Systematically derived reduced kinetics for hydrogen/ammonia gas-turbine combustion

Abstract

Starting with a detailed-chemistry description involving 20 elementary steps for hydrogen oxidation and 40 elementary steps for ammonia oxidation, it is shown that systematic application of sensitivity analyses of premixed flames under typical gas-turbine combustion conditions reduces the description to 12 elementary steps for hydrogen oxidation, 4 of them being reversible, and an additional 19 steps for ammonia oxidation, 6 of them being reversible, yielding reasonable predictions for auto-ignition and deflagration processes. Subsequent introduction of steady-state approximations for chemical intermediates, afforded by the high-pressure conditions existing in gas-turbine combustion chambers, effectively reduces the fuel-oxidation description in systems utilizing H2-NH3 fuel mixtures to two global steps for deflagrations, namely, 2H2 + O2 <-> 2H2O and 4NH3 ＋ 3O2 <-> 2N2 + 6H2O. Analytical expressions for the associated overall rates, involving the local temperature and the O2, H2, NH3, N2, and H2O concentrations, are derived through selective truncation of the steady-state expressions, resulting in a simplified chemistry description that can facilitate future numerical analyses based on direct-numerical and large-eddy simulations.