Energy & Fuels 1996,9, 984-994
984
Artificial Coalification of a Fossil Wood from Brown Coal by Confined System Pyrolysis FranCoise Behar Institut Franqais d u Pdtrole, 1-4 Avenue de Bois Prdau, Rueil-Malmaison, 92506 Cedex, France
Patrick G . Hatcher* Fuel Science Program, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 31, 1995. Revised Manuscript Received August 3, 1995@
Artificial coalification of a sample of fossil wood of lignitic rank by a confined system pyrolysis in gold tubes reveals that such a simulation only partially mimics natural coalification processes. Analysis of gaseous products by gas chromatography, liquid products by gas chromatography/ mass spectrometry, and solid products by elemental analysis, quantitative solid-state 13C NMR, and flash pyrolysis/gas chromatography/mass spectrometry allows mass balance calculations. It is clear that, for this sample, confined system pyrolysis reproduces well the natural transformations of aromatic rings, but the reactions of aliphatic structures, derived predominantly from the side-chain carbons of lignin, are not simulated by such an artificial maturation process. This partially explains the deviations from the normal van Krevelen diagram observed in most artificial maturation experiments. The primary gaseous product, COZ,appears to originate from carboxyl and carbonyl groups and the methane, generated during the most severe stages of thermal treatment, appears to derive from methyl substituents on aromatic rings. The main liquid products observed are phenols, alkylbenzenes, and naphthalenes, with the phenolic products predominating at low severity and the naphthalenes and alkylbenzenes at high severity.
Introduction Coalification or maturation is a process involving the diagenetic and catagenetic alteration of sedimentary organic remains of plants influenced by burial, microbial activity, and geothermal heating. Understanding the chemistry of such processes has been the objective of countless studies involving two basic approaches. The first presupposes that burial and heating for long periods of time can be simulated by heating samples at elevated temperatures over shorter time frames in the lab~ratory.'-~The second approach seeks to delineate chemical processes by examining closely related plant remains which have been buried for varying periods of geologic time and have undergone natural maturation. This latter approach is fraught with difficulties due to inappropriate comparisons among samples whose specific initial similarities are unknown and to inabilities to obtain adequate samples. The organic geochemical Abstract published in Advance ACS Abstracts, September 15, 1995. (1)Tissot, B. P.; Espitalie, J. Reu. Inst. Fr. Petr. 1975,24,470. (2) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985,8 (41, 275. (3)Lewan, M. D. Philos. Trans. R. SOC. London Ser. A 1985,350, 123. (4) Behar, F.; Kressmann, S.; Rudkiewicz, J. L.; Vandenbroucke, M. Org. Geochem. 1991,19 (1-31, 173. (5) Teerman, S. C.; Hwang, R. J. Org. Geochem. 1991,17 (61, 749. (6) Hatcher, P.; Wenzel, K. A.; Cody, G. D. ACS Symp. Ser. 1994, 570,112. (7) Landais, P. Org. Geochem. 1991,17 (6),705. (8)Landais, P.; Michels, R.; Elie, M. Org. Geochem. 1994,22(3-51, 617. (9) Hatcher, P.; Lerch, H. E.; Verheyen, V. T. Int. Jour. Coal. Geol. 1989,13,65.
community has recognized the importance of chemical comparisons among samples whose common origin can be verified optically. Thus, comparisons among coalified gymnospermous xylem t i s ~ u e ,bled ~ resin,lOJ1 fossil seeds,12J3leaves,14and other plant parts are beginning to provide important clues regarding chemical changes associated with maturation of these individual fossilized plant remains. Simulation of the overall maturation process circumvents the problem of ensuring genetic commonality and also allows for the detection of all intermediate products formed during maturation, some of which may have been removed from samples influenced by natural maturation. Thus, one can chemically examine all maturation products which allows for a better differentiation of maturation processes. Unfortunately, it has not been unequivocally determined that laboratory simulations adequately reproduce natural evolution. Some controversiesexist regarding methodologies which best reproduce natural p r o c e ~ s e s , ~ Jeven ~ J ~ though
@
(10) van Aarssen, B. G. K.; de Leeuw, J. W. Org. Geochem. 1992, 19 (4-6), 315. (11)Anderson, K.B. Org. Geochem. 1994,21(21, 209. (12) van Bergen, P. F.; Collinson, M. E.; Sinninghe Damst6, J. S.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1994,58,231. (13)van Bergen, P. F.; Goni, M.; Collinson, M. E.; Sinninghe Damste, J. S.; Barrie, P. J.; de Leeuw, J. W. Geochim. Cosmochim. Acta 1994,58, 3823. (14) Nip, M.; Tegelaar, E. W.; Brinkhuis, H.; de Leeuw, J. W.; Schenck, P. A,; Holloway, P. J. Org. Geochem. 1986,10, 769. (15) Vandenbroucke, M.; Behar, F.; San Torcuato, A,; Riillkotter, J. Org. Geochem. 1993,20,961. (16) Lewan, M. D. In Organic Geochemistry; Engel, M. H., Macko, S.A,, Eds.; Plenum Press: New York, 1993; p 419.
0887-0624/95/2509-0984$09.00/00 1995 American Chemical Society
Artificial Coalification of a Fossil Wood
Energy & Fuels, Vol. 9, No. 6, 1995 985
Table 1. Mass Balances, Molecular Composition of the Gaseous Fraction, and Atomic Composition of the Resi Obtained during Artificial Coalification of the Morwell Coal in a Confined System for Various Temperaturemime Conditionsa mass balances T("C) t ( h ) C0,COz HzO C1 C2-C5 initial 2 0 0 0 90 2 3 1 36 41 0 0 200 1 47 43 0 0 220 1 75 73 0 0 250 250 9 124 71 1 0 1 119 132 1 0 300 0 300 3 140 152 2 300 9 165 160 3 0 0 300 24 174 199 6 330 1 160 128 3 1 330 2 171 172 4 2 330 3 172 170 4 2 5 185 164 4 2 330 330 7 187 174 6 2 330 72 228 221 8 4 400 24 266 224 32 6 500 8 318 212 65 6
ext.C6+ 10 14 17 23 33 24 29 35 25 18 18 20 19 18 18 10 0
molecular composition of gases res.
