Biochemical Conversion

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Biochemical Conversion 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. Biomass from Animal Waste FYI ... How much electricity can be utilized from the average daily droppings of the three most common farm animals? Answer: • Cow 3.0 kWh • Pig 0.2 kWh • Chicken 0.012 kWh Table 6-3. Calorific values for different biomass fuelsSourceHeating Value* (MJ/kg)Sugar cane21.5Municipal solid waste13.0Garbage19.7Newspaper19.7Hardwood20.4Pinewood21.3Grass18.5Methane49.8Methanol20.0Ethanol27.0* Lower heating value 17 Office of Technology Assessment, “From Pollution to Prevention: A Progress Report on Waste Reduction,” Washington, D.C., 1987. 135 Chapter 6 - Biomass Energy 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, Ethanol Enigma Digging Deeper ... Ethanol is formed by fermenting sugar in grains such as corn, wheat, and rice, in much the same way that beer, wine, and liquors are made. The process is not new; in fact it has been around for many centuries. Before the Civil War, ethanol was used in the United States mainly as a lantern fuel and as a lubricant. In order to finance the war, US president Abraham Lincoln imposed a tax on liquors. Since ethanol was produced by fermentation of grains, it was taxed heavily and people turned to kerosene, which was poisonous and not suitable for drinking. The “Spirit Tax” was eventually repealed in 1906, and ethanol became an ideal fuel to run internal combustion engine once again. Fuel ethanol made a partial comeback, but the introduction of Prohibition in 1919 killed any chances of ethanol becoming a major fuel source. To make combustion more uniform, ethanol continued to be used as an additive, however. To increase market share, oil companies opted for lead additives, which soon proved to be a major health concern. In 1933, Prohibition ended and ethanol was put to new uses in making synthetic rubber and the production of ethanol soared again. A decade later, as WWII came to an end, grains found new market and ethanol production was curtailed one more time. Lead continued to be the primary additive for many years until the increasing level of emission from automobile exhaust forced new regulations that resulted in development of catalytic reactors, which were poisoned by the lead additives in gasoline. Even after lead was removed from gasoline, oil companies chose aromatics like benzene, toluene, and MTBE. Benzene was eventually phased out in 1990s and many states are now passing legislations to ban MTBE. Ethanol, as an alternative fuel or as an additive, has generated renewed interest. As we will show in Chapter 14, ethanol has a higher octane than gasoline and, because it is oxygenated, burns cleaner. The effect on production of the greenhouse gases is less certain. It was commonly presumed that, since ethanol is a biomass, no net greenhouse gases are produced. Recent studies, however, rebuke this claim on ground that to produce biofuels, virgin forests and grasslands must be cleared. About 2.7 times more carbon is stored in terrestrial soils and plant material than in the atmosphere, and this carbon is released when these areas are cleared (often by burning) and the soil is tilled. Cellulosic and sugar-based ethanol are considered best. The United States, under pressure from corn-producing states uses corn as source of its biofuel. Corn farming requires more land andonly yields 1/4 more energy out than the fossil energy you have to put in.i In addition, serious environmental problems, such as higher food prices, soil erosion and nutrient leaching, and substantial demand on the world’s land and water resources may not warrant production of biofuels in a large scale. i From Proceeding of the National Academy of Science of the USA, July 2006. 136 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.


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