Innovative biomass fuelled SOFC’s for polygeneration Xavier Fonoll Almansa

Innovative biomass fuelled SOFC’s
for polygeneration
Xavier Fonoll Almansa
Master of Science Thesis
KTH School of Energy and Environmental Technology
(Heat and Power Technology)
SE-100 44 STOCKHOLM
Master of Science Thesis EGI 2010
Innovative biomass fuelled SOFC’s for
polygeneration
Xavier Fonoll Almansa
Approved
Examiner
Supervisor
Date
Prof. Torsten Fransson
Bin Zhu/Rizwan Raza
Commissioner
Contact person
KTH School of Chemical Science
and Engineering
Torsten Fransson
Abstract
The supply of energy and the protection of the environment have been two of the most important
problems to have affected mankind in recent decades. The waste management is become a very crucial
environmental problem in the world, due to the ever increasing amount of waste material, domiciliary and
industrial, generated.
The main strategies for the waste management are the protection of the environment and used to produce
energy.
Lignin, the second most abundant component besides cellulose in biomass. In this study, lignosulfonate,
is used as fuel in the solid oxide fuel cell to produce a power and heat, which is a byproducts from the
wood pulp using sulfite pulping. To enhance the performance and conductivity of the biomass based fuel
cell, the lignin was treated with hydroxides (Li/K) and chlorides (Fe2Cl3) at different temperature 200 -500
°C. The maximum obtained power density was about 20 mW/cm2. And the calculated electrical efficiency
of the cell was about 30%.
1
Also, this kind of biomass fuel was used in single component fuel cell and results were compared with
three components fuel cell. The phase analysis, microstructure and conductivity of lignin was analysed by
XRD, SEM and AC impedance technique.
It has been concluded that a sulfonated lignin from waste of the paper industry can be used as a energy
sources with fuel cell operation.
“When it seems nothingis goingto work, you are usuallyjust a few millimiters awayfor make it happen.”
Anthony Robins
2
Acknowledgments
I am very grateful to many persons for their help and support for completing my research. I shall first
address my gratitude to Doctor Bin Zhu for gave me the opportunity to work in his laboratory of fuel
cells and for all the knowledge that he gave to me. Also it was very good for me to meet and work with
Rizwan Raza which help a lot in my experiments. I’m sure that he will be an excellent teacher and father in
the future.
I would also like to thank Dr. Haying Qin. I will never forget the conversations with her about my project
and other things. You were as an eldest sister for me here in Stockholm, thank you very much.
Thanks also goes to Fan, Dr. Wang (I will never forget you smile!), Inken, Manish and Quentin, the team
of the lab and to To all the Erasmus students that I met here: Alan, Antonio, Javi, Miguel, Dani, Aitor,
Luisja, Irene, Pedro, Itziar, Manuel, Paula, Alessio, Steffano, Mateo, Jeff, Lisa … Thanks to you, I had a
good stay here in KTH.
Thanks are due to the people from polymer department, specially to Raquel Bohn for your help in the
experiments in the polymer department. I’m sure that you will get your PHD and you will enjoy your stay
in Sweden during this period.
I’m also grateful to the people of EPRoBio program ( Ioanna, Evita, Melina, Spyros, Lazaros, Miriam,
Miguel, Maikol, Sofia and Liliana)
I express my thanks to Aribau 127 (Dani, Iñaki, Hugo and Marcos) and to the people from Palace barna
(Tocigol, Cavernicola, Latigo and Toni ) myfamily of Barcelona which supported me from there.
Thanks also to Laura Evangelio Araujo. Without you the weight of this work would crush me. Thank you
for your help during our long study days. And also to my friends of the university of Barcelona (Javi,
Pablo, Chus, Toreyonesa and Norma).
Thank you to the chemist team (Ariadna, Alba, Elvira and Irene) for the support that they gave me. And
thank you too to Laura Cabeza to stay with me during my stay in Sweden
It’s also very important for me to be grateful for the support of my friends from Andorra: Marc, Carlos,
Toni, Jose, Estel, Nunu, Carla, Lara…)
Specially thanks to my parents. They were always there for me and they gave me all the support that I
needed at any time. Also thanks to my sisters and to Marc, Carla and Marc petit.
Last but not least, I would like to thank Mireia Anqguita. You supported to me a lot during my period in
Sweden. You always were there for me and I always will be there for you.
3
Index
0 Introduction
7
0.1 Problematic
7
0.2 Motivation
9
0.3 Objective
9
1 The fuel cells
10
1.1 History of fuel cells
10
1.2 How generally does it works
11
1.3 Types of fuel cells
12
1.4 Fuel for fuel cells
13
1.5 The applications of fuel cells
17
1.6 Theory of the fuel cells
18
1.6.1 Thermodynamically basis
18
1.6.2 Thermodynamic for fuel cell
22
1.6.3 The losses of the fuel cell
24
1.6.4 The total efficiency of a fuel cell
26
2 SOFC
27
2.1 Principle of a solid oxide fuel cell
27
2.2 Constitution
28
2.2.1 Electrolyte
28
2.2.2 Anode
28
2.2.3 Cathode
28
2.3 designs of solid oxide fuel cells
30
2.4 Comparison of SOFC with other fuel cells
31
4
2.5 Applications of SOFC
32
2.6 Solid oxide fuel cells + gas turbine35
2.7 LTSOFC36
3 Biomass as a fuel
39
3.1 The biomass and bioenergy
39
3.2 SOFC fuelled with biomass
43
3.3 Lignin
44
3.3.1 Introduction: What’s lignin?
44
3.3.2 History of lignin
46
3.3.3 Lignin as a bio-resource
47
3.3.4 Lignosulfonate
48
4 Experimental
51
4.1The Fuel cell
51
4.1.1 Preparation of the electrolyte
51
4.1.2 Preparation of the electrode
51
4.2 Treated lignin as a fuel
51
4.2.1 Treatment with Iron chloride
52
4.2.2 Treatment with hydroxides
52
4.3 Lignin fuel cell fabrication
52
4.3.1Three component fuel cell
52
4.3.2 Single component fuel cell
53
4.3.3 Dry press method
53
4.4 Fuel cell test
54
4.5 X-ray diffraction test
55
4.6 Conductivity measurements
55
5
5 Results and discussions
5.1 fuel cells performance
56
56
5.1.1Lignosulfonate (Three component)
56
5.1.2 Lignin treated with hydroxides (Three component)
56
5.1.3 Lignin treated with Iron chloride (Three component)
58
5.1.4 Lignin treated with hydroxides (Single component)
60
5.2 XRD results
62
5.3 Conductivity measurements
64
5.4 Fuel cell efficiency
66
6 Conclusions
68
7 Bibliography
69
6
0 Introduction
0.1 Problematic
Today, the humanity recognizes that nature is not a good unalterable, is fragile, so their conservation is a
fundamental and urgent task. The Pollutants affect the air, the water, the soil and the animal and plant life.
The main reason of the air pollution is the burning of fossil fuels. As a direct problem of the burning
fuels, we can find:
- The smog, a photochemical effect that produces NOx, COV’s and increases the levels of ozone
in the troposphere.
- The increase of levels of SO2 and NOx that produces the acid rain damaging forests, water
systems, agriculture and civil engineering. It’s a very big problem of the Scandinavian countries
and the center of Europe.
- The increases of levels of CO and CO2 producing the greenhouse effect that increase the
Temperature of the world causing the sea level rise and other climatic catastrophes.
- The increase of levels of heavy metal
The above mentioned pollutants are the cause of several problems that
increase the vulnerability of
people and make them more prone to contagious respiratory and cardiovascular disease, including the
cancer development.
According to the European Commission, air pollution currently causes about 370,000 premature deaths in
the European Union.
Another problem is that the population in the world is increasing and the reserves of energy resources are
decreasing. As you can see in the Hubbert’s curve in the figure 0.1, right now we are in the peak oil and
since now, the amount of fuel cells will decrease. This produce that an increase of the cost of the fossil
fuels. In the European Union the unleaded 95 octane cost around 1,447 €.
7
Figure 0.1: Hubbert’s curve fromGerman-based energywatch group
Therefore, scientists are working on the creation of newfuels such as ethanol, methanol or hydrogen, and
even newpower generation systems based on chemical energy rather than combustion. The fuel cell is one
of the results of this work.
Figure 0.2: The effect of smogin Madrid. February2011
8
0.2 Motivation
The present work consists in the study of the use of biomass for the Low Temperature Solid Oxide Fuel
Cell (Forward mean as LTSOFC). I think that this studyis very important for the forward reasons:
-
The fuel cell has a very high efficiency.
-
The study of a fuel cell is very new for me. Also the possibility of using biomass as a fuel it’s
very interesting because we can reduce the amount of CO2 in the atmosphere. We can resolve
then the above mentioned problems.
-
With the study of a LTSOFC I’ll study a new polygeneration system that produces electricity
and heat which to cover the individual households need
-
With the study of renewable energies I think that I can give a little bit hope to this world with
the problem of pollution.
0.3 Objective
The objective of this project is to use lignosulfonated, a waste from the paper manufacturing as a fuel in
to the Low-Temperature solid oxide fuel cell. With this I will use a new kind of renewable energy, the
biomass, in order to do a good waste management in the paper industry.
The lignin can be used in the fuel cell directly or by pretreatment or by a steam reform process. In my
project, I will use the lignin directly and by some pretreatment in order to improve the efficiency.
Figure 0.3: Utilization of lignin in to the fuel cell
9
1 The fuel Cells
1.1 History of fuel cells
Christian Friedrich Schönbein was the first person to discover the principle of the fuel cells in 1838. After
that, in 1842 Sir William Robert Grove made the first fuel cell thanks to the work of Friedrich which
produced electrical energy by combining hydrogen and oxygen. He obtained a light current and the
hydrogen and oxygen were recombined producing water but at that time as shown in Figure 1.1, the highpowered steam engines were very popular and the fuel cell was forgotten. The fuel cell that he made used
similar materials to today's phosphoric-acid fuel cell.
