Ammonia (NH3) is a carbon-free energy carrier with an infrastructure for production, storage, and distribution. There is interest in direct NH3 combustion, but managing pollutant emissions is a key challenge, particularly nitric oxides (NOx) due to the fuel-bound nitrogen atom, nitrous oxide (N2O), which is a potent greenhouse gas, and unburned NH3, which is harmful to humans and the environment. Rich staged combustor concepts with extended primary residence times (τres,primary), like Rich-Relax-Quick-mix-Lean (RRQL), offer a viable pathway for direct NH3 combustion with low levels of NOx formation. However, minimizing secondary emissions such as N2O and unburned NH3 and hydrogen (H2) remains a critical challenge. Prior atmospheric-pressure studies have demonstrated that RRQL operation with sufficiently long τres,primary enables substantial NOx relaxation and promotes NH3 cracking to H2, if heat losses from the relaxation stage are limited. However, the combined influence of elevated pressure and long residence time on RRQL performance has not been explored. The present work examines RRQL operation at pressures up to 5 bar and elevated τres,primary. Exhaust measurements of NOx, NH3, and N2O are used to quantify the extent of NOx relaxation and NH3 cracking under nonadiabatic conditions. To contextualize and quantify the effects of heat losses in the experimental data, chemical reactor networks (CRNs) incorporating prescribed heat loss rates are employed to assess the sensitivity of emissions to thermal losses in the relaxation stage. Collectively, the results demonstrate that management and quantification of heat losses are essential to preserve NOx relaxation and limit NH3 and N2O emissions.
Cole et al. (Mon,) studied this question.