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Retention of botanical structure in anthracitic vitrinites carbonized at high temperatures
Retention of botanical structure in anthracitic vitrinites carbonized at high temperatures Fariborz
Goodarzi
and Duncan
G. Murchison”
Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 330333rd Street N. W., Calgary, Alberta, TZL 2AY, Canada * Organic Geochemistry Unit, Department of Geology, University of Newcastle, Newcastle-uponTyne, UK (Received 18 May 1987; revised 19 November 1987) Polished surfaces of anthracitic vitrinites, when viewed by reflected light using oil immersion lenses and crossed polars, frequently display remnant botanical structure. Such stucture may be retained, often with enhanced contrast between cell walls and cell fillings,even when the vitrinite is carbonized at temperatures as high as 2400°C.Although increases in aromatic-layer diameters and stack heights have occurred, albeit in the solid state, there is no apparent distortion of the cell structures. Reference is made to similar phenomena in vitrinites from other environments, viz. low-grade metamorphism and carbonization after laboratory or
natural oxidation processes. (Keywords: anthracite; vitrinite; botanical structure)
Anthracites, when viewed under appropriate optical conditions, may display a variety of botanical structures, similar to those seen in lower-rank coals. Early methods of revealing such structures predominantly relied on either flame-etching a polished surface of anthracite, or etching the polished surface with an oxidizing agent, e.g. hot chrome-sulphuric acid, a treatment that has been widely employed in petrographic studies in attempts to reveal the origin and composition of structureless macerals. The advent of microscope systems with minimal internal reflections from optical surfaces and fitted with high-intensity light sources has allowed the use of crossed or partially crossed polars with oil-immersion objectives to demonstrate with great clarity the petrographic structures both in high-rank coals and in cokes. Thus, Cook et al.’ illustrated crushed wood structure in Dadoxylon hendriksii in a meta-anthracitic vitrinite of Devonian age. Goodarzi and Murchison’ showed that after severe laboratory pre-oxidation of vitrinites, which inhibited any softening of the vitrinites during later carbonization, plant-cell structures were retained at temperatures in excess of 600°C. The same authors3 also demonstrated that plant-cell structures could be preserved in vitrinites of coking-coal rank when carbonized at temperatures as high as 600°C even when a mosaic structure had been generated. It seemed possible that these particular vitrinites had suffered some oxidation at the time of their deposition, and might well have been regarded as pseudo-vitrinites4 prior to carbonization. Jones and Creaney’ illustrated similar effects in naturally metamorphosed coals from northern England where the reflectances of the affected vitrinites ranged as high as 9% and mosaics had also developed. Goodarzi and Marsh6 found that cell structures were retained and mosaics developed in vitrinites carbonized under pressures of = 200 MPa, after the vitrinite had been pre-oxidized in the laboratory. Under certain conditions, macerals can thus be resistant to coalification or 1X316-2361/88/060831-03E3.00 0 1988 Butterworth & Co. (Publishers) Ltd.
carbonization processes which would have been expected to be sufficiently severe to destroy all traces of original botanical structure of the macerals. This paper relates further observations on the details of botanical structures retained in anthracite samples that were carbonized at temperatures up to 2400°C. The structures were observed fortuitously in a broader study of the optical properties of a range of coals that had been subjected to different heat treatments. EXPERIMENTAL Heat treatment
A concentrate (> 95 %) of vitrinite of anthracitic rank (carbon (dmmf)=94.2 %: R,i, = 4.10% at 546 nm) was crushed to pass BSS 72 mesh (> 210 pm) and stored in nitrogen at ambient temperature for the period of the pyrolysis experiments. Samples of this vitrinite, contained in silica boats, were carbonized in nitrogen using rectilinear heating rates of 1, 10 and 60°C min-’ up to a final temperature of 95O”C, which was held for 1 h controlled to +2”C, before cooling. Heat treatment to temperatures higher than 1000°C was carried out by reheating the samples, which had been pre-heated to 950°C in a graphite furnace using an argon atmosphere7. The samples were carbonized to several temperature levels within the range loOO-24OO”C, using a heating rate of 80°C min-’ and a ‘soak period’ of 10 min before cooling. Microscopical
preparation
and examination
The small amounts of sample remaining after each carbonization were mounted in plastic blocks and ground and polished by standard methods, Since high-rank typically show no anthracites, when carbonized, detectable changes in optical properties until the temperature has exceeded at least 6OO”C, only samples carbonized above this temperature level were examined. All photomicrographs were taken with Zeiss ‘Antiflex’ oil-immersion objectives (x 16/0.40 and x 40/0.65) in 1.
