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Low-temperature oxidation of brown coal. 1. Changes in internal surface due to oxidation
Low-temperature
oxidation of brown coal.
I. Changes in internal surface due to oxidation Philip D. Swarm*, David J. Atlardicet and David G. Evans* * Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia f Herman Central Scientific Laboratory, State Electricity Commission of Victoria, Howard Street, Richmond, Vie toria 3 121, Australia (Received 25 September 19731
The low-temperature oxidation of a Yallourn brown coal was found to decrease markedly the internal surface area of the coal. The surface areas were determined by applying the Dubinin-Polanyi approach to carbon-dioxide isotherms measured gravimetrically at 0°C.
The oxidation of dried brown coal at ambient temperatures significantly alters some of the important properties of the coal, such as briquettability and calorific value, and creates fondling problems owing to the risk of spontaneous ignition. This behaviour is well known in the industry and has led to the empirical development of suitable methods of sampling, handling and storing such coals. Nevertheless few systematic investigations of the oxidation of brown coal with molecular oxygen have been carried out; for example, the standard texts on coal by Lowry ‘and van Krevelen2 contain extensive sections on the low-temperature oxidation of coal, but make little’ or no2 reference to brown coals. This paper reports some results obtained during a larger investigation into the low-temperature oxidation of brown coals. As part of this investigation, internal surface areas were determined from carbon-dioxide adsorption isotherms at 0°C using the Dubinin-Polanyi method3-‘. These areas were measured in order to estimate the degree of surface coverage of the coal by adsorbed oxygen. However, the effect of oxidation on the surface areas measured proved to be so marked that the results warrant reporting in their own right. The remainder of this lowtemperature oxidation work will be analysed and reported in due course.
EXPERIMENTAL Sumpling and storage The brown coal investigated was taken from the Yallourn Open Cut located approximately 140 km east of Melbourne. This coal is currently used for large-scale electricity production and to a lesser extent for briquette and char manufacture. A bulk coal sample was obtained by cutting cubes with sides appro~mately O-3m from a freshly exposed face of coal, using a chain saw. These cubes were immediately stored in air-tight drums, under water drained from the coal face, to prevent premature oxidation. Although this deposit usually contains little adventitious mineral matter, the coal is not homogeneous in that it contains coalified plant remains of many different kind8. The
whole of the sample was therefore reduced in size under water to minus 6 B.S. mesh, and stored, still under water, in an 801itre air-tight drum. When sub-samples were required the slurry in this drum was stirred thoroug~y and 1 litre grab samples were withdrawn.
Predrying Preliminary experiments on bed-moist coal showed that oxidation was not detectable until the surface layer had dried out. Thus, in order to obtain measurable and reproducible oxidation conditions, it was necessary to predry the coal. The exact method used was critical because evacuation, even at the low temperatures used for the oxidation experiments, changed the structure of the coal’. Therefore a standard drying procedure was adopted for all experiments, which consisted of evacuating the coal for 24 hours, at the temperature which was to be used subsequently for the oxidation, and a pressure of 1.3 Pa (0.01 torr).
oxidation Coal samples, predried as described, were exposed tw pure molecular oxygen at a pressure of 100 kPa (1 bar) at temperatures of 35 and 70°C. Exposure was discontinued after 45 days because preliminary experiments had shown that mass changes after this period were not significant. Since the method of predrying involved pretreatment at the oxidation temperature, it was necessary to prepare blank samples for surface-area measurement which had undergone the thermal treatment but not the oxidation, These samples were prepared by evacuation at 35 and 70°C for 7 days. In order to gain complete control over the pretreatment of the coal samples, these were dried and oxidized in the same apparatus as was used for the surface-area measurements. The procedure adopted was to take a 1 g sample of coal from the 1 litre grab sample of minus 6 mesh wet coal, place it in a polypropylene bucket suspended on the quartz spring, and seal this in the microbalance case. The coal was
FUEL, 1974, Vol 53, April
85
then dried by evacuating the case at the desired temperature and pressure; then, with0ut changing the temperature, the coal was oxidized by introducing the oxygen, When the oxidation was completed the balance case was again evacuated, the temperature reduced to O”C, and doses of carbon dioxide were introduced in order to construct the isotherm required for the surface-area measurement.
