Heterogeneous reaction characteristics and its effects on homogeneous combustion of methane/air mixture in microchannels II. Chemical analysis

Catalytic combustion of methane/air mixture in various channels was numerically investigated to illuminate the chemical interaction mechanism between heterogeneous reactions (HTRs) and homogeneous reactions (HRs) with detailed gas-phase and surface reaction mechanisms. In the channels of different widths and with/without catalyst, the gaseous species, such as CH4, O2, CO, CO2, H, H2, and H2O and the reactions involving these species were monitored and the reaction rates along the streamwise direction were analyzed quantificationally. In the catalytic combustion, the HTR can be divided into prepare phase, weak HTR phase, violent HTR phase, second weak HTR phase, and completion phase according to the situation of gaseous species depletion on catalytic surface. The heat released by the homogeneous reaction and the consumption of O(s) can accelerate the absorption of CH4 on Pt surface in the violent HTR phase (phase III). Meanwhile, the stronger competition to CH4 of HR would inhibit the reactions of HTR involving CH4. The similar situation happens to O2, except the larger consumption of O2 by HTR than that of CH4 because of the larger stick coefficient of O2 on Pt surface. The additional reaction pathway of CO through HTR is favorable for the completeness of CO to CO2 during combustion. The consumption of H radical by HTR in the second weak HTR phase (phase IV) leads to the H2 release from Pt surface to the gas-phase and promotes the homogeneous reactions. The decrease of the width of the channel would suppress the HR intensity and enhance the HTR intensity. However, HR is still dominated in all the cases with a large amount of CH4 and O2 consumption and CO2 and H2O production.