Chemical Dynamics Simulation Of Methane Combustion Reaction Mechanism Under O₂/H₂O Atmosphere

The serious environmental problems associated with fossil fuel combustion have motivated a decades-long pursuit of clean combustion technologies with higher efficiencies and near-zero emissions.Oxyfuel combustion technology is currently one of the most effective clean combustion technologies.In order to achieve efficient low-carbon energy conversion,Oxy-Coal Combustion Steam System of Near-zero Emissions was proposed.Studies have shown that the concentration of water vapor in flue gas in an oxygen-rich atmosphere is as high as 15%-30%.The combustion of pulverized coal is affected by the presence of high concentrations of water vapor,but the specific mechanism is not clear.This article focuses on explores the mechanism of methane combustion reaction in O₂/H₂O atmosphere.The reaction OH+CH₄?CH₃+H₂O is one of the most important elementary reactions for methane combustion.The stationary point properties for the reactants,pre-and postreaction complexes,transition states,and products on the OH+CH₄?H₂O+CH₃ PES are determined using the MP2 and DFT,with the wB97XD,M06-2X and BHandHLYP functionals.The basis set,aug-cc-pVDZ,aug-cc-pVTZ,6-311++G?d,p?are used for both MP2 and DFT calculations.Using the energy optimized at the high computational level of CCSD?T?-F12a/aug-cc-pVTZ as the benchmark for calculating the standard deviation,it was found that the energy optimized by the M06-2X/aug-cc-pVTZ method was in good agreement with the benchmark.Then the potential energy surface of OH?H₂O?_n+CH₄?n=0-2?reaction was constructed by the preferred M06-2X/aug-cc-pVTZ calculation method.The effect of water molecules on the oxidation process of CH₄was studied.The article found that,in fact,the energy barrier of the reaction is affected by the water molecules.The study found that the water molecules actually led to a gradual decrease in the pre-coordination energy of the reaction,which was-1.3 kcal/mol,-1.6kcal/mol and-2.3 kcal/mol,respectively;the potential energy gradually increased to 2.6kcal/mol,9.5 kcal/mol and 20.6 kcal/mol,respectively.The central barrier is increased by approximately 0.6 kcal/mol.The reaction mechanism of OH?H₂O?_n+CH₄?H₂O+CH₃+n?H₂O??n=0-2?was studied by direct dynamics simulation.The trajectory calculation starts from the transition state and observes the reaction mechanism of each reaction channel after adding water molecules.It is found that there is no corresponding pre-or complex in the transition state,but the reactants or products are directly formed.This reaction mechanism is called direct mechanism and is a non-IRC?non-Intrinsic Reaction Coordinate?behavior,which is a phenomenon worthy of attention.When the water molecules are introduced into the system,the products of the trajectories are also different.The main products in the trajectory of the anhydrous system are water molecules and methyl radicals.After the introduction of water molecules,the main products of the system are water molecule complexes and methyl radicals.This has important value for predicting the existence of high concentration of water vapor to have an important impact on the combustion of pulverized coal,providing a reliable theoretical supplement and evidence for the experimental results.