Energy & Fuels 2000, 14, 117-126
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Maceral Effects in the Determination of Proximate Volatiles in Coals A. G. Borrego,* G. Marba´n, M. J. G. Alonso, D. A Ä lvarez, and R. Mene´ndez Instituto Nacional del Carbo´ n, CSIC, La Corredoria s/n. Ap. 73, 33080 Oviedo, Spain Received March 30, 1999. Revised Manuscript Received October 13, 1999
The proximate and ultimate analyses of coals are commonly regarded as sufficient for a satisfactory description of their chemical composition. This is no doubt true if coal is considered as a whole, but it has to be borne in mind that coals are actually composed of fairly different materials (macerals), and that the corresponding chemical characterization data are average values of the individual compositions of the macerals present in the coal. As regards the determination of coal maturity, the volatile matter content is widely used as a rank parameter, although this characteristic varies not only with coalification but also with maceral composition. A full description of the chemistry of coals, for the prediction of their behavior during industrial processing, requires that the characteristics of their maceral components is also known. This paper is an attempt to evaluate the relative contributions of vitrinite, liptinite, and inertinite to the volatile matter content of coals, over the entire coalification scale. Proximate and petrographic analyses were carried out on 39 coals, and data from 992 more coals and 83 coal fractions were obtained from the literature and coal databases. A best-fit strategy was run with these data, so that the volatile matter contents of the three maceral groups and their variations with rank were determined. The results showed qualitative trends similar to those reported in the literature, but a substantial improvement in their predicting ability was achieved, and the ranges of applicability were expanded to the entire coalification scale.
Introduction Coal is a heterogeneous rock derived from plant debris which has undergone physical and chemical changes during its burial. The extent of these transformations determines the rank of coals, while the variations in the depositional environment and basinal conditions account for their different maceral compositions. An accurate determination of coal rank has proven to be of a foremost importance for the prediction of coal behavior under the variety of environmental conditions to which it is submitted during its industrial processing. It is well-known that the volatile matter content of coals decreases with the increase of rank, and this relationship has been wide and successfully used for the estimation of coal rank. However, the broadening of coal markets has made available coals differing in parent flora, geological period, and depositional environment, and therefore with rather different maceral compositions, which severely reduces the usefulness of the above-mentioned relationship. The main transformations observed in coal with increasing rank, as described in textbooks,1,2 can be summarized as follows: (i) a drop in moisture and a marked decrease in oxygen content due to the loss of hydroxyl, carbonyl, and carboxyl * Corresponding author. Phone: +34 985280800. Fax: +34 985297662. E-Mail:
[email protected]. (1) Stach, E.; Mackowsky, M.-Th.; Teichmu¨ller, M.; Taylor, G. H.; Chandra, D.; Teichmu¨ller, R. Coal Petrology, 3rd ed; Gebru¨der Borntraeger: Berlin, 1982; p 535. (2) Taylor, G. H.; Teichmu¨ller, M.; Davis, A.; Diessel, C. F. K.; Littke, R.; Robert, P. Organic Petrology; Gebru¨der Borntraeger: Berlin, 1998; p 704.
groups occur in the first place, followed by (ii) a removal of aliphatic and alicyclic groups, which causes an important reduction of volatile matter content, with a parallel increase in aromaticity during the bituminous coal stages; (iii) the anthracite stage is characterized by a rapid fall of hydrogen content and a particularly strong increase in both the reflectance and the optical anisotropy. Attempts have been made to relate the variations in the chemical composition of coals brought about by the increase of rank with optical parameters such as vitrinite reflectance. Thus, acceptable correlations were found whenever dealing with homogeneous coals of similar origins and maceral compositions, such as those carried out on German,3 Canadian,4 EuroAmerican,5 Japanese,6 Australian,7 and Indian8 series of coals. However, when coals of fairly different maceral compositions are used, the correlation between vitrinite reflectance and volatile matter content becomes much poorer,9 and therefore of little applicability for the estimation of coal rank. The reflectance of vitrinite has been vindicated by coal petrographers as the best rank parameter, since it is (3) Bartenstein, H.; Teichmu¨ller, R. Fortschr. Geol. Rheinld. u. Westf. 1974, 24, 129-160. (4) Haquebard, P.; Cameron, A. Int. J. Coal Geol. 1989, 13, 207260. (5) Krevelen, D. W. van. Coal; Elsevier: Amsterdam, 1961; p 514. (6) Takahashi, R.; Aihara, A. Int. J. Coal Geol. 1989, 13, 437-453. (7) Stephens, J. F.; Leow, H. M.; Gilbert, T. D.; Philp, R. P. Fuel 1985, 64, 1531-1536. (8) Chandra, D.; Chakrabarti, N. C. Int. J. Coal Geol. 1989, 13, 413435. (9) Krevelen, D. W. van. Coal, 3rd ed; Elsevier: Amsterdam, 1993; p 979.
