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For a down‐fired furnace using a cascade‐arch‐firing low‐NOx and high‐burnout configuration (CLHC, which includes flue gas recirculation and is taken as a strengthened low‐NOx combustion solution), the gas/particle flow, coal combustion, and NOx emissions were investigated at various flue gas recirculation (FGR) ratios of 5%, 10%, 15%, and 20% (labeled as settings of FGR = 5%, 10%, 15%, and 20%, respectively). The aim was to disclose the FGR role on the strengthened low‐NOx combustion characteristics and meanwhile obtain an available FGR ratio. With increasing FGR, although the downward flame penetration increased continuously, the large local high‐temperature zone favoring combustion in the furnace shrunk. Increasing FGR lowered gas temperatures and strengthened the reductive atmosphere in the downstream of the reburning zone to restrain combustion and meanwhile weaken the NO‐reduction effect of reburning. Trends of the final performance indexes with the FGR ratio showed that NOx production increased continuously, while burnout loss generally undergone an increase‐to‐decrease trend. In view of the above findings and the gained optimal furnace performance (i.e., NOx emissions of 530 mg/m3 at 6% O2 accompanied by carbon content in fly ash at 5.28%), low FGR levels such as FGR = 5% with a hot‐air dilution in FGR should be preferred for the CLHC furnace where FGR is used to deliver the fine pulverized‐coal particles for reburning. In comparison to an advanced in‐service low‐NOx art, the CLHC reduced further NOx production by 41.4% while remained a high burnout achievement except a limited increase in CO emission. [ABSTRACT FROM AUTHOR]

A cascade-arch-firing low-NO x and high-burnout configuration (CLHC) is developed to strengthen low-NO x combustion and maintain high burnout in down-fired furnaces. Numerical simulations of the in-furnace flow field, coal combustion, and NO x formation are performed in a 600 MW e down-fired furnace to evaluate the staged-air location's effect on the low-NO x and high-burnout performance. The furnace, which is in service with a deep-air-staging multi-injection and multi-stage combustion technology (MIMSCT), was modified with the CLHC combustion system for the present simulations. Various staged-air location coefficients of C H = 0.65, 0.70, 0.75 and 0.80 were analysed. Moving downward staged-air to enlarge C H initially improves but then deteriorates the in-furnace flow-field symmetry. Similar changes in the gas temperature and species distribution patterns in the furnace are evident. As C H increases from 0.65 to 0.75, levels of the residual O 2 concentration, carbon content in fly ash, and NO x emissions at the furnace outlet decline continuously. The highest setting C H = 0.80 generates severely asymmetric combustion accompanied by poor performance indexes, raising the unburnt rate and NO x emissions by 20–30%. The staged-air location of C H = 0.75 achieves a symmetric flow-field pattern, satisfactory low NO x emissions of ~670 mg/m3 at 6% O 2 and high-burnout performance with carbon in fly ash of 5%. NO x emissions are reduced by 26% without affecting burnout in comparison to the currently advanced deep-air-staging MIMSCT. [Display omitted] • A cascade-arch low-NO x combustion solution includes staged-air arrangements. • The staged-air location evidently affects coal combustion and NO x emissions. • An available staged-air location is fixed for the low-NO x combustion solution. [ABSTRACT FROM AUTHOR]

