Furnace-height FGR location impact of a cascade-arch-firing configuration in ameliorating low-NOx combustion and eliminating hopper overheating for a large-scale arch-fired furnace

Abstract

This study analyses an alternative to the existing multiple-injection and multiple-staging combustion technique (MIMSCT) of large-scale arch-fired furnaces. The alternative approach is a cascade-arch-firing, low-NOx, and high-efficiency configuration (CLHC) with flue gas recirculation (FGR). The research is necessary as typical 600 MWe large-scale arch-fired furnaces produce high concentrations of NOx and have a high risk of overheating in their hoppers when firing low-volatile coals. Three furnace-height FGR locations were considered in descending order (i.e., FGR through (i) primary burners to mitigate fuel-NO (FGR-PB), (ii) tertiary air to restrain thermal-NO (FGR-TA), and (iii) auxiliary burners to reduce NO (FGR-AB)). Industrial-scale measurements and simulations were conducted in the existing MIMSCT furnace (which was used to verify simulations and compare with the subsequent CLHC furnace results), followed by numerical investigations of the CLHC furnace at the three FGR-locations. The FGR’s NO inhibition in the upstream zone decreased in the following order: FGR-PB > FGR-TA > FGR-AB. Initial combustion and NO generation in the reburning stage was most intense in the FGR-TA configuration (producing the highest NO amounts) whereas the FGR-AB configuration contributed the lowest amounts. As the FGR locations descended, burnout loss dropped a little, while NOx production was initially elevated and then descended. The FGR-PB was optimal among the three options, with a low NOx output of 604 mg/m3 (O2 = 6 %) and combustible level in fly ash of about 5 %. Compared to the MIMSCT, the CLHC not only enhanced the low-NOx environment and shortened the flame penetration but also improved combustion intensity and extended the overfire air penetration. This sharply reduced NOx without affecting burnout nor lowering apparent hopper temperatures. The above results demonstrate that a staged FGR consisting of primary FGR-PB and supplementary FGR-AB, is a promising solution to lower NOx while retaining high furnace burnout levels.