The impact of fragmentation on char conversion during pulverized coal combustion

Abstract

Synthetic char particles with controlled macroporosity are used to study char fragmentation behavior during pulverized coal combustion. The chars are burned in an atmospheric laminar flow reactor that permits the control of gas temperature and composition. Char particles are extracted from the reactor at selected residence times and characterized for extent of mass loss and particle size distribution. Chars of 23% and 36% porosity are subjected to environments containing 12 mol % O2 at nominally 1500 K. Number distributions show a large increase in the numbers of small particles during devolatilization, the higher porosity char exhibiting the larger increase. Results show that char burn off increases with porosity at any given residence time, demonstrating an impact of fragmentation on char burn off. The data indicate that both particle diameter and apparent density decrease during burn off and support power-law relations between char particle mass, apparent density, and diameter. A particle population balance model is developed and used to characterize the type of fragmentation that occurs during char oxidation and to quantify the rates of fragmentation events. The model allows for attrition-type behavior (in which only fines are produced), breakage-type behavior (in which particles break into two or three smaller particles), and percolation-type behavior (in which particles fragment into a distribution of smaller-size particles). Calculations using the model indicate that fragmentation during burn off is percolative in nature and char burning rate parameters determined from mass loss, size, and temperature measurements are too high if account is not made for the effects of particle fragmentation. Calculations also suggest that fragmentation during develatilization is percolative in nature and the extent of fragmentation increases with coal volatile yield. Fragmentation rates during devolatilization are estimated to be as high as five times the fragmentation rates during char oxidation.