Ind. Eng. Chem. Res. 1995,34, 4185-4192
4185
Analysis and Optimization of Chlorocarbon Incineration through Use of a Detailed Reaction Mechanism Wenpin HoJ Michael R. Booty: Richard S. MageeJ and Joseph W. Bozzelli*J Department of Chemical Engineering, Chemistry, and Environmental Science and Department of Mathematics and Statistics, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102-1982
Chemical species profiles are calculated by using a detailed reaction mechanism and a reactor code that simulates a well-mixed, three-zone incineration process. The chemical systems include CH3CYCH4 and CH2CldCH4 oxidation in air at fuel equivalence ratios 4 from 0.8 to 1.1,with additives injected at downstream positions. Combustion is characterized for temperature, principal organic hazardous constituent (POHC), and product of incomplete combustion (PIC) levels. Major PICs comprise C1, Cl2, CO, HOC1, and COCl:! and are calculated versus time, temperature, fuel equivalence ratio, and feed conditions. Steam, H202,02, air, and other species are injected as additives in the burnout region to discern changes in the combustion chemistry. Steam addition improves or decreases the CO/CO2 ratio at a n additive mole fraction of 0.1. Atomic C1 is the active radical species of highest concentration in the initial high-temperature reaction zone when CH3C1 is the POHC at a feed concentration above 1200 ppm and 4 I 1. Cl2 is found to be a major PIC under fuel-lean and stoichiometric conditions, while CO is a major PIC under fuel-rich conditions. Reduction of combined CO and Cl2 levels in the incinerator stack effluent is achieved by operation a t stoichiometric conditions or slightly fuel-lean with the controlled addition of high-temperature steam.
Introduction Incineration is an effective way of handling the disposal of many combustible wastes, such as solids, semisolids, sludges, and concentrated liquids. It dramatically reduces volume, which eliminates future environmental exposure, and often converts waste material into recoverable energy. Incineration is becoming a more viable disposal option with the increase in regulation applied t o more types of waste and the increased costs and prohibitions placed on disposal by landfill. Although the incineration of hazardous wastes presents a means of disposal, applications of the technology have been hindered by environmental concerns regarding the stack effluent of such systems. Two important issues here are the capability of incinerators t o reach the required high level of destruction of the original waste feed and the possible production during the incineration process of other hazardous species. Hazardous waste incineration of chlorine compounds deserves attention because chlorocarbons are a major component of waste solvents and because the behavior of chlorine is unique among the halogenated compounds. Organic chlorine compounds provide a source of chlorine atoms which readily abstract hydrogen atoms from other organic hydrocarbons, thereby extending the radical chain and accelerating the production of heavier hydrocarbons with the possible formation of soot. Concerns regarding the behavior of chlorine are not always easily resolved. For example, HC1 is often a desirable product of combustion, since it removes chlorine and can easily be neutralized or removed from the stack effluent by scrubbing, but it can also inhibit the combustion process through reactions like OH + HC1- H2O + C1, which depletes the free OH radical that is needed for CO burnout. In the analysis of incineration chemistry and systems Tsang (1990a) proposed that temperature is only one +
Department of Chemical Engineering, Chemistry, and
Environmental Science.
* Department of Mathematics and Statistics.
factor in the successful destruction of chlorinated hydrocarbons. Chuang and Bozzelli (1986a,b) have also noted the importance of the reaction mixture’s chemical makeup and the need for an integral or larger ratio of hydrogen to chlorine. Sufficient hydrogen is needed both to convert C1 t o HCl and to supply hydrogen for water formation, since the 0-H and H-OH bond strengths are greater than the H-C1 bond strength. Tsang (1990b) also demonstrated the importance of atomic hydrogen in the high-temperature destruction of organic compounds. Lyon (1990) suggested the presence of two different thresholds for effective combustion. The first threshold depends on the composition of the reaction mixture through its flammability limit. The second threshold depends on the radical pool experienced by the principal organic hazardous constituents (POHCs) which is created during the combustion process by the reaction of fuel and oxygen feed. He proposed that the second threshold can be improved by introducing a minimal amount of fresh fuel after the reaction process has started in order t o raise the total fuel concentration above the threshold level that is needed to create an active, Le., destructive, radical pool. This second active radical pool serves to promote further destruction of products of incomplete combustion (PICs). Cooper and Clausen (1991) studied the effect of H202 and O3 addition on the oxidation of heptane vapor in the temperature range from 773 to 973 K. They postulated that injection of hydrogen peroxide into the hot reacting gases would lead to increased free-radical levels, even in the absence of a flame, and observed that heptane destruction at 923 K and 0.9 s reaction time increased from 28% to 50% upon the addition of H202 at molar ratios HaOdheptane of 4:lO. In this work we perform calculations that simulate an incinerator with the following three reactors in series: (i)a perfectly stirred flame reactor (PSR, usually adiabatic), (ii) a nearly constant temperature burnout zone (plug flow reactor (PFR), adiabatic), and (iii) a cooldown plug flow reactor (PFR, temperature-programmed).
