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The effect of oxidation on the thermoplastic and coking properties of coal at elevated pressures
The effect of oxidation and coking properties elevated pressures
on the thermoplastic of coal at
K. Mark Thomas, Peter D. Green*
Michael
Andrew
P. Tytko*,
J. Mulligan*
and
Northern Carbon Research Laboratories, Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU UK *British Gas plc, London Research Station, Michael Road, London SW6 2AD, (Received 76 April 1991; revised 78 June 1991)
UK
on coal thermoplastic and coke properties have been investigated using a variety of techniques. The methods used include thermogravimetric analysis, high pressure dilatometry, constant shear rate plastometry and optical anisotropy. The apparent effects of oxidation are strongly influenced by the temperature and time of oxidation as well as the experimental conditions under which the coal carbonization behaviour is studied. Oxidation decreases swelling, the maximum rate of volatile evolution and optical anisotropy except under very mild conditions where usually little or no effect was observed. Oxidation can cause the shape of the dilatation versuspressure curves to change dramatically. This may result in the dilatation of an oxidized coal being higher at elevated pressure than at atmospheric pressure in contrast to the results for the fresh coal. However, under the same experimental conditions, the dilatation of the oxidized coal is either identical within experimental error or less than that of the original fresh coal. Comparison of coals with varying extents of oxidation has shown that an increase in the carbonization pressure tends to decrease the apparent effect of oxidation on the coal swelling and coke optical anisotropy index. There are good correlations between the changes in the swelling, maximum rate of devolatilization and coke optical anisotropy parameters, resulting from oxidation indicating the relationship between these concurrent processes during carbonization. The perturbation of the structure of a coal by oxidation in conjunction with the investigation of the dependence of caking, swelling, devolatilization and coke structure factors on experimental conditions provides a clear insight into the development of coal thermoplasticity and coke structure during carbonization and gasification. The effects of oxidation
(Keywords: coal; coke; oxidation)
As supplies of oil and natural will be a need for alternative
gas begin to decline
there
sources of these products and coal gasification will have an important part to play in providing options for the production of SNG and liquid products. In addition, coal gasification will have an important role in the generation of electricity by the combined cycle power process. In this case the increased efficiency and ability to meet stringent environmental regulations on high sulphur coals are important considerations. The long term future and need for coal gasification technology is therefore assured’. The relationship between coal properties and gasification behaviour is an important consideration in obtaining optimum gasifier performance2. Since most commercial processes operate at high pressure, the measurement of coal behaviour in the laboratory under simulated gasifier conditions is an important aspect of coal characterization for gasification processes. In general, little information is available in this area. A knowledge of the variation in coal properties in relation to pressure, heating rate, rank, etc. and the extent to which they can be modified3 by pretreatment and the inter-relationship between all these factors are essential to optimize a commercial process. Coals, with the exception of anthracites, and coal macerals other than inertinite are liable to change their 00162361/92/020169-13 ,c 1992 Butterworth-Heinemann
Ltd.
properties on oxidation in air. Very small quantities of oxygen, which cannot be detected by conventional analytical methods, can alter the caking and swelling properties significantly4. In extreme cases, freshly mined coal when exposed to air for a few days at ambient temperature will markedly decrease caking and swelling properties and calorific value. Extensive oxidation can be detected by spectroscopic and analytical methods. Details of the chemical changes responsible for the modifications in technological properties have been the subject of considerable discussion and are not known in detail, although some basic reaction processes, for example the loss of aliphatic groups and the formation of phenolic group in the coal macromolecules, are evident. Recent studies have shown’ that oxidation in air can lead to substantial modification of coal swelling in relation to pressure. Clearly, this possibility is an important consideration in the utilization of fine coal in a high pressure commercial gasifier where small particle sizes make the coal more susceptible to oxidation. This investigation was initiated to provide a general understanding of the relationship between coal thermoplastic and coke properties under elevated pressures and the effect of oxidation on these properties. The objective of the investigation was to carry out a systematic
FUEL, 1992, Vol 71, February
169
Thermoplastic
and coking properties
Table 1 Characterization data for the coals
Rank (NCB)
of coal: K. M. Thomas et al
used
NW
Ph
Wh
Ma
301a
402/502
502
702
Proximate analysis (wt% dry basis) 22.3 Volatile matter 5.4 Ash
32.1 16.0
36.5 3.7
38.0 3.7
Ultimate analysis (wt% dry basis) 85.2 Carbon 4.6 Hydrogen 0.7 Sulphur 0.02 Chlorine
70.3 4.9 2.3 0.05
81.3 5.0
79.4 5.2 1.6 0.23
Caking and swelling properties BS swelling number 5 G2 Gray-King coke Dilatometry 361 T, (“C) 433 T, (“C) 462 T3 (“C) 23 c (%) 43 d (%) Petrographic analysis Reflectance (max.) (%) Maceral analysis @) ’ Vitrinite Liptinite Inertinite
7 G9
6 G4
E
334 398 450 26 185
371 410 438 24 54
369 _ 429 35 _
1.24 54 3 43
0.42
0.83 79 6 15
0.85 14 12 14
1.5
0.65 80 5 15
investigation of the effect of oxidation on a wide range of coals at a range of temperatures from those found in stockpiles to likely pretreatment conditions, using high pressure dilatometry, plastometry, thermogravimetric analysis and optical anisotropy techniques to monitor the effect. EXPERIMENTAL Coals used The characterization data for the coals used in this study are given in Table 1. The coals had ranks 301a, 4021502, 502, 602 and 702 in the British Coal classification scheme. Preparation of samples
The coals were sampled in accordance with the guidelines6 outlined in BS1017. Coal with a particle size of -6 mm was spread out in a thin layer on a tray and oxidized for various times from 1 h to 200 days and at temperatures in the range of 50-200°C to examine oxidation effects over a wide range of conditions. Each sample of oxidized coal was then divided into representative samples for characterization studies. Thermogravimetric analysis (TGA)
The method described previously by Izuhara et al.’ was used. The thermal analysis measurements were carried out using a thermal analyser coupled to a computer for data collection. This allowed accurate time frame correlation between the weight and temperature measurements. The thermogravimetric data for the devolatilization of - 20 mg samples of coal with a particle size of < 212 pm were carried out at a heating rate of 10°C min-’ up to 750°C in a nitrogen atmosphere with a flow rate of 44 cm3 min- ‘. The weight loss, differential weight loss (dw/dt) and temperature results were recorded on a chart recorder as well as being stored on
170
FUEL,
1992,
Vol 71, February
floppy disc by the computer for further analysis. Graphs were produced from this data. High pressure dilatometry
The apparatus used has been described previously’. It operates at pressures up to 10 MPa and heating rates up to 60°C min-‘. In all other respects the dilatometer operating procedure and coal pencil manufacture are similar to BS1016 part 12 with the exception that the preliminary temperature stabilization was not used’. The repeatability of the dilatation and contraction values are estimated to be lo-15% and 5-lo%, respectively. The repeatability in the temperature measurements is lo-15°C. The carbonized residues from the dilatometer were collected for examination by polarized light microscopy. PlastometrJ
The plastometer used in this study has been described elsewhere”. The samples used in the study were ground to - 500 pm and 25 g was loaded into the plastometer for each run. The instrument was operated in air at atmospheric pressure at 10 rev min- ’ and with a heating rate of 3°C min- ‘. The torque developed as a function of temperature was recorded on a chart recorder and also digitized and stored in a computer for further data analysis. Optical anisotropy measurements
The carbonized residues produced in the high pressure dilatometer were crushed, mixed with epoxy resin and formed into discs. These discs were prepared as polished sections by grinding and subsequent polishing using various grades of silicon carbide paper followed by alumina powder. The polished blocks were examined by polarized light microscopy with a x 50/l .3 oil immersion objective with crossed polars and a plate to produce interference colours. The structural features were classified according to their appearance, size and shape. The following classifications were used: isotropic, mosaic (very fine (co.5 pm), fine (0.5-1.5 pm), medium (1.5-5 pm) and coarse (> 5 pm)), flow (granular and striated) and basic anthracitic anisotropy. A 300 point count was used for all the samples investigated. The error on the measurements is estimated to be better than 5%. The optical anisotropy index (OAI) was calculated according to the following equation: OAI= lvf+2f+3m+4c+5gf+6tl+7b where vf = very fine mosaic, f = fine mosaic, m = medium mosaic, c = coarse mosaic, gf = granular flow, fl = flow anisotropy and b = basic anisotropy. N.m.r. spectroscopy
The 13C n.m.r. spectra of the oxidized coals were measured using a spectrometer using cross-polarization and magic angle spinning modes. RESULTS
AND DISCUSSION
The role of thermal pretreatment and oxidation in relation to coal thermoplasticity and coke properties has been reviewed elsewhere4. Previously, the main reason for studying the effect of oxidation on coal properties was the use of preheated coal in the manufacture of blast
Thermoplastic
of coal: K. M. Thomas et al.
and coking properties
furnace coke. As a result, most of the studies have involved investigations appropriate to conditions pertaining in coke ovens. However, the developing interest in coal gasification has led to continuing interest in these effects. Most commercial gasification processes are operated at high pressure for economic considerations. Therefore, the effect of gas pressure on coal and coke properties is an important consideration in understanding gasifier performance. There have been a considerable number ofinvestigations into the effect of pressures9’ 1-24, heating rate8,“-‘5,23,24, gaseous atmosphere’2-‘4~22, additives’6-20 and particle size’* on coal thermoplastic properties. The studies of the coke properties are more limited being restricted to mainly strength and optical anisotropy measurements4~11~15~21~23~z4. However, very little research has been carried out on the effect of oxidation in relation to coal behavioural characteristics at high pressure. High pressure dilatometric studies on Pittsburgh coal have shown’ that oxidation could lead to the swelling of a coal at high pressure being higher than at atmospheric pressure. This result, at first sight, might seem surprising, but the complex behaviour of dilatation with respect to a range of variables, in particular, pressure and coal rank must be considered. The shape of the dilatation versus pressure curve changes with rank and a similar effect with oxidation could explain the observations. A detailed multifaceted approach was adopted to obtain an overall picture of the effect of oxidation. A series of bituminous coals were
selected to cover a wide range of rank and these coals were oxidized in air, the effect on coal thermoplastic and coke properties being monitored by a range of techniques.
