Influence of Maturation on the Pyrolysis Products ... - ACS Publications

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Energy & Fuels 1996, 10, 19-25

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Influence of Maturation on the Pyrolysis Products from Coals and Kerogens. 1. Experiment Sylvie Charpenay,* Michael A. Serio, Rosemary Bassilakis, and Peter R. Solomon Advanced Fuel Research, 87 Church Street, East Hartford, Connecticut 06108 Received July 18, 1995. Revised Manuscript Received September 28, 1995X

In order to evaluate the existence of general trends in pyrolysis products with maturation, three series of coals and three series of kerogens (type II-S and type III) of varying maturity were characterized by programmed open-system pyrolysis in a TG-FTIR (thermogravimetric analysis with Fourier transform infrared spectroscopy) apparatus. These series were chosen according to their degree of homogeneity of the precursors, the availability of good estimates of the thermal history, and extensive characterization data of the samples. For most series, similar gas and tar evolution trends during open-system pyrolysis were observed as a function of sample maturity. When increasing maturity, (1) the yields of pyrolytic CO2, H2O and CO decrease, (2) the yields of CH4 and tars go through a maximum, (3) the evolution rates of the oxygenated gases are lower in the whole pyrolysis temperature range, suggesting that during maturation all precursors are removed, regardless of their temperature of stability during pyrolysis, (4) the temperature for the maximum tar evolution presents a systematic shift to higher temperatures, consistent with Rock-Eval Tmax, and (5) in several series, the temperature for maximum CH4 evolution shows a shift toward higher temperatures. The yields of tar and CH4 were found to be series-dependent, while the oxygenated gas yields, for all series including the type II-S kerogens, were found to be very similar for comparable levels of maturation and may represent a seriesindependent criteria for maturity. This feature may be useful in estimating maturity in type II or II-S kerogens, when vitrinite reflectance measurements cannot be used. This study also confirms previous results that the tar amount can be used to estimate kerogen type, as type II (and type II-S) kerogens produce more tar. In conclusion, the pyrolysis volatile trends observed in the series investigated suggest that the same processes may be involved during the maturation of organic matter. TG-FTIR analysis provides multiple information which can be used for maturity estimation and kerogen type identification. The availability of multiple parameters may lift possible uncertainties in these estimations.

Introduction In order to ultimately predict oil and gas formation in sedimentary basins, the influence of maturation on the structure and decomposition behavior of organic matter needs to be estimated. Three important points should be considered when one studies the effect of maturation and attempts to predict its effect: (1) the influence of the origin and type of the sample on its behavior during maturation, (2) the assessment of the level of maturity through reliable parameters, and (3) the method used to estimate changes in structure or decomposition behavior due to maturation. The first point may be addressed by investigating multiple natural series of samples, each containing samples from different maturity levels, but from similar precursors. While the number of natural series responding to that criteria is limited, their study is important to estimate possible systematic variations in properties. Studies performed on coal and kerogen series have shown specific trends.1-6 However, it is difficult to establish, Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Welte, D. H.; Schaefer, R. G.; Stoessinger, W.; Radke, W. Mitt. Geol. Palaontol Inst. Univ. Hamb. 1984, 56, 263. (2) Requejo, G.; Gray, N. R.; Freund, H.; Thomann, H.; Melchior, M. T.; Gebhard, L. A.; Bernardo, M.; Pictroski, C. F.; Hsu, C. S. Energy Fuels 1992, 6, 203-214. (3) Whelan, J. K.; Solomon, P. R.; Deshpande, G. V.; Carangelo, R. Energy Fuels, 1988, 2, 65-73. X

