Abstract:

The behavior of volatile radicals is very important for regulating coal pyrolysis tar products, but it is difficult to obtain their chemical reactions directly by experimental methods. In this paper, two dominant mechanisms of stabilization of volatile radicals and condensation reactions in bituminous coal pyrolysis associated with volatile radicals were investigated by using the large-scale ReaxFF MD simulation combining with high-performance computing and cheminformatics based reaction analysis method. The number evolving trends for the competition of stabilization and condensation reactions with time under different temperature condition were obtained with the aid of the multilevel searching strategy. The results show that stabilization reactions of volatile radicals are dominant at relatively low temperature of primary pyrolysis stage while condensation reactions of volatile radicals dominate the late pyrolysis stage at high temperature. The compromise phenomenon in competition between stabilization and condensation reactions is observed when the temperature is a little higher than that of the stage transition from the primary pyrolysis to secondary pyrolysis. Importantly, the transition point from stabilization dominant stage to the intensive competition stage of stabilization and condensation corresponds to the highest yield of tar products, while the transition point from the intensive competition stage to condensation dominant stage corresponds to the starting point for char generation. The phenomenon indicates that such understanding of reaction competitions would be very helpful to modulate tar composition and obtain the high yield of tar products industrially.

Key words: molecular simulation, chemical reaction, mesoscale, coal pyrolysis, volatile radical, compromise in competition

摘要：

挥发分自由基的行为对调控煤热解焦油产物十分重要，但实验方法直接获得其化学反应存在困难。利用化学反应分子动力学 ReaxFF MD 模拟方法，结合基于GPU高性能计算和化学信息学分析，研究了煤基体中挥发分自由基（基本单元）及其涉及的分子群（系统）在热解过程中的演化规律，着重考察了挥发分自由基参与的被稳定反应和缩聚反应在煤热解过程中的竞争协调关系。大规模ReaxFF MD模拟结果表明，随温度升高，煤热解过程可分成三个区域：被稳定反应占主导的区域、被稳定和缩聚反应竞争区域和缩聚反应占主导的区域。其中被稳定反应占主导的区域主要发生在较低温或反应前期，使得焦油收率升高；缩聚反应占主导的区域出现在高温阶段或反应后期，使得煤热解结焦加剧。当热解体系处于从被稳定反应占主导到被稳定和缩聚反应竞争激烈的转折点时，焦油收率出现最大值；当处于从两类反应竞争激烈到缩聚反应占主导的转折点时，焦炭产物开始大量生成。煤热解中挥发分自由基参与的两类相互竞争的化学反应控制着煤热解过程中焦油收率的演化，可为调控焦油生成，以进一步获得高品质化学品提供理论指导。

关键词: 分子模拟, 化学反应, 介尺度, 煤热解, 挥发分自由基, 竞争协调