Energy & Fuels 1993, 7, 734-742
734
Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid-state C-13 NMR Spectroscopy T. H. Fletcher,***S. Bai,g R. J. Pugmire,? M. S. Solum,$ S. Wood,# and D. M. Grant$ Departments of Chemical Engineering and Chemistry, Brigham Young University, Provo, Utah 84602, and Departments of Chemical and Fuels Engineering and Chemistry, University of Utah, Salt Lake City, Utah 84112 Received April 8, 1993. Revised Manuscript Received August 18, 1 9 9 9
Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2 % Chemical structural features determined from 13C NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. 'H NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from lH NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.
.
Introduction As industry blends or switches coals for existing power plants, the combustion behavior of the coal becomes an important design consideration. A prominent goal is to quantify the relationship between coal structure and coal combustion behavior, allowing prediction of combustion and pollutant emission characteristics directly from chemical analyses of the fuel. A great deal of effort has gone into obtaining coal structure data that is directly applicable to modeling studies of coal devolatilization behavioral Structural characteristics have been determined for parent coals2and for chars collected at different stages of p y r o l y ~ i s .Recent ~ ~ ~ work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phen~mena.~ Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal' and the initial amount of pyridine extract^.^^^ Pyrolysis mass spectroscopy com-
* Author to whom correspondence should be addressed.
+ Department of Chemical and Fuel Engineering, University of Utah. t Department of Chemistry, University of Utah.
of Chemistry, Brigham Young University. of Chemical Engineering, Brigham Young Univerity. Abstract published in Advance ACS Abstracts, October 15, 1993. (1) Fletcher,T. H.; Kerstein,A. R.; Pugmire,R.J.;Grant,D. M.Energy Fuels 1992,6, 414. (2) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,3, 1 Department
1 Department
bined with multivariate data analyses shows that the early pyrolysisproducts are chemically distinct from the balance of the evolved coal tar and correspond roughly to bitumenlike materials.5J339 Coal extraction experiments are commonly used to determine in part the chemical structure of the parent coal.1°-13 Fong et al.14 used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh No. 8 coal. These experiments demonstrated that pyridine-extractable material initially increased with pyrolysis temperature, passed through a maximum, and then decreased as retrogressive reactions became dominant at the higher temperatures. Under moderate heating conditions (-500 K/s to 873K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. Erbatur et al." studied the l3C NMR characteristics of the extracts and residues of seven coals, showing preferential transfer of several moieties to either the extracts or residues, They also showed that the coal spectra could not be generated as a weighted combination of the corresponding spectra from the residue and extract. Erbatur's work emphasizes the need for quantitative determination of the accuracy of yields and extracts. They caution against using the chemical structural data obtained from extracts until the
(8) Chakravarty, T.;Windig,W.; Hill, G. R.;Meuzelaar, H. L. C. Energy Fuels 1988,2, 400-405. 187.. (9) Simmleit,N.;Schult.en,H.;Yun,Y.;Meuzelaar,H.L.C.InAduances (3) Fletcher,T.H.;Solum,M.S.;Grant,D.M.;Critchfield,S.;Pugmire, in Coal Spectroscopy; Meuzelaar,H. L. C. Ed.;Plenum Press: New York, 1992; pp 295-339. R. J. 23rd Symposium (International) on Combustion;The Combustion (10)Chang, H. K.; Nishioka, M.; Bartle, K. D.; Wise, S. A.; Bayona, Institute Pittsburgh, 1990; pp 1231. (4) Pugmire,R. J.;Solum,M.S.;Grant,D.M.;Critchfield,S.;Fletcher,J. M.; Markides, K. E.; Lee, M. L. Fuel 1988, 67, 45-57. (11) Erbatur, G.; Erbatur, 0.;Davis, M. F.; Maciel, G. E. Fuel 1986, T. H. Fuel 1991, 70,414. 65, 1265-1272. (5) Meuzelaar, H. L. C.; Yun, Y.; Chakravarty, T.; Metcalf, G. S. In (12) Buchanan, D. H.; Osborne, K. R.; Warfel, L. C.; Mai, W.; Lucae, Advances in Coal Spectroscopy;Meuzelaar, H. L. C., Ed.; Plenum Press: D. Energy Fuels 1988,2, 163-170. New York, 1992; pp 275-294. (13) Carbon, R. E.; Critchfield, S.;Vorkink, W. P.; Dong, J.; Pugmire, (6) Nikea, S. Energy Fuels 1991,5, 673-683. R. J.; Lee, M. L.; Zhang, Y.; Shabtai, J.;Bartle, K. D. Fuel 1992,71,19-29. (7) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; (14) Fong, W. S.; Peters, W. A.; Howard, J. Fuel 1986, 65,251. Deshpande, G. V. Energy Fuels 1988,2, 405-422.
