Energy & Fuels 1988,2, 9-13
9
DXRD Studies of Oil Shale Mineral Reactions W. J. Thomson,* K. A. Helling, and R. J. Rodriquez Department of Chemical Engineering, Washington State University, Pullman, Washington 99164-2710 Received May 18,1987. Revised Manuscript Received September 14, 1987
The technique of dynamic X-ray diffraction (DXRD) has been applied to a study of the mineral reactions accompanying high-temperature processing of oil shale. Isothermal experiments for a Green River Formation oil shale have been carried out at temperatures between 800 and 1173 K and under various CO2/N2environments. It was possible to separate the two-step decomposition of ankeritic dolomite, and it was found that the intrinsic rates are higher than previously reported and that the rate-determining step is the first step, i.e., the decomposition of ankeritic dolomite to calcite. It was also found that ankeritic dolomite decomposition could be prevented and inhibited by relatively low COz overpressures at temperatures less than 950 K. Silicates were observed to form rapidly (less than 1200 s) at temperatures of 1073 K or higher, but conversions were low. The primary source of silicon in these reactions was quartz, and it appeared to combine with the ankeritic dolomite to form calcium-magnesium silicates. At 1173 K quartz conversion was significant (31%) after only 500 s, and this was the only temperature at which calcium silicates were formed.
Introduction With the advent of second generation, above-ground oil shale processes, retorted shale is likely to be combusted at temperatures between lo00 and 1200 K. At these temperatures the mineral matrix of the shale will undergo a variety of chemical reactions including carbonate decomposition, sulfation, and recombination reactions to form silicates. This complex set of reactions can be very important to the optimum design of a retorted shale combustor. For example the net heat of combustion is very dependent on these Feactions since the carbonate mineral decomposition reactions are highly endothermic and some of the silication reactions are only mildly endothermic.' In addition, the combusted shale (ash) will have to be disposed and revegetated, and the environmental consequences of this process will be highly dependent on the mineral composition of the ash.2 The degree to which the mineral reactions influence these considerations will depend on the timetemperature history to which the shale is exposed. Thus it is important to have a knowledge of the kinetics of these reactions. Previous attempts to study these kinetics for Green River oil shale have been made at Lawrence Livermore Laboratories1ls and in our own l a b ~ r a t o r y . ~However, these studies all employed TGA techniques, and since there is usually more than one reaction occurring simultaneously, there is no way to distinguish between competing reactions. The difficulty here was very apparent in the previous work of McCarthy and his ~olleagues,~.~ which focused on the mineral reaction kinetics of Julia Creek oil shale. The occurrence of calcite decomposition5 along with silicate formation was impossible to study with TGA techniques alone and consequently postreaction wet chemistry had to be employed in order to obtain information on the (1)Burnham, A. K.;Stubblefield, C. T.; Campbell, J. H. Fuel 1980,
59. - -, an-877. - .- -
(2) Kuo, M. C.; Park,W. C.; Lindemanis, R. E.; Compton, L. E. Oil Shale Symp. R o c . 1979,12th,81-93. (3)Campbell, J. H.Lawrence Liuermore Lab., [Rep.] UCRL 1978, UCRL-52089-2. (4)Thompson, L. G.; Thomson, W. J. A C S Symp. Ser. No. 1983,No. 230. 613-628. (5) McCacthy, D. J. Fuel 1983,62,1238-1239. (6)McCarthy, D.J.; Poynton, H. J. Fuel 1984,63, 769-773.
