Time-resolved evolution of fly ash during pulverized coal combustion

The time resolved evolution of ash during the combustion of individual char particles has been studied using a Montana lignite, a North Dakota Beulah lignite, and a synthetic char prepared from doped sucrose. The three chars were selected to illustrate the effects of macroporosity and mineral matter content on the ash evolution. The synthetic char is distinguished from the lignites by being macropore-free. The ash content was varied from 17%(Montana) to 37%(Beulah). When the chars are reacted under external-diffusion-controlled conditions, the mineral matter is released as the carbon surface recedes. The ash particles on the surface of the char aggregate and grow in size with increasing char burnout. The rate of growth of particle size increases markedly as the extent of burnout increases because of the increase in surface concentration of released ash. The ash sizes are larger for the char with the higher mineral content. As complete burnout is approached, however, the macroporous lignite chars fragment yielding smaller final ash particles than the macropore-free sucrose char. A percolation model applied to the ash formation processes was used to demonstrate the major features of the dependence of the time resolved evolution of ash on macroporosity.