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Application of TG–FTIR to the determination of organic oxygen and its speciation in the Argonne premium coal samples
Fuel Processing Technology 87 (2006) 335 – 341 www.elsevier.com/locate/fuproc
Application of TG–FTIR to the determination of organic oxygen and its speciation in the Argonne premium coal samples J.A. MacPhee *, J.-P. Charland, L. Giroux CANMET Energy Technology Centre-Ottawa, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1 Received 1 April 2005; accepted 1 October 2005
Abstract During rapid pyrolysis of coal, TG – FTIR (thermogravimetry – Fourier transform infrared) technique can be effectively used to simultaneously detect and measure the three main O-containing gases, namely H2O, CO and CO2. Their sum corresponds to the quantitative amount of oxygen in the coal and is, in general, inherently more accurate than the Fby-difference_ values. In this paper, we first attempt to relate the Fby-difference_ values for %O reported for the Argonne premium coal samples (lignite to bituminous rank) (Argonne Users Handbook) to those determined from a TG – FTIR examination of the pyrolysis gases evolved. Another objective of the work is to relate the pyrolysis gases (H2O, CO and CO2) evolved to oxygen-containing functional groups found in coals as well as the evolution of these functional groups as a function of rank. Correlations are also developed between the TG – FTIR oxygen values and other parameters determined for the Argonne Premium Coals. In particular, comparisons of our results using TG – FTIR with analyses carried out by other workers on functional group analysis of acidic groups are considered. D 2005 Elsevier B.V. All rights reserved. Keywords: Argonne premium coals; Oxygen content; TG – FTIR; Oxygen functional groups
1. Introduction Knowledge of the amount of organic oxygen in a coal sample is important for many reasons. In coal conversion processes, it has a bearing on the process parameters necessary to obtain high conversion and determines the difficulty of upgrading liquid products to stabilized chemical entities. For coking coals, it may indicate undesirable weathering that will have a negative impact on coke quality. Combustion techniques are commonly used in the analysis of coal and organic compounds in general for the determination of elemental hydrogen, carbon and nitrogen according to ASTM D3176-89 [1]. For oxygen, the method in common practice involves the determination Fby-difference_ from directly determined values for moisture, ash, sulfur, hydrogen, carbon and nitrogen. In spite of the inherent errors of this approach, which may be significant, it must be recognized that, in many cases, oxygen values obtained Fby-difference_ are adequate; for others, such as studies of coal weathering, more accurate values are required. * Corresponding author. E-mail address: [email protected] (J.A. MacPhee). 0378-3820/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2005.10.004
We have shown recently [2] that the nature of oxygen functional groups is important in assessing the degree of oxidation of a coal. In many cases it is impossible to know whether a particular coal has suffered oxidation purely from the Fby-difference_ oxygen value because the overall oxygen content of the sample has not changed in a manner that is detectable by this approach. Some years ago, a pyrolysis technique was developed for the direct determination of oxygen using the Perkin Elmer 240 Microanalyser [3]. This technique was based on earlier work, which showed that all of the organic oxygen in coal could be liberated by pyrolysis as H2O, CO and CO2. Culmo showed that the passage of the pyrolysis gases over hot activated carbon converts all the oxygen to CO. Subsequent passage of the CO over cupric oxide forms CO2, which can be identified quantitatively by a suitable detector. The pyrolysis technique has been developed by other groups as well [4,5]. For a number of years, we have used this technique routinely in our work on coal oxidation where small changes in oxygen content are important [6,7]. However, although we have found this technique both useful and reliable, it does not provide information about the chemical speciation of
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J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
TGA/FTIR spectrometer consisting of a DuPont 951 TGA, multipass gas cell, Michelson 110 FTIR and microcomputer was used. The system can simultaneously control furnace temperature, record weight loss and capture IR spectra for up to 20 evolving species in real time. Typically, a 25– 35 mg sample was continuously weighed in the TG/Plus system while heated using the following temperature program: a drying step at 150 -C followed by pyrolysis in helium with a temperature ramp from 150 to 900 -C at 30 -C/ min. Gases and volatiles were entrained in a gas cell by a helium stream where the rate and amount of the IR-active species were quantitatively measured. During the thermal ramp program, infrared spectra are collected every 30 s. On-line analysis and post process manipulations of the data were performed by the TG/Plus and SpectraCalc software packages. In general, the spectra obtained from the coal samples showed absorption bands for tars, CH4, C2H4, NH3, CO, CO2, COS, SO2 and H2O. The TG/Plus quantitative analysis program
oxygen in coal. This has led us to consider the use of thermogravimetry coupled to gas analysis by infrared spectroscopy (TG –FTIR) to measure organic oxygen in coal directly. Although this technique, developed by Solomon et al. [8], has been extensively used by our group [9– 11] and others, it appears not to have been considered for this particular purpose. In this work, we report the application of TG – FTIR under pyrolysis conditions to measure total organic oxygen and examine oxygen speciation in the Argonne premium coal samples. 2. Experimental The technique has been described elsewhere in some detail [9]. Here follows a brief description. TG – FTIR — as mentioned above, the TG –FTIR technique has been developed and used extensively by Solomon et al. [8]. In the current configuration of the system, a Bomem TG/Plus
a
3.0
H2O CO CO2 Temperature
2.5
1000
800
600
T,°C
Wt%/min
2.0
1.5 400 1.0 200
0.5
0.0
0 0
10
20
30
40
50
60
70
Time, min
b
0.8 H2O CO CO2
Wt%/min
0.6
0.4
0.2
0 100
250
400
550
700
850
1000
T,°C Fig. 1. Illinois #6 coal: (a) evolution of H2O, CO, CO2 and thermocouple temperature vs. time; (b) evolution of H2O, CO and CO2 vs. temperature.
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Table 1 Analysis of Beulah-Zap and Wyodak coals (CETC-Ottawa) (wt.%, as analysed)
employs a database of calibrated IR spectra of gases/volatiles to obtain their relative amounts. All proximate and ultimate analyses were carried out according to ASTM D3176-89 [1].
