Simulation of a Three=Stage Chlorocarbon Incinerator through the Use of a Detailed Reaction Mechanism: Chlorine to Hydrogen Mole Ratios below 0.15 MICHAEL R. BOOTY,+ JOSEPH W. BOZZELLI,*f' WENPIN H O , * A N D RICHARD S. MAGEE' Department of Chemical Engineering, Chernistv, and Environmental Science and Department of Mathematics and Statistics, N e w Jersey Institute of Technology, University Heights, Newark, New Jersey 07102-1982
Introduction Incineration is an effective way of handling the disposal of many types of combustible waste. Among its benefits, the process dramatically reduces volume and often converts waste material into recoverable energy through the generation of heat. It also has the capability for near-complete conversion of hazardous materials into minerals, CO,, H20, and HC1. Although incineration of either hazardous or nonhazardous wastes presents a means of disposal, applications of the technology have been hindered by environmental concerns regarding the constituents of the stack effluent from such systems. Two important issues are (i) the capability of incinerators to reach the required level of destruction of the waste feed, e.g., principal organic hazardous constituent (POHC);and (ii) the possible production of hazardous species during incineration, e.g., products of incomplete combustion (PICs). Incineration of chlorocarbons and chlorine compounds deserves attention because chlorocarbons are a major component of waste solvents and because of the unique behavior of chlorine among the halogenated species. Organic chlorine compounds provide a source of chlorine atoms in the initial stages of the combustion process which then rapidly abstract hydrogen atoms from other organic hydrocarbons. This increases hydrocarbon radical levels, enhances chain propagation; and accelerates production of heavier hydrocarbons through HC radical combination and addition reactions. * Corresponding author e-mail address:
[email protected]; telephone: (201)596-3459,fax: (201)802-1946. ' Department of Mathematics and Statistics. + Department of Chemical Engineering, Chemistry, and Environmental Science.
0013-936X/95/0929-3059$09.00/0
@ 1995 American Chemlcal Society
Concerns regarding the behavior of chlorine in combustion processes are not, however, always easily resolved. For example,HClis often a desirable product of combustion, since it serves to remove chlorine and can easily be neutralized or removed by scrubbingfrom the stack effluent. On the other hand, it can inhibit the combustion process through reactions like OH HC1 H20 C1, which consumes the hydroxyl radical that is important for CO burnout via the reaction CO OH CO, H. The present study presents calculations using a detailed reaction mechanism in amode that simulates an incinerator with three reactors operatingin series: (i)a perfectly stirred flame reactor (PSR,adiabatic or near-adiabatic,well-mixed); (ii) a nearly constant temperature burn-out region (PFR, adiabatic, radially mixed); and (iii) a cool-down plug flow reactor (PFR, radially mixed). The choice of three stages is made to produce a model that captures the main features of a real incinerator but also allows identification of species profiles. The chemical systems include CH3ClICH4 and CH2C12/CH4oxidation in air at various fuel equivalence ratios, with the chlorocarbon POHC, hydrocarbon, and air being fed simultaneously to the flame zone inlet at chlorine to hydrogen mole ratios below 0.15. Predictions of POHC and PIC levels are presented as functions of temperature, fuel equivalence ratio, and the injection of selected additives that are mixed with the reacting gases immediately downstream of the PSR. Main findings are as follows. Given that both CO and Cl,, defined as Clz C1, can be formed during the combustion process and are undesirable products, on exit from the incinerator stack (i) CO is more abundant that C1, under fuel-rich and stoichiometric conditions, as expected, but the addition of steam can reduce CO emissions and leaves C1, emissions, which are already low, almost unchanged. (ii) Cl, is more abundant than CO under fuel-lean conditions, and at high temperature with q5 = 0.95, the addition of steam can reduce combined Cl, and CO emissions. The relative abundance of chlorine under fuel-lean conditions occurs because, while the reaction sets
+ +
-
-
+ +
+
CH,
+ CH3C1+ 3.750, - 0.5c1, + 2C0, + 3.5H20 (1) CH, + CH,Cl+ 3.50, - HCl + 2C0, + 3H,O (2)
are nearly equivalent in energy release, see Table 1, there is a more subtle competition between oxygen and chlorine for hydrogen under fuel-lean conditions. Since the H-0 and H-OH bond strengths are greater than the H-C1 bond strength, hydrogen tends to form H20 in preference to HCl under fuel-lean conditions. This leaves moderately high levels of uncombined C1, in the hot combustion gases. Under fuel-rich conditions, the accessibility of a strong H-C1 bond combinedwiththe competition between carbon and hydrogen for relatively scarce oxygen allows HCl to be formed in far greater excess than Cl,. HC1 concentrations are not reported in detail because of the relative ease with which HC1 can be scrubbed from
VOL. 29, NO. 12. 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
3059
Heactor
TABLE 1
Additive Injection
Thermodynamic Data for Reaction Sets 1 and 2
1
6.5 mwc.
