NEXT GENERATION CELLULOSIC ETHANOL

NEXT GENERATION CELLULOSIC ETHANOL
NEXT
GENERATION
TECHNOLOGIES
AND
CELLULOSIC
THEIR
ETHANOL
CONTRIBUTION
TO
A
SUSTAINABLE AFRICA
W.H. van Zyl1* • A.F.A. Chimpangho2 • R. den Haan1 • J.F. Görgens2 • P.W.C. Chirwa3
Departments of
1
Microbiology and
2
Process Engineering, University of Stellenbosch,
Stellenbosch, 7600, South Africa; 3Forest Science Postgraduate Programme, University of
Pretoria, Pretoria, 0002, South Africa
Abstract.
The world is currently heavily dependent on oil, especially in the transport sector. However,
rising oil prices, concern about environmental impact, and supply instability are among the
factors that have lead to greater interest in renewable fuel and green chemistry alternatives.
Lignocellulose is the only foreseeable renewable feedstock for sustainable production of
transport fuels. The main technological impediment to more widespread utilization of
lignocellulose for production of fuels and chemicals in the past has been the lack of low-cost
technologies to overcome the recalcitrance of its structure.
Both biological and
thermochemical second generation conversion technologies are currently coming online for
the commercial production of cellulosic ethanol concomitantly with heat and electricity
production. The latest advances in biological conversion of lignocellulosics to ethanol with a
focus on consolidated bioprocessing (CBP) are highlighted.
Furthermore, integration of
cellulosic ethanol production into existing bio-based industries also utilizing thermochemical
processes to optimize energy balances is discussed. Biofuels have played a pivotal yet suboptimal role in supplementing Africa energy requirements in the past. Capitalizing on SubSaharan Africa’s total biomass potential and utilizing second generation technologies merits a
fresh look at the potential role of bioethanol production towards developing a sustainable
Africa while addressing food security, human needs and local wealth creation.
Keywords
Cellulosic ethanol; consolidated bioprocessing; integrating bio-based industries; sustainable
biofuels in Africa
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1.
INTRODUCTION
The world is currently heavily dependent (97%) on oil, especially in the transport sector (IEA
2008). Rising oil prices, concern about environmental impact, and supply instability are
among the factors that have lead to greater interest in renewable fuel and green chemistry
alternatives. Renewable bioenergy, particularly biofuels, have played a pivotal role in Africa
in the past and could help address the need for energy expansion in the future (Amigun et al.
2008). It is estimated that 52% of the developing world and close to 80% of African countries
rely on this traditional system to meet their energy needs (Cotula et al. 2008). Smeets et al.
(2007) projected that, depending on the level of advancement of agricultural technology,
Africa has the largest potential for bioenergy production by 2050 in the world, namely 317 EJ
per annum. This could constitute a quarter of the projected total world potential of 1,272 EJ
per annum.
Biofuels should ideally retain the advantages of fossil fuels with regards to being relatively
cheap and rich in energy and should in addition provide a net energy gain, have
environmental benefits and be producible in large quantities without impacting on food
supplies (Hill et al. 2006).
Plant biomass is therefore the only foreseeable renewable
feedstock for sustainable production of renewable transport fuels. Lignocellulose is globally
recognised as the preferred biomass for the production of a variety of fuels and chemicals that
may result in the creation of a sustainable chemicals and fuels industry, with significant
benefits in agricultural development. Lignocellulose represents the most wide-spread and
abundant source of carbon in nature and is the only source that could provide a sufficient
amount of feedstock to satisfy the world’s energy and chemicals needs in a renewable manner
(Marrison & Larson 1996; Lynd et al. 2003). The main technological impediment to more
widespread utilization of lignocellulose for production of fuels and chemicals is the lack of
low-cost technologies to overcome the recalcitrance of its structure (Van Zyl et al. 2007).
Producing biofuels such as ethanol from cellulosic plant material has the potential to meet
capacity requirements without impacting directly on food production (Dale et al. 2010).
Capitalizing on Sub-Saharan Africa’s biomass potential and bringing back the focus on
agriculture merits a fresh look at the bioenergy potential of Africa. For Africa to realize its
potential for bioenergy production, advanced agricultural technologies and practices must be
employed in a sustainable way to serve the needs of rural and urban communities, foster
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development of the industrial sector, reduce greenhouse gas emissions, develop agricultural
infrastructure and lead to land restoration and ecologically healthy landscapes.
Due to the wide range of commercial opportunities for 2nd generation biofuels production,
in particular the opportunity for integration of production of such biofuels with existing
biomass, fermentation and energy-production industries, both biological and thermochemical
conversion processes (summarized in Figure 1) should be considered (Aden & Foust 2009;
Swanson et al. 2010). Although biochemical and thermal processes for lignocellulose
conversion have comparable efficiencies and economics, the selection of a preferred
technology on the basis of the particular industrial scenario for commercialisation is still
required.
This paper will summarize recent developments in second generation technologies for the
production of ethanol from lignocellulose, focusing on consolidated bioprocessing (CBP).
Integrating cellulosic ethanol into existing bio-based and industries and using of
thermochemical processes to maximize energy gains and potential electricity production will
also be discussed. Lastly, the potential role of bioethanol production towards developing a
sustainable Africa while addressing food security, human needs and local wealth creation will
be highlighted.
2.
NEXT GENERATION CELLULOSIC ETHANOL TECHNOLOGIES
Current technologies for biological conversion of biomass commences with a pretreatment
step during which physical and/or chemical processes are used to render the polymeric sugar
fractions more accessible to conversion by enzymatic processes (Stephanopoulos 2007). The
type of pretreatment defines the optimal enzyme mixture to be used in subsequent hydrolysis
steps and the composition of the hydrolysis products. Four biologically mediated events
occur during conversion of pretreated lignocellulose to ethanol via processes featuring
enzymatic hydrolysis: production of depolymerising enzymes (cellulases and hemicellulases),
hydrolysis of the polysaccharide constituents of pretreated biomass, fermentation of the
hexose sugars present, and fermentation of pentose sugars present (Lynd et al. 2002).
Improvements of biomass conversion technology generally entail the consolidation of two or
more of these steps. Hydrolysis and fermentation steps can be combined in simultaneous
saccharification and fermentation (SSF) of hexoses or simultaneous saccharification and cofermentation (SSCF) of both hexoses and pentoses. The ultimate objective would be a onestep ―consolidated‖ bioprocessing (CBP) of lignocellulose to bioethanol, where all four of
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these steps occur in a single reactor and a single microorganism or microbial consortium
converts pretreated biomass to a commodity product such as ethanol without added
saccharolytic enzymes.
CBP would represent a breakthrough for low-cost biomass
processing, due to economic benefits of process integration (Galbe et al. 2005; HahnHägerdal et al. 2007; Hamelinck et al. 2005; Robinson 2006) and avoiding the high costs of
enzymes that make the biochemical conversion route unattractive (Anex et al. 2010; Kazi et
al. 2010).
