Environ. Sci. Technol. 1993, 27, 1602-1610
Influence of Combustion Parameters on the Formation of Polychlorinated Dibenzo-pdioxins, Dibenzofurans, Benzenes, and Biphenyls and Polyaromatic Hydrocarbons in a Pilot Incinerator Ingrid Fangmark,'*t Bert van Bavel,t Steilan Marklund,?Birgitta Stramberg,* Niklas Serge,* and Christoffer Rappet Institute of Environmental Chemistry, University of Ume& S-90 1 87 Ume& Sweden, and Thermal Processes AB, Studsvik, S-611 82 Nykoping, Sweden
A laboratory-scale fluidized-bed incinerator was used to study the influence of several combustion parameters with respect to the emission of important aromatic contaminants including polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), polychlorinated benzenes (PCBz), polyaromatic hydrocarbons (PAHs),and polychlorinated biphenyls (PCBs). The parameters studied include bed temperature, 02-concentration,variations in HC1 and HzO, and temperature and residence time in the postcombustion zone. A two-levelfractional factorial design was used for planning the experiments. Flue gas samples were collected and analyzed on HRGC-HRMS. The most important parameter for the formation of the above chlorinated aromatics was found to be the residence time in the postcombustion zone. A substantial formation of the chlorinated compounds occurred during residence times as short as 1.6 s. PAH formation was found to be influenced by the oxygen concentration in the combustion air, and good combustion conditions favor low PAH emissions.
Introduction
A large variety of halogenated polycyclic aromatic compounds have been identified in the emission from municipal solid waste (MSW) incinerators (1). Chlorinated compounds are preferably formed in MSW incinerators due to the high chlorine content in the municipal refuse compared to other fuels and due to the presence of catalytic metals. Because of the high toxicity of some of the polychlorinated dibenzo-p-dioxins (PCDDs) and the dibenzofurans (PCDFs),they are the combustion products given the most attention, although they constitute only a minor part of the total chlorinated hydrocarbon emission. Other combustion-related sources releasing PCDDs and PCDFs into the air include metal refineries, the steel industry, and car exhausts (2). Polychlorinated benzenes (PCBzs) are another group of chlorinated substances identified in the emission from MSW incinerators (3). PCBzs are considered to be possible precursors to the PCDDs (4,5). Polycyclic aromatic hydrocarbons (PAH) are formed in a range of combustion processes, but wood burning and automobile combustion are considered to be the dominating sources for PAHs (6). Polychlorinated biphenyls (PCBs) have been identified in fly ash as well as in flue gas from MSW incinerators ~~~~
~
* Corresponding author.
+ University of UmeH.
* Thermal Processes AB. 1802 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
(7-10). Although PCBs are present in the waste to a lesser extent, they could also be formed by dimerization of PCBzs during combustion, as suggested earlier (4, 5). Much attention has been directed toward elucidating the correlation between PCDD and PCDF emission with combustion conditionsand fuel composition. A correlation with combustion parameters such as CO, 0 2 , and combustion temperature was reported by Hasselriis (11), Tysklind et al. (12), and Pitea et al. (13). Suggestions concerning the correlation between the HC1concentration in the flue gas, or the chlorine content of the refuse, and the emission of PCDDs and PCDFs have been made (1417) but no clear consensus has yet been reached (18). In other studies, low temperature surface-catalyzedreactions in the cooler parts of the incinerator were shown to be the dominant mechanism for the formation of PCDDs and PCDFs (19-22). A correlation between the emission of PAH and poor combustion conditions has been reported (23-25). However there are conflicting data regarding the correlation between the emissions of PCDD/Fs and PAHs (22-24). Conditions that favor the production of PAHs have been reported to suppress formation of PCDD/Fs (23, 24), though Benefanti et al. (22)have reported simultaneous increases of flue gas concentrations of PCDD/F, PCB, and PAH during the cooling of the flue gases. The majority of published data concerning the presence of PCDD/F, PCBz, PAH, and