reaction temperature A enthalpy A internal energy A entropy set (K) (kcal mol-') (kcal mol-') (cal mol-' K-l)
300 1500 300 1500
1 1 2 2
-352.9 -353.6 -346.1 -346.5
-353.0 -354.4 -346.4 -348.0
2.348 3.494 10.00 11.66
the stack effluent. In general, under fuel-lean conditions, HC1 emissions are 2 orders of magnitude greater than C1, emissions and about 4 orders of magnitude greater than CO emissions. Under fuel-rich conditions, HCl emissions are 5-8 orders of magnitude greater than C1, emissions and ca. 3 times greater than CO emissions. These results are summarized in Figures 6 and 7. Our calculations show the effect of adding steam at high temperature as a chemical reactant, where the hydrogen content may convert chlorine to HCl and the oxygen may convert CO to C02. High temperatures are, however, required because these reactions are endothermic, and reduction in chlorine emissions is generally less than reduction in CO emissions. Many incinerators are run with chlorine and HC1 scrubbers implemented in the downstream sections. Our calculations do not include the effects of scrubbing chlorine or HC1, but the results indicate the importance of implementing chlorine scrubbers under fuel-lean conditions. The predictions also indicate that combined CO and C1, pollutants in the stack effluent can be reduced by operation under near-stoichiometric conditions.
Kinetic Mechanism The mechanism that we use for the high-temperature oxidation of CH3Cl and CHZCl2 consists of 281 elementary reactions and 61 species, including C2 species, and is based on the mechanism described in detail by Ho et al. (1-3). Improvements to the mechanism described in refs 1-3 have been implemented in the version used here. The mechanism has been used extensively in the combustion community and is validated by quantitative comparisons to experimental data in the literature, which includes validation against flow reactor, flat flame, and opposed flame experiments over awide range of conditions (see, for example, refs 1- 10). Other mechanisms could be applied as well, and we mention those of Fisher et al. (11) and Karra et al. (12).Our aim is not to support a particular mechanism but to illustrate that the use of a mechanism can lead to further understanding and can facilitate optimization of an incineration process relative to the use of trial burns. One constraint that a mechanism used for industrial prediction should satisfy is that its rate constants obey thermodynamic constraints of the reactions. Mechanisms that are optimized without regard to thermodynamic constraints should not be used for industrial scale modeling where a reactor wall or furnace is not available to accept or provide significant amounts of energy.
Numerical Simulation and Code We use the INCIN program, which was developed by Ritter and Barat (13). This is a driver program for the widely used and referenced CHEMKIN suite of numerical integration 3060
#
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 12, 1995
Feed
/
Well-Stirred Flsmr Zonr
Burn-Out Zone
Feed
I 5 miec.
PSR Adinbatid
Adiabatic PFR
NDsr
100 msec
Cool-Down Zone
+
Temperature Programmed PFR ___.
codes for chemical kinetics and flame calculations (14).It runs in about 10 min on a 33 MHz 486(DX)IBM-compatible PC if a good initial estimate of the PSR temperature is given. The INCIN program is used to simulate three combustion chambers in series, as shown in Figure 1. Fuel and oxidizer enter a stabilized flame zone, which is modeled by a perfectly stirred reactor (PSR)operating under adiabatic or near-adiabatic conditions. On exit from the PSR, the gases enter a high-temperature burn-out zone, which is modeled by an adiabatic plug flow reactor (PFR). A plug flow reactor with a specified temperature distribution or heat loss is used to model cool-down of the gases in the incinerator stack before emission to the atmosphere.
