Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnaceboiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants. Its composition depends on what is being burned, but it will usually consist of mostlynitrogen (typically more than two-thirds) derived from the combustion of 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 a number of pollutants, such as particulate matter (like soot), carbon monoxidenitrogen oxides, and sulfur oxides.

Large amounts of flue gases are generated due to combustion of petroleum products in oil and gas industries. These gases are also created by wood fires and vehicle exhaust.

The contents of flue gases are quite variable. The composition depends on what is being burned, but usually consists of mostly nitrogen (typically more than two-thirds) derived from the combustion air, carbon dioxide and water vapor as well as excess oxygen (also derived from the combustion air). It further contains a small percentage of a number of pollutants, such as particulate matter (like soot), 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.

Flue gas desulfurization (FGD) involves the removal of sulfur dioxide (SO2) contained in gases produced by the combustion of fossil fuels such as coal, oil, municipal solid waste, automobile tires and many industrial processes such as gasoline refining, cement, paper, glass, steel, iron and copper production.

Sulfur dioxide emissions are a primary contributor of acid rain and have to be regulated by every industrialized nation in the world. Emissions at flues are routinely tested to determine whether or not the gases are being adequately scrubbed before they are vented into the environment. Scrubbing can lead to meaningful recovery of sulfur for further industrial use.

Flue gas analysis offers a means of determining pollutant concentrations and adjusting heating installations for maximum efficiency.

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