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The influence of pressure on the development of optical anisotropy during carbonization of coal
The influence of pressure on the development of optical anisotropy carbonization of coal John W. Patrick,
Peter
D. Green*,
K. Mark
Thomas*
during
and Alan Walker
Carbon Research Group, Department of Chemical Engineering, University of Technology, Loughborough, Leicestershire, LEl 1 3TU UK * British Gas plc, London Research Station, Michael Road, London, SW6 2AD, UK (Received 2 June 1988; revised 22 July 1988)
Semicoke residues from high pressure dilatometer tests on a series of bituminous coals, were examined by polarized light microscopy to determine the effects of pressure on the optical anisotropy developed during the plastic stage of the carbonization process. Similar examinations were also carried out with high and low rank coals which had been subjected to different levels of oxidation to determine the additional effects of this factor. The major part of the influence of pressure carbonization on the coke anisotropy takes place at pressures up to 2 MPa, the influence being dependent on the rank of the parent coal. The total volume anisotropy is increased significantly with coke from some low rank coals but the changes in the type or nature of the anisotropy are generally less well marked. In accord with the effects of oxidation on plasticity, oxidation progressively retards the anisotropic development more so with the cokes from lower rank coals, to such an extent that carbonization at pressures up to 4 MPa cannot overcome this effect, the cokes being predominantly isotropic. (Keywords:
anisotropy;
coke; bituminous
coal)
The optical anisotropy of cokes produced from coals has been the subject of considerable study over the years, and the classification of the anisotropy into several types based on their size, shape and appearance has provided a means of characterization of the coke--carbon texture’. However, this coke texture, which is coal rank dependent and has to be taken into account in considerations of coke reactivity and strength2, develops during carbonization at different rates dependent on the carbonization conditions3. Some of the effect of carbonization conditions such as rate of heating and soaking time have this emphasis reflecting the been well documented3, interests of the coking industry. Factors such as gas overpressure have received less
attention until recently4 when it has been shown that both the extent and the composition of the optical anisotropy are strongly pressure dependent. The motivation for studies on the influence of pressure during coal carbonization is to be found in the developing interest in coal gasification. Commercial considerations demand the use of high pressures in large scale gasifiers. In the gasification of coal in fixed bed gasifiers the counter current flow of gases and solids leads to coal in the upper levels of the gasifier being carbonized by the sensible heat of the hot gases produced in the gasification zone. The properties of this high pressure coke are of considerable significance in determining the optimum conditions for smooth and efficient operation of the fixed-bed gasifier5. Hence, as part of a programme of work to examine the relevant coke behaviour, a detailed study was made of the effects on the carbon texture of carbonization under various pressures using a series of bituminous coals 0016-2361/89/020149~6$3.00 0 1989 Butterworth & Co. (Publishers)
Ltd.
covering a wide range of rank. Also an examination was made of the influence of the additional factor of oxidation of the coal on the carbon textures developed during pressure carbonization. EXPERIMENTAL Cokes used
The cokes used were the 550°C residues from high pressure dilatometer tests6 carried out at a heating rate of 40°C min- ’ on the coals listed in Table 1. This table also contains analyses
the chemical analyses and the petrographic of the coals are given in Table 2.
Microscopic examination of carbon texture
The dilatometer test residues after crushing were prepared as polished blocks by previously described procedures4. The microscopic examinations in polarized light were carried out with the polars adjusted close to extinction using a x 40 objective to give an overall magnification of x 720. The different optically identifiable features were classified as described in Table 3 into isotropic, mosaic type anisotropy of various mosaic grain sizes, granular or coarse-flow anisotropy, flow type anisotropy and inerts. A point counting technique using 300 points was used to quantify the data which are quoted in the tables to one decimal place. It is recognized that the linal figure has little significance but it is used to reduce the rounding up errors involved in making the totals add up to 100 vol ‘A. The errors involved in the analysis are calculated to vary from + 1% at the 1 ~01% level of a component to f6% at the 50 ~01% level.
FUEL, 1989, Vol 68, February
149
Influence Table 1
of pressure
on the development
Coal analytical data
Coal
wt % (dry basis) Ash C
H
VM
BS swelling no.
Sn Gg Ma SW Wa Mn Cr Cd We NW Lw
4.3 3.4 3.7 5.6 6.3 5.0 2.3 2.2 9.7 5.4 12.2
5.4 5.4 5.4 5.7 5.5 5.6 5.4 5.6 n.d. 4.9 4.6
42.1 39.1 39.3 39.8 39.6 36.6 38.0 35.9 24.6 23.6 15.1
1 1.5 1.5 6 5.5 6 7.5 8.5 6 5 1.5
Table 2
of optical anisotropy during coal carbonization:
81.7 81.1 82.5 83.7 82.8 83.7 83.8 86.7 n.d. 90.1 90.2
Wt % daf
British Coal classification _~ 902 802 702 602 602 502 502 501 301a 301a 203
Petrographic analyses
(vol %)
Exinite (vol%)
Inertinite
Coal
(vol %)
Reflectance R,max (%)
