Energy & Fuels 1988,2,775-782
775
Effect of Pyrolysis Conditions on the Composition of Nitrogen-Containing Polycyclic Aromatic Compounds from a Bituminous Coal Mary J. Wornat, Adel F. Sarofim,* John P. Longwell, and Arthur L. Lafleur Department of Chemical Engineering and Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 23, 1988. Revised Manuscript Received August 12, 1988 We have used capillary column gas chromatography with nitrogen-specific chemiluminescence detection to monitor pyrolysis-induced changes in the ring number composition of nitrogen-containing polycyclic aromatic compounds (PACN) from a bituminous coal. Examining PACN from pyrolysis at 1100-1500 K and at average gas residence times of 0.25-0.75 s, we find that the ring number composition of the PACN correlates well with the total polycyclic aromatic compound (PAC) yield (or conversion) and depends on temperature or time only to the degree that these variables determine conversion. The mass distribution and rate of decay of the PACN follow the order: five ring > six ring > four ring > three ring > two ring. Comparison of (1) PACN to nonpolar PAC mass ratios for each ring number and (2) coal nitrogen and carbon distributions in the PAC and soot indicates that PAC conversion reactions leading to ring buildup, ring: rupture, and soot formation are accelerated (i.e., they occur faster) for the PACN relative to the nonpolar PAC. These observations are consistent with relative rates of pyrolysis of individual compounds and estimates of bond dissociation energies found in the literature. Elemental analysis shows that the N:C ratio in the soot drops with increasing PAC conversion. A fraction of this drop is due to liberation of soot nitrogen in gaseous form. The major factor, however, proves to be soot growth by conversion of PAC with successively lower nitrogen content. Our data do not permit us to determine what fraction of the converting PACN yields gaseous nitrogen directly and what fraction is f i s t incorporated into the soot. However, since the soot liberates nitrogen in gaseous form, the net effect of the conversion of the nitrogen in PAC is the production of gaseous nitrogen species, most probably HCN.
Introduction When coal is pyrolyzed, many of its constituent aromatic clusters are released as polycyclic aromatic compounds (PAC). The nitrogen present in coal-comprising about 1.4 mass % of the bituminous coal of this study-exists mainly as heteroatoms within aromatic rings.'+ Upon coal devolatilization, this nitrogen can either be retained in the char or released as aromatic vapors-the partitioning between the two depending on coal type, pyrolysis temperature, and residence time.If further exposed to pyrolytic conditions, the nitrogen-containing PAC (PACN) may decompose, losing their nitrogen in the form of HCN'9*l2 or other cyano c o m p ~ u n d s . ~ JPACN ~ composition can then be expected to vary with pyrolysis conditions. It is the goal of this study to examine the effects of pyrolysis temperature and residence time on the composition of (1)Freihaut, J. D.; Zabielski, M. F.; Seery,D. J. Symp. (Znt.)Combust. [Proc.] 1982,19th,1159-1167. (2)Whitehurst, D. D. In Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series 71;American Chemical Society: Washington, DC, 1978; Chapter 1. (3)Attar, A.; Hendrickson, G. G. Coal Structure; Meyers, R. A,, Ed.; Academic: New York, 1982;Chapter 5. (4)Pohl, J. H. Sc.D. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, 1976. (5)Solomon, P. R.; Colket, M. B. Fuel 1978,57,749-755. (6)Freihaut, J. D.; Seery, D. J. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1981,26(3),18-34. (7)Song, Y. H.;Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1982,28,177-183. (8)Pohl, J. H.;Sarofim, A. F. Symp. (Znt.) Combust. [Proc.] 1976, 16th, 491-501. (9)Homer, T. J.; Hull, M.; Alway, R. M.; Biftu, T. Int. J. Chem. Kinet. 1980,12,569-574. (10)Davies, R. A.; Scully, D. B. Combust. Flame 1966,10,165-170. (11)Axworthy, A. E.; Dayan, V. H.; Martin, G. B. Fuel 1978,57,29-35. (12)Blair, D.W.; Wendt, J. 0. L.; Bartok, W. Symp. (Int.)Combust. [ P ~ o c .1976,16th, ] 475-489. (13)Kausch, W. J., Jr.; Clampitt, C. M.; Prado, G.; Hites, R. A,; Howard, J. B. Symp. (Int.)Combust. [Proc.] 1981, 18th, 1097-1104.
