At present most methane from anaerobic digestion is used for cooking and in gas or dual fuel engines (diesel engines where part of the fuel is substituted by biogas) to generate electricity with an overall efficiency of 30—50%. In Western Europe part of the excess heat is used in residences and factories. In a few instances fuel cells are used to generate electricity and high — temperature process heat. It is, however, better to remove the carbon dioxide and to inject the gas into the natural gas grid. It can then be used to generate elec­tricity in 60% efficient combined cycle plants

In Europe and in several states of the United States there are requirements to gradually introduce biomass — derived fuels in the transport sector. Approximately 5 million cars currently run on compressed natural gas and could run on compressed methane from anaerobic digestion.

An alternative is to liquefy the gas and use the lique­fied gas as biofuel in vehicles. This is done in Snurrevar — den (Norway) and Gasum (Finland).

Anaerobic digestion and the use of compressed methane is more energy efficient than the hydrolysis of cellulose and hemicellulose to sugars and conversion of these sugars into alcohol. This alcohol has to be distilled in order for it to be used as a transportation fuel.

In areas where there is no natural gas infrastructure, methane in high-pressure bottles can replace bottled liq­uefied petroleum gas (LPG or propane). Energy den­sities of 20% of that of LPG bottles can be reached at a pressure of 4 MPa using bottles filled with activated carbon.