Numerical Study on Flame Stabilization and NOx Formation in a Novel Burner System for Sulfur Combustion

Numerical simulations have been conducted for a novel double-concentric swirl burner, which is specifically designed for combustion of sulfur with a high power density. The burner serves as a major component of an enclosed conversion cycle, which uses elemental sulfur as a carbon-free chemical energy carrier for storing solar energy. The focus of the work is to assess operability of the burner and NOx formation at fuel-lean conditions with an equivalence ratio of phi = 0.5, which is crucial regarding flame stabilization and evaporation. To quantitatively evaluate the NOx formation, a new reaction mechanism for sulfur combustion along with S/N/O and NOx reactions has been developed and used for the simulation. In comparison to our previous simulations using a higher phi, the flame is lifted slightly and the overall flame temperature is lowered in the current case, leading to a weakened evaporation performance. Accordingly, an increased share of sulfur droplets hitting the chamber wall and escaping the domain has been confirmed. The local NOx share has been shown to increase strongly with the flame temperature from a threshold value of approximately 1600 K. In addition, the NOx formation from the burner setup with a high swirl intensity (HSI) has been shown to be 2 times higher than that with a low swirl intensity (LSI). This is attributed to a higher flame temperature and longer residence time caused by a strong inner recirculation flow. However, the HSI setup yields a better evaporation performance and a reinforced flame stabilization. The results reveal a trade-off for operating the sulfur burner with different burner designs and equivalence ratios.