Environ. Sci. Technol. 2000, 34, 4370-4375
MSWI Fly Ash Native Carbon Thermal Degradation: A TG-FTIR Study† P. FERMO,‡ F. CARIATI,‡ S. SANTACESARIA,‡ S. BRUNI,§ M. LASAGNI,# M. TETTAMANTI,# E . C O L L I N A , # A N D D . P I T E A * ,# Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita` di Milano, via G. Venezian 21, 20133 Milano, Italy, Istituto di Scienze Matematiche Fisiche e Chimiche, Universita` dell’Insubria, via Lucini 3, 22100 Como, Italy, and Dipartimento di Scienze dell’Ambiente e del Territorio, Universita` di Milano-Bicocca, piazza della Scienza 1, 20126 Milano, Italy
The CO2 evolution curve from MSWI fly ash (FA) and model systems containing different carbon species was studied by means of the TG-FTIR technique. The number of peaks in the curves depends on the system studied: one peak is observed from activated carbon, Cact, in model mixtures with silica, SiO2, or whereas two from Cact-CuCl2SiO2. This, together with the higher Cact reactivity in the twopeak system indicates the catalytic effect of Cu ion. Moreover, this effect is dependent on the copper compound: adding copper as CuO or CuSO4 does not change the CO2 evolution curve in comparison with the uncatalyzed CactSiO2 system. Two peaks were also observed for the CactTTFA (Thermally Treated Fly Ash) and Cnat-TTFA systems (Cnat is the “native” carbon, i.e., the unburnt unextractable organic carbon from FA). The behavior of amorphous carbon systems, Camorphous-SiO2 and Camorphous-TTFA, was different as two peaks were observed in both cases; this was explained by comparing the Raman spectra of amorphous carbon with respect to those of the native and activated carbon. Finally, two peaks were observed in the CO2 evolution curves from raw FA and fractions obtained from it. Thus, it is possible to conclude that low temperature native carbon gasification is highly dependent on the catalyst added to the model mixtures as well as on the metal already present in the TTFA or raw FA. This conclusion supports previous findings from kinetic studies. The results obtained are also important to explain the formation reactions of organochlorinated micropollutants. The influence of initial organic carbon content is also discussed.
Introduction Literature data (1) clearly indicate that fly ashes collected in the energy recovery and environmental control units, called “cold zones” of MSWI, contain significant concentrations of * Corresponding author phone: +39-02-26603252; fax: +39-0264474300; e-mail:
[email protected]. † This paper follows two related papers which appeared in Environ. Sci. Technol., refs 6 and 7. ‡ Universita ` di Milano. § Universita ` dell’Insubria. # Universita ` di Milano-Bicocca. 4370
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PCDD and PCDF even if such pollutants are totally absent or present only in traces at the outlets of the combustion and postcombustion chambers. This observation suggests that these compounds are formed after the combustion zones, involving lower temperatures and probably two mechanisms on fly ash surface: (i) formation from precursors, i.e. chloroaromatic molecules such as polychlorobenzenes and polychlorophenols and (ii) de novo synthesis, starting from more or less complex non-chlorinated molecules, native carbon included, and a chlorine source such as Cl2, HCl or inorganic chlorides. In both cases, fly ash can act as a catalyst; at least in the latter mechanism, oxygen is essential. In a paper on the mechanism of de novo synthesis of PCDD/PCDF (2), Huang and Buekens report an