Energy & Fuels 1987,1, 167-172 tetralins) as well as decomposition of labile functional groups (e.g., carboxylic moieties) takes place around 300 "C during the early stages of decomposition of the Hiawatha coal. The main degradation step occurs between 500 and 600 "C when the "bulk" components of the coal start to decompose. A third stage consists of char-forming reactions characterized by the evolution of small, stable molecules and aromatic hydrocarbon moieties. Weatheririg has an effect on all degradation steps. The alkylnaphthalenes and alkyltetralins are likely to react with the network phase, p.resumably through "grafting" reactions, thereby becoming unable to evaporate a t low temperatures. The increase of oxygen-containing products indicates the formation of various oxygen functional groups (most likely carboxyl, carbonyl, and hydroxyl moieties) during weathering. On the other hand, the intensity of the
167
phenols is decreased in the weathered coal, indicating that the hydroxyl groups react with the network phase by condensation reactions and that the newly formed bonds are not broken at the maximum pyrolysis temperature (610 "C) used in the Py-MS experiments.
Acknowledgment. The work reported here was sponsored by U.S. DOE Contract No. DEFG22-84PC70798and several research contracts from Utah Power and Light Co. The authors wish to express their appreciation to Mike Brady (Utah Power and Light Co.) for helping to obtain a fresh Hiawatha seam coal sample from the Wilberg mine. The TG/MS experiments were carried out at the Laboratory for Inorganic Chemistry, Budapest, Hungary; Dr. T. Szekely, F. Till, and P. Szabo are acknowledged for the data.
Molecular Mobility during Pyrolysis of Ausltralian Bituminous Coals Richard Sakurovs,? Leo J. Lynch,*t T. Patrick Maher,$ and Rabindra N. Banerjee' CSIRO Division of Fossil Fuels, North Ryde, N S W 2113, Australia, and Joint Coal Board, Sydney, N S W 2001, Australia Received September 3, 1986. Revised Manuscript Received November 14, 1986
Bituminous coal "reactivity" during heating and pyrolysis has been assessed in terms of the extent and degree of molecular mobility attained a t the thermoplastic stage. Thirty-three Australian bituminous coals, including some maceral concentrates (8040% carbon), were examined in situ during heating at 4 K/min by pulsed proton nuclear magnetic resonance ('H NMR). The maximum fraction of mobile material achieved during the pyrolysis correlates strongly with hydrogen content, extrapolating to zero a t 4 % hydrogen. High inertinite (mainly semifusinite) coals are undifferentiated in this correlation. For vitrinite there is no significant molecular mobility below -600 K, but rapid mobilization occurs above this temperature. This latter transient mobilization is identified as the essence of coal thermoplasticity. As might be expected, exinite, which is probably composed predominantly of long-chain aliphatic material, acquires considerable mobility a t lower temperatures than vitrinite, though its maximum mobility is achieved at the thermoplastic stage of the coal (-700
K).
Introduction Maceral analysis is a technique extensively used in the prediction of coking ability of bituminous coals from laboratory tests. However established criteria based on macera1 analysis consistently underestimate the coking potential of many Australian Permian' or West Canadian Cretaceous2coking coals. This deviation is attributed to the presence of "reactive" materials that are grouped into the "inert" maceral group fraction by petrographic analysis of these coals and are rare or absent in the Carboniferous coals for which the criteria were established. Diessel' has examined the reactivity of macerals in Australian coals by means of ieflectance changes on carbonization and has established a criterion for reactivity based on the reflectance value of the macerals regardless of the maceral type. CSIRO Division of Fossil Fuels. *Joint Coal Board. 0887-0624/87/2501-0167$01.50/0
The thermoplastic property - of bituminous coais is also considered important for predicting coking potential, and there are a number of established test methods used to assess this property (e.g. Gieseler plastometry, crucible swelling index, dilatometry). A more recent technique for assessment of thermoplasticity is proton nuclear magnetic resonance (NMR) thermal analysis (PMRTA).3s4 PMRTA, which measures bulk properties of the specimen, can be used to distinguish hydrogen in rigid and in mobile molecular structures and therefore, to a reasonable approximation, to determine the fraction of molecular structure that is mobile during heating of a coal specimen through the thermoplastic stage, i.e. the extent of the (1)Diessel, C. F. K. Fuel 1983, 62,883-892. (2)Nandi, B.N.; Montgomery, D. S. Fuel 1975,54,193-196. (3)Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W. A.; Maher, T.P.Proc.-Znt. Conf. Coal Sci. 1985 1985,887-890. (4) Lynch, L. J.; Sakurovs, R.; Barton, W. A. Fuel 1986,65,1108-1111.
0 1987 American Chemical Society
Sakurovs et al.