Results and Discussion A mixture of chlorocarbonIhydrocarbon fuel and air is fed
at a temperature of 400 K into an adiabatic PSR of volume 250 cm3maintained at 1 atm. The temperature in the PSR is about 1950 K and povides near-complete conversion (299%) of the PSR feed under fuel-lean (4 < 1) and stoichiometric (4 = 1) conditions at residence times of 6.5 ms. On exit, the gases remain in the adiabatic PFR burnout region for 15 ms, and the temperature in the cooldown PFR zone decreases over a residence time of 0.2-2.0 s to a final temperature near 320 K. The fuel equivalence ratio 4 is defined as moles of chlorocarbon and hydrocarbon fuel to moles of oxygen, relative to values under stoichiometric conditions, for the reactions CH4 202 COP 2H20 and either 2CH&l+ 3 0 2 2C02 2H20 2HC1for chloromethane incineration or CH2C12 0 2 COz 2HC1 for dichloromethane incineration, at the same ratio of chlorine to hydrogen.
-
+ +
+
-
+
-
+ +
Incineration of CHJCI and CHa in Air POHC Destruction. Species concentrations resulting from the addition of CH3Cl at levels from 0 to 7000 M ppm to CHdair feed are shown in Figure 2 for the exit of the burnout zone and in Figure 3 for the low-temperature stack effluent. The system is fuel-lean, 4 = 0.8,while the chlorine to hydrogen ratio is varied. This example shows the extent to which high-temperature well-mixed incineration can destroy the waste feed. It is seen from Figure 2 that CO, H2, and the radicals H, OH, and HOz maintain nearly constant levels in the burnout zone as the quantity of CH3C1is increased to 1000 ppm. Significant decreases in these species only occur above CH3C1 feed levels of 1000 ppm. Molecular chlorine and phosgene concentrations are measured against the right-
1.OE-02
25
I
1 .OE-07
/ /
1.OE-03
1.OE-08 1.OE-09
C
.P 1,OE-04
P
1.OE-10
2
2
J
8 Q
N
U .
P
m -
2 1.OE-05
:.
1.OE-13
1.OE-06
1.5
2
l.OE-ll 1.OE-12 7
N
8 .
85'
8 cocl 2 ,,/'
1.OE-07 1
10
100 CH3CI (ppm)
1
1.OE-l4 1.OE-15
1000 0.5
FIGURE 2. Effect of CHJCIadded to CHJair PSR feed under fuel-lean conditions, 4 = 0.8, calculated at the burn-out zone outlet at about 1950 K. Mole fractions of CIz and COClz are measured on the righthand axis. Note log scales.
1 OE-03
0
0.01
0.05 0.15 0.25 Additive Mole Fraction
0.35
0.5
FIGURE 4. CO/COz ratio calculated at the stack exit (320 K) under fuel-rich conditions (PSR 0 = 1.5) versus mole fractions of additives.
1
HCI
1 .OE-04 1.OE-05
14.0
1.OE-CB
6 1.OE-07
2
1.OE-08
-
1.OE-09
lL 0
*COiM2 *COICO2
2 1.OE-10
a
1.OE-12 1 OE-14
1 OE-15
13.0
(CH3CliCH4= 1/41
2.5
1.OE-11 1 OE-13
[CH3CIICH4- lil0)
C C O (CHXI/CH4=1/4)
1
COCl2
".J
4 1
10
100 CH3CI (pprn)
ow
1
1
FIGURE 3. Effect of CHJCIadded to CHJair PSR feed under fuel-lean conditions, $ = 0.8, calculated at the low-temperature exit of the cool-down PFR or stack at 320 K.
