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Verification of dioxin formation in a catalytic oxidizer
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Chemosphere 72 (2008) 75–78 www.elsevier.com/locate/chemosphere
Verification of dioxin formation in a catalytic oxidizer John R. Hart * California EPA, Department of Toxic Substances Control, P.O. Box 806, Sacramento, CA 95812-0806, USA Received 5 November 2007; received in revised form 25 January 2008; accepted 28 January 2008 Available online 10 March 2008
Abstract Emissions and inlet concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) have been measured from a catalytic oxidizer and a thermal oxidizer. The catalyst inlet temperature was 427 °C. The thermal oxidizer operating temperature was 791 °C. Data of the toxic dioxin and furan congeners are reported. Important results of this field study are: (1) the catalytic oxidizer in this study produced an increase in PCDD/PCDF congener concentration of almost 10-fold from the inlet to the outlet (stack), thus verifying results of a previous study that evaluated only PCDD/PCDF emissions. All congeners increased from inlet to the stack. (2) The thermal oxidizer had little effect on PCDD/PCDF levels. There was a decrease in four of the congeners and an increase in 13 congeners. (3) Ambient air was the main source of PCDD/PCDFs in the stack emissions of the thermal oxidizer in this study. Laboratory investigations are needed to understand how PCDD/PCDFs are formed (and emitted) under conditions of this study. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Dioxins; Catalytic combustion; Oxidation; Chlorinated VOCs, chlorinated hydrocarbons
1. Introduction The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) has been a concern from all types of combustion devices. The low temperature (‘‘temperature window”) formation of dioxins with transition metal catalysts has been reported, but not the formation of PCDD/PCDFs with combustion catalysts with the exception of the recent work of Hart (2004), de Jong et al. (2004), and Cieplik et al. (2001). The fundamentals of catalytic combustion have been studied recently by Van den Brink (1999) and others (Koltsakis and Stamatelos, 1997; Corella et al., 2000; Deutschmann, 2000; Kissel-Osterrieder et al., 2000; Mantzaras et al., 2000; Trevin˜o et al., 2000; Van den Brink et al., 2000; Liljelind and Marklund, 2001; Padilla et al., 2002). Under certain conditions, catalytic combustion has been used to destroy PCDD/PCDFs, as discussed by Blanco et al. (1999) and *
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0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.01.058
Liljelind et al. (2001). This paper verifies results from a previous study by Hart (2004), which evaluated data from 57 PCDD/PCDF emissions field tests on catalytic and thermal oxidizers. In this study, PCDD/PCDFs were measured at the inlet and outlet of one catalytic oxidizer and one thermal oxidizer (both not part of the previous study), in order to verify whether PCDD/PCDFs are produced or destroyed by the oxidizers. 2. Description of field tests Many sites with soils contaminated with chlorinated VOCs are cleaned by soil vapor extraction (SVE), by applying a vacuum to an underground vapor well. The higher the compound’s vapor pressure, the more readily it will be transported to the gas phase from the soil. A blower causes the extracted gas to be transported from the soils to the wells and ducts. The contaminated gas may contain chlorinated and non-chlorinated VOCs, and is then burned in a catalytic oxidizer or thermal oxidizer, prior to release through a stack.
