Oscillatory Behavior in Methane Combustion: Influence of the Operating Parameters

Abstract

The influence of the main process parameters on the oscillatory behavior of methane oxidation was analyzed in conditions relevant for low-temperature combustion processes. The investigation was performed by means of direct comparisons between experimental measurements realized in two jet-stirred flow reactors used at atmospheric pressure. With the operating conditions of the two systems coupled, wide ranges of the inlet temperature (790–1225 K), equivalence ratio (0.5 < Φ < 1.5), methane mole fraction (X CH4 from 0.01 to 0.05), bath gases (i.e., He, N2, CO2, or H2O) and different overall mixture dilution levels were exploited in relation to the identification of oscillatory regimes. Although the reference systems mainly differ in thermal conditions (i.e., heat exchange to the surroundings), temperature measurements suggested that the oscillatory phenomena occurred when the system working temperature accessed a well-identifiable temperature range. Experimental results were simulated by means of a detailed kinetic scheme and commercial codes developed for complex chemistry processes. Simulations were also extended considering systems with different heat losses to the surroundings, thus passing from adiabatic to isothermal systems. Results highlighted the kinetic nature of the dynamic behavior. Because predictions were consistent with experimental tests, further numerical analyses were realized to identify the kinetics responsible for the establishment of oscillatory phenomena. Temperature oscillations were predicted for a significant reactor working temperature range, where oxidation and recombination kinetic routes, involving carbon C1–2 species as well as reactions of the H2/O2 sub-scheme, become competitive, thus boosting limit cycle behaviors. Oscillatory phenomena cease when the system working temperatures exceed characteristic threshold values with the promotion of faster oxidation routes that diminish the inhibiting effects of recombination reactions.

Alcohols as Energy Carriers in MILD Combustion

“…In general, this approach aims at interpreting the experimental results through a simplified but very effective methodology, reducing the nature of the problem to a zero-dimensional (0D) and/or one-dimensional (1D) elementary process. The features of the MILD reactive structure based on a complex interplay between chemical kinetic and fluid dynamic issues should be addressed through proper computational fluid dynamics (CFD) simulations, but turbulence–chemistry interaction models − and kinetic oxidation mechanisms − still represent significant issues to be addressed under MILD operative conditions. , This would be even more evident for the complex kinetics of alcohols.…”

Critical Issues of Chemical Kinetics in MILD Combustion

“…At the same time, methyl radicals recombine to ethane (CH 3 + CH 3 + M = C 2 H 6 + M), followed by C 2 dehydrogenation/pyrolytic routes. The competition between oxidation/recombination-pyrolytic routes, coupled with system heat exchange phenomena, promotes instabilities (de Joannon et al, 2005;Sabia et al, 2014;Lubrano Lavadera et al, 2018b). The enhanced role of the methyl recombination routes for these temperatures is emphasized also in other works (Zhukov et al, 2005;Li et al, 2014).…”

“…For higher T in (>1,250 K), the activation of further oxidative routes (CH 3 + OH + M→CH 3 OH→CH 2 OH→CH 2 O) followed by CH 2 O→HCO→CO→CO 2 , or CH 3 + OH→CH 2 (s)→CH 2 →CH→CO, relieves the system from the inhibiting effect of methyl radicals conversion to ethane; thus, the system reaches a steady stationary state (Lubrano Lavadera et al, 2018b).…”

“…MILD combustion processes present unique physical/chemical features, drastically different from traditional combustion systems, which should be discussed at basic levels for the fine comprehension of the process itself. The structure of the reactive region (Özdemir and Peters, 2001;de Joannon et al, 2012a,b;Van Oijen, 2013;Minamoto et al, 2014;Minamoto and Swaminathan, 2015;Sorrentino et al, 2019), the chemistry (de Joannon et al, 2005;Zhukov et al, 2005;Li et al, 2014;Sabia et al, 2014;Lubrano Lavadera et al, 2018b), and the interaction between chemistry-turbulence (Dally et al, 2004;Parente et al, 2008Parente et al, , 2016Isaac et al, 2013;Noor et al, 2013b) represent key points to address, with strong implications also on modeling activities (Dally et al, 2004;Parente et al, 2008Parente et al, , 2016Isaac et al, 2013;Noor et al, 2013b).…”

“…Among the issues to consider, the chemical aspect of the oxidation process represents a fundamental one. In fact, the high levels of dilution coupled with moderate working temperatures imply a drastic change of the kinetics involved during the fuel oxidation process with respect to flame chemistry, with relatively lower chemical characteristic times and heat release rates (Dally et al, 2004;de Joannon et al, 2005;Zhukov et al, 2005;Parente et al, 2008Parente et al, , 2016Isaac et al, 2013;Noor et al, 2013b;Li et al, 2014;Sabia et al, 2014;Lubrano Lavadera et al, 2018b). Furthermore, it occurs in presence of great amounts of non-inert species, such as carbon dioxide and steam, which can alter the kinetic routes by means of several effects, here reported: 1) Thermal: higher heat capacities with respect to N 2 , thus lower adiabatic flame temperature; 2) Kinetic: they participate directly in bimolecular reactions and enhance the role of third-molecular reactions because of higher third-body efficiencies with respect to N 2 .…”