Abstract:

Chemical looping combustion (CLC) is an important research orientation of clean energy processing and efficient conversion in the period of profound global changes. The reaction between solid fuels such as coal and oxygen carriers are one of the core scientific issues in CLC research. However, the evolution rules and the transformation mechanism are still unclear, especially the microscale description of the molecular structure evolution and functional group transformation of coal and other macromolecular solids in the fuel reactor from the atomic and molecular scale is lacking. Based on the macromolecular structure of coal and ReaxFF MD simulation, we study the chemical looping combustion process of Ningxia QH and YCW coal and nickel oxide oxygen carrier. Through the CLC process analysis of QH-NiO and YCW-NiO systems, the rules of total energy, total molecular number, product distribution and gas conversion are obtained. Based on the ReaxFF MD simulation process, the dynamic evolution process of coal macromolecular structure in CLC process is directly obtained. As the CLC reaction progresses, the macromolecular structure of coal gradually dissociates due to the breaking of old bonds and the generation of new bonds, resulting in various intermediate products, small molecules such as CO _x and H₂O. The regulation and mechanism of quantity and rate of oxygen release from oxygen carrier during CLC in QH-NiO and YCW-NiO systems are revealed. The results show that compared with QH coal, YCW coal has lower metamorphism and higher reactivity. Therefore, the YCW-NiO system has lower total potential energy, generates more molecular fragments and more non-hydrocarbon gas, which can attribute to the oxygen carrier release lattice oxygen earlier. With the progress of CLC reaction, the oxygen vacancy formed on oxygen carrier surface leads to the migration and release of lattice oxygen in the secondary outer and inner lattice to the surface.

Key words: molecular simulation, mesoscale, reaction, chemical looping combustion, coal molecular structure, oxygen carrier

摘要：

化学链燃烧（CLC）技术是全球深刻变革时期能源清洁加工和高效转化的重要研究方向。煤等固体燃料和载氧体的反应是CLC研究的核心科学问题之一。然而，其作用规律及转化机制仍不明确，尤其缺乏从原子分子尺度上理解煤等大分子固体在燃料反应器中分子结构演变和官能团转化的微观尺度描述。基于煤的大分子结构和反应力场分子动力学（ReaxFF MD）模拟，研究了宁夏庆华（QH）和羊场湾（YCW）煤与氧化镍载氧体的化学链燃烧过程。通过QH-NiO和YCW-NiO体系的CLC过程分析总结获得了体系总能量、总分子数、产物分布和气体转化率的规律。基于ReaxFF MD模拟过程直接获得了煤的大分子结构在CLC过程中的动态演化过程。随着CLC反应的进行，煤的大分子结构由于旧键断裂和新键生成逐步解离生成各种中间产物、CO _x 、H₂O等小分子。揭示了QH-NiO和YCW-NiO体系CLC过程中载氧体释氧量和释氧速率的规律和影响机制。结果显示，相较于QH煤，YCW煤的变质程度较低，反应活性较高。因此，YCW-NiO体系总势能更低、生成的分子片段数较多，且生成非烃气体的含量也较多，可使载氧体更早开始释放晶格氧。随着CLC反应的进行，载氧体表面形成氧空位导致次外层和内部的晶格氧向表面迁移并释放。

关键词: 分子模拟, 介尺度, 反应, 化学链燃烧, 煤分子结构, 载氧体