Environ. Sci. Technol. 1994, 28,624-629
Influence of Postcombustion Temperature Profiles on the Formation of PCDDs, PCDFs, PCBzs, and PCBs in a Pilot Incinerator Ingrid Fangmark,'vtv* Birgitta Stromberg,l Nlklas Berge,§ and Christoffer Rappet
Institute of Environmental Chemistry, University of Umea, S-90 1 87 Sweden, and Thermal Processes, Studsvik, S-611 82 Nykoping, Sweden
A laboratory-scale fluidized-bed reactor fueled by a synthetic waste was used to study the influence of the flue gas temperature profile after the combustor on the formation of chlorinated aromatic compounds. An experimental plan of full factorial design involving the variables temperature and residence time was chosen for the experiments. Flue gas samples with residence time in the cooling section of the reactor between 0.9 and 2.9 s were collected at temperatures between 260 and 510 "C. Response surface models describing the formation of toxic equivalents of polychlorinated dibenzo-p-dioxins, dibenzofurans, and benzenes as a function of temperature and residence time were constructed from the analytical results. The samples were also analyzed for non-ortho-polychlorinated biphenyls. The results show that all the above compounds exhibit a similar dependency on temperature and residence time. The highest levels were obtained at 340 "C and 2.9-s residence time, and the lowest levels were obtained under conditions of rapid quenching of the flue gas temperature t o 260 "C.
Introduction Since the discovery of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) emissions from considmunicipal solid waste (MSW) incinerators (1,2), erable efforts have been made to learn more about the formation and degradation of these compounds during combustion. Such information is essential in order to reduce the formation of chlorinated aromatics from combustion processes and, further, to apply appropriate abatement technologies. A variety of experimental designs have been used to simulate incinerator conditions in the laboratory (3).Heat treatment of fly ash from electrostatic precipitators has been applied to study the influence of temperature on the formation of PCDDs and PCDFs. This way it was shown that there is a substantial increase in the levels of PCDDs and PCDFs at temperatures around 300 "C (4-7). At higher temperatures, 600 "C, or under oxygen deficient conditions, degradation or dechlorination/hydrogenation reactions dominated (6, 7). Investigations in MSW incinerators revealed that levels of PCDD/Fs in the stack far exceed those measured in the furnace (8-13). These findings support the low-temperature surface-catalyzed reaction mechanisms suggested by Shaub and Tsang (14). The proposed mechanism requires a solid surface, such as fly ash, with catalytic sites (3). The fact that the formation of many chlorinated species is frequently influenced by reactive surfaces supports this theory (14). Today the
* To whom correspondence should be addressed. + University of Umea. t Present adress: National Defence Research Establishment, S-901 82 Umel, Sweden. Thermal Processes.
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Environ. Sci. Technol., VoI. 28, No. 4, 1994
importance of non-gas-phase surface reactions are generally accepted in kinetic models of PCDD and PCDF formation (15-17). In order to study full-scaleincinerators in the laboratory, it is of course vital to use relevant experimental conditions that mimic the real world. The simulations done in the laboratory using heated MSW fly ash demonstrate the importance of low-temperature catalytic formation of PCDDs and PCDFs. The substantial increase of PCDD/F formation at temperatures around 300 "C and decreases observed at higher temperatures suggest that PCDDs and PCDFs are most efficiently formed within a narrow temperature window around 300 "C (18). However formation rate calculations using data collected over hours in fly ash heating experiments cannot probe the secondscale formation events which occur in real incinerators (3,151. One reason for this is that this way it will be neither possible to simulate the fresh fly ash surface available in full-scale incinerators nor the continuous generation of new fly ash, which supplies reagents in a high quantity for chemical reactions. Furthermore, fly ashes used in heating experiments are often heat treated and solvent extracted, which also will affect surface properties. Another fact that has to be considered is that if the active sites on fly ash particles are being formed via condensation of vaporized metals or metal compounds, these will be primarily associated with the submicron fly ash particles passing the electrostatic precipitator (3,19). Fly ash composition will also vary widely with the nature of the fuel waste combusted, which makes data from different fly ashes difficult to use when evaluating fundamental parameters (3). In previous papers, we have reported on the formation of PCDDs, PCDFs, polychlorinated benzenes (PCBzs), polyaromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) studied in a laboratory-scale fluidizedbed reactor (20,21). Using this reactor the combustion conditions and the fly ash surface properties are similar to full-scale incinerators. The pilot reactor is more practical than full-scaleexperiments, which are prohibited by high costs, carryover problems between experiments, and difficulties in controlling and varying operating conditions. The operating conditions of the pilot incinerator simulate full-scale reactors, and combustion parameters are more easily reproduced. Since the composition of fly ash is an important parameter for the formation of chlorinated aromatics, the experiments described here were performed with a synthetic fuel. Previous screening experiments verify that the chlorinated aromatics considered in this report are formed downstream of the combustor and that the most important parameters are the temperature and residence time in the cooling section of the reactor (21). The objective of the present study is to analyze data that are relevant for short time-scale formation of PCDDs and PCDFs in the postcombustion environment of MSW incinerators. PCBzs 00 13-936X/94/0928-0624$04.50/0
0 1994 Amerlcan Chemical Society
FUEL
HEATER SP3 Tk5
Tk6
SP4 Tk7 OBSERVATION PORTS
AIR DETRIBUTER START UP GAS
Flgure 1. Pilot fluidized-bed reactor with thermocouple positions (Tf and Tk) and sampling points (SP1-SP4).
