Environ. Sci. Technol. 2000, 34, 4565-4570
Presence of Chlorine Radicals and Formation of Molecular Chlorine in the Post-Flame Region of Chlorocarbon Combustion CARLO PROCACCINI,‡ JOSEPH W. BOZZELLI,§ JOHN P. LONGWELL,‡ KENNETH A. SMITH,‡ AND A D E L F . S A R O F I M * ,†,‡ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102
This study is concerned with the formation and persistence of atomic and molecular chlorine in the late stages of chlorocarbon combustion under fuel-lean conditions as well as in postcombustion cooling stages, since chlorinecontaining hazardous organic pollutants including dioxin precursors can be formed through reactions involving these active forms of chlorine. In bench-scale experiments with a chlorine-containing fuel mixture (C2H4/CH3Cl or CH4/ CH3Cl, with Cl/C in the range of 0 to 2.2%) the fuel is oxidized in a relatively short time (10-20 ms). The major chlorine-containing product is HCl; however, the cooled combustion gases contain significant concentrations of Cl2 (up to 18% of the total chlorine load), with the exact amount depending on the fuel equivalence ratio, the residence time in the combustor, the H/C ratio of the fuel, and the rate of cooling of the gas products. Calculations indicate that the Cl2 measured in the cold exhaust gas is formed by recombination during the quenching of Cl radicals present at high temperature. The model predicts that, under conditions found in practice, Cl radicals can likewise be present in significant concentrations (1 to 5% of the total chlorine) in combustion products at the exhaust. Due to kinetic constraints, Cl radicals can then persist at surprisingly low temperatures (down to ca. 500 K) before recombining to form Cl2.
Introduction Cl2 is a powerful oxidant and a chlorinating agent, the emissions of which should be controlled. The presence of Cl2 in the combustion effluents of chlorinated wastes and fuels has also been linked to the homogeneous and catalytic formation of chlorinated dibenzo-p-dioxins and dibenzofurans and their precursors in the low-temperature section of combustors and incinerators (1). Molecular chlorine is * Corresponding author phone: (801)585-9258; fax: (801)585-5607; e-mail:
[email protected]. † Present address: Department of Chemical and Fuels Engineering, University of Utah, 110 Kennecott, 1495 East 100 South, Salt Lake City, UT 84112. ‡ Massachusetts Institute of Technology. § New Jersey Institute of Technology. 10.1021/es001051z CCC: $19.00 Published on Web 09/22/2000
2000 American Chemical Society
also difficult to scrub from the flue gas, due to its relatively low water solubility; therefore, the formation of Cl2 should be minimized during the combustion process because of both its role in the formation of chlorinated hydrocarbons and the difficulty of its capture relative to HCl. The present paper addresses the issue of the persistence of the Cl2 and its precursors, Cl radicals, in the effluents from combustion systems. The paper provides experimental validation over a range of fuel air ratios and cooling rates of an existing chemical kinetic scheme expanded to include reactions of importance at low temperatures, the identification of the pathways responsible for the persistence of Cl radicals and Cl2, and the application of the validated kinetic model to provide estimates of Cl2 emissions for the cooling rates encountered in practical combustion systems. Detailed descriptions of the elementary reactions of chlorinated compounds and radicals have been developed previously because of the importance of chlorine in incinerators and flame suppression systems. Chlorine participates both in chain termination reactions, which consume important oxidizers, and in the abstraction of H atoms from hydrocarbon molecules. As a consequence, fuel oxidation is inhibited (2-5), and the formation of heavier molecular weight species is promoted (6, 7). Chlorine chemistry is particularly important in the late stages of combustion by its impact on the final oxidation of CO and of trace byproducts which may have escaped the primary reactor zone. Chlorine is thought to inhibit these processes by consuming HO2 and O radicals (8) and to participate in the formation of chlorinated and oxychlorinated pollutants if the temperature is such as to favor recombination reactions (9-13). Relatively little research has focused on the persistence of Cl radicals and Cl2 to the temperature of importance for dioxin formation (500 to 650 K) or at the combust or exhaust, since the general perception was that HCl was the dominant product of the chlorine present in fuels or wastes. Equilibrium calculations under fuel-lean conditions show that HCl is the dominant product but that Cl radicals are present in significant concentrations (>1% of the total chlorine) under fuel-lean conditions above 1500 K. At lower temperatures (T < 900 K), Cl2 becomes favored by equilibrium, but the formation of Cl2 at low temperatures was considered to be unimportant since the reactions which transform HCl are inactive unless suitable catalysts are present (14). In contrast to this assumption, detailed kinetic calculations (15) have shown that significant concentrations of Cl2 may be found in the quenched products of the fuellean combustion of chlorohydrocarbons. In the present study, experimental confirmation of the model predictions is provided for a range of equivalence ratios, fuel hydrogen/ carbon ratio, and cooling rates. The model is applied to determine the conditions under which Cl2 and Cl persist in practical combustion systems. The results of the present study provide a basis for developing strategies to control Cl2 emissions from combustion sources.
