Environ. Sci. Technol. 1894, 28, 107-118
Role of Combustion and Sorbent Parameters in Prevention of Polychlorinated Dlbenzo-pdioxin and Polychlorinated Dibenzofuran Formation during Waste Combustion Brian K. Gullett' and Paul M. Lemleux Air and Energy Engineering Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1 James E. Dunn
Department of Mathematical Sciences, University of Arkansas, Fayetteviiie, Arkansas 72701 This research uses experimental data and a statistical approach to determine the effect of combustion- and sorbent-injection-relatedparameters on the mechanism of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD and PCDF, respectively) formation and prevention in waste combustors. The operation of a pilot-scale combustor was varied to effect different regimes of oxygen (Oz), hydrogen chloride (HCl),and chlorine (Clz) concentration; temperature; residence time; quench rate; and sorbent injection. The fly ash loading of a municipal waste combustor was simulated by postcombustion injection of fly ash collected from a full-scale facility. Downstream sampling and analysis indicated significant PCDD and PCDF formation, beyond concentrations on the preinjected fly ash, at rates conducive to explaining formation in full-scale facilities at particle/gas residence times ~315 "C, the The logit(?) m d e l is always negatively affected by addition of Ca(0H)Z reduces the yield to zero. Since d increases in TDUW.Higher values of T D Uwill ~ therefore [PCDD PCDFI/d[Ca(OH)zl - ?'DUCT (see Table 4, decrease the [PCDDI compared to [PCDFI. section c) and TDUWis never negative, the addition of Effect of HCI Concentration. [HCl] is a significant Ca(OH), will always decrease [PCDD PCDF]. The parameter,being included in predictors forallfourmdels.
... ...
m -
5
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-
I
+
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114 EMm. Sol. T M . . Vd. 28. No. 1. 1884
Discussion
postfurnace, downstream PCDD and PCDF formation. It also suggests that collected ESP fly ash is still active for formation and that sufficient organic precursors are present (either on the fly ash or in the natural gas combustion products) to form PCDD and PCDF. Tests without the addition of C1 as HC1 or Cl2 also show large increases in PCDD and PCDF, indicating that sufficient C1 precursors are also present on the fly ash to produce PCDD and PCDF. These findings are consistent with both the chloro-organic percursor (6,17)and de nouo (15) hypotheses. The volumetric concentrations of sampled PCDD and PCDF are, in many cases, several times higher than those obtained from field sampling trials on MWCs prior to flue gas cleaning equipment and without "good combustion practices" (GCPs). The facility from which the fly ash used in these tests was obtained had PCDD + PCDF levels up to 4000 ng/Nm3 prior to GCP facility modifications (4). GCPs typically include appropriate temperature control, air rates and distribution, and boiler structure modifications to improve mixing (54). The higher concentrations sometimes found in our work are likely due to any number of factors inherent in our simulation of a MWC. These might include a higher percentage of condensed organics on our collected fly ash than is representative of in situ fly ash. The time rate of PCDD and PCDF formation reached a maximum of 1160 ng (g of fly ash1-l s-l in run 17. This rate is apparently the highest value reported to date from an experimental system. Rate results from bench-top, fixed bed reactors have been typically more than 2 orders of magnitude less with the exception of recent results with a PCP precursor 1147 ng (g of fly ash)-' s-l, (19)l. Even this recent result requires a mechanism including particle deposition for sufficient residence time, reaction, and subsequent reentrainment or desorption to explain some field-observed (34)PCDD and PCDF concentrations. The rate of PCDD and PCDF formation observed in our work suggests that in-flight formation alone is sufficiently rapid to explain concentrations observed in field-scale operations. The effect of [ 0 2 1 over the range of our experiments was minimal except in determining the partition coefficient between [PCDDI and [PCDFI. [ 0 2 1 had a high value of R2sp(02,...I (0.5822) for logit((p)and, as such, is the most influential parameter in determining the partitioning between PCDD and PCDF. Increases in [ 0 2 1 will lead to greater fractions of the yield as PCDD, as shown on Figure 6, if TDUCT is sufficientlylow. Highvalues of TDUCT always requires lead to low values of logit(cp) while lower TDUCT low io21 (800 OC significantly reduced [PCDD + PCDF] (see Figure 14). This is true even during the "baseline- runs when no additional C1 source was added. The presence of Ca(OH)2reduces the yields expected from formation on particle surfaces where the only source of C1 is that which is found on the fly ash feed. These results also show that Ca(OH)2 interferes with formation on the particle itself, reducing PCDD and PCDF yields beyond those expected from gaseous HCl removal alone. At TDUCT > -280 OC (all but run 26), the presence of Ca(OH12 has an inhibitory effect upon [PCDD + PCDF] beyond that
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116 Environ. Sci. Technol., Vol. 28, No. I,1994
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Flgure 14. Effect of [HCI] on [PCDD PCDF] with and without [Ca(OH)2] ( 0 2 = 6 % , Clp = 20 ppm, Ca(OH), = 0 and 2.84 g/mln, fR= 1.7 s, 7 ' & ~ = 315 OC, QUENCH = 24 'C/S).
