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Show distribution control through minimization of particle size m a y be a useful method for fouling and slagging control. Size Distribution Effects on Emission Control Particulate emission technologies are also based on the Stokes number. In contrast to slagging and fouling, the objective here is to maximize particulate collection. In general, cyclones, electrostatic precipitators, and baghouses operate at satisfactory efficiencies for supermicron particles, but less so with decreasing size. Thus, the critical part of the size distribution for particulate cleanup is less than several microns. Although the submicron material does not comprise much of the total particulate mass, health effects and long term atmospheric dispersion are more related to number and surface area than total volume or mass. Furthermore, the size and cost of particulate removal equipment increases with decreasing particle size requirements (along with total volume of gas flow) and is less dependent on total mass of removal. Small particles are difficult to remove by common size classification techniques. Figure 5 shows the size distribution of a Montana lignite which was sieve-classified in an attempt to remove the small particles. The two curves with dark symbols are repetitive runs of the same pulverized fuel, while the curves with the open symbols represent repetitive data under combustion conditions. A number peak in the 30-70 /zm size range is apparent and more than 9 9 % of the particle mass is included in particles larger than 20 /zm. However, even though a significant number of particles has been removed in the intermediate size range of 5-20 /zm, the number density of smaller particles does not seem to have been markedly reduced by the classification process. Similar observations of aerodynamically classified pulverized coal have been previously reported (Holve et al., 1985), and it appears that aerodynamic separation is superior to sieve classification. Nevertheless, it is very difficult to remove the small particles for a friable material such as pulverized coals. Similar experience in clean-room environments (obtained by filtration) has been observed; particle counts increase rapidly with decreasing particle size (Gunawardena et al., 1985). SUMMARY W e have discussed the impact of broad particle size distributions on pulverized and coal/water slurry fuel combustion. In particular, w e have presented quantitative measurements of size evolution and used measured initial conditions to calculate the effects of size distribution and particle swelling on carbon conversion. The initial size distribution, from super- to sub-micron particles, greatly influences the size distribution of the residual flyash. W e summarize our ideas regarding the influence of size distribution as follows: 1. The upper end of the size distribution controls the time required for complete char burnout. 2. The mid-range of the size distribution (containing most of the mass) affects devolatilization, ignition, and formation of the residual char. 3. The mid-range of the flyash size distribution is the primary source for slagging and fouling, since total mass deposition is controlled by impaction and particle rebound on heat transfer surfaces. The ash particle sizes are in turn directly related to the initial size distribution. 7 |