FUEL, 1988, Vol 67, June
831
Retention
of botanical
structure in anthracitic
vitrinites:
plane-polarized light; and 2. using an analyser and a polarizer in the optical train with their vibration directions slightly uncrossed to allow rather greater reflection from the surfaces to the camera. RESULTS At all temperature levels sampled up to 2400°C telinitic cell structures were found in the residues of the vitrinite. A representative carbonized anthracitic selection of photomicrographs is given in Figures lug. Only photomicrographs of the carbonized vitrinites under partially crossed polars are shown, since little structure of any kind could be observed in planepolarized light. The photographs were taken with the particles rotated into their highest contrast position in relation to the vibration directions of the polars. The carbonized particles displayed a high but complicated anisotropy, since the degree of contrast between the fillings of the cell cavities and the substance of the cell walls could be radically altered by rotating the microscope stage. Occasionally strain lines could be seen in particular orientations. DISCUSSION Figure 1 illustrates that some particles of the high-rank anthracitic vitrinite, even when exposed to temperatures as high as 24OO”C, suffer little obvious morphological change as a result of severe heat treatment. There is no indication, for example, through rounding of the particle of any softening having occurred or of margins, mechanical disruption, even when the relatively high heating rate of 60°C min-* was employed. Plant-cell structures are undistorted, and appear to be similar to the cell structures in vitrinites that can be more readily observed in lower-rank coals on polished surfaces in reflected light or in thin sections in transmitted light. It is possible that this telinitic anthracitic vitrinite may have originally been a pseudovitrinite and hence less reactive than other associated vitrinites4. and apparent stability of cell The retention morphology might suggest that the anthracitic vitrinite did not undergo pronounced molecular change during high-temperature carbonization. Reflectance, however, shows an increasing, if irregular, rise to 24Oo”C*, well beyond the maximum oil reflectance of ~8.5 Y0 quoted by Goodarzi and Murchison’ for carbonized vitrinites at 95O”C, eventually reaching z 11.0 7;‘. Likewise, the oil bireflectance has risen markedly from its original level to 2 10.0 (Ref. 8), so on the basis of the behaviour of these parameters increased molecular ordering has occurred. X-ray diffraction studies of vitrinites”,” of widely varying rank have shown that parameters that estimate the aromatic-layer diameters and stack heights of carbonized products both show continuing rise with temperature beyond a temperature level of z 95O”C, representing gradually increasing molecular structural ordering. In the present case, the 002 diffraction band shown by the fresh anthracitic vitrinite had sharpened considerably at a temperature of 2400°C and 101 and 112 diffraction bands had made their appearance. Although a complete graphite diffraction pattern had certainly not developed, greater structural ordering has occurred. Molecular changes in vitrinites and other organic
832
FUEL, 1988, Vol 67, June
F. Goodarzi and 0. G. Murchison constituents of coals carbonized beyond a temperature of z 650°C take place in the solid state. What is surprising in the present circumstances is that, despite optical data and independent evidence from X-ray studies of rapid development in the growth and ordering of the aromatic structures of the carbonized vitrinite, principally through pyrolytic dehydrogenation, there is seemingly no disruption of the plant-cell ‘macro-structure’. Some changes might reasonably be expected with temperature rise, particularly where the heat input and level have begun to overcome the resistance of the anthracitic molecular structure to the graphitization process at = > 2000°C”. At the highest temperature employed (24Oo”C), the carbonized residues are soft to the touch and do resemble graphite in appearance. The contrast between the cell-cavity infillings and the cell-wall substances must represent an early compositional difference of the tissues, which was there before or introduced during peat deposition. Retention of this compositional difference in material carbonized at 2400°C can hardly be envisaged, yet the optical contrast is maintained, indicating a molecular-structural contrast. Jones and Creaney’ have drawn attention to the same effect in naturally metamorphosed coals. Several examples were given in the introduction to this paper of the retention of botanical structure in high-rank vitrinites or in vitrinites that had been oxidized prior to carbonization. The processes of vitrinite coalification to anthracitic level and pre-oxidation, either