Environ. Sci. Technol. 1992, 26, 1938-1943
Commerce, National Technical Information Service: Washington, DC, 1986; Vol. 1B. Spittler, T. M.; Clifford, W. S.; Fitch, L. G. National Water Well Association Meeting, Nov, Denver, CO, 1985; pp 236-246.
Kiang, P. H.; Grob, R. L. J. Environ. Sei. Health 1986,21, 71.
Griffith, T.J.; Robbins, G. A.; Spittler T. M. Proceedings, FOCUS Conference on Eastern Regional Water Issues, National Water Well Association, Sept, Stamford,CT, 1988; pp 223-248. Robbins, G. A.; Roe, V. D.; Stuart, J. D.; Griffith, T. J. Proceedings, NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water-Prevention Detection and Restoration, Nov, Houston, TX, 1987; pp 307-315. Stuart, J. D.; Wang, S.; Robbins, G. A. Second International
Symposium,Field Screening Methods for Hazardous Waste and Toxic Chemicals, Feb, Las Vegas, NV, 1991; pp 407-414.
Dietz, E. A,, Jr.; Singley K. F. Anal. Chern. 1979,51,1809. Hewitt, A. D.; Miyares, P. H.; Leggett, D. C.; Jenkins, T. F. SR91-4; USA Cold Regions Research and Engineering Laboratory, Hanover, NH, 1991. Lewis, T. E.; Crockett, A. B.; Siegrist, R. L.; Zarrabi, K. EPA/590/4-91/001; Technology Innovation Office, Office of Solid Waste and Emergency Response, U.S. EPA, Washington, DC, 1991. Jenkins, T. F.; Schumacher, P. W. SR87-22; USA Cold Regions Research and Engineering Laboratory, Hanover, NH, 1987. Verschueren, K. Handbook of Environmental Data on Organic Chemicals; Van Nostrand Reinhold: New York, 1983.
(16) McGovern, E. W.Ind. Eng. Chern. 1943,35, 1230. (17) Hansch, L.; Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biology; Elsevier: Amsterdam, 1979. (18) McDuffie, B. Chemosphere 1981,10, 73. (19) Hine, J.; Mookerjee, P. K. J . Org. Chern. 1976,40, 292. (20) Roberts, P. V.; Dandliker, P. G. Environ. Sci. Technol. 1983, 17,484. (21) Pavlostathis, S.G.; Jaglal, K. Enuiron. Sci. Technol. 1991,
25,214. (22) Spittler, T. M., personal communication, U.S. Environmental Protection Agency, Environmental ServicesDivision-Region 1,Lexington, MA, 1989. (23) Dixon, W. J. Biometrics 1953,March, 74. (24) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979,13,241. (25) Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983,17,227. (26) Boyd, S. A,; Sun, S. Environ. Sci. Technol. 1990,24,142. (27) Hewitt, A. D.; Miyares, P. H.; Leggett, D. C.; Jenkins, T. F. CR92-6; U.S. Cold Regions Research and Engineering Laboratory, Hanover, NH, 1992. (28) Smith, J. A.; Chiou, C. T.; Kammer, J. A.; Kile, D. E. Enuiron. Sci. Technol. 1990,24, 676. (29) Sawhney, B. L.; Gent, M. P. N. Clays Clay Miner. 1990, 38,14.
Received f o r review January 27,1992. Revised manuscript received M a y 13,1992.Accepted June 16,1992.Funding for this work was provided by the U.S. A r m y Toxic and Hazardous Materials Agency, Durant Graves, Project Monitor. This publication reflects the personal views of the authors and does not suggest or reflect policy, practices, programs, or doctrine of the U.S. A r m y or Government of the United States.
