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Heat of immersion in water of Wyodak No. 3 coal as a function of moisture content
Heat of immersion in water of Wyodak coal as a function of moisture content
No. 3
James 0. Glanville, Stephen T. Hall, Donald L. Messick, Kimberley L. Newcomb, Katherine M. Phillips, Francis Webster and James P. Wightman Chemistry VA 24061, (Received
Department, USA 4 December
Virginia Polytechnic
Institute
1984; revised 27 September
and State University,
Blacksburg,
1985)
The heat of immersion in water of a subbituminous coal, Wyodak No. 3, as a function of its moisture content has been determined. The heat of immersion decreases smoothly with increasing moisture content over the range from 1% to 15% moisture. Comparative analyses of coal moisture by gravimetric and Karl Fischer (KF) methods show good agreement for this coal. When exposed to a vacuum at room temperature, about 47; residual moisture is left on the coal independent of its moisture content prior to evacuation. The BET surface on the basisofwater adsorption,compared with 7.5 m2g-’ based areaofthecoalisestimated to be350m2g-’ on krypton adsorption.
(Keywords:
coal; moisture
content; heat of immersion)
It remains a complex problem to fully understand the nature of the water present in moist coal. Such moisture occurs in a wide variety of both physically and chemically bonded forms’, and its presence influences various coal properties. For example, Wightman and co-workers2-4 have investigated the effect of water on the interaction of coal with alcohols, and on coal dust phenomena. The moisture in coal also affects its oxygen content5, and has been used as a probe for pore size distribution6,‘. method for assessing the The most direct thermodynamics of the interaction of water with coal is to measure heats of immersion (wetting). For coals of low rank, such as lignites, the heat of wetting of partially dried specimens has been implicated as a factor in spontaneous heating idcidents 8,9. Despite the potential complicating factors, isosteric heats of wetting of various coalslss- lo appear to follow normal thermodynamic relationships”. Thus, measurements of heats of wetting are useful both as a general means of investigating the coal-water interaction, and because of their practical value in efforts to better understand spontaneous heating phenomena. An interesting collateral feature of the present work has been the calculation of the BET surface area of Wyodak No. 3 on the basis of water adsorption. This same technique has been extended to data already in the literature” for Dietz coal, of which the calculated surface area is 270 m2gm1. Compared with classical nitrogen BET surface area measurements, these values are very high. However, the discordant values of the BET surface areas of coals using different absorbates at different temperatures are well known’ 3 ’ 5. The most widely practiced method of routine analysis of moisture in coal involves oven-dried weight 10s~‘~ under precisely specified conditions. However, the continuing need for rapid yet accurate analysis has prompted research into alternative methods, such as pulsed n.m.r. spectroscopy”. Furthermore, it is well 0016-2361/86/050647-03S3.00 ‘1.~11986 Butterworth & Co. (Publishers)
Ltd.
