Langmuir 1993,9, 2726-2729
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Fractal Pore Surfaces in Brown Coal, Their Changes on Processing and Their Effect on Combustion Ian Snook* and Phillip McMahon Department of Applied Physics, Royal Melbourne Institute of Technology, G.P.O. Box 2476V, Melbourne, Victoria, Australia 3001 Received February 19, 1993. In Final Form: August 9, 199P Using small angle X-ray scattering, we show that the scattering pattern caused by the pore surfaces of dried coal may often be interpreted as arising from fractally rough surfaces. In practice a wide range of values of the surface fractal dimension, D,, was found covering virtually the whole allowable range. In the case of brown coal we find that as it is dried from the bed-moist state, the scattering curve changes from that characteristic of a mass fractal to that appropriate to a surface fractal. Furthermore, during upgrading of brown coal by deoxygenationat various processing temperatures, T, changesof the parameters characterizing the fractal surfaces may occur. Under some conditions D,and the surface measure, NO, change very slightly with increasing T. However, heating in hydrogen-rich atmospheres causes dramatic increases in D,and dramatic decreases in NOfor T above about 320 O C . The results seem to indicate that after a very slight initial smoothing of the pore surfaces, they become very rough and of very much smaller extent indicating pore collapse. This correlates well with other properties of the coal, and in fact we are able to relate the oxidation (combustion) behavior of these coals with the measured values of No.
Introduction The pore structure of coal has long been the subject of intense interest and investigation.’ This is due in large part to the importance of this pore structure to the passage of fluids in and out of coal which naturally must occur when coal is processed or burnt. Furthermore this structure also plays a major role in determining the mechanical behavior of coal, particularly for brown coal.2 Many experimental studies and much theoretical modeling has been carried out in order to attempt to describe this structure and to elucidate its role in determining the physical and chemical properties of coal.’“ However, so far no wholly satisfactory nor complete description of this -pore structure is available, although various models are able to explain some aspects of the physical structure of coalH albeit often only by the use of many parameter^.^?^ Recently, however, the methods of fractal geometry6 have been applied to interpret a variety of data which is dependent on the pore structure of solids.’ Of particular relevance to this paper is the fact that small angle X-ray scattering (SAXS)results for some porous solids may be readily interpreted as arising from fractal structures.s Recently the pioneering work of Bale and Schmidt8 on Beulah lignite has been extended to other porous solidsP12 8 Abstract published
in Advance ACS Abstracts, October 1,1993.
(1)APS Study Group on Coal Utilization and Synthetic Fuel Production Rev. Mod. Phys. 1981,53 (4), Part 11. (2)Evans, D. G.;Allardice, D. J. In Analytical Methods for Coal and CoalProducts; Karr, C., Jr., Ed.; Academic Press: New York, 1978;Vol. I, Chapter 3. (3)Grimes, W. R. The Physical Structure of Coal. In Advances in CoaZ Chemistry; Academic Press: New York, 1980; Vol. I, Chapter 2. (4)Bale, H. D.;Carbon, M. L.; Kalliat, M.; Kwak, C. Y.; Schmidt, P. W. In The Chemistry ofLow Rank Coal; Schobert, H. H., Ed.; American Chemical Society: Washington, D.C., 1984;pp 79-94. (5)Setek, M.; Snook, 1.K.; Wagenfeld, H.K. In The Chemistry ofLow Rank Coal;Schobert,H. H.,Ed.;AmericanChemicalSociety: Washington, D.C., 1984;pp 95-108. (6)Mandelbrot, B. B. The Fractal Geometry of Nature; Freeman: San Francisco, CA, 1982. (7)Avnir, D.;Pfeifer, P. J. Chem. Phys. 1983,79,3558.Avnir, D. In Better Ceramics Through Chemistry I& Brinker, C. J., Clarck, D. E.; Uhlrick, D. R.,Ede.;Materials and Science Society: New York, 1986. (8) (a)Bale, H. D.; Schmidt, P. W. Phys. Reo. Lett. 1984,53,596. (b) Pfeifer, P.; Schmidt, P. W. Phys. Rev. Lett. 1988,60, 1344.