Surface areas of the coals were determined from carbondioxide isotherms constructed ~avimetri~ally, using a quartz-spiral microbalance, at a temperature of 0°C and pressures up to 100 kPa. These isotherms were then expressed in terms of the Dubinin-Polanyi equation, in the form: log V = log V0 - D log2 (P#) where V is the amount of carbon dioxide adsorbed at equilibrium pressure P, C*ois the micropore capacity, fo is the saturated vapour pressure, and L) is a coefficient characteristic of the particular coal sample used. This coefficient contains a term which is a function of the microporesize distribution, its value increasing with increasing mean micropore diameter. To calculate surface areas, V0 is assumed to be the monolayer capacity and a value of 0.253 nm2 is taken as the cross-sectional area of the carbondioxide molecule. It has been shown, on a wide range of coals4*5*8,that this method gives surface areas in good agreement with values calculated from carbon-dioxide isotherms using the BET approach. Isotherms were measured for the untreated sample (merely predried by evacuation at the ro0m temperature of 22”C), samples which had been dried by evacuation for seven days at 35 and 70°C, but not oxidized, and samples which had been evacuated and then oxidized at 35 and 70°C for 45 days. The Dubinin-. Polanyi plots of these isotherms are shown in Figure I to demonstrate the rectilinearity of the plots; the surface areas and values of the coefficient L) obtained from these plots are given in Tab& I.
Log2
( PO/P
f
F&WE I ~~b~n~~-~~~anyi plats of the carbon-dioxide isotherms * unoxidized raw-coat sample 0 sample heat-treated at 35% for 7 days m sample heat-treated at 70% for 7 days 0 sample oxidized at 35% for 45 days * sample oxidized at 70% for 45 days
Tab/e I
Surface areas of brown-coax samples SlKfZE@
area Sample
(m*/g)
D
Wnoxidized raw coal (dried by evacuation at 22’Cf
290
0.118
Heat treated, 35°C for 7 days
262
0.105
254
@I12
Oxidized, 35*C for 45 days
187
o-112
Oxidized, 7O’C for 45 days
156
0.123
DISCUSSION The surface area of 230 m2/g for the raw coal compares well with the values of 238-268 m2/g reported for North American lignites’, using the same Dubinin-Polanyi carbondioxide isotherm method. It is also in good agreement with the value of 280 m2fg determined from water-s0rption isotherms on a similar Yallourn coal using the BET equation and assuming liquid packing in the adsorbed state”, which gives” a molecular cross-section of 0.105 nm2. AS seen from Table 1, mild heat treatment of the unoxidi~d coal results in a slight decrease in surface area. This effect can be attributed to thermal decomposition of functional groups on the coal surface. The release of cars bon dioxide and water from an evacuated sample of Yallourn coal with increasing temperature t2provides evidence of this phenomenon. This breakdown of functional groups probably causes some coftapse of micropores which would result in the observed decrease in surface area, and the slight drop in the mean micropore diameter (see values for the coefficient D). However, the effects of heat treatment alone are small
86
FUEL. 1974, Vat 53, April
under vacuum
Heat
treated,
70°C
for 7 days
under vacuum
compared with the effects of oxidation. As seen from Table 1 the area of the unoxidized coal was reduced by 35% by prolonged oxidation at 35”C, and by 45% at 70°C. It can be deduced from porosity mea~rementsg~13 that the surface area of low-rank coals is contained predominantly in micropores of 1.2 nm diameter or less. With pore diameters of this order, it can readily be envisaged that
P. D. Swann, D. J. Allardice
adsorbed oxygen would block the pores enough to restrict the access of the measuring gas, carbon dioxide, to the surface area contained in them. In a study of a higher-rank coal oxidized at higher temperatures, Marsh and Siemieniewska’ observed a similar decrease in measured surface area, which they also attributed to mechanical blocking of the pores by oxygen. It will be noted from the values of the coefficient D given in Table I that oxidation of the coal tends to increase the mean diameter of the micropores. This refers to the mean diameter of the pores actually penetrated by the carbon dioxide, and an increase would be expected if some of the smaller pores were closed off, as suggested above. Some consequences of this oxidation effect on the surface area of brown coals are:
(1)
(3
(3
Any interpretation of the kinetics of brown-coal oxidation at low temperatures must take account of the progressive reduction in the effective surface area caused by blockage of the pores. Since the surface areas of brown coals can be changed markedly by oxidation, precautions should be taken to store samples in air-tight containers under an inert fluid before conducting adsorption studies or surfacearea measurements using adsorbates other than oxygen. The reduction in surface area due to oxidation could reduce the effectiveness of the coal as a gas-cleaning or decolourizing agent. A similar effect may well occur if chars produced from the coal are oxidized.
