Numerical Study on the Combustion Properties of Ammonia/DME and Ammonia/DMM Mixtures

Ammonia (NH3) is considered a promising zero-carbon fuel and was extensively studied recently. Mixing high-reactivity oxygenated fuels such as dimethyl ether (DME) or dimethoxymethane (DMM) with ammonia is a realistic approach to overcome the low reactivity of NH3. To study the combustion characteristics of NH3/DMM and NH3/DME mixtures, we constructed a NH3/DMM chemical mechanism and tested its accuracy using measured laminar burning velocity (LBV) and ignition delay time (IDT) of both NH3/DMM and NH3/DME mixtures from the literature. The kinetic analysis of NH3/DMM flames using this mechanism reveals that the CH3 radicals generated from the oxidation of DMM substantially affects the oxidation pathway of NH3 at an early stage of flame propagation. We investigated the formation of nitrogen oxides (NOx) in NH3/DMM and NH3/DME flames and little difference can be found in the NOx emissions. Using NH3/DMM flames as an example, the peak NOx emissions are located at an equivalence ratio (phi) of 0.9 and a DMM fraction of 40% in the conditions studied. Kinetic analysis shows that NOx emission is dominated by NO, which primarily comes from fuel nitrogen of NH3. The addition of DMM at 40% significantly promotes the reactive radical pool (e.g., H, O, and OH) while the maintaining a high concentration of NO precursors (e.g., HNO, NO2, and N2O), which results in a high reaction rate of NO formation reaction and subsequently generates the highest NO emissions.