(TGâFTIR) to the Analysis of Oxygen Functional Groups - American

1988

Energy & Fuels 2006, 20, 1988-1996

Application of Thermogravimetric Fourier Transform Infrared Spectroscopy (TG-FTIR) to the Analysis of Oxygen Functional Groups in Coal L. Giroux,* J.-P. Charland, and J. A. MacPhee CANMET Energy Technology CentresOttawa, 1 Haanel DriVe, Ottawa, Ontario K1A 1M1, Canada ReceiVed February 28, 2006. ReVised Manuscript ReceiVed June 26, 2006

This paper attempts to relate oxygen-containing gases H2O, CO2, and CO evolved during pyrolysis of the Argonne premium coals to oxygen-containing functional groups as a function of rank. Our approach to functional group analysis of oxygen-containing species in coal has been to use a pyrolysis technique, thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR), involving thermogravimetric analysis with the measurement of the gaseous decomposition products via IR detection. Under suitable heating conditions, TG-FTIR pyrolysis of a coal sample in a stream of inert gas has been shown to expel quantitatively all of the organic oxygen in the form of H2O, CO2, and CO, and consequently, this technique can be effectively applied for determining the total oxygen content. Focusing on the Argonne premium coals, which cover a wide range in rank between lignite (Ro ) 0.25) and low-volatile bituminous (Ro ) 1.68), TG-FTIR provided complex pyrolysis profiles of oxygen-containing gases, which yield information on the sources of the different peaks observed in coal as a function of rank from a chemical-structure standpoint. Deconvolution of the complex profiles was performed to assign peaks to the different sources of oxygen-containing gases. Model polymers containing various oxygen functional groups in aliphatic and/or aromatic molecular environments were also pyrolyzed by TG-FTIR in an attempt to assign peaks in the gas evolution profiles of the Argonne premium coals. Although complex evolution profiles were observed for the three oxygen-containing gas species H2O, CO2, and CO in the Argonne premium coals, the strength of the TG-FTIR technique in revealing both similarities and differences in profiles depending upon the coal rank was evident. The findings in this investigation are compared to data published on oxygen functional group analysis for the Argonne premium coals made with various analytical techniques.

Introduction Functional group analysis of coal has mainly been accomplished through the use of chemical reagents that react with specific groups.1,2 Quantitative analysis of functional groups in coal has been recognized as being a very difficult task in large part because of the limited accessibility of the reagent to the coal structure. As summarized by van Krevelen,1 a large set of experimental investigations on the various oxygen-containing functional groups in coal found (1) the hydroxyl groups (-OH) in coals to be predominantly phenolic (or acidic in nature) and not alcoholic and to be more abundant in brown coals, Ro < 0.30, (2) the carboxyl groups (-COOH) to occur essentially in brown coals and lignites, (3) the methoxyl groups (-OCH3) to also more likely be present in brown coals and lignites, and (4) the carbonyl groups (R1R2CO) to be found in coals of different rank but being present in a lower amount the higher the rank. Using the chemical methods of Blom et al.3 and Schafer,4 Jung et al.2 measured the percent oxygen in Argonne premium coals * To whom correspondence should be addressed. Telephone: +1-613996-7638. Fax: +1-613-995-9728. E-mail: [email protected]. (1) van Krevelen, D. W. Coal: Typology, Physics, Chemistry, Constitution, 3rd ed.; Elsevier: Amsterdam, The Netherlands, 1993. (2) Jung B.; Stachel, S. J.; Calkins, W. H. Organic oxygen contents of Argonne premium coal samples. Prepr. Symp.-Am. Chem. Soc., DiV. Fuel Chem. 1991, 36, 869-876. (3) Blom L.; Edelhausen L.; van Krevelen D. W. Chemical structure and properties of coal XVIIIsOxygen groups in coal and related products. Fuel 1957, 83, 135-153. (4) Schafer, H. N. S. Carboxyl groups and ion-exchange in low-rank coals. Fuel 1970, 49, 197-213.

