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Show r-f PROSPECTIVE TECHNIQUES FOR CONTROLLING COMBUSTION GENERATED NOx EMISSIONS Vladimir M. Zamansky Energy and Environmental Research Corporation 18 Mason, Irvine, C A 92618 Phone:(949)859-8851 Fax:(949)859-3194 E-mail: vzamansky@eercorp.com EXTENDED ABSTRACT A ten-fold increase in global nitrogen oxide emissions since 1900 is mainly attributed to human activities utilizing combustion processes. Nitrogen oxides, formed from combustion air and fuel-bounded nitrogen, contribute directly or indirectly to acid rain, changes to the ozone layer, and the greenhouse effect. Controlling flue gas N O x emissions (NOx=NO+N02) is the major environmental problem faced by various industries utilizing combustion processes. For example, in the U S A , the 1990 Clean Air Act Amendments (CAAA) mandate a 2,000,000 ton per year decrease in N O x emissions. Similar regulations are established or being established in other developed countries, and restrictions for N O x emissions will become more stringent in the future. Well-known commercial technologies for N O x control from stationary sources include combustion modifications (e.g., L o w N O x Burners - L N B ) , reburning, Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). S C R is the onjj commercially available N O x control technology that can achieve N O x reductions above 80%. With SCR, N O x is reduced in the reaction with ammonia on the surface of a catalyst typically positioned at a temperature of about 650 K. The costs of applying S C R are high principally due to the cost of the catalyst and its periodic replacement. The catalysts are also toxic and pose disposal problems. Applications of S C R to incinerators are generally regarded as non-feasible because waste contains many trace impurities that can act as catalyst poisons. While S C R costs have decreased somewhat in recent years, the cost of N O x reduction with S C R is still much greater than lower efficiency N O x controls such as L N B , SNCR, and reburning. However, the achievable N O x control level for these methods, typically in the range of 40-60%, is not enough to meet future environmental regulations. Expectations of more stringent regulations and limitations of the available methods are driving the current research efforts to develop new and improved technologies for controlling nitrogen oxide emissions. Design of more efficient combustors and effective post-combustion N O x control concepts are active areas of combustion research. Novel approaches in this field are mainly based on recent achievements of chemical kinetics and combustion chemistry providing better understanding of high-temperature chemical reactions, in particular, the chemical mechanisms responsible for N O x formation and removal. Along with chemistry, an important factor for achieving efficient N O x control is the optimization of gas dynamics effects, such as mixing of reagents and turbulence. To meet future standards, several novel NOx control techniques are currently being developed. The main goal of this paper is to review the principles, performance, advantages/disadvantages, and development status of novel prospective N O x control techniques that have the potential to be commercially applied in the near future for reducing nitrogen oxide emissions. The second goal is to discuss future R & D needs in both process fundamentals and technological developments. |