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A co-oxidation model was constructed from available submechanisms for ammonia and ethanol oxidation. The ammonia submechanism validated for combustion at atmospheric pressure conditions was modified for the higher densities and lower temperatures (655-700 • C) of supercritical water. The ethanol submechanism had previously been tested and validated at supercritical water conditions. The initial model poorly reproduced experimental ammonia conversion data and was not able to consistently match nitrous oxide production as a function of temperature over a range from 655-700 • C at 246 bar. To improve model predictions, the low-pressure NH 2 + NO x submechanism was replaced with a submechanism that included the H 2 NNO x adduct species that are expected to be stabilized in the high-pressure supercritical water environment. Thermochemical and kinetic parameters for the adduct species were estimated with quantum chemical calculations using Gaussian 98 with

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in Wiley InterScience (www.interscience.wiley.com).The cooxidative effect of ethanol on ammonia oxidation in supercritical water was studied for a range of temperatures (655-7058C), initial ammonia (1-3 mM), ethanol (0-1.0 mM), and oxygen concentrations (0.7-5.0 mM), corresponding to fuel equivalence ratios ranging from 0.9 to 2.2 for the complete combustion of both organic species. With a stoichiometric amount of oxygen available for complete oxidation, the addition of ethanol on an equivalent molar basis was found to increase ammonia conversion from 20 to 65% at initial concentrations of 1 mM for each reactant, T ¼ 7008C, P ¼ 246 bar, and t ¼ 2.5 s. Nitrous oxide was produced in much larger quantities for ammonia-ethanol cooxidation than for ammonia oxidation. Based on fractional yields of nitrogen product, this amounted to 40-75% for co-oxidation with ethanol versus 4-13% without ethanol present.

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