Roles of inhibitors in global gas-phase combustion kinetics

Abstract

Roles of inhibitors in the global kinetics of methane combustion were investigated computationally, by employing the Arrhenius equation to express the global rate constant. When the concentrations ofthe fuel and the oxidizer in a combustible mixture are kept constant, the global reaction rate is controlled by the flame temperature Ti, the global frequency factor A, and the global activation energy E,. Changes in A and E, with the additions of various inhibitors were estimated using the relationship between these parameters and the laminar burning velocity. Inhibitors investigated are He, CO,, CF,, CHF,, C,HF,, CF,Br, CFJ, NaOH, and Fe(CO),. The global kinetic parameters were found to vary according to the inhibitors' actions, physical, non-catalytic scavenging, and catalytic scavenging. That is, while He and CO, caused negligible changes in both A and E,, CHF, and C,HF, caused significant reduction in A without remarkable changes in E,. In contrast, CF,Br, CFJ, NaOH, and Fe(CO), caused noticeable increase in E,. Although an increase in E, seems to be a general characteristic of catalytic scavengers, its magnitude showed complicated variation due to many factors including the combustion promotion at higher temperatures and the saturation effect, which is speculated to be dependent on the thermochemistry of key inhibitor species. The maximum increase in E, with CF,Br at T,=2000 K was found to be approximately twice as much as that with Fe(CO),, which can be attributed to the coexistence of all the key inhibitor species in the CF,Br-doped flames more equally than those in the Fe(CO),-doped flames at lower temperatures.