Analysis
3. Results and discussion Plots of the H2O, CO and CO2 evolved during pyrolysis are used to estimate the total organic oxygen in a coal sample. Typical plots for Illinois #6 and Beulah-Zap are given in Figs. 1a, b and 2a, b, respectively. Integration of these curves yield the wt.% of the various oxygen containing gases evolved from the coal from which the contribution to the total oxygen content is calculated. In Fig. 1b, there is a sharp peak in the CO2 evolution curve at 765 -C that corresponds to calcite decomposition in the Illinois #6 coal [8,12]. The shape of the water evolution curves for Illinois #6 and Beulah-Zap is important in the present context. The curve for the former indicates no water evolution below
a
337
Coal Beulah-Zap
Wyodak
Proximate Moisture Ash VM + FC
17.17 7.97 74.86
16.39 7.42 76.19
Ultimate C H N S O (Fby-difference_)
53.88 3.50 0.83 0.67 15.98 (12.32a)
56.40 4.01 0.89 0.57 14.32 (11.68a)
a
Argonne Users Handbook.
¨300 -C and is a fairly simple bell-shaped curve consisting of two or more major components, whereas the curve for the latter indicates small yet important amounts of water
1.5
H2O CO CO2 Temperature
1.25
1000
800
600
T, °C
Wt%/min
1
0.75 400 0.5 200
0.25
0
0 0
10
20
30
41
51
61
71
Time, min
b
1.5 H2O CO CO2
1.25
Wt%/min
1
0.75
0.5
0.25
0 100
250
400
550
700
850
1000
T, °C Fig. 2. Beulah-Zap lignite: (a) evolution of H2O, CO, CO2 and thermocouple temperature vs. time; (b) evolution of H2O, CO and CO2 vs. temperature.
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J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341 1.25 not heat-treated heat-treated, 24 h @ 75C heat-treated, 24 h @ 105C
Wt%/min
1
0.75
0.5
0.25
0 0
200
400
600
800
1000
T, °C Fig. 3. Beulah-Zap lignite — effect of heat treatment on H2O evolution vs. temperature.
evolution at surprisingly low temperatures, starting at ¨ 120 -C. How we interpret this phenomenon, this low-temperature water, is of some significance to this work and to the analysis of lignites in general. The question of whether this water is bulk water or pyrolysis water has to be considered since it determines whether this water originates as organic oxygen or not. We will indicate our interpretation of this below in some detail. In previous work that considered bituminous and subbituminous coals [7,9,10] but not lignites, the estimation of total oxygen via TG –FTIR was rather straightforward with total oxygen values close to our in-house by-difference values. In the present work, six of the eight Argonne samples behaved as expected but difficulties were encountered with Wyodak (sub-bituminous) and Beulah-Zap (lignite). For the latter two coals, the TG –FTIR values were several percentage points higher than the dry Fby-difference_ values reported in the Argonne Users Handbook, Vorres [12]. To address this problem, we first had Wyodak and BeulahZap analysed by our own characterization laboratory. Prior to analysis, the two coals were dried at ambient temperature overnight. We found that pre-drying of the sample (under nitrogen) was necessary to obtain reliable sample weights for analysis since both samples in their pristine state from the vials
had ¨30% moisture. Allowing the moisture to stabilize at ambient conditions made the analysis much simpler. Results are reported in Table 1 where some differences with the previously reported results are noted. In particular, the carbon values from our laboratory are ¨1% lower that the reported values making the O Fby-difference_ values higher by the same amount. These new values were still lower than the %O values determined by TG – FTIR. In an attempt to explain this occurrence, the TG –FTIR behaviour of Beulah-Zap pre-dried at 75 -C and 105 -C for 24 h in a nitrogen atmosphere was examined. The water evolution curves for the un-dried and pre-dried coals are shown in Fig. 3. It is clear that even at 75 -C the water evolution peak at ¨150 -C is drastically reduced, suggesting that this is bulk water which is not removed in conventional drying at 105 -C and consequently should not be considered as contributing to the overall organic oxygen of the sample. As a consequence of this finding, the water evolution peaks for both Beulah-Zap and Wyodak were integrated manually so as to eliminate this low-temperature contribution to the overall water evolution. The work of Bartholomew et al. [13] on measurements of surface and pore properties of Argonne National Laboratory and Pittsburgh Energy Technology Center coals found that
Table 2 Argonne premium coals — TG – FTIR data Coal
Rank
H2 O (wt.%; dba)
CO (wt.%; db)
CO2 (wt.%; db)
O (wt.%; db)
H2O (mmol/g; db)
CO (mmol/g; db)
CO2 (mmol/g; db)
O (mmol/g; db)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
lvb mvb hvAb hvAb hvCb hvBb subb C lig A
1.15 3.12 4.09 4.65 5.58 6.00 9.97 11.23
1.99 2.09 2.70 3.05 3.69 4.67 8.42 9.00
0.70 1.14 1.42 1.39 1.82 2.00 5.53 7.41
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
0.64 1.74 2.27 2.58 3.10 3.33 5.54 6.24
0.71 0.75 0.97 1.09 1.32 1.67 3.01 3.22
0.16 0.26 0.32 0.32 0.41 0.45 1.26 1.68
1.67 3.02 3.91 4.33 5.30 5.92 11.08 12.85
a
Dry basis.
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
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Table 3 Comparison of oxygen concentrations — Argonne and TG – FTIR
Table 4 Argonne premium coals — contributions to overall O content, wt.% (db)
Coal
Coal
O (H2O)
O (CO)
O (CO2)
O total (FTIR)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
1.02 2.79 3.66 4.15 5.01 5.34 8.88 10.00
1.14 1.20 1.55 1.75 2.13 2.67 4.82 5.16
0.51 0.84 1.04 1.02 1.34 1.46 4.03 5.40
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
O
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap a b
Argonne (wt.%; db)
TG – FTIR (wt.%; db)
2.36a 4.84a 6.74a 7.79a 8.65a 10.79a 17.13b 19.29b
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
Argonne Users Handbook. Determined at CANMET Energy Technology Centre-Ottawa.
containing carbonates was obtained by subtracting the inorganic CO2 contribution from the total CO2 measured.
