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Energy & Fuels 1999, 13, 396-400
Thermogravimetric and Rock-Eval Studies Of Coal Properties and Coal Rank He Huang, Shaojie Wang, Keyu Wang, M. T. Klein, and W. H. Calkins* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Alan Davis Coal and Organic Petrology Laboratories, The Pennsylvania State University, 105 Academic Projects Building, University Park, Pennsylvania 16802-2300 Received April 20, 1998
A wide range of coal ranks from lignite to anthracite has been investigated using thermogravimetric analysis (TGA). The derivative peak maximum for volatile matter evolution shows a strong correlation (r2 ) 0.998) with vitrinite reflectance, thereby providing a convenient measure of degree of coalification (coal rank) without requiring the equipment of and more time-consuming petrographic analysis. Tmax by the Rock-Eval method also shows a positive although somewhat poorer correlation (r2 ) 0.856) with vitrinite reflectance. TGA also reproduces the components of the proximate analysis of coal.
Introduction Coalification is the process by which plant material has been progressively altered through peat, lignite, subbituminous, and bituminous coals to anthracite. This entire process significantly changes the chemical composition by the loss of much of the hydrogen and oxygen (and some of the other heteroatoms) of the coal. These changes are illustrated in what is now known as the “van Krevelen diagram”,1 in which the hydrogen to carbon atomic ratio (H/C) is plotted against the oxygento-carbon ratio (O/C). Of course, these chemical changes are somewhat different for the various plant-derived components which made up the original peat (wood, spores, charcoal, etc.). The proportions of these various petrographic components can be measured in the maceral analysis.2 The significant changes (chemical, physical, thermal, etc.) undergone during coalification have been used by various workers,3 the British Standards,4 and the American Society for Testing and Materials (ASTM)5 to devise systems for ranking or classifying coals according to their properties. Coalification has also been thought of as occurring in steps,1,2 representing the kinds of chemical transformation which have occurred; the first of these is diagenesis in which the plant material is broken down mainly by bacterial action to form peat. The second stage is categenesis in which the peat is transformed into the (1) Krevelen, D. W. v. Coal Typology, Physics, Chemistry, Constitution, 3rd ed.; Elsevier: New York, 1993; pp 91-95. (2) Stach, E.; Taylor, G. H.; Mackowsky, M. Th.; Chandra, D.; Teichmuller, M.; Teichmuller, R. Stach’s Textbook of Coal Petrology, 2nd ed.; Gebruder Borntraeger: Berlin, 1975; pp 34-172. (3) Alpern, B.; Lemos de Sousa, M. J.; Flores, D. Int. J. Coal Geol. 1989, 13, 1-19. (4) British Standard Methods for analysis and testing of coal and coke Method BS 1016 Part 3, 1973. (5) ASTM D388-95: Standard Classification of Coals by Rank.
higher ranks of coal as shown in the van Krevelen diagram. These processes are largely controlled by the time and temperature by being subjected to elevated temperatures as a result of being exposed to the temperature gradients (often in the range of 3-4 °C/ 100 m of depth) as the coal is buried deep in the earth. Because categenesis primarily involves thermal processes, thermal analysis of coals of the various stages of coalification should show measurable differences, and indeed thermal methods of analysis has been used by the coal industry for many years. In recent years, computerization and improvements in thermal analytical equipment have made the various thermal analytical methods of coal and other materials increasingly precise and automatic. Warne has published a recent overview of trends, methods, and applications of thermal analysis in fossil fuels.6 Ottaway used thermogravimetry to provide an alternative measure of the proximate analysis of coals and cokes.7 Warne also showed the value of determining proximate analysis of coal, oil shale, and low-quality fossil fuels by thermogravimetry.8 Determination of the rank of a coal by one parameter or set of parameters allows one to make a reasonable estimate of what the other properties of a particular coal will be. Parameters which have been used as indices of coal rank include volatile-matter yield, carbon content, the carbon/hydrogen atomic ratio, and the heats of combustion. One generally accepted and reliable measure of coal rank is mean maximum vitrinite reflectance.9 An advantage of this technique is that unlike most chemical methods, it is performed on a single (6) Warne, S. St. J. Thermochim. Acta 1990, 166, 343-349. (7) Ottaway, M. Fuel 1982, 61, 713. (8) Warne, S. St. J. Trends Anal. Chem. 1991, 10, 195-199. (9) Stach, E.; Taylor, G. H.; Mackowsky, M. Th.; Chandra, D.; Teichmuller, M.; Teichmuller, R. Stach’s Textbook of Coal Petrology, 2nd ed.; Gebruder Borntraeger: Berlin, 1975; pp 263-276.
