Analysis of wall temperature characteristics of water-cooled platens in supercritical CFB boilers at low loads

When a power generating unit equipped with supercritical circulating fluidized bed (CFB) boilers participates in deep peak regulation of the grid, the inlet fluid to the water-cooled platen of the boiler is vapor-liquid two-phase under low loads. Resultingly, there is a marked maldistribution of the fluid mass flow among the parallel tubes in the platen, which leads to a large wall temperature deviation and is a prominent problem affecting the safe operation of the boiler at low loads. The water-cooled platen of a 350 MW supercritical CFB boiler is used as the object for analysis. First, the operation characteristics of the water-cooled platen at low loads are analyzed, pointing out that the water-cooled platen operates in the full-wet state below the lowest once-through load (30% BMCR), in the semi-dry state (wherein the wet and dry states co-exist in the tube platen) in the range of (30%−45%) BMCR, and in the full-dry state above 45% BMCR. Then, based on the establishment of wall temperature calculation model and the development of wall temperature calculation program, the flow, heat transfer and wall temperature characteristics of the water-cooled platen under low loads are studied in depth. The results show that: under a certain mass flow rate of the fluid, with the fluid in the tube continuously absorbing heat, the heat transfer coefficient in the tube increases and the wall temperature decreases in the wet section of the tube, while the two decrease and increase, respectively, in the dry section of the tube; There is no risk of over-temperature under the full-wet loads of the water-cooled platen, but under the semi-dry and full-dry loads, attention must be paid to whether the wall temperature at the fluid outlet of the water-cooled platen exceeds its limit. Under a certain mass flow rate deviation of the fluid, with the fluid in the tube continuously absorbing heat, the wall temperature difference between the neighboring tubes tends to decrease in the wet section, and tends to increase in the semi-dry and full-dry sections of the heating tube; The wall temperature difference between the neighboring tubes exceeding its limit occurs firstly at the fluid outlet of the tube platen, and the safety of wall temperature needs to be examined in the semi-dry and full-dry loads; in the full-dry loads, the wall temperature differences between the neighboring tubes are basically unchanged with the load increases. As the maximum allowable wall temperature difference between the neighboring tubes is 30 ℃, the corresponding maximum allowable mass flow rate deviation of the two tubes is ±23% at full-dry loads ((45%−55%) BMCR), and then increases to ±32% with the load decreasing to the semi-dry loads ((30%−45%) BMCR), and further increases with the load decreasing to the full-wet loads (< 30% BMCR). The results obtained are of great significance for the water-cooled platens in supercritical CFB boilers, both in the structural optimization design and the safe operation at low loads.