Equilibrium thermodynamics of ammonia oxidation in supercritical water

The sewage sludge contains a lot of nitrogen-containing organics whose intermediate product during the supercritical water oxidation (SCW) process is ammonia. A thermodynamic model was developed to predict the performance of ammonia oxidation in supercritical water. Based on Peng-Robinson EOS formulations and direct Gibbs free energy minimization, the equilibrium composition of the reaction system for different conditions was calculated. The result shows that as the reaction system reaches equilibrium, ammonia can be completely oxidized in the supercritical water, and most of which converts to N₂; NO₂ and NO are the minor by-products and N₂O is hardly produced in the system. Increasing temperature and excess oxygen can increase the equilibrium composition of NO_x. But the highest selectivity of NO_x is less than 2%. Comparing the calculated results with experimental results from other papers, it shows that the N₂O is only an intermediate product in the supercritical water oxidation of ammonia, and the degradation of ammonia is mostly affected by dynamic factors.