Impact of the hopper-air and overfire-air distribution on the low-NOx combustion performance within a 600 MWe staged arch-firing furnace

Abstract

Within a 600 MWe staged arch-firing furnace (SAF) having the staged hopper air (HA) and overfire air (OFA) as its last two air-staging layers, the present attention was focused on evaluating the impact of the HA:OFA distribution and determining an appropriate HA:OFA ratio used to improve the hopper environment and low-NO x performance. Accordingly, by varying the HA:OFA distribution at a total air ratio of 31%, four HA:OFA ratio settings of 11:20, 13.5:17.5, 16:15, and 18.5:12.5 were formed to compare the in-furnace flow, coal combustion, and NO x emissions. With increasing the HA flux to raise the HA:OFA ratio, all of combustion symmetry, the downward flame penetration, and the HA's combustion-aided effect on char firstly improved and then weakened, the OFA penetration became shallower, and the high-temperature flame in the upper furnace extended. Final performance indexes showed that both NO x emissions and burnout loss first declined and then ascended, meaning a rise-to-drop trend in the low-NO x performance as deepening the HA:OFA ratio. Among the four settings, the moderate HA:OFA = 13.5:17.5 achieved the optimal indexes with NO x emission of 543 mg/m3 (6% O2) and carbon in fly ash of 4.89%. Compared with its previous reference furnace (holding levels of 905 mg/m3 and 5.1% respectively in the two index aspects), the SAF furnace using such a HA:OFA distribution mode reduced significantly NO x by 40%, improved a little burnout, and dropped apparently the hopper temperatures by 500 K to eliminate its high overheating risk.