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Energy & Fuels 2008, 22, 3626–3630
Rate Limitations of Lime Dissolution into Coal Ash Slag Liza K. Elliott,*,† John A. Lucas,† Jim Happ,‡ John Patterson,§ Harry Hurst,§ and Terry F. Wall† School of Chemical Engineering, The UniVersity of Newcastle, UniVersity DriVe, Callaghan, Newcastle, New South Wales 2308, Australia, Jim Happ and Associate Pty. Ltd., and CSIRO DiVision of Energy Technology, 10 Murray Dwyer Cct, Mayfield West, Post Office Box 330, Newcastle, New South Wales 2300, Australia ReceiVed May 22, 2007. ReVised Manuscript ReceiVed September 3, 2008
The rate-limiting mechanisms of lime dissolution from a solid pellet into coal ash slag and synthetic slag was investigated using an experiment involving a rotating cylinder of lime in a liquid slag bath at temperatures of 1450-1650 °C. Scanning electron microscopy (SEM) analysis of the slag composition around the lime cylinder was used to determine the nature of the boundary layer surrounding the pellet and the calcium concentration profile. Predictions using shrinking core models of a cylindrical pellet were compared to experimental results, suggesting that diffusion through the slag boundary layer and the change of the phase of lime from solid to liquid in the boundary layer combine to limit the process. These results indicate that a combination of controlling steps: diffusion through the boundary layer and the phase change of lime from solid to liquid, must be considered when predicting lime dissolution rates.
Introduction The work presented in this paper is the initial investigation of the rate-limiting mechanisms for dissolution of lime into coal ash slag. Future papers will discuss the measurement of the rate of dissolution of this system, which will be used in modeling of slag flow in entrained flow gasifiers. Integrated coal gasification is a technology in which coal is gasified with steam and oxygen or air to create a product gas that can be combusted in a gas turbine, producing electricity. In entrained flow gasifiers, the mineral matter of the coal is heated to produce a slag, which flows to the base of the gasifier, allowing for efficient removal. However, many Australian coals have ash fusion temperatures in excess of the optimum operating temperature of entrained flow gasifiers and would be considered unsuitable for use in such gasifiers without the use of a flux to lower the ash melting temperature. Pulverised limestone is expected to be used in entrained flow gasifiers to improve slag flow properties because of its abundance and low cost. At the gasifiers operating temperatures (which are greater than 1400 °C) and pressures (greater than 2 MPa), limestone decomposes to lime (CaO) and carbon dioxide. The lime dissolves into the slag on the gasifier walls along with other ash components, resulting in a slag with a viscosity less than 15 Pa s.1 To predict the behavior of these gasifiers, models including the slag flow * To whom correspondence should be addressed: School of Chemical Engineering, The University of Newcastle, University Drive, Callaghan, Newcastle, New South Wales 2308, Australia. Telephone: +61-2-49217441. Fax: +61-2-49218692. E-mail:
[email protected]. † The University of Newcastle. ‡ Jim Happ and Associate Pty. Ltd. § CSIRO Division of Energy Technology. (1) Hurst, H. J.; Novak, F.; Patterson, J. H. Energy Fuels 1996, 10, 1215– 1219.
along the gasifier wall require an accurate measurement of the rate of lime dissolution. Review A reaction or a mass-transfer step may limit the rate of an entire process when the rate of that step is significantly slower than the rate of all other steps, such that the overall rate of the process is governed by the rate of the limiting step. The rate of dissolution processes are generally accepted to be limited by mass transfer through the boundary layer near the solid-liquid interface of the dissolving species. Boom et al.2 found the dissolution of CaO into steelmaking slags is mass-transfercontrolled. Similarly, Matsushima et al.3 showed a change in radius of a dissolving rotating cylinder with time is a function of the periphery velocity to the power of 2/3-4/5, for steelmaking and blast-furnace-type slags. Matsushima et al. states that this supports the assumption that the dissolution rate of solid lime into molten slag is controlled by diffusion of calcium through the liquid boundary layer. Natalie and Evans4 and Amini et al.5 showed that dissolution of lime from a cylinder rotating in slag increased with increasing rotational speed, suggesting that diffusion through the boundary layer does limit the dissolution process. However, Schlitt and Healy6 also determined the dissolution rate of lime for 0-30 wt % CaO/10-40 wt % SiO2/FeO slags. Finding the dissolution rate depended upon the physical structure (2) Boom, R.; Beisser, R. R.; Vanderknoop, W. In the 2nd International Symposium on Metallurgical Slags and Fluxes, the Metallurgical Society of the American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME), New York, 1984; pp1041-1060. (3) Matshshima, M.; Yadoomaru, S.; Mori, K.; Kawai, Y. Trans. Iron Steel Inst. Jpn. 1977, 17, 442–449. (4) Natalie, C. A.; Evans, J. W. Ironmaking Steelmaking 1979, 3, 101– 109. (5) Amini, S. H.; Brungs, M. P.; Jahanshahi, S.; Ostrovski, O. Metall. Mater. Trans. B 2006, 37 (5), 773–780. (6) Schlitt, W. J.; Healy, G. W. Ceram. Bull. 1971, 50 (12), 954–957.