0887-0624/96/2510-0019$12.00/0

for prediction purposes, how applicable these trends are to other series. To address this point, the present study included the analysis of three coal series and three kerogen series. These series were chosen according to the following criteria: if possible, a high degree of homogeneity of the precursors, a good estimate of the thermal history, and extensive characterization data of the samples. The second point mentioned above, concerns the assessment of maturity. Maturity has been correlated with parameters such as vitrinite reflectance,7,8 elemental analysis,7,9 and fluorescence.7,10 It is, however, difficult in certain cases to assess with certainty the level of maturity from single parameters, as the validity of each parameter can be limited to specific conditions.7 For example, vitrinite reflectance measurements can be made only on samples containing vitrinite, and very (4) Tissot, B. P.; Pelet, R.; Ungerer, P. Am. Assoc. Pet. Geol. Bull. 1987, 71 (12), 1445-1466. (5) Burnham A. K.; Oh, M. S.; Crawford, R. W.; Samoun, A. Energy Fuels 1989, 3, 42-55. (6) Reynolds, J. G.; Burnham, A. K. Energy Fuels 1993, 7, 610. (7) Whelan, J. K.; Thompson-Rizer, C. L. In Organic Chemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Press: New York, 1993. (8) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1984. (9) Van Krevelen, D. W. Coal Elsevier: New York, 1993. (10) Alpern, B.; Durand, B.; Espitalie, J.; Tissot, B. In Advances in Organic Chemistry, 1971; von Gaertner, H. R., Wehner, H., Eds.; Pergamon Press: Oxford, U.K.; 1972; pp 1-28.

© 1996 American Chemical Society

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mature samples are found to be difficult to analyze. Also, vitrinite measurements are subject to operator bias. Techniques involving the pyrolysis of the organic matter with the analysis of the products, such as RockEval11-13 (which includes pyrolysis with a flame ionization detector and a thermal conductivity detector) have led to the development of another maturity parameter, which is the peak temperature of product evolution Tmax. Rock-Eval Tmax is a good indicator of maturity for type II and III kerogens, but not for type I because in that case Tmax does not vary significantly with maturation.7,13 The third point concerns the various methods which can be used to determine changes in structure or decomposition behavior due to maturation. In addition to the Rock-Eval technique, pyrolysis with mass spectroscopy analysis (Py-MS),7,14 with gas chromatography analysis (Py-GC),7 or both (Py-GC-MS)2,7 have been used for the characterization of organic matter. These methods can estimate changes in decomposition behavior with the level of maturity of the sample. The present work also involved the use of pyrolysis with analysis of the products, and consists of programmed pyrolysis where thermogravimetric analysis is combined with the Fourier transform infrared spectroscopy analysis of evolved products (Py-TG-FTIR). This method has been shown to be useful for the analysis of petroleum source rocks, as it provides considerable geochemical information which can be used as an estimate of maturity.3 In particular, it provides the evolution rates and yields of the major gas species evolving during pyrolysis as well as the evolving, room-temperature condensible hydrocarbons (tars). The latter can be correlated with the S2 peak as measured by Rock-Eval. Py-TG-FTIR then constitutes a method to estimate the maturity of the sample as well as an analytical method to evaluate the influence of maturation on the decomposition behavior of organic matter. Since it provides detailed information on the formation of gas species during pyrolysis, the results from TG-FTIR can be analyzed to evaluate the kinetics of gas precursor removal during maturation and can be integrated into a maturation model. This was the object of the second part of our study, published elsewhere.15 The main goal of this study was to estimate the influence of maturation, as measured by Py-TG-FTIR, in the pyrolysis products of several series of coals and kerogens and determine if similar trends in gas evolution and yields are present throughout the series. In particular, potential differences (or similarities) between coals and kerogens were to be investigated. Experimental Section TG-FTIR. The apparatus used has been described extensively.16,17 The TG-FTIR gives a quantitative analysis of the evolution of tar or oil, CH4, CO, CO2, H2O, C2H4, SO2, NH3, HCN, COS, and many other species as a function of temper(11) Espitalie, J.; Deroo, G.; Marquis, F. Rev. l’inst. Fr. Pet. 1985, 40 (5), 563-579. (12) Boudou, J. P. Fuel 1984, 63, 430-431. (13) Ungerer, P. J.; Espitalie, J.; Marquis, F.; Durand, B. In Thermal Modeling in Sedimentary Basins; Burris, J., Ed.; Editions Technip: Paris, 1986. (14) Reynolds, J. G.; Crawford, R. W.; Burnham, A. K. Energy Fuels 1991, 5, 507-523. (15) Charpenay, S.; Bassilakis, R.; Serio, M. A.; Solomon, P. R. Energy Fuels, following paper in this issue. (16) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Gravel, D.; Baillargeon, M.; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 319.

