高煤级煤石墨化过程中微晶结构的演化及机理研究

The evolution of microcrystalline structure of coal during the coalification are always the forefront topics that need to be explored untiringly in the coal science field. The research on the evolution of microcrystalline structure of coal during the coalification from lignite to anthracite have made great progress so far, while the microcrystalline structure evolution of high-rank coal to the coal-based graphite with the further metamorphism still needs to be deeply explored. The present project is proposed to employ the methods utilized in the coal geology, mineralogy, and molecular simulations to reveal the evolution process of microcrystalline structure of high-rank coal during the graphitization through investigating the microcrystalline structure of high-rank coal, semigraphite, and graphite in the metamorphic zone located in the typical mining district in China. On the basis of study of the molecular structure of the high-rank coal, their 3D models are constructed. Then molecular dynamics simulations are performed on the models using the reactive and classical force fields to explore the evolution of the making-up and ordering of macromolecular aromatic carbon layer of high-rank coal during the evolution of microcrystalline structure under different coupling of pressure and stress at atomic scale. In addition, the thermodynamic mechanism and controlling factor for such evolution are explored. The research project is of greatly theoretical significance for completing the evolutionary sequence from high-rank coal to coal-based graphite and the ore-forming mechanism of the coal-based graphite.

煤的微晶结构在煤化作用过程中的演化始终是煤科学领域不懈探索的前缘课题。目前对褐煤变质成无烟煤过程中煤微晶结构的演化研究已取得重要进展，然而高煤级煤进一步变质形成煤系石墨的微晶结构演化依然有待深入研究。本项目拟以我国几个典型地区的煤系石墨矿床为研究对象，利用煤地质学、矿物学和分子模拟等的研究方法，通过对变质带中高煤级煤、半石墨和石墨微晶结构的研究，揭示高煤级煤石墨化的微晶结构演化过程。在高煤级煤分子结构分析的基础上，建立其3D结构模型，并对其进行基于反应力场和普通势能力场的分子动力学模拟，在原子尺度范围内探索高煤级煤在不同温度和应力耦合作用下的微晶结构演化过程中，其大分子芳香碳层拼叠和有序化的演化过程，以及该过程的热力学机理和控制因素。本课题研究对完善从煤到石墨的演化序列和煤系石墨的成矿机理具有重要的理论意义。

煤系石墨作为石墨资源的重要组成部分，是制备石墨烯和石墨烯量子点的潜在原材料，其成矿机制及晶体结构有序度的鉴别对煤系石墨的勘探和高值化利用具有重要意义。本项目以我国湖南鲁塘、新化，陕西凤县和内蒙古扎鲁特地区的煤系石墨和无烟煤变质带为研究对象，在变质带上系统采集了无烟煤、变质无烟煤、半石墨和石墨样品。采用XRD、拉曼光谱、扫描电镜和高分辨率透射电镜等测试方法对样品的微晶结构和芳香片层结构进行了系统研究，探明了无烟煤石墨化过程中芳香片层横向拼接、纵向堆叠，由二维有序向三维有序的演化过程。采用XRD(002)晶面层间距和衍射峰的不对称指数，拉曼光谱D1峰与G峰的面积比和G峰半高宽参数建立了煤系石墨晶体结构有序度的鉴别方法。采用元素分析、核磁共振碳谱、高分辨率透射电镜和X射线光电子能谱等测试方法揭示了无烟煤芳香片层、脂肪侧链的结构特征和杂原子的赋存状态，据此构建了无烟煤大分子平均结构模型和3D结构模型。采用基于反应力场(CHON-2019-ReaxFF)的分子动力学模拟，对构建的模型进行了不同温压作用下的分子动力学模拟，从分子尺度揭示了温度是引发无烟煤石墨化的主控因素，压力、剪切应力和时间效应也是自然界中无烟煤石墨化的重要因素。高温可促使芳香片层脂肪侧链、氢原子和杂原子官能团的脱除，致使其边缘产生断键，实现芳香片层的横向拼接。大尺寸芳香片层之间的范德华力是促使其纵向堆叠的主要驱动力。本研究对煤系石墨的成矿机理和开发利用，以及人工合成石墨都具有重要的理论与现实意义。