Sn Gg Ma SW Wa Mn Cr Cd We NW Lw
54 75 80 78 79 74 75 46 61 54 70
15 10 5 9 8 12 7 13 3 -
31 15 15 13 13 14 18 41 39 43 30
0.46 0.53 0.65 0.77 0.79 0.83 0.87 0.97 1.24 1.24 1.82
Vitrinite
J. W. Patrick et al.
than 0.5 %. This series of coals had carbon contents from 8 1 to 90 wt % (daf) and their volatile matter covered the range from 15 to 42 wt ‘A (daf). The maceral counts and the ash level varied widely but with no particular trends with regard to the coal rank. The results of the optical anisotropy determinations in Table 4 show that with the highest rank coals there was only a small effect attributable to the increased carbonization pressure but with the lower rank coals, increased pressure led to increased enhancement of the optical texture in terms of both the total anisotropic content and the increased grain size of the mosaic anisotropy. With two of the high rank coals, Lw and We, there was some evidence for a reduction in the total anisotropy at the higher pressures but this apparent behaviour should be treated with caution as some of the material counted as isotropic had an unusual appearance and was difficult to apportion to any one of the textural classes used. The trends are shown in Figure 1 where the total volume anisotropy on an inerts-free basis is plotted against the carbonization pressure. For each of the coals the anisotropic content reaches an apparent limiting value at around 4 MPa and with the exception of the lowest rank coal, coal Sn, this limit is in excess of 90 ~01% anisotropy. The effects of increased pressure on the type as well as the amount of anisotropy can be seen in Figure 2, which uses an optical anisotropy index derived according to the equation: OAI=I+2Mf+3Mm+4Mc+5Gf+6F+7B where, as vol ‘A on an inerts-free basis, I=isotropic;
Table 3
Classification of textural components
Component type
Appearance
Isotropic Mosaic fine medium coarse Granular/coarse flow
An optically-featureless material, often porous Composed of small rounded isochromatic areas: Mean size 0.5 pm Mean size. 0.7 pm Mean size 1.3 lrn An intermediate type between the coarse mosaic and flow types comprising rounded but elongated isochromatic areas, approximately > 2 x 1 pm Composed of elongated isochromatic areas often curved around pores. Can be subdivided into broad and striated types according to size A non-porous relatively featureless anisotropic material which does not show evidence of fusing to form softening components An isotropic material identifiable by its wood-like structure or, if small, by unfused sharp edges
Flow
Basic
Inerts
Coal oxidation
For the tests of the influence of oxidation, two coals, one a low rank, high volatile, weakly coking coke (Ma) and the other a high rank, low volatile, prime coking coal (NW) were used. They were exposed, as thin layers of ground material, to air at various temperatures up to 200°C and for periods of time up to 16 h prior to testing in the high pressure dilatometer.
m-
0
I
6y 50
RI
l
40-
30-
1
20*'a
F
10 ,A/
RESULTS AND DISCUSSIONS Variation with coal rank
The coals used covered a wide range of rank from coking steam coals with a reflectance (R,max) of about 1.8 % to high volatile non-caking coals of reflectance less
150
FUEL, 1989, Vol 68, February
OO
1
2
3
4
5
6
lest pressure, MPa Figure 1
Influence of test pressure on coke anisotropic content: cokes from various coals tested in high pressure dilatometer
influence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. T&e
4
Optical anisotropy of semicokes from high pressure dilatometer tests: 40°C min -
’ heating rate
Anisotropic composition, ~01%
Sample
Identification
Coal
Pressure, MPa
Isotropic
Fine mosaic
Sn
0.1 1 2 4 6
86.0 84.0 78.0 70.7 68.7
6.7 7.3 10.7 23.3 18.0
_ _ _ _ _
_ _ _ _ _
Gg
0.1 0.5 1 1.5 2 4 6
71.3 67.3 46.7 28.0 16.7 14.0 10.0
18.0 26.7 45.3 66.7 76.7 80.7 78.0
_
_ _ _ _ _ _ _
Wa
0.1 1 2 4 6
16.0 14.0 8.7 9.3 8.7
70.0 69.3 70.7 60.7 58.0
8.0 8.7 17.3 26.0 26.0
1.3 _ _ _
SW
0.1 1 2 4 6
12.0 10.0 11.3 8.0 11.3
72.0 45.3 38.0 34.7 16.0
8.0 37.3 44.7 54.7 70.0
_ _ _ _
Ma
0.1 0.5 1 2 4
88.7 86.0 62.3 40.0 8.7
6.0 12.0 35.0 57.7 88.3
_ _ _ _
0.1 1.5 2 4 6
12.0 7.0 9.3 6.0 4.0
64.7 43.0 46.0 26.0 24.0
16.7 45.0 38.7 60.7 68.7
Mn
0.1 2 4
10.0 8.0 11.3
44.7 22.0 14.0
Cd
0.1 1.5 2 4 6
8.7 1.3 4.7 _ _
_ _ _
We
0.1 0.5
_ -
2 4
13.3 12.7 16.0 14.0 24.7
0.1 1.0 2.5 4 6
13.3 10.7 8.7 8.0 4.7
15.3 2.0 3.3 4.0 6.0
0.1 2 4 6
;.0 2.0 12.7
3.3 2.7 7.3 8.0
Cr
1
NW
Lw
-
1.3
1.3 2.0
Medium mosaic
0.7 0.7 _ 2.0 8.7
Granular
Flow
_ _ _ _ _
Inerts 7.3 8.7 11.3 6.0 13.3 10.7 6.0 7.3 4.6 6.6 3.3 3.3 4.7 8.0 3.3 4.0 7.3
_ _ _ _ _
8.0 7.4 6.0 2.6 2.7
_ _ _ _
5.3 2.0 2.7 2.3 3.0
_ _
_ _ _ _ _
4.6 5.0 6.0 7.3 3.3
39.3 64.0 70.7
_ _
_ _ _
6.0 6.0 4.0
11.3 IO.7 7.3 10.0 7.3
76.7 76.7 80.7 83.3 84.7
_ _ _ _ _
3.3 11.3 7.3 6.7 6.7
14.7 6.0 8.0 10.0 7.3
62.0 72.7 64.7 64.7 60.7
_ _ _ _
10.0 8.6 11.3 10.0 5.3
22.7 2.7 2.7 0.7
23.3 50.0 42.0 37.3 32.0
-. 9.3 15.3 26.0 29.3
25.4 24.7” 25.3” 20.7” 24.7”
22.0 41.3 32.7 22.7
29.3 20.0 18.7 17.3
28.0” 26.0” 28.0 26.7”
2.0 7.3 2.0
2.0
’ Remainder is basic anisotropy
Mf= fine grain mosaic; Mm =medium grain mosaic; MC= coarse grain mosaic; Gf = granular flow; F = flow texture and B = basic anisotropy of the type found in high rank coals. The results clearly demonstrate that whatever the rank of bituminous coals, the nature of the coke carbon in high pressure cokes is quite strongly dependent on the pressure attained during carbonization but with the major part of any changes taking place under relatively low pressures of
1 to 2 MPa with an apparent limiting value at about 4 MPa. The extent of the textural development in relation to pressure varies with the coal rank in a fairly systematic manner but there is no direct correlation between the anisotropy and any individual rank parameter. However, it may be observed that dilatation also changes5*6 to a major extent in this pressure range with a limiting value attained at high pressure (> 4 MPa). Neither the chemical nor the petrographic analyses
FUEL, 1989, Vol 68, February
151
Jnffuence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al.
