult below 70C.
7.6
Performance of Engine Using Ethanol
The effect of speed on power output, brake specific fuel consumption (BSFC),
and thermal efficiency of an engine using ethanol is compared with gasoline engine, is shown in Figs. 7.2 through 7.5.
The observations are listed below:
1. The power output of the ethanol engine is higher, compared to a gasoline engine at all speeds.
2. The BSFC is improved with an ethanol engine, compared to a petrol
engine.
3. The maximum thermal efficiency of an ethanol engine is higher than
that of a petrol engine. The efficiency curve of an ethanol engine is
flat for a wide range of speeds, which indicates that the partial-load
efficiency is much better, compared with a petrol engine.
4. The engine torque is considerably higher for ethanol as compared to
a petrol engine.
Power (kW)
0.8
0.6
0.4
Ethanol
Rc = 10.2
Petrol
Rc = 8.2
0.2
0
1000
Figure 7.2 Effect of speed on power
3000
5000
7000
Speed (rpm)
9000
at different compression ratios.
Ethanol and Methanol as Fuels in Internal Combustion Engines
203
BSFC (kg/kWh)
80
60
Petrol
40
20
Ethanol
0
2000
4000
6000
Speed (rpm)
8000
Figure 7.3 Effect of speed on
BSFC (brake specific fuel consumption).
Thermal h
20
15
Petrol
Ethanol
10
5
Effect of speed on
thermal efficiencies.
Figure 7.4
0
1000
3000
1.6
5000
7000
Speed (rpm)
9000
Ethanol
Torque (kJ)
1.2
0.8
Petrol
0.7
0
Figure 7.5
0
2000
4000
6000
Speed (rpm)
8000
torque.
Effect of speed on the
204
7.7
Chapter Seven
Alcohols in CI Engine
Although the physical and thermodynamic characteristics of alcohols do
not make them particularly suitable for compression ignition (CI)
engines, with certain modifications, however, they can also be used in
CI engines. In heavy vehicles powered by CI engines, ethanol carburetion can be employed for bi-fuel operation of the engine with proportional
savings in diesel oil. The various methods for using alcohols with diesel
are fumigation, dual injection, and alcohol–diesel emulsions.
In a fumigation system the engine is fitted with a suitable carburetor and auxiliary ethanol tank. An ethanol-air mixture is carbureted
during the induction stroke to provide 50% of the total energy of the cycle
and the remaining energy is provided by diesel oil being injected in the
conventional manner near the end of the compression stroke. The materials of a fuel tank and fuel system must be compatible with alcohol. The
entire system can be used as a retrofit kit, as shown in Fig. 7.6.
Ghosh et al. [4] carried out an investigation on the performance of a
tractor diesel engine with ethanol fumigation (see Figs. 7.7 and 7.8). The
following observations were recorded:
1. The brake thermal efficiency decreases with an increase in ethanol
fumigation rate at a constant engine speed.
2. The BSFC decreases with an increase in ethanol fumigation rate at
a constant engine speed.
Air cleaner
Methanol
jet
Methanol
tank
Feed
pump
Venturi
Float
chamber
Methanol metering
valve
Overflow return
Diesel
tank
Fine filter
Feed
pump
Prefilter
Figure 7.6 Fuel circuit for fumigation.
FIP
Ethanol and Methanol as Fuels in Internal Combustion Engines
205
Figure 7.7 Experimental setup
of ethanol fumigation.
3. The diesel substitution and the energy replacement increases with
an increase in an ethanol fumigation rate at a constant engine speed.
4. The NOx emission level and the exhaust gas temperature decreases with
an increase in a ethanol fumigation rate at a constant engine speed.
5. The CO emission level increases with an increase in an ethanol fumigation rate at a constant engine speed.
6. The smoke level decreases with an increase in an ethanol fumigation
rate at a constant engine speed.
7. The fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal
for good engine performance.
Figure 7.8 Ethanol fumigation nozzle.
206
Chapter Seven
Ethanol fumigation in diesel engines can play a major role in environmental air pollution control, and ethanol is a viable alternative fuel
for diesel engines.
Ethanol is a very good SI engine fuel and a rather poor CI engine fuel.
Ethanol has a high octane rating of 90 and a low cetane rating of 8, and
will not self-ignite reasonably in most CI engines. Dehydrated ethanol
is fumigated into the air stream in the intake manifold of a 42-hp tractor diesel engine to improve its self-ignition quality. The performance
of the engine under dual-fuel (diesel and fumigated ethanol) operation
is compared with diesel fuel operation at various speeds (800, 900, 1000,
1100 rpm), loads (0, 4, 8, 12, 16 kgf ), and fumigation rates (0.00, 1.06,
1.45, 2.06 kg/h). Analysis of the results shows that ethanol fumigation
has the advantages of reduction in BSFC, NOx emission, and smoke level
and the disadvantage of slight reduction in brake thermal efficiency. The
fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal for
good engine performance.
It has been concluded that ethanol is a viable alternative fuel for
diesel engines. A dramatic reduction in the NOx and the smoke level suggests that fumigation, as an emission control technique in diesel engines,
can play a vital role in environmental air pollution control on a farm.
In the dual-injection method, two injection systems are used, one for
diesel and the other for alcohol. This method can replace a large percentage of diesel fuel. In this method, air is sucked and compressed, and
then methanol is injected through a primary injector. To ignite this, a
small amount of diesel is injected through a pilot injector. The relative
injection timing of alcohol and diesel is an important aspect of the system.
As two injection systems are required, two injectors are required on the
cylinder head, which limits the application of this method to large-bore
engines. An additional pump, fuel tank, and fuel line are also required,
making the system more complicated. But this method replaces 60% of
diesel at a partial load and 90% at a full load, and provides higher thermal efficiency.
7.7.1
Alcohol–diesel fuel solution
This method is the easiest but requires anhydrous ethanol, because
methanol has limited solubility. A maximum of 10% diesel can be substituted due to the lower solubility of methanol in diesel. No component
changes; only adjustments of injection timing and fuel volume delivery
are required to restore full power. Dodecanol is an effective surfactant
for methanol–diesel fuel blends. Straight-run gasoline is an economical
additive for ethanol–diesel blends.
Solubility of alcohols in diesel fuels is a function of (a) fuel temperature, (b) alcohol content, (c) water content, (d) specific gravity of diesel,
Ethanol and Methanol as Fuels in Internal Combustion Engines
207
(e) wax content, and (f) hydrocarbon composition. Methanol solubility
in diesel increases as the aromatic content goes up.
7.7.2
Alcohol–diesel fuel emulsions
Here, an emulsifier extends the water tolerance of alcohol–diesel blends.
In general, equal volumes of alcohols and emulsifiers are required for
suitable emulsions. No component changes, but injection volume and
timing are adjusted for diesel fuel with alcohol then solutions, i.e., up
to 35% diesel substitution is possible. Addition of ignition improvers, e.g.,
cyclohexanol nitrate, up to 1% helps increase the alcohol percentage up
to 35% while maintaining a cetane rating at permissible levels. Cost of
emulsifiers and poor low-temperature physical properties of emulsions
limit the use of this technique. Stable emulsion requires the use of costly
surfactants. Using higher-order alcohols improves the stability of blends
at temperatures as low as 20C.
7.7.3
Spark ignition
This technique replaces 100% diesel. The injection system can be retained
as is or replaced by carburetion or port-type fuel injection. A spark plug
is introduced in the combustion chamber, and the associated ignition
system is added. High compression ratio and positive ignition result in
smooth combustion, thereby improving thermal efficiency.
This approach is quite attractive as it uses the high latent heat of the
vaporization of alcohols and their octane rating to good advantage.
Power output is reduced due to lower heat content of alcohols. Changes
in engine operability are not noticeable with alcohol-fired SI engines, relative to the same engines using diesel fuel due to their similar torque.
The engines are as efficient as their diesel-fueled counterparts. In fact,
huge torque is available at engine speeds below 1400 rpm, which increases
engine flexibility and response in use. Converting an existing diesel
fleet to an SI technique involves engine modification. Space at the appropriate place must be available for spark plugs in the cylinder head.
Lubricants need to be added to alcohols to increase lubricity and prevent wear. Small amounts of cetane improvers may be added, but they
are not required. It is not easy to switch between fuels after conversion
to the SI technique.
7.7.4
Ignition improvers
Neat alcohols are used in diesel engines by increasing the cetane number
sufficiently using ignition improvers. This technique saves the expense
and complexity of engine component changes but adds the cost of ignition improvers. The cost of 10–20% ignition improvers is quite prohibitive.
208
Chapter Seven
The most effective ignition improvers are nitrogen-based compounds
which can aggravate exhaust emissions of NOx. Ethylene glycol nitrates
have shown promising trends at 5% concentration.
Engines operating on cetane-enhanced alcohol need a few changes, e.g.,
䊏
Injection volume and timing must be adjusted to obtain optimum
performance.
䊏
A large pump, fuel lines, and injectors are required to satisfy total fuel
requirements of the engine for the desired output.
䊏
A lubricant (generally castor oil used so far) is required to be added
to alcohols using improvers.
7.8
Methanol as an Alternate Fuel
Methanol behaves much like petroleum, so it can be stored and shifted in
the same manner. It is a more flexible fuel than hydrocarbon fuels, permitting wider variation from the ideal A:F ratio. It has relatively good lean
combustion characteristics compared to hydrocarbon fuels. Its wider
inflammability limits and higher flame speeds have shown higher thermal efficiency and less exhaust emissions, compared with petrol engines.
Methanol can be used directly or mixed with gasoline. Tests have
shown improvements in fuel economy by 5–13%, decreases in CO emission by 14–70%, and reductions in exhaust temperature by 1–9%, with
varying methanol in petrol from 5 to 30%. Depending on the gasoline–
methanol mixture, some changes in fuel supply are essential. Simple
modifications to the carburetor or fuel injection can allow methanol to
replace petrol easily. Some important features of methanol as fuel are
listed below:
1. The specific fuel consumption with methanol as fuel is 50% less than
a petrol engine.
2. Exhaust CO and HC are decreased continuously with blends containing higher percentage of methanol. But exhaust aldehyde concentration shows the opposite trend.
3. Like ethanol, methanol can also be used as a supplementary fuel in
heavy vehicles powered by CI engines with consequent savings in
diesel oil and reduced exhaust pollution. No undue wear of engine
components are encountered with methanol as a fuel, while engine
peak power improves and smoke density and NOx concentration in
exhaust is reduced.
Phase separation, vapor lock, and low-temperature starting difficulties
are the problems associated with the use of methanol or its blends as
Ethanol and Methanol as Fuels in Internal Combustion Engines
209
IC engine fuels. Availability from indigenous sources, ease of handling,
low emission, and high thermal efficiency obtainable with its use make
methanol a logical alternative fuel for vehicular engines.
7.8.1
Production of methanol
Methanol can be produced from resources such as coal, natural gas, oil
shell, and farm waste, which are abundant worldwide. But methanol
from natural gas is unlikely to provide a large greenhouse benefit, not
more than a 10% reduction in emissions with quite optimistic assumptions. It is not considered as a main raw material to produce methanol.
For countries having vast reserves of coal but small oil deposits,
methanol from coal can provide an indigenous substitute to oil. But this
method has an adverse effect on greenhouse gases and is very expensive, requiring capital investments that can increase the price by 50%.
In India, there is an abundant production of sugarcane. The government can divert this feedstock to produce methanol. The production of
methanol by using water and methane is shown in Fig. 7.9, and by
using methane and a catalyst in Fig. 7.10.
Producing methanol from methane with the technology available
today generally involves a two-step process. Methane is fuel reacted
with water and heat to form carbon monoxide and hydrogen—together
called synthesis gas. Synthesis gas is then catalytically converted to
methanol. The second reaction unleashes a lot of heat, which must be
removed from the reactor to preserve the activity of the temperaturesensitive catalyst. Efforts to improve methanol synthesis technology
Step 1
Pressure
Synthesis gas
Methane
Water
+
Carbon monoxide Hydrogen
Heat
Step 2
Pressure
Heat
+
Catalyst
Figure 7.9 Conversion of methane to ethanol.
Methanol
210
Chapter Seven
Methane
Methanol
Iron
Oxygen
Figure 7.10 Production of methanol by using methane and a catalyst.
focus on sustaining the catalyst life and increasing reactor productivity. As a novel alternative to the two-step method, a chemical catalysis
that mimics biological conversion of methane by enzymes is being developed. The iron-based catalyst captures a methane molecule, adds oxygen
to it, and ejects it as a molecule of methanol. If this type of conversion
could be performed on a commercial scale, it would eliminate the need
to first reform methane into a synthesis gas, which is a costly, energyintensive step. Conversion of coal to methanol is simpler and cheaper
as compared to its liquefactions to gasoline.
Advantages of methanol.
1. 1% methanol in petrol is used to prevent freezing of fuel in winter.
2. Tertiary-butyl alcohol is used as an octane improving agent.
3. Because of the excellent antiknock characteristics of the fuel, it is very
suitable for SI engines.
4. Isopropyl alcohol is used as an anti-icing agent in carburetos.
5. Addition of methanol causes a methanol–gasoline blend to evaporate
at a much faster rate than pure gasoline below its boiling point (bp).
6. Due to an increase in emission levels of conventional fuels, the percentage of O3 in the atmosphere is increasing. This increase in the
O3 in the atmosphere might cause biomedical and structural changes
in the lungs which might cause chronic diseases. O3 content of even
between 0.14 and 0.16 ppm temporarily affects lung function if
the person is exposed to it for 1–2 h. An annual crop yield is also
reduced if exposed to O3; some trees suffer injury to needles or leaves,
Ethanol and Methanol as Fuels in Internal Combustion Engines
211
Grams ozone per mile
0.8
0.7
5.3% reduction
0.637
0.6
0.5
0.396
0.4
40% reduction
0.301
0.3
0.236
0.2
0.1
0
GTMV
Gasoline
(Indolene)
Figure 7.11
FFV
Methanol
(M85)
Effects of methanol on O3 emission compared with petrol.
and lower productivity or even die. High content of O3 has disturbed
the natural ecological balance of species in national forests in
California. The effects of methanol on O3 emission as compared with
petrol is shown in Fig. 7.11.
7.8.2
Emission
Methanol-fueled vehicles emit less CO2 and other polluting gases compared to gasoline-fueled vehicles. Therefore, methanol use maintains
good air quality. For a higher compression ratio compared to gasoline,
a higher level of NOx can be achieved. But low flame temperature and
latent heat of vaporization tend to decrease NOx emissions. The overall effect is a lower level of NOx emissions.
7.8.3
Fuel system and cold starting
Methanol has high latent heat; therefore, some provision must be provided to heat the intake manifold, because cold starting problems are
often caused by A:F vapor mixture being outside the flammability range.
Specially, methanol in its pure form is much more inferior to petrol for
cold starting. Cold starting more or less becomes impossible with
methanol when the ambient temperature falls below system on chip
(SOC). Figure 7.12 shows the modification that is provided to avoid the
difficulty of cold starting. By preheating, methanol dissociates into CO 2H2 to obtain gaseous H2, which gives a broad flammability limit. While
cranking the engine, a rich gaseous A:F mixture of methanol is collected
near the spark plug, which enables good starting of the engine.
212
Chapter Seven
Carburetor
EGR valve
Engine coolant
heat exchanger
Air pump
Intake
manifold
Exhaust
manifold
Stock motor
Exhaust purifier
Figure 7.12 Modification provided to avoid the difficulty of cold starting.
7.8.4
Corrosion
Corrosion of the engine parts has been one of the main reasons for not
using alcohols as fuels. The problem of corrosion is severe during starting and idling; but once the engine starts and gets heated, corrosion does
not take place. Severe corrosion is noticed with Zn, Pb, Cu, Mg, and Al.
This problem has been solved by using a methanol-resistant filter before
the carburetor. Corrosion by methanol has been prevented by using the
corrosion inhibitor LZ541 manufactured by M/S Lubrizol India. Being
solvent, it swells or softens many parts of plastic or rubber commonly
used for gaskets or floats in the carburetor. This is solved by using elastomers instead of rubber or plastic. American Motors’ Gremlin model of
1970 has been used continuously for 9 years using pure methanol without facing any difficulty of corrosion. Two 1972 Plymouth Valiants have
been used for 7 years: one using pure methanol and the other using a
methanol blend without any difficulty. None of these vehicles has had
a failure of engine components or fuel system components.
7.8.5
Toxicity of methanol
Methanol is more toxic as compared to petrol, which creates difficulty
in its handling. The toxicity of methanol is reduced by adding chemical
emetics.
Ethanol and Methanol as Fuels in Internal Combustion Engines
7.8.6
213
Formaldehyde emission
The major problem with methanol is high levels of formaldehyde emission, which is negligible with conventional fuels. Formaldehyde emission levels with and without an electric heater are shown in Fig. 7.13.
The level with an electric heater is considerably lower compared with
its absence.
The performance characteristics compared with petrol engine are
considered as brake thermal efficiency versus air fuel (A:F) ratio, the
effect of speed power output and specific heat consumption. In addition, the performance characteristics also include the effect of A:F
ratio on exhaust emission. The effects of A:F ratio and speed on brake
power are shown in Figs. 7.14a and 14b. Another important characteristic is the effect of speed on volumetric efficiency, which is shown
in Figs. 7.15 and 7.16.
Both alcohols, as well as their blends, are studied as alternative fuels
for IC engines. The power can be increased from 6 to 10% with alcohols
or their blends. The use of a leaner mixture provides more O2, which
reduces the emission. Because of the high heat of vaporization of these
fuels compared to petrol, greater cooling of the inlet mixture occurs,
which gives higher thermal efficiency, less specific heat consumption,
and smooth operation. At higher speeds, the specific heat consumption
is lower than that of petrol. Methanol dissociates in the engine cylinder
forming H2. This H2 gas helps the mixture to burn quickly and increases
the burning velocity, which brings about complete combustion and
makes a leaner mixture more combustible. In a petrol engine, misfiring
Formaldehyde (mg/s)
30
Formaldehyde emission
with electric heater
Formaldehyde emission
without electric heater
20
10
0
0
2
4
6
8
Time after engine start (min)
Figure 7.13 Performance of methanol as an IC engine fuel.
10
214
Chapter Seven
60
Lean
Rich
50
Brake power (kW)
Brake power (kW)
60
Methanol
40
30
Petrol
20
Rc = 13
50
Rc = 8.2
40
Methanol
30
20
10
10
0
0.7
0.8
0.9 1.0 1.1 1.2
Equivalence ratio
(a)
1.3
Petrol (Rc = 8.2)
0
1000 2000 3000 4000 5000 6000
Speed (rpm)
(b)
Figure 7.14 Effect of (a) equivalence ratio and (b) speed on brake power.
BSFC (103 kJ/kWh)
14
Petrol (Rc = 8.2)
13
12
11
Rc
8.2
13
10
Methanol
9
Figure 7.15
8
2000
4000
Speed (rpm)
Volumetric h
0.9
6000
Effect of speed on
BSFC.
Methanol
0.8
Petrol
0.7
Full load
Effect of speed on
volumetric efficiency.
Figure 7.16
0
1000 2000 3000 4000 5000 6000
Speed
Ethanol and Methanol as Fuels in Internal Combustion Engines
8
215
Lean
Rich
CO (% Volume)
6
Methanol
4
Petrol
2
0.6
0.8
1.0
1.2
1.4
Equivalence ratio
Figure 7.17 Effect of equivalence ratio on CO.
occurs while operating at a lean A:F ratio, whereas in an engine using
alcohol, the engine can manage to handle leaner mixtures without any
misfire. Important objectionable emissions are CO, HC, NOx, and aldehydes. The effect of equivalence ratio on all these emissions for petrol
and methanol are shown in Figs. 7.17 through 7.20.
2000
Lean
Rich
1600
HC (ppm)
Petrol
1200
800
Methanol
400
0.6
0.8
1.0
1.2
1.4
Equivalence ratio
Figure 7.18 Effect of equivalence ratio on HC.
1.6
216
Chapter Seven
5000
Petrol
NOX (ppm)
4000
3000
2000
Methanol
1000
0.6
0.8
1.0
1.2
1.4
1.6
Equivalence ratio
Figure 7.19 Effect of equivalence ratio on NOx.
For all the above graphs, the engine details and compression ratio are
as follows:
1. Full throttle rpm 2500
2. Compression ratio
Methanol
Petrol
Rc 9
Rc 12.6
Rc 9
Regarding emission, ethanol and methanol are considered as clean
fuels, as emissions of CO, HC, and NOx are reduced by nearly 10–15%
compared with a petrol engine. The flame speed of alcohol mixtures is
higher than a petrol A:F mixture, and this helps in making the combustion more complete without misfiring.
Regarding the production of formaldehyde, its percentage in exhaust
is much higher, which is a great problem to extract methanol in pure form
Aldehydes (ppm)
300
Rc = 8.5 & rpm = 2000
Methanol
200
Petrol
100
Figure 7.20 Effect of equivalence
0.7
0.8
0.9
1.0
1.1
Equivalence ratio
1.2
1.3
ratio on aldehyde.
Ethanol and Methanol as Fuels in Internal Combustion Engines
217
as a replaceable fuel. To avoid this, blends (15–25%) of both alcohols are
preferred over pure ethanol or methanol. The properties of blends and
their effect lie in between pure alcohol and petrol. As we know, methanol
blends have lower stoichiometric air requirements compared to petrol.
Therefore, if we use a methanol–petrol blend without any modification
in the carburetor, we get more air for combustion, which will reduce the
emission of CO and HC as well as NOx as the engine works cooler with
the blend compared with a petrol engine.
Oxidizing catalytic devices can control aldehyde emissions.
Platinum–rhodium and platinum–palladium catalysts are considered
the most effective in tackling aldehyde emissions of methanol fueling.
Concerning the alcohol fuels, the following conclusions can be drawn:
1. Alcohol is potentially a better fuel than gasoline for SI engines.
2. Its use improves the thermal efficiency as a higher compression ratio
(12:16) can be used.
3. It can avoid knocking even at a higher compression ratio because of
the high octane number.
4. It provides better fuel economy and less exhaust emissions.
5. High latent heat of alcohol reduces the working temperature of the
engine.
6. It gives more power, specially when used as a blend.
7. Easy availability of raw materials.
8. Cost of production is low because of the price hike in crude petroleum.
In agricultural countries like India, we can get ethyl alcohol easily from
vegetables, agricultural material, and sugarcane waste at a much lower
cost compared with the cost of petrol today. Therefore, replacing petrol
with alcohol in a SI engine has a good future.
7.9
Comparison of Ethanol and Methanol
Most of the properties are similar, with differences of only 5–10%.
Ethanol is superior to methanol as it has a wider ignition limit (3.5–17)
than methanol (2.15–12.8). Its calorific value (CV) (26,880 kJ/kg) is considerably higher than methanol (19,740 kJ/kg).
Ethanol is a much more superior fuel for diesel engines as its cetane
number is 8 compared to the cetane number of 3 for methanol. There are
wide resources for manufacturing ethanol compared with methanol.
Therefore, ethanol is widely used as SI engine fuel in many countries.
Methanol is superior to ethanol in one respect: Its vaporization rate is much
higher than ethanol. Therefore, mixing with air rapidly forms a uniformly
218
Chapter Seven
vaporized mixture and also burns uniformly. One major drawback of
methanol is that it creates vapor locks because of the higher vaporization rate. Properties of ethanol and methanol as compared with petrol
are listed in Table 7.3.
7.10 Ecosystem Impacts Using
Alcohol Fuels
7.10.1
Aquatic system impacts
The biological consequences of alcohol spills or leaks into marine water
are sensitive to many factors such as scale and duration of the spill, tidal
patterns, water currents, flow rate, temperature, and available oxygen.
Marine life can tolerate low concentrations of alcohol.
In general, methanol and ethanol are significantly less toxic than
gasoline or crude oil. Because alcohols are miscible, volatile, and degradable, they are dispersed readily, and diluted and neutralized in aquatic
environments. The aquatic environment recovers more rapidly and completely from an alcohol spill than from a gasoline or crude oil spill of the
same volume.
7.10.2
Terrestrial system impacts
The direct exposure of soils to methanol spills results in immediate
damage of surface vegetation. The miscibility, volatility, and degradability of alcohols reduce the alcohol residence time in soil and minimizes the environmental impact. Fungal and bacterial populations, which
are important agents of nutrient cycling, exhibit 80–90% recovery with
3 weeks of exposure. Total recovery of the site occurs within a period of
weeks or months. In comparison, recovery of biodegradation by crude
oil and petroleum products takes months or years.
