A smokestack, stack, or chimney is a tall vertical pipe or channel used by power plants to exhaust combustion gases into the air. This height disperses pollutants over a wider area in order to minimize their impact. These gases are called flue gas, and are emitted when hydrocarbon based fuels such as coal, oil or natural gas are burned.
These flue gases originate in the boiler where the combustion occurs, and contain various pollutants depending on what kind of fuel is burned. Some of these pollutants include nitrogen oxides (NOx), sulfur oxides (SOx), carbon dioxide, carbon monoxide and particulate matter. All of these are harmful and have various effects on the environment, so many methods are employed to rid of as much of these as possible before the gas is sent into the atmosphere. Technologies such as wet and dry scrubbers, and electrostatic precipitators can remove up to 99% of certain pollutants.
Along with using these technologies, another technique is used which reduces harmful gases over a given area. This technique is to build the stack very high, because wind speeds are greater at high elevations, and the dispersion of the gases results in less gas remaining in a given area. Also, by building stacks higher than inversion layers the gas can be dispersed even more effectively. Some smokestacks are comparable to the largest buildings in the world, such as that in Figure 1, which is about as tall as the Empire State Building. Although dispersion of certain pollutants helps in some cases, this technique does not by any means solve the pollution problem of these power plants since these pollutants may cause problems downwind such as acid rain.
Visit the page on air pollution control devices to learn the specifics on how flue gases are cleaned before being sent to the smokestack.