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Coal thermoplasticity and coke structure as related to gasification
Coal thermoplasticity and coke structure related to gasif ication Effect of inorganic additives on high pressure dilatometric properties and reactivity towards Peter D. Green, Ian A. S. Edwards*, and Robert F. Watson*
Harry
Marsh*,
as
hydrogen
K. Mark
Thomas
British Gas plc, Research and Development Division, London Research Station, Michael Road, Fulham, London SW6 2AD, UK “Northern Carbon Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE7 7RlJ, UK (Received 16 January 1987; revised 5 June 1987)
The effects of potassium carbonate and ferric oxide on the thermoplastic properties of a weakly caking coal have been investigated using a high-pressure dilatometer. The inorganic additives decrease the swelling ofthe coal, the effect being most pronounced in the pressure range l-2 MPa. The effects of the additives on dilatometric properties are strongly influenced by heating rate and pressure. The residual chars from the dilatometry experiments were heat-treated further and their reactivity towards hydrogen was examined. Potassium carbonate did not increase the reactivity substantially except when the char was initially carbonized at atmospheric pressure. All the chars prepared with ferric oxide as additive showed higher reactivity than that from coal carbonized alone under identical conditions. The greater reactivity observed for the chars carbonized initially at atmospheric pressure was due to their surface areas being higher than those of chars prepared at higher pressures. (Keywords: coal; coke; thermoplastic properties)
Coals are heterogeneous materials with constituents taking the form of discrete mineral compounds within the coal and the organic part of the coal containing microscopic macerals. In addition, some elements may be directly bound to the organic macromolecular part of the coal in the form of organometallic compounds, heterocycljc compounds or functional groups. In the case of the former it is well established that small quantities of certain inorganic compounds and minerals can modify the behaviour of coal during gasilication’ and pyrolysis’, affecting (1) the rates and quantities of products formed ‘.‘; (2) the caking and swelling properties3; and (3) the coke/char structural properties4. Considerable interest has been shown in the use of additives for the modification of coal behaviour during gasification 4- 6 The two main areas of interest are points (1) and (2) listed above. In (1) the possibility of catalytic effects leading to lower process-operating temperatures can benefit overall efficiency and reliability of operation. One of the problems is to obtain a catalyst which is cheap enough for expensive recovery systems not to be required. In the case of (2), operational difficulties can occur with high-swelling coals in both fluidized and fixed bed gasiliers. Various methods proposed for the modification of coal behaviour can involve a partial oxidation step, and are accompanied by an economic penalty and/or operational problems. There is scope for the dry mixing of additives with lines to produce briquettes of the required properties and when thecoal-additivecontact and mixing need to be intimate’. Alternatively, the additive could be added directly with the coal or as a solution. The potential exists for 0016-2361/88.!03038947$3.00 0 1988 Butterworth & Co. (Publishers)
Ltd.
development of a cheap additive which could confer benefits in both areas, be economic and in addition provide greater operational flexibility with high swelling coals. of thermoplastic, Measurements pyrolysis and gasification properties of coals encounter problems related to the empirical nature of the techniques used and their dependence on experimental conditions such as heating rate, pressure, particle size, etc. An important aspect of the present investigation was to establish a direct link between the properties of coals plus additives under high pressure and the reactivity of resultant chars produced in swelling experiments. This approach eliminates ambiguities in relating two sets of data, and this is considered to be an important aspect of the study. In general, coal behaviour during gasification differs widely due to a number of factors (method of contacting gases and solids, the experimental conditions, raw product gas composition etc.). In the case of the product gas composition, a comparison of a typical series of steam/oxygen gasifiers that cover the main types of gasifiers shows that they can vary greatly with the following ranges being found: CO, 2 l@ 55 %; CO,, z 5-20%; H,, 2 l&45%; H,O, = 15-50”s;; CH,, = 5-20%. Even within gasifiers of a similar type, for example, lixed bed gasifiers, large differences in product gas composition can occur. In the British Gas/Lurgi slagging gasifier, high temperatures (2000°C) are produced in the raceway. These temperatures lead to this region being under mainly thermodynamic control rather than kinetic control. The steam/oxygen ratio is low, there is little or no excess steam resulting in low concentrations in the gasitier
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bed. The raw gas composition at the top of the slagging gasifier bed contains typically: CO, z 50%; COZ, z 2 %; H,O, ~15%; CH,, ~7%. At the HZ, ~25%; temperatures prevailing at the top of the gasifier bed the reactivity of steam and hydrogen are of interest. In comparison the raw gas composition in the dry ash Lurgi gasilier is typically: CO, z 12%; CO,, z 16%; H,, z 23 %; H,O, zy42 %; CH,, % 7 % and the reactivity of the char towards steam is much more important in this case. The object of the study was to investigate some of the factors which might affect the reactivity of the char produced in situ at the top of a fixed bed gasifier towards hydrogen. EXPERIMENTAL Samples used
The characterization data for the coal used in this study are given in Table I. The coal was stored under deoxygenated distilled water to prevent changes in its swelling properties during the period of the investigation.
Table 1
Characterization
data for the coal used in this study
NCB rank classification 702 Proximate analysis (wt % d.b.) Ash Volatile matter
3.7 38.0
Ultimate analysis (wt % daf) Carbon Hydrogen Chlorine Sulphur
82.4 5.4 0.2 1.7
Caking and swelling properties BS swelling number Dilatometry (BS1016) Tl (“C) T2 (“C) T3 (“C) c (%) d (%) Petrology B max (%)
lf 369 _ 429 35 _ 0.65
Maceral analysis (~01%) Vitrinite Exinite Semi-fusinite Fusinite Micrinite Macrinite Sclerotinite
80 5 14 1 Tr Tr _
Ash analysis (wt %) SiO, Al,& Na,O fW Fe@, TiO, Mn#a CaO MgO P,D, SO3
34.0 26.0 1.7 1.5 23.1 1.4 0.0 3.9 0.9 0.6 1.7
Ash fusion characteristics (oxidizing atmosphere “C) Def temperature Hem temperature Flow temperature Caloritic value kJ kg-’ daf
390
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P. D. Green et al.