10.1021/ef980088q CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
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Table 1. Analytical Data of the Coals Studied wt % daf basis no.
coal
rank
source
C
H
O
Sorg
N
MM-free Btu/lb
Ro mean max %
vit%maf
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Upper Freeport Wyodak-Anderson Illinois #6 Pittsburgh #8 Pocahontas #3 Blind Canyon Lewiston-Stockton Beulah-Zap DECS-1 DECS-2 DECS-3 DECS-11 DECS-12 DECS-18 DECS-19 DECS-21 DECS-22 DECS-25 PSOC-1516 PSOC-1523 DECS-8 DECS-9 DECS-10 DECS-17 DECS-26 DECS-27 PSOC-1486 PSOC-1515
mvb subC hvCb hvAb lvb hvBb hvAb lignite subC hvCb mvb ligA hvAb hvBb lvb an hvAb ligA lvb hvBb subC subB subB hvAb subB subA subB sa
Argonne Argonne Argonne Argonne Argonne Argonne Argonne Argonne Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State Penn State
85.50 75.01 77.67 83.20 91.05 80.69 82.58 72.94 74.27 78.11 87.37 73.14 83.32 79.08 89.87 90.33 84.57 74.60 88.83 83.69 74.43 75.54 78.19 81.61 75.48 77.84 73.30 88.07
4.70 5.35 5.00 5.32 4.44 5.76 5.25 4.83 5.64 5.44 5.88 4.46 5.69 5.80 4.90 4.01 5.84 5.22 4.68 5.40 5.23 5.18 4.22 6.21 6.11 5.34 5.91 3.91
7.51 18.02 13.51 8.83 2.47 11.58 9.83 20.34 17.45 9.73 4.32 20.59 8.37 8.89 3.31 4.30 5.59 18.30 3.29 8.48 18.49 17.94 15.21 10.33 16.92 14.71 18.29 6.07
0.74 0.47 2.38 0.89 0.50 0.37 0.65 0.70 1.02 2.67 0.57 0.43 0.81 2.09 0.55 0.48 0.69 0.40 0.87 0.82 0.50 0.33 0.54 0.44 0.38 0.51 0.84 0.68
1.55 1.12 1.37 1.64 1.33 1.57 1.56 1.15 1.46 1.32 1.74 1.00 1.37 1.44 1.14 0.80 1.77 1.07 1.64 1.58 1.00 0.90 1.05 1.29 1.02 1.28 1.33 1.13
15799 9363 13525 15083 15805 14046 14878 8382 8501 12522 15573 7676 14763 13421 15007 14760 15546 7781 14981 13550 8952 9811 9782 14205 9535 11060 10265 14245
1.16 0.32 0.46 0.81 1.68 0.57 0.89 0.25 0.36 0.52 1.28 0.35 0.87 0.56 1.71 5.19 0.77 0.23 1.73 0.92 0.37 0.38 0.42 0.59 0.29 0.46 0.38 2.80
92 89 85 85 89 87 73 74 78 87 94 74 83 86 89 87 30 74 90 62 79 88 74 80 86 74 90 91
maceral and so is largely independent of variations in petrographic composition. Not all laboratories are equipped to perform the precise optical measurements required for reflectance determinations, however, and alternative techniques for rank estimation are always of interest. Because of the many applications that have been developed for thermogravimetric analysis (TGA), TGA equipment is now available in many laboratories. Also, because of the rapid improvement and automation of TGA equipment, thermograms of coal are now comparatively easy and precise measurements to make. Differential thermal analysis (DTA, a relative of TGA) was used by Hollings and Cobb as early as 191410 to investigate coal rank. Somiya and Hirano11 used total volatiles yield of coals to follow changes in coal rank. Kopp and Harris12 used initial volatilization temperatures and average volatilization rates as a measure of coal rank. It is by using the temperature of maximum evolution rate of volatiles (the derviative of the weight loss curve, DTG), an easily defined maximum, as an index of coal rank that this paper is written. Another volatilization method also exists designed primarily for the study of kerogen in source rock as a means of determining their petroleum potential. This method, known as the Rock-Eval method,13,14 employs special instrumentation to follow the evolution of hydrocarbons and other organic materials and CO2 during the pyrolysis of kerogen. At the same time it measures the temperature maximum for volatiles evolution from kerogen. It does require special Rock-Eval equipment. Since this method does measure volatiles evolution from (10) Somiya, T.; Hirano, S. J. Chem. Soc. Ind. (Jpn.) 1930, 33, 737. (11) Hollings, H.; Cobb, J. W. J. Gas Light 1914, 126, 917. (12) Kopp, O. C.; Harris, L. A. Int. J. Coal Geol. 1984, 3, 333-348. (13) Espalite, J.; Laporte, J. L.; Madec, M.; Marquis, F.; Leplat, P.; Paulet, J.; Boutefeu, A. Rev. Inst. Fr. Pet. 1997, 32, 23-42. (14) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1978; pp 443-448.
