Lower-arch location effect on the flow field, coal combustion, and NOx formation characteristics in a cascade-arch, down-fired furnace

For a cascade-arch low-NOx and high-burnout configuration designated as a solution to the currently incompatible problem of the ultra-low NOx combustion and high burnout in down-fired furnaces, numerical simulations, which were verified by real-furnace measurements of a 600 MWe down-fired furnace with a multiple-injection and multiple-staging combustion technology, were performed to evaluate its in-furnace flow, coal combustion, and NOx formation at various lower-arch location setups of CH = 0.65, 0.6, 0.55, and 0.5 (CH representing the ratio of the lower arch height to the front/rear wall height in the lower furnace). The aim was to evaluate its lower-arch location effect on the furnace performance and confirm its superiority compared with the prior art. With asymmetric combustion developing at extreme settings of CH = 0.65 and 0.5, the combustion symmetry and furnace performance originally improved but then worsened with decreasing CH, resulting in the exhaust gas loss, carbon in fly ash, and CO and NOx emissions all decreasing first and then increasing. Consequently, the CH = 0.6 location setup achieved the optimal furnace performance characterized as NOx emissions of 707 mg/m3 at 6% O2 and carbon content in fly ash of about 5.50%. In comparison to the prior art, the new combustion configuration owned an improved symmetrical combustion pattern, strengthened deep-air-staging conditions, and lengthened flame travel by positioning hopper air, applying flue gas recirculation, and introducing fine pulverized-coal reburning, thereby achieving the apparently improved low-NOx and high-burnout performance with a NOx reduction of 22% without affecting burnout.