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As the economy and culture seek to expand energy alternatives and achieve a higher energy efficiency, the notion of renewable fuels has captured the public imagination. Among the renewable options like wind, solar and hydro-electricity is biogas. For decades, managing organic waste has served as a bane to farmers and home owners alike. Whether the source is livestock manure, dead flora, unused food and scraps, or animal carcasses, few will deny that strong odors emit from them. These are, in fact, gases and they are potent sources of energy generation. The key is in how to capture and utilize these emissions.
When organic substances ferment where air is absent, as inside a compost pile, biogas is produced by the disintegration of micro-organisms. This is primarily composed of methane and carbon dioxide. Methane (CH4) is a volatile gas already in use for stoves, ovens and some lighting fixtures. Carbon dioxide (CO2), meanwhile, is not only present in the combustion of organic compounds, it also exists in the air from which it gets absorbed by plants and vegetation. To be usable on a large scale, this mixture must be processed in a biogas plant.
The biogas plant has to eat: food scraps like eggshells and coffee grounds; potato peels and carrot stems; rendered fat and sludge can all serve as substrate upon which the chemical fermentation begins. Grass and leaf clippings as well as animal dung are also common inputs for a biogas plant. These are channeled into a fermenter — a light-less, oxygen-free chamber that is heated to about 104 degrees Fahrenheit. The micro-organisms then break down the substrate, facilitating the release of biogas. The resulting biogas contains not just CO2 and CH4, but also toxic hydrogen sulphide (HS). The biogas rises to the upper realm of the fermenter chamber and is then burned in the heat and biogas power plant. This is where electricity and heat are produced.
Biogas conversion can potentially work for any number of fuels as its end-product. The cleaned and purified methane can be used for ethanol, diesel, methanol, gasoline or liquified petroleum gas (LPG). Methanol, for example, can theoretically result from biomethane after the application of methanotrophic bacteria. Another way is to first chemically alter the biogas into synthesis gas, followed by the reformation of the syn-gas into methanol. Using biogas as the feedstock for methanol production enables the circular use of organic waste streams. Farms, food processors and wastewater utilities that produce biogas on-site can add methanol production as an additional revenue stream, improving overall project economics.
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