hand axis and are orders of magnitude lower, but these and HCl and C1 atom concentrations (left-hand axis) increase with the increase of CH3Cl to the feed. At the 7000 ppm feed level, the CH3Clconcentration on exit from the PSR is below 5 x of its initial value, i.e., 99.95% is destroyed within the PSR. On exit from the bumout PFR, the concentration of CH3Cl is less than 1.5 x lo-" of its initial value, i.e., about 10 nine's destruction. The mole fractions of Hz, H02, OH, HCl, C1, Clz, and COC12 at the exit of the cool-down PFR or stack versus CH3C1feed level are shown in Figure 3. Note that the Hz,OH, and HOz levels decrease with the level of CH3C1 feed while the HCl, C12, C1, and phosgene (C0Cl2)levels rise. The C1 atoms can convert to Cln, react with CO, or be scrubbed. Additives or combustion modifiers alter the effluent composition and temperature via their initial heat capacity and subsequent influence on the reaction process. We consider these effects for additives that are injected immediately downstream of the PSR. Effect ofAdditives under Fuel-Rich Conditions. Figure 4 shows the effect of injection of various additives at 400 K on the CO/C02 ratio at the exit of the incinerator stack under fuel-rich conditions, with a PSR fuel equivalence ratio of 1.5 and a fmed CH3C1 feed level of 10 000 ppm (1.0 mol %). Since the fuel-rich system is oxygen starved, CO levels are reduced by any additive that increases the concentration of oxidizer.
12.0
In
ui0
I
Q
0 .-c
8
8
2.0
11.0
8
2
8
10.0
T
&
5'
5 rn
5
1.5 9.0
1.o
8.0 0
0.1 0.2 0.3 0.4 Steam Mole Fraction
0.5
FIGURE 5. CO/COz ratio and CO mole fraction calculated at the stack exit (320 K) under fuel-lean conditions (PSR 4 = 0.8) versus mole fraction of steam additive.
Figure 4 shows that the addition of H202 reduces the CO/COz ratio, in agreement with the data of Cooper and Clausen ( 1 9 ,but pure oxygen has a more dramatic effect. The addition of H202 does not meet the improvement of 0 2 because one of the two oxygen atoms in each HZO2 molecule is needed to form H20, which acts as a sink for the hydrogen of the peroxide. Steam also reduces the CO/ COZratio but at the cost of HZ formation. The CO/C02 ratio has a minimum under these fuel-rich conditions at a steam additive mole fraction of about 0.15. Steam Additive under Fuel-Lean Conditions. The CO/ COZratio and CO mole fraction on exit from the stackversus the addition of steam at 400 K are shown in Figure 5 for fuel-lean conditions at @ = 0.8. Two different feed ratios of CH3Cl/CH4are run, 1:4 and 1 : l O . Compared to the fuel-rich case of Figure 4, the CO/COz ratio is again reduced by steam addition but by lower percentages. Relative to no additive, the ratio is reduced by a maximum of 18%for a CH3C1/CH4ratio of 1:lO or 9% for a ratio of 1:4. The CO/CO2 ratio decreases with the VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
3061
1 OE-01 1 OE 02 3
OE-03
1
9 1 OE-05 c DE-04 i ii
-w ' 1
Clx
f- co
I
OE-06 1
- - - - - - _---
E 0 7
1 OE 08 1 OE09 1
a
' ,
OE 10 08
1 OE-01
7llc-l
ro
, 085
m
09
m
095
1 OE-02
HCI
/f-----
1 OE-03
c 1 OE-04
0
2 1 OE-05
U
I \
IU 1 OE.06 0
. \
,
A
\
'
\
1
cix
1 OE-07
.
1 OE-08
'.- _
105
1 '
1 OE-09
115
Fuel Equivalence Ratio
FIGURE 6. Mole fractions of C I , CO, and HCI at the cool-down PFR or stack exit (320-400 K) from kinetic calculations with CH2C12 as POHC versus overall fuel equivalence ratio. Data are shown for two stack residence times, 0.2 s (solid lines) and 2.0 s (dashed lines). Also shown are the results of equilibrium calculations at 1100 K: triangles denote C I , and squares denote CO.
addition of steam up to mole fractions of about 0.08 but rises again at higher levels of additive. A greater improvement is found in the absolute CO mole fraction, which is reduced by up to one-half by the addition of steam at a mole fraction of 0.15 for a CH3ClICH4 ratio of 1:lO with a similar improvement at a steam mole fraction of 0.2 for a CHiCl/CHl ratio of 1:4. Under both fuel-lean and fuel-rich conditions, CO emissions are minimized at a specific steam mole fraction. Continued increase of steam additive quenches the overall reaction by incresing the system's mass and heat capacity and by diluting the concentrations of the more active reactants. Steam is not effective as an additive for reducing chlorine levels in the stack under significantly fuel-lean conditions; sufficient hydrogen is needed to convert Clz to HC1 and to supply hydrogen for water formation. The 0-Hand H-OH bond strengths are greater than the H-C1 bond strength so that hydrogen does not go to HCl at temperatures sufticient for complete reaction to H20 when fuel-lean conditions are maintained.