rocks, it has also been used in evaluating coal. Bostick and Daws of the U. S. Geological Survey15 undertook an extensive investigation of the Rock-Eval technique as a measure of coal properties. In the present paper, results obtained with the TGA method are compared to those of Bostick and Daws using Rock-Eval and with vitrinite reflectance determinations based on identical coal samples. Experimental Section The thermogravimetric analyzer was a model 51 TGA (TA Instruments, New Castle, Delaware). An approximately 30 mg sample of each coal was loaded in a quartz pan and mounted in the instrument. All of the TGA runs were carried out in nitrogen under atmospheric pressure. For coalification studies or rank determinations, the TG operating parameters were standardized as follows. Step 1: Heat approximately 30 mg of the sample in a quartz pan at 10 °C/min from ambient temperature to 150 °C and hold for 10 min in 100 cm3(STP)/min of nitrogen. (NOTE: because of their high moisture content, lignites are heated at a lower rate (e.g. 1 °C/min) to 120 °C and held for 60 min in 100 cm3(STP)/min of nitrogen to allow the moisture to be eliminated.) The moisture content is determined by the weight change for lignites and higher rank coals. Step 2: Continue to heat the sample at 10 °C/min to 950 °C and hold for 7 min in the presence of nitrogen at the same purge rate. The volatile matter (VM) is determined by the weight change. Step 3: Switch to oxygen (100 cm3(STP)/min) at that temperature (950 °C) and hold for 30 min. The weight change measures the fixed carbon (FC), and the weight of the residue gives the ash content. Step 4: The derivative of the weight-loss curve is printed out and superimposed on the weight-loss curve, and the temperatures of each peak in that curve are recorded by the instrument. (15) Bostick, N. H.; Daws, T. A. Org. Geochem. 1944, 1, 35.
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Table 2. Determined TGA and DTG Characteristics of the Coals Studied TGA parameters, wt % (dry)
DTG peak parameters (daf)
no.
coal
rank
VM
FC
ash
T, °C
height, wt %/min
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Upper Freeport Wyodak-Anderson Illinois #6 Pittsburgh #8 Pocahontas #3 Blind Canyon Lewiston-Stockton Beulah-Zap DECS-1 DECS-2 DECS-3 DECS-11 DECS-12 DECS-18 DECS-19 DECS-21 DECS-22 DECS-25 PSOC-1516 PSOC-1523 DECS-8 DECS-9 DECS-10 DECS-17 DECS-26 DECS-27 PSOC-1486 PSOC-1515
mvb subC hvCb hvAb lvb hvBb hvAb lignite subC hvCb mvb ligA hvAb hvBb lvb an hvAb ligA lvb hvBb subC subB subB hvAb subB subA subB sa
31.2 40.06 36.2 31.0 17.1 40.1 27.4 37.7 37.6 32.2 23.0 38.3 30.9 35.7 15.8 2.9 27.6 38.0 15.0 28.2 38.8 38.0 34.4 43.6 41.0 30.8 37.2 7.4
56.3 50.4 47.1 57.6 78.1 55.1 52.3 52.0 46.8 52.0 69.2 53.3 58.3 52.2 77.2 84.3 48.4 50.5 73.6 54.3 47.5 55.0 51.8 48.8 51.2 54.5 43.3 62.5
12.5 8.9 16.7 8.4 4.5 4.8 20.3 10.3 15.6 15.8 7.8 8.4 10.9 12.1 7.0 12.8 24.0 11.5 11.4 17.5 13.7 7.0 13.8 7.7 7.8 14.7 19.5 30.0
499 447 456 478 526 464 477 442 452 458 501 445 479 460 524 711 471 446 527 480 451 456 454 460 450 457 454 582
2.03 2.05 3.53 2.60 1.04 2.81 2.60 1.82 2.60 2.69 1.61 1.51 2.89 3.27 1.16 0.0995 3.78 1.62 1.11 3.12 2.38 2.26 2.27 3.94 2.58 2.45 2.34 0.363
