Research Formation of Chlorinated Dioxins and Furans in a Hazardous-Waste-Firing Industrial Boiler B R I A N K . G U L L E T T , * ,† ABDERRAHMANE TOUATI,‡ AND CHUN WAI LEE† U.S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory (MD-65), Research Triangle Park, North Carolina 27711 and ARCADIS Geraghty & Miller, Research Triangle Park, North Carolina 27709
This research examined the potential for emissions of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) from industrial boilers that cofire hazardous waste. PCDD/F emissions were sampled from a 732 kW (2.5 × 106 Btu/ h), 3-pass, firetube boiler using #2 fuel oil cofired with 2,4dichlorophenol or 1,2-dichlorobenzene and a copper naphthenate mixture. PCDD/F levels were significantly elevated when improved combustion conditions (reduced carbon monoxide, increased carbon dioxide) followed periods of flame wall-impingement and soot formation/deposition on the boiler tubes. Boiler tube deposits became a sink and source for PCDD/F reactants (copper and chlorine) and PCDD/F, resulting in continued formation and emissions long after waste cofiring ceased. The role of deposits in PCDD/F formation makes emissions dependent on current as well as previous firing conditions, resulting in uncertainty regarding prediction of emissions based solely on the type and rate of cofired hazardous waste.
Introduction Emissions of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) from industrial boilers that cofire hazardous waste are scantly characterized. This potential source of PCDD/F emissions will be receiving attention from the U.S. Environmental Protection Agency (EPA) as part of an effort to consider revising emission standards for hazardous waste combustors. It is common practice for combustible hazardous waste, typically liquid waste, to be cofired in industrial boilers. These boilers have a variety of designs (i.e., watertube, firetube, and stoker), types of primary fuels (i.e., coal, oil, natural gas), and amount and type of hazardous waste cofiring. Data from two of the 136 hazardous-waste-firing industrial boilers/furnaces had PCDD/F emissions averaging 1% 370, 750 30, 120
2.0d 2.4 1.7d 4.8d
12.2, 14.2 12.3 13.7, 13.7 8.0, 7.8
0 0 0 0
4, 5 ND 12, 10 4, 2
14, 18, 31 38
none none
none none
50, 57 67 75, 81 87, 93
1,2-diClBz 1,2-diClBz 2,4-diClPh none (natural gas firing)
Cu Naph Cu Naph Cu Naph
107, 114 120, 126 132 138 147
1,2-diClBz 2,4-diClPh 2,4-diClPh 2,4-diClPh 2,4-diClPh
Cu Naph Cu Naph Cu Naph Cu Naph Cu Naph
Formation Phase 535 240, 400 95 320, 480 165 165 416 19 451 12
1.5d 1.5d 1.3 2.9 2.9
15.4, 15.2 15.3, 15.4 15.4 13.8 12.5
0 10 0 0 0
300, 307 257, 139 287 1953 588
153, 158 553
none none
none none
Decontamination Phase 0 6, 7 0 8
3.1d 2.8
13.1, 12.4 14.0
29d 34
111, 134 1
558 564 577 581 586 592 598
HCl 2,4-diClPh 2,4-diClPh 2,4-diClPh 1,2-diClBz 1,2-diClBz 1,2-diClBz
none none Cu Naph Cu Naph Cu Naph Cu Naph Cu Naph
Verification Phase 218 8 466 5 411 4 375 140 538 5 97 7 408 7
2.7 2.7 3.1 1.4 3.1 2.9 3
14.5 14.3 14.2 15.3 14.5 14.1 14.0
0 0 0 0 0 0 99
4 6 51 98 73 11 20
a
All concentrations reported dry at 7% O2.
b
520 540 160 0
Italicized test hours are LRMS analyses. c Mono- to octa-CDD/F.
