Most thermal power plants generate electricity by burning fuel (most often pulverized coal or natural gas) in air to release its thermal energy. The heat boils water into steam which propels a turbine to generate electricity; the combustion generates CO2 and water vapor, which, along with nitrogen from the air, are major components of the flue gas. Flue gas also contains small amounts of sulfur dioxide and nitrogen oxides due to nitrogen from the air and sulfur from the fuel. Postcombustion capture, the most popular and intuitive method, removes CO2 from this gas mixture.