Thermal destruction of hazardous wastes The need for&nhnental chemical kinetic research By Selirn M. Senkan Large-scale field test burns, conducted primarily under the auspices of the U.S. Environmental Protection Agency, generally support the use of combustion and incineration as a suitable treatment method for managing a large spectrum of hazardous chemical wastes. Unfortunately, these studies were and still are conducted without accompanying fundamental research programs, which are vital to interpreting the results of field studies correctly and directing future test burns. Although large-scale field test burns over the years have resulted in the accumulation of a significant amount of very expensive empirical data, these tests have contributed very little to our understanding of the underlying chemical processes that take place in incinerators. As a direct consequence of this lack of fundamental understanding, there now exists a serious public skepticism concerning the broad utility of incineration in the treatment of hazardous wastes. This negative perception is not likely to change with additional field test burns. Unless substantial fundamental research programs are developed to support and direct large-scale studies, there will be more unanswered questions regarding the effectiveness and safety of incineration, no matter how intrinsically effective and safe incineration may be in destroying hazardous wastes. A tragic outcome of this may even be the complete abandonment of incineration as a treatment process because of growing public opposition. The current lack of understanding of the fundamentals of incineration exacerbates the high cost of this process and has an adverse effect on the international competitiveness of the chemical industry. For example, as part of an elaborate licensing procedure, incinerators must now meet somewhat arbitrary operating criteria and undergo extensive trial burns that may be unjustified. 368 Environ. Sci. Technol., Vol. 22. No. 4. 1988
L Selim M. Smkan
These elaborate procedures clearly manifest our ignorance of the fundamentals of incineration and are often criticized by all interest groups. If we had sufficient understanding of the basic chemical processes that take place during incineration, these regulations could have been established more rationally, protecting both the environment and the economy. In addition, if fundamental research programs were in progress and predictive models were available, then the thermal destruction behavior of different mixtures could be assessed under different sets of operating conditions through the use of computer simulations. Thus the need for redundant and very expensive field tests would be reduced. Unfortunately, this state of affairs is likely to continue in the near future because only a miniscule amount of related fundamental research is in progress today. EPA, the leading federal agency responsible for regulating incinerators, still has not adequately funded and coordinated fundamental research programs in keeping with its mission to expand our knowledge base on incineration. This is both disturbing and paradoxical, especially in view of EPAs own commitment to incineration as a major treatment process in the management of hazardous wastes ( I , 2).
The lack of adequate EPA funding for fundamental research in this area has been pointed out by the agency’s Science Advisory Board (3),by the National Academy of Sciences (4). and by Congress (5). At present, EPA assumes little responsibility in supporting fundamental research relevant to its mission, as epitomized by its small Office of Exploratory Research (OER). This program, which in 1987 received about $16 million in funds (approximately $3 million for the University Centers and $13 million for the Competitive Grants Program, $3 million of which was provided by Congress), represents about 0.3% of EPAs budget. It is significant that Congress has asked EPA to devote at least IS% of its budget to peer-reviewed long-term research in order to ensure scientific excellence (3. Yet the OER Competitive Grants Program, which supports EPAs peer-reviewed long-term research, is now nearing extinction because of lack of funding (6). Other mission-oriented agencies spend anywhere from 2% to 5% of their budgets in support of long-term research projects. To be effective, EPA must follow the pattern of other mission-oriented agencies and increase support of long-range fundamental research relevant to its mission through OER. Furthermore, this expansion should be done through the Competitive Grants Program in order to attract the most talented researcher’s into the field and to ensure scientific excellence. There Seems to be no question that EPA needs the infusion of fresh talent and more emphasis on scientific accountability to properly undertake its mission in protecting the environment.
