Environ. Sci. Technol. 1986, 20, 703-710
Laboratory Investigation of Thermal Degradation of a Mixture of Hazardous Organic Compounds. 1 John L. Graham," Douglas L. Hall, and Barry Dellinger
Research Institute, University of Dayton, Dayton, Ohio 45469
In this report, the effect of oxygen concentration on the thermal stability of the components of a mixture of carbon tetrachloride, monochlorobenzene, 1,1,2-trichloro-1,2,2trifluoroethane [Freon 113 (Du Pont)], trichloroethylene, and toluene and the formation of thermal reaction products is examined. Thermal decomposition studies were conducted in atmospheres in which combustion oxygen was in excess, stoichiometric, and absent (pyrolysis). The components were also run individually in atmospheres with stoichiometric and excess oxygen. Results indicate that decreasing oxygen concentration increased the stability of the mixture components except Freon 113 and carbon tetrachloride. Furthermore, with the exception of Freon 113, each component was less stable in the mixture as compared to pure compound data. The stability of Freon 113 remained unchanged regardless of reaction atmosphere. I t was found that the number and complexity of thermal reaction products increased with decreasing oxygen concentration. In all cases, products ranged from simple chlorinated aliphatics to complex polynuclear aromatics.
Introduction Controlled, high-temperature incineration has been identified as a desirable method of disposal of hazardous organic wastes (1). This approach avoids many of the problems associated with storage of hazardous materials in landfills or impoundments. Theoretically, incineration could result in the total conversion of hazardous organic compounds to innocuous thermodynamic end products, such as carbon dioxide and water, and other simple compounds which are easily scrubbed with existing pollution control equipment (2,3). In practice, total conversion to innocuous materials cannot be achieved without considerable expense, and for an incinerator of less than optimum design or operating conditions, the most thermally stable components in the waste feed may not be totally decomposed ( 4 , 5 ) . Also of concern is the formation of stable toxic combustion products (5). The emission of hazardous organic compounds from poorly designed or inadequately controlled incinerators represents a potentially significant threat to the environment. In addition, since hazardous organic compounds are also subjected to thermal degradation in sources not specifically designed or regulated for their disposal (e.g., boilers, cement plants, municipal incinerators, etc.), it is important that knowledge of the thermal decomposition behavior of hazardous organic materials is first obtained under controlled, environmentally safe laboratory conditions (6-8). Then, many of the problems associated with full-scale thermal destruction may be avoided. In the following paragraphs are the results of a laboratory study of the thermal degradation of a mixture of five hazardous organic compounds in three reaction atmospheres: oxygen starved, stoichiometric oxygen, and oxygen rich. The five compounds, from the U.S. EPA's list of regulated hazardous organic compounds, were selected on the basis of their frequency of occurrence in actual waste 00 13-936X186/0920-0703$0 1.5010
streams and predicted range of thermal stability ( 4 , 9). The behavior of the compounds in the mixture is compared to their behavior when tested as pure compounds. Thermal reaction products are identified on the basis of mass spectral (MS) interpretation and, in some cases, gas-chromatographic (GC) retention time. Thermal decomposition behavior is analyzed and related to elementary chemical reaction kinetics. Finally, the laboratory observations have been related to actual full-scale incineration conditions, and guidelines for application of laboratory results to full-scale incineration have been proposed.