known’ 8 that moisture determinations in low rank coals, particularly their equilibrium moisture analysis, present special difficulties due to hysteresis effects. The most direct method for moisture analysis is one in which moisture is explicitly measured, as with Karl Fischer (K-F) reagent”; such methods are in principle less ambiguous than gravimetric procedures. In the present work, K-F moisture analysis was used to determine the residual moisture in the coal after its evacuation at both room and elevated temperatures, as well as to determine the moisture content of humidity-equilibrated samples. For Wyodak No. 3, excellent agreement was found between the K-F method and vacuum weight loss at 120°C. Wyodak No. 3 is a subbituminous coal; the as-received sample was crushed and sieved to ~44 pm. The proximate and ultimate analyses of the sample are given in Tuble 1 20. The krypton BET surface area of the coal was determined to be 7.5 m2g-’ (Micromeritics, Norcross, Georgia). Samples for gravimetric analysis and K-F titration were equilibrated for no less than 8 days at different relative humidities (R.H.) over aqueous sulphuric acid solutions ofvarying concentration. A YSI 91 HC dew point hygrometer was used to measure R.H. Equilibrated coal samples were transferred to weighed custom sample holders, and the weight of moist coal was measured by difference. The samples were then maintained for two hours at < 1 x 10m4 torr at the desired temperature and reweighed. Moisture loss was determined by difference. To ensure a reproducible, constant weight it was necessary to place the sample holder in a dessicator over CaCl, before all weighings. K-F determinations were made with a Fisher model 395 automatic titrimeter. Coal samples (0.5 g) equilibrated at different R.H. and also after evacuation were transferred quickly to tared septum bottles, sealed, and reweighed. Approximately 30ml of methanol was accurately weighed, injected into each bottle, and agitated
FUEL,
1986,
Vol 65, May
647
Heat of immersion Table 1
Proximate
in water of Wyodak
and ultimate
analyses
No. 3 coal: J. 0. Glanville
of Wyodak
No. 3 coalZ”
Moisture Volatiles Fixed carbon Ash C’ltimurr Utrtrlrsis (wt”,. moistwwfwe 58.2 13.9 (by difference) 0.81 4.2 2.9 0.04
Carbon Oxygen Nitrogen Hydrogen Sulphur Chlorine
husk)
252
0
I
I
1
I
I
IO
30
50
70
90
Relative
Humidity
(%)
Figure 1 Moisture content of Wyodak No. 3 coal as a function of relative humidity asdetermined gravimetrically (0) and by Karl Fischer titration (A)
for at least 2 h. Aliquots of 34 ml were then withdrawn, weighed, and injected into the K-F apparatus. After titration the percentage moisture in the coal was calculated. Blank experiments were performed separately and a small correction was applied for the background moisture in the methanol Heat of immersion experiments were performed as follows: four coal samples (3&60mg) were weighed into custom Pyrex glass ampoules and evacuated at < 1 x 10B4 torr for 2 h at 26°C. Subsequently the samples were exposed to water vapour in a volumetric adsorption apparatus. Coal moisture content was varied by exposing the samples to water vapour for varying amounts of time. The ampoules were then sealed off in cucuo. A Calvet MS 70 microcalorimeter was used to determine the heat of immersion of the coal in water. Two of the waterequilibrated samples and the two empty ampoules were used in the calorimetric measurements which were performed as previously described3. All heats of immersion were exothermic and were corrected for the heat of empty bulb breaking. The moisture content of the two remaining samples and an unequilibrated control sample were determined by K-F titration. The results of moisture determination by gravimetric analysis and K-F titration for Wyodak No. 3 coal exposed to various humidities are shown in Figure 1; the two methods give good agreement for this coal. The calcuiated BET water surface area is 350 m2g ml based on a cross-sectional area of 0.104nm’ per water molecule. The recalculated data of Porter and Ralston” for water on Dietz coal when plotted as a BET isotherm yields a surface area of 270 m’g _ ‘. It is interesting that this value is in excellent agreement with modern values for similar low rank coals from the western US.
648
FUEL, 1986, Vol 65, May
I
et al.
The 7.5 m2g ’ BET krypton surface area of Wyodak No. 3 is much lower than the BET water surface area. By comparison, earlier Dubinin-Polyani surface area measurements13 of a Wyodak coal gave 2.6 m*g-’ with nitrogen and 308 m’g- ’ with carbon dioxide. An apparent water BET surface area of a Wyodak coal of 270 m’g-‘, increasing to 380m’g-’ after alkali treatment, has also been reported2’. Mahajan and Walker14 have discussed the possible reasons for differences in the BET surface area of Iignites measured with different adsorbates; similar arguments could be made with regard to the krypton/water discrepancy here reported. As shown in Tuble 2, after evacuation at 26 C, the residual coal moisture content is about 4% regardless of the initial moisture content of the coal prior to evacuation. This represents a surface coverage of about 2.5 monolayers based on the BET water surface area. It would be incautious to overstate this result, but it is well known that the first two or three monolayers of adsorbed water are more strongly bonded to the substrate than subsequent layer?. The heat of immersion of Wyodak No. 3 coal in water as a function of its moisture content is plotted in Figure 2. Table 2 Residual moisture of Wyodak room and elevated temperatures ~_ Residual Initial “/ moisture 3.4 6.8 7.6 18.8 22.1
No. 3 coal after evacuation
at
9; moisture
(26’C evacuation)
(120°C evacuation)
3.5
_
4.3
0.5 1.3 _
1.4
I
I
90
80
_
70
m \ T -
60
E .In G
50
E 40 Ys
Ifs
30
20
IO
0 0
I
I
5
IO
Moisture Content (%I Figure 2 Heat of immersion of Wyodak No. 3 coal in water function of moisture content
I5
as a
Heat of immersion
in water of Wyodak No. 3 coal: J. 0. Glanville et al.