Charpenay et al. Table 1. Original Names, Vitrinite Reflectance, and Oxygen Content for the Coal and Kerogen Samples Investigated

coal/kerogen series Mahakam coals

San Juan coals

Argonne coals

Ikpikpuk kerogens, Alaska Slope, type III

Monterey kerogens, type II-S Middle Valley kerogens, type III

a

original sample name 32362 46149 31590 31591 46156 CH5A CH5B GR3B KB5A KB5B SJ8B CU5A ND WY IL UT PITT UF POC #1 #2 #5 #7 #10 KG-8 KG-24 17R 23R 25-26R 31R 51R

name M1 M2 M3 M4 S1 S2 S3 S4 A1 A2 A3 A4 I1 I2 I3 I4 K1 K2 V1 V2 V3 V4 V5

Ro 0.45a 0.55a 0.61a 0.94a 0.44 0.52 0.66 0.82 0.93 1.24 1.30 0.28 0.31 0.46 0.50 0.72 0.99 1.42 0.55 0.60 0.78 No vitrinite 0.67 0.75 0.95 1.2 >2.48

oxygen, % (daf) 29.7 23.2 10.5 9.3

20.34 18.02 13.51 11.58 8.83 7.51 2.47

∼20 ∼10

Estimated values.

ature and time during pyrolysis. The samples were heated in the TG-FTIR system using a helium flow and a heating rate of 30 °C/min up to 900 °C. In this paper, only the major volatile species (tar or oil, CH4, CO, CO2, H2O) will be investigated. Samples. Three coal series (Mahakam, San Juan, and Argonne) and three kerogen series (Monterey, Middle Valley, and Ikpikpuk) were investigated. The vitrinite reflectance and oxygen content of the samples of each series are given in Table 1. In each series, the samples are organized in order of increasing maturity (as measured by vitrinite reflectance, or, when the latter is not available, oxygen content). The Mahakam Delta coals have been extensively studied, and have been shown to be derived from a homogeneous organic precursor.18 This special feature allows the study of the influence of maturation on the coals independently from other factors. The San Juan coals come from the San Juan basin in the southwestern United States and have been extensively characterized.6 While these coals do not come from a unique well, they come from the same region and are thought to derive from a similar precursor. The Argonne coals do not represent a homogeneous series from the same starting precursor. However, they have been extensively studied and characterized.16,19 The Ikpikpuk (Alaska North Slope) samples are isolated type III kerogens and have been described in detail in several publications.20-22 The Middle Valley samples are also (17) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Yu, Z. Z.; Charpenay, S.; Whelan, J. J. Anal. Appl. Pyrolysis 1991, 19, 1. (18) Boudou, J. P.; Durand, B.; Oudin, J. L. Geochim. Cosmochim. Acta 1984, 48, 2005. (19) Vorres, K. Prepr. Pap.sAm. Chem. Soc. Div. Fuel Chem. 1987, 32 (4), 221. (20) Bird, J. K. In Alaskan North Slope Oil/Rock Correction Study; Magoon, L. B., Claypool, G. E., Eds.; AAPG Spec. Stud. Geol.; AAPG: Tulsa, OK, 1985, Vol. 20, pp 3-29. (21) Whelan, J. K.; Farrington, J. W.; Tarafa, M. E. Org. Geochem. 1986, 10, 207.

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isolated kerogens of type III and have been subjected to high temperatures (from 120 °C up to 225 °C) and short times (∼250 000 years) due to a hydrothermal flow in their vicinity.23,24 All the Middle Valley samples have reached fairly high levels of maturity (see Table 1). The Monterey samples are from isolated kerogens of type II-S from the Naples and Lyons Head of the Monterey formation.25 They are relatively immature, having been subjected to fairly low temperatures. No reflectance data is available for these samples, since no vitrinite was present, and few characterization methods were successful at estimating the maturity of each of the two samples.25 From the vitrinite reflectance data given in Table 1, it can be observed that the Argonne coals have the widest range of maturity, followed closely by the San Juan coals. The Mahakam coals, as well as the Ikpikpuk kerogens, possess a narrower maturity range, with samples of moderate maturity. The Monterey kerogens include a very immature sample and a fairly matured sample (estimated using the oxygen content). Finally, the Middle Valley kerogens series covers a range of maturity from fairly matured to highly matured samples.

