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The catalytic gasification of carbon in incinerator fly ash
Carbon,
Vol. 31, No,. 6, pp. 977-986. 1993 Copyright
Printed rn Great Hnfam.
THE CATALYTIC GASIFICATION OF CARBON INCINERATOR FLY ASH MICHAEL
The Isermann Department
000X-h223193 $6.00 + .o(l 0 1993 Pergamon Press Ltd
IN
S. MILLIGAN and ELMAR ALTWICKER of Chemical Engineering, Rensselaer Troy. NY 12180-3590, U.S.A.
(Received 20 Jutwary 1993; mrepfed
Polytechnic
Institute,
in revised,form 29 Murch 1993)
fly ashes collected from electrostatic precipitators-three from municipal solid waste incinerators and one from a coal-fired power plant-were studied for their potential to catalyze carbon gasification reactions. A fixed-bed tubular reactor employing mixtures of oxygen and nitrogen was used to measure CO and COZ evolved from native carbon in fly ash in the temperature range 27S-350°C. Experiments using pure carbon were also run for comparison. MSWI fly ash was discovered to accelerate carbon gasification rates by at least an order of magnitude. At 3OO”C,gasification rates of native carbon in the different MSWI fly ash ranged from 1-8 mg-C/g-min, compared to pure carbon gasification rates of 0.03-0.2 mg-C/g-min. Activated carbon, having a high internal surface area, mixed to MSWI fly ash also showed an accelerated gasification rate, suggesting that the catalytic action was long-range. No catalytic activity was observed with coal fly ash. Apparent activation energies for gasification of native carbon in MSWI fly ash ranged from 25-34 kcalimole, while those for pure carbon ranged from IO-20 kcalimole. The apparent activation energy of gasification in coal fly ash was 14 kcalimole. The oxygen concentration dependence on gasification rate in fly ash was determined to be 0.54; that for pure carbon was 0.71. Abstract-Four
Key Words-Catalytic
1.
carbon gasification, fly ash
INTRODUCTION
It is well known that fly ash produced in municipal solid waste incinerators (MSWI) is capable of catalytically promoting reactions that form polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F)[ I-61. Of particular interest is an intriguing reaction whereby native carbon (i.e., unburned, unextractable, elemental carbon in MSWI fly ash) is somehow transformed into chlorinated compounds such as chlorobenzenes and PCDDiF at low temperatures indicative of conditions in the post-combustion regime of an MSWI[7-81. For example, we have physically mixed ‘“C-activated carbon with actual fly ash in a tubular reactor, flowed air at 300°C for 30 minutes, and made appreciable quantities of isotopically fabelled PCDD/F[9]. This phenomenon has been called de Nova synthesis, for the combination of unlike starting materials such as carbon, oxygen, chlorine, and hydrogen to form complex aromatic compounds. Current speculation is that de Novo synthesis of PCDD/F may be a major contributor to MSWI emissions. While investigating possible mechanisms for de Nouo synthesis, we made the discovery that MSWI fly ash is also active in accelerating native carbon gasification, that is, conversion of carbon to CO and CO,[ lo]. In addition, a correlation was noticed between the ability of a particular fly ash to promote both de Nouo synthesis and carbon gasification, suggesting a common catalytic link between these two mechanisms[ Ii]. Better understanding of the catalytic mechanism for carbon gasification in fly ash 977
could lead to a better understanding of PCDD/F formation mechanisms in MSWIs. Catalytic carbon gasification reactions have been extensively studiedf 12-171, and at least four mechanisms have been proposed: electron transfer, oxygen transfer, interfaciat, and spilloverj 18-2 I 1. In electron transfer. metal ion catalysts draw electrons from the carbon matrix, thus weakening carboncarbon bonds at the surface. These activated surface or edge carbon atoms can then participate in gasification events. In oxygen transfer, intermediates are formed between the catalyst particles and the oxidizing atmosphere, leading to gasification of adjacent carbon atoms. For example, oxygen may diffuse through the catalyst particle to adjacent active carbon sites where gasification occurs. The interfacial mechanism involves dissolution of carbon atoms at the carbon-catalyst interface, perhaps followed by carbon diffusion through the catalyst partitie to the oxidizing atmosphere at the surface. Spillover occurs when metal species present are effective in dissociating molecular oxygen into highly reactive oxygen atoms, which then diffuse to active carbon sites and react. While studying oxygen chemisorption on carbon, Ismail and Walker1221 found that the rates of carbon gasification in the temperature range 150-250°C were orders of magnitude greater than those predicted from extrapolation of higher-temperature data. They attributed these observations to the presence of super-active sites, perhaps dangling carbon atoms not in the condensed arm-chair or zig-zag structures, which at higher temperatures are rapidly
M. S.
978
MILLIGAN and E. ALTWICKER
annealed out. In this work we present evidence of catalytic gasification of carbon in incinerator fly ash at temperatures ranging from 275-350°C. The catalytic mechanism responsible for these observations may be acting on “super-active” sites. Baseline experiments using pure carbons, such as carbon black and activated carbon, were also run for comparison.
Mixtures of oxygen and nitrogen are then flowed, and gas-syringe samples are taken from the outlet at different time intervals for subsequent gas chromatographic analysis. Experiments done at flowrates of 80 ml/min and 20 ml/min of 10% 0, resulted in identical gasification rates, suggesting that mass transfer limitations were not apparent under these reaction conditions. To measure CO and CO, generated from fly ash or pure carbon, a technique described by AndersonL231 employing a catalytic methanizer reactor and a flame ionization detector (FID) is used. In our apparatus, a 1.00 ml sample loop is filled with a gas-syringe sample from the outlet of the reactor and injected into a Poropak Q stainless steel column (8’ x l/a”), where CO and CO, are separated. The outlet stream from the column is then mixed with hydrogen and passed over a Nii silica catalyst at 32o”C, where the CO and CO, are stoichiometrically reduced to methane. After the methanizer unit, the gas stream enters an FID. Tests with standard mixtures of CO and CO, confirm that all of the carbon is quantitatively reduced to CH,. Using this technique, we have been able to detect levels of CO and CO, as low as one ppm.
2. EXPERIMENTAL
All experiments are run in the fixed-bed reactor diagrammed in Fig. 1. In the middle of a 1.2-cm diameter and 45cm long tubular reactor is a glass frit, which holds one-half to one-gram samples of fly ash or carbon in place. Prior to experiments, all fly ashes are extracted with toluene and washed with hexane to remove adsorbed organic species. The fly ash is then air-dried and sieved to particle sizes smaller than 210 pm. The reactor is shrouded by a temperature-controlled Lindberg furnace providing a reactor temperature stability of 2 1.O”C. Helium or mixtures of oxygen and nitrogen are flowed using Tylan mass flow controllers at flowrates ranging from 20-100 ml/min. All ancillary tubing is maintained at a temperature of approximately 180°C with heating tape. In a typical experiment, a one-half to one-gram sample of fly ash or carbon is transferred to the fixed-bed reactor, and under a helium flow heated to the desired reaction temperature (250-350°C).
3. RESULTS
AND
3.1 Pure carbon gasification Before measuring carbon gasification rates from fly ash, baseline experiments were run using stan-
EXMAUST
BYPASS
FURNACE ‘\
I
SAMPLE
.
I FIXED-BED
Fig.
,:: :.,
::
,: I
,
GAS
1. Fixed-bed
DISCUSSION
FLY
ASH
REACTOR
reactor
apparatus.
