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利用ReaxFF MD方法对煤加氢及类煤体系热解的探索
Alternative TitleInvestigation On Coal Hydropyrolysis and Coal-related Pyrolysis Using ReaxFF MD Method
刘晓龙
Subtype工程硕士
Thesis Advisor李晓霞
2014-07
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword煤   高密度聚乙烯   热解   reaxff   反应机理
Abstract煤加氢热解是一种高效、清洁利用煤炭的方法,是我国能源领域的重大课题。实验原位检测技术缺乏使得其对机理的研究存在一定困难,而用分子模拟的方法能帮助更深入的理解煤加氢热解中发生的反应机理以及小分子的生成路径。本文将ReaxFF MD方法应用于煤加氢热解机理的探索。本文主要研究内容和结论如下: 为了在可接受的反应时间内观察到反应,我们采取了升温策略,升温幅度由参照类煤体系高密度聚乙烯(HDPE)热解的模拟研究确定。构建了含有8条聚合度为150的聚乙烯长链,共计7216个原子的HDPE体系。为研究温度的影响,对构建的HDPE体系用ReaxFF MD方法进行了2000 – 3000 K的NVT模拟250 ps。然后利用一个新开发的C++程序,VARxMD,对得到的分子轨迹文件进行了分析,得到了详细机理和主要气体小分子的生成路径。产物分布和主要气体小分子产量随温度演变趋势与用Py-GC/MS进行的HDPE热解实验得到的以及文献报告的结果定性相符。分析了C31以下产物生成的表观动力学特性并用于预测实验条件下HDPE热解失重率到达90%所需要的时间,与文献报告的数值非常接近。模拟获得的基于具体化学反应的反应机理和文献的报告有较好的符合。 基于对HDPE热解的模拟研究积累的经验,对柳林煤的加氢热解过程进行了研究。为了考察氢气的影响以及煤颗粒内部和表面热解特性的区别,本文以郑默等人提出的柳林煤模型化合物为基础,构建了常规柳林煤体系(4728个原子)、真空环境下的柳林煤体系(4728个原子)以及氢气气氛中的柳林煤体系(14028个原子)。将这些体系进行了1800 – 3000 K下NVT系综的ReaxFF MD模拟250 ps,用LAMMPS自带的Reax Tool分析获得了产物分布随温度演变的规律,并对三种体系中产物分布的异同进行了对比。结果显示,在2200 K之前,煤的惰性热解占主导地位,而氢气的作用并不十分显著;而在2200 K之后氢气的作用开始显现,主要是阻止煤炭发生结焦反应。这些结果与Suuberg的实验工作和郑默的模拟的结果定性一致。此外,本文还用VARxMD工具对获得的轨迹文件进行了详细机理和甲烷生成路径的分析。
Other AbstractCoal hydropyrolysis is a clean and efficient way of coal utilization and a major project of energy in China. However, knowledge of chemical reaction mechanisms in coal process is still limited due to the lack of experimental techniques for capturing the radicals. The Reactive molecular dynamics (ReaxFF MD) simulation is a promising approach to investigate large scale molecular systems with complex chemical reactions involved, thus employed in the thesis for a deeper understanding of the reaction mechanisms during coal hydropyrolysis and a coal-related system HDPE. In the simulation of HDPE pyrolysis, artificially increased simulation temperature is used to observe the reactions in acceptable simulation time. The temperature range used in the simulation is determined via the research of a coal-related system (HDPE) pyrolysis. An HDPE system with 8 PE chain (n = 150) contains 7216 atoms is constructed. A series of NVT ReaxFF MD simulation was then carried out at 2000 – 3000 K for 250 ps. Molecular trajectory of the simulation is analyzed via a newly developed C++ program VARxMD, detailed reaction mechanism and generation path of major gas products are obtained. Simulated product profile and temperature evolution tendency of major gas molecules qualitatively agree with experimental result obtained via Py-GC/MS. Apparent kinetic feature of product under C31 obtained in the simulation is analyzed and used to predict the time of 90% conversion in experimental condition and agrees well with the literature. Mechanism based on detailed reaction observed in the simulation is obtained. Most mechanism reported in the literature is obtained in this work. Study of Liulin coal hydropyrolysis is carried out in this work. Based on the structure of Liulin coal model purposed by Zheng et al, three coal model systems including a conventional Liulin coal system (4728 atoms), a Liulin coal system with hydrogen environment (14028 atoms) and a Liulin coal system with vacuum environment (4728 atoms) are constructed to investigate the influence of hydrogen in pyrolysis and the difference of pyrolysis behavior between inner coal particle and coal particle surface. A series of NVT ReaxFF MD simulation at 1800 – 3000 K were then performed on these models for 250 ps. Further analysis and comparison of product evolution in these 3 model systems with temperature are carried out via Reax Tool in LAMMPS. It is shown that inertia pyrolysis is the dominant process and the effect of hydrogen is not obvious when the simulation temperature is under 2200 K. When simulation temperature is over 2200 K, the effect of hydrogen is a significant factor preventing coal from coking. These results agrees qualitatively with Suuberg’s experiments. Detailed reaction lists were obtained based on the trajectory analysis using VARxMD and the generation paths of methane were analyzed.
Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/15568
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
刘晓龙. 利用ReaxFF MD方法对煤加氢及类煤体系热解的探索[D]. 中国科学院研究生院,2014.
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