Kinetic analysis of the mixture effect in supercritical water oxidation of ammonia/methanol

• As methanol concentration increased, ammonia conversion increased.
• After methanol complete oxidation, ammonia oxidation continued at much lower rate.
• Ammonia also accelerated oxidation of methanol and CO.
• Phenomenological findings can be explained by a radical chain mechanism.

Reaction kinetics of supercritical water oxidation (SCWO) of ammonia/methanol mixture was investigated at 530 °C, 25 MPa, and [NH3]0 = 2.9–3.0 mmol/L, both experimentally and computationally. Ammonia conversion increased with an increase in the initial methanol concentration. Ammonia oxidation was retarded after complete oxidation of methanol. The major product from ammonia oxidation was N2O. Methanol decomposition and CO oxidation were accelerated in the ammonia/methanol mixture under the conditions used. Calculations using an elementary reaction model show that ammonia and methanol mutually promote their oxidation through radical intermediates. The mixture effect in the SCWO of ammonia/methanol can be explained by a radical chain mechanism in which ammonia and methanol share the radicals generated and promote mutual oxidation reactions. The ammonia oxidation cycle initiated by methanol co-oxidation was maintained by nitrogen-containing species even after methanol had been completely oxidized.

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