OCR Text |
Show slightly overstate the amount of soot. The coal char used in the validation experiments was not prewashed and may contain some soot mixed with the large particles. Figures 6a, b, c show typical material produced by the refined fraction separation process. The bottom fraction, Figure 6a, is dominated by large porous char particles and no fines are seen on the filter membrane. The top fraction, Figure 56b, contains micron-sized spheres and fragments and clusters of ultrafine particles. Higher magnification, Figure 6c, shows that these clusters are composed of ultrafine primary spheres indicative of soot formation. The relative amounts of soot and char in the UT coal fly ash for the five combustion conditions and for the IL coal for two combustion conditions are shown in Figure 7. This graph shows a conservative estimate of the amount of soot. The method bias is shown as the lesser of the carbon in the top fraction or 5% of total fly ash mass. The method uncertainty is shown as the standard deviation of the replicate determinations (n= 2-6). The raw results of the quantitative separations in tabular form are presented elsewhere. [21] The amount of soot in the UT coal fly ash from the extreme low-NOx laboratory combustion conditions is at least 5% and may be as much as 15% of the total fly ash mass. Measurable carbon in the form of soot is also observed for low-NOx combustion with IL coal. The soot in the base case fly ash for U T and IL coal and in the fly ash from all combustion conditions using the N D lignite was within the experimental uncertainty and method bias and could not be quantified by the current method. Table 4 shows the soot and total carbon in the collected fly ash sample expressed as a percentage of the fuel carbon for the combustion conditions shown Figure 7. The lower bound includes the allowances for both method bias and statiatical uncertainty. The data for the STGr@1.5S combustion condition show conversion of between 0.2 and 0.6% of fuel carbon into soot. These data cannot be checked by material balance using the parent coal ash content since a large portion of the mineral ash remains in the furnace as bottom ash and is not represented in an exit gas sample. Char and Soot 1 1 September 4, 1998 |