Effects of local CO2 injection on the chemistry of methane inverse diffusion sooting flames: An experimental and numerical study

To investigate the combustion chemistry of methane inverse diffusion sooting flames with local CO2 injection, this study designs an axial local dilution (ALD) inverse diffusion flame (IDF) burner capable of precisely controlling the diluent injection rate and location along the flame centerline using a diluent injection pipe. Then the flame generated by the ALD-IDF burner is simulated by a modified model based on the CoFlame code. The distributions of flame temperature and major species mole fractions are measured using a thermocouple and online gas chromatography, and the measurements are compared with numerical results. The impact of different injection flow rates and locations on the flame structure is studied. Results show that injecting CO2 close to the high-temperature region significantly affects the oxidizer flow field. The predictions of the main trends of species mole fractions show good agreement with experimental results. The oxidizer stream structure variety caused by CO2 injection is the main reason for affecting the spatial distribution of species in the flame. The local CO2 injection inhibits the formation of some small molecules and radicals (H2, H, OH) as well as intermediate species (C2H2, C2H4, C4H2, A1) close to the flame centerline region. In addition, the inhibitory effect of CO2 on the aforementioned species increases with higher injection flow rates and heights. However, injecting CO2 at different heights results in moderate differences in the inhibition of A1 formation. Kinetic analysis of the A1 formation pathways indicates that this phenomenon is due to the varying degrees of suppression on the main A1 formation reaction (2C3H3 = A1) at different injection heights. This study presents a novel method for adding diluents within the flame and analyzes the underlying regulatory mechanisms. The obtained insights are expected to expand the existing pollutant control technologies.