Lignocellulosic plant biomass represents the largest source of renewable carbon and
consists of 40–55% cellulose, 25–50% hemicellulose and 10–40% lignin, depending on
whether the source is hardwood, softwood, or grasses (Sun & Cheng 2002). The main
polysaccharide present is water-insoluble cellulose that represents the major fraction of
fermentable sugars. Full enzymatic hydrolysis of crystalline cellulose requires synergistic
action of three major types of enzymatic activity (1) endoglucanases, (2) exoglucanases,
including cellodextrinases and cellobiohydrolases, and (3) -glucosidases (Figure 2A) (Zhang
& Lynd 2004). Endoglucanases are active on the non-crystalline or amorphous regions of
cellulose and their activities yield cellobiose and cellooligosaccharides as hydrolysis products.
Cellobiohydrolases are processive enzymes that are active on the crystalline regions of
cellulose and most yield almost exclusively cellobiose as their main hydrolysis product. In
turn,
-glucosidases convert cellobiose and some cello-oligosaccharides to glucose.
Hemicellulose refers to a number of heterogeneous structures, such as (arabino)xylan,
galacto(gluco)mannan, and xyloglucan (Sun & Cheng 2002). These chemically diverse
polymers are linked together through covalent and hydrogen bonds, as well as being
intertwined and can be chemically bound to the lignin fraction. Although many pretreatment
protocols remove variable amounts of hemicelluloses, it remains imperative from an
economic perspective that sugars contained in the hemicellulose fraction of lignocellulose are
also converted to ethanol (Hahn-Hägerdal et al. 2001). The compositions of the major and
minor types of hemicelluloses present in lignocellulosic feedstocks and the enzymes required
to hydrolyze them are reviewed elsewhere (Girio et al. 2010; Van Zyl et al. 2007).
While several microorganisms can be found in nature with the ability to produce the
required enzymes to hydrolyse all the polysaccharides found in lignocellulose, there is no
organism with the ability to directly hydrolyze these polysaccharides and ferment the
liberated sugar to a desired product such as ethanol at rates and titers required for economic
feasibility (Hahn-Hägerdal et al. 2006; Lynd et al. 2005). Strain development is therefore the
most important technical obstacle towards the conversion of lignocellulose to commodity
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products in a CBP configuration (Bothast et al. 1999; Alfenore et al. 2002). Organisms with
broad substrate ranges and cellulolytic and/or hemicellulolytic abilities generally suffer from
poor growth characteristics or poor product producing characteristics. These include poor
yield, titer and rate or producing mixtures of products where desirable products are produced
along with undesirables. In comparison, organisms with desirable product producing qualities
often suffer from limited substrate range including lack of cellulolytic ability, poor
fermentation qualities, and sensitivity to the inhibitors present in pretreated lignocellulosic
biomass. Two strategies have been followed to develop CBP organisms (Lynd et al. 2005).
The native cellulolytic strategy involves engineering naturally cellulolytic microorganisms to
improve product-related properties.
The recombinant cellulolytic strategy involves
engineering non-cellulolytic organisms with high product yields so that they express a
heterologous cellulase system to enable cellulose utilization (Figure 2B).
2.1. Engineering cellulolytic ability into eukaryotic process organisms
The yeast Saccharomyces cerevisiae has long been employed for the industrial production of
ethanol from hexose sugars (Kuyper et al. 2005; Nissen et al. 2000; Van Dijken et al. 2000).
However, this yeast has a number of shortcomings in terms of a CBP processing organism
such as its inability to hydrolyze cellulose and hemicellulose or utilize xylose or arabinose. A
number of research groups around the world have been working on improving the substrate
range of S. cerevisiae to include the monomeric forms of sugars contained in plant biomass
(Hahn-Hägerdal et al. 2001; Hahn-Hägerdal et al. 2007; Karhumaa et al. 2006; Kuyper et al.
2005). A S. cerevisiae strain that expressed the xylose isomerase gene from the fungus
Piromyces sp. E2 was further metabolically engineered to allow anaerobic growth on xylose
in synthetic media (Kuyper et al. 2004). Laboratory and industrial S. cerevisiae strains were
also engineered to co-ferment the pentose sugars D-xylose and L-arabinose (Karhumaa et al.
2006).
There have been many reports detailing the expression of one or more cellulase encoding
genes in S. cerevisiae (Van Zyl et al. 2007). Strains of S. cerevisiae were created that could
grow on and ferment cellobiose, the main product of the action of cellobiohydrolases on
cellulosic substrates, at approximately the same rate as on glucose in anaerobic conditions
(Van Rooyen et al. 2005). Recently the high affinity cellodextrin transport system of the
model cellulolytic fungus Neurospora crassawas was reconstituted into S. cerevisiae
(Galazka et al. 2010). This led to the efficient growth of a recombinant strain also producing
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an intracellular β-glucosidase on cellodextrins up to cellotetraose. Cho et al. (1999) showed
that for SSF experiments with a strain producing a -glucosidase and an enzyme with exoand endocellulase activity, loadings of externally added cellulase could be reduced. Fujita et
al. (2002; 2004) reported co-expression and surface display of cellulases in S. cerevisiae.
High cell density suspensions of a recombinant strain displaying the Trichoderma reesei
endoglucanase II, cellobiohydrolase II, and the Aspergillus aculeatus β-glucosidase were able
to directly convert 10 g/L phosphoric acid swollen cellulose (PASC) to approximately 3 g/L
ethanol. However, growth of this strain on the cellulosic substrate was not demonstrated. An
S. cerevisiae strain co-expressing the T. reesei endoglucanase 1 (cel7B) and the S. fibuligera
β-glucosidase 1 (bgl3A) was able to grow on and convert PASC to ethanol up to 1.0 g/L (Den
Haan et al. 2007b). Jeon et al. (2009) constructed a similar strain expressing the S. fibuligera
bgl3A and the Clostridium thermocellum cel5E endoglucanase genes that produced
significantly more endoglucanase activity than the strain reported by Den Haan et al. (2007b)
and notably improved conversion of PASC to ethanol was achieved. It has been hypothesized
that the addition of successful, high-level expression of exo-cellulases to these strains will
enable conversion of crystalline cellulose to ethanol. However, while there have been reports
of successful expression of CBH encoding genes in S. cerevisiae the titres achieved were
generally too low to allow CBP (Den Haan et al. 2007a).
Several other yeast strains have innate properties that make them attractive as possible
CBP organisms (Lynd et al. 2005). It would be advantageous if the biologically mediated
processing steps could occur at an elevated temperature as it would increase enzyme activity,
reduce the risk of contamination and decrease the amount of cooling required, thereby
decreasing cost.
There is subsequently a lot of interest in developing thermophilic or
thermotolerant organisms for CBP. Strains of the yeast Kluyveromyces marxianus can grow
at temperatures as high as 52°C, can convert a wide range of substrates, including xylose, to
ethanol and successful SSF with a variety of feedstocks at elevated temperatures has been
demonstrated (Rajoka et al. 2003; Fonseca et al. 2007; 2008). Thermotolerant
cellobiohydrolase, endoglucanase and β-glucosidase encoding genes were expressed in
combination in a strain of K. marxianus (Hong et al. 2007). The resulting strain was able to
grow in synthetic media containing cellobiose or carboxymethylcellulose as sole carbon
source but the hydrolysis of crystalline cellulose was not shown. Recently, a K. marxianus
strain was engineered to display T. reesei endoglucanase II and Aspergillus aculeatus βglucosidase on the cell surface (Yanase et al. 2010). This strain successfully converted 10 g/l
of a cellulosic β-glucan to 4.24 g/l ethanol at 48°C within 12 h.