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J.R. Hart / Chemosphere 72 (2008) 75–78
The catalytic oxidizer in this study is a King–Buck Model HDC-200EX containing a Degussa ‘HDC’ catalyst (0.15% Pt + 0.15% Pd on Alumina). The inlet vapor stream contained mainly trichloroethylene, but only total hydrocarbons were measured during the test, averaging 55 ppm V as methane. The inlet vapor stream is pre-heated by both an electric heater (25 kW) and a heat exchanger from the catalyst exhaust, resulting in a 427 °C gas at the inlet of the catalyst. The gas flow rate into and out of the catalytic oxidizer is 6 standard m3 min 1 (dry basis and at 20 °C and 760 mmHg). No combustion air is fed to this oxidizer, but the outlet gas contains an average oxygen concentration of 20.3%. Average emissions data also indicated 8.3 ppm V of total hydrocarbons as methane. The thermal oxidizer in this test is a Soil Therm Model CLR2002. The oxidizer includes a heat exchanger, venturi quench scrubber and a packed tower scrubber. The oxidizer was operated at 791 °C during the test. A caustic scrubber solution is used with a pH from 7 to 9. In the quench, the gas is cooled to about 71 °C. The oxidizer is fired with natural gas at a rate of 293 kW. The maximum inlet gas flow rate is a nominal 10 standard m3 min 1 at the SVE blower. However, the average blower flow rate during the tests was 2 dry standard m3 min 1 at the inlet and 5.5 dry standard m3 min 1 at the outlet. Combustion air is fed to the burner. The inlet sampling point is prior to the soil gas blower and combustion air feed locations. The inlet contained an average oxygen concentration of 6.3%, and an average hydrocarbon concentration of 599 ppm V as methane. Average emissions data also indicate 34 ppm V of CO, 0.2 ppm V of total hydrocarbons as methane, and 13.9% O2. The PCDD/PCDF sampling method used is CARB’s Method 428 (www.arb.ca.gov/testmeth/vol3/M_428.pdf). Similar to US EPA’s Method 23 sampling train (see www.epa.gov/ttnemc01/promgate/m-23.pdf), CARB Method 428 is an extractive adsorbent technique. The analytical methods are high resolution gas chromatography followed by high resolution mass spectroscopy. The method requires a gas correction to standard temperature (20 °C) and pressure (760 mmHg), but not for air dilution. The gas volume is presented at its sampled oxygen content, on a dry basis, as dry standard cubic meters (ds m3). Depending on the PCDD/PCDF congener, detection limits are on the order of 10 5 ng ds m 3. The methods incorporate internal standard recoveries and blanks to insure data quality. Besides PCDD/PCDFs, other products of incomplete combustion are not determined during this study.
Table 1 Catalytic oxidizer results – 17 PCDD/PCDF congeners Inlet (ng ds m 3) Data represent an average 2,3,7,8-TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDF 2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD
Outlet (ng ds m 3)
of 3 triplicate runs 5.07E – 04 23.1E – 04 0.72E – 04 8.26E – 04 5.70E – 04 17.2E – 04 1.58E – 04 19.8E – 04 2.19E – 04 26.5E – 04 <0.23E – 04 1.41E – 04 6.96E – 04 39.3E – 04 14.0E – 04 170E – 04 1.60E – 04 17.8E – 04 6.30E – 04 77.0E – 04 0.32E – 04 0.85E – 04 0.46E – 04 3.07E – 04 0.38E – 04 2.88E – 04 0.76E – 04 12.9E – 04 0.92E – 04 9.29E – 04 7.14E – 04 92.6E – 04 22.3E – 04 208E – 04
Increase % (if not ND) 356 1054 202 1153 1110 465 1114 1013 1122 170 570 662 1604 913 1197 833
< means ‘‘less than” number if value is non-detect.
Table 2 Thermal oxidizer results-17 PCDD/PCDF congeners Inlet (ng ds m 3) Data represent an average 2,3,7,8-TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDF 2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD
Outlet (ng ds m 3)
of 3 triplicate runs 3.13E – 04 4.91E – 04 1.51E – 04 4.43E – 04 6.42E – 04 8.72E – 04 2.30E – 04 5.74E – 04 1.57E – 04 5.06E – 04 0.86E – 04 1.30E – 04 5.46E – 04 7.33E – 04 11.7E – 04 14.8E – 04 1.75E – 04 2.72E – 04 9.97E – 04 8.58E – 04 0.43E – 04 0.82E – 04 0.60E – 04 1.75E – 04 0.69E – 04 1.82E – 04 1.45E – 04 2.81E – 04 12.3E – 04 3.59E – 04 14.4E – 04 8.12E – 04 47.4E – 04 17.1E – 04
Increase %
57 193 36 150 222 51 34 26 55 14 94 194 163 94 71 44 64
test the sum of the congener concentration increases by almost 10-fold. 3.2. Data from thermal oxidizer tests
3. Results 3.1. Data from catalytic oxidizer tests Concentration data from the catalytic oxidizer tests for each of the 17 toxic PCDD/PCDF congeners are contained in Table 1. The concentration of each of the 17 congeners increases from the inlet to the outlet for these tests. For this
Table 2 contains the congener concentration data for the thermal oxidizer tests. The concentration of 13 congeners increases from the inlet to the outlet for these tests. Four of the congeners decrease in concentration. The sum of the congener concentrations decreases by 18% for the thermal oxidizer, which is negligible considering the overall accuracy of the congener emissions values.