7.10.3
Occupational health impacts
Occupational heath risks associated with using alcohol fuels are lower
than those associated with conventional fuels. The relative toxicity of
alcohol fuels depends on the means of exposure, inhalation, and ingestion. Gasoline poses a greater occupational health risk than either
methanol or ethanol as carcinogens in gasoline can be readily absorbed
by the skin or inhaled.
7.10.4
Occupational safety impacts
Two major safety hazards of all fuels are fire and explosion, which can
occur because of improper fuel storage, spills, or vehicle accidents. The
properties of alcohols and gasoline that pertain to fire and explosion
Ethanol and Methanol as Fuels in Internal Combustion Engines
219
risks include the flash point, auto-ignition temperature, flammability
limits, and saturated vapor concentrations. While ethanol and methanol
have broader flammability limits than gasoline, gasoline poses a greater
risk of fire in open air. Because of the low flash point and auto-ignition
temperature of gasoline, gasoline is more likely to ignite and burn rapidly; therefore, the fire hazard is greater for gasoline.
Alcohol-fueled fire can be more readily contained than a gasoline-fueled
fire of equivalent volume because alcohols have a lower heat of combustion
than gasoline and less of the energy released is converted to radiant heat.
Therefore, energy release and potential damage from an explosion caused
by alcohol would be less than that of an explosion caused by gasoline.
7.10.5
Socioeconomic impacts
Substitution of alcohol fuels for conventional fuels will increase the number
of jobs in fuel production, distribution, and handling industries. Alcohol
fuels are expected to cost more than gasoline over the next 10 years.
As a result, vehicle-operating costs will be somewhat higher if alcohol blends are used. The price of alcohol blends varies significantly,
depending upon the type of alcohol and feedstock used. Blends containing methanol derived from coal are the least expensive. The most
expensive are alcohol blends containing ethanol produced from corn.
7.10.6 Transportation and infrastructure
impacts
The existing fuel distribution system must be modified and expanded
to accommodate the increasing use of alcohol fuels in the long run. The
changes required will include construction of new pipelines, storage
facilities, and retrofitting of existing facilities with alcohol-compatible
pumps, hoses, valves, and other components.
The vehicle support services such as refueling, maintenance, repairs,
and vehicle sales will be unaffected by the use of alcohol fuels. The use
of alcohol fuels is not expected to have a significant impact on the existing transportation system infrastructure.
References
1. A. Nag. Analytical Techniques in Agriculture, Biotechnology and Environmental
Engineering, New Delhi, India: Prentice-Hall of India, 2006.
2. A. Nag. Text book of Agriculture Biotechnology, New Delhi, India: Prentice-Hall of
India, 2007 (in press).
3. E. S. Lipinsky. Chemicals from biomass: Petrochemical substitution options, Science
212, 1465–1471, 1992.
4. B. B. Ghosh, E. V. Thomas, and S. Natarajan. The Performance of a Tractor Diesel
Engine with Ethanol Fumigation, Ph.D. Thesis, Mechanical Engineering Department,
Indian Institute of Technology, Kharagpur, India, 1992.
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Chapter
8
Cracking of Lipids for Fuels
and Chemicals
Ernst A. Stadlbauer and Sebastian Bojanowski
8.1
Introduction
Lipids [1] in the form of fat and edible oils are important energy sources
for humans due to the high calorific value of triacylglycerols (~37 kJ/g, or
~9 kcal/g) and the nutritional benefits of both essential fatty acids and
phosphate. In addition, energy stored in lipids may be technically realized
by either direct use in combustion or by upgrading into a more versatile
fuel. In this respect, lipids play an important role for providing lighting
and warmth.
Historically, whale oil lamps and tallow candles were gradually displaced by kerosene lamps and electric bulbs [2]. Nowadays, lipids are
attracting interest as a renewable source of fuels and chemical feedstock.
Therefore, segmentation in the marketplace for lipids is noticeable [3].
In emerging economies of eastern Asia, there is a demand for cheap,
edible commodity oils, such as soybean or palm oils. In developed
economies, a nutritionally led demand for niche oils, such as low-transfat oils, high-omega-3 oils, and enhanced lipophilic vitamins (especially
A and E), prevails. More recently, nonedible uses of lipids arise from the
proliferating demand for alternative fuels [4] to substitute liquid hydrocarbons derived from mineral oil [5]. Such strategies fall into four broad
categories. One is aimed at fueling diesel engines with pure vegetable oils
[6] or vegetable oil–fossil fuel blends [7]. The other focuses on biodiesel
(alkyl esters of fatty acids), which is mainly sourced from rapeseed and
palm oils [8–10]. Problems [11] associated with the more polar characteristics of vegetable oil and biodiesel in comparison to conventional
221
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
222
Chapter Eight
diesel has given rise to studies for cracking of lipids (vegetable
oils/animal fat) into nonpolar hydrocarbons [12] to be used as a base for
fuels or chemical commodities. Decomposition studies with and without
catalysts (metallic salts, metal oxides) have been performed. Finally,
lipids (and proteins) in dead cellular matter such as sewage sludge or
meat and bonemeal may be converted by natural catalysts present in
the substrate to oil having properties similar to diesel fuel [13].
In the following sections, basic processes of converting lipids into nonpolar hydrocarbons with alkanes, alkenes, and arenes as main constituents are discussed. Details of pure vegetable oils or biodiesel are
outlined elsewhere (see Chaps. 4, 5, 6).
8.2
Thermal Degradation Process
Thermal decomposition of vegetable oil was performed to prove the
theory of the origin of mineral oil from organic matter [14] as early as
1888. Literature up to 1983 has been reviewed by Schwab et al. [15]. In
many cases, inadequate characterization of products formed in pyrolysis of vegetable oils was found. Therefore, analytical data obtained by
gas chromatography–mass spectrometry (GC-MS) from thermally
decomposed soybean oil and high oleic safflower oil in the presence of
air or nitrogen were reported [15].
The ASTM standard method for distillation of petroleum products
D86-82 has been used for decomposition experiments. Catalytic systems
were excluded in this destructive distillation. The actual temperature of
the oil in the feeder flask was about 100C higher than the vapor temperature throughout the distillation. Under these conditions, GC-MS analysis showed that approximately 75% of the products were made up of
alkanes, alkenes, aromatics, and carboxylic acids with carbon numbers
ranging from 4 to more than 20 (see Table 8.1).
A comparison of fuel properties is given in Table 8.2. The carbonhydrogen ratio shows 79% C and 11.88% H for the pyrolyzate of soybean
TABLE 8.1
Composition Data of Pyrolyzed Oil
Percent by mass
high oleic safflower
Soy
Class of compounds
N2 sparge
Air
N2 sparge
Air
Alkanes
Alkenes
Alkadienes
Aromatics
Unresolved unsaturates
Carboxylic acids
Unidentified
37.5
22.2
8.1
2.3
9.7
11.5
8.7
40.9
22.0
13.0
2.2
10.1
16.1
12.7
31.3
28.3
9.4
2.3
5.5
12.2
10.9
29.9
24.9
10.9
1.9
5.1
9.5
12.6
Cracking of Lipids for Fuels and Chemicals
TABLE 8.2
ASTM
test no.
D613
D129
D130
D524
D1796
D482
D97
D445
DE191
223
Comparison of Fuel Properties
Specification
Cetane rating
Higher heating
value, BTU/Ib
Sulfur, %
Copper corrosion,
3 h at 50C
standard strip
Carbon residue
at 10% residium
Water and sediment,
% by volume
Ash, % by weight
Pour point, C
Viscosity, mm2/s
at 38C
Carbon, %
Hydrogen, %
Distilled
soybean oil
(N2 sparge)
No. 2
diesel fuel
Soybean
oil
High oleic
safflower
oil
43*
17,333
40 (min.)
19,572
37.9*
17,035
49.1
17,030
<0.005
1*
<0.5
<3
0.01
1*
0.02
1*
0.45%
<0.35%
0.27%
0.24
0.05
<0.05
Trace
Trace
0.015
7
10.21
<0.01
7C (max.)
1.9–4.1
<0.01
12
32.6
<0.01
21
38.2
79.00
11.88
86.61
13.20
—
—
—
—
*
ASTM test D613 with ignition delays observed visually.
oil. This indicates considerable amounts of oxygenated compounds in the
distillate. Consequently, methylation of these oils has revealed 9.6–12.2%
of carboxylic acids ranging from C-3 to over C-18. This is reflected in the
higher viscosity compared to diesel.
Mass-spectral fingerprints of the entire pyrolysis product slate from
tripalmitin, different vegetable oils, and extracted oils from microalgae
confirm that the decomposition of ester bonds in the absence of external
catalysts is extensive [16–18]. However, a great variability in primary
pyrolysis/vaporization product slates was observed [18].
Thermodynamic calculation in the degradation process shows that the
cleavage of C-O bond takes place at 288C and fatty acids are the main
product [19]. The actual pyrolysis temperature should be higher than
400C to obtain maximum diesel yield [20]. The mechanism of pyrolysis
of vegetable oil has been discussed by various authors [9, 15, 19]. Generally,
thermal decomposition proceeds through either a free-radical or carbonium
ion mechanism. The primary R-COO splits off carbon dioxide. The alkyl
radicals (R), upon disproportionation and elimination of ethene, give rise
to alkanes and alkenes. The formation of aromatics is facilitated by a
Diels-Alder addition of ethene to a conjugated diene formed in the pyrolysis reactions. However, the product mix and product quality are influenced
by many factors such as feed pretreatment, heating rate, and temperature.
As vegetable oils may contain trace elements, catalytic effects cannot be
completely excluded from any thermal degradation process [21].
224
Chapter Eight
8.2.1
Catalytic cracking (CC)
In 1979, a paper [22] from the petrochemical industry reported for the
first time that high-molecular-weight triglycerides such as corn oil
(C57H104O6) and castor oil (C57H104O9) were convertible to a high-grade
gasoline when passed over H-ZSM-5, a catalyst. The latter is a synthetic,
medium-pore, shape-selective acid catalyst. Lipids were fed with a piston
displacement pump at a rate of 2 mL/h with flowing hydrogen (300 mL/h)
over 2 mL of H-ZSM-5 catalyst (0.77 g, 14–30 mesh) contained in a vertical Pyrex reactor at atmospheric pressure and T 400–450C.
Paraffins, olefins, aromatics, and nonaromatics could be detected in the
product mixture. The distribution of hydrocarbons is similar to selective
conversion of methanol into hydrocarbon units with up to 10 carbon
atoms per molecule. In all cases, a high degree of BTX aromatics (benzene, toluene, and xylene) was achieved. The precondition for the catalytic
conversion is that the molecule penetrate the cavities of microporous
zeolite.
This new catalytic approach has paved the way for a variety of applications. A schematic diagram of experimental arrangements for pyrolysis and catalytic conversion is given in Fig. 8.1.
Conversion of different kinds of vegetable oils over medium-pore HZSM-5 have been investigated in detail [23–26]. Catalytic cracking of
by-products from palm oil mills with a selectivity of 51wt.% toward aromatic hydrocarbon formation has been reported [27]. To achieve higher
yields, this type of work was extended to pyrolysis and zeolite conversion of both whole algae and their major components as well as whole
seeds and selected vegetable oils [18, 28–31]. Hot vapors from solid
organic material (microalgae, seeds, etc.) or vaporized vegetable oils
were passed directly over the H-ZSM-5 catalyst. Products of different
Cooling water
NCG
TIR
TIR
Residual
solid
Substrate,
nitrogen
TIC
el.
Biocrude/Biofuel
Reaction water
Figure 8.1
General scheme of pyrolysis and catalytic conversion reactor.
Cracking of Lipids for Fuels and Chemicals
225
algae, seeds, or vegetable oils emerging from the passage showed a uniform, high-octane, aromatic gasoline product. Obviously, the molecular
pattern of products is insensitive to the nature of lipids used. This is in
contrast to pyrolysis without a catalyst [18].
Upgrading of crude tall oil to fuels and chemicals has been studied at
atmospheric pressure and in the temperature range of 370–440C, in a
fixed-bed microreactor containing H-ZSM-5 [32]. The oil was co-fed with
diluents such as tetralin, methanol, and steam. High oil conversions, in
the range of 80–90 wt.%, were obtained using tetralin and methanol as
diluents. Conversions under steam were reduced to 36–70 wt.%. The
maximum concentration of gasoline-range aromatic hydrocarbons was
52–57 wt.% with tetralin and steam, but only 39% with methanol. The
amount of gas product in most runs was 1–4 wt.% [32].
8.3 Vegetable Oil Fuels/Hydrocarbon Blends
100
[1.1]
TG (%)
80
60
40
20
0
Mass change: −21.24 %
Mass change: −78.23%
[2.1]
[1.1]
0
−2
−4
−6
−8
−10
−12
−14
−16
DTG (%/min)
At first glance vegetable oil offers a favorable CO2 balance. However,
when the extra N2O emission from biofuel production is calculated in
“CO2-equivalent” global warming terms, and compared with the quasicooling effect of “saving” emissions of fossil fuel derived CO2, the outcome is that production of commonly used biofuels can contribute as
much or more to global warming by N2O emissions than cooling by fossil
fuel savings [33]. In addition, widespread use of vegetable oil fuels is limited by high viscosity, low volatility, poor cold flow behavior, and lack of
oxidation stability during storage [6, 7]. Partial conversion of vegetable
oil to hydrocarbons offers the possibility to preserve the favorable environmental characteristics of vegetable oil-based fuels while improving
viscosity and cold flow behavior [34, 35]. Figure 8.2 depicts thermogravimetry of vegetable oil without pure oil (dashed line) and in the presence of a Y-zeolite (Koestrolith). The dotted line represents the first
derivative from the catalyzed conversion reaction.
100
200
300
400
500
600
Thermogravimetry of commercial vegetable oil fuel without pure oil (dashed
line) and in the presence of a Y-zeolite.
Figure 8.2
226
Chapter Eight
100.0
95
3006.44
2359.85
90
1633.78
1376.88
T(%)
85
1458.40
80
1096.13
721.85
75
2852.42
70
1159.96
65
2921.92
60
55.0
4000.0
1743.18
3000
2000
1500
1000
540.0
cm−1
IR spectrum of commercial vegetable oil fuel. The bands observed at around
2900 and 1740 cm1 are due to absorption of IR radiation, the absorbed energy causing
transitions between energy levels for the stretching vibrations of C-H (hydrocarbons)
and CO bonds (ester function R1COOR2), respectively.
Figure 8.3
The efficiency of the decarboxylation effect of Y-zeolite activity on
pure vegetable oil at T 450C may be seen by comparing the IR spectrum of pure vegetable oil fuel in Fig. 8.3 with the corresponding spectrum of the conversion product in Fig. 8.4. The carbonyl band at around
1700 cm1 is an indicator for conversion efficiency.
Table 8.3 summarizes physical and chemical parameters of vegetable
oil fuel and conversion products at different temperatures. The change
100.12
1035.93
1605.08
T (%)
874.00
90.0
676.01
1377.31
809.88
782.60
2870.01
768.63
1457.75
80.0
694.05
741.69
2955.69
77.49
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000
cm−1
728.16
800
540.0
IR spectrum of conversion product. The carbonyl function at 1720–1740 cm−1
is missing. Absorption bands between 1300 and 650 cm−1 are generally associated with
complex vibrational and rotational energy changes (fingerprint region) of the molecules.
Figure 8.4
Cracking of Lipids for Fuels and Chemicals
227
TABLE 8.3 Characteristics of Commercial Vegetable Oil Fuel and Its Y-Zeolite
Conversion Product
Parameter
Commercial
vegetable oil fuel
Y-zeolite
(koestrolith),
T 430C
Y-zeolite
(koestrolith),
T 450C
Yield, %
NCV, MJ/kg
Density, g/mL
Viscosity, mm2/s
C, %
H, %
N, %
S, %
—
37.0
0.91
32.87
77.04
12.0
0.29
<0.34
34.3
42.4
0.79
0.73
87.8
9.59
<0.14
<0.34
43.5
42.2
0.81
0.79
88.32
9.67
<0.14
<0.34
in viscosity is quite remarkable. In accordance with Fig. 8.2, a reaction
temperature of T 450C is preferred.
8.3.1
Refitting engines
Presently, vegetable oil is regarded as a niche application. One liter of
rapeseed oil substitutes for approximately 0.96 L of diesel. The annual
yield is 1480 L/ha. CO2 reduction in relation to the diesel equivalent
is about 80% [36]. However, this is questioned in newer literature [33]
in terms of global warming reduction considering the effects of extra
N2O entering the atmosphere as a result of using nitrogen-based fertilizers to produce crops for biofuels. Before unmodified vegetable oil
is used as a fuel, the engine must be refitted for the fuel to correspond
to the viscosity and combustion properties of vegetable oil. Refitting
concepts include preheating either the fuel and the injection system
or the equipment with a two-tank system. The engine is started with
diesel and changes to vegetable oil only when the operating temperature has been reached. Blends of pure vegetable oils and a conversion
product together with additives (antioxidants) increase oxidation stability, reduce viscosity, and give a better perspective for vegetable oil
fuel markets.
8.3.2
Tailored conversion products
The chemical nature of conversion products depends both on the structure
or type of the zeolite used and the reaction temperatures, because
restructuring occurs at the inner surface, which acts as a reaction vessel
at the molecular scale. Specific reactions depend on the diameters of
pores, the resident time of molecules within the pores or channels and
voids of the microporous zeolite, and the temperature. The penetration
of lipids into a zeolite is depicted in Fig. 8.5. The scheme is based on [22].
228
Chapter Eight
O
H2C
HC
O
O
C
C17H35
C17H35
C
O
H2C
O
C
O
C17H35
Diffusion
H35C17
H35C17
C
O O
O
C
O
H2
C
CH3
HC O C C17H35
C
H2
Reaction
O
CH3
CH3
Figure 8.5 Scheme of restructuring triglycerides with shapeselective H-ZSM-5 to aromatic hydrocarbons.
To demonstrate this influence of catalysts and reaction temperature
on yields and products, Table 8.4 considers a shape-selective zeolite
type H-ZSM-5, commercially available as Pentasil, PZ-2/50H, and Yzeolite (DAY-Wessalith). The physical characteristics of oils formed from
the conversion of animal fat (rendering plant) are depicted [56]. Yields
are between 30% and 70%, depending on the type of zeolite and temperature. Net calorific values are in the range of 40 MJ/kg compared to
TABLE 8.4 Yields and Physical Characteristics of Hydrocarbons from
Catalytic Conversion of Animal Fat Using Zeolite Types H-ZSM-5
(Pentasil, PZ-2/50H) and DAY-Wessalith at Different Temperatures
Parameter
H-ZSM-5
PZ-2/50H,
T 550C
H-ZSM-5
PZ-2/50H,
T 400C
DAY-Wessalith,
T 400C
Yield, %
NCV, MJ/kg
Density, g/mL
Viscosity, mm2/s
C, %
H, %
N, %
S, %
31.48
40.1
0.83
0.92
88.6
10.7
<0.14
<0.34
56.74
40
0.85
1.01
84.5
12.5
<0.14
<0.34
72.9
41.3
0.81
2.29
83.4
13.5
<0.14
<0.34
Cracking of Lipids for Fuels and Chemicals
229
35 MJ/kg of animal fat. All reaction products show relatively low viscosity and densities.
Products at T ⴝ 400ⴗC.
Again, the chemical nature of products formed
from animal fat was analyzed by spectroscopic methods (see Fig. 8.6).
The IR spectrum reveals the hydrocarbon nature of products. The strong
C-H stretching vibrations (frequencies) at 2900 cm1 is characteristic for
alkanes. Functional groups are widely missing. The comparison to diesel
from a commercial gas filling station (imprinted spectrum) shows a similar pattern [37].
Proton resonance spectroscopy depicts the chemical environment of protons in the product formed from the conversion of animal fat. Figure 8.7
shows the dominance of aliphatic protons at chemical shifts of 0.9–2.25
ppm. Aromatic protons absorb at 6.5–8 ppm. The inspection of the ratio of
the integral of absorptions reveals 5% aromatics for catalysis at T 450C.
This is also reflected in the 13C-NMR spectrogram (see Fig. 8.8). However,
with increasing temperature in the catalytic bed, the content aromatic
alkylbenzenes increase.
Using 13C-NMR spectroscopy in-depth mode (see Fig. 8.9), negative
signals at 30–20 ppm are characteristic for CH2-groups. The intensity
indicates the presence of long-chain hydrocarbons. Peaks between 140
and 120 ppm denote carbon atoms of aromatic systems. The low intensity reflects the low content. Obviously, catalytic cracking over a Yzeolite widely preserves hydrocarbon moiety in vegetable oil.
100.0
98
96
1711.1
1377.4
94
100
90
96
Transmission (%)
92
T (%)
88
86
84
2955.4
82
2852.9
88
84
2852.4
76
78
C-H stretching
frequency
76
74
1457.9
92
80
80
72.0
4000.0 3600
1461.8
4000
2920. 9
3200
2400
2922.3
3200
2800
2400
2000
1800 1600
cm−1
1400
1800
1400
1000
Wave number, cm−1
1200
1000
800
600
540
400.0
IR spectrum of hydrocarbons derived from animal fat at 400C (Y-zeolite
catalyst, DAY-Wessalith).
Figure 8.6
230
1.0010
0.9778
1.3807
1.7128
2.6022
2.4139
2.3586
2.1038
3.1752
2.9668
1.0010
0.9778
5.5183
5.4900
8.8232
1.3807
7.4471
7.2519
7.0856
6.8991
8.6196
IV CDCl3
9
Figure 8.7
1.0
1
8
8.8232
1.2
(ppm)
1.0000
Integral
1.4
7
6
5
4
ppm
3
2
*****Current Data Parameters****
NAME
: ok13gs
EXPNO
: 10
PROCNO : 1
***Acquisition Parameters***
AQ_mod : dqd
BF1
: 300.1300000 MHz
CPDPRG :
: Oct 13 2003
DATE_d
INSTRUM : spect
NS
: 16
O1
: 1650.71 Hz
O2
: 0.00 Hz
PULPROG : zg30
RG
: 20.2000008
SFO1
: 300.1316507 MHz
SFO2
: 300.1300000 MHz
SOLVENT : CDCl3
SW
: 20.5671 ppm
TD
: 32768
***Processing Parameters***
: 0.0000000
GB
LB
: 0.10 Hz
: 0.20 cm
MI
: 15.745 ppm
OFFSET
: 54.078 degree
PHC0
PHC1
: −0.996 degree
SI
: 32768
SW_p
: 6172.8395062
1
H-NMR spectrogram of hydrocarbons from animal fat at T 400C (Y-zeolite catalyst, DAY-Wessalith).
150
Figure 8.8
140
13
130
120
110
100
90
80
70
ppm
32.0140
29.8041
29.7583
29.5657
29.4557
29.1256
28.0436
22.7619
22.6610
22.2117
19.6717
19.2224
14.4084
14.1149
11.4007
77.4218
77.0000
76.5782
129.0374
128.2305
127.8821
126.0482
125.7822
IV CDCl3
60
50
40
30
20
***Current Data Parameters***
NAME
: ok13gs
EXPNO
: 11
PROCNO : 1
***Acquisition Parameters***
AQ_mod : qsim
BF1
: 754677190 MHz
CPDPRG : waltz16
DATE_d : Oct 13 2003
INSTRUM : spect
NS
: 4000
O1
: 7497.00 Hz
O2
: 1650.71 Hz
PULPROG : zgpg30
RG
: 4597.6000977
SFO1
: 75.4752160 MHz
SFO2
: 300.1316507 MHz
SOLVENT : CDCl3
SW
: 300.4395ppm
TD
: 32768
***Processing Parameters***
GB
: 0.0000000
LB
: 1.00 Hz
MI
: 0.20 cm
OFFSET
: 249.104 ppm
PHC0
: 141.257 degree
PHC1
: 58.984 degree
SI
: 32768
: 22675.7369615
SW_p
10
C-NMR spectrogram of hydrocarbons from animal fat at T 400C (Y-zeolite catalyst, DAY-Wessalith).
231
13.9509
31.7883
29.5643
29.5268
29.2273
22.5476
19.0430
78.0592
127.6925
125.6182
232
***Current Data Parameters***
NAME
EXPNO
PROCNO
: SE082S
: 109
: 1
***Acquisition Parameters***
AQ_PD
BF1
DEC_MMR
INSTRUM
NS
O1
O2
PULPROG
SFO1
SOLVENT
SW
TD
Figure 8.9
140
130
120
110
100
90
80
ppm
70
60
50
40
qseq
75.4700000 MHz
DO
AM
800
6141.61 HZ
: 4588.00 HZ
:
: 75.4761416 MHz
: Solvent
: 245.3804
: 65536
***Processing Parameters***
GB
: 0.0000000
LB
: 1.00 Hz
OFFSET
: 222.473 ppm
PHC0
: 18.722 degree
PHC1
: −5.040 degree
SI
: 32768
SW_p
: 18518.5185184
Mother nature’s synthetic achievement of long-chain
hydrocarbons are preserved with Y-zeolite at T = 400°C.