4759
The catalytic gasification of carbon in incinerator fly ash 0.80
I Q
T=XO’C
0
T=325”C
A 0
T=300”C T=275’C
0
T=250aC
0.00 0
5
10
15
20
25
t(min) Fig. 2. Carbon gasification rates vs. reaction time for activated carbon. T = 350-35O”C, IO% C& = 80 mlimin.
dard materials, such as activated carbon (Fisher: Activated Carbon, Darco G-60, 100 mesh) and carbon black (Fisher: Carbon, Lampblack). In each of these experiments, 0.50 g of carbon were charged to the fixed-bed reactor, and pretreated with a helium Row at the reaction temperature for 30 minutes. Figure 2 plots carbon gasification rates, reported as mg of carbon gasified per gram of initial carbon per minute, vs. time for activated carbon reacted with
Reaction temperatures ranged from 10% 0,. 250-350°C. Figure 3 shows the same for carbon black. The activated carbon exhibited steep initial rates, which leveled off after about 20 minutes. Carbon black, although qualitatively showing the same behavior, had lower overall gasification rates. The carbon burnoff after 30 minutes was less than I%. In Fig. 4. Arrhenius plots for both carbons at reaction times of I .5 minutes and 10% O? are given.
0.12 *
.s
0.08
-
0.04
-
T=350’C
0
T=325”C
A
T=300”C
0 0
T=275”C T=250’C
7 5 ? z
0.00 0
I 5
I 10
I
I 20
I 25
Fig. 3. Carbon gasification rates vs. reaction time for carbon black. T = 25%35O”C, 10% @ = X0 mlimin. CAR 31:6-F
M. S. MILLIGAN and E. ALTWICKER
980
o q
A
Q.__, &=I6
Activated Carbon (this study) Carbon Black (thls study) Saran Char (Ismail and Walker
(22))
kcal/mol
E,=
Q.001
) 1.40
/
I
1.60
/
a
I
1.80
I
2.00
I
/
I
2.20
2,
lO~O/T(K) Fig. 4. Carbon gasification rates vs. temperature for activated carbon and carbon black. Reaction time = 1.5minutes, 10% 0; = 80 mlimin. Also shown are previous results for Saran char from Ismail and WaIker[22], with reaction time = 10 minutes, and 21% Oz. Apparent activation energies (E,) are reported as kcab’mole.
The curvature of the data made extrapolation of initial rates difficult. Included are results from Ismail and Walker for Saran char at a reaction time of 10 minutes using pure oxygen[22]. Although Ismail and Walker’s data seem to fall directly in line with ours for activated carbon, care should be taken in comparison; their results are for a longer reaction time and a higher oxygen concentration. However, they are in general agreement. Evident in these results is the curvature in Arrhenius plots described by Ismail and Walker, resulting in a low prediction of rates extrapolated from higher-temperature data. Apparent activation energies for activated carbon shift from 11 to about 16 kcal/mole as temperature increases in these experiments; for carbon black the change is from 10 to 20 kcahmole. In this context, Ismail and Walker’s data at slightly lower temperatures, with I& = 9 kcalimole, agree quite well. Experiments were run to determine the apparent gas-phase oxygen order of reaction for activated carbon gasification. For these trials, carbon gasification rates vs. time from 0.50 g samples of activated carbon were found at four different oxygen concentrations ranging from 5-21%. The initial gasification rate can be written as R, = k,[W,
(1)
where [O,] is the concentration of oxygen. If we assume that the data at a reaction time of 1.5 minutes is agood representation of the initial rate of reaction, then the slope of In R, plotted against ln [O,] represents the global dependence on oxygen concentra-
tion, a. Figure 5 shows these results with a = 0.71. This oxygen dependence fits within the range of reported literature values from similar experiments. Karsner and Perlmutter studied the oxidation of coal particles in a fixed-bed reactor at 150-3OO”C, and found reaction orders for oxygen in the range of 0.70-0.871241. Kam et al., in their oxidation studies of bituminous coal in a fixed bed at 200-277°C found an oxygen concentration dependence of 0.66-0.92[25]. 3.2 Native carbon gasification injly ash Identical experiments were run using one-gram samples of exhaustively extracted fly ash. Four fly ashes collected from electrostatic precipitators (ESP) were used in this study, three from municipal solid waste incinerators (MSWI) and one from a coal-fired power plant. The source and carbon content of each fly ash are shown in Table 1. The analytical procedure to determine the carbon content in
Table 1. List of Ay ashes used in experiments and corresponding percentages of unextractable carbon; fly ashes A-C collected from electrostatic precipitators (ESP) from different municipal solid waste incinerators (MSWI) in North America and Europe; coal fly ash collected from an ESP of a coal-fired power plant (source NIST) Fly ash A B c Coal
Source MSWI MSWI MSWi Power
ESP ESP ESP plant
% Carbon 7.4 1.9 2.0 3.7
2 t r i
0.2 0.2 0.2 0.2
The catalytic
-0.80
981
of carbon in incinerator fly ash
gasification
-r
-1.00 R,= k0[02]0’71 .S
-1.20
: < 0 I
-1.40
1
-1.60
$ s -1.80
-2.00
-2.20
l-
1.20
1.60
2.00
2.40
In 02
2.80
3
io
(%I
Fig. 5. Effect of oxygen concentration on gasification rate of pure activated carbon. Reaction time = 1.5 minutes, T = 3OO”C, total flowrate = 80 mlimin. Data are plotted as In R, vs. In t&: the slope represents the global oxygen concentration dependence.
ator fly ashes were at least an order of magnitude greater than pure carbon, although the steep initial rates were not observed. Interestingly, native carbon in coal fly ash behaved much like pure carbon-no apparent catalytic activity was noticed. Two explanations come to mind: (a) There are com-
fly ash has been described elsewhere[26]. Figure 6 plots carbon gasification rate-normalized to carbon content in the fly ash (see Table I)-vs. time for each fly ash at 300°C. Included for comparison are the results for pure activated carbon and carbon black. Carbon gasification rates in the three inciner-
Native v
Native
A-4
Native
carbon carbon
carbon
!n fiy
ash
in fly
ash
Fig. 6. Carbon
(
0
gasification carbon
I 5
I 10
aah
A
”
0
C B
Carbon
0.001
in fly
”
t&T”)
1
block
I 20
/ 25
rates vs. reaction time for activated carbon. carbon in four fly ashes. T = 300°C. 10% 0: = 80 ml/min.
30
black,
and native
982
M. S. MILLIGAN and E. ALTWICKER 1000
350°c
:.:: j,
325’C
3OO’C
275’C
1.75
1.65
1.85
1000/T(K)
Fig. 7. Native carbon gasification rates vs. temperature for four fly ashes. Apparent activation energies (E,) are reported as kcal/mole. Reaction time = 1.5 minutes, T = 275-35o”C, 10% O2 = 80 ml/min.
in MSWI fly ashes not present in coal fly ash that catalyze carbon oxidation, or (b) a poison such as sulfur in coal inhibits the catalytic potential of coal fly ash. In Fig. 7, Arrhenius plots for the four fly ashes in the temperature range 275-350°C are shown for pounds
reaction times of 1.5 minutes. Again we see a marked difference between the MSWI fly ashes and the coal fly ash. Apparent activation energies for carbon gasification in MSWI lly ash range between 25 and 34 kcal/mole. However, the apparent activation energy for the coal fly ash is much lower (14
0.80
0.60
.G
0.40
: \” 0 I
0.20
1
0.00
$ 5
-0.20
-0.40
-0.60
r
1 .O(1
1.50
2.50 2.60 In 02 (w)
3.60
3. 0
Fig. 8. Effect of oxygen concentration on gasification rate of native carbon in fly ash B. Reaction time = 1.5 minutes, T = 3OO”C,total flowrate = 80 ml/min. Data are plotted as In R, vs. In 0,; the slope represents the global oxygen concentration dependence.
