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Heat of immersion of Virginia-C coal in water as a function of surface oxidation
Colloids and Surfaces, 21 (1986) l-8 Elsevier Science Publishers B.V., Amsterdam
1 -
Printed
in The Netherlands
Heat of Immersion of Virginia-C Coal in Water as a Function of Surface Oxidation*,** K.M. PHILLIPS,
J.O. GLANVILLE
and J.P. WIGHTMAN
Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Received 3 October 1985; accepted in final form 27 February
1986)
ABSTRACT The heat of immersion of Virginia-C coal in water has been determined following heating of the coal in air for 24 h between 125 and 320°C. A fifty fold increase in the heat of immersion was observed. The total oxygen content of each thermally treated coal was determined by elemental analysis and increased as the preheat temperature increased. Surface oxygen content was determined by ESCA analysis. The oxygen/carbon ratio calculated from the ESCA spectra paralleled values for the total oxygen content. Above 150°C there is a linear relationship between the heat of immersion and the O/C ratio. The increase in the O/C ratio is attributed to formation of more >C-0 surface groups.
INTRODUCTION
The state of oxidation of coal affects processes such as coal flotation [l], coking [2,3] liquefaction [4], and spontaneous combustion [5]. Invariably these processes involve the coal surface, and in some cases the interaction of a liquid with the surface. ESCA (electron spectroscopy for chemical analysis) and the heat of immersion (heat of wetting) are two quite different surface sensitive measurements which yield complementary chemical information applicable to the characterization and utilization of coal. Extensive reviews of the ESCA technique are available [6-81. Indeed, the use of ESCA in coal analysis has been reported [3,6,9-121. Elements except hydrogen within the top 5 nm of the coal surface can be distinguished, including trace elements, because of characteristic electron binding energies. Additionally, binding energy shifts are indicative of the chemical bonding state of surface species. Chessick and Zettlemoyer [13] have published an excellent review of the *The authors wish to note the passing on 22 April 1985 of Professor P.H. Emmett who remained active in research well past “retirement age” (see Ref. [30]). Professor Pa&t along with many others used an analysis of surface areas based on the Brunauer-Emmett-Teller equation. **Dedicated to the memory of Professor G.D. Parfitt.
0166-6622/86/$03.50
0 1986 Elsevier Science Publishers
B.V.
2
theory and application of immersional wetting measurements. The polarity and structure of both the liquid and the solid have been shown to influence the amount of heat released [ 14,151. Parfitt made a number of significant contributions to many areas of surface and colloid science. Of particular relevance to the present work, Parfitt and Tideswell reported [ 161 the heats of immersion of Graphon in binary mixtures of heptane and hexadecane in which case strong evidence was obtained for a highly ordered adsorbed layer of hexadecane against the Graphon surface. Parfitt and Rochester reviewed recently [ 171 the role of immersional calorimetry in studying the adsorption of small molecules from solution onto solids and commented on the critical need for further studies in this area. The heat of immersion of coals in liquids has been used to investigate pore structure [ 18-221, surface area [ 231, and moisture content [ 24,251. Factors such as storage temperature [26] and exposure to ultraviolet light [3] have been shown to affect coal oxidation. Furthermore, Perry and Grint [lo] found that the surface oxygen content varied with coal rank, decreasing from anthracite to lignite. Thus, it is useful to understand how the degree of oxidation affects other properties of coal. In the present work the heat of immersion of a bituminous coal in water was determined as a function of controlled oxidation of the coal. METHODS, TECHNIQUES
AND MATERIALS
STUDIED