Figure 1.1. Drawingof the first fuel cell
In 1958 appeared the first commercial use of the fuel cell thanks to W. Thomas Grubb and Leonard
Niedrach. They made a new type of fuel cell using a sulphonated polystyrene ion-exchange membrane as
electrolyte and platinum into the membrane as a catalyst for the oxidation of hydrogen and reduction of
oxygen. This cell was known as the “Grubb-Niedrach fuel cell” and it was used by the NASA and
McDonnell Aircraft. [1,2]
10
1.2 How generally does it works?
In the figure 1.2 it’s possible to see the production of hydrogen and air thanks to the electrolysis of the
water, but after that, it’s possible to change the power supply by one ammeter (like figure b) and see a
little amount of current by the recombination of hydrogen and oxygen:
2H2 + O2
2H2O
Figure 1.2: (a) The electrolysis of water. (b) The hydrogen and air are recombiningto get some current
Also, in the figure 1.2 is shown that the cell is constituted by the electrodes and the electrolyte. The
electrolyte is strictly necessary to separate the two kinds of fuels and allow the ion exchange between the
anode and the cathode. In the electrodes take place the reaction and they have to be porous to provide a
good pathway. The production of electricity and power depends on the kind of fuel used and how long
it’s provided. The life of the fuel cell depends on the kind of electrodes and electrolyte used.
At the anode:
At the cathode:
Overall reaction:
H2
2H+ + 2e-
2H+ + ½ O2 +2e2H2 + O2
H2O
2H2O
The oxygen reacts with electrons produced at the anode whose pass through an electrical circuit to the
cathode and H+ ions taken from the electrolyte. The mechanism can be seen in the figure 1.3. It’s very
important to consider that the reactions change with the type of fuel cell and the type of fuel.
11
Figure 1.3: Mechanismof a general fuel cell
Normally, the value of current is very low. The most important reasons are the low contact area between
the gas, the electrode and the electrolyte and the long distance between the electrodes. That’s why the fuel
cell usually has a thin layer of electrolyte between the electrodes and a porous surface in the electrodes to
give the maximum possible contact between the electrode, the fuel and the electrolyte. [1]
1.3 Types of fuel cell
It’s possible to classify the fuel cells according two parameters, the operating temperature and the type of
electrolyte. Actually, there are six classes of fuel cell as viable systems for the present and the near future.
These fuel cells are: The alkaline fuel cell, the proton exchange membrane fuel cell, the direct methanol
fuel cell, the phosphoric acid fuel cell, the molten carbonate fuel cell and the solid oxide fuel cell
The table 1.1 shows, as summarize, the different types of fuel cell.
12
Table 1.1: Summarize of the types of fuel cell.
As is possible to see on this table the Solid Oxide fuel cell (SOFC) has a very good advantage in
comparison of the others fuel cells, the fuel flexibility. That’s why the SOFC is used in this project.
1.4 Fuels for fuel cells
The most common fuels used in the fuel cells are the hydrogen and some hydrocarbons like coal gas,
natural gas….But also, it’s possible to find some liquid fuels like ethanol, methanol, liquefied petroleum
gas…. or even solid like the lignin, a waste from the paper manufacturing.
Normally, it is better to use hydrogen as fuel because it is good for the environment, but, sometimes other
fuels are used for some problems like the storage of hydrogen.
Hydrogen is the favourite fuel for the fuel cells because the only product that it gives is water, which is
environmentally benign. The hydrogen use to be a gas very flammable, it can burn in air at a very big
range of concentration (5-75%):
2H2 + O2
2H2O
13
There are many ways to produce Hydrogen. One of the ways to make Hydrogen is the electrolysis of
water. For this is necessary to give electrical power to two electrodes placed in the water:
Figure 1.4: Electrolysis of water
In the cathode takes place the formation of hydrogen and in the anode the formation of oxygen. So the
overall reaction is:
H2O(l)
H2(g) + O2(g)
Assuming ideal faraday efficiency, the amount of hydrogen generated is twice the number of moles of
oxygen, and both are proportional to the total electrical charge conducted by the solution. The electrolysis
of pure water requires a lot of energy due to the self-ionization of water, that’s why it’s very normal to add
one electrolyte (such as a salt, an acid or a base) to increase the efficiency.
It’s a very good idea to produce the electrical power by renewable energy (solar, wind, wave…)
The most economically way is using processes to remove the hydrogen from hydrocarbons like petroleum,
coal, coal gases, natural gas… One of the processes can be the steam reforming of natural gas at high
temperatures:
CnHm + H2O
nCO + (m/2 +n)H2
For the production of high purity hydrogen is normal to use the water-gas shift reaction in conjunction
with the steam reforming:
CO + H2O
CO2 + H2
These two reactions are carried out normally over a supported nickel catalyst at elevated temperatures,
typically above 500 ºC. Also is very important to know something about the physical and combustion
characteristics of the fuel and also his chemical composition. The chemical composition will determine the
14
type of fuel processing to produce hydrogen.
A similar method to produce hydrogen is the pyrolysis of the hydrocarbon. The pyrolysis consists on heat
the hydrocarbon fuels in the absence of air to decompose it into hydrogen and solid carbon.
But, before fed the cell with one of these fuels, is very important to do some process for the conversion
of the raw primary fuel into the fuel gas required by the stack. Each type of fuel cell stack has some
particular fuel requirements as you can see in the next table:
Table 1.2: The fuel requirements for the different types of fuel cells
For example, is very important to take out the sulphur with a hydrodesulphurisation reactor because it
deactivates the steam reforming catalysts or poisons the cell. [1]
Of course, is better for the environment to use renewable fuels for these processes like biofuels or wastes
instead fossil fuels. It’s possible to obtain energy from the biomass by different ways like:
- Direct combustion
- Conversion to biogas via anaerobic digestion
- Conversion to ethanol via fermentation
- Conversion to syngas
Also it is possible to get bio-fuels from the wastes. The combustible components of such waste may be
extracted chemically by conversion to gases, liquid distillates or biochemically by anaerobic digestion using
methane-forming bacteria. Of course, the biogas has high values of carbon monoxide making it
unattractive for use in gas engines, but, it can be used on MCFC or SOFC cells which are able to handle
very high levels of carbon oxides. Bioliquids like methanol and ethanol are also attractive for the fuel cells,
we can produce methanol from syngas made by biomass and ethanol by the fermentation of biomass.
15
The next picture shows the different ways for supply hydrogen in the fuel cells:
Figure 1.5: Howto supplyHydrogen in a fuel cell
In this work I will discuss about the SOFC. The SOFC has a superior approach that it can be use more
than one fuel. Particularly it is used for the hydrogen gas but later it shows that it can be used for the
liquid and solid also. It’s a good idea also to feed this cell with bio-ethanol or bio-methanol.
In North America, they get the ethanol from the corn and in Brazil from the sugar cane. If ethanol is
directly exposed to the environment by, for example, by one leakage, it is not harmful. The ethanol made
from the corn gives a 35% of energy and the ethanol made from the sugar cane gives 700 %, but the
conditions for the cultivation are more stringent. Also is more easy to transport and store than the
hydrogen. That's why the ethanol is a good alternative of fuel. [5,7]
The methanol is toxic and poison but not considers a carcinogenic and harmful for the environment.
At anode:
CH3OH + H2O
CO2 +6H+ + 6e16
3/2 O2 + 6H+ +6e-
At cathode:
CH3OH + 3/2 O2 + H2O
Overall Reaction:
3H2O
CO2 + 3H2O
It’s possible too to feed the fuel cell with carbon directly. Direct carbon fuel cell (DCFC), using molten
hydroxide as electrolyte, achieves 0,75 and 0,85 V of OCV with a peak power output of 180 mW/cm2 and
a maxim current density of 250 mA/cm2 at 600 ºC. [28]
1.5 The Applications of fuel cells
The potential applications of fuel cells are increasing due to the different benefits that it gives. The two
most important are the environmental considerations (no NOx, SOx, or hydrocarbon emissions and no
or much reduced CO2 emissions) and the efficiency considerations (The efficiency is better for a fuel cell
than for a thermal machine).
The fuel cells can be used in a wide range of applications due to the different types that exists and they can
also work from a few watts to megawatts. The fuel cells can be used in three different fields: Combined
heat and power systems and mobile power systems.
Mobile application
The fuel cells give higher power densities than batteries, that’s why is very important to use the fuel cell in
some devices where only few watts are required like palm-top, lap-top computers, mobile phones and
other portable electronic devices.
Also, thanks to reduced emissions of pollutants and for the high efficiency, it’s possible to use the fuel
cells on vehicles. This idea is very important for the urban areas where localised pollution is a major issue.
The good fuel cells for this transport application are the fuel cells which work at lowtemperature because
of the need for short warm-up and cool-down times (PEM fuel cell). The problem is that is very
expensive to fuel with hydrogen, but, In the case of public transport in cities, hydrogen is a realistic fuel
since buses can be refuelled with hydrogen at a central depot. There are some busses running with
hydrogen in some cities; the compressed hydrogen is stored in tanks in the roof, which can give a range of
up to 300 km and a top speed of 80 km/h. However, some companies are working for the fuel cell car.
Different companies like Toyota or Ford selected methanol as fuel for the car but other companies chose
the hydrogen like Daimler-Benz, which build a first car in 1994 fuelled by hydrogen.
17
Combined heat and power applications (CHP)
Here the fuel cells operate from more or less than 1 kW to several MW which covers individual
households and industries providing all the power and hot water from a single system. They offer greater
efficiency than the current situation where electricity is distributed from a small number of centralised
power stations and the heat is supplied by boiler units in each house. Normally the fuel cells used are
which work at high temperature where the heat produced is used to drive a gas turbine to produce more
electricity. It’s possible to use the fuel cells for small-scale, stand-alone and remote applications like gas
pipelines, farms, caravans… [1], [8]
Figure 1.6: Some applications of the fuel cells
1.6 Theory of the fuel cells
1.6.1 Thermodynamically basis
To understand the theory of fuel cells, I think it is necessary to reviewthe knowledge of the
thermodynamic. First I will discuss the first law of thermodynamics, which is also known as the law of
energy conservation, after this, I will see the second lawand the exergy term.
First principle
The first explicit statement of the first law of thermodynamics was given by Rudolf Clausius in 1850:
"There is a state function E, called ‘energy’, whose differential equals the work exchanged with the
surroundings during an adiabatic process."
18
The first lawsays that the total variation of the energy (kinetic, potential or internal) of one system is equal
to the difference between the transferred heat to the system and the work that the system gives. The
mathematical equation is:
1.1
dE = dQ + dW
So, this law says that is impossible to produce work by a cyclic machine without consume the same
quantity on energy.
I can see the energy as three forms, the kinetic energy K, the potential energy V, and the internal energy
U. This last one is due to molecular motions and intermolecular interactions.
E = K +V +U
Normally, in our experiments I consider K and V equal to 0, then:
1.2
dU = dQ + dW
Q is a kind of energy whose can be transferred by conduction, convection or radiation. When I work at
constant pressure, like our case, I can consider the heat like the enthalpy H, a state function.