150
:
:
:
:
:
30
20
10
DEPT-135 13C-NMR spectrogram of biofuel from animal fat at 400C (Y-zeolite catalyst, DAY-Wessalith).
233
1.0
1.0
0.5
0.5
0.0
0.0
0
5
10
15
Minutes
20
25
Volts
Volts
Cracking of Lipids for Fuels and Chemicals
30
GC pattern of Y-zeolite conversion product of animal fat at reaction temperature T 400C. GC-14A Shimadzu, column: FS-Supreme-5/H53, 30 m; temperature
program: 50C (5 min); 15C/min to 320C (10 min); FID detector at 320C.
Figure 8.10
1.0
1.0
0.5
0.5
0.0
0.0
0
5
10
15
Minutes
20
25
Volts
Volts
These spectroscopic findings are confirmed by gas chromatography (GC) [56]. Pyrolyzates (see Fig. 8.10) and commercial diesel
(see Fig. 8.11) have a similar GC pattern. However, crude conversion
products contain more volatile hydrocarbons.
GC separation on an OV101 capillary [column: 20 m 0.3 mm, split
1:25; temperature program: 25C (2 min), 4C/min to 320C] reveals
double peaks in more detail (see Fig. 8.12). The first peak is for the
alkene with a double bond of a given C number. The second peak is for
the alkane having the same C number.
30
Figure 8.11 GC pattern of commercial diesel. GC-14A Shimadzu, column: FS-Supreme5/H53, 30 m; temperature program: 50C (5 min); 15C/min to 320C (10 min); FID
detector at 320C.
peaks.
Figure 8.12
29.00
28.00
27.00
26.00
25.00
24.00
23.00
22.00
21.00
20.00
C16 -Aldehyde
C18 – Sre.
C16 –Sre.
C17 -Aldehyde
C18 : 0
C16 : 1
C17 : 1
C15 : 0
C17 : 0
2400000
19.00
C16 : 0
C15 : 1
1600000
18.00
C14 : 1
C13 : 1
1800000
17.00
C14 : 0
C13 :03
C12 :1
2000000
16.00
C10 – Sre.
C12 : 0
C9 : 1
1400000
15.00
14.00
13.00
12.00
11.00
C10 : 1
C4 -Benzenes
C11:0
C11 :1
C10 : 0
9.00
10.00
C3 -Benzenes
200000
8.00
C9:0
C2 -Benzenes
C8 : 1
C7 : 1
1200000
7.00
6.00
C8 : 0
C1 -Benzenes
800000
5.00
4.00
600000
C7 : 0
1000000
3.00
C5 : 0
C6 : 0
400000
2.00
1.00
Abundance
234
2800000
TIC Jun 14903. D/date. ms
2600000
2200000
Time
GC separation of products from degradation fuel from animal fat at T 400C (Y-zeolite catalyst, DAY-Wessalith) with characteristic double
Cracking of Lipids for Fuels and Chemicals
235
You may use these hydrocarbons as a base for biofuels. However, there are
markets for certain fractions of this hydrocarbon mixture. For example,
the C-12 to C-18 fraction is a raw material widely used for bulk commodities. As mineral oil prices increase, it is becoming more financially
viable to produce chemical feedstock for commodities and specialities
from wastes. Wastes are an energy and carbon source of the future.
Products at T ⴝ 550ⴗC. For a given H-ZSM-5-zeolite, the nature of conversion products of lipids (animal fat) shifts to more aromatic compounds as the temperature increases. This is demonstrated by different
NMR findings [56] for animal fat as a substrate at a reaction temperature
of T 550C (see Figs. 8.13 through 8.15). Especially, DEPT-135
13
C-NMR pattern of oil from catalytic conversion of animal fat at 550C
shows the dominance of aromatic protons and a very low amount of CH2
groups. Chromatographic separation revealed alkylbenzenes (especially
1,3,5-trimethybenzene) as main products [38].
7.2
8
7
1
6
7.0
5
ppm
4
2.2
3
2
1
ppm
ppm
Figure 8.13 H-NMR spectrogram of hydrocarbons from animal fat at T 550C with
the commercial catalyst H-ZSM-5 (Pentasil, PZ-2/50H).
236
140
Chapter Eight
120
100
80
60
40
ppm
20
13
Figure 8.14
C-NMR spectrogram of hydrocarbons from animal
fat at T 550C with the commercial catalyst H-ZSM-5 (Pentasil,
PZ-2/50H).
Heating oil and a conversion product from animal fat have been used
in a commercial burner (Buderus, Germany). Both oils resulted in emissions within legal limits (see Table 8.5).
A straightforward approach to apply vegetable oil in the most-talkedabout biomass-to-liquid-fuel scheme is to use it as a co-substrate in
mineral oil refineries. Advantages are low investments for peripheral
facilities such as loading and storage and use of an existing infrastructure for distribution and marketing. The processing of rapeseed oil as
a feed component in a hydrocracker was described in 1990 [39]. The
results are summarized in Table 8.6.
It is worth mentioning that rapeseed oil is converted in the hydrotreatment step to paraffins. The oxygen content of the vegetable oil causes an
increased consumption of hydrogen to form water. Changes in quality
140
120
100
80
60
40
20
ppm
13
Figure 8.15 DEPT-135 C-NMR spectrogram of hydrocarbons from
animal fat at T 550C with the commercial catalyst H-ZSM-5
(Pentasil, PZ-2/50H).
Cracking of Lipids for Fuels and Chemicals
237
TABLE 8.5 Comparison of Combustion Parameters: Heating Oil versus Oil Derived
from Y-Catalytic Conversion of Animal Fat (AF) at T ⴝ 400ⴗC
Parameter
Heating oil
Heating oil & oil
from AF 1:1
Oil derived
from AF
Limiting
value
NCV, MJ/kg
Kinetic viscosity, mm2/s
C, %
H, %,
N, %
S, %
NOX, mg/m3
SO2, mg/m3
Smoke pot no.
42.0
3.25
86.5
14.0
<0.14 (d/l)
<0.34 (d/l)
162
87
0.0
41.5
2.74
85.7
13.8
<0.14 (d/l)
<0.34 (d/l)
186
26
0.4
41.3
2.51
83.4
13.5
<0.14 (d/l)
<0.34 (d/l)
233
0
0.4
>42
<6.0*
—
—
—
<0.20*
250†
350†
1‡
*
*
DIN 51 603
TA Luft
1. BImSchV
d/l: detection limit
†
‡
occur in the middle distillate. A lower density and a higher cetane number
are a quality-enhancing advantage. A drawback is the susceptibility to
freezing point of the fuel. This kind of cold flow behavior would make its
use in winter impossible unless special additives are supplemented [40].
8.3.3 Feed component in FCC
In 1993, the influence of 3–30% rapeseed oil in vacuum distillate FCC
feed on product slate and quality both at laboratory and at a continuously operated bench-scale apparatus was reported for the first time
[41]. On the one hand, results showed decreasing yields of liquid hydrocarbons with increasing rapeseed oil concentrations. On the other hand,
TABLE 8.6 Product Quality of the Hydrocracker with 20% and without Rapeseed Oil
as a Feed Component
Fraction
Rapeseed oil, %
Density (15C), g/mL
Carbon, mass %
Hydrogen, mass %
Sulphur, ppm
Nitrogen, ppm
Oxygen, mass %
NCV, MJ/kg
Octane number (MOZ)
Cetane number
Pour point,C
*VGO, vacuum gas oil.
Total oil
0
0.815
86.04
14.01
284
<1
0.1
20
0.815
85.33
14.42
114
2
0.1
Gasoline
0
0.753
85.39
14.48
29
<1
0.05
43.9
63.2
20
0.759
85.31
14.64
39
0.5
0.1
44.0
61.4
Middle distillate
0
0.830
86.06
13.82
103
<1
<0.1
43.4
20
0.817
85.27
14.66
18
<1
0.06
44.0
48
35
64
3
VGO*
0
0.852
20
0.847
38
0.7
11
<1
238
Chapter Eight
the gasoline portion in the liquid product increased. Considering
propenes, butanes, and i-butenes as gasoline potentials, low rapeseed
oil portions in the FCC feed seem to result in an optimum yield of gasoline plus gasoline potentials. Most interestingly, the gasoline fraction
recovered from a 500-h bench scale run using a feed with 30% rapeseed
oil proved suitable for standardized gasoline blending. Calcium concentration c(Ca) > 2 ppm gradually decreases FCC catalyst activity.
Oxygen contained in the vegetable oil was mainly converted to water.
Moreover, traces of phenols and carboxylic acid were detected in the
liquid reaction product.
MAT with animal fat. In a laboratory scale, mixtures of vacuum gas oil
and up to 15% of animal fat were converted in a Micro-Activity Test
(MAT) unit [37]. Results are given in Figs. 8.16 and 8.17. Two aspects
are of special interest. First, yields of propene and butene increase with
animal fat as a co-substrate. This is an advantageous finding as C-3 and
C-4 are gasoline potentials. C-3 and C-4 liquefied petroleum gas can be
used for the manufacture of isoparaffins for motor gasoline through
alkylation and polymerization processes.
Second, a higher yield of gasoline fraction is observed. This is a consequence of the high hydrogen:carbon ratio of about 2 and the low heteroatom content. For this reason, biomaterials with a hydrocarbon-like
structure are particularly interesting candidates for conversion to lowmolecular-weight fuels or chemical raw materials. Problems to be investigated are possible calcium and phosphate deposits on the catalyst
particles which may impair catalyst activity and process stability of the
riser. Therefore, the process must include a regeneration step. The
market will decide whether or not animal fat can substitute a bit of nonrenewable resources in petroleum refining.
Liquid hydrocarbons
[wt.%]
64
62
60
58
56
54
52
50
0
2
4
6
8
10
Animal fat (%) in VGO
12
14
16
Figure 8.16 Cocatalytic cracking of animal fat and vacuum gas oil (VGO) in MAT
experiments. At around 7% feed component, the maximum yield of liquid hydrocarbons is found; weight–hourly space velocity (WHSV) 2 h1.
Cracking of Lipids for Fuels and Chemicals
239
Total C3 + C4 (LPG) [wt.%]
14
13
12
11
10
9
8
0
2
4
6
8
10
12
14
16
Animal fat (%) in VGO
Cocatalytic cracking of animal fat and vacuum gas oil (VGO) in MAT
experiments. Gasoline potentials show a maximum at low rapeseed portions around
7%; weight–hourly space velocity (WHSV) 2 h1.
Figure 8.17
8.4
Other Metal Oxide Catalysts
Cottonseed oil has been thermally decomposed at 450C using 1% Na2CO3
as a catalyst [42]. Pyrolysis produced a yellowish-brown oil with 70C yield.
The fuel properties of original and pyrolyzed cottonseed oil are summarized in Table 8.7. Results of ASTM distillation compared to diesel
are given in Table 8.8 showing a higher volatility for the conversion
product.
Rapeseed oil was pyrolyzed in the presence of about 2% calcium oxide
up to a temperature of 450C [43]. An oil was obtained with a heating
TABLE 8.7
Fuel Properties of Original and Pyrolyzed Cottonseed Oil and No. 2 Diesel
Fuel
Property
Diesel fuel
Pyrolyzed oil
Original oil
API gravity
Specific gravity (at 15.6C)
Kinetic viscosity (mm/s2) at 40C
Cetane index
Flash point, C
Sulfur content, wt.%
Pour point, C
Sediment content, wt.%
Calorific value, kJ/g
Water content, vol.%
Ash content, wt.%
Carbon residue, wt.%
35
0.8504
0.0213
33
96
0.04
0.0
Nil
45.57
Nil
Nil
0.01
35
0.8500
0.0178
28
53
Nil
>15
0.04
45.57
2.98
Nil
0.16
21.5
0.9246
0.0357
20
268
0.02
23
5.0
41.80
1.20
Nil
1.06
240
Chapter Eight
Results of ASTM Distillation of No. 2 Diesel
Oil and Pyrolyzed Cottonseed Oil as Volume Percent
TABLE 8.8
Temperature C
Parameter
Diesel oil
Distillate, %
0
10
20
30
40
50
60
70
80
90
98
Recovery, %
Residue, %
Loss, %
63
105
174
192
200
210
235
245
250
255
260
98
1
1
Pyrolyzed oil
55
79
116
131
157
178
186
220
247
269
–
90
9
1
value of 41.3 MJ/kg, a kinematic viscosity of 5.96 mm2/s, a cetane
number of 53, and a flash point of 80C. When tested on a diesel engine,
the thermal efficiency (␩th) and brake specific fuel consumption were
improved. The concentration of nitrogen oxide in the exhaust gas was
less than diesel. The absence of sulfur in the pyrolytic oil was seen as
an advantage to avoid corrosion problems and the emission of polluting
sulfur compounds from combustion.
Triolein, canola oil, trilaurin, and coconut oil were pyrolyzed over activated alumina at 450C and atmospheric pressure [44]. The products
were characterized by IR spectrometry and decoupled 13C-NMR spectroscopy. The hydrocarbon mixture contained both alkanes and alkenes.
These results are significant for the pyrolysis of lipid fraction in sewage
sludge as well as for wastes from food-processing industries [44].
Pyrolysis of rapeseeds, linseeds, and safflowers results in bio-oil containing oxygenated polar components. Hydropyrolysis at medium pressure in the presence of 1% ammonium dioxydithiomolybdenate
(NH4)2MoO2S2 can remove two-thirds to nine-tenths of the oxygen present in the seeds to generate bio-oils in yields up to 75% [45]. In addition, extraction with organic solvents including diesel oil gave yields up
to 40%.
The potential of liquid fuels from Mesua ferrea seed oil [46], Euphorbia
lathyris [47, 48], and underutilized tropical biomass [49] has been investigated in the search for “energy farms” involving the purposeful cultivation of selected plants to obtain renewable energy sources.
Cracking of Lipids for Fuels and Chemicals
8.5
241
Cracking by In Situ Catalysts
This method is applicable for cellular biomass containing lipids, e.g.,
sewage sludge or organic residues from rendering plants. The
European Union is looking for new markets for both materials. On the
one hand, treatment of municipal and industrial wastewaters generates huge quantities of sludge, which is the unavoidable by-product
especially if biological processes are used. Management of this residue
poses an urgent problem. The residue contains about 60% of bacterial
biomass and up to 40% of inorganic materials such as alumina, silicates, alkaline and alkaline earth elements, phosphates, and varying
amounts of heavy metals [56]. On the other hand, returning animal
meal (AM) or meat and bone meal (MBM) from the rendering plant into
the food cycle is forbidden by law since the BSE crisis [50, 51]. Besides
burning, low-temperature conversion (LTC) of these organic materials
offers an alternative disposal method [52–54]. LTC is a thermocatalytic process whereby organics react to hydrocarbons as the main
product [12].
The conversion of bacterial biomass or organic residues from rendering plants to oil may be formally defined by considering the starting
materials and the end products. The principal components of these substrates are proteins and lipids. They make up about 60–80% of this biomass. The average elemental composition of neutral lipids is C50H92O6.
An empirical formula for proteins is (C70H135N18O38S)x. From these compounds, nonpolar hydrocarbons of the general elemental composition
CnHm have to be produced [13, 55].
Obviously, LTC removes the heteroatoms from both principal components. In general, it splits off functional groups from complex biomass. The process operates at moderate temperatures (380–450C),
essential atmospheric pressure, and the exclusion of oxygen. Under
these conditions, heteroatoms from organics are removed as ammonia (NH3), dihydrogensulfide (H2S), water (H2O), and carbon dioxide
(CO2). This decomposition scheme may serve as a model for the formation of coal from primarily plant sources. Carbohydrates (C6H10O5)n
are the principal components in plants. The elimination of water from
carbohydrates produces elemental carbon, according to the following
reaction:
(C6H10O5)n – 5H2O → Cm
Consequently, carbohydrates of bacterial mass will be converted to
carbon, mainly in the form of graphite [56, 57]. Therefore, the formation of oil from complex biomass will always be accompanied by the formation of carbon. Figure 8.18 depicts the mechanism for the production
of oil from lipids by LTC [58].
242
Chapter Eight
O
R′
C
CH2
(CH2)n
R
+ R′
O
O
(CH2)n
R
O
C
R′
C•
C
CH2
CH2
CH2
(CH2)n
CH2• +
•CH2
(CH2)n
R
O
O
Chain
composition n+1
R'
CH2
O
Radical splitting
•CH2
CH2
Chain
decomposition n-1
R
R′
O
C
CH2
(CH2)n
R
O
O
Disproportionation
2•CH2
CH2
Figure 8.18
(CH2)n
R
CH2
CH
(CH2)n
R + CH3
CH2
(CH2)n
R
Mechanistic aspects of the formation of hydrocarbons by cracking of lipids [58].
It is worth mentioning that the ash content (Table 8.9) includes natural catalysts (e.g., alumina and silicates) that substantially influence
the yield and composition of LTC products. Table 8.9 shows results of
the conversion of these organic residues. Yields of oil, solid product,
water, volatile salts (NH4Cl, NaHCO3), and noncondensable gases
(NCG: CO2, H2, C-1–C-4 alkanes and different alkenes) are given in Fig
8-19. Digested sludge produces less oil than aerobically stabilized
sludge. This correlates with the carbon content in Table 8.9. The food
chain of anaerobic bacteria efficiently removes organic carbons as biogas
(CH4/CO2). Thus it is no longer available for the production of oil in subsequent LTC. AM shows higher yields of oil due to its higher content
of fat and proteins (Table 8.9). The viscosities of untreated oils at 40C
2
2
2
are as follows: DS, 14 mm /s; AS, 35 mm /s; AM, 27 mm /s; and MBM,
2
21 mm /s. In comparison, diesel from a filling station has a viscosity of
TABLE 8.9 Chemical and Physical Characteristic Substrates for LTC
Parameter
AS
DS
AM
MBM
Dry solids, %
Ash content, %
Protein, %
Fat, %
Calcium as Ca, %
Phosphorus as P2O5, %
NCV, MJ/kg
C, %
H, %
N, %
S, %
95.0
35.1
32.9
—
—
—
14.2
31.6
4.4
5.0
0.6
79.6
40.7
26.6
—
9.6
6.3
9.9
23.0
5.0
3.3
1.0
94.3
23.2
52.3
14.4
9.7
8.7
18.8
42.5
6.6
8.3
0.5
95.0
38.2
49.6
8.9
20.1
16.0
15.4
30.5
4.8
7.6
0.3
AS: aerobically stabilized sewage sludge; DS: digested sewage sludge;
AM: animal meal; MBM: meat and bone meal.
Cracking of Lipids for Fuels and Chemicals
243
80
Solid product
Oil
Reaction water
Salt
NCG
Product yield (mass%)
60
40
20
0
AS
DS
AM
MBM
Figure 8.19 Mass yield of LTC products from different substrates. AS: Aerobically stabilized sewage sludge; DS: digested sewage sludge; AM: animal meal; MBM: meat and
bone meal [60].
about 4 mm2/s. The solid products consist of carbon, nonvolatile salts
(e.g., CaKPO4), and metal oxides or sulfides. Especially in the case of
AM and MBM, the solid product is of commercial interest due to its high
content of phosphate. It is free of proteins [59].
As with natural crude oils, the hydrocarbon mixtures obtained by
LTC of lipids containing biomass are of a highly complex composition.
For example, Fig. 8.20 shows the gas chromatogram of oil derived from
C8
Goschromatogramm dos gesamtols aus
klarschlamm vaihingen
C9
C10 C11 C12 C13 C14 C15 C16 C17 C18
50°C
100°C
150°C
C20
200°C
250°C
Chromatographic separation of oil derived from sewage sludge; separation was performed on OV101 capillary column 20 m 0.3 mm, split 1:25; temperature program 25C (2 min), 4C/min to 320C.
Figure 8.20
244
Chapter Eight
2.26216
1.58385
1.24420
1.17452
1.12234
1.01315
1.00514
0.99071
0.08420
0.96417
0.92519
0.90611
0.83640
0.87784
0.85917
0.82083
0.79726
0.76995
0.75050
sewage sludge AS [61]. Peaks assigned by numbers correspond to the
aliphatic, unbranched saturated hydrocarbons. The peak appearing before
the n-alkane corresponds to the n-alkenes.
The predominant aliphatic nature of oils produced is readily ascertained by NMR spectroscopy. Figure 8.21 depicts the 1H-NMR spectrogram of oil from DS with about 5% of aromatic protons.
Infrared spectroscopy (see Fig. 8.22) reveals the presence of C-H1
stretching frequencies at 2850–3000 cm . In addition, the spectrum
provides clear evidence of hydrogen bonding due to a broad absorption
band of 3350 cm1. Thus, decarboxylation of lipids in the presence of
in situ catalysts is not complete. This is consistent with the higher viscosities in comparison to diesel. A special loop reactor for recycling
catalytic activity to overcome these problems has been designed [62].
Hydrocarbons are derived from both lipids and proteins in the sewage
sludge in the presence of in situ catalysts. However, oil produced from
proteins under anaerobic LTC conditions is high in nitrogen and sulfur:
Amines, purins, and mercaptanes are trace contaminants that are
formed. Consequently, this oil smells and is a nuisance, and upgrading (e.g., over H-ZSM-5 as catalyst) is essential [64]. The useful oil is
~95% alkanes
& alkenes
4.582
95.418
~5% arenes
9
Figure 8.21
8
1
7
6
5
ppm
4
3
H-NMR of oil from LTC of DS at T 400C.
2
1
0
Cracking of Lipids for Fuels and Chemicals
245
100
Air peak
Transmission (%)
96
3344
1707
92
88
2853
84
2923
80
4000
3200
2400
1800
1400
1000
540
Wave number, cm−1
Figure 8.22 Infrared spectrum of oil from DS shows associated –OH and –NH
bonds (3350 cm1) from the remaining carboxylic acids R-COOH or amides
R-CONH2 [63].
produced from lipids. When sewage sludge was spiked with triolein,
representative of unsaturated triglycerides, the compound did not survive the LTC [65]. As a result, sludge was extracted with toluene using
a Soxleth extraction method to yield 12 wt.% lipids. Pyrolysis of sewage
sludge lipids over activated alumina produced liquid hydrocarbons
containing mostly alkanes [65]. Even the carboxylic acid fractions of
the lipids that were separated were completely converted. This is in
contrast to direct sewage sludge LTC, where long-chain carboxylic
acids are detectable in the IR spectrum (see Fig. 8.22). The reason is
the lower content of catalytically active in situ material. Pyrolyzed
liquid products from sewage sludge lipids contain virtually no nitrogen or sulfur (see Table 8.10). Only this liquid has a potential for use
as a base for commercial fuels [65].
TABLE 8.10 Elemental Composition of Original Dried Sludge, Extracted
Lipids, and Pyrolyzed Liquid Product
Component
C, %
H, %
O, %
N, %
S, %
Ash, %
Original sludge
Extracted lipids
Liquid product
39.5
72.2
86.6
6.0
10.7
13.5
26.1
14.2
0.0*
6.00
0.35
0.08
0.7
0.85
0.22
20.5
0.6
0.0*
*
By difference.
246
8.6
Chapter Eight
Conclusion
The potential offered by lipids for alternative fuel and chemicals is
widely recognized. Various sources from plant seeds to animal fat are
commercially available. Cracking converts polar esters into nonpolar
hydrocarbons. Highly efficient conversion technology should include
use of catalysts, e.g., zeolites such as H-ZSM-5 or Y-type representatives.
At 380–450C, alkanes and alkenes are predominantly found in the
liquid product. With increasing temperatures up to 550C, the product
spectrum shifts to alkylbenzenes with 1,3,5-trimethylbenzene as the
main product. For commercial fuel production based on lipids, assessment of oxidation stability and deposit formation are essential.
Influences on regulated and nonregulated emissions have to be analyzed.
Attention should be paid both to the NOx content of exhaust gas and to
the particle size distribution with special focus on ultrafine particles. In
addition, mutagenic tests for potency of particulate matter extracts are
recommended. Finally, it has to be kept in mind, that the replacement
of fossil fuels by biofuels may not bring the intended climate cooling due
to the accompanying emissions of N2O from the use of N-fertilizers in
crop production. Much more research on the sources of N2O and the
nitrogen circle in connection with biofuels from lipids is needed.
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Chapter
9
Fuel Cells
A. K. Sinha
9.1
Introduction
Global primary energy consumption (i.e., energy used for space heating,
transportation, generating electricity, etc.) is expected to triple from about
400 exajoules (EJ 1018 joules) per year in 2000 to about 1200 EJ/yr
in 2050 at the present rate of increase in consumption. However, due to
increased energy efficiency of the devices, the actual increase is expected
to be about 800–1000 EJ.