Results The evolution rates of CO2, CO, H2O, tar, and CH4 were measured for the samples for each series during the programmed pyrolysis excursion and plotted versus temperature. The final yield was also plotted against a maturity parameter, the vitrinite reflectance Ro. This parameter was chosen as it is a standard measure for maturity.7,8 It should, however, be kept in mind that Ro is only one among several possible indicators of maturity and may not be applicable to all samples. In particular, the type II-S Monterey kerogens cannot be classified using Ro as no vitrinite is present in the samples. Oxygenated Gases. The yields of CO2, H2O and CO as a function of the vitrinite reflectance (i.e., degree of maturity) for all the samples investigated are given in Figure 1. While individual series may show some variations in the trends (in particular for the H2O yields), it is clear that the yields decrease with maturity. This is to be expected as the oxygen content decreases with increasing maturity. An interesting result, however, is that all series (coal, type II-S, and type III kerogen series) seem to behave similarly in terms of loss of oxygenated precursors, as the yields of oxygenated gases are similar for comparable levels of maturity. Also, the precursors of all three oxygenated gases appear to be removed in the same proportion as a function of maturation, which may imply similar kinetics for the removal of oxygen functional groups.15 As maturity increases, the evolution rates of CO2, H2O, and CO (see Figures 2, 3 and 4, respectively) are found to be lower in the whole pyrolysis temperature range for most of the series. This behavior is especially pronounced in the case of low and moderate maturation samples and implies that natural maturation is different from open pyrolysis where the loosely bound precursors are removed first. However, in the case of highrank (i.e., high maturity) coals of the Argonne series, the gas trends seem to follow a maturation path which resembles open pyrolysis, since the loosely bound pre(22) Farrington, J. W.; Davis, A. C.; Tarafa, M.; Whelan, J. K.; Hunt, J. R. Org. Geochem. 1988, 13, 303. (23) Davis, E.; Mottl, M.; Fisher, A.; et al. Init. Rpts., Leg 139, Ocean Drilling Program, College Station, TX, 1992; Vol. 1. (24) Whelan, J. K.; Seewald, J.; Eglinton, L.; Miknis, F. P. Proc. Ocean Drilling Program, Sci. Results. 1994, 139. (25) Isaacs, C. M. AAPG Hedberg Conf. Proc., New Orleans 1993.

Figure 1. Yields of volatile species obtained by TG-FTIR. (a) CO2, (b) H2O, (c) CO, (d) tar, (e) CH4. Open symbols are used for coals and filled symbols for kerogens.

Figure 2. CO2 evolution rate during pyrolysis as measured by TG-FTIR for the following series: (a) Argonne, (b) San Juan, (c) Ikpikpuk, (d) Mahakam, (e) Monterey, and (f) Middle Valley.

cursors are removed first. This may be explained by the fact that, for these high-rank coals, fairly high temperatures (150-200 °C) are reached in the geological environment. This behavior can also be observed, although not as clearly, in the Middle Valley kerogens which have also reached high levels of maturity. In the other series, the gas evolutions for the most matured samples show little variation, possibly due to the lower maturity level reached. It should be noted that the coals (except the Argonne coals) and kerogens samples may be slightly oxidized, which would impact the yields of early CO2 and CO. Also, since the kerogens have been isolated from the rock by acid treatment, the possible effect of exchanged cations on CO2 and CO yields may not be present.15

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Figure 3. H2O evolution rate during pyrolysis as measured by TG-FTIR for the following series: (a) Argonne, (b) San Juan, (c) Ikpikpuk, (d) Mahakam, (e) Monterey, and (f) Middle Valley.

Charpenay et al.

Figure 5. Tar evolution rate during pyrolysis as measured by TG-FTIR for the following series: (a) Argonne, (b) San Juan, (c) Ikpikpuk, (d) Mahakam, (e) Monterey, and (f) Middle Valley.

Figure 6. Tar Tmax as a function of vitrinite reflectance Ro. Open symbols are used for coals and filled symbols for kerogens.