Effect of Sulfur Dioxide on the Formation Mechanism of Polychlorinated Dibenzodioxin and Dibenzofuran in Municipal Waste Combustors Brian K. Gullett* Air and Energy Engineering Research Laboratory, US. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
Kevin R. Bruce and Laura 0. Beach Acurex Corporation, P.O. Box 13109, Research Triangle Park, North Carolina 27709
w The effect of sulfur dioxide on the formation mechanism of polychlorinated dibenzodioxin (PCDD) and polychlorinated dibenzofuran (PCDF) in the postcombustion, downstream region (500-300 "C) of a municipal waste combustor (MWC) was investigated. Laboratory experiments simulating the flue gases and particle environment of an MWC examined PCDD production under varying conditions. Effects on the concentration of an organicchlorinating constituent, Clz, through both homogeneous reaction with SOz and deactivation of a Cl,-forming catalyst [Cu(II)] were examined. Experimental results suggest that the reaction of Cu(I1) with SOpto form CuS04 renders the catalyst less active, decreasing PCDD formation. However, this inactivity is not a result of decreased Clz formation, but rather of reduced ability of Cu(I1) to promote a second catalytic step of biaryl synthesis. These findings suggest that the apparent lack of PCDD and PCDF in the emissions from coal-fired combustors may be due to the relatively high concentrations of SOz. 1. Introduction
While significant amounts of polychlorinated dibenzodioxin (PCDD) and polychlorinated dibenzofuran (PCDF) 1938
Environ. Sci. Technol., Vol. 26, No. 10, 1992
have been detected in the emissions of municipal waste combustors (MWCs), the same cannot be unequivocally said of emissions from coal-fired combustors. Despite the presence of chlorine and organic ring structures in both systems and the ability of coal fly ash to aid chlorination of organics ( I ) , tetrachlorinated dibenzodioxin (TCDD) was not detected in effluent sampling from a combined coal/municipal waste plant (2), nor were noteworthy amounts found on coal fly ash (3). These results, however, are inconsistent with laboratory studies ( 4 ) producing chlorodioxins from experiments with bituminous coal. It is possible that conditions within MWCs are sufficient to promote the in situ formation of PCDD and PCDF, while this is not true of coal-fired combustors. Examination of the similarities and differences between conditions in MWCs and coal-fired combustors will thus not only enlighten the mechanism of PCDD and PCDF formation but also may suggest potential methods for preventing the formation of PCDD and PCDF. Laboratory research has successfully produced PCDD and PCDF through simulating postfurnace conditions of MWCs. Experiments with MWC fly ash have shown substantial PCDD and PCDF formation when treated
0013-936X/92/0926-1938$03.00/0
0 1992 American Chemical Society
under temperature and gas composition conditions s i m i i to those of the downstream MWC environment (5). A theory has been proposed (6) and tested (7) that shows formation occurs due to de novo synthesis from compounb within the flue gas and fly ash. This synthesis involves the Deacon process reaction:
C"
2HC1+ yzOz
ClZ+ HzO
-
2HCI-k SO3
(2)
agent, Cl,, into a form (HC1) leas likely to undergo ammatic Substitution reactions to form PCDD and PCDF precursors. The second postulated role of S is to react with the Cu-based Deacon catalysts CUO + so, + y 2 0 ,
-
cuso4
EXHAUST
PROCESS
GAS
!
(1)
from which the Clz produced subsequently chlorinates aromatic ring structures through substitution reactions. A PCDD and PCDF source of chlorine has been confirmed as Clz (8), and the latter's derivation from the Deacon process has been determined (9) [an additional mechanism for aromatic chlorination involves surface-bound iron(II1) chloride as the chlorinating agent (IO),albeit a t low temperatures (80 "C)]. The carbon source is derived from unhurnt, particulate matter (5) reacting with Oz and Clz to form PCDD and PCDF. An alternative theory suggests that the carbon source is derived from high-temperature radical reactions during combustion ( 1 1 ) which reacts a t lower temperatures to form PCDD and PCDF (12, 13). This theory indicates the catalytic involvement of fly ash constituents in forming biaryl structures from chloroorganic compounds (12,14). This research has examined two possible mechanisms for limitation of PCDD production, both involving the role of sulfur species. The presence of S species (predominantly in the form of SOz) in the emissions of coal-fired utility power plants provides a significant distinction from those of MWCs. While a typical power plant burning a 2% S coal would be expected to have a postfurnace SOz concentration of -1500 ppm, most MWCs experience SOz emissions -1 order of magnitude less (-200 ppm). The fmt postulated role of S is to undergo homogeneous reaction 2, thereby converting the primary chlorinating Clz + SOz + HzO
.