J.R. Hart / Chemosphere 72 (2008) 75–78
4. Discussion of results 4.1. General aspects All of the data from this study and the previous study are field data. In other words, the studies are not controlled laboratory experiments, but studies that may have design or operating factors that affect the PCDD/PCDF formation and destruction chemistry. The main point of this study is that PCDD/PCDFs are formed in the catalytic oxidizer. 4.2. Increase over catalyst For the catalytic oxidizer, each congener increased dramatically. The highest increase was for 1,2,3,6,7,8HxCDD, which increased 1600%. The next highest group of congeners that increased over 1000% are 1,2,3,4,6,7,8HpCDD, 1,2,3,4,7,8-HxCDF, OCDF, 1,2,3,4,6,7,8HpCDF, 1,2,3,4,7,8,9-HpCDF, 1,2,3,6,7,8-HxCDF, and 1,2,3,7,8-PeCDF. The remaining group of the 17 congeners increased from 170% for TCDD to 900% for 1,2,3,7,8,9HxCDD. Thus, for the catalytic oxidizer, the congener with the highest toxicity increased the least. Keep in mind that this test occurred with a 427 °C temperature at the inlet of the catalyst. Although this is on the edge of the PCDD/PCDF ‘‘formation window” of 200–400 °C, there were significant concentrations of PCDD/PCDFs formed. In the previous study (Hart, 2004), PCDD/PCDF emissions increased exponentially with lower operating temperature. 4.3. Thermal oxidizer changes in PCDD/PCDF congeners For the thermal oxidizer, there was a negligible change in total congener concentration. There were increases at the stack for thirteen of the congeners. The congeners 1,2,3, 6,7,8-HxCDF, 1,2,3,7,8-PeCDD, and 1,2,3,7,8-PnCDF increased by 200%. Decreases were shown in OCDF,
Catalytic
1,2,3,7,8,9-HxCDD, 1,2,3,4,6,7,8-HpCDD, and OCDD. The greatest decrease in concentration was for OCDD, which accounted for most of the decrease in the total congener concentration. TCDD increased by 94%. Decrease in PCDD/PCDFs may be due in part to higher temperature, but also may be due to the natural gas flame or composition of the inlet gas stream. The inlet gas stream consisted of soil gas with benzene, vinyl chloride, 1,1dichloroethane, and other compounds. Van den Brink et al. (2000) found a reduction or elimination in by-products by removal of Cl from a catalytic surface by the addition of alkanes. In this study, consumption of Cl by the non-chlorinated VOCs and methane to form HCl may suppress chlorinated PCDD/PCDF formation in the thermal oxidizer without a catalyst. 4.4. Ambient air as source of inlet PCDD/PCDFs As the soil vapor extraction system pulls gas from the sub-surface wells, ambient air is drawn into the soil. Even if there is an asphalt cap over the contaminated site, as it was for the thermal oxidizer, air is drawn from around the cap to replace the gas that is extracted from the subsurface. Also, ambient PCDD/PCDFs will be entrained in to the oxidizers through combustion air and dilution air. Both of the oxidizers in this study were located in Southern California’s South Coast Air Basin. Ambient concentrations of PCDD/PCDFs have been measured in this air basin prior to this study by Hunt et al. (1990), and are comparable to the inlet values in this study. The comparisons of congener concentrations at the inlet for both tests are shown in Fig. 1. It is not clear how PCDD/PCDFs at these levels would be in the soil gas at the inlet to the oxidizers. The soil can act as an adsorbent for organic compounds, especially for those with low vapor pressures. The soil sorption equilibrium constant for 2,3,7,8-TCDD is quite high (Kd is equal to 26 259 according to US EPA – see http://www.epa.gov/ superfund/sites/npl/hrsres/tools/scdm.htm). There must be enough air exchanges through the soil to saturate the soil with PCDD/PCDFs from ambient air. Then, the PCDD/ PCDFs from ambient air pass through to the vapor extraction well and to the inlet of the oxidizer.