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Some strains of the methylotrophic yeast Hansenula polymorpha have a high capacity for
heterologous protein production, are able to grow at elevated temperatures ranging up to 48°C
and ferment glucose, cellobiose and xylose to ethanol (Ryabova et al. 2003). A recent report
highlighted the promise of H. polymorpha in biomass conversion when strains were
constructed that could ferment starch and xylan (Voronovsky et al. 2009). Pichia stipitis is
one of the best studied xylose-fermenting yeasts and has a substrate range including all the
monomeric sugars present in lignocellulose (Jeffries & Shi 1999). Some P. stipitis strains
produce low quantities of various cellulases and hemicellulases among which a β-glucosidase
that allows the yeast to ferment cellobiose, although it cannot utilize polymeric cellulose as
carbon source (Jeffries et al. 2007).
Endoglucanases were successfully produced in
H. polymorpha (Papendieck et al. 2002) and P. stipitis (Piotek et al. 1998). As these yeasts
are capable of growth on cellobiose these recombinant strains should theoretically have the
ability to hydrolyse amorphous cellulose although this aspect was not tested. The xylanolytic
ability of P. stipitis was enhanced by the co-expression of β-xylanase and β-xylosidase
encoding genes (Den Haan & Van Zyl 2003). The resulting strains displayed improved
biomass production on medium with birchwood glucuronoxylan as sole carbohydrate source.
Despite the fact that P. stipitis is a relatively poor fermentor, its ability to consume acetic acid
and reduce the furan ring in furfural and hydroxymethylfurfural (HMF) creates an opportunity
for this yeast to clean up some of the toxins in cellulosic biomass conversion (Agbogbo &
Coward-Kelly 2008).
2.2. Engineering prokaryotic organisms to hydrolyze polysaccharides
Although Escherichia coli cannot hydrolyze cellulose or produce ethanol at appreciable
quantities it has been shown to metabolize all major sugars present in plant biomass,
producing a mixture of organic acids and ethanol (Alterthum & Ingram 1989). Brau and
Sahm (1986) successfully modified E. coli metabolism by expressing the Z. mobilis pyruvate
decarboxylase at high levels. The resulting strain produced ethanol at levels comparable with
Z. mobilis. Subsequent work has focused on improving ethanol yields, growth rate, strain
stability and ethanol tolerance (Ingram et al. 1987; Ohta et al. 1991a; Ingram et al. 1991;
Chen et al. 2009; Da Silva et al. 2005; Yamano et al. 1998). The K. oxytoca casAB operon
coding for an enzyme IIcellobiose and a phospho-β-glucosidase was expressed in the ethanol
producing strain of E. coli enabling transformants to efficiently utilize cellobiose. Several
endoglucanases have been expressed in E. coli allowing it to hydrolyze amorphous and
8
soluble cellulose to shorter cello-oligosaccahrides (Da Silva et al. 2005; Seon et al. 2007;
Srivastava et al. 1995; Wood et al. 1997; Yoo et al. 2004; Zhou et al. 2001). Among these
are Cel5Z and Cel8Y from Erwinia chrysanthemi.
Zhou et al. (2001) successfully
reconstructed the type II secretion system, the predominant secretion system type in Gram
negative bacteria, encoded by the out genes from E. chrysanthemi, in E. coli. This enabled
E. coli to secrete more than 50% of the recombinant Cel5Z it produced.
Klebsiella oxytoca is a hardy prototrophic bacterium with the ability to transport and
metabolize cellobiose, cellotriose, xylobiose, xylotriose, sucrose, and all other monomeric
sugars present in lignocellulosic biomass (Zhou & Ingram 1999).
Four fermentation
pathways are present in K. oxytoca producing formate, acetate, ethanol, lactic acid, succinate
and butanediol (Ohta et al. 1991b). Through metabolic engineering and expression of the
Z. mobilis pdc and adhB genes it was possible for a recombinant K. oxytoca strain to produce
ethanol from soluble sugars at 95% of the maximum theoretical yield (Wood & Ingram 1992).
Unlike most other ethanol producing organisms K. oxytoca has the ability to ferment xylose
and glucose at equivalent rates (Ohta et al. 1991b). This significantly shortens the time
required to ferment the mixtures of glucose and xylose typically present in lignocellulosic
hydrolysates. Zhou and Ingram (1999; 2001) constructed a K. oxytoca strain expressing the
E. chrysanthemi cel8Y and cel5Z endoglucanase genes. By also introducing the genes that
encodes the type II secretion system from E. chrysanthemii, both Cel8Y and Cel5Z were
secreted effectively by K. oxytoca.
This strain was capable of fermenting amorphous
cellulose and producing a small amount of ethanol without the addition of cellulases.
Zymomonas mobilis is a well known fermenting bacterium that produces ethanol at high
rates but cannot ferment or utilize xylose as carbon source or hydrolyze polysaccharides
(Zhang et al. 1997). Zhang and co-workers (1997) engineered a Z. mobilis strain capable of
fermenting both xylose and arabinose, the major pentose sugars present in plant material. Cofermentation of 100 g/L sugar (glucose:xylose:arabinose - 40:40:20) yielded a final ethanol
concentration of 42 g/L in 48 hours.
Brestic-Goachet et al. (1989) expressed the E.
chrysanthemi cel5Z in Z. mobilis obtaining 1000 IU/L activity with 89% of the recombinant
endo-glucanase secreted to the extracellular medium. Expression of the Ruminococcus albus
β-glucosidase enabled Z. mobilis to ferment cellobiose to ethanol very efficiently in two days
(Yanase et al. 2005).
The thermophilic anaerobic bacterium Thermoanaerobacterium saccharolyticum is also
under development for biomass conversion. T. saccharolyticum grows in a temperature range
of 45 - 65°C and a pH range of 4.0 – 6.5 and is able to ferment a wide range of sugars present
9
in cellulosic biomass including cellobiose, glucose, xylose, mannose, galactose, and arabinose
(Shaw et al. 2008a). Unlike most organisms T. saccharolyticum metabolizes xylose and
glucose essentially at the same rate (Shaw et al. 2008a; Shaw et al. 2008b) but it produces
organic acids in addition to ethanol. Knockout mutants were created that produced almost
exclusively ethanol from xylose. Furthermore, a strain with hfs and ldh deletions exhibited an
increased ethanol yield from consumed carbohydrates (Shaw et al. 2009). T. saccharolyticum
naturally produces both a β-xylanase and a β-xylosidase (Lee et al. 1993a; Lee et al. 1993b)
enabling it to ferment xylan directly to ethanol. Furthermore, T. saccharolyticum was able to
produce as much ethanol from Avicel with 4 filter paper units (FPU) of externally added
enzyme as S. cerevisiae was with 10 FPU in SSF, the result of improved enzyme efficiency at
higher temperatures (Shaw et al. 2008b). This shows the potential of this thermophile as CBP
organism if a cellulolytic system can be established.