Virginia-C is a bituminous coal from a seam in Buchanan County, VA. It was received from the Island Creek Coal Company as a - 325 mesh ( < 44 pm) powder. The ash and moisture contents were 5.22% and 0.15%, respectively, and the ultimate analysis of the as-received coal, determined by Galbraith Laboratories is given in Table 1. The BET krypton surface area was determined by Micromeritics to be 1.9 m2 g-l. Oxidation was achieved by heating the coal for a 24-h period in a thermostatted air oven at temperatures ranging from 110 to 320 2 5’ C. For each temperature treatment a portion of coal was spread to a thin layer in a Pyrex dish, heated, removed from the oven, immediately covered with aluminum foil, and stored in a dessicator over CaCl,. Subsequent TABLE 1 Ultimate analysis of Virginia-C coal Element
Weight percent (MF basis)
Carbon Oxygen (by difference) Hydrogen Nitrogen Sulfur Chlorine
81.0 6.9 4.25 0.97 0.64 0.20
3 TABLE 2 Summary of ESCA and oxygen analysis results for Virginia-C coal Preheat temp. (“0
[O/Cl ESCA
110 150 175 210 225 250 300 320
0.053 0.065 0.087 0.11 0.16 0.18 0.22 0.27 0.28
Oxygen (wt%)
9.2
18.3 23.8 33.6
experiments were performed within two weeks of heat treatment. A control sample, tested 40 days after heating, yielded ESCA and calorimetric results no different than a sample from the same batch run 2 days after heating. Bulk C-H-N analyses of the coal after the thermal treatment at 110,210, 250, and 320” C were obtained from Multichem Laboratories, Inc. The percent oxygen was determined by difference, with the contribution of mineral matter assumed to be the same for all samples. The total oxygen content determined at these temperatures is listed in Table 2. Heats of immersion were determined using a Calvet MS-70 microcalorimeter and a previously reported procedure [ 191. A known weight of coal (20-40 mg), outgassed at < low4 torr for 2 h at room temperature in a Pyrex bulb, was immersed in 5 ml of distilled, deionized water. Four samples were run for each thermal treatment and the average value calculated. The precision of a typical heat of immersion was determined to be approximately 10%. All heats of immersion were exothermic. A correction was applied for the heat of empty bulb breaking, which was approximately 75 mJ ( < 4% of the total heat). ESCA analysis was performed with a Kratos XSAM-800 spectrometer using a 1253.6 eV MgK, X-ray source and a hemispherical analyzer at a background pressure of 1 x lo-’ torr. A thick coat of coal powder was mounted on the sample probe with double-stick tape. Wide scan spectra (0 to 1200 eV) were obtained for each sample followed by narrow scan spectra (approximately 25 eV wide) to achieve better sensitivity for each element. All binding energies were referenced to the C(1.s) photopeak at 285.0 eV. The following equation was used to calculate the atomic fraction (AF) of each element: AF= RAF/.ZRAF where the relative atomic fraction (RAF) is given by
4
RAF=A/(c-m-KE-S) with A representing the area under the photopeak, c and m the reported [27] electron cross-section and mean free path values, respectively, KE the kinetic energy of the photoejected electron, and S the number of spectral sweeps. Curvefitting of the C( 1s) photopeak was done with a peak synthesis program, which is part of the Kratos DS300X data system. The full width at half height (FWHH) of 1.9 eV was used for each subpeak. RESULTS AND DISCUSSION
Heat of immersion and oxidation The heat of immersion in water of coal heated to different temperatures is shown in Fig. 1. There is a nearly fifty-fold larger heat of immersion for coal heated at 320°C than for unheated coal. Such a dramatic increase in the heat of immersion bespeaks significant changes in the coal surface. The change in the heat .ohimmersion is modest until the coal is heated above 150 oC. In predicting spontaneous ignition of coal under non-adiabatic conditions, the auto-ignition temperature (AIT) is frequently used. The AIT is that temperature at which sensible heat begins to be produced by oxidation of the coal sample. A recent review [28] concludes that AITs of the U.S. coals are in the 120-140°C range. In related experiments in which oxygen-fed coal is heated in an oil bath, there comes a temperature at which the coal begins to heat above the bath temperature. This is the crossing point temperature. Nandy et al. [ 291 report crossing point temperatures in the 130-170°C. range for a variety of Indian coals. Both the AIT and the crossing point temperature are measures
Fig. 1. Heat of immersion (0) in water and oxygen to carbon ratio ( A ) as a function of pretreat temperature of Virginia-C coal.