FLOW CONTROL PANEL
(3)
to alter the form of the Cu-based catalyst and, presumably, its ability to produce C1, through reaction 1. Perhaps the first inference to this mechanism is in a US.patent (15) that claims inhibition of catalytic activity through introduction of S-based compounds (e.g., CSz and SOz) and subsequent reduction of PCDD formation. While the patent does not disclose a mechanism, it illustrates several tests in which CS, or C4SH, (thiophene) was added to fly ash and exposed to heat (150-400 "C) and pentachlorophenol. In both cases, production of tetra- to hexachlorodioxins was decreased. A third possibility for apparently low PCDD and PCDF emission levels in coal combustion is due to the relatively lower concentrations of Cu found in coal than municipal waste. Researchers (16) have found 36 times more Cu in refusederived fuel than in coal. However, since the majority of this Cu is found in the bottom ash, it will not reach the postfurnace zones where it can effect downstream PCDD and PCDF Synthesis. This is supported by model calculations and actual measurements which show, respectively, 99% and 89% of the Cu in municipal waste to end up in the residual bottom ash (17). This apparent insensitivity of PCDD and PCDF emissions to concen-
! TO DRY GAS
:... !
METER
_-
2% KI SOLUTION Flgura 1. Reactor system for
duction tests.
organic PCDD and inwganic CI,
pro-
trations of Cu in the fuel feed, also reflected by experimental work ( 1 3 , underscores that low emissions from coal combustion are not likely linked to, or limited by, the amount of Cu in the fuel. A fourth possibility is that the presence of SOz gas affects the PCDD and PCDF mechanism by either sulfonating the phenolic precursors, thereby preventing subsequent chlorination and hiaryl synthesis, or replacing the oxygen link(s) with S and forming polychlorodibenzothiophene (PCDT) and polychlorothianthrenes (PCTA), the S analogs of PCDD and PCDF (18). The research reported in this paper indirectly examines the apparently low PCDD and PCDF levels in coal-fired utility boilers through laboratory examination of the effect of SOz species upon the mechanism of PCDD synthesis in MWCs. Experiments were conducted in a reactor system that simulates the MWC postfurnace gas and temperature characteristics.
2. Experimental Section Tests were run in a concentric tube, quartz reactor inserted into the horizontally mounted, single-zone,electric furnace shown in Figure 1. The catalyst of interest (1g of CuO or CuSO,) was embedded in a quartz wool bed, placed in the center of the inner tube of the reactor, and then heated to the test temperature (300-500 OC for PCDD tests). Process gases were preheated by flowing through the outer quartz tube and into the inner tube reaction zone. The process gases were composed primarily of 10% O2in N,. The total process gas flow rate was set to yield a reador residence time of 10 s, while the run duration was 30 min. Low concentrations (2(Ht1000 ppm) of HC1, Cl,, or SOz from compressed gas cylinders were added to various experiments, as was vapor-phase phenol a t a level of -500 ppm. Phenol was added by passing N2 through a phenol-filled vessel submerged in a controlled-temperature water bath. Other tea& used a chlorophenol mix in toluene consisting of 6 X mol each of 2,3,4-trichlorophenol, 2,3,5-trichlorophenol, 2,3,6-trichlorophenol, 2,3,5,6-tetrachlorophenol, and pentachlorophenol that was added to the catalyst plug in the reactor. To minimize the heatup time of the reactants in these tests, the reactor furnace was preheated to the desired temperature and then the reador tube with chlorophenol-laden catalyst plug was inserted. Envhon. Sci. Technol.. VoI.