wtloss
1489 1430 1415 1351 1270 1246 1198 1168 1117 1211 1154 1153 1147 1133 1042 983 921
33 91 107 171 251 275 323 354 404 310 367 368 374 388 479 538 601
HZ C1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 4
0 0 0 1 2 2 4 4 8 5 6 6 6 8 9 24 34
CO 3 4 13 12 3 7 4 2 1 5 4 4 4 3 0 1
3
C02 97 96 87 87 95 91 91 93 90 89 88 89 89 87 89 71 58
C2-C5 0 0 0 0 0 0 0 0 0 1 1 1 1 1 2 2 1
residue (wt %) C 66 66.6 67.4 68.6 69.9 71.5 73.3 75.7 78.8 82.5 76.6 80.0 80.0 80.7 81.2 87.4 90.3 93.4
H 4.9 5.0 5.1 4.9 4.9 4.6 4.7 4.5 4.8 4.4 4.2 4.3 4.2 4.2 4.1 4.1 3.6 3.2
0 29 28.5 27.5 26.4 25.2 23.9 22.0 19.8 16.4 13.1 19.2 15.7 15.8 15.2 14.7 8.6 6.2 3.5
residue WC 0.90 0.89 0.90 0.86 0.83 0.78 0.77 0.72 0.73 0.64 0.66 0.65 0.63 0.62 0.61 0.56 0.48 0.41
OK 0.34 0.32 0.31 0.29 0.27 0.25 0.23 0.20 0.16 0.12 0.19 0.15 0.15 0.14 0.14 0.07 0.05 0.03
ext = extract. res = residue from extraction. Mass balances are in units of mg/g of orgC of initial sample. Molecular composition of gases are in relative %.
reproducibility for the production of certain products (e.g., petroleum hydrocarbons) may be adequate. It has recently been demonstrated that confined system pyrolysis in gold tubes closely simulates the maturation of bulk coal and kerogen samples through both the diagenetic and catagenetic p h a ~ e s . ~SimulaJ~ tion proficiencies using this technique have been established by comparison of artificially altered samples with natural samples using bulk chemical studies (elemental analysis, 13C NMR, FTIR) of residues and detailed analysis of gaseous and solvent extractable prodU C ~ S . ~ J Even ~ J ~ though many similarities exist between artificially and naturally altered samples, a detailed understanding of specific transformations is generally lacking, due primarily to the chemical complexity and heterogeneity of samples examined and the structural insensitivity of methods used. An approach which promises to overcome some of these problems, due in part to the fact it represents a chemically homogeneous plant part, has been the focus of our recent research. It involves the comparison of simulation experiments for a sample of xylem tissue, which is structurally uniform and well-defined, with a series of naturally coalified xylem which has become vitrinite. Combined use of elemental analysis, solidstate 13C NMR, and analytical pyrolysis provides sufficient structural detail t o elaborate on specific natural coalification reactions throughout diagenesis and catagenesis and to compare these with reactions of the xylem subjected to artificial coalification. The study reported herein compares the chemical transformations of a sample of brown coal xylem subjected to confined pyrolysis with a natural series of coalified woods under study for many years in our laboratory and with a series of hydrous pyrolysis experiments6 and open system pyrolysis experiments conducted17 on the same or similar coalified wood samples. Gaseous, liquid, and solid products are examined in detail to provide a complete accounting of the chemistry during artificial coalification. We chose a sample which had already undergone biological diagenesis because it had been (17)Behar, F.;Vandenbroucke, M.; Teerman, 5. C.; Hatcher, P. G.; Leblond, C.; Lerat, 0. AAPG Bull, in press.
coalified to the rank of brown coal or lignite B, had lost most of its cellulosic components as determined by 13C NMR, and represents selectively-preserved ligninderived structures. Thus, simulation experiments are designed to delineate processes beyond this rank level. We selected such a sample in preference to modern wood, because it is well-known that microbial degradation of wood during early diagenesis selectively alters and removes the cellulose, preserving the lignin components of wood in a nearly unaltered state.g This biological degradation cannot and should not be simulated by thermal treatment. We also chose a sample whose origin is from angiospermous wood. We would have preferred to conduct such experiments on gymnospermous wood; most of our previous studied8 of coalification reactions were focused on gymnospermous wood, but the large number of thermal experiments required large quantities of material and only angiospermous wood samples of sufficient quantities were available in our laboratory.
Experimental Section Sample Origin and Handling. The coalified wood sample was collected in 1985 from the Morwell Open Cut, Latrobe Valley, Victoria, Australia. Collected from a recently exposed face of the mine, the sample was placed in a large plastic bag and transported to the laboratory where it was refrigerated for shipment. The sample was later placed in a laboratory fume hood and air-dried to constant weight prior to packaging. We verified, in a separate 13C N M R experiment, that the airdrying did not oxidize the sample to any significant extent, as compared with freeze-drying. The dried sample was ground to a powder in a mortar and pestle and extracted with chloroform in a reflux apparatus to remove soluble lipid material. Geochemical data for the initial sample are reported in Table 1 and Figure 1. Confined System Pyrolysis. The pyrolysis device is a closed reactor heated isothermally at various temperatures (200-350 " C )and times ranging from 1to 72 h.19 The reactor is a gold tube with a 5 mm, o.d., 40 mm length and 0.5 mm thickness, sealed at both ends by welding under argon (18)Hatcher, P.G.Org. Geochem. 1990,16 (4-61, 959. (19)Behar, F.; Saint-Paul,C . ; Leblond, C. Rev. Inst. Fr. Pet. 1989,
44, 387.
