Hopper-air distribution impact on the low-NOx combustion and hopper environment in a 600 MWe staged arch-firing furnace

Abstract

In today's call for environmental protection and sustainable development, it is urgent to solve the environmental pollution problems caused by industrial coal-fired boilers. Aiming at the ultra-low NO x combustion against with poor burnout and hopper overheating of a 600 MWe arch-firing furnace, this article developed the allocation scheme of hopper air (HA) as the third air-staging layer under a staged arch-firing framework (SAF). Investigations were devoted to the staged HA distribution between the upper and lower HA jets (called the UHA and LHA, respectively). The flow field, coal combustion, NO x emissions, and hopper environment were compared comprehensively at five HA distribution setups of UHA:LHA = 4:6, 5:5, 6:4, 7:3, and 8:2. With increasing moderately UHA to raise the distribution ratio, the comprehensive benefits of the staged HA were obvious. That is, the UHA's function in promoting char combustion was strengthened and again, the roles of LHA to hold up the downward flame and protect hopper were guaranteed. The reductive atmosphere in the reburning zone was maintained at an optimal status added by the staged HA. While these positive improvements were reversed as raising excessively the distribution ratio. Trends of final performance indexes showed that with raising the distribution ratio, residual O2 and CO emission decreased in the whole, while both burnout loss in fly ash and NO output first descended and then ascended. Among the five setups, UHA:LHA = 6:4 gained the best low-NO x combustion merits with NO x output of 582 mg/m3 (O2 = 6%) and unburnt matter in fly ash of 4.97%. Compared with its previous deep-air-staging low-NO x furnace, the SAF incorporating with this optimal staged hopper-air distribution dropped further NO x emissions by 36%, improved combustion, and raised burnout, thereby achieving the simultaneous NO x reduction and burnout improvement. Again, the hopper environment was improved to be moderate-oxygen and low-temperature, alleviating the previous overheating problem to ensure the furnace's safe operations.