Senkan and Fuller” have reported a heat of immersion of 120 Jg-’ for a Wyodak coal after outgassing at 12O’C which is in reasonable agreement with the present results. It is concluded that whatever the actual complexities of moisture adsorption by coal, immersional thermodynamics follow established classical behaviour known for a wide range of inorganic mineral surfaces22. Furthermore, subbituminous coal appears to behave normally, inasmuch as the majority of the immersional heat is liberated during the formation of the first few monolayers of coverage. 12
ACKNOWLEDGEMENTS Funding for this work was provided by the Mining and Minerals Resources and Research Institute and the Center for Coal and Energy Research at Virginia Tech.
REFERENCES 1
2 3 4
Allardice, D. J. and Evans, D. G., ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr), Academic Press Inc., New York, 1978 Glanville, J. 0. and Wightman, J. P. Fuel 1980, 59, 557 Widyani, E. and Wightman, J. P. Colloids and .Su$~es 1982,4, 209 Wightman, J. P., Awad, H. and Widyani, E., paper presented at Intl. Conf. on Coal Chemistry, Pittsburgh, PA, 1983
13 14
15 16 17 18 19 20
21 22
Volborth, A., Miller, G. E., Garner, C. K. and Jerebek, P. A. Frre/ 1977. Xi. 209 Mraw, S. C. and O’Rourke. D. F. J. ColloirlInterfircr Sci. 1982,89, 268 Mraw, S. C. and Silbernagel, B. G., AIP Conf. Proc. No. 70, Chemistry and Physics of Coal Utilization, 1980, 332 Sondreal, E. A. and Ellman, R. C., Report RI7887, US Bureau of Mines, Washington. DC, 1974 Nordon, P. and Bainbridge. N. W. Furl 1983,62. 619 Porter, H. C. and Ralston, 0. C., Technical Paper No. 113, US Bureau of Mines, Washington, DC, 1916 Zettlemoyer, A. C., Chemistry and Physics and Chemistry of Interfaces, (Ed. S. Ross), American Chemical Society, Washington. DC, 1965, 189 Scholberg, H. M. MS. Thesis, University of North Dakota, Grand Forks, ND, 1932 Can, H.. Nandi. S. P. and Walker, Jr., P. L. Fuel 1972, 51, 277 Mahajan. 0. P. and Walker, Jr., P. L. ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr). Academic Press, Inc., New York, 1978 Reucroft, P. J. and Patel, K. B. Furl 1983. 62, 279 Test methods D-3302 and D-3 173, American Society for Testing and Materials, Philadelphia, PA. Periodically revised Riley, J. T. Amrricun Luhorator~ 1983, 15, 17 Luppens, J. A., paper presented at the 2nd Annual Coal Testing Conf., Lexington. Kentucky, 1982 Patzman, R. A. J. Coul Quulity 1984, 3, 38 Squires. A. M.,Taylor, L. T., Brown, R. S. and Hellgeth J. W.,‘The Occurence and Role of Organometallics in Coal Liquefactions’, Virginia Polytechnic Institute and State University, Blacksburg, VA 1983 Senkan, S. M. and Fuller, Jr., E. L. Furl 1979, 58, 729 Partyka, S., Rouquerol, F. and Rouquerol. J. J. Colloid Infr~$~. Sci 1979,68, 2 1
FUEL,
1986,
Vol 65, May
649
No. 3
James 0. Glanville, Stephen T. Hall, Donald L. Messick, Kimberley L. Newcomb, Katherine M. Phillips, Francis Webster and James P. Wightman Chemistry VA 24061, (Received
Department, USA 4 December
Virginia Polytechnic
Institute
1984; revised 27 September
and State University,
Blacksburg,
1985)
The heat of immersion in water of a subbituminous coal, Wyodak No. 3, as a function of its moisture content has been determined. The heat of immersion decreases smoothly with increasing moisture content over the range from 1% to 15% moisture. Comparative analyses of coal moisture by gravimetric and Karl Fischer (KF) methods show good agreement for this coal. When exposed to a vacuum at room temperature, about 47; residual moisture is left on the coal independent of its moisture content prior to evacuation. The BET surface on the basisofwater adsorption,compared with 7.5 m2g-’ based areaofthecoalisestimated to be350m2g-’ on krypton adsorption.