0887-0624/88/2502-0775$01.50/0
PACN from a high-volatile bituminous coal. Coal-derived PACN are of particular concern since, according to the literature, PACN from a variety of coal conversion processes exhibit significant levels of carcinogenicity14or mutagenicity.15-21 There have been several attempts to correlate mutagenicity with the various types of PACN: tertiary (sp2-hybridized)N-heterocycles, like pyridine; secondary (sp3-hybridized) N-heterocycles, like pyrrole; aromatics with amino, cyano, or nitro funct,ionalities. Tertiary PACN can exhibit appreciable levels of mutagenic a ~ t i v i t y , " J ~but ~ ~amino~ , ~ ~ and nitro-substituted PAC appear to be the most mutagenic PACN in fossil fuel p r o d u ~ t s . ' " ~ ~ In ~ ~PACN ~ - ~ ~with either ring (14)Garner, R. C.;Martin, C. N.; Clayson, D. B. In Chemical Carcinogens, 2nd ed.; Searle, C. E., Ed.; ACS Monograph 182; American Chemical Society: Washington, DC, 1984;Vol. 1, Chapter 4. (15)Ho, C.-H.; Clark, B. R.; Guerin, M. R.; Ma, C. Y.; Rao, T. K. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1979,24(1),281-291. (16)Wilson, B. W.; Pelroy, R.; Cresto, J. T. Mutat. Res. 1980, 79, 193-202. (17)Buchanan, M. V.; Ho, C.-H.; Guerin, M. R.; Clark, B. R. In Chemical Analysis and Biological Fate: Polynuclear Aromatic Hydrocarbom; Cooke, M., Dennis, A. J., Ed.; Battelle: Columbus, OH, 1981; pp 133-144. (18)Haugen, D. A.; Peak, M. J.; Suhrbler, K. M.; Stamoudis, V. C. Anal. Chem; 1982,54,32-37. (19)Later, D. W.; Lee, M. L.; Pelroy, R. A.; Wilson, B. W. In Polynuclear Aromatic Hydrocarbons: Physical and Biological Chemistry; Cooke. M.. Dennis, A. J., Fisher, G. L., Ed.; Battelle: Columbus, OH, 1982;pp 427-438. (20) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Anal. Chem. 1981,53,1612-1620. (21)Nishioka, M.; Smith, P. A.; Booth, G. M.; Lee, M. L.; Kudo, H.; Muchiri, D. R.; Castle, R. N.; Klemm, L. H. Prepn. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1986,30(4),93-98. (22)Dong, M.; Schmeltz, I.; LaVoie, E.; Hoffmann, D. In Carcinogenesis-A Comprehensiue Suruey. Volume 3. Polynuclear Aromatic Hydrocarbons; Jones, P. W., Freudenthal, R. I., Ed.; Raven: New York, 1978; pp 97-108. (23)Ho, C.-H.; Clark, B. R.; Guerin, M. R.; Barkenbus, B. D.; Rao, T. K.; Epler, J. L. Mutat. Res. 1981,85,335-345.
0 1988 American Chemical Society
776 Energy & Fuels, Vol. 2, No. 6, 1988 N or N-containing substituents, one of the biggest factors in determining mutagenic activity appears to be the number of aromatic rings.1"20*23p26Though primarily compounds of only up to five rings have been considered, the literature indicates that four- and five-ring compounds generally have higher mutagenicity than their lower ring number c o ~ n t e r p a r t s . ~ ~ - ~ ~ ~ ~ ~ ~ ~ ~ We have used high-pressure liquid chromatography (HPLC) to determine ring number composition of the nonpolar constituents of our coal-derived samples,26but PACN are polar compounds for which such an HPLC technique2' does not apply. PACN analysis is further complicated by the fact that for a given number of rings, there are many more isomers of PACN than of PAH (PAC containing only C and H). For example, there are 12 isomers of five-ring PAH having MW 278; there are 117 isomers of five-ring PACN having MW 279 with one ring N atom.2s This multiplicity of structural arrangements plus the low percentage of N in the source fuels lead to small concentrations of a large number of PACN species in coal products. In order to detect the small quantities of PACN present, analysis of PACN in fossil fuel mixtures often requires prior isolation of the PACN from other PAC by f r a ~ t i o n a t i o n . ~ " " f land/or ~ a sensitive detector that responds only to nitrogen compounds. Following the first option, Novotny et a1.33*34 have employed microcolumn liquid chromatography to analyze PACN of up to 10 rings in fossil fuel samples. Pursuing the latter option, Burchill et dm and Kausch et al.13 have used gas chromatography with nitrogen-specific detection to analyze respectively a low-temperaturecoal tar and products of a pyridine-doped benzene flame. Several researchers3a1@* report that the most abundant N-containing constituents in low-temperature coal tar are tertiary PACN, many of which contain alkyl substituents. None of these works, however, examines the question of how PACN composition varies with pyrolysis temperature and time-the question we wish to address here. In view of the influence of ring number on mutagenicity, we focus our attention on composition with respect to ring number. We employ capillary column gas chromatography (GC) with nitrogen-specific chemiluminescence detection.
