Bankside power station and St George the Martyr church - geograph

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, furnace,

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central ...

or steam generator. Quite often, the flue gas refers to the

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

exhaust gas produced at power plants. Its composition depends on what is being burned, but it will usually consist of mostly

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

(typically more than two-thirds) derived from the combustion of air,

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

(), and

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

as well as excess

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

(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 Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...

, nitrogen oxides, and sulfur oxides. Most fossil fuels are combusted with ambient air (as differentiated from combustion with pure

). Since ambient air contains about 79 volume percent gaseous

(N₂), which is essentially non-combustible, the largest part of the flue gas from most fossil-fuel combustion is uncombusted nitrogen.

Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

(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 nitrogen oxides (),

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...

(SO₂) and particulate matter. 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

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

-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

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many ...

s and the process furnaces in large refineries,

petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewabl ...

and chemical plants, and incinerators 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

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...

, fuel oil and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

. The data were obtained by stoichiometric calculations. 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).

Scrubbing

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

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 Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

or urea. 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 Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...

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. Technologies based on regenerative capture by amines 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

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...

, 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.

Composition of flue-gas emissions from fossil-fuel combustion

: 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.

References

{{Authority control Air pollution Combustion Environmental engineering Industrial gases

Scrubbing

Composition of flue-gas emissions from fossil-fuel combustion

See also

References