Table 2
profiles
Effect of oxidation
in air on thermogravimetric
Thermogravimetric
analysis
The method reported7 previously by Izuhara et al. was used. The results of the effect of oxidation on the volatile evolution profiles of coals Ph, NW and Wh are given in Table 2. From a typical plot (not shown) of the effect of oxidation on the volatile evolution profile plotted as the rate of change of weight (dw/dt) against temperature it is found that there is a decrease in the peak parameter ((l/w,)dw/dt),,, (where wd = original dry weight) with increase in the duration of the oxidation in air. The effect of oxidation on the peak parameter is also found to vary considerably with rank, the lower rank coals being more reactive towards oxidation. The weight loss also starts at a lower temperature and continues to a larger extent at the higher temperatures effectively broadening the evolution profile although overall this is a small effect and only readily observed at high degrees of oxidation. The changes in the volatile matter estimated in these measurements are small and clear trends are not easily detected bearing in mind the experimental errors involved. In general the volatile matter decreases as the normalized maximum rate of volatile evolution decreases with increasing extent of oxidation. However, previous work has shown that coals can gain and lose weight
volatile evolution
Thermogravimetric Oxidation Coal
Time
Coal Nw
Fresh
parameters
conditions Temperature
( ‘C)
(dn!dt),,, (mg min-’
Volatile rng-‘)
Water
(%)
matter
(O/o I
19.1
0.5
21.2
I7 days
50
16.6
0.3
20.5
23 days
50
16.5
0.3
20.5
29 days
50
16.5
0.3
20.2
36 days
50
15.7
0.8
20.3
50 days
50
15.X
0.6
20.3
57 days
50
16.2
0.4
20.4
64 days
50
15.9
0.5
20.6
85 days
50
15.9
0.2
20.4
IO0 days
50
15.7
II2 days
50
15.3
0.2
20.5
126 days
50
15.6
0.4
21.0
I40 days
50
16.4
0.7
?I .o
163 days
50
15.6
I.1
21.0
I82 days
50
15.1
I .5
20.X
202 days
50
15.4
0.4
21.4
211 days
50
15.0
I .2
20.6
20.3
5h
II0
18.1
0.2
20.X
I6 h
I IO
17.2
0.2
20.9
32 h
II0
16.4
0.0
20.6
64 h
I IO
13.7
0.4
19.3
96 h
I IO
12.7
0.2
19.4
I h
150
17.5
0.2
21.1
5h
150
15.1
0.2
70.6
16 h
150
14.5
19.5
FUEL,
1992,
Vol 71, February
171
Thermoplastic Table 2
and coking properties
of coal: K. M. Thomas et al.
continued Thermogravimetric Oxidation
parameters
conditions Volatile
@w/dt)ma, Coal
Time
Coal Ph
Fresh
Temperature
(“C)
(mg min-’
mg-‘)
Water
(X)
matter
(%)
29.8
I.1
30.9
12 days
50
31.1
0.8
31.7
17 days
50
30.9
0.6
31.2
23 days
50
28.9
0.6
29.7
29 days
50
28.8
0.7
30.6
36 days
50
28.7
0.0
29.4
50 days
50
28.8
0.8
30.3
57 days
50
28.6
0.6
30.
64 days
50
28.4
0.0
30.3
I
78 days
50
28.2
2.5
29.4
85 days
50
28.2
2.2
29.2
100 days
50
28.5
2.0
30. I
I12 days
50
28.7
0.7
31.0
126 days
50
28.0
0.8
29.8
140 days
50
27.1
0.8
30.8
163 days
50
28.4
0.0
31.5
I82 days
50
27.8
0.0
31.0
202 days
50
27.
I
0.8
30.5
21 I days
50
25.2
I .7
30. I
I h
110
30.2
0.0
30.7
5h
110
29.5
0.5
30. I
I6 h
II0
28.2
0.4
31.1
30 h
II0
26.2
0.5
30.2
48 h
II0
24.5
0.6
30.5
Ih
150
30.9
0.6
30. I
3h
150
23.3
0.6
28.6
6.5 h
150
21.1
0.8
30.3
Fresh
33.6
2.9
32.2
Ih
34.5
0.8
32. I
33.0
0.8
32.0
29.1
I .2
30.8
Coal Wh II0
5h
110
I6 h
during oxidation considered.
and
this is a factor
II0
which
must
be
Dilatometry The effect of pressure on dilatometry parameters has been the subject of several investigations. The results l-l * show that increasing pressure obtained previously 13%~ has the following effects on dilatometry parameters: the softening point decreases leading to an increased plastic range; the dilatation against pressure graph may increase, decrease or have a peak at - l-l.5 MPa, depending on the particular coal and this cannot be predicted from standard characterization data and atmospheric dilatometry measurements; the range of swelling for a series of coals is much less at high pressures (>4 MPa) than at atmospheric pressure.
The results of the effect of oxidation on dilatometry parameters measured at a heating rate of 40°C min-’
172
FUEL,
1992,
Vol 71, February
over a wide range of pressure (O-6 MPa) for NW, Wh, Ph and Ma coals are given in Table 3. The data show that oxidation usually has the following effects: 1. decreases swelling; 2. increases the softening point, resulting in a marked decrease in the plastic range; 3. increases the temperature of maximum contraction slightly. The results show that the effect of oxidation on swelling varies significantly with the rank of the coal, the decrease being more marked for the lower rank coals which are more reactive. The effect of oxidation on swelling also decreases with increasing temperature and time of oxidation. The effect of pressure on dilatation is complex and depends on the rank of coal. At pressures of 4 MPa and above, the swelling of a suite of coals has a much smaller range than at atmospheric pressure’,“. A comparison of the effect of oxidation on carbonization at atmospheric and 4 MPa shows that under a variety ofexperimental conditions, the change in swelling relative
Thermoplastic Table 3
and coking properties
of coal: K. M. Thomas et al.
continued
Heating rate (“C min-i)/pressureOxidation (MPa x 10)
7-cTI temperature
mastic range Oxidation
time
Ti (“C)
Tz (“C)
Ts (“(3
iv
c (X)
d (X)
Coal NW 40/40
Fresh
422
508
550
128
18
102
40140
50
29 days
421
511
550
139
21
98
40140
50
36 days
410
510
550
140
20
88
40140
50
43 days
425
498
538
113
20
90
40/40
50
50 days
410
502
538
128
19
85
40140
50
64 days
425
508
545
120
15
86
40140
50
78 days
431
520
558
127
16
74
40140
50
100 days
390
512
540
150
17
70
40140
50
163 days
430
512
552
122
15
63
40/40
50
182 days
430
524
562
132
14
52
40140
50
211 days
450
520
550
100
13
40
Fresh
232
40/o
460
505
550
90
7
40/o
50
29 days
453
495
541
88
5
228
40/O
50
36 days
430
493
532
102
11
223
40/o
50
43 days
430
505
543
113
9
196
40/o
50
50 days
440
500
539
99
9
190
40/o
50
64 days
450
506
548
98
6
190
40/o
50
78 days
442
498
530
88
8
152
40/o
50
100 days
441
508
546
105
6
136
40/o
50
163 days
455
518
550
95
8
85
40/o
50
182 days
465
521
550
85
6
73
40/o
50
211 days
455
517
550
85
10
30
40/40
110
lh
421
509
546
125
16
77
40140
110
5h
420
507
552
132
18
79
40140
110
16 h
425
521
559
134
16
74
40/40
110
32 h
415
505
550
135
18
70
40/40
110
64h
438
506
540
102
16
46
40140
110
96 h
420
525
545
125
19
14 176
40/o
110
16 h
450
503
545
95
6
40/o
110
64 h
455
520
550
95
6
40/o
110
96 h
445
550
550
105
13
40140
150
lh
425
531
560
135
7
40/40
150
5h
425
528
553
128
7
40/40
150
16 h
439
565
565
126
10
40140
175
lh
420
525
560
140
16
40140
175
5h
435
565
565
130
11
-11
40140
175
16 h
435
565
565
130
3
-3
40140
200
lh
445
565
565
120
17
-17
40140
200
16 h
436
556
556
130
4
-4
40140
175 (vacuum)
5h
420
508
545
125
18
84
40140
175 (vacuum)
10 h
430
525
554
124
19
86
50 -13 75 37 -10 37
40140
175 (vacuum)
16 h
445
524
569
124
15
79
40140
175 (vacuum)
22 h
427
516
552
125
10
56
Coal Ph 392
482
533
141
8
148
40/40
50
17 days
365
467
537
172
14
168
40140
50
29 days
385
480
536
151
19
153
40/40
50
36 days
380
478
540
160
19
143
40140
50
50 days
380
480
530
159
20
146
40140
50
43 days
376
460
526
150
10
133
40140
50
64 days
385
485
542
157
17
162
Fresh
40/40
FUEL,
1992,
Vol 71, February
173
Thermoplastic Table 3
and
coking properties of coal: K. Ad Thomas
et al.
continued
Heating rate (“C min _ 1)jpressureOxidatlon (MPa x lO)VC)
Tj-T, temperature Oxidation
time
T, (“C)
T, (“Cl
r, (“Cl
plastic range (“C)
c (%)
d (%)
40;40
50
78 days
380
485
546
156
15
160
40;40
50
85 days
375
480
540
165
21
144
40;40
50
100 days
405
480
528
123
16
I50
40/‘40
50
163 days
420
485
540
120
15
158
40;40
50
182 days
405
492
540
135
14
134
4oj40
50
211 days
410
485
540
130
16
100
Fresh
40;40
420
470
540
120
4
396
40/‘40
50
17 days
425
465
514
89
7
437
40/40
50
29 days
418
470
528
110
6
350
40/40
50
36 days
430
475
537
107
5
365
40/40
50
43 days
426
463
512
86
8
336
40/40
50
64 days
458
480
536
77
2
354
40140
50
85 days
430
470
so5
75
5
305
40140
50
100 days
430
485
540
1iO
5
348
40140
50
163 days
442
490
533
91
5
293
40/40
50
182 days
422
478
so5
83
8
225
40/40
50
211 days
420
487
520
100
4
120
Fresh
355
471
535
180
24
98
40:60
110
lh
340
455
535
195
25
93
40/60
110
Sh
345
475
535
190
25
89
40160
110
16 h
335
476
535
200
23
113
40/60
110
30 h
340
495
540
200
15
108
40/60
110
48 h
369
498
548
179
15
85
Fresh
40/60
40/25
410
475
542
132
8
175
40/25
110
lh
480
485
532
152
14
169
40/25
110
Sh
410
485
544
134
7
176
40/25
110
16 h
390
480
536
146
18
168
40/25
LlO
30 h
420
490
537
117
7
150
40125
110
48 h
427
489
540
113
9
123
Fresh
420
470
540
120
4
396
Ih
419
470
543
124
6
391
110
5h
420
464
526
106
6
375
11a
16h
435
484
540
105
5
309
4W
110
30 h
422
484
525
103
10
162
WO
110
4S h
460
506
555
95
2
25
40/o 40/o
110
40/o 40/o
40/60
Fresh
355
471
535
180
24
98
40/60
150
3h
405
512
550
135
19
95
40/60
150
6.5 h
455
505
540
85
14
19 175
40/25
Fresh
410
475
542
132
8
40/25
150
lh
379
460
518
139
20
146
40125
150
3h
440
495
545
105
13
140
40/25
150
6.5 h
450
505
540
90
I?