and three non-ortho-PCBs were also analyzed in the collected samples, in efforts toward seeking a broader understanding of the formation of chlorinated aromatic compounds. Response surface models that describe the influence of residence time and temperature on the formation of PCDDs, PCDFs, and PCBzs were determined.
Table 1. Properties and Elementary Composition of Synthetic Fuel, Based on Analysis of Dry Material Properties calorific heat value (MJ/kg) ash content (%) HzO content ( % )
17.6 14.4 7.7
Materials and Methods Combustion Reactor and Synthetic Fuel. A laboratory-scale fluidized-bed reactor, (Figure 1)was used for the experiments (22). The reactor was constructed for flexible operation within a region defined by full-scale incinerators. It has a high freeboard designed for efficient burn-out of volatile combustion products. The reactor has an internal diameter of 100 mm up to the height of 350mm, where it is expanded to 125mm in order to reduce elutriation of bed material. The fluidized bed height is 50 mm, and the freeboard height is 1.5 m. Dried filtered air is used as fluidizing media, and the gas velocity in the bed is 1m/s. Combustion air is supplied both as primary air to the fluid bed and above the bed and as secondary air. Propane is used as a fuel during the startup of the reactor. Olivine sand, with an original mean particle size of 0.33 mm, is used as bed material. During a test run, the average particle size will increase slightly due to elutriation of fines. By using the bed cooler, the bed temperature can be maintained at a constant level despite variations in load and air distribution. The reactor is constructed in stainless steel (17.5 % Cr, 12 % Ni, 2 % Mo) as is the rest of the unit. The outlet of the reactor freeboard is connected to a cooling section which simulates the boiler in a MSW incinerator. The flue gas temperature profile in the cooling section can be varied independently of the temperature of the combustor by cooling, by electric heating, or by adjusting insulation over the different parts of the cooler. Flue gas samples with different residence times in the combustion and postcombustion zones are collected at any of the four sampling ports (SP) available (SP1-SP4, Figure 1). In this respect, the reactor was modified compared to the earlier design (20-22). During the experiments, temperatures are monitored every 30 s in the bed, at three places in the freeboard (Tfl-TB), and at seven places in the cooling section (TklTk7) by thermocouples, as shown in Figure 1. 02,CO,
element
C
H N 0
S C1 Na
Elementary Composition concn (% ) element 42 6.0 1.6 35 0.21 0.69 0.2
K Ca
P
cu Fe A1
concn (%) 0.17 4.4 1.9 0.056 3.4 0.39
COz,and NO are also continuously monitored in the flue gas. The sampling point for the gases is at the end of the gas cooler. Total C1- content of the flue gas is monitored during each experiment by passing a flue gas sample through dilute NaOH. The chlorine content was then determined by titration with Ag(N03)~. The synthetic fuel has a composition that reflects the main components in Swedishhousehold waste. Bone meal, potato starch, and bleached and unbleached pulp are the main components used (22,23).The fuel has the following chlorine sources: PVC ( l % ) , AICL (0.5%), and CuC12 (0.25 % 1. Apart from Cu and Al, the metal fraction also include 5 % Fe. The fuel was prepared by extruding the mixed ingredients into small pellets (6 mm in diameter X 1-2 cm length). Properties and elementary composition of the synthetic fuel are given in Table 1. Experimental Design. The variables and levels used in the experimental runs are presented in Table 2. A full factorial experimental design, in the variables residence time and sampling temperature, at three levels complemented with three centerpoints was employed (24)(Table 2). The residence time is defined as the time spent by the flue gases from the freeboard exit to the sampling port used. For samples collected after SP2, two temperature regimes are therefore included in the residence time: a short quench regime (from the freeboard exit to SP2)where the flue gases are cooled from 740 OC to the sampling Environ. Scl. Technol., Vol. 28, No. 4. 1994 625