0888-5885/95/2634-4185$09.00/0 0 1995 American Chemical Society
4186 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
This is intended as an initial study t o illustrate the viability of computer modeling as a diagnostic for understanding, as well as for the improvement or optimization of, a particular combustion process under the following assumptions: (i) Ideal or perfect mixing occurs in the PSR and in the radical direction throughout the PFRs. (ii)The initial stages of incineration occur at high temperature (ca. 1900 K), with residence times in the PSR and adiabatic PFR that are sufficient t o insure complete or near-complete reaction at that temperature. Thus, the product species and compositions we observe result from a combination of equilibrium and kinetic limitations, with the kinetic limitations caused primarily by cool-down of the effluent gases in the downstream section of the model incinerator. We consider mixed chlorocarbodhydrocarbon oxidation in air for a range of fuel equivalence ratios, and we study the effects of selected additives such as H20, H202, CH4, CH20, and 0 2 which are injected into the reacting gases immediately after the PSR. The POHC chlorocarbons include CH3Cl and CH2C12; CHI is the hydrocarbon fuel. POHC and selected PIC concentrations are calculated in both the high-temperature burnout zone and the low-temperature effluent of the incinerator stack. Other variables that can readily be calculated but are not reported here include variable residence time in the PSR and PFRs, the use of POHCs with a higher chlorine to carbon or chlorine to hydrogen ratio, the use of other additives or combinations of additives, and additives supplied with the feed.
(c) Constraints of microscopic reversibility are applied to insure that forward and reverse rate constants are consistent with thermochemical principles. (d) Isomerization rate constants agree with transition state theory. (e) Quantum RRK theory (Dean (1985), Dean et al. (1991)) and modified strong collision theory (Gilbert et al. (1983)) are applied to the temperature and pressure analysis of chemically activated reactions (combination, addition, and insertion) and are used to rate falloff in unimolecular dissociation reactions. (0 Abstraction Arrhenius A factors as given in the literature (Kerr and Moss (19811, NIST (1993)) or generically derived. Abstraction activation energies as given by evaluation in the literature (Kerr and Moss (1981)) or as given by thermodynamic principles and Evans-Polanyi relations. (g) The model conforms with data in the literature, when available. (h) Thermochemical kinetic constraints are applied to all rate constants, thereby restricting the variation of rate constants by optimization. The presence of chlorocarbons has long been known to inhibit the oxidation rate of hydrocarbons from studies of flame velocity, temperature, and flame stability; see, e.g., Westbrook (1982). Here, we use a detailed reaction mechanism to determine species concentrations at various locations in the combustor and to evaluate the effects of additives.
Kinetic Mechanism
We have compared a number of different experimental results from our lab and from the literature with calculations that use the mechanism; see Ho et al. (1992~).The comparisons are satisfactory in all cases except for one, a microreactor molecular beam sampling mass spectrometric study by Hung and Pfefferle (19931, for which our mechanism gave only a fair fit to the data. A number of other researchers have also used the mechanism to compare its predictions with their data, and their experiments are listed in Table 1.