formation of silicates.6 Even then, data on individual silication reactions were not forthcoming, and larnite (pCazSi04)appeared to be the major silicate product. As will be discussed below, we have successfully applied a relatively new technique, "dynamic X-ray diffraction" ( D - m ) ,to the study of oil shale mineral reactions. Using typical retorted shale combustion conditions, we are able to obtain phase specific rate data on the decomposition of mineral carbonates as well the reactions of the carbonates and/or their oxides with quartz to form various silicates. Experimental Section While the details of the DXRD system are presented elsewhere,' Figure 1 shows a sketch of the hot stage, including the sample strip and the pyrometer used to measure sample surface temperatures. The hot stage is compatible with a Siemens D-500 powder X-ray diffractometer. Goniometer movement is under the control of a microprocessor and can be programmed by the operator. The approach is to systematically move the goniometer to diffraction peaks of interest, record the minimum data necessary for accurate integration and then repeat the process, keeping track of time. The data are dumped on to hard disks and can be recalled at the conclusion of the experiment for reduction and analysis. Anderson and Thomson" have shown that, with this system, it is possible to extract a solid-state concentration data point every 20 s. However, the complex nature of the oil shale diffraction patterns typically limited data collection here to one concentration measurement every 2 min, although some of the data were successfully acquired at 10 times this rate. The oil shale studied here was a retorted sample from the Parachute Creek Member, provided by UNOCAL Science and Technology. The retorted shale had an organic carbon content of 5.5 w t % and the ash contained 33% ankeritic dolomite, 20% feldspars (primarily albite and orthoclase), 15% quartz, 10% analcime, and 10% calcite. This analysis was rationalized on the basis of X-ray diffraction (with an internal standard), ICP data, and weight loss due to carbonate mineral decomposition (determined by TGA). It was estimated that approximately ll% of the retorted sample was amorphous. The shale was crushed and sieved to -400 mesh to produce a "master batch", and a thin layer (-60 pm thick) was deposited on a platinum strip that served as both a sample holder and a heating strip. The thin layers ~~~~
(7)Anderson, D.A.;Thomson, W. J. Ind. Eng. Chem. Res. 1987,26, 1628-1632.
0887-0624/88/2502-0009$01.50/00 1988 American Chemical Society
Vol. 2,No.1, 1988
10 Energy & Fuels,
Thomson et al.
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Figure 2. Inhibition of ankeritic dolomite decomposition (873 K).
NOT TO S C A L E
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Figure 1. Hot stage and sample holder. were necessary in order to insure that the solid sample temperature was equal to the heating strip tefnperature, which in turn was monitored with a Pt-Rh thermocouple. In most experiments an internal standard (10% corrundum) was used which, when combined with reference intensity ratios, could be used to obtain concentration measurements of the species of interest. The experimental procedure was to slurry the shale-corrundum mix with acetone and then deposit the slurry on the heating strip. The sample was dried by purging with nitrogen a t room temperature, and a careful X-ray scan was then taken of the dried sample. The dried sample was typically on the order of 30 mg and was approximately 60 pm thick. The dynamic measurements were then carried out by heating the strip to the desired temperature as rapidly as possible (-30 s), followed by the admission of the predetermined gas mixture to the hot stage. Typically, three peaks plus the corrundum standard were sampled during any one run. Experiments were conducted a t temperatures between 800 and 1200 K and at various concentrationsof C02-in-N2 a t a total pressure of 100 kPa.
Results The DXRD results must be interpreted in terms of the expected chemistry. Equations 1-5 show idealized stoidecarbonation
-
Ca(MgXFel-x)(CO& CaC03 + xMgO + (1- x)FeO + 2C02 (1) CaC03 + CaO calcium silicate formation 2Ca0
+ Si02
2Ca2Si04+ CaC03
-
-
-
+ C02
(2)
Ca2Si04
(3)
2Ca2Si04-CaC03
(4)
Ca2MgSi207+ 2C02
(5)
magnesium silicate formation 2CaC03 + MgO
+ 2Si02
chiometries for the decomposition of mineral carbonates as well as the formation of calcium and calcium-magnesium silicates. Whether the latter reactions occur as shown is debatable, and at least five different silicate products have been reported by previous investigator^.^,^^^ It is also important to point out that iron is incorporated in the ~~
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(8)Park,W. C.;Lindemanis, A. E.; Raab, G. A. In Situ 1979, 3, 353-381. (9) Smith, J. W.; Robb, W. A.; Young, N. B. Oil Shale Symp. Proc. 1978, Ilth, 100-112.
