VOLUME 16, NUMBER 6
NOVEMBER/DECEMBER 2002
© Copyright 2002 American Chemical Society
Articles Global Kinetic Rate Parameters for the Formation of Polycyclic Aromatic Hydrocarbons from the Pyrolyis of Catechol, A Model Compound Representative of Solid Fuel Moieties Elmer B. Ledesma, Nathan D. Marsh, Alyssa K. Sandrowitz, and Mary J. Wornat* Princeton University, Department of Mechanical and Aerospace Engineering, Princeton, New Jersey 08544 Received November 6, 2001
To obtain kinetic parameters on PAH formation relevant to solid fuels combustion, pyrolysis experiments have been conducted with catechol, a model fuel representing entities in coal and biomass. Catechol pyrolysis experiments were performed in a tubular-flow reactor at temperatures of 500-1000 °C and at a residence time of 0.4 s. PAH products were identified and quantified by high-pressure liquid chromatography with ultraviolet-visible diode-array detection and by gas chromatography with flame ionization and mass spectrometric detection. A pseudo-unimolecular reaction kinetic model was used to model the experimental yield/temperature data of 15 individual aromatics and of combinations of PAH grouped by structural class and ring-number. The modeling of the individual species’ yields showed that the pseudo-unimolecular model agreed very well with the experimental data. Ea values ranged from 50 to 110 kcal mol-1, generally increasing as the size of the aromatic product increased from one to five aromatic rings. The pseudounimolecular model also performed well in modeling the experimental yields of PAH grouped by structural class and ring number. The global kinetic analysis results for PAH grouped by ring number revealed that Ea values increased in the following order: 2-ring < 3-ring < 4-ring < 5-ring < 6-ring. Their yields followed the reverse order: 2-ring > 3-ring > 4-ring > 5-ring > 6-ring. These trends of increasing Ea and decreasing yield, as ring number is increased, are consistent with a mechanism for PAH growth involving successive ring buildup reactions.
Introduction The formation of polycyclic aromatic hydrocarbons (PAH) during the combustion of solid fuels such as coal and biomass stems from rapid pyrolytic reactions that result in the thermal degradation of the solid fuel and the liberation of fuel fragments.1-4 Such fuel fragments, * Author to whom correspondence should be addressed. Tel: (609)258-5278. Fax: (609)-258-6109. E-mail:
[email protected]. (1) Howard, J. B. Proc. Combust. Inst. 1990, 23, 1107.
which include high-molecular-weight species, can undergo further pyrolytic reactions that lead to the formation of PAH and soot.1-3 Due to the inherent carcinogenicity5 and mutagenicity6,7 of some PAH, their (2) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Proc. Combust. Inst. 1988, 22, 135. (3) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431. (4) Fletcher, T. H.; Solum, M. S.; Grant, D. C.; Critchfield, S.; Pugmire, R. J. Proc. Combust. Inst. 1990, 23, 1231.
