Flue gas

Bankside power station and St George the Martyr church - geograph.org.uk - 1607688

Air pollution by industrial chimneys

Flue gas is the gas that exits to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, furnace, boiler, 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 mostly nitrogen (N2) from the air, carbon dioxide (CO2), and water vapor (H2O) as well as excess oxygen (O2). It also contains a small amount of pollutants such as particulate matter, carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), and hydrocarbons (HC).

Composition

The exact composition of flue gas varies depending on the fuel type and combustion process. In general, flue gas from the combustion of fossil fuels contains:

• Carbon dioxide (CO2) - a greenhouse gas, which is the primary component of flue gas after nitrogen.
• Water vapor (H2O) - the amount depends on the hydrogen content in the fuel.
• Nitrogen (N2) - makes up the majority of the flue gas as it is the main component of air used for combustion but does not react.
• Oxygen (O2) - present if combustion is incomplete or excess air is supplied.
• Sulfur dioxide (SO2) - comes from the combustion of sulfur-containing fuels.
• Nitrogen oxides (NOx) - formed from the reaction between nitrogen and oxygen at high temperatures.
• Carbon monoxide (CO) - a product of incomplete combustion.
• Particulate matter - includes soot and other particles formed in the combustion process.
• Hydrocarbons (HC) - unburned or partially burned fuel.

Environmental Impact

Flue gas has a significant environmental impact, particularly due to the presence of CO2, NOx, and SO2. These gases contribute to air pollution, acid rain, and global warming. The particulate matter can also have adverse health effects on the respiratory system.

Flue Gas Treatment

To mitigate the environmental impact of flue gas, several treatment methods are employed:

• Flue gas desulfurization (FGD) - removes SO2 from flue gas.
• Selective catalytic reduction (SCR) - reduces NOx levels.
• Electrostatic precipitators and fabric filters - remove particulate matter.
• Carbon capture and storage (CCS) - a method to capture CO2 emissions for storage or reuse.

Regulations

Various international and national regulations aim to limit the emissions of harmful substances found in flue gas. These regulations mandate the use of specific technologies to reduce pollution and require regular monitoring of emissions.

See Also

• Air pollution
• Combustion
• Emission standard
• Environmental impact of the coal industry

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