35
40/o
Fresh
420
470
540
120
4
396
40/O
150
Ih
439
475
521
82
6
400
40/o
150
3h
450
79
10
150
6.5 h
491
493 _
529
40/O
550
59
15
90 -15
Coal Wh 40140
Fresh
382
474
523
141
20
136
40140
110
lh
395
485
540
I45
19
to3
40140
110
5h
400
488
545
145
5
122
40/40
110
16 h
390
500
540
150
16
76
174
FUEL, 1992,
Vol 71, February
Thermoplastic Table 3
and coking
properties
of coal: K. M. Thomas
al.
et
continued
Heating rate (“C min- ‘)/pressureOxidation (MPa x 10) (“C)
T,-T, temperature
40/o
plastic range Oxidation
time
d (%)
T, (“C)
T, (“C)
Ta (“C)
Fresh
435
475
532
97
5
341
lh
430
476
517
81
5
300
(“C)
--
c (%)
40/o
110
40/o
110
5h
445
498
540
95
17
148
40/o
110
16 h
445
509
530
85
I
0
Coal Ma 40140
Fresh
373
466
519
146
18
78
346
485
528
182
13
76
lh
386
490
540
154
I
72
lh
400
490
525
125
16
56
110
5h
435
520
545
110
3
110
16 h
435
550
550
115
9
40/o
110
lh
415
504
545
130
6
13
4015
110
lh
425
485
517
92
6
48
40115
110
lh
410
494
540
130
4
75
40120
110
lh
370
484
524
154
12
72
40140
150
lh
410
550
550
140
7
50
lh
40/40
80
40140
110
40140 40/40
40140
Coal Ma (heat treatment
26 -9
-7
in vacuum)
40140
175
5h
340
490
545
205
8
81
40140
175
12 h
340
505
525
185
10
67
40/40
175
20 h
365
502
540
175
12
56
to the original dilatation is much less at higher pressure. Figure I illustrates this change in dilatation resulting from oxidation as a function of pressure for coals NW, Ph and Wh. It is clear from the results that at high pressure the swelling of extensively oxidized coals is greater than the corresponding low pressure measurement for the coals studied. This can be rationalized by considering the change in shape of dilatation versus pressure curves in relation to rank. The dilatation of low rank coals usually increases with increasing pressure whereas that of higher rank and high swelling coals usually decreases with pressure. Also there are coals with British Coal rank 702, where there is a peak in the dilatation versus pressure curve at 1-2 MPa. The oxidation behaviour of the coals can be rationalized by considering oxidation as similar to lowering the rank of the coal and changing the shape of the dilatation versus pressure curve to being similar to that of a low rank coal. The results in Table 3 also show the effect of oxidation temperature and time on the dilatometric properties of a range of coals over a range of pressures. It is apparent that oxidation temperature has a more marked effect on dilatometric properties than oxidation time. A comparison of the dilatation observed under experimental conditions of heating rate 40°C min-’ and pressure 4 MPa for samples of coal Ma oxidized for 1 h at temperatures of 50, 80, 110 and 150°C illustrates the effect of oxidation temperature on coal thermoplastic properties. Plustomctry
The results of the effect of oxidation on the Brabender (constant shear rate) plastometry parameters measured at atmospheric pressure and a heating rate of 3°C min- ’
for NW, Ph and Wh coals are shown in Table 4. The different trends in coking intensity for coals Ph and Wh which increase with oxidation and NW which decreases, both reflect a change in plastic behaviour similar to that of a lower rank coal. However, during oxidation, coal NW does not go through a highly fluid phase similar to coal Ph. This can be rationalized by the lack of hydrogen-containing species available to contribute to generation of fluidity. The results illustrate the importance of rank in determining coal thermoplastic and coke properties. Plastometry measures the agglomerating characteristics of coal particles and changes in interparticle interactions. Oxidation of coal will occur initially at the surface of the particles. Hence plastometry is expected to be a technique which will be very sensitive to small amounts of oxidation. This is apparent from the comparison of the plastometry results with the corresponding dilatometry and TGA results obtained at atmospheric pressures. In particular, results obtained at low degrees of oxidation indicated that all three techniques are capable of detecting the effects of oxidation at broadly similar levels. Optical
anisotropy
The results of the optical anisotropy measurements on the series of oxidized Ph coals carbonized in the high pressure dilatometer are given in Table 5. Other results of optical anisotropy measurements on coals NW and Ma, subjected to varying degrees of oxidation, have been published separately15q2’ and these show similar trends. The fact that the carbonized residues from the dilatometry measurements have been examined means that a direct comparison of the results is appropriate. The main effect is that oxidation decreases the optical
FUEL,
1992,
Vol 71, February
175
Thermoplastic
and coking properties
of coal: K. M. Thomas et al
a
5
10
15
Oxidation time (hours)
40
80
Oxidation time (hours)
lb
Figure 1 Effect of oxidation on dilatation for gauge and 4 MPa for: a, coal NW; b, coal Ph; c, coal Wh
Oxidation time (hours)
anisotropic content of the cokes carbonized identical conditions. However, high pressure tends to increase the optical anisotropic content coals15-21 and this effect can also be seen in the oxidized coals in Figure 2. The effect is not as
176
FUEL,
1992,
Vol 71, February
under usually in fresh series of clear in
pressures
of 0
the oxidized coals as in the fresh coal because although the mosaic size increases it is still within the limits set for fine grain mosaic texture. However, it was evident that increased carbonization pressure was producing an increase in the mosaic sizes. The apparent effect of
Thermoplastic Table 4
Effect of oxidation
on plastometry
and coking
properties
of coal:
K. M. Thomas
et al.
parameters
T,-7-1 Oxidation time
Plastometry
temperature (“C)
temperatures
(“C) ~
Tl
T;
390
391
T';
T3
T4
range (“C)
401
409
438
48
405
409
423
21
-
1;
Caking I;
Max. coking 1,
1,
60
50
95
45
24
70
Coal NW Fresh 120 days
50
396
50
5h
110
388
392
406
420
435
41
50
66
51
84
16 h
110
391
400
415
420
438
41
72
62
88
32 h
110
405
_
412
420
438
33
55 _
46
30
15
64h
110
425
_
432
435
448
23
_
43
26
32
96 h
110
No thermoplastic
lh
150
398
420
430
442
44
61
93
415
421
13
52 _
64
418
41
25
87
405
3h
150
414
_
6h
150
No thermoplastic
properties
properties
Coal Ph Fresh 12 days
50
350
351
365
398
440
90
52
50
10
16
358
360
371
417
439
81
50
48
12
24
17 days
50
363
368
376
418
439
16
49
48
15
32
93 days
50
368
373
378
416
450
82
44
58
8
20
120 days
50
378
383
387
390
421
43
35
40
32
62
lh
110
355
360
313
400
438
83
44
49
8
21
5h
110
360
367
376
410
435
15
46
53
12
27
16 h
110
370
314
386
391
411
47
53
39
68
30 h
110
389
_
395
398
423
34
46 _
43
21
73
96 h
110
No thermoplastic
properties
lh
150
372
373
380
418
424
52
23
40
150
418
_
422
424
439
21
44 _
49
3h
42
41
80
6.5 h
150
No thermoplastic
395
398
400
418
441
46
32
50
24
46
lh
110
407
412
414
421
435
28
34
38
32
60
5h
110
414
416
420
427
436
22
36
46
44
72
16 h
110
No thermoplastic
properties
properties
Coal Wh Fresh
oxidation on coal thermoplastic behaviour is dependent on the conditions under which the coal is carbonized and tends to be less at high pressure. A similar trend is observed for optical anisotropy, the change in optical anisotropy index tends to decrease as carbonization pressure increases. This is illustrated in Figure 2 which compares the change in optical anisotropy of cokes derived from oxidized Ph coal carbonized at a heating rate of 40°C min - ’ and gauge pressures of 0, 2.5 and 6 MPa. Similar results have been reported15s21 for 301a and 702 rank coals. The effect of increasing pressure on coke optical anisotropy is usually to increase optical anisotropy although there is usually a limiting value at pressures >4 MPa. This can be explained2r by the following factors: 1. retention of low molecular weight volatiles during the carbonization phase; 2. a decrease in viscosity (increase in maximum fluidity)22; 3. increased plastic range due to a decrease in the softening point.
Oxidation tends to decrease the plastic range and this is evident in both the high pressure dilatometry and the plastometry results. The plastometry results which are measured at atmospheric pressure show that the torque at maximum fluidity increases on oxidation for coals Ph and Wh but decreases for coal NW. The coking intensity behaves in a similar manner for both types of coal. The effect of oxidation as seen from the changes in plastic properties is best described as similar to a decrease in rank. Clearly, oxidation has a complex effect on coal thermoplastic and coke properties which is dependent on rank. studies The oxidation of coals NW and Ma has also been monitored by i.r. and n.m.r. spectroscopy. The FT-i.r. spectra show 25 that the oxidation of both coals oxidized at 110 and 150°C for various times produces bands in the region 1600-1800 cm-’ which can be assigned to a C=O stretching mode and an increase in intensity in the O-H stretching region. The ratio of the bands due to C-H(A1) relative to C-H(Ar) decreases indicating that Spectroscopic
FUEL, 1992, Vol 71, February
177
Thermoplastic Table 5
Optical
Oxidation
and coking properties anisotropy
measurements
16hat
on chars prepared
conditions
110°C
110°C
30 h at 110°C
48 h at 110°C
3 hat
150°C
OAI
Isotropic
coal Ph
texture composition Very fine
(vol%) Fine
191
3.5
2
94.5
3/25
190
3
4
93
3160
195
1
3
96
3
93
4010
189
40110
196
4 2
40125
194
2
2
96
40/60
192
2
4
94
40/o
196
4
96
40/25
197
1
1
98
40/60
189
4
3
93
40/o
179
2
17
81
40/60
195
1
3
96
40/o
104
16
63
21
40125
194
6
94
40160
197
3
97
4010
85
44
27
29
155
4
37
59 90
_
_
98
40/60
190
10
40/o
190
10
90
40125
199
_
1
99
77
45
33
22
40125
180
_
20
80
40/60
192
_
8
92
40/o
30
72
26
2
40125
78
39
44
17
40160
114
18
50
32
150.
the aliphatic content of the coal decreases on oxidation. This is confirmed by the measurement of the 13C n.m.r. spectra of fresh and oxidized coals NW and Ma which indicate that oxidation causes an increase in the aromatic carbon relative to the aliphatic carbon. In the case of coal NW oxidation in air at 200°C causes f, to increase from 0.82 for the fresh coal to 0.84, 0.88 and 0.9 after oxidation for 1, 5 and 16 h, respectively. Oxidation of coal Ma in air for 16 h causes f, to increase from 0.68 to 0.69 and 0.84 at 110 and 2OO”C,respectively.
