Approximately 90 percent of natural gas is composed of methane, but it also contains small amounts of other gases such as propane and butane. Natural gas may also come from several other sources. One source is the methane gas found in coalbeds. Until recently, coalbed gas was just considered a safety hazard to miners, but now it is a valuable source of natural gas. Another source of natural gas is the gas produced in landfills.
Landfill gas is considered a renewable source of natural gas since it comes from decaying garbage.
The gas from coalbeds and landfills accounts for six percent of the total gas supply today - that could double by the year 2010. The gas recovered from landfills is usually burned on the landfill site to generate electricity for the facility itself. Altogether, the United States produces about 23 percent of the world's natural gas each year. In 1997, the United States imported 13 percent of its natural gas from other countriesmostly from Mexico and Canada. Usually by pipeline.