Reactive Molecular Dynamics Study On The Influence Of CO₂ And H₂O On The Combustion Of Zhundong Coal Char

Oxy-fuel combustion technology is regarded as one of the most promising combustion technologies due to the low cost,low pollution and high efficiency.Oxy-fuel combustion atmosphere is rich in CO₂ and H₂O,whose physicochemical properties are different from those of N₂.Therefore,the characteristics of oxy-fuel combustion are different from that of air combustion.Traditional experimental methods can only explain the combustion phenomenon from the macro level,but it is difficult to reveal the reaction mechanism at the microscopic level.Therefore,it is of great significance to study the influence of CO₂ and H₂O on the coal char combustion process at the molecular level.The reaction molecular dynamics method（ReaxFF-MD）can be used to simulate complex chemical reaction processes without presupposing reaction path,which provides a possibility for studying the mechanism of oxy-fuel combustion of coal char at the molecular level.Firstly,the coal char is prepared by acid washing and pyrolysis of Zhundong coal.The coal char is characterized by High-Resolution Transmission Electron Microscopy（HRTEM）,elemental analysis and X-ray photoelectron spectroscopy（XPS）.Based on coal composition,structural parameters and group distribution,the Zhundong coal char molecular structural model is constructed combined with classic coal char model.Thus,coal char combustion system under O₂/N₂,O₂/CO₂ and O₂/H₂O atmospheres was constructed.Then the combustion characteristics of coal char in O₂/N₂ and O₂/CO₂ atmospheres were investigated by thermogravimetric analyzer.The results show that the coal char weight loss rate is slower under O₂/CO₂ atmosphere,and the coal char comprehensive combustion characteristic index is lower.The kinetic analysis shows that the activation energy of coal char reaction is higher in O₂/CO₂ atmosphere,indicating that CO₂ inhibits the transformation of coal char.On the other hand,ReaxFF-MD simulation method was used to investigate the coal char conversion,product distribution and molecular structural evolution under O₂/N₂ and O₂/CO₂ atmospheres:At low temperatures,the coal char conversion rate is lower in O₂/CO₂ atmosphere.The opening rate of benzene ring structure in coal char is slower.At high temperatures,CO₂ gasification promotes the carbon consumption,resulting in rapidly opening of the benzene ring and production of CO.In the O₂/H₂O atmosphere,the combustion performance of coal char was investigated by thermogravimetric analysis and ReaxFF-MD method.Under the water-added atmosphere,the weight loss rate of coal char decreases and the activation energy is larger,indicating that H₂O inhibits the conversion of coal char.The results show that the presence of H₂O under low temperature and low oxygen conditions inhibits the conversion of coal char.At present,the transformation of coal char is controlled by the diffusion effect of O₂.At high temperature,the reaction of H₂O with H and O radicals promotes the formation of OH radicals.H₂O also reacts with coal char to promote carbon conversion and CO production.However,CO can also reacts with H₂O to form CO₂ and H₂,thereby the content of CO is reduced.