A
flue-gas stack is a type of
chimney, a vertical pipe, channel or similar structure through which combustion product gases called
flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is
combusted in an industrial furnace, a
power plant's steam-generating boiler, or other large combustion device. Flue gas is usually composed of
carbon dioxide (CO
2) and water vapor as well as
nitrogen and excess
oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as
particulate matter,
carbon monoxide,
nitrogen oxides and
sulfur oxides. The flue gas stacks are often quite tall, up to 400 metres (1300 feet) or more, so as to disperse the exhaust pollutants over a greater area and thereby reduce the
concentration of the pollutants to the levels required by governmental environmental policy and environmental regulation.