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   » » Wiki: Flue Gas
Tag Wiki 'Flue Gas'.

Flue gas is the exiting to the via a , which is a pipe or channel for conveying , as from a fireplace, oven, furnace, or steam generator. It often refers to the exhaust gas of at . Technology is available to remove pollutants from flue gas at power plants.

Combustion of is a common source of flue gas. They are usually combusted with ambient air, with the largest part of the flue gas from most combustion being , , and .

Flue gas is the exiting to the atmosphere via a , which is a pipe or channel for conveying from , as from a , , furnace, or steam generator.

Power plants
Quite often, the flue gas refers to the combustion exhaust gas produced at . Its composition depends on what is being burned, but it will usually consist of mostly (typically more than two-thirds) derived from the combustion of air, (), and as well as excess (also derived from the combustion air). It further contains a small percentage of a number of pollutants, such as particulate matter (like ), , , and . Fossil fuel combustion flue gases Milton R. Beychok, Encyclopedia of Earth, 2012.

At power plants, flue gas is often treated with a series of chemical processes and , which remove pollutants. Electrostatic precipitators or fabric filters remove particulate matter and flue-gas desulfurization captures the 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 or . In either case, the aim is to produce nitrogen gas, rather than nitrogen oxides. In the United States, there is a rapid deployment of technologies to remove mercury from flue gas—typically by absorption on sorbents or by capture in inert solids as part of the flue-gas desulfurization product. Such scrubbing can lead to meaningful recovery of sulfur for further industrial use. Sulfur C. Michael Hogan, Encyclopedia of Earth, 2011.

Technologies based on regenerative capture by for the removal of from flue gas have been deployed to provide high purity gas to the food industry, and for enhanced oil recovery. They are now under active research as a method for capture for long-term storage as a means of greenhouse gas remediation, and have begun to be implemented in a limited way commercially (e.g. the Sleipner West field in the , operating since 1996).

There are a number of proven technologies for removing pollutants emitted from power plants that are now available. There is also much ongoing research into technologies that will remove even more air pollutants.

Fossil fuels
Most are combusted with ambient air (as differentiated from combustion with pure ). Since ambient air contains about 79 volume percent gaseous (N2), Sulfur C. Michael Hogan, Encyclopedia of Earth, 2011. which is essentially non-combustible, the largest part of the flue gas from most fossil-fuel combustion is uncombusted nitrogen. (CO2), the next largest part of flue gas, can be as much as 10−25 volume percent or more of the flue gas. This is closely followed in volume by water vapor (H2O) created by the combustion of the hydrogen in the fuel with atmospheric oxygen. Much of the 'smoke' seen pouring from flue gas stacks is this water vapor, forming a cloud as it contacts cool air.

A typical flue gas from the combustion of fossil fuels contains very small amounts of (), (SO2) and particulate matter. Fossil fuel combustion flue gases Milton R. Beychok, Encyclopedia of Earth, 2012. The nitrogen oxides are derived from the nitrogen in the ambient air, as well as from any nitrogen-containing compounds in the fossil fuel. The sulfur dioxide is derived from any -containing compounds in the fuels. The particulate matter is composed of very small particles of solid materials and very small liquid droplets which give flue gases their smoky appearance.

The steam generators in large and the process furnaces in large , and , and burn considerable amounts of fossil fuels and therefore emit large amounts of flue gas to the ambient atmosphere. The table below presents the total amounts of flue gas typically generated by the burning of fossil fuels such as , and . The data were obtained by calculations.Citation error. See inline comment how to fix.

The total amount of wet flue gas generated by coal combustion is only 10 percent higher than the flue gas generated by natural-gas combustion (the ratio for dry flue gas is higher).

Composition of flue-gas emissions from fossil-fuel combustion
+Exhaust flue gas generated by combustion of fossil fuels (In SI metric units and in US customary units) ! align="left"Combustion data ! align="right" ! align="right" ! align="right"
, MJ/m343.01
, Btu/scf1,093
Gross caloric value, MJ/kg 43.50
Gross heating value, Btu/gal'' 150,000
Gross caloric value, MJ/kg 25.92
Gross heating value, Btu/lb 11,150
, °API 15.5
/ ratio by weight 8.1
weight % carbon 61.2
weight % hydrogen 4.3
weight % 7.4
weight % sulfur 3.9
weight % nitrogen 1.2
weight % ash 12.0
weight % moisture 10.0
Excess combustion air, %121520
Amount of wet , m3/GJ of fuel294.8303.1323.1
Amount of wet exhaust gas, scf/106 Btu of fuel11,60011,93012,714
in wet exhaust gas, volume %8.812.413.7
O2 in wet exhaust gas, volume %
Molecular weight of wet exhaust gas27.729.029.5
Amount of dry exhaust gas, m3/GJ of fuel241.6269.3293.6
Amount of dry exhaust gas, scf/106 Btu of fuel9,51010,60011,554
CO2 in dry exhaust gas, volume %10.814.015.0
O2 in dry exhaust gas, volume %
Molecular weight of dry exhaust gas29.930.430.7

m3 are standard cubic meters at 0 °C and 101.325 kPa, and scf is standard cubic feet at 60 °F and 14.696 psia.

See also
  • AP 42 Compilation of Air Pollutant Emission Factors
  • Carbon capture and storage
  • Emission standard
  • Flue gas stacks
  • Flue gas to fuel
  • Flue-gas desulfurization
  • Integrated gasification combined cycle (often referred to as IGCC)

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