5
of the onset of a more vigorous oxidation. Clearly, the work here reported confirms that an oxidation take-off temperature exists in the vicinity of 150°C by immersional calorimetry. ESCA and oxidation An independent technique which purports to account for this sudden change in the heat of immersion is ESCA analysis. The surface oxygen content of the thermally treated coals was determined by ESCA analysis. The values of atomic fraction based on the C(ls) and O(ls) photopeaks were determined for coal heated at seven different temperatures up to 320” C. The percentage of carbon and oxygen together account for between 96 and 99% of the surface elements. The values of the atomic fraction ratio of oxygen to carbon as calculated from the ESCA spectra are listed in Table 2 and are plotted in Fig. 1 as a function of preheat temperature. Again, there is only a modest rise in the O/C ratio up to 110°C but there follows a dramatic increase with increasing preheat temperature up to 300 oC. There is one feature in Fig. 1 which might suggest an increase in the effective surface area of the coal. Heating beyond 300’ C produces a significant increase in the heat of immersion but only a modest change in the O/C ratio. Ludvig et al. [ 301 found for a bituminous coal that heating at 350 oC in air resulted in a greater surface area as measured by CO, adsorption at 298 K. A larger effective surface area could produce a significant increase in the heat of immersion without a corresponding change in the O/C ratio. Since there is a close correspondence between the heat of immersion and the surface O/C ratio as determined by ESCA, the results in Fig. 1 are replotted in Fig. 2. The results fall into three distinct regions. First, there is only a modest increase in the heat of immersion in water of coal with a surface O/C ratio < 0.11 followed secondly by a linear increase up to 0.27. In the third region, the heat of immersion then increases dramatically for a minimal increase of the surface O/C ratio to 0.28. The total oxygen contents determined by difference from elemental analysis and the ESCA O/C ratios are shown in Table 2 and also plotted as part of Fig. 2. It can be seen that above llO”C, increasing surface O/C ratio parallels increasing total oxygen content. However, above 300’ C, total oxygen content increases more rapidly than the surface O/C ratio. This finding is not unlike the results of flotability studies where coal flotability and total oxygen content are related [ 11. The fact that surface sensitive properties such as immersion and flotation and surface O/C ratios vary in a similar manner to the total (bulk) oxygen content clearly suggests that coal cannot be viewed as an inert, inpenetrable, stable substrate. The complex nature of the coal surface is again revealed. Perry and Grint [lo] suggested that coal oxidation occurs initially via the external surface. It is perhaps not surprising then that controlled oxidation of
6
Fig. 2. Heat of immersion (0) in water and total oxygen content ( A ) as a function of the oxygen to carbon ratio for Virginia-C coal heated to different temperatures.
-
BINDING
ENERGY
(&I
-
Fig. 3. Curve fit C(1.s) photopeaks for Virginia-C coal heated to different temperatures. The large right-hand peak is from C-C, the smaller peaks to the left are C-0 bonds of various types (see text).
coal results in a higher heat of immersion in water and an increase in the surface oxygen concentration. However, this is the first documented study combining the results of the immersional calorimetry and ESCA analysis of oxidized coal. As noted above, carbon and oxygen accounted for > 96% of the coal as determined by ESCA analysis. Other elements detected in trace amounts were silicon, sulfur, nitrogen, aluminum and chlorine. The C(ls) photopeak for each thermally treated coal was curve fitted as described above and the results are shown in Fig. 3. The specific photopeaks were assigned [3,9,10] to the following functionalized forms of carbon: 285.0 eV =C-H; 286.8 eV =c-0; 288.9 eV=C=O; 290.8 eV=COOH; and 292.5 eV= x to 7c*shake-up satellite. In all cases, singly bonded C-O groups (ether and hydroxyls) contribute most to the total oxygen content, with carbonyls and carboxylates making lesser contributions. As the preheating temperature is raised, the C( 1s) photopeak becomes more and more asymmetric, due to an increase in the carbon-oxygen subpeaks. The concentration of C-O groups increases most upon heating, while C=O groups increase to a lesser extent. CONCLUSION
The increasing heat of immersion in water of coal heated up to 320 oC is due to the increasing number of surface >C-0 groups. This is a definitive result and demonstrates a further understanding of coal surfaces gained by combining thermodynamic and spectroscopic techniques. ACKNOWLEDGEMENTS
The authors acknowledge with appreciation the assistance of Steve McCartney with ESCA, and the help of Andy Mollick who provided custom glassblowing for calorimetric experiments. The help of Terry Hamrich at the Island Creek Coal Company in obtaining the coal samples was appreciated. 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
D.W. Fuerstenau, J.M. Rosenbaum and J. Laskowski, Colloids Surfaces, 8 (1983) 153. J. Gibson, in G.J. Pitt and G.R. Millward (Eds), Coal and Modern Coal Processing: An Introduction, Academic Press, New York, 1979, pp. 62-63. D.T. Clark and R. Wilson, Fuel, 62 (1983) 1034. D.C. Cronauer, R.G. Ruberto, R.G. Jenkins, A. Davis, P.C. Painter, D.S. Hoover, M.E. Starsinic and D. Schlyer, Fuel, 62 (1983) 1124.
