Carbon attrition during the combustion of a char in a circulating fluidized bed

Carbon attrition by abrasion significantly affects combustion efficiency and particulate emissions from bubbling fluidized bed combustors as shown by laboratory and pilot scale studies carried out at gas velocities up to 1.5 m/s, i.e. in the range of velocities typical of these units. The paper deals with the results of a work directed to extend gas velocities up to 6 m/s, i.e. to a range of interest to circulating fluidized bed combustors. Experiments have been carried out by means of a laboratory scale circulating fluidized bed combustor, whose riser is 41 mm ID and 6.1 m high. Char has been charged to the combustor batchwise. Experimental conditions have been such that attrition by abrasion could be isolated from other comminution phenomena leading to the multiplication of carbon particles in the bed, namely. primary and secondary fragmentation and fragmentation by percolation. Both nitrogen and air have been used as fluidizing agents in order to establish the relative importance of purely mechanical attrition and combustion assisted attrition in generating carbon fines. For given gas velocities and rates of solids circulation in the combustor loop, carbon load and particle size distribution in the riser and carbon attrition rates have been determined as a function of reaction time. As a first approximation, these variables have been correlated according to the rate equation previously found for attrition by abrasion in a bubbling fluidized bed combustor. The comparison between the rate equation constants points out some peculiarities of attrition by abrasion in circulating fluidized bed combustors. © 1990, Taylor & Francis Group, LLC. All rights reserved.