attributable to reduction in gas-phase [HCl] alone (injection of 2.84 g/min typically drops [HCll -70%). CPCDD + PCDFI with [Ca(OH)d = 2.84 g/min is much lower than for the equivalent [HCl] (300 ppm) with no added [Ca(OH)21. Table 5 suggests that, after [Ca(OH)21, TDUCT is the most significantparameter in describing [PCDDI, [PCDFI, and [PCDD + PCDFI. In addition, after 1021, TDUCT also is the second most significant parameter in describinglogit(cp). The results suggest that, in general, under normal operating conditions lower values of [PCDDI, [PCDFI, and [PCDD + PCDFI are found at higher values of TDUCT, = >350 "C. This may, however, be somewhat of an artifact from the manner in which the samples were collected. The runs with high TDUCT were in part due to their association with high values of Tom,the latter being within the commonly cited temperature region of PCDD and PCDF formation (300-400 "C). Thus, these samples may still have been undergoing appreciable reaction at the point (temperature) of sampling, resulting in lower amounts of collected PCDD and PCDF and, therefore, in being associated with higher values of TDUCT. This phenomenon combines with [HCl] in Figure 9 to show lower [PCDD + PCDFI values at higher [HCl]. These results suggest that the formation mechanism shifts with temperature. For instance, surface-bound C1 on fly ash may be more active at higher temperatures, while gaseous HC1 does not play a significant role until lower temperatures are reached. The partition coefficient, cp, is least affected by parameters related to C1 composition ([HClI, [Clzl, [Ca(OHM) but is rather almost solely affected by the parameters TDUCT and [Ozl. Figure 6 shows that lower [021 and higher TDUCT result in lower values of cp, meaning less [PCDD] in relation to [PCDF]. This suggests that formation of PCDD versus PCDF requires higher amounts of 0 2 and lower temperatures. The role of O2 in PCDD formation from particulate carbon has been noted earlier (18) and is likely due to the 0 dependency of both the Deacon reaction (eq 1) and the phenol (a PCDD precursor) formation.
Conclusions In-flight formation of PCDD and PCDF to levels representative of those observed in field sampling trials has been observed by reinjection of MWC fly ash and downstream sampling in a pilot-scale combustor. Formation of PCDD and PCDF to field-representative levels
can be explained sufficiently by a mechanism that involves reaction on an entrained particle at residence times less than 5 s. System parameters that significantly affect the PCDD: PCDF partitioning include [ 0 2 ] and TDUCT, This suggests that measures could be taken to control this partitioning, rather than the total yield of PCDD and PCDF, to effect the greatest reduction in health and environmental toxicity. Combustion modifications that lead to changes in the operating characteristics of the duct environment can result in reduced PCDD and PCDF formation. At high values of QUENCH, t R has little influence on yields. The converse is true at lower values of QUENCH, implying that more rapid quench rates in the duct system will result in lower PCDD and PCDF yields. In a related manner, higher average duct temperatures result in less formation of PCDD and PCDF. While [02]did not have a strong correlation with levels of PCDD or PCDF formation, variation of excess air still has an effect on yields; its significance was most prominent in explaining the partitioning between PCDD and PCDF. Intermediate levels of [021(=4-7 % ) tend to produce larger PCDD and PCDF yields than the extremes. The presence of high temperature (>BOO OC) Ca(OH)2 sorbent injection has a significant role in preventing PCDD and PCDF formation. The resultant reductions in the HC1 or Clz concentrations, especially at lower values of !D ' UCT, significantly decrease the formation of PCDD and PCDF. Sorbent addition, even in the absence of added gaseous C1 species, reduces PCDD and PCDF yields, implying suppression of a mechanism involving particlebound C1. The effect of combustor operating parameters on PCDD and PCDF yield during waste combustion is interactive, resulting in complicated interrelationships. This underscores the difficulties in drawing mechanistic conclusions from parametrically uncontrollable field tests. Despite their complexity, these pilot-scale results can be used to begin to understand the effect of sorbent injection and combustion operating conditions upon PCDD and PCDF yields and suggest the means for minimizing the potential for downstream formation of PCDD and PCDF. Nomenclature