This method has been shown to prevent any significant changes in swelling properties over a period of 3 years. The potassium carbonate (AR) and ferric oxide, used as additives, were obtained from BDH Chemicals. Preparation
of samples
The coal was sampled according to BS1017 (reference 8) and milled to - 212 pm (- 72 BS mesh). The particle size obtained with ultra centrifugal mill is partly controlled by the number of rotor blades, the screen size and speed of rotation. The additive was ground to the same maximum particle size 212 pm and added to the coal so that the concentration was 0.36mmolg-’ of either K or Fe. These powders were then mixed in a ‘Turbula’ mixer for 15 min to ensure uniform mixing. Each sample was used immediately after mixing to minimize any deterioration. The mixtures were made and studied in random order to avoid systematic errors. The coal pencils for the dilatometry experiments were made by the method described in BS1016: Part 12 (Ref. 9). The chars obtained from the dilatometer (HTT 823 K (550°C); soak time 0.1 h; heating rates 3 and 40°C min- ’ ; gauge pressures O-6 MPa) were heat-treated further to 1273 K in flowing nitrogen at atmospheric pressure at a heating rate of 5°C min- ’ and for a soak time of 1 h. The two heating rates used in the dilatometry studies were chosen because the lower heating rate (3°C min- ‘) is used in BS1016: Part 12 and previous work” has shown that the dilatation does not change significantly with increase in heating rates above 20°C min-‘. The resulting chars were ground and the size fractions between 500 and 250 pm were used for the reactivity measurements. Hence the chars used in the reactivity studies had been subjected to initial carbonization at high pressure followed by heat treatment at atmospheric pressure. Dilatometry
The high-pressure dilatometer used in this study and the experimental procedure have been described previously”. A coal pencil identical with that described in BS1016: Part 12 (Ref. 9) is used. Results identical within experimental error are obtained with the highpressure and BS dilatometers when both are operated under BS standard conditions (3°C min -I and atmospheric pressure). Reactivity
measurements
A schematic diagram of the apparatus used in this study is given in Figure 1. About 0.2 g of char was supported in a sintered silica sample holder. This was heated to the required reaction temperature at a heating rate of 30°C min- ’ under flowing nitrogen. When the reaction temperature (1266 K; 993°C) had been reached the nitrogen flow was replaced by hydrogen (150 ml min- ‘). The product gas was analysed at specified time intervals using a Perkin Elmer F17 chromatograph equipped with a 1 metre Poropak Q column and a flame ionization detector. The methane concentration tended to reach an initial maximum followed by a levelling off. Therefore the reactivity was measured at 0.5 wt ‘Acarbon weight loss, after the system had stabilized and in the region where consistent repeatable measurements could be obtained without noticeable effects due to sample burn-off.
Coal thermoplasticity
I
and coke structure
Rotameters
as related
to gasification:
P. 0. Green et al.
iample holder
Gas sampling
Traps
valve
-Al
Pen recorder
Figure 1
Schematic
diagram
of the reactivity
apparatus
Gas chromatograph
Surface area measurements Surface areas were determined from adsorption isotherms of carbon dioxide at 273 K using a McBain Spring apparatus. About 0.2 g of char (25&5OOpm) was outgassed for 6 h at 373 K and 0.1 Pa. Carbon dioxide was admitted to the system and extents of adsorption were monitored from the extension of the spring. Readings were taken every 4 h or overnight. The adsorption was studied at pressures up to 0.1 MPa. A value for the equivalent surface area was calculated using the Dubinin-Radushkevich equation assuming that the surface area covered by one molecule of carbon dioxide is 1.7x 10-19m2 (Ref. 11). RESULTS The dilatometry, char surface area and reactivity parameters obtained with a range of heating rates and pressures for three series: (a) coal, (b) coal + potassium carbonate and (c) coal + ferric oxide are given in Table 2. Figure 2 shows the variation of dilatation as a function of pressure for the three series, indicating that the additives cause the largest decrease in dilatation at a heating rate of 40°C min - ’ and at pressures in the range l-2 MPa, thus altering the shape of the dilatation uersus pressure curve. The dilatometry temperature parameters were also affected by the additives. The temperatures of maximum contraction (T2) and resolidification (T3) changed only slightly on the addition of either potassium carbonate or ferric oxide. However, the softening temperature (T,) was increased substantially by the addition of potassium carbonate, but with ferric oxide addition showed no consistent trend. Graphs of reactivity towards hydrogen against pressure of initial carbonization for the three series are shown in Figure 3 (heating rate 3°C min-‘) and Figure 4
I
I
2
4 Pressure
(MPa)
Figure 2 Variation of dilatation (d) of coal with pressure and additive concentration (contraction, c, values used at atmospheric pressure in some cases): 0, coal, 40°C min ’ : l , coal. 3°C min I ; V, coal + 2.5 w/w Y< K&O,, 40”Cmin-‘: v, coal+2Sw/w”; 3”Cmin-‘; A, coal+2.85w/w”,, Fe,O,, 40”Cmin-‘; K,CO,, A, coal+2.85w/wY/, Fe,O,, 3”Cmin-’
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Coal thermoplasticity and coke structure as related to gasification: P. 0. Green et al. Table 2 Dilatometry parameters, char (HTT 550°C; 823 K) surface area and rates of reaction of resultant function of heating rate and pressure for coal; coal + potassium carbonate; coal + ferric oxide
Dilatometry Series gauge (MPa)
pressure
(a) Coal 0.0 1 1.5 3 4
Heating rate (“C min-‘)
3 3 3 3 3
T, (“C)
T2 (“C)
T, (“Cl
380 373 368 354 357
414 408 406 402
435 430 435 434 438
40 40 40 40 40 40 40 40
454 445 442 434 427 426 358 335
(b) Coal + 2.5 ‘4 K&O, 0.0 1.5 4 0.0 1.5 4 6
3 3 3 40 40 40 40
383 382 362 458 429 410 355
(c) Coal + 2.85 y0 Fe,O, 0.0 1.5 4 0.0 1.5 4
3 3 3 40 40 40
385 365 326 435 424 332
0.0 0.5 1 1.5 2 2.5 4 6
512 478 463 463 466 466 469 462
527 510 518 501 515 511 519 517
chars in hydrogen,
parameters
T,-T, (“C) c (%I 55 57 67 80 81
41 35 41 34 38
73 65 76 67 88 85 161 182
4 13 4 5 6 15 22 31 36 42 39 33 28 15 31
408 -
434
72
470 482 469
498 524 507
69 114 152
418 413 -
445 446
80 120
472 465
510 508
86 176
-
23 42 42 29 20 25
at 993°C (1266 K) as a
Reaction rate at 0.5 % weight loss (ml’CH, &in-’ g-‘)
d (%)
-15 -8 7 11 10 41 88 112 100 85 63 47
-29 7 28 18
-33 -3 30 48
Surface area (carbon dioxide at 273 K) (m2 g-rj
0.161
60
0.095
10
0.118
<5
0.111
<5
0.087
<5
0.093 0.065
<5 <5
0.458 0.167 0.151 0.534 0.101 0.108 0.095
47 20 <5 47 15 11
0.615 0.268 0.297 0.513 0.383 0.348
24 21 10 33 <5 <5
0.8-
0.6 -
Heating rate 40°C min-’
0
I 2
I
4
6
Figure 3 Variation of reactivity of char towards hydrogen pressure at which initial carbonization took place (heating 3”Cmin-‘): 0, coal; V, coal+2.5w/w% K&O,; coal + 2.85 w/w 7; Fe,O,
392
FUEL, 1988, Vol 67, March
0
2
4
6
Pressure (MPa)
Pressure (MPa)
with rate A,
Figure 4 Variation of reactivity of char towards hydrogen pressure at which initial carbonization took place (heating 40”Cmin’): 0, coal; V, coal+2.5w/w% K&03; coal + 2.85 w/w % Fe,O,
with rate A,
Coal thermoplasticity
and coke structure
as related
to gasification:
P. 0. Green et al.
(heating rate 40°C mini). These show that while the chars produced by the carbonization of coal with potassium carbonate at atmospheric pressure were more reactive than those made under identical conditions from coal alone, the differences were much less at high pressures (> 1.5 MPa) and for a heating rate of 40°C min- ’ were not significant. In contrast, the reactivities of the chars produced by carbonization of coal with ferric oxide at both heating rates showed a sharp decrease up to 1.5 MPa, but remained virtually
Pressure (MPa)
A
-40
0
40
80
120
Dilatation d (%) Figure 5 Rate of reaction of char, at 0.5 y/‘,weight loss in hydrogen at 993°C (1266 K) versus dilatation: 0, coal; v, V, coalf2.5 w/w”; K,CO,; A, A, coal + 2.85 w/w % Fe,O,. Solid symbols: carbonization of coal + additive at atmospheric pressure
Figure 7 Variation in surface area of dilatometer chars (HTT 55o“C: 823 K) with carbonization pressure: heating rate 40°C min- ’ : 0. coal; V, coal+2.5 w/w% K,CO,; A, coa1+2.85 w/w “(, Fe,O,
unchanged thereafter at a level significantly higher than those of chars produced from coal alone under the same conditions. The chars produced with ferric oxide as additive at 40°C min - 1 and pressures > 1.5 MPa had higher reactivities than those made at 3”Cmin-’ and pressures > 1.5 MPa. Figure 5 shows the variation in rates of reaction at 0.5 wt % carbon weight loss at 993°C (1266 K) with dilatation during production of the char. When results obtained for samples initially carbonized at atmospheric pressure are excluded, series (a) and (b) showed a slight decrease in reactivity with increase in pressure whereas series (c) showed a slight increase. The results indicate that reactivity and dilatation are not related in a simple manner. Figures 6 and 7 show the variation of the surface area (CO, adsorption at 273 K) of the resultant dilatometer chars (HTT 823 K; 550°C) with pressure for heating rates of 3°C min- ’ and 40°C min - ’ respectively. The surface areas of the chars carbonized at high pressure (> 4 MPa) were <5m2 gg’. Generally the surface areas tended to decrease with increase in pressure, but all the chars from carbonization of coal without additive at 40°C min- * had surface areas of < 5 m2 g - ’ irrespective of carbonization pressure in the range O-6 MPa. DISCUSSION
Pressure (MPa) Figure 6 Variation in surface area of dilatometer chars (HTT 550°C; 823 K) with carbonization pressure: heating rate 3°C min-’ : 0, coal; V, coal + 2.5 w/w :/, K&O,; A, coal + 2.85 w/w oA Fe,O,
Effects of additives The caking and swelling of coal is a physical phenomenon but its origins are chemical. The interactions between coal and inorganic compounds involve both physical and chemical processes. The thermoplastic process is influenced strongly by the release
FUEL, 1988, Vol 67, March
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Coal thermoplasticity
and coke structure
as related to gasification:
of volatiles, which in turn is affected by the experimental conditions such as heating rate and pressure. High heating rates also tend to raise fluidity. Increasing pressure reduces the volume of gas evolved; it also decreases the volatility of decomposition products and increases their residence time in the plastic phase, so enhancing secondary reactions and leading to an increase in gas and coke yields but with a decreased tar yield”. Previous studies 10,12- l5 have shown that it is not possible to predict the effects of pressure on dilatation from dilatometry measurements carried out at atmospheric pressure. Causes of possible effects of additives on the thermoplastic properties of coal have been discussed previously”. The results obtained in this study clearly demonstrate that inorganic additives have a marked effect on the thermoplastic properties of coal, the effects being dependent on pressure and heating rate. Effects are sufficiently marked to change the shape of the dilatometry-pressure curve at a heating rate of 40°C min- ‘, with a sharp peak at 1.5 MPa, to one where the peak has disappeared and dilatation increases gradually with increase in pressure (Figure 2). The largest differences between the coal and coal + additive curves are at l-l .5 MPa. It is relevant to note that additions of tar or pitch to this coal enhance both the dilatation and optical anisotropy markedly, the maximum effect being in the same pressure range l-l.5 MPa with a heating rate of 40°C min-’ (Ref. 15). In contrast, the dilatation versus pressure curve obtained for the coal alone at a heating rate of 3°C min- ’ shows dilatation increasing with pressure but for the coal plus either additive there is a shallow minimum at 1.5 MPa followed by an increase in dilatation with further increase in pressure. High-pressure microdilatometry studies have shown that increases in pressure and heating rate reduce the effects of addition of potassium carbonate16-’ a and various calcium compounds’6,‘7,‘9 on dilatation. A larger concentration (20% w/w) was used in these studies compared with <3 y0 w/w used in the study described here. These differences result from the method of contacting the coal and additive. In this study the mixture was pressed into a briquette at 10MPa. For a rank range of coals with significant swelling differences at atmospheric pressure, increasing pressure to <4 MPa reduces markedly the differences in dilatation observed at atmospheric pressures10,12*15: hence, effects