Upgrading the strengthened low-NOx and high-burnout combustion performance by staging the hopper air in a down-fired furnace

Abstract

With respect to the unexpected hopper-air function of strengthening the deep-air-staging conditions in a 600 MWe down-fired furnace equipped with a cascade-arch low-NOx and high-burnout configuration (CLHC), the in-furnace flow field, coal combustion, and NOx formation are numerically evaluated at four hopper-air layout setups of the elevated hopper-air scheme (EHS), conventional hopper-air scheme (CHS), descended hopper-air scheme (DHS), and staged hopper-air scheme (SHS). The aim is to determine the hopper-air layout impact and establish an available hopper-air layout setup for upgrading the strengthened low-NOx and high-burnout combustion performance. At EHS, the combustion symmetry seems to be a little imperfect, with the shallow flame penetration and involuntarily strengthened combustion in the upper furnace incurring high NOx emissions and burnout loss. DHS forms asymmetric combustion with a poor combustion status appearing in the furnace's front-half side, corresponding to high levels of residual O2, poor burnout, and low NOx emissions at the furnace outlet. Both CHS and SHS achieve symmetrical combustion to improve the furnace performance. In comparison to CHS, SHS partitions the flux-increased hopper air into a staged form for achieving a perfect combination of (i) strengthening air staging to suppress NOx generation and (ii) extending the downward flame travel to promote burnout. Consequently, the furnace performance is upgraded further at SHS, showing the best indexes of the calculated NOx emissions of 632 mg/m3 at 6% O2 (amended to 604 mg/m3 at 6% O2 according to the NOx calculation error) and carbon content in fly ash of 5.4%. The generated further NOx reduction rate of 10.5% without affecting burnout (compared with CHS) thus confirms the availability of SHS in upgrading the CLHC’s strengthened low-NOx and high-burnout combustion performance. Compared with a currently used low-NOx combustion technology, CLHC with SHS reduces NOx emissions by 30% without affecting burnout.