Coal seams contain pockets of methane gas, and methane molecules also become attached through pressure and chemical attraction to the microscopic internal surfaces of the coal itself. The methane is released into the atmosphere when coal miners break open gas pockets in the coal and in coal-bearing rock. Crushing and pulverizing the coal also breaks open tiny methane gas pockets and liberates the methane adsorbed in the coal. It can take days or even months for this absorbed methane to escape from the mined coal. The amount of methane released per unit of coal depends on the type of coal and how it is mined.
Some coal seams contain more methane per unit of coal than do others.
Not only is coal buried under high pressure deep in the earth able to hold more methane, but underground mining techniques allow additional methane to escape from both the coal that is not removed and from the coal-bearing rock. The table below shows methane emissions from underground and surface mining for the ten countries with the highest emissions. These ten countries produce over 90 percent of both the world s coal and coal-related methane emissions. Based on the United Nations Energy Statistics and the U.S. Bureau of Mines Cement Manufacturing Data, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 1989. Estimates of CO2 from gas flaring were taken from this source.