Energy & Fuels 1987, 1,484-488
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Sulfation of Mineral Matter in New Brunswick Oil Shale D. Karman* and S. Kresta Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Received July 1, 1987. Revised Manuscript Received August 17, 1987
New Brunswick oil shale samples are studied by X-ray diffractometry and scanning electron microscopy coupled with X-ray microanalysis to characterize the sulfation behavior of the calcium content. Calcium is predominantly in the form of dolomite and forms CaS04 upon exposure to simulated combustion gas. Ca-bearing grains in the shale are of the order of 2-10 pm. Partially sulfated particles show higher sulfur concentration around the edges and cracks pointing to the importance of SO2 penetration into particles for determining the overall rate of sulfation.
Introduction The oil shale deposit in the Albert Mines region of New Brunswickl has recently attracted interest as a possible SO2 scavenger in the utilization of local high sulfur (6-8% S) coal. The Research and Productivity Council of New Brunswick have carried out pilot scale experimental work on the fluidized bed combustion and/or retorting of these shalese2 The New Brunswick Electric Power Commission have built a circulating fluidized bed coal combustor a t Chatham, NB, which can use limestone and/or shale for SO2 capture. In the Department of Chemical Engineering at UNB, we have completed a thermogravimetric study of the thermal decomposition and sulfation kinetics of shale from Albert Mines. The subject of this paper is our experience in using X-ray diffractometry and scanning electron microscopy to characterize the shale and monitor the progress of sulfation. Experimental Section The samples used in the study come from the core archive of Canadian Occidental Petroleum no. 5 hole in the Albert Mines region and cover the four major zones identified in this deposit as shown in Table I. After the oil shale was crushed and sieved, two size fractions were selected for use in both the thermogravimetric and analytical studies. In the TGA, runs were carried out by heating the samples from ambient temperature to 850 "C, in inert (nitrogen) or simulated combustion gas (79.8% N2, 2.9% 02, 16.7% COP,and 0.6% SO2) environments. Samples were then analyzed as described below. X-ray Diffractometry. Selected samples were further ground to powder form and fixed on slides by using acetone. A Rigaku-Miniflex instrument was used to obtain a trace of peak intensities in the 9' < 28 < 68O position range. The d spacing for the peaks is obtained from Braggs law3 nX = 2d sin B (1) with n = 1 and X = 1.5418 8, for the Cu K a radiation used. For quantitative analysis, NaCl was used as internal standard and calibrations were carried out by using mixtures of calcite, dolomite, NaC1, CaS04, and quartz. The calibration constant for a component is defined as3
Kn = In Ws/Is Wn
(2)
The major peaks for the components of interest and the corresponding calibration constants are given in Table 11. The (1) Macauley, G.;Ball, F. D.; Powell, T. G. Bull. Can. Pet. Geol. 1984, 32(1), 27-34. (2) Salib, P. F.; Barua, S. K.; Furimsky, E. Can. J.Chem. Eng. 1986, 64, 1001-1007. (3) Cullity, B. D. Elements of X-Ray Diffraction, 2nd ed.; Addison-
Wesley: Reading, MA,1978.
