Analysis of thermal deviations in L-shape platen superheaters of a supercritical 600 MW circulating fluidized bed boiler

Abstract

In circulating fluidized bed (CFB) boilers, L-shape platen superheaters often suffer from poor working conditions with a high risk of tube wall overtemperature. Thermal deviations of an L-shape platen superheater were studied by using a supercritical 600 MW CFB boiler as reference. Based on the prediction model of thermal deviations and a self-developed software, thermal deviations of the platen superheater were calculated and analyzed under three kinds of heat load distributions. The results show that the heat absorption deviations for steam are related to the heat load and heating area distribution, wherein the heat load is more effective than the heating area. The mass flow rate deviations for steam mainly depend on the header effect and the nonuniform tube length, while the mass flow rate deviations induced by thermal effects are negligible. The mass flow rates of each tube are nearly identical because the header effect offsets the effect of the nonuniform tube length. Therefore, the heat absorption deviations determine the final thermal deviations of the platen superheater. It is suggested to make complete use of the header effect on the mass flow rate distribution; thus, achieving good control of the thermal deviation of platen superheaters.