Energy & Fuels 1993,7, 357-361
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A Simple Multimode Size Exclusion Chromatographic Method for the Determination of the Degree of Thermal Treatment of Fossil Fuel Pyrolysis Products Arthur L. Lafleur,*Adel F. Sarofim, and Mary J. Wornat Center for Environmental Health Sciences, Department of Chemical Engineering, and Energy Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received September 9, 1992. Revised Manuscript Received February 5, 1993
Multimode size exclusion chromatography of complex mixtures from coal pyrolysis using a poly(divinylbenzene) column and dichloromethane mobile phase permitted the separation of the mixture into two fractions enriched in either substituted (2CPAC)or unsubstituted (HPAC) polycyclic aromatic compounds. The XPAC/HPAC ratio, a parameter easily obtained by this method, was found to be a useful indicator of the degree of thermal treatment of coal and other fossil fuels. The sensitivity of the technique allowed it to be applied to the characterization of microgram-level samples produced by small laboratory-scale pyrolysis furnaces.
Introduction The formation and transformation of polycyclicaromatic compounds under pyrolysis or during incomplete combustion of carbon-based materials have been the subject of many studies.l-9 Although much is understood about some areas of PAC formation such as their pyrosynthesis from simple precursors,10-12current knowledge in this field is still incomplete. Even less is known about the thermal reactions of PACs in polymeric fossil fuels such as coal or lignite. One reason why the study of thermal reactions of PACs in fossil fuels has been hampered is the difficulty in monitoring the formation or destruction of individual components in the complex mixtures resulting from the pyrolysis of fossil fuels. For coal, chemical analysis of the products has proven especially difficult because of the wide range of structures and functional groups present, especially in low-temperature tars. Even the simplest analyses often require tedious chromatographic separations and the use of complex instrumentation.13-16
* To whom correspondence should be addressed at Core Laboratory in Analytical Chemistry, Room 2OC-032,Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. (1)Collin, P.J.; Tyler, R. J.; Wilson, M. A. Fuel 1980,59,479-486. (2)Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980,59,819-820. (3)Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1.4.11-437. --- ~(4)Wornat, M. J.;Sarofim, A. F.; Longwell, J. P. Symp. Int. Combust., [Proc.], 22 1988,135-143. (5)Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuek 1988, 2,775-782. (6)Nelson, P. F.; Tyler, R. J. Symp. Int. Combust., [Proc.], 21 1986, 427-435. (7) Nelson, P. F.; Smith, J. W.; Tyler, R. J.; Mackie, J. C. Energy Fuek 1988,2,391-400. (8)Doolan, K. R.;Mackie, J. C.; Tyler, R. J. Fuel 1987,66,572-578. (9)Prado, G.; Garo, A.; KO,A.; Sarofim, A. F. Symp. Int. Combust., [Proc.], 20 1984,989-996. (10)Bittner, J. D.; Howard, J. B. In Particulate Carbon Formation During Combustion; Siegla, D. C., Smith, G. W., Eds., Plenum Press: New York, 1981;pp 109-142. (11)Lee, M. L.; Novotny, M. V.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981. (12)Grimmer, G. InEnuironmental Carcinogens: Polycyclic Aromatic Hydrocarbons; Grimmer, G., Ed.; CRC Press: Boca Raton, FL, 1983;pp 27-31. - I
Some analytical techniques have been used successfully to give useful bulk information without requiring extensive sample preparation. For example, nuclear magnetic resonance spectroscopy (NMR)in both the lH or 13Cmodes has been used to determine the relative amounts of aromatic and aliphatic H in coal t a r ~ . ~ p ~ JAlso, ' J ~ Fouriertransform infrared spectroscopy (FTIR) has proven useful for the determination of functional groups in fossil fuelsrelated complex m i x t u r e ~ . l ~ - ~ ~ In spite of these advances, however, the chemical characterization of complex fuel-derived samples remains laborious; and, with laboratory-scale pyrolyzers such as our drop tube furnace, the task becomes even more difficult because of the microgram amounts of products formed. A simple analytical method that provides data that is both characteristic of the sample as a whole and which reflects the extent of thermal treatment of the sample would be very useful for the monitoring of pyrolysis reactions. Some indication that such a method could be developed came from previous work on the size exclusion chromatography separation of mixtures of polycyclic aromatic compounds (PACs), species that make up the bulk of pyrolysis samples. In an SEC study of PAC retention on poly(diviny1benzene) (PDVB), we found that by proper choice of mobile (13)Bartle, K. D.; Collin, G.; Stadelhofer, J. W.; Zander, M. J.Chem. Tech. Biotechnol 1979,29,531-551. (14)Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Ed.; ACS Symposium Series No. 205,American Chemical Society: Washington, DC, 1982;pp 47-76. (15)Altgelt, K.H.;Gouw, T. H. Adu. Chromatogr. 1975,13,71-175. (16)Speight, J. G.The Chemistry and Technology of Coal; Marcel Dekker: New York, 1983. (17)Serio, M. A. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1984. (18)Fletcher,T. H.; Solum, M. S.;Grant, D. M.; Critchfield, S.;Pugmire, R. J. Symp. Int. Combust., [Proc.], 23, 1990,1231-1237. (19)Solomon, P. R.;Hamblen, D. G.; Carangelo, R. M. In Coal and Coal Products: Analytical Characterization Techniaues; Fuller, E. L., Ed.; ACS Symposium Series No. 205, American Chemical Society: Washington, DC, 1982;pp 77-131. (20)Wornat, M. J.; Nelson, P. F. Energy Fuels 1992,6,136-142. (21)Freihaut, J. D.; Seery, D. J. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1981,26(2), 133-148.
