User:Milton Beychok/Sandbox

Flue gas is gas that exits to the atmosphere via a flue, which is a pipe, channel or chimney for conveying combustion product gases from a fireplace, oven, furnace, boiler or steam generator.

Flue gases are produced when coal, fuel oil, natural gas, wood or any other fuel is combusted in an industrial furnace or boiler, a steam-generator in a fossil fuel power plant or other large combustion source.

Flue gas composition

Flue gas is usually composed of carbon dioxide (CO₂) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. Typically, more than two-thirds of the flue gas is nitrogen.

Table 1: Flue gas quantities generated by combustion of fossil fuels

Combustion Data	Fuel gas	Fuel oil	Coal
Fuel properties:
Gross heating value, MJ/m³	43.01
Gross heating value, MJ/kg		43.50	25.92
Molecular mass	18
Specific gravity		0.9626
Carbon/hydrogen ratio by weight		8.1
weight % carbon			61.2
weight % hydrogen			4.3
weight % oxygen			7.4
weight % sulfur			3.9
weight % nitrogen			1.2
weight % ash			12.0
weight % moisture			10.0
Combustion air:
Excess combustion air, %	12	15	20
Wet combustion flue gas:
Wet combustion flue gas, m³/GJ of fuel	294.8	303.1	323.1
Molecular mass of wet combustion gas	27.7	29.0	29.5
Dry combustion flue gas:
Dry combustion flue gas, m³/GJ of fuel	241.6	269.3	293.6
Molecular mass of dry combustion gas	29.9	30.4	30.7

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.

See also

• 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)