Table 2. Experimental Design, Variables, and Levels8 time (s)
temp ("C)
time (s)
temp ("C)
260 2.9 340 260 0.9 510 (430)b 510 260 1.4 340 2.9 430 340 a Time = residence time between the freeboard exit to the sampling port. Temp = sampling temperature. b430 "C at 0.9- and 2.9-5 residence times; 510 O C at 0.9 and 1.4 s. 0.9 1.4 2.9 0.9 1.4
Table 3. Average Combustion Conditions in Experimental Runs and the Standard Deviation Obtained between Experiments parameter
level f SD
bed temperature ("C) freeboard 1 temperature ("C) freeboard 2 temperature ("C) freeboard 3 temperature ("C) total air flow (std m3/h)a primary air flow (std m3/h)a secondary air flow (std m3/h)a fuel feed rate (kg/h)a
875 f 3 904 f 1 830 f 8 740 f 12 7.8 6.82 0.95 1.4
Parameter kept at a constant level. Table 4. Composition of Presampling Spike
250 ng of [13C&1,4-dichlorobenzene 250 ng of [13Csl-1,2,4-trichlorobenzene 250 ng of [13C&1,2,4,5-tetrachlorobenzene 250 ng of [13C~lpentachlorobenzene 250 ng of ["Cp,]hexachlorobenzene 5 ng of [l3C12]-2,3,7,8-TCDF 5 ng of [l3C121-2,3,7,8-TCDD 5 ng of [l3C12]-2,3,4,7,8-PeCDF 5 ng of [13C1~1-1,2,3,7,8-PeCDD 5 ng of [13C1~1-1,2,3,6,7,8-HxCDF 5 ng of [13C1~1-1,2,3,6,7,8-HxCDD 5 ng of [l3C1zl-1,2,3,4,7,8,9-HpCDF 5 ng of [13C121-1,2,3,4,6,7,8-HpCDD 5 ng of [13C1210CDD
temperature, followed by a longer isothermal zone. The sampling temperatures were selected to cover the range of experimental domain obtainable in the cooling section of the reactor. The maximum maintainable temperature throughout the entire cooling section was 430 "C. To provide the same output of precursors from the freeboard exit to the cooling section, the combustion parameters were set at the same levels in all experiments (Table 3). Sampling and Analysis. The reactor was preheated with propane as a fuel, before the synthetic fuel was introduced. At stable operating conditions, flue gas samples of between 80 and 400 L were collected isokinetically. Flue gas samples were collected in SP2, SP3, and SP4 to obtain the desired residence time interval (Table 2). The "cooled probe-polyurethane foam" sampling method was employed (25,26). With this technique the flue gases already in the probes are cooled rapidly to room temperature in order to quench further chemical reactions. Presampling spikes of 13C-labeled PCDD/Fs and PCBzs were used as shown in Table 4 to control and correct for sampling losses. The experiments were performed in a random order, and the run corresponding to the center point of the experimental plan was repeated in three separate experiments. Between experiments, the reactor and the cooling section were vacuum cleaned and the bed material was replaced. A blank sample was 626 Environ. Scl. Technol., Vol. 28, No. 4, 1994
Table 5. Parameters Monitored in Flue Gases, Averages and Standard Deviations between Experimental Runs parameter 0 2 (%)
coz ( % ) CO (ppm) NO (ppm)
HCl (mgistd m3)
level f SD
6.7 f 0.7 12.7 f 0.6 240 f 16 270 f 4 735 f 200