most of the internal surface area of Wyodak and Beulah-Zap coals consists of micropores having diameters less than 1 nm. Also, the low pore volumes reported by Bartholomew et al. [13] for these coals lends support to our attribution of the shoulder peak of the H2O evolution profile of Beulah-Zap in Fig. 3 to bulk water. In light of what has been stated above concerning the data treatment, the amounts of H2O, CO and CO2 and the corresponding total organic oxygen evolved from the Argonne Premium Coals using TG –FTIR are given in Table 2. The total organic oxygen values obtained for the Argonne coals by TG – FTIR and Fby-difference_ are given in Table 3 for comparison purposes. These values are plotted in Fig. 4 where the least squares line is constrained to pass through zero. It should be mentioned that the total organic oxygen measured via TG –FTIR and presented in this paper refers strictly to organic oxygen, although four of the eight Argonne coals, namely Pocahontas #3, Illinois #6, Blind Canyon and Beulah-Zap, were found to contain inorganic oxygen from the decomposition of calcite and also siderite in the case of Pocahontas #3. The fraction of inorganic oxygen present in these four coals was obtained through CO2 profile resolution and was based both on the evolution temperature for CO2 from decomposition of pure siderite and calcite via TG –FTIR [9,10]. In other words, the amount of organic CO2 listed in Table 2 for the Argonne coals identified as
3.1. Oxygen distribution as a function of rank The contributions from H2O, CO and CO2 to organic oxygen in the coals are shown in Table 4 and graphically in Fig. 5. As expected, there is a general monotonic increase in all quantities with decreasing carbon content. Extensive work on oxygen functional analysis has been reported and discussed in Van Krevelen [14]. The shapes of curves of functional group content versus carbon content provided in that reference are concave downward while those shown in Fig. 5 are all concave upward. The results cited in Van Krevelen are from a variety of sources and make use of a variety of chemical techniques. Van Krevelen points out that, except for peat and brown coal, the sum of the functional group oxygen and the total oxygen in the coal are in agreement. Consequently, it is doubtful that ‘‘unreactive’’ oxygen really exists. In the present work, it has been shown that all the oxygen in the coal sample is expelled during pyrolysis and measured by the amounts of the three gases, H2O, CO and CO2. The normalized organic oxygen content occurring in the three O-containing pyrolysis gases is given in Table 5 where it is seen, with the exception of Pocahontas #3, the highest rank coal in the series, that there is a remarkable similarity of the various contributions as noted previously [9,10].
25 y = 1.01x R2 = 0.98
20
O (TG-FTIR), wt% (db)
O (TG-FTIR), wt% (db)
25
15
10
5
0
20 O (Total)
15
10
O (H2O) O (CO)
5
O (CO2)
0
5
10
15
20
25
O (Argonne, by difference), wt% (db)
0 70
75
80
85
C (Argonne), wt% (dmmf) Fig. 4. Comparison of the TG – FTIR organic O (wt.%, db) with the Argonne by-difference values.
Fig. 5. O (TG – FTIR) vs. C (Argonne).
90
95
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341 7
Table 5 Argonne premium coals — normalised organic O wt.% Coal
O (H2O)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
O (CO)
38 58 58 60 59 57 50 49
O (CO2)
43 25 25 25 25 28 27 25
19 17 17 15 16 15 23 26
H2O (TG-FTIR), mmol/g (db)
340
6 y = 3.05x - 2.55 2 R = 0.65
5 4 3 2 1 0 1.0
3.2. Oxygen speciation
1.5
2.0
2.5
3.0
Ar-OH (Aida), mmol/g (db)
The concentrations of –OH and – CO2H functional groups occurring in the Argonne Premium Coals have been measured by Aida et al. [15] using a chemical method. His method involves the reaction of carboxyl functional groups in coal with n-Bu4NBH3 in pyridine. The amount of hydrogen evolved is assumed to be equivalent to the carboxyl concentration in the coal. Carboxylate groups are not detected by this method but, after acid washing, the total carboxyl content was measured to yield carboxyl and carboxylate amounts in the original coal. The underlying assumption is that this reagent (n-Bu4NBH3) reacts only with carboxyl groups and not with phenolic –OH groups. Corroborating evidence from solid-state 13C NMR supports the assumption. A further reagent, LiBH3 in pyridine, which reacts with both carboxyl and phenolic – OH generating hydrogen, was used to measure the concentrations of these two functional groups together. The concentration of phenolic –OH is then calculated from the difference between total acidic groups and the total carboxyl group concentrations. A direct comparison between the pyrolysis data of our work and Aida’s functional group analysis is therefore possible. We first consider the relationship between pyrolysis CO2 and total carboxyl plus carboxylate measured chemically by this author shown in Fig. 6. It is evident that the pyrolysis CO2 is equivalent to the – CO2H(M) measured by Aida. From a chemical point of view, this is reasonable since it is difficult to envisage the production of CO2 from any other functional group as pointed out by Schafer [4] for Australian brown coal decomposition. Fig. 6 therefore constitutes a validation of both
Fig. 7. H2O (TG – FTIR) vs. Ar – OH functionality (Aida et al. [15]).
the Aida and the pyrolysis (TG – FTIR) approaches to the measurement of carboxyl groups in coal. It is worth pointing out that the curves for the evolution of CO2 as a function of temperature are not simple, indicating more than one component. It is possible that carboxyl and carboxylate decompose to produce CO2 in different temperature ranges and that the relevant information may be extracted from the pyrolysis curves by deconvolution. This point will be examined in future work. The situation for acidic – OH groups is somewhat different. A plot of pyrolysis H2O as a function of –OH groups from Aida’s work indicates that there is more water evolved than can be accounted for by his estimation of phenolic – OH groups (Fig. 7). There exists a large body of work on phenolic – OH groups in coal estimated by a variety of techniques that was reviewed recently [16]. This author sums up the literature data on phenolic –OH groups by means of the following equation: ln Oph ¼ 130:3909 þ 96:124lnðcC Þ 134:133cC ðr ¼ 0:859Þ where c C is the fractional carbon content on a dry, ash-free basis. A comparison of phenolic –OH content generated by the above equation and the experimental work of Aida for the Argonne coals indicates that the latter are somewhat lower than the ‘‘consensus’’ values assembled by Gagarin [16]. Some of 7
1.75
H2O (TG-FTIR), mmol/g (db)
CO2 (TG-FTIR), mmol/g (db)
2
y = 1.12x - 0.22 2 R = 0.84
1.5 1.25 1 0.75 0.5 0.25 0
6 y = 1.07x + 0.04 R2 = 0.87
5 4 3 2 1 0
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
COOH+COOM (Aida), mmol/g (db) Fig. 6. CO2 (TG – FTIR) vs. COOH + COOM functionality (Aida et al. [15]).
