Investigation of agglomeration process of coal slime used in fluidized bed combustion

The circulating fluidized bed( CFB) combustion is one of the main ways of coal slime utilization.The agglomeration phenomenon of wet coal slime into the furnace has two sides,which is very important for the steady combustion of coal slime in the CFB.In order to explore the mechanism of coal slime agglomerating process and provide theoretical guidance for the efficient utilization of coal slime,from two perspectives of time and space,the changes of compressive strength,water,minerals and functional groups with time,and the changes in the outer,middle and inner parts of the slime column after coal slurry put into the furnace were studied systematically.The compressive strength is an important index to measure agglomeration strength.The results shows that the compressive strength of the slime column increases first and then decreases with the prolongation of the residence time.Based on the results of the observation of the front and the section of slime column in different time and the results of the water,minerals and the change of the functional group at different time,it is concluded that the increase of compressive strength of coal slime column is mainly related to the evaporation of water at 0-30 s.The change of the compressive strength may be related not only to the moisture,but also to the pyrolysis and thermal decomposition at the high temperature at 30-90 s.The rapid increase of the compressive strength at 90-210 s is related to the changes of the mineral and functional groups,but it may also be related to the mesophase produced in the process of volatilization.The temperature is an important factor affecting the agglomeration of coal slime.When the furnace temperature is low,the moisture and volatilization release slowly,which is not conducive to the development of mesophase,therefore,the compressive strength is relatively low.In a certain range,the compressive strength increases with the increase of furnace temperature,but when the furnace temperature is too high,which causes rapid volatile precipitation,may inhibit the development of mesophase and affect the compressive strength. Therefore,an appropriate furnace temperature is conducive to the formation of mesophase and promotes the increase of compressive strength.It can make the slime reach a relatively high compressive strength at 850 ℃.