Thermal
5. Conclusions
OCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8-PeCDD
Fig. 1. Inlet data from both tests.
1,2,3,4,7,8-HxCDD
OCDF
2,3,7,8-TCDD
1,2,3,4,7,8,9-HpCDF
2,3,4,6,7,8-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,7,8,9-HxCDF
1,2,3,6,7,8-HxCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,7,8-TCDF
The results of this field study indicate that:
1,2,3,7,8-PeCDF
Concentration, ng dsm-3
Comparison of Inlet Congeners 5.00E-03 4.50E-03 4.00E-03 3.50E-03 3.00E-03 2.50E-03 2.00E-03 1.50E-03 1.00E-03 5.00E-04 0.00E+00
77
The catalytic oxidizer in this study produced an increase in PCDD/PCDF congener concentration of almost 10fold from the inlet to the outlet (stack), thus verifying results of a previous study that evaluated only PCDD/ PCDF emissions. All congeners increased from inlet to the stack. The thermal oxidizer had little effect on PCDD/PCDF levels.
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Ambient air was the main source of PCDD/PCDFs in the stack emissions of the thermal oxidizer in this study. Acknowledgement The author wishes to thank staff of the California Air Resources Board for contracting the PCDD/PCDF sampling for this study. References Blanco, J., Alvarez, E., Knapp, C., 1999. Control dioxin emissions from combustion processes. Chem. Eng., October, 149–152. Cieplik, M.K., de Jong, V., Louw, R., 2001. Formation of dioxins from model compounds and micropollutant-like mixtures over supported Pt, iron ores and MSWI flyash. In: Poster Presentation at the Seventh International Congress on Toxic Combustion By-Products, Research Triangle Park, North Carolina. Corella, J., Toledo, J.M., Padilla, A.M., 2000. On the selection of the catalyst among the commercial platinum-based ones for total oxidation of some chlorinated hydrocarbons. Appl. Catal. B, Environ. 27 (4), 243–256. De Jong, V., Cieplik, M.K., Louw, R., 2004. Formation of dioxins in the catalytic combustion of chlorobenzene and a micropollutant-like mixture on Pt/c-Al2O3. Environ. Sci. Technol. 38, 5217–5223. Deutschmann, O., 2000. Catalytic combustion: state of the art and modeling needs. In: Second International Workshop on CHEMKIN in Combustion, Edinburgh, Scotland. Hart, J.R., 2004. Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans from catalytic and thermal oxidizers burning dilute chlorinated vapors. Chemosphere 54, 1539–1547.
Hunt, G., Maisel, B., Hoyt, M., 1990. Ambient Concentrations of PCDDs/PCDFs in the South Coast Air Basin. Report for the California Air Resources Board, Contract No. A6-100-32. Kissel-Osterrieder, R., Behrendt, F., Warnatz, J., 2000. Dynamic monte carlo simulations of catalytic surface reactions. In: 28th (Int’l) Symposium on Combustion, The Combustion Institute, pp. 1323– 1330. Koltsakis, G.C., Stamatelos, A.M., 1997. Catalytic automotive exhaust aftertreatment. Prog. Energy Combust. Sci. 23, 1–39. Liljelind, P., Marklund, S., 2001. Behaviour of PCDD/F in a catalytic converter following simulated waste combustion. In: Poster Presentation at the Seventh International Congress on Toxic Combustion ByProducts, Research Triangle Park, North Carolina. Liljelind, P., Unsworth, J., Maaskant, O., Marklund, S., 2001. Removal of dioxins and related aromatic hydrocarbons from flue gas streams by adsorption and catalytic destruction. Chemosphere 42, 615–623. Mantzaras, J., Appel, C., Benz, P., 2000. Catalytic combustion of methane/air mixtures over platinum: homogeneous ignition distances in channel flow configurations. In: 28th (Int’l) Symposium on Combustion, The Combustion Institute, pp. 1349–1357. Padilla, A.M., Corella, J., Toledo, J.M., 2002. Total oxidation of some chlorinated hydrocarbons with commercial chromia based catalysts. Appl. Catal. B, Environ. 22 (2), 107–121. Trevin˜o, C., Lin˜an, A., Kurdyumov, V., 2000. Autoignition of hydrogen/ air mixtures by a thin catalytic wire. In: 28th (Int’l) Symposium on Combustion, The Combustion Institute, pp. 1359–1364. Van den Brink, R.W., Louw, R., Mulder, P., 2000. Increased combustion rate of chlorobenzene on Pt/c-Al2O3 in binary mixtures with hydrocarbons and with carbon monoxide. Appl. Catal. B, Environ. 25, 229– 237. Van den Brink, R.W., 1999. Catalytic Combustion of Chlorinated Organics. Doctoral Thesis, Universiteit Leiden.
