Study Of The Nitrogen-containing Model Compounds In Coal Oxy-fuel Combustion Via ReaxFF Molecular Dynamics Simulation

NO_x and CO₂ produced from coal combustion cause serious damage to human health and ecological equilibrium.Oxy-fuel combustion is a novel technology for multiple pollutants emission control.During the process of oxy-fuel combustion,nitrogen transformation is affected by the main compositions?CO₂ and H₂O?of flue gas.Nitrogen within coal mainly exists in the forms of pyrrole and pyridine.Therefore,the study on the reaction mechanism controlling nitrogen transformation and formation can provide important theoretical and practical significance for the control of NO_x emission in the process of oxyfuel combustion.Reax FF-MD,a molecular reaction kinetics based on bond order,can describe the breakage and formation of chemical bond and provide a new method to understand the molecular-level reaction mechanism of pyrrole and pyridine nitrogen.In this study,the reaction process of pyrrole and pyridine combustion in O2/N2,O2/CO₂ and O2/H₂ O atmosphere was investigated using ReaxFF-MD reaction molecular dynamics simulation method.The microscopic reaction mechanism of pyrrole and pyridine combustion under different condicions?reaction temerpatures,oxygen concentrations,combustion atmosphere?was revealed through analyzing the change in the structure of intermediates,radicals and final products.Reactive molecular dynamics?MD?simulations employing the ReaxFF reactive force field have been performed to study the combustion of pyrrole and pyridine in O2/N2,O2/CO₂ and O2/H₂ O atmosphere.ReaxFF MD calculation results indicate that the initial reaction step of pyrrole combustion in air is dehydrogenation and oxygenation.During pyridine combustion,C-H bond is first broken to form H radicals,which subsequently react with O2 to form HO2.Molecular dynamic calculations based on ReaxFF-MD method are reliable.In the O2/CO₂ atmosphere,CO₂ does not participate in chemical reactions at lower temperatures.However,CO₂ is involved in the chemical reaction to inhibit the reaction of pyrrole and pyridine at higher temperatures.Compared with O2/CO₂ atmosphere,air atmosphere is favorable to achieve lower NO emission at fuel-rich and stoichiometric condition.At fuellean condition,O2/CO₂ atmosphere is unfavorable for NO formation compared to O2/N2 atmosphere.In the O2/H₂ O atmosphere,the higher concentration of H₂ O accelerates the reaction rate of pyrrole and pyridine.Compared with O2/H₂ O atmosphere,O2/N2 atmosphere inhibits NO formation under fuel-rich,stoichiometric and fuel-lean conditions.In the different atmosphere,the maximum concentration of nitrogen containing intermediates of pyrrole and pyridine is obtained within a shorter time when the reaction temperature increases.Pyrrole and pyridine are favorably converted to NO as the oxygen concentration increases.