0887-062419312507-O357$Q4.oO/O 0 1993 American Chemical Society
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358 Energy & Fuels, Vol. 7,No. 3, 1993
phase, multimode separation can be obtained for mixtures of PAC whereby substituted PAC (XPAC) elute according to molecular size while unsubstituted PAC (HPAC) exhibit nonsize behavior and elute beyond the totally permeated volume.22 Thus, the use of multimode SEC with PDVB permitted the separation of complex mixtures of PAC into two fractions: an early-eluting one containing W A C and a later-eluting one containing HPAC. At the same time, it was known that basic fossil fuels materials contain primarily highly-alkylated aromatics,12*23 whereas work here and elsewhere showed that their ultimate pyrolysis products often comprise a set of simple, unsubstituted PAC.3J2 For example, in a previous study we found that, when pyrolyzed, coal initially forms a complex mixture of PAC with most ring hydrogens substituted by alkyl or functional groups. Further thermal treatment of the complex mixture causes additional chemical change and a reduction of the extent of substitution.3 Elsewhere, NMR was used to show that increasing pyrolysis severity by either temperature or time effects an increase in aromaticity and a decrease in the presence of functional groups.17 Also, by employing FTIR spectroscopy, Solomon and co-workers found that the reduction of IR absorption characteristic of functional group attachments to aromatic rings in coal tars is associated with increasing temperature.19 Therefore, it became feasible that MMSEC data could provide a simple method for the determination of the degree of pyrolysis or thermal treatment of a particular fossil fuel product without the use of complex methods of chemical analysis. In this study, we have investigated the use of MMSEC for the determination of extent of thermal treatment of coal, lignite, and wood pyrolyzed in a laboratory-scale drop-tube furnace. Experimental Section Apparatus. For our pyrolysis studies, samples of interest were ground and sieved for fluidized bed feeding into a laminarflow drop-tube furnace described e l ~ e w h e r e . 3 Introduced , ~ ~ ~ ~ ~ in argon, samples were immediately vaporized at selected temperatures ranging from 1000 to 2000 K. An optical pyrometer was used to measure furnace temperature which can be set to values ranging from lo00 to 2000 K by adjustment of the electricalpower input. Average gas residence time, which varied from 0.25 to 0.75 a, was controlled by adjusting the vertical position of the collection probe. Pyrolysis products were collected on a fluorocarbon filter and extracted with dichloromethane with the aid of ultrasonic agitation. Liquid Chromatography System. The high performance liquid chromatography (HPLC) system used for wideband-UV data acquisition consisted of a Varian Model 5060A ternary pumping system coupled to a Hewlett-Packard Model 8450A diode-array spectrophotometer through a quartz flow cell having a path length of 10 mm and a circular aperture of 2.0 mm. With a flow rate of 2.0 mL/min and a data acquisition interval of 2.09, the elution volume resolution was 0.067 mL. The flow rate was confirmed using a volumetric flask. We selected a wideband wavelength interval of 236-500 nm for these HPLC measurements because, in an earlier work, we found that mass-based response factors vary surprisingly little for a wide range of polycyclicaromatic compounds (PAC) as long (22)Lafleur, A. L.; Wornat, M. J. Anal. Chem. 1988,60, 1096-1102. (23)B l u e r , M. Sci. Am. 1976,234, 35-45. (24)Nenniger, R.D.Sc.D. Thesis, Massachusetts Institute of Technology, 1988. (25)Wornat, M. J.Sc.D. Thesis, Massachusetts InstituteofTechnology, 1988.
