NOx Emission from a Circulating Fluidized Bed Boiler Cofiring Coal and Corn Stalk Pellets

Abstract

The NOx emission from a circulating fluidized bed (CFB) boiler with 50 MW rated thermal output (50 MWth) during the cofiring of anthracite coal and pelletized corn stalk biomass was investigated. Cofiring could improve combustion efficiency and, thus, the thermal efficiency of the boiler. However, contrary to most results found in bubbling fluidized bed (BFB) combustors and the one found in a CFB boiler burning rice husk, NOx concentration in flue gas and fuel nitrogen conversion ratio increased with the biomass mass fraction. Several causes for such a contradiction were discussed and the configuration in combination with feedstock location of biomass was regarded as the most important one. In a CFB boiler, fuel is mostly burnt above the dense bed, opposite to that in a BFB boiler, and, thus, the average oxygen concentration above the dense bed, where biomass was fed, was higher. High bed temperature and high primary air ratio used for burning the anthracite coal increased NOx emission and enhanced the variation trend. In addition, the catalyst-contained ash also could promote NOx formation. The results indicated that biomass is not necessarily an effective cofiring fuel to reduce NOx emission in a CFB boiler and in order to reduce NOx emission, low primary air ratio, and locating the feedstock ports of biomass below the secondary air should be considered.