Results and Discussion The four Baseline runs had nondetectable PCDD/F formation, even for the fourth run (38 h) when combustion was poor and [CO] >1%. This suggests that trace levels of Cu and Cl in #2 fuel oil are insufficient to result in detectable formation. Tests during the Sooting phase aimed to determine whether combustion conditions or type of simulated waste promoted high PCDD/F yields. Runs 50, 57, and 67 h, in which 1,2dichlorobenzene with copper naphthenate was cofired with #2 fuel oil, were conducted under relatively poor combustion conditions. These conditions resulted in highly sooting, broad flames (that impinged on the walls of the Pass one firing tube), relatively low CO2 levels, and high levels of PAHs. The first seven emission samples collected during the initial 67 h of NAPB run time (both Baseline and Sooting phases) did not result in formation of any significantly quantifiable (via LRMS) levels of PCDD/F. Extension of the analytical methods to HRMS for the next two runs (75 and 81 h) likewise did not indicate appreciable formation. This suggests that waste firing under poor combustion conditions leading to high CO levels, visibly sooting flames, and low CO2 does not necessarily lead to high PCDD/F yields. Runs at 87 and 93 h were conducted without waste cofiring and with #2 fuel oil replaced by methane (CH4) to determine if potential emissions from the sooting phase continued and, if so, how long these emissions would persist (the reader is reminded that samples were only analyzed between the last three test phases, so previous runs’ results were unknown). The low emissions from earlier runs at 75 and 81 h declined slightly, but did not cease, to 4 and 2 ng/dscm. Prior to sampling at run time 107 h, the fuel injection nozzle was realigned to be coaxial with the first pass, the atomization air was adjusted to result in lower CO, and the firing rate was reduced up to 20% to minimize flame impingement on the near-burner, inner wall of the first pass. This had the effect of improving the combustion conditions of the boiler, reflected by higher CO2 levels and (typically)
d
Average value for tests in row.
reduced CO, PAHs, and visible soot formation. Subsequent emission results (Figure 2) from 107 to 147 h showed significantly higher levels of PCDD/F formation. All but one run had PCDD/F emissions > 250 ng/dscm. The high PCDD/F formation rates are similar to those observed from previously mentioned work with a chlorinated waste fired through a Cu- and soot-laden tube (5). The highest formation, at 138 h, was 1953 ng/dscm, which exceeded 49 ng I-TEQ/dscm. This value is over 2 orders of magnitude higher than proposed regulations for hazardous waste incinerators in the U.S. The next test, at 147 h of firing time, was run under virtually identical conditions, although it was chronologically 10 weeks later. Its emissions dropped 3-fold, to 588 ng/dscm (≈15 ng I-TEQ/dscm). The deposits also showed extremely high levels of PCDD/F: samples from the Pass 2 deposits (138 h) exceeded 16 000 ng/g of PCDD/F (see Table 2). These test phases indicated that either (1) poor combustion conditions, resulting in highly sooting conditions, are not necessarily sufficient to produce PCDD/F levels of concern or (2) sufficient time of operation may be necessary to establish a surface deposit environment that is amenable to PCDD/F formation. Hypothesis #1 is consistent with the observation that sufficient O2 is necessary for PCDD/F formation (11-13). It is possible that (a) in an O2-limited environment, high concentrations of higher molecular weight hydrocarbons are formed which compete for Cl with PCDD/F precursors, (b) in a fuel-rich, O2-starved environment, the Cl contained in the dopant feed is converted mostly to HCl which is less likely to chlorinate organics, or (c) production of molecular chlorine (Cl2) or Cl radical via the Deacon reaction (14) is limited by O2 availability. In fuel-lean flames, although less hydrocarbon molecular growth and less products of incomplete combustion (PICs) are produced, the excess O2 increases the Cl2 (15), which in turn might have a positive effect on PCDD/F formation. Hypothesis #2 suggests that the presence of sufficient surface deposits acts as a sink and mediating surface for all reactants. Pass 2, as VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. PCDD/F concentrations in the emissions and Pass 2 deposits (five data points only) during the Sooting (75-93 h), Formation (107-147 h), and Decontamination (153-553 h) phases (results prior to 75 h showed little discernible formation and are omitted).
