Trends in Aromatic Ring Number Distributions of Coal Tars during

Trends in Aromatic Ring Number Distributions of Coal Tars during Secondary Pyrolysis ... Based on the trends observed for the various ring groups, ins...
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Energy & Fuels 1998, 12, 450-456

Trends in Aromatic Ring Number Distributions of Coal Tars during Secondary Pyrolysis Liya E. Yu and Lynn M. Hildemann* Environmental and Water Studies Program, Civil and Environmental Engineering Department, Stanford University, Stanford, California 94305-4020

Stephen Niksa Chemistry and Chemical Engineering Laboratory, SRI International, Menlo Park, California 94025 Received April 15, 1997

To characterize the ring number distribution of coal tars throughout secondary pyrolysis, tar samples from two coal types were fractionated via gravity flow column chromatography (GFCC), and the polycyclic aromatic compounds (PAC) in the toluene fraction were analyzed via high performance liquid chromatography (HPLC) to quantify ring number distributions. During the early stages of secondary pyrolysis, the measured ring number distributions reflect prominent features of the parent coals, but the influence of original coal structure on the ring number distribution diminishes as pyrolysis conditions become more severe. Based on the trends observed for the various ring groups, insights are gained regarding the orchestration among neutralization, polymerization, and ring rupture, as well as the predominance of various sooting pathways. During the early stages of secondary pyrolysis, neutralization represents a major mass transformation mechanism among PAC, while direct conversion of PAC to soot also appears to be a dominant pathway. As secondary pyrolysis proceeds, polymerization and ring rupture become significant, while the addition of acetylene appears to be important in contributing to soot growth. By the end of secondary pyrolysis, ring rupture appears to be the dominant mass transformation mechanism.

Introduction It is known that 40-70% of carbon in both subbituminous and bituminous coals exists in aromatic form, mainly consisting of one to four fused aromatic rings.1 Substantial amounts of these ring structures are released intact in the tar product when coals decompose during heating to combustion temperatures. In addition, it is known that pyrolysis products can undergo chemical transformations during severe thermal conditions, forming more mutagenic compounds.2 The toxicity of polycyclic aromatic compounds (PAC) can vary greatly with the number of fused rings; the 4- and 5-ring PAC have a strong tendency to be carcinogenic and/or mutagenic,3,4 while PAC composed of 6 or more rings have substantial mutagenicity in human cells.5 Since the tar generated during a combustion process tends to condense onto soot particles, these potentially toxic PAC can deposit in the human lung. To mitigate this risk, * Address correspondence to this author. E-mail: [email protected]. (1) Gavalas, G. R. Coal Science and Technology Vol. 4: Coal Pyrolysis; Elsevier Scientific Publishing Co.: New York, 1982. (2) Braun, A. G.; Wornat, M. J.; Mitra, A.; Sarofim, A. F. Environ. Health Perspect. 1987, 73, 215-221. (3) Ho, C.-H.; Clark, B. R.; Guerin, M. R.; Barkenbus, B. D.; Rao, T. K.; Epler, J. L. Mutat. Res. 1981, 85, 335-345. (4) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P.; Lafleur, A. L. Energy Fuels 1988, 2, 775-782. (5) Durant, J. L.; Busby Jr., W. F.; Lafleur, A. L.; Penman, B. W.; Crespi, C. L. Mutation Res. 1996, 371, 123-157.

it is important to understand how the ring distribution in coal tars is influenced by thermal conditions and coal structure. Although the change in the ring number distribution of PAC during certain stages of secondary pyrolysis has been studied,6,7 detailed measurements covering the entire range of secondary pyrolysis for various types of coal have not been reported to date. The study described in this paper used a novel coal flow reactor that enables organics to be collected at specific stages spanning the whole range of secondary pyrolysis. In addition, the analysis of tar samples from pyrolysis of two dissimilar types of coal, one subbituminous coal and one high-volatile (hv) bituminous coal, allows us to assess the effects of coal type on the composition of the organic emissions. The mechanisms of polymerization, neutralization, and ring rupture occurring among PAC at high temperatures have been mentioned by many researchers,6-16 and the relative prominence among isomerization, biaryl (6) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Symp. (Int.) Combust. [Proc.], 22 1988, 135-143. (7) Bissett, L. A.; Lamey, S. C. Energy Fuels 1988, 2, 827-833. (8) Graham, S. C.; Homer, J. B.; Rosenfeld, J. L. J. Proc. R. Soc. London 1975, A344, 259-285. (9) Bittner, J. D.; Howard, J. B. In Progress in Aeronautics and Astronautics; Bowman, C. T., Birkeland, J., Eds.; American Institute of Aeronautics and Astronautics: New York, 1978; pp 335-358. (10) Stein, S. E. J. Phys. Chem. 1978, 82, 566-570. (11) Longwell, J. P. Symp. (Int.) Combust. [Proc.], 19 1982, 13391350.