PCB in MSW incinerator flue gases have been obtained from samples collected after the air pollution control (APC) systems in full-scale incinerators. Such data reflect the combined effect of the combustion process, heat exchange in the boiler, and the APC systems. With full-scale experiments there is also a risk that memory effects between experiments will bias the results and obscure potentially important parameters. Laboratory-scaleflow reactors, with a fixed bed of fly ash, have been employed as an alternative study approach. The disadvantage with this experimental setup is that it is uncertain if a steady-state approach will adequately mimic incinerator conditions (26). Furthermore, it is impossible to simulate the continuous supply of fresh fly ash surface that is available in the postcombustor catalytic environment in a real incinerator. The goal of the present study is to reach a better understanding regarding which combustion parameters are important for the emission of PCDD/F, PCBz, PAH, and PCB from combustion processes. A laboratory-scale fluidized-bed reactor was used for the experiments. By this approach, the formation conditions and the fly ash surface properties will be similar to full-scale incinerators without the major drawbacks with full-scale experiments such as high costs, carry over between experiments, and 0013-936X/93/0927-1602$04.00/0
$3 1993 American Chemlcal Society
Table 1. Composition of Synthetic Fuel wwte fraction
component
weight fraction (% )
kitchen sweepings
bone meal potato starch bleached pulp unbleached pulp polyvinyl chloride (PVC) polyethylene tetramethylthiuram disulfide Fe Alc13 cuc12
25.1 25.1 17.5 17.5 1.0 7.5 0.5 5 0.5 0.25
paper plastics, rubber metal fraction PRIMAI1"A,P
+c ' - + ITARTUPCIS 1
Flgura 1. Pllot Ruldlred-bed reactor. T, = thermocouple for the bed temperature; T,-TB = freeboard temperature measurements; Tk,Tk, = thermocouple posiiions in the cooling sectbn.
difficulties to obtain large variations in operating conditions. In the laboratory-scale reador, variable operating conditions can be more easily controlled, simulated, and reproduced. A well-defined synthetic refuse was developed in order to eliminate variations due to inhomogenity in the fuel (27).An experimentalplanof afractionalfactorialdesign was used to study the influence of bed temperature ( T w ) , Orconcentration (Oz),fluegastemperatureatthesampliig port in the cooling section (Ts),residence time ( t ) in the cooler, and HC1 and HzO content of the fuel on the formation of PCDDIFs, PCBz, PAH, and PCBs. In an earlier phase of this study, it was verified that the combustion reactor and the synthetic fuel could be used to simulate municipal waste incinerators (27). This was seen from comparison of the isomeric pattern for the PCDD/Fs in the emission from the laboratory-scale reador compared to a full-scale incinerator. PCDDs and PCDFs were evaluated in the subsequent screening experiments (28). These initial results show that the formation of PCDDIFs is primarily influenced by the residence time in the cooling section of the pilot reactor. In the present study,we haveincludedanalysesofPCBz,PAH,andPCBs of the collected flue gas samples in addition to a more detailed analysis on the PCDDIF data. Materials and Methods Combustion Reactor and Synthetic Fuel. The laboratory-scale fluidized-bed reactor is shown in Figure 1. The reactor has a high freeboard connected to a cooling section which simulates the boiler in an MSW incinerator. The flue gas temperature profile in the cooling section can be varied independently of the temperature in the combustor by cooling or electric heating or by means of insulating the different parts of the cooler. There are two sampling porta available in the cooling section where flue gas samples with different residence times in the combustion and postcombustion zones could he collected. It is also possible to supply additional HC1 or H20 to the primary air. Olivine sand (Mg,Fe)SiOd was used as bed material. Temperatures are monitored in the sand bed ( T w )at three levels in the freeboard (Tn-Tn) and at four positions in the cooling section (TLI-TL.~. Figure 1. The concentrations of 0 2 , CO, Con, and NO are continuously monitored in the flue gases. The supplied air flow and the HC1 and H20 feed rates are monitored by rotameters.