10.1021/ef700261h CCC: $40.75 2008 American Chemical Society Published on Web 11/06/2008
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Figure 1. Position of the slag composition analysis relative to the lime pellet and an example of the analysis results. Table 1. Compositions of Slags Studied in the Experimental Program SiO2/Al2O3 (wt %)
CaOi (wt %)
FeO (wt %)
2.9 2 1.75
Synthetic Slags 25–30 30 30
0–15 0 0
3.22 3.03 2.17 2.02 1.74
Coal Ash Slags 22 25.5 8 20 10
2.6 7 8.4 7 8
of the lime and the slag lime concentration and was independent of time. No conclusion was drawn of the rate-controlling mechanism, but the lack of variation with time suggests that another mechanism other than diffusion controlled the process. The majority of researchers have published work in which the rate of dissolution has been measured but the controlling step of the dissolution process was not investigated. Several methods have been used by previous researchers to determine the rate of dissolution of a species in a slag. Each method requires contact of the slag with the dissolving species and a measurement of the change in concentration of either the dissolving or solvent phases. Wang7 studied the dissolution of lime (CaO) in slags containing SiO2-Al2O3-FeO-CaO by intimately mixing powdered components of the slag with lime in a crucible and heating for a set time, after which the sample was analyzed for unreacted or “free” lime. Oishi et al.8 dipped crystals of alumina, calcium aluminate, and fused silicate into slag consisting of CaO-SiO2-Al2O3 at 1400-1600 °C. After a set time, the crystals were removed and quenched. A radioactive Ca45O tracer in contact with a slag was used by Towers and Chipman9 and Johnston10 to determine the diffusion coefficient. (7) Wang, S. M. Masters Thesis, The University of Newcastle, Newcastle, New South Wales, Australia, May 1995. (8) Oishi, Y.; Cooper, A. R.; Kingery, W. D. J. Am. Ceram. Soc. 1973, 58 (2), 777–787. (9) Towers, H.; Chipman, J. Trans. AIME 1957, 769–773.
The most common technique used to study the rate of dissolution of refractory oxides is the “rotating cylinder or disk” method.3-5,11-15 In this method, a pellet or crystal is rotated in the slag at a set speed, with all surfaces of the pellet protected to prevent reaction, except for the base (rotating disk) or sides (rotating cylinder). At the end of the test, the pellet or crystal is removed from the slag and the change in size of the dissolving species is measured, clearly giving the amount of lime dissolved and the mass-transport flux, while the changing surface area of dissolving species is also known. This method assumes that the dissolving pellet does not swell or expand because of temperature changes, reactions, or natural expansion after its production. It is common in dissolution experiments for solid crystalline phases to form in the liquid slag, which may interrupt or slow diffusion. Because lime and magnesium oxide (or together as dolomite) are the main fluxing agents used in steelmaking production, these materials are most often studied. Natalie and Evans,4 Williams et al.,16 Masumi,17 Umakashi,15 White,18 and Coate et al.19 observed a dicalcium silicate layer when studying the dissolution behavior of lime. Some researchers observed these crystals floating in the slag away from the lime interface, while others observed them at the interface. Most researchers note that the presence of this crystal layer affected the rate of dissolution. Derge20 also observed 3CaO · Al2O3 formed in slags with high Al2O3 contents. In general, researchers assume a dissolution process is limited by the mass transfer of the dissolving species through the boundary layer and, in most cases, use a rotating cylinder or disk technique to measure the rate of dissolution. This study used a modified rotating cylinder technique to investigate the rate-limiting step of lime dissolution in synthetic and coal ash slags at temperatures between 1450 and 1650 °C. Experimental Technique The rate of dissolution of lime in slag was determined by contacting a pellet of lime with molten slag in a Haake ME1700 high-temperature “rotating bob” viscometer. The “bob” normally used to measure viscosity with the Haake viscometer was replaced by a cylindrical lime (CaO) pellet, 12 mm in diameter and 26 mm in length. The lime pellets were prepared by filling a hardened steel die with lime (CaO) particles, of less than 75 µm diameter, and then applying pressure (100 kN) to the die in a hydraulic press. Once the pellets were formed, they were sintered in a muffle furnace, with an oxidizing atmosphere, at 1550 °C for 2 h. The present study found that significant swelling of the lime pellet occurred both before and after it was sintered in a muffle furnace. The