Reactions of Nitrate Salts with Ammonia in Supercritical Water

Reactions involving nitrate salts and ammonia were investigated in supercritical water at temperatures from 450 to 530 °C and pressures near 300 bar. Reaction products included nitrite, nitrogen gas, and nitrous oxide. Observed reaction rates and product distributions provided evidence for a free-radical reaction mechanism with NO2, NO, and NH2 • as the primary reactive species at supercritical conditions. In the proposed elementary mechanism, the rate-limiting reaction step was determined to be the hydrolysis of MNO3 species, which resulted in the formation of nitric acid and subsequently NO2. A simple second-order reaction model was used to represent the data. In developing an empirical kinetic model, nitrate and nitrite were lumped as an NO x - reactant. Empirical kinetic parameters were developed for four MNO x /NH3 reacting systems, assuming first orders in both NH3 and NO x -. Observed MNO x /NH3 reaction rates and mechanisms suggest immediately a practical significance of these reactions for nitrogen control strategies in supercritical water oxidation processes.