Reaction Molecular Dynamics Study Of Sodium Carboxylate Migration And Conversion During Zhundong Coal Combustion

The high content of sodium is released during the combustion of Zhundong coal in Xinjiang,which leads to deposition,slagging and corrosion on the heating surface of large power generation boilers.It is difficult for the traditional experimental measurement methods to detect different forms of alkaline earth metals and free radicals in the coal combustion process to complete the change process.In recent years,Molecular Dynamics(Reax FF MD-Reactive Force Fields,Molecular Dynamics)method has developed rapidly,which can explore the migration and transformation mechanism of alkaline earth metals in complex systems during coal combustion at the molecular level.Based on this,this paper uses the Reax FF MD method to proceed as follows the study:Based on elemental analysis,industrial analysis,13C-NMR and other methods,and the molecular structure model of Zhundong coal containing sodium carboxylate was constructed and modified until it was consistent with the experimental analysis results.Secondly,the Reax FF force field was selected to determine the appropriate simulation parameters to simulate the reaction molecular dynamics of Zhundong coal pyrolysis.Then the sodium carboxylate coal sample was obtained with ion exchange method.Finally,the sodium carboxylate coal sample was analyzed by thermogravimetric analysis,and the weight loss rate of the thermogravimetric analysis was consistent with the Reax FF MD results,which verified the rationality of the molecular model of Zhundong coal.Reax FF MD was used to study the migration and transformation mechanism of sodium carboxylate during the pyrolysis of Zhundong coal.The results show that the volatile sodium increases with the temperature.At lower temperatures,it mainly exists in the form of sodium atoms,while it is mainly in the presence of inorganic sodium at higher temperatures.One way to release sodium atoms during coal pyrolysis is that sodium carboxylate in char can directly release sodium atoms,and the other way is that H radical attacks-COONa group to produce atomic sodium release.The atomic sodium released from the coal pyrolysis system will combine with the OH and O free radicals to form sodium oxide and sodium hydroxide.In the process of coal pyrolysis,the activation energy of volatile form sodium is 17.37 k J/mol,inorganic sodium is 40.52 k J/mol,and atomic sodium is 2.58 k J/mol.The char combustion system was constructed by extracting char macromolecules from the primary pyrolysis products of coal.Reax FF MD was used to study the influence of sodium on the char combustion.The results showed that the carbon conversion of char reaches 100% at 3000 K temperature,but the oxygen in the system still exists in large quantities.Compared with the previous stage of coal pyrolysis,the main product of combustion stage changed from char to inorganic gas.In the process of char combustion,sodium is mainly released in the form of volatile sodium: firstly,it appears in the form of atomic sodium,and then its release rate decreases rapidly with the gradual increase of temperature,while the release rate of inorganic sodium gradually increases with the temperature.The inorganic substances are mainly Na OH.According to the release kinetic parameters of sodium in the process of char combustion,the release activation energy of volatile sodium is 5.85 k J/mol,inorganic sodium is 8.23 k J/mol,and atomic sodium is 26.35 k J/mol.