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Figure 1. Calibration plot for the separation of complexmixtures of polycyclic aromatic compounds into substituted [XPAC] and unsubstituted [HPAC] fractions. The vertical line at 751 s corresponds to the elution time for 4H-cyclopenta[deflphenanthrene, a reference compoundused tomark the boundary between substituted and unsubstituted polycyclic aromatic compounds. Data were obtained using a Jordi-Gel poly(diviny1benzene) column with a dichloromethane mobile phase. as the entire range of wavelengths over which the PAC absorb are monitored.% In this study, the selectionof a lower wavelength limit of 236 nm was dictated by the strong absorbance of our mobile phase (i.e., dichloromethane) below this wavelength. For the single-wavelength studies (260 nm) the HPLC instrument consisted of Perkin-Elmer Series-4quaternary solvent delivery system coupled to a Model LC-85Bvariable-wavelength detector with a 1.4-pLflow cell. The output signal was processed by a Perkin-Elmer Model 3600 data station running Chromatographics I1 software. Sample injection was performed using a Rheodyne injector with either a 6- or 100-pL loop. The Jordi-Gel 500 column used in this study was obtained from Jordi Associates,Bellingham, MA. It was lOmm in diameter and 50 cm long and was packed with 500-A Jordi-Gel poly(divinylbenzene)material. The mobile phase was 100%dichloromethane and the flow rate was 2.0 mL/min. The use of this column and mobile phase combination for the separation of polycyclic aromatic compound types has been reported elsewhere.22 Chemicals. The dichloromethane used for the mobile phase and for sample preparation was Caledon distilled-in-glassgrade obtained from American Bioanalytical, Natick, MA.
Results and Discussion As stated earlier, the analytical basis for this study stems from our observation that the use of a Jordi-Gel poly(divinylbenzene) (PDVB) column with a dichloromethane mobile phase permita the separation of substituted PAC (XPAC) from unsubstituted PAC (HPAC).22 This is shown schematically in Figure 1. Here we see the calibration plot for the separation of complex mixtures of PAC into substituted [XPAC] and unsubstituted [HPAC] PAC fractions. A Jordi-Gel 500 PDVB column with a dichloromethane mobile phase (flow rate = 2.0 mL/min.) was used to obtain these data. The vertical line at 761s correspondsto the elution time for 4H-cyclopenta[deflphenanthrene(CpPh). This molecule, which by virtue of ita bay-region methylene bridge is slightly nonplanar, was found to elute midway between substituted and unsubstituted PAH. Ita elution volume, therefore, is used here as an empirically-derived marker (26)Lafleur, A. L., Monchamp, P. A., P l u m e r , E. F., Wornat, M. J. A d . Lett. 1987,20 (a), 1171-1192.
Determination of the Degree of Thermal Treatment
identifyingthe boundary between XPAC and HPAC. The XPAC region is defined as the elution interval between 500 and 750 s while the HPAC region covers the 7511o00-s interval. For a given chromatogram, XPAC/HPAC values were obtained by summing the absorbance over the XPAC elution interval (500-750 s) and dividing this value by the one obtained for the HPAC region (751-1000 8) *
The data points and calibration line in the XPAC region were obtained for a series of compounds known to elute with a size-dependent mechanism with this column and mobile phase combination. These included polystyrene reference standards, phthalate esters, and nitroalkanes.22 The MW calibration data are included to help illustrate the different elution volume regions where either sizedependent or nonsize elution will occur for PDVB/CH2Cl2; however, these data should not be applied to the determination of MW information. It should be kept in mind that the method used in this work, although utilizing a size exclusion chromatography column, is not intended to provide MW information for fuels pyrolysis products. A number of other methods have been developed for this p ~ r p o s e . ~The ~ - ~gathering ~ of MW information from fossil fuels and their products by SEC is a very specialized technique that requires