986 Energy & Fuels, Vol. 9, No. 6, 1995
Behar and Hatcher the oxygen balance as follows:
nryl.0
1
O,(water) = O,(initial) - O,(CO
300
200
LOO
0 ppm
Figure 1. Solid-state I3C NMR spectrum of the initial Monvell coal showing peaks for the various functional groups. atmosphere. The sample charge varies between 100 and 500 mg. The temperature is measured with an accuracy estimated to be &2 "C, by a thermocouple penetrating an empty cell inside the autoclave. Cooling the autoclave in water at 25 "C takes less than 2 min and this time is consequently of negligible importance. Extracted coal was introduced into the gold cell which was purged three times under vacuum and an argon atmosphere. Pyrolysis products were first fractionated according to their molecular weight.Ig The gold tube was placed in a vacuum line a t MPa and connected to a liquid nitrogen trap. After the extraction line from the vacuum pump was isolated, the gold tube was pierced with a needle, allowing the permanent gases to be volatilized into the line and condensable compounds to be trapped by liquid nitrogen. Permanent gases (H2, CO, C1, and Ar) were concentrated by a Toepler pump into a calibrated volume in order to quantify their total yield and to recover them for molecular analysis as described below. Then, the liquid nitrogen (2' = -173 "C) in the trap was replaced by a mixture of ethanol and liquid nitrogen (T = -106 "C), allowing condensable gases ((202, H2S and C2-C5) t o be recovered and quantified by the same procedure as that used for permanent gases. Molecular characterization and quantification of the total gas fraction was performed by gas chromatography with specific Porapak and alumina columns and thermal conductivity and flame ionization detectors. The C@-c13 fraction remaining in the vacuum line was dissolved in 1mL of dichloromethane injected through a selfsealing device, then recovered by disconnecting the trap from the vacuum line. Then, the gold cell was cut into small pieces, extracted with dichloromethane together with the c6-c13 fraction for 1h under reflux, and filtered. The soluble fraction was first injected into a gas chromatograph equipped with an autosampler and calibrated by external standards. This analysis allowed an absolute quantification of the c6-c13 fraction. For the c14+fraction, direct GC can quantify only products which could go through the column. Thus, in order to get an absolute quantification of the C14+,the solvent of the total c6+ extract is evaporated under nitrogen, after GC analysis, and the C14+ fraction is weighed. The quantification of the total c6+ extract was the sum of the CS-c13 fraction evaluated by GC and of the weighed fraction. The insoluble fraction was recovered, mixed with the gold pieces on the filter, and weighed. The filter and gold tube being previously weighed after welding, the amount of insoluble residue was calculated by difference. It is worth noting that in this analytical procedure, direct quantification of water released during the experiment is very difficult. Consequently, the amount of oxygen in water was calculated (in mg) from
+ CO, + residue)
Such calculations were not very accurate. Thus, water estimation might be considered as semi quantitative data. In order to evaluate the accuracy of the analytical procedure, it is possible t o calculate the carbon balance independently of the mass which must be equal to 100 wt % because water was calculated by difference. For all our experiments, we could account for between 98 and 99 wt % of the carbon. Elemental Analysis. The recovered residue was stored under an argon atmosphere in order to minimize contamination with atmospheric water. Direct elemental analysis was performed by sampling this insoluble residue under a nitrogen atmosphere. Elemental compositions were determined by standard combustion methods (Carlo Erba Model 1106 elemental analyzer) with oxygen determined independently and directly by a pyrolytic method using the same instrumentation. Analytical Pyrolysis. Both the original lignite and the solid residues from the reactors were analyzed by flash pyrolysis. The flash pyrolysis technique used was that published by Hatcher et aLZ0and Bates et a1.21 Using a Chemical Data System Pyroprobe 1000, approximately 1 mg of sample was loaded into a quartz capillary tube, and this tube was placed inside the coils of the pyroprobe. The probe and sample were then inserted into the injection port (temperature maintained a t 280 "C) of a Varian 2700 gas chromatograph and the sample pyrolyzed. The residue was first thermally desorbed a t 300 "C for 30 s and the gas chromatograph cycled to elute these volatiles from the column. The samples were then pyrolyzed. Flash pyrolysis conditions were as follows: temperature, 610 "C for 10 s with a heating rate 5 W m s . The pyrolysate was cryotrapped with liquid nitrogen prior to being chromatographed on a 25 m x 0.25 mm i.d. J&W DB-17 capillary column. The GC was temperature programmed from 30 to 280 "C at 4 "C/min. The effluent was swept into the source of a DuPont 490B mass spectrometer fitted with a Teknivent Vector/One data system for detection and compound identification. Compounds were identified by a combination of methods which included comparison of mass spectra to the NBSNiley library, to published mass spectra, and to authentic standards whenever possible. Solid-state 13C NMR. Solid-state 13C NMR spectra were obtained by the method of cross polarization and magic angle spinning (CPMAS)using the conditions previously given.22The spectrometer was a Chemagnetics, Inc. M-100 spectrometer operating a t 25.2 MHz carbon frequency. Cycle times of 1 s and contact times of 1 ms were chosen as the optimal conditions for quantitative spectroscopy. Spin counting measurements and Bloch decay experiments were used on selected samples t o verify that the spectra are quantitative. Because of the low mineral matter contents of coalified wood, few deviations are expected from quantitative behavior. The spectra were integrated by dropping vertical lines t o the baseline between chemical shift regions characteristic of the various types of functional groups. Gas ChromatographyAWass Spectrometry. Dichloromethane extracts of the residues from confined pyrolysis were chromatographed using a Kratos MS 80 RFA GC/MS system. The gas chromatograph was a Carlo Erba Model 500 fitted with a J&W dB17 column (30 m x 0.25 mm, i.d.) operating in the splitless mode of injection. After sample injection and a 5 min isothermal period a t 40 "C, the column was temperature programmed a t a rate of 4 "C/min t o 280 "C and held a t that temperature for 10 min. The mass spectrometer was scanned a t a rate of 0.6 s/decade from 30 to 500 amu. The acquired data were transferred to a Sun Sparc I worksta(20) Hatcher, P. G. Fuel 1988,67,
(21)Bates, A, L.; Hatcher, P. G.; Lerch, H. E.; Blaine, C. C.; Neuzil, S. G.; Supardi Org. Geochem. 1986,17 (11, 37. (22) Hatcher, P. G. Energy Fuels 1988,2(11,49.
Artificial Coalification of a Fossil Wood
Energy & Fuels, Vol. 9, No. 6, 1995 987
tion and processed with Kratos Mach 3 software. Mass spectral analysis allowed peak identifications by comparison with the NISTAViley library of mass spectra. Gas chromatographic peak areas for identified products were tabulated and reported as peak areas normalized to a sum of all identified products.
m 10
Results and Discussion
Closed System Pyrolysis Yields. Results from artificial maturation in the gold tube closed system are summarized in Table 1. Experiments have been performed at various temperatures and various heating in order to follow the generation of both early and late products. In fact, as the Morwell coal is a very immature sample, the experiments were started a t 200 "C for 1 h in contrast to other samples for which typical experimental conditions are 300-350 "C for some h o ~ r s . Then, ~ , ~ in ~~ order to follow the complete degradation of the coal, it was necessary to submit the sample to 500 "C for 8 h.4J7 For a good comparison of the results obtained for various temperatures and times, the atomic ratios WC and O/C of the residual coals were chosen as maturity p a r a m e t e r ~ . ~ , ~ J ~ s ~ ~ Table 1 includes mass balances, molecular composition of the gaseous fraction and elemental analysis of the insoluble residue. The mass balances are dominated by non-hydrocarbon species such as C02 and H2O a t all pyrolysis conditions. The liquid products represent only 2.5% of the original sample weight. For all the experiments below 330 "C, the hydrogen content was too low t o be accurately quantified. For higher temperature, the molecular percentage of hydrogen in the gaseous fractions does not exceed 4%, a negligible contribution. A very early production of C02 and H2O occurs at the onset of thermal cracking (200 and 220 "C for 1h). The yield of these two compounds increases with increasing severity and represents 320 and 230 mg/g of orgC, respectively, under the most severe conditions (500 "C/8 h). In terms of the carbon balance, the maximum contribution of the C02 is only 9 wt %. This is approximately the same as the content of both carbonyl and carboxyl groups present in the initial sample (7 wt % calculated from NMR spectra, Figure 1). The main point of C02 generation occurs at both 300 and 330 "C for which around 6 wt % is produced, representing 66 wt % of the maximum C 0 2 generated under the most severe heating. For water, although the initial sample was dried prior to pyrolysis, a small part may be adsorbed in the coal network and is released very early as indicated by the experiment carried out at 90 "C/2 h and at 200 "C/1 h. Thus, we can estimate that around 20-30 mg/g of orgC may be subtracted from the water yield obtained in all the experiments to account for this bound water. Thus, the maximum water yield from coal thermal degradation will be around 200 mg/g of orgC, a yield much lower than the C 0 2 production. But, due t o our method for estimating the amount of generated water, the quantifications are not very accurate leading to some inconsistent results such as 330 "C/9 h compared to 300 "CI9 h and 330 "CI7 h. Methane is present in very low amounts a t the beginning of thermal cracking and increases sharply with increasing severity. For example, the production (23) Van Krevelen, D. W. Fuel 1950,29, 269.
