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.)
- Lox-NOX Burners – NOX is formed when heat forces nitrogen atoms apart and they recombine with oxygen atoms. Any fuel that is burned in high enough temperatures will form NOX. One of the best ways to reduce NOX is to keep it from forming in the first place, so low-NOX burners, which burn fuels at lower temperatures and in stages are used. In these burners, the fuel and oxygen combine, rather than nitrogen and oxygen.
- Overfire Air – as the name indicates, overfire air is air injected into the burner above the main combustion zone. Where overfire air is used, the boiler is set up to use a lower air/fuel ratio, which maintains a lower combustion zone temperature.
- Reburning – is a technique that uses three separate burning zones in a boiler to minimize NOX formation. In the main zone, the majority of the fuel (85%) is burned in low excess air conditions. In the second "reburning” zone, the remaining portion of the fuel is injected to create a fuel-rich, reducing zone. Often a highly volatile fuel is used to ensure complete combustion. The third zone is an overfire air zone that provides the necessary air excess levels and completes combustion.
- Flue Gas Recirculation – is a process in which a portion of the flue gas is captured and re-routed back into the boiler as a means of reducing the peak flame temperatures in the boiler and modify oxygen content.
- Operational Modifications – changing the setup of a boiler can alter the conditions in the boiler that produce NOX. This is also known as "off-stoichiometric combustion”. Examples of varying boiler configurations include:
- Burners out of service: involves alternating boiler configurations to produce fuel rich, air rich, or air only staged zones within the boiler
- Overfire air: described above
- Low Excess Air: involves reducing excess air to the minimum levels that still allow complete fuel combustion
- Biased Firing: involves altering fuel levels from burners so that lower burners (typically) inject more fuel, while higher burners receive less fuel.
Post-Combustion Treatment (or, "Add on controls”)
- Selective Catalytic Reduction (SCR) – SCR involves injecting ammonia (NH3) or urea into flue gas and then passing it over a catalyst bed. The NH3 and NO2 react with oxygen and the catalyst to form N2 (nitrogen) and H2O (water). The catalysts are typically oxides of base metals (tungsten, vanadium, titanium, etc.) formed into a honeycomb shape through which the flue gas passes.
- Selective Noncatalytic Reduction (SNCR) – SNCR involves injected ammonia (NH3) or urea into a hot gas zone in the furnace. The high heat encourages thermal reactions between NOX, oxygen, and NH3 to form N2 and water (H2O)
Hybrid options also exist to mix SNCR and SCR, as well as low-NOX burners as a means of maximizing NOX reductions.
Links & Bibliography