lowered softening point5*$ and third, decreased viscosity as measured by the Gieseler fluidity’. The differences attributable to the coal rank are then a consequence of the balance between these pressure effects and the nature of the coal as exemplified for example, by the different volatile matter contents and plastic properties. The limit to the effects of pressure on the textural formation has been observed previously* for pitch materials and is attributed to the condensation of the low molecular weight species and to the physical effects of the resultant change in viscosity. Differences in viscosity were also used to account for enhanced mesophase growth observed in high pressure carbonization of pitches’, and for the smaller optical textures obtained in low pressure carbonizations”.
‘*.
500-
Influence of oxidation of the coal
g
Cr ___g--_
Mn .,.,,.....v...““““’ . .. . . . ... . . v . . .. . .
F i
.. .. ...* 4“
____z___‘p-_c-c
0
.*
/
SW Wa
o-
;__.-e-
H “y; 8 0
Y
0
,,/
*
The results of the optical anisotropy determinations of the dilatometer residues formed from the oxidized coals at 40°C min - ’ heating rate and 4 MPa pressure are given in Tables 5 and 6. The anisotropic development was systematically and progressively reduced with both increased time of oxidation and with increased temperature of oxidation. Not surprisingly, the effect is most marked and is particularly rapid with the more
sn *-
*-*-*
100
--.-----._
p---.-*+._ Vacuum
I
OO
2 4 3 Test .pressure , MPa
1
5
6
heated
175°C
60
Figure 2
Influence of test pressure on anisotropic composition of cokes from a range of coals: 40°C min- ’
appear to be of any great value to predict with any degree of accuracy the coke carbon texture attained during pressure carbonization. The process of the development of optical anisotropy in cokes requires two basic conditions to be met, namely, the presence of planar molecules of appropriate size and the existence of conditions that will ensure the mobility necessary for the ordering of the lamellae. The type of optically anisotropic species developed and the extent of their growth will thus depend on the chemical nature of the species present at the onset of anisotropic developments (and hence on the parent vitrain), and on the maintenance of the appropriate conditions within the carbonizing mass. For materials other than coal, carbonization under high pressures has been reported’*s to augment the size of the optical textures in the resultant cokes because of the retardation or even complete hindrance of volatilization of low molecular weight species that form a lower viscosity medium favouring the enhancement of the optically anisotropic texture. There is little doubt that the effects of the influence of pressure in the carbonization of coals can be explained in similar terms of first, increased retention within the plastic mass of low molecular weight species which can undergo further condensation reactions; second, increased plastic range resulting from a
152
FUEL,
1989,
Vol 68,
February
.-.-.a.-.-.-.-.
