Methane can be released during the production, transport and use of coal, oil, and gas. Various techniques can be used to reduce these emissions, some of which can be captured for use as an energy resource (Williams, 1993; IEA Greenhouse Gas R&D Programme, 1996, 1997; IGU, 1997, 2000; US EPA, 1993, 1999a).
With coal, methane is trapped in coal seams and surrounding strata in varying amounts and is released as a result of mining. Coal mines are ventilated to dilute the methane as it is released to prevent an explosive build up of the gas. The diluted methane is then normally released to the atmosphere. The emissions can be reduced substantially by capturing some of the methane in a more concentrated form in areas of old workings in a mine or by drilling into the coal seams to release the methane prior to mining, then using it as an energy source. Around 50% of emissions from coal mining could be prevented at costs in the range US$1-4/tCeq (IEA Greenhouse Gas R&D Programme, 1996).
With natural gas, leakage of methane occurs at exploration, through transportation to final use. In North America and Europe, the major source is from fugitive emissions, often leaked from above ground installations or old cast iron or steel pipelines that were originally installed for coal- and oil- derived town gas. Vulnerable networks can be replaced with polyethylene pipes. In Russia, the main sources of leakage are from exploration, compressors, pneumatic devices, and fugitive emissions. Techniques are being applied to reduce emissions including replacing seals, increasing compressor efficiencies, and replacing gas-operated pneumatic devices. Around 45% of global emissions from gas could be eliminated and produce a saving of US$5 billion (IEA Greenhouse Gas R&D Programme, 1997). A further 10% of gas emissions could be reduced at costs up to US$108/tCeq, and a further 15% at costs up to US$135/tCeq. However, the cost of emission reduction for old distribution systems remains very high, and the reduction potential will be reached mainly as networks are replaced.
Methane and other gases often occur with oil in the ground and are brought to the surface during extraction. If there was no market for the gas, it was normally vented to the air, but since the1960s methane has increasingly been utilized, compressed and reinjected into the oil field to aid oil production or flared rather than vented. Emissions can be reduced typically by 98% by such methods and these are now common. In Nigeria, where venting is still practised, Shell has made a commitment to end continuous venting by 2003 and continuous flaring by 2008, and has started to liquefy the gas for export.
Sulphur hexafluoride, SF6, used as an insulator in electrical transmission
equipment, is covered in Section 3.5.
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