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emission into the environment is of concern. Accordingly, there is a need to examine in detail PAH formation mechanisms and kinetics during the combustion of solid fuels, and consequently to develop predictive tools in order to model their formation. The literature2,3,8-21 reports several studies on the identification and quantification of PAH from solid fuels combustion and pyrolysis. However, no study has yet reported kinetic parameters for PAH formation from solid fuels combustion/pyrolysis. The majority of kinetic studies in relation to solid fuels combustion/pyrolysis have focused on a determination either of kinetic parameters for overall weight loss of the fuel or of kinetic parameters for the evolution of light gases such as CO, CH4, and H2.22-24 Global kinetic models have usually been employed in these studies to obtain the kinetic data.22,24 In contrast to the case of solid fuels combustion, in the area of gaseous fuel combustion, a number of studies involving detailed elementary reactions have been performed to model the formation of aromatic compounds and PAH from lighter hydrocarbon fuels such as methane,25,26 ethane,26 ethylene,27 acetylene,27,28 propane,29 and n-butane.30 However, these mechanisms are only partially applicable to the modeling of PAH formation from large-scale combustion devices, such as coal- and biomass-fired burners, where the nature of the (5) Dipple, A.; Moschel, R. C.; Bigger, C. A. H. In Chemical Carcinogens, 2nd ed.; Searle, C. E., Ed.; ACS Monograph 182; American Chemical Society: Washington, DC, 1984; Vol. 1, pp 41-164. (6) Durant, J. L.; Busby, W. F., Jr.; Lafleur, A. L.; Penman, B. W.; Crespi, C. L. Mutation Res. 1996, 371, 123. (7) Durant, J. L.; Lafleur, A. L.; Busby, W. F., Jr.; Donhoffner, L. L.; Penman, B. W.; Crespi, C. L. Mutation Res. 1999, 446, 1. (8) Mukherjee, J.; Sarofim, A. F.; Longwell, J. P. Combust. Flame 1994, 96, 191. (9) Masonjones, M. C.; Mukherjee, J.; Sarofim, A. F.; Taghizadeh, K.; Lafleur, A. L. Polycyclic Aromat. Compd. 1996, 8, 229-242. (10) Wornat, M. J.; Sarofim, A. F.; Lafleur, A. L. Proc. Combust. Inst. 1992, 24, 955. (11) Wornat, M. J.; Vriesendorp, F. J. J.; Lafleur, A. L.; Plummer, E. F.; Necula, A.; Scott, L. T. Polycyclic Aromat. Compd. 1999, 13, 221. (12) Wornat, M. J.; Vernaglia, B. A.; Lafleur, A. L.; Plummer, E. F.; Tagizadeh, K.; Necula, A.; Scott, L. T. Proc. Combust. Inst. 1998, 27, 1677. (13) Wornat, M. J.; Ledesma, E. B.; Sandrowitz, A. K.; Roth, M. J.; Dawsey, S. M.; Qiao, Y.-L.; Chen, W. Environ. Sci. Technol. 2001, 35, 1943. (14) Wornat, M. J.; Ledesma, E. B. Polycyclic Aromat. Compd. 2000, 18, 129. (15) Wornat, M. J.; Mikolajczak, C. J.; Vernaglia, B. A.; Kalish, M. A. Energy Fuels 1999, 13, 1092. (16) Nelson, P. F.; Tyler, R. J. Proc. Combust. Inst. 1986, 21, 427. (17) Mastral, A. M.; Callen, M. S.; Murillo, R.; Garcia, T. Polycyclic Aromat. Compd. 2000, 18, 1. (18) Mastral, A. M.; Callen, M. S. Environ. Sci. Technol. 2000, 34, 3051. (19) Mastral, A. M.; Callen, M. S.; Garcia, T. Fuel Process. Technol. 2000, 67, 1. (20) Pisupati, S. V.; Wasco, R. S.; Scaroni, A. W. J. Hazard. Mater. 2000, 74, 91. (21) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 2001, 35, 1716. (22) Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Prog. Energy Combust. Sci. 1992, 18, 133. (23) Saxena, S. C. Prog. Energy Combust. Sci. 1990, 16, 55. (24) Burnham, A. K.; Braun, R. L. Energy Fuels 1999, 13, 1. (25) Bo¨hm, H.; Kohse-Ho¨inghaus, K.; Lacas, F.; Rolon, C.; Darabiha, N.; Candel, S. Combust. Flame 2001, 124, 127. (26) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Castaldi, M. J.; Senkan, S. M. Combust. Sci. Technol. 1996, 116, 211. (27) Wang, H.; Frenklach, M. J. Phys. Chem. 1997, 110, 1173. (28) Miller, J. A.; Melius, C. F. Combust. Flame 1992, 91, 21. (29) Marinov, N. M.; Castaldi, M. J.; Melius, C. F.; Tsang, W. Combust. Sci. Technol. 1997, 128, 295. (30) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Vincitore, A. M.; Castaldi, M. J.; Senkan, S. M.; Melius, C. F. Combust. Flame 1998, 114, 192.