TABLE 2. Deposit Sampling Time and Results soot deposit PCDD/F concna (ng/g)
soot deposit PCDD/F concna (ng/g)
test hour Pass 1 Pass 2 Pass 3 test hour Pass 1 Pass 2 Pass 3 87 138 158 298 378 a
393 0 2 253 16203 1512 524 11549 3936 1669 18409 3660 210 8211 4509
548 558 564 577 598
190 227 241 113 97
1464 2778 238 196 165
435 1595 36 304 105
LRMS analyses.
shown later, is the region where the highest concentrations of PCDD/F, Cl, and Cu are found in the soot deposits. Thus, the PCDD/F emissions are shown to be dependent not only on a temperature window favorable to their formation but also on the soot deposit layer and copper/chlorine concentration. The extremely high emission values obtained during run 138 h (0.2000 ng/dscm) suggest that very high emission levels are possible from these boiler sources during moderate cofiring combustion conditions when a sufficient reactive surface is formed after a certain boiler operation time. These ideas are supported by similar findings by Lee et al. (5). While the potential for these emission levels are clearly specific to the operating conditions used, these results do suggest that boiler designs which have high potential for sooting will also have high potential for PCDD/F formation. The molar PCDD/F emission rate from the 138 h run is roughly equivalent to 1 × 10-6 times its molar feed rate of 2,4-diClPh. Despite fairly consistent run and feed conditions, the changes in PCDD/F emission levels during the Formation phase (up to 10×) suggest that formation conditions are actually fairly dynamic. The waste feed conversion rate varied up to 2 orders of magnitude throughout the test program, suggesting that observed emission levels are not a function solely of waste or Cl feed rate but, rather, are affected by other conditions such as combustion efficiency and/or surface effects. During the Sooting and Formation phases, the tetra- to octa-CDD/F homologue profiles shift with the onset of significantly large emissions. Observation of representative homologue emission profiles from 87 to 147 h (Figure 3) shows that the tetra-CDD-dominated profiles (87 h) gradually give way to profiles dominated by higher chlorinated DD homologues. Similarly, the PCDFs evolve to a profile dominated by the HxCDF and HpCDF homologues. The shift to higher chlorinated PCDD/F homologues with time was also observed in the related fixed bed tests of Lee et al. (5). The 2072
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profiles of the deposits reflect those of the emissions, despite large vapor pressure differences between the homologues. Even after stopping the chlorinated waste feed at 147 h, the 153 and 158 h emission homologue profiles show little change (not shown) and, in fact, are quite similar to the profile at 553 h. This suggests that there are no significant mechanistic changes in the absence of waste feed. These emission and deposit profiles persist throughout the remainder of the testing, despite large variations in PCDD/F formation. Subsequent runs were conducted without hazardous waste cofiring (no diClPh, diClBz, or Cu ) so that only #2 fuel oil was fired (the Decontamination phase). The reduced but significant levels of PCDD/F emissions indicated both the extent of the residual, or memory, effect on the PCDD/F emissions and the persistent reactivity of the deposits. After 11 h of run time, emissions had dropped considerably but persisted at >100 ng/dscm (tests 153 and 158 h in Figure 2). Indeed, approximate calculations suggest that the rate of post-waste-firing emissions (the average emissions from runs 153 and 158 h) should reduce the boiler deposit concentrations (taken from the 138 h run, Table 2, Pass 2) to zero within 100-200 h for PCDD and 60-70 h for PCDF, given no further PCDD/F formation reactions. However, the concentration of PCDD/F in the tube deposits, measured by sampling tube scrapings, showed continued high concentrations (samples at 158, 298, and 378 h) even though the chlorinated organic waste feed was ceased after 147 h. Over 150 h after cessation of chlorinated organic feed, the 298 h deposits indicate a PCDD/F concentration similar to that observed after the highest emitting test at 138 h (order of magnitude comparisons only are appropriate given the uncertainties in the soot deposit sampling procedures and analysis). Only when post-Cl-feed run time reached 300 h (run 548 h), did the Pass 2 solids concentration reduce below 2000 ng/g. These yield and homologue results from the gas and solid samples show conclusively that the solid deposits act as a sink and source for continued PCDD/F formation and that gas-phase emissions result from PCDD/F vaporization and/or surface mediated reactions and subsequent vaporization. The soot deposits may serve as the source of PCDD/F emissions. A recent study (16) suggested that PAHs, which are fuel byproducts often attached on soot particle surfaces, are a carbon source for formation of polychlorinated aromatics such as PCDF and polychlorinated naphthalenes. Alternatively, or concurrently, the soot deposits may serve as an adsorption substrate for any undestroyed diClPh or diClBZ which, in turn, may undergo condensation reactions to form PCDD/F (see ref 7).