Role of fundamental research During the past few years, several simplistic methods have been proposed to rank the relative ease or difficulty associated with the thermal destruction of single hazardous compounds. The proposed methods include the use of autoignition temperatures; heats of
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combustion; thermal oxidation and decomposition rates in the absence of flames (7);rates of free radical attack (8); and laminar flame velocities (9). Although each method has certain attractive features, the data necessary to evaluate their applicability frequently are not available. EPA currently uses the beat of combustion, a thermodynamic property, to rank the incinerability of hazardous compounds, although kinetics, not thermodynamics, are responsible for the formation of pollutants in incinerators (10). It must be recognized, however, that even the availability of a ranking scheme for single compounds would be of questionable practical value because mixtures are present in actual waste streams. That is, because of the inevitable catalytic and inhibitive interactions between species, the rates of reaction or destruction of hazardous compouods in a mixture can be considerably different from those observed under sioglecompound conditions. In addition, hydrocarbon fuels used to co-fire incinerators can interact with the hazardous substances, further affecting destruction rates. Therefore it is clear that any destruction criteria based upon single-compound studies would be overly simplistic as a tool for assessing the relative ease or difficulty of thermal destruction of real hazardous materials. From a practical point of view, catalytic interactions are desirable because they implicitly support the conclusions from singlecompound studies that rank the incinerability of species. That is, experiments with single compouods will result in the identification of conservative sets of conditions for the destruction of that species. In practical systems (Le., in mixtures), the extent of destruction should be even greater. On the other hand, if inhibitory interactions exist, single-component destruction data will be inadequate for assessing the behavior of the system. In this case, the extent of destruction of some species in the mixture will be less than that observed in singlesomponeot experiments. Consequently, additional data (i.e., actual destruction tests using the mixtures) will be needed to assess the Situation. However, the amount of data needed to clearly describe the behavior of all possible mixtures would be prohibitively large and costly. An important example of inhibitory interaction between species can be found in the oxidative destruction of highly chlorinated hydrocarbons in the presence of methane (seebox). Similar inhibitory interactions between species l i l y will exist in other mixtures; however, a priori prediction of such interactions would be impossible without a better understanding of the fundamental
chemical processes that take place during incineration. Recognizing this, we have been conducting systematic experimental and theoretical studies to develop detailed or microchemical kinetic mechanisms that will describe the high-temperature reactions of select chlorinated h y d m carbons in flames and other high-temperature processes (11-14). These mechanisms qwriratively describe the thermal destruction behavior of hazard-
Inhibitoryinteracti between species Figure 1 shows the destruction of trichloroethylene (C2HCI& a highly chlorinated hydrocarbon, in the This reaction begins presence of 02. at about 600-700 "C. However,
According to a recent mechanism Drooosed bv Chano " -et al - 111) ~ H C I desiuclion S in the pres&&
ous compounds individually, as well as in mixtures, by properly taking into account the formation of intermediates and by-products. Detailed chemical kinetic models represent numerical tools of exceptional generality and broad utility because of their fundamental bases. They contribute not only to the correlation experimental data on combustion, but also to the prediction of combustion behavior of the system under conditions
CpHC13 CzHCla
+
+ CI
C2Ch + 0 2
CzHC12 -+
+
+ CI
CzCl3 + HCI
COCI,
+ COCl
(1)
(2) (3)
COCl co + CI (4) where Equation 1 is the chain initiation step, and CI, C2C13,and COCl are the chain carriers. That is, although the initial rate of generation of CI via Equation 1 is slow, the rate of destruction of C2HC13is rapid as a consequence of the chain reactions in Equations 2-4. However, when methane is present in the system, it intercepts and removes the CI radical bv the following reaction: CH4 + CI CH3 + HCI (5) and senerates the much less rem -+
+
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for which prior data are not available. Consequently, the development of detailed chemical kinetic models would be of considerable usefulness in interpreting field measurements and in helping to direct future test burns in applied incineration research. In spite of their superior features, detailed chemical kinetic modeling will not have an impact on applied incineration research any time in the foreseeable future unless adequate and sustained funding of fundamental research becomes available. It takes a considerable amount of time, effort, and money, as well as the cooperative interaction of a broad spectrum of scientists and engineers, to generate the knowledge base needed for developing quantitative, detailed chemical kinetic models.