Figure 4. CO evolution rate during pyrolysis as measured by TG-FTIR for the following series: (a) Argonne, (b) San Juan, (c) Ikpikpuk, (d) Mahakam, (e) Monterey, and (f) Middle Valley.

Another feature to be noted in the evolution curves is the presence of sharp peaks, such as in the S1 and V1 samples. These usually correspond to mineral peaks which result from inherent mineral components or ionexchanged cations. The amounts of CO2 and CO appear to be very sensitive to the level of maturity of the sample, especially for very immature samples: the variations in CO2 and CO yield are the largest between Ro ) 0.3 and Ro ) 0.6. This observation may provide a “quick test” to determine the maturity of a sample in that maturity range. Tar. The tar yield gives an indication of the oil potential of the sample. In most of the series studied,

the tar amount goes through a maximum (see Figure 1), while the temperature for the maximum tar evolution presents a systematic shift to higher temperatures with increasing rank (see Figures 5 and 6). This behavior is consistent with previous findings on the variations of the hydrocarbon potential and Rock-Eval Tmax with maturity,7,12 and is especially clear for the coal series. In addition, the presence of a “shoulder” prior to the major tar peak occurs for coals of medium rank with a high tar yield. In the case of kerogens, this shoulder can be even more significant, as seen in the Ikpikpuk and Middle Valley series, and in certain cases represents an important amount of tar. High-maturity kerogens give almost no tar (they are “thermally spent”). In general, kerogens have a lower tar yield (on a dry ash free basis) than coals (see Figure 1). While only two samples are available, the type II-S Monterey kerogens also seem to follow the trend observed in the other series, i.e., an increase in tar yield between immature and moderately mature samples. The temperature of the maximum tar evolution varies slightly, which appears to be consistent with the other series when one considers the variation in the maturity of the samples as described by the oxygen content. These kerogens also produce a higher amount of tar than the type III kerogens (∼40-50% daf, as opposed to ∼15-25% for the Ikpikpuk kerogens and 0-12% for the Middle Valley kerogens), which is consistent with

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Figure 9. Ratio of the final CH4 yield versus the char yield measured at 550 °C (roughly corresponding to the onset of CH4 evolution) as a function of vitrinite reflectance for the three coal series.

Figure 7. CH4 evolution rate during pyrolysis as measured by TG-FTIR for the following series: (a) Argonne, (b) San Juan, (c) Ikpikpuk, (d) Mahakam, (e) Monterey, and (f) Middle Valley.

Figure 8. CH4 Tmax as a function of vitrinite reflectance Ro. Open symbols are used for coals and filled symbols for kerogens.

the fact that type II-S kerogens are principally oil producing while type III kerogens are gas producing. Methane. In all of the series studied, the CH4 evolution shows an increase and then a decrease in the maximum evolution rate (see Figure 7) and amount (see Figure 1) with increasing maturation. However, in coals the increase in yield and evolution rate is more significant than the subsequent decrease, while in kerogens the reverse is observed; i.e., the original increase is not as pronounced as the subsequent decrease, and the decrease occurs at lower maturity. Also, the CH4 yield is found to be larger in the case of coals than in the case of kerogens. A slight shift of the evolution curve toward higher temperatures is usually observed for highmaturity coal samples (Figures 7 and 8), but seems to be series-dependent. The shift is seen to be very pronounced in the case of the Middle Valley kerogen series. These results are in accord with previous studies.3 One possible explanation for the increase in pyrolytic CH4 lies in a concentration effect of the CH4 precursors in the coal through the loss of oxygen functionalities during maturation. However, it was estimated that the increase is much higher (at least in the case of coals) than what would be expected from mass loss alone. Another possible explanation for the increased CH4 would result from a larger char fraction available at the temperature of CH4 evolution, i.e., a larger amount of