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fuel fragments is substantially different and the reaction chemistry is coupled to transport processes. As an initial step to model PAH formation during coal/biomass combustion, global kinetic data on the formation of PAH from these fuels, or from model compounds representing these fuels, would be useful. To this end, we have performed a global kinetic analysis on a number of the aromatic products from pyrolysis of the model solid fuel catechol (ortho-dihydroxybenzene). As a predominant structural entity in lignin (a major component of wood31) and coal32sas well as a major component in biomass tars33scatechol is a suitable model compound for investigating the formation of PAH from the combustion and pyrolysis of solid fuels. The catechol pyrolysis experiments have been conducted in a flow reactor at temperatures of 500-1000 °C and a residence time of 0.4 s. The aromatic condensed-phase products of these experiments have been analyzed by gas chromatography with flame ionization and mass spectrometric detection, as well as by high-pressure liquid chromatography with diode-array ultravioletvisible absorption detection. Our previous paper34 provides the chromatographic and spectral evidence documenting the identification of the PAH produced in the catechol pyrolysis experiments; a companion paper35 reports the yields of these aromatic products as functions of pyrolysis temperature and residence time. In the following, we report the kinetic rate parameters for the formation of several single-ring and multi-ring aromatic products of catechol pyrolysisderived from applying a single first-order kinetic reaction model to our experimental product yield data. Experimental Method Fuel Vaporizer and Pyrolysis Reactor. Pyrolysis experiments are conducted in a tubular-flow reactor system,34,35 which consists of a fuel vaporizer, quartz reactor, productcollection filter, and a solvent trap. Experiments are carried out by loading catechol particles (> 99.5% pure) into a Pyrex tube fixed within the vaporizer, a constant-temperature oven held at 85 °C, for slight vaporization of the catechol. A flowing stream of ultrahigh purity nitrogen picks up the vapor-phase catechol, resulting in a 0.72 mol % carbon loading in the reactor feed gas. Upon exiting the vaporizer, the vaporized catechol/nitrogen mixture enters the laminar-flow reactor, which consists of a 2-mm (inner diameter) quartz tube insulated at both ends and maintained at uniform temperature by a three-zone electrically heated furnace. The reactor is operated at temperatures of 500-1000 °C and a residence time of 0.4 s. Product Sample Collection and Analysis Preparation. Exiting the reactor, the catechol reaction products are quenched to room temperature and collected on a Balston filter and in a dichloromethane (DCM) solvent trap that follows the filter. The collected condensed-phase products are combined, and full dissolution into DCM is facilitated by ultrasonic agitation. 10% of the product solution is removed and reserved for analysis by gas chromatography. The remainder is concentrated in a (31) Lee, S. Alternative Fuels; Taylor & Francis: Washington, DC, 1996. (32) Lynch, B. M.; Durie, R. A. Aust. J. Chem. 1960, 13, 567. (33) Elliott, D. C. In Pyrolysis Oils from Biomass; Soltes, J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 55-65. (34) Wornat, M. J.; Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K. Fuel 2001, 80, 1711. (35) Marsh, N. D.; Ledesma, E. B.; Sandrowitz, A. K.; Wornat, M. J. Energy Fuels, submitted.
