OCR Text |
Show A more general overview for comparing these effects is provided by Figure 18, which shows the cumulative volume distribution above 3 microns for all the slurries. As in the cumulative number distributions of Figure 13, the values have been normalized by the total mass flowrate of slurry #2 to give a uniform basis of comparison. The distinction between the chemically beneficiated low ash and unbeneficiated high ash slurries is made clear by the large increase in volume during the rapid heat-up region at 2-3 milliseconds. In contrast, the high ash slurries show a much smaller level of swelling which occurs later in the reaction process (6-10 ms). Comparison of the volume fraction at the longest residence times shows no clear trend although the finer grind materials appear to have a more rapid decrease in particle volume at about 20 milliseconds. Because these are the first known in situ measurements of absolute size distributions for coal/water slurries, there are no direct comparison experiments or theories. Recent sampling measurements of particle distributions in a small flow reactor (Solomon et al., 1982-83) show normalized distributions which increase in size in the early time reaction region, indicating particle swelling; the present results are consistent with these previous observations and go further in clarifying the rate of carbon burnout for different slurry mixtures. Other investigators, e.g. Boni et al., 1984, have obtained cascade impactor measurements at the outlet of a small combustor with a total residence time of 0.7 seconds, and found normalized bimodal mass (volume) distributions which have mass peaks in the 2-5 micron size range with a mass minimum in the 0.5 micron range. Our results (see Figure 14) show the same qualitative trend. Given the wide range of atomization characteristics for the slurries measured here, it is clear that any detailed investigation of combustion performance of various fuel mixtures and/or atomizers must adequately characterize both the distribution and absolute number density of the injected fuel spray. CONCLUSIONS These results show that atomization and particle growth at early times in the flame are important considerations in determining ignition and char burnout characteristics. The rapid onset of luminosity in the flame appears to derive from sodium-based additives which give a strong sodium emission. The high speed movies suggest a longer particle devolatilization delay (7-10 ms) which increases with particle size. Detailed particle size distribution measurements for five coal water slurry mixtures show the following major results: 1. Size distributions do not follow any simple distribution law, but rather exhibit several peak modes in the 0.3-120 micron range. 2. In the submicron range, high number densities of particles are formed (10°/cm3) in the atomization process. Though these particles are a small part of the total mass, they will be important contributors to the ignition process and flame stability. 3. In general, particle fragmentation is not observed for most of the slurries fuels studied here, although in one case it is possible that a limited amount of fragmentation may be occurring. |