Biogas Electricity (Small-scale)

Biogas is a combustible gas mixture produced during the anaerobic digestion of organic matter in an anaerobic biogas reactor (e.g. small-scale digester, biogas settler, digestion of organic waste, anaerobic baffled reactor, etc; see also anaerobic digestion, general factsheet). During anaerobic digestion, wastes are treated and degraded and biogas is produced. Anaerobic treatment also has the advantage over aerobic treatment of a smaller emission of greenhouse gases. Therefore, biogas is a renewable green energy source.

Biogas consists mostly of methane (CH₄, around 65-70%) carbon dioxide (CO₂, around 25-30%) and varying quantities of water (H₂O) and hydrogen sulphide (H₂S) and some trace amounts of other compounds, which can be found, especially in waste dump biogas (e.g. ammonia, NH3, hydrogen H₂, nitrogen N₂, and carbon monoxide, CO). The amount of each gas in the mixture depends on many factors such as the type of digester and the kind of organic matter. Diverse sludge composition requires diverse/specialised reactor designs to achieve a high conversion.

Methane is the valuable component under the aspect of using biogas fuel. The calorific value of biogas is about 6 kWh/m3, which corresponds to about half a liter of diesel oil and can be utilised directly as a heat source or to produce electricity. In all cases, the biogas must be dehumidified and purified before combustion; otherwise it can damage the gas engine.

Various technologies to generate electricity from biogas on a household level are available. In principle, the chemical energy of the combustible gases is converted to mechanical energy in a controlled combustion system by a heat engine. This mechanical energy then activates a generator to produce electrical power. The most common heat engines used in for biogas energy conversion are gas turbines and combustion engines. Combustion engines can be either internal combustion engine (e.g. reciprocating engine) or external combustion engine (e.g. Stirling engine).

For small-size heat engines, combustion engines are popular as they are more efficient and less expensive than small gas turbines. However, gas turbines may be more efficient when operating in a cogeneration cycle producing heat and electricity. Cogeneration or combined heat and power (CHP) describe the simultaneous generation of both electricity and useful heat. Heat engines (also thermal power plants) in general do not convert all of their thermal energy into electricity. In most cases, a bit more than half is lost as excess heat. By capturing the excess heat, CHP use heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants (WRAPAI 2009). This means that less fuel needs to be consumed to produce the same amount of useful energy. By-product heat at moderate temperatures (100-180°C) can also be used in absorption chillers for cooling (WRAPAI 2009). A plant producing electricity, heat and cold is sometimes called trigeneration or more generally a polygeneration plant.

Micro cogeneration is a so-called distributed energy resource (DER). Biogas is burned for running a generator (e.g. micro turbine). The installation is usually less than 5 kWe (Kilowatts-electrical, WRAPAI 2009). Instead of burning fuel to merely heat space or water, some of the energy is converted to electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid (WIKIPEDIA 2010). 

Mini cogeneration is a DER producing usually more than 5 kWe and less than 500 kWe (WRAPAI 2009) and the excess energy is generally fed into the electricity grid. To be viable a good base load for electrical demand and heat demand must exist (WIKIPEDIA 2010).

Current Micro- and Mini CHP installations use five different technologies: micro turbines, internal combustion engines, external combustion engines (stirling engines), steam engines and fuel cells.

Biogas systems are an environmental friendly way of energy production and have a positive impact on climate change. In fact, the contribution of a methane molecule (CH₄) to the greenhouse effect is 21 times greater than that of a carbon dioxide molecule (SUSANA 2009). Therefore burning methane, even though producing CO₂, reduces its impact on the environment.