The mechanism that we use is published; it is used in the combustion community and is well-validated by citations in the literature. A number of improvements have been made to the mechanism as a result of our continued research on chlorocarbon kinetics. The kinetic mechanism for the high-temperature oxidation of CH&l and CHzC12 consists of 281 elementary reactions and 61 species and is described in detail by Ho et al. (1992a-c). The mechanism will continue to be improved as we learn more about fundamental processes of the reaction kinetics. There are other mechanisms that could also be applied here, for example, those of Karra and Senkan (1988), Karra et al. (1988), and Fisher et al. (1990). Our aim is not to support a particular mechanism but to illustrate that the use of a detailed reaction mechanism may lead to further understanding and facilitate the optimization of a given incineration process relative to the use of trial burns. Since the mechanism is based on fundamental principles of thermochemical kinetics, its applicability should extend beyond the bounds of any experimental calibrations under which it was developed because of the validity of the thermochemicaland kinetic theories upon which it is based. Thus, the model is not an optimized fit to experimental data recorded over a limited parameter range. Some of the constraints that have been incorporated into the mechanism include the following: (a) Evaluated rate constants as given in the literature, where possible (e.g., see NIST (1993), Cobos et al. (1992), Allara and Shaw (1980)). (b) Accurate representation of thermodynamic variables for all reacting species.
Model Validation
Numerical Simulation and Code Ritter and Barat (1993) have developed the INCIN program (for Investigating Chemistry Important to Incineration),which is a flexible driver program for the widely used and referenced CHEMKIN suite of numerical codes for the study of chemical kinetics in flames and different types of reactor (Kee et al. (1989)). The program runs in about 10 min on a 486,33 MHz, IBMcompatible PC provided a reasonable initial estimate of the PSR temperature is provided. Here the INCIN program is used to simulate a series of three turbulent flow combustion chambers, as shown in Figure 1. The fuel and oxidizer enter a stabilized flame zone, which is modeled by a perfectly stirred reactor operating at either adiabatic or near-adiabatic conditions. On exit from the PSR, the gases enter a hightemperature burnout zone, which is modeled by an adiabatic plug flow reactor. A temperature-programmed plug flow reactor is then used t o model cool-down of the incinerator gases in the stack before emission t o the atmosphere. Perfect mixing is assumed throughout. We look to the future work of other researchers to improvements in incinerator simulations which include nonideal mixing effects in codes that run in reasonable time.
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4187 Table 1. List of Experiments for Which Data Is Compared to Mechanism Predictions experiment researcher species type (A) Comparison of Mechanism Predictions by NJIT to NJIT Data and Other Data Ho et al. (1992a) CH3CI/HdOz tubular reactor Ho et al. (199213) CHzC12/Hd02 tubular reactor Ho and Bozzelli (1992~) CH3CVCHJ02 tubular reactor Chiang and Bozzelli (1993) CH2CldCHdOz tubular reactor Karra and Senkan (1988) CH3CVO2 tubular reactor Tsao (1987) CH2C12/Hz tubular reactor Tavakoli and Bozzelli (1994) CHZC12/CHJAr tubular reactor Miller et al. (1984) CHaCYCHdair flat flame Karra et al. (1988) CH3CVCHJOdAr flat flame Qun and Senkan (1990) CHzC12/CHJOdAr flat flame (B) Comparison of Mechanism Predictions by Others flat flame Wang et al. (1994) CH3CVCHdair Wang and Barat (1995) CH3CVCHJair flat flame Miller et al. (1994) CHzCldCHJair flat flame Lee and P u n (1993) CH&VCHJair opposed flame Yang et al. (1993) CH3CYCHJair opposed flame Hammins (1993) CHzCldCHJair flame Roesler et al. (1992) CH3CVC0/02 tubular reactor Barat et al. (1990) CHaCVHdair PSR Brouwer et al. (1992) CH3CVCzHJair PSR
~~~
Additive Injection
1
6.5msec Feed Flame Zone
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Adiabatic Flame Temperature
Adiabatic Temperature
~
CH4iAir + POHC
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ISmsec
Cool Down Zone
Variable Temperature Profile
MODEL Temperature-Programmed PFR
1
Additive Injection
Figure 1. Scheme of incinerator simulation. An adiabatic perfectly stirred reactor (PSR) is followed by a high-temperature adiabatic plug flow reactor (PFR) modeling the burnout zone, which is followed by a temperature-programmed PFR cool-down zone. 2500 Adhbatie PPR
Combustion modifiers or additives are inlet immediately downstream of the PSR. Data on four sets of operating conditions are reported. In all cases the gas pressure is 1 atm.
