OCR Text |
Show above was for monoch10robenzene cofired with coal. All other combinations showed lower destruction efficiency under upset conditions. One of the purposes of the present tests was to examine the nature and extent of background emission variability or noise and hysteresis. The facility was initially free of POHC exposure and waste firing was introduced only after fuel baselines. The results showed that during the fuel baseline and the early days of cofiring, the POHC and PIC emission levels were very low, frequently nondetectab1e. As the upset operating settings were introduced and the deposits built up on the convective section, background emissions began routinely appearing. These background emissions were quantified by sampling after cofiring was stopped, with the unit operating on gas only. The level of the background appears to depend on the elapsed time since cofiring had been curtailed, on the types of upsets imposed prior to stopping cofiring, and on the total number of days of facility exposure to POHCs. The background, or hysteresis, thus appears to have both short-term and long-term components. Figures 8 and 9 show the residual emissions at various intervals after cofiring was curtailed. Figure 8 includes volatile chlorinated POHCs and PICs, while Figure 9 shows specific POHC results. In both cases, there is a general trend for the background to decay with elapsed time since cofiring. At the 15- and 65-hour samples, there are numerous data points on the abcissa that outweigh the outliers. It should be emphasized that this facility was operated on waste for 6 to 8 hours per day,S days per week, and on gas only during the remaining time. The deposits, therefore, had an opportunity to bake-out or desorb during nights and weekends. Alternate duty cycles could cause the hysteresis results to differ. Figures 10 and 11 show the historical variation of the hysteresis effect by comparing emissions sampled at 3 hours, or less, and 15 hours after cofiring cessation, but at different total number of days' operation of the facility on waste. These results show a general tendency for the background emission levels to increase with exposure of the facility to POHCs. Soot was blown from the tubes on day 50, which may account for the higher backgrounds around day 40. The effect of sootb10wing on emissions is shown in Figures 12 and 13. Figure 12 shows emissions during sootb10wing at 5 separate stations of the convective section, labeled as SI through S5. Sl was in the high-temperature entry zone of the convective section, and S5 was at the low-temperature exit. For comparison, S8 is a measurement after sootb10wing during normal operation. This indicates that significant organics were emitted during sootb10wing from the first three stations, producing emissions of up to an order of magnitude higher than normal. Figure 13 compares the sootb10wing results to normal cofiring emissions, labeled P10, many of which were taken with deposits on the tubes. These results indicate lower emissions after sootblowing than during normal operation. Hysteresis results, not shown, also were lower after sootb10wing than during operation with deposits on tubes. The identification and quantification of PICs generally showed a reasonable level of closure. Figures 14 and 15 show chromatograms for volatile organics from mini-Vost runs. The chromatograms on the left of the figures are from the flame ionization detector, which shows primarily 7 |