C
1
P
O
Pl
W C -1h
iI
I,
7
P1
330% -2h P1
"1
m
r
330% -5h
500
330°C -72h
500
1CCU
1500
204)
2uIo
SCAN
Figure 2. GCIMS (total ion current) traces of the total CS+ extracts recovered in closed pyrolysis experiments carried out for various temperaturehime conditions. Peak identification is given in Table 2.
at 400 "(3124 h is around 32 mg/g of orgC while it is only 6 mg/g of orgC at 300 "C/24 h. This production occurs mainly after the production of the liquid c6+extract and reaches a maximum of 65 mg/g of orgC under the most severe conditions. Only a small part of this methane can originate from secondary cracking of oil due t o the low production of soluble extract (35 mg/g of orgC). It is mainly produced from the insoluble residue by the preferential loss of the methyl groups as will be discussed below with NMR results. The soluble oil extract is produced mainly at 250 and 300 "C and its diminution starts a t 330 "C, due to secondary cracking reactions. The GC traces for a representative set of experiments are presented in Figure 2. The identified peaks are listed in Table 2. Results show that at low severity, Le., 250 "C/9 h, the chromatogram comprises only oxygenated aromatics such guaiacols and catechols. Then, with increasing
Behar and Hatcher
988 Energy & Fuels, Vol. 9, No. 6, 1995 Table 2. List of Peaks in Figures 2,7, and 8 Identified by Gas Chromatographyhfass Spectrometry peak code peak identification B benzene B1 to1uene B2 xylene, ethylbenzene B3 &-benzene isomers B4 C4-benzeneisomers C catechol c1 methylcatechol COMe methoxycatechol G guaiaco1 G1 methylguaiacol G2 ethylguaiacol G2= vinylguaiacol G2=o acetoguaiacone G3 propylguaiacol G3= eugenol and isoeugenol G3=o propioguaiacone G3=oo ferulic acid P phenol P1 cresol P2 Cn-phenol P3 C3-phenol P4 C4-phenol N naphthalene N1 methylnaphthalene N2 Cz-naphthalene N3 C3-naphthalene N4 C4-naphthalene S syringol s1 methylsyringol s2 ethylsyringol s2= vinylsyringol s3= allylsyringol s3=, acetosyringone S~OH syringyl alcohol
maturity, guaiacols disappear rapidly; and an increase of the relative percentage of both catechols and phenols is apparent. At the same time, aromatic species, benzenes and naphthalenes, emerge. Finally, at increasing thermal severity, phenols are degraded and the chromatogram is dominated by monoaromatics with associated alkylated homologs. It is worth noting that the naphthalenes start to be produced very late, Le., after the peak of oil generation. This means that these compounds are not primary products but are likely to be formed through secondary cracking reactions of both the oil and insoluble residue. Moreover, GCNS analyses reveal that there is no production of triaromatics and heavier homologs; this suggests that aromatic polycondensation reactions are minimal in the range of temperaturehime conditions under study, i.e., 330 "C/ 1-72 h. In summary, we note that the soluble products are dominated by aromatic structures a t all levels of maturity. At low severity, the aromatic compounds are oxygenated, for the most part, and they generally lose their oxygen over the course of maturation. If we consider that the yields of oil are generally within a factor of 2-3 of each other at all levels of maturation, except the most mature sample (500 "C/8 h), then it seems very likely that the oil itself is being deoxygenated over the course of thermal maturation. Thus, thermal processes are affecting both the macromolecular material as well as the oil that is generated during heating. It is important to point out that the oils produced from the Monvell coalified wood are not typical of oils generally associated with Type I11 coals, which are often dominated by paraffinic hydrocarbons of high chain length.13 However, if this wood is mixed with
other sources of organic matter such as waxes or cuticles, paraffinic hydrocarbons will be generated as well as aromatic structures. Elemental Analysis. The evolution of the atomic ratios WC and O/C of the residues was plotted on a van Krevelen Diagram in Figure 3 together with those derived from a natural series of C O ~ ~ With S increas. ~ ~ ~ ~ ~ ing maturity, the natural series undergoes first a strong decrease of the O/C from 0.35 to 0.10 while the WC decreases only from 0.90 to 0.75. In artificial maturation experiments, for the same decrease of O/C, the corresponding decrease of the WC is much higher from 0.90 to 0.58 than in natural samples. Then with further maturity, the artificial evolution of the residue seems to parallel the evolution curve for natural samples. This suggests that, although the general trend of artificial maturation follows the same trend as the natural one, some discrepancies occur a t least for the simulation of the early part of maturation involving primarily diagenesis reactions. In fact, natural diagenesis is characterized by the preferential loss of oxygenated functional groups which leads t o a higher decrease of the O/C vs WC. During this stage, the carbon skeleton of the coal is not degraded suggesting that there are few C-C bond cracking reactions. In confined pyrolysis as the O/C decreases, the WC ratios decrease at a slightly higher rate than is observed in the natural evolution series which could mean that there is a slightly greater loss of hydrogen-rich groups. These results were confirmed by NMR analyses as discussed below. Solid-state lSC NMR. The solid-state 13C NMR spectrum of the Morwell angiospermous xylem is shown in Figure 1. As explained in previous report^,^^,^^ coalified wood from brown coal exhibits NMR resonances similar t o those observed in lignin, primarily because the basic lignin framework constitutes the majority of the sample. Resonances at 56,115,133, and 148 ppm can be attributed to methoxyl, protonated aromatic, carbon-substituted aromatic, and oxygensubstituted aromatic carbons (dihydric or methoxy phenols), respectively, which are remnants of the methoxyphenolic structures common to lignin. A minor inflection in the resonance line at 153 ppm can be observed, and this resonance corresponds t o either monohydric phenolic structures or ether-linked aryl-0 carbons in lignin-derived structures. Resonances in the region of 60-90 ppm are remnants of the hydroxylated side-chain carbons in lignin, their reduced intensity in brown coal wood in comparison to lignin being indicative of dehydroxylation reactions and cleavage of aryl ethers during coalification. The region between 0 and 60 ppm, excluding that of methoxyl carbons, is attributable to alkyl carbons, some of which may be related to the dehydroxylated lignin side chains and some due to resinous substances present in wood. Peaks for carboxyl and carbonyl carbons a t 175 and 205 ppm, respectively, are probably representative of lignin structures which have been partially oxidized, most likely at side-chain sites. Spinning sidebands (ssb)for aromatic carbons are also observed in this spectrum centered at -25 and 275 PPm. Integration of the above spectrum provides estimates of the contributions the various carbons make t o the average structural composition. Thus, Table 3 shows -
~
(24) Bates, A. L.; Hatcher, P.G. Org. Geochem. 1989,14,609-617.