0
Vacuum
heated % 1 5°C
20 Oxidation
i
time - h
Figure 3 Influence of coal oxidation on total optical anisotropy of dilatometer semicokes (heating rate 40°C min-‘, pressure 4 MPa)
Influence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. Table 5
Effect of oxidation of coal NW on optical anisotropy of high pressure dilatometer semicoke: 40°C min - ’ heating rate, 40 atmospheres pressure Anisotropic composition, ~01%
Oxidation temp. “C/ time, h
Isotropic
Fine mosaic
Coarse mosaic
Medium mosaic
Granular flow
Flow
Basic 3.3
Inerts
4.0 2.7 5.3 2.0
0.7 1.3 3.3 4.1
_ _ _ 1.3
37.3 32.0 36.7 41.4
26.0 23.3 14.0 11.3
/:6
8.0 14.7 26.7
2.0 4.7 32.0
4.0 9.3 6.0
_ _ _
48.0 42.7 12.7
14.7 7.3 _
175/l I5 I16
11.3 18.0 42.0
3.3 16.0 16.7
10.0 6.7 5.3
_ _ _
40.7 29.3 11.3
8.7 4.0 2.0
200/l
18.7 36.7 63.4
22.7 19.3 14.0
10.7 7.3 1.3
_ _
16.3 13.3 3.3
10.3 4.7 _
_ _ _
21.3 18.7 18.0
0.1
2.7 2.7 3.3 2.7
_ _ _
37.3 42.7 42.0 42.0
26.0 23.3 20.7 24.0
5.3 1.3 2.0 0.6
26.0 24.7 26.0 27.3
Unoxidized 110/l ;:6 150/I
;:6
8.0
12.7 15.3 19.3
Unoxidized vacuum heated 11515 2.0 2.0 110 4.0 I16 2.1 122
Table6
3.3 2.0 0.7
Effect of oxidation ofcoal Ma on optical anisotropy of high pressure dilatometer semicoke: 40°C min-
_ 2.7 2.0
20.7 22.0 25.4 20.0 20.6 19.3 22.6 26.0 26.0 22.7
’ heating rate, 40 atmospheres pressure
Anisotropic composition, ~01% Oxidation temp., “C/ time, h Unoxidized
Isotropic 9.3
Fine mosaic
Medium mosaic
81.4
1.3
Coarse mosaic
Granular flow
Flow
Inerts
3.3
_
4.7
-
0.7 _ _
110/l
11.3
78.7
;:6
93.3 12.6
22.0 4.1
_ 0.7
__
_
_ _ _
150/l
80.0
13.3
;;6
88.0 82.7
11.3 3.3
_
92.0
5.3
_
2.0 2.7
_
200/l
92.7 89.3 ;:6 Unoxidized vacuum heated 12.0 17515 14.7 r12 10.7 120
80.0 77.3 81.3
0.7 _ _
reactive low rank coal, Ma (Table 6). The results obtained with the vacuum heat treatment of the unoxidized coal show that the effect is entirely attributable to the oxidation and not the heat treatment. The coke optical anisotropy was not significantly affected by vacuum heating of the coal to 175°C over long periods of time. Figures 3 and 4 respectively show the influence of oxidation time and temperature on the volume percentage anisotropic content on an inerts-free basis and on the nature of the anisotropy as represented by the anisotropy index. The effects of oxidation on the plastic properties of coals are well documented and these effects are clearly associated with the reduced anisotropic development. However, although the practical effects of oxidation are well established, the details of the mechanism of oxidation
9.3
2.0 _ 0.7 _ _ 0.7 _
_ _
_
2.0 4.7
_
4.7
_
8.0 6.0
_
2.7
_
4.7 8.0
_
7.3 8.0 6.0
2.0
are not completely resolved. Most hypotheses propose the formation of oxygen cross linkages of the ether type as the cause of the reduced plasticity, and these are often assumed to develop during carbonization from the oxygen functional groups postulated to be formed at the coal surface in the initial stages of oxidation. The cross linking causes two interacting effects. It limits the extent of the thermal depolymerization of the coal so diminishing the formation of the appropriately sized lamellar species required for the anisotropic development and this in turn reduces the fluidity or plasticity of the system. Hence both the basic conditions for the development of optically anisotropic species are effected in such a way as to retard or prevent the development of the optical anisotropy.
FUEL,
1989,
Vol 68, February
153
fnfiuence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. WV”
I
Coal
These effects hinder the three dimensional development that is associated with the optical texture and the effects of duration. The temperature of oxidation on the anisotropic textural development and the increased effects noted with the lower rank coal are in general accord with the numerous previous studies of the influence of oxidation on coking properties. As regards the relevance of these results to coal gasification at high pressure, it must be borne in mind that the conditions in the high pressure dilatometer cannot completely simulate the conditions at the top of a commercial gasification unit. Nevertheless, the results provide some insight into the influence of the effects of high pressure conditions likely to be encountered, on the nature of the coke carbon produced from coals of different rank.
NW Vacuum *T)-._._.*._._.+-
AL._
./*-
heated
175%
110°C
150% 200°C
ACKNOWLEDGEMENTS :
The authors would like to thank British Gas for permission to publish this paper and John W. Patrick and Alan Walker gratefully acknowledge the financial support provided by a research contract from British Gas.