0887-0624/93/2507-0734$04.00/00 1993 American Chemical Society
Structure of Pyridine Extracts of Coal effects of the extraction and washing procedures on yields and chemical structure features can be quantitatively determined. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. The Argonne Premium coal samples were selected because they have been the subject of many experiments and provide the basis for meaningful comparisons over long periods of time.15 Extraction experimenta were performed in duplicate to demonstrate repeatability, and total yields of extracts and residues are reported to show how much material is lost during the extraction procedure. The 13CNMR analyses of the coals, extracts, and residues are compared, and the 'H NMR data obtained from the extracts are compared with the tars formed at different stages of the pyrolysis process from similar coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.
Experimental Approach It is well-known that a considerable amount of material is extracted by pyridine from bituminous coals. However, a significant part of the extract seems to form colloidal dispersions that can be disruptive to analytical techniques such as proton and carbon NMR spectroscopy. It is also known that pyridine is imbibed into the structure of coal and is very difficult to remove. This imbibed pyridine makes it difficult to quantify the structural features of coals. Colloidal dispersions in extracts are minimized by performing extractions in the absence of oxygen?2and rinsing procedures have been proposed to minimize the amount of pyridine trapped in the extracts.12 The multiple-solvent Soxhlet extraction procedure for coal used by Buchanan et a1.12 was modified to a single exhaustive pyridine extraction. The sealed glass ampules containing the Argonne premium coal sample were broken in an Ar glovebag, and the samples were transferred to Soxhlet thimbles in Arflushed vials prior to weighing. A capillary tube was used to continuously introduce Ar into the liquid pyridine in the bottom of the Soxhlet apparatus to purge the solvent and maintain the Ar atmosphere during the extraction. No oxidationoccurs during the extraction with the Ar introduced through the capillary tube. Two 5-g coal samples were extracted in parallel with approximately 150 mL of Ar-purged pyridine. The Soxhlet extractions were continued for 5-7 days until the solvent in the Soxhlet funnel became colorless. The soluble fractions were filtered through a 0.45-pm Nylon 66 membrane filter, followed by removal of the solvent by rotary evaporation (water aspirator, 50 "C bath). The solid material collected on the Nylon filter was about 0.2 % of the original coal sample. The resulting pyridine-soluble fractions were sonicated with 120 mL of 80% methanol/water. The wash solvent was then removed using the rotary evaporator. The methanol/water washings were repeated three to four times. The resulting solid extracts were dried to constant weight in a largebore Abderhalden apparatus over PZOSat 0.03 Torr and room temperature. The insoluble residue was recovered from the extractionthimble and then washed and dried in the same manner as the extract. The pyridine level was lower than detectable by smell, and no evidence of pyridine was detected using FTIR analyses of either the extracts or residues. Coal extracts were dissolved in DMSOds solvent and analyzed by proton NMR spectroscopy; trace amounts of pyridine were still detected. In an effort to remove (15) Vorres, K. S. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990, 35(3), 774-778.
Energy &Fuels, Vol. 7, No. 6,1993 735 this trace amount of pyridine from the extracts, the MeOH/ water wash in the procedure was followed by washing with 10mL of 0.1 M NHdOH solution. However, the NHdOH washing did not remove any of the pyridine from the extracts, suggestingthat the acid-base type complex between pyridine and coal compounds are not the major mechanism by which pyridine molecules are trapped in solid extracts. Toluene washing, which uses toluene to replace the MeOH/water mixture in the first washing step,12 was also used in an attempt to remove the trace amounts of pyridine. The pyridine concentration in the extracts was decreased slightly by toluene washing. However, a trace amount of toluene was introduced to the extracts using this method. Therefore, only the MeOH/water washing was used to prepare extracts and residues, recognizing that trace amounts of pyridine are present. The carbon-13 NMR spectra of all extracts and extraction residues were obtained according to the procedure of Solum et al.,'J6 which involves cross-polarization (CP) magic angle spinning (MAS) and dipolar dephasing techniques modified for background suppression as in sequence (Id) from White and co-workers.l' There was enough sample to fill a rotor (350-370 mg) for all samples except for the Pocohontas extract, where only 35 mg was available. The signal-to-noiseratio was therefore high in the Pocahontas extract due to the small sample size. High-resolution lH NMR data were obtained on a Varian VXR500 spectrometer using dimethyl sulfoxideas solvent,in a manner reported p r e v i o ~ s l y . It ~ ~was ~ noted that the solubility of the extracts varied with the coal and that solubilization was not complete for any of the coals in DMSO. The lH NMR spectra exhibited the characteristics of high-resolution with line widths that would be typical of compounds with molecular weights of several hundred daltons. Elemental analyses (C, H, and N) of residues and extracts were performed by Coors Analytical Laboratories, as described by Fletcher and Hardesty.l* Ash compositions of residues and extracts were also performed using ASTM methods adjusted for small samples, although no ash was found in the residues (as expected). Moisture contents were also measured for the residues and extracts (1h at 105 "C), but were always lower than 1% and hence were ignored.
