Heat transfer and protection of high-temperature reheater of a 660 MW circulating fluidized bed boiler after black out

Abstract

• Heat transfer model of buried tube in external heat exchanger was established. • Safety of 660 MW circulating fluidized bed reheater was evaluated when black out. • Correlation between minimum working fluid flow and some key factors was analyzed. With the increase of boiler capacity and steam parameters, compared with other circulating fluidized bed (CFB) boiler, the high-temperature heating exchangers in a 660 MW CFB boiler may face severe potential safety problems after black out, especially the reheater in the external heat exchanger (EHE). In this study, a numerical simulation was conducted to predict the heating transfer process of an EHE after black out in a 660 MW ultra-supercritical CFB boiler. The tube temperature of the reheater inside the EHE was also estimated. It was found that metal tubes of the reheater are probably overtemperature. Therefore, to protect the metal tube from the overtemperature damaged, the additional cold water has to be given inside the reheater. The tube temperature rises first and then falls, and the highest temperature will be reached within tens to hundreds of seconds. The minimum flow rate of cold water to ensure the safety of the pipe was also estimated under different conditions. Some key parameters including the bed temperature of the EHE, thermal diffusivity of the bed materials and thermal deviation of the reheater were also analyzed, which are of great significance to the safety of the EHE and the economics of the emergency water system.