Enhanced combustion behavior and NOx reduction performance in a CFB combustor by combining flue gas recirculation with air-staging: Effect of injection position

Abstract

In circulating fluidized bed (CFB) boilers, the flue gas recirculation (FGR) is an important technical means to promote de-NOx combustion. However, the conventional injection position of recycled gas seriously hinders the combustion efficiency and the flexibility of load regulation. To solve this, thermal-state experiments are conducted on a lab-scale CFB combustor to explore the feasibility of adopting novel FGR and air staging technology to achieve low NOx emission and sufficient combustion efficiency. The effects of FGR/supplementary burnout air (SBA) injection positions and operating parameters on temperature profile, combustion efficiency and original NOx emission in the furnace are discussed in detail. The synergistic effect of FGR and SBA on combustion characteristics and NOx reduction is also analyzed. Eventually, a systematic approach to enhance NOx control and combustion efficiency in fluidized bed is proposed. The key results indicate that the NOx reduction rate reaches 46.15% by adopting middle FGR and top SBA, which exceeds the reduction performance of utility boilers with conventional FGR. Also, the furnace outlet temperature and combustion efficiency can be ensured to be over 860 °C and 98.4% respectively, thus to provide new technical basis for enhancing de-NOx combustion in CFB boiler at low load, as well as the prospective guidance for FGR retrofitting of various fluidized bed combustors.