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Kuderna-Danish apparatus and exchanged into dimethyl sulfoxide for analysis by high-pressure liquid chromatography. Benzene, the most volatile of the aromatic products reported here, is collected from separate experiments in a gas sampling bag and analyzed by gas chromatography.36 Product Analysis. Gas chromatographic analysis of the catechol products is performed using an Agilent 6890 gas chromatograph with a flame-ionization detector (GC-FID), in conjunction with an Agilent 5973 mass spectrometric detector (MSD). A 10-µL sample volume is injected onto an HP-5 capillary column (30 m × 0.25 mm × 0.1 µm). The column temperature is programmed to hold at 40 °C for the first 3 min, followed by a ramp at 4 °C min-1 to 300 °C, where it is held for 15 min prior to cooling. The GC-FID/MSD instrument is used to quantify the one- and two-ring aromatic products, which are identified by matching retention times and mass spectra with those of reference standards. PAH are analyzed by a Hewlett-Packard 1050 high-pressure liquid chromatograph coupled to a diode-array ultravioletvisible absorption detector (HPLC-UV). PAH component separation is achieved with a reversed-phase Vydac 201-TP octadecylsilica column and time-programmed mobile phases of water/acetonitrile, acetonitrile, and dichloromethane.10-14 The flow rate is 1.5 mL min-1, and the solvent temperature is kept at 25 °C. PAH species are unequivocally identified by matching each component’s retention time and UV absorbance spectrum with those of reference standards, as documented by Wornat et al.34
Global Kinetic Analysis In this study, we apply a global kinetic analysis technique to obtain global rates of formation of aromatic species from the pyrolysis of catechol. Thus, we are interested in the global reaction: catechol f aromatic product. Within the context of this study, we make no attempt to construct a detailed chemical kinetic model containing a variety of elementary reactions to model aromatics formation. Here, we assume pseudo-unimolecular kinetics in which the empirical rates of formation of all species are first-order in the species yet to be formed, as expressed by the following equation:22-24
(1)
where X is the yield of a species at time t, and Xf is the yield as t f ∞. k is a first-order formation rate constant which has the Arrhenius form,
( ) Ea RT
(2)
where A is the preexponential factor (s-1), Ea is the activation energy (cal mol-1), R is the universal gas constant, and T is the temperature (K). The integrated form of the rate expression, which can be expressed as
[
(
ments and a fixed residence time) and the experimentally observed maximum yield, Xf, to eq 3, to obtain values of A and Ea for the formation of each product species. A yield/temperature curve is then generated from the derived values of A and Ea, to examine how well the experimental data conform to the assumed firstorder behavior. It should be noted that since we have experimental data only at one residence time, conversion and temperature are thus highly correlated, and the computed activation energies are not really rigorously constrained by the data. It should also be noted that in this study, the modeling of each species’ yield is confined strictly to the temperature range in which there is a net increase in product yield. No attempt is made to model aromatic product depletion at the highest temperatures. Results and Discussion
dX ) k(Xf - X) dt
k ) A exp -
Figure 1. Comparison between experimental yield/temperature data (filled circles) and modeling results (curve) for the formation of benzene and the benzenoid PAH: naphthalene, phenanthrene, and anthracene.
( ))]
X ) Xf 1 - exp -At exp -
Ea RT
(3)
is used to model the experimental results using a standard Marquardt-Levenberg least-squares algorithm code. The modeling consists of fitting the product yield/ temperature data (obtained from isothermal experi(36) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. Proc. Combust. Inst., accepted.
Aromatic Product Identifications. HPLC-UV and GC-FID/MSD analyses of the condensed-phase products of the catechol pyrolysis experiments reveal that the aromatic products range in size from one to eight fused aromatic rings and comprise several compound groups: bi-aryls, indene benzologues, oxygen-containing aromatics, benzenoid PAH, fluoranthene benzologues, cyclopenta-fused PAH, ethynyl-substituted aromatics, polyacetylenes, alkylated aromatics, and vinyl aromatics. Detailed discussions on the identifications of these species and on the effects of temperature and residence time on PAH yields from our catechol pyrolysis experiments are presented in separate publications.34,35 In this study, we present the results from the global kinetic analyses of several individual aromatic species as well as of combinations of aromatics grouped according to certain structural characteristics. Individual Species Kinetics. Figures 1-4 show comparisons between the modeling results and the experimental data for the variation in yield with temperature of a number of single- and multi-ring aromatic products of catechol pyrolysis: benzene and benzenoid PAH, Figures 1 and 2; vinyl- and ethynyl-substituted aromatics, Figure 3; and cyclopenta-fused PAH, Figure
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Figure 2. Comparison between experimental yield/temperature data (filled circles) and modeling results (curve) for the formation of the benzenoid PAH: pyrene, benz[a]anthracene, chrysene, and benzo[a]pyrene.
