How They Work: “How Biogas Plant Works”

Many times, we generate lots of waste and leave it to decay at its own pace. Such waste may include human, animal and plant waste, such waste contains some elements of carbon, hydrogen and oxygen. It is these elements that sustain life of many forms. Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel. Biogas is produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and –more problematic- energy crops. Biogas comprises primarily methane and carbon dioxide and may have small amounts of hydrogen sulphide, moisture and siloxanes. The gases methane, hydrogen and carbon monoxide can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a fuel in any country for any heating purpose, such as cooking. It can also be used in anaerobic digesters where it is typically used in a gas engine to convert the energy in the gas into electricity and heat. Biogas can be compressed, much like natural gas, and used to power motor vehicles. In the UK, It is estimated to have the potential to replace around 17% of vehicle fuel. Biogas is a renewable fuel, so it qualifies for renewable energy subsidies in some parts of the world. Biogas can also be cleaned and upgraded to natural gas standards where it becomes bio methane. Biogas plant produces biogas and bio-fertilizer from biological wastes of agricultural and food industries as well as special energy crops by means of oxygen-free fermentation (anaerobic digestion). Biogas plant - Industrial biogas plant is a construction project which consists of equipment for 70-80%. Main part is closed reactors (or digesters, methane tanks, bioreactors) made of reinforced concrete or glass-fused-to-steel tanks. Most of substrates can be mixed with each other. The difference is only in substrate loading systems. For solid substrates there are screw loaders and for liquid substrates tanks with pumping stations. Facilities are of modular type with 8-24m in diameter and up to 9m in height. In case plant capacity increased the number of digesters is increased accordingly. Liquid biological waste is pumped to biogas plant by sanitary pump or extraction pipeline. Sewage pumping station (SPS) is located in a separate service room. Solid biological waste (manure, dung) delivered by belt conveyor, and in case of manure or dung storage, delivery made by tractor. Liquid wastes initially come to primary tank. In primary tank waste homogenized and heated (sometimes cooled) to required temperature. As a rule such tank has 2-3 days storage capacity. Solid waste can be loaded to that tank as well for homogenization or get into digester through screw loader. From homogenization tank and screw charger biomass (manure, dung or distillery waste) comes to digester (biological reactor, methane tank, and fermenter). Biological reactor is gas-proof tank made of acid-resistant concrete or glass-fused-to-steel tanks . Reactor is heat-insulated. The heat-insulation is calculated depending on the biogas plant site climate conditions. For microorganisms’ vital activity constant and even temperature inside the digester must be kept, usually it is mesophilic temperature mode (+30-41°С). In some cases termophilic mode of temperature is used (about 55°С). Biomass mixing inside the digester is made by several ways and depends on the type of raw material, its humidity and other features. Mixing can be done by inclined mixers and submersible mixers. Al mixer types are made of stainless steel. Digesters are heated by heat agent . Heating system is a network of pipes, which can be built-in to reactor wall or to be mounted to interior side of the digester wall. In case biogas plant equipped with co-generation unit, digester can be heated by generator cooling water. If biogas plant is purposed only for biogas production hot water is taken from a special biogas water boiler. Biogas plant self-energy and heat consumption usually makes from 5% to 10% of total power produced. The fermentation process is made by anaerobic microorganisms, which are injected into the digester only once during the biogas plant start up. Digester is a hermetically sealed container that is environment friendly. As end products we have: biogas and bio-fertilizer (solid and liquid). Biogas is stored at a gas holder. Inside the gas holder pressure and biogas composition is evened. From the gas holder by means of gas supply system biogas is constantly supplied to gas of diesel-gas combined heat and power system. Gas system has integrated systems for biogas drying, hydrogen sulphide removal and gas conditioning. Big biogas plants are equipped with biogas flares. In case plant owners do not need electricity as a final product, biogas plant can be equipped with CO2 removal system. Digested biomass is turned into fertilizer. Liquid fraction is separated from solid fertilizer fraction with the help separator and is stored in fertilizer storage. eddie@afrowebs.com