careful selection of columns and mobile phases. Moreover, fuels products have molecular configurations that differ greatly from those of common SEC calibration standards (e.g., polystyrenes) so special techniques must be utilized to calibrate SEC columns. For coal-derived complex mixtures, additional sample composition effects can also come into play so the relationship between elution volume and molecular size is often not straightforwardand satisfactory calibration curves can be difficult to construct.3c32 Figure 2 shows HPLC chromatograms of wood (sweet gum) pyrolysis products obtained at 1213,1323, and 1483 K. The small peak centered at 750 s corresponds to CpPh, the reference compound we used to mark the boundary between substituted and unsubstituted PAC. For this figure, as well as for Figures 3 and 4, individual traces were normalized to full scale and the vertical scale at the left gives the relative absorbance. The measured response is the total absorbance over the 236-500 nm wavelength interval. In this figure, it is seen that the chromatographic maximum moves from the XPAC region (400-750 s) to the HPAC region (750-1200 s) with increasing pyrolysis temperature. This shift from XPAC to HPAC results from a decrease in substituted PAC content that parallels the increased cleavage of PAC substituents with increasing temperature, as discussed earlier. Figure 3 shows three HPLC chromatograms of lignite pyrolysis products obtained at pyrolysis temperatures of 1060,1213, and 1483 K. Individual traces are normalized to full scale and the measured response is the total absorbanceover the 236-500nm wavelength interval. The smaller peak at 750 s is the CpPh XPAC/HPAC marker, (27) Edstrom, T.;Petro, B. A. J.Polym. Sci. Part C 1968,21,171-182. (28) Lewis, I. C.; Petro, B. A. J. Polym. Sci. 1976,14, 1975-1985. (29) Bartle, K. D.; Martin, T.G.;Williams, D. F. Fuel 1975,54,226235. (30) Ignaaiak, B. S.; Chakrabartty, S. K.; Berkowitz, N. Fuel 1978,57, 507-510. (31) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984,63,1556-1560. (32) Evans, N.; Haley, T. M.; Mulligan, M. J.; Thomas,K. M. Fuel 1985,65,694-703.
Energy & Fuels, Vol. 7, No. 3, 1993 369 1
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Figure 2. High performance liquid chromatogram of pyrolysis products from wood (sweet gum) at selected pyrolysis temperatures. Individual traces are normalized to full scale and the vertical axis gives relative absorbance. The measured response is the total absorbance over the 236-500 nm wavelength interval. The smaller peak at 751 s is 4H-cyclopenta[deflphenanthrene, an empirically-derived marker whose elution volume marks the boundary between XPAC and HPAC. - - -, 1213 K;-, 1323 K, --, 1483 K.
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Figure 3. High performance liquid chromatogram of pyrolysis products from lignite at selected pyrolysis temperatures. Individual traces are normalized to full scale and the vertical axis gives relative absorbance. The measured response is the total absorbance over the 236-500 nm wavelength interval. The smaller peak at 751 s is 4H-cyclopenta[deflphenanthrene,a reference compound used to mark the boundary between substituted and unsubstituted polycyclic aromatic compounds. ---, 1060 K; -, 1213 K,-, 1483 K.
as before. The chromatograms at 1213 and 1483 K for lignite are roughly comparable to those in Figure 2 for the wood pyrolysis products and show a broad distribution of structural types in both the XPAC and HPAC regions. Figure 4 shows three HPLC chromatograms of pyrolysis products of PSOC 997 bituminous coal at pyrolysis temperatures of 1223,1376, and 1473 K. Individual traces are normalized to fullscale and the measured response is the total absorbance over the 236-500 nm wavelength interval. The smaller peak at 750 s is the CpPh marker, as before. One striking feature of this set of data is the narrow elution range for both the low-temperature (1223 K)and high-temperature (1473 K)chromatograms, and, in contrast with the other fuel results, the low-temperature products elute nearly completelyin the XPAC region while
Lafleur et al.