More than 80% of the present primary energy requirements are met
by fossil fuels. The consequences of burning hydrocarbons at such a
large scale for our energy needs are already evident in the form of global
warming and its disastrous environmental effects. In order to permit stabilization of anthropogenic greenhouse gases, fossil fuel consumption
will have to be limited to about 300 EJ/yr by 2050. Hopefully, the concern about global warming, limit on fossil fuel supplies, and rise in their
prices will force us to gradually decrease the use of fossil fuels in the
future. Reducing hydrocarbon consumption to 300 EJ requires carbonfree energy sources to supply the difference ~700 EJ/yr. This shortfall
is a problem that requires immediate attention and proactive action for
sustainable development.
The need for an efficient, nonpolluting energy source for transportation,
large-scale generation, and portable devices has spurred the development of alternative energy sources. Fuel cells are a promising alternative
energy source that fits the above requirements [1–6]. A fuel cell is an
electrochemical device that converts the chemical energy of a fuel (hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen)
into electricity, with water and heat as by-products. Since no combustion
251
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252
Chapter Nine
is involved in the hydrogen fuel cell process, no NOx are generated. Since
sulfur is a poison to fuel cells, it has to be removed from fuel before feeding it to a fuel cell; therefore, no SO2 is generated in the fuel cell.
The trend toward portability and miniaturization of computing and
communication devices has created a requirement for very small and
lightweight power sources that can operate for long periods of time without any refill or replacement. Also, advances in the medical sciences are
leading to an increasing number of electrically operated implantable
devices like pacemakers, which need power supplies to operate for an
extremely long duration (years) without maintenance, as any maintenance would necessitate surgery. Ideally, implanted devices would be
able to take advantage of the natural fuel substances found in the body
[7–8]. The idea of a biofuel cell that can generate electricity based on various metabolic processes occurring in our own cells is very appealing. A
biofuel cell converts chemical energy to electrical energy by the catalytic
reaction of microorganisms. Most microbial cells are electrochemically
inactive, and electron transfer from microbial cells to the electrode requires
mediators such as thionine, methyl viologen, methylene blue, humic acid,
and neutral red. In recent years, mediatorless microbial fuel cells have
also been developed; these cells use electrochemically active bacteria
(Shewanella putrefaciens, Aeromonas hydrophila, etc.) to transfer electrons to the electrode. A major advantage of the biofuel cell over the hydrogen fuel cell is the replacement of expensive and precious platinum (Pt)
as a catalyst by much cheaper hydrogenase enzymes. A brief description
of the development and state of the art of hydrogen and biofuel cells is
presented in this chapter.
9.2
Fuel Cell Basics
Although fuel cells have been around for more than a century (William
Grove in 1839 first discovered the principle of the fuel cell), it was not
until the National Aeoronautics and Space Administration (NASA)
demonstrated its potential applications in providing power during space
flights in the 1960s that fuel cells became widely known and the industry began to recognize the commercial potential of fuel cells. Initially,
fuel cells were not economically competitive with existing energy technologies; but with advancements in fuel cell technology, it is now becoming competitive for some niche applications [6].
The main components of a fuel cell are anode, anodic catalyst layer,
electrolyte, cathodic catalyst layer, and cathode, as shown in Fig. 9.1.
The anode and cathode consist of porous gas diffusion layers, usually
made of high-electron-conductivity materials such as thin layers of
porous graphite. The most common catalyst is platinum for lowtemperature fuel cells. Nickel is preferred for high-temperature fuel
Fuel Cells
253
Electric current
e−
Depleted
fuel
e−
e−
Water
e−
H+
Η2Ο
H2
O2
H+
Air in
Fuel
Anode
Oxygen molecule
Cathode
Gas diffusion layer
Electrolyte
Gas diffusion layer
Cathode catalyst
layer
Anode catalyst layer
Figure 9.1
Hydrogen molecule
Water
Hydrogen ion (H+)
Generic H2-O2 fuel cell.
cells. Some other materials (Pt-Pt/Ru, Perovskites, etc.) are also used,
depending on the fuel cell type [3].
The electrolyte is made up of materials that provide high proton conductivity and zero or very low electron conductivity. The charge carriers
(from the anode to the cathode or vice versa) are different, depending
on the type of fuel cell. A fuel cell stack is obtained by connecting such
fuel cells in series/parallel to yield the desired voltage and current outputs (see Fig. 9.2). The bipolar plates (or interconnects) collect the electrical current and also distribute and separate reactive gases in the
fuel cell stack. Sometimes, gaskets for sealing/preventing leakage of
gases between anode and cathode are also used.
Load
Air
flow field
Hydrogen
flow field
Bipolar plates
End plate
Electrolyte
Cathode
End plate
Anode
Figure 9.2
A fuel cell stack.
254
Chapter Nine
The anode reaction in hydrogen fuel cells is direct oxidation of hydrogen. For fuel cells using hydrocarbon fuels, the anodic half reaction consists of indirect oxidation through a reforming step.
In most fuel cells, the cathode reaction is oxygen (air) reduction. The
overall reaction for hydrogen fuel cells is
H2 1
1
O S H2OwithG 5 2237.2 kJ/mol
2 2
where, G is the change in Gibbs free energy of formation. The reaction
product is water released at the cathode or anode, depending on the type
of fuel cell.
For an ideal fuel cell, the theoretical voltage E0 under standard conditions of 25C and 1 atm pressure is 1.23 V, whereas typical operating
voltage for high-performance fuel cells is ~0.7 V. Stack voltage depends
on the number of cells in a series in a stack. Cell current depends on the
cross-sectional area (the size) of a cell.
Fuel cell systems are not limited by Carnot cycle efficiency. Therefore,
a fuel cell system with a combined cycle and/or cogeneration has very high
efficiency (55–85%) as compared to the efficiency of about 30–40% of current power generation systems. In a distributed generation system, fuel
cells can reduce costly transmission line installation and transmission
losses. There are no moving parts in a fuel cell and very few moving parts
(compressors, fans, etc.) in a fuel cell system. Therefore, it has higher reliability compared to an internal combustion or gas turbine power plant.
Fuel cell-based power plants have no emissions when pure hydrogen
and oxygen are used as fuel. However, if fossil fuels are used for generating hydrogen, fuel cell power plants produce CO2 emissions. Compared
to a steam power plant, a fuel cell plant has very low water usage;
water/steam is a reaction product in a fuel cell. This clean water/steam
does not require any pretreatment and can be used for reactant humidification and cogeneration. Another advantage of the fuel cell power
plant is that it does not produce any solid waste and its operation is very
silent as compared to a steam/gas turbine power plant. The noise generated in a fuel cell power plant is only from the fan/compressor used
for pumping/pressurizing the fuel and the air supply to the cathode.
A fuel cell power plant has good load-following capability (it can
quickly increase or decrease its output in response to load changes). The
modular construction of fuel cell plants provides good planning flexibility
(new units can be added to meet the growth in electric demand when
needed), and its performance is independent of the power plant size
(efficiency does not vary with variation in size from W to MW size).
The major technical challenges in fuel cell commercialization at present are (1) high cost, (2) durability, and (3) hydrogen availability and
Fuel Cells
255
infrastructure. For fuel cells to compete with contemporary power generation technology, they have to become competitive in terms of the cost
per kilowatt required to purchase and install a power system. A fuel cell
system needs to cost ~$30/kW to be competitive for transportation applications and for stationary systems; the acceptable price range is
$400–$750/kW for widespread commercial application [9]. Fuel cell technology needs a few breakthroughs in development to become competitive with other advanced power generation technologies.
9.3
Types of Fuel Cells
Fuel cells are classified primarily on the basis of the electrolyte they use.
The electrolyte is the heart of the fuel cell as it decides the important
operating parameters such as the electrochemical reactions that take
place in the cell, the type of catalysts required, the temperature range of
cell operation, and the fuel (reactants) to be used, and therefore the
applications for which these cells are most suitable. There are several
types of fuel cells currently under development; a few of the most promising types include
䊏
Polymer electrolyte membrane fuel cells (PEMFCs)
䊏
Direct methanol fuel cells (DMFCs)
䊏
Alkaline electrolyte fuel cells (AFCs)
䊏
Phosphoric acid fuel cells (PAFCs)
䊏
Molten carbonate fuel cells (MCFCs)
䊏
Solid oxide fuel cells (SOFCs)
䊏
Biofuel cells
9.3.1 Polymer electrolyte membrane
fuel cells (PEMFCs)
The PEMFC uses a solid polymer membrane as an electrolyte. The main
components of this fuel cell are an electron-conducting anode consisting of a porous gas diffusion layer as an electrode and an anodic catalyst layer; a proton-conducting electrolyte, a hydrated solid membrane;
an electron-conducting cathode consisting of a cathodic catalyst layer
and a porous gas diffusion layer as an electrode; and current collectors
with the reactant gas flow fields (see Fig. 9.3).
In the PEMFC, platinum or platinum alloys in nanometer-size particles are used as the electrocatalysts with NafionTM (a DuPont trademark) membranes [3, 10–12]. The polymer electrolyte membranes have
some unusual properties: In a hydrated membrane, the negative ions
are rigidly held within its structure and are not allowed to pass through.
256
Chapter Nine
Electric current
e−
e−
t
e− Electric curren
e−
Depleted fuel
Water
Hydrogen molecule
H+
Oxygen molecule
H2O
H2
Water
Hydrogen ion (H+)
O2
H+
Air in
H2
Anode
Gas diffusion layer
Anode
catalyst layer
Figure 9.3
Cathode
Electrolyte
(PEM)
Gas diffusion layer
Cathode
catalyst layer
Polymer electrolyte membrane fuel cell.
Only the positive ions contained within the membrane are mobile and
free to carry a positive charge through the membrane. The proton
exchange membrane (PEM) is a good conductor of hydrogen ions (protons), but it does not allow the flow of electrons through the electrolyte
membrane. As the electrons cannot pass through the membrane, electrons produced at the anode side of the cell must travel through an
external wire to the cathode side of the cell to complete the electrical
circuit in the cell.
In the PEMFC, the positive ions moving through the electrolyte are
hydrogen ions, or protons. Therefore, the PEMFC is also called a proton
exchange membrane fuel cell. The polymer electrolyte membrane is also
an effective gas separator; it keeps the hydrogen fuel separated from the
oxidant air. This feature is essential for the efficient operation of a fuel cell.
The heart of a PEMFC is the membrane electrode assembly (MEA),
consisting of the anode–electrolyte–cathode assembly that is only a few
hundred microns thick [11].
All electrochemical reactions consist
of two separate reactions: an oxidation half reaction occurring at the
anode and a reduction half reaction occurring at the cathode.
Electrochemistry of PEM fuel cells.
Oxidation half reaction:
2H2 → 4H 4e
Reduction half reaction:
O2 4H 4e → 2H2O
Overall cell reaction:
2H2 O2 → 2H2O
Fuel Cells
257
The H2 half reaction. At the anode, hydrogen (H2) gas molecules diffuse
through the porous electrode until they encounter a platinum (Pt) particle. Pt catalyzes the dissociation of the H2 molecule into two hydrogen
atoms (H) bonded to two neighboring Pt atoms; here each H atom
releases an electron to form a hydrogen ion (H). These H ions move
through the hydrated membrane to the cathode while the electrons pass
from the anode through the external circuit to the cathode, resulting in
a flow of current in the circuit.
The O2 half reaction. The reaction of one oxygen (O2) molecule at the
cathode is a four-electron reduction process that occurs in a multistep
sequence. The catalysts capable of generating high rates of O2 reduction
at relatively low temperatures (~80C) appear to be the Pt-based expensive catalysts. The performance of the PEMFCs is limited primarily by
the slow rate of the O2 reduction half reaction, which is many times
slower than the H2 oxidation half reaction.
Electrolyte. The polymer electrolyte membrane is a solid organic polymer, usually poly-[perfluorosulfonic] acid. A typical membrane material
used in the PEMFC is Nafion [11, 12]. It consists of three regions:
1. The teflon-like fluorocarbon backbone, hundreds of repeating
–CF2–CF–CF2– units in length
2. The side chains, –O–CF2–CF–O–CF2–CF2–, which connect the molecular backbone to the third region
3. The ion clusters consisting of sulfonic acid ions, SO3 H
The negative ion SO3 is permanently attached to the side chain and
cannot move. However, when the membrane becomes hydrated by
absorbing water, the hydrogen ion becomes mobile. Ion movement occurs
by protons (H ) bonded to water molecules, hopping from one SO3 site
to another within the membrane. Because of this mechanism, the solid
hydrated electrolyte is an excellent conductor of hydrogen ions.
Electrodes. The anode and the cathode are separated from each other
by the electrolyte, the PEM. Each electrode consists of porous carbon to
which very small Pt particles are bonded. The porous electrodes allow the
reactant gases to diffuse through each electrode to reach the catalyst.
Both platinum and carbon are good conductors, so electrons are able to
move freely through the electrode [13, 14].
Catalyst. The two half reactions occur very slowly under normal conditions at the low operating temperature (~80C) of the PEMFC.
Therefore, catalysts are needed on both the anode and cathode to
increase the rates of each half reaction. Although platinum is a very
expensive metal, it is the best material for a catalyst on each electrode.
258
Chapter Nine
The half reactions occurring at each electrode can occur only at a high
rate at the surface of the Pt catalyst. A unique feature of Pt is that it is
sufficiently reactive in bonding H and O intermediates, as required to
facilitate the electrode processes, and is also capable of effectively releasing the intermediate to form the final product. The anode process
requires Pt sites to bond H atoms when the H2 molecule reacts; next,
these Pt sites release the H atoms, as follows:
2H 2e → H2
H2 2Pt → 2(Pt-H)
2(Pt-H) → 2Pt 2H 2e
This optimized bonding to H atoms (neither very weak nor very
strong) is a unique property of the Pt catalyst. To increase the reaction
rate, the catalyst layer is constructed with the highest possible surface
area. This is achieved by using very small Pt particles, about 2 nm in
diameter, resulting in an enormously large total surface area of Pt that
is accessible to gas molecules. The original MEAs for the Gemini space
program used 4 mg of platinum per square centimeter of membrane area
(4 mg/cm2). Although the technology varies with the manufacturer, the
total platinum loading has decreased from the original 4 mg/cm2 to
about 0.5 mg/cm2. Laboratory research now uses platinum loadings of
0.15 mg/cm2. For catalyst layers containing Pt of about 0.15 mg/cm2, the
thickness of the catalyst layer is ~10 m; the MEA with a total thickness
~200 m can generate more than half an ampere of current for every
square centimeter of the MEA at a voltage of 0.7 V between the cathode and the anode [2, 3, 10–12]. Recently, scientists at Los Alamos
National Laboratory, USA have developed a new class of hydrogen fuel
cell catalysts that exhibit promising activity and stability. The catalysts, cobalt-polypyrrole-carbon (Co-PPY-XC72) composite, are made of
low-cost metals entrapped in a heteroatomic-polymer structure.
The cell hardware. The hardware of the fuel cell consists of backing
layers, flow fields, and current collectors. These are designed to maximize the current that can be obtained from an MEA. The backing layers
placed next to the electrodes are made of a porous carbon paper or
carbon cloth, typically 100–300 m thick. The porous nature of the backing material ensures effective diffusion of the reactant gases to the catalyst. The backing layers also assist in water management during the
operation of the fuel cell; too little or too much water can halt the cell
operation. The correct backing material allows the right amount of
water vapor to reach the MEA and keep the membrane humidified.
Carbon is used for backing layers because it can conduct the electrons
leaving the anode and entering the cathode. A piece of hardware, called
Fuel Cells
259
a plate, is pressed against the outer surface of each backing layer. The
plate serves the dual role of a flow field and current collector. The side
of the plate next to the backing layer contains channels machined into
the plate. The plates are made of a lightweight, strong, gas-impermeable,
electron-conducting material; graphite or metals are commonly used,
although composite material plates are now being developed. Electrons
produced by the oxidation of hydrogen move through the anode, through
the backing layer, and through the plate before they can exit the cell,
travel through an external circuit, and reenter the cell at the cathode
plate. In a single fuel cell, these two plates are the last of the components making up the cell.
In a fuel cell stack, current collectors are the bipolar plates; they
make up over 90% of the volume and 80% of the mass of a fuel cell stack
[11, 15, 16].
Water and air management. Although water is a product of the fuel cell
reaction and is carried out of the cell during its operation, it is necessary that both the fuel and air entering the fuel cell be humidified.
This additional water keeps the polymer electrolyte membrane
hydrated. The humidity of the gases has to be carefully controlled, as too
little water dries up the membrane and prevents it from conducting
the H ions and the cell current drops. If the air flow past the cathode
is too slow, the air cannot carry all the water produced at the cathode
out of the fuel cell, and the cathode “floods.” Cell performance deteriorates because not enough oxygen is able to penetrate the excess
liquid water to reach the cathode catalyst sites. Cooling is required
to maintain the temperature of a fuel cell stack at about 80C, and the
product water produced at the cathode at this temperature is both
liquid and vapor.
Performance of the PEM fuel cell [3, 11, 16].
Energy conversion in a fuel
cell is given by the relation:
Chemical energy of the fuel electric energy heat energy
Power is the rate at which energy (E) is made available (P dE/dt, or
E Pt). The power delivered by a cell is the product of the current
(I ) drawn and the terminal voltage (V ) at that current (P IV watts).
In order to compute power delivered by a fuel cell, we have to know the
cell voltage and load current. The ideal (maximum) cell voltage (E ) for
the hydrogen/air fuel cell reaction (H2 1/2O2 → H2O) at a specific temperature and pressure is calculated from the maximum electrical energy
Wel G nFE
or
E
G
nF
260
Chapter Nine
where ∆G is the change in Gibbs free energy for the reaction, n is the
number of moles of electrons involved in the reaction per mole of H2, and
F (Faraday’s constant) 96,487 C (coulombs joules/volt). At a constant
pressure of 1 atm, the change in Gibbs free energy in the fuel cell process
(per mole of H2) is calculated from the reaction temperature (T ) and from
changes in the reaction enthalpy (H ) and entropy (S ).
G H T S
285,800 J (298 K)(163.2 J/K)
237,200 J
For the hydrogen–air fuel cell at 1 atm pressure and 25C (298 K), the
cell voltage is
E
G
nF
5 2a 2
237,200 J
b 5 1.23 V
2 3 96,487 J/V
As temperature rises from room temperature to the PEM fuel cell
operating temperature (80C or 353 K), the change in values of H and S
is very small, but T changes by 55C. Thus the absolute value of G
decreases. Assuming negligible change in the values of H and S,
G 285,800 J/mol (353 K)(163.2 J/mol K)
228,200 J/mol
Therefore,
E 5 2a 2
228,200 J
b 5 1.18 V
2 3 96,487 J/V
Thus, for standard pressure of 1 atm, the maximum cell voltage
decreases from 1.23 V at 25C to 1.18 V at 80C. An additional correction is needed for using air instead of pure oxygen, and also for using
humidified air and hydrogen instead of dry gases. This further reduces
the maximum voltage from the hydrogen–air fuel cell to 1.16 V at 80C
and 1 atm pressure. With an increase in load current, the actual cell
potential is decreased from its no-load potential because of irreversible
losses, which are often called polarization or overvoltage (h). These originate primarily from three sources:
䊏
Activation polarization (hact)
䊏
Ohmic polarization (hohm)
䊏
Concentration polarization (hconc)
Fuel Cells
261
The polarization losses result in a further decrease in actual cell voltage (V ) from its ideal potential E (V E potential drop due to losses).
The activation polarization loss is dominant at low current density. This
is because electronic barriers have to be overcome prior to current and
ion flows. Activation polarization is present when the rate of an electrochemical reaction at an electrode surface is controlled by sluggish electrode kinetics. Therefore, activation polarization is directly related to the
rates of electrochemical reactions. In an electrochemical reaction with
hact > 50 100 mV, activation polarization is described by a semiempirical equation known as the Tafel equation:
hact 5 a
RT
i
b ln a b
nF
i0
where is the electron transfer coefficient of the reaction at the electrode (anode or cathode), and i0 is the exchange current density. The Tafel
slope for the PEMFC electrochemical reaction is about 100 mV/decade
at room temperature. Thus there is an incentive to develop electrocatalysts that yield a lower Tafel slope [2, 3, 11, 12].
Ohmic losses occur because of the resistance to the flow of ions in the
electrolyte and resistance to the flow of electrons through the electrode
materials. Decreasing the electrode separation and enhancing the ionic
conductivity of the electrolyte can reduce the ohmic losses. Both the
electrolyte and fuel cell electrodes obey Ohm’s law; the ohmic losses can
be expressed by the equation: hohm iR, where i is the current flowing
through the cell and R is the total cell resistance, which includes ionic,
electronic, and contact resistance. See Figure 9.4.
Due to the consumption of reactants at the electrode by an electrochemical reaction, the surrounding material is unable to maintain the
700
Cathode loss
Polarization (mV)
600
500
Cathode loss (O2)
400
300
200
Electrolyte IR loss
100
Anode loss (H2)
0
Figure 9.4
200
400
600
Current density (mA/cm2)
800
Activation losses in a PEM fuel cell [1].
262
Chapter Nine
initial concentration of the bulk fluid and a concentration gradient is
formed, resulting in a loss of electrode potential. Although several
processes contribute to concentration polarization, at practical current
densities, slow transport of reactants and products to and from the electrochemical reaction site is a major contributor to concentration polarization. The effect of polarization is to shift the potential of the electrode:
For the anode,
Vanode Eanode |hanode|
and for the cathode,
Vcathode Ecathode |hcathode|
The net result of current flow in a fuel cell is to increase the anode
potential and to decrease the cathode potential. This reduces the cell
voltage. The cell voltage includes the contribution of the anode and
cathode potentials and ohmic polarization. See Figure 9.5.
Vcell Vcathode Vanode iR; or
Vcell Ecathode |hcathode| (Eanode |hanode|) iR; or
Vcell Ecell |hcathode| |hanode| iR
where Ecell Ecathode Eanode
The goal of fuel cell developers is to minimize the polarization losses
so that the Vcell approaches the Ecell by modifications to the fuel cell
Cell voltage
Theoretical EMF or ideal voltage
Region of activation polarization
(Reactive rate loss)
1.0
Total loss
0.5
Region of ohmic polarization
(Resistance loss)
Operating voltage V curve
0.0
0
200
400
600
800
Current density (mA/cm2)
Figure 9.5
curve [3].
PEM fuel cell voltage versus current density
Fuel Cells
263
design by improvement in the electrode structures, better electrocatalysts, more conductive electrolytes, thinner cell components, and so forth.
It is possible to improve the cell performance by modifying the operating conditions such as higher gas pressure, higher temperature, and a
change in gas composition to lower the gas impurity concentration [3].
9.3.2
Direct methanol fuel cells (DMFCs)
Direct methanol fuel cells are similar to the PEMFC as they also use a
polymer membrane as the electrolyte. However, it produces power by
direct conversion of liquid methanol to hydrogen ions on the anode side
of the fuel cell. In the DMFC, the anode catalyst draws hydrogen directly
from the liquid methanol, thus eliminating the need for a fuel reformer.
All the DMFC components (anode, cathode, membrane, and catalysts)
are the same as those of a PEMFC. A DMFC system is shown in Fig. 9.6.
Methanol diluted to a specified concentration is fed to the fuel cell stack.
During operation, the concentration of the methanol solution exiting the
stack is reduced. Therefore, pure methanol is added in the feed cycle to
restore the original concentration of the solution. A gas–liquid separator is used to remove carbon dioxide from the solution loop, and a compressor feeds air to the DMFC stack. Water and heat are recovered
by passing the outlet air through a condenser. A portion of the recovered water is returned to the fuel circulation loop. The stack temperature is maintained by removing the excess heat from the fuel
circulation loop using a heat exchanger. The DMFC can attain high
efficiencies of 40% with a Nafion-117 membrane at 60C, with current
CO2
Methanol
Bypass
Vapor
cooler
Heat
exchanger
Vent
Circulation pump
Sump tank
Figure 9.6
Air out Air supply
pump
Gas–liquid
separator
Sump
pump
A DMFC system.
Cold-start
heater
Methanol
sensor
Condenser
Mixer
Feed
pump
Pure
methanol
Air/water
return stream
264
Chapter Nine
density in the range of 100–120 mA/cm2. Studies have shown that DMFC
efficiency decreases with increasing methanol concentration. Therefore,
operating a fuel cell to maintain the maximum efficiency needs close control of methanol concentration and temperature. An online concentration sensor is used in the feedback loop for this purpose. Some of the
advantages of this system, relative to the hydrogen systems, are that
the liquid feed (methanol) helps in attaining the uniform stack temperature and maintenance of membrane humidity; it is also easy to
refill since the fuel (methanol) is in liquid form.