Experimental Equipment and Procedures T o produce the PAC of this study, 44-53-pm parbicles of PSOC 997, a high-volatile bituminous coal, are pyrolyzed in a laminar flow, drop-tube furnace described elsewhere.%s3' Temperatures vary from 1100 to 1500 K; average gas residence times vary from 0.25 t o 0.75 s. As the pyrolysis products exit the reaction zone, they are quenched by a stream of argon gas, causing the heavier (24) Nishioka, M.; Smith, P. A.; Booth, G. M.; Lee, M. L.; Kudo, H.; Muchiri, D. R.; Castle, R. N.; Klemm, L. H. Fuel 1986, 65, 711-714. (25) Later, D. W.; Wright, B. W. J. Chromatogr. 1984,289,183-193. (26) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Symp. (Int.)Combust. [ R o c . ] 1988, 2 2 4 in press. (27) Lafleur, A. L.; Monchamp, P. A.; Plummer, E. F.; Chang, N. T.; Wornat, M. J. J. Chromatogr. Sci. 1988,26, 337-344. (28) Lee, M. L.; Wright, B. W. J.Chromatogr. Sci. 1980,18,345-358. (29) Burchill, P.; Herod, A. A.; Pritchard, E. Fuel 1983, 62, 20-29. (30) Later, D. W.; Andros, T. G.; Lee, M. L. Anal. Chem. 1983, 55, 2126-2132. (31) Ruckmick S. C.; Hurtubise, R. J. J. Chromatogr. 1986, 321, 343-352. (32) Ostman, C. E.; Colmsjo, A. L. Fuel 1988, 67, 396-400. (33) Novotny, M.; Konishi, M.; Hirose, A.; Gluckman, J.; Wiesler, D. Fuel 1986, 64, 523-527. (34) Borra, C.; Wiesler, D.; Novotny, M. Anal. Chem. 1987, 59, 339-343. (35) Karr, C., Jr.; Chang, T.-C. L. J. Inst. Fuel 1958, 31, 522-527. (36)Nenniger, R. D. Sc.D. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, 1986. (37) Wornat, M. J.; Sarofi, A. F.; Longwell, J. P. Energy Fuels 1987, 1,431-437.
Wornat et al. Table I. Retention Behavior of Standards retn time: mol w t ring no. min compd quinoxaline 130 2 6.82 0.27 2,6-dimethylquinoline 157 2 10.93 phenanthridine 179 3 18.87 carbazole 167 3 19.04 9-ethylcarbazole 195 3 19.28 i 0.45 2-phenylindole 193 3 22.45 0.32 25.94 0.33 9-phenyl carbazole 243 4 l3i-dibenzo[a,i]carbazole 267 5 35.22 benzoacenaphthoquinoxaline 304 6 37.95 f 0.31
*
* *
Standard deviations given for standards tested a t least seven times. gases (two or more rings) to condense onto the surfaces of the soot and char. T h e products then enter an impactor for size separation of the solids. The larger char particles deposit on the first stages; soot ends up on the lowest impactor stages and the Millipore Teflon filter (hole size, 0.2 pm) following the impactor. Details of the product yield measurement and PAC extraction procedures have been previously p ~ b l i s h e d . ~After ' extraction, the char and soot are collected and reserved for elemental analysis. The coal, char, and soot are then subjected to simultaneous carbon and nitrogen analyses, performed by Huffman Laboratories of Golden, CO. The ring number compositions of the nitrogen-containing components of the coal-derived PAC samples are obtained by GC on a Perkin-Elmer Model 990 gas chromatograph, equipped with a Reatek Type RTX-5 methyl phenyl (5%) silicone ( f hthickness, 1.5 pm) capillary column (15 m X 0.53 mm i.d.). Sample volumes of 5-10 pL are introduced into the on-column injector, maintained at 240 "C. The column temperature is programmed from 60 t o 300 "C at 6 OC/min and held at 300 "C for 16 min. The detection system is comprised of a Thermo Electron Model 610 TEA Analyzer. Passing through a 300 "C interface, the effluent from the column enters a heated tube, where the PAC are oxidized a t 850 "C. A platinum wire inside the tube ensures that the nitrogen in the PAC samples is converted to NO, which is then oxidized by ozone to form NOz*. The chemiluminescence of this species is detected by a photomultiplier tube, and the signal is transferred t o a recorder. Before calibrating the GC, it is necessary t o determine the prevalent form of the nitrogen in the PACN. The possibilities are nitroaromatics, aminoaromatics, cyanoaromatics, and ring nitrogen compounds (both secondary and tertiary). Fourier transform infrared spectroscopy (FTIR) analyses, performed previously on our samples:' allow us to eliminate some of the possibilities. There is no indication of the presence of nitro functionalities in our samples since the FTIR spectra show no peaks at wavenumbers of 1510-1560 cm-', characteristic of the asymetric NOz stretch in nitro compound^.^^ Our lowest temperature sample (1125 K)shows a large signal between 3200 and 3450 cm-', the characteristic frequency of both N H stretch in amino groups and OH stretch in hydroxyl groups,% but this signal is most likely due t o hydroxyl groups since the same spectrum exhibits a large signal a t the frequency characteristic of CO stretch. In any case, the 3200-3450-cm-' signal is greatly attenuated in the 1223 K pyrolysis sample and does not exist in samples made at higher temperatures. Amino-substituted compounds, thus, do not appear to account for a significant portion of our PACN. Cyano compounds may be present in small amounts in our samples since at temperatures 11223 K, the samples show very small peaks at 2220 cm-', which can be attributed to aromatic nitriles.% Although our samples show hugh peaks a t 1590-1650 cm-' for six-membered ring nitrogen heterocyclic aromatics% and peaks between 700 and 800 cm-' for five-membered ring nitrogen heterocycles,% we cannot use this evidence as proof of the presence of aromatic ring nitrogen compounds since they cannot be separated from the signals characteristic of aromatic CH in-plane and out-of-plane deformation, respectively, of the nonheterocyclic aromatics in our samples. Since analyses on low-temperature coal (38) Pouchert, C. J. The Aldn'ch Library of Infrared Spectra, Edition IZI. Aldrich Chemical Co.: Milwaukee, WI, 1981.