as carboxyl, hydroxyl, carbonyl, and ether (by difference). Their analysis essentially agreed with van Krevelen,1 except for the greater variability in carbonyl with increasing rank. They also found appreciable amounts of ether oxygen in all coals. Also using a chemical method, Aida et al.5 measured the concentrations of -COOH and arylic -OH functional groups in the Argonne premium coals. Beside the use of chemical reagents, a number of different analytical methods have been used to examine the oxygencontaining functional groups in coals, in general, and in the Argonne premium coals, in particular. Using pyrolysis (4 °C/min) interfaced to triple-quadrupole mass spectrometry to investigate the chemistry of decomposition products of Argonne premium coals, Burnham et al.6 reported that H2O and CO2, whose amounts decreased significantly with rank, were generated via multiple reactions. The Beulah-Zap lignite and Wyodak sub-bituminous coals gave broad H2O peaks that resolved into at least two components for the higher rank coals. They suggested that the loss of H2O at T g 500 °C may originate from clay minerals and that the formation of CO2 at 550-600 °C for Illinois no. 6 high-volatile bituminous and higher rank coals could be due to the decomposition of siderite (5) Aida A.; Nishisu A.; Yoneda M.; Yoshinaga T.; Tsutsumi Y.; Yamanishi I.; Yoshida T. Chemical determination of oxygen-containing functionality in Argonne premium coal samples. Prepr. Symp.-Am. Chem. Soc., DiV. Fuel Chem. 2001, 46, 325-327. (6) Burnham, A. K.; Oh, M. S.; Crawford, R. W. Pyrolysis of Argonne premium coals: Activation energy distributions and related chemistry. Energy Fuels 1989, 3, 42-55.

10.1021/ef0600917 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006

TG-FTIR of Oxygen Functional Groups in Coal

(ferrous carbonate). Furthermore, this group reported several additional features in the CO2 evolution profiles: (1) the temperature of the maximum rate of CO2 evolution in the 300500 °C range tended to increase with rank, and (2) the coincidence of the hydrocarbon and CO2 evolution peaks below 500 °C could be related to the decomposition of carboxylic acids, salts, or esters in coal. Evolution profiles for CO revealed the following features: (1) the presence of a small peak below 500 °C was found to gradually shift to a higher temperature and to decrease in intensity with increasing rank; this peak evolved at a similar temperature as the largest CO2 peak; and (2) the majority of CO is evolved at higher temperatures, in the 600-800 °C range. Using 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, Solum et al.7 provided information as to the kinds of oxygen functionalities present in the Argonne premium coals. They found that only the Beulah-Zap lignite and Wyodak subbituminous coals contained a significant amount of carbonyl carbon and that other types of carbon associated with oxygen showed a decrease with increasing rank. Solomon et al.8 developed and pioneered the use of thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR) to investigate the pyrolysis of Argonne premium coals including the kinetics of functional-group decomposition in some detail. From TG-FTIR data, they assigned functional group descriptions or “pools” to oxygen-containing gases in terms of their kinetics and maximum evolution temperatures. For example, H2O peaks were labeled as (1) moisture, (2) extra loose, (3) loose, and (4) tight. In the same vein, CO2 peaks were labeled as (1) a very low temperature peak occurring only for the lowest rank coals and originating from the low-temperature tail of the extra loose CO2, (2) extra loose, associated with H2O, (3) loose, associated with tar and H2O evolution, for higher rank coals, (4) tight, for higher rank coals, and (5) evolution of CO2 in high-rank coals from carbonate minerals such as siderite and calcite. For CO peaks, these were labeled as (1) associated with “loose” CO2, (2) associated with “tight” CO2, and (3) hightemperature peak, with no accompanying H2O or CO2 peaks. The CO peak (3) is the only one present in high-rank coals, whereas low-rank coals showed the peaks labeled (1-3). The 1H CRAMPS NMR technique has also yielded some insight into oxygen functionalities present in the Argonne premium coals.9 On the basis of specific regions of H chemical shifts, ethoxy methyl and aliphatic ether methyl groups were tentatively assigned in Beulah-Zap lignite and either carboxylic or hydroxyl protons were identified in several coals including Upper Freeport, Beulah-Zap, and Wyodak. In addition, X-ray photoelectron spectroscopy (XPS) has been used to determine the organic oxygen functionalities present in Argonne premium coals.10 From curve fitting of the carbon (1s) spectrum, hydroxyl and carboxyl oxygen were found to be intense in the lower rank coals, Beulah-Zap and Wyodak. Hydroxyl groups were found to persist with an increasing coal rank up to medium-volatile bituminous, Lewiston-Stockton. (7) Solum, M. S.; Pugmire, R. J.; Grant, D. M. 13C solid-state NMR of Argonne premium coals. Energy Fuels 1989, 3, 187-193. (8) Solomon P. R.; Serio M. A.; Carangelo R. M.; Bassilakis R.; Gravel D.; Baillargeon M.; Baudais F.; Vail G. Analysis of the Argonne premium coal samples by thermogravimetric Fourier transform infrared spectroscopy. Energy Fuels 1990, 4, 319-333. (9) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. 1H cramps NMR study of the chemical functionality of Argonne premium coals. Fuel 1994, 73, 823-827. (10) Kelemen, S. R.; Kwiatek, P. J. Quantification of organic oxygen species on the surface of fresh and reacted Argonne premium coal. Energy Fuels 1995, 9, 841-848.