0
1
2
3
4
5
6
7
Ar-OH (van Krevelen), mmol/g (db) Fig. 8. H2O (TG – FTIR) vs. Ar – OH functionality (Van Krevelen [14]).
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
the work dates from Van Krevelen’s group in the 1950s [17,14]. It involved measurement of phenolic – OH groups by acetylation with acetic anhydride in pyridine for a relatively wide range of coal rank. From that work, it is possible to estimate the phenolic –OH content for the Argonne coals from the appropriate carbon content. A comparison of the pyrolysis H2O content of the Argonne coals with the phenolic – OH from van Krevelen’s work is given in Fig. 8. The correlation between these two values is excellent and, perhaps what is more significant, the slope is close to unity and the intercept of the least squares line is close to zero. This leaves us with the intriguing conclusion that the pyrolysis water originates mainly as phenolic – OH, although it is difficult to see how this can be the case since it is likely that the reaction mechanisms producing pyrolysis water are complex. 4. Observations and conclusions 1. TG –FTIR can be reliably used for the determination of the organic oxygen content of coals in general as indicated by the results reported here for Argonne premium coals. 2. Low rank coals contain water that is difficult to remove at 105 -C. 3. The normalized amounts of oxygen found in the gaseous species H2O, CO and CO2 remain remarkably constant with changes in rank. 4. Pyrolysis CO2 corresponds to the CO2H(M) functional group in coal. 5. The quantity of pyrolysis H2O is accounted for almost entirely by – OH functional groups (using the experimental results of Van Krevelen) indicating this functional group as its principal source. Acknowledgement The authors would like to thank the Canadian Carbonization Research Association for support of this work. References [1] ASTM Standards, Volume 05.06, Gaseous Fuels; Coal and Coke, ASTM International, 100 Barr Harbor Drive, PO Box C700, W. Conshohocken, PA 19428-2959, USA, 2004.
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[2] J.A. MacPhee, L. Giroux, J.-P. Charland, J.F. Gransden, J.T. Price, Detection of natural oxidation of coking coal by TG – FTIR — mechanistic implication, Fuel 83 (2004) 1855 – 1860. [3] R. Culmo, Microdetermination of oxygen in organic compounds with an automatic elemental analyzer, Microchimica Acta (1968) 811 – 815. [4] H.N.S. Schafer, Pyrolysis of brown coals: 2. Decomposition of acidic groups on heating in the range 100 – 900 -C, Fuel 58 (1979) 673 – 679. [5] B. Zischka, H. Stemming, Oxygen determination in solid fuels, Erdoel Kohle, Erdgas, Petrochem 38 (5) (1985) 221 – 224. [6] J.A. MacPhee, W.P. Hutny, L. Giroux, J.-P. Charland, J.T. Price, Characterization of coal weathering/oxidation by TGA – FTIR, Proc. Int. Conf. on Coal Science, Banff, Sept. 12 – 19, 1993, pp. 590 – 593. [7] J.A. MacPhee, L. Giroux, J.-P. Charland, J.T. Price, A study of the oxidation pathways of coking coal by thermogravimetric Fourier transform infrared spectroscopy (TG – FTIR), Proc. Int. Conf. on Coal Science, San Francisco, 2001. [8] P.R. Solomon, M.A. Serio, R.M. Carangelo, R. Bassilakis, D. Gravel, M. Baillargeon, F. Baudais, G. Vail, Analysis of the Argonne premium coal samples by thermogravimetric Fourier transform infrared spectroscopy, Energy and Fuels 4 (3) (1990) 319 – 333. [9] J.-P. Charland, J.A. MacPhee, L. Giroux, J.T. Price, M.A. Khan, Application of TG – FTIR to the determination of oxygen content of coals, Fuel Processing Technology 81 (2003) 211 – 221. [10] J.A. MacPhee, J.-P. Charland, L. Giroux, J.T. Price, W.P. Hutny, M.A. Khan, Determination of organic oxygen in coals with application to the study of coal weathering, ACS Division of Fuel Chemistry Preprints 46 (1) (2001) 340 – 342. [11] J.A. MacPhee, L. Giroux, J.-P. Charland, R. Sakurovs, J.T. Price, Interactions between coals in binary blends during heating as studied by thermal rheology, PMRTA, TG – FTIR and optical microscopy, Proc. 18th Ann. Pittsburgh Coal Conf., Newcastle Australia, Dec. 4 – 7, 2001. [12] K.S. Vorres, Users Handbook for the Argonne Premium Coal Sample Program: accessed December 9, 2003 at URL http://www.anl. gov/PCS/pcshome.html, 1993. [13] C.H. Bartholomew, W.E. White, D. Thornock, W.F. Wells, W.C. Hecker, L.D. Smoot, D.M. Smith, F.L. Williams, Surface and pore properties of ANL and PETC coals preprints, Fuel Chemistry Division, American Chemical Society 33 (3) (1988) 24 – 31. [14] D.W. Van Krevelen, Coal: Typology, Physics, Chemistry, Constitution, Third edR, Elsevier, Amsterdam, 1993. [15] A. Aida, A. Nishisu, M. Yoneda, T. Yoshinaga, Y. Tsutsumi, I. Yamanishi, T. Yoshida, Chemical determination of oxygen-containing functionality in Argonne premium coal samples, Fuel Chemistry Division Preprints 46 (1) (2001) 325 – 327. [16] S.G. Gagarin, Forms of Oxygen in Coal Organic Matter (Review) Coke and Chemistry, No. 10, 2001. [17] L. Blom, L. Edelhausen, D.W. van Krevelen, Chemical structure and properties of coal: XVIII. Oxygen groups in coal and related products, Fuel 36 (1957) 135 – 153.
