Numerical investigations on the aerodynamics and flue gas recirculation of cyclone-fired coal boiler

Abstract

Cyclone-fired boilers have gained wide applications due to their ability to burn low ash fusion temperature coals that are unsuitable for pulverized coal (PC) firing. In comparison to PC boilers, cyclone-fired boilers exhibit significantly different aerodynamics and the molten slag layer plays a critical role in the operation of boilers. However, by far there are still lack of CFD models that are able to provide effective guidance to the design and operation of cyclone-fired boilers. In this paper a CFD model that was designed to solve the practical problems of cyclone-fired boilers was developed in which the capture of coal particles by the molten slag layer was considered through a slag layer capture model to remove the captured particles from calculation and release the combustion heat of the remaining combustibles. This model was then employed to investigate the aerodynamics and flue gas recirculation (FGR) optimization design of a 550 MW cyclone-fired boiler. The results demonstrated the highly nonuniform characteristics of furnace aerodynamics of cyclone-fired boilers due to the strong swirling flows created by the cyclones that frequently leads to the formation of localized high temperature zones and severe boiler fouling problems. Thus, it is critical to adapt the FGR design with the nonuniform furnace flow and temperature distributions. The simulation results showed that with the adapted FGR design the high temperature zones in the furnace could be effectively eliminated. This study is instrumental for the design of FGR system of cyclone boilers to avoid the potential severe boiler fouling problems.