(Keywords:
coal; moisture
content; heat of immersion)
It remains a complex problem to fully understand the nature of the water present in moist coal. Such moisture occurs in a wide variety of both physically and chemically bonded forms’, and its presence influences various coal properties. For example, Wightman and co-workers2-4 have investigated the effect of water on the interaction of coal with alcohols, and on coal dust phenomena. The moisture in coal also affects its oxygen content5, and has been used as a probe for pore size distribution6,‘. method for assessing the The most direct thermodynamics of the interaction of water with coal is to measure heats of immersion (wetting). For coals of low rank, such as lignites, the heat of wetting of partially dried specimens has been implicated as a factor in spontaneous heating idcidents 8,9. Despite the potential complicating factors, isosteric heats of wetting of various coalslss- lo appear to follow normal thermodynamic relationships”. Thus, measurements of heats of wetting are useful both as a general means of investigating the coal-water interaction, and because of their practical value in efforts to better understand spontaneous heating phenomena. An interesting collateral feature of the present work has been the calculation of the BET surface area of Wyodak No. 3 on the basis of water adsorption. This same technique has been extended to data already in the literature” for Dietz coal, of which the calculated surface area is 270 m2gm1. Compared with classical nitrogen BET surface area measurements, these values are very high. However, the discordant values of the BET surface areas of coals using different absorbates at different temperatures are well known’ 3 ’ 5. The most widely practiced method of routine analysis of moisture in coal involves oven-dried weight 10s~‘~ under precisely specified conditions. However, the continuing need for rapid yet accurate analysis has prompted research into alternative methods, such as pulsed n.m.r. spectroscopy”. Furthermore, it is well 0016-2361/86/050647-03S3.00 ‘1.~11986 Butterworth & Co. (Publishers)
Ltd.
known’ 8 that moisture determinations in low rank coals, particularly their equilibrium moisture analysis, present special difficulties due to hysteresis effects. The most direct method for moisture analysis is one in which moisture is explicitly measured, as with Karl Fischer (K-F) reagent”; such methods are in principle less ambiguous than gravimetric procedures. In the present work, K-F moisture analysis was used to determine the residual moisture in the coal after its evacuation at both room and elevated temperatures, as well as to determine the moisture content of humidity-equilibrated samples. For Wyodak No. 3, excellent agreement was found between the K-F method and vacuum weight loss at 120°C. Wyodak No. 3 is a subbituminous coal; the as-received sample was crushed and sieved to ~44 pm. The proximate and ultimate analyses of the sample are given in Tuble 1 20. The krypton BET surface area of the coal was determined to be 7.5 m2g-’ (Micromeritics, Norcross, Georgia). Samples for gravimetric analysis and K-F titration were equilibrated for no less than 8 days at different relative humidities (R.H.) over aqueous sulphuric acid solutions ofvarying concentration. A YSI 91 HC dew point hygrometer was used to measure R.H. Equilibrated coal samples were transferred to weighed custom sample holders, and the weight of moist coal was measured by difference. The samples were then maintained for two hours at < 1 x 10m4 torr at the desired temperature and reweighed. Moisture loss was determined by difference. To ensure a reproducible, constant weight it was necessary to place the sample holder in a dessicator over CaCl, before all weighings. K-F determinations were made with a Fisher model 395 automatic titrimeter. Coal samples (0.5 g) equilibrated at different R.H. and also after evacuation were transferred quickly to tared septum bottles, sealed, and reweighed. Approximately 30ml of methanol was accurately weighed, injected into each bottle, and agitated