10.1021/ef990050t CCC: $19.00 © 2000 American Chemical Society Published on Web 01/17/2000
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applicable to the whole range of coalification, it is measured on individual particles, and it changes regularly with rank. In addition, and contrary to the bulk chemical analytical data, it is independent of the maceral composition, with the single exception of certain liptinite-rich coals, where suppressed vitrinite reflectance values have been reported.10-12 Different attempts have been made to physically separate the individual maceral components of coals for their chemical characterization. Most of the methods take advantage of the differences in bulk density between the three maceral groups, by the use of different separating media and/or centrifugation.13 These methods are not without disadvantages, among them the laborious sample preparation techniques, which limit the amount of sample to be obtained, and the need of acid demineralization if mineral matter-free fractions are to be obtained. These difficulties do not, however, affect all the maceral groups to the same extent. Thus, quite pure vitrinite fractions can be obtained by handpicking from the coal seam, over the entire coalification range. Also, relatively pure inertinite fractions can be achieved by a careful and expert selection of inertiniterich lithotypes, which could be eventually further enriched by density separation. However, data of acceptably pure liptinite fractions from coal are commonly reported only for low rank coals, as the increase in coalification results in an increased similarity between the physical properties of the macerals groups, thus making the separation more difficult to attain. Furthermore, the elastic properties of liptinite, particularly the sporinite, prevent it from being crushed to a particle size small enough for an efficient density separation. Despite these difficulties, since the publication of the first van Krevelen diagram14 in the 1950s, it is known that the three maceral groups consist mainly of H, C, O, and different but generally low amounts of heteroatoms, and that, for a given rank, liptinite has the highest H content, inertinite has the highest C content, and vitrinite is the richest in O up to the anthracite rank. This results in a variation of volatile matter contents in the following order: liptinite > vitrinite > inertinite, these differences being attenuated with the increase in rank. Most of the earlier attempts to correlate volatile matter and maceral composition were directly or indirectly addressed to predicting the volatile yields in the coke ovens, and this is why the obtained trends15,16 are restricted to the specific rank interval of the coking coals. Given the higher prices of these coking coals, other conversion processes such as liquefaction, gasification, and specially combustion, where an understanding of the mechanisms of volatiles formation and evolution is also crucial, preferentially make use of coals which lie outside that rank range. In this paper, an attempt is made to evaluate the relative contribution of the volatiles released from each (10) Hutton, A. C.; Cook, A. C. Fuel 1980, 59, 711-714. (11) Kalkreuth, W. D. Bull. Can. Petr. Geol. 1982, 30, 112-139. (12) Raymond, A. C.; Murchison, D. G. Fuel 1991, 70, 155-162. (13) Winans, R. E.; Crelling, J. C. In Chemistry and characterization of coal macerals; Winans, R. E., Crelling, J. C., Eds.; ACS Symposium Series 252, 1984; pp 1-20. (14) Krevelen, D. W. van. Fuel 1950, 29, 269-271. (15) Kro¨ger, C.; Pohl, A. Brennst.-Chem. 1957, 38, 102-107. (16) Diessel, C. F. K.; Wolff-Fischer, E. Int. J. Coal Geol. 1987, 9, 87-108.