and D. G. Evans: Low-temperature
oxidation
of brown coal (1)
ACKNOWLEDGEMENTS The oxidation studies were carried out in the laboratories of the University of Melbourne, and the surface-area measurements in the Herman Central Scientific Laboratory of the State Electricity Commission of Victoria. Thanks are due to both these bodies for provision of facilities and permission to publish this paper. The work was sponsored in part by the Australian Research Grants Committee, whose assistance is gratefully acknowledged.
REFERENCES 1
3”
4 5 6 I 8
9 10 11 12 13
Lowry, H. H. (Ed.) ‘chemistry of Coal Utilization’, Supplementary Volume, Wiley, New York, 1963 Van Krevefen, D. W. ‘Coal’, Elsevier, Amsterdam, 1961 Dubinin. M. M. ‘Cbemistrv and Phvsics of Carbon’. Vol.2 (Ed. P. L. Walker, Jr), Marcel Deiker, New York,’ 1966, pp 51-120 Lamond, T. G. and Marsh, J. Carbon 1964, 1, 281, 293 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1965, 44, 344 Francis, W. ‘Coal’, 2nd edn, Arnold, London, 196 1 Swann, P. D., Harris, J. A., Siemon, S. R. and Evans, D. G. Fuel, Lond. 1973,52, 154 Walker, P. L., Jr and Patel, R. L. Fuel, Land, 1970, 49, 91 Gan, H., Nandi, S. P. and Walker, P. L., Jr Fuel, tend. 1972, 51,272 Aiiardice, D. J. Phi) Thesis. University of Melbourne, 1968 McCIellan, A. L. and Hamsberger, H. F. J. Colloid Interface Sci 1967, 23, 571 Allardice, D. J. and Evans, D. G. Fuel, Lond. 1971, 50, 201 Evans, D. G. Fuel, Lond. 1973, 52, 156
FUEL,
1974, Vol 53, April
87
oxidation of brown coal.
I. Changes in internal surface due to oxidation Philip D. Swarm*, David J. Atlardicet and David G. Evans* * Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia f Herman Central Scientific Laboratory, State Electricity Commission of Victoria, Howard Street, Richmond, Vie toria 3 121, Australia (Received 25 September 19731
The low-temperature oxidation of a Yallourn brown coal was found to decrease markedly the internal surface area of the coal. The surface areas were determined by applying the Dubinin-Polanyi approach to carbon-dioxide isotherms measured gravimetrically at 0°C.
The oxidation of dried brown coal at ambient temperatures significantly alters some of the important properties of the coal, such as briquettability and calorific value, and creates fondling problems owing to the risk of spontaneous ignition. This behaviour is well known in the industry and has led to the empirical development of suitable methods of sampling, handling and storing such coals. Nevertheless few systematic investigations of the oxidation of brown coal with molecular oxygen have been carried out; for example, the standard texts on coal by Lowry ‘and van Krevelen2 contain extensive sections on the low-temperature oxidation of coal, but make little’ or no2 reference to brown coals. This paper reports some results obtained during a larger investigation into the low-temperature oxidation of brown coals. As part of this investigation, internal surface areas were determined from carbon-dioxide adsorption isotherms at 0°C using the Dubinin-Polanyi method3-‘. These areas were measured in order to estimate the degree of surface coverage of the coal by adsorbed oxygen. However, the effect of oxidation on the surface areas measured proved to be so marked that the results warrant reporting in their own right. The remainder of this lowtemperature oxidation work will be analysed and reported in due course.
EXPERIMENTAL Sumpling and storage The brown coal investigated was taken from the Yallourn Open Cut located approximately 140 km east of Melbourne. This coal is currently used for large-scale electricity production and to a lesser extent for briquette and char manufacture. A bulk coal sample was obtained by cutting cubes with sides appro~mately O-3m from a freshly exposed face of coal, using a chain saw. These cubes were immediately stored in air-tight drums, under water drained from the coal face, to prevent premature oxidation. Although this deposit usually contains little adventitious mineral matter, the coal is not homogeneous in that it contains coalified plant remains of many different kind8. The
whole of the sample was therefore reduced in size under water to minus 6 B.S. mesh, and stored, still under water, in an 801itre air-tight drum. When sub-samples were required the slurry in this drum was stirred thoroug~y and 1 litre grab samples were withdrawn.