As compared to the PEMFC, the DMFC has a very sluggish electrochemical reaction (significant activation over voltage) at the anode. It
therefore requires a high surface area of 50:50% Pt-Ru (a more expensive bimetal) alloy as the anode catalyst to overcome the sluggish reaction and an increase in catalyst loading of more than 10 times that for
the PEMFC. Even then, the output voltage on the load is only 0.2–0.4 V
with an efficiency of about 40% at operating temperatures between 60C
and 90C. This is relatively low, and therefore, the DMFC is attractive
only for tiny to small-sized applications (cellular phones, laptops, etc.)
[17]. Another potential application for the DMFC is in transport vehicles; as it operates on liquid fuels, it would greatly simplify the onboard
system as well as the infrastructure needed to supply fuel to passenger
cars and commercial fleets and can create a large potential market for
commercialization of fuel cell technology in vehicle applications.
9.3.3
Alkaline-electrolyte fuel cells (AFCs)
Alkaline-electrolyte fuel cells (see Fig. 9.7) are one of the most developed
fuel cell technologies. They have been in use since the mid-1960s for
Apollo and space shuttle programs [3, 6, 18, 19]. The AFCs onboard
these spacecraft provide electrical power as well as drinking water.
AFCs are among the most efficient electricity-generating fuel cells with
an efficiency of nearly 70%. The electrolyte used in the AFC is an alkaline solution in which an OH ion can move freely across the electrolyte.
The electrolyte used in the AFC is an aqueous
(water-based) solution of potassium hydroxide (KOH) retained in a
porous stabilized matrix. The concentration of KOH can be varied with
the fuel cell operating temperature, which ranges from 65 to 220C.
The charge carrier for an AFC is the hydroxyl ion (OH) that migrates
from the cathode to the anode, where they react with hydrogen to produce water and electrons. Water formed at the anode migrates back to
the cathode to regenerate hydroxyl ions.
Electrochemistry of AFCs.
Anode reaction: 2H2 4OH → 4H2O 4e
Cathode reaction: O2 2H2O 4e → 4OH
Fuel Cells
265
Electric current
e−
e−
e−
Hydrogen
Oxygen
e−
Water
Electrolyte
reservoir
Figure 9.7
Anode
Electrolyte
Anode catalyst
layer
Cathode
Cathode catalyst
layer
An alkaline-electrolyte fuel cell.
Hydroxyl ions are the conducting species in the electrolyte.
Overall cell reaction: 2H2 O2 → 2H2O heat electricity
In many cell designs, the electrolyte is circulated (mobile electrolyte)
so that heat can be removed and water eliminated by evaporation. Since
KOH has the highest conductance among the alkaline hydroxides, it is
the preferred electrolyte.
Electrolyte. Concentrated KOH (85 wt.%) is used in cells designed for
operation at a high temperature (~260C). For lower temperature
(<120C) operation, less concentrated KOH (35–50 wt.%) is used. The
electrolyte is retained in a matrix (usually asbestos), and a wide range
of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, and noble
metals). A major advantage of the AFC is the lower activation polarization at the cathode, resulting in a higher operating voltage (0.875 V).
Another advantage of the AFC is the use of inexpensive electrolyte
materials. The electrolyte is replenished through a reservoir on the
anode side. The typical performance of this AFC cell is 0.85 V at a current density of 150 mA/cm2. The AFCs used in the space shuttle orbiter
have a rectangular cross-section and weigh 91 kg. They operate at an
266
Chapter Nine
average power of 7 kW with a peak power rating of 12 kW at 27.5 V.
A disadvantage of the AFC is that it is very sensitive to CO2 present in
the fuel or air. The alkaline electrolyte reacts with CO2 and severely
degrades the fuel cell performance, limiting their application to closed
environments, such as space and undersea vehicles, as these cells work
well only with pure hydrogen and oxygen as fuel.
Electrodes. A significant cost advantage of alkaline fuel cells is that
both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. The most important characteristics of the catalyst structure are high electronic conductivity and
stability (mechanical, chemical, and electrochemical). Both metallic (typically hydrophobic) and carbon-based (typically hydrophilic) electrode
structures with multilayers and optimized porosity characteristics for the
flow of liquid electrolytes and gases (H2 and O2) have been developed. The
kinetics of oxygen reduction in alkaline electrolytes is much faster than
in acid media; hence AFCs can use low-level Pt catalysts (about 20% Pt,
compared with PEMFCs) on a large surface carbon support [20].
Performance. The AFC development has gone through many changes
since 1960. To meet the requirements for space applications, the early
AFCs were operated at relatively high temperatures and pressures. Now
the focus of the technology is to develop low-cost components for AFCs
operating at near-ambient temperature and pressure, with air as the oxidant for terrestrial applications. This has resulted in lower performance. The reversible cell potential for an H2 and O2 fuel cell decreases by
0.49 mV/C under standard conditions. An increase in operating temperature reduces activation polarization, mass transfer polarization, and
ohmic losses, thereby improving cell performance. Alkaline cells operated
at low temperatures (~70C) show reasonable performance.
Pure hydrogen and oxygen are required in order to operate an AFC.
Reformed H2 or air containing even trace amounts of CO2 dramatically
affects its performance and lifetime. There is a drastic loss in performance
when using hydrogen-rich fuels containing even a small amount of CO2
from reformed hydrocarbon fuels and also from the presence of CO2 in
the air (~350 ppm CO2 in ambient air). The CO2 reacts with OH (CO2 2OH → CO32 H2O), thereby decreasing their concentration and thus
reducing the reaction kinetics. Other ill effects of the presence of CO2 are:
䊏
Increase in electrolyte viscosity, resulting in lower diffusion rate and
lower limiting currents.
䊏
Deposition of carbonate salts in the pores of the porous electrode.
䊏
Reduction in oxygen solubility.
䊏
Reduction in electrolyte conductivity.
Fuel Cells
267
A higher concentration of KOH decreases the life of O2 electrodes when
operating with air containing CO2. However, operation at higher temperatures is beneficial because it increases the solubility of CO2 in the electrolyte. The operational life of air electrodes polytetrafluoroethylene
[PTFE] bonded carbon electrodes on porous nickel substrates) at a current
density of 65 mA/cm2 in 9-N KOH at 65C ranges from 4000 to 5500 h
with CO2-free air, and their life decreases to 1600–3400 h when air
(350-ppm CO2) is used. For large-scale utility applications, operating
times >40,000 h are required, which is a very significant hurdle to commercialization of AFC devices for stationary electric power generation.
Another problem with the AFC is that the electrodes and catalysts
degrade more on no-load or light-load operation than on a loaded condition,
because the high open-circuit voltage causes faster carbon oxidation
processes and catalyst changes. The AFC with immobilized KOH electrolyte
suffers much more from this as the electrolyte has to stay in the cells causing residual carbonate accumulation, separator deterioration, and gas cross
leakage during storage or unloaded periods if careful maintenance is not
carried out. In circulating an electrolyte-type AFC, the electrolyte is emptied from the cell during nonoperating periods. Shutting off the H2 electrodes from air establishes an inert atmosphere. This shutdown also
eliminates all parasitic currents and increases life expectancy. The
exchangeability of the KOH in a circulating electrolyte-type AFC offers the
possibility to operate on air without complete removal of the CO2 [20, 21].
9.3.4
Phosphoric acid fuel cells (PAFCs)
Phosphoric acid fuel cells (see Fig. 9.8) operate at intermediate temperatures (~200C) and are very well developed and commercially available today. Hundreds of PAFC systems are working around the world
in hospitals, hotels, offices, schools, utility power plants, landfills and
wastewater treatment plants, and so forth. Most of the PAFC plants are
in the 50- to 200-kW capacity ranges, but large plants of 1- and 5-MW
capacity have also been built; a demonstration unit has achieved 11 MW
of grid-quality ac power [3]. PAFCs generate electricity at more than 40%
efficiency and if the steam produced is used for cogeneration, efficiencies of nearly 85% can be achieved. PAFCs use liquid phosphoric acid
as the electrolyte. One of the main advantages to this type of fuel cell,
besides high efficiency, is that it does not require pure hydrogen as fuel
and can tolerate up to 1.5% CO concentration in fuel, which broadens
the choice of fuels that can be used. However, any sulfur compounds
present in the fuel have to be removed to a concentration of <0.1 ppmV.
Temperatures of about 200C and acid concentrations of 100% H3PO4 are
commonly used, while operating pressure in excess of 8 atm has been
used in an 11-MW electric utility demonstration plant [3, 22, 23].
268
Chapter Nine
Electric current
e−
e−
e−
Depleted
hydrogen out
Hydrogen molecule
Oxygen molecule
e−
Water
Water
H2
Hydrogen ion (H+)
H2O
H2
O2
Hydrogen in
Oxygen in
Anode
Cathode
Electrolyte
(Phosphoric acid) Cathode catalyst
Anode catalyst
layer
layer
Figure 9.8
Phosphoric acid fuel cell.
The electrochemical reactions occurring in
Electrochemistry of PAFCs.
a PAFC are
At the anode:
H2 → 2H 2e
At the cathode:
1
O 1 2H1 1 2e2 S H2O
2 2
The overall cell reaction:
1
O2 1 H2 S H2O
2
The fuel cell operates on H2; CO is a poison when present in a concentration greater than 0.5%. If a hydrocarbon such as natural gas is used
as a fuel, reforming of the fuel by the reaction
CH4 H2O → 3H2 CO
and shifting of the reformat by the reaction
CO H2O → H2 CO2
is required to generate the required fuel for the cell.
Fuel Cells
269
Electrolyte. The PAFC uses 100% concentrated phosphoric acid (H3PO4)
as an electrolyte. The electrolyte assembly is a 0.1- to 0.2-mm-thick matrix
made of silicon carbide particles held together with a small amount of
PTFE. The pores of the matrix retain the electrolyte (phosphoric acid)
by capillary action. At lower temperatures, H3PO4 is a poor ionic conductor and CO poisoning of the Pt electrocatalyst in the anode can
become severe. There will be some loss of H3PO4 over long periods,
depending upon the operating conditions. Hence, as a general rule, sufficient acid reserve is kept in the matrix at the beginning.
The PAFC (similar to a PEMFC) uses gas diffusion electrodes.
Platinum or platinum alloys are used as the catalyst at both electrodes.
In the mid-1960s, the conventional porous electrodes were PTFE-bonded
Pt black, and the loadings of Pt were about 9 mg/cm2. In recent years,
Pt supported on carbon black has replaced Pt black in porous PTFEbonded electrode structures. Pt loading has also dramatically reduced
2
2
to about 0.25 mg Pt/cm in the anode and about 0.50 mg Pt/cm in the
cathode. The porous electrodes used in a PAFC consist of a mixture of
the electrocatalyst supported on carbon black and a polymeric binder to
bind the carbon black particles together to form an integral structure.
A porous carbon paper substrate provides structural support for the
electrocatalyst layer and also acts as the current collector. The composite structure consisting of a carbon black/binder layer onto the carbon
paper substrate forms a three-phase interface, with the electrolyte on
one side and the reactant gases on the other side of the carbon paper.
The stack consists of a repeating arrangement of a bipolar plate, the
anode, electrolyte matrix, and cathode.
Electrode.
A bipolar plate separates the individual cells and electrically connects them in a series in a fuel cell stack. A bipolar plate has a
multifunction design; it has to separate the reactant gases in the adjacent cells in the stack, so it must be impermeable to reactant gases; it
must transmit electrons to the next cell (series connection), so it has to
be electrically conducting; and it must be heat conducting for proper heat
transfer and thermal management of the fuel cell stack. In some designs,
gas channels are also provided on the bipolar plates to feed reactant
gases to the porous electrodes and to remove the reaction products.
Bipolar plates should have very low porosity so as to minimize phosphoric
acid absorption. These plates must be stable and corrosion-resistant in
the PAFC environment. Bipolar plates are usually made of graphite–
resin mixtures that are carbonized and heat treated to 2700C to
increase corrosion resistance. For 100-kW and larger power generation
systems, water cooling has to be used and cooling channels are provided
in the bipolar plates to cool the stack.
Hardware.
270
Chapter Nine
Temperature and humidity
management are essential for proper operation of a PAFC. The PAFC
system has to be heated up to 130C before the cell can start working.
At lower temperatures, concentrated phosphoric acid does not get dissociated, resulting in a low availability of protons. Also, due to lower vapor
pressure of the concentrated acid, the water generated will not come out
with the reactant stream and the moisture retention dilutes the acid.
This causes an increase in acid volume, which results in acid oozing out
through the electrode. With the start of normal cell operation, its temperature increases and acid concentration gets back to its normal value
that causes acid volume to shrink, resulting in drying of the electrolyte
matrix pores if the acid is not replenished. Controlled stack heating at
start-up is achieved by using an insertable heater system. During operation, the temperature of the stack is maintained by controlling the air
flow in the oxidant channel. At high loading conditions, insertable coolers
may be used to remove excess heat from the stack. Large-power PAFC
systems use a water-cooling system.
Moisture generated at the cathode dilutes the acid on the cathode side
of the electrolyte matrix, causing higher vapor pressure. This results in
more moisture out with the oxidant stream. With the movement of protons from anode to cathode, moisture migration takes place at the cathode side also. This water evaporation results in an acid concentration
gradient from anode to cathode, causing low availability of protons and
a lower potential of the cell. Therefore, water management is needed to
maintain humidity of the anode stream gas at a sufficient level so that
the vapor pressure matches the acid concentration level at the operating temperature.
Temperature and humidity management.
Performance. For good performance, the normal operating temperature range of a PAFC is 180C < T < 250C; below 200C, the decrease in
cell potential is significant. Although an increased temperature increases
performance, higher temperatures also result in increased catalyst sintering, component corrosion, electrolyte degradation, and evaporation.
PAFCs operate in the current density range of 100–400 mA/cm2 at
600–800 mV/cell. Voltage and power limitations result from increased
corrosion of platinum and carbon components at cell potentials above
approximately 800 mV. Since the freezing point of phosphoric acid is
42C, the PAFC must be kept above this temperature once commissioned to avoid the thermal stresses due to freezing and thawing. Various
factors affect the PAFC life. Acid concentration management by proper
humidity control is very important to prevent acid loss and performance
degradation. A PAFC has a life of 10,000–50,000 h, commercially available (UTC Fuel Cells) PAFC systems operating at 207C have shown a
Fuel Cells
271
life of 40,000 h with reasonable performance (degradation rate ∆Vlifetime
(mV) 2 mV/1000 h) [3, 23].
9.3.5
Molten carbonate fuel cells (MCFCs)
The MCFC has evolved from work in the 1960s, aimed at producing a
fuel cell that would operate directly on coal [23, 24]. Although direct operation on coal is no longer a goal, a remarkable feature of the MCFC is
that it can directly operate on coal-derived fuel gases or natural gas and
is therefore also called a direct fuel cell (DFC). MCFCs operate at high
temperatures (600–650C) compared to phosphoric acid (180–220C) or
PEM fuel cells (60–85C). Operation at high temperatures eliminates the
need for external fuel processors that the lower temperature fuel cells
require to extract hydrogen from naturally available fuel. When natural gas is used as fuel, methane (the main ingredient of natural gas) and
water (steam) are converted into a hydrogen-rich gas inside the MCFC
stack (“internal reforming”) (see Fig. 9.9). High operating temperatures
also result in high-temperature exhaust gas, which can be utilized for
heat recovery for secondary power generation or cogeneration. MCFCs
can therefore achieve a higher fuel-to-electricity and an overall energy
use efficiency (>75%) than the low-temperature fuel cells. The MCFC
CO2
CO2
Internal
reforming
H2O
Anode
CH4 + 2H2O + Heat → 4H2 + CO2
H2+
CO32−
− → H2O + CO2 +
e−
2e−
Anode
catalyst
+ Heat
e−
e−
CO32−
I
e−
1/2 O2 + CO2 + 2e− − → CO32−
CO2
CO2
e−
Oxygen (Air)
Electrolyte
Cathode
catalyst
Cathode
Oxygen (Air)
Figure 9.9
Molten carbonate fuel cell.
272
Chapter Nine
is a well-developed fuel cell and is a commercially viable technology for
a stationary power plant, compared to other fuel cell types. A number
of MCFC prototype units in the power range of 200 kW to 1 MW and
higher are operating around the world. The cost and useful life issues
are the major challenges to overcome before the MCFC can compete with
the existing (thermal or other) electric power generation systems for
widespread use.
Electrochemistry of MCFC.
The electrochemical reactions occurring in
the cell are:
Anode half reaction. At the anode, hydrogen reacts with carbonate ions to
produce water, carbon dioxide, and electrons. The electrons travel through
an external circuit—creating electricity—and return to the cathode.
H2 CO32 → H2O CO2 2e
Cathode half reaction. At the cathode, oxygen from the air and carbon
dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte and transfer the current
through the fuel cell, completing the circuit.
1
O 1 CO2 1 2e2 S CO32
2 2
The overall cell reaction is
H2 1
1
O 1 CO2 scathoded S H2O 1 CO2 sanoded
2 2
If a fuel such as natural gas is used, it has to be reformed either externally or within the cell (internally) in the presence of a suitable catalyst to form H2 and CO by the reaction:
CH4 H2O → 3H2 CO
Although, CO is not directly used by the electrochemical oxidation, but
produces additional H2 by the water gas shift reaction:
CO H2O → H2 CO2
Typically, the CO2 generated at the anode is recycled to the cathode,
where it is consumed. This requires additional equipment to either transfer CO2 from the anode exit gas to the cathode inlet gas or produce CO2
by combustion of anode exhaust gas and mix with the cathode inlet gas.
Electrolyte. The MCFC uses a molten carbonate salt mixture as its
electrolyte. At operating temperatures of about 650C, the salt mixture
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273
is in a molten (liquid) state and is a good ionic conductor. The composition of salts in the electrolyte may vary but usually consist of
lithium/potassium carbonate (Li2CO3/K2CO3, 62–38 mol%) for operation
at atmospheric pressure. For operation under pressurized conditions,
lithium/sodium carbonate (LiCO3/NaCO3, 52–48 or 60–40 mol%) is used
as it provides improved cathode stability and performance. This allows for
the use of thicker Li/Na electrolyte for the same performance, resulting
in a longer lifetime before a shorting caused by internal precipitation.
The composition of the electrolyte has an effect on electrochemical activity, corrosion, and electrolyte loss rate. Li/Na offers better corrosion
resistance but has greater temperature sensitivity. Additives are being
developed to minimize the temperature sensitivity of the Li/Na electrolyte. The electrolyte has a low vapor pressure at the operating temperature and may evaporate very slowly; however, this does not have any
serious effect on the cell life. The electrolyte is suspended in a porous,
insulating, and chemically inert ceramic (LiAlO2) matrix. The ceramic
matrix has a significant effect on the ohmic resistance of the electrolyte.
It accounts for almost 70% of the ohmic polarization. The electrolyte
management in an MCFC ensures that the electrolyte matrix remains
completely filled with the molten carbonate, while the porous electrodes
are partially filled, depending on their pore size distributions.
Electrode. The anode is made of a porous chromium-doped sintered
Ni-Cr/Ni-Al alloy. Because of the high temperatures resulting in a fast
anode action, a large surface area is not required on the anode as compared to the cathode. Partial flooding of the anode with molten carbonate is desirable as it acts as a reservoir that replenishes carbonate in
the stack during prolonged use. The cathode is made up of porous lithiated nickel oxide. Because of the high operating temperatures, no noble
catalysts are needed in the fuel cell. Nickel is used on the anode and
nickel oxide on the cathode as catalysts. Bipolar plates or interconnects
are made from thin stainless steel sheets with corrugated gas diffusion
channels. The anode side of the plate is coated with pure nickel to protect against corrosion.
Performance. At the high operating temperatures of an MCFC, CO is not
a poison but acts as a fuel. In the MCFC, CO2 has to be added to oxygen
(air) stream at the cathode for generation of carbonate ions. The anode
reaction converts these ions back to CO2, resulting in a net transfer of
two ions with every molecule of CO2. The need for CO2 in the oxidant
stream requires that CO2 from the spent anode gas be separated and
mixed with the incoming air stream. Before this can be done, any residual hydrogen in the spent fuel stream must be burned. Systems developed in the future may incorporate membrane separators to remove the
hydrogen for recirculation back to the fuel stream to increase efficiency.
274
Chapter Nine
Internal reforming of natural gas and partially cracked hydrocarbons
is possible in the inlet chamber of the MCFC, eliminating the separate
fuel processing of natural gas or other hydrogen-rich fuels. The requirement for CO2 makes the digester gas (sewage, animal waste, food processing waste, etc.) an ideal fuel for the MCFC; other fuels such as
natural gas, landfill gas, propane, coal gas, and liquid fuels (diesel,
methanol, ethanol, LPG, etc.) can also be used in the MCFC system. The
elimination of the external fuel reformer also contributes to lower costs,
and high-temperature waste heat can be utilized to make additional electricity and cogeneration. MCFCs can reach overall thermal efficiencies
as high as 85%.
With the increase in operating temperature, the theoretical operating voltage for a fuel cell decreases, but increases the rate of the electrochemical reaction and therefore the current that can be obtained at
a given voltage. This results in the MCFC having a higher operating voltage for the same current density and higher fuel efficiency than a PAFC
of the same electrode area. As size and cost scale roughly with the electrode area, the MCFC is smaller and less expensive than a PAFC of comparable output. Another advantage of the MCFC is that the electrodes
can be made with cheaper nickel catalysts rather than the more expensive platinum used in other low-temperature fuel cells. Endurance of the
cell stack is a critical issue in commercialization, and MCFC manufacturers report an average potential degradation of 2 mV/1000 h over a
cell stack lifetime of 40,000 h. The high temperature limits the use of
materials in the MCFC, and safety issues prevent their application for
home use. MCFC units require a few minutes of fuel burning at the start
up to heat up the cell to its operating temperature and therefore are not
very suitable for use in automobiles. However, they are very good for stationary power applications and units with up to 2 MW have been constructed, and designs for units with up to 100 MW exist [3, 23–25].
9.3.6
Solid oxide fuel cells (SOFCs)
The SOFC has the most desirable properties for generating electricity
from hydrocarbon fuels. The SOFC uses a solid electrolyte and is very efficient. It can internally reform hydrocarbon fuels and is tolerant to impurities. The SOFC operates at a very high temperature (700–1000C) and
so does not require any cooling system for maintaining a fuel cell operating temperature. For small systems, insulation has to be provided to
maintain the cell temperature. In large SOFC systems, the operating
temperature is maintained internally by the reforming action of the
fuel and by the cool outside air (oxidant) that is drawn into the fuel cell.
At high operating temperatures, chemical reaction rates in the SOFC
are high and air compression is not required. This results in a simpler
Fuel Cells
275
system, quiet operation, and high efficiencies. Westinghouse has worked
at developing a tubular style of the SOFC that operates at 1000C (see
Fig. 9.10) for many years [1–3, 26, 27]. These long tubes have high electrical resistance but are simple to seal. Many other manufacturers are
now working on a planar SOFC composed of thin ceramic sheets which
operate at 800C or even less. Thin sheets offer low electrical resistance,
and cheaper materials such as stainless steel can be used at these lower
temperatures [3, 6, 26]. One big advantage of the SOFC over the MCFC
is that the electrolyte is a solid. Therefore, no pumps are required to circulate a hot electrolyte, and very compact, small planar SOFC systems
of a few kW range could be constructed using very thin sheets.
A major advantage of the SOFC is that both hydrogen and carbon
monoxide are used in the cell. Therefore, in the SOFC, many common
hydrocarbon fuels such as natural gas, diesel, gasoline, alcohol, and coal
gas can be safely used. The SOFC can reform these fuels into hydrogen
Interconnection
contact
Interconnection
Electrolyte
Air electrode
Fuel electrode
Tubular SOFC design
Anode
Electrolyte
Cathode
Electrolyte-supported planer SOFC
Figure 9.10
Tubular and planer solid oxide fuel cell.
Anode-supported planer SOFC
276
Chapter Nine
and carbon monoxide inside the cell, and the high-temperature waste
thermal energy can be recycled back for fuel reforming. During operation, the SOFC is at the same time a generator and a user of heat.
Heat is generated through exothermic chemical reactions and ohmic
losses, while it is absorbed by the reforming reaction. It is possible to
design the SOFC to be thermally balanced, thereby eliminating the
requirement for external insulation and heating. Small SOFC systems
are not thermally self-sustaining and may require an external heat
source to start and maintain operation. In large systems, the heat generated is not fully absorbed by fuel reforming, and the excess heat can
be used in gas turbines for generating electricity or for cogeneration.
