Flue gas is emitted from a flue or pipe when fossil fuels are burned. This gas is what makes up the discharge or exhaust from a fireplace, oven, boiler, furnace, etc. Flue gas cleaning is often associated with power plants or other facilities that emit a large amount of combustion exhaust.
Flue Gas Composition
The composition of flue gas depends on what is creating it. Typically, flue gas consists mostly of nitrogen. It also contains carbon dioxide, water vapor, excess oxygen and heavy metals (lead, mercury, cadmium). A small percentage is made up of pollutants such as carbon monoxide (CO), nitrogen oxides (NO and NO2), hydro-fluoric acid (HF), hydrochloric acid (HCI), sulfur dioxide (SO2) and particulate matter (dust or fly ash). As emission of gases into the atmosphere is strictly regulated by the EPA (Environmental Protection Agency), cleaning flue gas is very important.
Power plants often clean flue gas with a series of chemicals and scrubbers, which remove pollutants. Electrostatic precipitators or fabric filters remove particulates such as fly ash. Sodium bicarbonate is injected into the gases to neutralize acids and absorb heavy metals. Desulfurization removes sulfur dioxide produced. Nitrogen oxides are removed by high temperature or with ammonia or urea. The process converts the nitrogen from an oxide to a gas. A bag filter removes active carbon or lignite coke and fly ash.
There are several new technologies being developed to try and remove flue gas pollutants, such as sorbent absorption and capture in inert solids. While still works in progress, the creation and implementation of new cleaning processes, especially for large-output facilities, will help insure our air quality.