A biogas system is a technology that produces biogas. Biogas is a type of renewable energy formed by the anaerobic digestion of organic matter. Anaerobic digestion refers to bacteria-aided decomposition that occurs in an oxygen-free environment. The fuel may be used for heating or, in its concentrated form, for vehicle propulsion. A biogas system provides an airtight environment for the anaerobic digestion to occur and is usually composed of a digester and a gas holder.
The digester is an airtight tank in which organic waste is dumped and decomposed. Organic materials typically include animal waste, plant waste, and energy crops, or crops that are grown for the express purpose of fuel production. Bacteria within the digester tank breaks down the waste and, as it decomposes, gases such as carbon monoxide, methane, hydrogen, and nitrogen, are released.
The gas holder is another tank in the biogas system that harnesses the gases emitted by the decomposing waste, or slurry. Through a pressurized system, the gases released in the digester are conducted into a hole in the gas holder. The holder is specially designed to allow gas to flow freely into the holder while preventing any harnessed gas from escaping back into the digester or into the outside environment. This is important, both for efficiency and for safety, because many of these gases are combustible and may cause explosions when mixed with oxygen or other gases. Once the gas is harnessed, it can then be used as fuel.
To improve efficiency in the biogas system, the slurry in the digester should be kept at a slightly basic pH. The digester should also be kept at a temperature of 29-41 degrees Celsius (84.2-105.8 degrees Fahrenheit) to ensure fast decomposition with optimal gas production. The slurry should be stirred occasionally to prevent a hard crust from forming on top of the wastes, blocking the gases from traveling toward the holder.
Operating procedures may vary depending on the construction of the biogas system. An above ground biogas plant is easier to maintain and benefits from solar heating, but takes more care in construction. A below ground biogas system is cheaper to construct and easier to feed, but more difficult to maintain. Feeding, or adding organic materials to the digester, will vary based on design. A batch feeding biogas system decomposes mostly solid wastes that are added to the tank in installments, while continuous feeding models constantly feed mostly liquid wastes to the digester.
Biogas is often preferred to fossil fuels, such as coal or oil, because it is renewable, costs less, makes use of otherwise wasted materials, and has a lower carbon output. Carbon, in small amounts, is a vital component of a healthy atmosphere, but becomes problematic when too much is emitted. The carbon contained in fossil fuels has been buried for such a long time in ancient organic matter 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.