Results Extract Yields. The amount of extract obtained from each of the eight Argonne Premium coals is given in Table I. Duplicate extraction experiments were performed in parallel in order to ascertain the accuracy and repeatability of the yields. The average values of the yields of extracts and residues are shown in Figure 1 as a function of coal rank. As expected, the highest extract yields were obtained from the high-volatile bituminous coals (32 % for the Utah Blind Canyon coal), while negligible extract yields were obtained from the lignite (Zap)and low volatile bituminous coal (Pocahontas). Carlson et al.I3 recently reported THF extract yields from the Argonne Premium coals, and while it is well-known that pyridine should extract more material, the same trends in extract yields are noted (2.2% for Zap, 18.9% for Utah Blind Canyon, 16.8% for Illinois No. 6, and 0.8% for Pocahontas). One of the past criticisms of extraction experiments was the lack of repeatability and-the lack of mass closure data. Mass closures obtained in the present experiments of at least 94% were obtained on each sample, repeatable (16) Orendt, A. M.; Solum, M.S.; Sethi, N. K.; Pugmire, R. J.; Grant, D. M.In Advances in Coal Spectroscopy;Meuzelaar, H. L.C., Ed.; Plenum Press: New York, 1992; pp 215-254. (17) White, J. L.;Beck, L. W.; Ferguson, D. B.; Haw, J. F. J . Magn. Reson. 1992,100, 336-341. (18) Fletcher, T. H.; Hardesty, D. R. 'Milestone Report for DOES Pittsburgh Energy Technology Center"; contract FWP 0709, Sandia Report No. SAND92-8209, available from NTIS,1992.
736 Energy & Fuels,
Fletcher et al.
VoZ. 7, No.6, 1993
100
Pocahontas a0
m 0
e
w
U
Resklue Extrad
L
0
E
40
2!
5
LA" " " "
20
n "
n
o
*
>
N
S
o
~
m
Z
m *
o
*
c
-
1
o
3
u
JAviL
o
0,
Blind Canyon
f Figure 1. Yields of extracts and residues from pyridine extractions of the eight Argonne premium coals. Extractions were performed in a Soxhlet apparatus purged with argon. Table I. Pyridine Extraction Yields (Including Duplicate Data) extract residue tQtal coals yield (%)a yield (%)" recovery wyodak Blind Canyon Illinois #6 Pittsburgh #8
Stockton Upper Freeport Pocahontas
3.0 3.2 6.1 6.3 31.1 33.1 27.3 28.5 26.0 27.0 13.8 15.6 14.8 15.0 0.5 0.5
94.5 94.7 92.4 91.7 65.3 64.6 67.3 65.1 69.1 70.5 80.2 80.5 81.8 82.7 96.8 97.2
.
97.5 97.9 98.5 98.0 96.4 97.5 94.6 93.6 95.2 97.5 94.0 96.1 96.6 97.7 97.3 97.7
200
450
-50
PPM from TMS
-300
Figure 2. 1W CP/MAS spectra of the extracts from the eight Argonne coals. The spectrawere run with a 1-s pulse delay, 2-ms contact time, and 54-kHz decoupling power.