26, No. 10, 1992
IS39
Thermogravimetric tests were conducted with phenol to simulate the possible extent of chlorophenol vaporization during runs. While considerable vaporization of the phenol occurred, the residual molar amount was still 2 orders of magnitude greater than the maximum amount of PCDD formed in our experiments. Because chlorophenols have a higher boiling point, the expected vaporization will be even less. Thus, if the mechanism of reaction is somehow dependent upon substantial residence time of chlorophenol on the plug, we expect sufficient quantities of chlorophenol remaining on the catalyst to ensure that the reaction will not be limited by reactant supply. If this were not true, PCDD formation levels could only be compared between experiments at similar temperatures and, therefore, similar amounts of vaporization. Figure 1 shows alternate gas absorption trains for inorganic or organic sampling. The inorganic sampling train, designed for quantifying Clz, consists of two impingers filled with a buffered potassium iodide solution as described in Fisher et al. (29). C12reacts quantitatively with the KI solution to form I,, which is in turn titrated with a standardized sodium thiosulfate solution using a starch indicator. In the organic experiments using phenol, the reactor effluent was bubbled through two ice-cooled impingers flled with toluene. After a 30-min reactor run, the catalyst plug was cooled, removed from the reactor, and placed in an extraction thimble, where it was spiked with l3CI2-labeled internal PCDD standards. The reactor and all the lines of the sampling system were rinsed with toluene, and the rinsate was combined with the liquor from the impingers. All of this liquid was added to a Soxhlet extractor, in which the catalyst plug was extracted overnight. Following this step, the extract was concentrated and solvent-exchanged to hexane. The sample was then cleaned using, in order, acid/base silica, alumina, and carbon columns. It was then concentrated to 1mL, transferred to a 2-mL microconcentration tube, and spiked with a 6Clz-labeledTCDD recovery standard. The sample was then concentrated to a final volume of 100 pL. Injections of 2 p L were run on a Hewlett-Packard 5890 gas chromatograph (GC) equipped with a Hewlett-Packard 5970 maw selective detector and a 30-m DE35 column from J&W Scientific and quantified in accordance with the guidelines in method 8280 (20). Analyses monitored levels of PCDDs rather than PCDFs since use of phenol as an organic source preferentially produces PCDDs (21). Errors in experiments may arise from temperature control a t the catalyst plug location; uneven distribution of catalyst or organic reactants on the quartz wool plug; and sample loss due to incomplete reactor rinse, Soxhlet extraction, and column cleanup. X-ray diffraction (XRD) analyses for compound identification were performed on a Siemens Model D-500 diffractometer with a copper target source running a t 50 kV and 40 mA. The entrance aperture was 1.0" and the detector slit was 0.05". A scintillation detector with a graphite monochrometer was used. Spectra were identified by computer comparison with the Joint Committee for Power Diffraction Spectra (JCPDS) spectral files. 3. Results
The possible effect of SOz upon formation of Clz from HC1 (reaction 1, Deacon process) was examined both homogeneously and heterogeneously. The homogeneous reaction of Cl, and SO2 (reaction 2) was monitored in the reactor over temperatures ranging from 400 to 800 "C with a 10-s residence time. Gas concentrations for Clz and SO2 were 200 and 1000 ppm, respectively, while the H20con1940 Environ. Sci. Technoi., Vol. 26, No. 10, 1992
p- 0.6 -E 0
N 0
0.5 0.4
E x
"r 0.3
z
g
0.2
E5 -
2
0.1
0
0
0.0 300
340
380
420
460
TEMPERATURE
500
540
580
(OC1
Flgure 2. Temperature response of the Deacon process, reaction 1 (1 g of catalyst, 1000 ppm HCI, 10% 02,balance N2).
-
tent was 10%. The GC traces indicate the presence of a minor SO3 peak when temperatures reach 800 "C. Higher C12 concentrations (-1000 ppm) a t 800 "C were necessary to confirm the production of detectable SO3 formation. Although equilibrium calculations suggest that the reaction is favored over the full range of temperatures tested, the kinetics of reaction 2 may prevent observation of measurable product until testing at the higher temperatures. The effect of SO, upon the heterogeneous Deacon process reaction (reaction 1) was examined with Cu-based catalysts in the presence of HC1. Tests with CuO as the Deacon process catalyst and CuSO, functioning as a Spoisoned Deacon catalyst were conducted in the presence of nominal 1000 ppm HC1. CuSO, was assumed to have formed from the heterogeneous reaction, reaction 3. The results for CuO (Figure 2) show the temperature response typical of Deacon-active catalysts (9);conversion of HC1 to Clz peaks a t a temperature around 400 "C. The use of CuS04&s the S-poisoned Deacon process catalyst resulted in production of Clz concentrations quite similar to those with CuO. However, the peak temperature of formation increased significantly to -500 "C. XRD analysis of this catalyst plug indicated that the CuSO, was preservedDeacon activity was not due to decomposition to other CuO-related forms, as found with CuCl and CuC1, (21). This was also found to be true in the presence of phenol-a potential reducing agent for CuSO,. XRD testa on reactor catalyst/quartz wool plugs were conducted to verify the formation of CuSO, (through reaction 3) within the CuO catalyst matrix. XRD analysis of the reactor catalyst/quartz wool plugs tested at 300,500, 700, and 900 "C with a CuO catalyst and gaseous SOz(lo00 ppm) mixed with the HCl (1000 ppm) and Oz (10%)resulted in little detectable CuSO, formation except a t temperatures of 500 "C. Thermogravimetric analysis (TGA) tests on CuSO, indicate decomposition at temperatures in excess of 615 "C. This is consistent with others' findings (22) that the decomposition temperature of CuSO, was between 572 and 678 "C. The effect of S species upon the production of tetrathrough octachlorinated PCDD congeners was examined with reactor tests at three temperatures (300,400, and 500 "C) and three combinations of reactants (CuO/HCl/ phenol, CuO/HC1/S02/phenol, and CuSO,/HCl/phenol) for a total of nine permutations. The temperatures spanned the range of maximum PCDD production (300-400 "C) and Deacon process activity (400 "C for CuO and 500 "C for CuSO,). The CuO/HCl/phenol results (Figure 3) for 300, 400, and 500 "C verify the PCDD production mechanism
1 .OE' 1
w cuso,
, O E d
I
.-6
+
1.OE:
a, ,I-
-J
a 2
1.OEE
1
z 0 I0
TETRA
HEXA
HEPTA
OCTA
PCDD CONGENER CLASS
3 Q
Flgure 5. PCDD production by congener at 300 "C. All tests in duplicate, indicated by range of data (1 g of catalyst, 500 ppm CI, 10% O,, balance N2).