observed swelling was partially attributed to absorption of atmospheric water after it was sintered, but expansion of pellets encapsulated in epoxy resin was also observed, suggesting that the (10) Johnston, R. F.; Stark, R. A.; Taylor, J. Ironmaking Steelmaking 1974, 4, 220–227. (11) Gudenau, H. W. Stahl Eisen 1991, 111 (2), 89–94. (12) Olsson, R. G.; Koump, V.; Perzak, T. F. Trans. Metall. Soc. AIME 1966, 236, 426–429. (13) Cooper, A. R.; Kingery, W. D. J. Am. Ceram. Soc. 1964, 47 (1), 37–43. (14) Samaddar, B. N.; Kingery, W. D.; Cooper, A. R. J. Am. Ceram. Soc. 1964, 47 (5), 249–254. (15) Umakoshi, M.; Mori, K.; Kawai, Y. Trans. Iron Steel Inst. Jpn. 1984, 24, 532–539. (16) Williams, P.; Sunderland, M.; Briggs, G. Ironmaking Steelmaking 1982, 9 (4), 150–162. (17) Masui, A.; Yamada, K. Proceedings of the 4th Japan-USSR Joint Symposium on Physical, Chemical, and Metallurgical Processing, The Iron and Steel Institute of Japan (ISIJ), Tokyo, Japan, 1973; p 117. (18) White, J. Ironmaking Steelmaking 1974, 2, 115–117. (19) Coate, D. W.; Selmeczi, J. G. Electr. Furn. Proc. 1979, 37, 258– 262. (20) Derge, G. Trans. Metall. Soc. AIME 1967, 293, 1480–1489.
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Figure 2. Electron micrograph of the crystals formed on the lime surface.
away from the lime pellet, as shown in Figure 1. This figure also shows a sample of the experimental results. The rate of dissolution from each pellet was determined by calculating the flux from the pellet based on the area under the curve produced by the concentration boundary layer and above the bulk slag concentration, as per eq 1
j)
Figure 3. Steps involved in the process of lime dissolving into molten slag.
sintering process does not eliminate expansion of the pellet. Therefore, the rotating cylinder method used by previous authors in which the size of the pellet remaining after dissolution was measured and used to determine the rate of mass transfer may produce erroneous results. In each experiment, the pellet in the slag was rotated at speeds between 0.5 and 500 rpm. Both end surfaces of the cylindrical pellet were covered with molybdenum, which were cemented in place with zirconia high-temperature cement to ensure dissolution only occurred from the side walls of the pellet. Each pellet was completely immersed to within 10 mm of the base of the crucible. Pellets completely immersed in the solvent slag are not exposed to forces of surface tension, which would increase dissolution.13 A graphite crucible with a molybdenum liner was used to produce a slag of known composition. Slags produced in graphite crucibles alone were found to have different viscous behavior than slags produced in contact with molybdenum. It appeared that carbon was dissolving into or reacting with the slag. However, molybdenum was found to reduce some iron in the slag when the iron content was high (>12 wt %). Slags produced from both analytical-grade oxides and coal ash were studied. The compositions of the slags are listed in Table 1, where CaOi represents the lime content in the slag before dissolution occurred. All experiments were completed under a nitrogen curtain, with an oxygen partial pressure of 6 × 10–10 at 1340 °C. To overcome the impact of the expansion of the pellet, when the experiment was complete, the crucible was removed from the furnace with the pellet still in position and cooled quickly in a nitrogen gas stream to solidify the slag around the pellet, stopping the dissolution reaction. The slag sample was sectioned through the vertical axis of the pellet, and the sample was mounted and polished for analysis by scanning electron microscopy. The boundary layer of lime dissolving into the slag could be measured by analyzing the composition of the slag on a perpendicular line
Fslag
∫
rn
r0
r wt % CaO dr rt
(1)
where j is the flux from the pellet, r is the distance from the center of the pellet, and t is the time for dissolution. The value of the slag density, Fslag, was estimated from values provided in the slag atlas.21 The integral is solved by Simpsons rule.22 The rate constant for dissolution can then be simply determined on the basis of the rate-limiting step. That is, if diffusion through the boundary layer limits the dissolution process, the diffusion rate constant, kD, is calculated from j ) kD(Ci – Cb), where j is the flux of lime from the pellet and C is the lime concentration at the pellet slag interface (i) and in the slag bulk (b). In addition, if a reaction limits the process, the rate constant for the reaction, kR, can be determined from the reaction: j ) kRCR, where CR is the reactant concentration. The total amount of lime dissolved can be simply determined from the flux, the surface area of the pellet, and the time of dissolution.
