Thread Content
Some problems existing in the development of circulating fluidized bed boilers and an analysis of them: 1. Based on the currently operating circulating fluidized bed boilers in China, the main problems encountered are as follows: 1) The boiler’s evaporation capacity is below the designed value； 2) Coking in the high-temperature separator and material returner ； 3) Wear of refractory materials and heating surfaces ； 4) The boiler exhaust temperature is too high. 2. Among the four main issues mentioned above, the most fundamental one is that the rated evaporation capacity of the boiler does not reach the designed value. The reasons for the insufficient boiler output include a low fluidization speed and an inadequate fly ash circulation rate. Furthermore, the low efficiency of the fly ash separation device also results in an insufficient particle circulation flow within the bed. Therefore, the actual heat transfer performance differs significantly from the heat transfer coefficient values used in the design calculations. 1 The arrangement of the heating surfaces in the dense-phase bed and the suspension zone is inappropriate or contradictory; especially when burning low-quality coal, the lack of heating surfaces in the dense-phase bed leads to overheating within the bed when the boiler load is high, which indirectly limits the increase in the boiler load. 2 The screening range for coal is too wide ； Combustion fraction in the dense-phase bed, lean-phase bed ； The selection of the primary and secondary ratio air flow rates, as well as inconsistencies with the designed heat transfer coefficient values, are also among the reasons for insufficient boiler output. (In our plant, the ratio of primary to secondary air is 60:40.) The selection of auxiliary equipment such as blowers and exhaust fans, as well as improper choices regarding their flow rate and pressure head, can also affect the boiler’s output. To address the issue of insufficient evaporation capacity in circulating fluidized bed boilers, it is necessary to conduct a specific analysis of the particular circumstances. Generally, the following measures can be taken: 1) Increase the heating surface area within the bed; 2) Narrow the range of coal particle sizes used; 3) If the circulation rate of particles within the bed is low, the efficiency of the fly ash recovery system can be improved by reusing the ash filtered at the boiler’s rear end in the combustion process, or by adding fine ash from outside the system. 4) If the improper selection of blowers and exhaust fans is the cause, it is necessary to replace them with fans that have a higher head pressure and greater capacity. Choosing a slightly larger fan during design is beneficial for adjustments during the trial run.