Environ. Sci. Techno/. 1995, 29, 1156- 1162
Formation of Polychlorinated Dioxins, Furwts, Benzenes, and Phenols in the Post-Combustion R e g k of a Mewqeneous Combuster: Effect of Bed Matorid d Past-Cr#ilbWion Temperature S . B E H R O O Z GHORISHI A N D ELMAR R. ALTWICKER* The Howard Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180
The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (hereafter called PCDD/Fs), downstream from the combustion zone of a spouted bed combustor was heterogeneously catalyzed within seconds in the temperature range 430-390 "C and in the presence of fine sand particles. The PCDD/Fs formation was not observed at substantially lower temperatures or in the presence of fine quartz particles. The two different bed materials, sand and quartz, were used together with a fuel mixture of 10% (mole) 1,2-dichlorobenzene in n-heptane. Chlorobenzenes (CBs) other than the starting fuel and chlorophenols (CPhs) were also observed in the post-combustion region. Formation of trichlorobenzenes and dichlorophenols dominated in the presence of sand. Predictions of a homogeneous gas phase mechanism for the formation of PCDDs were compared to the experimental data to reveal possible influences of surface reactions on the formation of PCDD/Fs.
Introduction In the past decade, the emissions of chlorinated aromatic compounds from waste incinerators have been a subject of increased environmental concern. Of particular interest is a class of compounds notorious for their toxicities; polychlorinated dibenzo-p-dioxins and polychlorinated furans (PCDDlFs). For a number of years, investigators have been intrigued by the formation of PCDDlFs in the post-combustion region of municipal solid waste incinerators at low temperatures (200-400 "C). Numerous measurements on incinerators have reported such formations across air pollution control devices such as electrostatic precipitators, Le., (PCDDlFs),,, >> (PCDDI FS)in. If such formation occurs during the typical gas phase residence times experienced in these devices,the formation reactions must occur within seconds (short time scale), whether they occur in the gas phase or on the surface of the particles that move with the gas stream. The low temperature (250-350 "C)llong time scale (reaction times of minutes-hours) formation of PCDDlFs from fixed beds of fly ash is a well-studied and established process (1-4). This has been termed de novo synthesis. With few exceptions, these reactions lead to a PCDDlPCDF ratio that is less than 1. Using chemicallysimilar precursors, such as chlorophenols, PCDDs dominate the product mixture (4-6). The de novo reactions typically occur at rates less than 0.1pg of PCDD/F(gof fly ash)-' min-l, while the chlorophenols reactions can occur at rates as high as 0 (10) pg of PCDDlF(g of fly ash)-' min-' in the same temperature range (4). If formation of PCDDlFs occurred during the passage time of uncollected particles through the incinerators (Le., on the order of seconds), the above rates would appear to be too slow. Until recently, there was no evidence of the rapid formation of PCDDlFs from laboratory or pilot plant studies. However, Fhgrnark et al. (7) have reported the rapid formation of PCDDlFs in the cooler section of a pilot plant fluidized bed combustor, using a synthetic refused derived fuel (RDF),in the temperature range of 250-340 "C and gas phase residence time of 0.8-3 s. Research at the U.S. Environmental Protection Agency (8) has shown that substantial formation of PCDDlFs can occur on fly ash injected downstream of a natural gas burner at 300400 "C and at rates conducive to explaining the formation in full-scale combustion facilities at particlelgas residence times less than 5 s. While these studies focus on the heterogeneous formation of PCDDlFs in the presence of fly ash, a catalytically active surface, we reported in a preliminary investigation (9) on the effect of sand on the short time scale formation of PCDDlFs, from the combustion of 1,2-dichlorobenzene in the post-combustion region of a spouted bed combustor (residence time of 4-5 s). This paper reports on the continuation of that investigation and contrasts additional results in the presence of sand with those in the presence of quartz. Chlorobenzenes and chlorophenols are also quantified. Sand and quartz (noncombustible materials) can make up a significant portion of municipal solid wastes; therefore, Author to whom all correspondence should be addressed.