of additives on swelling at high pressures are likely to be small. Previous work6 has shown that alkali metal carbonates when added to coal in quantities up to 5 y0 w/w destroy the dilatation completely, the effect being virtually identical for a given number of moles of additive. The softening temperature also increased gradually with increase in alkali metal carbonate concentration. It was suggested6 that the hydroxyl functional groups in the coal reacted with the alkali metal carbonates to form phenolate salts, so reducing mobility of the lamellar units in the coal matrix. Carbon dioxide is evolved during the reaction and this evolution is inhibited by increase in pressure. With sodium carbonate, formation of the sodium salts leads to displacement of hydrogen from the carbonization system, so causing an increased coke and decreased tar yields, as observed experimentally. Similar trends are expected for the other alkali metal salts, for example those of potassium. Khan and Jenkins17 suggest that because potassium carbonate causes an increased 394
FUEL, 1988, Vol 67, March
P. D. Green et al
coke yield at the expense of tar-forming reactions, this results in decreased fluidity. Tromp et a/.‘* suggested from SEM studies that potassium carbonate causes the permeability of the walls of coal particles to be increased by the formation of small openings. The mechanism outlined above for potassium carbonate addition is not appropriate for ferric oxide6. It has been shown that addition of ferric oxide gave a lower weight loss during carbonization20, similar to the effect of some alkali metal salts, and that reduction of ferric oxide occurred21. The precise mechanism of the interaction with coal is not known. Reactivity of char towards hydrogen
Factors which influence rates of reaction of chars with gases are: 1. The surface area of the char available at reaction temperatures; 2. The concentration of active sites on the available surfaces; 3. Diffusion of gases to the active sites; 4. The structure and crystallinity of the char; 5. The presence of catalytic inorganic species. The rapid decrease in reactivity towards hydrogen at 1266 K for the coal, coal + potassium carbonate and coal + ferric oxide series with increase in pressure (Figures 3, 4) parallels the decrease in char surface areas (HTT 823 K (55O’C); Figures 6, 7). Differences in surface area with respect to pressure thus appear to contribute to changes in reactivity22. Analysis of the data of Table 2 suggests that, referred to the char surface areas, the reactivities of the chars from coal and coal plus potassium carbonate or ferric oxide are approximately 2-20 x 10e3, 3-10x 1O-3 and lO70x 10-3mlmin-‘m-2 respectively. Although the additives are modifying the carbonization chemistry and hence the active site concentrations of the char surfaces, the possibility of an associated effect of catalysis by potassium and iron has to be considered, although our investigation has not produced evidence for this. Pressure also increased the anisotropic content of the chars, in series (a) (heating rate 40°C min- ‘), 15 % linegrain mosaic after carbonization at atmospheric pressure to 90% at a pressure 4.0 MPa15. Fujita et a1.23 reported that mosaic textures were 65% less reactive towards carbon dioxide than isotropic textures. In the coal series the increase in anisotropic mosaic content could also contribute to the decrease in reactivity with increase in carbonization pressure. Potassium carbonate decreased the optical anisotropy of chars made at atmospheric pressure. The extent to which such a decrease would be affected by increasing carbonization pressure is unknown. However, it is likely that K2CO3 would further decrease the optical anisotropy, and this would be the reverse of the effect of pressure observed for this coal alone. The results of this work suggest that additives react chemically with coal during carbonization, and the extents to which the chemical, physical and structural properties of the resultant chars are modified depend on the experimental conditions. Additives used in this study modify char reactivity towards hydrogen by reacting with coal during the carbonization process to change the structure of the resultant char and the number and accessibility of the active sites.
Coal thermoplasticity
and coke structure
ACKNOWLEDGEMENTS R.F.W. acknowledges support of SERC and British Gas as a CASE student. The authors thank Miss Bridget A. Clow and Mrs Patricia M. Wooster for assistance with the manuscript. REFERENCES Wen, C. Y. Car. Ret’. Sci. Eng. 1980, 22, 1 Franklin. H. D.. Peters, W. A. and Howard, J. B. Furl 1982.61, 155 and 1213 Habermehl, D., Orywal, F. and Beyer, H. D. in ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), Second Supplementary Volume, Wiley-Interscience, 198 1, p. 3 17 Marsh, H. and Walker, P. L. Jr, Fuel Process. Technol. 1979. 2. 61 Crewe, G. F., Gat, U. and Dhir, V. K. Fuel 1975, 54, 20 Bexley, K., Green, P. D. and Thomas, K. M. Fuel 1986, 65, 47 Evans, R., Thompson, B. H., Hiller, H. and Vierrath, H. E. in ‘Proceedings of 5th International Conference and Exhibition on Coal Utilization and Trade (Coal Tech. 85) Dec. 1985’, 3. 659 ‘BSI 017: Methods for the Sampling of Coal and Coke’, Part 1, The sampling of Coal, British Standards Institution, 1977 ‘BS1016: Methods for the Analysis and Testing of Coal and
10 11 12
13 14 15 16
17 18 19 20 21 22 23
as related
to gasification:
P. 0. Green et al.
Coke’, Part 12, The Caking and Swelling Properties of Coal. British Standards Institution. 1980 Green, P. D. and Thomas, K. M. Furl 1986.64. 1423 Gregg, S. J. and Sing, K. S. W. ‘Adsorption, Surface Area and Porosity’, Academic Press, London. 1982 Green, P. D., Patrick, J. W., Thomas, K. M. and Walker. A. Preprints of 1986 International Gas Research Conference. 1986. 4. IO Beyer, H. D. ‘Caking and Coking Power of Bituminous Coals’. Report BMFT-FB-T 82-055 Khan, M. R. and Jenkins. R. G. F[,rl 1986,65, 725 Green, P. D., Patrick.J. W.. Thomas, K. M. and Walker, A. Fuel 1985, 64, 1431 Khan, M. R. and Jenkins, R. G. in ‘Proceedings of IEA 1983 International Conference on Coal Science. Pittsburgh, USA, August 1983’. p. 495 Khan, M. R. and Jenkins, R. G. Furl 1986, 65. 1291 Tromp. P. J. J.. Karstein, P. J. 4.. Jenkins. R. G. and Moulijn, J. A. Fuel 1986, 65, 1540 Khan, M. R. and Jenkins, R. G. Far/ 19X6, 65, 1203 Partington, R. G. and Sidebottom. R. J. J. Inst. Furl 1959, 32. 597 Gaines, A. F. and Partington. R. G. Fuel 1960, 39, 193 Fischer, B. and Hammer, H. in ‘Proceedings 4th International Carbon Conference, Carbon ‘86, Baden Baden, July 1986’. p, 164 Fujita, H.. Hijiriyama, M. and Nishida, S. Fuel 1983, 62. 875
FUEL, 1988, Vol 67, March
395
Harry
Marsh*,
as
hydrogen
K. Mark
Thomas
British Gas plc, Research and Development Division, London Research Station, Michael Road, Fulham, London SW6 2AD, UK “Northern Carbon Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE7 7RlJ, UK (Received 16 January 1987; revised 5 June 1987)
The effects of potassium carbonate and ferric oxide on the thermoplastic properties of a weakly caking coal have been investigated using a high-pressure dilatometer. The inorganic additives decrease the swelling ofthe coal, the effect being most pronounced in the pressure range l-2 MPa. The effects of the additives on dilatometric properties are strongly influenced by heating rate and pressure. The residual chars from the dilatometry experiments were heat-treated further and their reactivity towards hydrogen was examined. Potassium carbonate did not increase the reactivity substantially except when the char was initially carbonized at atmospheric pressure. All the chars prepared with ferric oxide as additive showed higher reactivity than that from coal carbonized alone under identical conditions. The greater reactivity observed for the chars carbonized initially at atmospheric pressure was due to their surface areas being higher than those of chars prepared at higher pressures. (Keywords: coal; coke; thermoplastic properties)
Coals are heterogeneous materials with constituents taking the form of discrete mineral compounds within the coal and the organic part of the coal containing microscopic macerals. In addition, some elements may be directly bound to the organic macromolecular part of the coal in the form of organometallic compounds, heterocycljc compounds or functional groups. In the case of the former it is well established that small quantities of certain inorganic compounds and minerals can modify the behaviour of coal during gasilication’ and pyrolysis’, affecting (1) the rates and quantities of products formed ‘.‘; (2) the caking and swelling properties3; and (3) the coke/char structural properties4. Considerable interest has been shown in the use of additives for the modification of coal behaviour during gasification 4- 6 The two main areas of interest are points (1) and (2) listed above. In (1) the possibility of catalytic effects leading to lower process-operating temperatures can benefit overall efficiency and reliability of operation. One of the problems is to obtain a catalyst which is cheap enough for expensive recovery systems not to be required. In the case of (2), operational difficulties can occur with high-swelling coals in both fluidized and fixed bed gasiliers. Various methods proposed for the modification of coal behaviour can involve a partial oxidation step, and are accompanied by an economic penalty and/or operational problems. There is scope for the dry mixing of additives with lines to produce briquettes of the required properties and when thecoal-additivecontact and mixing need to be intimate’. Alternatively, the additive could be added directly with the coal or as a solution. The potential exists for 0016-2361/88.!03038947$3.00 0 1988 Butterworth & Co. (Publishers)
Ltd.
development of a cheap additive which could confer benefits in both areas, be economic and in addition provide greater operational flexibility with high swelling coals. of thermoplastic, Measurements pyrolysis and gasification properties of coals encounter problems related to the empirical nature of the techniques used and their dependence on experimental conditions such as heating rate, pressure, particle size, etc. An important aspect of the present investigation was to establish a direct link between the properties of coals plus additives under high pressure and the reactivity of resultant chars produced in swelling experiments. This approach eliminates ambiguities in relating two sets of data, and this is considered to be an important aspect of the study. In general, coal behaviour during gasification differs widely due to a number of factors (method of contacting gases and solids, the experimental conditions, raw product gas composition etc.). In the case of the product gas composition, a comparison of a typical series of steam/oxygen gasifiers that cover the main types of gasifiers shows that they can vary greatly with the following ranges being found: CO, 2 l@ 55 %; CO,, z 5-20%; H,, 2 l&45%; H,O, = 15-50”s;; CH,, = 5-20%. Even within gasifiers of a similar type, for example, lixed bed gasifiers, large differences in product gas composition can occur. In the British Gas/Lurgi slagging gasifier, high temperatures (2000°C) are produced in the raceway. These temperatures lead to this region being under mainly thermodynamic control rather than kinetic control. The steam/oxygen ratio is low, there is little or no excess steam resulting in low concentrations in the gasitier
FUEL, 1988, Vol 67, March
389
Coal thermoplasticity
and coke structure
as related
to gasification:
bed. The raw gas composition at the top of the slagging gasifier bed contains typically: CO, z 50%; COZ, z 2 %; H,O, ~15%; CH,, ~7%. At the HZ, ~25%; temperatures prevailing at the top of the gasifier bed the reactivity of steam and hydrogen are of interest. In comparison the raw gas composition in the dry ash Lurgi gasilier is typically: CO, z 12%; CO,, z 16%; H,, z 23 %; H,O, zy42 %; CH,, % 7 % and the reactivity of the char towards steam is much more important in this case. The object of the study was to investigate some of the factors which might affect the reactivity of the char produced in situ at the top of a fixed bed gasifier towards hydrogen. EXPERIMENTAL Samples used
The characterization data for the coal used in this study are given in Table I. The coal was stored under deoxygenated distilled water to prevent changes in its swelling properties during the period of the investigation.
Table 1
Characterization
data for the coal used in this study
NCB rank classification 702 Proximate analysis (wt % d.b.) Ash Volatile matter
3.7 38.0
Ultimate analysis (wt % daf) Carbon Hydrogen Chlorine Sulphur
82.4 5.4 0.2 1.7
Caking and swelling properties BS swelling number Dilatometry (BS1016) Tl (“C) T2 (“C) T3 (“C) c (%) d (%) Petrology B max (%)
lf 369 _ 429 35 _ 0.65
Maceral analysis (~01%) Vitrinite Exinite Semi-fusinite Fusinite Micrinite Macrinite Sclerotinite
80 5 14 1 Tr Tr _
Ash analysis (wt %) SiO, Al,& Na,O fW Fe@, TiO, Mn#a CaO MgO P,D, SO3
34.0 26.0 1.7 1.5 23.1 1.4 0.0 3.9 0.9 0.6 1.7
Ash fusion characteristics (oxidizing atmosphere “C) Def temperature Hem temperature Flow temperature Caloritic value kJ kg-’ daf
390
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P. D. Green et al.