Another advantage of the SOFC is that expensive catalysts are not
required. However, a few minutes of fuel burning is required to reach
the operating temperature of the SOFC at the start. This time delay
is a disadvantage for an automotive application, but for stationary
electric power plants, this is not a problem as they run continuously for
long periods of time.
Hydrogen or carbon monoxide in the fuel
stream reacts with oxide ions (O2) from the electrolyte to produce water
or CO2 and to deposit electrons into the anode. The electrons pass outside the fuel cell, through the load, and back to the cathode, where
oxygen from the air receives the electrons and is converted into oxide
ions, which are injected into the electrolyte. In the SOFC, oxygen ions
are formed at the cathode. The reaction at the cathode is
Electrochemistry of SOFCs.
O2 4e → 2O2
At the operating temperature, the electrolyte offers high ionic conductivity and low electrical conductivity; therefore, oxygen ions migrate
through the electrolyte to the anode. The overall reaction occurring at
the anode is as follows:
The hydrogen in the fuel reacts with the oxygen ions to produce water
and releases two electrons.
H2 O2 → H2O 2e
Carbon monoxide present in the fuel causes a shift reaction to produce
additional fuel (H2).
CO H2O ↔ H2 CO2
The following internal reforming reaction for the hydrocarbon fuel takes
place on the anode side:
Cx Hy xH2O → xCO (x y
)H
2 2
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277
For methane-rich fuels, this reforming reaction is
CH4 H2O → CO 3H2
This reaction is generally not in chemical equilibrium, and the CO
shift reaction takes place to provide more hydrogen. The overall cell
reaction is
1
H2 O2 → H2O
2
Electrolyte. The use of a solid electrolyte in the SOFC eliminates the
electrolyte management problems associated with the liquid electrolyte
fuel cells and also reduces corrosion considerations to a great extent. In
the SOFC, it is the migration of oxygen ions (O2) through the electrolyte that establishes the voltage difference between the anode and
cathode. Therefore, the electrolyte must be a good conductor of O2 ions
and a bad conductor of electrons; it must also be stable at the high operating temperature. Some ceramics possess these properties and therefore are good candidates for this application. With the help of modern
ceramic technology and solid-state science, many ceramics can be tailored for electrical properties unattainable in metallic or polymer materials. These tailored ceramic materials are termed electroceramics, and
one group is known as fast ion conductors or superionic conductors.
These superionic conductors when used as a solid electrolyte allow easy
passage of ions from the cathode to the anode in an SOFC. The material
generally used as an electrolyte in the SOFC is dense yttria-stabilized
zirconia. It is an excellent conductor of negatively charged oxygen ions
at high temperatures (1000C), but its conductivity reduces drastically
with the drop in temperature. Other materials such as scandia-stabilized
zirconia (ScSZ), which shows good ionic conductivity at a lower temperature (800C), are also being investigated, but the electrolyte developed
with ScSZ-based materials is very expensive and they degrade very fast.
Electrode. The anode is made of metallic Ni and Y2O3-stabilized ZrO2
(YSZ). Ytrria-stabilized zirconia is added in Ni to inhibit sintering of the
metal particles and to provide a thermal expansion coefficient close to
those of the other cell materials [26]. Nickel structure is normally
obtained from NiO powders; therefore, before starting the operation for
the first time, the cell is run with hydrogen in an open-circuit condition
to reduce the NiO to nickel. The anode structure is fabricated with a
porosity of 20–40% to facilitate mass transport of the reactant and product gases. The Sr-doped lanthanum manganite (La1x Srx MnO3, x 0.10–0.15; known as LSM) is most commonly used for the cathode material. LSM is a p-type semiconductor. Similar to the anode, the cathode
is also a porous structure that permits rapid mass transport of the reactant and product gases.
278
Chapter Nine
Hardware. In the SOFC, both CO and hydrogen are used as direct fuel.
Therefore, it is important that the fuel and air streams are kept separate, and a thermal balance should be maintained to ensure that operating temperatures remain within an acceptable range. Several designs
of the SOFC (tubular and planer) have been developed to accommodate
these requirements. The SOFC is a solid-state device and shares certain
properties and fabrication techniques with semiconductor devices.
Individual cells in the stack are connected by interconnects, which
carry an electrical current between cells and can also act as a separator between the fuel and oxidant supplies. In high-temperature SOFCs,
the interconnects that are used are ceramic such as lanthanum chromite,
or if the temperature is limited to less than 1000C, a refractory alloy
based on Y/Cr may be used. The interconnects constitute a major proportion of the stack cost. Stack and other plant construction materials that
are used also need to be refractory to withstand the high-temperature
gas streams. Volatility of chromium-containing ceramics and alloys can
result in contamination of the stack components, and the presence of a
toxic material such as Cr6 requires special disposal procedures.
The high operating temperature (1000C) of the SOFC requires a significant start-up time. The cell performance is very sensitive to operating temperatures. A 10% drop in temperature results in an ~12% drop
in cell performance due to the increase in internal resistance to the flow
of oxygen ions. The high temperature also demands that the system
include significant thermal shielding to protect personnel and to retain
heat. Also, the materials required for such high-temperature operation,
particularly for interconnect and construction materials, are very expensive. Operating the SOFC at temperatures lower than 700C would be
very beneficial as low-cost metallic materials, such as ferritic stainless
steels, that can be used as interconnect and construction materials.
This will make both the stack and balance of a plant cheaper and more
robust. Using ferritic materials also significantly reduces the problems
associated with chromium. The other advantages of low/intermediatetemperature operation are rapid start-up and shutdown and significantly reduced corrosion rates.
However, to operate at reduced temperatures, several changes are
required in stack design, cell materials, reformer design and operation,
and operating conditions. With the reduction in operating temperature,
the ionic conductivity of the electrolyte decreases and the parasitic
losses due to the conductivity of the electrodes and interconnects
increase. This results in a rapid deterioration of the performance of the
SOFC. This can be overcome to some extent by reducing the thickness of
the electrolyte to compensate for its reduced ionic conductivity. The thickness reduction that is required to accommodate, say a 200C reduction
in the operating temperature, leads to impracticably thin membranes.
Fuel Cells
279
Some designs in which a thin, dense layer of the electrolyte is physically
supported on one of the electrodes (electrode-supported design) are suggested. This structure of a very porous support is difficult to manufacture, and an expensive thin-film deposition technique such as chemical
vapor deposition (CVD) is needed to manufacture these systems. Even
then, the mechanical strength of the structure (defined by the porous
electrode) is often poor, and the handling of the structure through subsequent processing and assembly is difficult. Another approach to
improve SOFC performance at low operating temperatures is to use
different materials for the electrolyte and the electrode. Several materials options are being investigated [2, 6, 26, 27].
9.3.7
Biofuel cells
A biofuel cell operation is very similar to a conventional fuel cell, except
that it uses biocatalysts such as enzymes, or even whole organisms
instead of inorganic catalysts like platinum, to catalyze the conversion
of chemical energy into electricity. They can use available substrates
from renewable sources and convert them into benign by-products with
the generation of electricity. As mentioned earlier, in recent years, medical science is increasingly relying on implantable electronic devices for
treating a number of conditions. These devices demand a very reliable
and maintenance-free (any maintenance that might require surgery)
power source. Biofuel cells can provide solutions to most of these problems. A biofuel cell can use fuel that is readily available in the body, for
example, glucose in the bloodstream, and it would ideally draw on this
power for as long as the patient lives. Since they use concentrated
sources of chemical energy, they can be small and light.
A biofuel cell can operate in two ways: It can utilize the chemical
pathways of living cells (microbial fuel cells), or, alternatively, it can use
isolated enzymes [7, 28]. Microbial fuel cells have high efficiency in
terms of conversion of chemical energy into electrical energy; however,
they suffer from the low volumetric catalytic activity of the whole organism and low power densities due to slow mass transport of the fuel
across the cell wall. Isolated enzymes extracted from biological systems
can be used as catalysts to oxidize fuel molecules at the anode and to
enhance oxygen reduction at the cathode of the biofuel cell. Isolated
enzymes are attractive catalysts for biofuel cells due to their high catalytic activity and selectivity. The theoretical value of the current that
can be generated by an enzymatic catalyst with an activity of 103 U/mg
is 1.6 A, a catalytic rate greater than platinum! However, practical
observed currents are much lower due to the loss of catalytic activity
from immobilization of the enzymes at the electrode surface and energy
losses of the overall system. A major challenge in the biofuel cell design
280
Chapter Nine
is the electrical coupling of the biological components of the system with
the fuel cell electrodes. Molecules known as electron-transfer mediators
are needed to provide efficient transport of electrons between the biological components (enzymes or microbial cells) and the electrodes of the
biofuel cell. Integrated biocatalytic systems that include biocatalysts,
electron-transfer mediators, and electrodes are under research and
development. Biofuel cells have much wider fuel options; enzymatic biofuel cells can operate on a wide variety of available fuels such as ethanol,
sugars, or even waste materials.
A basic microbial biofuel cell consists of two compartments, an anode
compartment and a cathode compartment, separated by a PEM as shown
in Fig. 9.11. Usually, Nafion-117 film (an expensive material) is used as
the PEM; it allows hydrogen ions generated in the anode compartment
to be transferred across the membrane into the cathode compartment [8].
Previously, graphite electrodes were used as the anode and cathode,
but they are now replaced by woven graphite felt as it provides a larger
surface area than a regular graphite electrode of similar dimensions.
This facilitates an increased electron transfer from the microorganisms.
A microorganism (e.g., Escherichia coli) is used to breakdown glucose
in order to generate adenosine triphosphate (ATP), which is utilized by
cells for energy storage. Methylene blue (MB) or neutral red (NR) is used
as an electron mediator to efficiently facilitate the transfer of electrons
from the microorganism to the electrode. Electron mediators tap into the
electron transport chain, chemically reducing nicotinamide adenine dinucleotide (NAD) to its protonated form NADH. The exact mechanism
by which the transfer of electrons takes place through these electron
mediators is not fully known [29]; however, it is known that they insert
themselves into the bacterial membrane and essentially “hijack” the
electron transport process of glucose metabolism of the bio-electrodes in
a biofuel cell. Their activity is very dependent on pH, and a potassium
phosphate buffer (pH 7.0) is used to maintain the pH value in the anode
compartment. The cathode compartment contains potassium ferricyanide,
Anode
Cathode
Microbial cell
Primary substrate
e−
x
e−
x Fuel product
O2
H2O
Membrane (PEM)
Figure 9.11
Biofuel cell.
Oxidized fuel
Fuel Cells
281
a potassium phosphate buffer (pH 7.0), and a woven graphite felt electrode. Potassium ferricyanide reaction helps in rapid electron uptake.
Hydrogen ions (H) migrate across the PEM and combine with oxygen
from air and the electrons to produce water at the cathode. The cathode compartment has to be oxygenated by constant bubbling with air
to promote the cathode reactions. It may be worth mentioning that the
electron transport chain occurs in the cell membrane of prokaryotes
(a unicellular organism having cells lacking membrane-bound nuclei,
such as bacteria), while this process occurs in the mitochondrial membrane of eukaryotes (animal cells). Therefore, attempts to substitute
eukaryotic cells for bacterial cells in a biofuel cell may present a significant challenge.
In a microbial fuel cell, two redox
couples are required in order to generate a current: (a) coupling of the
reduction of an electron mediator to a bacterial oxidative metabolism and
(b) coupling of the oxidation of the electron mediator to the reduction of
the electron acceptor on the cathode surface. The electron acceptor is
subsequently regenerated by the presence of O2 at the cathode surface.
The electrochemical reactions in a biofuel cell using glucose as a fuel are
Electrochemistry of microbial fuel cells.
At the anode:
C6H12O6 1 6H2O S 6CO2 1 24e2 1 24H1
At the cathode:
4FesCNd632 1 4e2 S 4FesCNd642
4FesCNd642 1 4H1 1 O2 S 4FesCNd632 1 2H2O
Complete oxidation of glucose does not always occur. One might often
get additional products besides CO2 and water. For example, E. coli forms
acetate, being unable to completely breakdown glucose, thereby limiting electricity production. Recently, an elegant approach to address this
long-standing problem of limited enzyme stability has been reported
[30]. It is suggested that the immobilization of enzymes in Nafion layers
to create a bio-anode results in stable performance over months.
Another way of using a microorganism’s ability to produce electrochemically active substances for energy generation is to combine a bioreactor with a biofuel cell or a hydrogen fuel cell. The fuel can be produced
in a bioreactor at one place and transported to a (H2 or bio-) fuel cell to
be used as a fuel. In this case, the biocatalytic microbial reactor produces
the fuel, and the biological part of the device is not directly integrated
with the electrochemical part (see Fig. 9.12).
282
Chapter Nine
Pump
Pump
Electric current
Anode
Cathode
e−
e−
Microbial cell
−
e−
Primary substrate
e
O2
H2O
Bioreactor
Figure 9.12
Fuel product
Oxidized fuel
Membrane
Bioreactor and biofuel cell combination.
The advantage of this scheme is that it allows the electrochemical part
to operate under conditions that are not compatible with the biological
part of the device. The two parts can even be separated in time, operating completely independently. The most widely used fuel in this
scheme is hydrogen gas, allowing well-developed and highly efficient
H2/O2 fuel cells to be conjugated with a bioreactor.
In recent years, ethanol has been developed as an alternative to the
traditional methanol-powered biofuel cell due to the widespread availability of ethanol for consumer use, its nontoxicity, and increased selectivity by alcohol. Ethanol fuel cells with immobilized enzymes have
provided higher power densities than the latest state-of-the-art methanol
biofuel cells. Open-circuit potentials ranging from 0.61 to 0.82 V and
power densities of 1.00–2.04 mW/cm2 have been produced.
Mediatorless microbial fuel cells. Most biofuel cells need a mediator molecule to speed up the electron transfer from the enzyme to the electrode.
Recently, mediatorless microbial fuel cells have been developed. These use
metal-reducing bacteria, such as members of the families Geobacteraceae
or Shewanellaceae, which exhibit special cytochromes bound to their
membranes. These are capable of transferring electrons to the electrodes directly. Rhodoferax ferrireducens, an iron-reducing microorganism, has the ability to directly transfer electrons to the surface of
electrodes and does not require the addition of toxic electron-shuttling
mediator compounds employed in other microbial fuel cells. Also, this
metal-reducing bacterium is able to oxidize glucose at 80% electron efficiency (other organisms, such as Clostridium strains, oxidize glucose at
only 0.04% efficiency). In other fuel cells that use immobilized enzymes,
glucose is oxidized to gluconic acid and generates only two electrons,
whereas in microbial fuel cells (MFCs) using R. ferrireducens, glucose
is completely oxidized to CO2 releasing 24 electrons. These MFCs have
a remarkable long-term stability, providing a steady electron flow over
Fuel Cells
283
extended periods. Current density of 31 mA/m2 over a period of more than
600 h has been reported [31]. MFCs using R. ferrireducens have the ability to be recharged, and have a reasonable cycle life and low capacity
loss under open-circuit conditions. They allow the harvest of electricity
from many types of organic waste matter or renewable biomass. This is
an advantage over other microorganisms in the family Geobacteraceae,
which cannot metabolize sugars.
Another recent development has been the use of microfibers rather
than flat electrodes and the enzyme-based electroactive coatings. The
anode coating used is glucose oxidase, which is covalently bound to a
reducing-potential copolymer and has osmium complexes attached to its
backbone. The cathode coating contains the enzyme laccase and an
oxidizing-potential copolymer. The osmium redox centers in the coatings
electrically “wire” the reaction centers of the enzymes to the carbon
fibers. This electrode design avoids glucose oxidation at the cathode
and O2 reduction at the anode, eliminating the need for an electrodeseparating membrane. This has led to miniature “one-compartment biofuel cells” for implantable devices within humans, such as pacemakers,
insulin pumps, sensors, and prosthetic units. Biofuel cells with two
7-m-diameter, electrocatalyst-coated carbon fiber electrodes placed in
1-mm grooves machined into a polycarbonate support with a power
output of 600 nW at 37C (enough to power small silicon-based microelectronics) have been reported [32].
Microbial fuel cells have a long way to go before they compete with
more established hydrogen fuel cells or electrical batteries. However, a
number of factors provide motivation for research into microbial fuel
cells for electricity production.
1. Bacteria are adapted to feeding on virtually all available carbon
sources (carbohydrates or more complex organic matter present in
sewage, sludge, or even marine sediments). This makes them potential catalysts for electricity generation from organic waste.
2. Bacteria are omnipresent in the environment and are selfreproducing, self-renewing catalysts; thus a simple initial inoculation
of a suitable strain could be cultured continuously in an MFC for longterm operation.
3. The catalytic core of conventional fuel cells uses very expensive precious metals such as platinum, and biocatalysts like bacteria may
become a serious cost-reducing alternative.
Although biofuel cells are still in an early stage of development and
work toward optimizing the performance of a biofuel cell system is needed,
the utilization of white blood cells as a source of electrons for a biofuel
cell could mark an important step in developing a perpetual power
284
Chapter Nine
source for implantable devices. There is still a lot of work to be done as
there are many unanswered questions; however, the feasibility of constructing commercially viable biofuel cell power supplies for a number
of applications is very promising.
9.4
Fuel Cell System
A fuel cell power system requires the integration of many components.
The fuel cell produces only dc power and utilizes only certain processed
fuels. Besides the fuel cell stack, various components are incorporated
in a fuel cell system. A fuel processor is required to allow operation with
conventional fuels; a power conditioner is used to tie fuel cells into
the ac power grid or distributed generation system; for high-temperature
fuel cells, a cogeneration or bottoming cycle plant is needed to utilize
rejected heat for achieving high efficiency. A schematic of a fuel cell
power system with interaction among various components is shown in
Fig. 9.13.
9.4.1
Fuel processor
A fuel processor converts a commercially available fuel (gas, liquid, or
solid) to a fuel gas reformate suitable for the fuel cell use. Fuel processing
involves the following steps:
1. Fuel cleaning—It involves cleaning and removal of harmful species
(sulfur, halides, and ammonia) in the fuel. This prevents fuel processor and fuel cell catalyst degradation.
2. Fuel Conversion—In this stage, a naturally available fuel (primarily hydrocarbons such as natural gas, petrol, diesel, ethanol,
methanol, biofuels [such as produced from biomass, landfill gas,
biogas from anaerobic digesters, syngas from gasification of biomass
and wastes] etc.) is converted to a hydrogen-rich fuel gas reformat.
Source fuel
Fuel
processor
Fuel in
Fuel cell stack
Water
management
Air system
Figure 9.13
Water out Exit
Air in Heat out
Power
conditioner Electrical
power out
Thermal
management
A fuel cell power system schematic.
Heat out
Exhaust out
Fuel Cells
285
3. Downstream processing—It involves reformate gas alteration by
converting carbon monoxide (CO) and water (H2O) in the fuel gas
reformate to hydrogen (H2) and carbon dioxide (CO2) through the
water gas shift reaction, selective oxidation to reduce CO to a few
parts per million, or removal of water by condensing to increase the
H2 concentration.
A schematic showing the different stages in the fuel-processing system
is presented in Fig. 9.14. Major fuel-processing techniques are steam
reforming (SR), partial oxidation (POX) (catalytic and noncatalytic),
and autothermal reforming (ATR). Some other techniques such as dry
reforming, direct hydrocarbon oxidation, and pyrolysis are also used.
Most fuel processors use the chemical and heat energy of the fuel cell
effluent to provide heat for fuel processing. This enhances system
efficiency.
Steam reforming is a popular method of converting light hydrocarbons
to hydrogen. In SR, heated and vaporized fuel is injected with superheated steam (steam-to-carbon molar ratio of about 2.5:1) into a reaction vessel. Excess steam ensures complete reaction as well as inhibits
soot formation. Although the steam reformer can operate without a catalyst, most commercial reformers use a nickel- or cobalt-based catalyst
to enhance reaction rates at lower temperatures. Although the water gas
shift reaction in the steam reformer reactor is exothermic, the combined SR and water gas shift reaction is endothermic. It therefore
requires a high-temperature heat source (usually an adjacent hightemperature furnace that burns a small portion of the fuel or the fuel
effluent from the fuel cell) to operate the reactor. SR is a slow reaction
and requires a large reactor. It is suitable for pipeline gas and light distillates using a fuel cell for stationary power generation but is unsuitable for systems requiring rapid start and/or fast changes in load.
In POX, a substoichiometric amount of air or oxygen is used to partially combust the fuel. POX is highly exothermic, and the resulting
high-temperature reaction products are quenched using superheated
steam. This promotes the combined water gas shift and steam-reforming
Air
Gaseous
fuel
Reactor
high °C
Sulfur
removal
350°C
Water
Figure 9.14
A fuel-processing system.
Hightemperature
shift
260–370°C
Lowtemperature
shift
200–260°C
CO
removal
200–260°C
286
Chapter Nine
reactions, which cools the gas. In a well-designed POX reformer with
controlled preheating of the reactants, the overall reaction is exothermic and self-sustaining. Both catalytic (870–925C) and noncatalytic
(1175–1400C) POX reformers have been developed for hydrocarbon
fuels. The advantage of POX reforming is that it does not need indirect heat transfer, resulting in a compact and lightweight reformer.
Also, it is capable of higher reforming efficiencies than steam reformers
[3, 6].
Autothermal reforming combines SR with POX reforming in the presence of a catalyst that controls the reaction pathways and thereby determines the relative extents of the POX and SR reactions. The SR reaction
absorbs part of the heat generated by the POX reaction, limiting the
maximum temperature in the reactor. This results in a slightly exothermic process, which is self-sustaining, and high H2 concentration. The
ATR fuel processor operates at a lower operating cost and lower temperature than the POX reformer, and is smaller, quicker starting, and
quicker responding than the SR.
Most of the natural hydrocarbon fuels, such as natural gas and
gasoline, contain some amount of sulfur, or sulfur-containing odorants
are added to them for leak detection. As the fuel cells or reformer catalysts do not tolerate sulfur, it must be removed. Sulfur removal is
usually achieved with the help of zinc oxide sulfur polisher, which
removes the mercaptans and disulfides. A zinc oxide reactor is operated at 350–400C to minimize bed volume. However, removing sulfurcontaining odorants such as thiophane requires the addition of a
hydrodesulfurizer stage before the zinc oxide polisher. Hydrogen (supplied by recycling a small amount of the natural gas-reformed product) converts thiophane into H2S in the hydrodesulfurizer. The zinc
oxide polisher easily removes H2S.
To reduce the level of CO in the reformat gas, it must be water gas
shifted. The shift conversion is often performed in two or more stages
when CO levels are high. A first high-temperature stage allows high
reaction rates, while a low-temperature converter allows a higher conversion. Excess steam is used to enhance the CO conversion. In a
PEMFC, the reformate is passed through a preferential CO catalytic oxidizer after being shifted in a shift reactor, as a PEMFC can tolerate a
CO level of only about 50 ppm.
A fuel processor is an integrated unit consisting of one or more of the
above stages, as per the requirements of a particular type of fuel cell.
High-temperature fuel cells such as the SOFC and MCFC are equipped
with internal fuel reforming and hence do not require a hightemperature shift, or low-temperature shift stage. The CO removal
stage is not required for the SOFC, MCFC, PAFC, and circulating AFC.
For the PEMFC, all the stages are required.
Fuel Cells
9.4.2
287
Air management
Besides fuel, a fuel cell also requires an oxidant (usually air). Depending
on the application and design, air provided to the fuel cell cathode can
be at a low pressure or a high pressure. High pressure of the air improves
the reaction kinetics and increases the power density and efficiency of
the stack. But increasing the air pressure reduces the water-holding
capacity of the air and therefore reduces the humidification requirements of the membrane (PEMFC). It also increases the power required
to compress the air to a high pressure and thereby reduces the net
power available. At present, most fuel cell stacks for stationary power
applications are designed for operating pressures in the range of 1–8
atm, while automotive fuel cell systems based on the PEMFC technology are designed to operate at lower pressures of 2–3 atm to increase
power density and improve water management.
9.4.3
Water management
Water management is critical for fuel cell operation. Water is a product
of the fuel cell reaction, and it must be removed from the exhaust gas
for use in various operations such as fuel reformation and humidifying
reactant gases (to avoid drying out the fuel cell membrane). For automotive applications, water condensed from the exhaust steam is recycled for reforming and reactant humidification in a closed cycle to avoid
periodical recharging with water.