2.5MPa
Comparison of techniques
::
O 100 1
01 0
20
40
6
Oxidation time (hours)
Figure 2 Variation of OAI with oxidation and 6 MPa for coal Ph
178
from fresh and oxidized
3/O
40/o
6.5 h at 150°C
dilatometer
Optical
40125
1 h at 150°C
et al.
in the high pressure
Dilatometer conditions (heating rate (“C min- ‘)/pressure (MPa x 10))
Fresh
5 hat
of coal: K. M. Thomas
FUEL,
1992,
at gauge pressures
Vol 71, February
of 0,2.5
This investigation has used a number of techniques to investigate the effect of oxidation on a range of coals varying in rank. The techniques used range from the monitoring of thermoplastic properties, such as dilatation and plastometry, to thermogravimetric and structural characterization methods. The empirical technological properties are usually the most sensitive to oxidation effects. Bearing in mind that some of the measurements are only carried out at atmospheric pressure, it is apparent from the results that Brabender plastometry dilatometry and TGA parameters have broadly similar sensitivities to oxidation (Tables 2-4) usually being able to detect changes in the plastic properties of coal before any substantial changes in chemical analysis and spectroscopic properties can be detected although this is not invariable. This observation
Thermoplastic
COAL m
400 t
I
II
110%
I
150%
A
I
0
I
0
0
300 t
2 .z 200 5 s z
100 -
o-
-301 0
10
20
30
of coal: K. M. Thomas et al.
and coking properties
ship between coking intensity and experimental conditions such as heating rate and pressure is complex due to effects on swelling and fluidityz4 and both of these parameters affect the coking intensity. Both coals show a decrease in plastic range with increasing oxidation. Figure 3 shows a graph of dilatation (40°C min- ‘, atmospheric pressure) against the (dwldt),,, obtained from the thermogravimetric measurements at 20°C min- ’ and atmospheric pressure. It is clear that there is a good correlation between these dilatometry and thermogravimetric parameters for the three coals oxidized over a range of temperatures (SO-15OC) for various times. There is no correlation between the thermogravimetric parameter measured at atmospheric pressure and the dilatation at high pressure. This is not surprising since pressure will have a considerable effect on the rate of release of volatiles. The apparatus for conducting the corresponding thermogravimetric measurements at high pressure was not available. Figure 4 shows a graph of dilatation versus OAI for coal Ph over a range of oxidation temperatures, times and carbonization pressures. It is clear that for these pressures, 0, 2.5 and 6 MPa, there is a good correlation between the changes in dilatation and OAI with oxidation. Figure 5 shows a graph of OAI for cokes carbonized at 40°C min- ’ and atmospheric pressure and the TGA maximum volatile release parameter (20°C min - ‘, atmospheric pressure). It is apparent that there is a good correlation between these parameters for a range of oxidation times and temperatures (50-l 50°C). The correlations between the results clearly suggest that there
TGA
Figure 3 Variation of dilatation (40°C min-‘, atmospheric pressure) with normalized maximum rate of weight loss from thermogravimetric measurements
can be explained by the nature of the measurements. Plastometry, being mostly concerned with the interaction between particles, is primarily affected by the surface of the particle whereas swelling is mainly an intraparticle effect. On this basis, the initial caking peaks would be expected to show marked changes on oxidation. This is observed in the increase in softening temperature on oxidation although the changes in caking intensity with increasing extent of oxidation are small. High pressure constant shear rate plastometry results have shown that the caking intensity is not sensitive to changes in heating rate and pressure 23-24. The increase in softening point is also observed in the dilatometry studies. It is clear from the plastometry studies that oxidation can cause different trends in the apparent maximum fluidity and coking intensities with increasing extent of oxidation. This can be rationalized by comparison with the plastometry curves obtained” for coals of various ranks. In coal Ph which exhibits very high swelling and Gieseler fluidity under standard conditions (3°C min- ‘, atmospheric pressure) carbonization results in porosity with large pores which give the coke a weak strength. Oxidation causes a decrease in the swelling and the size of the pores leading to an increased coking intensity in the plastometry curves. However, coal NW shows a decrease in coking intensity and dilatation with increasing oxidation. It is noteworthy that the high pressure dilatometry experiments were carried out at a much higher heating rate than the plastometry. The relation-
PRESSURE
400
300
s c 0 ‘i 200 % .z n
100
0
I
I
100
150
I
-30 0
50
L
200
OAI
Figure 4 Variation of OAI with dilatation and extents of oxidation for coal Ph
FUEL,
1992,
over a range of pressures
Vol 71, February
179
Thermoplastic
and coking properties
I o20
of coal: K. M, Thomas et al.
an increase in rank. Therefore the effect of oxidation on coal properties follows the trends expected for both increased oxygen content and heat treatment. The differences in thermoplastic behaviour and coke structure observed after oxidation are based on chemical reactions which alter the chemical structure possibly accompanied by physical changes in structure, for example, porosity. It is apparent that small chemical changes can produce large changes in thermoplastic and pyrolysis properties and the implications for high pressure gasification are significant. Further work is required to assess the changes in chemical and physical structure of coal, in particular, porosity.
. 25
30
35
dwldt Figure 5 Variation of OAI with maximum coal Ph: 0, 110°C; +, fresh; n , 150°C
rate of volatile release
for
are relationships between the release of volatiles, dilatation and the development of optical anisotropy and that these concomitant effects are being modified by structural changes induced by the oxidation process. The change in coal structure, for example, coal functionality and porosity, are factors which may affect the release of during volatiles, and subsequent coal behaviour carbonization. The reduction in the maximum rate of volatile release caused by oxidation will cause a reduction in the swelling because the liquid and gaseous products are able to escape more easily from the particles. The changes in volatile release profile with respect to temperature and also the decrease in plastic range limit the amount of fluid material present during the thermoplastic phase and hence the development of anisotropic structures is restricted by oxidation leading to reduction in OAI. Increasing the carbonization pressure modifies the release of volatiles from the coal and increases the plastic range and fluidity usually leading to an increase in OAI. Therefore, the apparent effect of oxidation on coke structure (OAI) is smaller at high pressures. The results suggest that oxidation modifies the coal structure by the introduction of the oxygen functionality and decreasing both the carbon and hydrogen aliphatic to aromatic ratios such that the release of volatiles in relation to temperature changes substantially. It is also possible that the coal macromolecular and porous structures are modified significantly during oxidation. The modifications in the rate of volatile release appear to account for the changes in swelling and the thermoplastic and optical anisotropic properties. The effects of oxidation on coal thermoplastic and coke anisotropy properties are similar but not identical to a decrease in rank. However, low rank coals are usually more reactive coals and have higher values of the TGA maximum rate of volatile release parameter whereas oxidation causes this parameter to decrease. Substantial changes in spectroscopic parameters are only observed in the samples which have been subjected to extensive oxidation. The results obtainedz5 using the oxidized coals used in this study clearly indicate that oxidation increases the oxygen functionality and decreases the amount of aliphatics present. Extensive oxidation of coal also causes the vitrinite reflectance to increase whereas weathering causes the reflectance to decrease26~27. The changes in aromatic to aliphatic carbon and hydrogen ratios and vitrinite reflectance caused by oxidation are similar to
180
FUEL, 1992,
Vol 71, February
CONCLUSIONS The application of a variety of characterization methods to monitor the oxidation of a wide range of coals which vary in rank, has illustrated the benefits of using a multi-technique approach. The data show good correlations between the maximum of release of volatiles during pyrolysis, the development of optical anisotropy and dilatometric properties, and that these changes induced by oxidation in air are concurrent effects which are related closely. The perturbation of the coal structure by oxidation is a useful method for studying changes in coal structure and its effect on carbonization since it allows the structure to also be changed without the introduction of other variables associated with changing coals which are difficult to quantify. The results show that a series of techniques are needed to follow the progress of coal oxidation since they are sensitive to different levels of oxidation and monitor different properties. The broad insight into the effect of oxidation on the carbonization process obtained by using a range of techniques has illustrated the dependence on the apparent effect of oxidation on rank, oxidation time and temperature and experimental conditions, in particular, pressure. Whether or not the changes in coal properties caused by oxidation are beneficial will depend on the nature of the particular process and the relative importance of the properties in determining gasifier behaviour. ACKNOWLEDGEMENTS The authors would like to thank British Gas plc for permission to publish this paper and Mr J. Williamson for the 13C n.m.r. spectra. REFERENCES Evans, R., Thompson, B. H., Hiller, H. er al. ‘Proc. 5th International Conference and Exhibition on Coal Utilisation and Trade (COAL TECH ‘85)‘, Vol. 3, 1985, p. 659 Thomas, K. M., White, A. and Williams, A. Paper presented at the ‘Gasification-Status and Prospects’ Conference, Institute of Fuel, Harrogate, May 1988 Thomas, K. M. in ‘Carbon and Coal Gasification-Science and Technology’ (Eds J. L. Figuiredo and J. A. Moulijn), NATO ASI, Series E105, Martinus Nijhoff, Dordrecht, 1986, p. 421 Habermehl, D., Orywal, F. and Beyer, H. D. in ‘Chemistry of Coal Utilisation’ (Ed. M. A. Elliott), 2nd Suppl. Volume, Wiley-Interscience, New York, 1981, p. 317 Kahn, M. R. and Jenkins, R. G. Fuel 1985, 64, 189 British Standards Institution. ‘BS1017: Methods for Sampling Coal and Coke, Part 1, The Sampling of Coal’, 1977 Izuhara, H., Tanibata, R. and Nishida, S. ‘Proc. Int. Conf. Coal Sci.‘, Sydney, Australia, 1985, p. 491 Green, P. D. and Thomas, K. M. Fuel 1985,64, 1423
Thermoplastic 9
10 11 12 13 14 15
16 17 18
British Standards Institution. ‘BSl016: Methods for the Analysis and Testing of Coal and Coke, Part 12, The Caking and Swelling Properties of Coal’, 1980 Mulligan, M. J. and Thomas, K. M. Fuel 1987,66, 1289 Green, P. D., Patrick, J. W., Thomas, K. M. and Walton, A. Fuel 1985,64, 1431 Beyer, H. D. ‘Caking and Coking Power of Bituminous Coals Under High Pressure’, Report BMFT-FB-T 82-55 Kahn, M. R. and Jenkins, R. G. Fuel 1984,63, 108 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 725 Green. P. D.. Patrick. J. W.. Thomas. K. M. and Walton. A. ‘Proc. of the 1986 International Gas Research Conference’, 1987, p. 1053 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 1203 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 1291 Bexley, K., Green, P. D. and Thomas, K. M. Fuel 1986,65,47
19 20 21 22 23 24 25 26 27
and coking properties
of coal: K. M. Thomas et al.
