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Methyl Chloride and Methylene Chloride Incineration in a Catalytically Stabilized Thermal Combustor Stephen L. Hung and Lisa D. Pfefferle"
Department of Chemical Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520 The catalytically stabilized thermal combustor has been demonstrated to be an effective burner for the thermal destruction of CH3C1and CH2C12. Propane was used as an auxiliary fuel. For all cases where stable combustion was achieved, destruction effectiveness of total hydrocarbons was beyond detector limits (corresponding to a greater than 99.994% destruction efficiency) using a 5-ms catalytic section residence time. Both Pt and a binary Cr203/Co304catalyst were tested, with the Cr203/Co304 catalyst observed to be the more effective in the presence of the chlorinated fuels. Lean stability limits for CH3C1/C3H8/airmixtures and CH2C12/C3H8/airmixtures each appear to be the same function of the total Cl/H ratio under the conditions studied.
Introduction The catalytically stabilized thermal (CST) combustor has been demonstrated to be a particularly effective high-throughput burner for minimizing the emission of fuel combustion byproducts. EPA-sponsored tests have shown that the CST combustor can operate over a wider range of operating conditions than conventional combustors and can reduce the emissions of CO, soot, NO,, and other products of incomplete combustion (1-4). The objective of this study was to explore the feasibility of chlorinated hydrocarbon (CHC) destruction in an appropriately configured catalytically stabilized thermal combustor. Methyl chloride (CH3C1) and methylene chloride (CH2C12)were used as the test chlorinated fuels and propane was used as an auxiliary fuel. By demonstrating that chlorinated hydrocarbon combustion can be stabilized in a plug flow configuration in a CST combustor and that high destruction efficiencies can be achieved, we have shown that catalytically stabilized thermal combustion may offer an attractive alternative to current methods of halogenated waste incineration. Major types of incinerators used include rotary kilns, liquid injection combustors, fixed hearths, and fluidizedbed combustors (5). These incinerators can meet current EPA regulations for incineration; however, to achieve combustion stability and high destruction efficiencies in the face of the flame-inhibiting properties of halogenated compounds, these systems require back-mixing (swirling), auxiliary fuel, staged burning, high temperatures of operation (producing NO,), and long average residence times of approximately 300-5000 ms (requiring large reactor volumes and consequent high capital costs) (6). In addition, avoiding PIC (product of incomplete combustion) 0013-936X/89/0923-1085$01.50/0
formation is difficult given wide actual residence time distributions, nonuniformitiesin mixing, flow disturbances, and cold reactor walls. Long residence times at high temperatures also increase the formation of NO,. With a CST combustor in a tubular flow configuration, however, the size of an effective incinerator may be reduced significantly by reducing the required average residence time. This results from catalytic stabilization overcoming halogen flame inhibition sufficiently to permit plug flow combustion (i.e., without significant back-mixing). With combustor size requirements significantly reduced, capital and operating costs could also be brought down and on-site or mobile incineration made more economic. A CST combustor is suitable for liquid or gas combustion. In addition, the reactor geometry allows for gas turbine electricity generation. Except for a preliminary report by the authors (7), there are no published works on the evaluation of CST combustion for the destruction of halogenated organic wastes. Many (e.g., ref 8-15) extensive studies have been made into the oxidation chemistry of CHC fuels under various conditions. Although the catalytic oxidation of CHC fuels has also been reported by several researchers at temperatures below 800 K (i-e., ref 16-18), no gas-phase reactions were reported.
