In-furnace flow field, coal combustion and NOx emission characteristics regarding the staged-air location in a cascade-arch down-fired furnace

Abstract

A cascade-arch-firing low-NOx and high-burnout configuration (CLHC) is developed to strengthen low-NOx combustion and maintain high burnout in down-fired furnaces. Numerical simulations of the in-furnace flow field, coal combustion, and NOx formation are performed in a 600 MWe down-fired furnace to evaluate the staged-air location's effect on the low-NOx and high-burnout performance. The furnace, which is in service with a deep-air-staging multi-injection and multi-stage combustion technology (MIMSCT), was modified with the CLHC combustion system for the present simulations. Various staged-air location coefficients of CH = 0.65, 0.70, 0.75 and 0.80 were analysed. Moving downward staged-air to enlarge CH initially improves but then deteriorates the in-furnace flow-field symmetry. Similar changes in the gas temperature and species distribution patterns in the furnace are evident. As CH increases from 0.65 to 0.75, levels of the residual O2 concentration, carbon content in fly ash, and NOx emissions at the furnace outlet decline continuously. The highest setting CH = 0.80 generates severely asymmetric combustion accompanied by poor performance indexes, raising the unburnt rate and NOx emissions by 20–30%. The staged-air location of CH = 0.75 achieves a symmetric flow-field pattern, satisfactory low NOx emissions of ~670 mg/m3 at 6% O2 and high-burnout performance with carbon in fly ash of 5%. NOx emissions are reduced by 26% without affecting burnout in comparison to the currently advanced deep-air-staging MIMSCT.