To date no ideal organism has been developed for CBP conversion of biomass. Bacteria
generally have a high growth rate but lack process robustness. Yeasts are often sufficiently
robust, but lack substrate range. Filamentous fungi often have a wide substrate range, but
grow relatively slowly and do not produce enough of a desirable product.
While the
advantages of using the yeasts S. cerevisiae, P. stipitis, K. marxianus and H. polymorpha are
well appreciated, the engineered cellulolytic ability of these strains are currently rudimentary.
None of the strains are as yet capable of utilizing crystalline cellulose and the high level
production of an exocellulase remains a requirement.
New information on secretion
pathways, chaperones and metabolic engineering should help alleviate this problem in future.
Compared to S. cerevisiae, all of the bacterial species discussed above are relatively sensitive
to inhibitors associated with lignocellulosic hydrolysates (Bothast et al. 1999; Yamano et al.
1998; Ohta et al. 1991b). E. coli and K. oxytoca strains capable of breaking down cellulose
could also be modified to produce other commodity products such as lactic acid, succinic
acid, acetic acid or 2,3-butanediol (Ji et al. 2009). It is likely that more than one organism
may eventually be used in various biomass conversion processes and the choice may depend
on the sugar composition of the feedstock, the pretreatment method used and the end product
required (La Grange et al. 2010).
3. PROCESS INTEGRATION TO IMPROVE ECONOMIC VIABILITY OF
SECOND GENERATION BIOFUELS PRODUCTION
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The cost disadvantage of second generation biofuels may be addressed through innovative
methods of process integration, in order to minimize the capital investment, maximize energy
efficiency and improve overall economics. Various scenarios regarding technical options for
process integration, to achieve more attractive financial returns, are presented below.
3.1. Energy integration within lignocellulosic conversion processes
Apart from biochemical conversion of lignocellulose to ethanol, discussed in section 2, three
thermochemical options are also available too: combustion, pyrolysis, and gasification.
Combustion entails burning of biomass in the presence of air, which generates hot gases at
temperatures of around 800-1000 C and energy that can be harvested as heat. Pyrolysis is the
conversion of biomass to liquid (bio-oil), solid (char) and gaseous fractions by heating the
biomass in the absence of air to about 500 C. Bio-oils can be upgraded to transport fuels, or
bio-oils and char can be gasified. Gasification is the conversion of biomass by partial
oxidation at temperatures typically in the range of 800-900 C to generation a combustible gas
(called syngas) that can be used for synthesis of different synthetic fuels (typically using the
Fischer-Tropsch process) or burnt for heat production (McKendry 2002).
Several studies have demonstrated the technical, environmental and economic benefits of
utilizing process integration within biological and thermochemical processes. Examples of
such process integration are the SSF, SSCF and CBP configurations for the production of
cellulosic ethanol by the biochemical route (see section 2). As an example, heat integration
within biological (Aden & Foust 2009; Kazi et al. 2010) and thermochemical routes for
second generation biofuels production have the potential to increase overall energy efficiency
by as much as 15% (Leibbrandt 2010) and can reduce capital and operational costs
substantially (Galbe et al. 2005).
In the biological process for lignocellulose hydrolysis-fermentation, large amounts of
energy remain in the non-fermentable lignin-rich residues in the distillation bottoms product.
Conversion of these residues through high-efficiency processes, such as a high pressure boiler
coupled with a multi-stage steam turbine (Aden & Foust 2009; Piccolo & Bezzo 2009), or
advanced Biomass-Integrated-Gasifier/Combined Cycle (BIG/CC) systems (Reith et al. 2002)
can provide all the heat and electricity needed for cellulosic ethanol production, together with
surplus electricity production for sale (Cardona & Sanchez 2007; Leibbrandt 2010; Reith et
al. 2002). Energy consumption in the biochemical process can be reduced further by
performing enzymatic hydrolysis and/or SSF processes at high substrate loadings, together
11
with recycling of the process streams, both of which have substantial benefits in terms of
process energy efficiency and economics (Wingren et al. 2003; Martin et al. 2010). Hot
vapour generated by evaporation steps for sugar concentration and/or water recovery can
provide heat for distillation (Reith et al. 2002), one example of how the energy needs of
ethanol distillation can be minimised through heat integration and pinch analysis (Piccolo &
Bezzo 2009). Anaerobic digestion for waste water treatment can be used to produce methanerich biogas that can be captured and used to generate electricity and/or process heating
(Banerjee et al. 2009). Similarly, the integrated production of synthetic biofuels and
electricity from lignocellulose in the gasification-synthesis process route will provide higher
energy efficiencies than production of synfuels alone (Leibbrandt 2010; Swanson et al. 2010).
3.2. Energy integration between lignocellulosic conversion processes and adjacent
industrial processes
In addition to performing process and energy integration within a particular second generation
biofuel production process, integration of second generation biofuels production with adjacent
industrial processes can address both energy efficiency and production costs for the
lignocellulose conversion process. Process integration with adjacent industrial processes can
be broadly classified as integration (i) with electricity production from biomass or fossil fuels,
(ii) with biomass processing for pulp or sugar production, (iii) of first and second generation
biofuels production by the biological route, (iv) of second generation biofuels production by
the thermochemical route with petrochemical processing, and (v) integration of biological and
thermochemical processing of lignocellulose to second generation biofuels. Examples of such
process integrations, and the associated economic benefits, are presented below.
3.2.1. Integration with electricity production from biomass or fossil fuels
Integration of second generation biofuels production with dedicated electricity production
from coal, natural gas or biomass can provide benefits in economies of scale (financial) and
process efficiency. Both the biological (hydrolysis-fermentation) and thermochemical
(gasification-synthesis) processes require the use of advanced, high efficiency equipment for
electricity production, to satisfy process requirements and provide surplus electricity for sale
(Larson et al. 2009; Laser et al. 2009a). However, advanced equipment such as BiomassIntegrated-Gasifier/Combined Cycle (BIG/CC) systems (Jin et al. 2009; Laser et al. 2009b;
12
Reith et al. 2002) have high capital costs per unit electricity (Larsson et al. 2010; Kazi et al.
2010).
Economies of scale achieved through integration of electricity production in second
generation biofuel processes with electricity production in adjacent industrial facilities can
reduce capital investment per unit of electricity substantially (Easterly 2002; Hahn-Hägerdal
et al. 2006; Laser et al. 2009a; Laser et al. 2009b; Sassner et al. 2008; Sims et al. 2008).
Integration and scale-up of electricity and steam production can be achieved by combining
feedstocks for electricity generation, such as lignin-rich residues from biological processing
or residual syngas from gasification synthesis, and using heat recovery/integration in both
biofuel and electricity generation for steam production and drying/evaporation (Easterly 2002;
Laser et al. 2009a; Sassner et al. 2008). The resulting maximisation of electricity production
will increase revenue to second generation biofuels production, since it minimizes
domestic/industrial heat production, for which limited markets are available (Eriksson &
Kjellström 2010). Increasing the overall energy efficiency of the combined biofuels-electricity
production processes will have substantial benefits in reducing the GHG emissions of
processes (Eriksson & Kjellström 2010). Opportunities for sharing of feedstock supply and
handling infrastructure and logistics will also exist when integrating second generation
biofuels production with electricity production from biomass. These opportunities are similar
to benefits in feedstock supply through integration considered in section 3.2.2 below.