Borrego et al. Table 1. Proximate Volatiles and Petrographic Analyses of Coalsa code
country
VM daf (%)
Rr (%)
FG2 ILL FG3 CAI CAS LEA FGI VDD INS GED DRA COS CER PT2 BMC WA2 BB2 WWI PT3 SA1 SA2 LEM CRO KEL CAN BEN LOH PHA CRB NOR BSE SBB FM3 FM1 HRR CRA SMK TAF DAN
CA USA CA BR BR BR CA CA IN UK AU CO CO ES NZ AU AU AU ES SA SA AU SA UK ES UK UK CA CA UK NZ ES CA CA DE CA CA UK DE
43.5 45.0 45.1 41.2 39.7 41.8 40.7 39.3 46.0 40.5 39.4 42.9 39.4 38.1 46.2 36.9 24.8 24.2 38.4 33.7 30.6 37.4 30.0 40.2 39.3 37.1 35.8 36.7 28.6 31.5 31.9 33.1 28.9 25.6 27.8 22.3 18.1 13.6 6.2
0.43 0.44 0.46 0.47 0.48 0.48 0.49 0.56 0.58 0.58 0.59 0.60 0.61 0.62 0.64 0.66 0.66 0.66 0.66 0.68 0.68 0.71 0.71 0.73 0.74 0.76 0.79 0.84 0.97 0.98 1.03 1.04 1.05 1.07 1.14 1.23 1.53 1.77 3.23
Vitrinite Inertinite Liptinite (vol mmf%) 79.4 78.9 79.2 69.4 58.2 59.2 60.6 68.8 92.0 74.3 50.1 84.4 81.8 69.6 85.8 57.2 1.6 1.2 65.6 27.2 30.6 65.9 25.4 66.4 90.2 81.9 85.2 80.4 61.2 73.1 96.2 93.4 55.0 55.6 77.6 53.0 57.8 71.6 61.8
18.6 12.8 19.6 28.0 33.2 28.8 36.0 28.6 3.8 11.1 40.7 13.2 17.0 26.4 2.4 39.2 97.2 98.8 21.4 67.6 66.0 29.1 68.4 16.7 4.6 10.4 9.0 9.4 38.2 17.1 0.0 3.6 45.0 44.0 17.8 47.0 42.2 28.4 38.2
2.0 8.3 1.2 2.6 8.6 12.0 3.4 2.6 4.2 14.6 9.2 2.4 1.2 4.0 11.8 3.6 1.2 0.0 13.0 5.2 3.4 5.0 6.2 16.9 5.2 7.7 5.8 10.2 0.6 9.8 3.8 3.0 0.0 0.4 4.6 0.0 0.0 0.0 0.0
a VM ) volatile matter, R ) random reflectance, vol ) volume, r daf ) dry-ash-free, mmf ) mineral matter-free.
maceral group to the proximate volatiles of coals over a wider rank range. Proximate and petrographic data from 39 selected coals of varying rank and maceral composition will be used, plus those of well-characterized coal sets obtained from the literature and coal databases. Both the maceral and the reflectance analysis of coals are time-consuming, make use of microscopy facilities of moderate to high cost, and require a trained operator to carry them out. For these reasons, the proximate volatiles test will always be the preferred method for the estimation of volatile matter contents, but the qualitative aspects of coal pyrolysis would no doubt benefit from a deeper knowledge about the pyrolysis behavior of each individual maceral. Samples and Data Thirty-nine coals of different provenance, geological age, and maceral composition, covering a wide range of maceral occurrences and ranks, have been analyzed for proximate, maceral, and vitrinite random reflectance standard analyses (Table 1). Proximate volatiles were obtained following the standard UNE 32-019-84 and the values were corrected for the CO2 evolved from carbonates calcination as described in ISO-168. The data are expressed in a dry, ash-free basis, and ash contents were
Determination of Proximate Volatiles in Coals
Energy & Fuels, Vol. 14, No. 1, 2000 119
Table 2. Source of Data from Literature Indicating the Reference and the Corrections Applied to the Data Set reference Crelling et
al.19
table
N
type
Tables 1, 3
18 coal
Furimsky et al.20
Tables 1, 2
13
Milligan et al.21
Tables 1, 2
9
Teichmu¨ller22
Table 2
19
Diessel & Wolf-Fischer23 Pugmire et al.24
Table 1 Table 2
6 9
Roy et al.25
Tables 1, 2
15
Fermont et al.26
Tables 1, 2, 4, 5, 6, 8
19
van Krevelen9
compendium
23
Kuehn et al.28 Table 1 SBN database Argonne Premium Coals34 Penn State database a
24 104 6 796
corrections applied
VM recalculated to dry-ash-free basis. Rmax corrected to Rr16 (LK and H series). coal VM recalculated to dry-ash-free basis. Rmax corrected to Rr16 (10 samples). 3 coals rejected because of their high ash contents, and 5 for their low Rr (