The future Having strong supportive fundamental research programs in incineration is paramount for the rational use of this technology in the management of hazardous wastes. Because such programs
currently do not exist, EPA must undertake a major initiative in this area and support high-quality scientific research that will create the knowledge base necessary to better accomplish its regulatory mission. This is most important, given the national and international impact of these regulations. It is also imperative that the public, the elected government officials, and the scientific and technical communities demand a scientifically strong EPA, so that environmental regulations can withstand rigorous scientific scrutiny.
References ( I ) Gerber, C. R. 3. Air Pollur. Control AsSOC. 1985.35. 749. ( 2 ) Oppelt, E. T. J . Air Pollur. Control APme. 1987.37, 558. (3) "Incineration of Liquid Hazardous Waster"; U.S. Environmental Protection Agency. Science Advisory Board: Washington, D.C.. April 1985. (4) Opportunities in Chemirrry: Pimentel. G.C.. Ed.; National Academy Press: Washington. D.C.. 1985. ( 5 ) Brown. G. E.; Byerly. R. Science 1981, 211. 1385. (6) Annual Report o/the Environmental Re-
search Grants Program: US. Environmental Protection Agency: Washington, D.C.. Fiscal Year 1987. (7) Graham, J . L.: Hall, D. L.; Dellinger, B. Environ. Sci. Technol. 1986.20, 703. (8) Tsang. W.; Shaub. W. In Detoxication o/ Hazardous Wosre; Exner. J . H . , Ed.; Ann Arbor Science: Ann Arbor. Mich.. 1982. (9) Valeiras. H.; Senkan. S. M. Combust. Sci. Technoi. 1984.36, 123. (IO) Fed. Regisr. Appendix VIII, Part 261, Jan. 22. 1981. (11) Chang. W. D.; Karra, S. B; Senkan, S . M. Combusr. Sci. Technoi. 1986.49. 107. (12) Senkan. S. M. Combust. Sri. Technol. 1984,38, 197. (13) Chang. W. D.; Senkan, S. M. Combust. Sci. Technol. 1985.43.49, (14) Chang. W. D.; Karra. S. B.; Senkan. S . M . Combust. Flome1987, 69. 113.
Selim M. Senkan is an associateprofessor of chemical engineering at the Illinois Institute of Technology in Chicago. He received his M.S. and Ph.D. from the Massachusetts Institute of Technology. One of his current research interests involves experimental and theoretical studies of the high-temperature oxidofion and pyrolysis of halogenated hydrocarbons.
Applying genetic ecology to environmental management By Betty H. Olson and Robert A. Goldstein During the last decade, research scientists have made advances in molecular genetics that are now ready to be applied to environmental problems facing the nation. Biotechnology based on genetic ecology provides a means for cleanup of toxic substances in situ. This
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new approach relies on a conceptual framework founded in genetics and ecology. Paramount among the disciplines and subdisciplines that will contribute a knowledge base to this endeavor is molecular microbial ecology. For the first time in history, successful methods have been developed for intervention at the genetic level to enhance pollution cleanup. Extrapolating
upon previous attempts at biological manipulation aimed at altering bacterial communities, this innovative method alters systems at the level of greatest control-the genes. This new approach does not involve the introduction of genetically engineered microorganisms (GEMS), but rather enhances of the capabilities of the natural community through gene amplification and increased expression. Genetic ecology can be applied to the management of potentially toxic chemicals in the environment. The prime objective for the management of organics is complete and rapid degradation in Situ. For metals, the objective is to control biogeochemical cycling by concentrating an element in a chemical species or an environmental compartment that minimizes availability and toxicity to biota. Management strategies based on microbial genetic systems may be considerably more effective, efficient, and less costly than conventional biotechnologies and chemical and physical processes for waste cleanup. Genetic ecology suggests control of an ecosystem compartment (i.e., soil,
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