CH4 precursors available for producing CH4 as opposed to being evolved with the tar. Figure 9 displays, for the coal series, the ratio of the CH4 yield over the char yield present at the onset of CH4 evolution (measured at a temperature of 550 °C for all series) as a function of vitrinite reflectance. The plateau starting at a value of Ro of about 0.6 (i.e., lower than Ro at the maximum of CH4 evolution seen in Figure 1, which is about 1.1), indicates that between Ro ) 0.6 and Ro ) 1.1, increases in CH4 yield may be related to increases in the available char yield. However, this effect cannot explain the substantial CH4 increase at low maturity. The overall analysis shows that the main increase in pyrolytic CH4 yield occurs between lignites and bituminous coals, while the yield stays probably constant (within 1% daf) for coals of higher ranks. The decrease in CH4 at very high rank is expected to derive from CH4 formation in the basin during maturation. The type II-S Monterey kerogens do not show the same trends in terms of CH4 evolution and yield than the type III kerogens and the coals, and possible explanations for this behavior are discussed below. Discussion For the six series studied, in most cases similar gas and tar evolution trends have been observed as a function of the degree of maturation. The general trends observed are as follows: as a function of increasing rank, the yields of CO2, H2O and CO during pyrolysis decrease, while the yields of CH4 and tar go through a maximum. In addition, the evolution rate of oxygenated gases appears to decrease in the whole pyrolysis temperature range; i.e., during maturation all the precursors are removed. Finally, the tar and CH4 evolution rates present a systematic shift toward high temperature when increasing maturity. The three coal series show the trends described above very clearly and give very similar volatile yields for the same maturity level. The kerogen series appear to follow the trends from the coal series fairly well for the oxygenated gas species, even if the data shows more scatter. However, the kerogen series differ from the coals series for the evolution of CH4 and tar. Oxygenated Gas Trends. Complex reactions, involving oxygenated functional groups (hydroxyls, carbonyls, carboxyls, ethers), are thought to lead to the formation of oxygenated gases during the pyrolysis of

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organic matter. Standard pyrolysis, as performed in the laboratory at high temperatures (above 200 °C) and with or without the presence of gas flow, usually depletes the loosely bound gas precursors first. The same behavior does not occur during natural maturation since, from the TG-FTIR characterization data, it can be seen that all the precursors, and not only the loosely bound, have been removed. In addition, the removal of oxygen gas precursors occurs extremely fast in low-maturity samples (faster than when predicted using laboratory-derived kinetics,15 even if these samples have seen only low temperatures, i.e., less than 100 °C, and often much less). The differences in decomposition behavior between laboratory pyrolysis and natural maturation have been suspected to derive mainly from different operating mechanisms and the presence of water in the seam.26-28 At high temperatures, radical reactions are preponderant, while at low temperatures ionic reactions may be dominant, especially if water is present. The presence of water has been observed to facilitate reactions such as ionic condensations, cleavages, and hydrolyses.27 For example, laboratory studies have shown that decarboxylation reactions occur at much lower temperatures under hydrous conditions than pure thermal conditions.28 Reactions involving water may facilitate the removal of oxygen functionalities which would be otherwise stable under purely thermal conditions. Unfortunately, it is difficult to elucidate natural maturation mechanisms from short-time, higher temperature laboratory experiments, as conclusions from laboratory simulations may not fully apply to mechanisms occuring under low temperature and long times. However, it seems likely that the qualitative conclusions obtained from laboratory studies would still hold for natural maturation. For very matured samples, the similarity in pyrolytic gas trends between naturally matured samples (see in particular the Argonne series) and samples “matured” using laboratory pyrolysis may be explained by the fact that very mature, natural samples contain little water (moisture) and have seen high temperatures, usually above 150 °C. Under these conditions, the reactions occurring may be different than the ones operating in the earlier stages of the maturation process and could be closer to the reactions found during laboratory pyrolysis. Finally, the presence of minerals, and in particular organically exchanged cations in low-maturity samples, has been shown to impact the CO2 and CO yields.15,29 Exchanged cations such as calcium are coordinated with carboxyl groups and contribute to the first CO2 peak and to the high-temperature CO peak. It was also observed that the presence of cations increases the cross-linking tendency of the coal during pyrolysis.15,29 During maturation, the carboxyl groups may be removed, with the remaining calcium possibly forming calcite. Tar Yield Variation with Maturity. In low-rank (i.e., low maturity) coals, early cross-linking during pyrolysis has been found to be partially responsible for (26) Michels, R.; Mansuy, L.; Elie, M.; Landais, P. Presented at the American Chemical Society Meeting, Spring Meeting, Geochemistry Division 1995. (27) Siskin, M.; Katritzky, A. R. Science 1991, 254, 231-237. (28) Siskin, M.; Brons, G.; Vaugh, S. N.; Katritzky, A. R.; Balasubramanian, M. Energy Fuels 1990, 4, 488. (29) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 577.

