Confirming the efficacy of a new arch-firing solution in safely strengthening low-NOx combustion within a large-scale furnace: Impact of the flue gas recirculation position in burners

Abstract

To address the persistently high NO x production and the heightened overheating risk in the hopper of a 600-MWe, deep-air-staging, arch-fired boiler furnace (i.e., the reference furnace), a solution was devised with a staged arch-firing framework (SAF) and flue gas recirculation (FGR). This required establishing an appropriate position for the burner-FGR and confirming the viability of the SAF for the furnace. Comprehensive industrial-scale physical tests and computer simulations were conducted using the reference furnace. Subsequently, the furnace with SAF was examined with FGR introduced sequentially, first using a fuel-rich mixture, then using an inner secondary-air flow, and finally using an outer secondary-air flow (i.e., denoted in turn as FGR-FR, FGR-IS, and FGR-OS). Given this FGR-location order, the FGR’s functions about combustion degradation and NO inhibition weakened, resulting in increased NO x emissions and continuously decreasing burnout loss. Considering the satisfactory burnout levels across all three configurations, the FGR-FR configuration demonstrated the best reduction in NO x emissions, achieving NO x output of about 600 mg/m3 (O2 = 6%) and an unburnt combustible rate in fly ash of about 5%. Comparing the conditions before and after implementing the SAF indicated that the SAF enhanced combustion intensity and improved the utilisation of overfire air and hopper air, resulting in a further 33.3% reduction in NO x emissions without compromising burnout efficiency. Additionally, the SAF effectively mitigated the overheating risk in the hopper by significantly lowering local temperature levels by 400 K.