S0887-0624(97)00059-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998

Ring Number Distributions of Coal Tars

formation, and condensation has been reported by Wornat et al.17 for a standard compound at the late stages of secondary pyrolysis. However, the relative importance of these mechanisms as a function of different coal types throughout secondary pyrolysis has not been fully characterized. Our previous work using gravity flow column chromatography (GFCC) reported the polarity distributions of coal tars generated from the early to late stages of secondary pyrolysis and showed that the mechanisms of secondary reactions among tar compounds involve neutralization as well as mass transformation.18 In this work, we utilize high-performance liquid chromatography (HPLC) to chemically characterize tar samples spanning the full range of secondary pyrolysis, focusing on the evolution mechanisms indicated from the ring-number distributions of the 2- to 5-ring PAC. Experimental Section Sample Preparation. Two different types of coal were pyrolyzed in order to observe the effects of coal rank on product compositions: Dietz coal (a subbituminous coal) contained a substantial amount of oxygen, while Pittsburgh No. 8 coal (a hv bituminous coal) was low in oxygen. In general, subbituminous coals are rich in the aromatic compounds composed of one to three condensed rings, while bituminous coals contain primarily 2- to 4-ring compounds.1 The elemental compositions and other features of these two types of coal have been published elsewhere.19 Tar samples were generated from our radiant coal flow reactor under conditions throughout secondary pyrolysis. The design and operating specifications of the radiant coal flow reactor used to generate tar samples for these analyses have been described in detail elsewhere.20,21 Briefly, pulverized coal particles are heated rapidly via near-black thermal radiation from an inductively heated graphite tube mounted around a quartz flow tube. By controlling the coal-particle loading, entrainment gas velocity, and heating rate (104 K/s), the furnace temperature can range from 1380 to 1660 K, generating samples spanning the early to late stages of secondary pyrolysis. In our laboratory, the organic aerosol trapped by five stages of glass filters were extracted via ultrasonication in tetrahydrofuran (THF). THF-soluble species were designated as tar compounds, while THF-insoluble carbonaceous aerosol was referred to as “soot” in order to be consistent with previously published work.18-20,22 The selection of the extraction solvent (THF) and the storage of tar samples have been described in other publications.18,20 Because the addition of acetylene (C2H2) to soot compensates for the weight loss due to CO elimination from tar, the sum of the total carbonaceous aerosol (12) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Fuel 1985, 64, 13941400. (13) Nelson, P. F.; Tyler, R. J. Symp. (Int.) Combust. [Proc.], 21 1986, 427-435. (14) Jacobson, J. M.; Gray, M. R. Energy Fuels 1988, 2, 316-320. (15) Sullivan, R. F.; Boduszynski, M. M.; Fetzer, J. C. Energy Fuels 1989, 3, 603-612. (16) Li, C.-Z.; Nelson, P. F. Energy Fuels 1996, 10, 1083-1090. (17) Wornat, M. J.; Sarofim, A. F.; Lafleur, A. L. Symp. (Int.) Combust. [Proc.], 24 1992, 955-963. (18) Yu, L. E.; Hildemann, L. M.; DaDamio, J.; Niksa, S. Fuel, in press. (19) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254-264. (20) Chen, J. C. Effects of Secondary Reactions on Product Distribution and Nitrogen Evolution From Rapid Coal Pyrolysis. Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Stanford, CA, 1991. (21) Chen, J. C.; Niksa, S. Rev. Sci. Instrum. 1992, 63, 2073-2083. (22) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264271.

Energy & Fuels, Vol. 12, No. 3, 1998 451 (tar plus soot) stays invariant throughout secondary pyrolysis.22 Consequently, the soot fraction (SF) of the total condensable aerosol product is a convenient measure for the extent of secondary pyrolysis and will be used throughout this paper. Because polar compounds will interfere with the separation and measurement of less polar PAC according to fused aromatic ring numbers, as a first step the tar compounds were separated according to their polarity and molecular size via GFCC; the elution used the following sequence of solvents: heptane, toluene, dichloromethane (DCM), methanol (MeOH), and THF. The toluene fraction, containing 43-70% of tar yield, was found to contain the largest portion of the tar mass for both coals, including PAC with two and more rings;18 in particular, the PAC consisting of four or more aromatic rings are of greatest health concern. In addition, the polarity distributions suggested that compounds in the toluene fraction are the predominant precursors to soot throughout secondary pyrolysis.18 By further segregating the tar compounds collected in the toluene fraction according to fused aromatic ring number via HPLC, additional insights building on this earlier work18 can be obtained regarding how the composition and reaction mechanisms among tar compounds correspond to various stages of secondary pyrolysis. HPLC Analysis. The solvent fractionation followed by gravimetric measurements of tar samples gave mass recoveries averaging 96%; this work has been described in detail elsewhere.18 Since the tar mass recovered in individual solvent fractions via GFCC requires that the samples be dried by a nitrogen flow, the tar compounds collected needed to be redissolved for further analyses. Because all the coal tars were extracted with THF during the sampling procedure and preserved in THF, the toluene fraction was redissolved in THF prior to injection. A Hewlett-Packard 1090 HPLC, consisting of a PV5 solvent delivery system coupled with a UV-vis diodearray detector, and controlled by an HP9000 model 300 workstation, was utilized for this work. Because of the complex composition of coal tars, it was impossible to individually separate the hundreds of aromatic compounds in the toluene fraction. Thus, the selection of a suitable analytical column for this work was mainly based on how well the PAC segregated according to their fused ring numbers. Although C18 and NH2-bonded silica have been commonly used as stationary phases for HPLC columns, we found their resolution ambiguous: the undistinctive grouping of our complex samples using C18 and the irreversible adsorption of compounds onto the stationary phase using the NH2 column23 adversely affected the ability of these two columns to resolve complicated tar samples according to fused aromaticring numbers. Our laboratory tests showed that while a column containing the stationary phase of dinitroanilinopropyl (DINAP) gave good resolution of PAC composed of small ring numbers, a cyano column gave equally good resolution of the low-MW PAC as well as better resolution for larger PAC ring groups; this was consistent with the evaluation of columns reported by Lafleur et al.24 Thus, a cyanopropyl column (25 cm × 4.6 mm, Alltech Associates), containing a pore diameter of 6 nm and a particle size of 5 µm, was used. A mixture of heptane and DCM, consistent with the solvents used for GFCC fractionation, was utilized as the mobile phase for this work. Heptane has the advantage of being transparent to the detection wavelength and can sequentially resolve aromatic compounds, while DCM, by providing greater solubility for the tar compounds in the toluene fraction, can ensure that more polar compounds will travel through the column. As a result, the eluent was composed of 98% heptane and 2% DCM for the first 15 min followed by 100% DCM for an additional 5 min in order to elute the residue. Tests in our (23) Boduszynski, M. M. Energy Fuels 1988, 2, 597-613. (24) Lafleur, A. L.; Monchamp, P. A.; Chang, N. T.; Plummer, E. F.; Wornat, M. J. J. Chromatogr. Sci. 1988, 26, 337-344.