1156 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5. 1995
0013-936X/95/0929-1156$09.00/0
@ 1995 American Chemical Society
-
t--5cm +
USA
-+-e=
Thermocouple; Turp
3
(post combustion outlet)
V8d8C
icrmocouplc; T12
(postcombustion inlet) 20cm
L
H2 line for ignition
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25cm
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1Rotameter
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FIGURE 1. Schematic of the experimental section.
their possible contribution to the emission of chlorinated aromatics through heterogeneous reactions could be sipficant and has to be identified and quantified. Studies with these materials may provide a baseline against which more active solids, such as fly ash and other catalysts, may be measured. 1,2-Dichlorobenzenewas chosen as the fuel since it has been shown to be a precursor to PCDDlFs (9,10). It has been reported in the stack gases from municipal solid waste incinerators (11) and is readily soluble in auxiliary fuels such as heptane and toluene. n-Heptane was chosen as the solvent for 1,2-dichlorobenzene to minimize the potential contribution of the solvent to the aromatic structure. Shaub and Tsang (12)proposed a gas phase mechanism for the formation of PCDDs, using chlorophenols as precursor. This mechanism is used in this study to predict the levels of PCDDs expected from gas phase reactions under the conditions existingin the post-combustionregion of the spouted bed combustor. These predictions are then compared to the experimental results to reveal possible influences of heterogeneous processes on the formation of PCDDs.
Experimental Procedures The experiments were performed in an all-quartz spouted bed combustor, which has been described in detail elsewhere (10,131. A system schematic is shown in Figure 1. The spouted bed reactor is topped by two postcombustion stages to provide additional residence time. These two stages, comprising the post-combustion region, provide a total residence time of 4-5 s based on the average gas superficial velocity. The two stages are quartz tubes of 80 and 60 cm long, 6.6and 5 cm inner diameter, respectively, and are powered by calm-shell furnaces (Figure 1). Either
a steep axial temperature gradient (430-125 "C) or a near isothermal condition (430-390 "C) could thus be imposed between inlet and outlet of this region. At the inlet to the first stage, a F'yrex sampling cross (SC; Figure 1) is located. At the end of the second stage (outlet of the system),another samplingport is located (USP,Figure 1). Sampling was conducted at these two downstream (from the flame) positions. Since earlier studies (9, 14) had indicated the existence of a radial concentration gradient at the sampling cross (inlet to the post-combustion region) under certain conditions for some species, sampling was conducted at both the centerline (SC,)and the wall ( S G ) of the samplingcross. Based on numerous radial concentration profile measurements at the sampling cross, a procedure was developed to calculate the average concentration at this sampling position, using the two values at the center and the wall (14). This study reports only the average concentration at the sampling cross and compares it to the concentration at the outlet of the post-combustion region. PCDD/F concentrations at the center and at the wall location of the sampling cross were reported in the earlier study (9). Sampling probes were maintained at 100 "C f 5 "C. Sampling rates of 2-4 Llmin and sampling times of 0.5-2 h were used. Probe velocities were varied from 1.2 to approximately 12 times isokinetic. It has previously been shown that the probe velocity has very little effect on the PCDDlFs concentration (9). The flow rate of oxidizer (air) was controlled by a mass flow controller at 23.5 L/min (STP conditions). The liquid fuel, 10%1,2-dichlorobenzenein n-heptane,was contained in a stainless steel tank. Helium pressure insured a constant fuel flow that was controlled and measured by a liquid needle valve and a rotameter, followed by a preheater (stainless steel cylinder filled with glass beads) in which VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1157
-
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Outlet of the Post Combustion Region medium temperature
0
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I
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"
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150 200 250 300 350 400 post combustion temperature ( C )
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FIGURE 2. Post-combustion temperature profile runs.