FUEL,
1986,
Vol 65, May
647
Heat of immersion Table 1
Proximate
in water of Wyodak
and ultimate
analyses
No. 3 coal: J. 0. Glanville
of Wyodak
No. 3 coalZ”
Moisture Volatiles Fixed carbon Ash C’ltimurr Utrtrlrsis (wt”,. moistwwfwe 58.2 13.9 (by difference) 0.81 4.2 2.9 0.04
Carbon Oxygen Nitrogen Hydrogen Sulphur Chlorine
husk)
252
0
I
I
1
I
I
IO
30
50
70
90
Relative
Humidity
(%)
Figure 1 Moisture content of Wyodak No. 3 coal as a function of relative humidity asdetermined gravimetrically (0) and by Karl Fischer titration (A)
for at least 2 h. Aliquots of 34 ml were then withdrawn, weighed, and injected into the K-F apparatus. After titration the percentage moisture in the coal was calculated. Blank experiments were performed separately and a small correction was applied for the background moisture in the methanol Heat of immersion experiments were performed as follows: four coal samples (3&60mg) were weighed into custom Pyrex glass ampoules and evacuated at < 1 x 10B4 torr for 2 h at 26°C. Subsequently the samples were exposed to water vapour in a volumetric adsorption apparatus. Coal moisture content was varied by exposing the samples to water vapour for varying amounts of time. The ampoules were then sealed off in cucuo. A Calvet MS 70 microcalorimeter was used to determine the heat of immersion of the coal in water. Two of the waterequilibrated samples and the two empty ampoules were used in the calorimetric measurements which were performed as previously described3. All heats of immersion were exothermic and were corrected for the heat of empty bulb breaking. The moisture content of the two remaining samples and an unequilibrated control sample were determined by K-F titration. The results of moisture determination by gravimetric analysis and K-F titration for Wyodak No. 3 coal exposed to various humidities are shown in Figure 1; the two methods give good agreement for this coal. The calcuiated BET water surface area is 350 m2g ml based on a cross-sectional area of 0.104nm’ per water molecule. The recalculated data of Porter and Ralston” for water on Dietz coal when plotted as a BET isotherm yields a surface area of 270 m’g _ ‘. It is interesting that this value is in excellent agreement with modern values for similar low rank coals from the western US.
648
FUEL, 1986, Vol 65, May
I
et al.
The 7.5 m2g ’ BET krypton surface area of Wyodak No. 3 is much lower than the BET water surface area. By comparison, earlier Dubinin-Polyani surface area measurements13 of a Wyodak coal gave 2.6 m*g-’ with nitrogen and 308 m’g- ’ with carbon dioxide. An apparent water BET surface area of a Wyodak coal of 270 m’g-‘, increasing to 380m’g-’ after alkali treatment, has also been reported2’. Mahajan and Walker14 have discussed the possible reasons for differences in the BET surface area of Iignites measured with different adsorbates; similar arguments could be made with regard to the krypton/water discrepancy here reported. As shown in Tuble 2, after evacuation at 26 C, the residual coal moisture content is about 4% regardless of the initial moisture content of the coal prior to evacuation. This represents a surface coverage of about 2.5 monolayers based on the BET water surface area. It would be incautious to overstate this result, but it is well known that the first two or three monolayers of adsorbed water are more strongly bonded to the substrate than subsequent layer?. The heat of immersion of Wyodak No. 3 coal in water as a function of its moisture content is plotted in Figure 2. Table 2 Residual moisture of Wyodak room and elevated temperatures ~_ Residual Initial “/ moisture 3.4 6.8 7.6 18.8 22.1
No. 3 coal after evacuation
at
9; moisture
(26’C evacuation)
(120°C evacuation)
3.5
_
4.3
0.5 1.3 _
1.4
I
I
90
80
_
70
m \ T -
60
E .In G
50
E 40 Ys
Ifs
30
20
IO
0 0
I
I
5
IO
Moisture Content (%I Figure 2 Heat of immersion of Wyodak No. 3 coal in water function of moisture content
I5
as a
Heat of immersion
in water of Wyodak No. 3 coal: J. 0. Glanville et al.