Application of TG–FTIR to the determination of organic oxygen and its speciation in the Argonne premium coal samples J.A. MacPhee *, J.-P. Charland, L. Giroux CANMET Energy Technology Centre-Ottawa, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1 Received 1 April 2005; accepted 1 October 2005
Abstract During rapid pyrolysis of coal, TG – FTIR (thermogravimetry – Fourier transform infrared) technique can be effectively used to simultaneously detect and measure the three main O-containing gases, namely H2O, CO and CO2. Their sum corresponds to the quantitative amount of oxygen in the coal and is, in general, inherently more accurate than the Fby-difference_ values. In this paper, we first attempt to relate the Fby-difference_ values for %O reported for the Argonne premium coal samples (lignite to bituminous rank) (Argonne Users Handbook) to those determined from a TG – FTIR examination of the pyrolysis gases evolved. Another objective of the work is to relate the pyrolysis gases (H2O, CO and CO2) evolved to oxygen-containing functional groups found in coals as well as the evolution of these functional groups as a function of rank. Correlations are also developed between the TG – FTIR oxygen values and other parameters determined for the Argonne Premium Coals. In particular, comparisons of our results using TG – FTIR with analyses carried out by other workers on functional group analysis of acidic groups are considered. D 2005 Elsevier B.V. All rights reserved. Keywords: Argonne premium coals; Oxygen content; TG – FTIR; Oxygen functional groups
1. Introduction Knowledge of the amount of organic oxygen in a coal sample is important for many reasons. In coal conversion processes, it has a bearing on the process parameters necessary to obtain high conversion and determines the difficulty of upgrading liquid products to stabilized chemical entities. For coking coals, it may indicate undesirable weathering that will have a negative impact on coke quality. Combustion techniques are commonly used in the analysis of coal and organic compounds in general for the determination of elemental hydrogen, carbon and nitrogen according to ASTM D3176-89 [1]. For oxygen, the method in common practice involves the determination Fby-difference_ from directly determined values for moisture, ash, sulfur, hydrogen, carbon and nitrogen. In spite of the inherent errors of this approach, which may be significant, it must be recognized that, in many cases, oxygen values obtained Fby-difference_ are adequate; for others, such as studies of coal weathering, more accurate values are required. * Corresponding author. E-mail address: [email protected] (J.A. MacPhee). 0378-3820/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2005.10.004
We have shown recently [2] that the nature of oxygen functional groups is important in assessing the degree of oxidation of a coal. In many cases it is impossible to know whether a particular coal has suffered oxidation purely from the Fby-difference_ oxygen value because the overall oxygen content of the sample has not changed in a manner that is detectable by this approach. Some years ago, a pyrolysis technique was developed for the direct determination of oxygen using the Perkin Elmer 240 Microanalyser [3]. This technique was based on earlier work, which showed that all of the organic oxygen in coal could be liberated by pyrolysis as H2O, CO and CO2. Culmo showed that the passage of the pyrolysis gases over hot activated carbon converts all the oxygen to CO. Subsequent passage of the CO over cupric oxide forms CO2, which can be identified quantitatively by a suitable detector. The pyrolysis technique has been developed by other groups as well [4,5]. For a number of years, we have used this technique routinely in our work on coal oxidation where small changes in oxygen content are important [6,7]. However, although we have found this technique both useful and reliable, it does not provide information about the chemical speciation of
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J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
TGA/FTIR spectrometer consisting of a DuPont 951 TGA, multipass gas cell, Michelson 110 FTIR and microcomputer was used. The system can simultaneously control furnace temperature, record weight loss and capture IR spectra for up to 20 evolving species in real time. Typically, a 25– 35 mg sample was continuously weighed in the TG/Plus system while heated using the following temperature program: a drying step at 150 -C followed by pyrolysis in helium with a temperature ramp from 150 to 900 -C at 30 -C/ min. Gases and volatiles were entrained in a gas cell by a helium stream where the rate and amount of the IR-active species were quantitatively measured. During the thermal ramp program, infrared spectra are collected every 30 s. On-line analysis and post process manipulations of the data were performed by the TG/Plus and SpectraCalc software packages. In general, the spectra obtained from the coal samples showed absorption bands for tars, CH4, C2H4, NH3, CO, CO2, COS, SO2 and H2O. The TG/Plus quantitative analysis program
oxygen in coal. This has led us to consider the use of thermogravimetry coupled to gas analysis by infrared spectroscopy (TG –FTIR) to measure organic oxygen in coal directly. Although this technique, developed by Solomon et al. [8], has been extensively used by our group [9– 11] and others, it appears not to have been considered for this particular purpose. In this work, we report the application of TG – FTIR under pyrolysis conditions to measure total organic oxygen and examine oxygen speciation in the Argonne premium coal samples. 2. Experimental The technique has been described elsewhere in some detail [9]. Here follows a brief description. TG – FTIR — as mentioned above, the TG –FTIR technique has been developed and used extensively by Solomon et al. [8]. In the current configuration of the system, a Bomem TG/Plus
a
3.0
H2O CO CO2 Temperature
2.5
1000
800
600
T,°C
Wt%/min
2.0
1.5 400 1.0 200
0.5
0.0
0 0
10
20
30
40
50
60
70
Time, min
b
0.8 H2O CO CO2
Wt%/min
0.6
0.4
0.2
0 100
250
400
550
700
850
1000
T,°C Fig. 1. Illinois #6 coal: (a) evolution of H2O, CO, CO2 and thermocouple temperature vs. time; (b) evolution of H2O, CO and CO2 vs. temperature.
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
Table 1 Analysis of Beulah-Zap and Wyodak coals (CETC-Ottawa) (wt.%, as analysed)
employs a database of calibrated IR spectra of gases/volatiles to obtain their relative amounts. All proximate and ultimate analyses were carried out according to ASTM D3176-89 [1].
