Implication of amino cross-reactions on the ignition characteristics of ammonia-blended typical small saturated and unsaturated fatty acid methyl esters

Abstract

Amino radical plays a central role in the pyrolysis and oxidation of ammonia. The practical utilization of pure ammonia as a fuel still faces several challenges. The dual-fuel combustion strategy, which involves blending low-reactivity NH3 with high-reactivity fuels, can effectively address these issues. In this work, we theoretically investigate the amino cross-reaction kinetics of the three saturated methyl esters including methyl formate (MF), methyl acetate (MA) and methyl propanoate (MP) (i.e., CnH2n + 1C(=O)OCH3, (n = 0, 1, 2)) and the three unsaturated methyl esters methyl acrylate (MAe), methyl butenoate (MB) and methyl crotonate (MC) (i.e., CmH2m - 1C(=O)OCH3, (m = 2, 3)). Comparing the energy barriers and reaction energies of these reactions calculated at two high-level electronic structure methods CCSD(T)/cc-pVxZ (x = T, Q) for MF, MA and MAe and CCSD(T)-F12/cc-pVTZ-F12 for MP, the M05-2X/jun-cc-pVTZ method has been selected due to the best performance with mean unsigned deviations (MUDs) from the CCSD(T) calculations of 0.23 kcal mol-1 (MF), 0.59 kcal mol-1 (MA), 0.55 kcal mol-1 (MP) and 0.38 kcal mol-1 (MAe). The rate constants of these reactions are calculated by using the multi-structural canonical variational transition state theory (MS-CVT/SCT) including the multi-dimensional small-curvature tunneling approximation, and the multiple-structure and torsional potential anharmonic effects at 500–2000 K. Our results are in good agreement with the available literature results and it can be found that the effect of different abstracting free radicals on the rate constants is greater than the effect of different fuels. Furthermore, based on our calculations, a combustion kinetic model has been proposed to elucidate the combustion mechanism of MAe/MP + ammonia mixtures. Kinetic analysis indicates that MAe generates reactive radicals in the initial stage due to its high reactivity, which disrupts the system and accelerates the consumption of NH3 through H-abstraction reactions. In the presence of MP, the important intermediate N2H2 is more likely to form N2H3 rather than NNH. This contributes to a deeper understanding of the combustion mechanism of ammonia/fatty acid methyl esters.