Senkan and Fuller” have reported a heat of immersion of 120 Jg-’ for a Wyodak coal after outgassing at 12O’C which is in reasonable agreement with the present results. It is concluded that whatever the actual complexities of moisture adsorption by coal, immersional thermodynamics follow established classical behaviour known for a wide range of inorganic mineral surfaces22. Furthermore, subbituminous coal appears to behave normally, inasmuch as the majority of the immersional heat is liberated during the formation of the first few monolayers of coverage. 12
ACKNOWLEDGEMENTS Funding for this work was provided by the Mining and Minerals Resources and Research Institute and the Center for Coal and Energy Research at Virginia Tech.
REFERENCES 1
2 3 4
Allardice, D. J. and Evans, D. G., ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr), Academic Press Inc., New York, 1978 Glanville, J. 0. and Wightman, J. P. Fuel 1980, 59, 557 Widyani, E. and Wightman, J. P. Colloids and .Su$~es 1982,4, 209 Wightman, J. P., Awad, H. and Widyani, E., paper presented at Intl. Conf. on Coal Chemistry, Pittsburgh, PA, 1983
13 14
15 16 17 18 19 20
21 22
Volborth, A., Miller, G. E., Garner, C. K. and Jerebek, P. A. Frre/ 1977. Xi. 209 Mraw, S. C. and O’Rourke. D. F. J. ColloirlInterfircr Sci. 1982,89, 268 Mraw, S. C. and Silbernagel, B. G., AIP Conf. Proc. No. 70, Chemistry and Physics of Coal Utilization, 1980, 332 Sondreal, E. A. and Ellman, R. C., Report RI7887, US Bureau of Mines, Washington. DC, 1974 Nordon, P. and Bainbridge. N. W. Furl 1983,62. 619 Porter, H. C. and Ralston, 0. C., Technical Paper No. 113, US Bureau of Mines, Washington, DC, 1916 Zettlemoyer, A. C., Chemistry and Physics and Chemistry of Interfaces, (Ed. S. Ross), American Chemical Society, Washington. DC, 1965, 189 Scholberg, H. M. MS. Thesis, University of North Dakota, Grand Forks, ND, 1932 Can, H.. Nandi. S. P. and Walker, Jr., P. L. Fuel 1972, 51, 277 Mahajan. 0. P. and Walker, Jr., P. L. ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr). Academic Press, Inc., New York, 1978 Reucroft, P. J. and Patel, K. B. Furl 1983. 62, 279 Test methods D-3302 and D-3 173, American Society for Testing and Materials, Philadelphia, PA. Periodically revised Riley, J. T. Amrricun Luhorator~ 1983, 15, 17 Luppens, J. A., paper presented at the 2nd Annual Coal Testing Conf., Lexington. Kentucky, 1982 Patzman, R. A. J. Coul Quulity 1984, 3, 38 Squires. A. M.,Taylor, L. T., Brown, R. S. and Hellgeth J. W.,‘The Occurence and Role of Organometallics in Coal Liquefactions’, Virginia Polytechnic Institute and State University, Blacksburg, VA 1983 Senkan, S. M. and Fuller, Jr., E. L. Furl 1979, 58, 729 Partyka, S., Rouquerol, F. and Rouquerol. J. J. Colloid Infr~$~. Sci 1979,68, 2 1
FUEL,
1986,
Vol 65, May
649