Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion. Chemical Effect of CO₂

Abstract

The oxidation process of CH 4 /O 2 mixtures diluted in CO 2 under MILD/Oxy-fuel 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 pre-heated. Particular attention was focused on the effects of CO 2 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 CO 2 in comparison to N 2 , 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 non-adiabatic 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 H 2 /O 2 sub-system reactions independently of diluent nature, while, for fuel rich mixtures diluted in carbon dioxide, the CO 2 decomposition to CO and CH 3 recombination to ethane are key reactions for the onset of temperature oscillations.

Numerical Study of Pulverized Coal MILD Combustion in a Self-Recuperative Furnace

“…The kinetic constants of this reaction (R1) are A = 4.84×10 7 and E a = 5.1×10 7 J/kg mol for brown coal case according to previous studies 18 whereas for black coal case these parameters are A = 5.03×10 11 and E a = 2×10 8 J/kg mol following Álvarez et al 43 . 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 17 of 59…”

Thermochemical oscillation of methane MILD combustion diluted with N2/CO2/H2O

“…Even though methane oscillatory behaviour in the MILD regime has been documented experimentally (de Joannon et al, 2004(de Joannon et al, , 2005Sabia et al, 2007Sabia et al, , 2013, very few studies addressed these issues from a kinetic modelling perspective (Sabia et al, 2015;Wada et al, 2010). To some extent, the scarcity of kinetic modelling studies is also the reason behind the lack of general agreement concerning methane oscillatory phenomena within the combustion community.…”

Thermal effects of CO2 on the NOx formation behavior in the CH4 diffusion combustion system

“…The nonlinear variation of chemical effects of CO2 on temperature with fraction of O2 is also given. Sabia et al [14] found the dynamic behavior of temperature oscillations. Especially for the fuel-rich mixtures diluted with carbon dioxide, it significantly alters both the CH4 oxidation and CH3 recombination to ethane which mainly induces the instabilities.…”