overview of catalyzed and uncatalyzed mechanisms of carbon gasification in the low-temperature range (200-500 °C). They suggest that in order to understand the de novo synthesis reaction, it is necessary to understand the carbon gasification reaction, especially the O2 mode of attack on graphitic carbon structures. In the past few years, thermal behavior of fly ash has been extensively studied (3, 4). Fly ash contains both organic compounds and unburnt unextractable carbon (native carbon). We decided to focus our attention on their reactions and did kinetic work on the thermal behavior of MSWI raw fly ash as well as model systems (5-7). A global parameter, Total Organic Carbon (TOC), was used to measure the decrease of reagent concentration in time. It was shown (6, 7) that two processes are active. Both are carbon oxidation reactions to CO2, and the oxidized carbon is not from adsorbed organic compounds but from the native carbon matrix of MSWI fly ash. The presence of two combustion reactions was also shown in a previous study performed by Differential Scanning Calorimetry, DSC (8). In ref 7, the experimental results were interpreted in terms of elementary reactions describing the mechanism details, and a generalized kinetic model for fly ash native carbon oxidation was developed. It was shown that the conversion of fly ash native carbon to CO2 is the result of two simultaneous processes taking place on the fly ash surface: the first process (rate constant k2) is the oxygen transfer from a metal oxide site to a vacant carbon active site leading to immediate carbon gasification and the second is dissociative oxygen chemisorption where C(O) complexes are formed (k1) followed by C(O) complexes uncatalyzed gasification (k3). The model was validated using the kinetic data from model systems and those reported in ref 1 for four different fly ashes. The rate constants, k1, k2, and k3, together with activation and thermodynamic parameters were calculated. For each fly ash, the rate determining step is the C(O) complexes gasification. It was shown that the nature of the interaction between native carbon and the fly ash surface is the key factor for carbon gasification and C(O) complexes formation and gasification. Moreover, to correlate kinetic results with fly ash properties, we studied fly ash structure and analyzed the inorganic species of a fly ash sample (9, 10). The goal of this work is to get further information for the validation of the kinetic model. To do this, the nature of fly ash native carbon was investigated. Its behavior in combustion is studied by analyzing the carbon dioxide loss and its micro-Raman spectrum.
Experimental Section Fly ash was collected from modern waste incinerator hoppers in Italy (AMSA, Milan, via Zama); this is the raw FA sample called FA3 in refs 6-8. 10.1021/es000062p CCC: $19.00
2000 American Chemical Society Published on Web 09/07/2000
TABLE 1. TOC Content of the Samples and Average Particle Diameter of Seven FA Fractions sample
TOC (ppm)
fly ash, FA3 fraction 1 fraction 2 fraction 3 fraction 4 fraction 5 fraction 6 fraction 7 Cact-TTFAa Camorphous-TTFAa Cnat-TTFAa Cact-SiO2 Camorphous-SiO2 Cnat-SiO2 Cact-CuCl2 (5%)-SiO2 Cact-CuCl2 (10%)-SiO2 Cact-CuO (10%)-SiO2 Cact-CuSO4 (10%)-SiO2
3520 ( 80 9440 ( 590 8870 ( 570 8530 ( 320 9850 ( 240 30200 ( 5100 125000 ( 12700 186000 ( 58800 1980 ( 50 2020 ( 75 1970 ( 55 1980 ( 35 2005 ( 60 1985 ( 40 1980 ( 35 1980 ( 35 1980 ( 35 1980 ( 35
a
av diameter (µm) 9.5 5.6 3.9 2.3 1.4 0.92 0.53
TTFA ) Thermally Treated Fly Ash.