Results and Discussion The fuel equivalence ratio 4 is calculated as the ratio of moles of chlorocarbon and hydrocarbon fuel to moles of oxidizer, relative to stoichiometric conditions at the same ratio of atomic chlorine t o hydrogen. The chlorocarbonhydrocarbodair mixture is fed a t a temperature of 400 K into the adiabatic PSR, which has a volume of 250 cm3 and a residence time of 6.5 ms. The temperature in the adiabatic PSR is about 1950 K for fuel-lean operation a t 4 = 0.8 and about 1970 K for fuel-rich operation at 4 = 1.5. The gases remain in the adiabatic PFR burnout zone for 15 ms, and the temperature in the cool-down PFR zone decreases to about 320 K during a residence time of 200 ms. A typical temperature profile is plotted in Figure 2. In the cool-down PFR zone, the line follows the programmed or set temperature. In order to improve convergence of the temperature-programmed PFR subroutine, this profile is not followed exactly; a t each integration step the subroutine “aims” for the temperature at selected points on the set temperature profile (marked by filled squares in the figure) based on zero net heat release of the reaction. The open circles in the figure mark the actual temperature at each step, including reaction heat release. In the first two data sets the chlorocarbon is CH3C1, and in the last two data sets the chlorocarbon is CH2Cl2. Methane, CH4, is the hydrocarbon.
Incinerator Condition Set 1: CHsCYCWAir Oxidation
Base Conditions, without Additive. In this set of calculations we study the effect of a varied amount of CH3C1 added to a CHdair feed. The CH3Cl concentration in the PSR feed is varied from 0 to 7000 ppm, while the fuel equivalence ratio is fixed at a fuel-lean value of cp = 0.8.
0
0
0.05
0.1
0.2
0.15
0.25
ReactionTime (sec)
Figure 2. Typical incinerator temperature profile. The line follows the programmed temperature in the cool-downPFR zone, and the circles mark the actual or calculated temperature.
coc,2 ,,,,,’ 1.OF07 0.1
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10
100
1.OE-14 1.OE-15
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CHIC1 Concentration (ppm)
Figure 3. Effect of CH3Cl added to the CHdair PSR feed under fuel-lean conditions (4 = 0.8), as calculated on exit from the burnout zone at 1950 K. Mole fractions of Clz and COClz are given on the right-hand axis; mole fractions of C1, etc., are given on the left-hand axis.
Species concentrations versus initial CH3C1 concentration are shown at the outlet of the burnout zone (1950 K) in Figure 3 and at the outlet of the cool-down PFR zone (320K) in Figure 4. The data of Figure 3 show that CO,H2, H, and the radicals OH and HO2 maintain nearly constant levels in the burnout zone as the quantity of CH3C1 in the feed is increased up to a level of 1000 ppm. Significant decreases in the concentrations of these species only occur above this CH3C1 feed level (note log scales). Cl2 and COCl2 (phosgene) concentrations, measured on the right-hand axis, and atomic C1 levels, measured on the
4188 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 8 additives
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CHIC1 Concentration (ppm)
Figure 4. Effect of CH3C1 added to the CHJair PSR feed under fuel-lean conditions (9 = 0.81, as calculated at the low-temperature exit of the cool-down PFR at 320 K.
Figure 6. CO/CO2 ratio calculated at the incinerator exit (320 K) under fuel-rich conditions (9 = 1.5 in PSR) versus mole fraction of various additives. 14.0
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6
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Figure 5. CO/CO2 ratio versus reaction time under fuel-rich conditions (9 = 1.5 in PSR) without additive injection and with 1%PSR mass flux of selected additives.