0
0
Coal
E
Ma Vacuum
heated
175%
REFERENCES 1 2
Coin, C. D. A. Fuel 1987,66,702 Patrick, J. W. in ‘Coal: Phoenix of the ‘~OS’,Proc. 64th CIC Coal Symposium (Ed. A. M. AI Taweel), Can. Sot. Chem. Eng., Ottowa, 1982, p. 550 Patrick, J. W., Reynolds, M. J. and Shaw, F. H. Carbon 1975,13, 509
1
0
I
I
I
I
I
4
8
12
16
20
Oxidation
;
time - h
Figure 4
Influence of coal oxidation on anisotropic composition of dilatometer semicokes (heating rate 40°C min-‘, pressure 4 MPa)
154
FUEL, 1989, Vol 68, February
6 I 8 9
10
Green, P. D., Patrick, J. W.,Thomas, K. M. and Walker, A. Fuel 1985,64, 1431 Thomas, K. M. in ‘Carbon and Coal Gasification’ @Is. J. L. Figueiredo and J. A. Moulijn), Martinus Nijhoff, Dorcrecht, 1086, p. 421 Green, P. D. and Thomas, K. M. Fuel 1985,&l, 1423 Sanada, Y., Furuta, T., Kumai, J. and Kimura, H. .I. Jap. Pet. Inst. 1975, 18, 113 Huttinger, K. J. and Rosenblatt, U. Carbon 1977,15,69 Kaiho, M. and Toda, Y. Fuel 1979,58,397 Makabe, M., Itoh, H. and Ouchi, K. Carbon 1976, 14,365
Peter
D. Green*,
K. Mark
Thomas*
during
and Alan Walker
Carbon Research Group, Department of Chemical Engineering, University of Technology, Loughborough, Leicestershire, LEl 1 3TU UK * British Gas plc, London Research Station, Michael Road, London, SW6 2AD, UK (Received 2 June 1988; revised 22 July 1988)
Semicoke residues from high pressure dilatometer tests on a series of bituminous coals, were examined by polarized light microscopy to determine the effects of pressure on the optical anisotropy developed during the plastic stage of the carbonization process. Similar examinations were also carried out with high and low rank coals which had been subjected to different levels of oxidation to determine the additional effects of this factor. The major part of the influence of pressure carbonization on the coke anisotropy takes place at pressures up to 2 MPa, the influence being dependent on the rank of the parent coal. The total volume anisotropy is increased significantly with coke from some low rank coals but the changes in the type or nature of the anisotropy are generally less well marked. In accord with the effects of oxidation on plasticity, oxidation progressively retards the anisotropic development more so with the cokes from lower rank coals, to such an extent that carbonization at pressures up to 4 MPa cannot overcome this effect, the cokes being predominantly isotropic. (Keywords:
anisotropy;
coke; bituminous
coal)
The optical anisotropy of cokes produced from coals has been the subject of considerable study over the years, and the classification of the anisotropy into several types based on their size, shape and appearance has provided a means of characterization of the coke--carbon texture’. However, this coke texture, which is coal rank dependent and has to be taken into account in considerations of coke reactivity and strength2, develops during carbonization at different rates dependent on the carbonization conditions3. Some of the effect of carbonization conditions such as rate of heating and soaking time have this emphasis reflecting the been well documented3, interests of the coking industry. Factors such as gas overpressure have received less
attention until recently4 when it has been shown that both the extent and the composition of the optical anisotropy are strongly pressure dependent. The motivation for studies on the influence of pressure during coal carbonization is to be found in the developing interest in coal gasification. Commercial considerations demand the use of high pressures in large scale gasifiers. In the gasification of coal in fixed bed gasifiers the counter current flow of gases and solids leads to coal in the upper levels of the gasifier being carbonized by the sensible heat of the hot gases produced in the gasification zone. The properties of this high pressure coke are of considerable significance in determining the optimum conditions for smooth and efficient operation of the fixed-bed gasifier5. Hence, as part of a programme of work to examine the relevant coke behaviour, a detailed study was made of the effects on the carbon texture of carbonization under various pressures using a series of bituminous coals 0016-2361/89/020149~6$3.00 0 1989 Butterworth & Co. (Publishers)
Ltd.
covering a wide range of rank. Also an examination was made of the influence of the additional factor of oxidation of the coal on the carbon textures developed during pressure carbonization. EXPERIMENTAL Cokes used
The cokes used were the 550°C residues from high pressure dilatometer tests6 carried out at a heating rate of 40°C min- ’ on the coals listed in Table 1. This table also contains analyses
the chemical analyses and the petrographic of the coals are given in Table 2.
Microscopic examination of carbon texture
The dilatometer test residues after crushing were prepared as polished blocks by previously described procedures4. The microscopic examinations in polarized light were carried out with the polars adjusted close to extinction using a x 40 objective to give an overall magnification of x 720. The different optically identifiable features were classified as described in Table 3 into isotropic, mosaic type anisotropy of various mosaic grain sizes, granular or coarse-flow anisotropy, flow type anisotropy and inerts. A point counting technique using 300 points was used to quantify the data which are quoted in the tables to one decimal place. It is recognized that the linal figure has little significance but it is used to reduce the rounding up errors involved in making the totals add up to 100 vol ‘A. The errors involved in the analysis are calculated to vary from + 1% at the 1 ~01% level of a component to f6% at the 50 ~01% level.
FUEL, 1989, Vol 68, February
149
Influence Table 1
of pressure
on the development
Coal analytical data
Coal
wt % (dry basis) Ash C
H
VM
BS swelling no.
Sn Gg Ma SW Wa Mn Cr Cd We NW Lw
4.3 3.4 3.7 5.6 6.3 5.0 2.3 2.2 9.7 5.4 12.2
5.4 5.4 5.4 5.7 5.5 5.6 5.4 5.6 n.d. 4.9 4.6
42.1 39.1 39.3 39.8 39.6 36.6 38.0 35.9 24.6 23.6 15.1
1 1.5 1.5 6 5.5 6 7.5 8.5 6 5 1.5
Table 2
of optical anisotropy during coal carbonization:
81.7 81.1 82.5 83.7 82.8 83.7 83.8 86.7 n.d. 90.1 90.2
Wt % daf
British Coal classification _~ 902 802 702 602 602 502 502 501 301a 301a 203
Petrographic analyses
(vol %)
Exinite (vol%)
Inertinite
Coal
(vol %)
Reflectance R,max (%)