The composition of the synthetic fuel was decided after a survey of the main components of Swedish household waste and their mass fractions (Table I). A mixture of the solid ingredients with water solutions of the salts was extruded to small pellets with a dimension of 6 mm in diameter by 1-2 cm length. The total chlorine content of the fuel is 1 '% ,and the water content is 7.7 '% . The reactor and the synthetic fuel have been described earlier (27, 28). Screening of Experimental Variables. To select a suitable experimental design depends on the complexity of the problem, the number of experimental variables involved, and the question posed. As we are in an early stage of the investigation, there are a large number of variables that could be expected to influence the experimental outcome. The most efficient strategy is therefore to perform the first experiments in such a way that each important variable will be identified, through an arrangement of the experiments such that the experimental variables are orthogonal to each other. In such screening experiments, the variables need onlyto be investigated on two levels. The experimentalplan can be reduced further only if a balanced fraction of the full experimental plan, Le., all combinationsof variables and levels, is performed. Such a fractional factorial design, as described by Box et al. (29).was selected for the experiments reported in this paper. Because a subset of the full factorial design is selected, some information will be lost. These will show up as confoundings between model parameters. For more details on two-level designs, see ref 30. The combustion parameters considered as potentidy important to study with the two levels selected are given in Table 11. The levels were chosen to be within the relevant domain for MSW incinerators and a t the same time to provide measwable differencesin emission between the high and low levels (Table 11). Since HC1 is the main product from thermal decomposition of PVC, the addition of HCltotheprimary air was deemed apracticalaltemative to using a second fuel with a higher PVC content. To identify those variables that have a significant influence on the emission of PCDD/Fs, PCDFs, PCBz, PAHs, and PCBs, a total of 16 experimental runs was performed following an experimental plan of a two-level fractional factorial design, ;2 (Table 111). This design was chosen since it is considered important to estimate the parameter describing the main effect of HC1 free of confounding with the main effect. The following main effectswillbeconfounded TWwithtH20,Ozwith T.H20, T. with 02H20, t with TwH20, and H20 with OzT. as well as Twt. Sampling and Analysis. Before the starting up of an individual experiment, the reactor and the cooling section Envtm. Sd. Tsdnol.. Vol. 27. No. 8. 1083 180s
Table 11. Levels for Combustion Variables with Standard Error, As Obtained in the Screening Experiments (-1 = Low Level, +1 = High Level) level variable
-1
+1
bed temp ("C), T w 02-concentration (%), 0 2 flue gas temp at sampling point ("0, T, HC1 added to primary air (g/h), HC1 H2O added to primary air (g/h),H2O residence time in cooler (s), tb
760 f 3 4.9 f 0.2 264 & 13
874 f 1.5 7.5f 0.2 456 f 6
00
10.2
00
136.5
0.6& 0.05
1.6f 0.1
In the low-level experiments, no additional HCl or HzO was intentionally supplied. The high level corresponds to burning a fuel with 2.2% PVC and 17% moisture content. Calculated from total air supply, gas composition, and local temperatures. (I
*
Table 111. Experimental Design (-1 = Low Level, +1 = High Level): Variables According to Table I1 variable and level expt no.