Chemosphere 72 (2008) 75–78 www.elsevier.com/locate/chemosphere
Verification of dioxin formation in a catalytic oxidizer John R. Hart * California EPA, Department of Toxic Substances Control, P.O. Box 806, Sacramento, CA 95812-0806, USA Received 5 November 2007; received in revised form 25 January 2008; accepted 28 January 2008 Available online 10 March 2008
Abstract Emissions and inlet concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) have been measured from a catalytic oxidizer and a thermal oxidizer. The catalyst inlet temperature was 427 °C. The thermal oxidizer operating temperature was 791 °C. Data of the toxic dioxin and furan congeners are reported. Important results of this field study are: (1) the catalytic oxidizer in this study produced an increase in PCDD/PCDF congener concentration of almost 10-fold from the inlet to the outlet (stack), thus verifying results of a previous study that evaluated only PCDD/PCDF emissions. All congeners increased from inlet to the stack. (2) The thermal oxidizer had little effect on PCDD/PCDF levels. There was a decrease in four of the congeners and an increase in 13 congeners. (3) Ambient air was the main source of PCDD/PCDFs in the stack emissions of the thermal oxidizer in this study. Laboratory investigations are needed to understand how PCDD/PCDFs are formed (and emitted) under conditions of this study. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Dioxins; Catalytic combustion; Oxidation; Chlorinated VOCs, chlorinated hydrocarbons
1. Introduction The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) has been a concern from all types of combustion devices. The low temperature (‘‘temperature window”) formation of dioxins with transition metal catalysts has been reported, but not the formation of PCDD/PCDFs with combustion catalysts with the exception of the recent work of Hart (2004), de Jong et al. (2004), and Cieplik et al. (2001). The fundamentals of catalytic combustion have been studied recently by Van den Brink (1999) and others (Koltsakis and Stamatelos, 1997; Corella et al., 2000; Deutschmann, 2000; Kissel-Osterrieder et al., 2000; Mantzaras et al., 2000; Trevin˜o et al., 2000; Van den Brink et al., 2000; Liljelind and Marklund, 2001; Padilla et al., 2002). Under certain conditions, catalytic combustion has been used to destroy PCDD/PCDFs, as discussed by Blanco et al. (1999) and *
Tel.: +1 800 700 5854; fax: +1 916 327 4494. E-mail address: [email protected]
0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.01.058
Liljelind et al. (2001). This paper verifies results from a previous study by Hart (2004), which evaluated data from 57 PCDD/PCDF emissions field tests on catalytic and thermal oxidizers. In this study, PCDD/PCDFs were measured at the inlet and outlet of one catalytic oxidizer and one thermal oxidizer (both not part of the previous study), in order to verify whether PCDD/PCDFs are produced or destroyed by the oxidizers. 2. Description of field tests Many sites with soils contaminated with chlorinated VOCs are cleaned by soil vapor extraction (SVE), by applying a vacuum to an underground vapor well. The higher the compound’s vapor pressure, the more readily it will be transported to the gas phase from the soil. A blower causes the extracted gas to be transported from the soils to the wells and ducts. The contaminated gas may contain chlorinated and non-chlorinated VOCs, and is then burned in a catalytic oxidizer or thermal oxidizer, prior to release through a stack.