Table 3. Elemental Analysis of Rock-Eval Samples of Penn State Coals wt %, daf basis
coal no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Psoc71 Psoc95 Psoc122 Psoc124 Psoc147 Psoc156 Psoc182 Psoc240 Psoc245 Psoc278 Psoc297 Psoc317 Psoc321 Psoc380 Psoc401 Psoc407 Psoc410 Psoc500
rank
C
H
N
S
O
Btu/lb (MM free)
Ro mean max%
hvCb hvAb hvAb hvAb sa subA hvAb subB lig hvAb hvAb lvb mvb sa hvAb lvb lvb hvBb
77.32 81.64 82.93 83.62 92.17 76.48 81.02 72.59 72.02 75.94 81.87 86.03 87.19 91.25 80.51 88.97 88.11 79.59
5.X2 6.06 6.67 6.99 3.88 5.33 5.42 5.79 5.13 6.06 5.99 4.54 5.05 3.75 6.54 4.58 4.60 6.05
1.67 1.05 1.37 1.38 2.10 0.08 0.79 1.41 0.13 1.60 2.52 1.55 2.08 1.02 1.52 1.7S 1.94 1.23
0.58 1.79 1.05 1.09 0.81 1.44 1.79 1.30 1.53 6.85 2.06 7.40 3.09 0.58 3.73 1.51 1.12 0.90
14.61 9.46 7.99 6.92 1.04 16.68 10.98 18.91 21.19 9.55 7.55 0.48 2.59 3.39 7.69 3.19 4.24 12.24
13600 15060 15497 15972 15350 13248 14606 12838 12077 14262 14782 15478 15582 15202 15149 15585 15502 14314
0.60 0.84 0.75 0.55 2.47 0.42 0.59 0.48 0.31 0.56 0.71 1.57 1.35 2.77 0.73 1.62 1.69 0.58
Note: Heating and gas flow rates should be controlled within 5%. Under these conditions, the temperature peak can be determined to (2 °C. Twenty eight coal samples obtained from the Argonne and Penn State Coal Sample Banks ranging in rank from lignite to anthracite were investigated. Table 1 shows the coals studied, their sources, rank assignments, and analytical data including vitrinite reflectance. The temperature peak and peak height from thermogravimetric analysis and the derivative differential thermal analysis are shown in Table 2. All of the coals studied in this work were high-vitrinite coals or low-rank coals containing mainly huminite vitrinite precursors. An additional 18 coals from the Penn State Coal Sample Bank which had been run by Bostick and Daws by the RockEval method were also run by the same TGA method described above. The analytical data for these coals are given in Table 3. TGA parameters are given in Table 4. It should be noted that many of these additional coals have been stored for a considerable period of time under less than optimal conditions.
The Tmax numbers by the Rock-Eval data are taken from Bostick and Daw’s paper.15
Results and Discussion A sample TGA thermogram for Illinois No. 6 coal, a high-volatile bituminous coal from the Argonne Premium Coal Sample Bank, is shown in Figure 1. The variation of the volatile matter (VM), fixed carbon (FC), and ash (proximate analysis) determined by TGA is less than (2%. The first derivative of the TG curve (DTG) is also illustrated in Figure 1 and highlights the various volatilization processes. There is a main pyrolysis peak which represents the major portion of the volatile matter in the coal. The peak temperature and peak height (the maximum volatiles evolution rate) represent the significant parameters which can be used for following the coalification and estimating the coal rank.
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Energy & Fuels, Vol. 13, No. 2, 1999 399
Figure 1. TG scan on Illinois No. 6 high-volatile bituminous coal. Figure 3. Correlation of the peak height with the vitrinite reflectance (Ro).