1.3
H = U + PV
At constant pressure it’s possible to write this:
1.4
dH = dQ = C p dT
Cp is the hat capacity, is the measurable physical quantity that characterizes the amount of heat required to
change a substance's temperature by a given amount.
For a reaction like aA +bB
cC+ dD I can write the enthalpy reaction as:
∆H = c∆H fC + d∆H fD − a∆H fA − b∆H fB
In this equation,
1.5
Hf is the formation enthalpy of the compound at one Temperature. Normally, this
kind of enthalpy is tabulated for 25 ºC as a reference temperature and is the enthalpy reaction for the
formation of one mole of the pure substance at this Temperature.
Also, for a constant pressure, the enthalpy reaction is equal to the heat which gives the reaction. If the
reaction is exothermic then ∆H<0 and for an endothermic reaction H>0. [9]
The work W is very similar to the heat, is not a state function is a way to transfer energy. There are many
types of work like, for example, the expansion-compression work:
dW = pdV
1.6
19
Another kind of work is the electrical work:
dW = − Edq
1.7
It’s possible to obtain some work when a potential difference causes an electrical charge flow. If I divide
by the time I obtain the power.
dw = − E
dq
= − Ei
dt
i is the current.
Second principle
The first theory of the conversion of heat into mechanical work is due to Nicolas Léonard Sadi Carnot in
1824. He was the first to realize correctly that the efficiency of this conversion depends on the difference
of temperature between an engine and its environment. But, the Carnot work doesn’t had much influence
at that moment, but after, in 1850, Rudolph Clausius corrected this work with the help of the first law of
thermodynamic and he was the first to formulate the second lawof thermodynamic in this form:
“Heat does not flow spontaneously from cold to hot bodies”.
After, Kelvin-Planck formulates the second lawby this way:
“Is impossible for any device that operates on a cycle to receive heat from a single reservoir and
produce a net amount of work “
With the second law, we can deduce the performance of a Carnot engine, according to the Carnot’s
principle:
“No engine can be more efficient than a reversible engine operating between the same high
temperature and low temperature reservoirs. All irreversible heat engines between two heat reservoirs
are less efficient than a reversible Carnot engine operating between the same reservoirs”
For deduce the rule it's important to see the figure 1.14:
20
Figure 1.7: The Carnot engine
In this figure, the engine receives heat from the high temperature reservoir and it gives work and a
quantity of heat to a low-temperature reservoir obeying the second law. The efficiency of this engine is:
η Carnot =
Wnet
Qh
1.8
According to the first principle, the energy conserves:
0 = Qh − Ql − Wnet
η Carnot =
T
T − Tl
Wnet Qh − Ql
Ql
=
= 1−
= 1− l = h
Qh
Qh
Qh
Th
Th
1.9
Exergy
As engineer I have to focus on the exergy, the maximum useful work. For example, if we burn a quantity
of combustible, we can get a hot mix of air and combustion products and the energy of the system is
conserved, but, the capacity to produce useful work is now lower than before. So, it’s very interesting to
get a property which quantifies the maximum useful work.
This property is the exergy. The maximum useful work that the system can give us will depend on the
initial state, final state and the type of process. It’s a state function.
The work will be maximum if the process is reversible and if the system is in thermodynamic equilibrium
21
with the environment at the end of the process. At the end of the process, the system will have energy but
it can’t give work, the exergyis 0.
There are always some differences between the real work that the system gives us and the exergy. For the
engineers, the objective is to minimize these differences. The exergy is destroyed by the irreversibilities
and it depends on the system and environment state. [10]
1.6.2 Thermodynamic in fuel cells
In the Carnot engines the thermal energy source comes from the spontaneous reaction between the fuel
and the oxidant. With this, we obtain the thermal energy which can be transforms in work.
But, there is a better way to transform the chemical energy into work deleting the irreversible steps due to
a non-controlled exchange process of electrons between the reactants. It’s possible to control this process
if we change the system of combustion by a fuel cell. So, the fuel cell can be represented as a device where
the fuel and the oxidant participate in a chemical reaction controlled giving the products of the reaction
and current directly.
It’s very important to know the chemical energy produced by the fuel cell. In the case of fuel cell the
chemical energy is the Gibbs free energy which can be defined as the energy available to do external work,
neglecting any work done by changes in pressure or volume. This external work is the movement of the
electrons round an external circuit and the exergy in this case, is all the external work that can be
extracted, including that due to volume and pressure changes.
So, the external work is the electrical work that we saw before in 1.7. It’s possible to demonstrate this if
we consider that the work is the movement of the electrons round an external circuit. For example, if we
use Hydrogen as a fuel, for each molecule of water produced I have 2 electrons which pass round the
external circuit. So, for a mole of hydrogen 2N moles of electrons pass round the circuit and N is the
Avogadro’s number. If e is the charge of one electron, the charge that flows is:
− 2 Ne = −2F
E is the voltage of the fuel cell, so, the electrical work is
dW = − Edq = − nFE
n is the number of electrons produced for each molecule of fuel used. Also:
∆G = dW = −nFE
1.10
For a general gas reaction with A and B as a reactants and C and D as products:
22
A +B
C+ D
The Gibbs free energy can be expressed like this with the standard Gibbs free energy:
1.11
Where P is the partial pressure of the reactants and products of the reaction and
Go is the gibbs
standard energy of the reaction. Now, it’s possible to calculate the ideal standard potential, E.
1.12
This equation is also called the Nerst equation.
Usually, the fuel cell has to burn the fuel to release their energy, so, it’s normal to express the efficiency of
the fuel cell like the comparison between the electrical energy produced and the heat that would be
produced by burning the fuel. The reaction enthalpy H is the heat delivered by the chemical reaction, so,
the efficiency is:
ε=
Welec ∆G − nFE
=
=
∆H
∆H
∆H
1.13
This equation represents the maximum efficiency limit of the fuel cell.
If the cell operates at low temperature, the voltage losses will increase, so, in practice, the fuel cells have
higher performance at high temperature. Also, at high temperature It’s possible to use the waste heat of
the fuel cell.
Therefore the efficiency of a fuel cell should be compared to the theoretical efficiency of a heat engine as
e.g. the internal combustion engine of our cars.
23
Figure 1.8: Comparison of the efficiencyof the fuel cell with other engines
The fuel cells have a better performance than the Carnot engines( Figure 1.8). However, the fuel cells have
some losses too. [1] [4]
1.6.3 The losses of the Fuel cell
It’s possible to knowthe voltage for a fuel cell working at 100 ºC with Hydrogen as a fuel:
E=
∆G f
nF
1.14
Gf is the Gibbs free energy of formation and his value is tabulated for each compound at different
Temperature.
For this fuel cell, the voltage is 1.2 V, however, when the cell works the voltage is less than 1.2 V. If I put
in a graph the Cell voltage in V versus the current density in mA/cm2, it’s possible to see that even the
OCV is less than the theoretical value; there is a rapid fall in voltage. After this, when the current density
increases, the voltage decreases less rapidly and more linearly than before. Nevertheless, it’s possible to
decrease the difference between the theoretical value and the actual operating voltage increasing the
Temperature.
The comparison is shown in the next figure.
24
Figure 1.9: The first graph showthe voltage and the current densityfor a low-Temperature fuel cell and the second for a
fuell workingat 800 ºC
25
Also, the initial fall in voltage is very small and the graph is more linear. That’s because at high
temperature the voltage drop is smaller.
This is due to four major irreversibilities:
•
Activation losses: The reaction takes a long time to take place on the surface of the electrodes.
That’s because a proportion of the voltage generated is lost in driving the chemical reaction that
transfers the electrons to the electrode.
•
Fuel crossover and internal currents: This energy loss takes place because sometimes the waste of fuel
and/or, to a lesser extent, the electrons can pass through the electrolyte.
•
Ohmic losses: These losses are related with resistance to the flow of electrons through the
electrodes and other interconnections and the resistance to the flow of ions through the
electrolyte. This lose is proportional to current density.
•
Mass transport or concentration losses: The concentration of the reactants affects voltage, so, these
losses come from the change in concentration of the reactants at the surface of the electrodes as
the fuel is used. This type of loss has a third name – ‘Nernstian’. This is because of its
connections with concentration, and the effects of concentration are modelled by the Nernst
equation.
1.6.4 The Total efficiency of a fuel cell
Since now, I just talked about the electrical efficiency, but also, and especially for fuel cells which work at
high temperature, it’s important to consider the heat efficiency. So the total efficiency of a fuel cell is the
electrical efficiency plus the heat efficiency. The total efficiency can be showalso as this equation:
η Total =
Qused + power
Q provided ,to , sustem + Energy, content , of , fuel
1.15
Qused is the heat that I can benefit from the output gases of the fuel cell. The system works at constant
pressure so, the heat it’s equal to the enthalpy variation. [1],[4]
26
2 The Solid oxide fuel cell
In this work I will use the solid oxide fuel cell to make my experiments, so, in this chapter I will talk about
the actuality of the SOFC.
2.1 Principle of a solid oxide fuel cell
The mechanism of this fuel cell is different to the generally mechanism. The oxygen is reduced to oxygen
ions which pass through the solid electrolyte to the anode where they reacts with the hydrogen or the
other fuels like carbon monoxide giving, water in the case of the hydrogen, and carbon dioxide in the case
of the carbon monoxide as well as electricity and heat.
Figure 2.1 Mechanismof the solid oxide fuel cell
With this kind of fuel cells, it’s normal to achieve a performance of 80%. The SOFC operates at elevated
temperatures, conventionally between 800-1000 °C, though there is considerable interest in lowering the
operating temperature of smaller SOFCs in particular to reduce costs. The good advantage that the high
temperature gives, it’s that is possible to feed directly the cell with hydrocarbon fuels without the need of a
complex and expensive external fuel reformer. [8] [4]
27
2.2 Constitution
Like the other fuel cells, the SOFC is constituted by an anode, a cathode and a solid oxide electrolyte. The
different components of the fuel cell should have good chemical and physical stability in their
environment, should be chemically compatible with the other components, and should have good
conductivity and similar thermal expansion coefficient to avoid problems like the delamination or cracking
during the fabrication or the operation.
