The Molecular Dynamics Simulation Of Initial Char Oxidation/Gasification Surface Reaction At The Nanoscale

Oxy-fuel combustion technology has been widely investigated due to it is considered as one of the most feasible the way to reduce the CO₂ emission in coal-fired power plants with a great value of industrial application.Many researches indicated that the char gasification reaction of CO₂ can affect the combustion characteristic during char combustion in O₂/CO₂atmosphere.Nevertheless,in case of the surface reaction of initial char combustion is very short and complicated,the researchers can not get enough information of chemical reaction through the macroscopic experiments and macroscopic simulation methods.In that case,it is necessary to further study the surface reaction of initial char combustion in O₂/CO₂ atmosphere under molecular scale.The ReaxFF molecular dynamic（ReaxFF-MD）simulation can simulate the char combustion process on molecular scale and obtain the chemical information which is hard to get through the macroscopic experiments and simulation methods,which can not only supply to the macroscopic experimental research but also build a more comprehensive combustion mechanism with the reaction kinetics results.Based on this,it is significant to the development of O₂/CO₂ combustion system and equipment.Firstly,A calculation model of char combustion for ReaxFF-MD simulation is established.The suitable force field parameters are selected.Additionally,the reliability and accuracy of the calculation model and method are verified.Then,the char combustion processes underφ=1.88andφ=0.94 O₂ atmosphere andφ=0.94 CO₂ atmosphere in 3000 K,3250 K and 3500 K temperature are simulated by MD-ReaxFF.Changes of main gaseous products（CO₂,CO,O₂and H₂O）and carbon-containing intermediate product with temperature and reaction equivalent are analyzed.Based on the changes in the number of chemical bonds such as C-C bond,C-O bond and C-H bond,the oxidation and gasification reaction mechanisms are discussed in detail,and the C-C bond consumption rates of char molecules are calculated.The results show that with the increase of O₂,the production of CO is decreased and the production of CO₂ is increased.Additionally,the production of gaseous small molecules（C₂^C₄）in char gasification is higher than that of char oxidation.The oxidation rate and gasification reaction rate of char molecules both follow the first-order reaction rate equation.The activation energy of oxidation reaction is 164 kJ/mol and the activation energy of gasification reaction is 217 kJ/mol,which is in agreement with the experimental results in the literature.The accuracy of the ReaxFF molecular dynamics simulation of the char oxidation and gasification reactions is further demonstrated.Secondly,the char combustion processes under 3000 K,3250 K and 3500 K temperature in oxygen concentration of 25%,50%and 75%O₂/CO₂ atmosphere are simulated by MD-ReaxFF.The effects of temperature and oxygen concentration on the production of CO₂,CO,H₂O,and carbon-containing intermediates are analyzed.The effects of temperature and oxygen concentration on consumption of C-C bonds in char molecules are studied.Based on the formation of C-O bonds in char molecules,the effects of temperature and oxygen concentration on the char oxidation and gasification reaction are discussed.The results show that with the increasing of oxygen concentration,the production of CO is decreased and the production of CO₂ is increased.At the same time,the production of gaseous molecule（C₂^C₄）of intermediate carbon products is reduced.The temperature increase can increase the production of CO,but there is no obvious effect on the yield of intermediate carbon products.For the consumption rate of char molecules,the activation energies of chemical reactions were 196 kJ/mol,190kJ/mol and 167 kJ/mol under 25%,50%and 75%oxygen concentrations,respectively.The temperature and oxygen concentration increase can make is faster.Additionally,the temperature effect on the char gasification is more sensitive than that on the char oxidation.Finally,the char combustion processes under 3000 K,3250 K and 3500 K in oxygen concentration of 25%,50%and 75%O₂/N₂ atmosphere are simulated by MD-ReaxFF.Several results are compared between the O₂/N₂ and O₂/CO₂ atmosphere,such as the consumption and production of main components（O₂,CO₂,CO）,the C-C bonds consumption of char molecules,the char molecular oxidation rate,and the change of system potential.It can be seen that the O₂ consumption rate under O₂/N₂ atmosphere is higher and the production of CO is lower.The char burnout rate is higher in O₂/CO₂ atmosphere than that in O₂/N₂ atmosphere under anoxic conditions.And the C-C bonds consumption rate of char molecules is smaller in O₂/CO₂atmosphere than O₂/N₂ atmosphere.The surface temperature during char intial combustion is lower in O₂/CO₂ atmosphere than O₂/N₂ atmosphere due to endothermic gasification reaction.Additionally,the temperature difference between two atmospheres gradually decreases with the oxygen concentration increase.