The catalytic
gasification
in incinerator fly ash
of carbon
kcalimole), and almost the same as that found for pure carbon (Fig. 4). The oxygen dependence for native carbon gasification in fly ash B was found as described above for activated carbon. Oxygen concentrations were varied from 5% to 21%, and the gasification rates were measured from 1.00-g samples of fly ash B at 300°C. The plot of In R, vs. In [0,] is shown in Fig. 8. Assuming the relationship shown in eqn (l), the resultant oxygen dependence calculated from the slope is -0.54-notably different from that of pure carbon (Fig. 5).
3.3 Gasi$cation of carbon mixed with fly ash Activated carbon and carbon black were mixed with fly ash B in fractions similar to the native carbon (about 2.0%) for gasification studies. To determine the resultant gasification rates from the added carbons, the amount of CO and CO, measured earlier from native carbon in fly ash was subtracted from the total found here, and the difference assumed to be the contribution from the added carbon. Figure 9 compares these resultant gasification rates at 300°C and 10% 0, with those for pure activated carbon, pure carbon black, and native carbon in fly ash B as a function of time. When carbon black was added to fly ash, its gasification rate, although fast initially, quickly lev-
Native
eled off to the carbon-black-only case. However, activated carbon added to fly ash showed a dramatic increase in gasification rate, and close to that found for native carbon in fly ash. The slowly increasing rate with time observed here may be due to mass transfer limitations resulting from the complex porosity of the added activated carbon. Carbon black is characterized as having a highly amorphous structure with low surface area. On the other hand, activated carbon has regions of localized crystallites and a very high surface area, giving rise to its excellent adsorptive properties[27-281. One technique to activate carbon, out of many, is to subject a sample of carbon black to an oxygen stream at 4SO”C[27]. In this treatment, a low-temperature oxidation essentially burns away the amorphous structure, leaving behind a highly porous matrix consisting of localized crystallite regions. We would expect that the length of time for activation is related to the degree of activation, and thus the porosity and surface area. Three samples of carbon black activated to different degrees were separately added to fly ash B, and the resultant carbon gasification measured at 300°C with 10% OZ. The first sample had no activation; the second a 1.5-hour activation, resulting in a weight loss of 3%; and the third an 18-hour activation, resulting in a weight loss of 50%. In each case, the added carbon comprised about 2.0% by weight of the total mixture.
carbon
u
in fly
Activated
carbon
Carbon
0.001
/ 0
ash
LJ
U
I
I
5
10
983
I 15 t(min)
" in fly
4
ash
black
I
/
20
25
:
Fig. 9. Carbon gasification rates vs. reaction time for pure activated carbon, pure carbon black, carbon in fly ash B. activated carbon mixed with fly ash B, and carbon black mixed with fly For activated carbon and carbon black mixed with fly ash, the gasification rates from native have been subtracted out (see text). The resultant rates are for the added carbons only. T = 10% O? = 80 ml/min.
native ash B. carbon 3Oo”C,
M. S. MILLIGAN and E. ALTWICKER
984
Figure 10 shows the resultant carbon gasification rates of the added carbons as a function of time and degree of activation. An increase in activation translated to an increase in gasification rate, and for the l&hour activation, the rate was similar to that found for native carbon (Fig. 6). These results suggest that native carbon in MSWl fly ash has a morphology and reactivity similar to activated carbon.
The results presented above may offer clues to understanding the mechanism(s) responsible for the apparent catalytic oxidation of carbon in fly ash. Three of the four catalytic carbon gasification mechanisms discussed earlier-electron transfer, oxygen transfer, and interfacial-all require intimate contact between the catalyst and carbon. The resultant action is localized to carbon atoms in the immediate vicinity. It is unlikely that these mechanisms are dominating in our experiments; when highly porous activated carbon-where most of the available surface is intraparticle-is added to MSWI fly ash, its gasification rates increased to nearly those found for native carbon in fly ash. Carbon black, with little internal structure, showed a slight increase in gasification rate when added to fly ash, and subsequent activation-where the intraparticle surface area was increased-resulted in a proportional increase in gasification rate. It would seem that the catalytic gasification mechanism acts on a long-
range scale, previously observed by Yang and Wongll91. Contact between the catalyst particle and carbon atoms is not required, as evidenced by the apparent gasification of intraparticle carbon atoms in activated carbon mixed with fly ash. If the catalytic mechanism were only acting on carbon atoms in contact with the catalyst, carbon black and activated carbon mixed with fly ash should show similar gasification rates, One possible explanation for the above observations is that microscopic or molecular clusters of catalyst are diffusing to the intraparticle regions of the carbon particles. The catalytic reactions requiring contact between catalyst and carbon atoms-the electron transfer, oxygen transfer, and interfacial pathways-can then occur. An important parameter in this catalyst transport process is the Tammann temperature of the catalyst-approximately one half the bulk melting point of the solid in degrees absolute-which estimates the mobility of microscopic catalyst particles[30,31] Above or near the Tammann temperature, molecular species or small clusters of the metal become mobile on the surface, and can diffuse to different regions of the carbon matrix, where catalytic gasification can occur. At the temperatures of these gasification reactions, around 300°C the Tammann temperature of potential metal catalysts in fly ash is probably not reached. Transition metals-known to be good carbon gasification catalysts[14,19,29,32]-are found in abundance in fly ash[33-351, and may be responsible for
Activated
(18
Activated
(1.5
u
0.001
0
I
5
hn)
hm)
Carbon
Carbon
U
I
10
Carbon
block
Carbon
block
I
block
black
in fly ash
in fly ash r
”
In fly ash
I
I
20
25
: 3
Fig. 10. Carbon gasification rates vs. reaction time for carbon black and activated carbon black mixed with fly ash. The gasification rates from native carbon have been subtracted out; the resultant rates are for the added carbons only. Also shown for comparison are previous results for pure carbon black. Carbon black was activated in air at 450°C for 1.5 and 18 hours. T = 3OO”C,10% 0, = 80 mlimin.