laboratory or natural, of a low-rank vitrinite differ, but they do have some similarity in that both can lead to a product which will not soften on carbonization, even at high temperatures. An anthracitic vitrinite of the normal coalification series has a structure essentially formed of large polycyclic aromatic molecules, the elements of which are sufficiently strongly cross-linked to prevent little or no disruption until high temperatures (> 2000”C)11 are reached. Oxidation of a vitrinite of low rank--or oxidation of an earlier precursor of vitrinite, even in the peat swamp-will diminish or destroy the ability of the vitrinite to soften during pyrolysis. Larsen et al. l2 suggested that the softening capacity of vitrinites can be destroyed in at least three different ways by mild oxidation. Citing studies by Painter et al.’ 3 and Havens et cr1.14, they suggest that easily donatable benzylic hydrogens are lost during oxidation to carbonyl species and carboxylic acids. The importance of these easily donatable hydrogens in the coking process has been discussed by Neavel” and their loss is the most probable cause of the reduced or destroyed softening capacity of vitrinites which have been mildly oxidized. This is the most likely explanation for the retention of cell structures reported from earlier pyrolysis experiments2,3. Whether the anthracitic vitrinite discussed here retained its botanical structure because of oxidation early in its history, prior to advanced coalification, or such structure can be preserved through the normal coalification process is difficult to resolve. The low frequency of occurrence of such structures, however, suggests that some predisposing factor, such as oxidation, probably operated prior to the main coalification. CONCLUSIONS 1. Remarkable stability and resistance to morphological change at carbonization temperatures as high as
Retention
of botanical
structure
in anthracitic
vitrinites: F. Goodarzi
Figure 1 Plant-cell structures retained in vitrinite of anthracitic rank car .bonized to different b, 750°C at 60”Cmin’: c, 950°C at 1”Cmin~‘; d, 2400°C at 80”Cmin-’ lO”Cmin-‘;
2400°C can be shown by plant-cell
structures retained in vitrinites of anthracitic rank. Substantial changes to the molecular structure of anthracitic vitrinites take place in the solid state as temperature rises beyond z 7OO”C, with an increase in the aromatic-layer diameters and in the stack heights of the vitrinites10~16. Nevertheless, despite these contrasts, molecular-structural changes, optical reflecting initial compositional differences between cell-wall material and cell-infillings, are maintained and enhanced. Retention of botanical structures in plant material that has been exposed to relatively severe laboratory heating or to rigorous natural processes in the Earth’s crust, is clearly not uncommon. Besides the present occurrence, retained botanical structure has now been observed in 1. meta-anthracitic vitrinites from lowgrade metamorphic terrains; 2. vitrinites carbonized under pressure under natural and laboratory conditions; 3. vitrinites of bituminous rank which have been carbonized after pre-oxidation; and 4. certain vitrinites that were probably oxidized to a degree during biochemical coaltfication, but which still retained a composition and a molecular structure that was appropriate to develop a mesophase, although little if any softening could have occurred during carbonization.
temperatures
and D. G. Murchison
at different
heating
rates: a, 700°C at
ACKNOWLEDGEMENT The authors manuscript.
thank
Mrs
Y. Hall
for
preparing
the
REFERENCES
5 6 7 8 9 10 11 12 13 14 15 16
Cook, A. C., Murchison, D. G. and Scott, E. Grol. J. 1972,8,83 Goodarzi, F. and Murchison, D. G. Fuel 1973,52, 164 Goodarzi, F. and Murchison, D. G. J. Microsc. 1976, 106, 49 Benedict. L. G.. Thompson. R. R., Shigo III, J. J. and Aikman, R. P. Fuel 1968. 47, 125 Jones, J. M. and Creaney, S. J. Microsc. 1977, 109, 105 Goodarzi, F. and Marsh, H. Fuel 1980, 59, 269 Marsh, H. and Wynne-Jones, W. F. K. J. Sri. Inst. 1965,42,7 10 Goodarzi, F. Fuel 1984,63, 820 Goodarzi, F. and Murchison, D. G. Fuel 1972,51, 322 Blayden, H. E., Gibson. J. and Riley, H. C. Proc. Con/: Ultrajiw Struct. Coals Cokes 1943, BCURA, London, 176 Franklin, R. E. Proc. R. Sot., Ser. A 1951,209, 196 Larsen, J. W., Lee, D., Schmidt, T. and Grint, A. Fuel 1986,65, 595 Painter, P. C., Snyder, R. W., Pearson, D. E. and Kwong, J. Fuel 1980,59, 282 Havens, J. R., Koening, J. L., Kuehn, D., Rhoads, C., Davis, A. and Painter, P. C. Fuel 1983,62, 936 Neavel, R. C. in ‘Coal Science Vol. 1’, (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, 1982 Diamond, R. Phil. Trans. R. Sot., Ser. A 1960. 252, 193
FUEL, 1988, Vol 67, June
833
Goodarzi
and Duncan
G. Murchison”
Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 330333rd Street N. W., Calgary, Alberta, TZL 2AY, Canada * Organic Geochemistry Unit, Department of Geology, University of Newcastle, Newcastle-uponTyne, UK (Received 18 May 1987; revised 19 November 1987) Polished surfaces of anthracitic vitrinites, when viewed by reflected light using oil immersion lenses and crossed polars, frequently display remnant botanical structure. Such stucture may be retained, often with enhanced contrast between cell walls and cell fillings,even when the vitrinite is carbonized at temperatures as high as 2400°C.Although increases in aromatic-layer diameters and stack heights have occurred, albeit in the solid state, there is no apparent distortion of the cell structures. Reference is made to similar phenomena in vitrinites from other environments, viz. low-grade metamorphism and carbonization after laboratory or
natural oxidation processes. (Keywords: anthracite; vitrinite; botanical structure)
Anthracites, when viewed under appropriate optical conditions, may display a variety of botanical structures, similar to those seen in lower-rank coals. Early methods of revealing such structures predominantly relied on either flame-etching a polished surface of anthracite, or etching the polished surface with an oxidizing agent, e.g. hot chrome-sulphuric acid, a treatment that has been widely employed in petrographic studies in attempts to reveal the origin and composition of structureless macerals. The advent of microscope systems with minimal internal reflections from optical surfaces and fitted with high-intensity light sources has allowed the use of crossed or partially crossed polars with oil-immersion objectives to demonstrate with great clarity the petrographic structures both in high-rank coals and in cokes. Thus, Cook et al.’ illustrated crushed wood structure in Dadoxylon hendriksii in a meta-anthracitic vitrinite of Devonian age. Goodarzi and Murchison’ showed that after severe laboratory pre-oxidation of vitrinites, which inhibited any softening of the vitrinites during later carbonization, plant-cell structures were retained at temperatures in excess of 600°C. The same authors3 also demonstrated that plant-cell structures could be preserved in vitrinites of coking-coal rank when carbonized at temperatures as high as 600°C even when a mosaic structure had been generated. It seemed possible that these particular vitrinites had suffered some oxidation at the time of their deposition, and might well have been regarded as pseudo-vitrinites4 prior to carbonization. Jones and Creaney’ illustrated similar effects in naturally metamorphosed coals from northern England where the reflectances of the affected vitrinites ranged as high as 9% and mosaics had also developed. Goodarzi and Marsh6 found that cell structures were retained and mosaics developed in vitrinites carbonized under pressures of = 200 MPa, after the vitrinite had been pre-oxidized in the laboratory. Under certain conditions, macerals can thus be resistant to coalification or 1X316-2361/88/060831-03E3.00 0 1988 Butterworth & Co. (Publishers) Ltd.
carbonization processes which would have been expected to be sufficiently severe to destroy all traces of original botanical structure of the macerals. This paper relates further observations on the details of botanical structures retained in anthracite samples that were carbonized at temperatures up to 2400°C. The structures were observed fortuitously in a broader study of the optical properties of a range of coals that had been subjected to different heat treatments. EXPERIMENTAL Heat treatment
A concentrate (> 95 %) of vitrinite of anthracitic rank (carbon (dmmf)=94.2 %: R,i, = 4.10% at 546 nm) was crushed to pass BSS 72 mesh (> 210 pm) and stored in nitrogen at ambient temperature for the period of the pyrolysis experiments. Samples of this vitrinite, contained in silica boats, were carbonized in nitrogen using rectilinear heating rates of 1, 10 and 60°C min-’ up to a final temperature of 95O”C, which was held for 1 h controlled to +2”C, before cooling. Heat treatment to temperatures higher than 1000°C was carried out by reheating the samples, which had been pre-heated to 950°C in a graphite furnace using an argon atmosphere7. The samples were carbonized to several temperature levels within the range loOO-24OO”C, using a heating rate of 80°C min-’ and a ‘soak period’ of 10 min before cooling. Microscopical
preparation
and examination
The small amounts of sample remaining after each carbonization were mounted in plastic blocks and ground and polished by standard methods, Since high-rank typically show no anthracites, when carbonized, detectable changes in optical properties until the temperature has exceeded at least 6OO”C, only samples carbonized above this temperature level were examined. All photomicrographs were taken with Zeiss ‘Antiflex’ oil-immersion objectives (x 16/0.40 and x 40/0.65) in 1.