FIGURE 3. Homologue profiles for PCDD/F emissions (lines) from the early and late stages of the Formation phase compared with solid deposits (bars, Pass 2).
FIGURE 4. PCDD/F concentrations in the emissions and Pass 2 deposits (four data points only) during the Verification phase (run 553 h is the last emission sample from the Decontamination phase). A final “Verification” phase was run to determine the reproducibility of the PCDD/F formation and to begin to isolate causal emission factors. Results from sequential introduction of Cl by source (HCl then diClPh), metal catalysts, alternate organic waste source (diClBz), and SO2 suppressant are shown in Figure 4. Introduction of HCl (558 h) and then 2,4-diClPh (564 h) both resulted in very minor increases in the PCDD/F yield, without yielding a distinction between the Cl sources. These results suggest that the reactant-depleted deposits did not immediately return to their previous reactivity with only the introduction of a chlorine source, HCl. However, addition of Cu naphthenate to the 2,4-diClPh (577, 581 h) resulted in significant increases in PCDD/F levels. This suggests that the PCDD/F formation is Cu-limited at this point, either by the ability of the Cu to act as a catalyst and/or its ability to sequester then serve as a transfer agent for Cl. Changing the organochlorine source to 1,2-diClBz (586 h) resulted in a likely insignificant change in PCDD/F formation. A 4-fold reduction in the 1,2-diClBz feed rate and Cu feed rate (592 h) resulted in a 4-fold drop in PCDD/F levels, suggesting that changes in waste feed rates can have a substantive, first order effect on PCDD/F emissions even in the presence of potential concurrent memory effects. This sensitivity to waste feed rate was not observed in the Formation phase, suggesting that reactivity conditions then, and the high PCDD/F levels observed, were more likely mediated by surface effects and less sensitive to changes in
the availability of gas-phase Cl or Cu. Return of the 1,2diClBz feed rate to the 586 h level plus addition of SO2 to 100 ppm (598 h) resulted in a 73% lower yield than the 586 h test, showing that SO2-induced prevention mechanisms (5, 1720) have at least a prompt (1-4 h) effect on formation. Verification of the deposit sink/source effect would suggest that emissions can be substantially affected by the influence of firing conditions (such as periodic SO2 increases) on deposit properties for some time prior to the current sampling test, as has been found in field scale municipal waste combustion (20). Overall, the emissions during this Verification phase did not reach levels observed throughout the Formation phase, suggesting that the presence of a Sooting phase and the subsequent build up of reactive deposits are important to subsequent PCDD/F formation rates. Prior to firing changes at 107 h, only one deposit sample was analyzed (at 87 h); it showed that PCDD/F formation (about 400 ng/g) occurred only in Pass 1 (see Table 2). With the change in firing conditions at 107 h of operation, the highest PCDD/F deposit concentrations were typically observed in Pass 2. Conditions for formation and/or deposition/adsorption appear to be optimal in this region perhaps due to the build up of the deposits. Pass 2 concentrations (up to 18 000 ng/g) were approximately 2-10 times the concentrations observed in Pass 3 and 20-60 times the concentrations observed in Pass 1. The flue gas temperature values in the three passes were greater than 600 °C for Pass VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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1, 600-300 °C for Pass 2, and 300-130 °C for Pass 3. Pass 2 appears to be in the oft-cited optimal temperature region for PCDD/F formation (21). The large differences in deposit PCDD/F concentrations in each pass are likely related to effects of the gas temperature on gas phase species and concentrations as well as the effect of surface temperature on adsorption and reaction of precursors. XRF analyses of soot deposit samples, taken after the emissions sample with the highest PCDD/F formation (138 h), showed that the primary metals were iron (Fe), likely from oxidation of the tube surfaces, and Cu, from the waste feed. ICP analyses indicated that the Pass 2 Cu concentrations was 83.70 mg/g, while those in Pass 1 and Pass 3 were 5.81 mg/g and 19.7 mg/g, respectively. XRD analyses identified these to be primarily CuCl, with some Cu(OH)Cl. XRF analysis verified that Cl followed the Cu trends. After an additional 400 h of undoped #2 fuel oil firing (test 548 h), the sampled Cu concentrations in the soot deposits in Passes 1, 2, and 3 were, respectively, unchanged, 6.13 mg/g, and 1.45 mg/g, reflecting significant reduction. Similar reduction in the Pass 2 Cl concentration from 54.2 to 6.6 kilocounts/s (kcps) was noted, confirming the correlation of Cu and Cl concentrations in the deposits with high PCDD/F concentrations.
