Methane (CH4) is a natural gas produced by the breakdown of organic material in the absence of oxygen in wetlands, termite mounds, and by some animals. In addition, methane is a greenhouse gas which is 23 times as effective as carbon dioxide, but because of its low concentration is only responsible for 15% of global warming. Mankind is also responsible for the release of methane through biomass burning, agri­culture (rice paddies), cattle, and release from gas exploration (Chapter 2, Table 2.5).

The reasons for considering methane (biogas) as a possible biofuel are as follows:

• Increases in the costs of waste disposal due to regulation and taxes have encour­aged the investigation of alternative methods of disposing of waste.

• The EU directive on use of renewable fuels and the Renewable Transport Fuel Obligation (RTFO) in the UK, and methane is a renewable fuel.

• The greater use of biomass in the UK.

• Improvements in air quality by the introduction of biofuels.

• Reduction in methane released into the atmosphere to comply with the Kyoto Protocol.

• Reduction in natural gas imports as much of the natural gas is supplied from unstable areas.

Methane is produced under anaerobic (no oxygen) conditions where organic material is broken down by a consortium of microorganisms. The three main sources of mate­rial for anaerobic digestion are given in Fig. 5.1.

In the UK, biogas is produced by anaerobic digestion of sewage sludge (190,000 t of oil equivalent) but landfill produces the bulk of the gas (1,320,000 t of oil equiva­lent) (Fig. 5.2). The bulk of the biogas is used to generate electricity. It has been estimated that the UK is capable of producing 6.3 million t of oil equivalent (Mtoe) as methane (NSCA, 2006).

Fig. 5.3. Uses of landfill biogas in the USA. (Redrawn from Themelis and Uloa, 2007.)

The generation of electricity also appears to be the main use in the USA (Fig. 5.3). In the USA, 3.7 billion Nm3 of methane is produced in landfill sites generating 1071 MW of power (Themelis and Ulloa, 2007).