Energy & Fuels, Vol. 20, No. 5, 2006 1989

Levels of carbonyl were low in all coals, and ether oxygen levels did not vary much with rank. The objective of the present work was to use TG-FTIR to examine the pyrolysis profiles of oxygen-containing gases H2O, CO2, and CO of the Argonne premium coals to yield information on the sources of the different peaks observed as a function of rank from a functional-group standpoint. Assignment of peaks in the evolution profiles of the oxygen-containing gases was based on the decomposition of model polymers containing specific oxygen functionalities in aliphatic and/or aromatic environments. Ultimately, a better understanding of the type of oxygen functionalities present in coals of different rank will improve our knowledge of the structure of coal as well as its reactivity during conversion processes. Experimental Section Fresh samples of Argonne premium coals11 (-100 mesh size) and model polymers (25-35 mg) were pyrolyzed under a constant flow of He (0.935 L/min) using a Bomem TG-FTIR instrument consisting of a DuPont 951 TGA, multipass gas cell, Michelson MB 110 FTIR, and microcomputer. For the purpose of the present study, all samples were heated at a constant rate of 30 °C/min between 105 and 1000 °C for measuring the amounts and evolution rates of the three main oxygen-containing gases, H2O, CO2, and CO, during pyrolysis. Upon reaching 1000 °C, a cooling ramp also at 30 °C/min was initiated in the program to bring the system back to room temperature. During the thermal ramp program, IR spectra (3300-900 cm-1 range at 4 cm-1 resolution) were collected every 30 s (1 scan every 3 s). Peak profile curve fitting was accomplished using the SpectraCalc software package. A minimum number of peaks were chosen to map the profile, and curve-fitting calculations were deemed acceptable upon attaining a sufficiently low residual (χ2) and convergence efficiency. Additional details on the TGFTIR technique used in this study have been reported elsewhere.12 Pyrolysis of model polymers was done for the purpose of gaining specific information on the chemical environments of oxygen functionalities in Argonne premium coals. Table 1 lists the model polymers used, along with their source (synthesis or supplier), chemical structure, and oxygen functionality along with the molecular environment. The minerals used in this investigation, namely, illite (063181), kaolinite (014685), and montmorillonite (063285), which are present in coal clays and potential sources of H2O, as well as siderite (064373) and calcite (065598), inorganic carbonates producing CO2, were obtained from the Geological Survey of Canada. These minerals were pyrolyzed using TG-FTIR, and peak evolution temperatures for H2O and CO2 were measured.

Results and Discussion In previous work on TG-FTIR pyrolysis of Argonne premium coals, we reported contributions from H2O, CO2, and CO to organic oxygen and noted the remarkable similarity of the various contributions with rank, with the exception of Pocahontas no. 3.13,14 More specifically and as expected, the data showed a progressive decrease in organic oxygen in the three oxygen-containing gases with increasing rank. Also, we found the total organic oxygen to have contributions from (1) H2O, 56 ( 4% from all coals except Pocahontas no. 3 at 38%; (11) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program, http://www.anl.gov/PCS/pcshome.html, and Argonne Premium Coal Sample, Analytical Data, http://www.anl.gov/PCS/report/ part2.html (accessed Dec 9, 2003). (12) Charland J.-P.; MacPhee J. A.; Giroux, L.; Price J. T.; Khan M. A. Application of TG-FTIR to the determination of oxygen content of coals. Fuel Process. Technol. 2003, 81, 211-221.

1990 Energy & Fuels, Vol. 20, No. 5, 2006

Giroux et al. Table 1. Model Polymers and Pyrolysis Products chemical structure

oxygen functionality and molecular environment

model polymer

source (synthesis or supplier)

Bakelite (a thermosetting polymer, which decomposes before softening)a

synthesized in house from condensation of phenol with formaldehyde in the presence of ammonium hydroxide

H2O, aromatic

diallylphthalate resinb

Struers, Inc., DK. impure, containing 30-60% glass fiber filler

CO2 and CO, aromatic

poly(methyl methacrylate)c

Sigma-Aldrich

CO2 and CO, aliphatic

poly(acrylic acid)d

Sigma-Aldrich

CO2, CO, and H2O, aliphatic

monomer

a Undergoes ∼45% weight loss during heating under inert gas to 1000 °C. b Impure resin quoted by the supplier as melting in the 60-100 °C range but only undergoes ∼40% weight loss during heating under inert gas to 1000 °C. c Average molecular weight, 996 000 (GPC); melting point, 300 °C. d Viscosity average molecular weight, 4 000 000; Tg, 106 °C.