Experimental Section The thermal decomposition characteristics of the test sample, designated as hazardous waste mixture 1 (HWM1), were studied by using the thermal decomposition unit-gas chromatographic system (TDU-GC) and the thermal decomposition analytical system (TDAS). The details of these systems have been discussed elsewhere, and only a brief description will be presented here (10). Both the TDU-GC and TDAS utilize a sophisticated, high-temperature reactor in which a flowing gas stream may be exposed to temperatures as high as 1100 "C and for mean residence times of 0.5-6.0 s. Samples are admitted to the reactor through a heated inlet chamber which vaporizes the material and mixes it with the reaction atmosphere. A variety of probes may be used for handling liquid-, gas-, or solid-phase samples. Heated transfer lines connect the inlet region to the reactor and the reactor to the analytical instrumentation located downstream. The analytical function of the TDU-GC is carried out by a Varian Vista 4600 programmed temperature gas chromatograph along with a CDS 401 computer data station. The TDAS uses a Varian 2400 programmed temperature gas chromatograph (modified for cryogenic operation) coupled to an LKB 2091 mass spectrometer. A minicomputer is used to aid data acquisition, storage, and reduction. The weight fraction of each principal organic hazardous constituent (POHC) which survives a high-temperature exposure can be obtained from TDU-GC data by comparing the integrated detector response for the POHC at the temperature in question with the integrated detector response at a nondestructive reference temperature. This can be summarized as IDR, WPR = -x 100 IDRI where WPR, IDRl and IDRz represent the weight percent remaining, the integrated detector response at the reference temperature, and the integrated detector response at the temperature of interest, respectively. Often, thermal decomposition data are expressed in terms of a destruction efficiency (DE). This DE is a measure of how much material is degraded by a hightemperature exposure rather than how much survives. The DE (percent) is found from TDU-GC data as
0 1986 American Chemical Soclety
Environ. Sci. Technol., Vol. 20, No. 7, 1986 703
-)
DE = (1 - IDRz IDR,
X
100
Finally, to quantitatively describe the different reaction conditions ranging from oxygen starved to oxygen rich, the equivalence ratio (4) is used. O2 required for complete combustion 4= (3) O2 available for combustion !-
As defined above, when 4 < 1,an oxygen-rich condition exists, and when 4 > 1, there is insufficient oxygen available of complete combustion. When 4 = 1, there is stoichiometric oxygen available for combustion. The mixture of organic compounds used in this study consisted of, by weight, 2.5% chlorobenzene, 2.5% carbon tetrachloride, 2.5% trichloroethylene, 2.5% 1,1,2-trichloro-l,2,2-trifluoroethane, (Freon 113),and 90% toluene for a total distribution of 84.6% C, 8.0% H, 6.6% C1,0.8% F. Stock samples were prepared by injecting the liquidphase organic components into 1-L spherical sample flasks which were filled with either room air or dry nitrogen. These are referred to as HWM-l/air and HWM-l/nitrogen, respectively. The total volume of organic injected for the HWM-l/air sample was such that 4 = 1.0. In a similar manner, individual samples of the pure components were also prepared. These were prepared in an oxygen/nitrogen atmosphere such that the gas-phase concentration was the same as the concentration in the HWM-1 mixture and with 4 = 1.0. Also included with the pure compound data are the results of earlier experiments which involved these components. However, unlike the present experiments where the 4 is fixed at 1.0, these data were taken in an atmosphere of flowing air such that 4 80-85 kcal/mol), their relative thermal stability may be estimated by resistance to electrophilic attack. Thermal Reaction Products. The thermal reaction chemistry of this mixture of hazardous organics is obviously very complex. Examination of Table I indicates that the majority of identified products are the result of pyrolysis type reactions regardless of the oxygen level in the reactor, although increased numbers and amounts of partial oxidation products are observed as the oxygen concentration increases. The major oxidation product is benzaldehyde, possibly resulting from the partial oxidation of toluene. The complexity and variety of structures of the observed reaction products is indicative of a formation mechanism based on recombination of free radical fragments formed from the degradation of the parent compounds. A detailed discussion of possible mechanisms for the formation of the observed reaction products is beyond the scope of this paper; however, a discussion of general pathways of product formation is in order. At the temperatures tested, initiation of thermal degradation of the mixture may occur through rupture of the weakest bond in the system (possibly the carbon-chlorine bond in carbon tetrachloride) or hydrogen abstraction by molecular oxygen (possibly the carbon-hydrogen bond on the methyl substituent of toluene). Both of these initiation