Molecular Dynamics Simulation Of CH₄ Combustion In O₂/CO₂ And O₂/H₂O Environments

Oxyfuel combustion technology is a promising approach for carbon capture. CO2 and H2 O are the main components in combustion atmospheres during a wet flue gas recycling. The different characteristics between CO2, H2 O and N2 would affect the combustion process. Most recent researches focused on the differences in gas properties including heat capacity and radiative properties between CO2, H2 O and N2. There is still no clear understanding about the chemical role of CO2 and H2 O in oxyfuel combustion. However, the influence of molar heat capacity and radiative heat transfer could not be eliminated by experimental methods. In addition, combustion reaction is a rapid and drastic chemical reaction process, involving the free radical initiation at high temperature in an extremely short period of time. An atomistic description of the details of the critical initiation reactions of oxy-fuel combustion is significant.Three models of CH4/O2/N2, CH4/O2/CO2 and CH4/O2/H2 O were built using the Reax FF molecular dynamics simulation method. The reaction rate of CH4, the generation of free radicals, the distribution of intermediates and final products were studied under different conditions. So an atomistic description of the details of the critical initiation reactions of CH4 combustion in O2/CO2 and O2/H2 O environments was well-understood.First, CH4 combustion under O2/N2 atmosphere at different temperatures was studied. The main reaction path of CH4 obtained by reactive molecular dynamics simulation was2234?????COCOCHOOCHCHCH. This is consistent with previous studies of CH4 combustion mechanism which verify the accuracy of theReax FF method.Secondly, CH4 combustion in O2/CO2 atmosphere was studied and was compared with the combustion in O2/N2 atmosphere. The results showed that the role of CO2 advanced the oxidation of CH4 because CO2 could take part in the chemical reaction in high temperature. CO2 mainly takes part in the reaction of CO2 + H → CO + OH which promotes the generation of CO molecules and OH radicals. CO2 reactivity reduced the H/OH ratio by converting H to OH which enhanced H2 O production but inhibited H2 formation in O2/CO2 combustion. Effect of oxygen concentration was also studied. It was shown that the reaction rate of CH4 could be enhanced with the increase of oxygen concentration at low temperature; but the oxygen concentration had no obvious effect on the reaction rate of CH4 combustion at high temperature.Finally, the chemical effects of a high H2 O concentration in CH4 combustion were studied in this paper. The results showed that the role of H2 O advanced the oxidation of CH4 because of H2 O reactivity in high temperature. The amount of OH radicals and H2 molecules in O2/H2 O combustion was noticeably higher than that in air combustion which suggested that H2 O molecules probably take part in the reactions through H2 O + H → H2 + OH. The role of OH radicals reduced the CO/CO2 ratio by converting CO to CO2 through the reaction of CO + OH → CO2 + H.