Green, P. D., Edwards, I. A. S., Marsh, H., Thomas, K. M. and Watson, R. F. Fuel 1988, 67, 389 Tromp, P. J. J., Karstein, P. J. A., Jenkins, R. G. and Moulijn, J. A. Fuel 1986,65, 1540 Patrick, J. W., Thomas, K. M., Walker, A. and Green, P. D. Fuel 1989,68, 149 Kaiho, M. and Toda, Y. Fuel 1979,58, 397 Chan, M. L. and Thomas, K. M. Am. Chem. Sot. Div. Fuel Sci. Prepr. 1989, 34 (3), 915 Chan, M. L, Parkyns, N. D. and Thomas, K. M. Fuel 1991, 70,447 Thomas, K. M. and Bradshaw, D. I. unpublished results Bend, S. L. PhD Thesis University ofNewcastle upon Tyne, 1989 Bend, S. L., Edwards, I. A. S. and Marsh, H. Am. Chem. Sot. Div. Fuel Sci. Prepr. 1989, 34 (3), 923
FUEL, 1992,
Vol 71, February
181
on the thermoplastic of coal at
K. Mark Thomas, Peter D. Green*
Michael
Andrew
P. Tytko*,
J. Mulligan*
and
Northern Carbon Research Laboratories, Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU UK *British Gas plc, London Research Station, Michael Road, London SW6 2AD, (Received 76 April 1991; revised 78 June 1991)
UK
on coal thermoplastic and coke properties have been investigated using a variety of techniques. The methods used include thermogravimetric analysis, high pressure dilatometry, constant shear rate plastometry and optical anisotropy. The apparent effects of oxidation are strongly influenced by the temperature and time of oxidation as well as the experimental conditions under which the coal carbonization behaviour is studied. Oxidation decreases swelling, the maximum rate of volatile evolution and optical anisotropy except under very mild conditions where usually little or no effect was observed. Oxidation can cause the shape of the dilatation versuspressure curves to change dramatically. This may result in the dilatation of an oxidized coal being higher at elevated pressure than at atmospheric pressure in contrast to the results for the fresh coal. However, under the same experimental conditions, the dilatation of the oxidized coal is either identical within experimental error or less than that of the original fresh coal. Comparison of coals with varying extents of oxidation has shown that an increase in the carbonization pressure tends to decrease the apparent effect of oxidation on the coal swelling and coke optical anisotropy index. There are good correlations between the changes in the swelling, maximum rate of devolatilization and coke optical anisotropy parameters, resulting from oxidation indicating the relationship between these concurrent processes during carbonization. The perturbation of the structure of a coal by oxidation in conjunction with the investigation of the dependence of caking, swelling, devolatilization and coke structure factors on experimental conditions provides a clear insight into the development of coal thermoplasticity and coke structure during carbonization and gasification. The effects of oxidation
(Keywords: coal; coke; oxidation)
As supplies of oil and natural will be a need for alternative
gas begin to decline
there
sources of these products and coal gasification will have an important part to play in providing options for the production of SNG and liquid products. In addition, coal gasification will have an important role in the generation of electricity by the combined cycle power process. In this case the increased efficiency and ability to meet stringent environmental regulations on high sulphur coals are important considerations. The long term future and need for coal gasification technology is therefore assured’. The relationship between coal properties and gasification behaviour is an important consideration in obtaining optimum gasifier performance2. Since most commercial processes operate at high pressure, the measurement of coal behaviour in the laboratory under simulated gasifier conditions is an important aspect of coal characterization for gasification processes. In general, little information is available in this area. A knowledge of the variation in coal properties in relation to pressure, heating rate, rank, etc. and the extent to which they can be modified3 by pretreatment and the inter-relationship between all these factors are essential to optimize a commercial process. Coals, with the exception of anthracites, and coal macerals other than inertinite are liable to change their 00162361/92/020169-13 ,c 1992 Butterworth-Heinemann
Ltd.
properties on oxidation in air. Very small quantities of oxygen, which cannot be detected by conventional analytical methods, can alter the caking and swelling properties significantly4. In extreme cases, freshly mined coal when exposed to air for a few days at ambient temperature will markedly decrease caking and swelling properties and calorific value. Extensive oxidation can be detected by spectroscopic and analytical methods. Details of the chemical changes responsible for the modifications in technological properties have been the subject of considerable discussion and are not known in detail, although some basic reaction processes, for example the loss of aliphatic groups and the formation of phenolic group in the coal macromolecules, are evident. Recent studies have shown’ that oxidation in air can lead to substantial modification of coal swelling in relation to pressure. Clearly, this possibility is an important consideration in the utilization of fine coal in a high pressure commercial gasifier where small particle sizes make the coal more susceptible to oxidation. This investigation was initiated to provide a general understanding of the relationship between coal thermoplastic and coke properties under elevated pressures and the effect of oxidation on these properties. The objective of the investigation was to carry out a systematic
FUEL, 1992, Vol 71, February
169
Thermoplastic
and coking properties
Table 1 Characterization data for the coals
Rank (NCB)
of coal: K. M. Thomas et al
used
NW
Ph
Wh
Ma
301a
402/502
502
702
Proximate analysis (wt% dry basis) 22.3 Volatile matter 5.4 Ash
32.1 16.0
36.5 3.7
38.0 3.7
Ultimate analysis (wt% dry basis) 85.2 Carbon 4.6 Hydrogen 0.7 Sulphur 0.02 Chlorine
70.3 4.9 2.3 0.05
81.3 5.0
79.4 5.2 1.6 0.23
Caking and swelling properties BS swelling number 5 G2 Gray-King coke Dilatometry 361 T, (“C) 433 T, (“C) 462 T3 (“C) 23 c (%) 43 d (%) Petrographic analysis Reflectance (max.) (%) Maceral analysis @) ’ Vitrinite Liptinite Inertinite
7 G9
6 G4
E
334 398 450 26 185
371 410 438 24 54
369 _ 429 35 _
1.24 54 3 43
0.42
0.83 79 6 15
0.85 14 12 14
1.5
0.65 80 5 15
investigation of the effect of oxidation on a wide range of coals at a range of temperatures from those found in stockpiles to likely pretreatment conditions, using high pressure dilatometry, plastometry, thermogravimetric analysis and optical anisotropy techniques to monitor the effect. EXPERIMENTAL Coals used The characterization data for the coals used in this study are given in Table 1. The coals had ranks 301a, 4021502, 502, 602 and 702 in the British Coal classification scheme. Preparation of samples
The coals were sampled in accordance with the guidelines6 outlined in BS1017. Coal with a particle size of -6 mm was spread out in a thin layer on a tray and oxidized for various times from 1 h to 200 days and at temperatures in the range of 50-200°C to examine oxidation effects over a wide range of conditions. Each sample of oxidized coal was then divided into representative samples for characterization studies. Thermogravimetric analysis (TGA)
The method described previously by Izuhara et al.’ was used. The thermal analysis measurements were carried out using a thermal analyser coupled to a computer for data collection. This allowed accurate time frame correlation between the weight and temperature measurements. The thermogravimetric data for the devolatilization of - 20 mg samples of coal with a particle size of < 212 pm were carried out at a heating rate of 10°C min-’ up to 750°C in a nitrogen atmosphere with a flow rate of 44 cm3 min- ‘. The weight loss, differential weight loss (dw/dt) and temperature results were recorded on a chart recorder as well as being stored on
170
FUEL,
1992,
Vol 71, February
floppy disc by the computer for further analysis. Graphs were produced from this data. High pressure dilatometry
The apparatus used has been described previously’. It operates at pressures up to 10 MPa and heating rates up to 60°C min-‘. In all other respects the dilatometer operating procedure and coal pencil manufacture are similar to BS1016 part 12 with the exception that the preliminary temperature stabilization was not used’. The repeatability of the dilatation and contraction values are estimated to be lo-15% and 5-lo%, respectively. The repeatability in the temperature measurements is lo-15°C. The carbonized residues from the dilatometer were collected for examination by polarized light microscopy. PlastometrJ
The plastometer used in this study has been described elsewhere”. The samples used in the study were ground to - 500 pm and 25 g was loaded into the plastometer for each run. The instrument was operated in air at atmospheric pressure at 10 rev min- ’ and with a heating rate of 3°C min- ‘. The torque developed as a function of temperature was recorded on a chart recorder and also digitized and stored in a computer for further data analysis. Optical anisotropy measurements
The carbonized residues produced in the high pressure dilatometer were crushed, mixed with epoxy resin and formed into discs. These discs were prepared as polished sections by grinding and subsequent polishing using various grades of silicon carbide paper followed by alumina powder. The polished blocks were examined by polarized light microscopy with a x 50/l .3 oil immersion objective with crossed polars and a plate to produce interference colours. The structural features were classified according to their appearance, size and shape. The following classifications were used: isotropic, mosaic (very fine (co.5 pm), fine (0.5-1.5 pm), medium (1.5-5 pm) and coarse (> 5 pm)), flow (granular and striated) and basic anthracitic anisotropy. A 300 point count was used for all the samples investigated. The error on the measurements is estimated to be better than 5%. The optical anisotropy index (OAI) was calculated according to the following equation: OAI= lvf+2f+3m+4c+5gf+6tl+7b where vf = very fine mosaic, f = fine mosaic, m = medium mosaic, c = coarse mosaic, gf = granular flow, fl = flow anisotropy and b = basic anisotropy. N.m.r. spectroscopy
The 13C n.m.r. spectra of the oxidized coals were measured using a spectrometer using cross-polarization and magic angle spinning modes. RESULTS
AND DISCUSSION
The role of thermal pretreatment and oxidation in relation to coal thermoplasticity and coke properties has been reviewed elsewhere4. Previously, the main reason for studying the effect of oxidation on coal properties was the use of preheated coal in the manufacture of blast
Thermoplastic
of coal: K. M. Thomas et al.
and coking properties
furnace coke. As a result, most of the studies have involved investigations appropriate to conditions pertaining in coke ovens. However, the developing interest in coal gasification has led to continuing interest in these effects. Most commercial gasification processes are operated at high pressure for economic considerations. Therefore, the effect of gas pressure on coal and coke properties is an important consideration in understanding gasifier performance. There have been a considerable number ofinvestigations into the effect of pressures9’ 1-24, heating rate8,“-‘5,23,24, gaseous atmosphere’2-‘4~22, additives’6-20 and particle size’* on coal thermoplastic properties. The studies of the coke properties are more limited being restricted to mainly strength and optical anisotropy measurements4~11~15~21~23~z4. However, very little research has been carried out on the effect of oxidation in relation to coal behavioural characteristics at high pressure. High pressure dilatometric studies on Pittsburgh coal have shown’ that oxidation could lead to the swelling of a coal at high pressure being higher than at atmospheric pressure. This result, at first sight, might seem surprising, but the complex behaviour of dilatation with respect to a range of variables, in particular, pressure and coal rank must be considered. The shape of the dilatation versus pressure curve changes with rank and a similar effect with oxidation could explain the observations. A detailed multifaceted approach was adopted to obtain an overall picture of the effect of oxidation. A series of bituminous coals were
selected to cover a wide range of rank and these coals were oxidized in air, the effect on coal thermoplastic and coke properties being monitored by a range of techniques.