Predrying Preliminary experiments on bed-moist coal showed that oxidation was not detectable until the surface layer had dried out. Thus, in order to obtain measurable and reproducible oxidation conditions, it was necessary to predry the coal. The exact method used was critical because evacuation, even at the low temperatures used for the oxidation experiments, changed the structure of the coal’. Therefore a standard drying procedure was adopted for all experiments, which consisted of evacuating the coal for 24 hours, at the temperature which was to be used subsequently for the oxidation, and a pressure of 1.3 Pa (0.01 torr).
oxidation Coal samples, predried as described, were exposed tw pure molecular oxygen at a pressure of 100 kPa (1 bar) at temperatures of 35 and 70°C. Exposure was discontinued after 45 days because preliminary experiments had shown that mass changes after this period were not significant. Since the method of predrying involved pretreatment at the oxidation temperature, it was necessary to prepare blank samples for surface-area measurement which had undergone the thermal treatment but not the oxidation, These samples were prepared by evacuation at 35 and 70°C for 7 days. In order to gain complete control over the pretreatment of the coal samples, these were dried and oxidized in the same apparatus as was used for the surface-area measurements. The procedure adopted was to take a 1 g sample of coal from the 1 litre grab sample of minus 6 mesh wet coal, place it in a polypropylene bucket suspended on the quartz spring, and seal this in the microbalance case. The coal was
FUEL, 1974, Vol 53, April
85
then dried by evacuating the case at the desired temperature and pressure; then, with0ut changing the temperature, the coal was oxidized by introducing the oxygen, When the oxidation was completed the balance case was again evacuated, the temperature reduced to O”C, and doses of carbon dioxide were introduced in order to construct the isotherm required for the surface-area measurement.
Surface areas of the coals were determined from carbondioxide isotherms constructed ~avimetri~ally, using a quartz-spiral microbalance, at a temperature of 0°C and pressures up to 100 kPa. These isotherms were then expressed in terms of the Dubinin-Polanyi equation, in the form: log V = log V0 - D log2 (P#) where V is the amount of carbon dioxide adsorbed at equilibrium pressure P, C*ois the micropore capacity, fo is the saturated vapour pressure, and L) is a coefficient characteristic of the particular coal sample used. This coefficient contains a term which is a function of the microporesize distribution, its value increasing with increasing mean micropore diameter. To calculate surface areas, V0 is assumed to be the monolayer capacity and a value of 0.253 nm2 is taken as the cross-sectional area of the carbondioxide molecule. It has been shown, on a wide range of coals4*5*8,that this method gives surface areas in good agreement with values calculated from carbon-dioxide isotherms using the BET approach. Isotherms were measured for the untreated sample (merely predried by evacuation at the ro0m temperature of 22”C), samples which had been dried by evacuation for seven days at 35 and 70°C, but not oxidized, and samples which had been evacuated and then oxidized at 35 and 70°C for 45 days. The Dubinin-. Polanyi plots of these isotherms are shown in Figure I to demonstrate the rectilinearity of the plots; the surface areas and values of the coefficient L) obtained from these plots are given in Tab& I.