the fuel was vaporized and mixed with the airstream such that the equivalence ratio, @ = (fuellair),,t,~/(fuellair),toichiometry, was 0.55. A low equivalence ratio, @ = 0.55, was chosen to favor the formation of PCDDlFs, chlorobenzenes, and chlorophenols (9).A hydrogen pilot flame (Figure 1) ignited the fuel/air mixture, and a conical flame was established at the top of the bed of the spouted bed reactor. Two different bed materials were used; sand and quartz. A brief description of each material is given below: Sand. A building sand (Whitco no. 2 sand) was sieved to obtain a particle size range of 0.84-1.00 mm. The composition of different types of sand has been reported by Pettijohn (15). On average, sand contains 75% silica, 7% alumina, and 1-3% oxides of Fe, Mg, Ca, Na, K, Ti, and Mn. Quartz. Crushed GE 214 clear fused quartz purchased from National Scientific Company was sieved to a particle size range identical to that of sand. The composition of GE 214 quartz is reported as 99.995% silica, with trace metal oxides concentrations of 0.002% or less. Bed materials were replaced after each run to ensure constant combustor hydrodynamics and to minimize any possible contamination. During circulation, bed materials in the spouted bed combustor experience abrasion and thermal strain, which lead to attrition and formation of fine particles (13). These processes serve as a continuous source of fine particles in the post-combustion region along with the flue gases and provide fresh surfaces that appear to influence the formation of PCDDIFs, chlorobenzenes, and chlorophenols. Three different axial post-combustion temperature profiles (low, medium, and high; Figure 2) were imposed by changing the power input to the furnaces surrounding the post-combustion region. The temperature at the inlet of the post-combustion region and also the temperature upstream of this location (flame and the bed region of the spouted bed combustor) were the same for all three profiles since all the experiments were conducted under identical conditions, Le., same fuel, equivalence ratio, and bed temperature. The nature of bed material had no effect on the temperature of the bed and the flame. The high boiling point chlorobenzenes (CBs), cNorophenols (CPhs), and PCDDlFs were sampled using a specially designed volatile organic sampling train. Details of the sampling and further cleanup of the samples for CBs, CPhs, and PCDDlFs are described elsewhere (9). Samples were analyzed usinga HP 5890 gas chromatograph coupled to a HP 5971 mass spectrometer with a mass 1158
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29,
selective detector. The details of these analyses are described elsewhere (3, 10).
NO. 5 , 1 9 9 5
The concentration of PCDDlFs together with chlorobenzenes and chlorophenols at the inlet of the post-combustion region (SC; Figure 1)and at the outlet of this region (USP; Figure 1) were measured as a function of the bed material and the post-combustion temperature profiles (Figure 2). While keeping allthe other operating parameters constant, each of these temperature profile experiments was performed with sand and with quartz as the bed material. The results of the PCDD/Fs analyses (expressed in ng/ m3) are presented in Table 1. This table contains the experimental results obtained at the outlet of the postcombustion region (downstream) as a function of three temperature profiles and two bed materials and at the inlet of the post-combustion region (upstream). The upstream results are comparable for all post-combustion temperature profiles runs (low, medium, and high). Tetra (T4CDD and T4CDF) to octa (08CDDand 08CDF) congeners of dioxins and furans together with their sums (CPCDD and CPCDF) are given in this table. The CPCDD and CPCDF are also plotted in Figure 3. Experimental results with "&" are based on duplicate and triplicate runs; those with "I" are equal or less than the detection limit of the GUMS for that specific congener. Based on these experiments and previous results (9), the reproducibility of the experimental results is considered to be well within the acceptable range. For the low and medium temperature profiles, both in the presence of sand and quartz, there is very little change in PCDDlFs concentrations between the upstream and downstream positions. However, only in the presence of sand particles, the high (near isothermal) temperature profile (430-390 "C) leads to a drastic increase in the PCDDlF concentration, confirming previously reported results. In this temperature range, the formation of PCDDI Fs has occurred during the residence time in the postcombustion region (4 s). Results in Table 1 and Figure 3 also indicate that the ratio of CPCDD to CPCDF is less than 1 in all cases, which is in agreement with literature data (1, 16') and frequent incinerator observations. Results with quartz particles show only a very small PCDDlF increase for the high temperature profile, suggesting that quartz particles cannot enhance the formation of these compounds within the optimum temperature range (420-390 "C). The apparent catalytic effect of sand on the formation of PCDD/F perhaps may be attributed to the higher concentration of metal oxides. A difference between sand and quartz has been observed in the combustion of other hydrocarbons such as toluene (I 7 ) .
