Trends of the low-NOx and high-burnout combustion characteristics in a cascade-arch, W-shaped flame furnace regarding with the staged-air angle

Abstract For a cascade-arch-firing low-NOx and high-burnout configuration (CLHC) designated for solving the incompatible problem of strengthened low NOx combustion and good burnout in W-shaped flame furnaces, numerical simulations verified by real-furnace results of a 600 MWe furnace with the multiple-injection multiple-staging combustion technology (MIMSCT), were performed to evaluate trends of the CLHC’s in-furnace flow field, combustion, and NOx production at different staged-air angles of θ = 15°, 20°, 25°, and 30°. The flow-field symmetry and combustion performance first improved and then worsened with θ, generating a same change trend in the residual O2 at the furnace outlet, carbon content in fly ash, and emission levels of CO and NOx. The 20° setting finally established the optimal performance indexes labeled as NOx emissions of 669 mg/m3 at 6% O2 and carbon content in fly ash of 5.1%. In the NOx formation aspect along the flame travel in the CLHC, it was found that NOx was initially inhibited obviously in the preceding combustion stage but then surged in the primary combustion stage before finally reduced sharply by the reburning process. As a replacement of the prior MIMSCT, the CLHC achieved a strengthened low-NOx combustion performance with NOx reduced by 26% without affecting burnout. The prominent improvement was attributed to the combination of improving symmetrical combustion pattern, strengthening deep-air-staging conditions, lengthening flame travel by positioning hopper air, and introducing the flue gas recirculation and reburning process.