Pocahontas
-
Upper Freeport
inL,L
Weight percent on a dry basis.
within 2% on an absolute scale. The high degree of repeatability and the high mass closure in these experiments are important for subsequent data interpretation. The low amountof colloidal material collected on the filters and the lack of evidence of colloidal material in the subsequentNMR spectra distinguishthis set of data from previously published extraction data. 1SC NMR Analyses. The complete set of chemical structure data obtained from 13C NMR analyses of the parent Argonne premium coalswere published previously.2 Related structural data on other coals and chars at various extents of pyrolysiswere alsopublished.3A1* The 13CNMR analyses of the extracts and residues are presented in Tables I1 and I11along with the CP/MAS spectra in Figures 2 and 3. Figure 4 portrays the fraction of aromatic carbons that are protonated in the pyridine extractsand extraction residues. A higher percentage of aromatic carbons is protonated in the extract than in the residue, inacating that the residue contains a higher fraction of substituted carbons than the extract. The percentages of protonated aromatic carbons in the extracts and residues generally increase as a function of coal rank. Probable attachments to aromatic carbons are protons, carbons, and oxygens, so that a high percentage of proton attachments indicates a low degree of connection between aromatic clusters. A more clear indication of the connectivity between aromatic clusters comes from the examination of the types
Illinois #6
450
200 -50 PPM from TMS
-300
Figure 3. 1W CP/MAS spectra of the residues from the eight Argonne coals. The spectra were run with a 1-spulse delay, 2-ms contact time, and 54-kHz decoupling power.
of carbon attachments to aromatic clusters. As shown in Figure 5, the coordination number (i.e., the total number of attachments per cluster, or c 1) of the residues is higher than the extracts, with the single exception of the Illinois No. 6 in which no distinction can be made. Attachments on clusters are subdivided into two groups: (1)bridges and loops, and (2) side chains. Side chains are assumed to terminate with a methyl group. The fraction of intact bridges is therefore calculated from the total number of oxygenated and alkylated aromatic carbons minus the fraction of methyl groups, as follows:
+
Energy & Fuels, Vot. 7,No. 6, 1993 737
Structure of Pyridine Extracts of Coal Table 11. Zap WYO
UTBC I11 No. 6 Pitt. No.8
LS UF POC
lW NMR Analyses of Pyridine Extracts from the Argonne Premium Coals.
fa
fat
fac
faH
faN
fap
fas
55 59 61 72 73 76 77 82
48 53 57 68 69 73 74 74
7
17 17
30 34 36 42 41 42 41 44
7 7 7 7 7 7 5 5
13 15 14
6 4 4 4 3 3
18
23 23 26 29 32
8
faB -
17 17
10 12 15 17 17 18
18
18
15
24
18
fal
falH
45 41 39 28 27 24 23 18
33 28 27 19
fal*
fal0 ~
18
17 14 10
0.21 0.23 0.26 0.25 0.25 0.25 0.24 0.32
12 13 12 9 9 7 9 8
10 11
13 12 12 12 12 16
~~~~
4.2 4.6 4.8 4.4 4.1 3.9 3.7 4.3
~
0.40 0.41 0.43 0.64 0.63 0.71 0.61 0.60
~~
1.7 1.9 2.1 2.8 2.6 2.8 2.3 2.6
2.5 2.4 2.7 1.6 1.5 1.1
1.4 1.7
a Fractions carbon (error estimate): f a = total sp2-hybridizedcarbon (10.03); 4fat = aromatic carbon (10.04); fac = carbonyl, 6 > 165 ppm (*0.02); fa' = aromatic with proton attachment (10.03); faN = nonprotonated aromatic (10.03); fap = phenolic or phenolic ether, 6 = 150-165 ppm (10.02); fas = alkylated aromatic 6 = 135-150 ppm (10.03); faB = aromatic bridgehead (f0.04); fal = aliphatic carbon (10.02); falH = CH or CH2 (10.02); fd* = CH3 or nonprotonated (10.02); fdo = bonded to oxygen, 6 = 50-90 ppm (10.02).
Table 111. fa
f&t
1W NMR Analyses of Pyridine Extraction Residues from the Argonne Premium Coals.