Log2
( PO/P
f
F&WE I ~~b~n~~-~~~anyi plats of the carbon-dioxide isotherms * unoxidized raw-coat sample 0 sample heat-treated at 35% for 7 days m sample heat-treated at 70% for 7 days 0 sample oxidized at 35% for 45 days * sample oxidized at 70% for 45 days
Tab/e I
Surface areas of brown-coax samples SlKfZE@
area Sample
(m*/g)
D
Wnoxidized raw coal (dried by evacuation at 22’Cf
290
0.118
Heat treated, 35°C for 7 days
262
0.105
254
@I12
Oxidized, 35*C for 45 days
187
o-112
Oxidized, 7O’C for 45 days
156
0.123
DISCUSSION The surface area of 230 m2/g for the raw coal compares well with the values of 238-268 m2/g reported for North American lignites’, using the same Dubinin-Polanyi carbondioxide isotherm method. It is also in good agreement with the value of 280 m2fg determined from water-s0rption isotherms on a similar Yallourn coal using the BET equation and assuming liquid packing in the adsorbed state”, which gives” a molecular cross-section of 0.105 nm2. AS seen from Table 1, mild heat treatment of the unoxidi~d coal results in a slight decrease in surface area. This effect can be attributed to thermal decomposition of functional groups on the coal surface. The release of cars bon dioxide and water from an evacuated sample of Yallourn coal with increasing temperature t2provides evidence of this phenomenon. This breakdown of functional groups probably causes some coftapse of micropores which would result in the observed decrease in surface area, and the slight drop in the mean micropore diameter (see values for the coefficient D). However, the effects of heat treatment alone are small
86
FUEL. 1974, Vat 53, April
under vacuum
Heat
treated,
70°C
for 7 days
under vacuum
compared with the effects of oxidation. As seen from Table 1 the area of the unoxidized coal was reduced by 35% by prolonged oxidation at 35”C, and by 45% at 70°C. It can be deduced from porosity mea~rementsg~13 that the surface area of low-rank coals is contained predominantly in micropores of 1.2 nm diameter or less. With pore diameters of this order, it can readily be envisaged that
P. D. Swann, D. J. Allardice
adsorbed oxygen would block the pores enough to restrict the access of the measuring gas, carbon dioxide, to the surface area contained in them. In a study of a higher-rank coal oxidized at higher temperatures, Marsh and Siemieniewska’ observed a similar decrease in measured surface area, which they also attributed to mechanical blocking of the pores by oxygen. It will be noted from the values of the coefficient D given in Table I that oxidation of the coal tends to increase the mean diameter of the micropores. This refers to the mean diameter of the pores actually penetrated by the carbon dioxide, and an increase would be expected if some of the smaller pores were closed off, as suggested above. Some consequences of this oxidation effect on the surface area of brown coals are:
(1)
(3
(3
Any interpretation of the kinetics of brown-coal oxidation at low temperatures must take account of the progressive reduction in the effective surface area caused by blockage of the pores. Since the surface areas of brown coals can be changed markedly by oxidation, precautions should be taken to store samples in air-tight containers under an inert fluid before conducting adsorption studies or surfacearea measurements using adsorbates other than oxygen. The reduction in surface area due to oxidation could reduce the effectiveness of the coal as a gas-cleaning or decolourizing agent. A similar effect may well occur if chars produced from the coal are oxidized.
and D. G. Evans: Low-temperature
oxidation
of brown coal (1)
ACKNOWLEDGEMENTS The oxidation studies were carried out in the laboratories of the University of Melbourne, and the surface-area measurements in the Herman Central Scientific Laboratory of the State Electricity Commission of Victoria. Thanks are due to both these bodies for provision of facilities and permission to publish this paper. The work was sponsored in part by the Australian Research Grants Committee, whose assistance is gratefully acknowledged.
REFERENCES 1
3”
4 5 6 I 8
9 10 11 12 13
Lowry, H. H. (Ed.) ‘chemistry of Coal Utilization’, Supplementary Volume, Wiley, New York, 1963 Van Krevefen, D. W. ‘Coal’, Elsevier, Amsterdam, 1961 Dubinin. M. M. ‘Cbemistrv and Phvsics of Carbon’. Vol.2 (Ed. P. L. Walker, Jr), Marcel Deiker, New York,’ 1966, pp 51-120 Lamond, T. G. and Marsh, J. Carbon 1964, 1, 281, 293 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1965, 44, 344 Francis, W. ‘Coal’, 2nd edn, Arnold, London, 196 1 Swann, P. D., Harris, J. A., Siemon, S. R. and Evans, D. G. Fuel, Lond. 1973,52, 154 Walker, P. L., Jr and Patel, R. L. Fuel, Land, 1970, 49, 91 Gan, H., Nandi, S. P. and Walker, P. L., Jr Fuel, tend. 1972, 51,272 Aiiardice, D. J. Phi) Thesis. University of Melbourne, 1968 McCIellan, A. L. and Hamsberger, H. F. J. Colloid Interface Sci 1967, 23, 571 Allardice, D. J. and Evans, D. G. Fuel, Lond. 1971, 50, 201 Evans, D. G. Fuel, Lond. 1973, 52, 156
FUEL,
1974, Vol 53, April
87