Biogas is a biofuel produced by the anaerobic decomposition of organic materials. The digester and gas container are the main components of most biogas plants. The gas can be used for heating or vehicle propulsion. Biogas design varies according to materials and scale. Biogas is preferred over fossil fuels for environmental and economic reasons. Critics argue that food crops grown for biogas production can create a global food shortage and cause deforestation, water pollution, soil erosion, and a negative impact on oil-producing nations.
Biogas design differs according to available materials and production scale. 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 specifically for fuel production. The gas formed from this process can be used for heating purposes or, in its concentrated form, for vehicle propulsion. Although biogas design differs in details, most biogas plants have similar main components, including a digester and gas container.
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. To facilitate rapid decomposition with optimal gas production, the tank is maintained between 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 give off more acidic carbon dioxide, a desired decomposition product.
Through a pressurized system, the gases released in the digester are led into a hole in the gas holder drum. The biogas design of the gas container is specialized to allow gas to flow freely in the container, preventing trapped gas from escaping into the digester or external environment. This is important, both for efficiency and safety, because many of these gases are combustible and can cause explosions when mixed with oxygen or other gases. The slurry inside the tank also needs to be stirred occasionally 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.
The biogas design can vary depending on the amount of gas needed, the amount of waste at hand and whether the digester is built for batch feeding or continuous feeding. Batch feed systems primarily use solid waste which is added to the tank in installments and continuous feed models continuously feed liquids to the digester. A biogas project can place a plant above or below ground, although there are advantages and disadvantages to both designs. An above ground biogas plant is easier to maintain and can benefit from solar heating, but requires more construction investment 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 clean and repair.
Biogas is often preferred over fossil energy sources, such as oil or coal, for both environmental and economic reasons. The increasing concentration of carbon in the atmosphere has become a central issue in the global warming problem. Although both biogas and fossil fuels emit carbon, fossil fuels release carbon that has been buried for many years in ancient biomass and effectively removed from the carbon cycle. The carbon released during biogas production and use has only recently been stored in the form of organic matter and is still part of the cycle. Therefore, when it is released, it does not cause a change in the carbon concentration in the atmosphere.
Proponents of biogas production also prefer biogas to fossil fuels because it is a low-cost renewable energy source that uses otherwise wasted materials. Biogas design can cater for small-scale sites, making it a viable option for developing country regions. Critics of biogas design argue that food crops grown for the purpose of biogas production will create a global food shortage. Biofuels can also cause deforestation, water pollution, soil erosion and a negative impact on oil-producing nations.
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