Table 2. Model PolymerssOxygen-Containing Products and Evolution Temperatures model polymer Bakelite diallylphthalate resin poly(methyl methacrylate) poly(acrylic acid)

oxygencontaining product

evolution temperature (°C)

H2O, aromatic CO2, aromatic CO, aromatic CO2, aliphatic CO, aliphatic CO2, aliphatic CO, aliphatic H2O, aliphatic

214, 468, 582 (main) 381 (main), 724, 985 414, 732, 985 (main) 425 (main), 813, 987 987 340 (main), 388 444 270, 380 (main)

(2) CO, 26 ( 1% except from Pocahontas no. 3 at 43%; and (3) CO, 18 ( 3%. The purpose of the present investigation is to resolve the complex curve profiles of the three oxygen-containing species into identifiable peaks and to make an attempt at assigning these peaks to functional groups within the coal structure based on evolved peak temperature positions for H2O, CO2, and CO resulting from the decomposition of model polymers. Pyrolysis of Model Polymers. Oxygen-containing pyrolysis products along with a specific molecular environment (aliphatic or aromatic) from the four model polymers used in this work are listed in Table 2. TG-FTIR peak evolution temperatures for these pyrolysis products obtained through profile curve fitting are also given in this table. On the basis of the evolution temperatures of the oxygencontaining gases from the model polymers, tentative assignments were made to peaks observed in the H2O, CO2, and CO pyrolysis profiles of the Argonne premium coals. These assignments are discussed below. Pyrolysis of Argonne Premium Coals: H2O Evolution. Figure 1 presents H2O evolution profiles during pyrolysis of the Argonne premium coals. The yield of H2O progressively (13) MacPhee, J. A.; Charland J.-P.; Giroux L.; Gransden, J. F.; Price, J. T. Application of TG-FTIR to the determination of organic oxygen and its speciation in the Argonne premium coal samples. Proceedings of the 12th International Conference on Coal Science: Cairns, Australia, 2003. (14) MacPhee, J. A.; Charland J.-P., Giroux L. Application of TGFTIR to the determination of organic oxygen and its speciation in the Argonne premium coal samples. Fuel Process. Technol. 2006, 87, 335341.

decreases with increasing rank, with Beulah-Zap producing the largest amount and Pocahontas no. 3 producing the lowest amount. Also, the start of H2O evolution and the temperature of the maximum rate of evolution are delayed as the coal rank increases. Most of the H2O is evolved between 350 and 700 °C. Table 3 lists H2O peaks resolved for the eight Argonne premium coals with the corresponding maximum evolution temperatures during pyrolysis and relative peak intensities. It also gives average temperatures of evolution for the respective peaks. The H2O evolution profiles of the Argonne premium coals were successfully resolved into three peaks (Illinois no. 6, Pittsburgh no. 8, Lewiston-Stockton, and Pocahontas no. 3) and four peaks (Beulah-Zap, Wyodak, Blind Canyon, and Upper Freeport). Figure 2 presents the fitted H2O peak profile and assignment (Table 3) for Beulah-Zap. Pyrolysis water for Beulah-Zap and Wyodak, the lowest rank coals in this series, actually consisted of only two peaks (peaks 4 and 6), with the reason being that the two low-temperature, low-intensity features at 166 °C (peak 1) and 241 °C (peak 2) are assigned by us to “bulk” and not pyrolysis H2O.13,14 Lending support to this assignment is the work of Bartholomew et al.,15 who found that most of the internal surface area of Beulah-Zap and Wyodak coals consists of micropores having diameters less than 1 nm and thus of low pore volume, 0.08-0.14 cm3/g.16 Such small pores are therefore responsible for trapping some of the water in these very high-moisture coals. The evolved H2O peaks 1 and 2 are found to be rank-dependent. Peak 4 at 465 °C and peak 6 at 606 °C in Figure 2 are tentatively assigned to H2O originating from aromatic sites, namely, arylic-OH (phenol). This assignment is based on the temperature of evolution of the two main H2O peaks at 468 and 582 °C during pyrolysis of Bakelite, a thermosetting resin containing arylicOH groups. In their TG-FTIR work on Argonne premium coals, Solomon et al.8 reported the simultaneous occurrence of a prominent H2O peak with the tar peak and suggest that the (15) Bartholomew, C. H.; White, W. E.; Thornock, D.; Wells, W. F.; Hecker, W. C.; Smoot, L. D.; Smith, D. M.; Williams, F. L. Surface and pore properties of ANL and PETC coals. Prepr. Symp.-Am. Chem. Soc., DiV. Fuel Chem. 1988, 33, 24-31. (16) Deevi, S. C.; Suuberg, E. M. Physical changes accompanying drying of western US lignites. Fuel 1987, 66, 454-460.



