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Show corresponding ~o velocities of 1.25 mls and 5.0 m/s. O~ing to particle fragmentation, only a fraction of the particles ~ere large enough to impact the tube at the lo~er velocity, although almost all the particles impacted it at the higher velocity. The curves sho~ a sharp drop in the efficiency of capture, from almost 100 percent sticking at 450 milliseconds residence time, to essentially zero at 600 milliseconds. Using kinetic model calculations for comparison, it can be seen that the existence of a molten phase is responsible for the adhesion tendency. The duration from complete sticking to no sticking is almost completely accounted for by the variation in the feed particle size (53-63 ~m). Examination of the substrate (Figure 8) sho~s that particles impacting at short residence times form a completely molten deposit. Ho~ever, the extent of coalescence in the deposit decreases as the particle residence time in the furnace increases. This is apparent in the last micrograph, ~hich shows a deposit with only a few individual particles forming a sparse coating on the tube surface. Pyrite Fragmentation In previous experiments with Kentucky No.9 coal, it was observed that the final ash particle size distribution was finer than the starting mineral size distribution, strongly suggesting fragmentation of the minerals during combustion. As a large portion of the mineral matter in Kentucky No. 9 is pyrite, this suggests that pyrite itself may be fragmenting. The size evolution of pyrite particles must therefore be considered to fully understand the transformation process. Figure 7 shows the particle size distribution of both raw (53 - 63 ~m) and partially oxidized pyrite particles, as obtained from cascade impactor sampling. Almost 18 percent of the total mass in the oxidized sample is present in the 0.9 to 4.0 micron range, indicating that significant particle breakup occurred during combustion. Examination of scanning electron micrographs in Figure 5 suggests that particle degradation can occur following the pyrite decomposition step, and could be a result of 10 |