2a
n n
PENTA
1.OE4
1.0~4
2
1 .OE3
1 .OE2
TETRA PENTA
HEXA HEPTA
OCTA
PCDD CONGENER CLASS Flgure 3. PCDD production by congener at 300, 400, and 500 "C (from top to bottom, respectively). Tests wlth three or more repeats indicated by the average and standard deviation bars. Tests with duplicates indicated by range of data [ 1 g of catalyst, 1000 ppm HCI, 10% O,, 1000 ppm SO, (where applicable), balance Ne. BDL, below detection limit]. TETRA 1.OE7
0 CuO with
3
,U
n .K
1.OE5
f
SO
CuSO, with HCI
A
0
I
0
t I
1.OE2
400 TEMPERATURE (OCI
HEPTA
OCTA
Figure 6. PCDD production by congener at 400 "C. All tests with three or m e repeats, Indicated by the average and standard devlatlon bars (1 g of catalyst, 500 ppm C12, 10% O,, balance N,).
4, where the total PCDD production from Figure 3 is
1 .OE3
300
HEXA
PCDD CONGENER CLASS
HCI
A CuO with HCI and
1.OE6
PENTA
500
F w e 4. Total PCDD productlon at 300, 400, and 500 "C. Tests with three or more repeats indicated by the average and standard devlatlon bars. Tests wtth duplicates indicated by range of data [ 1 g of catalyst, 1000 ppm HCI, 10% O,, 1000 ppm SO, (where appllcable), balance N21.
through the successful synthesis of these materials under conditions simulating downstream MWC flue gas. The results also show maximum formation in the 300 and 400 "C tests and little formation a t 500 "C,consistent with research of others (5). The CuO/HC1/S02/phenol tests similarly exhibit PCDD production at 300 and 400 "C. However, the results show little apparent change in formation from the above tests without SOz. Very little PCDD is noted a t 500 "C. The CuS04/HCl/phenol results indicate reduced formation of PCDD a t all three temperatures, especially a t 300 and 500 "C. While CuS04 tests a t 500 "C have been shown to produce Clz (Figure 2), the lack of PCDD production in Figure 3 a t 500 "C is not surprising, since previous results (8) indicate only minor synthesis a t this temperature. These results are more clearly seen in Figure
combined. The catalytic activity, albeit reduced from that of CuO, is entirely attributable to CuS04 since XRD analysis of the plug found no other Cu species present. While definitive results require an integrated study from 500 to 300 "C (representing the cooling flue gases), our present experimental capabilities only allow for isothermal testing. Nonetheless, it is possible to examine the synthesis of PCDD under the presumption of upstream formation of Clz from a CuO or CuS04 catalyst. Tests were performed in which the Deacon process Clz production was decoupled from the integrated mechanism of Clz production and PCDD synthesis. Tests simulating upstream Clz formation via the Deacon process (HC1 in the process gas was replaced with C12)were conducted at 300 and 400 "C with CuO/Clz/phenol and CuS04/C12/ phenol. Results (Figures 5 and 6) indicate significant (albeit variable) PCDD production in the CuO testa, with noticeably reduced levels in the CuS04 tests a t 300 "C (Figure 5) and minor reductions in the CuS04tests at 400 "C (Figure 6). Blank tests run under similar reactor conditions but with only an inert quartz wool plug (no Cu-based catalyst) resulted in little detectable PCDD production. The last series of tests decoupled the PCDD synthesis mechanism further by eliminating both the Clz production step and the step in which C1 binds to the aryl structure. The results of testing the chlorophenol mix with CuO and CuS04 show the effect of catalyst choice on the PCDD yield. Figure 7 indicates that the PCDD yield from the Environ. Sci. Technol., Vol. 26, No. 10, 1992 1941
1 OE7
rinating agent through direct homogeneous reaction with SOZ.
