Dynamic behaviors in methane MILD and oxy-fuel combustion. Chemical effect of CO₂

Abstract

The oxidation process of CH4/O2 mixtures diluted in CO2 under moderate or intense low-oxygen dilution (MILD) and oxy-fuel combustion conditions was numerically investigated in a perfectly stirred flow reactor at atmospheric pressure. The analysis aimed to investigate the kinetics involved in fuel oxidation in systems highly diluted and strongly preheated. Particular attention was focused on the effects of CO2 on oxidation routes because it can significantly alter the kinetic pathways participating directly in key reactions or indirectly in termolecular reactions as a third body species. Furthermore, adiabatic flame temperatures are lower with respect to air conditions because of the higher thermal heat capacity of CO2 in comparison to that of N2, thus modifying the kinetics promoted by temperature in combustion processes. The analyses were realized as a function of main system parameters, systematically changing inlet temperatures and mixture compositions. Results showed the establishment of complex dynamic behaviors in terms of temperature oscillations for both lean and rich fuel mixtures in both nonadiabatic and adiabatic conditions. Further numerical analyses were performed to highlight the kinetic aspects of the problem. Simulations suggested that in fuel-lean conditions, the dynamics observed are related to the H2/O2 subsystem reactions independent of diluent nature, while for fuel-rich mixtures diluted in carbon dioxide, the CO2 decomposition to CO and CH3 recombination to ethane are key reactions for the onset of temperature oscillations.