Methane emissions from coal mines

Coal mining releases methane by underground and surface mining as well as by coal handling (post mining) activities. Since methane can create an explosive mixture with oxygen it must be removed for safety reasons by large-scale ventilation systems. They move massive quantities of air through the mines. Only the gas contains methane in very low concentration. Underground mining is the most important source of fugitive mine methane by far with about 70 percent of all coal mining related emissions. Methane is also produced from degasification systems (gas drainage systems) that employ wells to recover methane.

The background of methane from coal is as follows: Methane in coal was produced when vegetation was converted during the coalification process in earlier periods of Earth's history. It was stored under high pressure within coal seams and surrounding rock. It is liberated when mined. The quantity of the methane emitted depends on the carbon content in the coal (coal rank) and the coal depth. Coals such as anthracite have the highest carbon content and release most methane. Lignite is lowest in methane deliberation. Specific methane emissions range from about 0.7 to over 310 m3 per ton, e.g. in the U.S. mining industry.

Globally, coal mine methane accounts for 8 percent of total methane emissions resulting from human activities. In 2000, worldwide emissions totaled 380 Mio t CO2-eq. By 2020, the world's coal mines are expected to produce annual emissions of 450 Mio t CO2-eq./a.

More than 90 percent of the world's methane from coal mines is from only 11 countries. Amongst them China (40 percent) and USA (14 percent) as the biggest coal producers, together are emitting more than 50 percent of the world's total. Other significant emittents include Russia and Ukraine (7 percent each), Northern Korea (6 percent) and Australia (4 percent). India, Germany, Kazakhstan, South Africa, and the United Kingdom contribute 2 to 3 percent each. In some countries, such as Mexico and Vietnam, emissions of methane from mines are low on a national level, but some of their coal mines produce substantial volumes (EPA, 2007e), and are worthy of note therefore.

For decades methane in coal mines was considered a burden and a costly safety hazard. But recent projects have proven that it can provide many benefits to the mine and the environment. A variety of uses is possible. The optimal use at a given location is dependent on the quality of the methane containing gas, the availability of end-use options and the project economy. Possible uses include natural gas pipeline injection, electric power production, district heating, co-firing in boilers, mine heating, coal drying, vehicle fuel, flaring, and manufacturing or industrial uses such as feedstock for carbon black, methanol, and dimethyl ether production. Low concentration methane can be oxidized and the resulting thermal energy can be used to produce heat, electricity, and refrigeration.

The following examples will give an overview of the situation in the most important countries and initiatives to reduce mining methane emissions.

• In China it is estimated that there are more than 26,000 coal mines, mostly (90 percent) underground, which produce about 1.4 billion t of coal (2003). 50 percent of the large mines are emitting gas; they account for nearly 50 percent of coal production and about 86 percent of coal mine methane emissions. By 2004 over 200 mines were equipped with methane drainage systems. Over 1.5 billion m3 of methane had been recovered with approximately 40 percent of drained gas utilized. The gas is used for town gas, power generation, industrial applications, and vehicle fuel (EPA, 2007f).

• An example of a successful Chinese project is the Jincheng Anthracite Mining Group which produces anthracite coal at several highly gassing mines. They started methane capturing in 1995. The gas was used to fuel a 1.6 MW power station. Capacity was increased to 4 MW in 2002. The effect on GHG emissions is estimated to be about 40,000 t CO2-eq. per year. As a next step a 120 MW power station using the latest technology will be established, which by 2008 will serve the total power needs of the mine as well as 90,000 households and commercial and industrial consumers in the area. The project will save emissions of 7 Mio t CO2-eq. annually.

• U.S. mining industry accounts for about 10 percent of man-made methane emissions. In 2003 4.9 billion m3 were liberated from which one-quarter was recovered and used. GHG effects from U.S. coal mining methane declined by 170 Mio t CO2-eq. from 1990 to 2005. About 15 projects are in operation at active mines, and more than 20 in abandoned mines. The gas is used for mine heating, pipeline injection and power generation. Gas sales generate about US$ 90 Mio in revenues annually.

• For further improvement of the situation on an international level a nonprofit organization "Methane to Markets initiative" was established in recent years. All important coal producing countries and regions are members, including USA, EU, and China. It aims to improve awareness of emission reduction opportunities and the value of the recovered methane as well as to implement projects worldwide. A data submission form for project feasibility studies (EPA, 2007e) as well as a summary of methane technologies for mitigation and utilisation was prepared to support decision making processes (EPA, 2007e).

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