8 5 6
8 9 10
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
30
J.O. Glanville, L.H. Haley, Jr and J.P. Wightman, in G.D. Botsaris and Y. Glazman (Eds), Interfacial Phenomena in Coal Technology, Marcel Dekker, in press. D.C. Frost, B. Wallbank and W.R. Leeder, in C.Karr, Jr (Ed.), Analytical Methods for Coal and Coal Products, Vol. I, Academic press, New York, 1978, pp. 349-376. L.A. Casper and C.J. Powell (Eds), Industrial Applications of Surface Analysis, Am. Chem. Sot. Ser. 199, Am. Chem. Sot., Washington, DC, 1982. D.T. Clark, Adv. Polymer Sci., 24 (1977) 125. J.R. Brown, B.I. Kronberg and W.S. Fyfe, Fuel, 60 (1981) 439. D.L. Perry and A. Grint, Fuel, 62 (1983) 1024. R.B. Jones, C.B. McCourt and P. Swift, in Proceedings Int. Conf. on Coal Sci., Dusseldorf, Verlag Gluckauf, Essen, 1981, p. 657. H. Marsh, P.M.A. Sherwood and D. Augustyn, Fuel, 55 (1976) 97. J.J. Chessick and AC. Zettlemoyer, Adv. Catal., 11 (1959) 263. A.C. Zettlemoyer and J.J. Chessick, J. Phys. Chem., 62 (1958) 1217. D.H. Everett, and G.H. Findenegg, Nature, 223 (1969) 52. G.D. Parfitt and M.W. Tideswell, J. Colloid Interface Sci., 79 (1981) 518. G.D. Parfitt and C.H. Rochester, in G.D. Parfitt and C.H. Rochester (Eds), Adsorption from Solution at the Solid/Liquid Interface, Academic Press, London, 1983, pp. l-47. J.O. Glanville and J.P. Wightman, Fuel, 59 (1980) 557. E. Widyani and J.P. Wightman, Colloids Surfaces, 4 (1982) 209. E.L. Fuller, Jr, J. Colloid Interface Sci., 75 (1980) 577. J.P. Wightman, H. Awad and E. Widyani, Proc. Int. Conf. on Coal Chemistry, PETC, Pittsburgh, PA, 1983, p. 245. J.O. Glanville, K.L. Newcomb and J.P. Wightman, Fuel, 65 (1986) 485. O.P. Mahajan and P.L. Walker Jr, in C. Karr (Ed.), Analytical Methods for Coal and Coal Products, Vol. I, Academic Press, New York, 1978, pp. 140-141. P. Nordon and N.W. Bainbridge, Fuel, 62 (1983) 619. J.O. Glanville, ST. Hall, D.L. Messick, K.L. Newcomb, K.M. Phillips, H.F. Webster and J.P. Wightman, Fuel, 65 (1986) 647. R. Bouwman and I.L.C. Freriks, Fuel, 59 (1980) 315. J,H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. J.M. Kuchta, V.R. Rowe and D.S. Burgess, Report RI 8474, U.S. Bureau of Mines, Washington, DC, 1980. D.K. Nandy, D.D. Banerjee and R. Chakravorty, J. Mines Met. Fuels, 20 (1972) 41. M.M. Ludvig, G.L. Gard and P.H. Emmett, Fuel, 62 (1983) 1393.