dolomite phase (reaction 1)and, more properly, should be viewed as a solid solution (“ankeritic dolomite”) rather than as a stoichiometric compound. Under typical combustor conditions reaction 2 is reversible and will not occur at Pco22 30 kPa at temperatures below 1100 K. Reactions 3-5 reflect the observations we have made during the course of this investigation; viz, larnite (or bredigite) and spurrite were the only calcium silicates formed, and akermanite is representative of the calcium-magnesium silicates that were formed. Ankeritic Dolomite Decomposition. Ankeritic dolomite was observed to decompose in a N2 atmosphere starting at a temperature of about 825 K. However, as Figure 2 dramatically shows, the decomposition of ankeritic dolomite was totally inhibited at 873 K when exposed to a C02 pressure of 10 kPa. Pure dolomite equilibrium considerations lead to the conclusion that a C02pressure of 5.8 MPa would be necessary to prevent decomposition at this temperature.8 However, it should be kept in mind that we are really dealing with a solid solution and its equilibrium properties are not known. Furthermore, once the decomposition did occur, it was not possible to reverse the process via C02 recarbonation. It should also be pointed out that similar behavior was also observed by Thompson and Thomson4 with an oil shale from the Geokinetics site in northeast Utah. Although a systematic attempt to quantify the Pco2-T relationship has yet to be made, it does appear to be very temperature sensitive, similar to the temperature dependency of the calcite decomposition equilibrium ons st ant.^ Measurements of the decomposition rates of ankeritic dolomite were also influenced by the presence of C02. In fact, at low temperatures (below 850 K), care had to be taken that the sample thickness was thin enough to avoid problems with C02 diffusion out of the sample. Consequently the decomposition rates were also measured as a function of temperature at a C02pressure of 10 kPa as well as in pure N2. Once ankeritic dolomite begins to decompose to its oxides, there is a question as to whether the reaction path is a two-stage series reaction (as suggested by eq 1and 2) or a single, separate path. In order to answer this question, two experimental approaches were taken. In one, the disappearance of the ankeritic dolomite peak was used to follow the decomposition (the MgO product was amorphous below 1050 K) during experiments conducted in N2 at temperatures between 800 K and 1000 K. Reaction 2 was also monitored in these experiments by following the calcite and lime peaks (see Figure 3). The rate of disappearance of the ankeritic dolomite peak followed fiit-order
Oil Shale Mineral Reactions
02
Energy &Fuels, Vol. 2, No. 1, 1988 11
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KINETIC MODEL, Ea 6
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D X R D DATA
Table I. Kinetic Parameters (k= A e x p [ - E I R T ] ) E, 102k(1000K), Ao, 5-l kJ/mol 5-1 Ankeritic Dolomite this work, Pco, = 0 2.0E7C 170 2.6 Anvil Points" 2.4E10 239 0.83
lb
this work Anvil Points" Julia Creekb
Calcite 8.1E7 2.4E10 7.OE9
175 239 244
OReference 1. bReference 5. c2.0E7 = 2.0
TIME (ksec)
Figure 3. Concentration variations during ankeritic dolomite decomposition (in N2 at 873 K).
I
C, = Cd) exp(-k,t)
I
1
kdCdO +(exp(-kdt) - exp(-k,t)) kc - kd
(6)
where c, and cd are concentrations of calcite and dolomite, respectively. The calcite decomposition rate constant, k,, was then determined as a function of temperature by using the independently derived dolomite rate constant, kd, and nonlinear regression analysis." In the second method, ankeritic dolomite was decomposed in Pco, = 10 kPa at temperatures above 950 K. Under these conditions ankeritic dolomite will decompose to calcite (reacti6n 1)but (10) Hill, C. G . An Introduction to Chemical Engineering Kinetics and Reactor Design; Wiley: New York, 1977. (11)Helling, K. A. M.S.Thesis, Washington State University, 1987.
