Abstract The secondary reactions of volatile compounds have great effects in determining the yield of products in coal pyrolysis process. While secondary reactions of volatiles under different heating conditions have been extensively investigated over the last few decades, reaction mechanism of volatiles in cooling process is still not well understood. The effect of cooling rate on the reaction of volatiles from low-rank coal pyrolysis is studied using ReaxFF molecular dynamics simulations. First, a series of coal pyrolysis simulations are carried out at different temperatures. The newly formed volatile compounds containing radical fragments are extracted from pyrolysis products to construct the model of nascent volatiles. Then, the system of volatiles is cooled down to 300 K at different cooling rates ranging from 5 to 500 K/ps. The results show that the secondary reactions of volatiles still occur even at 500 K/ps, and the amount of gas and coke increase with decreasing cooling rate. The reaction mechanisms of volatiles in cooling process are revealed by analyzing the main elementary reactions. In rapid cooling, only the agglomeration of tar fragments is observed. While, the reactions of tar in slow cooling condition undergoes two stages: agglomeration of tar fragments resulting in the formation of coke compounds, and subsequently reactions of the coke with tar fragments leading to the growth of coke weight. Finally, the activation energy for the secondary reactions of nascent tar is determined by fixed-temperature simulations from 1800 to 2800 K and is found to be 24.4 kcal/mol, which is in good agreement with recent experimental results.

中文翻译：

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冷却速率对低阶煤热解挥发物反应的影响：使用 ReaxFF 的分子动力学模拟

摘要 煤热解过程中挥发性化合物的副反应对产物的收率有很大影响。虽然在过去的几十年里，挥发物在不同加热条件下的二次反应得到了广泛的研究，但挥发物在冷却过程中的反应机理仍不清楚。使用 ReaxFF 分子动力学模拟研究了冷却速率对低阶煤热解挥发物反应的影响。首先，在不同温度下进行一系列煤热解模拟。从热解产物中提取新形成的含有自由基碎片的挥发性化合物，构建新生挥发物模型。然后，挥发物系统以 5 到 500 K/ps 的不同冷却速率冷却至 300 K。结果表明，即使在500 K/ps下，挥发分的二次反应仍然发生，气体和焦炭量随着冷却速度的降低而增加。通过对主要元素反应的分析，揭示了挥发物在冷却过程中的反应机理。在快速冷却中，仅观察到焦油碎片的团聚。而焦油在缓冷条件下的反应经历两个阶段：焦油碎片团聚形成焦炭化合物，随后焦油与焦油碎片反应导致焦炭重量增加。最后，新生焦油二次反应的活化能由 1800 至 2800 K 的固定温度模拟确定，发现为 24.4 kcal/mol，这与最近的实验结果非常吻合。