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

NO_x 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 NO_x formation at these conditions. It was also found that a significant reduction in the NO_x 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 NO_x 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 NO_x 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 NO_x in the recycled-CO₂ can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NO_x 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.