Discussion The results discussed so far and the existing data in the literature suggest that the formation of PCDD/Fs can be greatly enhanced if two favorable conditions exist simultaneously in the post-combustion region, namely: (1) The presence of an active surface: active surface includes surfaces that contain mixed metal oxides (especially transition metals) such as fly ash and sand as opposed to the so-called"inactive"surface of compounds like quartz, which contain negligible amounts of metal oxides. (2) The presence of an optimum temperature window: In the case of sand, this study shows that the optimum temperature range for the formation of PCDD/Fs is around
TABLE 1
Concentrations of PCDDs and PCDFs at Inlet (Upstream) and W e t (Downstream) of Post-Combustion Region (Figure 1): Effect of Bed Material and Post-Combustien Temperature downstream low T runb sand quartz
downstream medium T runb sand quartz
downstream high T runb sand quartz
compound (nglm?
upstream. sand quartz
T4CDDs P5CDDs H6CDDs H7CDDs 08CDD ZPCDDs
51.3 51.8 52.4 3.0 53.0 511.5
51.3 51.8 3.8 4.5 53.0 514.4
51.3 51.8 3.3 f 0.9 2.6 f 0.1 53.0 5 13.0
5.4 4.8 3.9 1.7 53.0 518.8
51.3 51.8 2.5 3.3 53.0 512.0
2.0 5.0 5.0 4.5 4.0 20.5
6.5 f 1 7.8 f 1 17.8 f 3 11.5 f 1 3.6 f 0.6 48.0 f 6
1.4 4.7 6.4 5.9 3.9 22.3
T4CDFs P5CDFs H6CDFs H7CDFs 08CDF CPCDFs
6.0 51.0 51.5 51.8 53.0 513.3
7.8 51.0 51.5 51.8 53.0 515.5
10.6 f 1 51.0 51.5 51.8 53.0 517.9
11.4 1.5 1.9 1.5 53.0 5 19.3
9.0 51.0 51.5 51.8 53.0 516.3
7.0 1.5 51.5 3.0 5.0 521.3
34.4 f 3 19.7 f 5 12.8 f 2 3.2 dz 1.2 53.0 74.5 i 10
7.3 4.2 2.5 7.6 7.3 28.9
Upstream conditions are the same for all post-combustion temperature runs (low, medium, and high); measured at SC (Figure 1). For actual change in post-combustion temperature refer to Figure 2.
concentration (ng/m3)
80
60
40
PCDFs CDDs
-
0 " quartz sand
quartz sand
quartz sand
upstream
downstream l w T run
downstream medium T run
downstream high T run
430-29O'C
4 30-390' C
430-13O'C
quartz sand
FIGURE 3. Total dioxins (PCDD) and furans (PCDF) concentration at the inlet (upstream) and the outlet (downstream) of the post-combustion region, effect of bed material and post-combustion temperature.
390-420 "C. Literature data report an optimum temperature range of 250-350 "C for fly ash (4, IS). This difference in the optimum temperature range may be an indication to the lower catalytic activity of sand as compared to fly ash. The catalytic effect of sand particles for the high (near isothermal) temperature profile is more pronounced for the lower chlorinated congeners (tetra, penta, and hexa) of PCDDs and especially PCDFs. This observation may suggest that the heterogeneous reactions occurring on the surface of sand particles promote the formation of the lower chlorinated congeners, and subsequent chlorination of
these compounds to higher chlorinated congeners (hepta and octal may perhaps occur in the gas phase. The experimental data presented in Table 1 show consistently higher concentrations of T4CDF as compared to the other congeners. The high concentration of T4CDF is observed for both bed materials (sand and quartz) and for all three temperature profiles. A possible explanation for this high concentration may be as follows: The amount of 1,2-dichlorobenzenein the combustor and in the post-combustion region is in excess (relative to other chlorinated aromatics), since it is the fuel in our studies. Based on the measured post-flame destruction VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1159
TABLE 2
Is (Cplw) at Outlet of Post-Combustien Region Concentmh 4 ewt (W) ml (Figure 1): Effect of Bed Material a d Pest-Comhtion Temperature (,ug/m3)
medium T runa
low T runa
compound
high T runa
sand
quartz
sand
quartz
sand
quartz
T3CBs T4CBs P5CB H6CB ZCBS
472 & 74 3.0 f 0.2 0.1 f .02 50.05 475 f 75
15 1.2 0.3 50.05 16.5
333 2.2 0.1 50.05 336
17 1.2 0.4 50.05 18.6
325 f 11 5.2 f 1 1.2 f 0.3 0.2 f .03 332 f 12
11 f 2 1.5 f 0.3 0.6 f 0.1 50.05 13.1 f 2
DCPhs T3CPhs T4CPhs PCPh ZCPhs
104 f 11 57 f 10 17f2 1.6 f 0.4 180 f 24
41 87 53 3.1 184
73 65 16 2.0 156
57 f 1 63 f 11 17f3 2.9 f 0.1 140 f 15
23 f 9 63 f 32 56 f 6 2.7 f 0.1 145 f 47
37 126 87 6.0 256
For actual change in post-combustion temperature, refer t o Figure 2.