5.8
0.83d 0.13 X
lo7, etc. dInert
atmosphere, predicts same k for dolomite and calcite. calcite will not decompose. Once the dolomite had completely reacted, the temperature was lowered to 600 K, the COPwas replaced by N2, and then the temperature was rapidly raised to the value desired for the kinetic study of calcite decomposition. Again, good first-order plots were obtained, and the activation energy was essentially the same as was obtained from the nonlinear regression method discussed above. However the rate constants were about 50% lower. It is possible that the calcite formed by reaction 1 is sufficiently different than the "free" calcite initially present and decomposes at somewhat lower rates. The kinetic parameters for these two reactions are given in Table I, with the calcite decomposition rate constant based on eq 6. The latter was chosen to be more appropriate since it correctly predicts an absence of a maximum in the calcite concentration. This is illustrated in Figure 3, where the kinetic model predictions are also compared with the experimental data. Note that model predictions are not shown for the lime product since the latter was formed amorphously and continued to crystallize throughout the experiment. The kinetic parameters in Table I are also compared with the values obtained by Burnham et al.' and McCarthy et al.,5 the latter for Julia Creek oil shale. Our rates are not only higher than those reported by Burnham et al.' but we have concluded that, in an inert environment, calcite decomposition is faster than dolomite decomposition. If this were not the case, the calcite concentration would have built up to a maximum during decomposition in N2but this was never observed (see Figure 3). Note also that the calcite rate constants measured for the Julia Creek shale5 are more than an order of magnitude lower than those measured here although its activation energy is similar to that reported by Burnham et al. Since Julia Creek shale contains almost no dolomite, it is not too surprising that its calcite decomposes at a rate significantly different from that of the calcite in Green River oil shale; particularly since two-thirds of the latter is incorporated with ankeritic dolomite. Finally it should be noted that a series of experiments to study the decomposition of ankeritic dolomite in C02 atmospheres was also carried out. When the COzpressures were below the total inhibition point, the decomposition rates were found to be much slower than in the absence of COP A number of attempts were made to analyze these results by using conventional kinetic methodology, but consistent results could not be obtained. The inability to derive a suitable kinetic model to describe this phenomenon is consistent with the inability to recarbonate the ankeritic dolomite. It is likely that a more appropriate model should be based on phenomena associated with complex solid solution behavior. Silication. As pointed out by previous investigator^'^^^^, reactions of CaC03, CaO, and MgO with quartz to form silicates are not only possible but can occur under short time combustion conditions, particularly in the presence
Thomson et al.
12 Energy & Fuels, Vol. 2, No. 1, 1988 0061
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1
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Figure 5. Quartz and calcite concentrations (1173 K, Pco2= 100
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of steam. In the work reported here, silicates were observed to form under most conditions but at concentrations that were too low to be able to extract kinetic rate data. Nevertheless some interesting observations were made regarding the sequencing of the silication reactions as well as short-term rate data which gives some insight into the time scales involved. First of all, neither silicates formation nor quartz consumption were observed in either N2 or COPat 973 K for reaction times up to 1h. This is consistent with all of the previous work in a dry a t m ~ s p h e r e A . ~t 1073 ~ ~ ~K ~ in Nz, the quartz concentration again remained invariant for up to 1-h reaction times, and trace quantities of akermanite (CazMgSizO,) were identified. This latter species was separately confirmed by electron microprobe analysis since XRD could not distinguish between akermanite and gehlenite (Ca2A12Si07).It is interesting to note that dehydrated analcime (NaAlSi206)also disappeared under these conditions, and it is possible that amorphous intermediates were formed that might be precursors to the formation of gehlenite at higher temperatures as reported by Park et ala8 The situation was somewhat different when the identical experiment was carried out in pure COZ at 100 kPa pressure. In this case, there was a 13% conversion of quartz and the analcime disappeared, but the only silicate that formed by the end of 1 h was augite (Ca(Mg,Fe)Si206). In this case the COPoverpressure is sufficient to prevent calcite decomposition, and since no CaO was ever observed during dynamic monitoring, it is likely that the ankeritic dolomite was reacting directly with the silicon present in the quartz. Indeed, all of these reactions proceeded rapidly (