Theoretical The rate-limiting step of lime dissolving into coal ash slag was investigated by the method described by Levenspiel,23 in which experimentally determined conversions are compared to conversion rates predicted by a model. A shrinking core model was chosen because crystals around the lime pellet were observed during examination under the scanning electron microscope, and these were considered to be a layer of the product of a reaction. Figure 2 shows an electron micrograph of the lime surface showing the crystals observed. To determine the limiting step, the process is broken into several steps, as shown in Figure 3, with a schematic of the pellet-slag interface. As dissolution occurred from a cylindrical pellet, equations of a shrinking core model for a cylindrical pellet were derived, as shown in Table 2.22 If the experimental results match the conversion predicted by the appropriate equation, the step is said to limit the process. (21) Slag Atlas; Eisenhuttenleute, V. D., Ed.; Verlag Stahleisen mbH: Dusseldorf, Germany, 1981; pp 57-178. (22) Elliott, L. K. Ph.D. Thesis, The University of Newcastle, Newcastle, New South Wales, Australia, 1999. (23) Levenspiel, O. Chemical Reaction Engineering; John Wiley and Sons: Brisbane, Australia, 1972; pp 361-368, 372.
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Table 2. Equations for a Shrinking Core Model for a Cylinder When Each Step Limits the Process, as Shown in Figure 3a process step
time for complete conversion, τ
time, t
change in phase t) diffusion through crystal layer t)
F(R - r) kPCi
[
reaction to form a solid t) diffusion through the boundary layer t)
FR kPCi
t ) 1 - (1 - X)1/2 τ
F R2 D(Cx - Cl) 4
t ) X + (1 - X)ln(1 - X) τ
FR bksCr
t ) 1 - (1 - X)1/2 τ
FR 2kD(Cl - Cb)
t )X τ
τ)
]
r2 R2 r2 F r2 ln r - + - ln R D(Cx - Cl) 2 4 4 2
τ)
F(R - r) bksCr
τ)
[ ]
FR r2 1- 2 2kD(Cl - Cb) R
τ)
(t/τ)
a The following symbols are used in the table: F is the molar density; r is the radius from the center of the pellet; and R is the original radius of the pellet. The rate constant, k, for phase change (P), reaction to form a solid (S), and diffusion through the boundary layer (D) are included, as is the diffusion coefficient, D, for diffusion through the crystal layer. Several lime concentrations, C, are named: the lime concentration at the interface (I); the concentration at the slag-crystal interface (x) and crystal-pellet interface (l); and the concentration in the slag bulk (b). X refers to the proportion of lime converted.
Figure 5. Experimental time divided by the time for complete conversion plotted against the ratio t/τ predicted by the shrinking core model for the combined resistances of the change of phase of lime from solid to liquid and diffusion through the boundary layer limiting the entire process. Figure 4. Shrinking core model determining the limiting step of lime dissolving into molten slag, where X is the proportion of conversion, t is the time for dissolution, and τ is the time required for complete conversion.
In this study the change in phase was considered a “reaction”, with one reactant and one product, as shown: lime(solid) w lime(liquid).