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Figure 1. Calibration curve of PAC based on elution volume vs fused ring number. laboratory using a standard mixture consisting of 16 aromatic standard compounds showed that a constant flowrate of 1 mL/ min achieved satisfactory separation. Calibration Curve. In order to consistently separate PAC according to their ring numbers via HPLC analysis, a calibration curve showing the relationship between elution solvent volume and fused ring numbers was needed. Since PAC composed of the same number of fused rings were expected to either coelute or elute within a short time interval, the time when the first compound in a certain ring group eluted was designated as the starting point of the elution time range. Although the end point of the elution for a specific ring group is not definitely certain, the starting point can be examined by testing various possibly-first-eluted compounds. Figure 1 shows the elution volumes measured for various ring groups of PAC; those volumes were calculated by multiplying the measured retention time by the flow rate of 1 mL/min. The PAC standards, which were the first eluted compounds in their corresponding ring groups, were identified both with chromatograms and the measured spectrum. Figure 1 shows that the elution volume (Ve) relates to the fused aromatic ring number (N) according to the following empirical polynomial equation (where R2 ) 0.991):

Ve ) 2.309 + 1.323N - 0.273N2 + 0.025N3 It should be noted that this calibration curve is only applicable for nonpolar PAC compounds eluting under the conditions used in this study. However, preliminary tests on tar samples for this work showed chromatograms that were comparable to the standard mixtures used to obtain the calibration curve. In addition, because 3-ring PAC eluted right after naphthalene, overlapped elution between 2- and 3-ring classes was expected. Thus, instead of vaguely identifying a cutoff point between 2-ring and 3-ring PAC, the two ring classes were grouped together for quantification in what follows. Resolution Sequence. Unsubstituted PAC containing the same number of fused aromatic rings were expected to elute within the same range of time. However, substituted PAC (sub-PAC) are also likely to be present in real tar samples and can differ significantly in elution order from their unsubstituted analogues. As a result of the GFCC prefractionation step, these samples should not contain highly polar PAC (such as PAC with carboxylic groups); polar PAC should have been recovered in the DCM and/or MeOH fractions, rather than the toluene fraction.18 Thus, model compounds consisting of moderately polar sub-PAC standards were injected to further evaluate the applicability of the calibration curve. It was found that, as shown in Figure 2, nonpolar substituents like alkyl and phenyl groups do not alter the retention time of their parent PAC, while more polar sub-PAC were