Energy & Fuels, Vol. 2, No. 6, 1988 777
Effect of Pyrolysis Conditions 0.301
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-~~ ~
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21.2 29.2 37.9
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W$la* have shown the major nitrogen-containingconstituents to be those with ring nitrogen, and since the latter are among the more thermally stable PACN?' we choose ring nitrogen PACN to calibrate our GC for our high-temperature coal pyrolysis samples. Since the cyano group is not a very large functional group, any errors in using a ring nitrogen calibration for aromatic nitriles should not be very large. Representing a variety of structures and molecular weights (130-304 g/mol), the nine PACN standards used for calibrating the GC are listed, along with their retention times, in Table I. The retention times of these standards as well as the sample chromatograms themselveslead to the ring number/retention time relationships in Table 11. The nature of the means of detection is such that the response is directly proportional to the number of moles of nitrogen. Determination of the response factor (area/mol of N) for the nitrogen detector comes from the results, in Table 111, for five standard solutions, each prepared at 0.036 mg/mL. For comparisons on a mass basis, the response factor of 1.50 X 10l6area units/mol of N is divided by the average M W of each ring number compound (and multiplied by the number of moles of N per mole of compound) to obtain the response factors in terms of mass for each ring number.
Results and Discussion Assuming an average MW of each ring number PACN and one N atom per PACN molecule, we have used the results of the above GC technique to calculate the mass yield of coal going to each PACN ring number group, as depicted in Figure 1. Parts a and b correspond respectively to experiment set 1 (variable temperathe and constant average gas residence time of 0.75 s) and set 2 (variable time and constant temperature of 1375 K). As a rise in pyrolysis shown earlier for total PAC temperature or residence time brings about a fall in PACN yield. The PACN mass distribution and rate of decay follow the order five ring > six ring > four ring > three ring > two ring for all conditions observed, but the six-ring
Table 111. Response Factors of Standards moles of (area/mol mol inj X N inj X of N inj) X
compd quinoxaline 9-ethylcarbazole
2-phenylindole 9-phenyl carbazole benzoacenaphthoquinoxaline
mean f std dev
109 1.73 1.15 1.16 0.817 0.740
10s 3.46 1.15 1.16 0.817 1.48
10-18 1.48 1.64 1.52 1.51 1.37
1.50 k 0.10
PACN may be slightly underestimated since some six-ring PACN may be too heavy to be volatilized in the GC. Previous work shows that total PAC composition with respect to functional group substitution3' and nonpolar PAC composition with respect to ring number26 are functions solely of PAC yield (or conversion) and depend on temperature or time only to the extent that these variables determine conversion. Figure 2, a combination of the data from the set 1and set 2 experiments, demonstrates that PACN composition with respect to ring number adheres to the same behavior. Within the range of pyrolysis conditions examined, a certain degree of total PAC conversion corresponds to a certain PACN ring number composition-regardless of the time/temperature pathway leading to that conversion. As deduced before for the other PAC conversion reaction^,^^^^^ a narrow distribution of activation energies is suggested for the reactions governing the ring size distributions of the PACN. It is interesting to note the contrast in ordering between the PACN (Figure 2) and the nonpolar PAC (Figure 3)the latter results coming from HPLC.= At low conversions, two- and three-ring species comprise significantly larger portions of the nonpolar PAC than they do of the PACN. The very gradual decays of the two- and three-ring PACN resemble the behavior of the two- and three-ring nonpolar PAC a t higher conversions. This observation, along with Hayatsu et a l . ' listing ~ ~ ~ of one- to three-ring structures as nitrogen-containing moieties in coal, suggests that at PAC conversions less than we measure, the lower ring (39) Hayatsu, R.;Scott, R. G.;Moore,L. P.; Studier, M. H. Nature '(London)1976,257,378-380.