0743-7463/93/2409-2726$04.00/0
In this paper we give an overview of some of this work on these materials with particular emphasis on brown coal and products derived from brown coal.
Fractal Geometry and Scattering The subject of fractal geometry covers an enormously wide range of topics;6Js however, here we are only concerned with the use of a small subsection of fractal methods and concepts. In fact we will only make use of the concepts of self-similarity and fractal dimension.16A structure is said to be self-similar if it “looks the same” on all length scales, e.g. under all magnifications when observed with a microscope. To be more precise, normal Euclidean geometric measures of structure such as surface area, S,or mass density, p, will scale according to the length of the probe used to measure the structure and, thus, will not be constants. The exponents in these scaling relationships are termed fractal dimensions, Le. for a surface fractal S
Ng2-’#
(1)
and for a mass or volume fractal p
= p@md
(2)
where r is the radius of the yardstick used to measure the surface area S and R is the radius of the sphere over which the density p is sampled and D, is the surface fractal dimension and D, is the mass fractal dimension which have the following limits (9)Ciccariello, S.;Benedetti, A.; Polizzi, S. Euophys. Lett. 1987,4, 1279. (10)Reich, M. H.; Rueso, S. P.; Snook, I. K.; Wagenfeld, H. K. J. Colloid Interface Sci. 1990,135,353. (11)Reich, M. H.; Snook, I. K.; Wagenfeld, H. K. Fuel 1992,71,669. (12)Johnston, P. R.; McMahon, P.; Reich, M. H.; Snook, 1. K.; Wagenfeld, H. K. J. Colloid Interface Sci., in press. (13)Renouprez, A.; Avom, J. In Characterization of Porous Solids; Unger, K. K., Bahrans, D., Kral, E&.; Elsevier Science Publishers B.V.: Amsterdam, 1988; p 49. (14)Kaiser, H.; Gethner, J. S.In ZnternationaZ Conference on Coal Science, London; Internation Energy Agency, 1983;p 300. (15)Feder, J. Fractals; Plenum: New York, 1988. (16)Mandelbrot, B. B., ref 1, pp 123,124,157,349-350,367-365.
0 1993 American Chemical Society
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Fractal Pore Surfaces in Brown Coal 2 1 D , 5 3 and OlD,13 (3) Physically D, may be thought of as giving a measure of surface roughness and Dm a measure of the nonuniformity of mass distribution. Further, in the above scaling relationships (1) and (2) NOand PO are the true nonEuclidean probe length independent measures of the surface extent and mass distribution, respectively, as extensively discussed by Mandelbrot.6 Only when D, = 2 or D, = 3 can we use the normal Euclidean ideas of constant surface area and of constant material characteristic density, respectively.6 There have been a wide variety of ways which have been devised to test if natural structures obey (1) or (21, for example gas absorption, density measurement, and the scattering of radiation and the scattering of particle~.~J'J~ In particular these scattering techniques has been quite fruitfully and extensively applied to mass fractal structures" and more recently the number of studies of surface fractals."12 If certain conditions are fulfilled, then it may be shown that the intensity, I, of radiation scattered through an angle 9 is given by, for a surface fractal with D, # 3WOJ7
I(h) = ~ I $ ~ ( A p ) ~ r-(D,) 5 sin(D, - 1)
(4a)
or if D, = 3 thenEbJO
I(h) a h4 and for a mass fractal17
(4b)
I(h) a h-D, (5) where h is the magnitude of the scattering vector given by 4?r h = 1;;sin(t9/2) X being the wavelength of the radiation used. It should be noted that the limiting case of D, = 3, i.e. eq 4b, is a
subtle and complicated one which has been discussed at length in, for example, ref 8b and 10. Hence, a plot of log Z verses log h (or equivalently log t9 at small angles) will give a straight line whose slope is related to the appropriate fractal dimension (if D, # 3) and whose intercept with the I axis is related to NOfor a surface fractal (if D, # 3) or po for a mass fractal. We do not quote values of NOfor the systems studied here as we have only made relative measurements of I and of NO,the experimental uncertainties in NOare much larger than in D8,10-12 and the trends in NOmirror those in D,. Ideally one would like to be able to put upper and lower bounds on the range of validity of the fractality of the structures. However, this is very difficult to do in an entirely meaningful way from scattering measurements as the formulas used to analyze the results are of an asymptotic nature and they provided no unambiguous way to determine these limits.1° Often8Jo the upper and lower limits are estimated by Bragg's law from the smallest and largest values, respectively, of the scattering vector h for which the plot of log I verses log 9 is linear. This assumes the validity of Bragg's law and identifies the points of the breakdown of the asymptotic form of the scattering law with the values of the limits on fractal behavior. By use of this rough estimate in our experiments, the lower and upper limits are approximately 200 and 2000 Atlo respectively. In the next section we will set out the details of the technique used and the results of measurements made on (17) Teixeira, J. In On Growth and Form: Stanley, H. E., Ostrowsky, N., E&.; Martinus Nijhoffi Dordrecth, 1986; p 146.