This method has been shown to prevent any significant changes in swelling properties over a period of 3 years. The potassium carbonate (AR) and ferric oxide, used as additives, were obtained from BDH Chemicals. Preparation
of samples
The coal was sampled according to BS1017 (reference 8) and milled to - 212 pm (- 72 BS mesh). The particle size obtained with ultra centrifugal mill is partly controlled by the number of rotor blades, the screen size and speed of rotation. The additive was ground to the same maximum particle size 212 pm and added to the coal so that the concentration was 0.36mmolg-’ of either K or Fe. These powders were then mixed in a ‘Turbula’ mixer for 15 min to ensure uniform mixing. Each sample was used immediately after mixing to minimize any deterioration. The mixtures were made and studied in random order to avoid systematic errors. The coal pencils for the dilatometry experiments were made by the method described in BS1016: Part 12 (Ref. 9). The chars obtained from the dilatometer (HTT 823 K (550°C); soak time 0.1 h; heating rates 3 and 40°C min- ’ ; gauge pressures O-6 MPa) were heat-treated further to 1273 K in flowing nitrogen at atmospheric pressure at a heating rate of 5°C min- ’ and for a soak time of 1 h. The two heating rates used in the dilatometry studies were chosen because the lower heating rate (3°C min- ‘) is used in BS1016: Part 12 and previous work” has shown that the dilatation does not change significantly with increase in heating rates above 20°C min-‘. The resulting chars were ground and the size fractions between 500 and 250 pm were used for the reactivity measurements. Hence the chars used in the reactivity studies had been subjected to initial carbonization at high pressure followed by heat treatment at atmospheric pressure. Dilatometry
The high-pressure dilatometer used in this study and the experimental procedure have been described previously”. A coal pencil identical with that described in BS1016: Part 12 (Ref. 9) is used. Results identical within experimental error are obtained with the highpressure and BS dilatometers when both are operated under BS standard conditions (3°C min -I and atmospheric pressure). Reactivity
measurements
A schematic diagram of the apparatus used in this study is given in Figure 1. About 0.2 g of char was supported in a sintered silica sample holder. This was heated to the required reaction temperature at a heating rate of 30°C min- ’ under flowing nitrogen. When the reaction temperature (1266 K; 993°C) had been reached the nitrogen flow was replaced by hydrogen (150 ml min- ‘). The product gas was analysed at specified time intervals using a Perkin Elmer F17 chromatograph equipped with a 1 metre Poropak Q column and a flame ionization detector. The methane concentration tended to reach an initial maximum followed by a levelling off. Therefore the reactivity was measured at 0.5 wt ‘Acarbon weight loss, after the system had stabilized and in the region where consistent repeatable measurements could be obtained without noticeable effects due to sample burn-off.
Coal thermoplasticity
I
and coke structure
Rotameters
as related
to gasification:
P. 0. Green et al.
iample holder
Gas sampling
Traps
valve
-Al
Pen recorder
Figure 1
Schematic
diagram
of the reactivity
apparatus
Gas chromatograph
Surface area measurements Surface areas were determined from adsorption isotherms of carbon dioxide at 273 K using a McBain Spring apparatus. About 0.2 g of char (25&5OOpm) was outgassed for 6 h at 373 K and 0.1 Pa. Carbon dioxide was admitted to the system and extents of adsorption were monitored from the extension of the spring. Readings were taken every 4 h or overnight. The adsorption was studied at pressures up to 0.1 MPa. A value for the equivalent surface area was calculated using the Dubinin-Radushkevich equation assuming that the surface area covered by one molecule of carbon dioxide is 1.7x 10-19m2 (Ref. 11). RESULTS The dilatometry, char surface area and reactivity parameters obtained with a range of heating rates and pressures for three series: (a) coal, (b) coal + potassium carbonate and (c) coal + ferric oxide are given in Table 2. Figure 2 shows the variation of dilatation as a function of pressure for the three series, indicating that the additives cause the largest decrease in dilatation at a heating rate of 40°C min - ’ and at pressures in the range l-2 MPa, thus altering the shape of the dilatation uersus pressure curve. The dilatometry temperature parameters were also affected by the additives. The temperatures of maximum contraction (T2) and resolidification (T3) changed only slightly on the addition of either potassium carbonate or ferric oxide. However, the softening temperature (T,) was increased substantially by the addition of potassium carbonate, but with ferric oxide addition showed no consistent trend. Graphs of reactivity towards hydrogen against pressure of initial carbonization for the three series are shown in Figure 3 (heating rate 3°C min-‘) and Figure 4
I
I
2
4 Pressure
(MPa)
Figure 2 Variation of dilatation (d) of coal with pressure and additive concentration (contraction, c, values used at atmospheric pressure in some cases): 0, coal, 40°C min ’ : l , coal. 3°C min I ; V, coal + 2.5 w/w Y< K&O,, 40”Cmin-‘: v, coal+2Sw/w”; 3”Cmin-‘; A, coal+2.85w/w”,, Fe,O,, 40”Cmin-‘; K,CO,, A, coal+2.85w/wY/, Fe,O,, 3”Cmin-’
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Coal thermoplasticity and coke structure as related to gasification: P. 0. Green et al. Table 2 Dilatometry parameters, char (HTT 550°C; 823 K) surface area and rates of reaction of resultant function of heating rate and pressure for coal; coal + potassium carbonate; coal + ferric oxide
Dilatometry Series gauge (MPa)
pressure
(a) Coal 0.0 1 1.5 3 4
Heating rate (“C min-‘)
3 3 3 3 3
T, (“C)
T2 (“C)
T, (“Cl
380 373 368 354 357
414 408 406 402
435 430 435 434 438
40 40 40 40 40 40 40 40
454 445 442 434 427 426 358 335
(b) Coal + 2.5 ‘4 K&O, 0.0 1.5 4 0.0 1.5 4 6
3 3 3 40 40 40 40
383 382 362 458 429 410 355
(c) Coal + 2.85 y0 Fe,O, 0.0 1.5 4 0.0 1.5 4
3 3 3 40 40 40
385 365 326 435 424 332
0.0 0.5 1 1.5 2 2.5 4 6
512 478 463 463 466 466 469 462
527 510 518 501 515 511 519 517
chars in hydrogen,
parameters
T,-T, (“C) c (%I 55 57 67 80 81
41 35 41 34 38
73 65 76 67 88 85 161 182
4 13 4 5 6 15 22 31 36 42 39 33 28 15 31
408 -
434
72
470 482 469
498 524 507
69 114 152
418 413 -
445 446
80 120
472 465
510 508
86 176
-
23 42 42 29 20 25
at 993°C (1266 K) as a
Reaction rate at 0.5 % weight loss (ml’CH, &in-’ g-‘)
d (%)
-15 -8 7 11 10 41 88 112 100 85 63 47
-29 7 28 18
-33 -3 30 48
Surface area (carbon dioxide at 273 K) (m2 g-rj
0.161
60
0.095
10
0.118
<5
0.111
<5
0.087
<5
0.093 0.065
<5 <5
0.458 0.167 0.151 0.534 0.101 0.108 0.095
47 20 <5 47 15 11
0.615 0.268 0.297 0.513 0.383 0.348
24 21 10 33 <5 <5
0.8-
0.6 -
Heating rate 40°C min-’
0
I 2
I
4
6
Figure 3 Variation of reactivity of char towards hydrogen pressure at which initial carbonization took place (heating 3”Cmin-‘): 0, coal; V, coal+2.5w/w% K&O,; coal + 2.85 w/w 7; Fe,O,
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FUEL, 1988, Vol 67, March
0
2
4
6
Pressure (MPa)
Pressure (MPa)
with rate A,
Figure 4 Variation of reactivity of char towards hydrogen pressure at which initial carbonization took place (heating 40”Cmin’): 0, coal; V, coal+2.5w/w% K&03; coal + 2.85 w/w % Fe,O,
with rate A,
Coal thermoplasticity
and coke structure
as related
to gasification:
P. 0. Green et al.
(heating rate 40°C mini). These show that while the chars produced by the carbonization of coal with potassium carbonate at atmospheric pressure were more reactive than those made under identical conditions from coal alone, the differences were much less at high pressures (> 1.5 MPa) and for a heating rate of 40°C min- ’ were not significant. In contrast, the reactivities of the chars produced by carbonization of coal with ferric oxide at both heating rates showed a sharp decrease up to 1.5 MPa, but remained virtually
Pressure (MPa)
A
-40
0
40
80
120
Dilatation d (%) Figure 5 Rate of reaction of char, at 0.5 y/‘,weight loss in hydrogen at 993°C (1266 K) versus dilatation: 0, coal; v, V, coalf2.5 w/w”; K,CO,; A, A, coal + 2.85 w/w % Fe,O,. Solid symbols: carbonization of coal + additive at atmospheric pressure
Figure 7 Variation in surface area of dilatometer chars (HTT 55o“C: 823 K) with carbonization pressure: heating rate 40°C min- ’ : 0. coal; V, coal+2.5 w/w% K,CO,; A, coa1+2.85 w/w “(, Fe,O,
unchanged thereafter at a level significantly higher than those of chars produced from coal alone under the same conditions. The chars produced with ferric oxide as additive at 40°C min - 1 and pressures > 1.5 MPa had higher reactivities than those made at 3”Cmin-’ and pressures > 1.5 MPa. Figure 5 shows the variation in rates of reaction at 0.5 wt % carbon weight loss at 993°C (1266 K) with dilatation during production of the char. When results obtained for samples initially carbonized at atmospheric pressure are excluded, series (a) and (b) showed a slight decrease in reactivity with increase in pressure whereas series (c) showed a slight increase. The results indicate that reactivity and dilatation are not related in a simple manner. Figures 6 and 7 show the variation of the surface area (CO, adsorption at 273 K) of the resultant dilatometer chars (HTT 823 K; 550°C) with pressure for heating rates of 3°C min- ’ and 40°C min - ’ respectively. The surface areas of the chars carbonized at high pressure (> 4 MPa) were <5m2 gg’. Generally the surface areas tended to decrease with increase in pressure, but all the chars from carbonization of coal without additive at 40°C min- * had surface areas of < 5 m2 g - ’ irrespective of carbonization pressure in the range O-6 MPa. DISCUSSION
Pressure (MPa) Figure 6 Variation in surface area of dilatometer chars (HTT 550°C; 823 K) with carbonization pressure: heating rate 3°C min-’ : 0, coal; V, coal + 2.5 w/w :/, K&O,; A, coal + 2.85 w/w oA Fe,O,
Effects of additives The caking and swelling of coal is a physical phenomenon but its origins are chemical. The interactions between coal and inorganic compounds involve both physical and chemical processes. The thermoplastic process is influenced strongly by the release
FUEL, 1988, Vol 67, March
393
Coal thermoplasticity
and coke structure
as related to gasification:
of volatiles, which in turn is affected by the experimental conditions such as heating rate and pressure. High heating rates also tend to raise fluidity. Increasing pressure reduces the volume of gas evolved; it also decreases the volatility of decomposition products and increases their residence time in the plastic phase, so enhancing secondary reactions and leading to an increase in gas and coke yields but with a decreased tar yield”. Previous studies 10,12- l5 have shown that it is not possible to predict the effects of pressure on dilatation from dilatometry measurements carried out at atmospheric pressure. Causes of possible effects of additives on the thermoplastic properties of coal have been discussed previously”. The results obtained in this study clearly demonstrate that inorganic additives have a marked effect on the thermoplastic properties of coal, the effects being dependent on pressure and heating rate. Effects are sufficiently marked to change the shape of the dilatometry-pressure curve at a heating rate of 40°C min- ‘, with a sharp peak at 1.5 MPa, to one where the peak has disappeared and dilatation increases gradually with increase in pressure (Figure 2). The largest differences between the coal and coal + additive curves are at l-l .5 MPa. It is relevant to note that additions of tar or pitch to this coal enhance both the dilatation and optical anisotropy markedly, the maximum effect being in the same pressure range l-l.5 MPa with a heating rate of 40°C min-’ (Ref. 15). In contrast, the dilatation versus pressure curve obtained for the coal alone at a heating rate of 3°C min- ’ shows dilatation increasing with pressure but for the coal plus either additive there is a shallow minimum at 1.5 MPa followed by an increase in dilatation with further increase in pressure. High-pressure microdilatometry studies have shown that increases in pressure and heating rate reduce the effects of addition of potassium carbonate16-’ a and various calcium compounds’6,‘7,‘9 on dilatation. A larger concentration (20% w/w) was used in these studies compared with <3 y0 w/w used in the study described here. These differences result from the method of contacting the coal and additive. In this study the mixture was pressed into a briquette at 10MPa. For a rank range of coals with significant swelling differences at atmospheric pressure, increasing pressure to <4 MPa reduces markedly the differences in dilatation observed at atmospheric pressures10,12*15: hence, effects of additives on swelling at high pressures are likely to be small. Previous work6 has shown that alkali metal carbonates when added to coal in quantities up to 5 y0 w/w destroy the dilatation completely, the effect being virtually identical for a given number of moles of additive. The softening temperature also increased gradually with increase in alkali metal carbonate concentration. It was suggested6 that the hydroxyl functional groups in the coal reacted with the alkali metal carbonates to form phenolate salts, so reducing mobility of the lamellar units in the coal matrix. Carbon dioxide is evolved during the reaction and this evolution is inhibited by increase in pressure. With sodium carbonate, formation of the sodium salts leads to displacement of hydrogen from the carbonization system, so causing an increased coke and decreased tar yields, as observed experimentally. Similar trends are expected for the other alkali metal salts, for example those of potassium. Khan and Jenkins17 suggest that because potassium carbonate causes an increased 394