168 Energy & Fuels, Vol. 1, No. 2, 1987 Table I. Selected Properties of the Australian Bituminous Coal Suite Studied” coal sample no. 1 2 3 4 5 6 7 8 9 10
TmH$b
K
755 750 760 760 743 737 745
11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
730 729 727 732 725 726 713 725 720 725 728 723 733 720 725 718 715 713 726
%
mm 0 32 34 26 16 29 15 43 40 38 13 10 24 14 10 34 7 27 41 38 26 58 32 59 60 56 36 34 38 40 30 32 14 7
Tmm,b Tzmm,d K pS M3me M6$ 188 157 739 91 218 166 744 90 226 171 738 82 214 167 738 78 200 156 730 90 216 161 742 93 199 161 720 109 219 154 710 73 222 154 718 119 211 152 688 115 197 145 712 90 197 147 702 130 203 134 708 98 203 146 693 123 190 143 692 104 208 138 63 189 141 708 138 209 153 703 165 210 144 704 183 210 143 695 101 209 140 700 224 213 118 689 148 210 133 703 179 216 126 715 150 219 127 696 192 216 124 711 141 147 699 134 215 143 700 103 206 160 696 129 217 153 700 120 219 152 698 145 216 153 698 99 198 144 141 700 100
MTWe Mmi$ 153 140 139 144 145 138 144 116 116 114 132 149 117 136 137 105 140 123 95 99 120 59 104 69 71 72 105 104 111
98 112 105 131 131
114 109 126 141 123 130 105 107 107 127 114 127 139 105 116 92 92 115 58 103 66 66 71 101 103 104 96 110 105
ultimate anal. (daf), % C H 0 VM 91.2 3.58 2.4 11.1 89.2 4.95 3.6 23.9 89.0 5.26 3.6 26.6 88.8 4.82 4.6 23.1 88.3 4.60 4.8 24.1 88.0 5.13 4.9 26.9 87.5 4.65 6.1 24.4 87.2 5.17 5.6 31.3 87.0 5.34 4.8 32.2 86.5 5.07 6.4 30.9 85.7 4.60 7.3 28.1 85.7 4.40 7.8 24.4 85.4 5.07 7.1 34.7 85.1 4.80 8.1 31.0 84.9 4.40 8.7 26.4 84.8 5.57 6.8 35.9 84.7 4.20 9.2 25.5 84.5 5.10 8.4 33.1 84.4 5.34 7.7 37.8 84.3 5.31 7.8 38.4 84.2 5.10 8.4 33.9 84.1 5.83 7.4 43.3 84.0 5.40 8.1 37.3 83.8 6.32 6.9 44.1 83.8 6.01 7.2 44.4 83.7 5.91 7.4 44.4 83.4 5.23 9.2 37.4 83.4 5.63 8.4 39.5 83.1 5.25 9.3 40.0 82.8 5.48 9.2 42.2 82.6 5.26 10.1 38.7 82.5 5.44 9.7 38.9 81.7 4.50 12.0 32.1 81.4 4.48 11.7 30.4
L x ,
%
2.60 1.32 1.31 1.32 1.09 1.14 1.32 1.03 1.10 0.92 0.70 1.09 0.80 0.95 0.64 0.83 0.74 0.69 0.65 0.68 0.73 0.67 0.67 0.67 0.70 0.73 0.76 0.72 0.73
maceral anal.,’ Vit Ex Mac ID 59 0 1 9 68 1 0 9 0 0 5 89 34 0 5 15 1 1 17 12 50 3 3 20 0 1 26 14 53 3 1 11 1 7 76 2 40 4 2 15 6 6 2 30 4 1 24 10 29 8 1 18 27 7 1 17 5 7 1 24 1 9 71 4 2 28 21 11 24 5 1 20 34 10 2 19 1 18 33 11 1 30 19 12 0 7 67 11 34 11 1 13 1 6 64 13 1 6 64 13 64 13 1 6 51 6 1 14 8 3 6 0 3 65 4 0 6 0 5 78 6 8 9 6 0 2 77 5 0 15 1 19 24 5 8 6 2 28
%
SF Fu 31 2 19 3 5 1 40 6 64 3 20 2 50 7 26 5 11 2 35 4 50 4 59 2 39 4 45 3 58 3 11 3 33 5 44 5 28 5 30 5 36 3 12 3 37 4 12 2 12 2 12 2 26 2 7 1 20 4 8 2 3 0 10 2 44 4 51 4
a NMR results are the average of duplicates. Estimated error: f4 K. Estimated error: f2%. Estimated error: f 1 0 ps. e The MW values are in units of kHz2, with an estimated error of *2 kHz2. fMaceral analysis is given as percent by volume on a mineral-matter-free basis: Vit, vitrinite; Ex, exinite; Mac, macrinite; ID, inertodetrinite; SF, semifusinite; Fu, fusinite.
mobility. It can also give a measure of the degree of this mobility by quantifying the average molecular mobility of the mobile fraction. For the purposes of this study, molecular mobility as determined by PMRTA is taken to be a measure of coal thermoplasticity and hence “reactivity” insofar as the pyrolysis behavior of coals and their constituent macerais is concerned. The relationships between the molecular mobility attained during pyrolysis and the maceral content, rank parameters, and chemical composition of a range of Australian bituminous coals are investigated. A particular effort is made to distinguish the contribution to coal “reactivity” of the major maceral groups.
