次烟煤热解的分子动力学模拟及反应机理研究

The energy efficiency in the process of upgrading and conversion of low rank coal to produce fuel oil, gas and other chemicals can be significantly improved by pyrolysis. However, the problem about high content of heavy tar obtained by subbituminous pyrolysis still can't been solved. The main reason is that the molecular transformation and reaction mechanism of pyrolysis process is not clear and the nature of the regulation of the transfer and reaction processes has not been fully understood. Therefore, this project will investigate the pyrolysis process of subbituminous coal by using reactive molecular dynamics simulation based on ReaxFF force field. The following two crucial issues are mainly focused on: 1) the formation mechanism of heavy tar in subbituminous coal pyrolysis; 2) the influence of tar component distribution regulated by addition of free radical. Macromolecular structures which are close to the true nature of the coal organic matter are built by computer-aided molecular design method. Reactive molecular dynamics modeling approach is then applied to explore the process of coal pyrolysis, which can track the generation and polymerization of free radicals to reveal the formation pathways of heavy tar. The law of effect for tar component distribution by the addition of free radicals is studied in detail by combining with reactive molecular dynamics and density functional theory methods, to elucidate the molecular orbital interactions between molecules in free radical reactions. Theoretical calculations of this project will provide a reliable theoretical guidance not only for designing the regulation method of pyrolysis process, but also for controlling the yield of heavy tar.

热解可显著提高煤提质转化制备燃料油、燃气及化学品等过程的能源效率，但是次烟煤通过热解获得的焦油产物中重质组分含量高的问题至今仍然无法解决。主要是由于人们对热解过程的分子转化及反应机理的认识还处于模糊的水平，对自由基反应过程调控的本质认识不够。鉴于此，本项目拟采用基于ReaxFF力场的反应分子动力学方法模拟次烟煤的热解反应过程，重点研究焦油产物中重质组分的形成机理以及探索外加自由基对于调控焦油组分分布的影响。利用计算机辅助分子设计方法构建接近真实煤的有机质组分的大分子结构，采用反应分子动力学方法模拟热解反应过程，追踪自由基的产生和聚合反应并揭示焦油重质组分的形成路径，并结合密度泛函理论方法研究外加自由基调控焦油组分分布的影响规律，阐明自由基反应过程中分子轨道相互作用及电子转移机理。本项目的理论模拟研究可为热解过程调控方法的设计以及调节焦油重质组分的产率提供可靠的理论指导。

煤及油页岩等固体化石燃料通过热解可以转化制备燃料油、燃气及化学品，但是目前普遍存在热解效率低，产品品质差等问题，其原因主要是由于人们对于热解过程的分子转换及反应机理的认识还处于模糊的水平，对热解反应过程调控的本质认识不够。鉴于此，本项目对次烟煤及油页岩有机质的三维结构的建立，热解过程的反应分子动力学模拟，产物分布及自由基反应机理这个几方面内容进行了详细研究。基于热解实验前后对反应物、产物的结构分析，构建了依兰次烟煤的三维大分子结构，并采用分子模拟方法优化结构，从而建立通过NMR实验与模拟数据的对比评估结构可靠性的方法。分别针对依兰次烟煤、Hatcher次烟煤、绿河油页岩干酪根的热解过程进行了反应分子动力学模拟，考察不同升温速率、热解温度、体系尺度、密度等因素对有机质热解的热解断键行为和产物分布的影响规律，并分析典型油气产物组分的形成机理，进而探索外加自由基对于调控油气组分分布的影响。模拟结果表明随着升温速率的加快，大分子片段的热解失重峰向温度高的方向移动，与热重实验结果的趋势一致。在热解过程中不同类型化学键断裂基本遵循能量低先断裂的原则，其顺序依次为醚键，芳基碳碳键，脂肪族碳碳键，硫醚键。化学键的断裂顺序还会受到取代基的影响而发生变化，一旦发生均裂断键后形成的自由基会影响其周围电子发生重排，使相邻的化学键容易断裂。根据化学键的断裂特征和产物的生成特点可以将热解过程分为三个阶段，分别为：1）弱键断裂形成重油片段；2）大片段裂解产油阶段；3）深度裂解，自由基重排富产气体阶段。本项目的研究成果对于指导热解过程的调控，提高特定产物的产率及品质具有重要的意义。