i OE6
0 -
FZ v.= 3
n
1 OE5
2 :a.
a 2
1
OE4
0
o g
V
a
i.OE3
1
OE2
TETRA
PENTA
HEXA
HEPTA
OCTA
PCDD CONGENER CLASS
Flgure 7. PCDD production by congener at 400 O C . All tests with three or m e repeats, indicated by the average and standard deviation balance bars (1 g of catalyst, chlorophenol mix precursors, 10% 02, NE).
chlorophenol mix and the CuS04catalyst is significantly reduced (-2 orders of magnitude) from that of the CuO catalyst. Levels of PCDD produced with the CuSO, are quite similar to those of the blank. 4. Discussion
The experiments measuring PCDD synthesis affirm the role of Cu catalysts, HCl, O,, an organic source (e.g., phenol), and temperature for the production of these compounds in a simulated MWC flue gas environment. The lack of PCDD production during noncatalytic runs (C12 and phenol with a quartz wool plug) as a methods check indicates the necessity of the Cu-based catalyst for formation. Addition of SOz to these "baseline" synthesis conditions appears to have little, if any, effect on the production of PCDD a t all three temperatures, 300,400, and 500 "C. While some graphical evidence might infer slight specific PCDD congener declines (Figure 3) upon SOz addition (most notably a t 300 "C), the difficulty of the production and analytical methods does not provide sufficient statistical evidence to make this claim. The 45 data conditions in Figure 3 have four instances [below detection limit (BDL) measurements not included] in which the standard deviation exceeds 0.5 order of magnitude. Thus, data trends can begin to be inferred when the value differences exceed 1 order of magnitude. Figure 4 shows very little difference in concentrations for total PCDD congener production as compared to the CuO/HCl/phenol baseline experiments. These limited testing conditions suggest that neither reaction 2 nor phenol sulfonation may be responsible for apparently low PCDD/PCDF formation in coal-fired plants. Tests further exploring the negative effect of SOz upon PCDD formation indicate that the homogeneous reaction (reaction 2) of Cl, with SOz to form HC1 (a less likely chlorinating agent than C12)is not measurable below 800 OC. This is not apparent from thermodynamic calculations of the free energy change, AG, which suggest that the reaction is favorable over the total temperature range expected in a MWC. The 800 "C temperature is much higher than that (430 "C) in which the Deacon process activity, Clz production, is maximized (9). The concentration of C12(lo00 ppm) necessary to c o n f i i the presence of SOBis also -2 orders of magnitude higher than expected in MWCs. In the declining temperature flue gas environment, it follows then that the apparently minimal amounts of PCDD and PCDF detected in coal-fired combustors are not attributable to depletion of the Clz chlo1942 Environ. Scl. Technol., Vol. 26, No. 10, 1992
The apparent reduction in PCDD production during tests using CuSO, as the process catalyst confirms the inhibition of the baseline CuO catalyst. Figure 4 shows that total PCDD production is lower for a11 three test temperatures. The upstream formation of CuS04 in coal-fired combustors may possibly be responsible for the insignificant amounts of PCDD and PCDF noted in their emissions. Thermodynamic calculations suggest that formation of CuS04is favorable at temperatures below approximately 870 "C, while TGA results showed CuSO, decomposition occurs a t temperatures in excess of 615 "C. Our XRD analyses of the CuO catalyst plug from the SOz test (run without phenol) at 500 "C indicated the presence of minor CuSO, formation within the CuO catalyst. This suggests that reaction of SO2 with a CuO catalyst to form CuSO, may be responsible for decreased PCDD production. The lack of CuSO, product at other temperatures is likely due LO unfavorable equilibria and decomposition temperatures (the 700 and 900 "C tests) and slow reaction kinetics (the 300 "C test). Calculations suggest that SO, concentrations of -450 ppm