Table 2
profiles
Effect of oxidation
in air on thermogravimetric
Thermogravimetric
analysis
The method reported7 previously by Izuhara et al. was used. The results of the effect of oxidation on the volatile evolution profiles of coals Ph, NW and Wh are given in Table 2. From a typical plot (not shown) of the effect of oxidation on the volatile evolution profile plotted as the rate of change of weight (dw/dt) against temperature it is found that there is a decrease in the peak parameter ((l/w,)dw/dt),,, (where wd = original dry weight) with increase in the duration of the oxidation in air. The effect of oxidation on the peak parameter is also found to vary considerably with rank, the lower rank coals being more reactive towards oxidation. The weight loss also starts at a lower temperature and continues to a larger extent at the higher temperatures effectively broadening the evolution profile although overall this is a small effect and only readily observed at high degrees of oxidation. The changes in the volatile matter estimated in these measurements are small and clear trends are not easily detected bearing in mind the experimental errors involved. In general the volatile matter decreases as the normalized maximum rate of volatile evolution decreases with increasing extent of oxidation. However, previous work has shown that coals can gain and lose weight
volatile evolution
Thermogravimetric Oxidation Coal
Time
Coal Nw
Fresh
parameters
conditions Temperature
( ‘C)
(dn!dt),,, (mg min-’
Volatile rng-‘)
Water
(%)
matter
(O/o I
19.1
0.5
21.2
I7 days
50
16.6
0.3
20.5
23 days
50
16.5
0.3
20.5
29 days
50
16.5
0.3
20.2
36 days
50
15.7
0.8
20.3
50 days
50
15.X
0.6
20.3
57 days
50
16.2
0.4
20.4
64 days
50
15.9
0.5
20.6
85 days
50
15.9
0.2
20.4
IO0 days
50
15.7
II2 days
50
15.3
0.2
20.5
126 days
50
15.6
0.4
21.0
I40 days
50
16.4
0.7
?I .o
163 days
50
15.6
I.1
21.0
I82 days
50
15.1
I .5
20.X
202 days
50
15.4
0.4
21.4
211 days
50
15.0
I .2
20.6
20.3
5h
II0
18.1
0.2
20.X
I6 h
I IO
17.2
0.2
20.9
32 h
II0
16.4
0.0
20.6
64 h
I IO
13.7
0.4
19.3
96 h
I IO
12.7
0.2
19.4
I h
150
17.5
0.2
21.1
5h
150
15.1
0.2
70.6
16 h
150
14.5
19.5
FUEL,
1992,
Vol 71, February
171
Thermoplastic Table 2
and coking properties
of coal: K. M. Thomas et al.
continued Thermogravimetric Oxidation
parameters
conditions Volatile
@w/dt)ma, Coal
Time
Coal Ph
Fresh
Temperature
(“C)
(mg min-’
mg-‘)
Water
(X)
matter
(%)
29.8
I.1
30.9
12 days
50
31.1
0.8
31.7
17 days
50
30.9
0.6
31.2
23 days
50
28.9
0.6
29.7
29 days
50
28.8
0.7
30.6
36 days
50
28.7
0.0
29.4
50 days
50
28.8
0.8
30.3
57 days
50
28.6
0.6
30.
64 days
50
28.4
0.0
30.3
I
78 days
50
28.2
2.5
29.4
85 days
50
28.2
2.2
29.2
100 days
50
28.5
2.0
30. I
I12 days
50
28.7
0.7
31.0
126 days
50
28.0
0.8
29.8
140 days
50
27.1
0.8
30.8
163 days
50
28.4
0.0
31.5
I82 days
50
27.8
0.0
31.0
202 days
50
27.
I
0.8
30.5
21 I days
50
25.2
I .7
30. I
I h
110
30.2
0.0
30.7
5h
110
29.5
0.5
30. I
I6 h
II0
28.2
0.4
31.1
30 h
II0
26.2
0.5
30.2
48 h
II0
24.5
0.6
30.5
Ih
150
30.9
0.6
30. I
3h
150
23.3
0.6
28.6
6.5 h
150
21.1
0.8
30.3
Fresh
33.6
2.9
32.2
Ih
34.5
0.8
32. I
33.0
0.8
32.0
29.1
I .2
30.8
Coal Wh II0
5h
110
I6 h
during oxidation considered.
and
this is a factor
II0
which
must
be
Dilatometry The effect of pressure on dilatometry parameters has been the subject of several investigations. The results l-l * show that increasing pressure obtained previously 13%~ has the following effects on dilatometry parameters: the softening point decreases leading to an increased plastic range; the dilatation against pressure graph may increase, decrease or have a peak at - l-l.5 MPa, depending on the particular coal and this cannot be predicted from standard characterization data and atmospheric dilatometry measurements; the range of swelling for a series of coals is much less at high pressures (>4 MPa) than at atmospheric pressure.
The results of the effect of oxidation on dilatometry parameters measured at a heating rate of 40°C min-’
172
FUEL,
1992,
Vol 71, February
over a wide range of pressure (O-6 MPa) for NW, Wh, Ph and Ma coals are given in Table 3. The data show that oxidation usually has the following effects: 1. decreases swelling; 2. increases the softening point, resulting in a marked decrease in the plastic range; 3. increases the temperature of maximum contraction slightly. The results show that the effect of oxidation on swelling varies significantly with the rank of the coal, the decrease being more marked for the lower rank coals which are more reactive. The effect of oxidation on swelling also decreases with increasing temperature and time of oxidation. The effect of pressure on dilatation is complex and depends on the rank of coal. At pressures of 4 MPa and above, the swelling of a suite of coals has a much smaller range than at atmospheric pressure’,“. A comparison of the effect of oxidation on carbonization at atmospheric and 4 MPa shows that under a variety ofexperimental conditions, the change in swelling relative
Thermoplastic Table 3
and coking properties
of coal: K. M. Thomas et al.
continued
Heating rate (“C min-i)/pressureOxidation (MPa x 10)
7-cTI temperature
mastic range Oxidation
time
Ti (“C)
Tz (“C)
Ts (“(3
iv
c (X)
d (X)
Coal NW 40/40
Fresh
422
508
550
128
18
102
40140
50
29 days
421
511
550
139
21
98
40140
50
36 days
410
510
550
140
20
88
40140
50
43 days
425
498
538
113
20
90
40/40
50
50 days
410
502
538
128
19
85
40140
50
64 days
425
508
545
120
15
86
40140
50
78 days
431
520
558
127
16
74
40140
50
100 days
390
512
540
150
17
70
40140
50
163 days
430
512
552
122
15
63
40/40
50
182 days
430
524
562
132
14
52
40140
50
211 days
450
520
550
100
13
40
Fresh
232
40/o
460
505
550
90
7
40/o
50
29 days
453
495
541
88
5
228
40/O
50
36 days
430
493
532
102
11
223
40/o
50
43 days
430
505
543
113
9
196
40/o
50
50 days
440
500
539
99
9
190
40/o
50
64 days
450
506
548
98
6
190
40/o
50
78 days
442
498
530
88
8
152
40/o
50
100 days
441
508
546
105
6
136
40/o
50
163 days
455
518
550
95
8
85
40/o
50
182 days
465
521
550
85
6
73
40/o
50
211 days
455
517
550
85
10
30
40/40
110
lh
421
509
546
125
16
77
40140
110
5h
420
507
552
132
18
79
40140
110
16 h
425
521
559
134
16
74
40/40
110
32 h
415
505
550
135
18
70
40/40
110
64h
438
506
540
102
16
46
40140
110
96 h
420
525
545
125
19
14 176
40/o
110
16 h
450
503
545
95
6
40/o
110
64 h
455
520
550
95
6
40/o
110
96 h
445
550
550
105
13
40140
150
lh
425
531
560
135
7
40/40
150
5h
425
528
553
128
7
40/40
150
16 h
439
565
565
126
10
40140
175
lh
420
525
560
140
16
40140
175
5h
435
565
565
130
11
-11
40140
175
16 h
435
565
565
130
3
-3
40140
200
lh
445
565
565
120
17
-17
40140
200
16 h
436
556
556
130
4
-4
40140
175 (vacuum)
5h
420
508
545
125
18
84
40140
175 (vacuum)
10 h
430
525
554
124
19
86
50 -13 75 37 -10 37
40140
175 (vacuum)
16 h
445
524
569
124
15
79
40140
175 (vacuum)
22 h
427
516
552
125
10
56
Coal Ph 392
482
533
141
8
148
40/40
50
17 days
365
467
537
172
14
168
40140
50
29 days
385
480
536
151
19
153
40/40
50
36 days
380
478
540
160
19
143
40140
50
50 days
380
480
530
159
20
146
40140
50
43 days
376
460
526
150
10
133
40140
50
64 days
385
485
542
157
17
162
Fresh
40/40
FUEL,
1992,
Vol 71, February
173
Thermoplastic Table 3
and
coking properties of coal: K. Ad Thomas
et al.
continued
Heating rate (“C min _ 1)jpressureOxidatlon (MPa x lO)VC)
Tj-T, temperature Oxidation
time
T, (“C)
T, (“Cl
r, (“Cl
plastic range (“C)
c (%)
d (%)
40;40
50
78 days
380
485
546
156
15
160
40;40
50
85 days
375
480
540
165
21
144
40;40
50
100 days
405
480
528
123
16
I50
40/‘40
50
163 days
420
485
540
120
15
158
40;40
50
182 days
405
492
540
135
14
134
4oj40
50
211 days
410
485
540
130
16
100
Fresh
40;40
420
470
540
120
4
396
40/‘40
50
17 days
425
465
514
89
7
437
40/40
50
29 days
418
470
528
110
6
350
40/40
50
36 days
430
475
537
107
5
365
40/40
50
43 days
426
463
512
86
8
336
40/40
50
64 days
458
480
536
77
2
354
40140
50
85 days
430
470
so5
75
5
305
40140
50
100 days
430
485
540
1iO
5
348
40140
50
163 days
442
490
533
91
5
293
40/40
50
182 days
422
478
so5
83
8
225
40/40
50
211 days
420
487
520
100
4
120
Fresh
355
471
535
180
24
98
40:60
110
lh
340
455
535
195
25
93
40/60
110
Sh
345
475
535
190
25
89
40160
110
16 h
335
476
535
200
23
113
40/60
110
30 h
340
495
540
200
15
108
40/60
110
48 h
369
498
548
179
15
85
Fresh
40/60
40/25
410
475
542
132
8
175
40/25
110
lh
480
485
532
152
14
169
40/25
110
Sh
410
485
544
134
7
176
40/25
110
16 h
390
480
536
146
18
168
40/25
LlO
30 h
420
490
537
117
7
150
40125
110
48 h
427
489
540
113
9
123
Fresh
420
470
540
120
4
396
Ih
419
470
543
124
6
391
110
5h
420
464
526
106
6
375
11a
16h
435
484
540
105
5
309
4W
110
30 h
422
484
525
103
10
162
WO
110
4S h
460
506
555
95
2
25
40/o 40/o
110
40/o 40/o
40/60
Fresh
355
471
535
180
24
98
40/60
150
3h
405
512
550
135
19
95
40/60
150
6.5 h
455
505
540
85
14
19 175
40/25
Fresh
410
475
542
132
8
40/25
150
lh
379
460
518
139
20
146
40125
150
3h
440
495
545
105
13
140
40/25
150
6.5 h
450
505
540
90
I?
