|NOx Reduction Technologies|
NOX Reduction Technologies
(Those interested in other clean coal technologies should also check out our Clean Coal Technologies Issues page, the numerous articles on clean coal technologies in past issues of American Coal magazine (see our online articles about NOX control as well), as well as our fact sheets on environmental technologies associated with coal use..)
NOX is the generic term used to refer to nitrogen oxides (NO and NO2), which are primarily produced during the combustion of hydrocarbons in the presence of air. Nitrogen and oxygen gases in the air combine to form oxides of nitrogen when exposed to extremely high heats (>2800° F). When NOX, gases are released into the atmosphere, they dissolve in the natural moisture and form a weak nitric acid solution (acid rain). When NOX mixes with volatile organic compounds and reacts with sunlight they form photochemical smog.
While NOX can be formed naturally during lightning strikes, the primary anthropogenic sources of these gases are from gasoline combustion in automobiles and the combustion of fossil fuels during the generation of electricity.
NOX reduction technologies (as they relate to electricity generation) are aimed at reducing the temperatures in boilers to keep heat below the levels at which NOX is formed. They also will decrease or increase the amounts (or percentages) of excess air in the boiler to control NOX production. "NOX production is highest (B) at fuel-to-air combustion ratios of 5–7% O2 (25–45% excess air). Lower excess air levels starve the reaction for oxygen, and higher excess air levels drive down the flame temperature, slowing the rate of reaction.” By reducing nitrogen oxides produced in the boiler, utilities or industrial users can reduce the release of nitrogen oxides into the atmosphere.
Many information resources recognize two primary categories of NOX reduction technologies: combustion modifications and post-combustion processes. Examples of these categories – combustion modifications and post-combustion processes include:
(This information is excerpted from http://www.netl.doe.gov/technologies/coalpower/ewr/nox/NOx-reduct.html, http://www.epa.gov/apti/bces/module6/nitrogen/control/control.htm
Engineered coal fuels can also play a role in reducing NOX emissions through improving combustion efficiencies. See the Engineered coal fuels technologies Issues page on this website for more information.)
Post-Combustion Treatment (or, "Add on controls”)
Hybrid options also exist to mix SNCR and SCR, as well as low-NOX burners as a means of maximizing NOX reductions.
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