2.2.1 Electrolyte
The function of the electrolyte is to separate the two gas atmospheres and to transport the oxygen-ions
from the cathode to the anode. It should be fully dense to stop short-circuiting of reacting gases and It has
to be resistant to the oxidizing conditions. The transport of the oxygen-ions is due to the vacancies in the
oxygen sub lattice.
Also, the electrolyte has to be as thin as possible to minimize the voltage losses. The thickness of the
electrolyte depends on the cell design and in the mechanical stability of the electrolyte.
In the actuality, the electrolyte used is yttria-stabilised zirconia which has good oxygen ion conductivity,
good stability in reducing and oxidizing atmospheres, a relatively low cost and is abundant and easy to
fabricate.
2.2.2 Anode
Here, the fuel will be oxidized in the presence of oxygen. It’s very important that the anode must not be
oxidized under the operating conditions. The majority of fuel cells have Ni as metal for the anode because
it can works at high Temperature and has a low cost if I compare with other metals which can work at
high temperature like Co or noble metals. Also, the metal must have a porous structure which can resist
the higher temperatures.
This property, can be obtained if the electrode is mixed with electrolyte forming a cermet composite
which maintains the porosity , prevent the sintering of the nickel and gives a thermal expansion coefficient
very similar to the electrolyte.
2.2.3 Cathode
28
Like the anode, the cathode has to be very conductor and also it must have a porous structure but it has to
be stable in an oxidizing atmosphere.
As I said before, the SOFC works at very high Temperature so, this temperature restricts the material
selection in to the noble metals or electronically conducting oxides, but, the noble materials are excluded
for their high prices. Of course, the cathode must have the same thermal expansion coefficient than the
electrolyte and it doesn't have to react with the electrolyte. The most used material for the cathode is the
Strontium-doped lanthanum manganite (LSM) a perovskite material with intrinsic p-type conductivity.
The cathode is produced with a lanthanum deficiency to avoid the disintegration with the formation of
La2O3.
The cathode is made by two layers. Normally, one layer is the mixture between the LSM with the yttriastabilised zirconia to make a cermet like with the anode. This method, improves the thermal match of the
cathode with the electrolyte, the porosity and the resistance to sintering. The second layer is only pure
LSM and is called current collect layer. It’s possible to improve cell performance if I add at the cathode
some quantity of platinum.
The table belowshows the different materials used in the actually of SOFC.
Table 2.1: Materials used in the SOFC
Component
Anode
ca. 1965
Porous pt.
ca. 1975
Ni/ZrO2 cermet
Current Status
Ø Ni/ZrO2 cermet
Ø Deposit slurry, EVD fixed
Ø 12.5 ×10-6 cm/cm oC
Ø
150 µm thickness
Ø 20-40% porosity
Cathode
Porous pt.
La1_xSrxMnO3 mixed with
Ø Doped Ianthanum manganite
electrolyte
Ø Extrusion, sintering
Ø
2 mm thickness
Ø 11 × 10-6 cm/cm oC expansion
from room temperature to
1000oC
Ø 30-40% porosity
Electrolyte
Yttria stabilized
ZrO2
0.5-mm thickness
Yttria stabilized ZrO2
Ø Yttria stabilized ZrO2 (8mol%
Y2O3)
Ø EVD
Ø 10.5 × 10-6 cm/cm oC expansion
from room temperature to
1000oC
29
Ø 30-40-µm thickness
Cell Interconnect Pt
Mn doped cobalt chromite
Ø Doped Ianthanum manganite
Ø Plasma spray 10 x 10-6 cm/cm oC
Ø
100 µm thickness
2.3 Designs of solid oxide fuel cell
There are two types of design, the tubular and the flat plate shapes.
Tubular shape
It was developed by Siemens-Westinghouse and it’s the best design for the SOFC. The thickness of the
cathode, electrolyte and anode layer are around 2 mm, 40 and 100 um respectively. Cells with a length of
150 cm normally have 1036 cm2 of active area! The cells are connected in series by using ceramic LaCrO3interconnection, and in parallel by using Ni-felts. The cells and bundles have been demonstrated for more
than 25 000 h of operation, the power density achieved is around 0,2 W/cm2 and the fuel utilization is
almost 90 % at 1000 ºC.
The major disadvantage in the cell design that limits the performance is the ohmic loss due to the long
current pathways in the thin and porous layers.
The cell is supported bythe cathode mainly.[ 4]
Flat plate shape
The cell consisting of anode, electrolyte and cathode layer is sandwiched between the
interconnection plates. The current flow is perpendicular to the cell surface resulting in short current
pathways in a stack.
The interconnection has two functions, one is to be the electrical contact between the elements and the
other one is to supply fuel gas and air to the anode and the cathode. Here, the cell can be supported by the
cathode or by the electrolyte.
In the figure below, you can see the two types of design.
30
Figure 2.2: The two different shapes of the SOFC
2.4 Comparison of SOFC with other fuel cells
One of the most different parameter between the SOFC and the other fuel cells is the Temperature.
Normally, the SOFC works at 800-1000 ºC and this allows the internal reforming inside the cell, at the
anode, also it’s possible to use the cell for warm the water and the efficiency increases a lot. It’s possible to
see in the figure 2.4 the comparison of the efficiency of SOFC with the efficiency of other cells.
Also the durability of the SOFC is higher than the other cells and is very friendly with the environment. It
has a good fuel flexibility that allows the use of biomass as a fuel. The comparison of the different cells
emissions for the same fuel is shown in the table 3.2
Table 2.2: Comparison of different cells emissions
31
Figure 2.3: Comparison of the efficiencyof different fuel cells
2.5 Applications of SOFC
One of the most important applications is the polygeneration. The term ‘polygeneration’ is most widely
used to describe the generalisation of the idea of ‘cogeneration’ (i.e. the thermodynamically efficient use of
fuel) in the form of systems which simultaneously produce more than one form of energy to the final user,
for example: electricity, heating and cooling
According to [2004/8/EC], “‘cogeneration’ shall mean the simultaneous generation in one process of
thermal energy and electrical and / or mechanical energy”. On the basis of this definition, ‘polygeneration’
would be the simultaneous generation in one process of more than two energy carriers. The European
Commission published in 2004 the directive on promotion of cogeneration [2004/8/EC], with the
purpose to “facilitate the installation and operation of electrical cogeneration plants in order to save
energy and combat climate change.”
The main benefit of polygeneration is to save primary energy sources, e.g. to actively use the ‘waste’ heat
of the power generation process. Figure 2.5 illustrates this fact considering the simple example of
32
cogeneration and showing how much primary energy would be used in single plants to deliver the same
amount of electrical and thermal energy of an equivalent cogeneration plant.
Figure 2.4: Comparison between a CHP systemwith a system with separate generation
Communities are thus choosing polygeneration for two main reasons:
• Optimisation of the use of primary energy sources: one unit of primary energy source can
be used to provide more useful energy than a single plant without polygeneration. In this
way, the energy losses of the single processes are reduced and the energy balance of the
community is greatly improved. In other words, there is no reason to get rid of the heat
generated in a thermoelectric process if biomass has to be burned again to cover the
heating needs.
• Polygeneration can be used to convert local available energy or material resources into
other forms of energy or materials, which ideally should also be used at local level. Primary
energy sources, high temperature heating energy or by-product of other processes can be
used. The community should thus first check if any resources are already available at
community level in order to cover community energy needs, before importing additional
primary energysources.
33
The high flexibility of solid oxide fuel cells (SOFC) on fuels allows it for typically use with a large spam of
fuels and make it easy for the future. SOFC coupled with highly efficient energy transformation, this
characteristic could become an important basis for plants allot to the reduction of greenhouse gas
emissions, and for the widespread use of residential renewable fuels.
Figure 2.5: Polygeneration of the SOFC fuelled bydifferent fuels
Siemenes-Westinghouse installed a test plant of 100 kW in Netherlands which operated for more than 16
000 hours in total. The hot off-gas from the stack is used to preheat the air and produce steam prior to
being supplied to the local district heating system. The system has an electrical efficiency of 43%.
The particular advantage of stationary SOFC systems is that the hot SOFC off-gas can be used in a gas or
steam turbine in a combined cycle.
Recently, SOFC systems for domestic/Small business application have been attracting considerable
interest. The first system currently in the field test phase is the Hexis system developed by Sulzer for a
CHP with a power output of 1 kW. The electrical efficiency is 35%. [11], [12], [13]
Also, since 1998, SOFC systems for mobile applications have been under development. Targeted
applications in the automotive area are auxiliary power units for passenger cars or trucks.
34
2.6 Solid oxide fuel cells + gas turbine
The solid oxide fuel cell appears better suited to power generation in a hydrocarbon fuel economy.
Moreover, due to the synergistic effects, hybrid SOFC and gas turbine technology has been identified to
be a superior power-generation technology to many other options. For a small-scale, electrical powergeneration systems consisting of advanced microturbines and SOFC stacks, with a power rating of 250
kW, an electrical efficiency of 65% can be expected.
Some author did the model of a system with a hybrid SOFC-GT shown in the next figure:
Figure 2.6: The systemof IRSOFC-GT
The system consist in an internal-reforming SOFC stack, a combustor, a GT and a power turbine, two
compressors and tree recuperators. The mechanical work done by the GT is used to run the two
compressors and the sole mechanical power available for electrical power generation derived from the
power turbine. The fuel is preheated thanks to the recuperators 1 and 2 prior to entering in to the
IRSOFC stack and the recuperator 3 warms water. [14]
This system was modelling by the software tool MATLAB and the results are shown in the table 3.3
35
Table 3.3: Results of the modulation of SOFC-GT system
2.7 Low-Temperature solid oxide fuel cell
In the present solid oxide fuel cell (SOFC) R&D, many efforts are contributed to reduce the operation
temperature either by using thin film technology for yttrium stabilized zirconia (YSZ) electrolyte or by
using alternative electrolyte materials, such as various ionic doped ceria and lanthanum gallate electrolytes,
etc. That can make the SOFCs function at lower temperatures.
All these efforts have, however, limitations due to the deficiency of technology and the instability of the
material.
Low temperature operation (300-600 ºC) for SOFCs is realized based on a research breakthrough in
innovative nano- and hybrid conducting ceria-based composite materials. These materials were prepared
usually by mixing different cationic doped ceria, e.g. gadolinium doped ceria (GDC), yttrium doped ceria
(YDC) and samaria doped ceria (SDC) with various salts, e.g. chlorides, fluorides, hydroxides and
carbonates, or by two rare-earth oxides based on the ceria. The materials contain at least two phases, e.g.
the ceria phase and the salt phase, or the ceria phase and other rare-earth oxide phase different from ceria.
