Environ. Sci. Technol. 1996, 30, 998-1008
Polychlorinated Benzene, Phenol, Dibenzo-p-dioxin, and Dibenzofuran in Heterogeneous Combustion Reactions of Acetylene KENNETH L. FROESE† AND OTTO HUTZINGER* Ecological Chemistry and Geochemistry, University of Bayreuth, D-95440 Bayreuth, Germany
Acetylene was reacted with HCl/air under heterogeneous combustion conditions between 300 and 600 °C. Model catalyst mixtures of SiO2/metal oxides (Al2O3, Fe2O3, TiO2, and CuO) were compared with municipal waste incinerator (MWI) fly ash. Chlorinated benzenes (ClxBz), chlorinated phenols (ClxPh), and PCDD/F were detected in both gas-phase products as well as catalyst-adsorbed products. ClxBz production increased exponentially to 600 °C (HCB: 4 × 104 ng/g of C2H2), while ClxPh production (2.5 × 104 ng/g of C2H2) was maximized at 500 °C (ca. 104105 ng/g of C2H2). PCDF displayed a dramatic peak at 500 °C (TCDF: 2 × 106 pg/g of C2H2), in contrast to the generally accepted de novo and aromatic precursor formation temperature of 300 °C. Comparison of gasphase ClxBz congener distribution patterns revealed that the CuO-catalyzed reaction closely matched the ClxBz pattern produced in the reactions with MWI fly ash, providing further evidence of the importance of Cu in the fly ash matrix. Comparison of fly ash-catalyzed gas-phase ClxBz with MWI flue gas and 300 °C de novo synthesized ClxBz demonstrated the relevance of mid-temperature (600 °C) heterogeneous combustion reactions of C2 aliphatics in real combustion systems.
Introduction Polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) are classes of compounds commonly found in all areas of the environment (1). They are formed as byproducts in commercial chemical production of chlorinated phenols and phenolbased herbicides and in the synthesis of PCB mixtures (2), waste products from industrial processes [paper pulp industry effluents, coke regeneration in oil refineries (3)], and as products of incomplete combustion in the incinera†
Present address: Aquatic Toxicology, Pesticide Research Center, Michigan State University, East Lansing, MI 48824.
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tion of municipal wastes, in the burning of fossil fuels (e.g. coal, diesel), in chemical storage facility fires, and in forest fires (1, 2). As a broad class of substances and as individual groups, these compounds have been studied intensively. Many hypotheses and much data have been gathered with respect to their formation via combustion processes. Under favorable conditions of time and temperature, it is likely that any combination of C, H, O, and Cl can yield some PCDD/F or similar products (4). To date, the most researched pathways are the formation through de novo synthesis from particulate carbon or from chlorinated aromatic precursors. De novo synthesis at 300 °C in the post-combustion zone has been postulated by Vogg and Stieglitz (5) and Stieglitz et al. (6) as the principal pathway of formation of PCDD/F in municipal waste incineration (MWI) processes. Catalytic reactions of chlorinated aromatic precursors (chlorinated phenols) on the fly ash surface in the post-combustion zone has been reported by Hutzinger et al. (7) and Karasek and Dickson (8) as being the major PCDD/F formation pathway. Chlorinated benzenes and chlorinated phenols in addition to PCDD/F have been observed in the combustiuon of particulate carbon in fly ash and fly ash model mixtures (9, 10). There is, therefore, a possibility of a fundamental link between the two formation theories. The possible sources of the chlorinated benzenes or phenols as precursors in the post-combustion zone must be considered. Potential sources include pentachlorophenol (PCP)-based wood preservatives or PCP-contaminated textile materials (11) in the waste fuel or chlorinated aromatics as comtaminants in the fuel stock. One assumption that must follow is that such aromatics (e.g., PCP) survive intact through the high-temperature combustion zone and are then available to catalytically react with the fly ash in the 300 °C post-combustion zone. The de novo synthesis of PCDD/F from particular carbon, proposed by Vogg and Stieglitz (5), is independent of aromatic precursors coming into the incineration system as a part of the fuel. The assumption here is that particulate organic carbon can reform to dioxin-type structures in the (300 °C) postcombustion region. At this point, we might look at these concepts in graphical form, such as those presented in Figure 1, a simplified schematic of conceivable de novo and precursor PCDD/F formation pathways. For de novo synthesis, the following the mechanistic pathways have been postulated: (i) Carbon gasification, i.e., the transformation from particulate C to CO and CO2, and further to organic C (12, 13). However, recent work (14) provides evidence that, under conditions identical to de novo studies (12), CO and CO2 are not precursors to ClxBz or PCDD/F formation in fly ash-catalyzed systems at 300 °C. (ii) Aromatic structures in elemental C: oxidative decoupling and direct chlorination of benzene-like rings in the carbon matrix may lead to dioxin-type structures (9, 10, 14-16). The de novo and precursor pathways describe important aspects in the formation of PCCD/F in combustion systems. Before considering the third reaction pathway shown in
0013-936X/96/0930-0998$12.00/0
1996 American Chemical Society
FIGURE 1. Potential PCDD/F formation routes based on recent literature. The type of arrows represent our subjective and qualitative indication of the degree of speculation in the various pathways.
