Biogas design differs based on the materials at hand and the scale of production. Biogas is a type of biofuel formed by the anaerobic, or oxygen-free, decomposition of organic materials by bacteria. Organic materials typically include animal and plant waste, and energy crops, or crops that are grown for the express purpose of fuel production. The gas formed from this process may be used for heating purposes or, in its concentrated form, for vehicle propulsion. While biogas design differs in the particulars, most biogas plants have similar principle components, including a digester and 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. To facilitate fast decomposition with optimal gas production, the tank is kept between the temperatures of 29°C and 41°C (84.2°F-105.8°F). It is best to keep the slurry in the tank at a slightly basic pH to ensure faster decomposition. In an attempt to neutralize the slurry, the material will tend to emit more acidic carbon dioxide, a desired product of decomposition.
Through a pressurized system, the gases released in the digester are conducted into a hole in the drum of the gas holder. The biogas design of the gas holder is specialized 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. The slurry within the tank must also be occasionally stirred to prevent a hard crust from forming on top of the wastes. A crust can trap the gases within the slurry and impede the machinery’s ability to harness the gases.
Biogas design may vary depending on the amount of gas needed, the amount of waste at hand, and whether the digester is constructed 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 liquids continuously to the digester. A biogas design may place a plant either above or below ground, though there are advantages and disadvantages to both models. An above ground biogas plant is easier to maintain and able to benefit from solar heating, but takes more investment 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 more difficult to clean and repair.
Biogas is often preferred to fossil fuel energy sources, such as oil or coal, for both environmental and economic reasons. The rising concentration of carbon in the atmosphere has become a central issue in the problem global warming. Though both biogas and fossil fuels emit carbon, fossil fuels release carbon that has been buried for many years in ancient biomass and effectually removed from the carbon cycle. Carbon released during biogas production and use has been stored in the form of organic matter only recently and is still part of the cycle. Therefore, it does not cause as much of an upset in the carbon concentration in the atmosphere when released.
Proponents of biogas production also prefer biogas to fossil fuels because it is a low-cost, renewable source of energy that uses otherwise wasted materials. Biogas design can cater to small-scale sites, making it a viable option for regions in developing nations. Critics of biogas design argue that food crops grown for the purposes of biogas production will create a global food shortage. Biofuels may also cause deforestation, water pollution, soil erosion, and a negative impact on oil-producing nations.