collected in a separate experiment after cleaning the reactor using propane as a fuel. All samples were extracted and cleaned up according to Marklund (26). Each extract was then equally divided into two aliquots. The first part was examined for seven toxic PCDDs and 11toxic PCDFs; the sum of each congener group was estimated. Three planar PCBs (IUPAC numbers 77,126, and 169) were also determined. The second aliquot was analyzed for 10 PCBz isomers. Cleanup and analyses of PCBz and PCB were performed according to van Bavel et al. (27). Analyses of PCDDs, PCDFs, PCBz, and PCBs were made on high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS, VG 70E) at the Institute of Environmental Chemistry, Umel, Sweden. The analytical results were corrected for sampling losses relative to the recovery of the sampling spike. Data Analysis. The data analysis and the response surface modeling were performed with the statistical package MODDE (Umetri, S-90124 Umel, Sweden). Due to the skewed distribution of the analytical data, logarithmic transformation was performed (24). This transformation will make the distribution of the analytical data resemble the normal distribution more closely. In the analysis of the factorial design, the relationship between the measured responses (emission levels of chlorinated aromatics = Y) and the corresponding setting of the variables, residence time ( t ) and sampling temperature (Ts),will be modeled as
Y = bo + blt
+ b,T, + b,,Ts + bflt2+ bi2c+ e
(1)
Here bo is the model intercept, bl and b2 are the regression coefficients which express the main effects oft and T,, and e is the experimental and model error. The coefficient b12 expresses the conjugate interaction between t and T,. The quadratic terms allow a curved response surface to be fitted to the measured responses. The coefficients of the model are estimated by multiple regression. For a more detailed description of the analysis of factorial designs, see Box et al. (24).
Results and Discussion Combustion Conditions and Temperature Profiles. The average levels of the combustion parameters and the standard deviation obtained between the experimental runs are given in Table 3, and those monitored in the effluent are given in Table 5. The combustion parameter with the largest variation is the 0 2 concentration with a relative standard deviation of 10% of the average value. This variation was considered to be unimportant because earlier results from the pilot reactor showthat no difference in emission levels of chlorinated aromatics was obtained between 4.9% and 7.5% 02 (21). Temperature profiles in the cooling section, from representative runs, are shown in Figure 2 as the average temperature reading from thermocouples during the
800 '0°
7
t---l
*0°
1
[-*-51w;
O
L TI3
Tkl (SPO
+ ,
- * -430°C
100
~
Tk2
Tk3
Tk4 (SP2)
Isothermal part
Tk5 (SP3)
Tk8
Tk7 (SP41
Thermocouple
Figure 2. Temperature profiles in the cooling section starting with the freeboard exit temperature (Tf3) for some representative runs.
sampling period. The two upper curves represent samples collected at the high temperatures (430 and 510 "C), the middle is a sample collected at 340 "C, and the lower curve is a sample collected at the low temperature (260 "C). Differences in cooling rates are shown in Figure 2. Levels. The resulting flue gas concentrations of PCDDs, PCDFs, PCBzs, and PCBs in the experiments are presented in Tables 6-9. The concentrations of toxic isomers of PCDDs and PCDFs and total sums for each congener group, corrected for sampling losses, are given in Tables 6 (PCDD) and 7 (PCDF). Emission levels expressed as toxic equivalents, calculated according to the international model (I-TEQ) are also included in Table 7. As shown by Tables 6 and 7 there is a dominance of PCDFs over PCDDs in the flue gases. In most samples, the PCDFs account for around 80% of the total I-TEQ