Chlorinated Hydrocarbon Combustion It is well-known that chlorinated hydrocarbon (CHC) fuels are combustion inhibitors when added to a predominantly hydrocarbon flame and that CHC flames are harder to stabilize than their hydrocarbon counterparts (8, 9). Although CHC fuels inhibit combustion, the initiation steps for the combustion of CHC fuels are faster than for their analogous hydrocarbon fuels. This can be explained by the C-C1 bond found in CHC fuels, which is weaker than the C-H bonds of their hydrocarbon counterparts. For example, the C-C1 bond strength of methyl chloride is 84 kcal/mol compared to a bond strength of 104 kcal/mol for a C-H bond in methane. Although the initial decomposition of the parent fuel is faster for a CHC fuel than for its analogous hydrocarbon, the presence of chlorine in the reaction mixture can significantly reduce the rate of the combustion propagation steps. This occurs by the chlorine scavenging of hydrogen from the reaction mixture. Karra and Senkan (IO)have shown in CH3C1/CH4/02/Arsystems that the scavenging of hydrogen occurs mainly through the following mechanisms:
0 1989 American Chemical Society
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+ CH3Cl = CH3 + HCl H + HCl = H2 + C1
H
(A) (B)
These reactions compete for hydrogen radicals with the chain-branching reaction: H
+ O2= OH + 0
(C)
Because reactions A and B compete with reaction C, the concentration of both H and OH are reduced by the presence of chlorine. The inhibition of the combustion reactions by chlorine is especially noticeable for CO oxidation that proceeds via reaction with OH. CO
+ OH = COP+ H
(D)
CO oxidation is inherently slow and is significantly inhibited by the presence of chlorine in the reaction mixture. Chang et al. (11) have shown in a kinetic model of CO/ Hz/Clz combustion that the loss of H radicals from the reaction mixture is likely due to the following reactions from a sensitivity analysis of their model:
+ HCl = H,+ C1
(E)
+ C1+ M = HC1+ M
(F)
H H
Bose and Senkan (12) have also observed trichloroethylene flames to be extended and to exhibit two distinct combustion zones: the bulk of the fuel is oxidized to CO in the first zone followed by CO oxidation in the second zone. This property of chlorine to significantly inhibit CO oxidation has resulted in the use of CO measurements as an indicator of incomplete combustion in CHC flames. Because a chlorinated fuel’s inhibition effects occur after the C-C1 bond is broken, the inhibition effects are more a function of the total chlorine content of the mixture than of the chlorinated hydrocarbon parent fuel structure. It has been found that the laminar flame burning velocities of chlorinated methanes decrease with increasing fuel chlorine content (13).
Background on the CST Combustor The catalytically stabilized thermal (CST) combustor is a novel burner design that uses catalytic surface oxidation reactions to help ignite and sustain homogeneous gas-phase reactions (19-21). Because the CST combustor utilizes gas-phase combustion, it is capable of achieving high rates of destruction (per unit volume) not achievable with only catalytic surface reactions. As a gas-phase combustor, the CST burner can be stabilized over a wider range of operating conditions than a conventional burner while reducing pollutant emissions (hydrocarbon, CO, and NO,). It is suitable for high throughput applications as in a gas turbine combustor. The catalytically active hot surfaces of the CST combustor can also improve the stability of combustion to flow upsets and prevent PICs from bypassing the combustion zone due to quenching effects at the relatively cold combustor walls of conventional incinerators. The CST combustor is sometimes also referred to as a “catalytic combustor” or a “catalytically stabilized combustor” in literature and should not be confused with catalytic oxidation reactors. This can be confusing because catalytic oxidation reactors are also frequently referred to as “catalytic combustors” and “catalytic incinerators”. Catalytic oxidation reactors are low-throughput, moderate-temperature, high residence time devices, whereas CST combustors are high-throughput, high-temperature (1200-1550 K or more), low residence time devices. Catalytic oxidation reactors (e.g., the catalytic converter in a 1086 Environ. Sci. Technol., Vol. 23, No. 9, 1989
car) utilize primarily heterogeneous catalytic surface reactions and lower operating temperatures than CST combustors. Because reaction on the catalytic surface is the primary oxidation mechanism in catalytic oxidation reactors, their overall conversion and throughput are limited by the mass transport rate of the oxidation reactants and products to and from the catalyst surface. Because of this limitation, a well-designed catalytic oxidation reactor for chlorinated hydrocarbon incineration typically obtains POHC (Principle Organic Hazardous Component) conversions of approximately 95-99% as compared to the much higher conversions obtained in hazardous organics thermal incinerators. In addition, even with the most effective catalysts, selectivity to extremely toxic oxygenated products can be high enough to result in unsafe levels in the product gases. The CST combustor also has advantages over conventional means of toxic organic flame stabilization. By using heterogeneous surface reactions to ignite and sustain gas-phase reactions, CST combustors may be designed to approximate a plug flow reactor with combustion stability maintained without back-mixing. In a tubular geometry, the average residence time distribution of the flow is much narrower than in a back-mixed burner (well-stirred configuration). This is especially important for hazardous waste incinerators where high degrees of conversion are required, because even low levels of the primary chlorinated fuel or partial combustion byproducts formed may be toxic. A narrow residence time distribution allows the high conversion to be more reliably achieved. Hence, the average residence time and the size the burner may be decreased without compromising the degree of conversion or safety. In addition, the hot walls eliminate the possibility of organics bypassing the combustion region due to cold wall quenching.
