Encyclopedia of Renewable Energy. James G. Speight
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a dominant component of cereal crops such as corn and wheat, is comprised wholly of glucose. Starch may be subjected to an additional processing in the form of an acid- or enzyme-catalyzed hydrolysis step to liberate glucose using a single family of enzymes, the amylases, which makes bioconversion relatively simple. Downstream processing of sugars includes traditional fermentation, which uses yeast to produce ethanol; other types of fermentation, including bacterial fermentation under aerobic and anaerobic conditions, can produce a variety of other products from the sugar stream.
Forest biomass or agricultural residues are almost completely comprised of lignocellulosic molecules (wood), a structural matrix that gives the tree or plant strength and form. This type of biomass is a prime feedstock for combustion, and indeed remains a major source of energy for the world. The thermochemical platform utilizes pyrolysis and gasification processes to recover heat energy as well as the gaseous components of wood, known as synthesis gas (or syngas) which can then be refined by the Fischer-Tropsch process into synthetic fuels such as hydrocarbon liquids, methanol, and ethanol.
Lignocellulose is a complex matrix combining cellulose, hemicellulose, and lignin, along with a variable level of extractives. Cellulose is comprised of glucose, a six-carbon sugar, while hemicellulose contains both five- and six-carbon sugars, including glucose, galactose, mannose, arabinose, and xylose. The presence of cellulose and hemicellulose therefore makes lignocellulose a potential candidate for bioconversion. The ability of the bioconversion platform to isolate these components was initially limited, as the wood matrix is naturally resistant to decomposition. Recent advances, however, have made this process more commercially viable. Costs remain higher than for starch-based bioconversion, but there is added potential for value-added products that can utilize the lignin component of the wood.
In order to incorporate all aspects of biofuel production, including the value of co-products and the potential of the industry to diversify their product offering, we employ the biorefinery concept. The biorefinery concept is important because it offers many potential environmental, economic, and security-related benefits to society. Biorefineries provide the option of co-producing high-value, low-volume products for niche markets together with lower-value commodity products, such as industrial platform chemicals, fuels, or energy, which offsets the higher costs that are associated with processing lignocellulosic materials.
The two technological platforms being explored for the lignocellulose-based biorefinery are complementary. Each technological platform provides different intermediate products for further processing. It is the range of these intermediates that dictates the types of end products that are likely to be successful in a commercial sense.
See also: Bioalcohols, Bioconversion Platform, Biodiesel, Biogas, Fischer-Tropsch Process, Thermochemical Platform, Vegetable Oil.
Biofuels – Production
There is some concern related to the energy efficiency of biofuel production. Production of biofuels from raw materials requires energy (for farming, transport, and conversion to final product), and it is not clear what the overall efficiency of the process is. For some biofuels, the energy balance may even be negative.
Since vast amounts of raw material are needed for biofuel production, monocultures and intensive farming may become more popular, which may cause environmental damages and undo some of the progress made toward sustainable agriculture.
See also: Bioconversion Platform, Thermochemical Platform.
Biofuels – Properties, Variations with Source
The quality and composition of a biofuel depends on the source of the biomass/feedstock as well as the types of processing and conversion techniques utilized in its manufacture. Biomass feedstock composition ultimately decides the yield from the chemical or biochemical conversion processes, which in turn, affects the economics involved. There are many plant varieties which are used as biofuel sources - the geography, weather conditions, soil composition, and legislation of a location normally dictates what types are grown specifically for biofuel production. Ethanol, biodiesel, and butanol are the main types of commercially produced biofuels.
The soil organic matter content contributes greatly to the grain and stover, and hence carbohydrate content of maize plants. Lignin and cell-wall cross-linking also affect the ethanol production. Selection for reduced lignin and increased cellulose in stover can potentially be expected to increase mechanical strength as well as ethanol yield. Although pretreatment and enzyme hydrolysis constitute two of the more costly steps in cellulosic ethanol production, stover with reduced lignin may still need to be treated before being subjected to enzyme hydrolysis. It seems unlikely that the cost savings in pretreatment from reduced lignin can be fully realized because of an accompanying reduction in biomass. However, for ethanol production to be commercially viable, improvements must not only be made to the efficiency of ethanol production per unit dry mass, but also per unit land area.
Biomass feedstock composition ultimately decides the yield from the chemical or biochemical conversion processes, which in turn, affects the economics involved. There are many plant varieties which are used as biofuel sources – the geography, weather conditions, soil composition, and legislation of a location normally dictates what types are grown specifically for biofuel production. Ethanol, biodiesel, and butanol are the main types of commercially produced biofuels.
The soil organic matter content contributes greatly to the grain and stover, and hence carbohydrate content of maize plants. Lignin and cell-wall cross-linking also affects the ethanol production. Selection for reduced lignin content and increased cellulose content in stover can potentially be expected to increase mechanical strength as well as ethanol yield. Although pretreatment and enzyme hydrolysis constitute two of the more costly steps in cellulosic ethanol production, stover with reduced lignin may still need to be treated before being subjected to enzyme hydrolysis.
The production of biofuels from lingo-cellulosic feedstocks can be achieved through two very different processing routes which are (i) the biochemical route in which enzymes and other micro-organisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol and (ii) the thermochemical route in which pyrolysis and gasification technologies produce a synthesis gas (carbon monoxide, CO, and hydrogen, H2) from which a wide range of long chain biofuels, such as synthetic diesel or aviation fuel, can be reformed. One key difference between the biochemical and thermochemical routes is that lignin component is a residue of the enzymatic hydrolysis process and can be used for heat and power generation.
Genetics as well as environmental factors affect the chemical composition of the various parts of the plant, and it was found that husk, followed by rind and pith, has the highest sugar (glucan + xylan) content. The term glucan represents diverse glucose polymers that differ in the position of glycosidic bonds, which can be short or long, branched or unbranched, alpha or beta isomers, and soluble or insoluble. On the other hand, the term represents a group of hemicellulose derivatives.
The variation in the structure of the glucan derivatives and the xylan derivatives (Figure B-2) is due to differences in the amounts of the main chemical constituents of biomass (cellulose, hemicelluloses, and lignin, all of which have different uses) being present in different proportions in the various parts of the plant.
Figure B-2 Structure of xylan from hardwood.
The production of fuels from ligno-cellulosic feedstocks can be achieved through two different processing routes. These are (i) biochemical, whereby enzymes and other microorganisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol and (ii) the thermochemical route, where pyrolysis and gasification technologies