Assessment of a staged and inclined hopper-air injection in enhancing low-NOx combustion and establishing a safe hopper environment for a large-scale arch-fired furnace

Abstract

In response to the high NO x emissions and hopper overheating problem of a 600-MWe arch-fired furnace, a staged arch-firing framework (SAF) with an upper/lower staged hopper-air pattern was proposed to replace the original low-NO x technique. With implementing such a staged hopper-air configuration to enhance air staging and reduce the downward flame penetration, it was required to (i) affirm the SAF's effectiveness related to the mentioned problems and (ii) assess the upper hopper-air (UHA) angle effect for obtaining a reasonable UHA angle. Accordingly, the in-furnace flow, combustion, and NO x generation were respectively investigated under five UHA angles of θ = 0°, 20°, 30°, 35°, and 40°. Meanwhile, real-furnace tests and simulations in the original furnace were conducted to make comparisons with the SAF furnace. As θ increased, the flame penetration progressively deepened. The primary hopper-air function in favoring char combustion and overall combustion performance first strengthened and then weakened, while the production of CO and NO and combustible matter in fly ash first dropped and then ascended. The setup with θ = 35° demonstrated the most favorable low-NO x performance, exhibiting NO x levels of about 600 mg/m3 (O2 = 6%) and combustible matter of about 5% in fly ash. A comparison before and after the SAF application indicates that, despite reducing downward flame penetration, the SAF bolstered primary combustion and increased air utilization to facilitate char combustion in the hopper and the burnout zone. Consequently, a further 33.5% NO x reduction was achieved, along with slight burnout improvement and notably reduced hopper temperatures.