9.4.4
Thermal management
The reaction products of the electrochemical reaction in a fuel cell are
water, electricity, and heat. The heat energy released in a fuel cell stack
is approximately equal to the electrical energy generated and must be
managed properly to maintain the fuel cell stack temperature at the
optimal level. If this thermal energy (waste heat) is properly utilized,
it will considerably increase the efficiency of a fuel cell system. In lowtemperature (<200C) fuel cells (PEMFC, AFC, and PAFC), the stack is
cooled by supplying excess air in low power (<200-W) systems, whereas a
liquid coolant (deionized water) is used for large-size systems. The waste
heat carried out by the coolant is utilized for cogeneration (space heating, water heating, etc.). In high-temperature (<600C) fuel cell (MCFC
and SOFC) systems, all the heat of reaction is transferred to the reactants to maintain the stack temperature at the optimal level. The thermal energy of the high-temperature exhaust may be utilized to preheat
the incoming air stream, or in internal or external fuel reformer. The
high-temperature exhaust may also be used for cogeneration or electricity generation in a downstream gas turbine system.
288
Chapter Nine
9.4.5
Power-conditioning system [33]
The power-conditioning system is an integral part of a fuel cell system.
It converts the dc electric power generated by the fuel cell into regulated
dc or ac for consumer use. The electrical characteristics of a fuel cell are
very far from that of an ideal electric power source. The dc output
voltage of a fuel cell stack varies considerably with the load current (see
Fig. 9.15), and it has very little overload capacity. It needs considerable
auxiliary power for pumps, blowers, and so forth, and requires considerable start-up time due to heating requirements. It is slow to respond
to load changes, and its performance degrades considerably with the age
of the fuel cell. The various blocks of a fuel cell power-conditioning
system are shown in Fig. 9.16.
The dc voltage generated by a fuel cell stack is usually low in magnitude (<50 V for a 5- to 10-kW system, <350 V for a 300-kW system) and
varies widely with the load. A dc–dc converter stage is required to regulate and step up the dc voltage to 400–600 V (typical for 120/240-V ac
output). Since the dc–dc converter draws power directly from the fuel cell,
it should not introduce any negative current into the fuel cell and must
be designed to match the fuel cell ripple current specifications. A dc–ac
conversion (inverter) stage is needed for converting the dc to ac power
at 50 or 60 Hz (see Fig. 9.17). Switching frequency harmonics are filtered
out using a filter connected to the output of the inverter to generate a
high-quality sinusoidal ac waveform suitable for the load.
9.5
Fuel Cell Applications
900
800
700
600
500
400
50
45
V-I curve
P-I curve
40
35
300
200
100
30
25
0
Figure 9.15
fuel cell.
5
10
15
20
Current (A)
25
30
35
Fuel cell voltage (V)
Power (W)
The major applications for fuel cells are as stationary electric power
plants (including cogeneration units), as a transportation power source
20
Voltage-current and voltage-power characteristics of a typical
Fuel Cells
High-frequency
dc–dc converter
Fuel
cell
DC link &
battery Inverter
dc–ac
backup
Output
filter
+
_
V0
I0
DSP
Figure 9.16
120/240-V,
60-Hz
ac output
LOAD
48 V
289
Schematic of a fuel cell power-conditioning system.
for vehicles, and as portable power sources, besides an electric power
source for space vehicles or other closed environments.
Stationary power applications are very favorable for fuel cell systems.
Stationary applications mostly require continuous operation, so startup time is not a very important constraint. Thus, high-temperature fuel
cells such as the MCFC and SOFC systems are also suitable for this
application in addition to the PAFC and PEMFC systems. The fuel
source for stationary applications is most likely to be natural gas, which
is relatively easy to reform in the internal reformer of high-temperature
fuel cells or in the external reformer for low-temperature fuel cells. An
advantage of using natural gas is that the distribution infrastructure
for natural gas already exists. Promising applications for stationary fuel
cell systems include premium power systems (high-quality uninterruptible/
back-up power supply systems); high-efficiency cogeneration (heat and
electricity) systems for residences, commercial buildings, hospitals, and
+
+
Q1
Vdc
2
Vdc
Q3
Q5
A
+ N
B
Q4
Vdc
Q6
Q2
C
Output filter
L
C
2
−
−
Figure 9.17
Three-phase inverter for an ac load or grid connection.
AC load/
grid
connection
290
Chapter Nine
industrial facilities; and distributed power generation systems for utilities. Although some demonstration and commercial stationary fuel cell
power plants in sizes from a few kilowatts to 11 MW are in operation,
widespread commercialization can be expected only if their installation
cost drops down from the present cost of $4000/kW to about $400–700/kW
(or about $1000/kW for some premium applications).
The recent surge of interest in fuel cell technology is because of its
potential use in transportation applications, including personal vehicles.
This development is being sponsored by various governments in North
America, Europe, and Japan, as well as by major automobile manufacturers worldwide, who have invested several billion dollars with the
goal of producing a high-efficiency and low-emission fuel cell power
plant at a cost that is competitive with the existing internal combustion
engines. With hydrogen as the onboard fuel, such vehicles would be
zero-emission vehicles. With fuels other than hydrogen, an appropriate
fuel processor to convert the fuel to hydrogen will be needed. Fuel cellpowered vehicles offer the advantages of electric drive and low maintenance, because of the few critical moving parts. The major activity in
transportation fuel cell development has focused on the polymer electrolyte fuel cell (PEFC), and many of the technical objectives related to
the fuel cell stack have been met or are close to being met. The current
development efforts are focused on decreasing cost and resolving issues
related to fuel supply and system integration.
Besides exotic areas of applications such as space vehicles or submarines, another very promising area of application for fuel cells is
portable power systems. Portable power systems are small, lightweight
systems that power portable devices (e.g., computers, laptops, cellular
phones, and entertainment electronic devices), camping and recreational
vehicles, military applications in the field, and so forth. These devices
need power in the range of a few watts to a few hundred watts. Fuel cell
systems based on DMFC or PEMFC technology are well suited for many
of these applications. The convenience of transporting and storing liquid
methanol makes DMFC systems very attractive for this application. A
small container of methanol or a cylinder of compressed hydrogen can
be used as a fuel supply. When the fuel is depleted, a new fuel container
may be installed in its place after removing the old one.
In recent years, there has been a lot of interest in electric power generation using renewable energy sources such as wind energy, solar
energy, and tidal energy. A major problem with these energy sources is
that all are intermittent in nature. Combining the renewable energybased power generation system with a fuel cell system would solve this
problem to a great extent. A hybrid wind/solar energy–fuel cell system
can use wind/solar power for generating hydrogen using the electrolysis of water, and store it in cylinders at high pressure. This hydrogen can
Fuel Cells
291
then be used as the fuel for the fuel cell stack. The stored hydrogen
can also be used to fuel the fuel cell vehicles and so forth. In a gridconnected wind/solar energy–hydrogen system, wind/solar power
whenever available provides electricity for hydrogen production. The
grid power is used during off-peak periods for low-cost electricity and
hydrogen production; whereas during peak-demand periods or no/low
wind/solar energy periods, the fuel cell can generate electricity using the
stored hydrogen. These hybrid systems could be configured in several
ways.
9.6
Conclusion
Fuel cell systems are one of the most promising technologies to meet our
future power generation requirements. Fuel cell systems provide a very
clean and efficient technology for electrical and automotive power systems. With cogeneration efficiencies higher than 80%, fuel cells promise to reduce primary energy use and environmental impact. Fuel cells
are a very good alternative for rural energy needs, especially in remote
places where there are no existing power grids or power supply is unreliable. The application of fuel cells into the transportation sector will
reduce greenhouse emissions considerably; if fuels from renewable
energy sources are used, it would nearly eliminate greenhouse gas emissions. Utility companies are beginning to locate small, energy-saving
power generators closer to loads to overcome right of way problems and
transmission line costs. The modular design of fuel cells suits this distributed generation strategy very nicely as new modular units can be
added when the demand increases. This reduces the financial risk for
utility planners. Biofuel cells are very attractive for implant devices as
they can use glucose in blood to power these devices, eliminating the
need for surgery for maintenance and battery replacement. Use of
digester gas as a fuel in biofuel cells makes them very attractive for
power generation from garbage and other organic waste. This will also help
in waste disposal, a big problem in the agriculture and food industry.
All fuel cell technologies (PEMFC, DMFC, AFC, PAFC, MCFC, SOFC,
and MFC) discussed in this chapter are in a very advanced stage of
development and are very near to commercialization. Although a
number of demonstration units of different types of fuel cells are operating all over the world and many PAFC and AFC units have been commercially sold and are successfully operating, fuel cells are still awaiting
widespread commercialization due to their high cost and limitation in
the choice of the fuel used. These barriers will be overcome in the next
few years, and fuel cells will become a preferred power source with
widespread applications.
292
Chapter Nine
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Appendix
Relevant Definition
of Energy/Work Units
British thermal unit. Heat energy necessary to raise the temperature of 1 lb of water 1F.
Btu
Calorie or gram calorie. Heat energy required to raise the
temperature of 1 mL of water 1C (from 15 to 16C).
cal or gcal
1.6 1012 erg 1.6 1019 J 23.06 kcal/mol. Energy
gained by an electron passing through a potential of 1 V.
electron volt
foot pound
0.138 kg m.
ft lb. Work energy needed to raise 1 lb to a height of 1 ft force Correct force definition can be obtained from the second law of
Newton stating that the inertia is disturbed by unbalanced force, which
causes acceleration on a body directly proportional to the force (F ) and
inversely proportional to the mass of the body F K mf (F Kmf where
m is mass and f is acceleration). If all are reduced to unity, the unit of
force becomes pound foot per second per second (poundal) or gram centimeter per second per second (dyne).
joule Work energy to raise 1 kg to a height of 10 cm 0.1 kg m 0.74 ft lb.
joule (electrical) 0.239 cal. Energy developed when 1 C of electrons
(10.364 106 mole) passes through a potential of 1 V.
kcal/einstein Energy of a mole of a photon (einstein) of wavelength (in
) 28589.7 1 kcal/mol.
power
Rate of doing work, P W/t.
J/s W or ft lb/s
or horsepower 550 ft lb/s or 33,000 ft lb/min.
295
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
296
Appendix
So, 1 W 107 ergs/s 1 J/s 0.239 cal/s
1 hp 550 ft lb/s 33,000 ft lb/min
1 hp 746 W 178 cal/s
1 kW 1000 W 1.34 hp
1 kWh 3.6 106 J 860 kcal/h 3413 Btu/h
1 ft lb/s 1.356 W 0.324 cal
Wavelength eV 1239.8 1
Wave number cm1 eV 1239.8 104 cm1
quantum
units
1 erg 1 dyne 1 cm (dyne 1 g cm s2)
1 J 107 ergs 0.74 ft lb 0.239 cal
1 ft lb 1.3549 J
work, W
Force distance (both in the same direction on a body).
Index
Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use.
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Acetic acid, 80
Acid hydrolysis, 74, 78–80
enzymatic hydrolysis versus, 97–98
for sugar conversion to alcohol, 196
Acid hydrolyzates, detoxification of,
79–80
Acid transesterification, 177–181
Acid value (biodiesel), 156
Aerobes, in biohydrogen production,
41–42
Aerodynamic generation, 4
AFCs (see Alkaline-electrolyte fuel cells)
Alcaligenes eutrophus, in biohydrogen
production, 41–42
Alcohol, production of, 10
Alcohol fuels, 193–219
as blends, 200–201
in compression ignition engines,
204–209
ecosystem impacts of, 218–219
ethanol, 193–200
in compression ignition engines,
204–206
in gasahol, 201–202
methanol versus, 217–218
performance of engine with,
202–203
petrol versus, 201
production of, 194–198
properties of, 198–200
methanol, 208–218
advantages of, 210–211
and cold starting, 211–212
in compression ignition engines,
206–208
emissions with, 211
ethanol versus, 217–218
formaldehyde emission with, 213
and parts corrosion, 212
petrol versus, 213–217
problems associated with, 208–209
Alcohol fuels, methanol (Cont.):
production of, 209–210
properties of, 198–200
toxicity of, 212
properties of, 198–200
pure, modifications for use of, 200
Algae:
biomass yield of, 28
heterocystous blue-green, 26
in mixed cultures, 30
reduced organic compounds from, 25
Alkali transesterification, 177–181
Alkaline-electrolyte fuel cells (AFCs),
264–267
electrochemistry, 264–266
electrodes, 266
performance, 266–267
Allanblackia oil, 133
Allylic position equivalent (APE), 158
AM (see animal meal)
Amylopectin, 74
Amylose, 74
Anaerobes:
in biohydrogen production, 42–43
in biorefinery process, 65–66
Anhydrous alcohol, 197
Animal fats:
conversion in Micro-Activity Test unit,
238–239
conversion products, 228–237
iodine value, 157
tallow, 157, 168
engine performance with, 186–187
engine performance with esters of,
131–132
Animal meal (AM), 241
Animals and animal products:
animal fats, 131–132
conversion in Micro-Activity Test unit,
238–239
conversion products, 228–237
299
300
Index
Animals and animal products, animal
fats (Cont.):
iodine value, 157
tallow, 131–132, 157, 168, 186–187
energy from, 9–10
low-temperature conversion, 241–245
Anodes (fuel cells), 252, 253
Anodic catalyst layer (fuel cells), 252,
253
Antennas, plant bodies as, 10
APE (see allylic position equivalent)
Aquatic systems:
impact of alcohol fuels on, 218
types of, 9
Artificial photosynthesis, 2
Artocarpus hirsute, 58–59
ATR (see Autothermal reforming)
Australia, biodiesel tax exemptions in,
109
Automated oil stability index (OSI), 156
Automobiles:
efficiency of, 1
fuel-cell powered, 290
Autothermal reforming (ATR), 285, 286
Autotrophs:
as biomass producers, 22
solar energy used by, 8
Autoxidation, 156–157
Babassu, 55
Bacillus licheniformis, in biohydrogen
production, 41
Bacteria:
in ethanol fermentation, 87–88
photosynthetic, 25
Bahapilu oil, 110–111
BAPE (see bis-allylic position equivalent)
Batch fermentation processes, 90–91
Batteries, plant leaves as, 10
Benemann, John, 23
Biodiesel, 107–109
advantages of, 62
economic viability of, 108, 109
engine performance
with different waste oils, 130–131
vegetable oil processing and, 165–219
fuel and physical properties of,
149–160
cetane number and exhaust
emissions, 152–154
cold-flow properties, 154–156
determined by structure of fatty
esters, 149–151
Biodiesel, fuel and physical properties of
(Cont.):
oxidative stability, 156–159
legal framework regulating, 132
raw materials for, 109–139
allanblackia oil, 133
animal fats, 131–132
bahapilu oil, 110–111
bitter almond oil, 133, 134
cardoon oil, 124–125
castor oil, 111–113
chaulmoogra oil, 134–135
cottonseed oil, 113–114
cuphea oil, 114–115
Ethiopian mustard oil, 125–126
gold-of-pleasure oil, 126–127
Jatropha curcas oil, 115–116
karanja seed oil, 116–117
linseed oil, 117–119
low-cost edible oils, 124–129
mahua oil, 119–120
nagchampa oil, 120
neem oil, 121–122
nonedible oils, 109–124
oils for future research, 132–139
papaya oil, 135–136
rubber seed oil, 122–123
sal oil, 136–137
tigernut oil, 127–129
tonka bean oil, 123–124
tung oil, 137–138
ucuuba oil, 138–139
used frying oils, 129–131
standards for, 149
tax exemptions for, 108–109
transesterification for, 62
use of term, 62
vegetable oil processing and engine
performance, 165–219
degumming of vegetable oils,
169–177
engine performance with esters of
tallow and frying oil, 186–187
engine performance with esters of
vegetable oil, 184–186
enzymatic transesterification of oils,
181–184
for reducing pollutant emissions, 165
studies of, 166–169
transesterification of oils by acid or
alkali, 177–181
Biodiesel blends, 62
Bioenergetics (see Biological energetics)
Index
Bioenergy, 9–11
Bioenergy cell models, 18–21
Bioethanol, 69–102
alternative recovery and purification
processes, 100
conversion of simple sugars to, 83
distillation, 98–100
ethanol dehydration, 101
ethanol recovery, 98
fermentation concepts, 82
fermentation processes, 89–98
batch, 90–91
continuous, 92–94
enzymatic versus acid hydrolysis for
lignocellulosic materials, 97–98
fed-batch, 91–92
microorganisms related to, 86–89
separate enzymatic hydrolysis and
fermentation, 95–96
series-arranged continuous flow, 94
simultaneous saccharification and
fermentation, 96–97
strategies for enzymatic
lignocellulosic hydrolyzates
fermentation, 94
global market for, 69–71
lignocellulosic materials
characterization of, 76–77
enzymatic lignocellulosic
hydrolyzates fermentation
strategies, 94
enzymatic versus acid hydrolysis
fermentation processes for, 97–98
sugar solution from, 77–82
production
from hexoses, 83–85
overall process for, 72
from pentoses, 85
of sugars from raw materials, 73–76
Biofuel cells, 252, 279–284
bioreactors with, 281–282
electrical coupling in, 280
electrochemistry, 281–282
mediatorless microbial fuel cells,
282–284
“one-compartment,” 283
types of, 279
Biofuels, 25–27 (See also specific fuels)
Biogas, 28–30, 65 (See also Gobargas)
Biogeographical distribution, 9
Biogeological distribution, 9
Biohydrogen production, 38–43
aerobes, 41–42
301
Biohydrogen production (Cont.):
anaerobes, 42–43
cyanobacteria, 39–40
enzymatic route, 39
facultative anaerobes, 42
Klebsiella oxytoca, 39
major routes, 38
photosynthetic bacteria, 40–41
Biological energetics (bioenergetics),
11–16
anabolic photosynthesis, 15
energy conservation and efficiency
factor, 13–15
energy relations, 11–13
nitrogen fixation, 15–16
Biological energy, 5
Bioluminescence, 35–36
Biomass, 3, 34
from animals and animal products, 9–10
(See also Gobargas)
conversion process, 61
defined, 46
direct combustion of, 64
energy efficiency of, 1
products from, 60–66
gaseous, 62
liquid, 62–63
solid, 63–66
as renewable energy, 45–46
sources of, 34
yield of specific crops, 28
(See also Photosynthetic plants)
Biomethanation, 38 (See also Biogas;
Gobargas)
Biophotolysis, 25–26
Bioreactors (for fuel cells), 281–282
Biorefinery process, 64–66
Biosphere, 9
Biostil, 93–94
Bis-allylic position equivalent (BAPE), 158
Bitter almond oil, 133, 134
Black box fuel cells, 5
Blackman’s reaction (dark reaction), 49
Blends:
with alcohols, 200–201, 217
biodiesel, 62
of vegetable oil fuels/hydrocarbons,
225–239
engine performance, 165–219
feed component in FCC, 237–239
refitting engines for, 227
tailored conversion products,
227–237
302
Index
Branched esters, 155
Brazil:
ethanol production, 71, 197, 198
ethanol use, 62
sugarcane production, 194
British Petroleum, 70
British thermal units (Btus), 294
Buffalo gourd oil, 56
Butane (LPG), comparison with other
fuels, 12
C. beljerinscki, in biohydrogen
production, 42
C. butyricum, in biohydrogen production,
42
C. pasturianum, in biohydrogen
production, 42
C. welchii, in biohydrogen production,
42
Calories (cal), 295
Calotropis procera, 59
Calvin, Melvin, 27
Canada, fuel cell technology, 290
Canola oil, 240
Carafoli, Ernesto, 18
Cardoon oil, 124–125
Cassava, 55, 195
Castor oil, 111–113, 166, 184
Catalytic cracking (CC), 224–225
Cathodes (fuel cells), 252, 253
Cathodic catalyst layer (fuel cells),
252, 253
Cattle dung:
comparison of other fuels and, 12
gobargas yield of, 32
in mixed cultures, 30
(See also Gobargas)
Causuarina, 53, 54
CC (see Catalytic cracking)
Cellulose, 60, 76, 77
acid hydrolysis by-products, 79
enzymatic hydrolysis of, 82
in ethanol production, 196–197
Cetane number (CN), 150–154
CFPP (see Cold filter plugging point)
Charcoal, comparison of other
fuels and, 12
Chaulmoogra oil, 134–135
Chemical cells, 16–18
Chemical hydrolysis, of lignocellulosic
materials, 78–80
Chemical reaction, 5–7
Chemostat, 92
Chromatophores, 25
CI engines (see Compression ignition
engines)
Climate change, 45
Cloud points (CPs), 154, 155
CN (see Cetane number)
Coal conversion, 5, 210
Coal gas, comparison of other fuels and, 12
Coconut oil, 240
Cold filter plugging point (CFPP),
154, 155
Cold starting, with methanol, 211–212
Cold-flow properties (biodiesel), 154–156
Colligative motion energy, 5
Compression ignition (CI) engines:
alcohol fuels, 204–209
reducing emissions from, 165
Concentrated-acid hydrolysis, 78
Concentration cells, 17
Conservation of energy supplies, 2
Consumers, 8
Continuous fermentation processes,
92–94
Corn, biomass yield of, 28
Corrosion, with alcohol fuels, 200, 212
Cottonseed oil, 113–114
engine performance, 184
enzymatic transesterification, 182, 183
physical and chemical properties, 168
pyrolyzed, 239–240
Cow dung (see Cattle dung)
CPs (see Cloud points)
Cuba, sugarcane production in, 194
Cuphea oil, 114–115
Cyanobacteria, in biohydrogen
production, 39–40
Danish Co., 197
Danish Distilleries, Ltd., 94
Dark reaction, 47–49
DAY-Wessalith, 228
Deforestation, 3
Degumming of vegetable oils, 169–177
blend performances, 172
effect of loads on emissions, 172–177
effect of timing, 170
performance and emission
measurement, 170
Dehydration, ethanol, 101
Dependence on energy, 2
Developing countries, energy
administration in, 3
Development assistance, 2
Index
Diesel, Rudolf, 165
Diesel fuel:
alternatives to, 62, 107 (See also
Biodiesel)
in comparison of fuel properties, 223
ethanol and petrol compared to, 194
Dilute-acid hydrolysis, 78–80, 97
Direct methanol fuel cells (DMFCs),
263–264, 290
Distillation:
alcohol, 198
bioethanol, 98–100
petroleum products, 222
Diversification of energy supply, 2
DMFCs (see Direct methanol fuel cells)
Dry-biomass conversion process, 61
Dry-milling process, 75
Dye-sensitized solar cells, 1
Dynamic viscosity, 158
Ecosystems, energy-dependence of, 7–9
Edible oils, 124–129
cardoon oil, 124–125
Ethiopian mustard oil, 125–126
gold-of-pleasure oil, 126–127
low-cost edible oils, 124–129
tigernut oil, 127–129
tonka bean oil, 123–124
used frying oils, 129–131
(See also Lipids)
Efficiency of energy supplies, 2
Electricity:
comparison with other fuels, 12
renewable energy for power generation,
290–291
Electrochemical cells, with living
thylakoids, 21
Electrodes, 17, 18
alkaline-electrolyte fuel cells, 264–266
biofuel cells, 280
microbial fuel cells, 282, 283
molten carbonate fuel cells, 273
phosphoric acid fuel cells, 269
polymer electrolyte membrane fuel
cells, 257
solid oxide fuel cells, 277
standard potentials, 17, 18
Electrolyte concentrations, 17
Electrolytes, 252, 253
alkaline-electrolyte fuel cells, 264–266
direct methanol fuel cells, 263
molten carbonate fuel cells, 272–273
phosphoric acid fuel cells, 267, 269
303
Electrolytes (Cont.):
polymer electrolyte membrane fuel
cells, 255–257
solid oxide fuel cells, 274–277, 279
Electron volts, 295
Embden-Meyerhof pathway (EMP), 84, 85
(See also Glycolysis)
Energy, 1–43
bioenergy, 9–11
bioenergy cell models, 18–21
biofuels, 25–27
biogas, 28–30
biological energetics (bioenergetics),
11–16
bioluminescence, 35–36
biomass, 3, 34
chemical cell, 16–18
in common substances, 6
ecosystems dependent on, 7–9
energy and work units, 295–296
in Europe and South America, 3
gasification and pyrolysis, 34–35
global consumption, 251
gobargas, 30–34
hydrogen, 37–38
inefficient forms of, 1
of living cells, 21–22
microbial conversion for, 38–43
per capita requirement for, 1
of plant cells, 22–25
plant hydrocarbons, 27–28
renewable sources of, 2
resources remaining to be
developed/commercialized, 3–4
search for renewable/infinite
sources of, 3
solar light energy technologies, 1–2
thermodynamics, 5–7
12th Congress of World Energy, 2–3
types of, 5
in United Kingdom, 3
units of, 5
(See also specific sources of energy)
Engines:
compression ignition
alcohol fuels, 204–209
reducing emissions from, 165
diesel
alcohol fuels, 107
plant fuels, 62, 107 (See also
Biodiesel)
internal combustion
alcohol fuels (see Alcohol fuels)
304
Index
Engines, internal combustion (Cont.):
biodiesel, 108 (See also Biodiesel)
petroleum-based fuels, 191–192
performance
with ethanol, 202–206
with methanol–diesel fuel blends,
206–207
vegetable oil fuels/hydrocarbon
blends, 165–219
pollutants from, 191–192
refitting, for vegetable oil
fuels/hydrocarbon blends, 227
spark ignition, 193, 199
Enterobacter, in biohydrogen production,
42
Enthalpy, 7
Entropy, 7
Environment:
care of, 2
microenvironments, 9
Enzymatic biohydrogen production, 39
Enzymatic hydrolysis:
lignocellulosic materials, 80–82
starch, 75–76
Enzymatic processes:
fermentation processes coupled with, 63
for lignocellulosic processes, 63, 95–98
Epilimnon systems, 9
Epimarine systems, 9
Equilibrium state, 7
Equistar, 70
Escherichia coli, in biohydrogen
production, 42
Estuarine systems, 9
Ethanol, 69, 193–200
for biofuel cells, 282
biologically produced (see Bioethanol)
compared to petrol and diesel, 194
compared to petrol and gasohol, 201
in compression ignition engines,
204–206
current use of, 62
fermentation, 12, 86–89
in gasahol, 201–202
methanol versus, 217–218
performance of engine, 202–203
petrol versus, 201
prices, 70
production of, 62–63, 70, 71, 194–198
properties of, 198–200
purity of, 99
total world production, 70
Ethanol dehydration, 101
Ethanol recovery:
alternative processes, 100
bioethanol, 98
Ethiopian mustard oil, 125–126
Ethylene, 70
EU (see European Union)
Eucalyptus:
biomass yield of, 28
as renewable energy source, 53
Europe:
biomass fuel development in, 3
cetane number standards, 152
fuel cell technology, 290
oxidative stability standard, 157
European Union (EU):
biodiesel tax exemptions, 108
markets for sewage sludge and
organic residues, 241
support for renewable energy
research in, 45
Exergonic reaction, 6
Facultative anaerobes, in biohydrogen
production, 42
Fatty acids, 149–151
Fatty esters, 149–151
Fed-batch fermentation processes, 91–92
Fermentation, 82
in biorefinery process, 65
ethanol, 12, 86–89
lactic, 12–13
microorganisms related to, 86–89
of organic substrates, 27
simultaneous saccharification and, 96–97
Fermentation processes (ethanol
production), 89–98
batch, 90–91
continuous, 92–94
enzymatic lignocellulosic hydrolyzates
fermentation strategies, 94
enzymatic process coupled with, 63
enzymatic versus acid hydrolysis for
lignocellulosic materials, 97–98
fed-batch, 91–92
separate enzymatic hydrolysis and
fermentation, 95–96
series-arranged continuous flow, 94
simultaneous saccharification and
fermentation, 96–97
Fermentative ethanol (see Bioethanol)
Ficus elastica, 58–59
Filamentous fungi, in ethanol
fermentation, 88–89
Index
Firewood, comparison of other fuels
and, 12
Force, defined, 295
Formaldehyde emission (methanol), 213
Formic acid, 80
Fossil fuels, 45, 107
hydrocarbons from, 251
petroleum-based, 191
(See also Hydrocarbon fuels;
Petroleum-based fuels)
Foster, J. S., 2
Frictional energy, 5
Frying oils:
for biodiesel, 129–131
engine performance with, 186–187
Fuel cell power plants, 254
Fuel cells, 251–291
alkaline-electrolyte, 264–267
electrochemistry, 264–266
electrodes, 266
performance, 266–267
applications for, 288–291
biofuel, 279–284
electrochemistry, 281–282
mediator-less microbial fuel cells,
282–284
challenges for commercialization,
254–255
direct methanol, 263–264
molten carbonate, 271–274
electrochemistry, 272–273
electrode, 273
performance, 273–274
phosphoric acid, 267–271
electrochemistry, 268–269
electrode, 269
hardware, 269
performance, 270–271
temperature and humidity
management, 270
polymer electrolyte membrane,
255–263
electrochemistry, 256–259
performance, 259–263
water and air management, 259
power system for, 284–288
air management, 287
fuel processor, 284–286
power-conditioning system, 288
thermal management, 287
water management, 287
principles and components of,
252–255
305
Fuel cells (Cont.):
solid oxide, 274–279
electrochemistry, 276–277
electrode, 277
hardware, 278–279
types of, 255
Fuels:
biofuels, 25–27 (See also specific fuels)
blends
with alcohols, 200–201, 217
biodiesel, 62
vegetable oil fuels/hydrocarbon,
225–239
comparison of, 12
diesel
alternatives to, 62, 107
in comparison of fuel properties, 223
ethanol and petrol compared to, 194
fossil, 45, 107
hydrocarbons from, 251
petroleum-based, 191
gasoline
ethanol versus, 202–203
methanol blends with, 208
sulfur odorants in, 286
hydrocarbon, sulfur odorants in, 288
petrol
compared to ethanol and diesel, 194
compared to ethanol and gasohol, 201
effects of methanol compared with, 211
methanol blends with, 208
methanol versus, 213–217
petroleum-based
demand versus supply of, 191, 192
pollutants from, 191, 192
substitutes for, 192
use of stored energy for, 9
Fungi, filamentous, 88–89
Furfural, 80
Fusel oil, 98
Fusion of thermonuclear devices, 4
Gaseous biomass products, 62
Gasification, 34–35
Gasohol, 201–202
Gasoline (petrol):
compared to ethanol and diesel, 194
compared to ethanol and gasohol, 201
effects of methanol compared with, 211
ethanol versus, 202–203
methanol blends with, 208
methanol versus, 213–217
sulfur odorants in, 286
306
Index
Gcals (gram calories), 295
Genencor International, 63
Geothermal energy sources, 4
Global energy consumption, 251
Global warming, 45, 251
Glycolysis, 84, 85
Gobargas, 29–34
comparison with other fuels, 12
positive and negative factors with,
31–32
production of, 32–34
Gobargas plants, 30, 32–33
Gold-of-pleasure oil, 126–127
Gram calories (gcals), 295
Groundnut oil, 166
Grove, William, 252
Guaiacyl lignins, 77
Guaiacylsyringyl lignins, 77
Heat, as wasteful form of energy, 1
Heat content, 7
Hemicelluloses, 76, 77, 79
Hemp, 58
Heterocystous blue-green algae, 26
Heterotrophs, solar energy used by, 8
Hexoses:
ethanol production from, 83–85
fermentation of, 90
Hill reaction (light reaction), 48–49
(See also Light reaction)
Hone oil, 168
Human excreta, 34
Human population, energy requirement
for, 1
Hydrocarbon fuels, sulfur odorants in,
288
Hydrocarbon gases, in ethanol
production, 196–197
Hydrocarbons:
from fossil fuels, 251
plant, 27–28
vegetable oil fuel blends with, 225–239
feed component in FCC, 237–239
refitting engines for, 227
tailored conversion products,
227–237
Hydrodynamics, 4
Hydrogen:
for biofuels, 25–27
combustion of, 37
as fuel, 5, 37–38
production of
by microbial conversion, 38–43
Hydrogen, production of (Cont.)