left-hand axis, are orders of magnitude lower but grow algebraically with the increase of CH3C1 to the feed. At the 7000 ppm feed level, the CH3C1 concentration on exit from the PSR is below 1/2000 of its initial value; i.e., 99.95% is destroyed within the PSR. On exit from the burnout zone the concentration of CH3C1 is less than 1.5 x of its initial value; i.e., about ten 9’s destruction is achieved. Similar levels of destruction are achieved a t the 1000 ppm feed level. Figure 4 shows the OH, C1, Cl2, H2, HO2, and COCl2 mole fractions versus CH3C1 feed level at the lowtemperature end of the cool-down PFR or outlet of the incinerator (320 K). The Cl2, Cl, and COCl2 levels increase with CH3C1 feed, while the H2 and radical OH and HO2 levels decrease. Effect of Additives on CO/C02 Ratio. The CO/CO2 ratio and CO emission level are often used in the combustion community for the determination of combustion efficiency. We evaluate the CO/CO2 ratio under both fuel-rich and fuel-lean conditions with no additive and with additives of steam, H202, CH20, and 0 2 injected at the inlet,of the burnout zone. CO/CO2 Ratio, Fuel-Rich Conditions. Figures 5 and 6 illustrate the effects of injection of various additives under fuel-rich conditions. The fuel-rich system with C#J = 1.5 in the PSR is oxygen starved, so that it benefits from any additive that increases the overall content of oxidizer. The concentration of CH3C1 in th PSR feed is held fured a t 10 000 ppm, i.e., 1.0% mole fraction. In Figure 5, the concentration of additive injected to the burnout PFR is fured at 1%of the PSR mass flux and the CO/CO2 ratio is plotted versus total
Figure 7. CO/CO2 ratio and CO mole fraction calculated at the incinerator exit (320 K) under fuel-lean conditions (9 = 0.8) versus mole fraction of steam additive.
residence time. In Figure 6, the concentration of additive is varied and the CO/C02 ratio at the incinerator exit is plotted against the additive mole fraction. Figures 5 and 6 show that the addition of pure oxygen has the most dramatic effect on the CO/CO2 ratio, while H202 has a significant benefit, in agreement with the data of Cooper and Clausen (1991). The addition of H202 does not, however, equal the improvement of 0 2 because one of the two oxygen atoms in each H202 molecule is needed to form H20, which acts as a sink for the hydrogen content of the peroxide. Steam is also of some benefit to the CO/CO2 ratio but at the expense of Hz formation, whereas formaldehyde(CH20) inhibits combustion. Although the improvements given by 0 2 addition are to be expected under fuel-rich conditions, they provide a check on the model and indicate that it is representative of a real combustion system. The predictions that we describe may not achieve complete quantitative accuracy, but the qualitative features should be valid, and an incinerator operator can verify the model’s predictions by optimizing the operating conditions of a real incinerator. CO/CO2 Ratio, Fuel-Lean Conditions. The effects of steam additive at concentrations from 0.0 to 0.5 mole fraction on the CO/CO2 ratio and CO mole fraction at the incinerator outlet (320 K) are shown in Figure 7 under fuel-lean conditions at a fuel equivalence ratio of C#J = 0.8. Results for two different initial molar ratios of CH3Cl/C& are given, 1:4 and 1:lO. The CO/CO2 ratio is generally about 15%higher for the larger CH3CYCH4 ratio. A significant reduction in the CO/CO2 ratio occurs as steam is added. The CO mole fraction is also decreased,
Ind. Eng. Chem. Res., Vol. 34,No. 12,1995 4189 1.OEOl
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Figure 10. Clz mole fraction on exit fmm each of the three reactor sections under initially stochiometric conditions (8 = 1.0 in PSR) with 1%(PSR mass flux)cornhinations of fuel additive injected a t the burnout zone inlet.
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by up to one half as the additive mole fraction approaches a value of about 0.15,but then rises with the further increase of additive. The CO/COz ratio shows a similar minimum and begins its increase at lower additive concentrations. Both here and in the fuel-rich case above, the continued increase of steam additive inhibits combustion by increasing the system’smass and heat capacity and by diluting the concentrations of the more active species.