Analysis
3. Results and discussion Plots of the H2O, CO and CO2 evolved during pyrolysis are used to estimate the total organic oxygen in a coal sample. Typical plots for Illinois #6 and Beulah-Zap are given in Figs. 1a, b and 2a, b, respectively. Integration of these curves yield the wt.% of the various oxygen containing gases evolved from the coal from which the contribution to the total oxygen content is calculated. In Fig. 1b, there is a sharp peak in the CO2 evolution curve at 765 -C that corresponds to calcite decomposition in the Illinois #6 coal [8,12]. The shape of the water evolution curves for Illinois #6 and Beulah-Zap is important in the present context. The curve for the former indicates no water evolution below
a
337
Coal Beulah-Zap
Wyodak
Proximate Moisture Ash VM + FC
17.17 7.97 74.86
16.39 7.42 76.19
Ultimate C H N S O (Fby-difference_)
53.88 3.50 0.83 0.67 15.98 (12.32a)
56.40 4.01 0.89 0.57 14.32 (11.68a)
a
Argonne Users Handbook.
¨300 -C and is a fairly simple bell-shaped curve consisting of two or more major components, whereas the curve for the latter indicates small yet important amounts of water
1.5
H2O CO CO2 Temperature
1.25
1000
800
600
T, °C
Wt%/min
1
0.75 400 0.5 200
0.25
0
0 0
10
20
30
41
51
61
71
Time, min
b
1.5 H2O CO CO2
1.25
Wt%/min
1
0.75
0.5
0.25
0 100
250
400
550
700
850
1000
T, °C Fig. 2. Beulah-Zap lignite: (a) evolution of H2O, CO, CO2 and thermocouple temperature vs. time; (b) evolution of H2O, CO and CO2 vs. temperature.
338
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341 1.25 not heat-treated heat-treated, 24 h @ 75C heat-treated, 24 h @ 105C
Wt%/min
1
0.75
0.5
0.25
0 0
200
400
600
800
1000
T, °C Fig. 3. Beulah-Zap lignite — effect of heat treatment on H2O evolution vs. temperature.
evolution at surprisingly low temperatures, starting at ¨ 120 -C. How we interpret this phenomenon, this low-temperature water, is of some significance to this work and to the analysis of lignites in general. The question of whether this water is bulk water or pyrolysis water has to be considered since it determines whether this water originates as organic oxygen or not. We will indicate our interpretation of this below in some detail. In previous work that considered bituminous and subbituminous coals [7,9,10] but not lignites, the estimation of total oxygen via TG –FTIR was rather straightforward with total oxygen values close to our in-house by-difference values. In the present work, six of the eight Argonne samples behaved as expected but difficulties were encountered with Wyodak (sub-bituminous) and Beulah-Zap (lignite). For the latter two coals, the TG –FTIR values were several percentage points higher than the dry Fby-difference_ values reported in the Argonne Users Handbook, Vorres [12]. To address this problem, we first had Wyodak and BeulahZap analysed by our own characterization laboratory. Prior to analysis, the two coals were dried at ambient temperature overnight. We found that pre-drying of the sample (under nitrogen) was necessary to obtain reliable sample weights for analysis since both samples in their pristine state from the vials
had ¨30% moisture. Allowing the moisture to stabilize at ambient conditions made the analysis much simpler. Results are reported in Table 1 where some differences with the previously reported results are noted. In particular, the carbon values from our laboratory are ¨1% lower that the reported values making the O Fby-difference_ values higher by the same amount. These new values were still lower than the %O values determined by TG – FTIR. In an attempt to explain this occurrence, the TG –FTIR behaviour of Beulah-Zap pre-dried at 75 -C and 105 -C for 24 h in a nitrogen atmosphere was examined. The water evolution curves for the un-dried and pre-dried coals are shown in Fig. 3. It is clear that even at 75 -C the water evolution peak at ¨150 -C is drastically reduced, suggesting that this is bulk water which is not removed in conventional drying at 105 -C and consequently should not be considered as contributing to the overall organic oxygen of the sample. As a consequence of this finding, the water evolution peaks for both Beulah-Zap and Wyodak were integrated manually so as to eliminate this low-temperature contribution to the overall water evolution. The work of Bartholomew et al. [13] on measurements of surface and pore properties of Argonne National Laboratory and Pittsburgh Energy Technology Center coals found that
Table 2 Argonne premium coals — TG – FTIR data Coal
Rank
H2 O (wt.%; dba)
CO (wt.%; db)
CO2 (wt.%; db)
O (wt.%; db)
H2O (mmol/g; db)
CO (mmol/g; db)
CO2 (mmol/g; db)
O (mmol/g; db)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
lvb mvb hvAb hvAb hvCb hvBb subb C lig A
1.15 3.12 4.09 4.65 5.58 6.00 9.97 11.23
1.99 2.09 2.70 3.05 3.69 4.67 8.42 9.00
0.70 1.14 1.42 1.39 1.82 2.00 5.53 7.41
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
0.64 1.74 2.27 2.58 3.10 3.33 5.54 6.24
0.71 0.75 0.97 1.09 1.32 1.67 3.01 3.22
0.16 0.26 0.32 0.32 0.41 0.45 1.26 1.68
1.67 3.02 3.91 4.33 5.30 5.92 11.08 12.85
a
Dry basis.
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
339
Table 3 Comparison of oxygen concentrations — Argonne and TG – FTIR
Table 4 Argonne premium coals — contributions to overall O content, wt.% (db)
Coal
Coal
O (H2O)
O (CO)
O (CO2)
O total (FTIR)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
1.02 2.79 3.66 4.15 5.01 5.34 8.88 10.00
1.14 1.20 1.55 1.75 2.13 2.67 4.82 5.16
0.51 0.84 1.04 1.02 1.34 1.46 4.03 5.40
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
O
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap a b
Argonne (wt.%; db)
TG – FTIR (wt.%; db)
2.36a 4.84a 6.74a 7.79a 8.65a 10.79a 17.13b 19.29b
2.67 4.83 6.25 6.92 8.48 9.47 17.73 20.56
Argonne Users Handbook. Determined at CANMET Energy Technology Centre-Ottawa.
containing carbonates was obtained by subtracting the inorganic CO2 contribution from the total CO2 measured.