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Figure 2. Elution order of substituted PAC via HPLC analysis: (a) 1,2,4-trimethylbenzene; (b) 2-methylnaphthalene; (c) 9,10-diphenylanthracene; (d) rubrene; (e) 9-fluorenone; (f) 1-cyanonaphthalene; and (g) 1,5-dinitronaphthalene. substantially hindered in their elution from the column. For example, the elution time ranges of 1-cyanonaphthalene (a 2-ring sub-PAC) and 9-fluorenone (a 3-ring sub-PAC) corresponded to that of 8- and 9-ring PAC, respectively. One of the more polar compounds tested, 1,5-dinitronaphthalene (a two-ring sub-PAC) was retained in the column even longer than the 10-ring PAC. Similar tests have been reported elsewhere24 for 1-cyanonaphthalene and 9-fluorenone in a cyanopropyl column; however, the elution sequence observed differed from this work. Figure 2 shows that 9-fluorenone eluted before 1-cyanonaphthalene, which indicates that fused ring number was not the dominant factor controlling elution behavior among polar sub-PAC. In general, it was observed that any PAC with moderately polar substituents eluted from the column either along with or after the 6-ring PAC. Thus, we concluded that all the compounds eluting before the 6-ring PAC group, which should consist of less than six rings and might contain neutral substituents, would follow the elution order shown in the calibration curve (Figure 1). Furthermore, although PAC with sulfur and oxygen heteroatoms eluted through the column according to their fused ring numbers, PAC with nitrogen heteroatoms were observed to be retained in the HPLC column until the mobile phase consisted of 100% DCM, in agreement with the elution sequence reported by other researchers.24,25 Therefore, the tar compounds in the toluene fraction can be divided into three classes using this separation technique: (1) I-PAC: neutral PAC composed of two to five fused aromatic rings, including PAC with alkyl and phenyl substituents, as well as sulfur and oxygen existing as heteroatoms; all the PAC in this class eluted in an order corresponding to their fused ring numbers within the first 6.00 min; (2) II-PAC: PAC consisting of six or more aromatic rings, along with PAC containing moderately-polar substituents, such as cyano and carbonyl groups; they eluted between 6.00 and 20.00 min; (3) III-PAC: N-containing heterocyclic-PAC and/or PAC with oxygenated substituents, such as the hydroxyl group; these were only eluted by 100% DCM after more than 20.00 min. Because 1,2,4-trimethylbenzene eluted as a separate peak very early on, its elution did not interfere with the measurements of the real tar samples. Thus, for quality assurance, 1,2,4-trimethylbenzene was added in every injection sample as an internal standard to provide a consistent reference point for defining the window of elution time and adjusting the response factor. Response Factor and Quantification. With three other bandwidths used as references, a wavelength of 280 nm with a bandwidth of 120 nm was chosen for detection because it (25) Wise, S. A.; Chesler, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 49, 2306-2310.

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Energy & Fuels, Vol. 12, No. 3, 1998 453

Table 1. Earliest Elution Times and Response Factors for Individual Ring Groups fused ring no.

earliest elution time (min) (n ) 4)a

response factorb (area counts/(µg/mL)) (n ) 3)

2, 3 4 5 6 7 8d 9d 10

3.964 ((0.003)c 4.667 ((0.003) 5.644 ((0.007) 5.990 ((0.009) 6.215 ((0.024) 8.054d 10.090d 12.965 ((0.134)

11.03 ((0.23)c 18.38 ((0.38) 17.63 ((0.38) 15.26 ((0.32) 14.85 ((2.75) 12.01d 10.25d 7.98 ((0.47)

compounds in standard mixture naphthalene, acenaphthene, acenaphthylene, anthracene, fluorene, phenanthrene fluoranthene, chrysene, pyrene, benz[a]anthracene benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene indeno[1,2,3-cd]pyrene, benzo[ghi]perylene coronene decacyclene

n ) number of measurements averaged. b Response factor, mean value of three measurements at 220-340 nm using 1 mL/min of 98% heptane + 2% dichloromethane. c Standard deviation for n injections. d Standards are not available; earliest elution times are interpolated based on calibration curve (see Figure 1), while response factors are interpolated using the following equation as response factor vs fused ring number (N), based on three sets of data for 4-, 5-, 6-, 7-, and 10-ring PAC: response factor(area counts/(µg/mL)) ) 26.062 - 1.757N, R2 ) 0.969. a

can reliably detect PAC composed of various ring sizes. Since tar compounds were expected to elute as groups, a semiquantitative method was used to quantify each ring group in the I-PAC class. A standard suite (Chem. Science Inc. and Aldrich) containing 2- to 10-ring PAC was injected via HPLC. The resolution of this suite showed reproducible and consistent elution times referenced to the individual PAC standards. Table 1 lists the earliest elution time and the average response factor (area/(µg/mL)) found for each ring-number group. Since the response factor was found to depend on not only the sample concentration within the linear range of the detector but also the fused aromatic ring number, a least-squares regression was established to relate the response factor to the ring number. The 2- and 3-ring PAC were grouped together due to their overlapping elution, and an averaged response factor of the tested 2- and 3-ring PAC standards was used to quantitatively estimate the corresponding peaks. It should be noted that the concentrations of individual compounds in tar samples may differ substantially from the estimation based on the averaged response factors listed in Table 1, depending on the composition and proportions of compounds for certain ring groups. The II-PAC class was quantified primarily according to the response for neutral PAC composed of more than six rings. It should be noted that the II-PAC class could also in theory contain PAC with moderately-polar monosubstituents (such as cyano and carbonyl groups) which would not elute solely according to fused ring number. However, the PAC with moderately polar substituents appeared to be insignificant among eluted II-PAC peaks based on their elution behavior. Because the II-PAC for the two coal types examined in this work eluted earlier than 9-fluorenone (the smallest carbonylsub-PAC tested), these II-PAC peaks could be neutral PAC composed of more than 6 rings and/or 2- and 3-ring PAC with moderately-polar substituents. Since Wornat et al.6,26 reported that all sub-PAC, especially those composed of two to three rings, suffered from drastic depletion during the second half of secondary pyrolysis, while the II-PAC observed in our analyses did not appear until SF reached over 50%, it appears that the eluted II-PAC group mostly consist of nonpolar PAC with more than six aromatic rings. This is supported by Wornat et al., who reported an increase in larger PAC composed of more than 6 rings during the second half of secondary pyrolysis for a Pittsburgh No. 8 coal6 and the model compound anthracene.17 In addition, because the eluted IIPAC mainly appeared between 6.30 and 10.00 min, corresponding to where 7- and 8-ring PAC eluted (see Table 1), the elution time ranges and response factors for the 7- and 8-ring PAC were used to group and quantify the II-PAC class. While this approach should be considered only semiquantitative, it will identify the trends of II-PAC with increasing extents of (26) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431-437.