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Table I. Identification of S a m d e s Used" sample no. depth from surface, m zone I, I1 80 surface zone 111, IV 206 laminated marlstone v , VI 343 clay marlstone VII, VI11 557 dolomite marlstone Samples I, 111, V, and VI1 are -9 to +20 mesh; samples 11, IV, VI, and VI11 are -60 to +115 mesh. Table 11. Major Peaks and Calibration Constants for XRD peak used for calibration component quantification (ZO), dea const NaCl 31.6 1.0 dolomite 30.8 0.48 i 0.05 calcite 29.3 0.51 i 0.05 quartz 26.6 0.17 i 0.02 CaS04 25.5 0.76 f 0.08 Table 111. Calibration R u n Details for Quantitative XRD no. of runs mixture composition by w t 7 10% NaCl, 90% 1:4 do1omite:quartz 6 10% NaCI, 90% 1:9 do1omite:quartz 3 10% NaC1, 90% 1:1:8 do1omite:calcite:quartz 6 10% NaC1, 90% 1:l CaS04:quartz calibration constants were obtained through a set of runs with mixtures prepared as outlined in Table 111. Neither the different do1omite:quartz ratios nor the presence of calcite in the mixture affected the calibration constant for the dolomite to an extent that could be determined with the precision attainable. The reported constants for dolomite in Table I1 are therefore the average for all runs in Table 111. Scanning E l e c t r o n Microscopy. A Cambridge S4-10 instrument with secondary and backscatter electron detectors, coupled with a Tracor Northern NS880 energy dispersive X-ray system was used to examine samples that had been set in epoxy moulds and polished. The backscattered electron images were analyzed to obtain information on the size of grains and or laminae with different composition that make up the shale particles. The X-ray system was used to obtain dot maps and/or line scans for calcium and sulfur in fresh and sulfated shale samples.
Results Fresh samples shows XRD peaks that were identified as dolomite, calcite, and various components of a clay matrix such as quartz, illite, albite, micas, and montmorillonite. The clay matrix is not of immediate interest for the present purpose. The carbonaceous material was predominantly in dolomite form with only minor peaks for calcite in samples I11 and IV. Samples that had been decomposed in nitrogen at 850 "C showed predominantly quartz peaks and no carbonate 0 1987 American Chemical Society
Energy & Fuels, Vol. 1, No. 6,1987 485
Sulfation of Mineral Matter Table IV. Calcium Content in Fresh and Sulfated Samples w t % Ca sample as carbonate w t % CaS04 % no. in fresh shale in sulfated shale utilization I 10.1 42 5.5 I1 12.2 53 5.1 I11 13.7 65 4.6 IV 16.1 74 4.6 V 6.7 38 4.3 8.2 70 VI 2.9 7.7 26 VI1 6.9 VI11 15.1 53 7.0
peaks, confirming total decomposition at this temperature. Samples decomposed at 850 "C in a simulated combustion gas showed the presence of sulfur in the form of CaS04 only. The sulfation of the magnesium content is not thermodynamically favorable in this environment anyway, but the absence of CaS03and other forms was significant in simplifying the interpretation of weight gain observed in sulfation runs with the thermogravimetric apparatus. The results of quantitative XRD analysis are summarized in Table IV. Although the precision attainable is not very high (see Table 11),comparison of results for fresh and sulfated samples gave a clear trend for higher conversion of calcium from the carbonate to the sulfate form in the smaller size fraction, a trend independently conf m e d by the relative weight loss data from the TGA runs. The absolute levels of utilization are not particularly significant in themselves since the time-temperature history mentioned above is not a realistic one for the target application; however, the cases showing 65-70% utilization under these conditions compare quite favorably with the Ca utilization values generally achieved with limestone. The striking feature of the data in Table IV is the relatively low Ca content in a form suitable for sulfur capture for all samples. The varying oil yields of these samples must also be considered in an overall evaluation for coutilization with coal, but higher carbonate contents are obviously required for shale to compete with limestone for sulfur capture. The scanning electron microscope images that are discussed below are all for sample I of Table I, fresh or sulfated as indicated, under the conditions mentioned above. Figure l a shows kerogen laminae of approximately 100 pm thickness within particles that have characteristic dimensions on the order of 1000 pm. Some particles have significantly more of these laminae. The calcium dot map in Figure l b is a very clear demonstration of the heterogeneity in the sample being viewed. Although the resolution of the X-ray microprobe cannot distinguish variations due to the kerogen laminae a t this magnification, the variation in Ca content among individual particles is indicated by the relative intensity of the images. Line scans for Ca and S can be carried out to get information about the size of grains important in sulfur capture and to monitor the progress of the sulfation. Care needs to be exercised in interpreting the variation in signal intensity in such single-element line scans since the background component of the signal may be influenced strongly by the presence of other elements! Of particular concern in this study were interactions in the signals from Ca, S, and Si, the first two being of direct interest and the last one due to Si being present in large concentrations in the shale matrix in the form of quartz. For each Ca and S line scan, several points along the path were chosen for (4) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Fion, C.; Lifshin, E. Scanning Electron Microscopy and X-Ray Microanalysis; Plenum: New York, 1981.