are in equilibrium with CuS04 a t 627 "C (10% O,)-below this temperature, even lower SO, eoncentrations will continue to drive (given sufficient time) the reaction to the CuSO, product. The reaction rate of CuO with SOz is sufficiently rapid and complete that a pilot-scale flue gas desulfurization process with CuO sorbents has reported (typically) 7 0 4 0 % removal of SOz at 1/1 stoichiometry at temperatures around 400 "C (23). There is substantial evidence for chemisorption of compounds of S on Cu catalysts and subsequent reduction of catalytic activity (24). Catalytic activity will stop when equilibrium is reached between the poison (in this case, SO,) in the reactant stream and the catalyst surface. The mechanism of deactivation may be one of covering the active sites, which could otherwise adsorb reactant molecules (25), Because PCDD production and the Deacon process (Clz production) with CuO or CuSO, each has distinct regions of maximum temperature sensitivity, definitive comparisons of the results of Figure 3 are difficult. In each test permutation, the Clz concentration reaches different levels according to the catalyst type and test temperature. Since the production of PCDD has been shown to increase with higher Clzconcentration (211, each test should be expected to effect different levels of PCDD production. Thus, the results of bypassing the Deacon process through supply of a constant concentration of C1, will provide a more adequate comparison of PCDD production with CuO and cuso4. Tests with supplied, constant Clz concentration (Figures 5 and 6) indicate that the production of PCDD is promoted by and dependent on the type of catalyst present, completely independent of the catalyst's role in the Deacon process. This is further verified by insignificant PCDD production levels when an inert substrate (quartz wool) was tested. Slight decreases in the levels of PCDD produced with CuSQ, versus CuO indicate that the effect of CuSO, is not only to alter the effective temperature range of the Deacon process (as noted earlier in Figure 2) but also to reduce the potential for PCDD formation. The mechanism of the CuSO, catalyst deactivation may occur through inhibition of either the d u d ring formation (likely through condensation reactions) or the substitution reaction of Cl, onto the ring structure. The latter is unlikely in view of previous work (8) which has shown that C12
substitution onto the aromatic occurs readily a t elevated temperature without the presence of a catalyst or a Lewis acid (26). The former possibility is examined through the chlorophenol precursor results in Figure 7. Clearly, there is a large difference between the catalytic activity of CuO and CuS04. The former yields PCDD in amounts exceeding 2 orders of magnitude greater than that of the latter, suggesting that CuS04,as compared to CuO, is relatively ineffective a t promoting biaryl synthesis. These yield differences are maintained despite accounting for the presence of PCDT and PCTA. The maximum yield of PCDT (found only in the CuS04tests) was less than 40% of the PCDD yield, insufficient to account for the 2 orders of magnitude difference observed in Figure 7. These chlorophenol results are seemingly unsupportive of those in which the complete mechanism (CuO or CuS04catalyst with HC1 and phenol) yields less than 1 order of magnitude difference in PCDD yield (see Figure 4). Resolution of this point requires further investigation.