Energy & Fuels, Vol. 9,No. 6, 1995 989
Artificial Coalification of a Fossil Wood 1.30
1.10
I .m
Morwell
0.90
s*
'1* Lo
o'80 0.70
0.60 0.50 0.40
t
Confined pyrolysis
D
Vitrinite coalification series
Vitrinite coalification series"
0.30
o'lo om
20
A 0.05
0.10
0.15
0.25
0.20
0.35
0.30
0.40
atomic O/C Figure 3. Comparison of the atomic compositions of coals on a van Krevelen diagram between a natural seriesz0Sz1and artificial maturation in a confined system (Table 1). Table 3. Quantitative Data Obtained by Solid-state C NMR Analysis for the Morwell Coals Recovered in Confined Pyrolysis Experiments for Various Temperaturemime Conditions T wtloss C=O COOH phenol cate. aro ether OCH3 ali fa OCHdAr ("C) t ( h ) WC (mdsoforgC) (%) (%) (%) (%) (%) (%) (%) (%) (%) (xloo) ~r-o/~r init. 0.90 0 4.0 8.0 14.8 39.5 4.0 3.1 23.1 62.3 4.97 0.37 2.9 200 220 250 300 300 300 330 327
1 1 1 1
9 24 1 12
0.90 0.86 0.83 0.77 0.72 0.64 0.66
nd
91 107 171 275 354 404 310
nd
2.8 3.1 3.3 2.6 1.8 1.6 2.2 0.9
4.0 4.8 4.5 3.9 3.0 4.1 3.9 2.3
7.4 9.8 8.3 11.1 10.3 10.5 12.8 10.0
17.8 19.5 19.2 17.1 10.3 0.0 0.0 0.0
40.0 38.5 40.9 43.5 56.4 66.5 62.9 72.8
3.1 2.2 1.7 2.8 2.2 2.1 2.7 0.0
3.1 1.3 0.0 0.0 0.0 0.0 0.0 0.0
21.1 20.4 21.1 18.1 15.1 14.0 14.6 12.9
65.2 67.8 68.4 71.6 77.1 77.0 75.7 82.7
4.78 1.90 0.00 0.00 0.00 0.00 0.00 0.00
0.39 0.43 0.40 0.39 0.27 0.14 0.17 0.12
a nd: not determined. C=O: ketonelaldehyde carbons, 180-220 ppm. COOH: carboxyl carbons, 160-180 ppm. phenol: phenolic carbons, 150-160 ppm. cate.: catechol-like carbons, 140-150 ppm. aro.: aromatic carbons, not O-substituted, 90-140 ppm. ether: etheric carbons, includes alcohols, 60-90 ppm. OCH3: methoxyl carbons, 50-60 ppm. ali: aliphatic carbons, not O-substituted, 0-60 ppm. Ar: total aromatic carbons, includes phenol, cate., and aro. fa: aromaticity.
calculated area percentages contributed by carbonyl (C=O),carboxyl (COOH), monohydric phenols (phenol), dihydric phenols or catechols (cate), aromatic carbons other than the oxygen-substituted ones (aro), aliphatic ethers other than methoxyls (ether), methoxyl carbons (OCH3), and alkyl carbons (ali). It is clear from the integrations that most of the oxygen-bearing carbons are distributed in aromatic systems, consistent with what is expected from lignin-derived structures. From the data in Table 3, we can determine that between 2 and 3 carbons per aromatic ring have an attached oxygen and that only 0.3 of these are from an attached methoxyl group. This implies that the predominant structural units of this coalified wood are dihydric phenols (catechols) and possibly methoxycatechols. Morwell coalified wood subjected t o confined pyrolysis over a range of different severities undergoes a series of changes which can be depicted by the series of spectra shown in Figure 4. The quantitative data obtained from these spectra are shown in Table 3. It is important to point out that most residues were analyzed by NMR but some samples provided such low quantities of residue that NMR studies were impossible. Also, the most
highly altered samples induced probe arcing and led to nonquantitative NMR behavior, precluding our ability to obtain meaningful data. The most noticeable change occurring at low severity is the immediate loss of methoxyl groups at 56 ppm, diminishing from 3% to below detection levels when heated at 250 "C for 1 h. Such a change is consistent with changes observed in natural coalification of ~ o o d . ~The J ~relative amount of intensity attributable to aryl-0 carbons, the peak at approximately 150 ppm, appears to remain reasonably constant at 1519% over the course of the loss of methoxyl carbon. When the amounts of oxygen substituted aromatic carbons are normalized to total aromatic carbons, the ratio aryl-O/aryl remains constant over the course of methoxyl loss, suggesting that demethylation is the predominant means by which methoxyl groups are lost. This is again consistent with observed changes during natural coalifi~ation.~ Other apparent changes observed during low-severity treatment are the losses for aliphatic alcohols/ethers (alkoxy carbon) associated with lignin side chains and aliphatic carbons. The loss of alkoxy carbon is expected based on trends observed in the natural coalification of
Behar and Hatcher
990 Energy & Fuels, Vol. 9, No. 6, 1995 80 1
1
ZOO W l h r
n i. ""
I
fa
natural series
A
M
I 40
00
153
- 1
L 01
0.2
03
04
OS
\
OIC
300
200
100
Oppm
300
200
100
Oppm
Figure 5. Comparison between aromaticity (fa)determined on natural coalsz0 and those on coals recovered in confined pyrolysis experiments (this study, Table 3).