b
Figure 1. Fresh shale particles viewed in backscatter mode (a) and associated calcium dot map (b).
point analysis, which gives the signal intensity as a function of beam energy enabling the identification of peaks due to elements of different atomic number. By this procedure, it was possible to observe the intensity of peaks attributed to a particular element, relative to the background due to the presence of others. Figure 2a shows differences in calcium content among particles of the same sample by using a line scan. Of the three particles that the scan covers, the leftmost one has significantly more calcium than the one in the center while the rightmost one has the medium Ca content. With increasing magnification in Figure 2b-d, the Ca line scan provides grain size information within a particle. The kerogen layer in the center of Figure 2b shows a lower average Ca content relative to that in the layers to either side. In Figure 2c,d the Ca bearing grain size is seen to be as low as 2 pm, although there are significantly larger grains in smaller numers. The sulfur content of fresh New Brunswick shale is around 0.5%.2 Sulfur scans on fresh samples showed a detectable signal only very rarely; Figure 3 shows a sulfur-bearing grain of the same size as those seen for Ca in Figure 2c. The grain is identified as pyrite following a point analysis. Calcium and sulfur scans along the same line for a sulfated sample provide evidence on the nature of partial sulfation of a particle. Figure 4 shows a more or less uniform distribution of Ca throughout the particle but a sulfur profile that is concentrated at the edges of the particle. Point analysis confirm that the line scans do reflect a relatively uniform distribution of Ca throughout
486 Energy & Fuels, Vol. 1, No. 6, 1987
Karman and Kresta
Figure 2. Fresh shale particle(s) viewed in backscatter mode a t different magnifications with superimposed calcium line scans.
a
Figure 3. Fresh shale particle with sulfur line scan superimposed on backscatter image.
the particle while the S is concentrated at the edges. The XRD study mentioned earlier shows that the Ca as carbonate decomposes totally to CaO at 850 "C, but sulfation to CaS04 is dependent on particle size and ranges from 26 to 74% based on Ca. The Ca and S line scans in the SEM study suggest that the conversion to CaS04is taking place to a greater extent at the edges of the particles. The same phenomenon is observed around the internal crack of a particle in Figure 5, which is at twice the magnificaion of Figure 4. At even greater magnification in Figure 6, the individual grains of CaS04 in a sulfated sample can be pinpointed, first by either the Ca or S scans and later by verification by point analyses.
Discussion The sulfation characteristics of CaO obtained from the decomposition of limerock has been the subject of numerous ~ t u d i e s . ~The modeling effort for the sulfation reaction is concentrated on characterizing the physical structure of the calcine, given a relatively well-defined chemical composition. The conversion versus time behavior in this case can be interpreted reasonably well over narrow ranges of solid conversion but it is still difficult to predict such behavior over large ranges of solid conversion, starting from physical properties that can be easily mea(5) Kocaefe,D.; Karman, D.; Steward,F. R.Can. J. Chem. Eng. 1985,
63,971-976. (6) Kocaefe, D.; Karman, D.; Steward, F. R. AIChE J., in press.