The catalytic gasification of carbon in incinerator the observed catalytic gasification. However, their melting points, and those of their oxides and carbides, are usually well above 1000°C. In addition, silica and alumina-major constituents of fly ash-have melting points greater than 1600°C. The Tammann temperatures of these compounds are much higher than the reaction temperatures of these experiments. A long-range mechanism involving transport ofcatalyst fragments may not be important in these fly ash reactions. The remaining catalytic mechanism, spillover, occurs when the catalyst is active in dissociating 02, thus generating oxygen atoms that can migrate through the fly ash matrix. Evidence of this phenomenon has been observed at temperatures as low as lOPC[lS]. Our observations, where the catalytic action seems to be long-range, are consistent with this mechanism. Atomic oxygen generated on the catalyst surface can diffuse into the pores of the added activated carbon and activated carbon black in fly ash, and react with active carbon sites. Wong and Yang[36] showed that gasification reactions involving graphitic carbon and oxygen atoms can occur at temperatures as low as 150°C. At the relatively low temperatures of these reactions (275-350°C). fast reaction with the un-annealed “super-active” sites proposed by Ismail and Walker[22] may be occurring. The significant changes in activation energy and oxygen dependence when pure carbon gasification is compared to native carbon gasification in fly ash (Figs. 4 and 7) suggest that different ratecontrolling reactions describe the fly ash reactions. An oxygen dependence of near one half for native carbon gasification in fly ash (Fig. 8) supports an oxygen dissociation pathway. Table 2 compares COJCO ratios in our experiments for pure carbon, native carbon in fly ash, and activated carbon mixed with fly ash. For activated carbon and carbon black, the ratios found here agree well with literature values in this temperature regime[25]. However, for native carbon in fly ash and activated carbon added to fly ash. the COJCO ratio is much higher; oxygen atoms may be facilitating the oxygenation of surface CO complexes. Yang and Wong[ 191have also shown that certain transition metal carbides, such as WC, TaC, and MO& may act as dissociation centers for molecular oxygen. They speculated that the similar electronic structures between transition metal carbides and
Table 2. COJCO ratios during gasification of pure activated carbon, pure carbon black, native carbon in fly ash B, and activated carbon added 10% 02 = 80 mlimin, reaction
to fly ash B. T = 30o”C, time = 1.5-30 minutes coz/co
Activated carbon Carbon black Native carbon in fly ash Activated carbon in fly ash
1.9 2.1 10.3 9.0
2 t + +
0.1 0.1 0.8 1.0
fly ash
985
platinum result in similar catalytic actions; platinum is known to be a good catalyst for dissociating H-H and O-O bonds. The major metal species in fly ash are Si, Al, and Ca; however, transition metals, such as Ti, SC, V, Cr. Mn, Ni, Co, Cu. Zn, Y, and Cd are found in some MSWI fly ashes in concentrations greater than 100 pg/g[33]. Due to the oxidative incinerator environment, most of these metals probably end up in some oxide form[37,381. In wastes having high chlorine content, metal chlorides may also appear[34]. However, in the high-temperature combustion regime, pyrolytic oxygen-lean pockets might offer conditions favorable to metal carbide formation; the high carbon content of most MSWI fly ashes implies that reducing environments are present. Cocke and Owens[39] showed that in the presence of carbon or hydrocarbons, surface carbides can form from the decomposition of transition metal oxides in oxygen-lean environments. Similar pathways for metal carbide formation have been described by Toth[40]: reaction of metal oxide and carbon in a protective or reducing atmosphere, and reaction of a carbonizing gas such as a hydrocarbon or carbon monoxide with the metal. Transition metal chloride precursors can also form transition metal carbides under favorable conditions. For example, Zhao ct 01.[4 I ] made submicron tungsten-carbide particles from WCI, and CH, precursors using a hydrogen-chlorine flame and temperatures ranging from 1000-1600 K. Agrofiotis et rr/.(42] made silicon nitride-silicon carbide composites from combustion reactions between silicon, silicon nitride, and carbon powders. Titanium is the most abundant transition metal found in most MSWI fly ashes (up to 10% total by weight)[33], and titanium (IV) chloride, with a boiling point of 136”C, could act as an easily vaporized intermediate to titanium carbide. In addition, transition metal carbides may be present initially in solid waste, and end up as residual fly ash. At present, the existence of transition metal carbides in MSWI is speculative, and further work is required to prove their presence. 4. CONCLUSIONS Experiments in a fixed-bed tubular reactor suggest that municipal solid waste incinerator (MSWI) fly ash can catalyze carbon gasification reactions at temperatures around 300°C. Data from actual incinerators and other laboratory experiments show that fly ash also catalyzes dioxin formation reactions in this same temperature regime. These two mechanisms may be linked, and the study of carbon gasification reactions in fly ash could lead to insights into dioxin formation pathways. Coal fly ash did not catalyze carbon gasification reactions; sulfur from coal combustion may have poisoned catalytic sites, or the compounds responsible for catalytic gasification in MSWI fly ash may be missing in coal fly ash. Gasification rates of native carbon in MSWI fly ash at 275-350°C using 10% 0, were more than one
986
M. S. ~ILLIGAN and E. ALTWICKER
order of magnitude greater than those found for pure activated carbon and carbon black under identical conditions. Apparent activation energies for carbon gasification in fly ash ranged from 25-34 kcal/mole; those found for the pure carbons were quite different, and ranged from lo-20 kcalimole. The dependence of gasification rate on oxygen concentration differed for the two cases: Native carbon in fly ash had an oxygen dependence of -0.54, and pure activated carbon -0.71. Different mechanisms of gasification seem to be acting in these two cases. Catalytic mechanisms of gasification requiring contact between the catalyst particle and active carbon sites, such as electron transfer, oxygen transfer, and interfacial, may not dominate in these fly ash reactions. Activated carbon, with a complex internal structure and high surface area, when added to fly ash, experienced a substantial acceleration in gasification rate. In contrast, carbon black-with little internal structure and very low surface area-added to fly ash showed only a modest increase in gasification rate. Activated carbon black added to fly ash had gasification rates that seemed to reflect the degree of activation. The catalytic mechanism responsible for carbon gasification in fly ash appears to act long-range, as evidenced by the ability to gasify active carbon sites in the intrapartitle regions of activated carbon. The Tammann temperature-which provides a measure of the mobility of microscopic catalyst particles-of potential gasification catalysts in fly ash are probably much higher than the temperature range studied here, precluding the transport of catalyst fragments to the intraparticle regions of added activated carbon. Our observations might be explained by the spillover mechanism, where oxygen atoms are generated on molecular oxygen dissociation sites in the fly ash. These oxygen atoms can diffuse through the fly ash and carbon matrices, and react with active carbon sites. The high COJCO ratio measured from gasification in fly ash also suggests that oxygen atoms are oxidizing surface CO complexes.
10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
Acknowledgements-Support of this work by the New York State Energy Research and Development Authority and the United States Environmental Protection Agency is gratefully acknowledged.
34. 35. 36.
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1. L. Stieglitz and H. Vogg, Chemosphere 16, 1917 (1987). 2. H. Hagenmaier, M. Kraft, H. Brunner, and R. Haag, Environmental Science & Tech&. 21, 1080 (1987). 3. L. C. Dickson and F. W. Karasek, Journul ofchromatography 389, 127 (1987). 4. B. J. Ross, K. P. Naikwadi. and F. W. Karasek. Chemosphere 19, 291 (1989). 5. E. R. Ahwicker, J. S. Schonberg, R. K. N. V. Konduri, and M. S. Milligan, Hazardous Waste & Hazardous Materials 7, 73 (1990).
37. 38. 39. 40. 41. 42.
