What’s a biogas plant?

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A biogas plant is an anaerobic digester that produces biogas from animal waste or energy crops. Energy crops are low-cost crops grown for the purpose of biofuel, rather than food. Biofuels are liquid, gaseous or solid fuels made up of living 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 by anaerobic, or oxygen-free, digestion of organic matter by bacteria. A biogas plant consists of a digester and a gas holder.

The digester is an airtight container in which the waste is discharged and decomposed, and the gas container is a tank that traps the gases emitted by the sewage. The bacteria inside the digester tank break down the waste, and as it decomposes, gases such as carbon monoxide, methane, hydrogen and nitrogen are released. Through a pressurized system, the gas container conducts the flow of these gases upwards into a hole in its drum. The hole is specially designed to allow gases to pass freely into the holder while preventing gas from escaping into the digester. In a controlled environment, the gases are then burned, or reacted, with oxygen to create an energy source for processes such as heating and vehicle propulsion.

The construction of a biogas plant can vary depending on the amount of gas needed, the amount of waste on hand and whether the digester is designed for batch or continuous feed. Batch feed systems primarily use solid waste which is added to the tank in installments, while continuous feed models feed primarily liquids to the digester. A biogas plant can be built both above and below ground, with advantages and disadvantages for both models. An above ground biogas plant is easier to maintain and benefits from solar heating, but is more carefully constructed because it must be built to handle the internal pressure of the digester. An underground biogas plant is cheaper to build and easier to fuel, but more difficult to maintain.

To facilitate rapid decomposition with optimal gas production, digesters are often maintained between temperatures of 29°C and 41°C (84.2°F-105.8°F). In an attempt to neutralize the slurry, more acidic carbon dioxide will be created, which is a desired gas. The slurry inside the tank also needs to be stirred frequently to prevent a hard crust from forming on top of the waste. A crust can trap gases within the slurry and impede the machinery’s ability to harness the gases.

Biogas is increasingly preferred over fossil fuels, or fuels made from ancient organic matter such as coal or oil. Carbon, in small amounts, is a vital component of a healthy atmosphere, but becomes problematic when too much is added into the circulation. The carbon in fossil fuels has been buried for so long that it is no longer part of the carbon cycle. When it is released through the burning of fossil fuels, it increases the carbon concentration. Biogas, however, comes from living or recently dead organisms whose carbon content is still within the cycle, so burning these fuels affects the carbon concentration in the atmosphere less.

In addition to carbon production, biogas is often preferred over fossil fuels because it is a low-cost renewable energy source and uses otherwise waste materials. Biogas is also a valuable energy source for developing countries, as it can be produced on 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 can also cause deforestation, water pollution, soil erosion and a negative economic impact on oil-producing nations.

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