5. Conclusions Formation of PCDD between 300 and 500 OC under simulated MWC conditions is dependent upon a catalyst (e.g., CuO) that produces Cl,, the primary chlorinating agent of the aromatic structures, reaching a maximum a t -400 OC. While gas-phase SO2has little apparent effect upon the production of PCDD, the presence of S as an upstream reaction product, CuS04,results in minor (less than 1 order of magnitude) reduction of the overall PCDD production mechanism. Although CuS04also catalytically forms C12,the temperature of maximum C12 production shifts upward to -500 OC. However, the decreased ability of the CuS04catalyst to produce PCDD is not necessarily linked to the production of C12. Rather, the 2 order of magnitude difference in PCDD production between CuO and CuSOl catalysts in the presence of chlorophenol precursors suggests a second catalytic role for Cu species-the CuS04 appears to have lower activity than CuO in a biaryl synthesis reaction. Additional understanding of the PCDD and PCDF formation mechanism is necessary before apparently lower levels of PCDD and PCDF in coal-fired utility boilers can be definitively attributed to CuS04 formation alone. Acknowledgments
We thank George R. Gillis and Frank E. Briden (US. EPA, AEERL) for equipment support and XRD analyses, respectively. Significant analytical and time efforts from Ann M. Drago, Bryant K. Harrison, and Jeff V. Ryan of Acurex Corp. are gratefully acknowledged. Registry No. Tetrachlorodibenzo-p-dioxin,41903-57-5;pentachlorodibenzep-dioxin,36088-22-9;hexachlorodibenzo-p-dioxin, 34465-46-8; heptachlorodibenzo-p-dioxin,37871-00-4; octachlorodibenzo-p-dioxin, 3268-87-9; sulfur dioxide, 7446-09-5;
copper, 7440-50-8; 2,3,5-trichlorophenol, 933-78-8; 2,3,4-trichlorophenol, 15950-66-0; 2,3,6-trichlorophenol, 933-75-5; 2,3,5,64etrachlorophenol, 935-95-5;pentachlorophenol, 87-86-5; chlorine, 7782-50-5; hydrogen chloride, 7647-01-0. Literature Cited (1) Eiceman, G. A.; Hoffman, R. V.; Collins, M. C.; Long, Y.-T.; Lu, M.-Q. Chemosphere 1990,21, 35-41. (2) Junk, G. A.; Richard, J. J. Chemosphere 1981, 10, 1237-1241. (3) Kimble, B. J.; Gross, M. L. Science 1980, 207, 59-61. (4) Mahle, N. H.; Whiting, L. F. Chemosphere 1980,9,693-699. ( 5 ) Stieglitz, L.; Vogg, H. Chemosphere 1987,16, 1917-1922. (6) Griffin, R. D. Chemosphere 1986, 15, 1987-1990. (7) Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987,21, 1080-1084. ( 8 ) Gullett, B. K.; Bruce, K. R.; Beach, L. 0. Waste Manage. Res. 1990,8, 203-214. (9) GuUett, B. K.; Bruce, K. R.; Beach, L. 0. Chemosphere 1990, 20, 1945-1952. (10) Hoffman, R. V.; Eiceman, G. A.; Long, Y.-T.; Collins, M. C.; Lu, M.-Q. Environ. Sci. Technol. 1990,24,1635-1641. (11) Ballschmiter, K.; Braunmiller, I.; Niemczyk, R.; Swerev, M. Chemosphere 1988, 17, 995-1005. (12) Dickson, L. C.; Karasek, F. W. J . Chromatog. 1987, 389, 127-137. (13) Karasek, F. W.; Dickson, L. C. Science 1987,237,754-756. (14) Bruce, K. R.; Gullett, B. K.; Beach, L. 0. Presented at 1991 Conference on Thermal Treatment of Radioactive Hazardous Chemical Mixed and Medical Waste, Knoxville, TN, May 13-17,1991. (15) Karasek, F. W.; Dickson, L. C.; Hutzinger, 0. Incineration of waste materials, U.S. Patent 4,793,270, Dec 27, 1988. (16) Norton, G. A.; Malaby, K. L.; DeKalb, E. L. Environ. Sci. Technol. 1988,22, 1279-1283. (17) Barton, R. G.; Clark, W. D.; Seeker, W. R. Combust. Sci. Technol. 1990, 74, 327-342. (18) Buser, H. R. Abstracts; 11th International Symposium on Chlorinated Dioxins and Related Compounds: Dioxin '91; School of Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC, 1991; p s18. (19) Fisher, R.; Marks, M.; Jett, S. TAPPI J. 1987,70 (4), 97-102. (20) Method 8280. The Analysis of Polychlorinated Dibenzop-dioxins and Polychlorinated Dibenzofurans. In Test Methods for Evaluating Solid Waste, 3rd ed.; Report EPA/SW-846; NTIS NO. PB88-239223; 1986; Vol. 1B. (21) Bruce, K. R.; Beach, L. 0.;Gullett, B. K. Waste Manage. 1991,11, 97-102. (22) Mu, J.; Perlmutter, D. D. Ind. Eng. Chem. Process Des. Dev. 1981,20,640-646. (23) Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Environ. h o g . 1986, 4, 223-228. (24) Harriott, P.; Markussen, J. M. Ind. Eng. Chem. Res. 1992, 31, 373-379. (25) Vogg, H.; Stieglitz, L. Chemosphere 1986,15, 1373-1378. (26) Solomons, T. W. G. Organic Chemistry; John Wiley & Sons, Inc.: New York, 1976; p 452. Received for review March 2,1992. Revised manuscript received June 5, 1992. Accepted June 15, 1992.
Environ. Scl. Technol., Vol. 26, No. 10, 1992
1843
