Fuel NOx is much more sensitive to stoichiometry than to thermal conditions. For this reason, traditional thermal treatments, such as flue gas recirculation and water injection, do not effectively reduce NOx emissions from liquid and solid fuel combustion. http://arator.fi/
NOx emissions can be controlled either during the combustion process or after combustion is complete. Combustion control technologies rely on air or fuel staging techniques to take advantage of the kinetics of NOx formation or introducing inerts that inhibit the formation of NOx during combustion, or both. Post-combustion control technologies rely on introducing reactants in specified temperature regimes that destroy NOx either with or without the use of catalyst to promote the destruction.
The simplest of the combustion control technologies is low-excess-air operation–that is, reducing the excess air level to the point of some constraint, such as carbon monoxide formation, flame length, flame stability, and so on. Unfortunately, low-excess-air operation has proven to yield only moderate NOx reductions, if any.
Three technologies that have demonstrated their effectiveness in controlling NOx emissions are off-stoichiometric combustion. low-NOx burners, and combustion temperature reduction. The first two are applicable to all fuels, while the third is applicable only to natural gas and low-nitro-gen-content fuel oils.