SCATTERING ANGLE (mRAD)
Figure 1. Scattering curve for alkali-digested solar dried coal slurry. Table I. Fractal Pore Dimensions of Some Unprocessed Coals Meaeumd by SAXSd coal rank fractal dmensione Spring Creek black 2.08O Durian black 2.2P Loy Yang brown 2.3P Beulah lignite 2.54O Liddel high volatile subbituminous 2.57O Yallourn brown, air-dried 2.600 Taroom high volatile subbituminous 2.6P Morwell brown 2.860 Yallourn brown, bed moist 2.556 0 Surfacefractal dimension,D,. Maes fractaldimension,D, The estimated error bars on these measurements are, at worst 10.1 and at best a0.02, see refs 10, 11, and 12 for details. d All these measurements were made at R.M.I.T.
various coal samples which illustrate the extreme utility of the use of fractal concepts in describing the scattering patterns arising from complex porous solids.
Experimental Rssults and Discussion The experimental apparatus used was as described previously in the literature,1° that is an Anton Paar Kratky geometry SAXS camera, Cu anode X-ray tube, proportional counter, and associated electronics. Data collection and subsequent analysis were made by the associated electronic control equipment and a dedicated microcomputer system. Samples were prepared by grinding the solids, seizing by sieving, and then packing into a sample cell with removable mica windows.5 Data were collected at 50 different scattering angles and the data corrected for background scattering and absorption and deconvoluted to allow for f i i t e beam size as described previously.1° A typical scattering curve is shown in Figure 1 for a dried brown coal slurry and as can be seen the log I verses log 9 plot is sensibly linear over a wide range of scattering angles, i.e. a wide range of different probe lengths, and it should also be noted that the data represent a very large change in scattering intensity. In Table I we have collected together the measured results for D,for a variety of different coal samples. The estimated uncertainties in D, are, at worst fO.l and at best f0.02.1° As can be seen the values of D,span virtually the whole allowable range of values given by inequality 3. It is interesting to note that all the dried coal samples have scattering curves which may be interpreted as arising from surface fractals. However, for bed-moist brown coal we have found that the scattering in this case may be interpreted as arising from a mass fractal.1' The scattering
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curve, however, changes to that characteristic of a surface fractal upon drying this coal in air." This also correlates with the change in mechanical and other properties of brown coal upon drying: and this result would suggest that there is some type of pore collapse suffered by brown coal upon drying." Processing can have a profound effect on the scattering curves and thus, by inference, on the pore structure of coal. It has been shown, for example, that oxygenating black coal by heating in a stream of air tends to smooth and open up the pores.1° Two of the biggest disadvantages of brown coal as a fuel are (a) it contains up to 60 % moisture and (b) it has a very high proportion of oxygen present. Both of these aspects make brown coal very awkward to handle and use, give it a low calorific value, and make it uneconomicto transport. Thus, there have been attempts to deoxygenate dried brown coal in order to produce a fuel of higher calorific value, for example by heating dried brown coal in various atmospheres at fixed temperatures followed by a very rapid quench to room temperature.1a20 In these studies four distinct processes were used: (a) heating in an inert N2 atmosphere; (b) heating in a CO/ HzO, i.e. hydrogen rich, atmosphere; (c) heating an Cuimpregnated sample under a Hz atmosphere; (d) heating an acid-washedsample in a CO/H20 atmosphere. All these processes produced fuels of lower oxygen content and higher calorific value. However, the physical properties of the products varied with the processing conditions used. A t low processing temperature, T, the products were similar to the starting material, that is, powdery solids for which 0,and NO vary little with