and removal efficiency (DRE)of this species (around 96%), its post-combustion concentration is about 2 x 106pg/m3. T4CDF can be considered as a condensation product of 12-dichlorobenzene and trichlorophenols that could be produced by hydroxylation and subsequent chlorination of dichlorobenzene. The concentration of trichlorophenols in the post-combustion region is also relatively high (approximately 60pg/m3; Table 2). h o t h e r possible path for the formation of T4CDF is the condensation of trichlorobenzenes and dichlorophenols, both of which are present at relatively high concentrations in the post-combustion region (Table 2). Based on these arguments, the observed high concentration of T4CDF may be attributable to the high concentration of a species such as 1,2-dichlorobenzene in our experimental system; therefore, one may conclude that 1,2-dichlorobenzene and its immediate hydroxylation and chlorination products (T3CBs, DCPhs, and T3CPhs) can be considered as precursors to T4CDF, through a condensation mechanism which may have a relatively high yield even in the presence of a socalled inactive surface such as quartz. Under the favorable conditions for the formation of PCDD/Fs, dioxins appear to peak with the hexa congener (H6CDD)and furans appear to peakwith the tetra congener (T4CDF),suggesting that different precursors and perhaps different mechanisms are responsible for the formation of PCDDs as opposed to PCDFs. Chlorobenzenes and Chlorophenols. In part of this study, we focus on the formation of chlorobenzenes and chlorophenols as potential precursors to the PCDDIFs.The experimental results at the outlet of the post-combustion region (downstream) for tri-, tetra-, penta-, and hexachlorobenzene and di-, tri-, tetra-, and pentachlorophenols (expressed in pg/m3) are presented in Table 2 for the temperature profiles and bed materials employed. The concentrations of these species at the inlet of the postcombustion region (downstream) are very similar to the values at the outlet for the two bed materials and, therefore, are not presented here. Levels of chlorobenzenes in the presence of sand particles for all three temperature profiles are more than 1 order of magnitude higher than in the presence of quartz particles. This apparent catalytic activity as discussed before may be attributed to the presence of metallic species in sand. The effect of sand appears to be more pronounced for the lower chlorinated benzenes (especially trichlorobenzenes). Bed material does not have a strong effect 1180 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5. 1995
on the amount of ZCPhs in the post-combustion region; however, it appears that sand particles tend to increase the amount of lower chlorinated congener of chlorophenols (DCPhs)but not the higher chlorinated ones (tri, tetra, and penta). As shown in Table 2, with the exception of trichlorobenzenes in the case of sand, the concentrations of chlorophenols are consistently higher (in some cases up to 1 order of magnitude) than chlorobenzenes. Since the starting compound in our experiments is a chlorobenzene, higher levels of chlorophenols tend to suggest that hydroxylation of chlorobenzenes to form chlorophenols and the subsequent chlorination of chlorophenols is a faster process than chlorination of chlorobenzenes, and therefore chlorophenols can be produced readily in an oxidative environment from chlorobenzenes. Modeling Results and Their Comparison to Experimental Data. To consider the possibilitythat the formation of PCDD/Fs in the post-combustion region involves heterogeneous reactions, modeling calculations were performed using a PCDD gas phase mechanism under the conditions existing in the post-combustion region of the spouted bed combustor. Such a homogeneous gas phase mechanism-optimized for the formation of PCDDs using chlorophenols as precursors-has been proposed by Shaub and Tsang (12). The CHEMKIN package (19) was used to handle the PCDD kinetic mechanism. The output PCDD concentration from the model was compared to the CPCDD experimental results obtained at the outlet of the postcombustion region to reveal possible influences of heterogeneous processes responsible for the formation of PCDDs. As written (121,this mechanism does not apply to PCDF formation. In order to obtain the model output, the followinginput parameters have to be known: Species. The initial concentrations of the following species are needed at the inlet of the post-combustion region: total chlorophenols,total dioxins,the concentration of unburned fuel, 0 2 , HCl, and OH, H20. Concentrations of HC1, 0 2 , and H20 can be obtained based on the experimentally measured conversion of the fuel (10% 1,2-dichlorobenzenein n-heptane) at the equivalence ratio of 0.55 and the assumption of complete conversion of chlorine in the fuel to HC1. Concentrations of 0 2 , H20, and HC1were evaluated as 0.093,0.079,and 2.1 x mole fraction, respectively. On the basis of an assumption of a local thermodynamic equilibrium (121,
TABLE 3
Output Predictions of Gas Phase Model for PCDDs mole fraction
low Trunl
(CPCDDs)in 1.9 x lo-'' (CPCDDS)~,~ 1.9 x lo-'' a
medium Truna
high Truna
1.9 x IO-'' 1.9 x IO-"
1.9 x lo-'' 1.9 x lo-'*
For actual change in temperature refer to Figure 2.