FUEL, 1988, Vol 67, March
P. D. Green et al
coke yield at the expense of tar-forming reactions, this results in decreased fluidity. Tromp et a/.‘* suggested from SEM studies that potassium carbonate causes the permeability of the walls of coal particles to be increased by the formation of small openings. The mechanism outlined above for potassium carbonate addition is not appropriate for ferric oxide6. It has been shown that addition of ferric oxide gave a lower weight loss during carbonization20, similar to the effect of some alkali metal salts, and that reduction of ferric oxide occurred21. The precise mechanism of the interaction with coal is not known. Reactivity of char towards hydrogen
Factors which influence rates of reaction of chars with gases are: 1. The surface area of the char available at reaction temperatures; 2. The concentration of active sites on the available surfaces; 3. Diffusion of gases to the active sites; 4. The structure and crystallinity of the char; 5. The presence of catalytic inorganic species. The rapid decrease in reactivity towards hydrogen at 1266 K for the coal, coal + potassium carbonate and coal + ferric oxide series with increase in pressure (Figures 3, 4) parallels the decrease in char surface areas (HTT 823 K (55O’C); Figures 6, 7). Differences in surface area with respect to pressure thus appear to contribute to changes in reactivity22. Analysis of the data of Table 2 suggests that, referred to the char surface areas, the reactivities of the chars from coal and coal plus potassium carbonate or ferric oxide are approximately 2-20 x 10e3, 3-10x 1O-3 and lO70x 10-3mlmin-‘m-2 respectively. Although the additives are modifying the carbonization chemistry and hence the active site concentrations of the char surfaces, the possibility of an associated effect of catalysis by potassium and iron has to be considered, although our investigation has not produced evidence for this. Pressure also increased the anisotropic content of the chars, in series (a) (heating rate 40°C min- ‘), 15 % linegrain mosaic after carbonization at atmospheric pressure to 90% at a pressure 4.0 MPa15. Fujita et a1.23 reported that mosaic textures were 65% less reactive towards carbon dioxide than isotropic textures. In the coal series the increase in anisotropic mosaic content could also contribute to the decrease in reactivity with increase in carbonization pressure. Potassium carbonate decreased the optical anisotropy of chars made at atmospheric pressure. The extent to which such a decrease would be affected by increasing carbonization pressure is unknown. However, it is likely that K2CO3 would further decrease the optical anisotropy, and this would be the reverse of the effect of pressure observed for this coal alone. The results of this work suggest that additives react chemically with coal during carbonization, and the extents to which the chemical, physical and structural properties of the resultant chars are modified depend on the experimental conditions. Additives used in this study modify char reactivity towards hydrogen by reacting with coal during the carbonization process to change the structure of the resultant char and the number and accessibility of the active sites.
Coal thermoplasticity
and coke structure
ACKNOWLEDGEMENTS R.F.W. acknowledges support of SERC and British Gas as a CASE student. The authors thank Miss Bridget A. Clow and Mrs Patricia M. Wooster for assistance with the manuscript. REFERENCES Wen, C. Y. Car. Ret’. Sci. Eng. 1980, 22, 1 Franklin. H. D.. Peters, W. A. and Howard, J. B. Furl 1982.61, 155 and 1213 Habermehl, D., Orywal, F. and Beyer, H. D. in ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), Second Supplementary Volume, Wiley-Interscience, 198 1, p. 3 17 Marsh, H. and Walker, P. L. Jr, Fuel Process. Technol. 1979. 2. 61 Crewe, G. F., Gat, U. and Dhir, V. K. Fuel 1975, 54, 20 Bexley, K., Green, P. D. and Thomas, K. M. Fuel 1986, 65, 47 Evans, R., Thompson, B. H., Hiller, H. and Vierrath, H. E. in ‘Proceedings of 5th International Conference and Exhibition on Coal Utilization and Trade (Coal Tech. 85) Dec. 1985’, 3. 659 ‘BSI 017: Methods for the Sampling of Coal and Coke’, Part 1, The sampling of Coal, British Standards Institution, 1977 ‘BS1016: Methods for the Analysis and Testing of Coal and
10 11 12
13 14 15 16
17 18 19 20 21 22 23
as related
to gasification:
P. 0. Green et al.
Coke’, Part 12, The Caking and Swelling Properties of Coal. British Standards Institution. 1980 Green, P. D. and Thomas, K. M. Furl 1986.64. 1423 Gregg, S. J. and Sing, K. S. W. ‘Adsorption, Surface Area and Porosity’, Academic Press, London. 1982 Green, P. D., Patrick, J. W., Thomas, K. M. and Walker. A. Preprints of 1986 International Gas Research Conference. 1986. 4. IO Beyer, H. D. ‘Caking and Coking Power of Bituminous Coals’. Report BMFT-FB-T 82-055 Khan, M. R. and Jenkins. R. G. F[,rl 1986,65, 725 Green, P. D., Patrick.J. W.. Thomas, K. M. and Walker, A. Fuel 1985, 64, 1431 Khan, M. R. and Jenkins, R. G. in ‘Proceedings of IEA 1983 International Conference on Coal Science. Pittsburgh, USA, August 1983’. p. 495 Khan, M. R. and Jenkins, R. G. Furl 1986, 65. 1291 Tromp. P. J. J.. Karstein, P. J. 4.. Jenkins. R. G. and Moulijn, J. A. Fuel 1986, 65, 1540 Khan, M. R. and Jenkins, R. G. Far/ 19X6, 65, 1203 Partington, R. G. and Sidebottom. R. J. J. Inst. Furl 1959, 32. 597 Gaines, A. F. and Partington. R. G. Fuel 1960, 39, 193 Fischer, B. and Hammer, H. in ‘Proceedings 4th International Carbon Conference, Carbon ‘86, Baden Baden, July 1986’. p, 164 Fujita, H.. Hijiriyama, M. and Nishida, S. Fuel 1983, 62. 875
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