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Show impact on PCDD/PCDF formation and emissions. Fly ash hold up in convective passes or APCDs operating in this temperature range can result in the formation of PCDD/PCDF at levels well above those exiting the combustion zone. Disposal of this fly ash then becomes a problem. In addition, the chlorinated trace organics formed in the fly ash may volatilize and be emitted from the system. Analyses indicate nearly 50% of the PCDD/PCDF formed in the 570F filter sample system are captured as gas phase species. PCDD/PCDF Phase Studies. In the above tests, two types of fly ash were captured by the particulate filter, very small fines packed into a filter cake and large (diameter > , mm) black flyers. The filter cakes had brown hues ranging from a tannish to a darker reddish brown. The fines appeared to be mainly composed of inorganic material although there were undoubtably some organics present. The large black fly ash were very light and sat atop the filter cake. The black color suggested the large fly ash had a much higher carbon content than the fines. The large fly ash were easily separated from the fines with teflon tweezers. It was hoped that separate analyses of the two fly ash types and the other sample train components would provide insight into mechanisms of PCDD/PCDF formation. Concentration of the PCDD/PCDF in the large carbonaceous fly ash would suggest that organic precursors are found in the fly ash matrix. Concentration of the trace organics in the high surface area fines would suggest gas phase species are involved in a formation mechanism. It may be that gas phase organic precursors are catalyzed by inorganic constituents in the fines or solid phase organic precursors are more readily available for reaction with gas phase species such as chlorine or oxygen. Probe capture, filter capture, and train components downstream of the filter from simultaneous 250F filter oven and 570F filter oven MM5 sample trains were recovered and analyzed separately. The large black carbonaceous fly ash captured by the filter were separated from fines. It was not possible to separate the fly ash captured in the probe. The results of the train component analyses are shown in Figure 4. Examination of the data shows that gas phase species in the 570F filter train accounted for over 45°A> of the total while less than 10% of the species from the 250F filter train were gas phase. PCDD/PCDF vapor pressures are significantly higher at 570F than at 250F. PCDD/PCDF in the 250F filter train, that formed in the furnace, is found in both the large carbonaceous fly ash and in the fines, formation in neither fly ash type is dominant. Data from the 570F filter train also show similar PCDD/PCDF levels in both the large and fine fly ash. The downstream formation occurs in both fly ash groups. These data do not show either in-furnace or downstream PCDD/PCDF formation occurring predominately in either the large carbonaceous fly ash or the fines. The source of the volatile species, either large or fine fly ash or a combination of both, is not known. The high vapor pressure of the PCDO/PCOF in the 570F filter sample train suggest high temperature particulate control devices may not be very effective for the control of PCDO/PCDF emissions. Importance of Fly Ash Composition. To further investigate the influence of fly ash composition on PCOD/PCOF emissions a series of tests were run to compare emissions of fly ash carbon to PCOD/PCDF emissions. Simultaneous flue gas samples were taken using a standard MM5 train (filter T = 250F) and an EPA Method 5 (M5) particulate sampling train. M5 samples were recovered with distilled water. MM5 train components were analyzed for PCOO/PCDF and the M5 train components were analyzed for carbon and ash content using ASTM Method 03178-84. Fly ash samples were over 90% 4 |
