Hydrates

CH4 hydrates (also known as clathrates) are thought to be responsible for emissions of 5-10 Tg CH4/year to the atmosphere. Globally, there are huge amounts of CH4 stored as hydrates - it is estimated that there is about 3000 times more CH4 locked up as hydrates (~107 t) than is currently found in our atmosphere.

CH4 hydrates occur as solid deposits in deep marine sediments and in some polar regions. They are made up of a mixture of CH4 and water (~70% CH4) that can quickly break down due to changes in temperature and pressure, to release the trapped CH4.

Large-scale decomposition of CH4 hydrate deposits has been blamed by some researchers

19%

11%

5%

24%

□ Energy

□ Ruminants

□ Rice

□ Landfills

□ Waste

□ Biomass burn

■ Other

Fig. 9.2. Anthropogenic sources of methane (CH4).

Fig. 9.2. Anthropogenic sources of methane (CH4).

also produced during fossil fuel combustion, with sources such as fossil-fuelled power stations, transport and domestic heating all being significant contributors to atmospheric CH4 concentrations. The total CH4 emissions estimate from energy-related sources is ~100 Tg/year.

At 30-50 Tg CH4/year, coal mining constitutes a large source of anthropogenic CH4 (despite the decline of coal mining in the UK). The bulk of global CH4 emissions due to coal mining actually comes from just a few, relatively deep mines around the world. During the geological process of coal formation, CH4 is formed and some of this remains trapped until it is released by mining operations. Generally, the deeper the coal seam, the greater is the amount of CH4 that is trapped. In shallow and opencast mines, trapped CH4 is often released directly to the atmosphere during mining, whereas in deeper mines it is often released deliberately via ventilation shafts, so as to prevent potentially explosive concentrations building up.

Strategies to reduce CH4 emissions from coal mines include recovery of CH4 during mine construction, during coal extraction and even after the mine is closed down. Such recovery can be economically viable, particularly in deep mines where high concentrations mean that the recovered CH4 can be used as a fuel source, and can substantially reduce emissions to the atmosphere.

As with coal, the geological formation of oil can result in large CH4 accumulations. During oil drilling and extraction, this trapped CH4 can be released to the atmosphere. Targeted collection of the CH4 associated with oil can vastly reduce emissions from this source, with the collected CH4 being either flared off (as CO2 and water) or used as an additional fuel source.

The loss of CH4 during natural gas extraction is obviously something that has both a direct GHG and an economic cost. Nevertheless, significant amounts are lost during its extraction and aboveground transfer. It is estimated that in the 1990s ~6% of the CH4 piped across Russia was lost due to leaks. More efficient collection techniques, improved targeting of buried CH4 deposits and better-maintained transfer pipelines could all help reduce incidental CH4 emissions from this source. Leaks at the end of the supply network - in homes and businesses - also represent a potentially significant source of emissions.

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