Figure 4. Comparison between experimental yield/temperature data (filled circles) and modeling results (curve) for the formation of cyclopenta-fused PAH: acenaphthylene, acephenanthrylene, and cyclopenta[cd]pyrene.
Figure 3. Comparison between experimental yield/temperature data (filled circles) and modeling results (curve) for the formation of vinyl- and ethynyl-substituted aromatics: styrene, phenylacetylene, 2-vinylnaphthalene, and 2-ethynylnaphthalene.
4. In each of these figures, the filled circles correspond to measured product yields from the catechol pyrolysis experiments. The solid curves in the figures correspond to the modeling results, which are obtained by fitting the experimental yield data (X,T) to the assumed firstorder kinetics form, eq 3. In eq 3, t is fixed at 0.4 s and Xf is the maximum measured yield of the product. The kinetic parameters A and Ea are the outputs of the model. Above 700 °C, the experimental results presented in Figures 1 and 3 clearly indicate a rapid increase in yield for benzene, naphthalene, styrene, and phenylacetylene. The rapid production of these one- and two-ring aromatics from 700 °C coincides with the rapid decomposition of catechol between 700 and 800 °C (at 700 °C, catechol conversion is 22%; at 800 °C, 88% w/w).36 This observation indicates that catechol decomposition results in the formation of aromatic products. As Figure 1 shows, benzene is the most abundant aromatic produced from our catechol pyrolysis experiments, attaining a maximum yield of 6.7% w/w at 900 °C. In addition to
benzene, other aromatics found in abundance are naphthalene and indene (result presented elsewhere35). Figures 1-4 also show that the larger PAH, such as pyrene, benz[a]anthracene, chrysene, and benzo[a]pyrene, and the cyclopenta-fused PAH, commence formation above 800 °C and peak in yield at 950 °C. The variety of product structures and the differences in yield/temperature behavior, as demonstrated in Figures 1-4, indicate that a variety of reaction mechanisms are responsible for the formation of the aromatic products of catechol pyrolysis. For example, the displacement of hydroxyl groups by H atom is likely responsible36 for benzene formation from catechol at low temperaturessa pathway suggested for benzene formation from phenol pyrolysis.37,38 At higher temperatures, however, decomposition of the catechol ring occurs, as evidenced by the high degree of catechol conversion. This high catechol conversion, coupled with the increase in benzene yield at temperatures > 800 °C, indicates that reactions other than OH replacement by H are responsible for benzene formation at the higher temperatures. Though we do not report yield data for light hydrocarbon gases in this paper, we have measured36 several light hydrocarbon gasessmethane, ethane, ethylene, propylene, propadiene, acetylene, 1,3-butadiene, and propynesthat could contribute to aromatic growth in the catechol pyrolysis environment. Experimental and modeling studies with other fuels27,28 have suggested that growth of aromatic products and PAH formation can occur through a reaction sequence involving successive H-abstraction and C2H2-addition. Such a se(37) Brezinsky, K.; Pecullan, M.; Glassman, I. J. Phys. Chem. A 1998, 102, 8614. (38) Horn, C.; Roy, K.; Frank, P.; Just, T. Proc. Combust. Inst. 1998, 27, 321.