value. This is in agreement with data reported from MSW incinerator flue gases (28, 29). As in the earlier experiments, the isomeric pattern of PCDDs and PCDFs in these samples resembles that normally found from MSW incinerators (22). The congener profile is very similar in all samples (Tables 6 and 7),and no significant difference was detected. These findings disagree with the annealing experiments in which a shift toward the lower chlorinated PCDDs was observed at higher temperatures (6). This difference verifies that conclusions valid for the second time-scale formation events cannot be drawn from experiments performed during hours. Also for the PCBzs and PCBs (Tables 8 and 9), the degree of chlorination is very similar in all samples. The blank values reported in Tables 6-9 are higher than those observed earlier (21). The reason for this might be that this sample was collected during the startup procedure, before reaching the desired temperatures in the freeboard and cooling section. Experience has showed that running the reactor for the time needed for temperature stability will result in decreased blank levels. The sampling of microcomponents in flue gases is subjected to anumber of errors occurring during sampling, cleanup, and analysis of the samples. The total experimental variability encountered will also include variations in the reactor performance and the ability to repeat the combustion conditions. Calculations, using the triplicate runs in the center point of the experimental plan, show that the total experimental variability, expressed as standard error for PCDDs and PCDFs, is between 15 and 30% relative concentration, depending on the levels. For
Table 6. Flue Gas Concentrations of PCDDs (ng/std ma, 10% COz) in Experiments temp ("C) time (8) 2,3,7,8-TCDD sum of TCDD 1,2,3,7,8-PeCDD sum of PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-H~CDD 1,2,3,7,8,9-H~CDD sum of HxCDD 1,2,3,4,6,7,8-HpCDD sum of HpCDD OCDD sum of PCDDs
260 0.9 0.04 1.2 0.3 2.9 0.5 0.5 0.9 6.1 4.9 11 12 33
260 1.4 0.1 8.1 0.6 7.1 0.6 0.7 0.6 9.3 3.8 6.9 7.3 57
260 2.9 0.3 24 0.9 18 1.1 1.4 1.3 24 6.7 14 8.3 88
340 0.9 0.2 9.2 0.9 11 1.1 1.2 1.2 18 6.2 12 9.9 60
340 1.4 0.1 1.8
0.7 4.7 0.9 1.2 1.3 13 6.3 12 9.1 41
340 2.9 0.8 34 4.1 49 5.0 6.1 7.1 68 32 61 42 250
430 0.9 0.1 3.6 0.4 5.5 0.3 0.4 2.9 11 2.1 4.9 3.3 28
510 0.9 0.1 2.5 0.5 5.8 0.7 0.9 0.7 7.8 4.9 9.6 6.8 33
510 1.4 0.1 3.8 0.9 9.7 1.2 1.9 2.6 17 11 20 14 65
430 2.9 0.1 7.5 0.5 12 0.4 0.5 1.0 21 3.7 9.1 4.0 54
340 1.4 0.1 0.9 0.3 2.6 0.5 0.4 0.7 4.9 3.5 7.3 9.3 23
340 1.4 0.4 5.6 0.9 12 0.9 1.6 1.4 15 8.9 18 15 70
blank
510 1.4 0.7 49 5.4 4.7 45 7.3 6.1 0.4 6.7 55 27 3.4 43
430 2.9 1.0 48 3.5 2.5 43 4.6 3.5 0.3 2.4 30 13 2.0 20 5.0 150 3.3
340 1.4 0.2 13 1.4
340 1.4 0.5 27 2.8 1.9 47 4.0 4.6 1.0 3.1 46 20
blank
0.1 0.1 0.1 1.2
0.1 0.4 0.3 3 3.2 8 5.9 18
Table 7. Flue Gas Concentrations of PCDFs (ng/std ma, 10% C02) in Experiments. temp ("C) time (8) 2,3,7,8-TCDF sum of TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF sum of PeCDF 1,2,3,4,7&HxCDF 1,2,3,6,7,&HxCDF 1,2,3,7,8,9-H~CDF 2,3,4,6,7,8-HxCDF sum of HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF sum of HpCDF OCDF sum of PCDFs I-TEQ
260 0.9 0.03 2.5 0.13 0.08 1.9 0.30 0.25 0.04 0.30 2.8 2.9 0.40 5.9 3.4 17 0.23
260 1.4 0.5 68 1.9 2.2 40 2.4 3.4 0.5 2.9 28 10 1.3 18 3.8 160 2.9
260 340 340 340 2.9 0.9 1.4 2.9 0.8 0.6 1.1 4.4 40 340 110 54 3.3 5.7 21 6.6 4.0 3.1 5.1 21 54 380 130 110 8.0 4.4 6.2 34 7.7 4.3 6.9 47 1.0 0.5 1.3 3.1 1.9 6.4 5.4 13 67 75 50 300 19 30 23 170 2.3 3.0 3.3 13 35 42 41 230 5.6 7.5 11 36 350 200 290 1300 6.5 5.6 4.9 28 a TCDD equivalents (I-TEQ)were calculated from Tables 6 and 7.