E. R. Altwicker and M. S. Milligan, Chemosphere (in press). H. Vogg and L. Stieglitz, Chemosphere 15, 1373 (1986). K. Jay and L. Stieglitz, Chemosphere 22, 987 (1991). M. S. Milligan and E. R. Altwicker, llfh International Symposium on Chlorinated Dioxins and Related Compounds, September 23-27, Research Triangle Park, NC (1991). M. S. Milligan and E. R. Altwicker, The Fall Technical Meeting ufthe Combustion Institute, The Combustion Institute, 48-l to 48-4 (1991). M. S. Milligan and E. R. Ahwicker, environmental Science and ~echn~~#g~ (in press). T. Inui, T. Otowa, K. Tsutchihashi, and Y. Takegame, Carbon 3, 213 (1982). K. Otto, C. Lehman, L. Bartosiewicz. and M, Shelef, Carbon 3, 243 (1982). R. T. K. Baker, J. J. Chludzinski, Jr., N. C. Dispenziere, and L. L. Murrell, Carbon 6, 579 (1983). Y. Nishiyama, T. Haga, 0. Tamura, and N. Sonehara, Carbon 28, 185 (1990). Z. J. Pan and R. T. Yang, Journal ofCatalysis 130, 161 (1991). R. Meijer, M. Weeda, F. Kapteijn, and J. A. Moulijn, Carbon 29, 929 (1991). R. B. Levy and M. Boudart, Journai of Catalysis 32, 304 (1974). R. T. Yang and C. Wong, Jar~r~ra~ of Cat&is 85, 154 (1984). P. L. Goethel, J. A. Tsamopoulos, and R. T. Yang, AIChE Journal 35,686 (1989). J. M. Ranish and P. L. Walker, Jr., Carbon 28, 887 (1990). I. M. K. Ismail and P. L. Walker, Jr., Carbon 27,549 (1989). S. L. T. Andersson, J. Chrom. Science 23, 17 (1985). G. G. Karsner and D. D. Perlmutter, AIChE Journal 27, 920 (1981). A. Y. Kam, A. N. Hixson, and D. D. Perlmutter, Chemical Engineering Science 31, 821 (1976). M. Metzeer. FreseniusZAnaf. Chem. 327.726(1987). M. Rozwadowski and R. Wojsz, Carbon 26, 111 (1988). H. F. Stoeckli, Carbon 28, 1 (1990). Z. J. Pan and R. T. Yang, Journal of Catalysis 130, 161 (1991). R. T. K. Baker, Journal of Catalysis 78, 473 (1982). D. Gozzi, Materials Chemistry and Physics 21, 377 (1989). P. B. Weisz and R. 8. Goodwin, Journal of Catalysis 6, 227 (1966). D. J. Mitchell, LR815, Warren Spring Laboratory, Hertfordshire, U.K (1989). W. R. Seeker, Twenty-Third Symposium (International) on Combustion, pp. 867-885 (1990). W. S. Hinton and A. M. Lane, Chemosvhere 22.473 (1991). C. Wong and R. T. Yang, Journaf of Chemical Physics 78, 332.5 (1983). R. E. Desrosiers, J. W. Riehl, G. D. UIrich, and A. S. Chiu, Seventeenth Symposium (~nternationai) an Combustion, pp. 1395-1403 (1978). R. L. Davison, D. F. S. Natusch, and J. R. Wallace, Environmental Science & Technology 8, 1107 (1974). D. L. Cocke and M. S. Owens, Journal of Colloid and Interface Science 131, 166 (i989). ” L. E. Toth, In Transition Metal Carbide and Nitrides. Academic Press, New York (1971). G. Y. Zhao, V. V. S. Revankar, and V. Hlavacek, Journal of the Less-Common Metals 163,269 (1990). C. C. Agrafiotis, J. Lis, J. A. Puszynski, and V. Hlavacek, J. Am. Cerum. Sot. 73, 3514 (1990).
Vol. 31, No,. 6, pp. 977-986. 1993 Copyright
Printed rn Great Hnfam.
THE CATALYTIC GASIFICATION OF CARBON INCINERATOR FLY ASH MICHAEL
The Isermann Department
000X-h223193 $6.00 + .o(l 0 1993 Pergamon Press Ltd
IN
S. MILLIGAN and ELMAR ALTWICKER of Chemical Engineering, Rensselaer Troy. NY 12180-3590, U.S.A.
(Received 20 Jutwary 1993; mrepfed
Polytechnic
Institute,
in revised,form 29 Murch 1993)
fly ashes collected from electrostatic precipitators-three from municipal solid waste incinerators and one from a coal-fired power plant-were studied for their potential to catalyze carbon gasification reactions. A fixed-bed tubular reactor employing mixtures of oxygen and nitrogen was used to measure CO and COZ evolved from native carbon in fly ash in the temperature range 27S-350°C. Experiments using pure carbon were also run for comparison. MSWI fly ash was discovered to accelerate carbon gasification rates by at least an order of magnitude. At 3OO”C,gasification rates of native carbon in the different MSWI fly ash ranged from 1-8 mg-C/g-min, compared to pure carbon gasification rates of 0.03-0.2 mg-C/g-min. Activated carbon, having a high internal surface area, mixed to MSWI fly ash also showed an accelerated gasification rate, suggesting that the catalytic action was long-range. No catalytic activity was observed with coal fly ash. Apparent activation energies for gasification of native carbon in MSWI fly ash ranged from 25-34 kcalimole, while those for pure carbon ranged from IO-20 kcalimole. The apparent activation energy of gasification in coal fly ash was 14 kcalimole. The oxygen concentration dependence on gasification rate in fly ash was determined to be 0.54; that for pure carbon was 0.71. Abstract-Four
Key Words-Catalytic
1.
carbon gasification, fly ash
INTRODUCTION
It is well known that fly ash produced in municipal solid waste incinerators (MSWI) is capable of catalytically promoting reactions that form polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F)[ I-61. Of particular interest is an intriguing reaction whereby native carbon (i.e., unburned, unextractable, elemental carbon in MSWI fly ash) is somehow transformed into chlorinated compounds such as chlorobenzenes and PCDDiF at low temperatures indicative of conditions in the post-combustion regime of an MSWI[7-81. For example, we have physically mixed ‘“C-activated carbon with actual fly ash in a tubular reactor, flowed air at 300°C for 30 minutes, and made appreciable quantities of isotopically fabelled PCDD/F[9]. This phenomenon has been called de Nova synthesis, for the combination of unlike starting materials such as carbon, oxygen, chlorine, and hydrogen to form complex aromatic compounds. Current speculation is that de Novo synthesis of PCDD/F may be a major contributor to MSWI emissions. While investigating possible mechanisms for de Nouo synthesis, we made the discovery that MSWI fly ash is also active in accelerating native carbon gasification, that is, conversion of carbon to CO and CO,[ lo]. In addition, a correlation was noticed between the ability of a particular fly ash to promote both de Nouo synthesis and carbon gasification, suggesting a common catalytic link between these two mechanisms[ Ii]. Better understanding of the catalytic mechanism for carbon gasification in fly ash 977
could lead to a better understanding of PCDD/F formation mechanisms in MSWIs. Catalytic carbon gasification reactions have been extensively studiedf 12-171, and at least four mechanisms have been proposed: electron transfer, oxygen transfer, interfaciat, and spilloverj 18-2 I 1. In electron transfer. metal ion catalysts draw electrons from the carbon matrix, thus weakening carboncarbon bonds at the surface. These activated surface or edge carbon atoms can then participate in gasification events. In oxygen transfer, intermediates are formed between the catalyst particles and the oxidizing atmosphere, leading to gasification of adjacent carbon atoms. For example, oxygen may diffuse through the catalyst particle to adjacent active carbon sites where gasification occurs. The interfacial mechanism involves dissolution of carbon atoms at the carbon-catalyst interface, perhaps followed by carbon diffusion through the catalyst partitie to the oxidizing atmosphere at the surface. Spillover occurs when metal species present are effective in dissociating molecular oxygen into highly reactive oxygen atoms, which then diffuse to active carbon sites and react. While studying oxygen chemisorption on carbon, Ismail and Walker1221 found that the rates of carbon gasification in the temperature range 150-250°C were orders of magnitude greater than those predicted from extrapolation of higher-temperature data. They attributed these observations to the presence of super-active sites, perhaps dangling carbon atoms not in the condensed arm-chair or zig-zag structures, which at higher temperatures are rapidly
M. S.
978
MILLIGAN and E. ALTWICKER
annealed out. In this work we present evidence of catalytic gasification of carbon in incinerator fly ash at temperatures ranging from 275-350°C. The catalytic mechanism responsible for these observations may be acting on “super-active” sites. Baseline experiments using pure carbons, such as carbon black and activated carbon, were also run for comparison.