High ionic conductivity, varying from 0.01 to 1 S cm-1 between 300 and 700 ºC, has been achieved for the
hybrid and nano-ceriacomposite electrolyte materials, demonstrating a successful application for advanced
low temperature solid oxide fuel cells (LTSOFCs). The performance of 0.15–0.25 W cm-2 was obtained in
temperature region of 320- 400 ºC for the ceria-carbonate composite electrolyte, and of 0.35-0.66 W cm-2
in temperature region of 500-600 ºC for the ceria-lanthanum oxide composites. The cell could even
function at as lowas 200 ºC. The cell has also undergone a life test for several months.
36
The excellent LTSOFC performance is resulted from both functional electrolyte and electrode materials.
The electrolytes are two phase composite materials based on the oxygen ion and proton conducting
phases, or two rare-earth oxides. The electrodes used were based on the same composite material system
having excellent compatibility with the electrolyte. They are highly catalytic and conductive thus creating
the excellent performances at lowtemperatures. These innovative LT materials and LTSOFC technologies
would open the door for wide applications, not onlyfor stationary but also for mobile power sources. [15]
In Kungliga Tekniska Högskolan, Dr. Bin Zhu and his team, use NSDC as electrolyte for the LTSOFC.
The material consist of two phases, one host face of SDC (Ce0.8Sm0.2O1.9) and a second amorphous salt
phase of Na2CO3. The nanocomposite electrolyte shows super ionic conductivity above 300oC, where the
conductivity reaches over 0.1 S cm-1. Compared with the single phase electrolyte (SDC, YSZ…),
composite electrolyte contains lots of interface regions between the two constituent phases. The interface
supplies high conductivity pathway for ionic conduction, which have the capacity to increase mobile ion
concentration than that of the bulk. On the other hand, amorphous Na2CO3 plays an important role in
superionic conductivityof the electrolyte. [26]
It’s possible to obtain a lot of advantages with the LTSOFC:
•
It increases the conductivityof the material even at lowtemperature.
•
The size of the particles also reduces at nano levels.
•
It gives more homogenous and smooth composite structure.
•
It also improves the ceria-carbonate conductivity.
•
Martial handling and making cell is better than the one phase.
2.8 Single Component
Currently, the fuel cells are very expensive to produce. The conventional fuel cell consist in three
components, the cathode, the electrolyte and the anode, but now, Dr. Bin Zhu have achieved a
breakthrough in fuel cell technology by developing a electrolyte-free-single-layer fuel cell (Single
component). The device consists of a homogeneous composite layer of an ionic conductor and a metal
oxide which works as a biocatalyst.
Dr. Zhu, compared two devices. One was a three component fuel cell with LiNiO2-GDC as anode and
GDC as electrolyte and the other one was only a layer of LiNiO2-GDC (Single component). At 550 C, the
three component fuel cell achieved a maximum power density of 408 mW/cm2 and the single component
450 mW/cm2 with hydrogen as a fuel. It is surprising that the single component device displays no short
37
circuit at all and achieves even higher OCV and better performance. The single component device can
perform the same function as fuel cell and convert the chemical energy of H2 to electricity continuously.
at H2 side,
2H2 4H+ + 4e−
at air (O2) side
2O2 + 4e− 2O2−
Overall reaction
2H2+ O2
H2O
During this process, the H2 contacting side acts as a “functional” anode to release electrons by forming
H+, and the air O2 contacting side as a “functional” cathode to receive electrons. All reactions and
processes are completed on the surfaces of the particles in the material through direct combination
between H+ and O2− ions without involving the electrolyte process. At this stage, detailed mechanisms for
the single component device are not yet clear. More works have to be done to investigate the detailed
mechanism of the single component fuel cell.
Figure 2.7: Comparison between conventional three component fuel cell with the single component
38
3 Biomass
3.1 The Biomass and Bioenergy
The biomass is one kind of renewable energy source used for generate electricity or produce heat. It’s the
organic compounds produced from the activities of living creatures which can be used as energy or
material feedstock like the wood, organic waste…
Normally the way to use biomass is burning the biological material, but, it can be used too for the
production of fibers or chemicals (bioproducts). It’s possible to use forest residues (as dead trees,
branches, tree stumps…), agricultural wastes (Animal slurry, Grass stilage, Straw…) or
biodegradable
wastes (as food waste, garbage, landfill gases, industrial waste …) as biomass.
Figure 3.1: Different sources of biomass
39
The most important physical qualities for the biomass are humidity, which has the greatest effect on
heating value, particle size (which influences the treatment of the biofuel), and density. The most
important chemical qualities are the heating values, especially the net calorific heating value as received.
On a dry-weight basis, heating values range from about 17.5 GJ/t (herbaceous masses) to about 20 GJ/t
(woody masses), this is low compared to coal (30-35GJ/t) [23]. Heating value is defined as the amount of
energy released when a fuel is burned completely in a steady-flowprocess and the products are returned to
the state of the reactants.
With the last years, and due to the problems of pollution and the energy cost, the consumption of biomass
has increase.
Figure 3.2: Consumption of biomass in the world
The biomass can be converted by different ways in to bioenergy giving biofuels or producing heat and
electricity. This kind of energy can help us to reduce the amount of CO2 in the atmosphere. Of course,
during the production of energy through the biomass there is a production of CO2 because we burn it,
but, this CO2 can be used after by the source of biomass (Wood, crops….).
A major part of this biomass use is non-commercial and relates to charcoal, wood and manure used for
cooking and space heating, generally by the poorer part of the population in developing countries. Modern
bioenergy use (for industry, power generation, or transport fuels) is making already a significant
contribution, and this share is growing. The waste-energy comes from the municipal waste, manufacturing
40
waste and landfill gas and the alcohol fuels comes primarily from sugarcane and corn and they can be used
directly as a fuel or as an additive gasoline. These alcohols are mostly biodiesel or bioethanol.
In addition to fuels and power, renewable biomass resources can also be used to produce an array of
energy-related products, including chemicals and other materials such as glues, cleaners, solvents, and
plastics.
Figure 3.3: The cycle of the CO2 in the utilization of biomass
Woddy biomasses
One kind of biomass is very important for my work, the woody biomasses. The most important structural
materials in wood are cellulose, hemi cellulose and lignin. Lignin binds the wood’s fibres together and
gives the wood its mechanical strength. In wood there are also small amounts of extractives, for example
resin consists of them. There are lots of vaporizing substances in wood, so its flame is long and needs a
large space for combustion. The dry part in wood is about 99% and carbon, hydrogen and oxygen are the
mostly elements.
41
Table 3.1: Elements in woodyboifuels
The net calorific heating value of the dry matter in wood is usually 18.3-20 MJ/kg and the humidity of live
wood is usually high, 40% – 60%. This makes drying an essential part of producing wood-based biofuels.
It’s possible to get 3 kinds of fuels from the wood: The fuel wood, the charcoal and black liquor.
The fuel wood is the traditional forest product derived from forests and tress. This kind of fuel is very
used in the developing countries as a source of household energy. In Pakistan and the Philippines, for
example, fuel wood covered 58 and 82% respectively.
The Charcoal is the dark grey residue consisting of impure carbon obtained by removing water and other
volatile constituents from animal and vegetation substances.
The black liquor is a by-product from the paper and pulp industry. In North America and Europe, black
liquors are widely used for fuelling the heat and power plants of the large pulp and paper mills. Almost all
of their energy needs are met by black liquors and, in some cases, surplus electricity is sold to the public
grid.
Woody Residues are currently the main sources of bioenergy and this will continue to be the case in the
short to medium term, with dedicated energy forestry/crops playing an increasing role in the longer term.
They are a quite under-exploited potential energy resource, and present many opportunities for better
utilisation. However, there are a number of important factors that need to be addressed when considering
the use of residues for energy. Firstly, there are many other alternative uses, e.g. animal feed, erosion
control, use as animal bedding, use as fertilisers (dung), etc. Secondly, there is the problem of agreeing on
a common methodology for determining what is and what not a recoverable residue is. As woody residues
we have: Forestry residues (by-products of the pulp and paper industry), logging residues (treetops,
branches and refuse logs), whole tress and stemwood, stumps, cutter chips...
42
3.2 SOFC fuelled with biomass
Most experiments in the actuality for the fuel cell fuelled with biofuels use bioethanol as a fuel.
On 2010, Dr. Bin Zhu and his team did, in Kungliga Tekniska Högskolan some experiments with solid
oxide fuel cell fuelled with bio-ethanol and glycerol.
As electrolyte they used nanocomposite based Nano-structured on samarium-doped ceria and a second
phase of Na2CO3. The cathode and the anode was a mixture of carbonates (Li2CO3, CuCO3 (OH)2,
NiCO3) and electrolyte.
The results of the fuel cell test with one cell were: 148 mW/cm2 for the bioethanol and 215 mW/cm2 for
the glycerol with a very lowdeposition of carbon. .[24]
This cell can be used for a polygeneration application at low Temperature thanks to the nanocomposite.
The heat produced by the fuel cell can be used in turbine to produce further power.
This study may be very helpful for the different applications including poly-generation and hybrid system
especially for transport and stationary section, because liquid fuel is very easy to carry; no other fuel cells
have such kind of results with the liquid fuels.
Y. Shiratori used biogas in a fuel cell too. He used A solid oxide fuel cell with Scandia-stabilized-Zirconia
(ScSZ) for the electrolyte, NiO-ScSZ for the anode and (La0.8Sr0.2)0.98MnO3 -ScSZ for the cathode. At the
high operating temperature of 1000 ºC with a cell voltage above 0.9 V and a current density of 200
mA/cm2 was stably obtained over 50 h of performance without carbon deposition. [25]
Another kind of biomass which can be fuelled to the fuel cell is the lignin, however, the study of this
device is very newand there isn’t a lot of information.