fac
faH
faN
fap
fas
faB
fal
fdH
fd*
falo
c
Xb
a+l 6.2 5.6 5.4 4.4 5.1 4.7 4.8 4.7
Po
BL
sc
2.0 0.67 29 8 10 0.31 15 42 9 15 18 21 12 17 71 59 2.1 6 0.27 0.63 24 9 13 11 17 39 9 15 15 33 67 56 2.8 0.48 29 11 5 0.29 14 37 7 14 16 40 60 55 5 18 1.7 0.61 7 0.25 12 33 24 9 16 67 62 5 23 39 7 16 2.0 0.61 3 0.31 15 30 21 9 44 6 17 21 70 67 3 23 1.5 0.68 19 8 3 0.26 13 4 26 43 7 18 18 27 LS 73 69 1.7 3 0.65 0.32 16 21 13 8 3 29 5 18 24 79 76 47 UF 1.2 2 0.34 0.74 9 6 17 2 32 51 4 19 28 15 POC 85 83 a Fractions carbon (error estimate): fa = total sp2-hybridized carbon (10.03); fat = aromatic carbon (10.04); fac = carbonyl, 6 > 165 ppm (i0.02); fa' = aromatic with proton attachment (10.03); fa' = nonprotonated aromatic (10.03); fa' = phenolic or phenolic ether, 6 = 150-165 ppm (10.02);fas = alkylated aromatic 6 = 135-150 ppm (10.03);fa' = aromatic bridgehead (f0.04); fal = aliphatic carbon (10.02); fdH = CH or CH2 (10.02); fd* = CH3 or nonprotonated (10.02);fd0 = bonded to oxygen, 6 = 50-90 ppm (10.02). Zap Wyo UTBC IU#6 Pitt#8
L
a
C
v)
-" $ 3
n
a
N
o
$
o
-
E =
w
* )
21s22 collected tar samples from five of the coals from the DOE Direct Utilization AR&TD program. Tar samples were collected in a laminar flow reactor with gas temperatures of either 1050 or 1250 K in nitrogen, with particle heating rates of lo4K/s. The samples collected in the 1050K environment were shown to be relatively free of secondary tar reactions in the gas phase, while tar samples from the 1250 K environment showed evidence of significant secondary tar reactions which increased in severity with residence time. The NMR analyses of the chars and tars from these experiments have been reported p r e v i ~ u s l y . ~ ?Proton ~J~ NMR data are available for tars obtained in the 1050 K condition from three coals that are similar to coals in the Argonne Premium sample bank: Illinois No. 6 hvc bituminous coal (PSOC-1493D); Pittsburgh No. 8 hva bituminous coal (PSOC 1451D); and Zap lignite (PSOC 1507D). The PSOC number refers to the Penn State data base designation, and the "D" designates the DOE DU/ AR&TD suite of size-classified coals.18 The lH NMR analyses of the earliest available coal tars are compared with the analyses of the extracts from the Argonne Premium coals in Figures 16-18. This comparison shows the similarity between the proton structural features of the tars and extracts as a function of coal rank. Additional data regarding the extent of mass release due to devolatilization and the estimated tar yields for these samples are given in Table IV. The earliest tars evolved from a coal particle should correspond to the extractable material, since no labile bonds must be ruptured to liberate this trapped material.'^^' The Pittsburgh No. 8 coal tar in Figure 17 was taken from a later residence time than the other two coal tars and hence does not compare as well with coal extract data. In particular, the extent of yhydrogen in the extract is much larger than in the Pittsburgh No. 8 coal tar; decreases in the y-hydrogen are the first evidence of secondary reaction of tarseen in proton NMR of coal tars obtained at higher temperatures and
+
(21)Fletcher, T. H. Combust. Flame 1989, 78, 223. (22) Fletcher, T. H. Combust. Sci. Technol. 1989,63, 89.
Energy & Fuels, Vol. 7, No. 6, 1993 741
Structure of Pyridine Extracts of Coal 40
SO'
-n 5
Zap lignite
d)
-n
30
v)
Illinois
f
#6
C
a cn
tn
-
' C
30
E
20
U
-
>.
I
E Q) CI)
2
40
.-C
Q)
20
0
10
ae
10
r
0
ae
0
m I
m I
c;'
c;'
z
E
-
n 6
? E
m C I
L
0 L
F
m
C
L I.
N
0
m
C
L
m
6
s
m u
n
Figure 16. Comparison of lH NMR analyses of (a) pyridine extracts of the Argonne Premium Illinois No. 6 coal with tar samples from a similar Illinois No. 6 coal from the PETC DU/ AR&TD suite (PSOC-1493D). This tar sample was obtained at 91 ma in a transparent-wall drop tube reactor with a heating rate of lo" K/s at 1050 K in N2.
Figure 18. Comparison of 1H NMR analyses of (a) pyridine extracts of the Argonne Premium Zap lignite with tar samples from a similar Zap lignite from the PETC DU/AR&TD suite (PSOC-1493D). This tar sample was obtained at 160 ma in a transparent-wall drop tube reactor with a heating rate of 10.'K/s at 1050 K in N2.
