Energy & Fuels 1999, 13, 965-968
965
Influence of Fractal Pores on the Oxidation Behavior of Brown Coal P. J. McMahon,*,† I. K. Snook,‡ S. D. Moss,§ and P. R. Johnston| School of Physics, University of Melbourne, Parkville, Victoria 3052, Australia, Department of Applied Physics, Royal Melbourne Institute of Technology, Box 2476V, Melbourne, Australia 3001, School of Applied Science, University of Tasmania, P.O. Box 1214, Launceston, 7250 Tasmania, Australia, and Chemical Engineering Department, Monash University, Clayton Campus, Clayton, 3168 Victoria, Australia Received October 8, 1998. Revised Manuscript Received June 1, 1999
We show that both chemical and physical changes induced in Victorian brown coal through thermal processing in the presence of a range of gases has a profound effect on its oxidation behavior. In particular, the appearance of a low-temperature DTA peak which is believed to be related to the propensity of the coal to spontaneously combust is strongly correlated with the extent of the pore surface as determined by the fractal surface measure obtained from smallangle X-ray scattering measurements.
Introduction One of the most vital characteristics of coal is its combustion behavior and low-rank coals, e.g., Victorian brown coal, differs fundamentally from high-rank coals in that they exhibit low-temperature oxidation and often have a tendency to spontaneously combust at relatively low temperatures. There have been, of course, an enormous number of studies of coal combustion, and there is a corresponding large literature. It is clear that some of the most important factors affecting spontaneous combustion are1,2 (a) particle size, (b) bed voidage, (c) reaction-rate parameters, (d) porosity, and (e) surface area. Although a very large number of methods have been used to investigate coal combustion, very convenient methods are those of differential thermal analysis (DTA), differential thermo gravimetric analysis (DTG), and combined DTA/DTG.3,4 In particular, DTA gives a combustion profile, or more strictly an oxidation profile, of dried low-rank coal which shows two large peaks, with features at about 300 °C which are unique to low-rank coal and another feature at around 400 °C which is common to all coals.4,5 The lower-temperature peak is thought to be related to the tendency of coal to spontaneously combust, and recently it has been shown that this low-temperature oxidation peak is strongly affected by (a) the cation content and (b) the chemistry of the functional groups present.5,6 * Author to whom correspondence should be addressed. † University of Melbourne. ‡ Royal Melbourne Institute of Technology. § University of Tasmania. | Monash University. (1) Brooks, K.; Svanas, N.; Glasser, D. Fuel 1988, 67, 651. (2) Evans, D. G.; Allardice, D. J. In Analytical Methods for Coal and Coal Products, Vol. 1; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Chapter 3, pp 102-103. (3) Marinov, V. N. Fuel 1977, 56, 153, 158. (4) Ghetti, P.; De Robertis, U.; D’Antone, S.; Villani, M.; Chiellini, E. Fuel 1985, 64, 950.
Combined DTA and gas adsorption measurements have shown that the surface area has a profound effect on the high-temperature combustion of coal.3,4 Recently we have shown that a useful method of characterizing the pore surfaces of brown coal is small-angle X-ray scattering (SAXS) which provides a measure of fractal geometry.7-9 This approach to the study of porous solids allows a simple method of investigation into change in porosity from complex solids such as coal, without the need for complex models of pore structure or advanced computation.9 The scattering approach takes a measure of the log intensity versus log scattering angle to extract the fractal dimension from the gradient of these data which is a measure of the space-filling character of the material under study. Further, an additional parameter may be taken from the intercept of the intensity axis termed the surface measure which is related to the surface area; however, it is a weighted average of surface area over the range of length scales measured in the experiment, unlike a real parameter of interest such as surface area extracted from gas adsorption, which however dependent upon the gas species can be related directly to chemical/physical behavior. The surface measure and fractal dimension are best suited as a comparative measure with other chemical techniques of analysis. Thus, in this paper, we use this method combined with the results of recent DTA/DTG measurements to investigate the effect of pore structure on the lowtemperature oxidation and, thus, potentially on com(5) Christie, G. B. Y.; Mainwaring, D. E. Fuel 1992, 71, 4, 443 (6) Johnson, P. R.; Jackson, W. R.; Mathews, J. F.; Mainwaring, D. E.; Bachelor, F. W. Proc. Chemica ’89; Gold Coast, Queensland, Australia, 1989; pp 76-80. (7) Reich, M. H.; Russo, S. P.; Snook, I. K.; Wagenfeld, H. K. J. Colloid Interface Sci. 1990, 135, 353. (8) Reich, M. H.; Snook, I. K.; Wagenfeld, H. K. Fuel 1992, 71, 669. (9) Johnston, P. R.; McMahon, P.; Reich, M. H.; Snook, I. K.; Wagenfeld, H. K. J. Colloid Interface Sci. 1993, 155, 146.