Figure 1, some additional background information is necessary. Kinetic modeling was used by Shaub and Tsang (17) to consider gas-phase formation of PCDD/F in an incinerator environment. Those authors concluded that gas-phase formation of PCDD/F is of little importance and suggested gas-surface reactions as a potentially critical process. One should note that they assumed a temperature of 1200 °C (i.e., the temperature of the incineration zone) in their model calculations. Gas-phase reactions alone are likely not the major pathways to PCDD/F formation (4). For gas residence times of 1-2 s, which are realistic for a post-combustion region, gas-phase reactions probably account for only a small percentage of PCDD/F formation. Particulate surfaces, especially in the mid to high temperatures in the post-combustion zone, may be highly important with respect to the formation of chlorinated aromatics. Let us consider a broader perspective of the combustion process. If we envision a generalized flame combustion system in which the fuel is a complex mixture of organic, metallic, and mineralic materials, a complete oxidation to CO2 + H2O is seldom realized. A combustion environment is most often turbulent and inhomogeneous, with dynamic variables such as temperature gradients, quench zones, and oxygen-depleted pockets (18, 19). While larger molecules, including most aromatics, will begin to break down through oxidation and pyrolysis reactions, the decomposition is not likely to procede to completion, i.e., the production of only CO2 and H2O. C1 and C2 alkyl olefins and radicals will be formed along with other products of incomplete combustion (18-20). The post-combustion
zone immediately following the incineration zone is a potentially rich reaction environment, containing excess O2, metallic compounds, and catalytically active fly ash particles. This is an ideal reaction environment for the C1 and C2 compounds exiting the incinerator. Calculations of the rate of PCDD/F formation in incineration systems also point to the importance of this region in a municipal waste incinerator (12, 21). Based on an investigation of PCDD/F production in 5-30-min reactions, de novo reactions could explain the PCDD/F found in fly ash particles collected from electrostatic precipitators (ESP) (12). However, the de novo formation rate is not sufficient to explain the PCDD/F extracted from uncollected fly ash particles (i.e., those not trapped by the ESP; residence time 1-2 s). Investigations of de novo synthesis (6, 9, 10, 15, 16) in which 1-2-h reaction times were used failed to consider the issue of fast (1-2-s) production of chlorinated aromatic compounds. Therefore, fast, heterogeneous reactions of shortchain aliphatic and chlorinated aliphatic combustion products in the high-temperature post-combustion zone are believed to be critical in the formation of the first aromatic rings. Recent investigations of high-temperature gas-phase pyrolysis of chlorinated and non-chlorinated C1 and C2 aliphatics with and without external chlorine sources (13, 22-25) indicate that aromatic and chlorinated aromatic compounds can form from aliphatic precursors. In the only combustion study using acetylene that is relevant to this investigation, Miller and Melius (26) investigated the flame combustion of acetylene with O2/Ar. They developed a chemical kinetic model to predict the growth of more
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complex hydrocarbons and concluded that 1CH2 is central in forming C3 and C4 compounds that ultimately lead to ring formations. They state that the “first ring” is most likely formed by the reaction of two propargyl radicals and that similar reactions of butadienyl radicals are not effective in producing aromatic compounds in flames. The formation of chlorinated aromatic compounds from chlorinated and non-chlorinated aliphatics, within the larger framework of dioxin formation (Figure 1), is of fundamental importance in understanding thermal production of dioxin-type compounds as a whole. Such fundamental information is not limited to municipal waste incinceration processes, but may be applied to combustion processes in general. There are no quick ways to elucidate all of the pathways and mechanisms of chloroaromatic formation in combustion systems. The multitude of papers published in the past 15 years on this topic attest to the complexities of the problem. For this reason, we believe it is important to focus on one fundamental process in the formation of chlorinated aromaticssthe pathways from short-chain (C2) aliphatics to simple chlorinated aromatics (chlorinated benzenes). Our premise for this work is 2-fold: first, we believe that the formation of aromatic compounds from alphatic PICs is an important process in combustion systems. Secondly, we believe that these initial aromatics are formed at high temperatures (g400 °C) and that they may play a critical role in the formation of toxic and persistent chlorinated and non-chlorinated aromatic compounds, including PCDD/Fs and other dioxin-like chlorinated compounds, in the lower temperature (300 °C) zone downstream from the combustor exit. This investigation provides merely a starting point; more extensive investigations, including pilot plant scale testing, will be needed to gain more complete knowledge of the mechanisms and their potential application in combustion technologies. Many investigations of the potential mechanisms of PCDD/F formation from aromatic precursors are reported and reviewed in the literature (1, 27-31). Although we will discuss the formation of chlorinated aromatics from aliphatic reagents within the applied context of PCDD/F formation in municipal waste incinerator systems, detailed mechanistic discussion and review of the actual formation of PCDD/F was not included in the original intentions of the experimental work presented here and goes beyond the scope of this paper. This paper is the first in series examining heterogeneous combustion reactions of the non-chlorinated C2 aliphatics. In preliminary heterogeneous work with trichloroethylene, a clear temperature dependence in the production of chlorinated benzenes and chlorinated phenols was observed (32). These encouraging results led to further work with non-chlorinated C2 aliphatics.
