Effects of flue gas recirculation on combustion and heat flux distribution in 660 MW double-reheat tower-type boiler

Abstract

The ultra-supercritical double-reheat boiler has attracted more attention because of high parameters (steam with high temperature and pressure), low pollution and large capacity. However, there are some difficulties in the development of this technology, such as adjusting the reheat steam temperature. In this study, computational fluid dynamics simulation is used to analyze a 660 MW double-reheat tower-type pulverized coal-fired boiler. The influence of flue gas recirculation (FGR) on heat transfer characteristics and combustion process in the furnace (including heating surfaces in the upper parts of the furnace) are evaluated. The user-defined function approach predicates the CO reduction effect on NOx. The results show that the flow at the horizontal section through the centerline of recirculating flus gas nozzles is rotating. The velocity distribution changes into an elliptical rotating flow when FGR ratio is 20%. At higher FGR ratios, the high-temperature area (1565–1700 K) shrinks and both the NOx concentration at the low-temperature superheater outlet and the O2 concentration in the burner zone descend. The O2 concentration at the low-temperature superheater outlet first increases and then decreases. In the main combustion zone, the heat flux peak of water-cooled wall is about 330 kW m−2. As FGR ratio increases from 0% to 20%, the rate of heat absorption of water-cooled wall to that of total boiler decreases by 3.50%. These rates for reheater and superheater increase by 2.53% and 2.13%, respectively.