10.1021/ef980215i CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
966 Energy & Fuels, Vol. 13, No. 5, 1999
McMahon et al.
Table 1. Porosity and Combustion Characteristics of Coals Studied
a
coal
Dsa
Nob
1. Loy Yang untreated 2. Loy Yang sieved to 53 µm 3. Loy Yang Cu-treated 4. Taroom as-received 5. Loy Yang N2-upgraded 200 °C 300 °C 400 °C 6. Loy Yang CO/H2O-upgraded 200 °C 300 °C 400 °C
2.47 ( 0.07 2.37 ( 0.07 2.43 ( 0.07 2.62 ( 0.16
5.0 ( 1.1 7.3 ( 1.9 6.9 ( 1.5 0.69 ( 0.17
2.37 ( 0.07 2.24 ( 0.07 2.27 ( 0.08
7.2 ( 1.8 11.7 ( 3.4 9.9 ( 3.1
2.28 ( 0.07 2.13 ( 0.09 2.39 ( 0.09
9.3 ( 2.7 7.6 ( 2.4 1.1 ( 0.3
low-temperature peakc 1.2 1.3 2.8 absent 1.4 1.2 1.0 1.3 1.0 absent
Fractal dimension of pore surfaces. b Surface measure, arbitrary units. c Height of major first energy release, arbitrary units.
bustion and spontaneous combustion of Victorian brown coal in its raw and upgraded states. 8,9 Experimental Methods The samples used were air-dried and sized to between 53 and 106 µm. Acid washing was performed with 0.1 M H2SO4 using the method of Redlich.10,11 The Cutreated samples were produced by washing the raw coal with appropriate amounts of copper acetate, and the upgraded coals were made by heating in a nitrogen atmosphere or a CO/H2O atmosphere as described before.10,11 The porosity was measured by use of a Kratky camera and a Cu anode generator as detailed previously,7 and the resultant scattering curves were analyzed as before7 in terms of fractal geometry which yields a description of the pore structure in terms of the surface fractal dimension, Ds that gives a measure of the irregularity or roughness of pore surfaces and the fractal surface measure, No, which is the true scale-independent measure of the interface.7-9 Both DTA/DTG and SAXS measurements were made on a variety of samples in order to identify the effect of the surface and other properties on oxidation as discussed in the next section. Results and Discussion In Table 1 we have collected together a summary of the SAXS porosity measurements and the height of the first main combustion peak. To contrast the oxidation behavior of low-rank and high-rank coal we show, in Figure 1, the DTA oxidation curves of raw Loy Yang coal and that of Taroom coal which is a highly volatile sub-bituminous coal. As can be seen for the low-rank coal (Figure 1a) there are two main oxidation peaks, one at around 300 °C which may be related to spontaneous combustion and one around 400 °C. By contrast, the high-rank coal (Figure 1b) shows only a peak at around 400 °C. Similar results were obtained for two other highrank coals, namely Millmerran and Liddell coals.12 (10) Johnston, P. R.; Mathews, J. F.; Jackson, W. R. Proc. Aust. Coal Sci. Conf.; Adelaide, Australia, 1988; p. A4: 6.1-6.7. (11) Johnson, P. R.; Mathews, J. F. Chemica ’88; Australian Bicentennial International Conference for the Process Industries, Sydney, Australia, 1988; pp 246-250. (12) Johnston, P. R. Upgrading of Loy Yang Coal. Ph.D. Thesis, Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia, 1992.
Figure 1. (a) DTA curve of untreated Loy Yang brown coal. (b) DTA curve of high-volatile sub-bituminous Taroom coal.