Figure 1. H2O evolution as function of the temperature for the Argonne premium coals. Table 3. Argonne Premium CoalssH2O Peaks and Evolution Temperatures during Pyrolysisa peak 1b

peak 2b

peak 3

peak 4

peak 5

peak 6

peak 7

peak 8

T (°C)

T (°C)

T (°C)

T (°C)

T (°C)

T (°C)

coal

Roc (%)

rank

T (°C)

T (°C)

Beulah-Zap Wyodak Illinois no. 6 Blind Canyon Pittsburgh no. 8 Lewiston-Stockton Upper Freeport Pocahontas no. 3 average T of evolution (°C)

0.25 0.32 0.46 0.57 0.81 0.89 1.16 1.68

lig A subb C hvCb hvBb hvAb hvAb mvb lvb

1656 1684

2377 2453 3726

465100 464100 46013 46665 487100 49696

4065 166

241

389

4713 473

60637 59848 543100 549100 54283 563100 525100 54634 545

7774 66762 617100 61193 61289 595100 606

75614 667

767

a

Subscript x in the peak temperature, Tx, denotes the relative intensity, where 100 is the most intense peak. b Rank dependent. c Vitrinite reflectance, a measure of the coal rank.11

Figure 2. Fitted H2O peak profile and assignment for Beulah-Zap.

chemistry responsible for this peak relates to either the free radicals produced or the increase in fluidity, thus improving the mobility for bimolecular interactions. On the basis of their suggestion, the formation of H2O in the 460-620 °C range observed in our investigation could then arise from the condensation of phenols.

Only two coals, Blind Canyon and Upper Freeport, show the presence of a feature (peak 3) of low intensity at an average temperature of 389 °C. This peak is tentatively assigned to H2O originating from an aliphatic site. This assignment is based on the temperature of evolution of the main H2O peak at 380 °C during pyrolysis of poly(acrylic acid).


Giroux et al.

Figure 3. CO2 evolution as function of the temperature for the Argonne premium coals.

The majority of coals, except Upper Freeport, reveal a peak (peak 4) at about 473 °C; this peak is small in the case of Pocahontas no. 3. Peak 4 is the major H2O peak for BeulahZap, Wyodak, and Pittsburgh no. 8. As discussed, peak 4 is assigned to H2O originating from an aromatic site. Also of note about peak 4 for H2O is its comparable average evolution temperature, 473 °C, to peak 2 for CO2, 473 °C, and peak 1 for CO, 482 °C. Similarly, all coals, except for the two of lowest rank, show a feature (peak 5) centered at 545 °C. Peak 5 is the major H2O peak for Illinois no. 6, Blind Canyon, Lewiston-Stockton, and Upper Freeport. This peak is also assigned to H2O arising from an aromatic site. Peak 5 for H2O does not have corresponding peaks of either CO2 or CO evolving around a temperature of 545 °C. Six of the eight coals exhibit a peak (peak 6) around 606 °C. Peak 6 is the major H2O peak for Pittsburgh no. 8 (equivalent intensity to peak 4 for this coal) and Pocahontas no. 3 and is assigned to an aromatic site such as an arylic-OH producing H2O during pyrolysis. In the case of Beulah-Zap and Wyodak, this peak is of lower intensity compared to that of the four highest rank coals in the series. Peak 6 for H2O at 606 °C is evolved at a similar temperature to peak 4 for CO2, 615 °C, and peak 2 for CO, 619 °C. Only Blind Canyon gives a peak (peak 7) at 667 °C, which is of appreciable intensity, and Illinois no. 6 and Upper Freeport yield a minor feature (peak 8) at about 767 °C. Currently, Peaks 7 and 8 remain unassigned, although peak 8 evolved at similar temperatures to peak 6 for CO2, 770 °C, and peak 3 for CO, 766 °C. Formation of Pyrolysis H2O from Clay and Clay-like Minerals. Coal is largely constituted by organic matter but also contains inorganic matter in the form of naturally deposited minerals. Oxygen, generally considered to be mostly organic in coal, is also present in inorganic forms including water, oxides, carbonates, sulfates, and silicates.17 Therefore, certain minerals in coal can be sources of oxygen-containing gases upon decomposition. In their combined pyrolysis-mass spectrometry investigation, Burnham et al.6 suggested that clay minerals including iron

