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Show ash content of the parent coal. For the N D lignite the carbon content is uniformly low for both the high-NOx B A S E condition and for the two staged combustion conditions. Visual examination of the size-segregated samples collected with the Andersen cascade impactor showed a consistent color pattern. The preseparator and stage 0, which held the largest particles, had dark gray or black deposits. O n the succeeding stages the color became progressively lighter, changing to gray or brown, as the particle size became smaller. The final filter, which collects particles smaller than 0.4 urn aerodynamic diameter, had a darker or even black deposit. This indication of a bimodal distribution of carbon in the fly ash was quantified by Leco carbon analysis of composite samples from adjacent impactor stages. Table 3 shows typical data for the carbon concentration vs. particle size range for the STGr@1.5S condition with the three fuels. Data for U T coal under all five combustion conditions, also showing a minimum carbon content on impactor stages 5 and 6, was published previously. [16] An initial, but incorrect, interpretation of the carbon distribution was that the carbon in the large particles was only char and that the fine particle carbon was only due to soot. This hypothesis was rejected by S EM examination of the particles. Figure 4 is a 100 X S E M image of the material directly deposited in the preseparator of the Andersen impactor from Illinois coal at the STGr@1.5S combustion condition. Short-duration sampling was used to obtain a partial layer of particles for imaging while avoiding changes to the particle morphology by transfer and handling. The image shows porous char particles, spheres of mineral ash, and large, elongated fine-grained aggregates. At high magnification, these aggregates are shown to be composed of loosely-packed 10-50 nm diameter primary particles, indicative of soot. The submicron aerodynamic diameter particles deposited on stage 7 of the cascade impactor contained only clusters and small aggregates of the ultrafine particles mixed with the mineral ash spheres and an occasional irregular fragment. Under the laboratory conditions tested by cascade impactor sampling, large soot aggregates are deposited with the particles over 10 jim aerodynamic diameter and dispersed soot is deposited with the Char and Soot 9 September 4, 1998 |