Mixtures of oxygen and nitrogen are then flowed, and gas-syringe samples are taken from the outlet at different time intervals for subsequent gas chromatographic analysis. Experiments done at flowrates of 80 ml/min and 20 ml/min of 10% 0, resulted in identical gasification rates, suggesting that mass transfer limitations were not apparent under these reaction conditions. To measure CO and CO, generated from fly ash or pure carbon, a technique described by AndersonL231 employing a catalytic methanizer reactor and a flame ionization detector (FID) is used. In our apparatus, a 1.00 ml sample loop is filled with a gas-syringe sample from the outlet of the reactor and injected into a Poropak Q stainless steel column (8’ x l/a”), where CO and CO, are separated. The outlet stream from the column is then mixed with hydrogen and passed over a Nii silica catalyst at 32o”C, where the CO and CO, are stoichiometrically reduced to methane. After the methanizer unit, the gas stream enters an FID. Tests with standard mixtures of CO and CO, confirm that all of the carbon is quantitatively reduced to CH,. Using this technique, we have been able to detect levels of CO and CO, as low as one ppm.
2. EXPERIMENTAL
All experiments are run in the fixed-bed reactor diagrammed in Fig. 1. In the middle of a 1.2-cm diameter and 45cm long tubular reactor is a glass frit, which holds one-half to one-gram samples of fly ash or carbon in place. Prior to experiments, all fly ashes are extracted with toluene and washed with hexane to remove adsorbed organic species. The fly ash is then air-dried and sieved to particle sizes smaller than 210 pm. The reactor is shrouded by a temperature-controlled Lindberg furnace providing a reactor temperature stability of 2 1.O”C. Helium or mixtures of oxygen and nitrogen are flowed using Tylan mass flow controllers at flowrates ranging from 20-100 ml/min. All ancillary tubing is maintained at a temperature of approximately 180°C with heating tape. In a typical experiment, a one-half to one-gram sample of fly ash or carbon is transferred to the fixed-bed reactor, and under a helium flow heated to the desired reaction temperature (250-350°C).
3. RESULTS
AND
3.1 Pure carbon gasification Before measuring carbon gasification rates from fly ash, baseline experiments were run using stan-
EXMAUST
BYPASS
FURNACE ‘\
I
SAMPLE
.
I FIXED-BED
Fig.
,:: :.,
::
,: I
,
GAS
1. Fixed-bed
DISCUSSION
FLY
ASH
REACTOR
reactor
apparatus.
4759
The catalytic gasification of carbon in incinerator fly ash 0.80
I Q
T=XO’C
0
T=325”C
A 0
T=300”C T=275’C
0
T=250aC
0.00 0
5
10
15
20
25
t(min) Fig. 2. Carbon gasification rates vs. reaction time for activated carbon. T = 350-35O”C, IO% C& = 80 mlimin.
dard materials, such as activated carbon (Fisher: Activated Carbon, Darco G-60, 100 mesh) and carbon black (Fisher: Carbon, Lampblack). In each of these experiments, 0.50 g of carbon were charged to the fixed-bed reactor, and pretreated with a helium Row at the reaction temperature for 30 minutes. Figure 2 plots carbon gasification rates, reported as mg of carbon gasified per gram of initial carbon per minute, vs. time for activated carbon reacted with
Reaction temperatures ranged from 10% 0,. 250-350°C. Figure 3 shows the same for carbon black. The activated carbon exhibited steep initial rates, which leveled off after about 20 minutes. Carbon black, although qualitatively showing the same behavior, had lower overall gasification rates. The carbon burnoff after 30 minutes was less than I%. In Fig. 4. Arrhenius plots for both carbons at reaction times of I .5 minutes and 10% O? are given.
0.12 *
.s
0.08
-
0.04
-
T=350’C
0
T=325”C
A
T=300”C
0 0
T=275”C T=250’C
7 5 ? z
0.00 0
I 5
I 10
I
I 20
I 25
Fig. 3. Carbon gasification rates vs. reaction time for carbon black. T = 25%35O”C, 10% @ = X0 mlimin. CAR 31:6-F
M. S. MILLIGAN and E. ALTWICKER
980
o q
A
Q.__, &=I6
Activated Carbon (this study) Carbon Black (thls study) Saran Char (Ismail and Walker
(22))
kcal/mol
E,=
Q.001
) 1.40
/
I
1.60
/
a
I
1.80
I
2.00
I
/
I
2.20
2,
lO~O/T(K) Fig. 4. Carbon gasification rates vs. temperature for activated carbon and carbon black. Reaction time = 1.5minutes, 10% 0; = 80 mlimin. Also shown are previous results for Saran char from Ismail and WaIker[22], with reaction time = 10 minutes, and 21% Oz. Apparent activation energies (E,) are reported as kcab’mole.
The curvature of the data made extrapolation of initial rates difficult. Included are results from Ismail and Walker for Saran char at a reaction time of 10 minutes using pure oxygen[22]. Although Ismail and Walker’s data seem to fall directly in line with ours for activated carbon, care should be taken in comparison; their results are for a longer reaction time and a higher oxygen concentration. However, they are in general agreement. Evident in these results is the curvature in Arrhenius plots described by Ismail and Walker, resulting in a low prediction of rates extrapolated from higher-temperature data. Apparent activation energies for activated carbon shift from 11 to about 16 kcal/mole as temperature increases in these experiments; for carbon black the change is from 10 to 20 kcahmole. In this context, Ismail and Walker’s data at slightly lower temperatures, with I& = 9 kcalimole, agree quite well. Experiments were run to determine the apparent gas-phase oxygen order of reaction for activated carbon gasification. For these trials, carbon gasification rates vs. time from 0.50 g samples of activated carbon were found at four different oxygen concentrations ranging from 5-21%. The initial gasification rate can be written as R, = k,[W,
(1)
where [O,] is the concentration of oxygen. If we assume that the data at a reaction time of 1.5 minutes is agood representation of the initial rate of reaction, then the slope of In R, plotted against ln [O,] represents the global dependence on oxygen concentra-
tion, a. Figure 5 shows these results with a = 0.71. This oxygen dependence fits within the range of reported literature values from similar experiments. Karsner and Perlmutter studied the oxidation of coal particles in a fixed-bed reactor at 150-3OO”C, and found reaction orders for oxygen in the range of 0.70-0.871241. Kam et al., in their oxidation studies of bituminous coal in a fixed bed at 200-277°C found an oxygen concentration dependence of 0.66-0.92[25]. 3.2 Native carbon gasification injly ash Identical experiments were run using one-gram samples of exhaustively extracted fly ash. Four fly ashes collected from electrostatic precipitators (ESP) were used in this study, three from municipal solid waste incinerators (MSWI) and one from a coal-fired power plant. The source and carbon content of each fly ash are shown in Table 1. The analytical procedure to determine the carbon content in
Table 1. List of Ay ashes used in experiments and corresponding percentages of unextractable carbon; fly ashes A-C collected from electrostatic precipitators (ESP) from different municipal solid waste incinerators (MSWI) in North America and Europe; coal fly ash collected from an ESP of a coal-fired power plant (source NIST) Fly ash A B c Coal
Source MSWI MSWI MSWi Power
ESP ESP ESP plant
% Carbon 7.4 1.9 2.0 3.7
2 t r i
0.2 0.2 0.2 0.2
The catalytic
-0.80
981
of carbon in incinerator fly ash
gasification
-r
-1.00 R,= k0[02]0’71 .S
-1.20
: < 0 I
-1.40
1
-1.60
$ s -1.80
-2.00
-2.20
l-
1.20
1.60
2.00
2.40
In 02
2.80
3
io
(%I
Fig. 5. Effect of oxygen concentration on gasification rate of pure activated carbon. Reaction time = 1.5 minutes, T = 3OO”C, total flowrate = 80 mlimin. Data are plotted as In R, vs. In t&: the slope represents the global oxygen concentration dependence.
ator fly ashes were at least an order of magnitude greater than pure carbon, although the steep initial rates were not observed. Interestingly, native carbon in coal fly ash behaved much like pure carbon-no apparent catalytic activity was noticed. Two explanations come to mind: (a) There are com-
fly ash has been described elsewhere[26]. Figure 6 plots carbon gasification rate-normalized to carbon content in the fly ash (see Table I)-vs. time for each fly ash at 300°C. Included for comparison are the results for pure activated carbon and carbon black. Carbon gasification rates in the three inciner-
Native v
Native
A-4
Native
carbon carbon
carbon
!n fiy
ash
in fly
ash
Fig. 6. Carbon
(
0
gasification carbon
I 5
I 10
aah
A
”
0
C B
Carbon
0.001
in fly
”
t&T”)
1
block
I 20
/ 25
rates vs. reaction time for activated carbon. carbon in four fly ashes. T = 300°C. 10% 0: = 80 ml/min.
