Numerical Study on Flame Stabilization and NO x Formation in a Novel Burner System for Sulfur Combustion.

Abstract

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 NO x formation at fuel-lean conditions with an equivalence ratio of ϕ = 0.5, which is crucial regarding flame stabilization and evaporation. To quantitatively evaluate the NO x formation, a new reaction mechanism for sulfur combustion along with S/N/O and NO x reactions has been developed and used for the simulation. In comparison to our previous simulations using a higher ϕ, 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 NO x share has been shown to increase strongly with the flame temperature from a threshold value of approximately 1600 K. In addition, the NO x 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.