For achieving the strengthened low‐NOx combustion plus high burnout in W‐shaped flame furnaces, a cascade‐arch low‐NOx and high‐burnout configuration (CLHC) is realized by positioning two burner layers in a 600 MWe furnace. The gas/particle flow, combustion, and NOx emission characteristics in the furnace are numerically evaluated at four upper/lower furnace depth ratios of CW = 0.458, 0.500, 0.529, and 0.558 (CW signifying the furnace throat shrinkage level) so as to disclose the impact of the furnace throat shrinkage level on the overall furnace performance and meanwhile establish an available CW setup. Increasing CW initially updates but then deteriorates the low‐NOx combustion performance. At the narrowest CW = 0.485 setup, the gained slightly asymmetric combustion shows relatively lower burnout loss but higher NOx emissions, while the widest CW = 0.558 setup forms severely asymmetric combustion with apparently raised carbon in fly ash but relatively lower NOx emissions. At moderate setups of CW = 0.500 and 0.529, a symmetrical combustion pattern develops, showing the low‐NOx and high‐burnout performance characterized as NOx emissions of 700–707 mg/m3 (6% O2) and carbon content in fly ash of 5.4–5.5%. In comparison to CW = 0.500, the larger downward flame penetration, higher flame fullness, and lower CO emission at CW = 0.529 suggest that a reasonable upper/lower furnace depth ratio prefers CW = 0.529. Compared with a currently advanced low‐NOx combustion technology, the CLHC lowers again NOx emissions by about 200 mg/m3 (6% O2) while maintaining high burnout, which confirms the CLHC's availability. [ABSTRACT FROM AUTHOR]

• Incompatible low-NO x combustion and high burnout in down-fired furnaces. • Proposing a low-NO x combustion solution for this problem. • Confirming its availability in deepening low-NO x and high-burnout combustion. • Evaluating the hopper-air layout's impact on the low-NO x combustion. • Determining a staged hopper-air form for the low-NO x solution. With respect to the unexpected hopper-air function of strengthening the deep-air-staging conditions in a 600 MW e down-fired furnace equipped with a cascade-arch low-NO x and high-burnout configuration (CLHC), the in-furnace flow field, coal combustion, and NO x 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-NO x 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 NO x 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 O 2 , poor burnout, and low NO x 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 NO x 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 NO x emissions of 632 mg/m3 at 6% O 2 (amended to 604 mg/m3 at 6% O 2 according to the NO x calculation error) and carbon content in fly ash of 5.4%. The generated further NO x reduction rate of 10.5% without affecting burnout (compared with CHS) thus confirms the availability of SHS in upgrading the CLHC's strengthened low-NO x and high-burnout combustion performance. Compared with a currently used low-NO x combustion technology, CLHC with SHS reduces NO x emissions by 30% without affecting burnout. [ABSTRACT FROM AUTHOR]

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 MW e 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 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 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. • Incompatible low-NO x combustion and high burnout in W-shaped flame furnaces. • Proposing a low-NO x combustion solution for this problem. • Confirming its availability in deepening low-NO x and high-burnout combustion. • Evaluating the staged-air angle effect on its low-NO x combustion. • Determining a reasonable staged-air angle for the low-NO x combustion solution. [ABSTRACT FROM AUTHOR]

• Incompatible problem of low-NO x combustion and high burnout in down-fired furnaces. • Proposing a new combustion configuration as a solution for this problem. • Confirming its availability in deepening low-NO x and high-burnout combustion. • Evaluating its lower-arch location effect on the low-NO x combustion. • Determining a reasonable lower-arch location for the combustion configuration. For a cascade-arch low-NO x and high-burnout configuration designated as a solution to the currently incompatible problem of the ultra-low NO x combustion and high burnout in down-fired furnaces, numerical simulations, which were verified by real-furnace measurements of a 600 MW e down-fired furnace with a multiple-injection and multiple-staging combustion technology, were performed to evaluate its in-furnace flow, coal combustion, and NO x formation at various lower-arch location setups of C H = 0.65, 0.6, 0.55, and 0.5 (C H 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 C H = 0.65 and 0.5, the combustion symmetry and furnace performance originally improved but then worsened with decreasing C H , resulting in the exhaust gas loss, carbon in fly ash, and CO and NO x emissions all decreasing first and then increasing. Consequently, the C H = 0.6 location setup achieved the optimal furnace performance characterized as NO x emissions of 707 mg/m3 at 6% O 2 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-NO x and high-burnout performance with a NO x reduction of 22% without affecting burnout. [ABSTRACT FROM AUTHOR]