Experimental Section Materials and Reagents. Gaseous reagents and carrier gas were purchased at thee highest available purity. Synthetic air (99.999%, Linde AG), composed of 80% N2 and 20% O2, was used as the carrier for all experiments. HCl (99.8%, Linde AG) was received in 0.3-L lecture bottles. A special wall-mounted, stainless steel pressure regulator (Linde Model SMD 650-85) equipped with an inert gas purge valve was used to deliver this gas. For acetylene (99.8%, Linde AG), a one-way safety valve was installed just beyond the pressure regulator.
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Municipal waste incinerator (MWI) electrostatic precipitator fly ash was dried overnight and sieved to e1-mm particle size. It was then Soxhlet extracted with toluene (2 × 48 h); rinsed with acetone; heated (annealed) in a synthetic airstream (ca. 50 mL/min) at 600 °C for 24 h; rewashed with toluene, dichloromethane, and methanol; and annealed for another 24 h at 550 °C. While this treatment potentially alters the catalytic characteristics of the fly ash, our greater concern was for a carbon-free catalytic material. The fly ash preparation process was continued in stages until the extracts from blank runs at different temperatures showed no detectable chlorinated aromatic compounds. Surface and elemental aspects of the fly ash have been described previously (32). The fly ash was stored in a desiccator; 0.5 g was used for each reaction. Silica gel (200-500 mesh, 60 Å, ICN Biomedicals GmbH) was heated to 550 °C in a synthetic airstream (ca. 50 mL/ min) for 14 h; washed with toluene, hexane:dichloromethane (1:1 v:v), and methanol; and reheated to 550 °C for 14 h. Washing with methanol proved to be the key step in proper cleaning; methanol displaces even very strongly adsorbed organic compounds by occupying all of the adsorption sites. For model catalytic materials, many investigators have focused primarily on metal chlorides, such as FeCl3 and CuCl2 (33-36). For this study, however, metal oxides were chosen because they are most likely in a similar state as the metals in fly ash (37), particularly after the annealing process. Catalyst mixtures of the cleaned SiO2 were prepared with the appropriate quantity of metal oxide (Aldrich; >99.99% purity) to provide the following ratios: SiO2/Al2O3, Al ) 10% (wt); SiO2/Fe2O3, Fe ) 10% (wt); SiO2/ TiO2, Ti ) 0.6% (wt); SiO2/CuO, Cu ) 0.8-1% (wt). The metal oxides were used as received. A 0.5-g sample of the catalyst mixture ws used for each reaction. All solvents used in this investigation were commercial pesticide-grade analytical solvents (Mallinckrodt Nanograde, Promochem GmbH, Germany). Reaction Apparatus. The experimental apparatus was constructed on-site and has been described previously (32). The gas mixing chamber (glass; ∼30 cm × 20 mm i.d.) was equipped with threaded, septum-sealed gas inlets and Teflon stopcocks, enabling on/off control of a carrier gas and two reagent gases. A 90° elbow and a reducing adaptor served to connect the mixing chamber to the quartz reaction tube. All glass parts were connected via ground glass joints, sealed with Teflon liners. The glass reducing adaptor and Teflon seal was exchanged for a cleaned set after each set of reactions. Quartz reaction tubes (18 cm × 8 mm o.d., 6.7 mm i.d.) were cleaned and heated overnight at 300 °C. Catalyst material was filled into the lower part of the tubes using glass wool to keep it in place. A custom-made tubular furnace, employed previously in related mechanistic investigations (36), was used for all reactions in this project. An electronic thermostat provided temperature control from 50 to 600 °C. At 500 °C, under experimental conditions, the error was measured to be