the concentration of OH radical was estimated to be 6 x mole fraction. A sensitivity analysis with regard to OH indicated that under the conditions of our postcombustion region a large decrease or even an increase in the value of OH concentration has no effect on the predicted output value of PCDDs. Based on the experimentally measured conversion of both fuels (n-heptane and 1,2-dichlorobenzene), the inlet concentration of unburned fuel in the gas phase mechanism was estimated to be around mole fraction. The inlet concentrations of chlorophenols and dioxins were obtained from the experimental data (Tables 1 and 2) as 2.5 x lo-* and 1.9 x mole fraction, respectively. Temperature. Experimentally measured temperature profiles shown in Figure 2 were used. Residence T i e . Residence time in the post-combustion region depends on the temperature. For example, for the high temperature run (Figure 21, residence time can be estimated as follow: For this temperature profile, an average temperature of 400 "C was used. The flow rate through the reactor is the same for all experiments and is equal to 23.5 Llmin at STP conditions. Assuming ideal gas behavior at 400 "C, the flow rate is equal to 23.5 x (673l273) = 57.9 Llmin (9.65 x m3/s). The linear velocity through the postcombustion regions is equal to the flow rate divided by the cross-sectional area of the post-combustion regions, Le., 0.28 mls for the first stage and 0.49 mls for the second stage. Residence time ( t )in the post-combustion region is equal to the length of the post-combustion region divided by the linear velocity, Le., t = (0.8l0.28) (0.6l0.49) = 4.1 s at 400 "C. The gas phase model for the formation of PCDDs was run using these input values for the three temperature profiles; the output values for (CPCDD),,, are presented in Table 3. For the sake of comparison, input values for dioxins, (CPCDD)i,, are also included in this table. It is clear that under the conditions of the post-combustion region, the gas phase mechanism predicts neither formation nor destruction of any of the XPCDDs entering this region regardless of the post-combustion region temperature profiles, Le., (XPCDD)in = (XPCDD),,,. Comparing this finding to the experimental data, at the low and medium temperature profiles for both sand and quartz and at the high temperature profde for quartz only, the amount of XPCDD along the post-combustion region does not change significantly,which is in agreement with the prediction of the gas phase model, indicating that under these conditions the effects of possible surface reactions are negligible. However, the drastic change in the amount of CPCDD at the high temperature profile and in the presence of sand cannot be explained by the gas phase model, indicating that under these conditions the increase in the amount of PCDDs along the post-combustion region is mediated by the sand surface.
+
These results further support the conclusion that sand particles provide active surfaces that can catalytically enhance the formation of PCDDs in a suitable temperature range. It is important to note that a packed bed of sand has been found to be inert in the temperature range 250350 "C (2, 6, 18). The fine sand particles formed in our studies have not been fully characterized. As indicated earlier (91,these particles are likely to be smaller than 5pm.
