Combustion Reaction Mechanism in Coaxial Methane/Air Diffusion Flame

Abstract

We measured temperature and the concentrations of stable species in a methane/air coaxial laminar diffusion flame, and simulated the chemical reactions taking place in this flame by means of a detailed chemical reaction model based on the experimental results.
The air is entrained towards the center line just above the burner rim p roducing a premixed gas mixture as shown in Fig.9. Fig.4 shows the reaction mechanism in this premixed region. Methane is oxidized here through the Cc-route which consists of the steps of CH₄→CH₃O→HCH0→CHO→CO→CO₂. The active species and the thermal energy produced in a very short time hold the whole flame on the burner rim.
In thel uminous region, methane is pyrolyzed through t he C₂-route, which progresses as CH₄→CH₃→C₂H₄→C₂H₃→C₂H₂ Consequently more than 90% of methane is once converte d to acetylene and then oxidized as shown in Fig.6. Fig.8 shows the dominance of the C₂-route in this flame, and Fig.7 also suggests the important role of acetylene because the region of its presence is overlapped with the luminous region.
The oxygen, which was entrained near the nozzle and left unused here, produces hydroxyl radicals in the luminous region. The hydroxyl radicals are consumed by the dehydrogenation of methane. The entrained oxygen hence accelerates the first step of methane combustion as shown in Fig.10 and, therefore, activates all the reactions occurring in this flame including soot formation.