Unburnt unextractable carbon (in the following indicated as “native” carbon, Cnat) was manually isolated with tweezers from raw FA; its purity (99% C) was calculated comparing the expected TOC value with the experimental one. The residual FA was heated for 3 h at 650 °C in oxygen atmosphere to completely eliminate the carbon; the residual TOC was lower than 50 ppm, i.e., the limit of detection of the TOC instrument. In the following this support will be indicated as Thermally Treated Fly Ash, TTFA. Model mixtures with a carbon content similar to that of FA3 were prepared mixing appropriate amounts of reagents with a support. The reagents were native carbon Cnat, activated carbon Cact (DARCO G-60, 100 mesh, powder), amorphous carbon Camorphous (Aldrich, carbon 12C, amorphous 99.95 atom % 12 C), CuCl2 (cupric chloride, minimum 99% Carlo Erba), CuO (copper(II) oxide, minimum 99% Carlo Erba), and CuSO4‚5H2O (cupric sulfate, minimum 99% Carlo Erba). Supports were Thermally Treated Fly Ash (TTFA) and silica, SiO2 (Merck, grade 9385, 60 Å). From the raw FA sample, seven fractions with particles of different dimensions were obtained as described in a previous work (10). The raw FA and fraction TOCs are in Table 1 together with the average particle diameter of each fraction. To obtain an amount of each fraction sufficient for the subsequent studies, several runs were necessary; therefore, it was not possible to determine the weight percentage distribution. To obtain an estimate of the mass contribution of the fractions to the total FA mass, a separate experiment determined that more than 75% of the raw FA weight had a particle diameter higher than 150 µm; as fraction 1 had an average particle diameter of 9.5 µm, the total mass of the seven fractions was a very low mass percentage of the raw FA (supposedly lower than 10%). Moreover, the absolute mass of fractions 1 to 4 was much lower than that of fractions 5 to 7, and it was not possible to analyze them furthered. Ten mixtures were prepared: Cact-TTFA, Camorphous-TTFA, Cnat-TTFA, Cact-SiO2, Camorphous-SiO2, Cnat-SiO2, Cact-CuCl2 (5%)SiO2, Cact-CuCl2 (10%)-SiO2, Cact-CuO (10%)-SiO2, and CactCuSO4 (10%)-SiO2. Using a ball mill (Retsch, Model S1) operating with three 10 mm-L balls and two 20 mm-L ones at 80 rpm for 25 min each sample was homogenized, then dried, and used for the experiments without further treatment. The TOC content of each sample was measured using a Dohrmann instrument assembled with the Standard Module,
FIGURE 1. Fly ash FA3 curves and related y-axis units (in parentheses): (a) TG analysis (weight loss %); (b) DSC (heat flow); TG-FTIR analysis (absorbance, in arbitrary units); (c) carbon dioxide evolution curve; and (d) calculated deconvolution curves. DC-90, the Purgeable Organics Module, PRG-1, and the Sludge Sediment Sampler Accessory, S/SS. Details of the analytical method were already reported (11-13). The TOC of each mixture is reported in Table 1. The oxidative degradation of the native carbon was studied by monitoring the CO2 evolution at 2361 cm-1 using a JASCOFTIR spectrophotometer Model 360 with a resolution power of 0.5 cm-1 coupled to a Dupont Thermogravimetric analyzer Model 951. During the sample reaction, the gases evolved were transferred through a stainless steel tube heated at 110 °C to a 15 cm long multipass White cell equipped with KBr windows. This way it was possible to reach an optical path of 120 cm. An infrared spectra was collected automatically for each thermogravimetric analysis every 60 s: the total number of spectra was dependent on the temperature scanning and range. The experimental conditions used for the TG/FTIR analyses were as follows: reaction atmosphere, O2; gas flow rate, 60 mL/min; temperature scanning of 10 °C/min in the temperature range 50-700 °C for the raw FA and FA fractions from 5 to 7; and temperature scanning of 10 °C/min in the temperature range 50-200 °C followed by 2 °C/min in the range 200-700 °C for the model mixtures. Every TG/FTIR analysis was performed at least three times, to control the reproducibility of the experiment. The continuous monitoring of absorbance at the frequency of 2361 cm-1, that corresponds to the strongest band in the carbon dioxide infrared spectrum, produced an evolution curve of carbon dioxide as a function of time. The thermogravimetry (TG), DSC (8), and CO2 evolution curves as a function of temperature and time for FA3 are reported in Figure 1. The CO2 evolution curves for FA fractions and model mixtures are reported in Figures 2 to 5. To locate the maxima a deconvolution procedure was applied, assuming that the experimental curves result from one or the sum of two Gaussian functions. The number of Gaussian functions (one or two) and the parameters were determined by minimizing the residual sum of squares. The “best” values of the parameters were calculated iteratively starting from guess values. The deconvoluted Gaussian functions and their sum are reported in Figures 1 to 5. The peak at the lower temperature was called Peak 1, the one at the higher temperature was called Peak 2. From the deconvoluted Gaussian functions, the temperatures (T1 and T2) and times (t1 and t2) of the maxima and the area of each peak (A1 and A2) were calculated together with the A2(%), the percentage of A2 with respect to the total area of the two peaks, A1+A2. These values are reported in Table 2. VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. CO2 Evolution Curves: Temperatures and Times of the Maxima and A2 (%), Percentage of A2 with Respect to the Sum A1+A2 peak 1 sample fly ash, FA3 fraction 5 fraction 6 fraction 7 Cnat-SiO2 Cact-SiO2 Camorphous-SiO2 Cnat-TTFAc Cact-TTFAc Camorphous-TTFAc Cact-CuCl2 (5%)-SiO2 Cact-CuCl2 (10%)-SiO2
peak 2
T1 (°C) t1 (min) T2 (°C) t2 (min) A2 (%) 420 315 329 352
39a 26a 30a 35a
508 296 303 339 274 271
153b 36b 42b 65b 20b 19b
580 394 463 511 418 531 602 360 376 396 344 319
58a 35a 40a 45a 108b 170b 200b 80b 90b 99b 73b 63b
69.57 72.04 81.74 88.03 60.48 72.44 39.73 39.46 71.28 40.73
a Scanning velocity, 10 °C/min. b Scanning velocity, 2 °C/min. c TTFA ) Thermally Treated Fly Ash.