76
J.R. Hart / Chemosphere 72 (2008) 75–78
The catalytic oxidizer in this study is a King–Buck Model HDC-200EX containing a Degussa ‘HDC’ catalyst (0.15% Pt + 0.15% Pd on Alumina). The inlet vapor stream contained mainly trichloroethylene, but only total hydrocarbons were measured during the test, averaging 55 ppm V as methane. The inlet vapor stream is pre-heated by both an electric heater (25 kW) and a heat exchanger from the catalyst exhaust, resulting in a 427 °C gas at the inlet of the catalyst. The gas flow rate into and out of the catalytic oxidizer is 6 standard m3 min 1 (dry basis and at 20 °C and 760 mmHg). No combustion air is fed to this oxidizer, but the outlet gas contains an average oxygen concentration of 20.3%. Average emissions data also indicated 8.3 ppm V of total hydrocarbons as methane. The thermal oxidizer in this test is a Soil Therm Model CLR2002. The oxidizer includes a heat exchanger, venturi quench scrubber and a packed tower scrubber. The oxidizer was operated at 791 °C during the test. A caustic scrubber solution is used with a pH from 7 to 9. In the quench, the gas is cooled to about 71 °C. The oxidizer is fired with natural gas at a rate of 293 kW. The maximum inlet gas flow rate is a nominal 10 standard m3 min 1 at the SVE blower. However, the average blower flow rate during the tests was 2 dry standard m3 min 1 at the inlet and 5.5 dry standard m3 min 1 at the outlet. Combustion air is fed to the burner. The inlet sampling point is prior to the soil gas blower and combustion air feed locations. The inlet contained an average oxygen concentration of 6.3%, and an average hydrocarbon concentration of 599 ppm V as methane. Average emissions data also indicate 34 ppm V of CO, 0.2 ppm V of total hydrocarbons as methane, and 13.9% O2. The PCDD/PCDF sampling method used is CARB’s Method 428 (www.arb.ca.gov/testmeth/vol3/M_428.pdf). Similar to US EPA’s Method 23 sampling train (see www.epa.gov/ttnemc01/promgate/m-23.pdf), CARB Method 428 is an extractive adsorbent technique. The analytical methods are high resolution gas chromatography followed by high resolution mass spectroscopy. The method requires a gas correction to standard temperature (20 °C) and pressure (760 mmHg), but not for air dilution. The gas volume is presented at its sampled oxygen content, on a dry basis, as dry standard cubic meters (ds m3). Depending on the PCDD/PCDF congener, detection limits are on the order of 10 5 ng ds m 3. The methods incorporate internal standard recoveries and blanks to insure data quality. Besides PCDD/PCDFs, other products of incomplete combustion are not determined during this study.
Table 1 Catalytic oxidizer results – 17 PCDD/PCDF congeners Inlet (ng ds m 3) Data represent an average 2,3,7,8-TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDF 2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD
Outlet (ng ds m 3)
of 3 triplicate runs 5.07E – 04 23.1E – 04 0.72E – 04 8.26E – 04 5.70E – 04 17.2E – 04 1.58E – 04 19.8E – 04 2.19E – 04 26.5E – 04 <0.23E – 04 1.41E – 04 6.96E – 04 39.3E – 04 14.0E – 04 170E – 04 1.60E – 04 17.8E – 04 6.30E – 04 77.0E – 04 0.32E – 04 0.85E – 04 0.46E – 04 3.07E – 04 0.38E – 04 2.88E – 04 0.76E – 04 12.9E – 04 0.92E – 04 9.29E – 04 7.14E – 04 92.6E – 04 22.3E – 04 208E – 04
Increase % (if not ND) 356 1054 202 1153 1110 465 1114 1013 1122 170 570 662 1604 913 1197 833
< means ‘‘less than” number if value is non-detect.
