NOx formation from ammonia, and its effects on oxy-combustion of hydrocarbon fuels under supercritical-CO2 conditions

Abstract

NOx formation from ammonia impurities and its effects on fuel oxidation is not well understood for high-pressure supercritical CO2 oxy-combustion conditions. This effect is investigated computationally and experimentally in the present work. A chemical kinetic analysis revealed that the reaction between NH radical and CO2 plays an important role in determining the rate of NOx formation at these conditions. It was also found that a significant reduction in the NOx formation from ammonia oxidation occurs with sCO2 oxy-combustion at 300 atm when compared to traditional gas turbine conditions. A chemical reactor network simulation of realistic sCO2 cycle conditions confirmed the reduced NOx emissions. Monte Carlo simulations used to study the sensitivity of model input variables on the emissions showed that CO2 cooling impacts the CO emissions while most of the NOx are generated in the flame zone. Uncertainty analysis showed that the reaction between NH and CO2 to form HNO radical is an important contributor to the model uncertainty under sCO2 oxy-combustion conditions.The presence of combustion-generated NOx in the recycled-CO2 can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NOx on ignition, high-pressure shock tube ignition delay time experiments were performed at conditions relevant to sCO2 oxy-combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition under supercritical conditions using CO2 as the bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes ignition at these conditions. The effect is more pronounced for CH4 compared to syngas. Nitric oxide acts as a chemical catalyst to promote ignition by increasing the combustion radical pool. The catalytic cycle involves the conversion of NO to NO2 which also contributes to CH4 oxidation by H-atom abstraction to generate CH3 radical.