Experimental Setup Methyl chloride (CH3C1)and methylene chloride (CHzC1,) were burned in excess air in a laboratory-scale CST combustor. Propane was used both to start the burner and to act as a supplementary fuel. Propane was chosen because there is a large pool of published papers characterizing the combustion of propane in the CST combustor (e.g., ref 22-24). A schematic diagram of the CST combustor setup is shown in Figure 1. The air was preheated electrically to 650 K (measured at the catalyst section and kept constant to within & l o K) and was mixed thoroughly with the fuel mixture in a packed-bed mixing chamber. CH3Cl has a high enough vapor pressure to be used as a gas directly from a lecture bottle held at ambient room temperatures, but the liquid CHzClzwas heated so that it could be introduced into the mixing chamber as a gas. Immediately downstream of the mixing chamber was a flow distribution section approximately 15 cm in length and 5.0 cm in diameter that was packed with stainless steel mixing screens to improve mixing and to give the flow a uniform radial velocity profile. Downstream of this section was a piece of noncatalytic ceramic honeycomb monolith 2.5 cm in length (total diameter open to flow was 5.0 cm). This monolith was positioned approximately 0.1 cm upstream of the catalytic section to shield the catalytic section from thermal radiation losses. The catalytic section that followed was 7.6 cm long and 5.0 cm in diameter and was not insulated on the downstream side. The downstream end of the catalytic section was open to the atmosphere and accessible to sampling probes. The catalytic section was made with a ceramic mullite (3A1203/2Si02)monolith support provided by Corning
Insulating Ceramic Blanket
Fuel Inlets
Thermocouples
i.
Alr-
c
Combustion Products
ted ycombed d Monollths Mixing Screens
Insulating Honevcombed Monolith
Figure 1. Schematic diagram of experimental apparatus.
Figure 2. Schematic diagram of the honeycomb ceramic monoliths used for the catalyst support.
