Biogas is produced when organic matter decomposes without oxygen. Composed mainly of methane and carbon dioxide, it can be used in the same applications as natural gas to produce heat and electricity, or to be upgraded for use as vehicle fuel.
Biogas can be produced in plants via anaerobic digestion or fermentation of biomass. There are many different types of application for anaerobic digestion. On farms, it is used to treat manure, other agricultural residues and energy crops. Industry applies it to process food and beverage or pulp and paper waste. By municipalities, it is used to treat sewage sludge or the organic part of municipal solid waste.
Moreover, the solid material (digestate) which is left over as a bi-product of the process can be used as fertiliser.
Alternatively, gas can be collected at landfill sites where organic waste decomposes under increasing pressure, producing landfill gas.
Production on the rise
Biogas has become more attractive as an alternative source of energy as fossil fuel prices rise. In 2007, 5.9 million tonnes of oil equivalent (Mtep) was produced in the EU, a fifth more than the previous year, according to the Biogas Barometer. Around half of this was landfill gas and 15% came from waste treatment plants, with the remainder primarily coming from agricultural biogas units, which are driving growth in the EU by developing energy crops.
Moreover, climate goals are also driving use of renewable gases. Biogas has double credentials for cutting greenhouse gas emissions: it can be used to substitute polluting fossil fuels while reducing emissions of methane – 21 times more potent a greenhouse gas than carbon dioxide – which would otherwise be released into the atmosphere from landfill or traditional manure management, for instance.
The greenhouse gas savings achieved with biogas compared to fossil fuels depend on the source used and the efficiency of the production processes. Estimates put greenhouse gas emission reductions at between 50% and 150% compared to fossil fuels.
Heat and electricity from biogas
Heating and power generation are the most common applications for biogas worldwide.
Burning biogas in water and steam boilers is an established method of generating heat to serve the needs of industrial installations, or to feed into district heating systems. However, only a small number of plants use biogas as a stand-alone fuel. Most of the time, it is used in conjunction with power generation in combined heat and power units (CHP), although it is also used to produce steam for a number of industrial processes.
The green gas is also used to generate electricity in combustion engines, turbines or fuel cells. The most efficient way to do this is by using combined heat and power, employing a gas engine to generate electricity and capture the heat generated in the process to be used in heating systems.
Most biogas plants are now equipped with a CHP unit, and 58.4% of the European electricity produced from biogas in 2007 resulted from co-generation.
Tackling transport emissions
Although heating and power remain the primary uses of biogas, more and more attention is being paid to using biogas in the transport sector, where it is promoted as a means of controlling spiralling transport emissions.
Leading countries in biogas for transport include Sweden and Switzerland.
Biogas can be used as a biofuel in road transport once it has been cleaned to raise its methane content above 95% and compressed to ensure that it functions properly in vehicle engines.
The chemical composition of upgraded biogas is very similar to compressed natural gas, and the two can therefore be used interchangeably or mixed together. The biggest difference is that natural gas is a fossil fuel, so cars running on it produce roughly the same amount of CO2 emissions as a modern diesel car, while biogas both cuts CO2 emissions and reduces dependence on energy imports.
Compared to petrol, biogas can reduce greenhouse gas emissions by close to 100%, making the green fuel almost climate neutral. When produced using manure, the emission savings can become even greater.
In addition, use of biogas-fuelled vehicles improves air quality in congested cities due to lower nitrous oxide and particulate matter emissions. They are also quieter than petrol cars.
The downsides of biogas include expensive infrastructure and the cost of converting vehicles to run on gas, as well as higher maintenance costs. However, biogas is cheaper than traditional fuels and often subject to lower taxes.
Another major problem is that the network of filling stations remains insufficient throughout Europe, but it is improving.
Biogas is gaining clout in public transport, where the number of buses and waste trucks fuelled by the green alternative is increasing significantly. The gas has been taken up by several municipalities, most notably in Sweden and Lille in France, and to a lesser extent in a handful of other countries.
Stockholm, which has set itself the goal of running all its buses on renewable fuels by 2025, already has the world's largest fleet of ethanol-driven buses, and the first biogas buses started to operate in the streets of the Swedish capital in 2004. Biogas cars are exempt from Stockholm's congestion charges.
An increasing number of countries, including Sweden, the Netherlands, Switzerland, Germany and France, also allow upgraded biogas to be injected into the natural gas distribution grid in order to expand production. For this purpose, they have created national standards that biogas has to fulfil in order to prevent the contamination of pipelines.
Sweden has had national standards for biogas as a vehicle fuel since 1999, which determine its required composition and also apply to injecting it into the natural gas grid. Germany, on the other hand, requires producers to submit an additional safety data sheet describing the health hazards related to handling biogas.
Policy priorities determine use
Biogas use is determined by national frameworks such as the tax system, subsidies and the availability of gas and heating grids.
The green fuel is mainly used to produce electricity in Europe, as several countries have put in place more generous feed-in tariffs for electricity from renewable sources.
Germany is a case in point as its feed-in tariffs have spurred the use of biogas for electricity production. It has quickly built up the largest biogas production levels of all EU countries by developing methanisation units at small farms.
Another large producer is the UK, where a green certificate system, the Renewables Obligation Certificate (ROC), has given biogas production a boost, particularly at landfills. Biogas is used mainly to produce electricity and heat, while the availability for natural gas-fuelled vehicles and refuelling infrastructure remains very limited.
Sweden, on the other hand, has invested in biogas as a transport fuel due to relatively low electricity prices. It has put in place several incentive systems in favour of biogas, including carbon tax exemptions, investment subsidies for the construction of biogas units and incentives to buy cars that run on biofuels.