Previously it has been shown5,6 that ions are able to effectively catalyze the low-temperature oxidation of low-rank coal. That this effect is largely due to cations is supported by comparing the results for untreated raw Loy Yang coal with that of Cu-treated Loy Yang coal. These two coals have virtually identical surface properties (Table 1); however, they differ substantially in their combustion characteristics, particularly in the lowtemperature region as can be seen by comparing Figures 1a and 2b where the low-temperature peak is shifted to lower temperatures and is more pronounced and the high-temperature peak is also shifted to lower temperatures and more broad. Examination of Table 1 and Figures 1a and 2a suggest that the surface measure, No which is an indicator of coal surface area and the oxidation behavior of raw Loy Yang are not affected by sieving the coal sieved to less than 53 µm and display results very similar to the untreated Loy Yang coal. Recently it has been shown10-12 that brown coal may be upgraded by heating in either inert N2 atmosphere
Fractal Pores and the Oxidation Behavior of Brown Coal
Energy & Fuels, Vol. 13, No. 5, 1999 967
Figure 2. (a) DTA curve of untreated Loy Yang coal sieved to less than 53 µm. (b) DTA curve of Cu-treated Loy Yang coal.
Figure 3. DTA curves of Loy Yang coal preheated under a 6 MPa N2 atmosphere. (a) 300 °C, (b) 400 °C.
or a hydrogen-rich atmosphere. Although the resultant product from either process has a calorific value comparable with black coal, the physical characteristics, particularly the pore structure, depend strongly on the process used.9,11 The N2-upgraded coals retain the powdery texture of the raw coal and have a large surface extent, whereas the CO/H2O at heated coals have a small surface extent when treated at temperatures above 325 °C and are hard shiny lumps indicating a significant change in coal structure. Furthermore, as indicated by Ds (Table 1) the fractal character of these two products is also similar, the former being of lower fractal dimension and large surface extent, whereas those treated in CO/H2O-rich atmosphere at high temperature have a higher fractal dimension as well as a very small value of No as can be seen from the table.9 From a comparison of the oxidation curves shown in Figures 3 and 4 it may be seen that this surface behavior is strongly reflected in these curves. The highsurface-area N2-upgraded coals have oxidation curves (Figure 3, parts a and b) almost unaffected by the temperature of upgrading, i.e., they remain very similar to untreated Loy Yang brown coal. This behavior is exactly mirrored in the behavior of their surface area measure versus treatment temperature, see Table 1.9 By contrast, the CO/H2O-upgraded coals have combustion curves which change dramatically from those typical of low-rank coal for low-temperature upgrading to those resembling high-rank coal curves for treatment temperatures above 325 °C. This is once again mirrored in the behavior of the fractal surface measure versus treatment temperature. In fact the combustion curve shown in Figure 4b is remarkably similar to that of the high-rank Taroom coal shown in Figure 1b.
Figure 4. DTA curves of Loy Yang coal preheated under a 6 MPa CO/H2O atmosphere. (a) 300 °C, (b) 400 °C.
Conclusions It has been previously conjectured that there are strong chemical influences on the spontaneous combustion behavior of brown coal, viz, the presence of cations, by comparing the combustion behavior of coals which essentially differ only in cation content and not in
968 Energy & Fuels, Vol. 13, No. 5, 1999
surface area (or strictly fractal surface measure).7 Furthermore, we show that a change in surface measures induced by gas treatment coincides with significant changes to the low-temperature oxidation peak in the DTA curve which is believed to be related to the tendency of coal to spontaneously combust. Although the surface measure is a useful parameter for comparison between samples undergoing chemical/ physical changes, it should be noted that this parameter is a weighted average over all length scales corresponding to the q range of the SAXS apparatus used. The data obtained using SAXS need not correlate with observations of chemical exchange or reactions at the molecular level, which correspond to large angles of scatter only (large q). In this regard, what is seen as fractally rough over a range of length scales should not be extrapolated to the molecular level as SAXS is prone to noise in this range; however, as a preliminary guide to changes in pore structure induced by external influences perhaps some insights may be taken. An example of particular interest is the case of upgraded brown coal which shows that the different treatment methods produce coals which are similar in calorific value but totally different in combustion behavior.
McMahon et al.
N2-upgraded coals are always similar to raw brown coal, but coal upgraded in CO/H2O at temperatures above about 325 °C are very similar to high-rank coal in combustion behavior. This work does not shed any light on the typical hightemperature combustion peaks since we have not determined the surface area measure as a function of temperature as was done by Ghetti.4 This would require quenching the samples at various temperatures and would provide an interesting extension of the work reported in this paper. One final point that should be emphasized is that it is the use of fractal geometry that enables the ready, unambiguous interpretation of pore surface structure to be made from the scattering curves of these coals and coal products. Without the use of fractal geometry it would be difficult to derive quantitative measures about pore structure from the SAXS measurements. Acknowledgment. We acknowledge the help and advice of Dr. David Mainwaring, formerly of the Department of Colloid Chemistry, Swinburne Institute of Technology. EF980215I