most of the internal surface area of Wyodak and Beulah-Zap coals consists of micropores having diameters less than 1 nm. Also, the low pore volumes reported by Bartholomew et al. [13] for these coals lends support to our attribution of the shoulder peak of the H2O evolution profile of Beulah-Zap in Fig. 3 to bulk water. In light of what has been stated above concerning the data treatment, the amounts of H2O, CO and CO2 and the corresponding total organic oxygen evolved from the Argonne Premium Coals using TG –FTIR are given in Table 2. The total organic oxygen values obtained for the Argonne coals by TG – FTIR and Fby-difference_ are given in Table 3 for comparison purposes. These values are plotted in Fig. 4 where the least squares line is constrained to pass through zero. It should be mentioned that the total organic oxygen measured via TG –FTIR and presented in this paper refers strictly to organic oxygen, although four of the eight Argonne coals, namely Pocahontas #3, Illinois #6, Blind Canyon and Beulah-Zap, were found to contain inorganic oxygen from the decomposition of calcite and also siderite in the case of Pocahontas #3. The fraction of inorganic oxygen present in these four coals was obtained through CO2 profile resolution and was based both on the evolution temperature for CO2 from decomposition of pure siderite and calcite via TG –FTIR [9,10]. In other words, the amount of organic CO2 listed in Table 2 for the Argonne coals identified as
3.1. Oxygen distribution as a function of rank The contributions from H2O, CO and CO2 to organic oxygen in the coals are shown in Table 4 and graphically in Fig. 5. As expected, there is a general monotonic increase in all quantities with decreasing carbon content. Extensive work on oxygen functional analysis has been reported and discussed in Van Krevelen [14]. The shapes of curves of functional group content versus carbon content provided in that reference are concave downward while those shown in Fig. 5 are all concave upward. The results cited in Van Krevelen are from a variety of sources and make use of a variety of chemical techniques. Van Krevelen points out that, except for peat and brown coal, the sum of the functional group oxygen and the total oxygen in the coal are in agreement. Consequently, it is doubtful that ‘‘unreactive’’ oxygen really exists. In the present work, it has been shown that all the oxygen in the coal sample is expelled during pyrolysis and measured by the amounts of the three gases, H2O, CO and CO2. The normalized organic oxygen content occurring in the three O-containing pyrolysis gases is given in Table 5 where it is seen, with the exception of Pocahontas #3, the highest rank coal in the series, that there is a remarkable similarity of the various contributions as noted previously [9,10].
25 y = 1.01x R2 = 0.98
20
O (TG-FTIR), wt% (db)
O (TG-FTIR), wt% (db)
25
15
10
5
0
20 O (Total)
15
10
O (H2O) O (CO)
5
O (CO2)
0
5
10
15
20
25
O (Argonne, by difference), wt% (db)
0 70
75
80
85
C (Argonne), wt% (dmmf) Fig. 4. Comparison of the TG – FTIR organic O (wt.%, db) with the Argonne by-difference values.
Fig. 5. O (TG – FTIR) vs. C (Argonne).
90
95
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341 7
Table 5 Argonne premium coals — normalised organic O wt.% Coal
O (H2O)
Pocahontas #3 Upper Freeport Pittsburgh #8 Lewis-Stockton Illinois #6 Blind Canyon Wyodak Beulah-Zap
O (CO)
38 58 58 60 59 57 50 49
O (CO2)
43 25 25 25 25 28 27 25
19 17 17 15 16 15 23 26
H2O (TG-FTIR), mmol/g (db)
340
6 y = 3.05x - 2.55 2 R = 0.65
5 4 3 2 1 0 1.0
3.2. Oxygen speciation
1.5
2.0
2.5
3.0
Ar-OH (Aida), mmol/g (db)
The concentrations of –OH and – CO2H functional groups occurring in the Argonne Premium Coals have been measured by Aida et al. [15] using a chemical method. His method involves the reaction of carboxyl functional groups in coal with n-Bu4NBH3 in pyridine. The amount of hydrogen evolved is assumed to be equivalent to the carboxyl concentration in the coal. Carboxylate groups are not detected by this method but, after acid washing, the total carboxyl content was measured to yield carboxyl and carboxylate amounts in the original coal. The underlying assumption is that this reagent (n-Bu4NBH3) reacts only with carboxyl groups and not with phenolic –OH groups. Corroborating evidence from solid-state 13C NMR supports the assumption. A further reagent, LiBH3 in pyridine, which reacts with both carboxyl and phenolic – OH generating hydrogen, was used to measure the concentrations of these two functional groups together. The concentration of phenolic –OH is then calculated from the difference between total acidic groups and the total carboxyl group concentrations. A direct comparison between the pyrolysis data of our work and Aida’s functional group analysis is therefore possible. We first consider the relationship between pyrolysis CO2 and total carboxyl plus carboxylate measured chemically by this author shown in Fig. 6. It is evident that the pyrolysis CO2 is equivalent to the – CO2H(M) measured by Aida. From a chemical point of view, this is reasonable since it is difficult to envisage the production of CO2 from any other functional group as pointed out by Schafer [4] for Australian brown coal decomposition. Fig. 6 therefore constitutes a validation of both
Fig. 7. H2O (TG – FTIR) vs. Ar – OH functionality (Aida et al. [15]).
the Aida and the pyrolysis (TG – FTIR) approaches to the measurement of carboxyl groups in coal. It is worth pointing out that the curves for the evolution of CO2 as a function of temperature are not simple, indicating more than one component. It is possible that carboxyl and carboxylate decompose to produce CO2 in different temperature ranges and that the relevant information may be extracted from the pyrolysis curves by deconvolution. This point will be examined in future work. The situation for acidic – OH groups is somewhat different. A plot of pyrolysis H2O as a function of –OH groups from Aida’s work indicates that there is more water evolved than can be accounted for by his estimation of phenolic – OH groups (Fig. 7). There exists a large body of work on phenolic – OH groups in coal estimated by a variety of techniques that was reviewed recently [16]. This author sums up the literature data on phenolic –OH groups by means of the following equation: ln Oph ¼ 130:3909 þ 96:124lnðcC Þ 134:133cC ðr ¼ 0:859Þ where c C is the fractional carbon content on a dry, ash-free basis. A comparison of phenolic –OH content generated by the above equation and the experimental work of Aida for the Argonne coals indicates that the latter are somewhat lower than the ‘‘consensus’’ values assembled by Gagarin [16]. Some of 7
1.75
H2O (TG-FTIR), mmol/g (db)
CO2 (TG-FTIR), mmol/g (db)
2
y = 1.12x - 0.22 2 R = 0.84
1.5 1.25 1 0.75 0.5 0.25 0
6 y = 1.07x + 0.04 R2 = 0.87
5 4 3 2 1 0
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
COOH+COOM (Aida), mmol/g (db) Fig. 6. CO2 (TG – FTIR) vs. COOH + COOM functionality (Aida et al. [15]).