Incinerator Condition Set 2 C&CVCH4/Air Oxidation with Varied PSR Temperature For this data set the burnout PFR zone is removed, and the effects of a decrease in PSR temperature from the adiabatic value of 1950 K down to 1650 K are investigated at a fixed fuel equivalence ratio of 0.95 and a chlorine to hydrogen ratio of l:lO,i.e., a CH3C1 feed level of 36 000 ppm. All other conditions, including the PSR residence time (6.5ms) and cool-down PFR residence time (200 ms), are as above. Species concentrations on exit from the PSR and at the outlet of the cool-down PFR zone versus PSR temperature are illustrated in Figures 8 and 9, respectively. The data of Figure 8 for the POHC and radical pool show that the time scale of reaction in the PSR begins to slow down to a value comparable to the PSR residence time as the temperature decreases below about 1750 K However, for temperatures between 1750 and 1950 K the changes in species concentrations are small; e.g., although there is a %fold increase in the exit CH3Cl mole fraction over this temperature range, its so that absolute value is always less than 1.0 x
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Figure 11. CO/COI ratios on exit from rnrh of the three reactor sections under initially stoirhiometricronditions10= 1.0 m PSR with 1’%(PSRmassfluxIcumbinatronaoffurladditive rnjrcted at the burnout zone inlet
between 99.86% and 99.95% of the POHC is destroyed within the PSR flame zone. Changes in the radical pool concentrations are similar. On exit from the cool-down PFR, changes of emuent composition in the 1750-1950 K PSR temperature range are even smaller, as shown in Figure 9. Chlorine and HCI levels are almost constant, and the most noticeable change occurs in the relatively low emissions of CO. CHjCl emissions are zero, to within machine precision and estimated error of the integration codes, at PSR temperatures down to 1650 K.
Incinerator Condition Set 3 CH2C12/C&/Air Oxidation Initially Stoichiometric Conditions with Fuel Additives. Chlorine mole fractions and CO/COz ratios in the adiabatic PSR flame zone, at the exit from the adiabatic PFR burnout zone, and at the low-temperature exit of the cool-down PFR are calculated under stoichiometric conditions for a PSR CHzCIdCH4 molar feed ratio of 1:4. The results are illustrated in Figures 10 and 11 for combinations of fuel additive injected at the inlet of the burnout PFR at a level of 1% of the PSR mass flux. The three different additives selected are CHzCI? and CH, in the same ratio of 1:4as in the PSR feed, pure CHzCIz, and CH2CIzand CzH4in equal moles. All of these additives make the system slightly fuel-rich, but the last two change the system in such a way that most of the Clz at the outlet is converted to HCI.
4180 Ind. Eng. Chem. Res., Vol. 34,No. 12,1995
However, this dramatically increases the CO/C02 ratio and CO emissions, as shown in Figure 11. These data provide the first indication of an important effect in chlorocarbon incineration. When incineration occurs at the relatively high temperature of these calculations, ca. 1950 K, C12 can be a major product under fuel-lean conditions, which needs to be converted before effluent from the incinerator stack. In contrast, it is well-known that CO is a major product under fuelrich conditions, and this is one reason why most incinerators operate fuel-lean. Thus, it appears that operation at stoichiometric conditions or slightly fuellean with the injection of steam to convert the Cl2 after the initial stages of reaction may lead to reduced PIC emissions. We note that, under conditions that are sufficient for the reaction of hydrocarbon and chlorocarbon fuels to be almost complete within the PSR, the injection of hydrocarbon or oxyhydrocarbon fuels as additives which are intended to reduce chlorine emissions while still maintaining fuel-lean operation yields no benefit. With excess oxygen and temperatures leading to complete reaction in the PSR, the additives are converted to C02 and H20, while chlorine emissions remain unaltered. Different findings may result if the initial hightemperature flame zone is subject to constraints of mixing or heat loss, when fuel addition may increase the temperature and radical pool to enhance the overall conversion.