temperature. For processes a and d, this behavior continued to the highest values of T used; see Figure 2. However, for non-acidwashed samples treated in a hydrogen-rich atmosphere, i.e. processes b and c, for temperatures above about 320 "C the values of D, (the surface roughness) increase dramatically, which may be seen in Figure 3. This is accompanied by a similarly large decrease in No with temperature which mirrors the behavior of D,with T, and as the uncertainties in NOare much larger than those in D,, we do not show them here. The data appear to indicate pore collapse and are consistent with the change in appearance of the solid from a dull, powdery solid to black shiny lumps with a concomitant increase in density of about 50% .la20 On further interesting observation is that the oxidation curves (which are related to,the combusion behavior) as measured by differential thermal analysis show results typical of those for low rank coal, Le. an energy release peak at about 300 "C and one at about 450 O C , for the samples produced by process a but the curves for the samples from process b for T > 320 "C change to those characteristic of high rank coal, i.e. they consist of a single peak at about 500 "C. Conclusions
We have shown that fractal geometry can give a good, few parameter description of the scattering data from many porous solids. Furthermore the two parameters obtained (18) Johnston, P.R.;Mathews, J. F.; Jackson, W. R.Proc. Aust.Brown Cool Conf.,Adelaide,Australia,CoalCorporationofVictoritoria:Melbourne, Australia, 1988, pp A 4 6.1-6.7. (19) Johnston, P.R.;Mathewe, J. F. Chemica '88,Australia Bicentennial Intermtiom1 Conference for the ROCe88 Industriee, Sydney, Australia, The Australian Inetitute of Engineers: Melbourne, Australia, 1988, pp 246-260. (20)Johnston, P. R.; Jackson, W. R.; Mainwaring, D. E.; Bachelor, F. W. Proceeding8 Chemica '89, Gold Coast, Australia, The Australian Institute of Engineers: Melbourne, Australia, 1989 pp 76-80.
Snook and McMahon
I
I 6
1 P
1
500
200
TEMi;RATURE40;
C )
Figure 3. Fractal dimension verses the processing temperature for Cu-impregnatedbrown coal heated in a HZatmosphere, A, and brown coal heated in a CO/H20 atmosphere, 0 .
from this procedure, the surface fractal dimension, D,, and the surface measure, NO, both have well-defined physical meanings; i.e. they are a measure of the surface roughness and the extent of surface, respectively. The values of D,and to a lesser extent NO(owing to much larger experimental uncertainties in measuring NO) provide not only a means of describing the scattering and other data used to characterize these surfaces but may also provide some insight into the changes in pore structure that occur with processing and may be correlated with other properties of the porous solids, e.g., physical appearance, density, and combustion characteristics. More work involvingcorrelation of fractal parameters with other physical and chemical properties of porous solids would
Fractal Pore Surfaces in Brown Coal clearly be valuable, as would more work in providing a theoretical link between structure and properties. Finally it would be interesting to make measurements of D,and No using a variety of techniques on the same samples. This would be useful in order to see if each technique is probing the same structure and in order to have data for overlapping probe lengths. In particular SAXS probes total (open and closed pores) but gas adsorption only probes open (or accessible)pores, and the difference between these two could be significant, both quantitatively and qualitatively.
Langmuir, Vol. 9, No. 10, 1993 2729
Acknowledgment. We thank all those involved in organizing the international symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids and in particular Professor W. Rudzinski and Professor B. W. Wojciechowaki. Secondly we wish to acknowledge our colleagues Dr. M. Reich, Professor H. Wagenfeld, and Mr. S. Russo of R.M.I.T., Dr. P. Johnston of the Chemical Engineering Department, Monash University, and Mr. W. Stacy and the late Mr. Lois Kiss of the SEC Victoria without whose work and help none of this work would have been started nor finished.