1 -
Printed
in The Netherlands
Heat of Immersion of Virginia-C Coal in Water as a Function of Surface Oxidation*,** K.M. PHILLIPS,
J.O. GLANVILLE
and J.P. WIGHTMAN
Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Received 3 October 1985; accepted in final form 27 February
1986)
ABSTRACT The heat of immersion of Virginia-C coal in water has been determined following heating of the coal in air for 24 h between 125 and 320°C. A fifty fold increase in the heat of immersion was observed. The total oxygen content of each thermally treated coal was determined by elemental analysis and increased as the preheat temperature increased. Surface oxygen content was determined by ESCA analysis. The oxygen/carbon ratio calculated from the ESCA spectra paralleled values for the total oxygen content. Above 150°C there is a linear relationship between the heat of immersion and the O/C ratio. The increase in the O/C ratio is attributed to formation of more >C-0 surface groups.
INTRODUCTION
The state of oxidation of coal affects processes such as coal flotation [l], coking [2,3] liquefaction [4], and spontaneous combustion [5]. Invariably these processes involve the coal surface, and in some cases the interaction of a liquid with the surface. ESCA (electron spectroscopy for chemical analysis) and the heat of immersion (heat of wetting) are two quite different surface sensitive measurements which yield complementary chemical information applicable to the characterization and utilization of coal. Extensive reviews of the ESCA technique are available [6-81. Indeed, the use of ESCA in coal analysis has been reported [3,6,9-121. Elements except hydrogen within the top 5 nm of the coal surface can be distinguished, including trace elements, because of characteristic electron binding energies. Additionally, binding energy shifts are indicative of the chemical bonding state of surface species. Chessick and Zettlemoyer [13] have published an excellent review of the *The authors wish to note the passing on 22 April 1985 of Professor P.H. Emmett who remained active in research well past “retirement age” (see Ref. [30]). Professor Pa&t along with many others used an analysis of surface areas based on the Brunauer-Emmett-Teller equation. **Dedicated to the memory of Professor G.D. Parfitt.
0166-6622/86/$03.50
0 1986 Elsevier Science Publishers
B.V.
2
theory and application of immersional wetting measurements. The polarity and structure of both the liquid and the solid have been shown to influence the amount of heat released [ 14,151. Parfitt made a number of significant contributions to many areas of surface and colloid science. Of particular relevance to the present work, Parfitt and Tideswell reported [ 161 the heats of immersion of Graphon in binary mixtures of heptane and hexadecane in which case strong evidence was obtained for a highly ordered adsorbed layer of hexadecane against the Graphon surface. Parfitt and Rochester reviewed recently [ 171 the role of immersional calorimetry in studying the adsorption of small molecules from solution onto solids and commented on the critical need for further studies in this area. The heat of immersion of coals in liquids has been used to investigate pore structure [ 18-221, surface area [ 231, and moisture content [ 24,251. Factors such as storage temperature [26] and exposure to ultraviolet light [3] have been shown to affect coal oxidation. Furthermore, Perry and Grint [lo] found that the surface oxygen content varied with coal rank, decreasing from anthracite to lignite. Thus, it is useful to understand how the degree of oxidation affects other properties of coal. In the present work the heat of immersion of a bituminous coal in water was determined as a function of controlled oxidation of the coal. METHODS, TECHNIQUES
AND MATERIALS
STUDIED
Virginia-C is a bituminous coal from a seam in Buchanan County, VA. It was received from the Island Creek Coal Company as a - 325 mesh ( < 44 pm) powder. The ash and moisture contents were 5.22% and 0.15%, respectively, and the ultimate analysis of the as-received coal, determined by Galbraith Laboratories is given in Table 1. The BET krypton surface area was determined by Micromeritics to be 1.9 m2 g-l. Oxidation was achieved by heating the coal for a 24-h period in a thermostatted air oven at temperatures ranging from 110 to 320 2 5’ C. For each temperature treatment a portion of coal was spread to a thin layer in a Pyrex dish, heated, removed from the oven, immediately covered with aluminum foil, and stored in a dessicator over CaCl,. Subsequent TABLE 1 Ultimate analysis of Virginia-C coal Element