Figure 3. Ring number distribution of PAC for Dietz coal tars. secondary pyrolysis. However, one should not apply the response factor in this work for conditions where PAC with polar substituents are expected to be present. Compounds belonging to the III-PAC class measured via HPLC only dissolved in pure DCM, which strongly responded to detection wavelengths and substantially interfered with the absorbance measurements. Therefore, it was not considered appropriate to attempt quantification of the III-PAC class. Although the separation achieved via HPLC has been grouped into three major categories (I-PAC, II-PAC, and IIIPAC), it should be noted that aliphatic compounds collected in the toluene fraction were not included because they do not respond to UV detection. Additional limitations in achieving mass closure were expected, because the mass of each ring number group was estimated using an average response factor. However, based on the recovery of the internal standard used, the mass recovery was estimated to always exceed 80%. In addition, the amount of ring compounds quantified via this HPLC work was quite consistent with reported findings6,7 on similar types of coal tars using different approaches. Nevertheless, rather than emphasizing quantitative measurements, our focus here is on the trends seen in aromatic ring number distributions as a function of the extent of pyrolysis. On the basis of these trends, the relative importance of the various transformation mechanisms can be evaluated. However, it should be noted that some polymerization reactions do not change the number of fused rings, so the assessment of polymerization will be limited.

Results and Discussion Figures 3 and 4 show the trends of each aromatic ring group recovered in the toluene fraction throughout secondary pyrolysis for Dietz and Pittsburgh No. 8 coal tars, respectively. The solid lines in each figure show the total yields (wt % of coal, dry ash-free) of I-PAC and

454 Energy & Fuels, Vol. 12, No. 3, 1998

Figure 4. Ring number distribution of PAC for Pittsburgh No. 8 coal tars.

II-PAC. The 2- to 5-ring PAC comprising the I-PAC class are plotted as individual dashed lines; the polynomial-fit curves are intended to show trends only. It should be noted that the unmeasurable soot yield (0% SF) for the Pittsburgh No. 8 coal tar (in Figure 4) actually represents measurements at a late stage of primary devolatilization, rather than showing early secondary pyrolysis, because the total tar yield has not yet reached its ultimate value of 30 daf wt %.19 Early Stages of Secondary Pyrolysis. At the early stages of secondary pyrolysis (before 30% SF), although the total mass collected in the toluene fraction for Dietz coal tars increases,18 Figure 3 shows that the I-PAC yield decreases, while no II-PAC is observed. Thus, the increased tar mass measured in the toluene fraction must be primarily composed of the III-PAC class, and/ or aliphatic compounds which could be partially recovered in the toluene fraction yet not be detected via HPLC. A drastic increase in aliphatic compounds at the early stages of secondary pyrolysis is unlikely because the heptane fraction collected via GFCC, which solely consists of aliphatic compounds, is actually depleted severely along with the more polar solvent fractions.18 This suggests that the enhanced mass collected in the toluene fraction is likely due to a drastic increase in IIIPAC. Since the tar compounds are transformed from more polar solvent fractions (DCM and/or MeOH fractions) into compounds found in the toluene fraction very early during secondary pyrolysis,18 they must be converted mainly into PAC containing N-heteroatom and/ or oxygenated substituents. This scenario increases the mass of the toluene fraction despite the loss of I-PAC. Since the nitrogen content of Dietz coal is less than 2%, the concentration of N-containing polycyclic PAC is small relative to the concentration of PAC with oxygenated substituents in the Dietz coal. In addition, other researchers have observed substantial amounts of PAC with hydroxyl substituents during the very early stages of secondary pyrolysis.14,27 Therefore, it is likely that most of the transferred compounds are PAC with oxygen-containing substituents. It is known that cleavage of methylated PAC occurs early in secondary pyrolysis,12-16,28-30 resulting in meth(27) Surygala, J.; Sliwka, E. Fuel 1994, 73, 1574-1577. (28) Santoro, R. J.; Glassman, I. Combust. Sci. Technol. 1979, 19, 161-164. (29) Lafleur, A. L.; Sarofim, A. F.; Wornat, M. J. Energy Fuels 1993, 7, 357-361.