Sn Gg Ma SW Wa Mn Cr Cd We NW Lw
54 75 80 78 79 74 75 46 61 54 70
15 10 5 9 8 12 7 13 3 -
31 15 15 13 13 14 18 41 39 43 30
0.46 0.53 0.65 0.77 0.79 0.83 0.87 0.97 1.24 1.24 1.82
Vitrinite
J. W. Patrick et al.
than 0.5 %. This series of coals had carbon contents from 8 1 to 90 wt % (daf) and their volatile matter covered the range from 15 to 42 wt ‘A (daf). The maceral counts and the ash level varied widely but with no particular trends with regard to the coal rank. The results of the optical anisotropy determinations in Table 4 show that with the highest rank coals there was only a small effect attributable to the increased carbonization pressure but with the lower rank coals, increased pressure led to increased enhancement of the optical texture in terms of both the total anisotropic content and the increased grain size of the mosaic anisotropy. With two of the high rank coals, Lw and We, there was some evidence for a reduction in the total anisotropy at the higher pressures but this apparent behaviour should be treated with caution as some of the material counted as isotropic had an unusual appearance and was difficult to apportion to any one of the textural classes used. The trends are shown in Figure 1 where the total volume anisotropy on an inerts-free basis is plotted against the carbonization pressure. For each of the coals the anisotropic content reaches an apparent limiting value at around 4 MPa and with the exception of the lowest rank coal, coal Sn, this limit is in excess of 90 ~01% anisotropy. The effects of increased pressure on the type as well as the amount of anisotropy can be seen in Figure 2, which uses an optical anisotropy index derived according to the equation: OAI=I+2Mf+3Mm+4Mc+5Gf+6F+7B where, as vol ‘A on an inerts-free basis, I=isotropic;
Table 3
Classification of textural components
Component type
Appearance
Isotropic Mosaic fine medium coarse Granular/coarse flow
An optically-featureless material, often porous Composed of small rounded isochromatic areas: Mean size 0.5 pm Mean size. 0.7 pm Mean size 1.3 lrn An intermediate type between the coarse mosaic and flow types comprising rounded but elongated isochromatic areas, approximately > 2 x 1 pm Composed of elongated isochromatic areas often curved around pores. Can be subdivided into broad and striated types according to size A non-porous relatively featureless anisotropic material which does not show evidence of fusing to form softening components An isotropic material identifiable by its wood-like structure or, if small, by unfused sharp edges
Flow
Basic
Inerts
Coal oxidation
For the tests of the influence of oxidation, two coals, one a low rank, high volatile, weakly coking coke (Ma) and the other a high rank, low volatile, prime coking coal (NW) were used. They were exposed, as thin layers of ground material, to air at various temperatures up to 200°C and for periods of time up to 16 h prior to testing in the high pressure dilatometer.
m-
0
I
6y 50
RI
l
40-
30-
1
20*'a
F
10 ,A/
RESULTS AND DISCUSSIONS Variation with coal rank
The coals used covered a wide range of rank from coking steam coals with a reflectance (R,max) of about 1.8 % to high volatile non-caking coals of reflectance less
150
FUEL, 1989, Vol 68, February
OO
1
2
3
4
5
6
lest pressure, MPa Figure 1
Influence of test pressure on coke anisotropic content: cokes from various coals tested in high pressure dilatometer
influence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. T&e
4
Optical anisotropy of semicokes from high pressure dilatometer tests: 40°C min -
’ heating rate
Anisotropic composition, ~01%
Sample
Identification
Coal
Pressure, MPa
Isotropic
Fine mosaic
Sn
0.1 1 2 4 6
86.0 84.0 78.0 70.7 68.7
6.7 7.3 10.7 23.3 18.0
_ _ _ _ _
_ _ _ _ _
Gg
0.1 0.5 1 1.5 2 4 6
71.3 67.3 46.7 28.0 16.7 14.0 10.0
18.0 26.7 45.3 66.7 76.7 80.7 78.0
_
_ _ _ _ _ _ _
Wa
0.1 1 2 4 6
16.0 14.0 8.7 9.3 8.7
70.0 69.3 70.7 60.7 58.0
8.0 8.7 17.3 26.0 26.0
1.3 _ _ _
SW
0.1 1 2 4 6
12.0 10.0 11.3 8.0 11.3
72.0 45.3 38.0 34.7 16.0
8.0 37.3 44.7 54.7 70.0
_ _ _ _
Ma
0.1 0.5 1 2 4
88.7 86.0 62.3 40.0 8.7
6.0 12.0 35.0 57.7 88.3
_ _ _ _
0.1 1.5 2 4 6
12.0 7.0 9.3 6.0 4.0
64.7 43.0 46.0 26.0 24.0
16.7 45.0 38.7 60.7 68.7
Mn
0.1 2 4
10.0 8.0 11.3
44.7 22.0 14.0
Cd
0.1 1.5 2 4 6
8.7 1.3 4.7 _ _
_ _ _
We
0.1 0.5
_ -
2 4
13.3 12.7 16.0 14.0 24.7
0.1 1.0 2.5 4 6
13.3 10.7 8.7 8.0 4.7
15.3 2.0 3.3 4.0 6.0
0.1 2 4 6
;.0 2.0 12.7
3.3 2.7 7.3 8.0
Cr
1
NW
Lw
-
1.3
1.3 2.0
Medium mosaic
0.7 0.7 _ 2.0 8.7
Granular
Flow
_ _ _ _ _
Inerts 7.3 8.7 11.3 6.0 13.3 10.7 6.0 7.3 4.6 6.6 3.3 3.3 4.7 8.0 3.3 4.0 7.3
_ _ _ _ _
8.0 7.4 6.0 2.6 2.7
_ _ _ _
5.3 2.0 2.7 2.3 3.0
_ _
_ _ _ _ _
4.6 5.0 6.0 7.3 3.3
39.3 64.0 70.7
_ _
_ _ _
6.0 6.0 4.0
11.3 IO.7 7.3 10.0 7.3
76.7 76.7 80.7 83.3 84.7
_ _ _ _ _
3.3 11.3 7.3 6.7 6.7
14.7 6.0 8.0 10.0 7.3
62.0 72.7 64.7 64.7 60.7
_ _ _ _
10.0 8.6 11.3 10.0 5.3
22.7 2.7 2.7 0.7
23.3 50.0 42.0 37.3 32.0
-. 9.3 15.3 26.0 29.3
25.4 24.7” 25.3” 20.7” 24.7”
22.0 41.3 32.7 22.7
29.3 20.0 18.7 17.3
28.0” 26.0” 28.0 26.7”
2.0 7.3 2.0
2.0
’ Remainder is basic anisotropy
Mf= fine grain mosaic; Mm =medium grain mosaic; MC= coarse grain mosaic; Gf = granular flow; F = flow texture and B = basic anisotropy of the type found in high rank coals. The results clearly demonstrate that whatever the rank of bituminous coals, the nature of the coke carbon in high pressure cokes is quite strongly dependent on the pressure attained during carbonization but with the major part of any changes taking place under relatively low pressures of
1 to 2 MPa with an apparent limiting value at about 4 MPa. The extent of the textural development in relation to pressure varies with the coal rank in a fairly systematic manner but there is no direct correlation between the anisotropy and any individual rank parameter. However, it may be observed that dilatation also changes5*6 to a major extent in this pressure range with a limiting value attained at high pressure (> 4 MPa). Neither the chemical nor the petrographic analyses
FUEL, 1989, Vol 68, February
151
Jnffuence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al.