reactions result in the formation of reactive free radicals (Cl, HOz, CCl,, and C7H7). These free radicals may subsequently attack other unreacted neutral molecules or combine with other free radicals. As the mechanism progresses, the concentration of free radicals increases through chain-branching reactions until the principal modes of destruction of the feed materials are from attack by reactive free radicals (i.e., OH, 0, H, Cl) or low-energy bond rupture. At this point, initiation reactions are relatively unimportant in the destruction of the feed materials, and product formation is by radical attack on neutral species or free radical recombination. In addition to OH, 0, H, and C1, many organic free radicals are also present such as formyl (CHO), methyl (CH,), ethyl (CHzCH3), phenyl (C,H,), etc. A myriad of reaction products can be generated by recombination of these free radicals. Our discussions will concentrate on the thermally stable reaction products generated from the pyrolysis and 9 = 1.0 oxidation. Common to both reaction conditions are the products benzene and some of its simple derivatives. This is not suprising considering the large quantities of toluene and chlorobenzene in the original sample. Phenyl radical, formed by bond rupture or electrophilic addition followed by elimination, is evidently a common precursor species under both 4 = 1.0 and pyrolytic conditions. Fluorine atoms would be formed from the decomposition of Freon 113, and ethyl radical may be formed as a degradation product from most of the feed constituents. Thus, radical recombination is a viable mechanism for the formation of fluorobenzene and ethylbenzene. Benzene is observed to be a major reaction product under pyrolytic conditions and at 4 = 1.0. However, when excess oxygen was introduced to the reactor, roughly equal quantities of chlorophenol and benzaldehyde were observed. The presence of these latter two oxidative products may be explained through consideration of a shift in the equilibrium distribution of reactive radical species. Under
pyrolytic conditions, hydrogen atom will be the highest concentration reactive radical present in the system. Thus, phenyl radical is most likely to be quenched by a hydrogen atom to form benzene. Equilibrium calculations indicate that OH is present in greater concentrations than hydrogen atom for 4 = 1.0. Therefore, OH attack on chlorobenzene and toluene is likely to predominate, resulting in the formation of chlorophenol and benzaldehyde, respectively. The presence of the hydroxyl radical and oxygen atoms at 4 = 1.0 and 0.06 may, in fact, determine what products are observed. Although significant levels of products were observed at 1000 "C for pyrolysis conditions, no products were observed above 900 "C for 4 = 1.0 or above 850 "C at 4 = 0.06. Furthermore, significant quantities of halogenated and nonhalogenated polynuclear aromatic hydrocarbons were observed under pyrolytic conditions but not under oxidative conditions. Instead, simple chlorinated species such as tetrachloroethylene and dichloroethylene were observed as products under oxidative conditions. Reactive oxygenated species may affect product formation in two ways. The formation of high molecular weight polynuclear aromatics must proceed by a complex multiple reaction sequence with each step subject to competing oxidative reactions. The presence of reactive oxygenated species results in an increased rate of oxidation toward thermodynamic completion (COz,HzO, and HCl), reducing the number of radical "building blocks" available for radical-radical recombination or radical addition to form the polynuclear aromatics. In contrast, simple degradation products such as tetrachloroethylene and dichloroethylene that are very resistant to attack by oxidizing free radicals are virtually unaffected. The overall result is that the initial reaction products and intermediates (which are amenable to oxidative pathways) are destroyed at lower temperatures than they are under pyrolytic conditions while decomposition products with lower susceptibility to oxidative decomposition remain, regardless of the conditions. The propensity of formation of complex molecules such as observed under pyrolytic conditions is striking and, as discussed in the following paragraphs, may have a large bearing on the makeup of the effluent of full-scale systems regardless of their specific, nominal operating conditions. Application to Full-scale Thermal Destruction. While the immediate goal of this study was to generate laboratory data that could be used to guide full-scale thermal destruction of this mixture, a longer range goal is to correlate the results of this study with field studies and determine if general trends may be inferred that can be used to guide the incineration of other hazardous waste streams and ensure their efficient destruction. To this end, it is necessary to understand how laboratory test conditions correlate with conditions in full-scale systems. We have previously discussed how parameter distribution functions affect the composition of incinerator effluents (13, 26). I t appears on theoretical grounds that "worst case" or "upset" conditions control the composition and concentration of hazardous organics in the effluent from the system. This point may be illustrated by using data in this paper for a hypothetical full-scale system, we assume mean operating conditions with a plug flow calculated gas-phase residence time of 2.0 s, a centerline postflame temperature of 900 "C, and 200% excess air (corresponding to 4 = 0.33). Examination of Figures 1, 2, and 7 reveal that the fraction of the waste feed and thermal reaction products that are subjected to these conditions are completely destroyed. The fraction of the waste feed that experiences
