Biogas Electricity (Large-scale)

Biogas is a combustible gas mixture produced during the anaerobic digestion of organic matter in an anaerobic biogas reactor (e.g. large-scale digester, biogas settler, digestion of organic waste, etc.; see also anaerobic digestion, general factsheet).  Biogas can be recovered and used either directly for cooking and lighting or it can be transformed into heat in a gas heater system or into combined heat and power (CHP) in a cogeneration unit (MES et al. 2003; JENSSEN et al. 2004; WRAPAI 2009). 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.

Typically biogas is composed of methane (50–75%), carbon dioxide (25-50%) and varying quantities of water (H₂O) and hydrogen sulphide (H2S) (PIPOLI 2005; GTZ 2009). Other compounds can also be found, especially in waste dump biogas: ammonia (NH₃), hydrogen (H₂), nitrogen (N₂) and carbon monoxide (CO) (SCHALLER 2007). The exact composition and amount of each gas in the mixture depends on many factors such as the type of digester and the kind of organic matter, which is digested. Different sludge compositions require different/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 (ISAT/GTZ 1999 Vol. I).

There are different kinds of technology to convert the chemical energy in the biogas into mechanical and thermal energy and finally electricity. 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 for biogas energy conversion are gas turbines, which is an internal combustion engine (e.g. gas turbines, reciprocating engines) and external combustion engines (e.g. Stirling engines).

For small-size heat engines, external 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 uses 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 available (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.

Large-scale biogas conversion plants are almost always cogeneration plants based on gas turbines (internal combustion engines). The mechanical energy produced in such plants comes always from the pressure related to the change in temperature of a gas. The temperature gap required can lie either in a high temperature range or a low one. Topping-cycle plants for instance primarily produce electricity from a steam turbine based on a low temperature cycle (WRAPAI 2009): The heat from the combustion of the biogas activates a steam turbine, the exhausted steam is condensed and the low temperature heat released from this condensation is utilised for district heating.
Bottoming cycle plants produce high temperature heat for industrial processes. Then, a waste heat recovery boiler feeds a steam electrical plant (WRAPAI 2009). Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing, so they are less common.

Cogeneration plants are commonly found in district heating systems of big towns, hospitals, prisons, oil refineries, paper mills, wastewater treatment plants and industrial plants with large heating needs (WRAPAI 2009). Typical large-scale biogas conversion CHP plants are: Gas turbine generating electricity and using the waste heat; Gas engines using a reciprocating engine; Combined cycle power plants (CCPP) adapted for CHP; Steam turbine CHP plants that use the heating system as the steam condenser for the steam turbine; Molten-carbonate fuel cells have a hot exhaust, very suitable for heating (WRAPAI 2009).

Biogas systems are an environmentally 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.