In 1984, H.Howard used sulfonated lignin and Kraft lignin as a fuel in a small fuel cell. The fuel cell
consisted of two compartments separated byPyrex fine glass frit impregnated with 2% agar
dissolved in 1M of KCl. The oxidation side of the cell was maintained under N2 while the reduction side
was maintained under pure O2 or air. The oxidation side was maintained at pH 12 with NaOH additions
as necessary and the reduction side was maintained at pH 1. The results were 1 kWh for each 2-3 lb of
sulfonated lignin and 5-8 lb of black liquor. [21]
In 2010, Raquel Bohn used as a fuel for fuel cell, lignosulfonate and Kraft lignin, either directly in their
commercial forms, mixed with active carbon or altering their structures by a pH adjustment to pH 10. The
43
OCV was 0,7 V and the current density was 85 mA/cm2 with the lignin mixed with active carbon and 75
mA/cm2 for the lignin with pH 10. She used a direct carbon fuel cell (DCFC), the only fuel cell capable
of converting solid fuel into electricity without a reformation process with low pollution. The tests was
made at 550 ºC and the oxidation side of the cell was maintained under N2 while the reduction side was
maintained under pure O2 or air.[22]
Figure 3.4: Fuell cell made with lignin byRaquel Bohn
3.3 Lignin
3.3.1 Introduction: What’s lignin?
Cotton and wood are both fibrous plant-materials that have high tensile strengths. Cellulose is a main
functional compound of both, but in spite of this, the properties of the two materials are very
different; cotton is soft, flexible and absorbs water up to 10 times its weight, whereas wood is a stiff
material with lowwater adsorption. What is the chemical background to these differences?
The answer is that wood ,in contrast to cotton, the wood contains large amounts of lignin (15–35 % for
softwoods and around 20 % for hardwoods), a hydrophobic polymer that fills up between the cellulose
micro fibrils and hemicelluloses fixating them towards each other and thus giving the cell wall its “woody”
properties.
Lignin is neither a polysaccharide, a lipid, a protein, nor a nucleotide. It has the most complex structure
among naturally occurring polymers with a mixture of aromatic and aliphatic moieties. It is not a linear
polymer as cellulose, or a branched polymer as the hemicelluloses or pectin but rather a three-dimensional
web, with the monomers (i.e., the building blocks) connected with a number of different ether (C-O-C)and carbon-carbon (C-C) bonds that are randomly distributed, and the first impression of the covalent
structure may appear chaotic
44
Figure 3.5: Lignin’s structure
The structure and properties of lignin are of great interest for the pulp and paper industry, since the
chemical pulping and bleaching of pulp are mainly based on chemical reactions on lignin and how the
lignin released during chemical pulping. Lignins are polymerized from mainly three monomers called
monolignols, namely, p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Monolignol-ratios for
different lignins are shown in Table 3.2.
Figure 3.6: The monomers formingthe lignin structure
45
Table 3.2: Compositions of monolignons in different plants
The lignin has four biological functions:
•
Lignin gives stiffness to the cell walls: The composite nature of wood with its mixture of
cellulose, hemicellulose and lignin, makes the fibers relatively stiff and rigid
•
Lignin glues different cells together in woody tissues
•
Lignin makes the cell wall hydrophobic: This is a prerequisite for the development of cells for
efficient water- and nutrition transport
•
Lignin is a protection against microbial degradation of wood
Lignin does not have a regular structure like cellulose, but is a physically and chemically
heterogeneous material, although the exact chemical structure is unknown. [16]
3.3.2 History of Lignin
The history of lignin science stretches over a period of one hundred years and many scientists had made
efforts effectively to exclude the lignin from wood in order to extract cellulose in the pulping process. The
idea of the utilisation of the wood feedstock was exploited already in 1898 by Simonsen in his efforts to
convert sawdust to ethanol. Lignin has been considered as an unwelcome by-product and attempts have
been made to cultivate wood species having a small amount of lignin content using newbiotechnology.
The physical chemical nature of lignin as a representative biopolymer has been neglected for a long time.
Goring first shed light on the polymeric nature of lignin in early 1960. He measured the intrinsic viscosity
of lignins and concluded that conformation of lignins is between an Einstein sphere and a non-free
draining random coil in a solvent. He also showed that the lignosulfonate is in the form of spherical
particles of a wide range of sizes according to the electron micrograph of high molecular weight fraction
of sodium lignosulfonate (LS). He also reported that the softening temperature (Ts) of lignins varied from
127 to 193ºC in dry state and from 72 to 128ºC in moist lignins.
46
However, for practical approaches of effective utilization of lignins, it is important to pay attention to
industrial lignins such as by-products of pulping processes and bio-ethanol production
processes.
In
pulping processes, Kraft and sulfite pulping methods are usually used. In bio-ethanol production
processes, acid hydrolysis is commonly used. [17]
3.3.3 Lignin as a bio-resource
Lignin is one of the most important bio-resources for the rawmaterial of the synthesis of environmentally
compatible polymers. Lignins are derived from renewable resources such as trees, grasses, and agricultural
crops.
Production of lignin as a by-product of pulping process in the world is over 30 million tons per year.
However, it should be noted that this value is only an estimate, since there are no reliable statistics on
lignin production because it is mostly burnt as a fuel immediately after production. Commercial lignins are
obtained as by-products of the pulping or bio-ethanol industries.
Figure 3.7: Total utilization for bio-resources fromwood
In direct analogyto a petroleum refinery, which produces fuels and chemicals from crude oil, a
biorefinery is a facility that produces multiple products, including fuel, power, and bulk or fine
chemicals, from biomass. In addition, valorization of all components of lignocellulosic biomass is essential
for an economically viable biorefinery. Since lignin is a principle component of biomass, the biorefinery
receives and processes enormous quantities of lignin, and conversion of this component also to fuels and
chemicals is imperative for economic profitability. [18]
47
Figure 3.8: Utilization of the lignin as a bio-resource
3.3.4 Lignosulfonate
Lignosulfonates are by-products of the sulphite pulping process for the manufacture of specialty
dissolving pulps and paper. The wood chips are digested with acidic calcium bisulphite for 6-10 hours at
100-300 ºC in a batch-wise cooking process. During this process, the native lignin is broken down through
the degradation of the randomly distributed ether bonds throughout the
structure. The fragments are
solubilised in the cooking liquor through covalent addition of sulfonic acid groups at various positions in
the lignin structure. The sulphonic acid groups are stabilised by the presence of calcium ions.
After the competition of the cooking stage, the insoluble cellulose is separated from the solubilized lignin
by filtration. Further processing of the cooking liquor involves precipitation of the sulfonated
lignin
through the addition of excess calcium hydroxide, the evaporation of water and residual sulfite (in the
form of sulfur dioxide) and dilution with fresh water followed by ultrafiltration to remove low-molecularweight fractions and sugar monomers. The purified calcium lignosulfonate is a pH-adjusted to a specific
pH and evaporated to a drymatter content suitable for spray drying and packaging. [19]; [20]
48
Figure 3.9: The sulfite process
(The following structures do not specify the structure since lignin and its derivatives are complex mixtures: the purpose is to
give a general idea of the structure of llignosulfonates.)
Figure 3.10: Theoretical modification of lignin in the sulphate process
The overall structure of a lignosulfonate is not known. Several models have been proposed over the years
suggesting that lignosulfonate behaves as a coiled or expanded polyelectrolyte at either high or low
concentration.
49
Figure 3.11: Structure suggested for the lignosulfonate
The by far largest utilization of lignosulfonates worldwide which accounts for as much as 90% of the
worldwide production is as concrete admixtures and for energy production during the pulping process.
50
4 Experimental part
4.1 The Fuel cell
In this work, a low temperature solid oxide fuel cell (LTSOFC) was assembled by using Ce0.8Sm0.
O2− (SDC)–Na2CO3 (NSDC) nanocomposite as ionic conductor and LiNiCuZnO mixed with NSDC as
electronic conductor.
4.1.1 Preparation of the electrolyte
The NSDC was prepared by one-step co-precipitation process. Stoichiometric amounts of
Ce(NO3)3·6H2O and Sm(NO3)3·6H2O were mixed and dissolved in de-ionic water to form a 0.5M
solution. The solution was stirred at 80 oC for 30 minutes. Then a Na2CO3 solution of 0.5M was used as a
precipitation agent. It was added into the solution with a molar ratio of
(Ce3+ + Sm3+ ): CO32− = 1:2 under vigorous stirring to form a white precipitation. The precipitate was
then filtrated and dried in an oven at 120
C for 12 h. Finally, the NSDC precursor was sintered in a
furnace at 800 oC for 4 h. The resulting material was ground completely to obtain homogenous NSDC
nanocomposite powders.
4.1.2 Preparation of the electrode
The electrode was prepared mixing 1,5g of Li2CO3 (≥98%, Sigma–Aldrich, USA), 7 g of Ni(NO3)2·6H2O
(≥98.5%, Sigma–Aldrich, USA), 2 g of Cu(NO3)2·3H2O (99–104%, Sigma–Aldrich, USA) and 7g of
Zn(NO3)2·6H2O (98%, Sigma–Aldrich, USA). The mixture was put in the furnace at 800 oC during 4 h.
The resulting material was ground completely to obtain homogenous LiNiCUZnO powder.
4.2 Treated lignin as a fuel
In order to achieve a better performance and conductivity of the fuel cell, the lignosulphonate was treated
with some hydroxides/chlorides. Then the treated lignin was sintered at different temperatures (200 -500
oC)
in the laboratory furnace. The detailed of the treated lignin for my experiments was described as
below:
51
4.2.1 Treatment with Iron chloride
It has been demonstrated that the doped sulfur lignin has the properties of a semi-conductive polymer.
The conductivity of the sulfonated lignin is 1,2×10-10 S/cm, but after doping with FeCl3, its conductivity
increased as 0.006 S/cm. [27]
Using the same idea, I used 5ml solution of Iron chloride (FeCl3) and mixed with 1 g of lignin in a crucible
and covered with aluminum foil. Then the sample was heated in the furnace for one hour at different
Temperatures (200 -500 ºC). The obtained dried powder/material was ground completely.
4.2.2 Treatment with hydroxides
In order to compare the results of different treated lignin, the lignin was treated with hydroxides too. The
0,4 g of NaOH was mixed in the 100 ml of de-ionized water to get a solution. The 8g of lignin was used in
a 80ml solution of the hydroxide. The sample was dried at 150 ºC for 24 hour. After that, this dreid
powder was divided in 4 parts and sintered each part at different temperatures (200, 300, 400, 500oC)
using the aluminium coverd crucible to avoid the the oxidation.To get the homogenity of the sample, the
resultant powder was ground in a morter.
4.3 Lignin Fuel cell Fabrication
Two kinds of fuel cell was made for measurements using treated lignin.
i.
three component fuel cell
ii. single component fuel cell
4.3.1 Three component fuel cell
The pure lignosulfonate was tested with a conventional three component fuel cell (Anode-electrolyteCathode). The electrolyte used on the cell was SDC. The Anode was a mixture of lignosulphonate with
LiNiCuZnO and SDC with the same volume ratio, the cathode was a mixture of LiNiCuZnO with SDC
also with the same volume ratio.