Wornat et al.
778 Energy & Fuels, Vol. 2, No. 6,1988
compd
Table IV. Kinetic Parameters of Model Compound Pyrolysisa E,, kcal/mol A, s-l
PAH benzene
naphthalene phenanthrene secondary PACN pyrrole
indole
carbazole tertiary PACN pyridine quinoline Reference 40.
ln
kllooKb
112 107 108
5.48 x 1019 6.32 X 10l8 4.55 x 1018
-5.792 -5.664 -6.450
74.3 56.9 33.7
7.20 X 10ls ' 1.49 X 1OO 2.60 X lo6
-2.086 -2.608 -2.950
73.9 77.9
7.32 X 10l2 1.59 x 1013
-4.189 -5.243
k was measured in s-'.
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Figure 2. Variation of PACN ring number composition with total PAC yield open symbols, set 1 experiments (0.75 s); filled symbols, set 2 experiments (1375 K); half-filled symbols, both 0.75 s and 1375 K. number PACN comprise larger fractions of the PACN. It would appear that significant changes in the PACN composition occur even under relatively mild conditions. A plot of each ring number's ratio of PACN to nonpolar PAC mass appears in Figure 4. For a given level of PAC yield, the PACN:PAC mass ratio is higher for the larger ring number species. Since, at least for PAC within the gas chromatographable range, the mass distribution of the PACN is shifted to higher ring numbers than that of the nonpolar PAC, we conclude that the PAC ring buildup reactions are accelerated for the PACN relative to the nonpolar PAC, which consist primarily of PAH. The fact that PACN appear to be more reactive compared to PAH is not too surprising, in view of the recent model compound pyrolysis experiments of Bruinsma et al.40Table IV presents the first-order kinetic parameters they extract from their data on PAH and PACN pyrolyzed at 1000-1200 K for 5 s. We see that the tertiary PACN are indeed substantially more reactive than the PAH, and the secondary PACN are much more reactive than both other groups. The activation energy for the benzene reaction, 112 kcal/mol, is so close to the bond dissociation energy of Ph-H (110 kcal/mo141) that Bruinsma et al.
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(40) Bruinsma, A. S. L.; Tromp, P. J. J.; de Sauvage Nolting, H. J. J.; Moulijn, J. A. Fuel 1988, 67,334-340.
Figure 4. Effect of pyrolysis severity on the ratio of PACN to nonpolar PAC.
Energy & Fuels, Vol. 2, No. 6,1988 779
Effect of Pyrolysis Conditions
I
0
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'
20 30 40 R e t e n t i o n Time (mini
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Figure 6. Evolution of fine structure in the PACN chromatograms: pyrolysis temperature, 1375 K; residence times, (a) 0.25, (b) 0.5, and (c) 0.75 8.
conclude that the rate-determining step in their pyrolysis reactions is abstraction of an aryl hydrogen atom. It is this reason, they assert, that the activation energies are considerably lower for the secondary PACN: abstraction of the H from the N-H of the pyrrole series requires less energy than abstraction from an aryl C-H bond. Bruinsma et al.4 propose that subsequent to the aryl radical formation is the formation of the biaryl aryl species-much in keeping with the PAH ring buildup mechanism of which exhibits consistency with our nonpolar PAC data.26 Bruinsma et al.'s proposal is certainly reasonble for the tertiary PACN since pyrolysis of pyridine at temperatures of 600 to 850 "C has been to yield H2 and the bipyridyls-primarily the 2,2' isomer. Ruhemannu and Houser et al.g also point out, however, that, a t higher temperatures (900 "C), ring scission becomes prevalent, resulting in HCN production. Returning to Figure 4, we see that the two-ring PACN, unlike any of the other PACN, exhibit a small rise in proportion to their non-nitrogen-containing analogues as conversion proceeds. This observation is consistent with the results of vapor-phase model compound experiments by Johns, et al.,48who report quinoline to be slightly more thermally stable than naphthalene. Additional information can be derived from the samples' chromatograms themselves, some of which are pictured in Figure 5. As pyrolysis severity is increased by a rise in either pyrolysis temperature or time, the chromatograms become less complex and divided into more pronounced peaks-each clustered around a retention time corre(41) Morrison, R. T.;Boyd, R. N. Organic Chemistry, 4th ed.; Allyn and Bacon: Boaton, MA, 1983. (42) Badger, G. M. In Rogress in Physical Organic Chemistry; Cohen, 5.G., Stxeitwieaer, A., Jr., Taft, R. W., Ed.;Interscience: New York, 1965; Vol. 3, Chapter 1. (43) Knunholz, P. Sel. Chim. 1949,8, 1-15. (44) Ruhemann, S. Braunkohle (Duesseldorf) 1929,28, 749-756. (46) Meyer, H.; Hofmann-Meyer, A. J. Prakt. Chem. 1921, 202, 281-294. (46)Johns, I. B.; McElhill, E. A.; Smith, J. 0. J. Chem. Eng. Data 1962, 7, 277-281.
