After coal and oil, natural gas is the third type of fuel most widely used for electricity generation. The main component of natural gas is methane (85–90%); it also contains propane, ethane, butane, some inert gases such as nitrogen, helium and carbon dioxide, and trace amounts of other gases.
Natural gas is the most rapidly growing energy source in the world, and is considered to be the cleanest fossil fuel. Most of the air pollutant emissions from natural gas combustion processes are nitrogen oxides and greenhouse gases (mainly CO2).
Other emissions are small amounts of particulate matter, sulfur oxides, and trace amounts of metals. When natural gas is burned, it produces only half as much CO2 as an equivalent amount of coal (in thermal energy terms); therefore, if this cleaner fuel were substituted for coal, a reduction in emissions could be achieved. However, this is sometimes economically and/or strategically infeasible [45, 46, 48].