30
black,
and native
982
M. S. MILLIGAN and E. ALTWICKER 1000
350°c
:.:: j,
325’C
3OO’C
275’C
1.75
1.65
1.85
1000/T(K)
Fig. 7. Native carbon gasification rates vs. temperature for four fly ashes. Apparent activation energies (E,) are reported as kcal/mole. Reaction time = 1.5 minutes, T = 275-35o”C, 10% O2 = 80 ml/min.
in MSWI fly ashes not present in coal fly ash that catalyze carbon oxidation, or (b) a poison such as sulfur in coal inhibits the catalytic potential of coal fly ash. In Fig. 7, Arrhenius plots for the four fly ashes in the temperature range 275-350°C are shown for pounds
reaction times of 1.5 minutes. Again we see a marked difference between the MSWI fly ashes and the coal fly ash. Apparent activation energies for carbon gasification in MSWI lly ash range between 25 and 34 kcal/mole. However, the apparent activation energy for the coal fly ash is much lower (14
0.80
0.60
.G
0.40
: \” 0 I
0.20
1
0.00
$ 5
-0.20
-0.40
-0.60
r
1 .O(1
1.50
2.50 2.60 In 02 (w)
3.60
3. 0
Fig. 8. Effect of oxygen concentration on gasification rate of native carbon in fly ash B. Reaction time = 1.5 minutes, T = 3OO”C,total flowrate = 80 ml/min. Data are plotted as In R, vs. In 0,; the slope represents the global oxygen concentration dependence.
The catalytic
gasification
in incinerator fly ash
of carbon
kcalimole), and almost the same as that found for pure carbon (Fig. 4). The oxygen dependence for native carbon gasification in fly ash B was found as described above for activated carbon. Oxygen concentrations were varied from 5% to 21%, and the gasification rates were measured from 1.00-g samples of fly ash B at 300°C. The plot of In R, vs. In [0,] is shown in Fig. 8. Assuming the relationship shown in eqn (l), the resultant oxygen dependence calculated from the slope is -0.54-notably different from that of pure carbon (Fig. 5).
3.3 Gasi$cation of carbon mixed with fly ash Activated carbon and carbon black were mixed with fly ash B in fractions similar to the native carbon (about 2.0%) for gasification studies. To determine the resultant gasification rates from the added carbons, the amount of CO and CO, measured earlier from native carbon in fly ash was subtracted from the total found here, and the difference assumed to be the contribution from the added carbon. Figure 9 compares these resultant gasification rates at 300°C and 10% 0, with those for pure activated carbon, pure carbon black, and native carbon in fly ash B as a function of time. When carbon black was added to fly ash, its gasification rate, although fast initially, quickly lev-
Native
eled off to the carbon-black-only case. However, activated carbon added to fly ash showed a dramatic increase in gasification rate, and close to that found for native carbon in fly ash. The slowly increasing rate with time observed here may be due to mass transfer limitations resulting from the complex porosity of the added activated carbon. Carbon black is characterized as having a highly amorphous structure with low surface area. On the other hand, activated carbon has regions of localized crystallites and a very high surface area, giving rise to its excellent adsorptive properties[27-281. One technique to activate carbon, out of many, is to subject a sample of carbon black to an oxygen stream at 4SO”C[27]. In this treatment, a low-temperature oxidation essentially burns away the amorphous structure, leaving behind a highly porous matrix consisting of localized crystallite regions. We would expect that the length of time for activation is related to the degree of activation, and thus the porosity and surface area. Three samples of carbon black activated to different degrees were separately added to fly ash B, and the resultant carbon gasification measured at 300°C with 10% OZ. The first sample had no activation; the second a 1.5-hour activation, resulting in a weight loss of 3%; and the third an 18-hour activation, resulting in a weight loss of 50%. In each case, the added carbon comprised about 2.0% by weight of the total mixture.
carbon
u
in fly
Activated
carbon
Carbon
0.001
/ 0
ash
LJ
U
I
I
5
10
983
I 15 t(min)
" in fly
4
ash
black
I
/
20
25
:
Fig. 9. Carbon gasification rates vs. reaction time for pure activated carbon, pure carbon black, carbon in fly ash B. activated carbon mixed with fly ash B, and carbon black mixed with fly For activated carbon and carbon black mixed with fly ash, the gasification rates from native have been subtracted out (see text). The resultant rates are for the added carbons only. T = 10% O? = 80 ml/min.
native ash B. carbon 3Oo”C,
M. S. MILLIGAN and E. ALTWICKER
984
Figure 10 shows the resultant carbon gasification rates of the added carbons as a function of time and degree of activation. An increase in activation translated to an increase in gasification rate, and for the l&hour activation, the rate was similar to that found for native carbon (Fig. 6). These results suggest that native carbon in MSWl fly ash has a morphology and reactivity similar to activated carbon.
The results presented above may offer clues to understanding the mechanism(s) responsible for the apparent catalytic oxidation of carbon in fly ash. Three of the four catalytic carbon gasification mechanisms discussed earlier-electron transfer, oxygen transfer, and interfacial-all require intimate contact between the catalyst and carbon. The resultant action is localized to carbon atoms in the immediate vicinity. It is unlikely that these mechanisms are dominating in our experiments; when highly porous activated carbon-where most of the available surface is intraparticle-is added to MSWI fly ash, its gasification rates increased to nearly those found for native carbon in fly ash. Carbon black, with little internal structure, showed a slight increase in gasification rate when added to fly ash, and subsequent activation-where the intraparticle surface area was increased-resulted in a proportional increase in gasification rate. It would seem that the catalytic gasification mechanism acts on a long-
range scale, previously observed by Yang and Wongll91. Contact between the catalyst particle and carbon atoms is not required, as evidenced by the apparent gasification of intraparticle carbon atoms in activated carbon mixed with fly ash. If the catalytic mechanism were only acting on carbon atoms in contact with the catalyst, carbon black and activated carbon mixed with fly ash should show similar gasification rates, One possible explanation for the above observations is that microscopic or molecular clusters of catalyst are diffusing to the intraparticle regions of the carbon particles. The catalytic reactions requiring contact between catalyst and carbon atoms-the electron transfer, oxygen transfer, and interfacial pathways-can then occur. An important parameter in this catalyst transport process is the Tammann temperature of the catalyst-approximately one half the bulk melting point of the solid in degrees absolute-which estimates the mobility of microscopic catalyst particles[30,31] Above or near the Tammann temperature, molecular species or small clusters of the metal become mobile on the surface, and can diffuse to different regions of the carbon matrix, where catalytic gasification can occur. At the temperatures of these gasification reactions, around 300°C the Tammann temperature of potential metal catalysts in fly ash is probably not reached. Transition metals-known to be good carbon gasification catalysts[14,19,29,32]-are found in abundance in fly ash[33-351, and may be responsible for
Activated
(18
Activated
(1.5
u
0.001
0
I
5
hn)
hm)
Carbon
Carbon
U
I
10
Carbon
block
Carbon
block
I
block
black
in fly ash
in fly ash r
”
In fly ash
I
I
20
25
: 3
Fig. 10. Carbon gasification rates vs. reaction time for carbon black and activated carbon black mixed with fly ash. The gasification rates from native carbon have been subtracted out; the resultant rates are for the added carbons only. Also shown for comparison are previous results for pure carbon black. Carbon black was activated in air at 450°C for 1.5 and 18 hours. T = 3OO”C,10% 0, = 80 mlimin.