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Table 1. Arrhenius Parameters for Individual PAH Species name
log A (s-1)
Ea (kcal mol-1)
benzene naphthalene phenanthrene anthracene pyrene benz[a]anthracene chrysene benzo[a]pyrene styrene phenylacetylene 2-vinylnaphthalene 2-ethynylnaphthalene acenaphthylene acephenanthrylene cyclopenta[cd]pyrene
10.9 14.7 16.4 18.1 16.4 18.1 16.4 16.6 14.5 14.0 16.6 14.8 15.4 15.2 20.5
54.1 74.3 84.3 92.9 86.6 94.4 85.9 87.9 71.8 70.8 83.9 77.8 79.5 80.0 110
quence of reactions may play a role in producing some of the PAH observed from our catechol pyrolysis experiments. C2H2, an abundant product of our catechol experiments,36 can add to benzene or phenyl radicals to produce styrene and phenylacetylene, the two singlering products shown in Figure 3. The cyclopenta-fused PAH of Figure 4 can result from the addition of C2H2 to naphthalene, phenanthrene, pyrenesor to their corresponding aryl radicals.11,12,14,39 As suggested by Wang and Frenklach,27 succession of H-abstraction, C2H2addition reactions can also contribute to the production of the benzenoid PAH of Figures 1 and 2: naphthalene, phenanthrene, pyrene, etc. The observed higher yields of the smaller aromatics, compared to those of the larger ones, provide supportive evidence for such a ring buildup mechanism; Figures 1-4 show that yields decrease in the following order of ascending ring number: benzene > naphthalene > phenanthrene > pyrene > benzo[a]pyrene. The good agreement between the curves and data points in Figures 1-4 demonstrates that the assumed pseudo-unimolecular reaction model of eq 3 models well the formation of the fifteen aromatic products plotted in Figures 1-4. For all of these species, Figures 1-4 show that the model reproduces well the rapid increase in formation for temperatures greater than 700 °C. It is not within the scope of the present work to model PAH destruction reactions, so no attempt is made, at this point, to model the decay in aromatic yields exhibited in Figures 1-4 at the highest temperatures. Nevertheless, in the temperature regime of aromatic product formation, the good agreement between the model and the experimental data shows that the psuedo-unimolecular reaction model employed in this study is valid for the system investigated. Table 1 lists the Arrhenius parameters derived from the modeling for all the aromatics in Figures 1-4. The results in Table 1 show that the larger aromatic species generally exhibit higher Arrhenius parameter values than the smaller ones. For example, A and Ea values increase in the following order: benzene < naphthalene < phenanthrene < pyrene < benzo[a]pyrene. Although the Arrhenius parameters are global values (which represent the overall kinetic parameters for the global reaction: catechol f aromatic product), the higher Ea values for the compounds of higher ring number, (39) Marsh, N. D.; Wornat, M. J. Proc. Combust. Inst. 2000, 28, 2585.
Figure 5. Comparison between experimental yield/temperature data (filled circles) and modeling results (curve) for the formation of PAH, summed by structural class: benzenoid PAH, fluoranthene benzologues, and cyclopenta-fused PAH.
compared to those of smaller ring numbersand the experimental observation that the smaller aromatics have higher yields than the larger onesssuggest a successive ring buildup mechanism for the production of PAH. Kinetics of PAH Grouped by Structural Class. Figure 5 shows comparisons between modeling results (solid curves) and experimental data (filled circles) for the variation in yield with temperature of three PAH classes: benzenoid PAH, fluoranthene benzologues, and cyclopenta-fused PAH. Each class yield is composed of the sum of the yields of the individual species in that class and includes species plotted individually in Figures 1-4 as well as species whose yields are too small to portray in individual plots. (A full list of the 23 benzenoid PAH, 6 fluoranthene benzologues, and 10 cyclopenta-fused PAH making up these classes is published elsewhere.34) Just as in the case for the individual species’ model curves in Figures 1-4, the PAH classes’ model curves in Figure 5 are generated by fitting the experimental yield/temperature data of each class to the assumed first-order kinetics represented in eq 3. The experimentally obtained PAH class yields, plotted as the filled circles in Figure 5, exhibit temperature effects that are similar to those for the individual species’ yields in Figures 1-4. Rapid production for the benzenoid PAH and cyclopenta-fused PAH occurs above 700 °C, whereas fluoranthene benzologue formation occurs above 800 °C. All three PAH classes peak in yield at 950 °C. The good agreement between the solid curves and experimental points in Figure 5 demonstrates that the assumed first-order kinetic model of eq 3 models well the formation of classes of PAHsjust as it did the formation of individual PAH, as demonstrated in Figures 1-4.
