Selective Reduction of Nitrogen Oxides In Combustion Flue Gases

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Abstract
The body of Information presented in this paper is directed to those Individuals concerned with the removal of NOx in combustion flue gases. A catalytic process for the selective reduction of nitrogen oxides by ammonia has been investigated. Efforts were made toward the development of catalysts resistant to SOx poisoning. Nitrogen oxides were reduced over various metal oxide catalysts in the presence or absence of SOx(SO2 and SO3). Catalysts consisting of oxides of base metals (for example, Fe2O3) were easily poisoned by SO3, forming sulfates of the base metals. A series of catalysts which are not susceptible to the SOx poisoning has been developed. The catalysts possess a high activity and selectivity over a wide range of temperatures, 250—450°C. The catalysts were tested in a pilot plant which treated a flue gas containing 110-150 ppm NOx, 660-750 ppm SO2, and 40-90 ppm SO3. The pilot plant was operated at 350°C and at a space velocity of 10,000 h-1. The removal of nitrogen oxides was more than 90% for several months. A mechanism of the NO-NH3 reaction has also been investigated. It is found that NO reacts with NH3 at a 1:1 mole ratio in the presence of oxygen and the reaction is completely inhibited by the absence of oxygen. The experimental data show that the NO-NH3 reaction in the presence of oxygen is represented byNO + NH3 + 1/4 O2 = N2 + 3/2 H2O.

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