Glass Works. This monolith consists of small parallel channels packed together in a square honeycomb configuration (Figure 2). The square channels have walls that are approximately 0.15 cm thick and have a cross-sectional packing density of 31 channels/cm2 (with an open crosssectional area of approximately 70%). Control tests made with clean (no catalyst coating) mullite monoliths found them to have no observable catalytic activity under the conditions of these experiments. The monoliths were first coated with a film of y-alumina and calcined to 1200 K for 4 h to increase the surface area of the mullite support before the catalyst was applied. In this study, both platinum-coated catalytic monoliths and binary chromium oxide (Crz03)/cobaltoxide (Co304)(3 to 1 ratio by weight) coated monoliths were tested for effectiveness at stabilizing chlorinated hydrocarbon combustion. This ratio of the transition-metal oxides was used by Prasad et al. (22-24) and found to be a good oxidation catalyst for propane combustion. A metal loading of 2.2 X g/cm2 (per macroscopic measurable geometric surface area) was used for the platinum-coated monolith and a loading of 10 X g/cm2 was used for the transition-metal oxides. The transition oxide catalysts were impregnated onto the mullite ceramic monoliths as metal nitrates dissolved in solution and platinum was applied as HzPtCI, in solution. Both were dried and then calcined to 1200 K for 4 h prior to use in the burner. The concentrations of these catalyst precursor solutions were controlled in order to give the desired loadings. The metal and metal oxide loadings stated above are the actual loadings determined by weight difference of the monolith before and after impregnation and calcination. Although very little platinum is used, we have reported in an earlier study (25) that, because of the high activity of platinum toward hydrocarbon oxidation, this loading of platinum produces an overall rate of reaction that is well into the mass transfer controlled regime. The combustor monolith wall temperature was monitored by Pt/Rh type B thermocouples placed every 1.2 cm in the axial direction at the center of the catalytic monoliths. These thermocouples have an accuracy of approximately h10 K, but have been referenced to a common thermocouple to have a relative precision of f 2 K. Combustion emission exhaust gas samples were taken with a
stainless-steel water-cooled sampling probe to quench the reaction mixture at the exhaust end of the burner. The gas samples were then analyzed by gas chromatography (GC) techniques. By use of 100/120 Carbosieve S-I1 packing in a 0.3-cm-diameter column at ambient and 463 K oven temperatures, CHI, CzHs, CzH4, C2H2,C02, and CO may be identified and quantified. Both columns could also be bypassed to identify the total hydrocarbon (including CHC) content by using the GC’s flame ionization detector (FID). Simple chlorinated hydrocarbons (CH3C1, CHZClz,CHCl,, and CC14) were also separated and identified by use of a 3% SP-150080/120 Carbopack B packing in a 0.3-cm-diameter column at 463 K. The GC is also equipped with a thermal conductivity detector (TCD) for the identification of CO and C02. The total flow rate of the inlet gases is characterized by the reference inlet velocity ( Vref). This is the calculated average velocity of the inlet gases at the entrance to the catalytic bed before any reaction or thermal expansion has occurred (i.e., the cold gas velocity). The reference velocity was maintained at 97 cm/s for all runs, providing an average residence time of approximately 20 ms over the length of the 7.6-cm-long catalytic bed. The fuel/air ratio of the inlet feed gases is specified by using the equivalence ratio. The equivalence ratio is defined to be the molar fuel/air ratio actually in the feed divided by the molar fuel/air ratio that would give stoichiometric combustion (no excess air or fuel) to form the complete combustion products. The “complete” combustion products for hydrocarbon fuels are C02and HzO. For CH3Cl and CH2C12,the “complete” combustion products are COz, HzO, HC1, and Clz. Hydrogen will first be consumed to form HC1. The “excess” hydrogen will form HzO. Thermodynamically, some C12will be stable at the adiabatic flame temperatures; however, the Clz concentration will be low (e.g., for CH3C1 combustion at an adiabatic flame temperature of 1500 K, the mole fraction of Clz present has been estimated to be only 5.0 X and the assumption that C1 will be predominantly converted to HC1 is valid. This definition of the equivalence ratio for chlorinated fuels has also been reported elsewhere (8, 9). The equivalence ratio is more useful than just correlating the fuel/air ratio, because it normalizes the fuel/air ratio of different fuels to the amount of air each fuel needs to achieve the “complete”combustion products (i.e., an equivalence ratio of 1.0 gives rich fuel conditions). The fuel lean stability limits of various mixtures of CH3Cl/propane/air and CHzClz/propane/air mixtures in the CST combustor were determined experimentally. In order to have complete combustion, there must be a stoichiometric or greater amount of oxidizer present. For this reason, the behavior of the CST combustor was Environ. Sci. Technol., Vol. 23, No. 9, 1989
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studied only under lean fuel conditions (equivalence ratio,