lowered softening point5*$ and third, decreased viscosity as measured by the Gieseler fluidity’. The differences attributable to the coal rank are then a consequence of the balance between these pressure effects and the nature of the coal as exemplified for example, by the different volatile matter contents and plastic properties. The limit to the effects of pressure on the textural formation has been observed previously* for pitch materials and is attributed to the condensation of the low molecular weight species and to the physical effects of the resultant change in viscosity. Differences in viscosity were also used to account for enhanced mesophase growth observed in high pressure carbonization of pitches’, and for the smaller optical textures obtained in low pressure carbonizations”.
‘*.
500-
Influence of oxidation of the coal
g
Cr ___g--_
Mn .,.,,.....v...““““’ . .. . . . ... . . v . . .. . .
F i
.. .. ...* 4“
____z___‘p-_c-c
0
.*
/
SW Wa
o-
;__.-e-
H “y; 8 0
Y
0
,,/
*
The results of the optical anisotropy determinations of the dilatometer residues formed from the oxidized coals at 40°C min - ’ heating rate and 4 MPa pressure are given in Tables 5 and 6. The anisotropic development was systematically and progressively reduced with both increased time of oxidation and with increased temperature of oxidation. Not surprisingly, the effect is most marked and is particularly rapid with the more
sn *-
*-*-*
100
--.-----._
p---.-*+._ Vacuum
I
OO
2 4 3 Test .pressure , MPa
1
5
6
heated
175°C
60
Figure 2
Influence of test pressure on anisotropic composition of cokes from a range of coals: 40°C min- ’
appear to be of any great value to predict with any degree of accuracy the coke carbon texture attained during pressure carbonization. The process of the development of optical anisotropy in cokes requires two basic conditions to be met, namely, the presence of planar molecules of appropriate size and the existence of conditions that will ensure the mobility necessary for the ordering of the lamellae. The type of optically anisotropic species developed and the extent of their growth will thus depend on the chemical nature of the species present at the onset of anisotropic developments (and hence on the parent vitrain), and on the maintenance of the appropriate conditions within the carbonizing mass. For materials other than coal, carbonization under high pressures has been reported’*s to augment the size of the optical textures in the resultant cokes because of the retardation or even complete hindrance of volatilization of low molecular weight species that form a lower viscosity medium favouring the enhancement of the optically anisotropic texture. There is little doubt that the effects of the influence of pressure in the carbonization of coals can be explained in similar terms of first, increased retention within the plastic mass of low molecular weight species which can undergo further condensation reactions; second, increased plastic range resulting from a
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FUEL,
1989,
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.-.-.a.-.-.-.-.