35
40/o
Fresh
420
470
540
120
4
396
40/O
150
Ih
439
475
521
82
6
400
40/o
150
3h
450
79
10
150
6.5 h
491
493 _
529
40/O
550
59
15
90 -15
Coal Wh 40140
Fresh
382
474
523
141
20
136
40140
110
lh
395
485
540
I45
19
to3
40140
110
5h
400
488
545
145
5
122
40/40
110
16 h
390
500
540
150
16
76
174
FUEL, 1992,
Vol 71, February
Thermoplastic Table 3
and coking
properties
of coal: K. M. Thomas
al.
et
continued
Heating rate (“C min- ‘)/pressureOxidation (MPa x 10) (“C)
T,-T, temperature
40/o
plastic range Oxidation
time
d (%)
T, (“C)
T, (“C)
Ta (“C)
Fresh
435
475
532
97
5
341
lh
430
476
517
81
5
300
(“C)
--
c (%)
40/o
110
40/o
110
5h
445
498
540
95
17
148
40/o
110
16 h
445
509
530
85
I
0
Coal Ma 40140
Fresh
373
466
519
146
18
78
346
485
528
182
13
76
lh
386
490
540
154
I
72
lh
400
490
525
125
16
56
110
5h
435
520
545
110
3
110
16 h
435
550
550
115
9
40/o
110
lh
415
504
545
130
6
13
4015
110
lh
425
485
517
92
6
48
40115
110
lh
410
494
540
130
4
75
40120
110
lh
370
484
524
154
12
72
40140
150
lh
410
550
550
140
7
50
lh
40/40
80
40140
110
40140 40/40
40140
Coal Ma (heat treatment
26 -9
-7
in vacuum)
40140
175
5h
340
490
545
205
8
81
40140
175
12 h
340
505
525
185
10
67
40/40
175
20 h
365
502
540
175
12
56
to the original dilatation is much less at higher pressure. Figure I illustrates this change in dilatation resulting from oxidation as a function of pressure for coals NW, Ph and Wh. It is clear from the results that at high pressure the swelling of extensively oxidized coals is greater than the corresponding low pressure measurement for the coals studied. This can be rationalized by considering the change in shape of dilatation versus pressure curves in relation to rank. The dilatation of low rank coals usually increases with increasing pressure whereas that of higher rank and high swelling coals usually decreases with pressure. Also there are coals with British Coal rank 702, where there is a peak in the dilatation versus pressure curve at 1-2 MPa. The oxidation behaviour of the coals can be rationalized by considering oxidation as similar to lowering the rank of the coal and changing the shape of the dilatation versus pressure curve to being similar to that of a low rank coal. The results in Table 3 also show the effect of oxidation temperature and time on the dilatometric properties of a range of coals over a range of pressures. It is apparent that oxidation temperature has a more marked effect on dilatometric properties than oxidation time. A comparison of the dilatation observed under experimental conditions of heating rate 40°C min-’ and pressure 4 MPa for samples of coal Ma oxidized for 1 h at temperatures of 50, 80, 110 and 150°C illustrates the effect of oxidation temperature on coal thermoplastic properties. Plustomctry
The results of the effect of oxidation on the Brabender (constant shear rate) plastometry parameters measured at atmospheric pressure and a heating rate of 3°C min- ’
for NW, Ph and Wh coals are shown in Table 4. The different trends in coking intensity for coals Ph and Wh which increase with oxidation and NW which decreases, both reflect a change in plastic behaviour similar to that of a lower rank coal. However, during oxidation, coal NW does not go through a highly fluid phase similar to coal Ph. This can be rationalized by the lack of hydrogen-containing species available to contribute to generation of fluidity. The results illustrate the importance of rank in determining coal thermoplastic and coke properties. Plastometry measures the agglomerating characteristics of coal particles and changes in interparticle interactions. Oxidation of coal will occur initially at the surface of the particles. Hence plastometry is expected to be a technique which will be very sensitive to small amounts of oxidation. This is apparent from the comparison of the plastometry results with the corresponding dilatometry and TGA results obtained at atmospheric pressures. In particular, results obtained at low degrees of oxidation indicated that all three techniques are capable of detecting the effects of oxidation at broadly similar levels. Optical
anisotropy
The results of the optical anisotropy measurements on the series of oxidized Ph coals carbonized in the high pressure dilatometer are given in Table 5. Other results of optical anisotropy measurements on coals NW and Ma, subjected to varying degrees of oxidation, have been published separately15q2’ and these show similar trends. The fact that the carbonized residues from the dilatometry measurements have been examined means that a direct comparison of the results is appropriate. The main effect is that oxidation decreases the optical
FUEL,
1992,
Vol 71, February
175
Thermoplastic
and coking properties
of coal: K. M. Thomas et al
a
5
10
15
Oxidation time (hours)
40
80
Oxidation time (hours)
lb
Figure 1 Effect of oxidation on dilatation for gauge and 4 MPa for: a, coal NW; b, coal Ph; c, coal Wh
Oxidation time (hours)
anisotropic content of the cokes carbonized identical conditions. However, high pressure tends to increase the optical anisotropic content coals15-21 and this effect can also be seen in the oxidized coals in Figure 2. The effect is not as
176
FUEL,
1992,
Vol 71, February
under usually in fresh series of clear in
pressures
of 0
the oxidized coals as in the fresh coal because although the mosaic size increases it is still within the limits set for fine grain mosaic texture. However, it was evident that increased carbonization pressure was producing an increase in the mosaic sizes. The apparent effect of
Thermoplastic Table 4
Effect of oxidation
on plastometry
and coking
properties
of coal:
K. M. Thomas
et al.
parameters
T,-7-1 Oxidation time
Plastometry
temperature (“C)
temperatures
(“C) ~
Tl
T;
390
391
T';
T3
T4
range (“C)
401
409
438
48
405
409
423
21
-
1;
Caking I;
Max. coking 1,
1,
60
50
95
45
24
70
Coal NW Fresh 120 days
50
396
50
5h
110
388
392
406
420
435
41
50
66
51
84
16 h
110
391
400
415
420
438
41
72
62
88
32 h
110
405
_
412
420
438
33
55 _
46
30
15
64h
110
425
_
432
435
448
23
_
43
26
32
96 h
110
No thermoplastic
lh
150
398
420
430
442
44
61
93
415
421
13
52 _
64
418
41
25
87
405
3h
150
414
_
6h
150
No thermoplastic
properties
properties
Coal Ph Fresh 12 days
50
350
351
365
398
440
90
52
50
10
16
358
360
371
417
439
81
50
48
12
24
17 days
50
363
368
376
418
439
16
49
48
15
32
93 days
50
368
373
378
416
450
82
44
58
8
20
120 days
50
378
383
387
390
421
43
35
40
32
62
lh
110
355
360
313
400
438
83
44
49
8
21
5h
110
360
367
376
410
435
15
46
53
12
27
16 h
110
370
314
386
391
411
47
53
39
68
30 h
110
389
_
395
398
423
34
46 _
43
21
73
96 h
110
No thermoplastic
properties
lh
150
372
373
380
418
424
52
23
40
150
418
_
422
424
439
21
44 _
49
3h
42
41
80
6.5 h
150
No thermoplastic
395
398
400
418
441
46
32
50
24
46
lh
110
407
412
414
421
435
28
34
38
32
60
5h
110
414
416
420
427
436
22
36
46
44
72
16 h
110
No thermoplastic
properties
properties
Coal Wh Fresh
oxidation on coal thermoplastic behaviour is dependent on the conditions under which the coal is carbonized and tends to be less at high pressure. A similar trend is observed for optical anisotropy, the change in optical anisotropy index tends to decrease as carbonization pressure increases. This is illustrated in Figure 2 which compares the change in optical anisotropy of cokes derived from oxidized Ph coal carbonized at a heating rate of 40°C min - ’ and gauge pressures of 0, 2.5 and 6 MPa. Similar results have been reported15s21 for 301a and 702 rank coals. The effect of increasing pressure on coke optical anisotropy is usually to increase optical anisotropy although there is usually a limiting value at pressures >4 MPa. This can be explained2r by the following factors: 1. retention of low molecular weight volatiles during the carbonization phase; 2. a decrease in viscosity (increase in maximum fluidity)22; 3. increased plastic range due to a decrease in the softening point.
Oxidation tends to decrease the plastic range and this is evident in both the high pressure dilatometry and the plastometry results. The plastometry results which are measured at atmospheric pressure show that the torque at maximum fluidity increases on oxidation for coals Ph and Wh but decreases for coal NW. The coking intensity behaves in a similar manner for both types of coal. The effect of oxidation as seen from the changes in plastic properties is best described as similar to a decrease in rank. Clearly, oxidation has a complex effect on coal thermoplastic and coke properties which is dependent on rank. studies The oxidation of coals NW and Ma has also been monitored by i.r. and n.m.r. spectroscopy. The FT-i.r. spectra show 25 that the oxidation of both coals oxidized at 110 and 150°C for various times produces bands in the region 1600-1800 cm-’ which can be assigned to a C=O stretching mode and an increase in intensity in the O-H stretching region. The ratio of the bands due to C-H(A1) relative to C-H(Ar) decreases indicating that Spectroscopic
FUEL, 1992, Vol 71, February
177
Thermoplastic Table 5
Optical
Oxidation
and coking properties anisotropy
measurements
16hat
on chars prepared
conditions
110°C
110°C
30 h at 110°C
48 h at 110°C
3 hat
150°C
OAI
Isotropic
coal Ph
texture composition Very fine
(vol%) Fine
191
3.5
2
94.5
3/25
190
3
4
93
3160
195
1
3
96
3
93
4010
189
40110
196
4 2
40125
194
2
2
96
40/60
192
2
4
94
40/o
196
4
96
40/25
197
1
1
98
40/60
189
4
3
93
40/o
179
2
17
81
40/60
195
1
3
96
40/o
104
16
63
21
40125
194
6
94
40160
197
3
97
4010
85
44
27
29
155
4
37
59 90
_
_
98
40/60
190
10
40/o
190
10
90
40125
199
_
1
99
77
45
33
22
40125
180
_
20
80
40/60
192
_
8
92
40/o
30
72
26
2
40125
78
39
44
17
40160
114
18
50
32
150.
the aliphatic content of the coal decreases on oxidation. This is confirmed by the measurement of the 13C n.m.r. spectra of fresh and oxidized coals NW and Ma which indicate that oxidation causes an increase in the aromatic carbon relative to the aliphatic carbon. In the case of coal NW oxidation in air at 200°C causes f, to increase from 0.82 for the fresh coal to 0.84, 0.88 and 0.9 after oxidation for 1, 5 and 16 h, respectively. Oxidation of coal Ma in air for 16 h causes f, to increase from 0.68 to 0.69 and 0.84 at 110 and 2OO”C,respectively.