Incinerator Condition Set 4 CHnCldCHJAir Oxidation with Varied Fuel Equivalence Ratio The above results suggest that a study focusing on chlorine and CO emissions at the low-temperature exit of the cool-down PFR versus fuel equivalence ratio might elucidate the emission characteristics for these species in chlorocarbon incineration. For these calculations the high-temperature burnout PFR is removed. The single cool-down PFR has a fixed residence time of 200 ma and a constant, uniform heat loss such that the temperature at its outlet is near 400 K. As above, the PSR has volume 250 cm3 and is fed with reactants at 400 K and 1 atm pressure at a mass flux of 9.14g s-l. However, the PSR is now maintained a t a constant heat loss of 1200 cal s-l, which implies a PSR temperature of about 1800-1900 K and a residence time close to 5 ma. The ratio of atomic chlorine to hydrogen in the PSR feed is fixed a t l:lO,while the fuel equivalence ratio is varied. Figure 12 shows the mole fraction of CO and Cl,, as moles of Cl2 plus moles of atomic C1, a t the PFR outlet versus the fuel equivalence ratio. This illustrates that chlorine can be a major product under fuel-lean conditions, for which the conversion of CO to C02 is of high efficiency. Chlorine mole fractions are reduced by 4 orders of magnitude under fuel-rich conditions, with the chlorine being converted to HCl, but CO emissions are now increased by the same 4 orders of magnitude. Under stoichiometric conditions with no additive the mole fraction of CO emitted is slightly greater than that of Cl,, and there is a fairly rapid change in the levels of CO and Cl, emissions as the fuel equivalence ratio 4 is varied about 1.0. As 4 is increased, CO emissions and Cl, increase to a mole fraction of about 2 x emissions, which consist of ca. 2-596 moles of Cl2, decrease to Under fuel-lean conditions the CO and the C1, mole mole fraction is reduced to about
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Figure 12. Mole fraction of CO and C2,at the low-temperature PFR outlet (400 K)versus fuel equivalence ratio (solid lines). Also shown are the results of isothermal equilibrium calculations at 1100 K, squares denote CO, and triangles denote Cl,.
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Figure IS. Mole fractions of CO and C1, from isothermal equilibrium calculations a t 800 and 1100 K
fraction, which consists of ca. 90% moles of Cl2, is increased to 2.5 x Mole fractions of Cl, and CO under conditions of isothermal chemical equilibrium at 800 and 1100 K are shown in Figure 13,where all species in the mechanism are included. The data of Figures 12 and 13 show that the high concentrations of Cl, for 4 < 1and of CO for4 > 1 predicted by the mechanism calculations are near those of equilibrium. However, the equilibrium predictions of CO for 4 < 1 and of Cl, for 4 > 1 are much lower than those of the mechanism calculations, and this indicates the presence of kinetic limitations in the determination of PIC levels. In an attempt to provide a reduction of combined Cl, and CO emissions, the additives injected at the inlet stage of the PF'R are changed to a combination of steam and air. The additive mass flux is varied up to a maximum of 10% of the initial PSR mass flux with additive temperatures from 320 to 1000 K at a range of s t e d a i r ratios that lead to overall fuel equivalence ratios from 4 = 0.93 to 1.07. The effect on emissions is such that, a s a representative example, the addition of 4.5% steam and 4.5% air at a temperature of 1000 K to the PFR conditions of Figure 12 with 4 = 0.95 leads to a reduction in the CO mole fraction by about 72% (i.e., to 8.83 x lo-'), while the relatively from 3.11 x high chlorine mole fraction is reduced by 36% (from 1.87 x 10-4 to 1.19 x 10-4). Operation of the incinerator a t near-stoichiometric conditions with the addition of high-temperature steam appears to be one means of minimizing both Cl, and CO emissions, but the CO emissions that result can be
Ind. Eng. Chem. Res., Vol. 34,No. 12,1995 4191 higher than those achieved without additives at lower fuel equivalence ratios. The most extreme conditions tested, of 9% steam additive a t a temperature of 2200 K, lead t o an increase in CO emissions by a factor of 63 and a decrease in Cl, emissions by a factor of 10. The effects of H2 as an additive are also considered. The PSR is run under fuel-lean conditions at 4 = 0.9, while hydrogen is added at 320 K in varied amounts such that the overall fuel equivalence ratio lies in the range from 4 = 0.95 to 0.97. Relative to the use of air alone as an additive, at the same temperature and fuel equivalence ratio, there is a reduction in CO emissions of about 50%. However, a similar reduction of the relatively high concentrations of chlorine is not attained, with reduction of the chlorine mole fraction being at most 6%.