Charpenay et al.

the low tar evolution. Cross-linking reactions have been correlated with low-temperature CO2 (and/or H2O) gas evolution.30,31 This finding can be related to the low tar evolution in samples with high CO2 and H2O yields (see Figure 1). When the CO2 and H2O precursors are removed during natural maturation, the cross-link sites which form during normal pyrolysis will not be present, potentially increasing the tar yield. For moderate rank (bituminous) coals, the absence of low-temperature cross-linking, a small cluster size, and an abundance of labile bridges in the coal lead to high tar yields. When increasing the level of maturation, reactions of depolymerization of the organic matter start to occur. Bridges of increasing stability cleave, leading to the formation and expulsion of low molecular weight products, and forming a progressively more stable macromolecular network. This corresponds to the “oil window”. Past that stage, the structure of high-rank coals has been shown to be formed of large, mostly aromatic clusters, with no weak bridges left in the structure.32 At this stage of maturation, all the labile bridges have been removed, and condensation and aromatization processes have increased the average cluster size, thus limiting the tar amount and increasing the tar evolution temperature. While the above processes have been identified for coals, they may also apply to type III kerogens as kerogen samples present the same tar evolution trends as coal samples. However, the lower tar yields in kerogens may imply that kerogens contain more fixed carbon and possibly less aliphatic materials than coals. In addition, the tar yields from the two type III kerogen series studied (Middle Valley and Ikpikpuk) are found to be different. While this difference may result from a variation in maturity (the Middle Valley samples are almost all more mature than the Ikpikpuk samples), it seems likely that kerogens may produce various amounts of tar. This result suggests that, in contrast with oxygenated gas species, the pyrolysis tar yield is not series-independent. The increase in temperature for the maximum tar evolution rate is consistent with Rock-Eval Tmax results7,12 and could be also used as a maturity parameter. This feature may be especially useful for high-maturity samples, where the temperature shift becomes significant. As mentioned in the Results section, the tar yield in the type II-S Monterey kerogens is found to be much higher than in the type III kerogens and coals. An interesting observation can be made concerning these kerogens: standard maturity characterization methods, such as thermal alteration index, vitrinite reflectance, and quantitative fluorescence, were unsuccessful at estimating the maturity of each of the two samples,25 mainly due to the fact that no vitrinite is present in these samples. Other methods, using maturity indicators based on the extractable hydrocarbons or elemental composition, had to be used to estimate the maturity of the samples.25 However, the TG-FTIR results show clearly that these type II-S kerogens follow the CO2 and tar trends with maturity found in type III kerogens and coals. This feature, if reproducible in other kerogen (30) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668. (31) Solomon, P. R.; Serio, M. A.; Deshpande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42. (32) Stock, L. M.; Muntean, J. V. Energy Fuels 1993, 7, 704.

