Numerical Simulation of Combustion and NOx Formation Characteristics in a 330 MW Tangentially Fired Boiler Retrofitted with Gasification-Combustion Technology

(Motivation) The rising penetration of renewable energy requires coal-fired units to operate flexibly under deep low-load conditions, while maintaining stable combustion and minimizing NOx emissions under conditions of wide-load remain a big challenge. (Significance) Gasification-combustion technology shows promising potential for stable combustion and NOx control, yet its industrial-scale performance has not yet been clarified. (Aim) This paper aims to comparatively investigate the characteristics of in-furnace combustion and NOx formation before and after retrofitting under both high and low load conditions with numerical simulation. (Methodology) The object was a 330 MW coal-fired unit using gasification-combustion technology, in which pulverized coal was partially gasified into high-temperature mixture of gas and char, then injected into the furnace via a novel burner. NOx formation is predicted using a combined model consisting of fuel-NOx and thermal-NOx based on the Zeldovich mechanism, coupled with global reaction kinetics of N-containing intermediates. (Conclusion) The simulation results indicated that under full-load operation, the high-velocity gas flow field was significantly optimized after retrofitting, and the high-temperature zone in the furnace was reduced, maintaining a burnout rate above 99% and lowering NOx emissions by 14.45% owing to the contraction of high-temperature region and the reducing atmosphere of syngas. Under low-load conditions, the retrofit enhanced thermal uniformity within the main combustion zone and achieved a 49.0% decrease in NOx concentration in the exit of furnace. (Specific application) This study provided important theoretical basis and thermal engineering guidelines for the application of gasification-combustion technology in existing coal-fired units. It could also present a viable solution for the flexible operation and wide-load stable combustion of coal-fired power units.