Figure 4. Solid-state 13CNMR spectra for the Monvell coals recovered in confined pyrolysis experiments for various tem-
130
peratureltime conditions.
wood.ls However, in the natural series, loss of alkoxy carbon parallels an increase in alkyl carbon, suggesting that alkoxy groups are reduced to corresponding alkyl groups5 The diminution of alkyl carbon during artificial maturation (Table 3), along with the diminution of alkoxy1 carbon, implies that pyrolytic reactions are altering the lignin side chains, unlike the reductive processes observed for natural coalification. With increasing thermal severity, additional changes are observed, particularly in the aromatic region of the spectra. One of these involves the loss of signal from the 145 ppm region, the aryl-0 carbons mostly in dihydric phenols. The peak remaining in this region is one having a resonance at 153 ppm, that of monohydric phenolic carbon. The separation between the resonances of monohydric phenols and dihydric phenols is not sufficient to make a clear quantitative estimation of relative contribution. Nonetheless, if we assign resonance regions in a consistent fashion for each of the samples, we can estimate areas for each of these two types of aryl-0 carbons (Table 3). The loss of dihydric phenols is indicated as the area of the peak at 145 ppm diminishes from approximately 19 to 10% over the course of heating to a temperature of 300 "C. At the same time the peak for monohydric phenols, the one at 153 ppm, increases from a low of 7.4% to a high of 12.8%. When the areas of all aryl-0 carbons, those of both monohydric and dihydric phenols, are summed and reported on the basis of total aromatic carbon (aryl-O/ aryl), the values decrease from approximately 0.40 or 2.4 oxygens per ring t o 0.12 or less than 1 oxygen per ring. This is indicative of a transformation involving the loss of more than one oxygen substituent per ring, entirely consistent with the trend observed for the naturally coalified wood series.16 Thus, with regard to the aromatic region of the spectra, the artificial coalification series mimics well the natural coalification. The loss of aliphatic carbons during artificial maturation is well reflected by the change in carbon aromaticity (fa).As the severity of thermal alteration increases, the fa increases systematically in much the same way as is observed for natural coalification. The unaltered brown coal wood has an fa value of 62.3%. At a thermal stress of 327 "C/12 h, an fa of 82.7%is observed in the residue. Comparison of this data with fa of natural coalified wood
200
100
0 PPm
.
Figure 6. Solid-state NMR spectrum of the coalified plant fragments isolated from the Ambrosia lake mudstone.26
samples of Hatcher22can be made by plotting fa against the O/C ratio. Figure 5 shows a plot of this comparison. What does not follow well the natural series is the amount of aliphatic alkyl carbon. At the outset of confined pyrolysis, 23.1% of the carbons are alkyl; when heated to 327 "C for 12 h the amount of alkyl carbon drops precipitously to 12.9%. Comparison of natural coalified wood at an equivalent maturation level, as determined by the atomic O/C ratio, indicates that the artificially coalified sample is severely deficient in alkyl carbon. Figure 6 shows a spectrum of the coalified wood from the Morrison Formation, New which has an atomic O/C ratio of 0.149. Note that the aromatic region is similar to that of the pyrolyzed samples above 300 "C but that the aliphatic region is much more intense. It is clear that artificial coalification induces pyrolytic cleavage of the side-chain carbons of the lignin, probably during the initial stages of heating where the side chain is primarily substituted with hydroxyl groups. Perhaps hydroxyl substitution activates pyrolytic cleavage of the side chain. Loss of alkyl carbons during artificial coalification is entirely consistent with the observed deviation from the evolution of natural vitrinites on the van Krevelen diagram discussed above. Alkyl groups carry a substantial proportion of the elemental hydrogen content. The fact that the evolution line for closed system pyrolysates on the van Krevelen diagram is deficient in atomic WC ratio is consistent with diminished levels of alkyl substituents. Methyl carbons at about 15 ppm appear to be the primary remnants of aliphatic carbons in spectra of more severely stressed samples. These methyl groups (25) Hatcher, P. G . ;Spiker, E. C.; Orem, W. H.; Romankiw, L. A,; Szeverenyi, N. M. AAPG Bull. Stud. G'eol. 1986,22, 171.
Energy & Fuels, Vol. 9, No. 6,1995 991
Artificial Coalification of a Fossil Wood
GI
I
Wlhr
C
1 in
, I
Wlhr
i
Bl
C
I
4 .
4 30 40 Time (min)
Figure 8. GCMS (total ion current) trace of the total c6+ pyrolysate obtained in open flash pyrolysis system on the initial Morwell coal. Peak identification is given in Table 2. B1
1
I
PI I
P
1
LO
I
I
I
20
I
I
M
Retention time (min.) Figure 7. GC/MS (total ion current) traces of the total c6+ pyrolysates obtained by open flash pyrolysis system on the Morwell coals recovered from confined pyrolysis experiments carried out a t various temperaturehime conditions. Peak identification is given in Table 2.
are probably attached directly to aromatic carbons, although the presence of some methylene carbons at 30 ppm is indicative of the fact that some alkyl groups may have one or more methylenes. It is probable that these methyl carbons play an important role in the generation of methane under conditions of high-severity thermal treatment. l5 Carbonyl and carboxyl groups appear as distinct but broad peaks in the samples subjected to low-severity heating, but a general diminution in the amounts of these functionalized carbons is apparent as the severity of thermal stress increases. This decrease in relative abundance follows to some degree the evolution of C02 (Table 1). Thus, decarboxylation reactions could explain production of C02, but an insufficient diminution of carboxyl carbon is observed to account for all the C02 observed. It is possible that some additional C02 derives from reaction of water with carbonyl carbons according to the scheme proposed by Petitz6 and Song et al.2' Analytical Pyrolysis. Residues were subjected to analytical pyrolysis and the resulting pyrograms are shown in Figure 7 for a representative series, and the
peaks are identified by letters and subscripts (Table 2). The pyrogram for the untreated Morwell sample (Figure 8) shows intense peaks for guaiacol (G), 4-methylguaiacol (Gl), 4-ethylguaiacol (G2), 4-vinylguaiacol (G2=), syringol (S),4-methylsyringol (Sl),as well as catechol (C), the two methylcatechol isomers (Cl),and methoxycatechol (COM~). The presence of these peaks is consistent with pyrolysis results from other coalified wood samples from the Morwell Open and is indicative of the presence of lignin and altered lignin residues from angiospermous wood. The abundance of the catechols is indicative of lignin residues which have undergone demethylation reaction^.^,^^ Dimethoxyphenols (syringols) are observed, consistent with the origin of this sample being fiom angiospermous wood. Minor amounts of phenol (P),cresols (Pl),and C2-phenols (P2) are also observed. The presence of the three eugenol isomers (G3=)in the abundances observed is a good indication that much of the lignin units have relatively intact sidechains, although peaks for acetoguaiacone (G2=0),guaiacylprop-2-one (G3=o),and guaiacyl acetic acid (G~COOH) are an indication that some of the side-chains have undergone oxidation. Thermally altered residues display pyrograms characteristic of the chemical changes they have undergone. The pyrograms (Figure 8) of the residues show tranformations which correspond well to the NMR data. Lignin-related compounds diminish in relative intensity while catechols retain their initial intensity, but these, too, diminish with increasing severity. The retention of intensity for catechols is characteristic of the initial demethylation discussed above. Loss of catechols is consistent with the observed loss of dihydric phenolic carbon signals in the NMR spectra. The most altered sample analyzed by flash pyrolysis, 300 "C/9 h, contains peaks only for phenols, cresols, alkylphenols, alkylbenzenes (B), and alkylnaphthalenes (N). Calculations of the relative concentrations of various groups of compounds, notably the benzenes (including ~~
(26)Petit, J. C.Fuel 1991,70, 1053. (27) Song, C.; Saini, A. K.; Schobert, H. H. Energy Fuels 1994,8, 301.