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Table 2. Arrhenius Parameters for PAH Classes name
log A (s-1)
Ea (kcal mol-1)
benzenoid PAH fluoranthene benzologues cylcopenta-fused PAH
14.9 16.5 12.3
75.8 86.8 64.1
Arrhenius parameters derived for the PAH classes are presented in Table 2. The low values of A and Ea obtained for the cyclopenta-fused PAH, relative to those for the other classes, are due to the fact that there is less uniformity, in the yield/temperature data, among the species within the cyclopenta-fused PAH than among species within the other classes. For example, there is greater variation in the steepness of the yield profiles of the three cyclopenta-fused PAH in Figure 4 than there is in those of the four benzenoid PAH in Figure 2. Because of species-to-species variations within a given PAH class, therefore, one should exercise caution in using group-derived kinetic parameters for predicting the behavior of an individual species within that group. Kinetics of PAH Grouped by Ring Number. Figure 6 shows comparisons between modeling results (solid curves) and experimental data (filled circles) for the variation with temperature, of PAH yields summed by aromatic ring number. Each ring-number group includes all of the species of that ring numbers substituted, unsubstituted, and even oxygen-containings that have been identified34 in our catechol pyrolysis products. Single-ring species, the most abundant of all the aromatic products, are not included in Figure 6, since the focus here is on the multi-ring species. The experimental results (filled circles) in Figure 6 show that the 2-ring species are the most abundant PAH products at temperatures up to 900 °C. The 2-ring PAH peak at 900 °C, whereas the maximum yields for the 3- to 6-ring PAH occur at 950 °C. As shown in Figure 6, the pseudo-unimolecular reaction model (solid curves) exhibits good agreement with the experimental data for the formation of each ringnumber class of PAH. The derived Arrhenius parameters, presented in Table 3, show an increase in the value of Ea as the ring number increasesssimilar to the Ea results derived for the formation of individual species from catechol pyrolysis. This trend, exhibited by the kinetic results for both the ring-number classes and the individual PAHsalong with the trend of decreasing yield with increasing ring numbersprovides strong support for a successive ring buildup mechanism. As mentioned earlier, one of the main aims of this study has been to determine kinetic parameters on PAH formation from combustion/pyrolysis of a solid fuel or
Figure 6. Comparison between experimental yield/temperature data (symbols) and modeling results (curves) for the formation of PAH, summed by aromatic ring number: 2-, 3-, 4-, 5-, and 6-ring PAH. Table 3. Arrhenius Parameters for PAH Grouped by Aromatic Ring Number ring number
log A (s-1)
Ea (kcal mol-1)
2 3 4 5 6
9.92 12.9 15.7 19.6 23.9
48.6 66.3 82.0 104 128
model compound that could be used as an initial step toward modeling the formation of PAH from coal and biomass combustion. The Arrhenius parameters presented in Tables 1-3 can be used for such a purpose, to predict the formation of PAH from solid fuels combustion. Furthermore, since PAH can serve as precursors to soot,1,10,27 the Arrhenius parameters obtained here for PAH formation may also be instrumental in predicting soot production from solid fuels. Acknowledgment. We gratefully acknowledge Philip Morris, Inc., and the National Science Foundation for their support of this research. EF010261+