Pyrolysis Products from Coals and Kerogens. 1

series, may represent a useful method to estimate maturity in type II-S kerogens which do not contain vitrinite. CH4 Yield Variation with Maturity. While it is likely that multiple, complex reactions lead to CH4 formation, the experimental trends observed in the coal series can be explained in general terms as follows. In the case of coal, the two main sources of CH4 during pyrolysis are proposed to be hydroaromatics (i.e., cycloalkanes) and aryl methyls (i.e., methyls on aromatic rings) through multiple reaction sequences involving radical and hydrogen transfer.33 It appears that hydroaromatics are mostly responsible for the low-temperature CH4 and aryl methyls for the high temperature CH4. At the lignite stage, few hydroaromatics and aryl methyls are present,34 which explains the low CH4 yield during pyrolysis. However, long aliphatic chains have been observed to be present in lignites and subbituminous coals.35 During maturation between the lignite and bituminous stage, these aliphatic chains may undergo ring closure, increasing the concentration of hydroaromatics. A small increase in aryl methyls concentration has also been measured in that maturation range.34 Consequently, the CH4 yield during pyrolysis is increased. At the anthracite stage, the concentration of aryl methyls present has increased dramatically34 (possibly through processes of chain shortening and ring opening) but the hydroaromatics have been probably partially or completely consumed (either by decomposition or aromatization, the latter having been observed during laboratory hydrous pyrolysis36 ), which may explain the decrease in CH4 amount and the peak shift. While the same general trends in CH4 yield variation and evolution are found in the type III kerogen series (Ikpikpuk and Middle Valley), the yields are found to be lower than in the coal series and different between the two kerogen series. This is consistent with the trends observed in the case of tar, and may also be due to a higher fixed carbon content and/or a smaller aliphatic fraction in the case of kerogens. The decrease in CH4 is also seen to occur at lower reflectance values than in the coal series, which suggests a more efficient removal of CH4 precursors in the case of kerogens. In addition, the shift in peak temperature with increasing maturity is very pronounced. This feature has been previously found to correlate with maturity over a wide range of maturities.3 The type II-S Monterey kerogens have been found not to follow the trends described above. This is consistent with the fact that CH4 is assumed to primarily derive from aryl methyl groups and hydroaromatics33 which are less present in these kerogens than in type III kerogens or coals and may not be formed as readily during maturation. The presence of sulfur may also (33) Stock, L. M. Report, TSR program with Advanced Fuel Research, 1995. (34) Stock, L. M. personal communication, 1995. (35) Calkins, W. H.; Hagaman, E.; Zeldes, H. Fuel 1984, 63, 11131118. (36) Miknis, F. P.; Netzel, D. A.; Surdam, R. C. presented at the American Chemical Society Meeting, Spring Meeting, Geochemistry Division, 1995.

Energy & Fuels, Vol. 10, No. 1, 1996 25

possibly interfere with the formation of CH4 precursors during maturation or with the formation of CH4 during pyrolysis. Conclusions From this work, the following conclusions may be drawn: 1. With increasing rank, the trends of decreasing CO2, CO, and H2O, and increasing and decreasing CH4 and tar have been observed in almost all series of coal and kerogen samples. In all series (i.e., coal, type II-S and type III kerogen series), the oxygenated gas yields are found to be very similar for comparable levels of maturity. This feature, if confirmed in other series, may prove to be a series-independent criteria for maturity. The yields of tar and CH4 are, however, seen to be lower in the case of type III kerogens than coals, possibly due to a higher fixed carbon fraction and/or a lower aliphatic fraction. The tar and CH4 yields also seem to be seriesdependent. 2. The evolution rates and yields of CO2 may be a useful parameter for the determination of maturity for immature samples, while those of tar and CH4 Tmax, in accord with previous studies, can be used for highmaturity levels. The trends with maturity are, however, not as clear with kerogens as with coals. While a single parameter may be inaccurate in the estimation of maturity, multiple parameters such as the ones obtained by TG-FTIR may lift possible uncertainties. This may avoid errors in estimating maturity. 3. The type II-S kerogens used in this study appear to follow the same trends as the type III kerogens and coals in terms of oxygenated gas precursor removal during maturation and increase in pyrolysis tar yield. Maturity in this type of kerogen, which may sometimes be difficult to determine using standard methods, could be estimated using TG-FTIR data. 4. In agreement with previous work, the available kerogens series showed a large difference in tar amount between type II-S kerogens and type III kerogens. The tar yield provides a mean to easily differentiate type II-S and type III kerogens. These observations may provide a basis for understanding the chemistry of maturation for coals and kerogens. Acknowledgment. This work was supported under grant No. III-9203467 from the National Science Foundation. The authors thank Dr. Patrick Landais of CREGU (France), Total Indonesie, Pertamina, and Inpex for providing the Mahakam samples, Dr. Alan Burnham of the Lawrence Livermore National Laboratory for providing the San Juan samples, Dr. David Baskin of Chevron for providing the Monterey kerogens, and Dr. Jean Whelan for providing the Ikpikpuk and Middle Valley kerogen samples. The authors thank Prof. Eric Suuberg of Brown University for many useful discussions and insights, and Dr. Leon Stock of the Argonne National Laboratory for his contribution to the identification of possible mechanisms for gas formation during maturation as well as pyrolysis. EF950149+