2015

Experiments and simulations of NOx formation in the combustion of hydroxylated fuels

Experiments and simulations of NOx formation in the combustion of hydroxylated fuels

M.D. Bohon, M. El-Rachidi, S.M. Sarathy, and W.L. Roberts
Combust. Flame, 162:6,2322-2336, (2015)

M.D. Bohon, M. El-Rachidi, S.M. Sarathy, and W.L. Roberts
NOx, Alcohol, Swirl, Thermal, Prompt, Hydroxylated
2015
‚ÄčThis work investigates the influence of molecular structure in hydroxylated fuels (i.e. fuels with one or more hydroxyl groups), such as alcohols and polyols, on NOxNOx formation. The fuels studied are three lower alcohols (methanol, ethanol, and n-propanol), two diols (1,2-ethanediol and 1,2-propanediol), and one triol (1,2,3-propanetriol); all of which are liquids at room temperature and span a wide range of thermophysical properties. Experimental stack emissions measurements of NO/NO2, CO, and CO2 and flame temperature profiles utilizing a rake of thermocouples were obtained in globally lean, swirling, liquid atomized spray flames inside a refractory-lined combustion chamber as a function of the atomizing air flow rate and swirl number. These experiments show significantly lower NOxNOx formation with increasing fuel oxygen content despite similarities in the flame temperature profiles. By controlling the temperature profiles, the contribution to NOxNOx formation through the thermal mechanism were matched, and variations in the contribution through non-thermal NOxNOx formation pathways are observed. Simulations in a perfectly stirred reactor, at conditions representative of those measured within the combustion region, were conducted as a function of temperature and equivalence ratio. The simulations employed a detailed high temperature chemical kinetic model for NOxNOx formation from hydroxylated fuels developed based on recent alcohol combustion models and extended to include polyol combustion chemistry. These simulations provide a qualitative comparison to the range of temperatures and equivalence ratios observed in complex swirling flows and provide insight into the influence of variations in the fuel decomposition pathways on NOxNOx formation. It is observed that increasing the fuel bound oxygen concentration ultimately reduces the formation of NOxNOx by increasing the proportion of fuel oxidized through formaldehyde, as opposed to acetylene or acetaldehyde. The subsequent oxidation of formaldehyde contributes little to the formation of hydrocarbon (HC) radicals. Ultimately, by reducing the contributions to the HC radical pool, NOxNOx can be effectively reduced in these fuels through suppression of non-thermal NOxNOx formation pathways.