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yl groups and parent PAC which appear to be more stable than PAC with substituents. Since the elution order and absorptivity of I-PAC via HPLC are little affected by alkyl substituents,23-25 the absence of alkylated groups should not significantly contribute to the observed reduction in I-PAC. Furthermore, since PAC with furan-type (oxygen-containing) rings appear to be insensitive to thermal conditions at the early stages of pyrolysis,1,13,14 they should not be responsible for the decreasing concentrations seen for I-PAC. Therefore, the reduction in I-PAC concentrations during early secondary pyrolysis, as shown in Figure 3, appears to be mainly caused by direct conversion of ring compounds to soot. This agrees with measurements of the polarity distributions of tar compounds in our companion GFCC study;18 the neutral tar compounds collected in the toluene fraction appear to account for soot production through direct conversion during very early secondary pyrolysis. Once the SF exceeds 25%, the toluene fraction decreases monotonically,18 yet Figure 3 shows a contradictory increase of I-PAC for the Dietz coal tar. This indicates that the toluene fraction diminishes mainly due to the depletion of substituents (such as alkyl and/ or hydroxyl groups), rather than due to ring rupture. This is not surprising, because the substituents appear to be less stable than the parent ring structure and can be depleted in early secondary pyrolysis, as observed using Fourier transform infrared (FTIR) measurements31 taken during the corresponding pyrolysis stages. On the other hand, the increasing trend seen within each ring group for I-PAC could originate from any of three processes: (1) compounds originally in more polar solvent fractions whose polar substituents are neutralized subsequently elute earlier in the toluene fraction via GFCC; (2) III-PAC in the toluene fraction whose polar substituents are eliminated are transferred to the I-PAC group and elute earlier via HPLC; and/or (3) polymerization among tar compounds within and/or into the I-PAC group. The first two processes may be relatively more significant than the third process, because other research1,13,32 has reported that the increased production of water, CO2, and CO during this pyrolysis stage indicates the decomposition of carboxylic, hydroxyl, carbonyl substituents, and/or ether bridges. According to the data reported by Chen et al.,22 who used the same pyrolysis system as this work, CO and H2 production surpassed the production of light hydrocarbons that might encourage polymerization via addition. Therefore, neutralization of more polar PAC appears to be a major mechanism during the first half of secondary pyrolysis. Tar mass not only moves from the more polar solvent fractions into the toluene fraction18 but also may be transformed within the toluene fraction from III-PAC to I-PAC. Similar to the trends seen for the individual ring groups of Dietz coal tars, Figure 4 shows that at the early stages of secondary pyrolysis of Pittsburgh No. 8 coal tars, the total concentration of I-PAC is mainly (30) Hayashi, J.-I.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 57-66. (31) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, Jr., E. L., Ed.; American Chemical Society: Washington, DC, 1982; pp 77131. (32) Fitzgerald, D.; van Krevelen, D. W. Fuel 1959, 38, 17-37.

Ring Number Distributions of Coal Tars

determined by the PAC composed of two and three rings. However, relative to the Dietz coal tar, the depletion of 2- and 3-ring PAC for the Pittsburgh No. 8 coal tar occurs earlier during secondary pyrolysis. This faster decay has also been observed for oil yields from Pittsburgh No. 8 coal.22 We hypothesize that the larger loadings of Pittsburgh No. 8 coal during secondary pyrolysis increases the radiant heat absorption and results in slightly higher gas temperatures in the coal flow reactor, thereby promoting the disruption of oil and lighter tar compounds.22 Before the SF reaches 30%, Figure 3 shows that more than 70% of I-PAC consists of compounds composed of two or three rings, which reflects the fact that Dietz coal is rich in one to three condensed rings. On the other hand, Pittsburgh No. 8 coal is richer in two to four condensed rings,1 and Figure 4 indicates that the Pittsburgh No. 8 coal tar contains a larger proportion of 4-ring PAC than the Dietz coal tar. Since the polarity distributions of both the Dietz and Pittsburgh No. 8 coal tars reflect the features of their parent coals during early secondary pyrolysis,18 it is not surprising that the composition of ring compounds for both coal tars also reflects the features of the original coal structures. This agrees with the result reported by Teo and Watkinson,33 who used a different approach to study ring size during early secondary pyrolysis. Later Stages of Secondary Pyrolysis. Before 70% SF, Figure 3 shows that, for Dietz coal tars, 4-ring PAC replace the smaller PAC as the dominant ring size in the I-PAC group, while 2- and 3-ring PAC decrease monotonically. This indicates that, as shown in Figure 3, the major structural components of tar seem to change around 50% SF. The smaller PAC (2- and 3-ring) decay rapidly while the larger PAC (4- and 5-ring) exhibit higher stability as secondary pyrolysis becomes more severe; this is consistent with the findings of other researchers.6,16,34 Stein10 generalized these observations in terms of the thermochemical properties of PAC at high temperatures: PAC smaller than the critical ring size would tend to degrade, while larger PAC would continue to grow through polymerization. In addition, the contrast observed here between the trends for 2- and 3-ring PAC vs the larger PAC agrees with the observations by Lewis and Edstrom35 that three fused rings appears to be the fate-determining size; our data indicate as well that this divergence in trends occurs around 50% SF. Since previous work18 examining the trends in polarity distributions suggested that rings open around 50% SF, the degradation of 2- and 3-ring PAC may further corroborate the significance of the ring rupture mechanism in the middle stages of secondary pyrolysis. The drastic growth of acetylene levels at the expense of other light aliphatic compounds halfway through secondary pyrolysis22 is consistent with cleavage of smaller PAC. In addition, the gas production observed during secondary pyrolysis by others further supports the occurrence (33) Teo, K. C.; Watkinson, A. P. Fuel 1987, 66, 1123-1132. (34) Sarofim, A. F.; Longwell, J. P.; Wornat, M. J.; Mukherjee, J. In Soot Formation in Combustion; Springer Ser. Chem. Phys., 59; Bockhorn, H., Ed.; Springer-Verlag: Berlin, Germany, 1994; pp 485499. (35) Lewis, I. C.; Edstrom, T. Fifth Conf. Carbon [Proc.] 1963, 413430.

