Biomethane feedstock and production technologies

The use of crops for production of biofuels has been questioned a lot (the food vs. fuel debate) and this discussion will continue. The classification of the land used to grow the crops has an influence. When comparing the energy efficiency of different biofuels the relevant comparison basis is the fuel energy content per hectare of land used, not the yield per tonne of feedstock used.

• Landfill gas typically has a methane content of around 45 %, balance CO2 and sometimes also nitrogen (5-15 %).
• Raw biogas typically has a methane content of around 65 % (varies between 50 and 75 %, depending upon feedstock), balance CO2.
• Synthetic biomethane from gasification will be almost pure methane.

Different upgrading technologies will result in marginally different methane contents. Most will leave 2-3 % CO2 in the produced biomethane, and also leave any nitrogen in the landfill gas in the produced biomethane. Some processes may produce almost 100 % pure biomethane, e.g. cryogenic upgrading and various chemical processes. The typical composition of biomethane is 97-98 % methane and 2-3 % CO2. This is a quality which is chemically identical to Russian natural gas, the most common H-gas quality in Europe.

Biomethane delivered directly to CNG refuelling stations, or injected into the natural gas grid, is odorized and typically holds 97-97 % methane. If the NG grid at the point of injection delivers a gas quality with a higher energy content (due to minor contents of ethane, propane, butane, or pentane), propane is usually added when injecting biomethane to provide a gas mix with the same energy content per cubic metre.

The green gas concept - allows injection of biomethane into the NG grid, and the withdrawal  of an equivalent energy volume at another point of the grid (just like green electricity):