Combustion\nEnhancement of Ammonia by Cofiring Oxymethylene\nEthers (OME<sub><i>n</i></sub>, <i>n</i> = 0–2):\nAn Experimental and Kinetic Modeling Investigation

Ammonia (NH<sub>3</sub>) is a promising\nzero-carbon fuel\nwith an\nexceptionally high hydrogen density. However, the feasibility of employing\nammonia as a future fuel faces several obstacles including low combustion\nintensity. Co-firing reactive carbon-neutral fuels, such as oxymethylene\nethers (OME<sub><i>n</i></sub>) with NH<sub>3</sub> emerges\nas an effective approach to enhance NH<sub>3</sub> combustion. This\nwork investigates the laminar flame propagation of NH<sub>3</sub> cofired\nwith dimethyl ether (DME), dimethoxymethane (OME<sub>1</sub>), and\nmethoxy(methoxymethoxy)methane (OME<sub>2</sub>) using a high-pressure\nhigh-temperature constant-volume combustion vessel. Laminar burning\nvelocities (LBVs) are measured at an initial temperature of 423 K\nand pressures of 1–10 atm. A kinetic model for NH<sub>3</sub>/OME<sub><i>n</i></sub> combustion is developed and validated\nagainst the measured LBVs in this study, as well as LBVs and speciation\ndata in literature. Both the experimental and kinetic modeling studies\nindicate the positive effect of cofiring of OME<sub><i>n</i></sub> on ammonia combustion enhancement. The LBV levels of the NH<sub>3</sub>/OME<sub><i>n</i></sub> mixture can be similar to\nthat of methane. The effects of cofiring fuel compositions, equivalence\nratios, and pressures are investigated using modeling analysis and\nthe modified fictitious diluent gas method. In mixture combustion,\nthe reaction pathways of ammonia, DME, OME<sub>1</sub>, and OME<sub>2</sub> remain almost unchanged compared to single fuel combustion,\ndespite the slight contribution of C–N interaction. Combustion\nenhancements result from both chemical effects and thermal effects\nand their contribution ratios vary according to equivalence ratios\nand fuel compositions. At ϕ = 1.6, the contribution of chemical\neffects increases in the order 50%NH<sub>3</sub>/50%DME, 50%NH<sub>3</sub>/50%OME<sub>1</sub>, and 50%NH<sub>3</sub>/50%OME<sub>2</sub>. Though there are similar LBVs for DME, OME<sub>1</sub>, and OME<sub>2</sub>, the mixture LBVs follow the sequence of 50%NH<sub>3</sub>/50%DME < 50%NH<sub>3</sub>/50%OME<sub>1</sub> < 50%NH<sub>3</sub>/50%OME<sub>2</sub>, which can be attributed to the influence\nof their lower heating values. A quasi-square relationship between\nnormalized LBVs and energy fraction can be derived by using the correction\nof the energy fraction for the three NH<sub>3</sub>/OME<sub><i>n</i></sub> mixtures.