High-temperature industrial processes often produce excessive NOx emission levels, especially in instances where preheated air is used to increase efficiency. Different measures are used to reduce NOx formation, such as fuel and/or air staging and flue-gas recirculation. Flue-gas recirculation and mixing before the flame reduces NOx formation even further. Two examples are the Flox (flameless oxidation) and the Gaft (gas dynamic abated flame) burners. In radiant tubes, Gaft burners are reported to have NOx emissions of 25-45 ppm at a furnace temperature of 900C (1650F) using preheated air . Flameless oxidation occurs at extremely high recirculation; the flame temperature is reduced and more evenly distributed resulting in NOx formation. However, the combustion process requires a furnace temperature higher than approximately 850C (1560F) .
Catalytic cleaning is used in processes where the NOx emissions are very high, such as power plant boilers, glass melting and in combustion engines. Catalytic cleaning has been tested in the Netherlands using an automotive three-way catalyst, but is not a common technology in radiant-tube burners . NOx reductions exceeding 90% are reported using an automotive three-way catalyst, but high flue-gas temperatures and high thermal mass in the catalyst are possible reasons for catalyst degradation (failure). A NOx reduction of 40 to 50% was achieved using an alumina catalyst developed by Osaka Gas at the burner exit .