Table 4. H2O Evolution Temperatures from TG-FTIR Decomposition of Clay Minerals mineral


illite kaolinite montmorillonite

225, 395, 500, 580 532 582, 704

silicates identified in some of the Argonne premium coals by Wertz18 might be responsible for the loss of H2O above 500 °C. Table 4 lists temperatures of evolution of H2O from pure illite, kaolinite, and montmorillonite clay minerals measured in our laboratory by TG-FTIR. Ward et al.19 have conducted an X-ray diffraction study of the mineral matter in the Argonne premium coals. Mineralogy of the low-temperature ash (LTA) samples identified illite, illite/ smectite, and kaolinite clay minerals as the main potential sources of inorganic H2O. Their work provided data on LTA percentages of the coals and weight percentages of illite and kaolinite minerals. On the basis of their data and taking into account the percentages of H2O present in illite (4.5%), kaolinite (14.0%), and montmorillonite (5.0%), as well as our determination of H2O peak evolution temperatures of illite and kaolinite (Table 4), we concluded that pyrolysis H2O attributable to the decomposition of any of these aluminosilicate clay minerals present in low amounts in the Argonne premium coals is below the TG-FTIR detection limits. CO2 Evolution. Figure 3 presents CO2 evolution profiles during pyrolysis for the Argonne premium coals. It shows the progressive decrease of the CO2 yield with rank. The temperature evolution range for CO2 is wide, between 300 and 850 °C. Table 5 lists CO2 peaks resolved for the eight Argonne premium coals with the corresponding maximum evolution temperatures during pyrolysis and relative peak intensities. It also gives average temperatures of evolution for the respective (17) Elliot, M. A., Ed. Chemistry of Coal Utilization: Second Supplementary Volume; J. Wiley and Sons: New York, 1993. (18) Wertz, D. L. X-ray analysis of the Argonne premium coals. 1. Use of absorption/diffraction methods. Energy Fuels 1990, 4, 442-447. (19) Ward, C. R.; Taylor, J. C.; Matulis, C. E.; Dale, L. S. Quantification of mineral matter in the Argonne premium coals using interactive Rietveldbased X-ray diffraction. Int. J. Coal Geol. 2001, 46, 67-82.



Table 5. Argonne Premium CoalssCO2 Peaks and Evolution Temperatures during Pyrolysisa coal

Ro (%)

rank


0.25 0.32 0.46 0.57 0.81 0.89 1.16 1.68


peak 1

peak 2b

peak 3

peak 4

T (°C)

T (°C)

T (°C) 50234 50512

36113 34610 37013 36927 361

418100 439100 45351 46826 49034 49598 51014 51074e 473

504

peak 5

peak 6

peak 7

T (°C)

T (°C)

T (°C)

T (°C)

62414 64025 56766 611100 61440 63289 62026

69415c

75510

714100 72326c

76587c

615

685100c 704

776100 77957 782100 76575 770

98843 99718 99936 1000100 100031d 98975 996

a Subscript x in the peak temperature, T , denotes the relative intensity, where 100 is the most intense peak. b Rank dependent. c Assigned to inorganic x CO2 from calcite. d Evolved at a slightly higher temperature than 1000 °C. e Assigned to inorganic CO2 from siderite.

Figure 4. Fitted CO2 peak profile and assignment for Illinois no. 6.

peaks. The CO2 evolution profiles of Argonne premium coals (Table 5) were successfully fitted into three peaks (Wyodak), four peaks (Blind Canyon), and five peaks (Beulah-Zap, Illinois no. 6, Pittsburgh no. 8, Lewiston-Stockton, Upper Freeport, and Pocahontas no. 3). Figure 4 presents the fitted CO2 peak profile for Illinois no. 6. Contrary to the assignment made by Solomon et al.8 to CO2 extra loose associated with H2O in the CO2 pyrolysis profile of Wyodak around 200 °C, our work carried out on fresh samples does not show evidence of their observation. However, TGFTIR tests performed on oxidized samples of Beulah-Zap and Wyodak revealed the presence of low-temperature CO2 at 165 °C, which is tentatively formed through O2 adsorption, resulting in weakly bound carboxylic acid groups on the coal surface.20 The low-temperature CO2 resulting from oxidation is evolved along with peak 1 for H2O, assigned to physical water.13,14 The four highest rank coals in the series reveal a small peak (peak 1) at an average temperature of 361 °C, which does not appear to be associated with peak 3 for H2O, found at 389 °C for only two coals. Peak 1 for CO2 can tentatively be assigned to an aromatic CO2 site by virtue of its comparable evolution temperature as the main CO2 peak, 381 °C seen in the decomposition of diallylphthalate resin. (20) Chaffee, A. L. Private communication, 2006.

