Technology
Biogas plant
2017 December, SweetcrudeReports Edited by Theodore Obiako
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Biogas, a renewable alternative for cooking gas, electricity and organic fertiliser
T
he rising demands for energy from a growing population have led to the burning of increasing amounts of fossil fuels, which generate carbon dioxide emissions. These emissions, along with other greenhouse gas emissions, are leading to global warming - which have very serious consequences for the environment. With Fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly considered to be a possible alternative for the future. Biofuels are produced from natural products, often plant biomass containing carbohydrate. Some biofuels are produced by using microorganisms to anaerobically ferment carbohydrate in the plant material, as is the case with bioethanol and biogas production (each process uses different microorganisms). Biogas is a major biofuel being increasingly used as alternative to fossil fuels. Biogas is mixture of different gases produced by the breakdown of organic matter in the absence of oxygen referred to as anaerobic digestion. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste.. Biogas is primarily methane (CH4) and carbon dioxide (CO ) and may have small amounts of hydrogen sulfide (H2S), moisture and siloxanes. The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It can also be used in a gas turbines to convert the energy in the gas into electricity and heat. Biogas can be compressed, the same way as natural gas is compressed to CNG, and used to power motor vehicles 2
Biogas Digesters Biogas is produced commercially using a Biodigester plant. A biogas plant (air-tight tanks with different configurations) is an anaerobic digester that treats organic wastes from homes and farms. These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. During t h e p r o c e s s , t h e microorganisms transform biomass waste into biogas (mainly methane and carbon dioxide) and digestate. Internal combustion engines such as gas turbines are then used for the conversion of biogas into both electricity and heat. The digestate is the remaining inorganic matter that was not transformed into biogas. It can be used as an agricultural fertilizer. From 1 kg of decommissioned kitchen bio-waste, 0.45 mÂł of biogas can be obtained. There are several types of biogas systems and plants that have been designed to make efficient use of biogas. While each model differs depending on input, output, size, and type, the biological process that converts organic waste into biogas is uniform. Biological breakdown To produce biogas, organic
matter ferments with the help of bacterial communities. Four stages of fermentation move the organic material from their initial composition into their biogas state. 1. The first stage of the digestion process is the hydrolysis stage. In the hydrolysis stage insoluble organic polymers (such as carbohydrates) are broken down, making it accessible to the next stage of bacteria called acidogenic bacteria. 2. Second stage, the acideogenic bacteria convert sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. 3. At the third stage the acetogenic bacteria convert the organic acids into acetic acid, hydrogen, ammonia, and carbon dioxide, allowing for the final stage- the methanogens. 4. Fourth stage, the methanogens convert these final components into methane and carbon dioxide- which can then be used as a flammable, green energy.
Fig 1. Biological breakdown of biomass to biogas CONTINUES ON PAGE 40