I
Table IV. Tar Data from the Sandia Coal Devolatilization Laboratory1* Illinois Pittsburgh No.6 No.8 &P hvc bitum hva bitum lignite rank 63-75 75-106 size fraction (pm) 106-125 200 160 91 residence time (ma) mass release (% daf)* 28 33 47 estimated tar yield (% daf)* 5 29 13 total volatiles yield (% daf)c 51 53 53 a Determined from tracer analyses of the char (see Fletcher and Hardestyla). b From mass balance and collection efficiency. These estimates have large errors (-&lo), especially for the Illinois No.6 sample. From the 1250 K gas condition at -200 ma (see Fletcher and Hardesty18).
604 Pittsburgh
#8
Extract
C
a cn
2
U
30
h
I r
0
2o
ae 10
times were not available for lH NMR analyses. Experiments are currently underway to provide tar/char sets from low temperature experiments for more quantitative comparison of early tars and extracts.
n
u
n
Figure 17. Comparison of lH NMR analyses of (a) pyridine extracts of the Argonne Premium Pittsburgh No. 8 coal with tar samples from a similar Pittsburgh No. 8 coal from the PETC DU/AR&TDsuite (PSOC-1493D). This tarsample was obtained at 200 ma in a transparent-wall drop tube reactor with a heating rate of lo" K/s at 1050 K in N2.
longer residence t i m e ~ . ~ *The ~ J general ~ trend observed in Figures 16-18 is that the best agreement between structural features of the extracts and coal tars occurs at the earliest residence times. This same trend can be seen by comparing reported proton NMR analyses of tarsfrom these same three coals obtained at longer residence time~.3*~J~ Certainly some of the disagreement between the tars and extracts are coal dependent; the tars are not from the Argonne premium coals, and the agreement between tar features and extract features may be rank dependent as well as coal-dependent. Unfortunately,tar samplesderived from these three coals at earlier residence
Conclusions The Soxhlet extraction experiments performed using pyridine blown with argon showed little evidence of colloidal dispersions. The washing procedure removed the pyridine below the smell level and lower than detectable with FTIR. The lH NMR analyses of the extracts, however, showed that a trace amount of pyridine remained. Duplicate experiments showed that the mass closure of extract plus residue accounted for 94-98 % of the original coal mass, and that yields were repeatable to within 2 % . To our knowledge, this is the first reported quantitative mass closure data on extractions from coal. Comparing the 13C NMR data on the extracts and residues, it becomes apparent why material is extracted from the parent coal. While the CP/MAS spectra of both the extracts and residuesare quite similar,an examination of the detailsof the structure provide the subtledifferences in structural detail that are important in describing the extraction process. While one must recognize that only
742 Energy & Fuels, Vol. 7, No. 6,1993
averages of all the components present in the extracts are observed, the extracted material appears to have many of the structural features that are observed in the macromolecular structure of the coal. The differences lie in the fact that the number of cross links is reduced and the number of substituents on the aromatic rings is lower in the extracts than in the residues. Hence, these data are consistent with the fact that this material can be extracted since it is not extensivelyincorporated by means of covalent bonds into the macromolecular structure. Comparison of the structural features of the parent coals with those of the corresponding extracts and residues shows that most properties of the coal can be reconstructed as a composite of residue and extract. The discrepancy between coals and composite values reported by Erbatur et al." are only seen here for some features of the lignite, such as the number of attachments per cluster and the number of bridges and loops per cluster. The mass closure data reported from the current experiments (Table I and Figure 1)indicate that the amount of lost material is 2-6 ?6 of the coal, which is why large differences are not seen between major structural parameters of the coal and the composite of extracts and residues. The proton NMR data show that the amount of 2- and 3-ring components present in the extracts increases with
Fletcher et al. the rank of the coal. The aliphatic region of the proton NMR data indicates that (a) the amount of a-methyl groups is essentiallythe same in all extracts, (b)the amount of y-methyl groups reaches a minimum in the LewisStockton coal, and (c) the amount of a-hydrogen goes through a maximum a t the high-volatile bituminous rank range. The proton NMR data for the extracts of the Argonne Premium coals agree quite well with reported proton NMR data for early residence time coal tars from related coals, obtained in the Sandia Coal Devolatilization Laboratory. This agrees with the concept that the early tars directly correspond with the extractable material. The data presented here on the extracts will be used to compare the pyridine-extractable material that is obtained as these coals are pyrolyzed. Experiments that are now underway in our laboratories together with the results presented herein will be useful in our future work on modeling the transformations that occur during devolatilization and char formation.
Acknowledgment. This work was supported by the National Science Foundation through the Advanced Combustion Engineering Research Center at Brigham Young University and the University of Utah.