Conclusions Calculation of reactor operating Conditions and emission characteristics through the use of a detailed reaction mechanism and the assumption of ideal mixing is shown t o be a reasonable method for the analysis of incinerator operation. It is shown that when the reaction is nearly complete at high temperature, chlorine is a major product of chlorocarbodhydrocarbon incineration under fuel-lean conditions. Chlorine emissions are dramatically reduced by operation under fuel-rich conditions but at the expense of a similar increase in CO emissions. Operation under conditions that are stoichiometric or slightly fuel-lean with the controlled addition of hightemperature steam after the initial stages of reaction is found to yield a lower combination of CO and chlorine emissions. Under fuel-rich conditions, the addition of steam, hydrogen peroxide, and oxygen all improve the CO/C02 emission ratio. Of these combustion additives, oxygen is the most effective, whereas methane and formaldehyde increase CO levels. The results that we show are specific to the conditions of this study. We look to future calculations to indicate how further reductions of combined CO and Cl, emissions can be achieved.
Acknowledgment The authors gratefully acknowledge discussions on the mechanism with John Roesler and Gary Miller and discussions on the INCIN program with Ed Ritter. This work is funded by the New Jersey Institute of Technology NSF IndustryNniversity Hazardous Substance Research Management Center under Grant No. NJ 92240050,from the USEPA Northeast Regional Research Center under Grant No. R819679, and from the NSF under Grant No. DMS-9403798.
Nomenclature 4 = fuel equivalence ratio PIC = product of incomplete combustion POHC = principal organic hazardous constituent PFR = plug flow reactor PSR = perfectly stirred reactor
Appendix Reactions Added to Ho et al. (1992a-c). Rate coefficients in the form k = A P exp(-E,/RT). Units
are moles, cubic centimeters, seconds, Kelvin, and calories per mole. A
reaction
+
C2H3C1+ 0 = CHCHCl OH CzH&l+ 0 = CHzCCl+ OH CHClCHCl+ O = CHClCCl+ OH CHClCHCl 0 = CHCl2 CHO CHClCHCl 0 = CHClCHO C1 CHzCClz 0 = CHCCl2 OH CHzCCl2 CH3 = CHzCCl+ CH3C1 CHzCCl CH4 = C2H3C1+ CH3 C2HC13 + 0 = c&13 OH C2H3C1+ C1= CHCHCl HCl CHzCCl2 + C1= CHCClz HC1 CHClCHCl C1= CHClCCl HC1 C2HCl3 + C1= czc13 HCl CHClCO OH = CClCO H2O CHClCO 0 = CHCl COz CHClCO H = CHzCl+ CO CHClCO + C1= CClCO HC1 CHClCO + 0 = CHO CO C1
+ + +
+ + +
+ +
+
+ +
+ + + + + + +
+
+ + +
6.02 x 3.00 x 4.58 x 8.50 x 8.50 x 6.02 x 6.00 x 2.90 x 3.10 x
n
E,
loi
1.57 7190 1.57 6190 0.00 14610 0.00 1950 0.00 1950 1.57 7190 0.00 15390 0.00 11000 0.00 7000 8500 0.00 8500 0.00 0.00 8500 7500 0.00 0.00 2000 1350 0.00 5.00 x 1OI2 0.00 5900 3.00 x 1013 0.00 2800 2.00 x 10'2 0.00 5000
loi 1014 lo1' 1011 loi 10l1 10l1 10l2 5.00 x 1013 2.00 1013 5.00 x 1013 1.70 1013 7.50 x 1OI2 1.74 x 1OI2
Corrected Typographical Errors in Ho et al. (1992a-c). Rate coefficients in the form k = AT" exp(-E,/RT). Units are moles, cubic centimeters, seconds, Kelvin, and calories per mole. A
reaction
+ + +
CH3 0 2 = CH300 CH3 + 02 = CH30 0 CH3 02 = CHzOOH CH3 02 = CH2O + OH CHzCl+ 0 2 = CH2O C10 CHzCl+ CH3 = CzH4 HC1 CzH&l= CzH4 HC1 CzHsCl+ H = HC1+ C2H5 C2H3 0 2 = HCO CHzO CHClz H = CHzCl C1
+
+ +
+
+ +
+
+
5.78 2.86 8.04 x 4.35 1.91 x 3.50 x 7.81 x LOO 3.97 x 1.25
1036
1015 1014 1013 1014 1019
1014 1012
1014
n
E,
-7.82 -0.32 -2.35 -0.45 -1.27 -4.49 -2.00 0.00 0.00 -0.03
7500 30860 17720 17260 3810 9180 60660 7900 -250 570
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IE950232K
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Abstract published in Advance A C S Abstracts, November
15, 1995.