Varied Fuel Equivalence Ratio: CH&12/CH4/Air and CH3CI/CH4/Air Incineration at a Constant CI/H Mole Ratio of 0.1 C1, and CO emissions on exit from the cool-down PFR are considered for a range of fuel equivalence ratios near stoichiometry (@= 1). For these calculations, the adiabatic burn-out PFR is removed, so that the PSR delivers its products directly to the cool-down PFR or stack. The PSR is run with a constant heat loss of 1200 cal s-I, which leads to typical PSR residence times of about 5 ms and temperatures near 1900 K. This provides near-complete conversion (299%) of the feed within the PSR. PSR volume, pressure, and feed temperature are as above. The cooldown PFR is now run at a constant, uniform heat loss such that the outlet temperature is in the range from 320 to 400 K after a fixed residence time of either 0.2 or 2.0 s. CHzClz/CH4/Air Oxidation. Mole fractions of Cl,, CO and HC1 at the PFR outlet versus fuel equivalence ratio with CH2C12 as POHC are illustrated in Figure 6. This shows that C1, can be a significant product under fuel-lean conditions (@ < 1) while the conversion of CO to COz is high. By way of contrast, under fuel-rich conditions (@ > 3062 ENVIRONMENTAL SCIENCE &TECHNOLOGY i VOL.
29. NO. 12. 1995
I
E
1 OE'O
08
085
09
095 1 105 Fuel Equivalence Ratio
11
115
FIGURE 7. Kinetic calculations as in Figure 6 with CHJCI as POHC and 0.2 s stack residence time. Bars on the CO emissions curve indicate variation of mole fraction with PSR temperature between 1650 and 1950 K. Changes in CI, and HCI mole fractions with temperature in this range are less than 10%.
l), C1, emissions are lessened by about 4 orders of magnitude, since chlorine is converted to HCl, but a significant increase occurs in emissions of CO. This illustrates two important effects in chlorocarbon incineration. The first is well-known, fuel-rich operation leads to high CO levels, and this is one reason why incinerators are often run fuel-lean. The second requires further investigation: with C1, mole fractions at or above under the fuel-lean conditions reported here, i.e., when conversion of the reactants exiting the PSR is near-complete and occurs at high temperature, operators may need to consider measurement and/or scrubbing of chlorine as a PIC. Our studies also show that, while maintaining fuellean oxidation under these conditions, hydrocarbon fuel additives introduced at the inlet of the cool-down PFR tend to react more favorably with the excess O2to form C 0 2 and H20, leaving C1, levels almost unchanged. Operation under slightlyfuel-lean conditions (4 = 0.95) with the addition of high-temperature steam (1000 K) appears to be a means of minimizing combined Cl, and CO emissions, but resulting CO emissions can, in some cases, be higher than those achieved at lower fuel equivalence ratios. The most extreme conditions tested, 9% steam additive at a temperature of 2200 K with @ = 1.0, led to an increase in CO emissions by a factor of 63 and a decrease in C1, emissions by a factor of 10 relative to no additive. Figure 6 also depicts mole fractions of C1, (triangles) and CO (squares) that result from conditions of isothermal equilibrium at 1100 K. The data show concentrations of CO when 4 1 and Cl, when 4 < 1 predicted by the mechanism are near these equilibrium values. The concentration of CO at equilibrium when @ < 1 is ca. 2 orders of magnitude lower than given by kinetic calculations, while the concentration of C1, at equilibrium when @ > 1 compared to kinetic calculations depends on cool-down time. This is a comparison between the results of kinetic calculations at a final temperature between 320 and 400 K with equilibrium calculations at 1100 K. CH3ClICH4IAlr Oxidation. Mole fractions of Cl,, CO, and HCl at the PFR outlet versus 4 with CH3Clas POHC for a chlorine to hydrogen mole ratio of 0.1 and 0.2 s cooldown PFR residence time are illustrated in Figure 7. The similarity between the mechanism predictions of Figures 6 and 7 indicates that the emission characteritsics for Cl,, CO, and HCl as described above are robust for chlorocarbon incineration under the conditions of this study. For these
calculations, the PSR temperature was reduced from 1950 to 1650 K, in 100 K steps. Even at 1650 K, the PSR temperature and residence time (6.5 ms) provide nearcomplete ( ~ 9 9 % conversion ) of the feed within the PSR. Changes in outlet mole fractions of HCl and C!, are less than 10% and are not perceivable on the figure scale. Vertical bars on the CO emissions curve show the change in CO mole fraction over this temperature range; increased temperature under fuel-lean conditions slightly increases oxidation of CO to Con.