Energy & Fuels, Vol. 12, No. 3, 1998 455

of ring opening; for example, Fitzgerald and van Krevelen32 suggested that CO is mainly released from heteroatoms in tar compounds during more severe burning conditions. An abrupt increase in CO yields after 50% SF reported for both the Dietz and Pittsburgh No. 8 coals suggests that oxygen heteroatoms are mostly released from within the rings.22 This is expected to be caused by the opening of ring structures, because releasing additional CO through the disruption of furantype rings would not occur until higher temperatures are imposed,1,13,14 as during the later stages of secondary pyrolysis in our experiments. The buildup of 4- and 5-ring PAC (in I-PAC) in the Dietz coal tar, as shown in Figure 3, may result from the mechanism of polymerization during the middle stages of secondary pyrolysis. The enhanced concentration of these larger PAC under severe thermal conditions may be attributable to the buildup of existing PAC via addition of light hydrocarbons (such as C2H2) and/ or via successive condensation of small PAC.9,10,36 Since the opening of rings has been observed in the middle stages of secondary pyrolysis,18 resulting in the depletion of smaller PAC species, it is not surprising that the buildup of larger PAC species via addition of C2H2 might be more substantial, particularly after 50% SF. Interestingly, Figure 3 shows that II-PAC (mainly larger nonpolar PAC composed of more than six rings) do not appear until 50% SF, and reach their maximum yield during the second half of secondary pyrolysis. Thus, the increase of PAC composed of four, five, and six or more rings indicates the prominence of polymerization during the middle stages of secondary pyrolysis, particularly for SF over 50%. Although both ring rupture and polymerization have been observed at different pyrolysis conditions,6,10,30 they apparently especially influence the trends seen here in Dietz coal tar composition during the middle stages of secondary pyrolysis. Similar to the ring number distribution of Dietz coal tars, Figure 4 shows that 4-ring PAC in Pittsburgh No. 8 coal tars replace 2-/3-ring PAC at around SF 50% as the dominant ring size in the total I-PAC. This is of special concern because among the PAC focused upon in this work, the 4- and 5-ring PAC tend to include the most toxic of aromatic ring compounds.3,4 In addition, this shows that, regardless of coal type, the toxicity of tar compounds may be enhanced around halfway through secondary pyrolysis, which is supported by the observation of Braun et al.2 Although the substantial increase in 5-ring PAC and the severe degradation of 2- and 3-ring PAC indicate that ring buildup and ring rupture occur more aggressively for Pittsburgh No. 8 coal tars than for Dietz coal tars, a transition in dominant ring size among the ring groups in I-PAC also appears halfway through secondary pyrolysis. In addition, it is noteworthy that the abrupt increase in II-PAC for Pittsburgh No. 8 coal tars also happens after SF reaches over 50%, similar to what was measured for Dietz coal tars. Taken together, these observations show that around this crucial pyrolysis stage (50% SF), the characteristics of tar ring number distribution undergo dramatic changes due to the exertion of both ring rupture and polymerization for both types of coal. Insights on how different mechanisms predominate during various thermal conditions and affect soot growth