Weight percent (MF basis)
Carbon Oxygen (by difference) Hydrogen Nitrogen Sulfur Chlorine
81.0 6.9 4.25 0.97 0.64 0.20
3 TABLE 2 Summary of ESCA and oxygen analysis results for Virginia-C coal Preheat temp. (“0
[O/Cl ESCA
110 150 175 210 225 250 300 320
0.053 0.065 0.087 0.11 0.16 0.18 0.22 0.27 0.28
Oxygen (wt%)
9.2
18.3 23.8 33.6
experiments were performed within two weeks of heat treatment. A control sample, tested 40 days after heating, yielded ESCA and calorimetric results no different than a sample from the same batch run 2 days after heating. Bulk C-H-N analyses of the coal after the thermal treatment at 110,210, 250, and 320” C were obtained from Multichem Laboratories, Inc. The percent oxygen was determined by difference, with the contribution of mineral matter assumed to be the same for all samples. The total oxygen content determined at these temperatures is listed in Table 2. Heats of immersion were determined using a Calvet MS-70 microcalorimeter and a previously reported procedure [ 191. A known weight of coal (20-40 mg), outgassed at < low4 torr for 2 h at room temperature in a Pyrex bulb, was immersed in 5 ml of distilled, deionized water. Four samples were run for each thermal treatment and the average value calculated. The precision of a typical heat of immersion was determined to be approximately 10%. All heats of immersion were exothermic. A correction was applied for the heat of empty bulb breaking, which was approximately 75 mJ ( < 4% of the total heat). ESCA analysis was performed with a Kratos XSAM-800 spectrometer using a 1253.6 eV MgK, X-ray source and a hemispherical analyzer at a background pressure of 1 x lo-’ torr. A thick coat of coal powder was mounted on the sample probe with double-stick tape. Wide scan spectra (0 to 1200 eV) were obtained for each sample followed by narrow scan spectra (approximately 25 eV wide) to achieve better sensitivity for each element. All binding energies were referenced to the C(1.s) photopeak at 285.0 eV. The following equation was used to calculate the atomic fraction (AF) of each element: AF= RAF/.ZRAF where the relative atomic fraction (RAF) is given by
4
RAF=A/(c-m-KE-S) with A representing the area under the photopeak, c and m the reported [27] electron cross-section and mean free path values, respectively, KE the kinetic energy of the photoejected electron, and S the number of spectral sweeps. Curvefitting of the C( 1s) photopeak was done with a peak synthesis program, which is part of the Kratos DS300X data system. The full width at half height (FWHH) of 1.9 eV was used for each subpeak. RESULTS AND DISCUSSION
Heat of immersion and oxidation The heat of immersion in water of coal heated to different temperatures is shown in Fig. 1. There is a nearly fifty-fold larger heat of immersion for coal heated at 320°C than for unheated coal. Such a dramatic increase in the heat of immersion bespeaks significant changes in the coal surface. The change in the heat .ohimmersion is modest until the coal is heated above 150 oC. In predicting spontaneous ignition of coal under non-adiabatic conditions, the auto-ignition temperature (AIT) is frequently used. The AIT is that temperature at which sensible heat begins to be produced by oxidation of the coal sample. A recent review [28] concludes that AITs of the U.S. coals are in the 120-140°C range. In related experiments in which oxygen-fed coal is heated in an oil bath, there comes a temperature at which the coal begins to heat above the bath temperature. This is the crossing point temperature. Nandy et al. [ 291 report crossing point temperatures in the 130-170°C. range for a variety of Indian coals. Both the AIT and the crossing point temperature are measures
Fig. 1. Heat of immersion (0) in water and oxygen to carbon ratio ( A ) as a function of pretreat temperature of Virginia-C coal.
