Soot formation characteristics in laminar coflow flames with application to oxy-combustion

WashU affiliated authors: Benjamin M. Kumfer (Dept. of Energy, Environmental, and Chemical Engineering)

Abstract: Oxy-combustion is an effective carbon capture technology. Many oxy-combustion technologies utilize recycled flue gas (RFG) for dilution to control temperature and heat flux. As the concentration of dilution gas on the fuel and oxidant side changes, stoichiometric mixture fraction (Zst) and flame temperature will change and significantly impact the flame structure and soot formation characteristics. In this work, the effects of stoichiometric mixture fraction and flame temperature on soot formation characteristics in laminar diffusion flames are studied by diluting the fuel and changing the oxygen concentration. CO2 is used as a dilution gas to simulate the RFG in oxy-combustion. The numerical calculation combines gas reaction kinetics with a soot formation model. Soot nucleation, surface growth, oxidation processes are considered, as well as the distributions of temperature, soot concentration, and key substances. The experimental flame appearances and spectral radiation data are imaged by a hyperspectral imager, and the temperature and soot concentration are reconstructed. The results of experimental measurement and numerical calculation are compared to evaluate the applicability of the soot formation mechanism to oxy-combustion with elevated Zst and using CO2 as diluent. As the Zst increases, the flame changes into a blue flame, soot concentration and temperature in flames decrease, because nucleation and surface growth are both inhibited and oxidation is enhanced. As the flame temperature increases, the flame becomes brighter, numerical results indicate that soot formation and oxidation are both enhanced, while the promoting effect of temperature increase on surface growth is stronger than that of oxidation, resulting in an increase in soot concentration. This study provides a fundamental understanding of the effects of RFG utilization (fuel dilution and oxygen enhancement) on flame structure, temperature distribution, soot concentration and formation characteristics in non-premixed oxy-combustion systems.