Study On The Pyrolysis And Combustion Of Pyrrole And Pyridine Compounds In The Coal Via Molecular Dynamics Simulation

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.