Numerical investigation on combustion and NOx emissions of a down-fired 350 MWe utility boiler with multiple injection and multiple staging: Effect of the air stoichiometric ratio in the primary combustion zone

A new down-fired combustion technology based on multiple-injection and multiple-staging was previously developed for a down-fired 350 MWe utility boiler suffering from severely asymmetric combustion and particularly high NOx emissions. To evaluate its furnace performance with respect to the new technology, coal combustion process and NOx formation within the furnace were numerically investigated at four settings (i.e., air stoichiometric ratios in the primary combustion zone (SRp) of 1.02, 0.96, 0.90, and 0.84, respectively). Numerical results uncovered that with the new technology replacing the prior art, asymmetric combustion removals and significant NOx reductions (nearby 50%) were validated for all four settings. With decreasing SRP, both gas temperature at the furnace outlet and O2 in flue gas decrease continuously, carbon in fly ash generally increases, while NOx emissions decrease initially but then increase. In considering relatively appropriate values established simultaneously in gas temperature at the furnace outlet, O2 in flue gas, carbon in fly ash, and NOx emissions when SRp = 0.96 and 0.90, the optimum operation condition for the furnace after a retrofit with the technology was recommended at SRp = 0.90–0.96.