【作者】 刘佳；

【导师】 郭欣；

【作者基本信息】 华中科技大学 ， 热能工程， 2015， 硕士

【摘要】 吡咯类氮与吡啶类氮是煤中氮最主要的两种存在形式,研究其热解燃烧机理对于理解煤在热解与燃烧过程中氮的转化与生成具有重要的作用和意义。由于热解燃烧过程的反应速度非常快,现有的实验手段很难完全检测到所有的中间体和产物分布,量子化学计算却不便于模拟大型分子体系在不同实验条件下的反应变化,而传统力场的分子动力学,则不能模拟化学反应中键的断裂和生成。本文采用基于ReaxFF反应力场的分子动力学,通过模拟吡咯与吡啶在热解燃烧过程中化学键形成断裂的反应过程,研究了不同的反应条件对两者热解和燃烧过程的影响,着重考察了煤中氮元素在热解燃烧过程中的转化路径以及影响因素。构造和优化具有不同初始状态的吡咯吡啶体系,模拟了不同的温度、密度(压力)、升温速率下吡咯与吡啶的热解过程。通过分析热解过程中各类中间体和产物随时间的变化趋势,研究了不同热解条件对吡啶吡咯热解过程的影响,并分析了不同的热解条件对氮氧化物生成机理的影响。模拟过程中得到的所有热解、燃烧产物和中间体均能被实验数据所证实,说明基于ReaxFF反应力场的分子动力学方法模拟吡咯和吡啶的热解过程是合理且可信的。结果表明,温度的升高会促进吡咯和吡啶分子的分解,其主要热解产物为烃类和腈类物质,其中HCN为最主要的含氮产物。而密度(压力)的升高会促使总热解产物数目减少,且N2生成的时间也越来越早,生成的量也越来越多,这是由于高的热解压力下,热解产物更倾于以大分子形式的存在,HCN也越容易向N2转化。此外,较高的升温速率会增加热解处在高温区的时间,从而提升吡咯和吡啶热解的速度以及热解产物的数目。同时,由于热解开环方式的不同,吡啶分子要比吡咯分子更加稳定。为了研究不同的温度以及化学当量比对吡啶和吡咯燃烧过程的影响,本文分别模拟两者的燃烧过程,并分析了主要的燃烧产物随着反应时间的变化趋势。模拟结果表明,温度的升高会加快燃烧过程中吡咯和吡啶分解的速度和主要产物的生成速度;在燃烧过程中,H2O、CO2、N2、NO和NO2等主要的燃烧产物的量会随着反应的进行不断增加然后趋于平衡,到达平衡的时间也随着温度升高而越来越早;而随着化学当量比的升高,燃烧产物H2O、CO2、NO、NO2的量迅速增加;CO和HCN由于更容易转化为CO2和HCN,而逐渐减少。此外,结果表明氧气浓度对富燃的影响作用要明显高于贫燃的情况。更多还原

【Abstract】 The emission of oxides of nitrogen(NOx) from power generation using coal contributes to the formation of acid rain and photochemical smog, in addition to the direct effects on human health, which has been and will continue to be a major environmental concern. Significant amounts of nitrogen in coal is in the form of aromatic heterocyclic structures containing pyrrole and pyridine ring systems. These heterocycles may form nitrogen precursors of NOx during pyrolysis and combustion process so that the chemical mechanism from representative nitrogen compounds will determine NOx formation in the combustion of coal. However, chemical reactions take place because of the free radical initiation at high temperature in an extremely short period of time, which are hard to detect and replicate in the laboratory, and due to the limits of the available measurement methods, only limited species can be analyzed accurately and very few information about the intermediates and free radicals formed can be obtained during the pyrolysis and combustion process. Quantum mechanical(QM) calculations can provide accurate transition states and reaction rate constants for chemical kinetic modeling, but this method is too computationally expensive in providing a detailed, dynamic description of the complex pyrolytic reactions for a large-scale system exposed to a variety of temperature and pressure transients. On the other hand, molecular dynamics(MD) simulation with conventional force fields can not describe the processes of bond breaking and bond formation in the chemical reactions. Therefore, the greatest challenge is to find a reasonable method to fully characterize the pyrolysis and combustion process of the nitrogen compounds.The chemical processes of pyrolysis and combustion for pyrrole and pyridine systems were simulated by employing the MD simulations with the reactive force field(Reax FF). We not only studied how the pyrolysis and combustion process of the two nitrogen compounds depended on temperature, heating rate, density/pressure and oxygen ratio but also investigated the pathways of the NOx formation.The effect of temperature, density/pressure and heating rate on the pyrolysis process of pyrrole and pyridine molecules was studied by carrying out the Reax FF MD simulations on the two systems with different pyrolysis conditions. All the products and intermediates observed during the simulation can be demonstrated from the experiment, indicating Reax FF MD simulation can provide reasonable results of the pyrolysis process. It is found that more species and products were formed as the temperature increased, but when the density/pressure increased, the number of total fragments decreased and larger amount of N2 were produced, since the products were more likely to be larger molecules and HCN was more easily transformed to N2 at greater density. In addition, higher heating rate increased the decomposition of pyrrole and pyridine molecules, owing to that the time for simulation proceeding at higher temperature was increased with a larger heating rate. Pyridine molecule was more stable than pyrrole because of the different ring-open ways.In order to study the effect of temperature and oxygen ratios on the combustion process of pyrrole and pyridine molecules, a series of Reax FF MD simulations were performed on the two systems. The results show that the number of primary products, such as H2 O, CO2, N2, NO and NO2, increased as time evolved and then kept balance at a stable constant. Higher temperature promoted the formation of products and the decomposition of pyrrole and pyridine systems. As the oxygen ratio increased, the number of H2 O, CO2, NO and NO2 increased, but the number of CO and HCN decreased because they were more easily to transform to CO2 and NOx at a larger oxygen ratio.更多还原