Table 2 Thermal oxidizer results-17 PCDD/PCDF congeners Inlet (ng ds m 3) Data represent an average 2,3,7,8-TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDF 2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD
Outlet (ng ds m 3)
of 3 triplicate runs 3.13E – 04 4.91E – 04 1.51E – 04 4.43E – 04 6.42E – 04 8.72E – 04 2.30E – 04 5.74E – 04 1.57E – 04 5.06E – 04 0.86E – 04 1.30E – 04 5.46E – 04 7.33E – 04 11.7E – 04 14.8E – 04 1.75E – 04 2.72E – 04 9.97E – 04 8.58E – 04 0.43E – 04 0.82E – 04 0.60E – 04 1.75E – 04 0.69E – 04 1.82E – 04 1.45E – 04 2.81E – 04 12.3E – 04 3.59E – 04 14.4E – 04 8.12E – 04 47.4E – 04 17.1E – 04
Increase %
57 193 36 150 222 51 34 26 55 14 94 194 163 94 71 44 64
test the sum of the congener concentration increases by almost 10-fold. 3.2. Data from thermal oxidizer tests
3. Results 3.1. Data from catalytic oxidizer tests Concentration data from the catalytic oxidizer tests for each of the 17 toxic PCDD/PCDF congeners are contained in Table 1. The concentration of each of the 17 congeners increases from the inlet to the outlet for these tests. For this
Table 2 contains the congener concentration data for the thermal oxidizer tests. The concentration of 13 congeners increases from the inlet to the outlet for these tests. Four of the congeners decrease in concentration. The sum of the congener concentrations decreases by 18% for the thermal oxidizer, which is negligible considering the overall accuracy of the congener emissions values.
J.R. Hart / Chemosphere 72 (2008) 75–78
4. Discussion of results 4.1. General aspects All of the data from this study and the previous study are field data. In other words, the studies are not controlled laboratory experiments, but studies that may have design or operating factors that affect the PCDD/PCDF formation and destruction chemistry. The main point of this study is that PCDD/PCDFs are formed in the catalytic oxidizer. 4.2. Increase over catalyst For the catalytic oxidizer, each congener increased dramatically. The highest increase was for 1,2,3,6,7,8HxCDD, which increased 1600%. The next highest group of congeners that increased over 1000% are 1,2,3,4,6,7,8HpCDD, 1,2,3,4,7,8-HxCDF, OCDF, 1,2,3,4,6,7,8HpCDF, 1,2,3,4,7,8,9-HpCDF, 1,2,3,6,7,8-HxCDF, and 1,2,3,7,8-PeCDF. The remaining group of the 17 congeners increased from 170% for TCDD to 900% for 1,2,3,7,8,9HxCDD. Thus, for the catalytic oxidizer, the congener with the highest toxicity increased the least. Keep in mind that this test occurred with a 427 °C temperature at the inlet of the catalyst. Although this is on the edge of the PCDD/PCDF ‘‘formation window” of 200–400 °C, there were significant concentrations of PCDD/PCDFs formed. In the previous study (Hart, 2004), PCDD/PCDF emissions increased exponentially with lower operating temperature. 4.3. Thermal oxidizer changes in PCDD/PCDF congeners For the thermal oxidizer, there was a negligible change in total congener concentration. There were increases at the stack for thirteen of the congeners. The congeners 1,2,3, 6,7,8-HxCDF, 1,2,3,7,8-PeCDD, and 1,2,3,7,8-PnCDF increased by 200%. Decreases were shown in OCDF,
Catalytic
1,2,3,7,8,9-HxCDD, 1,2,3,4,6,7,8-HpCDD, and OCDD. The greatest decrease in concentration was for OCDD, which accounted for most of the decrease in the total congener concentration. TCDD increased by 94%. Decrease in PCDD/PCDFs may be due in part to higher temperature, but also may be due to the natural gas flame or composition of the inlet gas stream. The inlet gas stream consisted of soil gas with benzene, vinyl chloride, 1,1dichloroethane, and other compounds. Van den Brink et al. (2000) found a reduction or elimination in by-products by removal of Cl from a catalytic surface by the addition of alkanes. In this study, consumption of Cl by the non-chlorinated VOCs and methane to form HCl may suppress chlorinated PCDD/PCDF formation in the thermal oxidizer without a catalyst. 