Tw
0 2
T,
HCl
HzO
t
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
-1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +I -1 +1 -1 +1
-1 -1 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1
-1 -1 -1 -1 +1 +1 +1 +1 -1 -1 -1 -1 +1 +1 +1 +1
+1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 -1
+1 +1 -1 -1 -1 -1 +1 +1 +1 +1 -1 -1 -1 -1 +1 +1
-1 +1 +1 -1 +1 -1 -1 +1 -1 +1 +1 -1 +1 -1 -1 +1
were vacuum cleaned and the bed material was replaced. By this procedure, it has been shown earlier that carryover between experiments could be kept on a tolerable level (28). The reactor was heated to the desired temperature, using propane as a fuel, before the synthetic fuel was introduced. All operating parameters were allowed to stabilize on the desired levels before sampling during typically 0.5 h. Flue gas volumes between 60 and 100 L were collected isokinetically. The cooled probe-polyurethane foam sampling method was employed (31,32).With this technique, the flue gases are cooled rapidly in order to quench further chemical reactions. A series of 13Clz-labeled 2,3,7,8-substituted PCDD/Fs were used as presampling spikes, according to the protocol described by Marklund (32). This is to control losses during sampling. Duplicate samples were collected consecutively in each experiment. To check for memory effects between experiments, one blank sample was collected in the middle of the experimental series after the normal cleaning of the reactor. Propane was used as a fuel in the experiment when the blank sample was collected. All samples were extracted and cleaned up according to Marklund (32). Each extract was then equally divided into two aliquots. The first part was used for analysis of seven toxic isomers of PCDDs and 11 PCDFs, the sums 1604 Envlron. Scl. Technol., Vol. 27, No. 8, I993
of each congener group was estimated, and three planar PCBs (IUPAC nos. 77, 126, and 169) were determined. The second part was analyzed for 13 PAHs and 10 PCBz. Analyses of PCDDs and PCDFs were made on highresolution gas chromatography/high-resolutionmass spectrometry (HRGC/HRMS,VG 70E). Cleanup and analyses of PAH, PCBz, and PCBs were made according to van Bavel et al. (33). HRGC and low-resolution mass spectrometry (HRGC/LRMS, VG 12-250) was used for the analyses of PAH and PCBz. The reported levels were corrected for losses during sampling and cleanup. Principal Component Analysis. A data table comprised of measured responses (columns),e.g., amounts of PCDDs formed under different experimental conditions, contains two kinds of variation: between-experiment variation of each response and within-experiment variation of the responses. Principal component analysis (PCA) separates these variations into two entities, scores and loadings (see below). PCA is primarily used to extract the dominant patterns of variation in large data tables (34, 35). A geometrical description of the principles of PCA is as follows: From a data table with n experiments described by m responses, each experiment can be described as a point in a m-dimensional space defined by the m response variables. The n experiments will then constitute a swarm of points in this space. The first principal component (PC) loading vector defines the direction through the swarm of data points in such a way that the largest variance of the distribution will be described. (If X is the average centered data table, the first principal component loading vector p1 is an eigenvector to the variance-covariance matrix X'X.) The next PC dimension is calculated orthogonal to the first and describes the next largest variation. This is continued until the systematic variation is exhausted. The number of significant PCs is determined by crossvalidation. Full details on cross-validation has been given by Wold (36). The scores are the positions of the experimental points projected onto the loading vector. Experiments with similar outcome lie close to one another in the m-dimensional space and will, therefore, have similar score values. Accordingly, experiments with divergent outcome will be apart in the m-dimensional space and, hence,have different score values. The scores can thereby be used to compare experiments with each other and to evaluate how the experimental variables influence the pattern of variation of the observed responses. Interpretations of the PCA is facilitated by having the dominant trends visualized in plots. These plots are used to study relationships among objects, score plots, and between variables, loading plots. The data reduction capability of PCA is useful when trying to elucidate which experimental variables are responsible for the gross systematic variation of many measured response variables. From a data table containing analytical data for similar compounds, such as all isomers and sums of PCDDs, a single response value could be calculated by PCA, as described by Carlson et al. (37). The new response values are the score values from PCA, because they portray the systematic variation of analyses over the set of experiments. Calculations were made by the SIMCA 3B and MODDE softwares (Umetri, UmeA, Sweden).
