Numerical Study of the Combustion Characteristics of Propane-Oxyfuel Flames with CO_2 Dilution

摘要

Oxyfuel combustion with carbon capture could be employed to reduce CO2 emissions and eliminate thermal NOx emissions from combustion systems. High temperatures associated with the use of pure oxygen as an oxidizer in combustion systems would require recycling CO2 from flue gases to be used as a diluent, safeguarding the structural and material safety of the systems. The CO2 in the O-2/CO2 oxyfuel oxidizer mixture, being diluent, lowers the temperature as well as flame speed and, consequently, affects the combustion characteristics. In this study, we investigated, numerically, the effect of CO2 dilution level on nonpremixed, swirl-stabilized, propane-oxyfuel flames in terms of the flame's macrostructure, temperature, and emissions. Results show that the flame transitions from a jet-like to a V-shaped flame consequent to fuel jet vortex interaction and that this interaction can be employed in swirl-stabilized flame characterization. The jet-like flames obtained at low dilution levels were found to have the same nondimensional vortex strength of 0.2. Also, the flame transition coincides with a sudden increase in the vortex strength value from 0.2 to about 0.3 and continued to increase linearly with increase in CO2 dilution level. The CO emissions increase with CO2 dilution level due to the combined effect of cooling, low residence time, and CO2 dissociation, up to the threshold of 50% CO2 dilution level, beyond which it decreases due to drastic decrease in CO2 dissociation that is attributed mainly to the cooling effect.