sponding to an unsubstituted tertiary PACN isomer group. In view of Hurd and Simon's47finding that methyl pyridines show a considerably lower thermal stability than pyridine under pyrolysis conditions, it is reasonable to speculate that the emergence of fine structure in the PACN chromatograms signifies a loss of primarily alkylated PACN and perhaps less stable unsubstituted PACN, as seen earlier by HPLC and GC-MS for the nonpolar PAC.26*37 A plentitude of peaks exists even at the highest levels of conversion because of the huge number of isomers of PACN of a given ring number-especially the larger ring numbers, which show less development of the pronounced peaks and valleys. A major conclusion of our previous ~ o r k ,based ~ ~ on * ~ ~ the constancy of summed PAC and soot yields, is that PAC, as a whole, convert directly to soot. The question remains: as PACN are destroyed, what happens to the nitrogen? If it is retained in the aromatic ring structures, then one would expect it to be incorporated in the soot, and the sum of nitrogen in the PACN and soot would remain constant. If nitrogen is liberated from the rings, however, then that sum would decrease as conversion of PAC increases. Since the GC response is directly proportional to the number of moles of nitrogen, we can calculate the percentage of the coal nitrogen in each PACN ring number group without having to assume an average molecular weight of each ring number group or a number of N atoms per molecule. Particularly illustrative for our purposes here is the percentage of coal nitrogen going to all the PACN, which, along with that going to soot (obtained by elemental analysis), is plotted in Figure 6a. As a contrast, Figure 6b portrays the percentages of coal carbon going to PAC and soot. The first thing to notice is that at low PAC conversions, the percentage of coal N in the soot is higher than the percentage of coal C in the soot. This observation indicates that, just as the ring buildup reactions appear to be accelerated for the PACN relative to the nonpolar PAC, so do the reactions involving PAC conversion to soot-at least at very low PAC conversions. Also apparent from Figure 6 is that a smaller fraction of coal N goes to summed soot and PAC than of coal C. An increase in pyrolysis severity enhances this distinction. As Figure 6 depicts, although a loss of carbon from the PAC is compensated by a gain of carbon in the soot, a loss of nitrogen in the PACN is not offset by a gain of nitrogen in the soot. In fact, as PAC conversion proceeds, there is a decline in the fraction of coal nitrogen going to either PACN or soot. These observations indicate that as pyrolysis conditions become more severe, rupture of the nitrogen-containing rings in PACN and/or soot becomes more prevalent. Since PAC and soot carbon does not adhere to this same behavior, we infer that the C-N bonds within the coal's aromatic rings are.not as strong as the C-C bonds. This deduction is consistent with Sanderson's48calculations of the bond dissociation energies for C-C, C-N (tertiary PACN), and C-N (secondary PACN) bonds within aromatic rings: 122.3, 112.4, and 73.7 kcal/mol, respectively. The coal nitrogen that does not go to the PACN or soot must either remain in the char or be liberated as gaseous We have not measured nitrospecies such as HCN.1*9-12 gen-containing product gases, but yields and elemental analyses on our char samples permit us to determine the fraction of coal nitrogen that is retained in the char. Figure (47) Hurd, C. D.; Simon, J. I. J. Am. Chem. SOC.1962,84,4519-4524. (48) Sanderson, R. T. Chemical Bonds in Organic Compounds; Sun and Sand: Scottsdale, AZ, 1976; Chapter 8.
Wornat et al.