~~
(28) Hatcher, P.; Wilson, M. A.; Vassallo, A. M. Int. J. Coal. Geol. 1989,13, 99. (29)Stout, S. A,; Boon, J. J.; Spackman, W. Geochzm. Cosmochim. Acta 1988,52,405.
992 Energy & Fuels, Vol. 9, No. 6, 1995
Behar and Hatcher
-- I
Y
0 benzenes
Y
0
phenols
40
lignin phenols A catechols
.-
2OO/lh
250/lh
300/lh
300/9h
Thermal severity for residue
Figure 9. Production curves for aromatic compounds of the total c6+pyrolysates obtained by open flash pyrolysis system on the Morwell coals recovered from confined pyrolysis experiments carried out at various temperaturehime conditions.
alkylbenzenes), the phenols, the lignin-derived phenols (guaiacols and syringols), and the catechols (including alkylcatechols and methoxycatechol), allow tracing of the artificial coalification trends (Figure 9). It is apparent that increasing thermal severity causes a total diminution of the lignin phenols as expected from studies of natural coalification of wood.g,28 The diminution of lignin phenols leads t o an increase in catechols as would be expected from the demethylation reactions observed from natural coalification series9,28~29 as well as artificial coalification ~ e r i e s . ~The , ~ benzenes ~ , ~ ~ and phenols remain constant during the earliest stages of heating while lignin phenols are decreasing. The fact that the relative increase in catechols mirrors the loss of lignin phenols is clear indication that demethylation reactions are responsible for these early transformations and that demethoxylation to form phenols is not the predominant process. Interestingly, the rate of loss of lignin-derived compounds in pyrolysates is not as great as the rate of loss of methoxyl carbons in the NMR spectra. Methoxyl carbons in the NMR spectra are virtually undetectable in residues heated at 300 "C for 1 h, whereas significant amounts of lignin-derived compounds are present in pyrolysis data of the same residues. This could simply be indicative of the fact that the NMR spectrum is not as sensitive to detection of methoxy cabons as pyrolysis is to detection of ligninderived compounds. Below a 1%level, it is difficult to reliably integrate NMR spectra. Alternatively, the technique of flash pyrolysislgas chromatographylmass spectrometry is more sensitive for lignin than it is for altered residue. The peak in catechol production occurs in the residue treated for 1 h a t 250 "C (Figure 9). Further heating causes a decrease in the catechol yields followed by an increase in the yields of phenols and benzenes. Eventually, catechol disappears and the residue becomes enriched in phenols and benzenes. A similar trend has been observed in pyrolysis data for coalified wood samples and c 0 a 1 . ~The ~ ~ catechol-like ~ structures are simply unstable at higher thermal stress and are degraded either by ring opening or by dehydroxylation reactions.31 It is unlikely that ring opening occurs because this would yield aliphatic structures and these (30) Ohta, K.; Venkatesan, M. I. Energy Fuels 1992,6,271-277. (31) Ross, D. S.; Loo, B. H.; Tse, D. S.; Hirschon, A. S. Fuel 1991, 70, 289-295.
actually diminish over the course of heating as deduced from the NMR data discussed-above. Dehydroxylation reactions are most likely responsible for the loss of ~ a t e c h o l s .Hatcher ~~ et a1.6 have recently proposed a condensatiodpyrolysis mechanism for the loss of catechols. This involves condensation of catechols to diphenolic ethers which subsequently are unstable at elevated temperatures and undergo pyrolytic degradation involving the cleavage of the ether bond. This process releases a phenol and a catechol. The catechol and phenol can condense further and later cleave again. Eventually, all the catechols would disappear and the phenols would condense to form diphenyl ethers, relatively stable components. Flash pyrolysis of diphenyl ether-type compounds would produce phenols and benzenes, both with associated alkyl groups depending upon the thermal stabilities of side-chain carbons throughout this thermal maturation. This would explain the rise observed in the yields of phenols and benzenes as the coalified wood is stressed to higher temperatures. The NMR and pyrolysis data clearly show that residues from confined maturation simulations show progressive changes related t o specific reactions taking place at oxygenated aromatic centers.6 The unheated sample is dominated by lignin-derived phenols which typify angiospermous wood coalified to the rank of Progressive heating produces a residue brown which becomes depleted of lignin-derived phenols. Some of these pyrolyze and become entrained in solvent extractable phases as methoxyphenols. The greatest proportion, however, undergo transformations similar to those observed during increased coalification t o higher ranks. These include primarily demethylation reactions which enrich the residue in catechol-like structures. Once catechol formation ceases, another series of reactions transforms the catechols, dihydric phenols, t o monohydric phenols andor diphenyl ethers. Throughout the course of this reaction sequence, the oxygen contents of the residues diminish continuously as the average number of oxygen substituents on aromatic rings diminishes. The overall process of maturation at this point can be described as a condensatiodpyrolysis reaction. The reaction sequence shown in Scheme 1 depicts a probable series of events.6 Once the catechol-like structures (1) form from ligninderived structures via demethylation, the catechols undergo condensation with an ether bridge being formed between two catechol units (2). Due to the fact that this type of condensed catechol is known to be unstable at high temperature and to undergo C - 0 bond we would expect homolytic cleavage of the ether to form a catechol radical (3) and a phenol radical (4). The phenol radical can be capped by a hydrogen radical to form phenol (5). The catechol radical can either be capped by a hydrogen radical to produce a catechol (1) or be capped by a methyl radical t o produce a guaiacol (6). This might explain why guaiacols persist in the artificially matured residues throughout the growth and depletion of catechols. The phenol radical (4) can also be capped by a methyl radical to produce methyl phenols (7). We do not necessarily infer from the above reaction scheme that hydrogen radicals are persistent components of our maturation experiments. The hydrogen can (32) Siskin, M.; Katritzky, A. K. Science 1991,252,231.
Energy & Fuels, Vol. 9, No. 6, 1995 993
Artificial Coalification of a Fossil Wood Scheme 1 R
R
R
on
on
on
?