Figure 4. Calcium (a) and sulfur (b) line scans superimposed on backscatter image for shale particle sulfated in simulated combustion gas.
sured. When shale rock is considered as a sulfur-capturing agent, the picture is significantly more complicated due to the presence of very different components and a heterogeneous structure. In this study we have used XRD and SEM to obtain a qualitative and semiquantitative characterization of New Brunswick oil shale in terms of the carbonaceous content and sulfation behavior. The results of this study enable us to interpret thermogravimetric data in terms of dolomite decomposition and subsequent sulfation to form CaS04. Quantitative XRD shows a dependence of sulfation on particle size, in
Energy & Fuels, Vol. 1, No. 6, 1987 487
Sulfation of Mineral Matter
" I
b
Figure 5. Calcium (a) and sulfur (b) line scans superimposed on backscatter image for a cracked shale particle sulfated in simulated combustion gas.
agreement with TGA data. However, there is significant uncertainty in doing quantitative work with the mentioned apparatus. The SEM results show that the Ca which can act as the SO2scavenger in combustion applications is distributed throughout the shale particles in the form of grains of micrometer size range. The sulfur profiles of sulfated samples provide evidence that the penetration of SO2into the shale particles will play a significant role in determing the rate of sulfation. The observed sulfation behavior is very similar to that reported in the pioneering work of Hartmann and Coughlin78on the use of limerock for sulfur capture. A major consideration in the reaction of solid reactants with SO2is that the solid product formed has a larger molar volume than the solid reactant and hence causes a decrease in particle porosity and in the rate of diffusion of SOzinto the particle. If the solid phase is pure solid reactant, then the conversion may be limited by the value corresponding to zero porosity. For example the ratio 3.09 for the molar volume of CaS04to CaO would suggest that for an initial porosity of 50% the conversion a t zero porosity is 48%.5 On the other hand if CaO forms 10% of an otherwise inert solid, total conversion would decrease the porosity by only 20%. This latter situation is closer to the case of oil shale and suggests that higher utilization (7) Hartman,M.; Coughlin, R. W . Ind. Eng. Chem. Process Des. Deu. 1974,13,248-253. (8) Hartman, M.; Coughlin, R. W . AIChE J. 1976, 22, 490-498.
Figure 6. Calcium (a) and sulfur (b) line scans superimposed on backscatter image of the edge of a shale particle sulfated in simulated combustion gas.
of the Ca in oil shale should be possible. Another factor that affects the utilization of the solid reactant is the relative rates of diffusion of SO2into the particle (which is influenced by the porosity of the particle) and diffusion of SO2through the product layer form on the fresh solid reactant. The size and shape of grains of solid reactant on the pores within particles have been used in numerous models6to interpret conversion versus time behavior in terms of the physical changes occurring within a particle as reaction proceeds. If the porosity drops to zero before complete conversion, sharp concentration gradients of the solid product along a direction perpendicular to the particle surface can be expected. On the other hand, when porosity is not affected strongly by the conversion of solid reactant, gradients of solid product may reflect the slower diffusion of gaseous reactant through the pore plus inert solid matrix of the particle to get a t the grains of solid reactant. The sulfation behavior of shale rock observed in this study suggests that Hartmann and Co~ghlin's'*~ model of an inert fraction within the active limerock component would be a good starting point. For modeling of the sulfation behavior of shale rock, their model of an inert fraction within the active limerock component would seem to offer a good starting point.
Glossary line spacing for XRD, A I relative peak intensity for XRD tracers calibration constant for quantitative XRD, defined K by eq 2 W weight fraction d
Energy & Fuels 1987, I , 488-496
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x e
wavelength of radiation, A diffraction angle, deg
Subscripts n component n S internal standard Acknowledgment. We are indebted to E. Yildirim of CanOxy Ltd. for the shale samples and to D. Ball of 3-D
Geoconsultants, Fredericton, NB, Canada, for their selection and identification. The invaluable advice on XRD work provided by J. Vahtra of the Geology Department a t the UNB, and the cooperation of H. DeSouza from the SEM unit at the UNB are deeply appreciated. S.K. was funded by an NSERC Undergraduate Summer Research Scholarship during this work. Registry No. SOz, 7446-09-5; calcite, 13397-26-7;dolomite, 16389-88-1.