FUEL, 1988, Vol 67, June
831
Retention
of botanical
structure in anthracitic
vitrinites:
plane-polarized light; and 2. using an analyser and a polarizer in the optical train with their vibration directions slightly uncrossed to allow rather greater reflection from the surfaces to the camera. RESULTS At all temperature levels sampled up to 2400°C telinitic cell structures were found in the residues of the vitrinite. A representative carbonized anthracitic selection of photomicrographs is given in Figures lug. Only photomicrographs of the carbonized vitrinites under partially crossed polars are shown, since little structure of any kind could be observed in planepolarized light. The photographs were taken with the particles rotated into their highest contrast position in relation to the vibration directions of the polars. The carbonized particles displayed a high but complicated anisotropy, since the degree of contrast between the fillings of the cell cavities and the substance of the cell walls could be radically altered by rotating the microscope stage. Occasionally strain lines could be seen in particular orientations. DISCUSSION Figure 1 illustrates that some particles of the high-rank anthracitic vitrinite, even when exposed to temperatures as high as 24OO”C, suffer little obvious morphological change as a result of severe heat treatment. There is no indication, for example, through rounding of the particle of any softening having occurred or of margins, mechanical disruption, even when the relatively high heating rate of 60°C min-* was employed. Plant-cell structures are undistorted, and appear to be similar to the cell structures in vitrinites that can be more readily observed in lower-rank coals on polished surfaces in reflected light or in thin sections in transmitted light. It is possible that this telinitic anthracitic vitrinite may have originally been a pseudovitrinite and hence less reactive than other associated vitrinites4. and apparent stability of cell The retention morphology might suggest that the anthracitic vitrinite did not undergo pronounced molecular change during high-temperature carbonization. Reflectance, however, shows an increasing, if irregular, rise to 24Oo”C*, well beyond the maximum oil reflectance of ~8.5 Y0 quoted by Goodarzi and Murchison’ for carbonized vitrinites at 95O”C, eventually reaching z 11.0 7;‘. Likewise, the oil bireflectance has risen markedly from its original level to 2 10.0 (Ref. 8), so on the basis of the behaviour of these parameters increased molecular ordering has occurred. X-ray diffraction studies of vitrinites”,” of widely varying rank have shown that parameters that estimate the aromatic-layer diameters and stack heights of carbonized products both show continuing rise with temperature beyond a temperature level of z 95O”C, representing gradually increasing molecular structural ordering. In the present case, the 002 diffraction band shown by the fresh anthracitic vitrinite had sharpened considerably at a temperature of 2400°C and 101 and 112 diffraction bands had made their appearance. Although a complete graphite diffraction pattern had certainly not developed, greater structural ordering has occurred. Molecular changes in vitrinites and other organic
832
FUEL, 1988, Vol 67, June
F. Goodarzi and 0. G. Murchison constituents of coals carbonized beyond a temperature of z 650°C take place in the solid state. What is surprising in the present circumstances is that, despite optical data and independent evidence from X-ray studies of rapid development in the growth and ordering of the aromatic structures of the carbonized vitrinite, principally through pyrolytic dehydrogenation, there is seemingly no disruption of the plant-cell ‘macro-structure’. Some changes might reasonably be expected with temperature rise, particularly where the heat input and level have begun to overcome the resistance of the anthracitic molecular structure to the graphitization process at = > 2000°C”. At the highest temperature employed (24Oo”C), the carbonized residues are soft to the touch and do resemble graphite in appearance. The contrast between the cell-cavity infillings and the cell-wall substances must represent an early compositional difference of the tissues, which was there before or introduced during peat deposition. Retention of this compositional difference in material carbonized at 2400°C can hardly be envisaged, yet the optical contrast is maintained, indicating a molecular-structural contrast. Jones and Creaney’ have drawn attention to the same effect in naturally metamorphosed coals. Several examples were given in the introduction to this paper of the retention of botanical structure in high-rank vitrinites or in vitrinites that had been oxidized prior to carbonization. The processes of vitrinite