360 Energy & Fuels, Vol. 7, No. 3, 1993
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Figure 4. High performance liquid chromatogram of pyrolysis products of PSOC 997 bituminous coal at selected pyrolysis temperatures. Individual traces are normalized to full scale and the vertical axis gives relative absorbance. The measured response is the total absorbance over the 236-500 nm wavelength interval. The smaller peak at 751 s is 4H-cyclopenta[deflphenanthrene,a referencecompound used to mark the boundary between substituted and unsubstituted polycyclic aromatic compds. - - -, 1223 K; -, 1376 K; ---, 1473 K. 20,
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the high-temperature products elute sharply and completely in the HPAC region. When XPAC/HPAC data for the different fuels were plotted as a function of temperature over the 1100-1500 K temperature interval, the plots in Figure 5 were obtained. The data for both wood and lignite correspond quite well and fall on the same linear regression curve. The data for bituminous coal also show a linear decrease in XPACI HPAC values with increasing temperature, but give a different slope and intercept. In a parallel set of experiments, we obtained XPAC/ HPAC data for a set of nine bituminous coal samples obtained for seven different temperatures over the 11001500 K range. In this experiment, a single wavelength was monitored (260nm) in contrast to the broad wavelength range (236-500nm) employed for the wood, lignite, and bituminous samples. The use of a single wavelength was tested in order to determine the possibly of further simplifying our method so that it could be used with the simplest of HPLC instruments. Figure 6 shows the results obtained in this experiment. The data obtained from 260-nm chromatograms are seen
Figure 6. Plot of XPAC/H-PAC for soot-associated PAC obtained from the pyrolysis of bituminous coal. XIH values, plotted as squares, were derived from single-wavelength (260 nm) HPLC chromatograms. to approximate those obtained with the wideband data and to produce similar linear regression curves. This finding is expected since both substituted and unsubstituted PAC are highly likely to be based on the same set of PAC parent compound^;^^^^ and, because the substituents are primarily alkyl groups which have a minimal effect on the absorptivity of a PAC chromophore, the molar absorbance of either species will be nearly the same. Although using a wavelength of 260 nm gave successful results for the bituminous samples, some preliminary testing should be done before selecting a single wavelength for other fossil fuels samples, especially wood, lignite or low-rank coals, because of the possibility that their pyrolysis products might be more abundant in lower ringnumber species that do not absorb appreciably at 260 nm. The data in Figure 6 could be fitted to the following regression line
X/H= 7.607 - 5.12 X 10-3T r = 0.981 (1) where the parameter XIH is the ratio of substituted (XPAC) to unsubstituted PAC (HPAC) and T is the absolute temperature in degrees kelvin. The correlation coefficient ( r )of 0.981 indicates a good fit. A third-order regression calculation gave a better fit to the data, as seen below: XIH = -235 + 0.56T - 4.4 X
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1.12 X lO-'P r = 0.997 (2) Although the third-order fit is good, as indicated by the value of 0.997for r, it is expected that, if the temperature range could be expanded, the XPAC/HPAC curve would actually reach asymptotic limits at both ends of the temperature range. The reason for this hypothesis rests on the fact that, at lower temperatures (e.g., 300-1000 K), the bulk of the PAC are in the form of substituted species, and an increase in temperature over the low-temperature range results primarily in the reduction in size of the larger alkyl substituents into smaller ones with the result that the XPAC/HPAC parameter would remain constant at its maximum value.3J2J7 The presence of small amounts of unsubstituted PAC and certain other components eluting in the HPAC region combinedwith integrator offset errors and HPLC solvent absorption prevents the upper asymptotic limit from reaching large values. A t higher temperatures, (e.g., >1400 K)the bulk of the substituted PAC have been converted to unsubstituted
Determination of the Degree of Thermal Treatment
PAC and an increase in temperature in this regime leads not to further XPAC/HPAC conversion but only to the reduction in ring number of unsubstituted speciesalready present.3J2J7 Thus, a minimum value for XPAC/HPAC is reached and the instrumental factors operational in limiting the magnitude of the upper asymptotic limit also prevent the lower limit from reaching zero. Because our research goal has been to shed light on the formation of unsubstituted PAC, we have not investigated pyrolysis conditions that give other products ae endpoints. It is known that the pyrolysis of some organic substances under milder conditionsthan we employ give methylarenes as final products,ll whereas in our work, methylarenes would be considered intermediates. For these lowtemperature pyrolysis studies, our method has the potential to provide data on thermal treatment comparable to that presented here, but additionalwork may be required to develop a suitable method. We have demonstrated in previously reported work22that monomethylated arenes can be separated from larger alkylated arenes by our MMSEC method; but if the products from low-temperature pyrolysis are polymethylated species, our simple HPLC arrangement might not provide adequate separa-
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tion between starting materials and products. For these m e a , it may be necessary to employ higher-resolution column sets.
Summary and Conclueions 1. Multimode SEC of coal pyrolysis products with PDVB + CHzClz permits the separation of the mixture into two fractions enriched in either substituted (XPAC) or unsubstituted (HPAC) PAC. 2. The XPAC/H.PAC ratio, a parameter easily obtained by this method, was found to be a useful indicator of the degree of thermal treatment of coal and other fossil fuels. 3. The senaitivityof the technique allowsit to be applied to the characterization of microgram-level samples produced by small laboratory-scale pyrolyzers.
Acknowledgment. This investigation was supported by National Institute of Environmental Health Sciences Center Grant NIH-SP30-ES02109-13and Health Effects of Fossil Fuela Combustion Program Grant NIH-6PO1ES01640-14.