Table 6. CO2 Evolution Temperatures from TG-FTIR Decomposition of Carbonates mineral


siderite calcite

554 743

All eight coals reveal a peak (peak 2) at an average temperature of 473 °C; it is interesting to find that its evolution temperature is rank-dependent, as observed by Burnham et al.6 Its rank dependence lends support to the suggestion of Solomon et al.8 about it being associated with tar evolution. Burnham et al.6 also suggest that CO2 evolved below 500 °C could be formed via the decomposition of coal carboxylic acids, salts, or esters. Here, it is worth noting that peak 2 is the most intense one for the two lowest rank coals. In the case of Pocahontas no. 3, the highest rank in the series, we assign this peak to inorganic CO2 from siderite based on its rather sharp appearance and close evolution temperature to that of pure siderite, 554 °C (Table 6). The presence of siderite in this coal has tentatively been assigned by Solomon et al.8 and confirmed by Ward et al.19 The latter group also identified a small amount of siderite in Blind Canyon coal, but our work could not unequivocally make this assignment. Ward et al.,19 as well, found the mineral ankerite Ca(Fe, Mg, Mn)(CO3)2 in Blind Canyon and Pocahontas no. 3 coals, but once again, our investigation could not clearly


Giroux et al.

Figure 5. CO evolution as function of the temperature for the Argonne premium coals. Table 7. Argonne Premium CoalssCO Peaks and Evolution Temperatures during Pyrolysisa

a

peak 1b

peak 2b

peak 3

peak 4

peak 5

coal

Ro (%)

rank

T (°C)

T (°C)

T (°C)

T (°C)

T (°C)


0.25 0.32 0.46 0.57 0.81 0.89 1.16 1.68


46920 45512 4564 4615 49110 52211 5222

60822 60249 60732 61442 61236 62315 63530 64927 619

758100 773100 780100 770100 759100 757100 766100 768100 766

94415 9557

482

96036 953

98412 98210 98132 98625 98145 9945 985

Subscript x in the peak temperature, Tx, denotes the relative intensity, where 100 is the most intense peak. b Rank dependent.

distinguish this additional source of inorganic CO2. As pointed out earlier, peak 2 for CO2 is evolved at a temperature comparable to peak 4 for H2O and peak 1 for CO. Only the two lowest rank coals exhibit a peak (peak 3) centered at 504 °C, which could originate from the pyrolysis of carboxylic acids or esters.6 This peak is approximately 3 times more intense for Beulah-Zap relative to Wyodak. All coals, except the highest rank Pocahontas no. 3, show a feature (peak 4) centered at 615 °C. This peak evolves at temperatures comparable to peak 6 for H2O and peak 2 for CO. Its assignment to a specific carboxyl functional group is not obvious because of the lack of corresponding evidence from the decomposition of the model polymers considered. Four coals, irrespective of rank, reveal a peak (peak 5) at 704 °C. For three of these coals, including Beulah-Zap, Blind Canyon, and Pocahontas no. 3, peak 5 is assigned to inorganic CO2 from calcite. Its intensity is lowest for Beulah-Zap and highest for Pocahontas no. 3. Once again and as observed for the CO2 feature coming from the decomposition of siderite in Pocahontas no. 3, the rather sharp appearance for CO2 peaks originating from inorganic matter compared to that from organic matter is noteworthy. Also, its average evolution temperature of 717 ( 27 °C is close to our measurement of 743 °C via TG-FTIR (Table 6) for pure calcite mineral. Five of the coals, essentially of highest rank, give a feature (peak 6) at 770 °C, with its source being from calcite for Illinois no. 6 as shown in Figure 4. Our TG-FTIR work identifying Beulah-Zap, Illinois no. 6, Blind Canyon, and Pocahontas no. 3 yielding some CO2 via the decomposition of calcite is

supported by the XRD study of Ward et al.19 These authors also identified calcite in Pittsburgh no. 8 and Upper Freeport coals, which was not detected in our work. Peak 6 for CO2 appears to be associated with peak 8 for H2O and peak 3 for CO on account of comparable evolution temperatures. The six highest rank coals exhibit a peak (peak 7) at the relatively high temperature of 996 °C, which is comparable to temperatures around 985 °C measured for CO2 in both aliphatic and aromatic sites from the pyrolysis of diallylphthalate and poly(methyl methacrylate) polymers. Peak 7 for CO2 appears to be associated with peak 5 for CO, 985 °C. The temperature program in Figure 4 is purposely extended down to about 800 °C (in the cooling period) from the maximum of 1000 °C to show complete evolution of peak 7 for CO2 of Illinois no. 6 at 988 °C. Otherwise, choosing a temperature limit of 1000 °C would not show the complete evolution of the highest temperature CO2 peak. CO Evolution. Figure 5, presents CO evolution profiles during pyrolysis of the Argonne premium coals. It shows the progressive decrease of the CO yield with rank, with the exception being that the CO yield from higher rank Blind Canyon is higher than that of Illinois no. 6. CO is evolved between about 400 and 900 °C. Table 7 lists CO peaks resolved for the eight Argonne premium coals with the corresponding maximum evolution temperatures during pyrolysis and relative peak intensities. It also gives average temperatures of evolution for the respective peaks. The CO evolution profiles for all of the Argonne premium