4.4. Ambient air as source of inlet PCDD/PCDFs As the soil vapor extraction system pulls gas from the sub-surface wells, ambient air is drawn into the soil. Even if there is an asphalt cap over the contaminated site, as it was for the thermal oxidizer, air is drawn from around the cap to replace the gas that is extracted from the subsurface. Also, ambient PCDD/PCDFs will be entrained in to the oxidizers through combustion air and dilution air. Both of the oxidizers in this study were located in Southern California’s South Coast Air Basin. Ambient concentrations of PCDD/PCDFs have been measured in this air basin prior to this study by Hunt et al. (1990), and are comparable to the inlet values in this study. The comparisons of congener concentrations at the inlet for both tests are shown in Fig. 1. It is not clear how PCDD/PCDFs at these levels would be in the soil gas at the inlet to the oxidizers. The soil can act as an adsorbent for organic compounds, especially for those with low vapor pressures. The soil sorption equilibrium constant for 2,3,7,8-TCDD is quite high (Kd is equal to 26 259 according to US EPA – see http://www.epa.gov/ superfund/sites/npl/hrsres/tools/scdm.htm). There must be enough air exchanges through the soil to saturate the soil with PCDD/PCDFs from ambient air. Then, the PCDD/ PCDFs from ambient air pass through to the vapor extraction well and to the inlet of the oxidizer.
Thermal
5. Conclusions
OCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8-PeCDD
Fig. 1. Inlet data from both tests.
1,2,3,4,7,8-HxCDD
OCDF
2,3,7,8-TCDD
1,2,3,4,7,8,9-HpCDF
2,3,4,6,7,8-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,7,8,9-HxCDF
1,2,3,6,7,8-HxCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,7,8-TCDF
The results of this field study indicate that:
1,2,3,7,8-PeCDF
Concentration, ng dsm-3
Comparison of Inlet Congeners 5.00E-03 4.50E-03 4.00E-03 3.50E-03 3.00E-03 2.50E-03 2.00E-03 1.50E-03 1.00E-03 5.00E-04 0.00E+00
77
The catalytic oxidizer in this study produced an increase in PCDD/PCDF congener concentration of almost 10fold from the inlet to the outlet (stack), thus verifying results of a previous study that evaluated only PCDD/ PCDF emissions. All congeners increased from inlet to the stack. The thermal oxidizer had little effect on PCDD/PCDF levels.
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Ambient air was the main source of PCDD/PCDFs in the stack emissions of the thermal oxidizer in this study. Acknowledgement The author wishes to thank staff of the California Air Resources Board for contracting the PCDD/PCDF sampling for this study. References Blanco, J., Alvarez, E., Knapp, C., 1999. Control dioxin emissions from combustion processes. Chem. Eng., October, 149–152. Cieplik, M.K., de Jong, V., Louw, R., 2001. Formation of dioxins from model compounds and micropollutant-like mixtures over supported Pt, iron ores and MSWI flyash. In: Poster Presentation at the Seventh International Congress on Toxic Combustion By-Products, Research Triangle Park, North Carolina. Corella, J., Toledo, J.M., Padilla, A.M., 2000. On the selection of the catalyst among the commercial platinum-based ones for total oxidation of some chlorinated hydrocarbons. Appl. Catal. B, Environ. 27 (4), 243–256. De Jong, V., Cieplik, M.K., Louw, R., 2004. Formation of dioxins in the catalytic combustion of chlorobenzene and a micropollutant-like mixture on Pt/c-Al2O3. Environ. Sci. Technol. 38, 5217–5223. Deutschmann, O., 2000. Catalytic combustion: state of the art and modeling needs. In: Second International Workshop on CHEMKIN in Combustion, Edinburgh, Scotland. Hart, J.R., 2004. Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans from catalytic and thermal oxidizers burning dilute chlorinated vapors. Chemosphere 54, 1539–1547.
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