2.5MPa
Comparison of techniques
::
O 100 1
01 0
20
40
6
Oxidation time (hours)
Figure 2 Variation of OAI with oxidation and 6 MPa for coal Ph
178
from fresh and oxidized
3/O
40/o
6.5 h at 150°C
dilatometer
Optical
40125
1 h at 150°C
et al.
in the high pressure
Dilatometer conditions (heating rate (“C min- ‘)/pressure (MPa x 10))
Fresh
5 hat
of coal: K. M. Thomas
FUEL,
1992,
at gauge pressures
Vol 71, February
of 0,2.5
This investigation has used a number of techniques to investigate the effect of oxidation on a range of coals varying in rank. The techniques used range from the monitoring of thermoplastic properties, such as dilatation and plastometry, to thermogravimetric and structural characterization methods. The empirical technological properties are usually the most sensitive to oxidation effects. Bearing in mind that some of the measurements are only carried out at atmospheric pressure, it is apparent from the results that Brabender plastometry dilatometry and TGA parameters have broadly similar sensitivities to oxidation (Tables 2-4) usually being able to detect changes in the plastic properties of coal before any substantial changes in chemical analysis and spectroscopic properties can be detected although this is not invariable. This observation
Thermoplastic
COAL m
400 t
I
II
110%
I
150%
A
I
0
I
0
0
300 t
2 .z 200 5 s z
100 -
o-
-301 0
10
20
30
of coal: K. M. Thomas et al.
and coking properties
ship between coking intensity and experimental conditions such as heating rate and pressure is complex due to effects on swelling and fluidityz4 and both of these parameters affect the coking intensity. Both coals show a decrease in plastic range with increasing oxidation. Figure 3 shows a graph of dilatation (40°C min- ‘, atmospheric pressure) against the (dwldt),,, obtained from the thermogravimetric measurements at 20°C min- ’ and atmospheric pressure. It is clear that there is a good correlation between these dilatometry and thermogravimetric parameters for the three coals oxidized over a range of temperatures (SO-15OC) for various times. There is no correlation between the thermogravimetric parameter measured at atmospheric pressure and the dilatation at high pressure. This is not surprising since pressure will have a considerable effect on the rate of release of volatiles. The apparatus for conducting the corresponding thermogravimetric measurements at high pressure was not available. Figure 4 shows a graph of dilatation versus OAI for coal Ph over a range of oxidation temperatures, times and carbonization pressures. It is clear that for these pressures, 0, 2.5 and 6 MPa, there is a good correlation between the changes in dilatation and OAI with oxidation. Figure 5 shows a graph of OAI for cokes carbonized at 40°C min- ’ and atmospheric pressure and the TGA maximum volatile release parameter (20°C min - ‘, atmospheric pressure). It is apparent that there is a good correlation between these parameters for a range of oxidation times and temperatures (50-l 50°C). The correlations between the results clearly suggest that there
TGA
Figure 3 Variation of dilatation (40°C min-‘, atmospheric pressure) with normalized maximum rate of weight loss from thermogravimetric measurements
can be explained by the nature of the measurements. Plastometry, being mostly concerned with the interaction between particles, is primarily affected by the surface of the particle whereas swelling is mainly an intraparticle effect. On this basis, the initial caking peaks would be expected to show marked changes on oxidation. This is observed in the increase in softening temperature on oxidation although the changes in caking intensity with increasing extent of oxidation are small. High pressure constant shear rate plastometry results have shown that the caking intensity is not sensitive to changes in heating rate and pressure 23-24. The increase in softening point is also observed in the dilatometry studies. It is clear from the plastometry studies that oxidation can cause different trends in the apparent maximum fluidity and coking intensities with increasing extent of oxidation. This can be rationalized by comparison with the plastometry curves obtained” for coals of various ranks. In coal Ph which exhibits very high swelling and Gieseler fluidity under standard conditions (3°C min- ‘, atmospheric pressure) carbonization results in porosity with large pores which give the coke a weak strength. Oxidation causes a decrease in the swelling and the size of the pores leading to an increased coking intensity in the plastometry curves. However, coal NW shows a decrease in coking intensity and dilatation with increasing oxidation. It is noteworthy that the high pressure dilatometry experiments were carried out at a much higher heating rate than the plastometry. The relation-
PRESSURE
400
300
s c 0 ‘i 200 % .z n
100
0
I
I
100
150
I
-30 0
50
L
200
OAI
Figure 4 Variation of OAI with dilatation and extents of oxidation for coal Ph
FUEL,
1992,
over a range of pressures
Vol 71, February
179
Thermoplastic
and coking properties
I o20
of coal: K. M, Thomas et al.
an increase in rank. Therefore the effect of oxidation on coal properties follows the trends expected for both increased oxygen content and heat treatment. The differences in thermoplastic behaviour and coke structure observed after oxidation are based on chemical reactions which alter the chemical structure possibly accompanied by physical changes in structure, for example, porosity. It is apparent that small chemical changes can produce large changes in thermoplastic and pyrolysis properties and the implications for high pressure gasification are significant. Further work is required to assess the changes in chemical and physical structure of coal, in particular, porosity.
. 25
30
35
dwldt Figure 5 Variation of OAI with maximum coal Ph: 0, 110°C; +, fresh; n , 150°C
rate of volatile release
for
are relationships between the release of volatiles, dilatation and the development of optical anisotropy and that these concomitant effects are being modified by structural changes induced by the oxidation process. The change in coal structure, for example, coal functionality and porosity, are factors which may affect the release of during volatiles, and subsequent coal behaviour carbonization. The reduction in the maximum rate of volatile release caused by oxidation will cause a reduction in the swelling because the liquid and gaseous products are able to escape more easily from the particles. The changes in volatile release profile with respect to temperature and also the decrease in plastic range limit the amount of fluid material present during the thermoplastic phase and hence the development of anisotropic structures is restricted by oxidation leading to reduction in OAI. Increasing the carbonization pressure modifies the release of volatiles from the coal and increases the plastic range and fluidity usually leading to an increase in OAI. Therefore, the apparent effect of oxidation on coke structure (OAI) is smaller at high pressures. The results suggest that oxidation modifies the coal structure by the introduction of the oxygen functionality and decreasing both the carbon and hydrogen aliphatic to aromatic ratios such that the release of volatiles in relation to temperature changes substantially. It is also possible that the coal macromolecular and porous structures are modified significantly during oxidation. The modifications in the rate of volatile release appear to account for the changes in swelling and the thermoplastic and optical anisotropic properties. The effects of oxidation on coal thermoplastic and coke anisotropy properties are similar but not identical to a decrease in rank. However, low rank coals are usually more reactive coals and have higher values of the TGA maximum rate of volatile release parameter whereas oxidation causes this parameter to decrease. Substantial changes in spectroscopic parameters are only observed in the samples which have been subjected to extensive oxidation. The results obtainedz5 using the oxidized coals used in this study clearly indicate that oxidation increases the oxygen functionality and decreases the amount of aliphatics present. Extensive oxidation of coal also causes the vitrinite reflectance to increase whereas weathering causes the reflectance to decrease26~27. The changes in aromatic to aliphatic carbon and hydrogen ratios and vitrinite reflectance caused by oxidation are similar to
180
FUEL, 1992,
Vol 71, February
CONCLUSIONS The application of a variety of characterization methods to monitor the oxidation of a wide range of coals which vary in rank, has illustrated the benefits of using a multi-technique approach. The data show good correlations between the maximum of release of volatiles during pyrolysis, the development of optical anisotropy and dilatometric properties, and that these changes induced by oxidation in air are concurrent effects which are related closely. The perturbation of the coal structure by oxidation is a useful method for studying changes in coal structure and its effect on carbonization since it allows the structure to also be changed without the introduction of other variables associated with changing coals which are difficult to quantify. The results show that a series of techniques are needed to follow the progress of coal oxidation since they are sensitive to different levels of oxidation and monitor different properties. The broad insight into the effect of oxidation on the carbonization process obtained by using a range of techniques has illustrated the dependence on the apparent effect of oxidation on rank, oxidation time and temperature and experimental conditions, in particular, pressure. Whether or not the changes in coal properties caused by oxidation are beneficial will depend on the nature of the particular process and the relative importance of the properties in determining gasifier behaviour. ACKNOWLEDGEMENTS The authors would like to thank British Gas plc for permission to publish this paper and Mr J. Williamson for the 13C n.m.r. spectra. REFERENCES Evans, R., Thompson, B. H., Hiller, H. er al. ‘Proc. 5th International Conference and Exhibition on Coal Utilisation and Trade (COAL TECH ‘85)‘, Vol. 3, 1985, p. 659 Thomas, K. M., White, A. and Williams, A. Paper presented at the ‘Gasification-Status and Prospects’ Conference, Institute of Fuel, Harrogate, May 1988 Thomas, K. M. in ‘Carbon and Coal Gasification-Science and Technology’ (Eds J. L. Figuiredo and J. A. Moulijn), NATO ASI, Series E105, Martinus Nijhoff, Dordrecht, 1986, p. 421 Habermehl, D., Orywal, F. and Beyer, H. D. in ‘Chemistry of Coal Utilisation’ (Ed. M. A. Elliott), 2nd Suppl. Volume, Wiley-Interscience, New York, 1981, p. 317 Kahn, M. R. and Jenkins, R. G. Fuel 1985, 64, 189 British Standards Institution. ‘BS1017: Methods for Sampling Coal and Coke, Part 1, The Sampling of Coal’, 1977 Izuhara, H., Tanibata, R. and Nishida, S. ‘Proc. Int. Conf. Coal Sci.‘, Sydney, Australia, 1985, p. 491 Green, P. D. and Thomas, K. M. Fuel 1985,64, 1423
Thermoplastic 9
10 11 12 13 14 15
16 17 18
British Standards Institution. ‘BSl016: Methods for the Analysis and Testing of Coal and Coke, Part 12, The Caking and Swelling Properties of Coal’, 1980 Mulligan, M. J. and Thomas, K. M. Fuel 1987,66, 1289 Green, P. D., Patrick, J. W., Thomas, K. M. and Walton, A. Fuel 1985,64, 1431 Beyer, H. D. ‘Caking and Coking Power of Bituminous Coals Under High Pressure’, Report BMFT-FB-T 82-55 Kahn, M. R. and Jenkins, R. G. Fuel 1984,63, 108 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 725 Green. P. D.. Patrick. J. W.. Thomas. K. M. and Walton. A. ‘Proc. of the 1986 International Gas Research Conference’, 1987, p. 1053 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 1203 Kahn, M. R. and Jenkins, R. G. Fuel 1986,65, 1291 Bexley, K., Green, P. D. and Thomas, K. M. Fuel 1986,65,47
19 20 21 22 23 24 25 26 27
and coking properties
of coal: K. M. Thomas et al.
Green, P. D., Edwards, I. A. S., Marsh, H., Thomas, K. M. and Watson, R. F. Fuel 1988, 67, 389 Tromp, P. J. J., Karstein, P. J. A., Jenkins, R. G. and Moulijn, J. A. Fuel 1986,65, 1540 Patrick, J. W., Thomas, K. M., Walker, A. and Green, P. D. Fuel 1989,68, 149 Kaiho, M. and Toda, Y. Fuel 1979,58, 397 Chan, M. L. and Thomas, K. M. Am. Chem. Sot. Div. Fuel Sci. Prepr. 1989, 34 (3), 915 Chan, M. L, Parkyns, N. D. and Thomas, K. M. Fuel 1991, 70,447 Thomas, K. M. and Bradshaw, D. I. unpublished results Bend, S. L. PhD Thesis University ofNewcastle upon Tyne, 1989 Bend, S. L., Edwards, I. A. S. and Marsh, H. Am. Chem. Sot. Div. Fuel Sci. Prepr. 1989, 34 (3), 923
FUEL, 1992,
Vol 71, February
181