The catalytic gasification of carbon in incinerator the observed catalytic gasification. However, their melting points, and those of their oxides and carbides, are usually well above 1000°C. In addition, silica and alumina-major constituents of fly ash-have melting points greater than 1600°C. The Tammann temperatures of these compounds are much higher than the reaction temperatures of these experiments. A long-range mechanism involving transport ofcatalyst fragments may not be important in these fly ash reactions. The remaining catalytic mechanism, spillover, occurs when the catalyst is active in dissociating 02, thus generating oxygen atoms that can migrate through the fly ash matrix. Evidence of this phenomenon has been observed at temperatures as low as lOPC[lS]. Our observations, where the catalytic action seems to be long-range, are consistent with this mechanism. Atomic oxygen generated on the catalyst surface can diffuse into the pores of the added activated carbon and activated carbon black in fly ash, and react with active carbon sites. Wong and Yang[36] showed that gasification reactions involving graphitic carbon and oxygen atoms can occur at temperatures as low as 150°C. At the relatively low temperatures of these reactions (275-350°C). fast reaction with the un-annealed “super-active” sites proposed by Ismail and Walker[22] may be occurring. The significant changes in activation energy and oxygen dependence when pure carbon gasification is compared to native carbon gasification in fly ash (Figs. 4 and 7) suggest that different ratecontrolling reactions describe the fly ash reactions. An oxygen dependence of near one half for native carbon gasification in fly ash (Fig. 8) supports an oxygen dissociation pathway. Table 2 compares COJCO ratios in our experiments for pure carbon, native carbon in fly ash, and activated carbon mixed with fly ash. For activated carbon and carbon black, the ratios found here agree well with literature values in this temperature regime[25]. However, for native carbon in fly ash and activated carbon added to fly ash. the COJCO ratio is much higher; oxygen atoms may be facilitating the oxygenation of surface CO complexes. Yang and Wong[ 191have also shown that certain transition metal carbides, such as WC, TaC, and MO& may act as dissociation centers for molecular oxygen. They speculated that the similar electronic structures between transition metal carbides and
Table 2. COJCO ratios during gasification of pure activated carbon, pure carbon black, native carbon in fly ash B, and activated carbon added 10% 02 = 80 mlimin, reaction
to fly ash B. T = 30o”C, time = 1.5-30 minutes coz/co
Activated carbon Carbon black Native carbon in fly ash Activated carbon in fly ash
1.9 2.1 10.3 9.0
2 t + +
0.1 0.1 0.8 1.0
fly ash
985
platinum result in similar catalytic actions; platinum is known to be a good catalyst for dissociating H-H and O-O bonds. The major metal species in fly ash are Si, Al, and Ca; however, transition metals, such as Ti, SC, V, Cr. Mn, Ni, Co, Cu. Zn, Y, and Cd are found in some MSWI fly ashes in concentrations greater than 100 pg/g[33]. Due to the oxidative incinerator environment, most of these metals probably end up in some oxide form[37,381. In wastes having high chlorine content, metal chlorides may also appear[34]. However, in the high-temperature combustion regime, pyrolytic oxygen-lean pockets might offer conditions favorable to metal carbide formation; the high carbon content of most MSWI fly ashes implies that reducing environments are present. Cocke and Owens[39] showed that in the presence of carbon or hydrocarbons, surface carbides can form from the decomposition of transition metal oxides in oxygen-lean environments. Similar pathways for metal carbide formation have been described by Toth[40]: reaction of metal oxide and carbon in a protective or reducing atmosphere, and reaction of a carbonizing gas such as a hydrocarbon or carbon monoxide with the metal. Transition metal chloride precursors can also form transition metal carbides under favorable conditions. For example, Zhao ct 01.[4 I ] made submicron tungsten-carbide particles from WCI, and CH, precursors using a hydrogen-chlorine flame and temperatures ranging from 1000-1600 K. Agrofiotis et rr/.(42] made silicon nitride-silicon carbide composites from combustion reactions between silicon, silicon nitride, and carbon powders. Titanium is the most abundant transition metal found in most MSWI fly ashes (up to 10% total by weight)[33], and titanium (IV) chloride, with a boiling point of 136”C, could act as an easily vaporized intermediate to titanium carbide. In addition, transition metal carbides may be present initially in solid waste, and end up as residual fly ash. At present, the existence of transition metal carbides in MSWI is speculative, and further work is required to prove their presence. 4. CONCLUSIONS Experiments in a fixed-bed tubular reactor suggest that municipal solid waste incinerator (MSWI) fly ash can catalyze carbon gasification reactions at temperatures around 300°C. Data from actual incinerators and other laboratory experiments show that fly ash also catalyzes dioxin formation reactions in this same temperature regime. These two mechanisms may be linked, and the study of carbon gasification reactions in fly ash could lead to insights into dioxin formation pathways. Coal fly ash did not catalyze carbon gasification reactions; sulfur from coal combustion may have poisoned catalytic sites, or the compounds responsible for catalytic gasification in MSWI fly ash may be missing in coal fly ash. Gasification rates of native carbon in MSWI fly ash at 275-350°C using 10% 0, were more than one
986
M. S. ~ILLIGAN and E. ALTWICKER
order of magnitude greater than those found for pure activated carbon and carbon black under identical conditions. Apparent activation energies for carbon gasification in fly ash ranged from 25-34 kcal/mole; those found for the pure carbons were quite different, and ranged from lo-20 kcalimole. The dependence of gasification rate on oxygen concentration differed for the two cases: Native carbon in fly ash had an oxygen dependence of -0.54, and pure activated carbon -0.71. Different mechanisms of gasification seem to be acting in these two cases. Catalytic mechanisms of gasification requiring contact between the catalyst particle and active carbon sites, such as electron transfer, oxygen transfer, and interfacial, may not dominate in these fly ash reactions. Activated carbon, with a complex internal structure and high surface area, when added to fly ash, experienced a substantial acceleration in gasification rate. In contrast, carbon black-with little internal structure and very low surface area-added to fly ash showed only a modest increase in gasification rate. Activated carbon black added to fly ash had gasification rates that seemed to reflect the degree of activation. The catalytic mechanism responsible for carbon gasification in fly ash appears to act long-range, as evidenced by the ability to gasify active carbon sites in the intrapartitle regions of activated carbon. The Tammann temperature-which provides a measure of the mobility of microscopic catalyst particles-of potential gasification catalysts in fly ash are probably much higher than the temperature range studied here, precluding the transport of catalyst fragments to the intraparticle regions of added activated carbon. Our observations might be explained by the spillover mechanism, where oxygen atoms are generated on molecular oxygen dissociation sites in the fly ash. These oxygen atoms can diffuse through the fly ash and carbon matrices, and react with active carbon sites. The high COJCO ratio measured from gasification in fly ash also suggests that oxygen atoms are oxidizing surface CO complexes.
10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
Acknowledgements-Support of this work by the New York State Energy Research and Development Authority and the United States Environmental Protection Agency is gratefully acknowledged.
34. 35. 36.
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