Characterizing the Dimethyl Ether and NO Interaction in Outwardly Propagating Spherical Flame: Insights into the Roles of H and CH _<i>x</i> Radicals in the NO Reduction Mechanism

The cofiring of ammonia and dimethyl ether (DME) has emerged as a promising strategy for ammonia combustion in internal combustion engines and gas turbines. Given that NOx emissions are strongly associated with ammonia combustion, cofiring with DME underscores the need to deeply understand the DME and NOx interaction under flame conditions for effective NOx control, especially nitric oxide (NO). This work experimentally investigates the laminar burning velocities of DME/NO under ambient conditions using an oxygen-free, outwardly propagating spherical flame method in a constant-volume combustion vessel. Experimental observations demonstrate that DME and NO mixtures can form stable flame propagation, which enables the laminar burning velocity (LBV) measurements over a wide range of equivalence ratios from 0.4 to 2.0. LBV of the DME/NO mixture under the equivalent conditions is quite smaller than that of the DME/air mixture. A kinetic model for the DME/NO combustion is developed and validated against the newly obtained experimental data. Based on the kinetic analysis, it is explored that the NO reduction mainly proceeded through nonhydrocarbon and hydrocarbon NO mechanisms. The nonhydrocarbon NO reduction mechanism involving H radical and hydrocarbon NO reduction mechanism involving CHx are found to be the significant contributors to the laminar flame propagation of DME/NO mixtures. Specifically, NO + H = N + OH and CH3 + NO = HCN + H2O are among the most critical reactions for the nonhydrocarbon and hydrocarbon NO reduction mechanisms respectively, while reactions of oxygenated radicals with NO, such as those involving CH3O and HCO, have minimal influence on the DME/NO flames. The laminar burning velocity measurements obtained in this work serve as a highly sensitive validation for these NO reduction mechanisms in DME/NO mixtures, particularly for NO + H = N + OH.