Experimental Section The coals studied range in mean maximum vitrinite reflectance (R,) between 0.6 and 1.4% and their ultimate analysis, maceral analysis, and volatile matter (VM) analysis are included in Table I. For comparison, a semianthracite coal was also studied (sample 1, Table I). For petrographic analysis, coal samples were crushed to under 1mm size and mixed with synthetic resin and catalyst in synthetic rubber molds to form blocks with approximate dimensions 20 mm X 20 mm X 10 mm. The blocks were then ground on silicon carbide abrasive papers and polished on aluminium oxide powder and finally on magnesium oxide powder of 0.05-pm size. The maceral analyses of the blocks of coal were carried out in white light by using a Leitz Orthoplan microscope attached t o a Swift point counter. The immersion medium was oil of refractive index 1.518. The total magnification was approximately X500. A grid step of 1mm between traverses was used to cover the whole block. The maceral composition was determined by counting more
than 500 macerals in each block of coal. For the NMR measurements, coal samples were ground to pass a 0 . 2 5 ” mesh sieve and then acid washed to remove non-pyritic iron by stirring in 20% HC1 for 30 min a t 343 K and then washing with distilled watera4 The samples were dried overnight a t 378 K under nitrogen prior to pyrolysis. Samples (-200 mg) in open glass tubes were pyrolyzed a t a heating rate of 4 K/min to temperatures of -875 K in an NMR furnace probe.5 During pyrolysis the samples were kept under a flow of nitrogen to inhibit oxidation and remove volatile material from the sample tube. Solid echo measurements of the proton NMR transverse relaxation signal, I ( t ) , 3 were made on the specimens a t regular intervals during pyrolysis and on the residual semicokes during cooling and a t room temperature. The NMR signals stimulated by the 90,-~-90, pulse sequence with 7 = 6 fis were in excellent agreement with corresponding signals stimulated by a single pulse. The nature of these measurements for bituminous coals has been discussed by us e l ~ e w h e r e . ~ The ,~ intensity of the signals is empirically corrected to account for the temperature sensitivity of the NMR measurement.6
Results Stacked plots of NMR signals obtained in PMRTA pyrolysis experiments with a high-volatilebituminous coal ( 5 ) Webster, D. S.; Cross, L. F.; Lynch, L. J. Reo. Sci. Instrum. 1979, 50, 390-391.
(6) Lynch, L. J.; Webster, D. S.; Bacon, N. A.; Barton, W. A. In Magnetic Resonance: Introduction, Advanced Topics and Applications t o Fossil Energy; Eds. Petrakis, L., Fraissard, J. P., Eds.; NATO AS1 Series C124; D. Reidel: Dordrecht, The Netherlands, 1984;pp 617-628. (7) Barton, W. A.; Lynch, L. J.; Webster, D. S. Fuel 1984, 65, 1262-1266.
Pyrolysis of Australian Bituminous Coals
Energy & Fuels, Vol. I , No. 2, 1987 169 100
1
100
80
““1
-
2
III
0
0
TemperatureIKI
0
20
LO 60 Timeips1
80
1W
Figure 2. Plot of the apparent residual hydrogen content ( X ) and mobile hydrogen component (0) of sample 22 during pyrolysis at 4 K/min. The differential plot of the apparent hydrogen content is also shown (-). The definitions of T,, TmH,and % mm are also indicated.
0 1
300
500
,
I
700
,
- 0 930
Temperature IKI
Figure 3. Plot of the Mm (X) and P2(0) pyrograms of sample 22 during pyrolysis at 4 K/min. The definitions of Tz” and M0
20
LO 60 Time ips1
80
100
Figure 1. Stacked plots of signals obtained from NMR thermal analysis of (a) a high-volatile bituminous coal (sample 22) and
(b)a relatively unreactive inertinite maceral concentrate (sample
11). The signals have been interpolated to 10 K intervals. The thermoplastic event is clearly delineated in Figure la.