3.2.2. Integration with biomass processing for pulp or sugar production
Integration of biofuels production from lignocellulose with existing biomass processes such
as pulp-and-paper industries or sugar production can provide efficiency and economic
benefits due to potential for feedstock supply and/or energy integration (Goh et al. 2010;
Hahn-Hägerdal et al. 2006; Soccol et al. 2010). Both the cost of raw material and the capital
costs of raw material handlingr substantially to the total production cost of second generation
biofuels, even though lignocellulose is often considered to be inexpensive (Aden & Foust
2009; Anex et al. 2010; Gnansounou & Dauriat 2010; Hahn-Hägerdal et al. 2006; Kazi et al.
2010; Piccolo & Bezzo 2009).
Feedstock costs can be reduced by co-locating and integration of feedstock supply for
biofuels production into existing industrial processes, due to economies of scale benefits, the
potential availability of biomass (e.g. residues), and lower transportation costs (Goh et al.
2010; Galbe et al. 2007). For both pulp mills and sugar production mills, the potential
13
availability of residues, not suitable or useful in primary biomass processing, presents an
attractive opportunity for feedstock supply. Furthermore, sharing of pre-existing logistics,
supply chain and infrastructure for feedstock supply with an existing biomass processing
operation can reduce capital and operational cost substantially, especially when considering
the significant contribution to production costs from these costs (Cardona & Sanchez 2007;
Sims et al. 2008; Soccol et al. 2010). The Brazilian sugar industry, an example for Africa, is
prioritising the co-location of lignocellulose conversion with existing sugarcane processing
plants for cost minimization (Seabra et al. 2010), which may include the combination of
surplus bagasse from a number of nearby mills for economies of scale, and/or using sugarcane
agricultural residues (SCAR) to provide energy to primary sugarcane milling, thus liberating
additional bagasse for conversion (Seabra et al. 2010; Soccol et al. 2010).
Although bagasse liberated from sugar milling can be considered free of transportation
costs (Soccol et al. 2010), the feedstock does have economic value, while supply is often
limited. Although highly efficient sugar mills can liberate up to 50% of the bagasse present in
cane supply as surplus (Botha & von Blottnitz 2006), the availability of bagasse at present day
mills is highly variable and often limiting. Many conventional sugar mills in Africa designed
to dispose of bagasse residues by inefficient burning, resulting in energy inefficient operations
compared to international state-of-the-art mills (Seabra et al. 2010), and limited availability of
surplus bagasse. The economic cost of sugarcane bagasse is therefore coupled to the cost of
capital
investments
required
to
improve
the
energy
efficiency,
and/or
the
harvesting/transportation cost of replacing bagasse with another source of biomass for energy
production, e.g. SCAR. Design and construction of a sugar mill with co-located second
generation biofuels as an integrated, greenfields project is considered beneficial, and likely to
provide bagasse at low cost.
Co-location of second generation biofuels with pulp or sugar production will also provide
economic benefits in terms of energy supply (steam, electricity) to second generation biofuels.
Co-generation of electricity from sugar and pulp mills is widely considered to be an attractive
option for a sustainable energy future. The integration of such co-generation between primary
biomass processing and second generation biofuels production can provide substantial
economies of scale, making the capital costs of highly efficient electricity generation
affordable, and can reduce the cost of second generation biofuels production by 20 percent in
Sweden (Hahn-Hägerdal et al. 2006; von Sivers & Zacchi 1995).
3.2.3. Integration of biological first and second generation biofuels production
14
Integration of second generation biofuels production by the hydrolysis-fermentation of
lignocellulose with production of the same biofuels from sugar/starch with first generation
technology will provide technical, environmental and economic benefits in addition to those
considered above (Sims et al. 2008), reduce capital costs and investor risk, and increase
economic attractiveness (Gnansounou & Dauriat 2010). Particular aspects of process
integration would be feedstock supply, fermentation, water and nutrient recycle, distillation,
and additional opportunities for utilities/energy integration to provide process demand (Galbe
et al. 2007; Easterly 2002). Sugar-rich crops for first generation ethanol production, such as
sugarcane, sweet sorghum and sugarbeet, are particularly attractive for integrated first and
second generation processes, being able to supply feedstock to both processes and sharing the
cost of feedstock production and logistics, with associated environmental benefits
(Gnansounou & Dauriat 2010; Sims et al. 2008). These crops also allow construction of a
flexible manufacturing process, capable of making both crystallized sugar and ethanol from
the extracted plant juices, thus allowing switching between products according to market
conditions, which is practiced in some Brazilian sugar mills, and has also been suggested for
sweet sorghum processing in Northern China (Gnansounou & Dauriat 2010). Potential
benefits of combined fermentation-distillation processes for ethanol production from
lignocellulose may include, (i) replacing exogenous nutrient supplements with sugar juice
and/or molasses, which are rich in nutrients (Banerjee et al. 2009), (ii) mixing of sugars from
juice and lignocellulose to increase ethanol concentrations at the end of the cellulose
fermentation, and (iii) scale-up of ethanol purification/distillation to achieve economies of
scale and improve energy efficiency (Soccol et al. 2010). Similar integration possibilities
also exist in grain (small grains, corn, etc.) fermentation, where ethanol production from
starch may be supplemented with sugars from bran (starch fibre) and polysaccharide-rich
waste streams such as thin stillage (Cardona & Sanchez 2007; Linde et al. 2010).
3.2.4. Integration of thermochemical second processes with fossil fuel processing
Conversion of lignocellulose into biofuels by thermochemical processes such as gasification
and pyrolysis may be integrated with existing fossil fuel processing. Examples of cogasification of biomass with coal, and development of pyrolysis oil as feedstock to oil
refineries are considered here.
15
The gasification-synthesis route for production of biofuels from lignocellulose is based on
a similar process for fuels and chemical production from coal, which has been in commercial
operation at Sasol (South Africa) for more than 35 years (Anex et al. 2010). The synthesis and
downstream processing parts of this process route are well established, and the key to biomass
conversion is therefore the production of syngas of acceptable quality (Laser et al. 2009c;
Sims et al. 2008). One attractive means to integrate biomass processing into existing coal
gasification-synthesis processes is through co-gasification of biomass with coal, which may
provide substantial synergies in terms of both gas and liquids yields (Sonobe et al.; 2008).
Capital cost benefits of biomass co-gasification with coal in existing gasifiers, may be
complemented with syngas production from biomass in stand-alone gasifiers, both utilizing
existing synthesis-purification equipment to produce a ―blended‖ synthetic fuel.
Pyrolysis of biomass, in particular fast pyrolysis for the production of pyrolysis oils is
rapidly developing in terms of its potential to produce low cost transportation fuels from
lignocellulose (Anex et al. 2010). Whereas the production of bio-oil is a well-established,
low cost process (Leibbrandt 2010), the upgrading of bio-oil to transportation fuels through
catalytic hydrogenation and/or decarboxylation is not as well developed (Anex et al. 2010;
Wright et al. 2010). Refining of upgraded bio-oils may be integrated with existing oil
refineries, to further reduce the costs of transportation fuel production. Pyrolysis products
may also be gasified for syngas production, either in stand-alone units or by co-gasification
with biomass.