coalification to anthracitic level and pre-oxidation, either laboratory or natural, of a low-rank vitrinite differ, but they do have some similarity in that both can lead to a product which will not soften on carbonization, even at high temperatures. An anthracitic vitrinite of the normal coalification series has a structure essentially formed of large polycyclic aromatic molecules, the elements of which are sufficiently strongly cross-linked to prevent little or no disruption until high temperatures (> 2000”C)11 are reached. Oxidation of a vitrinite of low rank--or oxidation of an earlier precursor of vitrinite, even in the peat swamp-will diminish or destroy the ability of the vitrinite to soften during pyrolysis. Larsen et al. l2 suggested that the softening capacity of vitrinites can be destroyed in at least three different ways by mild oxidation. Citing studies by Painter et al.’ 3 and Havens et cr1.14, they suggest that easily donatable benzylic hydrogens are lost during oxidation to carbonyl species and carboxylic acids. The importance of these easily donatable hydrogens in the coking process has been discussed by Neavel” and their loss is the most probable cause of the reduced or destroyed softening capacity of vitrinites which have been mildly oxidized. This is the most likely explanation for the retention of cell structures reported from earlier pyrolysis experiments2,3. Whether the anthracitic vitrinite discussed here retained its botanical structure because of oxidation early in its history, prior to advanced coalification, or such structure can be preserved through the normal coalification process is difficult to resolve. The low frequency of occurrence of such structures, however, suggests that some predisposing factor, such as oxidation, probably operated prior to the main coalification. CONCLUSIONS 1. Remarkable stability and resistance to morphological change at carbonization temperatures as high as
Retention
of botanical
structure
in anthracitic
vitrinites: F. Goodarzi
Figure 1 Plant-cell structures retained in vitrinite of anthracitic rank car .bonized to different b, 750°C at 60”Cmin’: c, 950°C at 1”Cmin~‘; d, 2400°C at 80”Cmin-’ lO”Cmin-‘;
2400°C can be shown by plant-cell
structures retained in vitrinites of anthracitic rank. Substantial changes to the molecular structure of anthracitic vitrinites take place in the solid state as temperature rises beyond z 7OO”C, with an increase in the aromatic-layer diameters and in the stack heights of the vitrinites10~16. Nevertheless, despite these contrasts, molecular-structural changes, optical reflecting initial compositional differences between cell-wall material and cell-infillings, are maintained and enhanced. Retention of botanical structures in plant material that has been exposed to relatively severe laboratory heating or to rigorous natural processes in the Earth’s crust, is clearly not uncommon. Besides the present occurrence, retained botanical structure has now been observed in 1. meta-anthracitic vitrinites from lowgrade metamorphic terrains; 2. vitrinites carbonized under pressure under natural and laboratory conditions; 3. vitrinites of bituminous rank which have been carbonized after pre-oxidation; and 4. certain vitrinites that were probably oxidized to a degree during biochemical coaltfication, but which still retained a composition and a molecular structure that was appropriate to develop a mesophase, although little if any softening could have occurred during carbonization.
temperatures
and D. G. Murchison
at different
heating
rates: a, 700°C at
ACKNOWLEDGEMENT The authors manuscript.
thank
Mrs
Y. Hall
for
preparing
the
REFERENCES
5 6 7 8 9 10 11 12 13 14 15 16
Cook, A. C., Murchison, D. G. and Scott, E. Grol. J. 1972,8,83 Goodarzi, F. and Murchison, D. G. Fuel 1973,52, 164 Goodarzi, F. and Murchison, D. G. J. Microsc. 1976, 106, 49 Benedict. L. G.. Thompson. R. R., Shigo III, J. J. and Aikman, R. P. Fuel 1968. 47, 125 Jones, J. M. and Creaney, S. J. Microsc. 1977, 109, 105 Goodarzi, F. and Marsh, H. Fuel 1980, 59, 269 Marsh, H. and Wynne-Jones, W. F. K. J. Sri. Inst. 1965,42,7 10 Goodarzi, F. Fuel 1984,63, 820 Goodarzi, F. and Murchison, D. G. Fuel 1972,51, 322 Blayden, H. E., Gibson. J. and Riley, H. C. Proc. Con/: Ultrajiw Struct. Coals Cokes 1943, BCURA, London, 176 Franklin, R. E. Proc. R. Sot., Ser. A 1951,209, 196 Larsen, J. W., Lee, D., Schmidt, T. and Grint, A. Fuel 1986,65, 595 Painter, P. C., Snyder, R. W., Pearson, D. E. and Kwong, J. Fuel 1980,59, 282 Havens, J. R., Koening, J. L., Kuehn, D., Rhoads, C., Davis, A. and Painter, P. C. Fuel 1983,62, 936 Neavel, R. C. in ‘Coal Science Vol. 1’, (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, New York, 1982 Diamond, R. Phil. Trans. R. Sot., Ser. A 1960. 252, 193
FUEL, 1988, Vol 67, June
833