A biogas plant is an anaerobic digester that produces biogas from animal wastes or energy crops. Energy crops are cheap crops grown for the purpose of biofuels, rather than food. Biofuels are liquid, gaseous, or solid fuel made from live or recently dead organic material known as biomass, as opposed to fossil fuels, which are composed of ancient biological materials. Biogas is a type of biofuel created via anaerobic, or oxygen-free, digestion of organic matter by bacteria. A biogas plant is composed of a digester and a gas holder.
The digester is an airtight container in which the waste is dumped and decomposed, and the gas holder is a tank that harnesses the gases emitted by the slurry. Bacteria within the digester tank breaks down the waste and, as it decomposes, gases such as carbon monoxide, methane, hydrogen, and nitrogen, are released. Through a pressurized system, the gas holder conducts the flow of these gases upward into a hole in its drum. The hole is specially designed to allow gases to pass freely into the holder while prohibiting any gases from escaping back into the digester. In a controlled environment, the gases are later combusted, or reacted, with oxygen to create an energy source for such processes as heating and vehicle propulsion.
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Construction of a biogas plant may vary depending on the amount of gas needed, the amount of waste at hand, and whether the digester is designed for batch feeding or continuous feeding. Batch feeding systems use mostly solid wastes that are added to the tank in installments, and continuous feeding models feed mostly liquids to the digester. A biogas plant may be constructed either above or below ground, with advantages and disadvantages to both models. An above ground biogas plant is easier to maintain and benefits from solar heating, but takes more care in construction because it must be built to handle the internal pressure of the digester. A below ground biogas plant is cheaper to construct and easier to feed, but is more difficult to maintain.
To facilitate fast decomposition with optimal gas production, digesters are often kept between the temperatures of 29°C and 41°C (84.2°F-105.8°F). In an attempt to neutralize the slurry, more acidic carbon dioxide, which is a desired gas, will be created. The slurry within the tank must also be frequently stirred to prevent a hard crust from forming on top of the waste. A crust can trap the gases within the slurry and impede the machinery’s ability to harness the gases.
Biogas is increasingly preferred to fossil fuels, or fuels made from ancient organic matter like coal or oil. Carbon, in small amounts, is a vital component of a healthy atmosphere, but becomes problematic when too much is added into circulation. The carbon contained in fossil fuels has been buried for such a long time that is no longer part of the carbon cycle. When it is released through burning of fossil fuels, it raises the carbon concentration. Biogas, however, comes from live or recently dead organisms whose carbon content is still within the cycle, so burning these fuels does less to upset the carbon concentration in the atmosphere.
In addition to carbon output, biogas fuel is often preferred to fossil fuels because it is a low cost, renewable source of energy and it uses otherwise wasted materials. Biogas is also a valuable energy source for developing nations, as it can be produced in small-scale sites. Biogas fuel, however, also has its critics. Some argue that energy crops detract from food agriculture and will create a global food shortage. Biofuels may also cause deforestation, water pollution, soil erosion, and a negative economic impact on oil producing nations.