The micro-Raman spectra of native carbon, activated carbon, and activated carbon preheated in nitrogen at 700 °C were recorded by using a JASCO spectrophotometer model TRS-300 equipped with an Olympus optical microscope. The 488 nm emission of a Spectra Physic Kr+ laser was used as exciting line. The spectra are reported in Figure 6.
Results The TG/FTIR analyses were performed under a strong O2 excess and in the same temperature range as the kinetic studies (6). But the kinetic runs were executed in isothermal conditions, whereas the thermogravimetric analyses were performed by heating the sample very slowly (Table 2) to obtain optimal carbon dioxide evolution curves with respect to time. The carbon dioxide evolution curve of the raw FA3 sample (Figure 1) shows two maxima at 420 °C and 580 °C. It was found that the seven fractions TOC content (Table 1) increased with the decrease of the particle dimensions. The TOC was 8500-10000 ppm for fractions from 1 to 4 (particle diameter between 9.5 and 2.3 µm) and became higher for the fractions 5 (30000 ppm; particle diameter, 1.4 µm), 6 (125000 ppm; particle diameter, 0.92 µm), and 7 (186000 ppm; particle diameter, 0.53 µm). Taking into account that the mass percentage of fractions from 1 to 7 is less than 10% of the total mass, these TOC values are consistent with the TOC of FA3 (3520 ppm). Fractions 5 to 7 were analyzed by using the TG/FTIR technique (Figure 2). Unlike the raw FA, the decomposition process did not show two well resolved peaks. The carbon dioxide evolution curves showed a peak and a shoulder which were resolved in two peaks with the deconvolution procedure. The peak temperature (or the directly related time value) was assumed as an indirect estimate of reactivity: all other conditions being constant, lower temperature indicates an higher reactivity. For fractions 5 to 7, the decomposition times and temperatures of the two peaks increased with the decrease of the particle dimensions (Table 2). This trend may be related to differences in the support composition. In fact, in previous studies, the concentration of several metals in fractions 1 to 5 was determined (9-10). It was shown that the total metal distribution among the fractions depends on the metal speciation and possibly on the average particle dimensions. The results obtained in the experiments on the mixtures prepared by adding activated carbon, amorphous carbon, or native carbon to TTFA samples are reported in Figure 3 4372
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FIGURE 2. Carbon dioxide evolution curves (‚‚‚) and calculated deconvolution curves (s) for fly ash fractions 5 to 7. together with the deconvoluted curves and their sum. Once again the oxidation process of the carbon involved two reactions each at lower temperatures with respect to raw FA. The native carbon reacted faster than the activated carbon and the amorphous carbon was the less reactive. To better understand the behavior of these model mixtures kinetic studies are being performed. To go deeper into the nature of the processes, the support influence on the carbon oxidation process was studied on mixtures obtained by adding carbon with different structural arrangements to an inert SiO2 support. The experimental results are reported in Figure 4. A one-step decomposition process was observed both for the native and activated carbon mixtures, whereas a two-step process was observed for the amorphous carbon mixture. Both the reactions observed for activated