Deng Haixing, Wang Sihao, Fu Li, Ning Hongbo
Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China.
School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China.
Phys Chem Chem Phys. 2025 May 21;27(20):10787-10803. doi: 10.1039/d5cp00958h.
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 NH 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) (, CHC(O)OCH, = 0, 1, 2), and three unsaturated methyl esters, namely, methyl acrylate (MAe), methyl butenoate (MB) and methyl crotonate (MC) (, CHC(O)OCH, = 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 (MF), 0.59 kcal mol (MA), 0.55 kcal mol (MP) and 0.38 kcal mol (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 OCH 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/NH system, abundant reactive radicals are generated during the initial stage through unimolecular decomposition and H-abstraction reactions, which accelerates the NH ignition process and significantly reduces the ignition delay time. Elevated temperatures suppress MAe consumption while promoting NH conversion to NH radicals. For the MP/NH 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 NH preferentially forms NH rather than NNH. As the equivalence ratio increases, the concentration of NH radicals produced HNN shows a distinct decreasing trend, likely due to altered reaction pathway selectivity that suppresses the generation of these crucial NH radicals. This contributes to a deeper understanding of the combustion mechanism of ammonia/fatty acid methyl esters.
氨基自由基在氨的热解和氧化过程中起着核心作用。将纯氨作为燃料实际应用仍面临诸多挑战。双燃料燃烧策略，即将低反应活性的NH与高反应活性燃料混合，能够有效解决这些问题。在本工作中，我们从理论上研究了三种饱和甲酯，即甲酸甲酯（MF）、乙酸甲酯（MA）和丙酸甲酯（MP）（，CHC(O)OCH， = 0, 1, 2）以及三种不饱和甲酯，即丙烯酸甲酯（MAe）、丁烯酸甲酯（MB）和巴豆酸甲酯（MC）（，CHC(O)OCH， = 2, 3）的氨基交叉反应动力学。通过使用两种高水平电子结构方法计算这些反应的能垒和反应能量——MF、MA和MAe采用CCSD(T)/cc-pVxZ（x = T, Q），MP采用CCSD(T)-F12/cc-pVTZ-F12——由于M05-2X/jun-cc-pVTZ方法与CCSD(T)计算的平均无符号偏差（MUDs）表现最佳（MF为0.23 kcal mol，MA为0.59 kcal mol，MP为0.55 kcal mol，MAe为0.38 kcal mol），所以选择了该方法。这些反应的速率常数通过使用多结构正则变分过渡态理论（MS-CVT/SCT）计算得出，该理论包括多维小曲率隧道近似以及在500 - 2000 K下的多结构和扭转势非谐效应。结果表明，在OCH位点的氢提取反应速率常数对碳链延长不敏感，但对不饱和甲酯中CC双键的位置变化高度敏感。此外，基于我们的计算，提出了一个燃烧动力学模型来阐明MAe/MP + 氨混合物的燃烧机理。动力学分析表明，在MAe/NH系统中，初始阶段通过单分子分解和氢提取反应产生了大量活性自由基，这加速了NH的点火过程并显著缩短了点火延迟时间。温度升高会抑制MAe的消耗，同时促进NH转化为NH自由基。对于MP/NH系统，MP的氢提取反应对当量比变化的敏感性较低，表现出相对稳定的特性。在有MP存在的情况下，关键中间体NH优先形成NH而不是NNH。随着当量比增加，由HNN产生的NH自由基浓度呈现明显下降趋势，这可能是由于反应途径选择性改变抑制了这些关键NH自由基的生成。这有助于更深入地理解氨/脂肪酸甲酯的燃烧机理。