For the treated lignin, a conventional fuel cell was fabricated with three layers: Anode-Electrolyte-Cathode
using a layer of lignin which acts as a fuel layer.
The electrolyte used in the cell was pure NSDC and the layer which acts as fuel was a mixture of lignin
treated with pure sulphonated lignin, electrolyte and electrode with a volume ratio of Lignin treated: pure
lignin: electrode: electrolyte = 2:2:1:1
The layer of Anode and Cathode was the same, a mixture of electrode and electrolyte with the same
volume ratio. In each case, the Anode side was covered with Nickel foam and the cathode side was
painted paste with silver, both used as a current collector. To build the cell with lignin sintered at 400 oC
and 500 oC, I had to mix it before with binder to have a good mechanical strength.
52
The diameter of the cell was 1.3 cm, the thickness was 0.15 cm and the active area was 0.64 cm2. The
schematic structure of the prepared cell is shown in figure 4.1
Figure 4.1: Three component fuel cell with lignin treated
4.3.2 Single component fuel cell
I also tried to use lignin in a single component fuel cell which is recently introduced by our group. The
Single component was a mixture of treated lignin with pure sulphonated lignin, electrode and electrolyte.
The volume ratio of the single layer was:
Lignin treated: Pure lignin: electrode: electrolyte = 2:2:1:1
One side of the cell was cover with Nickel foam and the other side was pasted with silver, both used as a
current collector. To build the cell with lignin sintered at 400 and 500 oC, I had to mix it before with
binder to have a good mechanical strength.
The diameter of the cell was 1.3cm and thickness 0.1cm and using the active area 0.64 cm2. The
schematic structure of the single component cell is shown in figure 4.2
Figure 4.2: Single component fuel cell with lignin treated
4.3.3 Dry press method
Dry power press method was used for the preparation of cell pellet. With this method it was possible to
attach the different layers of the fuel cell from the powders of each used materials. The pressing power
require for the manufacturing of fuel cell is 25 Mpa. Before testing, the cell I painted the cathode side with
silver paste to improve the electrical contact.
53
Figure 4.3: Machine used for the drypress method
4.4 Fuel cell test
Testing device consist of two u-shaped pipes. The fuel cell was fixed in the device, and one of the two
pipes was used for the transportation of air to the cathode side. Here, the fuel is attached in the anode
side.
Figure 4.4: Device used for the fuel cell test
For the test, the device was put in the furnace and heats it up to require temperature. The fuel cell was
measured at 550 ºC under a variable resistance load, which adjusts the outputs of cell voltage and power.
The values of the voltage and current were recorded by using the fuel cell testing unit (L43-China). The
54
test takes almost 15 minutes. After collecting data of the cell voltage and current under each resistance
load, current density-V (voltage) or current density–P (power density) curve was drawn from the
calculation results using the collected data.
Figure 4.5: Furnace used for the fuel cell test with and without the device
4.5 X-ray diffraction test
In order to know more about the structure of lignosulfonate and the treated lignins I did and X-ray
diffraction test in the chemical department of Kungliga Tekniska Högskolan.
Powder X-ray diffraction (XRD) patterns of the samples were collected on a Philips X-ray Diffractometer
with Cu K
radiation ( =1.5418 Å) for phase analysis. With this technique, it’s possible to see if the
lignin presents some crystal structure or not.
4.6 Conductivity measurements
To know more about the lignin, the conductivity of the lignin was measured with the VersaSTAT
potentiostat machine connected to a computer. For this, I made a pellet of pure lignin only by the dry
press method. To build the pellet with lignin sintered at 400oC and 500oC, I had to mix it before with
binder to have a good mechanical strength. Each pellet was painted in both side with silver paste, which
plays the role of current collector, and the pellet was put in the cylindrical furnace. I measured the
conductivity with the help of the machine and I recorded the impedance results in a computer thank’s to
the VersaStudio Software. The measurements were made it at 500 and 550 oC due to that the resistance of
the lignin at lowtemperature was very high.
55
5 Results and discussion
5.1 Fuel cells performance
5.1.1 Lignosulfonate (Three component)
The fuel cell test was made with a fuel cell with 3 layers. The anode was a mixture of lignosulfonated with
LiNiCuZnO and SDC and the cathode was a mixture of electrode and electrolyte at 50 % of volume. Also
Ni foam was put on the anode side of the cell and I painted with silver painted the cathode side to
enhance the conductivity. The Open circuit voltage (OCV) was 0,09 V at 550oC, the maximum current
density at these temperatures was 11 mA/cm2 with an area of 0,64 cm2.
The results were very bad, that’s why I decided to do some treatments to the lignin.
5.1.2 Lignin treated with hydroxides (Three component)
The fuel cell test was made with a fuel cell with 4 layers. One of the layers is a mixture of lignin treated
with hydroxides, pure lignin, NSDC (electrolyte) and LiNiCuZn (electrode) with this ratio: 2;2;1;1. This
layer is attached to a three component fuel cell, I mean, a conventional fuel cell constituted of Anode,
electrolyte and Cathode. The anode and the cathode have the same constitution, a mixture of electrode
and electrolyte at 50 % of volume. Also I put Ni foam on the anode side of the cell and I painted with
silver painted the cathode side to enhance the conductivity.
I tested the lignin sintered at 4 different temperatures (200, 300, 400 and 500 ºC). The results and the
operating conditions are shown in the table below:
Table 5.1: Performance and operatingconditions of the cells
Cell A
Cell B
Cell C
Cell D
Sintering temperature
(oC)
200
300
400
500
Operating temperature
(oC)
521
546
542
480
OCV
(V)
0,23
0,2
0,074
0,42
Current density
(mA/cm2)
78,12
46,87
67,18
132,81
Power density
(mW/cm2)
4,21
2,53
1,32
8,2
56
0,25
4,5
4
3,5
3
2,5
2
1,5
1
0,5
0
Voltage (V)
0,2
0,15
0,1
0,05
0
0
20
40
60
Power density
(mW/cm^2)
Cell A
80
Current density (mA/cm^2)
Figure 5.1: Performance of Cell A
Cell B
3
0,25
Voltage (V)
2
0,15
1,5
0,1
1
0,05
Power density
(mW/cm^2)
2,5
0,2
0,5
0
0
0
10
20
30
40
50
Current density (mA/cm^2)
Figure 5.2: Performance of Cell B
0,08
0,07
1,4
0,06
0,05
1
1,2
0,8
0,04
0,03
0,02
0,6
0,4
Power density
(mW/cm^2)
Voltage (V)
Cell C
0,2
0,01
0
0
0
10
20
30
40
50
60
70
Current density (mA/cm^2)
Figure 5.3: Performance of Cell C
57
Voltage (V)
0,4
0,35
0,3
0,25
0,2
0,15
0,1
0,05
0
0
20
40
60
80
100
120
9
8
7
6
5
4
3
2
1
0
140
Power density
(mW/cm^2)
Cell D
Current density (mA/cm^2)
Figure 5.4: Performance of Cell D
In this case, this kind of lignin gives better results than the pure lignin. Especially the lignin sintered at 500
oC due to
the carbon layer made by the sintering.
5.1.3 Lignin treated with Iron chloride (Three components)
The fuel cell test was made with a fuel cell with 4 layers. One of the layers is a mixture of lignin treated
with hydroxides, pure lignin, NSDC (electrolyte) and LiNiCuZn (electrode) with this ratio: 2;2;1;1. This
layer is attached to a three component fuel cell, I mean, a conventional fuel cell constituted of Anode,
electrolyte and Cathode. The anode and the cathode have the same constitution, a mixture of electrode
and electrolyte at 50 % of volume. Also I put Ni foam on the anode side of the cell and I painted with
silver painted the cathode side to enhance the conductivity.
I tested the lignin sintered at 4 different temperatures (200, 300, 400 and 500 ºC). The results and the
operating conditions are shown in the table below:
Table 5.2: Performance and operatingconditions of the cells
Cell A
Cell B
Cell C
Cell D
Sintering temperature
(oC)
200
300
400
500
Operating temperature
(oC)
415
415
480
500
OCV
(V)
0,126
0,195
0,08
0,183
Current density
(mA/cm2)
59,37
46,87
0
48,43
Power density
(mW/cm2)
1,8
2,53
0
1,42
58
Cell A
0,14
2
1,5
0,1
0,08
1
0,06
0,04
0,5
Power density
(mW/cm^2)
Voltage (V)
0,12
0,02
0
0
0
10
20
30
40
50
60
Current density (mA/cm^2)
Figure 5.5: Performance of Cell A
Cell B
3
0,25
Voltage (V)
2
0,15
1,5
0,1
1
0,05
Power density
(mW/cm^2)
2,5
0,2
0,5
0
0
0
10
20
30
40
50
Current density (mA/cm^2)
Figure 5.6: Performance of Cell B
0,2
1,6
1,4
0,15
1,2
1
0,8
0,6
0,1
0,4
0,2
0
0,05
0
0
10
20
30
40
Power density
(mW/cm^2)
Voltage (V)
Cell D
50
Current density (mA/cm^2)
Figure 5.7: Performance of Cell D
Now, this kind of lignin gives better results than the pure lignin but worse than the lignin treated with
hydroxides. Here, the lignins sintered at 200 ºC and 300 ºC give the best performance.
59
Table 5.3: Performance and operatingconditions of the cells
Operating temperature
(oC)
520
518
535
545
OCV
(V)
0,484
0,48
0,35
0,345
Current density
(mA/cm2)
82,81
81,25
73,43
156,25
Power density
(mW/cm2)
16,15
8,59
6,73
8,73
5.1.4 Lignin treated with hydroxides (Single component)
In this case, the fuel cell test was made with a Single component. This cell was made it only with one layer
constituted by a mixture of lignin treated with hydroxides, pure lignin, electrolyte (NSDC) and electrode
(LiNiCuZn). Also, in on side of the cell I put Nickel foam and in the other side I painted with Silver paste.
