OCR Text |
Show 100% · - - ,... ~ ~ ~ - ~ - ~ ~ V V V · VV VV ~ V V V - v V V 80% V V V V V · V V V V V V · / V / V / V · / V ~ "=i 60% :~ =-- / ,/ · / ,/ V ,/ V V · V V V V V V · V V V V · v v i ~ v ~ - - -~- - ... 1/ i 40% ~ 0 20% 0% ~ e. m e - e ~ e ... e ott e A e<e~e£= e ~ e bo e ~ ciI :z: ..:. ~ 4> U .:. u .oCl. ~ ~ ~ F CIl ;J ~ :I: t).Q ~ :z: ~ u u CIl :I: ~ Increasing Volatility ~ Bottom Ash II ESP Capture ~ Emitted Fly Ash l:J Emitted Vapor Figure 3. Comparison between the measureddata of Meij and the metals partitioning model. Measured data are followed by the letter "m", as in U-m for measured uranium species. most of the selenium will be condensible particulate matter, the experimental evidence shows that over 25 percent escapes in the vapor phase. This apparent failure of the model could be attributable to insufficient thermodynamic data on the speciation of selenium compounds. However, closer analysis of the model shows that uncertainty in the experimental conditions is more likely to explain the discrepancy. For the conditions of the Meij data, selenium is predicted to exist primarily as Se02. Between temperatures of about 400 of and 212 of , however, the fraction of Se02 in the vapor phase will drop from 100 percent to essentially zero. Figure 4 shows predictions of the vapor pressures of mercury, selenium, uranium and boron as a function of temperature in the flue gas of a coal-fired combustor. These result are intended to illustrate how steep the vapor pressure curve is for selenium at temperatures near those of particulate control devices. It is clear that selenium undergoes a transition from nearly all vapor (P yap = ca. 1000 ppm) at about 205°C (400 OF) to nearly completely condensed (P yap = ca. 0.1 ppm) at 100°C (212 OF), corresponding approximately to four orders of magnitude decrease in vapor pressure. Because of the potential for both temporal and spatial stratification of the flue gas temperature within the ESP, the discrepancy between model and data is more satisfactorily explained in this way. 9 |