Abstract: Circulating fluidized bed boilers (CFBs) with high circulation rate have been proven to bring significant advantages in achieving efficient and clean combustion, maximizing the in-situ desulfurization performance, re-ducing the emission of nitrogen oxides, lessening the wear of the heat exchange surfaces, and increasing the relia-bility and availability of the boiler.However, the recent operations with several such boilers had frequently expe-rienced slagging in the standpipe of solids return when the boilers were fed with wet coal slime from the boiler top, which seriously affected the boilers operation reliability and durability.By comparison, this slagging phenomenon did occur neither in the coal slime firing CFBs with medium and low circulation rates, nor in the CFBs with high circulation rates when granular coal particles were combusted.To understand this unique slagging mechanism in the CFBs of high circulation rate when wet coal slime was fed from the boiler top, the slag samples were collected and subjected to chemical composition, TG-DSC and scanning electron microscopy analysis.The thermodynamic equilibrium calculations were also performed using the FactSage software.The results identified that the slagging in the standpipe of the high circulation CFBs is related to the boilers unique operation characteristics.Firstly, when the wet coal slime is fed from the boiler top, the slime lumps simultaneously undergo processes of drying, water evaporation, thermal explosion, pulverization, and agglomeration while falling down by gravity against the hot upflowing flue gas.The fine powders produced from aforementioned processes, which could not completely be burned out in the furnace, keep burning in the standpipe, leading the temperature there to go over 900 ℃.Secondly, because of high separator efficiency the limestone particles used for in-situ desulfurization are effectively collected for recirculation, resulting the CaO content in the circulation ash increased to 10%-30% by weight.Thermodynamic calculations indicate that when the CaO content in the circulation ash is less than 13.89%, the liquid slag formation is mainly affected by alkali metals, while as the content of CaO is more than 13.89%, the effect of CaO on the ash fusion becomes more prominent.Both experimental and analytical data revealed that under above CaO contents and temperatures of > 910 ℃ with the ash compositions investigated slags with liquid content of more than 20% are favorably produced.The liquid slags enhance the adhesion force of the particles and make the fine circulating ash particles more easily adhere to the wall, thus creating conditions for the accumulation of larger particles.It is found that high circulating ash rate promotes the slag formation rate.Initiated from condensation of sticky alkaline-based components and enhanced by the high reactivity of the fine ash particles, the liquid slag nucleus conglutinated the surrounding fine ash particles and gradually developed to form large slag, eventually forcing the CFB system to shutdown.With this understanding, some practical countermeasures were proposed, namely, including changing the position of slurry feeding, optimizing the size of coal slime feeding into the furnace, optimizing the size of desulfurized limestone particles, and optimizing the design of riser structure, to prevent the slagging from occurring in the standpipe of the high circulation, coal slime top feeding CFBs so that the boilers safe and stable operations can be ensured.