Summary Simulations of high-temperature chlorocarbon combustion in a three-stage model incinerator using a detailed reaction mechanism show that chlorine may need to be considered as a product under fuel-lean conditions, although it is well below HCl levels. Chlorine emissions are significantly reduced under fuel-rich conditions, but at the expense of a similar increase in CO. Operation slightly fuel-lean with the controlled addition of high-temperature steam at conditions representative of the first stage reactor exit is generally found to yield a lower combined total of C1, and CO emissions.
Acknowledgments Funding is from the New Jersey Institute of Technology NSF Industry/University Hazardous Substance Research Management Center under Grant NJ 92-240050, from the U.S. EPA Northeast Regional Research Center under Grant R819679, and from the NSF under Grant DMS-9403798. The thermodynamic data and kinetic mechanism are
available on diskette or via e-mail on request at bozzelli@ tesla.njit.edu.
Literature Cited (1) Ho, W.; Yu, C.; Bozzelli, J. W. Combust. Sei. Technol. 1992, 85, 23-63. (2)Ho, W.; Barat, R. B.; Bozzelli, J. W. Combust. Flame 1992, 88, 265-295. (3) Ho, W.; Bozzelli, J. W. Twenty-FourthSymposium (International) on Combustion;Combustion Institute: Pittsburgh, 1992;pp 743748. (4) Miller, D. L.; Senser, D. W.; Cundy, V. A,; Matula, R. A. Hazard. Waste 1984, 1 , 1-18. (5) Roesler, J. F.; Yetter, R. A,; Dryer, F. L. Combust. Sei. Technol. 1992, 82, 87-100. (6) Yang, M. H.; Lee, K. Y.; Puri, I. K. Combust. Flame 1993, 92, 419-439. (7) Lee, K. Y.; Puri, I. K. Combust. Flame 1993, 92, 440-455. (8) Miller, G. P.; Cundy, V. A.; Lester, T. W.; Bozzelli, J. W. Combust. Sci. Technol. 1994, 98, 123-136. (9) Wang, L.; Jalvy, P.; Barat, R. B. Combust. Sei. Technol. 1994, 97, 13-36. (10) Wang, L.; Barat, R. B. Hazard. Waste Hazard. Muter. 1995, 12, 51-61. (11) Fisher, E. M.; Koshland, C. P.; Hall, M. J,; Sawyer, R. F.; Lucas, D. Twenty-Third Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1990; pp 895-901. (12) Karra, S. B.; Gutman, D.; Senkan, S. M. Combust. Sei. Technol. 1988, 60, 45-62. (13) (a) Ritter, E. R. Villanova University, Villanova, PA, personal communication. (b) Mao, F. Ph.D. Thesis, NJIT, 1995. (14) Kee, R. J.; Rupley, F. M.; Miller, J. A. CHEMKIN-II Computer Code, SAND89-8009B. UC-706, supplied by Sandia National Labs., Livermore, CA. (15) Cooper, C. D.; Clausen, C. A. 1.Hazard. Muter. 1991, 27, 273285.
Received for review August 10, 1995. Revised manuscript received September 10, 1995. Accepted September 20, 1995.
ES950585A
VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY rn 3063