430 0.9 0.6 39 2.7 1.8 34 3.4 2.5 0.1 2.0 24 10 1.5 17 4.1 120 2.7
510 0.9 0.6 29 2.9 2.7 43 4.8 3.4 0.3 3.8 33 14 2.1 25 4.9 140 3.5
8.1
200 6.3
1.1
22 3.4 2.8 0.1 2.1 26 21
2.0 37 9.1 97 2.2
2.1
32 7.0 160 3.5
0.2 4.5 1.0 0.9 12 0.05 1.6 0.2 0.3 16 9 0.9 14 0.6 47 1.8
Environ. Sci. Technol., Vol. 28, No. 4, iSg4
627
~~
~
Table 8. Flue Gas Concentrations of PCBzs (pg/std ma,10% CO2) in Experiments temp ('C) time ( 8 )
260 260 260 340 340 0.9 1.4 2.9 0.9 1.4 0.15 4.9 6.8 2.8 0.92 1,2,3-tri 0.11 2.0 2.6 1.1 0.88 1,2,4-tri 0.002 0.051 0.12 0.03 0.04 1,3,5-tri 3.9 1.9 7.0 9.5 gum of trichloro 0.25 1.6 0.90 3.5 3.9 1,2,3,4-tetra 0.07 2.2 3.0 0.96 0.62 1,2,3,5-tetra+ 1,2,4,5-tetra 0.05 5.7 6.9 2.6 1.5 sum of tetrachloro 0.12 7.6 0.85 37 12 24 pentachloro hexaehlora 3.2 0.81 0.68 0.12 2.0 39 57 19 12 sum of PCBz (tri-hexal 1.3
340 430 430 510 510 340 340 blank 2.9 0.9 2.9 0.9 1.4 1.4 1.4 16 1.2 2.4 2.9 3.6 0.95 1.4 0.03 4.9 0.51 1.1 1.4 1.3 0.56 0.48 0.20 0.11 0.003 0.004 0.03 0.02 0.011 0.010 0.60 21 1.7 4.3 3.5 4.9 1.52 1.9 0.8 9.9 0.73 2.0 2.7 2.1 1.2 1.5 0.31 5.9 0.58 1.9 1.7 1.5 0.96 1.6 0.94 16 1.3 3.9 4.4 3.6 2.2 3.0 1.2 63 6.1 13 19 15 14 12 7.7 4.1 0.42 1.2 1.4 1.1 1.5 1.0 0.66 104 10 22 29 24 19 17 10
Table 9. Flue Gas Concentrations of Coolanar PCBs (IUPAC Numbers 77. 126, and 169) in Experiments (ng/std mJ, 10% Cod
temp
time
PCB77
PCB126
PCB169
260 260 260 340 340 340
0.9 1.4 2.9 0.9 1.4 2.9
3.7 NAa 26 5.9 3.2 26
5.6 NA NA 8.2 3.6 41
2.0 NA 8.6 3.2 2.1 15
430
1.4
2.0
1.4
0.57
Figure 4. Response surlace lor the formation 01 tri- to hexachlorcPCBr(rrglstdm', 10% COdasa IuncHOnoftemperatueandresaenca time in the cooling section 01 the reactor. :-'
Figure 3. Response surface lor the formation 01 toxic equivalents (I-TEQ) of PCDDS and PCDFs (ng Istd m3, 10% Cop).
thePCBzs, thevariability i s a r o u d 15%. Reponsesurface modeling is expected to extract the maximum available information from the experiments in the presence of noise. Response Surface Models. A response surface model was fitted to the measured emission of toxic equivalents of PCDDiF (I-TEQ) as a function of residence time between 260 and 430 OC (Figure 3). The rapid increase of PCDDs and PCDFs with residence time is demonstrated by Figure 3. The influence of residence time is, however, most pronounced within a temperature range around 340 "C. Onlytowardthe highend ofthe temperature interval, around 440 OC, residence time seems to be unimportant. The model shows that formation of PCDDs and PCDFs in flue gases will be reduced at conditions of rapid quenching of the flue gas temperature, below 260 "C, and a short residence time. The tri- to hexachlorinated PCBzs were used as an estimate of the total sum of PCBz, since the dichloroPCBzs were not considered reliable due to low sampling spike recovery. The resulting response surface model for thePCBzs,giveninFigure4, hasanalmostidenticalshape 628
Environ. Sci. Technol., Vol. 28. No. 4, 1994
'.. ... ...,.-,,-
-__._.-
Figure 5. Flue gas concantrations of PCB 169 (nglstd mS, 10% COS) at different temperature and residence time in the cooling sectbn of the
reactor.