3.2.5. Integration of biological and thermochemical processing of lignocellulose
to second generation biofuels
Whereas biological and thermochemical processing of lignocellulose are often considered as
competing technologies, integration of these processes may achieve improved energy
efficiency and economic returns for second generation biofuels production. For example,
conversion of carbohydrates in lignocellulose to ethanol by the biological route, may be
combined with thermochemical (gasification-synthesis) processing of the non-fermentable
lignin-residues, for which overall energy efficiencies as high as 70% have been demonstrated
for future mature technology scenarios (Laser et al. 2009c; Laser et al. 2009b). Scenarios that
integrate biological and thermochemical processing enable waste heat from the
thermochemical process to power the biological process, resulting in higher overall process
efficiencies (Laser et al. 2009c).
Alternative scenarios for process integration to improve
16
economies of second generation biofuels production in Africa should be considered through
rigorous process modelling coupled with economic analysis (Anex et al. 2010; Leibbrandt
2010), taking location-specific factors into account.
4. SUSTAINABLE BIOENERGY PRODUCTION IN AFRICA
Africa still remains a large consumer of traditional sources of energy, mainly fuel wood and a
greater proportion of its population faces energy insecurity (ICSU 2007). The availability and
access of socially and environmentally acceptable sources of energy is still very low and
disproportionate between rural and urban areas. With the exception of fuel-wood, other
energy sources (coal, crude oil, and more recently biofuels) have been the major sources of
power driving the transport and industry sectors.
Several conversion paths are being studied for total conversion of biomass into biofuels, in
particular for production of bioethanol from cellulosic feedstocks. The benefits of the total
conversion of cellulosic feedstock to bioethanol are different in different geographical
regions. In developed countries, the main thrust for ethanol production is the replacement of
fossil fuel in the transportation sector. The situation is different for developing countries such
as those in the Southern African Development Community (SADC) region because of their
unique socio-economic needs especially the chronic food and energy insecurity, extreme
poverty, high unemployment rate and degradation of the natural environment. Therefore,
biofuels in Africa have increasingly received attention for not only for their potential to
reduce green-house-gas (GHG) emissions and increase energy supply but also to open new
markets for agricultural surplus (thus additional revenue for farmers), and provide
employment opportunities and local economic development opportunities in rural areas, just
to mention a few (Meyer et al. 2008).
4.1. Biofuels production potential in Africa
The biofuels industry in Africa is being developed gradually in most African countries with
assistance from international agencies (Darkwah et al. 2007). Some of the major biofuels and
technologies that have been reported in Africa include: biogas, thermal gasification, biodiesel,
bioethanol and most recently, albeit at the research and/or developmental level, the second
generation biofuels devoted to total biomass conversion.
17
4.2. Ethanol production
In SADC, sugarcane production, an important feedstock for bioethanol production is growing
steadily (2.5% per annum) (Darkwah et al. 2007). Most of this potential in biofuels for the
region is in domestic markets especially in the transport sector. This has been attributed to the
contribution from rehabilitation programs in post-conflict countries (Angola and
Mozambique). The region has great potential to produce and meet the growing demand for
lead phase out programs in fuel for transport. Current figures for cultivated land (6%) are very
low and suggest that availability of land may not be a constraint to increasing production of
biomass for fuel production (Gnansounou et al. 2007). For bioethanol, most plants in Africa
are in SADC and active participants include South Africa, Malawi, Swaziland, Mauritius, and
Zimbabwe. There are also substantial amounts of sugarcane and a large potential for doubling
current production in the region (Darkwah et al. 2007).
Bioethanol production requires biomass with significant starch or sugars, which is
fermented through enzymatic biological processes to generate liquid biofuel (Cotula et al.
2008). The current major feedstock in the production of biofuels in the world is starchy
biomass, which accounts to nearly 53% of all bioethanol production. Maize, wheat, sorghum
and other starchy materials are the main starchy feedstocks used in bioethanol production.
The second method uses sugarcane and sugarbeet biomass, the feedstock that is already in
sugar form and the rest of the processes are the same as in starchy biomass; while the last
method uses biomass from cellulosic materials such as bagasse, straw and wood biomass
(McKendry 2002).
While the technology associated with the first two feedstocks (starchy biomass &
sugarcane) is available and can be replicated, maize and other starchy biomass feedstocks
have a very important role in food security in the sub Saharan African. To some extent, the
use of these feedstocks (maize included) in the promotion of biofuel production makes it less
attractive for most parts of Africa. On the other hand, secondary products from, for example,
processing of sugar from sugarcane generates co-products like bagasse, molasses, and fibre,
which can be used to generate electricity and provide additional revenue if exported (Jumbe et
al. 2009). Countries like Mauritius have successfully used this technology and supplied
electricity to the national grid contributing up to 40% of all domestic power consumption
(Deepchand 2005). Molasses, another form of wastes from crystalline sugar production can
also be used as feedstock in bioethanol production. This pathway has a very high unexploited
18
potential in Africa. For example, in Tanzania, only 30% of the molasses produced from sugar
production are exported and used as animal feed while 70% goes to waste (Gnansounou et al.
2007). Hence, in light of current debates on the potential negative impact of increasing biofuel
production to food security, sugarcane molasses offer a viable option.
4.3. Energy Crops
Several potential energy crops have been highlighted for biofuels production in Sub-Sahara
Africa (Jumbe et al. 2009). Ethanol is the most promising biofuel product that can be
produced from different raw materials in many African countries (see Table 1) with most of
the ethanol coming from molasses. On the other hand, Jatropha and oil seeds are the main
feedstocks for producing biodiesel, which is used to run stationary generators for electricity
generation and as a diesel substitute for transportation.
4.4. Biofuels production in Africa are hampered by economic and technical factors
Examples of first generation biofuels production plants with attractive economic returns
can be found in Africa, in particular in least developed countries (LDCs) with preferred access
to developed world markets. However, the commercial production of second generation
biofuels is constrained by economic and technical concerns (Sims et al. 2008). Due to the
recalcitrance of lignocellulose to biological degradation, the cost of production of a second
generation biofuels is dominated by investment costs, for examples second generation ethanol
by the biological route requires substantially higher capital investment than first generation
ethanol (Gnansounou & Dauriat 2010). Such economic concerns place second generation
technologies at an even greater disadvantage than first generation technologies, despite its
potential for improved environmental and socio-economic benefits, sinc they are not deemed
economically viable without government subsidies.
In addition, while methods for
lignocellulose pre-treatment/fractionation are available, these have not been optimized for
local substrates and novel African bio-energy crops.
