Non-catalytic Reduction of Nitrogen Monoxide with Ammonia and Oxidation of Ammonia with Oxygen under Coexistence of Hydrogen

Abstract

The reduction of NO with NH₃ and the oxidation of NH₃ with O₂ under coexistence of H₂ were investigated to establish the relations among gas composition, temperature, residence time, fractional conversion of NO, and the reaction mechanism in non-catalytic reduction of NO with NH₃. The experiments were carried out by using a flow reactor under an atmospheric pressure and at 600-1000° C. The composition of the inlet gas was 0-1500 ppm, NO-0-1500 ppm, NH₃-0-5000 ppm, H₂-0-5%, 0₂-0-13%, H₂O-N₂ and the residence time 0.37-2.9 Nsec.
The results obtained are as follows:
(1) Temperature range both for the reduction of NO and oxidation of NH₃ was reduced by adding H₂, and the higher the H₂ concentration, the larger became the effect. However, the fractional conversion of NO showed maxima at a certain temperature which depended on H₂ concentration. Above this temperature, by adding H₂ the oxidation of NH₃ to NO was promoted and the fractional conversion of; NO decreased considerably. It was difficult to obtain the high fractional conversion of NO under a coexistence of H₂.
(2) The same apparent stoichiometric reaction was obtained as in the absence of H₂: NO+ NH₃+ (1/4) 0₂ N2+ (3/2) H₂0.
(3) A chain reaction mechanism where OH radical was a main carrier was discussed as follows:
Since the oxidation of H₂ with 0₂ (H2+0 → H+ OH, H₂+ OH → H+ H₂0, H+02 → OH+ 0) occurred at a low temperature, and the oxidation of NH₃ (NH₃ → NH₂ → NH) with OH, H, and 0 radicals occurred at a low temperature (ca.600° C). And, it was deduced that NO was reduced to N₂ by the reaction NO+NH → N2+0H, and that above the temperature at which the maximum fractional conversion of NO occurred, the formation rate of NO became high in terms of the reaction NH+0₂ → NO+0H.