higher temperatures or oxygen levels is also totally destroyed. Therefore, positive deviations from the mean value would appear to have little bearing on the composition of the system's effluent. But what if a fraction of the waste feed experiences a lower temperature, due to quenching by cool combustion air,wall effects, or poor heat transfer to particulate matter, and the effective temperature for this fraction is 100 "C lower (i.e., 800 "C), then 1% of the chlorobenzene and 0 . 2 4 4 % of the trichloroethylene escape destruction (see Figure 2). Furthermore, the thermal reaction product benzene may be emitted (see Figure 6). The effect of the fraction of the waste feed decomposing under pyrolytic conditions is even more drastic. If the combustion temperature is 900 "C, 2% of the trichloroethylene escapes destruction and over 50% of the chlorobenzene. For the fraction of the waste feed experiencing pyrolytic conditions at 800 OC, more than 90% of the chlorobenzene, 50% of the trichloroethylene, and a few percent of the toluene escape undestroyed. In addition, the production of thermal reaction products such as benzene is near its maximum. Overall, worst case conditions control the composition of the system's effluent. The fraction of the waste feed subjected to nominal conditions is totally destroyed and does not contribute to the emissions. Once the worst case conditions are characterized, the appropriate laboratory data may be used or developed to predict emissions. For example, it is probable that there will be a distribution of oxygen levels between absolute pyrolysis and 4 = 1.0; thus, the effluent is likely to contain material produced under both conditions. In determining which laboratory data to give greatest weight, one may use process data from the incinerator to determine important upset modes. For the EPA's current regulatory approach, based on POHC stability, chlorobenzene would appear to be an excellent choice for monitoring in this mixture. If thermal reaction products are also considered, then benzene should also be monitored due to the fact that it was produced in relatively large quantities and appeared to be thermally stable at all oxygen levels studied. Clearly, additional correlations of laboratory and field studies are needed to determine how to best use laboratory-generated data to predict field results. Only limited data for comparison are available from past studies, but results are encouraging. Laboratory studies have correctly predicted the relative thermal stability of components of the feed material and the formation of thermal reaction products from the full-scale thermal destruction of polychlorinated biphenyls, kepone, hex wastes, and DDT (17). Comparative testing of hazardous waste samples under laboratory conditions and on the pilot scale at the U.S. EPA's Combustion Research Facility offers, the best opportunity to further elucidate the complexities of hazardous waste incineration (26). Such studies must be interactive in order to guide the development of pilot and full-scale test burn protocols and define conditions of future studies,
Summary The results of testing of a mixture of toluene, carbon tetrachloride, trichloroethylene, Freon 113, and chlorobenzene indicate that chlorobenzene followed by trichloroethyleneare most likely to challenge an incinerator's destruction efficiency. In contrast, Freon 113, which has been proposed elsewhere as a tracer or surrogate compound for DRE determination, appears to be more readily destroyed (18). The laboratory testing also indicates that Environ. Scl. Technol., Vol. 20, No. 7, 1986
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a very complex mixture of combustion products can be formed that includes simple products such as tetrachloroethylene and far more complex species including chlorinated and nonchlorinated polynuclear aromatic compounds. Benzene was observed to predominate as a thermal reaction product. Benzaldehyde and chlorophenol were major products in oxygen-containing atmospheres. Ethylbenzene was also observed to be a major reaction product under a variety of conditions with traces of fluorobenzene also being detected. Naphthalene and chlorinated polynuclear aromatics were observed under pyrolytic conditions. Acknowledgments
We gratefully acknowledge the laboratory efforts of our colleagues Debra Tirey and Fay Stamatiades. Registry No. Freon 113,76-13-1;carbon tetrachloride, 56-23-5; monochlorobenzene, 108-90-7;trichloroethylene, 79-01-6; toluene, 108-88-3.
