Combustion Enhancement of Ammonia by Cofiring Oxymethylene Ethers (OMEn, n = 0–2): An Experimental and Kinetic Modeling Investigation

Abstract

Ammonia (NH₃) is a promising zero-carbon fuel with an exceptionally high hydrogen density. However, the feasibility of employing ammonia as a future fuel faces several obstacles including low combustion intensity. Co-firing reactive carbon-neutral fuels, such as oxymethylene ethers (OME_n) with NH₃ emerges as an effective approach to enhance NH₃ combustion. This work investigates the laminar flame propagation of NH₃ cofired with dimethyl ether (DME), dimethoxymethane (OME₁), and methoxy(methoxymethoxy)methane (OME₂) using a high-pressure high-temperature constant-volume combustion vessel. Laminar burning velocities (LBVs) are measured at an initial temperature of 423 K and pressures of 1–10 atm. A kinetic model for NH₃/OME_n combustion is developed and validated against the measured LBVs in this study, as well as LBVs and speciation data in literature. Both the experimental and kinetic modeling studies indicate the positive effect of cofiring of OME_n on ammonia combustion enhancement. The LBV levels of the NH₃/OME_n mixture can be similar to that of methane. The effects of cofiring fuel compositions, equivalence ratios, and pressures are investigated using modeling analysis and the modified fictitious diluent gas method. In mixture combustion, the reaction pathways of ammonia, DME, OME₁, and OME₂ remain almost unchanged compared to single fuel combustion, despite the slight contribution of C–N interaction. Combustion enhancements result from both chemical effects and thermal effects and their contribution ratios vary according to equivalence ratios and fuel compositions. At ϕ = 1.6, the contribution of chemical effects increases in the order 50%NH₃/50%DME, 50%NH₃/50%OME₁, and 50%NH₃/50%OME₂. Though there are similar LBVs for DME, OME₁, and OME₂, the mixture LBVs follow the sequence of 50%NH₃/50%DME < 50%NH₃/50%OME₁ < 50%NH₃/50%OME₂, which can be attributed to the influence of their lower heating values. A quasi-square relationship between normalized LBVs and energy fraction can be derived by using the correction of the energy fraction for the three NH₃/OME_n mixtures.