Results The experimental conversion (i.e., the amount dissolved from the pellet compared to the total amount in the pellet) and the conversion predicted by the equations in Table 2 are plotted in Figure 4 against the ratio of dissolution time and the time for complete conversion predicted by the model (t/τ). To account for differences in the dimensions of the experimental pellets, it is necessary to divide the experimental time by the time for complete conversion. However, the time required to dissolve the entire pellet (or the time for complete conversion) was not found experimentally; therefore, the time predicted by the model, τ, was used to plot the experimental results. Therefore, the experimental results are repeated on the figure because each set has a different value of the time for complete conversion (τ). The rate-controlling step will have experimental results and model predictions that coincide. Several reactions to form crystals were considered, but only the model for the reaction to form dicalcium silicate is shown
in Figure 4. Various crystalline species were observed at the lime-slag interface under SEM EDAX examination, but dicalcium silicate was the most prolific. The model does not change for other reactions (such as the formation of tricalcium silicate and tricalcium aluminate). The ratios (texp/τ) predicted by the experimental results for these other reactions provide a poorer fit to the theoretical curve than experimental results for the formation of dicalcium silicate. Predicted trends for the reaction and the phase change limiting the process are exactly the same and are therefore seen on the graph as one line. The experimental results based on the reaction as the limiting step do not fall near the trend predicted by the model. Similarly, results based on diffusion through the crystal region limiting the process do not coincide with the model prediction and are very scattered. However, experimental results of both diffusion through the boundary layer and change of phase of lime fall close to the predictions of the models for these steps. The ratecontrolling step could not be distinguished between these two steps. Because a significant change in the radius of the dissolving lime pellet can have a significant impact on the final results, an attempt was made to ensure only a small percentage of the pellet dissolved. Limiting the amount of dissolution from the pellet meant that the results above can only cover a small range of the time for complete conversion, and as such, the results of this analysis are inconclusive.
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Table 3. Equations for a Shrinking Core Model for a Cylinder When Diffusion through the Boundary Layer and Change of Phase Combine to Limit the Process2 time for complete conversion, τ
time, t combined control (diffusion through the boundary layer and phase change)
2
t)
FR (Ci - Cb)2kD
[( ) 1-
kD r2 r +2 12 kP R R
(
)]
τ)
(
kD FR 1+2 (Ci - Cb)2kD kP
)
t ) τ
[
(t/τ) X+2
kD (1 - (1 - X)1/2) kP kD 1+2 kP
]
The nomenclature used in this table is the same as in Table 2.
Both results for diffusion through the boundary layer and the change in the phase of lime from solid to liquid have similar rates, suggesting that these two steps combine to limit the process. Both steps must be considered together if the conversion is to be described, as shown in Table 3.22 The derived expression of conversion with time for combined resistance requires knowledge of the ratio of the rate constants for each step, kD/kP, which can be obtained by combining the equations for mass flux because of diffusion and phase change and the equation of resistances kD j r -1 (2) ) kP kT(Cs - Cb) R The concentration term, Cs, is a saturation concentration, which occurs because diffusion through the crystal layer is assumed to be fast, such that no concentration gradient exists through this region. Therefore, diffusion through the boundary layer and the reverse reaction of phase change are assumed to occur from this intermediate concentration. Because the prediction relies heavily on the value of the intermediate concentration, without a measurement of this, there is no accurate way to determine the ratio of the rate constants. An estimate of this concentration was made by assuming the CaO concentration in the slag within the crystal layer was the lime content of the equilibrium liquidus composition at the experimental temperature, which was estimated using “FACT”,24 a thermodynamic package, and the determined value compared to phase diagrams.21
(
)
Using this technique to determine the ratio of rate constants, the experimentally and theoretically derived ratio of t/τ could be obtained, as shown in Figure 5. This graph suggests that both steps, diffusion through the boundary layer and change of phase, combine to limit the process. The average value of the ratio of rate constants, kD/kP, was approximately 2.52, although the value varied significantly. Conclusions Lime dissolution in coal ash slag was investigated using a shrinking core model to determine the rate-limiting step. Mass transfer in the experimental system studied appears to be limited by a combination of steps: diffusion through the boundary layer and the phase change of lime from the solid to liquid phase. The concentration profile surrounding the pellet in the slag can be used to measure the flux from the pellet, but measurement of the pellet size can be deceptively erroneous. Further investigation is required to determine the validity of an assumption in the model, which assumed that the lime concentration at the lime-slag interface was the equilibrium concentration. EF700261H (24) Facility for the Analysis of Chemical Thermodynamics (FACT). Ecole Polytechnique de Montreal, Royal Military College, Montreal, Quebec, Canada, July 1996.