(sample 22, Table I) and a relatively unreactive inertinite concentrate (sample 11, Table I) are shown in parts a and b of Figure 1, respectively. A number of parameters useful for semiquantitative comparisons of aspects of the composition and molecular dynamics of the coal specimens during pyrolysis can be obtained from these signals. The initial intensity, I(O),of the transverse relaxation signal (which is taken a t the peak of the solid echo3) gives an estimate of the residual hydrogen content of the specimen6 and is depicted in Figure la. The recorded NMR signals, I ( t ) ,are each characterized by the empirical second moment, M ~ Tof, a truncated frequency absorption spectrum-as distinct from the power spectrum as has been the case in other related studies6-obtained by Fourier transformation of I@).This truncation is made a t a frequency of 40 kHz in preference to truncation where the amplitude of the absorption spectrum has fallen to a fixed percentage of its on-resonance peak ~ a l u ebecause , ~ ~ ~ in principle the M2T values so obtained can be linearly related to the other measured NMR parameters. Pyrograms depicting the temperature dependence of apparent residual hydrogen content and M2T values are included in Figures 2 and 3, respectively. By resolving the proton NMR signals I ( t ) into a slowly relaxing exponential or “mobile” component and rapidly relaxing Gaussian or ”rigid” components, the fraction of hydrogen that is mobile in the coal specimen a t any temperature can be estimated. This was done by using an
are also indicated.
exponential least-squares fit to the tail of the signal where the Gaussian (i.e. “rigid”)components have relaxed.6 Two useful parameters obtained from this analysis are (i) the fraction of the hydrogen content of the original coal that is so defined as mobile and (ii) the exponential time constant, P 2 ,which is a measure of the average molecular mobility of the mobile fraction. The temperature dependences of these parameters are plotted in Figures 2 and 3, respectively. (The variation in P 2between 300 and 450 K is due to sorbed water initially in this specimen.) Whereas the residual hydrogen content pyrogram defines the region of main pyrolytic decomposition and loss of volatiles, the MzT, mobile hydrogen, and T*2pyrograms, which are sensitive to molecular mobility, delineate the thermoplastic region. Each of these latter quantities passes through an extremum a t about 700 K, indicating that both the maximum extent and maximum degree of molecular mobility occur together for thermoplastic or “reactive” coals. A number of useful secondary parameters can be defined from these pyrograms. The mobile hydrogen and T*2 pyrograms yield (i) the maximum mobile hydrogen content attained or maximum extent of molecular mobility (% mm) (Figure 2), (ii) the temperature a t which this maximum occurs (Tmm)(Figure 2), and (iii) the time constant for the mobile signal component at this maximum extent (Figure 3). The M2T pyrogram (Figure of mobility (TZmm) 3) yields (iv) Mm, Mm, etc., the M m at 300 K, 400 K, etc., and (v) M-, the minimum Mm attained by the specimen. The temperature at which the rate of loss of hydrogen reaches a maximum (TmH)can be readily obtained from a differential form of the residual hydrogen pyrogram (Figure 2). Values for some of these parameters are listed in Table I.
Sakurovs et al.
170 Energy &Fuels, Vol. 1, No. 2, 1987 Table 11. Correlation Matrixa
I
a b c d e f g h i ] k l m n o p q r s (a) TnH
0 . 8 p . P P . 8 . 6 6 8 . P P . . .
(b)
. 0 . 5 6 ~ 8 9 . 9 . 6 . 5 ~ . 5 6 .
%mm
( c ) ,,T
8.oP.4P.7.547.4....
( d ) Tzmm
p 5 P o . 5 7 6 P 5 . 6 P . 5 . . . .
( e ) M300
. 6 . . 0 . p P . 5 p p . 7 . . 6 6 ~
M650
P p 4 5 . 0 5 4 P p p 4 6 . 8 . . . .
(f)
(9) M700
P l ? P 7 ~ 5 0 9 P 8 . 9 4 P 5 . P 4 .
(h) %in
. 9 . 6 P 4 9 o P 9 . 7 P P 4 . P 4 .
(i) C (Zdaf)
0 . 7 P . P P P o . 9 5 8 . 4 . . . .
(1) H (Xdaf)
. 9 . 5 5 ~ 8 9 . 0 . 7 . 5 p . 6 6 ~
(k) 0 (%daf)
6.5.pp..9.op6.p....
(1) VM (Xdaf)
6 6 4 6 ~ 4 9 7 5 7 ~ 0 5 P 4 p P 4 p
(m)
%lax ( % )
8 . 7 P . 6 4 P 8 . 6 5 0 . 7 . . . .
(n)
Vit(%)
. 5 . . 7 . P P . 5 . P . o . P 8 9 4
(0)
Ex ( % )
0
0
/ I
I
I
L
5
6
I
H (%daf I
Figure 4. Plot of maximum mobile hydrogen (% mm) vs. hydrogen content (% , dry ash-free basis) for the coals listed in Table I: ( X ) vitrinite >25%; (0) vitrinite