Table IV. Average Levels and Standard Error of Monitored Combustion Parameters, Not Included in Experimental Design. combustion parameter
level
combustion efficiency (% ) NO (ppm)
99.776 -+ 0.04 182 f 6 883 f 9 827 9 718 9 585 k 16 5.1 (2.6-9.4)
Tn ('0
* *
Tm ('C) Tp3 ("'2) Tu ("C) total air flow (std m3/h) primary air flow (std m3/h) secondary air flow fuel feed rate (kg/h)
1.ob
4.2 (1.8-8.1) 1.0-1.5
a The values given for the air flows are the average, with the lowest and highest values between parentheses. Parameter held constant during the exDeriment.
*
Results and Discussion The averages of the operation parameters, varied in the screening experiments, are reported in Table 11, and those monitored during the experiments are presented in Table IV. Levels. The resulting flue gas concentrations in the experiments, in the blank sample, and in the levels in the fuel are given in Tables V-IX. A comparison between the levels of PCDD/Fs, as toxic equivalents (I-TEQ) from Table VI, shows that thevariation between the two samples collected in each experiment could be as high as a factor of 2, although in most cases around 30%. The highest variation, between 1.2 and 8.4 ng/std m3, 10% COz, was obtained in experiment 10. Because the recordings of the combustion parameters were lost for the second sample, (10:2), any variation caused by combustion problems cannot be verified. The emission levels of PCBz and PAHs (Tables VI1 and VIII) are in the pg/std m3 range while PCDDs, PCDFs, and PCBzs (Tables V, VI, and IX) are 3 orders of magnitude lower (ng/std m3). These levels are in accor-
dance with earlier measurements on incinerator flue gases (7,24).For PCDD/Fs, they are in the same range as has been found in incinerators operating without any APC device. Calculation based on fuel content of PCDD/Fs, PCBz, PAH, and PCB total air flow show that all of the above substances must originate from reactions occurring during the combustion process or during coolingof the flue gases. Isomeric Distribution, The isomeric distribution of PCDDs and PCDFs is similar to what can be found from MSW incinerators, as reported earlier (27). The distribution between the total of PCDD/Fs is, however, shifted toward a higher relative content of the PCDFs to what is normally found in MSW incinerator raw gases. Among the PCBzs, pentachlorobenzene was found to be the dominating isomer, followed by the tri-, tetra-, and hexachlorobenzenes. The main isomer of trichlorobenzenes was the 1,2,3-substituted, and of the tetrachlorobenzenes it was the 1,2,3,4-substituted. This dominance of pentachlorobenzene was also reported by Taylor et al. (7) from an MSW incinerator and by Stieglitz et al. (38) from a flow reactor experiment. From a similar substitution pattern of the trichlorobenzenes, Jay and Stieglitz suggested a radical chlorination mechanism (39). For PAHs there is a dominance of the lower molecular weight compounds, with the highest concentration of naphthalene. The isomeric distribution of the three planar PCBs resembles what is found in fly ash samples (33)with PCB 77 and 126 in the same order and 169 somewhat lower and adding up to about 10% of their congener group (tetra, penta, hexa). Screening Experiments. Relationship between Parameters. The outcome of the screening experiments performed resulted in a data matrix with 32 experiments each described by 57 measured responses, i.e., the levels of the six operating parameters and the resulting flue gas concentrations of PCDDs, PCDFs, PAHs, PCBzs, and PCBs (11PCDDs, 14 PCDFs, 13 PAHs, 10 PCBzs, and 3 PCBs). In this paper, this information is separated into Tables 11, 111, and V-IX.
Table V. Flue Gas Concentrations of PCDDs (ng/std m3, 10% COz) in Experiments and Levels in Synthetic Fuel (pg/g) experimentsample 2,3,7,8-TCDD sum TCDD 1,2,3,7,8-PeCDD sum PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7&HxCDD 1,2,3,7,8,9-H~CDD sum HxCDD 1,2,3,4,6,7,8-HpCDD sum HpCDD OCDD
1:l
1:2
2:l
2:2
31
3:2
41
42
51
52
6:l
6:2
7:l
7:2
8:l
82
