Introduction to biogas and biomethane
Biogas is produced by the anaerobic digestion (decomposition in the absence of air) of organic matter such as dead animal and plant material, manure, sewage, organic waste, etc. Bio-gas normally contains about 40% CO2 and 60% methane. Biomethane is the term used for bio-gas which has been upgraded to a high purity methane content (<95%) with unwanted contaminants removed and is suitable for use as a vehicle fuel.
Biomethane is completely interchangeable with natural gas use in a vehicle.
The major benefit of biomethane is that it is carbon neutral. The carbon dioxide emitted during the combustion of the gas is equal to that absorbed by the organic matter during its growth. This makes biomethane a sustainable and renewable fuel. Biomethane tends to be more expensive to produce than natural gas, however the carbon savings compared with diesel, on a well-to-wheel basis start at around 60% and can go to over 100% depending on the feedstock used to produce the biomethane. Savings of over 100% are possible with a feedstock such as liquid manure, which when left to decay naturally would emit a large amount of methane to the atmosphere which is a much more powerful greenhouse gas than CO2.
The UK is a large producer of biogas from landfill and other organic waste streams; however the majority of this biogas is currently used to produce renewable electricity. However, support mechanism, such as the RTFO and the RHI, are encouraging the increased production of biomethane to support transport applications.
Green Gas Certification schemes allow a mechanism for fleets to purchase the green credentials of biomethane (injected into the grid elsewhere) when drawing gas from their local grid connection point. For more information see http://www.cngservices.co.uk/green-gas-certification-scheme/ or http://greengastrading.co.uk/.
Introduction to natural gas
Natural gas is formed over millions of years from the decomposition of organic matter. Natural gas consists mainly of methane (CH4) along with smaller quantities of other hydro-carbon fuels such as ethane, propane and butane. Methane has the lowest carbon:hydrogen ratio of any hydrocarbon (1:4), which means it has the lowest carbon intensity of any hydrocarbon.
The UKs extensive national gas grid provides an abundant source of methane which can be extracted, compressed and used in vehicles as a fuel. Compressed natural gas (CNG) is stored on the vehicle in high-pressure tanks at circa 200 to 250 bar.
A natural gas liquefaction plant at Avonmouth, operated by the National Grid, is currently the sole UK source of liquefied natural gas (LNG) supply to vehicle refuelling stations. On the vehicle LNG has a higher energy density than CNG which means more fuel can be stored in the same space, extending range and reducing refuelling frequency. The Avonmouth plant is due to be decommissioned in 2018. A new LNG terminal being constructed at the Isle of Grain (due to open during 2015) will have the capacity to fill 36 gas delivery road tankers per day to support the growing domestic market for LNG road fuel.