456 Energy & Fuels, Vol. 12, No. 3, 1998

have been discussed by many researchers.8,37 Since it is reported that 10% of total soot yield is due to soot inception,38,39 the increase of soot yield as secondary pyrolysis becomes more severe should mainly result from subsequent growth of soot nuclei via addition of C2H2 and/or condensation of larger PAC compounds. Although no unified consensus has been achieved, under a condition rich in aromatic compounds, the competition between two mechanisms, ring buildup (polymerization) and ring rupture (thermal dissociation), is generally acknowledged; the dominance of either one leads to a different sooting pathway. On the one hand, ring opening accelerates the depletion of small PAC, which hinders the buildup of larger PAC via successive ring condensation. On the other hand, it is not surprising that the increase in C2H2 due to cleavage of rings encourages the growth of soot mass via direct addition. The enhanced participation of C2H2 in soot growth as secondary pyrolysis becomes more severe further supports the measurements of Chen and co-workers,22 who concluded that at least 20% of the added soot mass during secondary pyrolysis is due to carbon addition from the C1 to C3 species. Thus, although direct conversion of tar compounds into soot is the dominant pathway in the early stages of secondary pyrolysis,18,22 addition of C2H2 appears to be a more substantial pathway for growth of soot mass during the second half of secondary pyrolysis. Final Stages of Secondary Pyrolysis. At the latest stages of secondary pyrolysis, all ring groups decrease drastically for both the Dietz and Pittsburgh No. 8 coal tars. Other researchers6,7 also have observed that all the ring classes decay monotonically during the final stages of secondary pyrolysis. Although larger PAC might still be formed due to polymerization, their additional yield would be masked by the more vigorous depletion expected under such severe pyrolysis conditions. Thus, ring rupture appears to dominate the secondary reactions among tar compounds at the latest stages of secondary pyrolysis. While both ring rupture and direct conversion of tar compounds to soot via successive condensation of small PAC can result in a drastic decrease in the concentration of ring compounds, the dominant soot formation pathways during this late stage of pyrolysis cannot be clearly discerned. It should be noted that at the end of secondary pyrolysis, both types of coal tars contain almost the same composite proportion of 2- to 5-ring I-PAC, in addition to showing similar trends in ring number distributions throughout the whole range of secondary pyrolysis. This is consistent with earlier work18 showing that the polarity distribution for both coal tars by the very late stages of secondary pyrolysis tends to be almost identical. In addition, Wornat et al.17 concluded from pyrolyzing a standard compound that at the higher temperatures, the PAC product distribution is dictated by thermal conditions, rather than by the structure of the parent fuel, while Chen et al.22 reported that the behavior of noncondensables during coal pyrolysis con(36) Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Herod, A. A.; Stokes, B. J.; Kandiyoti, R. Fuel 1993, 72, 1381-1391. (37) Mar’yasin, I. L.; Nabutovskii, Z. A. Kinet. Catal. 1969, 10, 800806. (38) Toqan, M.; Farmayan, W. F.; Bee´r, J. M.; Howard, J. B.; Teare, J. D. Symp. (Int.) Combust. [Proc.], 20 1984, 1075-1081. (39) Lahaye, J. Carbon 1992, 30, 309-314.

Yu et al.

ditions similar to this work is insensitive to coal type. Therefore, by the end of secondary pyrolysis, the characteristics of organic aerosols depend more on pyrolysis conditions than on the structure of the raw coals. Conclusions In order to characterize organic emissions during secondary coal pyrolysis according to ring number distributions, both subbituminous and hv bituminous coal tars were analyzed via HPLC. Three classes of PAC in the toluene fraction were categorized. The trends of ring groups observed in the PAC classes reveal the orchestration among various mechanisms as well as the relative dominance of certain sooting pathways throughout secondary pyrolysis. At the early stages of secondary pyrolysis, the impact of neutralization is apparent as a mass transformation from more polar to more neutral PAC. In addition, the ring number distributions for both coal tars reflect the features of their parent coals. At this early stage, direct conversion of tar appears to be the major pathway for soot formation. As secondary pyrolysis proceeds, both polymerization and ring rupture become evident as significant transformation mechanisms. The SF value of 50% appears to be a critical point of transition for the dominant ring sizes, the prominence of ring opening and polymerization, and the predominant sooting pathways. PAC composed of four or more fused rings replace 2- and 3-ring PAC as the dominant ring groups. The ring rupture of 2- and 3-ring PAC may contribute to the generation of light hydrocarbons, the buildup of larger PAC, and the formation of soot through direct addition of C2H2. Near the end of secondary pyrolysis, both coal tars contain a similar composition of 2- to 5-ring PAC, indicating that the pyrolysis conditions rather than the original coal structures are ultimately primarily responsible for the characteristics of coal tar emissions. Acknowledgment. The authors appreciate the financial support of the U.S. Department of Energy under its University Coal Research program (grant no. DEFG22-91PC91284). This work has also been partially supported by National Science Foundation through its Presidential Young Investigator program (grant no. BCS-9157905). In addition, the sampling work done by Mr. John DaDamio is gratefully acknowledged. Registry No. (furnished by the author). 1,2,4Trimethylbenzene, 95-63-6; naphthalene, 91-20-3; fluorene, 86-73-7; pyrene, 129-00-0; benzo[ghi]perylene, 19124-2; coronene, 191-07-1; decacyclene, 191-48-0; 2-methylnaphthalene, 91-57-6; 9,10-diphenylanthracene, 1499-10-1; rubrene, 517-51-1; 9-fluorenone, 486-25-9; 1-cyanonaphthalene, 86-53-3; 1,5-dinitronaphthalene, 605-71-0; acenaphthene, 83-32-9; acenaphthylene, 20896-8; anthracene, 120-12-7; phenanthrene, 85-01-8; fluoranthene, 206-44-0; benz[a]anthracene, 56-55-3; benzo[a]pyrene, 50-32-8; benzo[b]fluoranthene, 205-992; benzo[k]fluoranthene, 207-08-9; indeno[1,2,3-cd]pyrene, 193-39-5; chrysene, 218-01-9. EF9700590