Exploration of NH3 and NH3/DME laminar flame propagation in O2/CO2 atmosphere: Insights into NH3/CO2 interactions

Abstract

Cofiring with reactive hydrocarbon and oxygenated fuels is an effective approach to enhance ammonia (NH3) combustion, which raises growing interest in understanding CN interactions. Among these fuels, dimethyl ether (DME, CH3OCH3) has attracted extensive attention due to its renewable nature and potential for large-scale synthesis from CO2 feedstock. This work reports an experimental and modeling investigation on the laminar flame propagation of NH3 and NH3/DME in O2/CO2 atmosphere, with an emphasis on evaluating the enhanced NH3/CO2 interactions under flame conditions. Laminar burning velocities (LBVs) were measured at elevated pressures up to 5 atm in a constant-volume combustion vessel. A kinetic model was developed and can well reproduce the measured LBVs in this work, as well as the LBVs and speciation data in literature. Modeling analyses and the modified fictitious diluent gas method were used to explore the effects of DME cofiring, pressure and CO2 dilution on LBVs. The results reveal that the chemical effect plays a crucial role in promoting the laminar flame propagation of NH3 with DME cofiring, while the reduction of LBVs with CO2 dilution is predominantly influenced by the thermal effect. LBVs of NH3/DME present a stronger pressure dependency than that of pure NH3, which is attributed to the enhanced pressure-sensitive three-body reactions, including CH3 + H (+M) = CH4 (+M) and H + O2 (+M) = HO2 (+M). Phenomenological analysis of LBVs was conducted by varying the diluent gas from pure N2 to pure CO2, complemented by modeling analyses with adjusted rate constants, to provide insights into NH3/CO2 interactions. The results show that CO2 reduction reactions, including NH + CO2 = HNO + CO and CO + OH = CO2 + H, compete for reactive radicals like H and NH with chain-branching reactions H + O2 = OH + H and consequently prohibit the laminar flame propagation.