Experimental Section The experimental combustor is comprised of two stages, a jet-stirred reactor (JSR) and a plug-flow reactor (PFR), both running at atmospheric pressure. A detailed description of the apparatus is given in ref 16 and in the Supporting Information. The fuel components (methyl chloride, ethylene, and methane) are premixed in known amounts with air and additional nitrogen. The feed stream is introduced to the JSR through 32 lateral jets that generate an intense turbulent VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mixing, which allows the JSR to be modeled as a perfectly stirred reactor (PSR) (3). After 10 ms in the JSR at 1500 K, the combustion gas is directed into the subsequent adiabatic PFR, from which samples of the combustion products are quenched and collected at different residence times. The combustion gas is sampled through a water-cooled probe, the inner surface of which is of Teflon, to minimize surfacecatalyzed reactions. The influence of temperature history on the final composition of the quenched gas was studied by the use of probes with three cooling rates. Probe I has the smallest internal diameter (1.6 mm) and a cooling rate, β, of -4.5 × 105K/s, based on gas temperature measurements made with a fine thermocouple wire (Type K, Omega Engineering) during the sampling of reactor gases initially at 1500 K. Probe II has an internal diameter three times larger than Probe I (4.8 mm) and a slower cooling rate of -1.8 × 105K/s. Probe III has the same internal diameter as Probe II but a different inlet geometry, characterized by the presence of an adiabatic tube (alumina) upstream of the cooled section. The cooling near the inlet of Probe III is -6.8 × 104K/s, i.e., slower than in Probe II, since the presence of the inlet extension permits the formation of a pipe flow before the gas enters the cooled section of the probe. The chemical species in the quenched combustion products were analyzed for Cl2 (by I2 displacement from a KI solution), for HCl (by adsorption in a buffered solution and measurement of the Cl ion using a selective ion electrode taking care to correct for the Cl formed from Cl2), for fixed gases (CO, CO2, and O2) and unburned hydrocarbons (C1 and C2 species) by gas chromatographic analyses of gases collected in sampling bottles.
FIGURE 1. Fractional conversion of total chlorine to Cl2, measured in the quenched reactor gas, versus the fuel equivalence ratio. Experimental data (symbols) are compared to model calculations (lines) for three values of the cooling rate, each corresponding to the use of one of the gas probes described in the Experimental Section.
Modeling The JSR of the experimental reactor is modeled as a perfectly stirred reactor (PSR) and the post-flame PFR as a plug flow reactor, respectively. In both stages, temperature and pressure are set constant and equal to the operating values in the experimental reactor. The mole fractions of the products calculated at the outlet of the PSR are used as input for the plug flow reactor simulating the post-flame zone. The quench-probe is modeled as an atmospheric pressure PFR, in which the temperature is an assigned function of the residence time. Chemical reactions are modeled by a detailed kinetic mechanism for methyl chloride combustion comprised of 50 species and 239 reaction steps, reported in the Supporting Information. The mechanism is a reduced version of the one published in Ho et al. (15, 17) from which hydrocarbon species with more than three carbon atoms were excluded. Reactions of an oxygen-catalyzed cycle of Cl-radical recombination were added to the model, because they could play an important role at low temperature (16).
Results and Discussion Experimental Validation of the Kinetic Model. Three sets of experiments were carried out for model validation: the first varying the fuel ratio in the JSR and the cooling rate in the probe; the second varying the residence time in the plug flow reactor from which the gases were sampled; and the third varying the hydrogen/carbon ratio of the fuel. In the first series of experiments, the reactor fuel was comprised of ethylene and methyl chloride, with a Cl/C ratio of 0.022. The fuel-to-air ratio of the feed is characterized by the fuel equivalence ratio (φ), calculated as φ ) (3[C2H4] + 3/2[CH3Cl])/O2]. During the experiments, φ was varied between 0.7 and 1.05. This was accomplished by replacing oxygen with equal volumes of nitrogen in the reactor feed, thus keeping the volumetric feed rate and the JSR residence time (10 ms) constant. The reactor temperature was equal 4566
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FIGURE 2. Fractional conversion of total chlorine to HCl, measured in the quenched reactor gas, versus the fuel equivalence ratio. Experimental data (symbols) are compared to model calculations (lines) for three values of the cooling rate, each corresponding to the use of one of the gas probes described in the Experimental Section. to 1500 K, and the mole fraction of methyl chloride in the feed was constant at 1.54 × 10-3. The high-temperature JSR effluent was directed into the nearly isothermal PFR, from which, after a residence time of 2 ms, samples of the combustion products were quenched and collected. In Figure 1, the fractional conversions of total chlorine to Cl2 are plotted versus the equivalence ratio. At the highest equivalence ratio tested (φ ) 1.05), the fractional conversion of total chlorine to Cl2 is very low (