4.5. Markets opportunities for bioenergy in Africa
19
The outlook of market potential for biofuels in Africa is varied with Sub-Saharan Africa
having the most potential and North Africa having the least potential. The potential value of
biofuels for Sub-Saharan Africa by 2010 to 2013 as estimated by global growth consultancy
Frost and Sullivan in the Africa Review of Business Technology, March 2008 was between
US$ 1.54bn to US$ 1.83. However, if the next generation technologies unlock the potential of
converting all cellulosic biomass, the potential value could be significantly higher. Figure 3
compares the potential biofuels production from agricultural and forestry residues, invasive
plants and energy crops in South Africa, in relation to the current fossil fuel and the Industrial
Biofuels Strategy’s target for 400 ML/annum. In this case, when considering the use of only
50-70% of this plant biomass with second generation biochemical and thermo-chemical
technologies, South Africa could very well exchange the bulk of its current liquid fossil fuel
usage (currently 21.2 BL/annum) with renewable biofuels. One of the factors stimulating
domestic demand on the continent is the implementation of the lead phase out programs in
gasoline. In West Africa, a market opportunity study for biofuels indicated that locally
produced anhydrous ethanol favourably competed with petrol (UEMOA 2008). Ethanol
programmes that produce a blend of ethanol and gasoline (gasohol) for use in existing fleets
of motor vehicles have been implemented in Malawi, Zimbabwe and Kenya (Amigun et al.
2008).
Notwithstanding, sugarcane producing countries have great potentials in the ethanol gel
fuel industry (Darkwah et al. 2007), which can be directed for local consumption at household
level. The ethanol gel and specially adapted ethanol cooking stoves such as superBlu stoves
(Lambe 2008; Robinson 2006) can lessen the burden of women and girl children spending
most of their time fetching fuelwood for their households.
Such technologies provide
alternatives to paraffin or open fire cooking and heating that are associated with fire hazards,
indoor pollution and inefficient conversion (in case of fuelwood). Johnson and Matsika
(2008) estimated a market of 10 billion litres is needed to substitute for 30% of all cooking
fuel in sub-Saharan Africa. Despite offering more socio-economic and environmental benefits
compared to traditional cooking and heating energy, the uptake of clean fuel cooking and
heating technologies in rural areas of Africa has been very low mainly because of lack of
distribution infrastructure, high costs and lack of awareness (Schlag & Zuzarte 2008).
However, several pilot projects run by NGOs and private companies such as Millennium
Gelfuel Initiative (MGI), which began in 2000 as a public-private partnership, have
demonstrated some level of acceptance of the gelfuel technology at household in Malawi,
South Africa and Zimbabwe (Utria 2004).
20
4.6. Biofuels and high-value chemicals in a biorefinery
Food and first generation biofuels are already produced from sugar, starch and oil-rich food
crops. When second generation technologies come to fruition, the appropriate entry point
could be the use of agricultural and forestry residues. This would allow the roll-out of the
necessary technologies while establishing the biofuels value chain.
Simultaneously,
agronomists and environmentalists can assist in identifying energy crops and how to utilise
invasive plants in a cost-effective way. The economics of the conversion of starch, sugar,
lignocellulosics and vegetable oil raw materials into biofuels can be improved by the
integration of various processing technologies in a single production plant, or ―biorefinery‖,
based on the conditions in a particular local industry and region (Lynd et al. 2003; Hatti-Kaul
2010). Biorefineries represent a technological solution that can substantially improve
economic feasibility, especially considering the highly volatile nature of agricultural raw
material costs and market prices for transportation fuels due to exposure to international
economic and political pressures.
4.7. Bioenergy production and the environment
According to the Intergovernmental Panel on Climate Change (IPCC), agricultural production
and access to food in many regions may be severely compromised by climate variability and
change (IFAD 2008). The area suitable for agriculture, the length of growing seasons and the
yield potential of some crops mainly in arid areas are expected to decrease. The adverse
impacts of mitigation measures being taken under the Kyoto Protocol such as carbon sinks,
the expansion of mono-crop plantations for biofuels (e.g. palm oil, soya, sugar cane, jatropha)
have been associated with undermining small-scale traditional livelihoods of indigenous
peoples (e.g. rotational agriculture, pastoralism, hunting and gathering), which usually have
higher biodiversity as opposed to the monocrops. However, on whether biofuels decrease or
increase the emissions, it will be important to appraise the entire energy chain when
comparing options and it is equally important to analyse the production and emissions based
on best practices, including innovative ways to manage crops and soils, such as zero-tillage
approaches; and also examine forestry management that includes judicious forest use without
burning and other activities that generate high emissions.
21
In dry areas, the production of fast growing biofuel crops will naturally be associated with
the competition for water between food and fuel crops and may become the overriding issue
in the fuels vs. food debate. Improvement in crop productivity as well as the shift from high
water-use bio-fuel crops (such as sugarcane) to drought-tolerant crops (such as sweet
sorghum) and Jatropha can be used as options to address the issue of water scarcity. Despite
what it is often said about production of biofuel crops on dry and marginal lands, irrigation in
low-rainfall ecologies is required for optimal yields. This may have the undesirable water
salinity problem in many regions (IFAD 2008).
With biofuels and the carbon markets, it has been suggested that ways need to be found to
link small-scale farmers to the global carbon market. This should occur without creating
bureaucracies or additional burdens for them while also establishing clear indicators for
accounting for carbon and providing payments to poor farmers for such environmental
services. Additionally, options for financing are much broader and are emerging rapidly. The
growing market for carbon projects and activities, through both the Clean Development
Mechanism (CDM) and voluntary markets, demonstrates that the sequestration of carbon
could offer opportunities for smallholder agriculturalists to gain from the mitigation potential
of the agriculture sector. However, in the global carbon market the participation of developing
countries, particularly the poorest communities within them, has been extremely challenging,
because of the costs, modalities and procedures for CDM verification.
4.8. Policy and institutional framework for bioenergy industry development
Political commitment and support for the development of the necessary regulatory
instruments for advancement of bioenergy in Africa is very important.
A number of
governments in Africa have made some progress in coming up with definitive policy
strategies that have unveiled the economic benefits of renewable energy.
For instance,
operating bioethanol programmes that blend ethanol and gasoline (gasohol) for motor
vehicles exist in Southern African countries such as Malawi, Zimbabwe and South Africa
(Amigun et al. 2008). However, most of the policy instruments are embedded within the
energy policies for the countries, which vary from country to country. Consequently, policies
in particular specific for second generation biofuels are generally lacking. While most of the
biofuels policies and regulatory frameworks have centred on the first generation of
commercially viable biofuels, due to the advancement in R&D and the sector’s associated
22
expansion and growth, future policies are likely to extend to other second generation biofuels
(UEMOA 2008).
However, the implementation structures for bioenergy policies are another source of
confusion with some pieces of mandates scattered in different departments falling under
different ministries (Amigun et al. 2008). The cooperation and interaction of all relevant
stakeholders, including civil society and private sector, can lead to the development of a
conducive policy instrument that can foster significant growth in biofuels development. An
example of such multistakeholder partnerships is The Competence Platform on Energy Crop
and Agroforestry Systems for Arid and Semi-arid Ecosystems (COMPETE) that aim at
developing cross sectoral work packages for evaluating Africa’s potential for sustainable
provision of bioenergy and develop innovative tools for financing national and local and
strategic policy mechanisms for developing bioenergy systems
(Janssen et al. 2009).
Furthermore, one of the suggestions that have been proposed as fundamental in the ―bioenergy revolution‖ has been the organization of smallholder farmers and producers in order to
facilitate their access to markets and enable them to commercially interact with large private
entities engaged in the energy markets (IFAD 2008).
