OCR Text |
Show and operating conditions tested and prevailed even when the boilers were intentionally operated in off-design upset conditions such as substoichiometric air, malfunctioning atomizers, and waste flow transients. In fact, the destruction efficiencies were so uniformly high that it was not generally possible within the constraints of plant operation to identify and test conditions where the destruction was not acceptable. This result, in part, supported EPA's decision to waive a trial burn when the boiler is operated within a specified window. Although the field test data base supporting the proposed standard indicates uniformly high destruction efficiencies, additional information and analysis is needed to interpret several patterns observed in the data and to generalize the effects of operation on destruction efficiency. One pattern observed in the field tests was variability in ORE results of up to an order of magnitude, which was apparently not due to artifacts in the sampling and analysis scheme or to contamination. Additionally, the variations did not correlate with any boiler operational settings or with boiler performance indicators such as CO or NOx emissions or smoke. It was also observed at several sites that the stack concentrations of waste species continued for several hours after waste firing was curtailed. This hysteresis effect may be attributable to deposition of trace concentrations of waste species on boiler surfaces, particularly on areas with carbonaceous deposits from oil or coal firing. A potential role of soot deposits in the observed hysteresis and stack emission variability was also indicated by limited sootb10wing results that showed higher emissions during routine sootb10wing from the boiler convective section than during normal operation. To gain more confidence in the field test data and assist in the data interpretation, data are needed to isolate the presence and extent of emission variability and to relate these data to operational settings and the conditions of the boiler heat exchange surfaces. A second key issue from the field tests is a more comprehensive characterization of the types and quantities of PICs emanating from the POHCs, or from the conventional fuel. Several tests showed PIC emissions in boilers as a higher fraction of the organic compounds in the flue gas than were observed in incinerators. More data are also needed on the effects of operating conditions on the relative concentrations of PICs and POHCs. A third issue is the degree to which nonsteady or off-design boiler operation affects destruction efficiency. The field tests were somewhat limited in the types and severity of imposed upsets but showed a broad operational window with high destruction efficiency. To generalize this observation, more data are needed on the effects on destruction of various types of upsets imposed with more severity than was possible in the field. An additional issue, not reported on in this paper, is the form and fate of heavy metals contained in the waste or fuel. The present pilot-scale tests addressed the above issues. The pilot-scale approach was selected because of wider operational flexibility, tighter experimental control, and lower cost compared to field testing. 3 |