and native carbon on SiO2 were slower compared with those on TTFA. Consequently, it is possible to conclude that the carbon oxidation process is highly dependent on the nature of the support. With every support, the amorphous carbon showed a twostep process. However, a support effect was evident. The CO2 evolution peaks appeared at 508 °C and 602 °C on the SiO2 and at 339 °C and 396 °C on the TTFA (Table 2). As for model mixtures with TTFA, kinetic studies are also underway on model mixtures with SiO2. To understand the role of metal species present in the support (2, 14-18), we studied four mixtures of activated carbon, SiO2, and a copper compound. The model mixtures were Cact-CuCl2 (5%)-SiO2, Cact-CuCl2 (10%)-SiO2, Cact-CuO (10%)-SiO2, and Cact-CuSO4 (10%)-SiO2. As shown in Figure 5, the evolution curves for the mixtures containing CuO or CuSO4‚5H2O have a single peak and are very similar to that for Cact-SiO2. On the contrary, the deconvoluted curves for the mixtures containing CuCl2 show two peaks. Finally, as different carbon structural arrangements exhibit a characteristic Raman spectrum (19), the native carbon of the FA3 sample as well as the activated carbon and the activated carbon preheated in nitrogen at 700 °C were analyzed by micro-Raman spectroscopy. The spectra recorded are reported in Figure 6 and compared with those of
FIGURE 4. Carbon dioxide evolution curves (‚‚‚) and calculated deconvolution curves (s) for the model mixtures Cnat-SiO2, CamorphousSiO2, and Cact-SiO2.
FIGURE 3. Carbon dioxide evolution curves (‚‚‚) and calculated deconvolution curves (s) for the model mixtures Cnat-TTFA, CamorphousTTFA, and Cact-TTFA. some standard carbon samples such as diamond, pyrolitic graphite, anthracite, and amorphous carbon.
Discussion For raw FA3, the carbon dioxide evolution curve shows two maxima at 420 °C and 580 °C (Figure 1), while only one oxidation reaction peak is observed in the CO2 evolution curve from native and activated carbon mixed with SiO2 (Figure 4). To rationalize the experimental results, first of all, we compare the activated carbon reactivity in model mixtures with SiO2 or with a copper compound and SiO2 (Figure 5). It must be emphasized that, to enhance the results observed, the Cu concentration in the model systems studied is about 2 orders of magnitude higher than in the raw FA. Moreover, the copper concentration, the speciation and the physical state of the catalyst, and the method of catalyst addition may have significant influences on its activity. We will go deeper into the effects of these variables in future experiments. The effect of Cu(II) as a potential catalyst depends on the copper compound added: 10% copper such as CuO or CuSO4 does not substantially vary the CO2 evolution curve in comparison with the uncatalyzed Cact-SiO2 system, whereas two peaks are observed in the corresponding curves of the Cact-CuCl2-SiO2 systems (Figure 5): both reactions in the latter
FIGURE 5. Carbon dioxide evolution curves (‚‚‚) and calculated deconvolution curves (s) for the model mixtures: Cact-CuCl2 (10%)SiO2; and Cact-CuCl2 (5%)-SiO2. systems are faster as the peaks are observed at lower temperatures in comparison with the Cact-SiO2 system (Table 2). Moreover, both the maxima for the 10% CuCl2 system lies at temperatures lower than those for the 5% CuCl2 system, supporting the catalytic role of CuCl2. VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Plot of A2 (%) as a function of TOC0 for raw fly ash and fractions 5 to 7.