Acknowledgments The authors wish to thank Shirley Wasson (U.S. EPA/NRMRLRTP) for the XRD and XRF analyses and Dennis Tabor and Bill Preston (Arcadis G&M) for the HRGC/LRMS analyses.
Literature Cited (1) U.S. EPA. Combustion Emissions Technical Resource Document, EPA/530-R-94-014; Office of Solid Waste and Emergency Response: Washington, DC, May 1994. (2) U.S. EPA. The Inventory of Sources of Dioxin in the United States, Review Draft; EPA/600/P-98/002Aa (NTIS PB98-137037); Office of Research and Development: Washington, DC, April 1998. (3) De Fre, R.; Rymen, T. Chemosphere 1989, 19, 331-336. (4) Halonen, I.; Tuppurainen, K.; Ruuskanen, J. Chemosphere 1997, 34(12), 2649-2662.
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(5) Lee, C. W.; Kilgroe, J. D.; Raghunathan, K. Environ. Eng. Sci. 1998, 15(1), 71-84. (6) U.S. EPA. Test Method 0023A, “Sampling Method for Polychlorinated Dibenzo-p-Dioxins and Polychlorinated Dibenzofuran Emissions From Stationary Sources, in Test Methods for Evaluating Solid Waste; SW-846 (NTIS PB88-239223); Environmental Protection Agency, Office of Solid Waste and Emergency Response: Washington, DC, December 1996. (7) Ghorishi, S. B.; Altwicker, E. A. Haz. Waste Haz. Materials 1996, 13, 11-22. (8) Method 23. In Title 40 Code of Federal Regulations Part 60, Appendix A; U.S. Government Printing Office: Washington, DC, 1991. (9) Method 8290. In Test Methods for Evaluating Solid Waste, Volume 1B: Laboratory Manual Physical/Chemical Methods, 3rd ed.; EPA-SW-846 (NTIS PB88-239223); U.S. Environmental Protection Agency: Washington, DC, 1986. (10) Gullett, B. K.; Ryan, J. V.; Tabor, D. Organohalogen Compds. 1999, 40, 121-124. (11) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 1373. (12) Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987, 21, 1080-1084. (13) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29(6), 15861590. (14) Tilly, J. J. Chem. 1981, 55, 2069-2075. (15) Procaccini, C. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1999. (16) Iino, F.; Imagawa, T.; Takeuchi, M.; Sadakata, M. Environ. Sci. Technol. 1991, 33, 1038-1043. (17) Lindbauer, R. L.; Wurst, F.; Prey, T. Chemosphere 1992, 25, 14091414. (18) Raghunathan, K.; Gullett, B. Environ. Sci. Technol. 1996, 30(6), 1827-1834. (19) Gullett, B. K.; Raghunathan, K.; Dunn, J. E. Environ. Eng. Sci. 1998, 15(1), 59-70. (20) Gullett, B. K.; Dunn, J. E.; Raghunathan, K. Environ. Sci. Technol. 2000, 34, 282-290. (21) Stieglitz, L.; Zwick, G.; Beck, J.; Roth, W.; Vogg, H. Chemosphere 1989, 18(1-6), 1219-1226.
Received for review October 19, 1999. Revised manuscript received February 10, 2000. Accepted February 29, 2000. ES991196+