From a rural development perspective, at the microeconomic level, bio-fuel policy
development must aim at contributing to the larger developmental goals but not at the expense
of more pertinent issues like food and social security. According to Sulle and Nelson (2009),
most of the biofuels production systems in Africa are characterised by large scale and
concentrated ownership operations, which in the African context, would affect the land tenure
system and rural development policies. Thus, biofuel development could, without appropriate
policy guidelines, increase pressure on land to the disadvantage of poor rural people and
deprive the locals of their customary land rights. Notwithstanding, secure access to land
tenure is a much broader issue in most developing countries that generally affects agricultural
production and so biofuels are not its main driver. Foreign investors that involve the locals in
the biofuels value chains such as the locals participating as outgrowers, formation of village
land trust and establishment of equity based ventures represent the positive models for
improving local livelihoods and the environment (Sulle & Nelson 2009).
5. CONCLUDING REMARKS
Great strides have been made in the development of second generation technologies for
cellulose conversion to bioethanol and other biofuels. New opportunities using agricultural
23
residues, energy crops or invasive plant species for biofuel production potentially allow for
the sustainable production of biofuels in significant quantities, while at the same time
stabilizing food production by providing alternative markets to farmers, as well as addressing
human development specifically in rural communities. First generation biofuels production
has been modest in Africa (Table 1), compared to the huge potential of Africa to produce
biomass (Smeets et al. 2007).
Although the geographical potential of Africa to produce biofuels is at least as large as any
other continent, recent developments in second generation technologies or the potential
applications thereof have had no impact on Africa at large. Partly, this is because Africa faces
pressing human challenges associated with an interconnected set of issues involving poverty,
food security, economic development, gender issues, and health and energy security. The
limited resources of many African economies, together with fragmented trade and economic
policies, apparently limit the abilities of governments to provide financial incentives for
second generation biofuels production.
However, the positive considerations of second generation biofuels production, like GHG
emission reductions, renewability, absence of competition with food/feed, negligible land use
impacts and sustainability should translate into a greater willingness by governments and
consumers to accept higher prices for second generation biofuels, compared to those produced
by first generation technologies (Gnansounou & Dauriat 2010). Policies to enable such price
differentials in the market place have not been implemented in Africa. Fossil fuels remain
generally cheap, and the development of a sufficient policy environment, that can provide
attractive economic returns for second generation biofuels through incentives, subsidies and
carbon taxes, mandated blending, carbon emissions legislation, and the financial benefits of
carbon trading, is lacking in the African context.
To actively take a step in the right direction, actions need to be considered to ensure that
Africa benefits along the full value chain of biofuel production and utilisation. Some worthy
actions could include:
i.
Proper analysis, understanding, and consensus on the potential of bioenergy, including
biofuels production with second generation technologies to realize a sustainable
Africa;
ii.
African scalable demonstration projects using second generation technologies for
learning perspectives e.g. training to strengthen local manpower. This could also show
best practices in energy efficiency and resource protection in transport, electricity
supply, cooking and other household needs; and
24
iii.
Alignment of international, regional and local policies on trade, aid, land tenure, and
development need to facilitate integrated value chains of agriculture and forestry for
food and bioenergy in Africa.
Biofuels present one of the most cost effective solutions for a global sustainable low carbon
energy future. This future demands sustainable agriculture and forestry in Africa to supply
food and bioenergy in support of Africa and the world. A sustainable globe requires a
sustainable Africa and it is for Africa to step up to this challenge.
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35
Table 1: Biofuels potential in selected African countries in megalitres (ML)
Country
Raw material
Biodiesel (ML)
Ethanol (ML)
Benin
Cassava
-
20
Burkina Faso
Sugarcane
-
20
Ivory Coast
Molasses
-
20
Ghana
Jatropha
50
-
Guinea Bissau
Cashew
-
10
Mali
Molasses
-
20
Malawi
Molasses
-
146
Kenya
Molasses
-
413
Ethiopia
Molasses
-
80
Niger
Jatropha
10
-
Nigeria
Sugarcane
-
70
Sudan
Molasses
-
408
Swaziland
Molasses
-
480
Senegal
Molasses
-
15
Tanzania
Molasses
-
254
Togo
Jatropha
10
-
Uganda
Molasses
-
119
Source: Jumbe et al. (2009).
36
Figure 1: Alternative process routes for conversion of lignocellulose to bio-energy
products. Biological (hydrolysis-fermentation) and thermo-chemical (combustion,
pyrolysis, gasification) routes result in different products, including cellulosic
ethanol, electricity, pyrolysis oils, charcoal, surplus heat and other transportation
fuels.
Figure 2: (A) Schematic representation of the hydrolysis of amorphous and
microcrystalline cellulose by cellulase systems (Lynd et al. 2002). The filled squares
represent reducing ends and the open squares non-reducing ends. Amorphous and
crystalline regions are indicated.
(B)
A graphic illustration of lignocellulose
conversion to bioethanol by in a single bioreactor by a CBP microorganism (adapted
from Van Zyl et al. (2007):
The enzymatic hydrolysis of the cellulose and
hemicellulose fractions to fermentable hexoses and pentoses requires the production
of both glycosyl hydrolases (cellulases and hemicellulases) and the subsequent
conversion of the hexoses and pentoses to ethanol requires the introduction of pentose
fermenting pathways.
Figure 3: Potential biofuels production from lignocellulosic biomass available in
South Africa (Lynd et al. 2003) (assuming only 50-70% were utilized) when advance
second generation biochemical and thermo-chemical technologies are available.
Optimal biofuels yields estimated when the appropriate technologies are available,
include (i) biochemical processing of maize-to-ethanol = 460 L/ton (Overend 2007)
or lignocellulosic-to-ethanol = 280 L/ton (only polysaccharide fraction) (Gibbs
1998); and (ii) thermo-chemical upgrade of bio-oils from fast pyrolysis = 310 L/ton
(Huber et al. 2006) and thermo-chemical biomass-to-liquid (BtL) = 570 L/ton
(Leibbrandt 2010).
37
Enzymatic
hydrolysis
Pretreatment
Fermentation
to ethanol
Cellulosic
ethanol
Distillation
Biological route
to cellulosic
ethanol
Residues for thermochemical conversion
High pressure
steam
Lignocellulose
feedstock or
residue
Steam
turbine
Combustion in
boiler
Thermochemical
routes
Electricity
Surplus heat
Pyrolysis
Pyrolysis oils
Catalytic
Upgrading
Refining
Charcoal
Gasification
Transportation
fuels
Product gas
or syngas
Product
gas
Syngas
Combustion for
heat/electricity
Synthesis
Refining
Figure 1.
38
A
B
Crystalline Amorphous Crystalline
Glycosyl
Hydrolases
O
Hexoses
Glucose
O
Pentoses
Cellobiose
Cello-oligosaccharides
Endoglucanase
Exoglucanase
(eg. CBHI)
-Glucosidase
Exoglucanase
(eg. CBHII)
P
YFG
T
Ethanol + CO2
Figure 2.
39
30000
Petrol
Diesel
25000
Ethanol (70% of feedstock)
Volumes in ML
Upgraded bio-oil (70% of feedstock)
20000
BtL (50% of feedstock)
15000
10000
Fossil fuel
Total
Energy
crops
Invasive
plants
Forest
biowaste
Agricultural
biowaste
0
Maize
5000
Figure 3.
40
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