Physicochemical and Biochemical Approaches for Treatment of Gaseous Emissions Containing NOx

Abstract

Nitrogen oxides (NOx) are the cause of severe environmental problems such as acid rain, smog formation, an increase in ground-level ozone, depletion of the ozone layer, and global warming and can indirectly affect human and animal health. Considering its severe polluting aspects, many approaches have been utilized so far for the development of a technology that efficiently removes NOx from the industrial gaseous emissions. These control techniques can be broadly classified as primary and secondary techniques. Primary control techniques modify the existing combustion methods to limit the production of NOx, which includes various physical and chemical approaches, whereas the secondary NOx control techniques involve chemical reduction of NOx in flue gas using a chemical reducing agent such as ammonia or urea, reacting on a specially engineered catalyst surface or by absorption of the NOx into a special chelating liquid, and then reducing the chelate-NOx complex to regenerate the chelate using chemical and biochemical approaches, which involve compost biofilters, trickling bed biofilters, packed bed reactors, and several other types of bioreactors. The overall efficiency of the process depends on the absorption efficiency of the chelating agent, the denitrification capacity of the microorganism and the process parameters and physicochemical conditions. The authors highlight the essential features of various physicochemical and biochemical NOx control strategies and techniques. Further, extensive research and development efforts are recommended to improve existing technology for effective NOx control.