Biogas is composed mostly of methane and carbon dioxide produced from organic material. Like natural gas, it is a versatile fuel and can be used directly to generate electricity, provide low- or high-temperature heat or to power vehicles. For transportation, it can be compressed and used in a vehicle in the same way that compressed natural gas is used. The advantage of biomethane is that it can use existing natural gas vehicle transport and fueling infrastructure, after the biometh-ane is cleaned and upgraded.
The key challenges for biogas are to grow the market and reduce costs. The use of biogas requires natural or biogas-based fuelling infrastructure and flex-fuel or dedicated natural/biogas vehicles. Alternatively, existing vehicles can be converted to run on biomethane, but at a cost and with a loss in storage space and range to accommodate the compressed biogas storage tank.
The two most promising routes for the production of bi-ogas for transportation are anaerobic digestion (AD) of organic matter and the gasification of woody biomass to produce synthetic biogas. AD is commercially mature and is already used around the world to produce biogas from organic wastes (e.g. refuse, sewage and other ef- fluents) which is upgraded for use in transport vehicles, often local buses or transport fleets. An emerging technology under demonstration in Germany is the power-to-gas technology. This uses renewable electricity from solar or wind to produce hydrogen by electrolysis, which is blended with carbon dioxide to produce "solar-methane" or "wind-methane" depending on the source of electricity. This could help smooth electricity demand and at the same time provide additional biomethane for transport applications.
AD converts biomass feedstocks with a high moisture content into a biogas. AD is a naturally occurring process and can be harnessed to provide a very effective means to treat organic materials, including energy crops, residues and wastes from many industrial and agricultural processes and municipal waste streams (Table 6.1). AD is most commonly operated as a continuous process and thus needs a steady supply of feedstock. The feedstock needs to be strictly checked and usually requires some form of pretreatment to maximise methane production and minimise the possibility of destroying the natural digestion process. Co-digestion of multiple feedstocks is most commonly practised to achieve the best balance of biogas yield and process stability.
Table 6.1: Waste feedstocks and appropriate digesters and characteristics
Source: Centre for Climate and Energy Solutions, 2012.
The two main AD products are biogas and a residue digestate. After appropriate treatment, the resdiue can be used as a bio-fertiliser. Biogas is primarily a mixture of methane (CH4) and carbon dioxide (CO2). There are some other minor constituents including nitrogen, ammonia (NH3), sulphur dioxide (SO2), hydrogen sulphide (H2S) and hydrogen.
Biogas is readily used as a fuel in power or combined heat and power (CHP) units and has the potential to be used as a vehicle fuel in the transport sector after appropriate cleaning and upgrading (IEA Bioenergy, 2011).
Germany, with 7 090 digesters, was the leading country in Europe in mid-2011 in both number and installed capacity of AD (Linke, 2011). These have been built to take advantage of the German feed-in tariff for biogas for power generation and are associated with total installed electrical capacity of 2 394 MW. Virtually all of this electrical capacity is located in the agricultural sector where maize sillage, other crops and animal slurry are used. In Germany and the rest of the world, virtually all biogas production destined for the transport sector comes from waste, as this is the cheapest feedstock.