Theoretical Analysis on the Criteria of MILD Coal Combustion

MILD combustion is featured with a uniform heat flux and low NOx emission, and is thus a promising technology for clean coal utilization. Establishing the theoretical criteria of coal combustion modes is essential to guide the design and organization of MILD coal combustion. In this work, different coal combustion modes were classified theoretically based on the analysis of time scale, heterogeneous ignition, heat transfer, and flue gas entrainment. The predicted coal combustion modes agreed well with the experimental results from the literature. The effects of the structural and operational parameters on coal combustion modes were also discussed. As the nozzle diameter increases, the critical inlet Reynolds number switching from traditional combustion to MILD combustion increases, while the corresponding jet speed decreases. When the size of the furnace cross section increases, the critical inlet Reynolds number increases. As the coal particle diameter increases, the critical inlet Reynolds number decreases. As the secondary air temperature increases, the critical inlet Reynolds number decreases. The classification method of coal combustion modes was further applied to instruct the organization of MILD coal combustion on a Hencken burner. The experimental results proved that the theoretical analysis is reasonable.