James G. Speight - Encyclopedia of Renewable Energy

See also: Bioalcohols, Bioconversion Platform, Biodiesel, Biogas, Fischer-Tropsch Process, Thermochemical Platform, Vegetable Oil.

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.

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, H 2) 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-2Structure 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 produce a synthesis gas (carbon monoxide and hydrogen) from which a wide range of long chain biofuels, such as synthetic diesel or aviation fuel, can be reformed. In terms of the biochemical route, much remains to be done related to (i) improving feedstock characteristics, (ii) improving the efficiency of enzymes, and (iii) improving overall process integration. 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.

In terms of the production of biodiesel, transesterification optimization is desired, which would depend on the chemical composition of alkyl esters of vegetable oils, animal fats, and cooking oil. Most common feedstocks possess fatty acid profiles consisting mainly of five C16 and C18 fatty acids, namely, palmitic (hexadecanoic), stearic (octadecanoic), oleic (9( Z )-octadecenoic), linoleic (9( Z ),12( Z )-octadecadienoic), and linolenic (9( Z ),12( Z ),15( Z )-octadecatrienoic) acids, with the exception of a few oils such as coconut oil, which contains high amounts of saturated acids in the C12 to C16 range. Changing the fatty acid profile can be achieved by physical means, genetic modification of the feedstock, or use of alternative feedstocks with different fatty acid profiles, such as wood-derived fatty-acids and related compounds.

The production of biogas is essentially achieved by wastewater treatment facilities which use anaerobic digestion to reduce the organic content of sewage sludge and animal wastes. The variation in the composition of biogas will depend on the organic content of the biomass as well as the pretreatment and processing. In addition to the main components (methane and carbon dioxide), biogas can also contain a variety of contaminants and impurities, such as sulfur compounds (hydrogen sulfide, mercaptans), halogens, ammonia, and dust particles.

Concerning biodiesel production, transesterification optimization is desired, which would depend on the chemical composition of alkyl esters of vegetable oils, animal fats, and cooking oil. Most common feedstocks possess fatty acid profiles consisting mainly of five C 16and C 18fatty acids, namely, palmitic (hexadecanoic), stearic (octadecanoic), oleic (9( Z )-octadecenoic), linoleic (9( Z ),12( Z )-octadecadienoic), and linolenic (9( Z ),12( Z ),15( Z )-octadecatrienoic) acids, with the exception of a few oils such as coconut oil, which contains high amounts of saturated acids in the C 12to C 16range or others. Changing the fatty acid profile can be achieved by physical means, genetic modification of the feedstock, or use of alternative feedstocks with different fatty acid profiles, such as wood-derived fatty acids and related compounds.