0
1
2
3
4
5
6
7
Ar-OH (van Krevelen), mmol/g (db) Fig. 8. H2O (TG – FTIR) vs. Ar – OH functionality (Van Krevelen [14]).
J.A. Macphee et al. / Fuel Processing Technology 87 (2006) 335 – 341
the work dates from Van Krevelen’s group in the 1950s [17,14]. It involved measurement of phenolic – OH groups by acetylation with acetic anhydride in pyridine for a relatively wide range of coal rank. From that work, it is possible to estimate the phenolic –OH content for the Argonne coals from the appropriate carbon content. A comparison of the pyrolysis H2O content of the Argonne coals with the phenolic – OH from van Krevelen’s work is given in Fig. 8. The correlation between these two values is excellent and, perhaps what is more significant, the slope is close to unity and the intercept of the least squares line is close to zero. This leaves us with the intriguing conclusion that the pyrolysis water originates mainly as phenolic – OH, although it is difficult to see how this can be the case since it is likely that the reaction mechanisms producing pyrolysis water are complex. 4. Observations and conclusions 1. TG –FTIR can be reliably used for the determination of the organic oxygen content of coals in general as indicated by the results reported here for Argonne premium coals. 2. Low rank coals contain water that is difficult to remove at 105 -C. 3. The normalized amounts of oxygen found in the gaseous species H2O, CO and CO2 remain remarkably constant with changes in rank. 4. Pyrolysis CO2 corresponds to the CO2H(M) functional group in coal. 5. The quantity of pyrolysis H2O is accounted for almost entirely by – OH functional groups (using the experimental results of Van Krevelen) indicating this functional group as its principal source. Acknowledgement The authors would like to thank the Canadian Carbonization Research Association for support of this work. References [1] ASTM Standards, Volume 05.06, Gaseous Fuels; Coal and Coke, ASTM International, 100 Barr Harbor Drive, PO Box C700, W. Conshohocken, PA 19428-2959, USA, 2004.
341
[2] J.A. MacPhee, L. Giroux, J.-P. Charland, J.F. Gransden, J.T. Price, Detection of natural oxidation of coking coal by TG – FTIR — mechanistic implication, Fuel 83 (2004) 1855 – 1860. [3] R. Culmo, Microdetermination of oxygen in organic compounds with an automatic elemental analyzer, Microchimica Acta (1968) 811 – 815. [4] H.N.S. Schafer, Pyrolysis of brown coals: 2. Decomposition of acidic groups on heating in the range 100 – 900 -C, Fuel 58 (1979) 673 – 679. [5] B. Zischka, H. Stemming, Oxygen determination in solid fuels, Erdoel Kohle, Erdgas, Petrochem 38 (5) (1985) 221 – 224. [6] J.A. MacPhee, W.P. Hutny, L. Giroux, J.-P. Charland, J.T. Price, Characterization of coal weathering/oxidation by TGA – FTIR, Proc. Int. Conf. on Coal Science, Banff, Sept. 12 – 19, 1993, pp. 590 – 593. [7] J.A. MacPhee, L. Giroux, J.-P. Charland, J.T. Price, A study of the oxidation pathways of coking coal by thermogravimetric Fourier transform infrared spectroscopy (TG – FTIR), Proc. Int. Conf. on Coal Science, San Francisco, 2001. [8] P.R. Solomon, M.A. Serio, R.M. Carangelo, R. Bassilakis, D. Gravel, M. Baillargeon, F. Baudais, G. Vail, Analysis of the Argonne premium coal samples by thermogravimetric Fourier transform infrared spectroscopy, Energy and Fuels 4 (3) (1990) 319 – 333. [9] J.-P. Charland, J.A. MacPhee, L. Giroux, J.T. Price, M.A. Khan, Application of TG – FTIR to the determination of oxygen content of coals, Fuel Processing Technology 81 (2003) 211 – 221. [10] J.A. MacPhee, J.-P. Charland, L. Giroux, J.T. Price, W.P. Hutny, M.A. Khan, Determination of organic oxygen in coals with application to the study of coal weathering, ACS Division of Fuel Chemistry Preprints 46 (1) (2001) 340 – 342. [11] J.A. MacPhee, L. Giroux, J.-P. Charland, R. Sakurovs, J.T. Price, Interactions between coals in binary blends during heating as studied by thermal rheology, PMRTA, TG – FTIR and optical microscopy, Proc. 18th Ann. Pittsburgh Coal Conf., Newcastle Australia, Dec. 4 – 7, 2001. [12] K.S. Vorres, Users Handbook for the Argonne Premium Coal Sample Program: accessed December 9, 2003 at URL http://www.anl. gov/PCS/pcshome.html, 1993. [13] C.H. Bartholomew, W.E. White, D. Thornock, W.F. Wells, W.C. Hecker, L.D. Smoot, D.M. Smith, F.L. Williams, Surface and pore properties of ANL and PETC coals preprints, Fuel Chemistry Division, American Chemical Society 33 (3) (1988) 24 – 31. [14] D.W. Van Krevelen, Coal: Typology, Physics, Chemistry, Constitution, Third edR, Elsevier, Amsterdam, 1993. [15] A. Aida, A. Nishisu, M. Yoneda, T. Yoshinaga, Y. Tsutsumi, I. Yamanishi, T. Yoshida, Chemical determination of oxygen-containing functionality in Argonne premium coal samples, Fuel Chemistry Division Preprints 46 (1) (2001) 325 – 327. [16] S.G. Gagarin, Forms of Oxygen in Coal Organic Matter (Review) Coke and Chemistry, No. 10, 2001. [17] L. Blom, L. Edelhausen, D.W. van Krevelen, Chemical structure and properties of coal: XVIII. Oxygen groups in coal and related products, Fuel 36 (1957) 135 – 153.