Literature Cited Gerber, C. R. J. Air. Pollut. Control Assoc. 1985, 35, p. 749-752. Niessen, W. R. Combustion and Incineration Processes; Marcel-Dekker: New York and Basel, 1978. Edwards, J. b. Combustion: The Formation and Emission of Trace Species;Ann Arbor Science: Ann Arbor, MI, 1974. Dellinger, B.; Torres, J. L.; Rubey, W. A.; Hall, D. L.; Graham, J. L.; Carnes, R. A., Hazard. Waste 1984, 1, 137-157. Trenholm, A.; Hathaway, R. Proceedings of the Tenth
Annual Research Symposium-Incineration and Treatment of Hazardous Wastes;U.S. EPA, HWERL. U.S. Government Printing Office: Washington, DC, 1984; EPA-600/ 9-84-022, P B 85-116291, p p 84-103. Castaldini, C.; Mason, H. B.; Dehosier, R. J.; Unnasch, S. Hazard. Waste 1984, 1, 159-165.
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Peters, J. A.; Hughes, J, W. Proceedings of the Ninth Annual Research Symposium-Incinerationand Treatment of Hazardous Wastes;U.S. EPA, HWERL. U.S. Government Printing Office: Washington, DC, 1983; EPA-600/ 9-84-015, P B 84-234525, p p 210-224. Eiceman, G. A. Anal. Chem. 1981,53, 955-959 “Guidance Manual for Hazardous Waste Incineration Permits”; OSW Report SW-966, 1983; U.S. EPA. Rubey, W. A.; Carnes, R. A. Rev. Sei. Instrum. 1985,56, 1795-1798. Dellinger, B.; Graham, J, L.; Hall, D. L.; Rubey, W. A., Proceedings of the Eleventh Annual Research Symposi-
um-Zncineration and Treatment of Haardous Wastes;U.S. EPA, HWERL. U.S. Government Printing Office: Washington, DC, 1985; EPA-600/9-85-013, P B 85-196376, pp 160-170. Dellinger, B.; Rubey, W. A.; Hall, D. L.; Graham, J. L. Hazard. Waste Hazard. Mater., in press. Benson, S. W. ThermochemicalKinetics, 2nd ed.; Wiley: New York, NY, 1976. Kerr, J. A,; Trotman-Dickinson, A. F. In CRC Handbook Of Chemistry and Physics, 64th ed.; Chemical Rubber Company: Cleveland, OH, 1983; pp F-187-F-193. Samuelson, G. S., In Advances in Environmental Sciences and Technology;Pitts, J . N.; Metcalf, R. L., Eds.; Wiley: New York, 1975; pp 219-323. Carnes, R. A.; Whitmore, F. C. Hazard Waste 1984, I, 225-236. Dellinger, B.; Torres, J. L.; Rubey, W. A,; Hall, D. L.; Graham, J. L.; Carnes, R. A. Determination of the Thermal DecompositionProperties of 20 Selected Hazardous Organic Compounds;U S . EPA. U.S. Government Printing Office: Washington, DC, 1984; IERL Report, EPA-BOO/ 52-84-13, P B 84-232487. Chem. Eng. News 1984, 62(5),24.
Received for review August 9, 1985. Accepted March 5, 1986. This work was performed under the partial sponsorship of the U.S. EPA under Cooperative Agreement CR-810783.
