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

Abstract

Amino radicals play 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 three saturated methyl esters, namely, methyl formate (MF), methyl acetate (MA) and methyl propanoate (MP) (i.e., C_nH_2n+1C(O)OCH₃, n = 0, 1, 2), and three unsaturated methyl esters, namely, methyl acrylate (MAe), methyl butenoate (MB) and methyl crotonate (MC) (i.e., C_mH_2m−1C(O)OCH₃, m = 2, 3). Upon comparing the energy barriers and reaction energies of these reactions calculated using 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 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. The results demonstrate that the rate constants of H-abstraction reactions at the OCH₃ site are insensitive to carbon chain elongation but highly sensitive to the positional variation of CC double bonds in unsaturated methyl esters. Furthermore, based on our calculations, a combustion kinetic model has been proposed to elucidate the combustion mechanism of MAe/MP + ammonia mixtures. Kinetic analysis reveals that in the MAe/NH₃ system, abundant reactive radicals are generated during the initial stage through unimolecular decomposition and H-abstraction reactions, which accelerates the NH₃ ignition process and significantly reduces the ignition delay time. Elevated temperatures suppress MAe consumption while promoting NH₃ conversion to NH₂ radicals. For the MP/NH₃ system, the H-abstraction reactions of MP demonstrate lower sensitivity to equivalence ratio variations, exhibiting relatively stable characteristics. In the presence of MP, the key intermediate N₂H₂ preferentially forms N₂H₃ rather than NNH. As the equivalence ratio increases, the concentration of NH₂ radicals produced via H₂NN shows a distinct decreasing trend, likely due to altered reaction pathway selectivity that suppresses the generation of these crucial NH₂ radicals. This contributes to a deeper understanding of the combustion mechanism of ammonia/fatty acid methyl esters.