5
of the onset of a more vigorous oxidation. Clearly, the work here reported confirms that an oxidation take-off temperature exists in the vicinity of 150°C by immersional calorimetry. ESCA and oxidation An independent technique which purports to account for this sudden change in the heat of immersion is ESCA analysis. The surface oxygen content of the thermally treated coals was determined by ESCA analysis. The values of atomic fraction based on the C(ls) and O(ls) photopeaks were determined for coal heated at seven different temperatures up to 320” C. The percentage of carbon and oxygen together account for between 96 and 99% of the surface elements. The values of the atomic fraction ratio of oxygen to carbon as calculated from the ESCA spectra are listed in Table 2 and are plotted in Fig. 1 as a function of preheat temperature. Again, there is only a modest rise in the O/C ratio up to 110°C but there follows a dramatic increase with increasing preheat temperature up to 300 oC. There is one feature in Fig. 1 which might suggest an increase in the effective surface area of the coal. Heating beyond 300’ C produces a significant increase in the heat of immersion but only a modest change in the O/C ratio. Ludvig et al. [ 301 found for a bituminous coal that heating at 350 oC in air resulted in a greater surface area as measured by CO, adsorption at 298 K. A larger effective surface area could produce a significant increase in the heat of immersion without a corresponding change in the O/C ratio. Since there is a close correspondence between the heat of immersion and the surface O/C ratio as determined by ESCA, the results in Fig. 1 are replotted in Fig. 2. The results fall into three distinct regions. First, there is only a modest increase in the heat of immersion in water of coal with a surface O/C ratio < 0.11 followed secondly by a linear increase up to 0.27. In the third region, the heat of immersion then increases dramatically for a minimal increase of the surface O/C ratio to 0.28. The total oxygen contents determined by difference from elemental analysis and the ESCA O/C ratios are shown in Table 2 and also plotted as part of Fig. 2. It can be seen that above llO”C, increasing surface O/C ratio parallels increasing total oxygen content. However, above 300’ C, total oxygen content increases more rapidly than the surface O/C ratio. This finding is not unlike the results of flotability studies where coal flotability and total oxygen content are related [ 11. The fact that surface sensitive properties such as immersion and flotation and surface O/C ratios vary in a similar manner to the total (bulk) oxygen content clearly suggests that coal cannot be viewed as an inert, inpenetrable, stable substrate. The complex nature of the coal surface is again revealed. Perry and Grint [lo] suggested that coal oxidation occurs initially via the external surface. It is perhaps not surprising then that controlled oxidation of
6
Fig. 2. Heat of immersion (0) in water and total oxygen content ( A ) as a function of the oxygen to carbon ratio for Virginia-C coal heated to different temperatures.
-
BINDING
ENERGY
(&I
-
Fig. 3. Curve fit C(1.s) photopeaks for Virginia-C coal heated to different temperatures. The large right-hand peak is from C-C, the smaller peaks to the left are C-0 bonds of various types (see text).
coal results in a higher heat of immersion in water and an increase in the surface oxygen concentration. However, this is the first documented study combining the results of the immersional calorimetry and ESCA analysis of oxidized coal. As noted above, carbon and oxygen accounted for > 96% of the coal as determined by ESCA analysis. Other elements detected in trace amounts were silicon, sulfur, nitrogen, aluminum and chlorine. The C(ls) photopeak for each thermally treated coal was curve fitted as described above and the results are shown in Fig. 3. The specific photopeaks were assigned [3,9,10] to the following functionalized forms of carbon: 285.0 eV =C-H; 286.8 eV =c-0; 288.9 eV=C=O; 290.8 eV=COOH; and 292.5 eV= x to 7c*shake-up satellite. In all cases, singly bonded C-O groups (ether and hydroxyls) contribute most to the total oxygen content, with carbonyls and carboxylates making lesser contributions. As the preheating temperature is raised, the C( 1s) photopeak becomes more and more asymmetric, due to an increase in the carbon-oxygen subpeaks. The concentration of C-O groups increases most upon heating, while C=O groups increase to a lesser extent. CONCLUSION
The increasing heat of immersion in water of coal heated up to 320 oC is due to the increasing number of surface >C-0 groups. This is a definitive result and demonstrates a further understanding of coal surfaces gained by combining thermodynamic and spectroscopic techniques. ACKNOWLEDGEMENTS
The authors acknowledge with appreciation the assistance of Steve McCartney with ESCA, and the help of Andy Mollick who provided custom glassblowing for calorimetric experiments. The help of Terry Hamrich at the Island Creek Coal Company in obtaining the coal samples was appreciated. 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
D.W. Fuerstenau, J.M. Rosenbaum and J. Laskowski, Colloids Surfaces, 8 (1983) 153. J. Gibson, in G.J. Pitt and G.R. Millward (Eds), Coal and Modern Coal Processing: An Introduction, Academic Press, New York, 1979, pp. 62-63. D.T. Clark and R. Wilson, Fuel, 62 (1983) 1034. D.C. Cronauer, R.G. Ruberto, R.G. Jenkins, A. Davis, P.C. Painter, D.S. Hoover, M.E. Starsinic and D. Schlyer, Fuel, 62 (1983) 1124.
