Percolative fragmentation and spontaneous agglomeration

Captive particle imaging experiments were performed on over 200 coal and ch ar particles in the pulverized size range from four coals of various rank a t oxygen concentration from 3-19 mol-% and at gas temperatures of about 125 0 K. Despite wide variations in single-particle behavior, the data set reve als two clear trends that provide new information on the nature of char com bustion. First, the low-rank coal chars are observed to maintain their high reactivity through the late stages of combustion, thus avoiding the near-e xtinction events and long burnout tails observed for bituminous coal chars. Secondly, percolative fragmentation in the late stages of combustion is a rare event under these conditions. Some particles reach a percolation thres hold late in combustion, but typically undergo spontaneous agglomeration ra ther than liberation of the incipient fragments. It is concluded that perco lative fragmentation behavior in the pulverized size range is determined no t only by solid-phase connectivity, but also by a real competition between disruptive and cohesive forces present at the time of formation of the coll oidal-sized incipient fragments. (C) 1998 by The Combustion Institute.