I treated the lignin sintered at 4 different temperatures (200, 300, 400 and 500 ºC). The results and the
operating conditions are shown in the table below:
0,6
18
16
14
12
10
8
6
4
2
0
0,5
0,4
0,3
0,2
0,1
0
0
20
40
60
Power density
(mW/cm^2)
Cell A
Voltage (V)
Cell A
Cell B
Cell C
Cell D
Sintering temperature
(oC)
200
300
400
500
80
Current density (mA/cm^2)
Figure 5.8: Performance of Cell A
60
0,6
10
0,5
8
0,4
Power density
(mW/cm^2)
Voltage (V)
Cell B
6
0,3
4
0,2
2
0,1
0
0
0
20
40
60
80
Current density (mA/cm^2)
Figure 5.9: Performance of Cell B
0,4
0,35
8
7
0,3
0,25
0,2
0,15
6
5
4
3
0,1
0,05
0
2
1
0
0
20
40
60
Power density
(mW/cm^2)
Voltage (V)
Cell C
80
Current density (mA/cm^2)
Figure 5.10: Performance of Cell C
Cell D
10
8
0,3
0,25
0,2
0,15
6
4
0,1
0,05
0
Power density
(mW/cm^2)
Voltage (V)
0,4
0,35
2
0
0
50
100
150
Current density (mA/cm^2)
Figure 5.11: Performance of Cell D
With the single component device I get the best results, especially with the lignins sintered at 500oC due to
the carbon layer.
The OCV in this case is higher than the others experiments. This can be due to the ohmic losses. With a
device with three components the ohmic losses can be higher than this cell where we only have one
component.
61
5.2 XRD results
Pure active carbon
HT 500
HT 400
Intensity (a.u.)
HT 300
HT 200
HT 100
Pure lignin
75-1621 C
27-0711 NaOH
21-0645 KOH
01-1038 KNaCO3
10
20
30
40
50
60
70
80
2θ (degree)
Figure 5.12: XRD results of lignin treated with hydroxides
In the figure 5.12 are shown the results of the XRD made of the 5 kinds of lignin treated with
hydroxides, the lignosulfonate (pure lignin) and the active carbon.
The result of the pure lignin can confirm that is not an inorganic compound, is an organic compound,
because it has an amorphous structure. Also, all the results have one thing in common, the peak of the
carbon structure at 24 degree.
It’s normal too to see the differences between the pure lignin and the other lignin. The lignins sintered at
100, 200 and 300 ºC present peaks at 31, 38 and 48 degree of NaOH and at peak at 32 degree of KOH.
Of course this is very normal because we treated before with KOH and NaOH.
I noticed that the peaks disappear when the lignin is sintered at 400 or 500ºC. However, newpeaks appear
after this at 29, at 32, at 34, and 57 due to the formation of KNaCO3 at 400 and 500ºC by this way:
62
2NaOH + CO2
Na2CO3 + H2
The KNaCO3 is present as a crystal structure, but maybe, it can stay as an amorphous structure too. This
could explain why, with the lignin sintered at 500oC we have a good performance. An amorphous
structure of KNaCO3 can increase the ionic conductivity because it can produce an interphase between
the electrolyte and the KNaCO3. [26]
Also, if I see the results from 64 to 80 degree I can note that the form of the curve of lignin is similar to
the pure active carbon. That’s because the lignin sintered at high temperature had covered with a layer of
carbon. We can think that also this layer of carbon gives good values of OCV and current because the
lignin sintered at 500 ºC gives the best performance.
Pure active carbon
HT 500
HT 400
Intensity (a.u.)
HT 300
HT 200
HT 100
Pure lignin
75-1621 C
74-1658 FeCl3
24-1005 FeClO
33-0664 Fe 2O3
10
20
30
40
50
60
70
80
2θ (degree)
Figure 5.13: XRD results of lignin treated with Iron chloride
The results of the XRD of the lignin treated with Iron chloride are shown in the figure 5.13. If I compare
63
this kind of lignin with the other one I can see that the structure of this lignin is not so amorphous like
the other one. This is due to the structure of the Iron chloride.
I can see with the peaks at 33 and 55 degree that the lignin sintered at 400 and 500 ºC presents some
traces of Fe2O3 and FeClO. Maybe, I get this with this reaction:
FeCl3 + H2O
Fe(OH)3
2Fe(OH)3 + FeCl3
Fe2O3
3FeClO + 3H2O
Maybe, that’s why the lignin sintered at 200 ºChas the current density, because they have a big amount of
FeCl3 in comparison with the others lignin and this enhanced the conductivity.
5.3 Conductivity Measurements
In the next table I showthe results of the conductivity tests of lignin treated with Iron chloride:
Table 5.4: Conductivityresults of lignin treated with Iron chloride
Resistance in
Temperature in ºC
Conductivity( S/m)
Thickness
Type of lignin
500
530
500
530
1,5
100 F
5
0,17
4,6875
137,8676
1,5
200 F
545
500
545
500
31,3
60
1
300F
500
545
900
211
0,8
400F
500
550
310
90
1
500F
510
560
127
33
0,748802 0,390625
500
545
0,017361 0,074052
500
550
0,040323 0,138889
510
560
0,123031 0,473485
The lignin sintered at 100 ºC and treated with Iron chlorides gives the best conductivity. I think the reason
is that this lignin has a good amount of Iron chloride and no other compounds like Fe2O3 or FeClO and
also this Iron chloride gives a good conductivity to the lignin. Also, we have good values with the lignin
sintered at 200 ºC, for the same reasons, and at 500 ºC. At 500oC, the lignin doesn’t have much Iron
chloride like the others lignins, but, with the sintering, this lignin has obtained a layer of pure active
carbon which gives a good conductivity.
64
So, the reason that I have the best current density with the lignin sintered at 200 ºC (as you can see above
in the results of the fuel cell tests) can be the presence of the Iron chloride.
The treatment was successful because the conductivity is higher than the pure lignin.
In the next table I showthe results of the conductivity tests of lignin treated with hydroxides:
Table 5.5: Conductivityresults of lignin treated with Hydroxides
Resistance in
Temperature in ºC
Thickness Type of lignin
1,5
100 OH
1
200 OH
0,5
300 OH
1
400 OH
1
500 OH
Conductivity( S/m)
500
550
500
550
3900
1260
0,00601
0,018601
500
550
500
550
970
370
0,016108
0,04223
500
550
500
550
2600
240
500
550
500
550
1400
500
0,011161
0,03125
500
550
500
550
1050
250
0,014881
0,0625
0,003005 0,032552
I can see that the lignin which has the best conductivity is the lignin sintered at 500oC. This should be
normal because as is shown in the XRD results this kind of lignin has a layer of activated carbon, and also
maybe it has KNaCO3 as amorphous structure, which gives good values of conductivity. The worse value
of conductivity is given by the lignin sintered at 100oC. This is because, at this sinter temperature, the
lignin didn’t made the carbon layer like the lignin sintered at high Temperature.
If I compare the conductivity results with the results of the fuel cell test it’s possible to see that they are
related. I obtained good results of current density with the lignin sintered at 500oC to both three
components and single component.
Here the results were successful too. With the treatment the lignin has more conductivity than before.
If we compare the two treatments, the lignin treated with hydroxides gives better results than the lignin
treated with Iron chloride. This can be due to the relation between the lignin treated with Iron chloride
and the mixture of electrolyte and electrode, or, maybe the problem is that there are some interactions
between the iron chloride and the carbon which is the fuel. This hypothesis can be solving with an IR test
to see the functional groups of the lignin treated with Iron chloride.
65
Also, the lignin treated with Iron chloride which has a pH lower than the lignin treated with hydroxides,
so, is more hydrophilic. The hydrophilicity is a critical factor for the fuel cell performance. If one
compound is more hydrophilic than other, the performance of the cell will be worse because the
electrolyte of the SOFC works bad with the presence of water. This can be a reason of the good results
gave by the experiments with the lignin treated with hydroxides.
5.4 Fuel cell efficiency
I will calculate the electrical efficiency of the cell with the higher value of current, the cell D of the single
component (E= 0,304 V ; I= 53,18 mA/cm2 ; P= 16,15 mW/cm2). To calculate the efficiency, we have to
followthe system of the photo. I don’t knowexactly if the fuel used in the fuel cell is mostly carbon, but I
assume that is true. Is necessary to do some studies in the future to see if this model is true.
Electrical efficiency
Anode reaction:
C + 2O2-
O2 + 4e-
Cathode reaction:
Overall reaction:
C + O2
The electrical efficiency is:
Thermal Heat =
CO2 + 4e-
ε=
2O2CO2
− nFE
∆H
H = ∑ H Produced −∑ H Reaction
66
H O2 = ∆H fO2 + C p (T2 − T1 ) = 0 + 29,35 × (550 − 25) = 15408
J
mol
H CO2 = ∆H fCO2 + C p (T2 − T1 ) = −393509 + 37,11× (550 − 25) = −374081
H C = ∆H fC + C p (T2 − T1 ) = 0 + 8,527 × (550 − 25) = 4476,6
J
mol
J
mol
∆H = H CO2 − H O2 − H C = −374081 − 15408 − 4476,6 = −393965,6
J
mol
The electrical efficiency is:
ε=
− nFE − 4 × 96485 × 0,304
× 100 = 30%
=
− 393965,6
∆H
It couldn’t be possible to calculate the heat efficiency of the fuel cell because we need to know a lot of
things about the lignin to continue. I don’t knowexactly which the products of the lignin are after burning
at 550 oC and the heat values of the lignosulfonated. Of course, this has to be done in the future to
continue the studies.
67
6 Conclusions
It has been demonstrated with this project that the solid biomass can be used as a fuel for the Solid Oxide
Fuel Cell for polygeneration applications (Combined heat and power generation) using nanocomposites
materials to work at low Temperature. This is very important for the waste management of lignin in the
paper manufacturing.
The treatments made to the lignin were very successful. The conductivity and fuel cell performance in
each case increase if we compare with the pure lignin.
Also, with this kind of fuel cell is possible to use a renewable energy to reduce the amount of CO2 in the
atmosphere with an electrical efficiency of 30 % and a power density of 16 mW/cm2. The best way to use
it is doing a treatment with hydroxides before.
It’s also possible to use the lignin with the newSingle component fuel cell with a very good performance.
However, the study doesn’t finish here. The electrical efficiency or the power density of the fuel cell can
be improved if we change some parameters in the experiments like, for example, the quantity of FeCl3 in
the treatment, the pH of the lignin, the ratio in the fuel layer or in the electrolyte…
Also is very important to know more about the behaviour and the structure of lignin in order to improve
the results and calculate the thermal efficiency. We should do some IR, SEM and TEM test to study the
structure of the pure lignin and the treated lignin. To do some experiments in order to know all the
important products in the combustion of lignin and it heat values.
This project makes the LTSOFC more applicable in different fields as compared to others fuel cell.
68
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