8 5 6
8 9 10
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J.O. Glanville, L.H. Haley, Jr and J.P. Wightman, in G.D. Botsaris and Y. Glazman (Eds), Interfacial Phenomena in Coal Technology, Marcel Dekker, in press. D.C. Frost, B. Wallbank and W.R. Leeder, in C.Karr, Jr (Ed.), Analytical Methods for Coal and Coal Products, Vol. I, Academic press, New York, 1978, pp. 349-376. L.A. Casper and C.J. Powell (Eds), Industrial Applications of Surface Analysis, Am. Chem. Sot. Ser. 199, Am. Chem. Sot., Washington, DC, 1982. D.T. Clark, Adv. Polymer Sci., 24 (1977) 125. J.R. Brown, B.I. Kronberg and W.S. Fyfe, Fuel, 60 (1981) 439. D.L. Perry and A. Grint, Fuel, 62 (1983) 1024. R.B. Jones, C.B. McCourt and P. Swift, in Proceedings Int. Conf. on Coal Sci., Dusseldorf, Verlag Gluckauf, Essen, 1981, p. 657. H. Marsh, P.M.A. Sherwood and D. Augustyn, Fuel, 55 (1976) 97. J.J. Chessick and AC. Zettlemoyer, Adv. Catal., 11 (1959) 263. A.C. Zettlemoyer and J.J. Chessick, J. Phys. Chem., 62 (1958) 1217. D.H. Everett, and G.H. Findenegg, Nature, 223 (1969) 52. G.D. Parfitt and M.W. Tideswell, J. Colloid Interface Sci., 79 (1981) 518. G.D. Parfitt and C.H. Rochester, in G.D. Parfitt and C.H. Rochester (Eds), Adsorption from Solution at the Solid/Liquid Interface, Academic Press, London, 1983, pp. l-47. J.O. Glanville and J.P. Wightman, Fuel, 59 (1980) 557. E. Widyani and J.P. Wightman, Colloids Surfaces, 4 (1982) 209. E.L. Fuller, Jr, J. Colloid Interface Sci., 75 (1980) 577. J.P. Wightman, H. Awad and E. Widyani, Proc. Int. Conf. on Coal Chemistry, PETC, Pittsburgh, PA, 1983, p. 245. J.O. Glanville, K.L. Newcomb and J.P. Wightman, Fuel, 65 (1986) 485. O.P. Mahajan and P.L. Walker Jr, in C. Karr (Ed.), Analytical Methods for Coal and Coal Products, Vol. I, Academic Press, New York, 1978, pp. 140-141. P. Nordon and N.W. Bainbridge, Fuel, 62 (1983) 619. J.O. Glanville, ST. Hall, D.L. Messick, K.L. Newcomb, K.M. Phillips, H.F. Webster and J.P. Wightman, Fuel, 65 (1986) 647. R. Bouwman and I.L.C. Freriks, Fuel, 59 (1980) 315. J,H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 (1976) 129. J.M. Kuchta, V.R. Rowe and D.S. Burgess, Report RI 8474, U.S. Bureau of Mines, Washington, DC, 1980. D.K. Nandy, D.D. Banerjee and R. Chakravorty, J. Mines Met. Fuels, 20 (1972) 41. M.M. Ludvig, G.L. Gard and P.H. Emmett, Fuel, 62 (1983) 1393.