780 Energy & Fuels, Vol. 2, No. 6,1988 25 a
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Mass P e r c e n t o f C o a l as PAC
Figure 7. Distribution of coal nitrogen among the pyrolysis products. 7 depicts the coal nitrogen distributed among the various pyrolysis products. That going to gases is calculated by difference and also includes the nitrogen in PACN too large to be analyzed by gas chromatography. (This latter quantity is probably very small since, on the basis of our HPLC work on nonpolar PAC,26we estimate that -30% of the total PACN would not be vaporizable and since the maximum percentage of coal nitrogen found in the gas chromatographable PACN is only 3.3%.) As Figure 7 portrays, the percentage of coal nitrogen in the char shows an abrupt initial decline, due primarily to a drop in char yield, and then a leveling off with further PAC conversion. The plateau value of -40% corresponds approximately to the percentage of the coal as char in this regime, but it would be expected to fall below the char yields with a further increase in pyrolysis temperature.8 Figure 7 also demonstrates the rising level of coal nitrogen going to gases
and the dwindling roles of the PAC and soot as receptors of coal nitrogen as pyrolysis conditions become increasingly more severe. A similar plot for another Appalachian high-volatile bituminous coal can be found in the work of Freihaut et al.,l who conduct their experiments on a heated grid, for which gas-phase secondary pyrolytic reactions are minimized. Consequently, their coal nitrogen is apportioned among the char, gases (HCN), and tar. It is interesting to note that like us, they observe a maximum of -60% of the coal nitrogen in the char under their least severe pyrolysis conditions and a maximum of -40% of coal nitrogen in the gases under their most severe pyrolysis conditions. However, throughout their pyrolysis experiments, they observe much higher proportions (by a factor of 2) of coal nitrogen as tar than do we as summed PACN and soot. Freihaut et al.’ also report that, for the three types of coal they consider, the fraction of coal nitrogen in the tar is the same as the fraction of coal that yields tar-leading them to conclude that nitrogen serves as a good tracer for coal’s primary pyrolysis products. Since our ratio of nitrogen in the PACN and soot to that in the coal falls from 21 to 11% while our ratio of PAC and soot to coal stays constant at 21%,37we find that we cannot extend Freihaut et ale’sconclusion into the regime where secondary pyrolysis reactions become significant. Because soot and char yields change with pyrolysis conditions, Figure 7 does not permit us to see how the nitrogen contents of the soot and char change relative to the carbon contents as pyrolysis proceeds. However, Figure 8a, a plot of the results of the elemental analyses, shows how the nitrogen to carbon ratios in the soot and char vary with pyrolysis severity. We see that as pyrolysis conditions become more severe (PAC yield decreases), the N:C ratio in the char decreases-consistent with previous experimental resultss~6~8 that show an increase in the liberation of HCN from char as either temperature or residence time is raised. The situation of the soot is more complicated. Its nitrogen to carbon ratio can change not only from liberation of HCN but also from the production of soot from PAC of successively lower nitrogen contents. An attempt to separate these two effects follows: Having experimentally determined the amounts and compositions of the PAC that
Energy & Fuels, Vol. 2, No. 6, 1988 781
Effect of Pyrolysis Conditions L
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the data curve of Figure 8b from the maximum and minimum curves, respectively. From these values, we calculate the two soot curves in Figure 8c, which portray the upper and lower bounds for the drop in soot nitrogen content due solely to nitrogen liberation from the soot. The experimentally derived char curve is included in Figure 8c for comparison. The lower bound soot curve corresponds to the case that all the nitrogen in the converting PACN is incorporated into the soot. To offset this gain in nitrogen and accommodate the overall drop in soot nitrogen observed by elemental analysis, the rate of loss of soot nitrogen to the gas phase must be relatively fast, as is evident from the steep slope of the lower bound curve. The upper bound corresponds to the case that the nitrogen in the converting PACN is liberated as gases and is thus not incorporated into the soot. In this latter case, the rate of loss of nitrogen from the soot, as seen from Figure 8c, is much lower, comparable to that from the char. Our experimental results enable us to determine the net changes in the soot N C ratio from the gain of nitrogen due to conversion of PACN and the loss of nitrogen due to liberation of gaseous nitrogen. Comparison of the calculated soot curves in Figure 8b to those in Figure 8c indicates that conversion of PAC of successively lower nitrogen contents (minimum change: 0.024-0.015 = 0.009) has a larger effect on bringing down the soot N C ratio than does the liberation of soot nitrogen (maximum change: 0.024-0.017 = 0.007). However, since the gain of soot nitrogen from the conversion of PACN depends on the degree to which nitrogen is retained in the rings during conversion, additional data would be necessary for a better delineation of the relative importance of the two processes. Conclusions Using a capillary column gas chromatograph with a nitrogen-specific detector, we have been able to determine pyrolysis-induced changes in the ring number composition of nitrogen-containing polycyclic aromatic compounds from coal. The mass distribution and rate of decay with respect to temperature or time follow the order: five ring > six ring > four ring > three ring > two ring. The ring number composition of the PACN correlates well with total PAC yield, as shown before for nonpolar PAC composition with respect to ring number26and total PAC composition with respect to sub~titution.~' A fine structure develops in the GC chromatograms of the PACN, which may signify a preferential loss of alkylated PACN and perhaps some unsubstituted PACN of lower thermal stability. Comparison of (1)PACN to nonpolar PAC mass ratios for each ring number and (2) coal nitrogen and carbon distributions in the PAC and soot indicates that PAC conversion reactions leading to ring buildup, ring rupture, and soot formation are accelerated (i.e., they occur faster) for the PACN relative to the nonpolar PAC. These observations are consistent with relative rates of pyrolysis of individual compounds40and estimates of bond dissociation energies48found in the literature. Soot formed under our least severe conditions contains a higher fraction of the coal nitrogen than of the coal carbon-marking an accumulationof nitrogen from PACN under conditions not severe enough to cleave aromatic C-N bonds. Results from elemental analysis of the soot and the N/C composition of the converting PAC allow us to place limits on the two factors governing the soot's N/C composition: (1) soot growth by conversion of PAC and (2) liberation of soot nitrogen. We find that soot growth by conversion of PAC with successively lower N contents contributes more to the lowering of the soot's N:C ratio than does the converstion of soot nitrogen to gaseous ni-
782
Energy & Fuels 1988,2, 782-786
trogen species. Although we cannot definitively separate the contributions of the PACN and soot to the yield of gaseous nitrogen, the data suggest that, in the PAC conversion regime of our measurements, the net effect of the conversion of the nitrogen in PAC is the production of gaseous nitrogen species, most probably HCN.