by photoheterotrophic bacteria, 26–27
in two-stage photolysis system, 23–24
in wind/solar power fuel cell systems,
290–291
Hydrogen energy, 37–38
Hydrogen fuel cells, 5, 252, 254
Hydrolysis:
for ethanol production, 74–76
of lignocellulosic materials
chemical, 78–80
enzymatic, 80–82
Hypolimnon systems, 9
H-ZSM-5, 228
Ideal energy crops, characteristics of,
52–53
Ignition Quality Tester (IQT), 153–154
Implanted medical devices, 252, 279
India:
methanol production, 209
solar energy, 2
sugarcane production, 194
Industrial waste waters, low-temperature
conversion of, 241–245
Infrastructure, impact of alcohol fuels on,
219
Injury potential (living cells), 21
Innovation, 2
Integrated biorefinery process,
64–65
Internal combustion engine fuels:
alcohols (see Alcohol fuels)
biodiesel, 108
petroleum-based, 191–192
International cooperation, in energy
development, 2
Iodine value (IV), 157–158
Iogen Corp., 63
IQT (see Ignition Quality Tester)
Isolated enzyme fuel cells, 279, 280
Isopropyl esters, 155
IV (see Iodine value)
Japan:
fuel cell technology in, 290
Research Institute of Innovative
Technology for the Earth (RITE),
88
Jatropha oil, 56–57, 115–116, 170–177
Jojoba oil, 56
Joules (electrical), 295
Joules (work energy), 295
Index
Karanja oil, 170–177
Karanja seed oil, 116–117
Kcal/Einstein, 295
Ken seed oil, 168
Kerosene, comparison with other
fuels, 12
Kinematic viscosity, 158
Kinetic energy, 5
Klebsiella oxytoca, in biohydrogen
production, 39
Krantz anatomy, 51
Krebs cycle, 84, 85
Lactic fermentation, 12–13
Latex-bearing plants, 59
Levulinic acid, 80
Life, energy-dependence of, 7–9
Light reaction, 22, 47, 48
Lignin, 60, 76, 77, 79
Lignocellulosic hydrolyzates:
detoxification of, 79–80
fermentation of, 89–90
batch processes, 90–91
continuous processes, 93
enzymatic, 95
fed-batch process, 92
separate enzymatic hydrolysis and
fermentation, 95–96
Lignocellulosic materials:
advantages of, 63
cellulose, 76, 77
characterization of, 76–77
enzymatic fermentation
acid hydrolysis fermentation versus,
97–98
of lignocellulosic hydrolyzates,
strategies for, 94
for heat, 9
hemicellulose, 76, 77
lignin, 76, 77
sugar solution from, 77–82
Linseed oil, 117–119, 240
Lipids, 221–246
cracking
catalytic, 224–225
by in situ catalysts, 241–245
metal oxide catalysts, 239–240
thermal degradation process, 222–225
in vegetable oil fuels/hydrocarbon
blends, 225–239
feed component in FCC, 237–239
refitting engines for, 227
tailored conversion products, 227–237
307
Liquid biomass products, 62–63
Living cells, energy of, 21–22
Low-temperature conversion (LTC),
241–245
Low-temperature flow test (LTFT),
154, 155
LPG (butane), comparison with other
fuels, 12
LTC (see Low-temperature conversion)
LTFT (see Low-temperature flow test)
Lubricity (biodiesels), 159
Magnetohydrodynamic generators, 4
Mahua oil, 119–120, 169
Maize oil, 184
Malt, 196
Manioc, 197
Mash, 98
MAT (see Micro-Activity Test)
MBM (see meat and bone meal)
MCFCs (see Molten carbonate fuel cells)
Meat and bone meal (MBM), 241
Mediatorless microbial fuel cells,
282–284
Melle-Boinot fermentation, 90
Membrane potential (living cells), 21
Membrane technology, ethanol
purification, 101
Mesolimnon systems, 9
Mesua ferrea seed oil, 240
Metal oxide catalysts, 239–240
Methanation, 30, 31
Methane:
for biofuels, 25
production of, 10
in biorefinery process, 65
by microbial conversion, 38
from oxidation ponds, 27
in two-stage photolysis system, 23–24
Methanobacterium (Methanotrix), in
biohydrogen production, 42
Methanogens, 30
Methanol, 208–218
advantages of, 210–211
for biofuel cells, 282
and cold starting, 211–212
in compression ignition engines,
206–209
emissions with, 211
ethanol versus, 217–218
formaldehyde emission, 213
and parts corrosion, 212
petrol versus, 213–217
308
Index
Methanol (Cont.):
problems associated with, 208–209
production of, 209–210
properties of, 198–200
toxicity of, 212
Methyl esters, 155
Methylomonas albus, in biohydrogen
production, 42
Methylosinus trichosporium, in
biohydrogen production, 42
MFCs (see Microbial fuel cells)
Micro-Activity Test (MAT), 238–239
Microbial conversion, 38–43
Microbial fuel cells (MFCs), 279–284
electrochemistry of, 281–282
mediatorless, 282–283
Mimosa, 54–55
Miscanthus, 57
Molasses, 195, 197
Molecular sieve adsorption, 101
Molten carbonate fuel cells (MCFCs),
271–274
electrochemistry, 272–273
electrode, 273
performance, 273–274
stationary power applications, 289
Municipal solid wastes:
low-temperature conversion, 241–245
properties and uses of, 63
Nagchampa oil, 120
Neem oil, 121–122, 169
Nonedible oils, 109–124
bahapilu oil, 110–111
castor oil, 111–113
cottonseed oil, 113–114
cuphea oil, 114–115
Jatropha curcas oil, 115–116
karanja seed oil, 116–117
linseed oil, 117–119
mahua oil, 119–120
nagchampa oil, 120
neem oil, 121–122
rubber seed oil, 122–123
tonka bean oil, 123–124
Novozymes, Inc., 63
Nutrients, use of stored energy for, 9
Nutristat, 92
Occupational health, impact of alcohol
fuels on, 218
Occupational safety, impact of alcohol
fuels on, 218–219
Oil shale and oil sand, 4
Oil stability index (OSI), 156
Oils, plant:
for future research, 132–139
allanblackia oil, 133
bitter almond oil, 133, 134
chaulmoogra oil, 134–135
papaya oil, 135–136
sal oil, 136–137
tung oil, 137–138
ucuuba oil, 138–139
low-cost edible, 124–129
cardoon oil, 124–125
Ethiopian mustard oil, 125–126
gold-of-pleasure oil, 126–127
tigernut oil, 127–129
tonka bean oil, 123–124
nonedible, 109–124
bahapilu oil, 110–111
castor oil, 111–113
cottonseed oil, 113–114
cuphea oil, 114–115
Jatropha curcas oil, 115–116
karanja seed oil, 116–117
linseed oil, 117–119
mahua oil, 119–120
nagchampa oil, 120
neem oil, 121–122
rubber seed oil, 122–123
tonka bean oil, 123–124
as renewable energy source, 55–57
vegetable oil fuels/hydrocarbon blends,
lipids in, 225–239
vegetable oil processing and engine
performance, 165–219
degumming of vegetable oils,
169–177
engine performance with esters of
tallow and frying oil, 186–187
engine performance with esters of
vegetable oil, 184–186
enzymatic transesterification of oils,
181–184
for reducing pollutant emissions, 165
studies of, 166–169
transesterification of oils by acid or
alkali, 177–181
OSI (see oil stability index)
Overliming, 80
Oxidation half reactions, 256, 257
Oxidation ponds, 27
Oxidative phosphorylation, 18, 19
Oxidative stability (biodiesel), 156–159
Index
P. furiosus, in biohydrogen production, 42
Packer, Lester, 18, 22
PAFCs (see Phosphoric acid fuel cells)
Palm oil, 56, 166, 179, 180
Panicum, 57, 58
Papaya oil, 135–136
Partial oxidation (POX), 285–286
Pathogenic prions, 131
Peanut oil, 56, 167, 168, 184
Peat formation, 30
PEMFCs (see Polymer electrolyte
membrane fuel cells)
Pentasil, 228
Pentose phosphate pathway (PPP), 50,
83–85
Pentoses:
ethanol production from, 85
fermentation of, 90
Peroxide value (biodiesel), 156
Petrol (see Gasoline)
Petroleum-based fuels:
demand versus supply of, 191, 192
pollutants from, 191, 192
substitutes for, 192
pH specificity (living cells), 21
Phauxostat, 92
Phenolic compounds, in hydrolyzates, 80
Philippines, alcogas program in, 197
Phosphoric acid fuel cells (PAFCs), 267–271
electrochemistry, 268–269
electrode, 269
hardware, 269
performance, 270–271
stationary power applications, 289
temperature and humidity
management, 270
Photobiolysis, 25–26
Photofermentation, 26–27
Photolysis:
electron flow during, 22, 23
two-stage system, 23–24
of water, 19–21, 48 (See also light
reaction)
Photophosphorylation, 8
cyclic and noncyclic, 15
in light reaction, 49
Photosynthesis:
artificial, 2
in bioenergy cell models, 18–21
in biofuel production, 25–27
biosynthesis of higher carbohydrates, 22
C3 metabolism, 50, 52
C4 metabolism, 51–52
309
Photosynthesis (Cont.):
efficiency of, 49–52
factors in, 8–9
light dependent step (light reaction),
22, 47
mechanism and efficiency of, 46–47
photosynthetic process, 47–52
temperature dependent step (dark
reaction), 47–48
Photosynthetic bacteria, 25
in biohydrogen production, 40–41
photofermentation by, 26–27
Photosynthetic plants, 45–66
Artocarpus hirsute, 58–59
babassu, 55
Calotropis procera, 59
cassava, 55
Causuarina, 53, 54
eucalyptus, 53
Ficus elastica, 58–59
growing cycles of, 52–60
growth conditions for, 59, 60
harvesting plants for bioenergy, 60–61
hemp, 58
mechanism/efficiency of photosynthesis
in, 46–47
mimosa, 54–55
Miscanthus, 57
oil-bearing crops, 55–57
Panicum, 57, 58
photosynthetic process in, 47–52
products from, 61–66
sorghum, 55
sugarcane, 55
yields of, 59–60
Photosynthetic systems, characteristic
energy limitations of, 10
Physic nut oil, 56–57
Plant cells, energy of, 22–25
Plant hydrocarbons, 27–28
Plants:
as antennas and batteries, 10
as biomass (see Photosynthetic plants)
oil-bearing (see Oils, plant)
Plastoquinones, 19, 21
Pollution:
with animal fat fuels, 132
and cetane number, 152–154
with ethanol, 216
fuel type and diesel exhaust emissions,
165–169
with gobargas, 31
with methanol, 211, 213, 216, 217
310
Index
Pollution (Cont.)
from petroleum-fueled vehicles,
191–192
and replanting of biomass, 46
Polymer electrolyte membrane fuel cells
(PEMFCs), 255–263
electrochemistry, 256–259
electrodes, 257
performance, 259–263
for portable power systems, 290
stationary power applications, 289
transportation applications, 290
water and air management, 259
Pongamia oil, 180, 181
Population, world, 3
Portable power systems, 290
Potatoes, crop area required for, 195
Potential energy, 5
Pour points (PPs), 154, 155
Power, defined, 295–296
Power-conditioning system (fuel cells),
288
POX (see Partial oxidation)
PPP (see Pentose phosphate pathway)
PPs (see Pour points)
Predator-prey relationship, 8
Primary consumers, 8
Producers, 8
Proton exchange membrane fuel cells, 256
(See also Polymer electrolyte
membrane fuel cells)
Purification (bioethanol):
alternative processes, 100
by distillation, 98–100
Putranjiva oil, 170–177
Pyrolysis, 35
PZ-2/50H, 228
Quality of life, 2–3
Quantum, 296
Rancimat method, 157
Rapeseed oil, 236, 239–240
as diesel substitute, 227
pyrolysis, 240
transesterification by acid or alkali, 180
Reduction half reactions, 256, 257
Renewable energy:
defined, 45
for electric power generation, 290–291
photosynthetic plants for (see
Photosynthetic plants)
sources of, 2–4
Rhodopseudomonas, in biohydrogen
production, 41
Rhodospirillum, in biohydrogen
production, 41
Rice bran oil, 168
RITE (Research Institute of Innovative
Technology for the Earth), 88
Rubber plants, 27–28, 61
Rubber seed oil, 122–123, 168
Ruminococcus albus, in biohydrogen
production, 42
SADAF, 70
Safflower oil, 167–168, 223, 240
Sal oil, 136–137
Saponification, 129
Sasol, 70
Secondary consumers, 8
Seed-based oils, 55
Self-reliance, 2
Separate enzymatic hydrolysis and
fermentation (SHF), 95–96
Series-arranged continuous flow
fermentation, 94
Sewage sludge, low-temperature
conversion of, 241–245
SHF (see Separate enzymatic hydrolysis
and fermentation)
SI engines (see Spark ignition engines)
Simultaneous saccharification and
fermentation (SSF), 63, 95–97
Sludge, 66, 241–245
Slurry, 66
Socioeconomic impact of alcohol fuels, 219
SOFCs (see Solid oxide fuel cells)
Soft coke, comparison of other fuels and, 12
Solar constants, 8
Solar energy, 8
dependence of biotic species on, 10
efficiency of, 45
for electric power generation, 290–291
energy forms converted from, 45
green plants’ use of, 45–46
limiting factors for use of, 10
Solar light energy technologies, 1–2
Solar photovoltaic panels, 1
Solar thermal power, 1
Solid biomass products, 63–66
Solid oxide fuel cells (SOFCs), 274–279
electrochemistry, 276–277
electrode, 277
hardware, 278–279
stationary power applications, 289
Index
Sorghum, 28, 55, 195
South America, biomass fuel development
in, 3
Soybean oil, 56, 166–169
in comparison of fuel properties,
222, 223
engine performance, 184–186
enzymatic transesterification,
182–184
transesterification by acid or alkali,
178–180
Spark ignition (SI) engines, 193, 199,
206, 207
SR (see Steam reforming)
SSF (see Simultaneous saccharification
and fermentation)
Standard state, 7
Starch, conversion to sugars, 195
Starch crops:
for ethanol production, 71
sugar production from, 73–76
(See also specific crops, e.g.: Sorghum)
Stationary fuel cell systems, 289–290
Steam reforming (SR), 285, 286
Stillingia oil, 168
Submarine systems, 9
Subterrestrial systems, 9
Sugar beets, 28, 194, 195
Sugar crops, for ethanol production,
71
Sugarcane:
anhydrous alcohol from, 197
biomass yield of, 28
crop area required for, 195
as renewable energy source, 55
as source of alcohol, 194
Sugars:
from lignocellulosic materials,
77–82
chemical hydrolysis, 78–80
enzymatic hydrolysis, 81–82
pretreatment prior to enzymatic
hydrolysis, 80–81
production of, 73–76
simple, conversion to ethanol, 83
Sulfite waste liquor, in ethanol
production, 196
Sunflower oil, 166–169
engine performance, 184–185
transesterification by acid or alkali,
178–180
SunFuel, 62
Sweden, 197–198
311
Tallow, 157, 168
engine performance with, 186–187
engine performance with esters of,
131–132
Tapioca, 197
Tappeiner, H., 28
TCA (see Tricarboxylic acid cycle)
Terrestrial systems:
impact of alcohol fuels on, 218
types of, 9
Thermal degradation (vegetable oils),
222–225
Thermodynamics, 5–7
Thylakoids, 19–21
Tigernut oil, 127–129
Tonka bean oil, 123–124
Transesterification, 62
by acid or alkali, 177–181
enzymatic, 181–184
variables influencing, 109
Transportation:
fuel cell technology, 290
impact of alcohol fuels on, 219
Tricarboxylic acid cycle (TCA), 84, 85
Trilaurin, 240
Triolein, 240
Tung oil, 137–138
Turbidostat, 92
12th Congress of World Energy, 2–3
Ucuuba oil, 138–139
United Kingdom, 33
United States:
biodiesel tax exemptions, 108
biomass energy, 46
cetane number standards, 152, 153
ethanol production, 71
ethanol use, 62
fuel cell technology, 290
gasohol, 201
Vegetable oil fuel/hydrocarbon blends,
225–239
feed component in FCC, 237–239
refitting engines for, 227
tailored conversion products, 227–237
at T = 400°C, 229–235
at T = 550°C, 235–237
Vegetable oils:
Buffalo gourd oil, 56
as diesel fuel alternatives, 55 (See also
Biodiesel)
edible (see Edible oils)
312
Index
Vegetable oils (Cont.):
engine performance and processing of,
165–219
degumming, 169–177
enzymatic transesterification,
181–184
with esters of tallow and frying oil,
186–187
with esters of vegetable oil,
184–186
transesterification by acid or alkali,
177–181
for illumination, 9
iodine value, 157
jojoba oil, 56
palm oil, 56
peanut oil, 56
physic nut oil, 56–57
for reducing pollutant emissions, 165
soybean oil, 56
studies of, 166–169
thermal decomposition of, 222–225
Vegetable oils (Cont.):
used frying oils, 129–131
(See also Nonedible oils)
Viscosity (biodiesel), 156, 158
W (work), 296
Water:
in biorefinery process, 65
photolysis of, 19–21
Wet-biomass conversion process, 61
Wet-milling process, 75, 76
Wheat, crop area required for, 195
Wind energy, for electric power
generation, 290–291
Winter rape oil, 167, 168
Winterization, 155–156
Wood, in ethanol production, 196
Work (W), 296
Xylans, 77
Yeasts, in ethanol fermentation, 86–87
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