Figure 6. Fitted CO peak profile and assignment for Pittsburgh no. 8.

coals (Table 7) were successfully fitted into four peaks. Figure 6 presents the fitted CO peak profile for Pittsburgh no. 8. All coals, except the highest rank Pocahontas no. 3, show a minor feature (peak 1) centered at 482 °C, whose maximum evolution temperature increases with rank. As discussed in the sections on H2O and CO2, peak 1 for CO is evolved at very similar temperatures to peak 4 for H2O and peak 2 for CO2. Its rank dependence suggests its association with tar evolution.8 Our findings on peak 1 for CO support those of Burnham et al.,6 who also report this peak to shift gradually to higher temperatures, to decrease in intensity with increasing rank, and to evolve at a similar temperature as the major CO2 peak (peak 2). The next peak (peak 2) centered at 619 °C is present in the pyrolysis profiles of the eight coals. It is associated with peak 6 for H2O and peak 4 for CO2. All eight coals reveal a peak (peak 3) around 766 °C; it is important to note that this peak corresponds to the major source of CO for all of the Argonne coals. Burnham et al.6 also find most of the CO to be evolved in the 600-800 °C range. Also, peak 3 for CO appears to be associated with peak 8 for H2O, 767 °C, and peak 6 for CO2, 770 °C. The two lowest rank coals, Beulah-Zap and Wyodak, and the highest rank coal, Pocahontas no. 3, yield a feature (peak 4) at 953 °C. Except for the two lowest rank coals, all coals show a peak (peak 5) centered at 985 °C. While it is minor for Illinois no. 6, Blind Canyon, and Pocahontas no. 3, it appears to be associated with peak 7 for CO2 at 996 °C. Peak 5 for CO is tentatively assigned as arising from an aromatic CO site in coal by virtue of the main CO peak observed at 985 °C in the pyrolysis profile of diallylphthalate resin. As in the case of Figure 4, the temperature program in Figure 6 is also purposely extended down to about 800 °C (in the cooling period) from the maximum of 1000 °C to show complete evolution of peak 5 for CO of Pittsburgh no. 8 at 981 °C. Otherwise, choosing a temperature limit of 1000 °C would not show the complete evolution of the highest temperature CO peak.

Distinction between Noncoking and Coking Coals via Pyrolysis Profiles. The Argonne premium coals consist of two types of coals, noncoking and coking. By definition, noncoking coals do not exhibit agglomerating or caking properties, whereas coking coals do cake and undergo softening followed by swelling and resolidification into metallurgical coke when heated in the absence of air.1 Differences in the H2O, CO2, and CO evolution profiles and more specifically the start of evolution of these gases for the two noncoking coals, Beulah-Zap and Wyodak, in this reference series, with respect to those of the six coking coals11 (Figures 1, 3, and 5), are the result of factors such as a higher overall oxygen content, a higher porosity, a higher aliphatic carbon content, and a more open pore structure. More specifically, Figures 1, 3, and 5 show that H2O for BeulahZap and Wyodak is first produced around 125 °C, CO2 at 150 °C, and CO at about 260 °C. In the case of the coking coals, H2O and CO2 only begin evolution around 275 °C and CO at about 325 °C. Conclusions Complex curve profiles of the three oxygen-containing gases evolved during pyrolysis of the Argonne premium coals were resolved into distinct peaks. An attempt was made at assigning these peaks to specific chemical environments within the coal structure based on evolved peak temperatures for H2O, CO2, and CO from the decomposition of model polymers. Most of the assignments to oxygen-containing peaks in coal were made using Bakelite and diallyphthalate, both thermosetting polymer resins, which have the advantage of undergoing decomposition without melting. In summary, this investigation found the following: (1) The H2O pyrolysis profiles were resolved into either three or four peaks, although a total of eight peaks were identified for the complete Argonne premium coals set. The two lowest-temperature H2O peaks (peaks 1 and 2), only found in Beulah-Zap and Wyodak, were assigned to “bulk” and not pyrolysis H2O. Peak 3 was tentatively assigned to H2O originating from an


aliphatic site. Peaks 4-6 are assigned to H2O arising from aromatic sites; peaks 4 and 6 are evolved along with CO2 and CO peaks. Peaks 7 and 8 remain unassigned. (2) The CO2 pyrolysis profiles were resolved into three, four, or five peaks but seven peaks in total were identified for the suite of coals. No evidence was found for the presence of low-temperature CO2 peaks,

(TGâFTIR) to the Analysis of Oxygen Functional Groups - American

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