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

摘要

For a cascade-arch-firing low-NO_x and high-burnout configuration (CLHC) designated for solving the incompatible problem of strengthened low NO_x 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 NO_x production at different staged-air angles of 0 = 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 O_2 at the furnace outlet, carbon content in fly ash, and emission levels of CO and NO_x. The 20° setting finally established the optimal performance indexes labeled as NO_x emissions of 669 mg/m3 at 6% O_2 and carbon content in fly ash of 5.1%. In the NO_x formation aspect along the flame travel in the CLHC, it was found that NO_x 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-NO_x combustion performance with NO_x 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.