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Flue gas is gas that exits to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, it refers to the combustion exhaust gas produced at power plants. Its composition depends on what is being burned, but it will usually consist of mostly nitrogen (typically more than two-thirds) derived from the combustion air, carbon dioxide (CO₂) and water vapor as well as excess oxygen (also derived from the combustion air). It further contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides.

At power plants, flue gas is often treated with a series of chemical processes and scrubbers, which remove pollutants. Electrostatic precipitators or fabric filters remove particulate matter and flue gas desulfurization captures the sulfur dioxide produced by burning fossil fuels, particularly coal. Nitrogen oxides are treated either by modifications to the combustion process to prevent their formation, or by high temperature or catalytic reaction with ammonia or urea. In either case, the aim is to produce nitrogen gas, rather than nitrogen oxides. In the US there is a rapid deployment of technologies to remove mercury from flue gas - typically by adsorption on sorbents or by capture in inert solids as part of the flue gas desulfurization product.

Technologies based on regenerative capture by amines for the removal of CO₂ from flue gas have been deployed to provide high purity gas to the food industry. They are now under active research as a method for CO₂ capture for long-term storage as a means of greenhouse gas remediation.

There are a range of emerging technologies for removing pollutants emitted from power plants. As yet, there is very little performance data available from large-scale industrial applications of such technologies and none has achieved significant penetration of the enormous worldwide market.

• Combustion
• Emission standard
• Exhaust gas
• Flue gas emissions from fossil fuel combustion
• Flue gas stacks
• Flue gas desulfurization (often referred to as FGD)
• Integrated Gasification Combined Cycle (often referred to as IGCC)
• Nitrogen oxides emission (often referred to as NOx)