Biochemical Conversion
From Thermal-FluidsPedia
Biochemical conversion could be either anaerobic or aerobic. In anaerobic conversion, biomass is put in anaerobic digesters, where it is exposed to microorganisms in the absence of oxygen and allowed to decay. The oldest form of biochemical conversion is the fermentation of grapes, corn, and barley by microscopic yeast to produce wine, beer, and other alcoholic beverages. The same process can be utilized to produce ethanol (also called grain alcohol) and other synthetic fuels. High-moisture herbaceous plants and marine crops are most suitable for biological digestion. Fuel alcohol is produced by cooking the biomass and converting the starch into sugar. The sugar is then allowed to ferment. Ethanol can be removed by distillation and can be used directly, or be blended with gasoline and used as fuel in internal combustion engines. New processes use enzymes to break down the cellulose part of the plants, allowing the entire plant (and not just the starch) to be utilized.
The main source of bioethanol is sugar-containing plants, corn in the United States and sugar cane in Brazil. Sugar from these plants is fermented by yeast and bacteria to reduce carbohydrate to ethanol and carbon dioxide according to:
C6H12O6 -> 2 C2H5OH + 2 CO2
Some energy of course is needed to produce the plant, to harvest it and ferment it into a biofuel. The ratio of the amount of energy produced when biofuel is burned to the amount of energy used to make the biofuel is the fossil energy replacement ratio (FER). FER is around 1.2-1.4 for corn ethanol, and about 8 for sugarcane. Therefore, it can be argued that not only can corn ethanol be economically viable substitute for fossil fuels, its impact on the production of greenhouse gases would be minimal.
Another important byproduct of anaerobic digestion is biogas. Biogas is a mixture of methane, carbon monoxide, and carbon dioxide gas which occurs naturally in the bottom of swamps, marshes and landfills, or is produced by the fermentation of human and animal wastes. Biogas is a well-known fuel for cooking and lighting in a number of countries. Biogas can be converted to liquid biofuels, which can potentially replace petroleum fuels.
Biochemical conversion is also possible in presence of air or oxygen (aerobic conversion). These processes occur at much higher temperatures, and generally do not produce appreciable amount of useful fuel gases.
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Further Reading
Sims, R., Bioenergy Options for a Cleaner Environment in Developed and Developing Countries, Elsevier, 2003.
Tillman, D., Combustion of Solid Fuels & Wastes, Academic Press, 1991.
Biofuels for Transport: Global Potential and Implications for Energy and Agriculture, The Worldwatch Institute, 2007.
Biomass and Bioenergy, Science Direct Elsevier Science Publishing Company.
External Links
National Renewable Energy Laboratory: Biomass Research (http://www.nrel.gov/biomass).
US Department of Energy (http://www1.eere.energy.gov/biomass).
Biomass Energy Research Association (http://www.bera1.org).
American Bioenergy Association (http://www.biomass.org).