Thermodynamic Modeling of Oil Shale Pyrolysis J. M. Charlesworth Materials Research Laboratories, Department of Defence, Ascot Vale, Victoria, 3032 Australia Received June 15, 1987. Revised Manuscript Received August 24, 1987 Theoretical calculations on the high-temperature stability of a range of compounds have been performed and the results compared with the experimentally measured composition of pyrolysis products from model compounds and that of shale oil derived from type I and type I1 kerogen. Acceptable agreement is found for a range of isomers of aromatic and heteroaromatic compounds; however, evidence is found indicating that kinetic factors play a role in determining several isomer distribution patterns. The mineral matter in Rundle shale ash assists in promoting equilibrium, as also does a long residence time at high temperature. Large-scale model calculations allowing for elemental balance and unrestricted reaction between species have so far not predicted many of the most characteristic features of the composition of shale oil. Structural units present in the original kerogen molecule appear to be preserved in the pyrolysis products to a significant extent following retorting a t 5 "C min-l. Introduction In order to understand the relationship between shale oil and kerogen, it is desirable that the thermally induced reactions occurring during pyrolysis be characterized. With this aim in mind, theoretical calculations on the likely stability of organic compounds a t the temperatures involved in oil shale retorting could prove valuable and should assist in the optimization of retorting conditions to enable the most favorable product distribution to be achieved. Thermodynamic calculations have long been used in the petroleum refining industry to predict the composition of gasoline range material produced by high-temperature cracking of crude oi1.l In this process it is accepted that a network of chemical equilibria is established and that alkyl aromatics are present in ratios determined by their free energies of formation.2 It is observed that reactions producing low molecular weight compounds via the decomposition of high molecular weight alkanes, naphthenes, and aromatics containing long-chain alkyl side groups are the thermodynamically probable ones. From the free energy viewpoint cracking reactions should therefore be most favorable a t high temperatures; however, kinetic factors may prevent the attainment of equilibrium. Thermodynamic modeling studies have been applied in a limited way to a study of the composition of shale oil n a ~ h t h ain , ~which it was shown that the experimentally (1) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. E. The Chemical Thermodynamics of Organic Compounds J . Wiley and Sons, New York, 1969. (2) Draeger, A. A.; Gwin, G. T.; Leesemann, C. J. G.; Morrow, M. R. Pet. Refin. 1951, 30, 71.
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measured distribution of the C2- and C3-substituted benzenes could be predicted by using free energy data. Digital computer methods, utilizing the Gibbs energies of formation, were first used to perform calculations on equilibria involving benzene and all 12 of the methylsubstituted benzenes, ranging up to hexamethylbenzene? A method of successive approximations was employed to solve seven independent equations for the seven unknown concentrations, consisting of individual components and groups of di-, tri-, and tetramethyl isomers. Relative amounts within a group of isomers were then calculated from their respective free energies of formation. Since these early investigations, refinements in analytical methods have enabled a very detailed and accurate picture of the composition of complex hydrocarbon mixtures to be obtained. Furthermore, advances in computer technology have allowed systems with a large number of species to be modeled. This has recently been exemplified by the thermodynamic equilibrium calculations on the C-H system by Linton and T ~ r n b u l l . ~These workers performed computer calculations for over 700 equilibrium distributions, with the intention of defining the optimum conditions for processes such as the conversion of coal to liquids. The computer program was able to accommodate up to 100 species simultaneously. Computed amounts were tabulated for compounds that accounted for more than 0.0001% of the carbon in the system. Temperature, pressure, and total hydrogen-carbon atom ratio were (3) Thorne, H. M.; Murphy, W. I. R.; Ball, J. S.; Stanfield, K. E.; Horne, J. W. Ind. Eng. Chem. 1951,43, 20. (4) Hastings, S. H.; Nicholson, D. E. J. Chem. Eng. Data 1961, 6 , 1. (5) Linton, M.; Turnbull, A. G. Fuel 1984, 63, 408.
0 1987 American Chemical Society