as the one obtained for the PCDDs and PCDFs (Figure 3). Thus, PCBzs, PCDDs, and PCDFs appear to exhibit a similar dependence on residence time and temperature in the cooling section of the reactor. This is also demonstrated by the correlation coefficient between the levels of PCDDs and PCDFs (I-TEQ) and the sum of PCBzs in the flue gases, which is 0.948. It was not possible to obtain a good fit of the PCBs to a response surface model. The resulting flue gas concentrations of the non-ortho-PCBs are therefore illustrated as a bar graph, showing PCB169 as an example (Figure 5). A similar dependence on temperature and residence time for PCDDs, PCDFs, and PCBze between 260 and 430 "C is suggested by a comparison of Figure 5 with the response surfacesinFigures3and4. Formationofthesechlorinated aromatics may therefore occur from a common precursor or by consecutive or concerted reaction mechanisms, affected in a similar way by temperature and residence time.
For all the analyzed compounds, the flue gas levels at 510 "C are higher than those sampled at 430 "C. The response surface models, however, indicate that very low emissions of PCDDs, PCDFs, and PCBzs should be expected at temperatures above 440 "C. Further experiments have to be made to verify this trend, although it is in agreement with the findings of Schwartz et al. (30). They report a second maximum for the formation of organochlorine compounds at 470 "C. Rapid quenching of flue gases seems to be a method to minimize the emission of PCDDs, PCDFs, and PCBzs. Quenching must, however, be done to a temperature below 260 "C since further reactions of precursors to form chlorinated aromatics is taking place within seconds at this temperature. From extrapolation of the response surface models, a flue gas level of PCDDs and PCDFs below 0.1 ng (I-TEQ)/std m3, 10% COZ,is expected by quenching the flue gas temperature to a temperature around 200 "C. To identify the temperature profile associated with the minimal formation of chlorinated aromatics, further studies are needed in the low-temperature region of the model. Conclusions
Response surface modeling of the formation of PCDDs, PCDFs, and PCBzs shows that these chlorinated aromatics exhibit a similar dependence on temperature and residence time between 260 and 430 "C. The highest formation rate is obtained around 340 "C and levels off at higher temperatures. A similar dependence on temperature and residence time was also observed for the PCBs. Formation of these chlorinated aromatics may therefore occur from a common precursor or by side reactions affected in a similar way by temperature and residence time. In order to minimize the formation of chlorinated aromatics, the flue gases should be cooled rapidly below 260 "C because at isothermal conditions formation still occurs at this low temperature. To identify the temperature profile associated with the minimal formation of chlorinated aromatics, further studies are needed in the temperature region below 260 "C. Acknowledgments
The authors thank Dr. Douglas Zook for valuable comments on the manuscript. Bo Strandberg, Gunilla Soderstrom, and Per Andersson are acknowledged for skilful assistance with cleanup and analysis of samples. Financial support from The Swedish National Board for Industrial and Technical Development (NUTEK)and the Center for Environmental Research (CMF) is gratefully acknowledged.
Abbreviations: PCDDs, polychlorinated dibenzo-pdioxins; PCDFs, Polychlorinated dibenzofurans; MSW, municipal solid waste; PCBz, polychlorinated benzenes; PAHs, polyaromatic hydrocarbons, PCBs, polychlorinated biphenyls; SP,sampling port; Tf,temperature monitoring place in the freeboard; Tk, temperature monitoring place in the cooling section; HRGC/HRMS, high-resolution gas chromatography/high-resolutionmass spectrometry; t residence time in the cooling section of the reactor; T,, sampling temperature; I-TEQ, toxic equivalents, calculated according t o the international model.
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Received for review June 2, 1993. Revised manuscript received December 29, 1993. Accepted December 29, 1993.'
* Abstract published in Advance ACSAbstracts, February 1,1994.
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