R
R
R
R
on
8
be abstracted from coal structures and the effect is generally similar to abstracting a hydrogen radical. With increasing maturation or heating, the phenols produced can also undergo a condensation reaction similar to that of catechols. In this instance, diphenyl ethers ( 8 ) are the products. We can speculate that the ethers would be a rather stable product and would undergo pyrolytic degradation with only great difficulty under thermal stress. Condensation of the phenols to ethers is a process which would effectively remove oxygen, thereby diminishing the O/C ratio of the residues. The residues subjected to flash pyrolysis would very likely produce both phenols and benzenes due to homolytic cleavage of the diphenyl ethers. This is the result which we observe in the flash pyrolysis data. The reaction scheme presented here is somewhat different from that presented by Ross et al.31 in that the condensation reactions effectively lead to the removal of oxygenated species and water. Ross et al.31 suggested that loss of water was primarily through dehydroxylation reactions, though they offered that such a reaction mechanism was not clear. They also suggested that condensation (polymerization) of aromatic structures was primarily by carbon cross-links. It is difficult to envision that catechols would be depleted as rapidly as observed by these processes. The mechanism proposed above explains both the removal of oxygen and condensation as one process of condensatiodpyrolysis.6
Conclusions Confined pyrolysis has traditionally been an excellent technique for simulation of maturation processes. Product yields and chemical analyses of volatile and liquid products by gas chromatography have provided a great deal of information which has been compared with gas and oil production in natural systems. One aspect of previous studies that has been lacking is a detailed characterization of solid residues from confined pyrolysis. Solid-state 13C NMR is well suited as a technique capable of providing quantitative information on the
types of carbon structures in original samples and pyrolyzed residues and complements well the gas and liquid product characterizations. Thus, as we have demonstrated in this study, we can maintain a mass balance of carbon during pyrolysis to ascertain the fate of all carbon species. Such a mass balance is shown in Table 1. The main product from confined pyrolysis of the Morwell coalified wood is insoluble residue which undergoes specific transformations which can be compared with coalified wood transformations in the natural system. The small quantities of gas and liquid products derive from pyrolytic degradation that increases in severity. CO2 is the main gaseous product. It likely derives from decarboxylation reactions for the most part. However, the increase in C02 that occurs with increasing severity cannot derive entirely from loss of carboxyl carbon in NMR spectra (Table 3). The remainder must come from loss of carbonyl carbon, perhaps via a process involving reaction with water as suggested by Petit26 and Song et al.27 No doubt sufficient water is present to allow such a reaction. Of course, the water itself derives from both tightly adsorbed water (the samples were dried thoroughly before pyrolysis) and from water formed by dehydration reactions taking place during heating. Likely sites for dehydration are hydroxylated side-chain carbons of lignin: Unfortunately, we cannot discern whether such dehydration occurs because there would be no significant changes in the oxygen-substituted carbon regions of NMR spectra if hydroxyl groups condense to form ethers. Methane is the next most abundant gaseous product while the C2 to C g gases are negligible or minor. Methane appears t o evolve only at higher temperatures and its yield appears to be exponentially increasing (Table 1). At the highest levels of thermal severity, methane production is 65 mg/g of orgC of initial coal. Such a high level of methane cannot derive from cracking of liquid products which, at maximum, only account for 35 mg/g of orgC.17 Furthermore, the liquid products are primarily phenols, alkylbenzenes, and akylnaphthalenes, all of which produce little, if any, methane during increased heating. It is clear that the methane derives from the residue, where there is more than sufficient amounts of methyl and alkyl carbon (Table 31, even a t higher levels of heating, to generate methane. The liquid products generated during confined pyrolysis are water, mentioned above, and extractable hydrocarbons. When yields are plotted with increasing severity, a peak in the generation of oil is observed at midlevels of severity not unlike that observed as the “oil window” during natural maturation. The increase in oil has traditionally been viewed as pyrolytic release of hydrocarbons from the kerogen or coal matrix,33as is likely the case in our studies. However, the oil released is primarily phenols, alkylbenzenes, and alkylnaphthalenes. The diminution past the peak generation has traditionally been explained as further cracking of the “oil” to form methane.34 As explained above, this is (33) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer Verlag: Berlin, 1984; 537 pages. (34)Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L.Reu. Inst. Fr. Pet. 1991, 46, 151.
Behar and Hatcher
994 Energy & Fuels, Vol. 9, No. 6, 1995
unlikely. What is likely is the condensation of phenolic liquid products into the residue, a process known in the coal liquefaction community as “retrogressive react i ~ n . ”Production ~~ of an “oil window” during artificial pyrolysis of coalified wood does follow the traditional view of natural evolution of Type I11 kerogen or coa1,33,36-38 but the nature of products is different than what has traditionally been expected or measured. The oil is reflective of the chemical nature of the coal residue, being primarily phenolic and generally aromatic. Since no sigdicant presence of long-chain aliphatic structures can be expected, few paraffinic hydrocarbons are generated as oil during confined pyrolysis of the Morwell coalified wood sample. The generation of long-chain paraffins in oils from previous artificial maturation studies of Type I11 kerogen or coa15,39,40 must derive from the incorporation of macerals other than those derived from the lignin in wood. Cutinite and alginite are the probable contenders. There are some important transitions that occur in the Morwell wood’sresidues heated a t various temperatures, and we can compare these to the natural series of coalified woods to establish whether confined system
pyrolysis adequately mimics natural evolution. The results from this comparison clearly show significant deviation from natural evolution; however, the deviation is not as large as it is for open-system pyrolysis17 or aquathennolysis presented in a previous report.6 Nonetheless, it is possible, as a close approximation, to mimic natural coalification by confined system pyrolysis. Differences between natural and artificial coalification relate to the state of preservation of aliphatic structures. Aromatic structures appear t o undergo similar structural changes as the natural series with regard to loss of oxygenated substituents, except for the fact that the aromatic rings are more substituted by akyl groups, most likely methyl groups. Knowing the pyrolytic lability of highly oxygenated side chains of lignin, it is not surprising that they are cleaved in the artificial coalification experiments. In contrast, the natural series definitely show preservation of the side chain by reactions which remove hydroxyls but preserve the alkyl chain.6 Such reactions are probably ionic rather than thermolytic, and they take place in a water-saturated medium during the early stages of coalification, certainly before the subbituminous stage.
(35)McMillen, D. F.; Malhotra, R.; Hum, G. P.; Chang, S.-J.; Nigenda, S. E. Int. Conf. Coal. Sci., Sydney, Australia. 1985, 91. (36) Durand, B.; Paratte, M.; Bertrand, P. Reu. Inst. Fr. Petr 1983, 38, 6, 709. (37) Fowler, M. G.; Goodarzi, F.; Gentzis, T.; Brooks, P. W. Org. Geochem. 1991, 17 (61, 681. (38)Bertrand, P.; Behar, F.; Durand, B. Org. Geochem. 1986, 10, 601. .~~ (39) Mukhopadhyay, P. K.; Hatcher, P.; Calder, J. H. Org. Geochem. 1991, 17 (61, 765. (40) Fowler, M. G.; Gentzis, T.; Goodarzi, F.; Foscolos, A. E. Org. Geochem. 1991, 17 (61, 805.
Acknowledgment. We thank M. Vandenbroucke (Institut FranGaise du PBtrole) and two anonymous reviewers for their constructive comments and C. Leblond (Institut FranGaise du PBtrole) for her analytical assistance. We also thank T. V. Verheyen (Coal Corporation of Victoria, Australia) for his assistance in sample collection. EF950061W