Figure 2. Correlation of the peak temperature (Tpeak) with the peak height. Table 4. Rock-Eval and TGA Data of Penn State Coals no.
coal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Psoc71 Psoc95 Psoc122 Psoc124 Psoc147 Psoc156 Psoc182 Psoc240 Psoc245 Psoc278 Psoc297 Psoc317 Psoc321 Psoc380 Psoc401 Psoc407 Psoc410 Psoc500
rock-Eval method TGA method Ro mean max% rank Tmax (°C) Tpeak °C) hvCb hvAb hvAb hvAb sa subA hvAb subB lig hvAb hvAb lvb mvb sa hvAb lvb lvb hvBb
435 445 442 454 541 434 484 428 427 423 432 476 475 543 432 483 484 436
463.99 475.53 471.79 484.73 569.89 458.05 465.38 452.39 444.98 451.79 465.29 545.02 526.97 594.36 472.85 529.08 533.31 457.62
0.60 0.84 0.75 0.55 2.47 0.42 0.59 0.48 0.31 0.56 0.71 1.57 1.35 2.77 0.73 1.62 1.69 0.58
For this illustrative example, peak temperature and peak height were 456 °C and 2.71 wt %/min, respectively. It has been recognized for a long time that the rate of evolution of volatiles in coal pyrolysis increases with coal rank and then goes through a maximum and declines as rank increases further as shown in Figure 2, in which peak height (evolution rate) (DTG) is plotted against vitrinite reflectance, an accepted rank param-
Figure 4. Correlation of the peak temperature (Tpeak) with the vitrinite reflectance (Ro).
eter. Figure 3 shows that the peak height (evolution rate) is also a function of the peak evolution temperature, an easily measurable value by TGA. A plot of peak evolution temperature (TGA) against vitrinite reflectance (Figure 4) shows a strong correlation (r2 ) 0.998). This indicates that one can estimate vitrinite reflectance and therefore coal rank with some degree of confidence by TGA. This is generally quite true for vitrinite coals (such as Carboniferous coals of the Northern Hemisphere), but there may be anomalous relationships for vitrinite-poor coals such as durains and sapropelic coals. Figure 5 is a plot of vitrinite reflectance vs the DTG volatiles evolution peaks by TGA and vs the Rock-Eval peak maximum Tmax. While an excellent correlation (r2 ) 0.9544) is obtained for the DTG maximum vs reflectance, the correlation is not quite as good for Rock-Eval Tmax vs reflectance (r2 ) 0.8562). TGA is also capable of providing other useful information about coals. The DTG curve for Illinois No. 6 high-volatile bituminous coal in Figure 1 shows a pattern which is more complex than that of many of the other Argonne and Penn State coals. The smaller pyrolysis peaks are indicative of other volatilization processes taking place. The main volatile matter peak
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Summary and Conclusions Thermogravimetric analysis (TGA) using the DTG maximum volatiles evolution rates and the proximate analysis offers an alternative and very simple and reproducible method for assignment of coal rank for vitrinitic coals. As such, it provides a way of coal-rank determination that is more reproducible than the ASTM criteria when optical equipment for reflectance measurements are not available. Rock-Eval measurements of Tmax also give an indication of coal rank but are not quite as reproducible as TGA for coal-rank determination.
Figure 5. Peak temperatures of TGA and Rock-Eval vs Reflectance.
starts at 350-400 °C and actually consists of several weight-loss processes. The well-defined peak at 571 °C is tentatively identified as due to pyrite decomposition because this peak is absent in coals containing little or no pyrite. Also, the decomposition temperature corresponds closely to that reported for coal-derived pyrite.16 This serves as a qualitative indication of the presence of pyritic sulfur in the coal. Other small peaks are not yet identified but are fairly broad and indicative of distinct processes.
Acknowledgment. The support of this work by the Department of Energy under Grant No. DE2293PC93205 is gratefully acknowledged. The use of Argonne Premium Coal Samples (Dr. Karl Vorres) and the Penn State Coal sample bank (Dr. David Glick) is also acknowledged. The Rock-Eval results and the coal samples provided by Dr. Neely H. Bostick was an important contribution to this research. Additional funds for the purchase of thermal analysis equipment was provided by the University of Delaware. EF980088Q (16) Maes, I. I.; Yperman, J.; Van den Rul, H.; Franco, D. V.; Mullens, J.; Van Poucke, L. C.; Gryglewicz, G.; Wilk, P. Energy Fuels 1995, 9, 950-955.