0
Vacuum
heated % 1 5°C
20 Oxidation
i
time - h
Figure 3 Influence of coal oxidation on total optical anisotropy of dilatometer semicokes (heating rate 40°C min-‘, pressure 4 MPa)
Influence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. Table 5
Effect of oxidation of coal NW on optical anisotropy of high pressure dilatometer semicoke: 40°C min - ’ heating rate, 40 atmospheres pressure Anisotropic composition, ~01%
Oxidation temp. “C/ time, h
Isotropic
Fine mosaic
Coarse mosaic
Medium mosaic
Granular flow
Flow
Basic 3.3
Inerts
4.0 2.7 5.3 2.0
0.7 1.3 3.3 4.1
_ _ _ 1.3
37.3 32.0 36.7 41.4
26.0 23.3 14.0 11.3
/:6
8.0 14.7 26.7
2.0 4.7 32.0
4.0 9.3 6.0
_ _ _
48.0 42.7 12.7
14.7 7.3 _
175/l I5 I16
11.3 18.0 42.0
3.3 16.0 16.7
10.0 6.7 5.3
_ _ _
40.7 29.3 11.3
8.7 4.0 2.0
200/l
18.7 36.7 63.4
22.7 19.3 14.0
10.7 7.3 1.3
_ _
16.3 13.3 3.3
10.3 4.7 _
_ _ _
21.3 18.7 18.0
0.1
2.7 2.7 3.3 2.7
_ _ _
37.3 42.7 42.0 42.0
26.0 23.3 20.7 24.0
5.3 1.3 2.0 0.6
26.0 24.7 26.0 27.3
Unoxidized 110/l ;:6 150/I
;:6
8.0
12.7 15.3 19.3
Unoxidized vacuum heated 11515 2.0 2.0 110 4.0 I16 2.1 122
Table6
3.3 2.0 0.7
Effect of oxidation ofcoal Ma on optical anisotropy of high pressure dilatometer semicoke: 40°C min-
_ 2.7 2.0
20.7 22.0 25.4 20.0 20.6 19.3 22.6 26.0 26.0 22.7
’ heating rate, 40 atmospheres pressure
Anisotropic composition, ~01% Oxidation temp., “C/ time, h Unoxidized
Isotropic 9.3
Fine mosaic
Medium mosaic
81.4
1.3
Coarse mosaic
Granular flow
Flow
Inerts
3.3
_
4.7
-
0.7 _ _
110/l
11.3
78.7
;:6
93.3 12.6
22.0 4.1
_ 0.7
__
_
_ _ _
150/l
80.0
13.3
;;6
88.0 82.7
11.3 3.3
_
92.0
5.3
_
2.0 2.7
_
200/l
92.7 89.3 ;:6 Unoxidized vacuum heated 12.0 17515 14.7 r12 10.7 120
80.0 77.3 81.3
0.7 _ _
reactive low rank coal, Ma (Table 6). The results obtained with the vacuum heat treatment of the unoxidized coal show that the effect is entirely attributable to the oxidation and not the heat treatment. The coke optical anisotropy was not significantly affected by vacuum heating of the coal to 175°C over long periods of time. Figures 3 and 4 respectively show the influence of oxidation time and temperature on the volume percentage anisotropic content on an inerts-free basis and on the nature of the anisotropy as represented by the anisotropy index. The effects of oxidation on the plastic properties of coals are well documented and these effects are clearly associated with the reduced anisotropic development. However, although the practical effects of oxidation are well established, the details of the mechanism of oxidation
9.3
2.0 _ 0.7 _ _ 0.7 _
_ _
_
2.0 4.7
_
4.7
_
8.0 6.0
_
2.7
_
4.7 8.0
_
7.3 8.0 6.0
2.0
are not completely resolved. Most hypotheses propose the formation of oxygen cross linkages of the ether type as the cause of the reduced plasticity, and these are often assumed to develop during carbonization from the oxygen functional groups postulated to be formed at the coal surface in the initial stages of oxidation. The cross linking causes two interacting effects. It limits the extent of the thermal depolymerization of the coal so diminishing the formation of the appropriately sized lamellar species required for the anisotropic development and this in turn reduces the fluidity or plasticity of the system. Hence both the basic conditions for the development of optically anisotropic species are effected in such a way as to retard or prevent the development of the optical anisotropy.
FUEL,
1989,
Vol 68, February
153
fnfiuence of pressure on the development of optical anisotropy during coal carbonization: J. W. Patrick et al. WV”
I
Coal
These effects hinder the three dimensional development that is associated with the optical texture and the effects of duration. The temperature of oxidation on the anisotropic textural development and the increased effects noted with the lower rank coal are in general accord with the numerous previous studies of the influence of oxidation on coking properties. As regards the relevance of these results to coal gasification at high pressure, it must be borne in mind that the conditions in the high pressure dilatometer cannot completely simulate the conditions at the top of a commercial gasification unit. Nevertheless, the results provide some insight into the influence of the effects of high pressure conditions likely to be encountered, on the nature of the coke carbon produced from coals of different rank.
NW Vacuum *T)-._._.*._._.+-
AL._
./*-
heated
175%
110°C
150% 200°C
ACKNOWLEDGEMENTS :
The authors would like to thank British Gas for permission to publish this paper and John W. Patrick and Alan Walker gratefully acknowledge the financial support provided by a research contract from British Gas.
0
0
Coal
E
Ma Vacuum
heated
175%
REFERENCES 1 2
Coin, C. D. A. Fuel 1987,66,702 Patrick, J. W. in ‘Coal: Phoenix of the ‘~OS’,Proc. 64th CIC Coal Symposium (Ed. A. M. AI Taweel), Can. Sot. Chem. Eng., Ottowa, 1982, p. 550 Patrick, J. W., Reynolds, M. J. and Shaw, F. H. Carbon 1975,13, 509
1
0
I
I
I
I
I
4
8
12
16
20
Oxidation
;
time - h
Figure 4
Influence of coal oxidation on anisotropic composition of dilatometer semicokes (heating rate 40°C min-‘, pressure 4 MPa)
154
FUEL, 1989, Vol 68, February
6 I 8 9
10
Green, P. D., Patrick, J. W.,Thomas, K. M. and Walker, A. Fuel 1985,64, 1431 Thomas, K. M. in ‘Carbon and Coal Gasification’ @Is. J. L. Figueiredo and J. A. Moulijn), Martinus Nijhoff, Dorcrecht, 1086, p. 421 Green, P. D. and Thomas, K. M. Fuel 1985,&l, 1423 Sanada, Y., Furuta, T., Kumai, J. and Kimura, H. .I. Jap. Pet. Inst. 1975, 18, 113 Huttinger, K. J. and Rosenblatt, U. Carbon 1977,15,69 Kaiho, M. and Toda, Y. Fuel 1979,58,397 Makabe, M., Itoh, H. and Ouchi, K. Carbon 1976, 14,365