FIGURE 6. Raman spectra of (a) native carbon; (b) activated carbon preheated in nitrogen at 700 °C; (c) activated carbon; (d) amorphous carbon; (e) anthracite; (f) pyrolitic graphite; (g) graphite; and (h) diamond. These results give a preliminary contribution to understanding the catalysis mechanism in these reactions. To act as a catalyst, copper has to be present in a free ionic form and not bound as in CuO. However, Cu is ionic in CuCl2 and CuSO4; the differences observed in the reactivity of the two corresponding mixtures could be due to the stability of CuSO4 as anhydrous salt as well as to the greater mobility and lesser shielding effect of Cl- ions with respect to SO42- ions. At this stage of our knowledge, we are not able to do any other hypothesis about the catalysis mechanism. In any case, this behavior is also important in order to explain the formation reactions of organochlorinated micropollutants. In fact, laboratory experiments (20-22) showed that the catalyst Cu2+ becomes less active when it is present as CuSO4 leading to a decrease in PCDD formation. Considering the reactivity of activated carbon mixed with TTFA, once again two peaks are observed in the CO2 evolution curve (Figure 3) at temperatures lower than those observed for Cact-SiO2 mixture. The same trend is observed for the Cnat-TTFA system (Figure 3) compared to the Cnat-SiO2 system (Figure 4). These results are consistent with those obtained in the kinetic runs and confirm the hypothesis (7) that native carbon gasification (eq 6 in ref 7), that was uncatalyzed in the Cact-SiO2 system, becomes catalyzed on TTFA surface. The behavior of amorphous carbon mixtures is different: a two-step process is always detected (Figures 3 and 4). This is probably due to the particular structural arrangement of this type of carbon. However, a catalytic effect of TTFA is evident, since reactions are faster and the maxima of the peaks are detected at lower temperatures in comparison with the Camorphous-SiO2 system. These differences are confirmed taking into account the Raman spectra recorded for the different carbon structural arrangements (Figure 6). The Raman spectrum of the native carbon is similar to that of activated carbon and very similar 4374
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to that of activated carbon preheated in nitrogen at 700 °C but quite different from that of amorphous carbon where there is a maximum which lies almost in the same range of that of graphite. A general trend of carbon reactivity in gasification is that carbons with a higher degree of graphitization tend to be less reactive (23). So, the lower reactivity observed in the Camorphous-TTFA in comparison with that in the Cnat-TTFA and Cact-TTFA systems can be explained on this basis. In conclusion, low temperature carbon gasification is strongly dependent on the catalyst either if it is added to the model mixtures or if it is already present in the TTFA. The behavior of these systems is consistent with that observed for raw FA and its fractions with respect to the number of processes involved as well as the reaction rates (Table 2). Finally, the peak areas A1 and A2 (Table 2) can be correlated to the importance of each of the two reactions. In fact, as the TOC of the solid residue is lower than 50 ppm, the total peak area, A1 + A2, is a measure of the CO2 developed during the complete oxidation of the initial carbon. If we plot the values of A2 (%) as a function of TOC0 we observe a very good linear agreement for raw FA and its fractions (Figure 7): passing from fractions with a lower TOC0 to fractions with a higher TOC0, reaction 2 becomes more important. It seems possible to explain this experimental evidence by considering that in the fractions with higher TOC0 and lower average particle diameter the interaction of carbon with catalytic sites becomes more difficult due to the lowering of the ratio catalytic sites/carbon: thus, the process corresponding to peak 2 is the dissociative oxygen chemisorption followed by the uncatalyzed gasification of the intermediate C(O) complexes (eqs 5 and 7 in ref 7). The same explanation holds for the decrease in reactivity observed for model mixtures when passing from CamorphousTTFA to Camorphous-SiO2 and from Cact-CuCl2 (10%)-SiO2 to Cact-CuCl2 (5%)-SiO2. Even if all TOC0 values are around 2000 ppm, peak 2 appears at higher temperatures, and calculated A2 (%) values are higher (Table 2). All the experimental results support previous findings from kinetic studies (6, 7) and give further information that will be used in the studies underway on the native carbon gasification reaction mechanism.
Acknowledgments We thank Massimo Ferri for his valuable work in the laboratory, the “Fondazione Lombardia per l’Ambiente” and the Italian CNR (National Research Council) for financial support (Grant no. 99.00788.CT13).
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Received for review March 20, 2000. Revised manuscript received July 17, 2000. Accepted July 19, 2000. ES000062P
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