Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 143

Biochemical conversion is the use of (i) fermentation or (ii) anaerobic digestion to produce fuels and chemicals from organic sources. In the general sense, fermentation refers to any chemical change of organic material that is accompanied by effervescence, normally without the participation of oxygen. The important differences between fermentation and anaerobic digestion are the nature of the product produced and the character of the biological contribution. Fermentation produces a liquid product in the presence of enzymes, while anaerobic digestion yields a gaseous product as a result of the metabolic activity of bacteria (Table B-3).

Ethanol is the principal product of the fermentation processes appropriate to biomass conversion, although other alcohols, as well as organic acids, ketones, and aldehydes, may be produced either as main products or as by-products. Anaerobic digestion is the decomposition of any organic material by the metabolic action of bacteria without the participation of atmospheric oxygen. Methane and carbon dioxide are the main products of the decomposition. The source of the oxygen in the carbon dioxide is the combined oxygen in the organic molecules and in the water.

Table B-3 Biomass conversion processes.

Process	Biomass feedstock	Scale*	Product
Combustion	Wood, municipal solid waste, grasses, crop residue	Small, large	Heat, steam, electricity
Gasification	Wood, municipal solid waste (grasses, crop residue)	Large	Low-heat content gas, synthesis gas ethanol
Pyrolysis	Wood, sewage sludge	Large	Medium-heat content gas tar
Fermentation	Grain and sugar crops	Small, large	Ethanol
*Small implies domestic or farm application; large is industrial-scale processing of up to 1,000 t/d of biomass.

Bacterial digestion is in effect accomplished by enzymes. Further, certain bacteria produce acids and alcohols as the principal degradation products. In some cases, it is not clear whether the degradation proceeds as a result of bacterial metabolism, or whether it can be achieved by non-growing cells. Nevertheless, the distinction between the two processes is convenient for presentation purposes, and should not cause confusion in classifying the important biochemical processes currently in contention.

Biomass fermentation to produce ethanol is similar to glycolysis (the fermentation that occurs in muscle tissue and converts glucose to lactic acid with the release of energy), but the use of different enzymes results in different end products.

See also: Aerobic Digestion, Anaerobic Digestion, Bioconversion, Fermentation.