Conclusion Formation of polychlorinated-p-dioxins(PCDDs) and polychlorinated furans (PCDFs)in the post-combustion region of a heterogeneous spouted bed combustor can be catalyticallyenhanced by fine sand particles (formed by attrition from the bed material) and a temperature range of 430390 "Cwithin seconds. This enhancement was not observed with quartz particles or at substantially lower temperatures. Based on the experimental results and literature data, two conditions that promote the formation of PCDDlFs in the post-combustion region are (1) the presence of an active surface that contains metallic species and (2)the presence of an optimum temperature window, which in the case of sand is around 430-390 "C. Modeling calculations using a proposed gas phase mechanism for the formation of PCDDs (12) lead to the conclusion that the gas phase formation of PCDDs under the conditions of the post-combustion region is negligible and that the surface processes have to be the primarypaths responsible for the observed increase in the levels of PCDDs in the post-combustion region. Experimentalresults suggest that condensation reactions of 1,2-dichlorobenzene and trichlorophenols (immediate hydroxylation and chlorination product of 1,2-dichlorobenzene) can lead to tetra congeners of furan (T4CDF), which were observed at relativelyhigh concentration under all the experimental conditions; 1,2-dichlorobenzene and trichlorophenols may be precursors to T4CDF. Catalytic activity of sand was also observed in the formation of chlorinated benzenes, especially trichlorobenzenes (T3CBs) from 1,2-dichlorobenzene. Experiments performed with sand as the spouted bed material lead to 1 order of magnitude higher concentration of T3CBs as opposed to quartz using 1,2-dichlorobenzeneas the fuel.
literature Cited (1) Stieglitz, L.; Vogg, H. Chemosphere 1987,16, 1917. (2) Dickson, L.; Karasek F. 1.Chromatogr. 1987,389, 127. (3) Milligan, M. Ph.D. Dissertation, Rensselaer Polytechnic Institute, 1994. (4) Milligan, M.; Altwicker, E. R. Environ. Sci. Technol. 1993,27, 1595. (5) Ross, E. J.; Naikwadi, K. P.; Karasek, F. W. Abstracts ofPapers; Dioxin 90, Bayreuth, Germany; Eco-Informa Press: Bayreuth, 1990; Vol. 111, p 143. (6) Karasek, F. W.; Dickson, L. C. Science 1987,237, 754. (7) Fangmark, I.; Marklund, S.; Rappe, C.; Stromberg, E.; Berge, N. Abstract of Papers; Dioxin 92, Tampere, Finland; Eco-Informa Press: Tampere, 1992; Vol. VIII, p 245. (8) Gullet, K. B.; Lemieux, P. M.; Dunn, J , E. Environ. Sci. Technol. 1994,28, 107. (9) Altwicker, E. R.; Konduri, R.; Lin, C.; Milligan, M. Chemosphere 1992,25 (121, 1935. (10) Altwicker, E. R.; Konduri, R.; Lin, C.; Milligan, M. Combust. Sci. Tech,nol 1993,88, 349.
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(11) Combustion and Emission Testing at the Oswego County Municipal Solid Waste Incinerator; Report 90-10; New York State Research and Development Authority: Albany, 1990. (12) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17, 721. (13) Altwicker, E. R.; Konduri, R. Combust. Sci. Technol. 1992, 87, 173. (14) Ghorishi, B. Rensselaer Polytechnic Institute, unpublished results, 1994. (15) Pettijohn, F. J.; Potter, P. E.; Siever, R. Sand and Sandstone, 1st ed.; Springer-Verlag: Berlin, 1971; p 60. (16) Takacs, L.; McQueen, A.; Moilanen, G. L. J. Air Waste Manage. 1993, 43, 889. (17) Ghorishi, B.; Altwicker, E. R. Combust. Sci. Technol. in press.
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(18) Dickson, L. Ph.D. Dissertation, University of Waterloo, 1987. (19) Kee, R. J.; Rupley, F. M.; Miller, J. A. CHEMKIN-11 a Fortran Chemical KineticsPackage for theAnulysis of &-Phase Chemical Kinetics; Sandia Report S A N D89-8009.UC-401;Sandia: Livermore, CA, 1989.
Received for review M a y 9,1994. Revised manuscript received October 25, 1994. Accepted January 27, 1995.@
E339402765 @
Abstract published in Advance ACS Abstracts, March 1, 1995.