TOUT
tR
[Ca(OH)21 CC121 [HClI 1021
[PCDD] fraction of total yield, defined in eq 2 median particle diameter, pm temperature quench rate, "C/s model coefficient of determination,unitless semipartial correlation of the model set of predictors including parameter P, unitless temperature of the duct, "C temperature of the particulate filter, O C temperature at fly ash injection point, "C temperature at sampling point, "C gas- and particle-phase residence time, s concentration of Ca(OH)2,g/min concentration of Clz, ppmv concentration of HC1, ppmv concentration of 02, %
[PCDD + PCDF] total yield of [PCDD] and [PCDFI, ng/ m3 [PCDDI yield of [PCDD], ng/m3 [PCDFI yield of [PCDF], ng/m3 (P [PCDDl/[PCDD + PCDF], unitless logit(& defined in eq 2 Y Acknowledgments
The authors gratefully acknowledge the field-related information from James D. Kilgroe (US.EPA/AEERL); the mechanical support of George R. Gillis (US.EPA/ AEERL); the sampling efforts of C. Courtney, A. Drago, W. Hansen, and R. Srivistava (Acurex Environmental); and the analytical efforts of R. Harris, B. Harrison, and J. Ryan (Acurex Environmental). Literature Cited (1) Shaub,W.; Tsang,W. Environ. Sci. Technol. 1983,17,721730. (2) Shaub, W.; Tsang, W. Physical and chemical properties of dioxins in relation to their disposal. In Human and Environmental Risks of Chlorinated Dioxins and Related Compounds; Tucker, R. E., Young, A. L., Gray, A., Eds.; Plenum Press: New York, 1983; pp 731-748. (3) Commoner, B.; Shapiro, K.; Webster, T. Waste Manage. Res. 1987,5, 327-346. (4) EnvironmentCanada. Environmental Characterizationof Mass Burning Incinerator Technology at Quebec City; Summary Report; National Incinerator Testing and Evaluation Program: Ottawa, June 1988; EPS 3/UP/5. (5) Goldfarb, T. D. Chemosphere 1989, 18, 1051-1055. (6) Dickson, L. C.; Karasek, F. W. J. Chromatogr. 1987,389, 127-137. (7) Eiceman, G.; Rghei, H. Chemosphere 1982, 11, 833-839. (8) Rghei, H. 0.;Eiceman, G. A. Chemosphere 1982,11,569576. (9) Rghei, H. 0.;Eiceman, G. A. Chemosphere 1984,13,421426. (10) Rghei, H. 0.;Eiceman, G. A. Chemosphere 1985,14,259265. (11) Vogg, H.; Metzger, M.; Stieglitz, L. Waste Manage. Res. 1987,5, 285-294. (12) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 1373-1378. Stieglitz, L.; Vogg, H. Chemosphere 1987,lS; 1917-1922. Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987,21, 1080-1084. Lustenhouwer,J. W. A.; Olie,K.; Hutzinger,0.Chemosphere 1980,9,501-522. Stieglitz, L.; Zwick, G.; Beck, J.; Roth, W.; Vogg, H. Chemosphere 1989,18, 1219-1226. Karasek,F. W.; Dickson, L. C. Science 1987,237,754-756. Dickson,L. C.;Lenoir,D.; Hutzinger, 0. Chemosphere1989, 19,77-82. Dickson, L. C.; Lenoir, D.; Hutzinger, 0. Environ. Sci. Technol. 1992,26, 1822-1828. Deacon, H. W. BritishPatent 1403/1863;U.S.Patent 85370/ 1868; U S . Patent 141.33. Griffin, R. D. Chemosphere 1986, 15, 1987-1990. Bruce, K. R.; Beach, L. 0.;Gullett, B. K. Waste Manage. 1991, 11, 97-102. Yang,M.; Karra,S.B.; Senkan, S. M. Hazard WasteHazard. Mater. 1987, 4, 55-68. Eiceman, G. A,; Rghei, H. 0. Environ. Sci. Technol. 1982, 16, 53-56. Gullett, B. K.; Bruce, K. R.; Beach, L. 0. Waste Manage. Res. 1990, 8, 203-214. Solomons,T. W. G. Organic Chemistry;John Wiley&Sons, Inc.: New York, 1976; p 452. de Leer, E.; Lexmon, R.; de Zeeuw, M. Chemosphere 1989, 19, 1141-1152. Sakai, S.;Hiraoka, M.; Takeda,M.; Nie,P.; Ito, T. Formation and degradation of PCDDs/PCDFs in a laboratory scale Environ. Scl. Technol., Vol. 28, No. 1, Wg4
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