Acknowledgment. We gratefully acknowledge the National Institute of Environmental Health Sciences
(Grants NIH 5 P30 ES02109 and NIH 5 PO1 ES01640)for support of this research. We also express our appreciation to Anthony Modestino and Peter Monchamp for help with the experiments and analyses. Registry No. Quinoxaline, 91-19-0; 2,6-dimethylquinoline, 877-43-0;phenanthridine, 229-87-8; carbazole, 86-74-8;9-ethylcarbazole,86-28-2;2-phenylindole,948-65-2; 9-phenylcarbazole, 1150-62-5; 13H-dibenzo[a,i]carbazole,239-64-5.
Wettability Measurements of Coal Using a Modified Washburn Technique Geatesh K. Tampy,?Wen-Jia Chen, Michael E. Prudich,” and Robert L. Savage Department of Chemical Engineering, Ohio University, Athens, Ohio 45701 Received August 17, 1987. Revised Manuscript Received July 21, 1988
The Washburn technique, which involves the wetting of a packed powder bed of solid by a liquid, was modified by using the fundamental equations of fluid transport through a packed bed. The modified version eliminates some of the inadequacies of the original technique: ambiguous calibration steps to estimate “orientation factors” or “equivalent radii” are not needed. Instead, the wetting of the powder by the liquid is quantified in terms of the bed porosity, a sphericity factor, and a tortuosity constant. The porosity is easily measured; the sphericity and tortuosity are obtained from the literature. Free energy changes during wetting and contact angles can be measured by this modified technique. The experimental procedure for making the measurements was also modified. The apparatus was altered to allow for the application of an external pressure, which acted in conjunction with the surface forces in causing the liquid to wet the solid. Thereby the particle mean diameters could also be measured. The wetting of three Ohio coals by three hydrocarbon oils was studied by using the modified Washburn technique. The interfacial free energy changes of wetting, contact angles, and particle mean diameters were measured. The values of the particle diameters were similar to those obtained by another technique. The interfacial free energy of wetting, which can be measured by this technique, is a property of fundamental importance in wettability studies. This technique is also simple to perform, is sensitive, and is reliable. It should therefore prove to be a useful technique in studies involving surface characterization of powdered coal.
Introduction The performance of several physical coal cleaning processes (e.g., oil agglomeration,flotation, etc.) is dependent on the surface properties of coal. Surface characterization is not easy because coal is heterogeneous; also, most of these processes utilize pulverized coal. This work addresses the need for a simple method that can properly characterize the interactions of liquids with polydisperse, heterogeneous solids like coal in powder form. One of the ways of characterizing solid-liquid interactions like those occurring in coal beneficiation is by the computation of the contact angle. Two of the older methods for measuring the contact angle in powder systems are the Bartell plug method’ and the Washburn capillary rise method? More recently, Heertjes and Kossen3 have suggested a different approach for contact angle measurement, which involves making a compressed pellet out of the powdered solid and measuring the height of a drop of liquid placed on the pellet. However, Neumann and Good4have indicated that pelletization changes the ‘Present address: Ames Laboratory, Iowa State University, Ames, Iowa 50011. 0887-0624/88/2502-0782$01.50/0
nature of the solid surface. The Bartell method is experimentally cumbersome. The Washburn technique, which also involves a plug of the solid powder, is much simpler to perform. However, the conventional application of this technique suffers from certain inadequacies that are explained in the next section. In this work, a modified version of the Washburn technique, which does not suffer from many of the limitations of the original technique, is presented. It is also shown that along with the contact angle, the free energy changes during wetting can be directly computed by this method. The free energy change being a fundamental thermodynamic property, its measurement is of greater significance than many of the empirical parameters that may be obtained by the several other popular surface characterization techniques in use today. Many of the implications of the free energy measurements that have been made by this technique, howBartell, F. E.; Merril, E. J. J. Phys. Chem. 1932,36,1178.Bartell, Whitney, C. E. Zbid. 1932,36,3115. Washburn, E. W. Phys. Rev. 1921,17, 273. Heertjes, P. H.;Kossen, N. W. F. Powder Technol. 1967,2, 33. Neumann, A. W.; Good, R. J. In Surface and Colloid Science; R. J., Stromberg, R. R., Eds.; Plenum: New York, 1979;Vol. 11.
0 1988 American Chemical Society
