u = longitudinal velocity, cm/s u = transverse velocity, cm/s or solvent flux, gal/ft2 day V = dimensionless transverse velocity, eq 20b x = longitudinal coordinate, cm
Technology," J. E. Flinn, Ed., p 47, Plenum Press, New York, N.Y., 1970. Brown, C. E.,Tulin, M. P., Van Dyke, P., Chem. Eng. frog. Symp. Ser., 67, No. 144, 174 (1971). Charlwood, P. A., J. Phys. Chem., 57, 125 (1953). Creeth, J. M., J. Biochem., 51, 10 (1952). Doherty, P.. Benedek, G. B., J. Chem. Phys., 5426 (1974). Gill, W. N., Derzansky, L. J.. Doshi, M. R., "Surface and Colloid Science," Vol. IV, E. Matijevic, Ed., p 262, Wiley, New York, N.Y., 1971. Grieves, R. B., Bhattacharyya, D., Schomp, W. G., Bewley, J. L., AlChEJ., 19, 766 (1973). Keller, K. H., Canales, E. R., Yum, S. I., J. Phys. Chem., 75, 379 (1971). Kozinski, A. A., Lightfoot, E. N., AlChEJ., 18, 1030 (1972). Michaels, A. S., Chem. Eng. frog., 64 (12), 31 (1968). Phillies, G. D. J., J. Chem. Phys., 60, 976 (1974). Phillies, G. D. J., Benedek, G. B., Mazer, N. A,, J. Chem. Phys., 65, 1883 (1976). Porter, M. C., Ind. Eng. Chem., Prod. Res. Dev., 11, 234 (1972). Scatchard, G., Batchelder, A. C., Brown, A., J. Am. Chem. SOC., 68, 2320 (1946).
y = transverse coordinate, cm
Greek Letters thickness of concentration boundary layer, cm dimensionless transverse coordinate, eq 8a absolute viscosity of the solution, dyn/cm2 s dimensionless viscosity, eq 22 T = shear stress, dyn/cm2
6 = q = p = p =
Subscripts g = a t gelling condition lim = at limiting flux condition w = atmembranewall 00 = a t bulk or feed condition
Received for review April 12, 1976 Accepted June 29,1977
Literature Cited Acrivos, A., Chem. Eng. Sci., 17, 457 (1962). Anderson, J. L.. Reed, C. C., J. Chem. Phys., 64, 3240 (1976). Blatt. W. F., Dravid, A,, Michaels, A. S., Nelson, L., "Membrane Science and
T h i s research was supported by t h e Office o f Water Research a n d Technology under G r a n t No. 14-31-0001-7506. One o f us (J.S.) was also supported t h r o u g h a N a t i o n a l Science F o u n d a t i o n Energy Traineeship.
Fluidized-Bed Coal Combustion with Lime Additives. The Phenomenon of Peaking of Sulfur Retention at a Certain Temperature Ralph T. Yang,' C. R. Krlshna, and M. Stelnberg Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973
Preliminary kinetic results and the mechanistic implications of the reactions between calcium sulfate and coal ash are presented. Calcium silicates were identified in the reaction products. These reactions also produce SO2 and have been shown to have a high temperature dependency. Taken with the published kinetic data on the sulfation reaction of limestone, this leads to a possible contributory mechanism toward the phenomenon of peaking of sulfur retention at a certain temperature in fluidized-bed coal combustion with lime additives.
Introduction In this communication, we present some of the results obtained in the experimental study of the following reaction Cas04
+ coal ash
-
silicates
+ SO2 + V 2 0 2
(1)
This reaction can be of some importance in the combustion of coal in a fluidized bed of limestone. The consequences of such a reaction occurring there along with the following reaction of the sulfating of lime CaO
+ SO2 + l / 2 0 2
-
Cas04
will be discussed after presentation of the experimental results. Experimental Section The rates of reaction 1 were measured gravimetrically. Detailed procedures and the apparatus used for the measurements have been described elsewhere (Yang and Steinberg, 1975, 1976). An alumina sample holder was used in all the experiments. The total amount of the sample mixture was about 0.5-1 g. The reactant sample mixture con-
tained approximately 1 CaS04:1.5 Si02 (molar) in all cases. A gas mixture was passed over the packed sample surface a t approximately 1cm/s. The composition of the gas mixture was 2.9% H20,3% S02,5% 0 2 and the balance N2. The water vapor content was so chosen that direct comparisons can be made with the published results on reaction 2 (Yang et al., 1975). Rates are expressed as (l/W)(dM/dt) where W is the instantaneous mass of the remaining Cas04 in grams, M is the number of gram-moles of SO2 evolved, and t is the time in seconds. The initial rates were obtained by the method described previously (Yang et al., 1975). Drierite (manufactured by Hammond Co.) was used as the Cas04 without purification. The coal ash was obtained by oxidizing Illinois No. 6 bituminous (Hvbb) coal with air at 1000 "C for 12 h. A representative analysis of the ash is given in Table I. The materials were ground, sized, and mixed evenly at the desired proportions for rate measurements. Two series of experiments were performed with two different ash sizes with the same size of Cas04 of the size range of 710-1000 p. The sizes of the ash particles were 250-425 p in the first series and less than 75 p in the second. Contact between the ash and Cas04 particles was obviously more intimate with the Ind. Eng. Chem., Fundam., Vol. 16, No. 4, 1977
465
Table I. Coal Ash Analysis
10-1
Constituent
Percentage (by wt)
Si02 Fez03
54.7 21.0 15.5 1.9
A1203
CaO Ti02
1000
ll0OT
900
p206
0.3 0.8
BOOT
lo-21
1
a
1.2
MgO Nan0 KZO
so3
,-
0.2 2.1 1.0
Table 11. X-ray Diffraction Analysis of Partially Reacted Products (Reaction 2 at 870 "C with Ash Sieved Out) Crystal phase Ca6Si3012"20 Cas04 (anhydrous) Ca6Si6017(OH)z 3CaO-Fez03.3Si02 3CazSi0~2H20 CaO CaAlzSizOs (hex.) Calcium aluminates Calcium ferrites and ferrates
Major Major Minor Possible minor Possible minor Possible trace Possible trace None None
smaller-size ash. The size of Cas04 is similar to that used in fluidized-bed combustion of coal. The gases were all of prepurified grades and were used without further purification. For x-ray diffraction analyses of the reaction products, the finer-sized coal ash was sieved out and the partially reacted Cas04 particles were picked and separated from the residual ash particles. In this manner, a great number of confusing diffraction lines associated with the ash were eliminated. The particles were ground to a size smaller than 62 p for the x-ray powder patterns. The patterns were obtained by the routine film techniques using Cu K a radiation. Results a n d Discussion Figure 1summarizes the results of the rate measurements for reaction 1and also includes the rates of reaction 2 taken from Yang et al. (1975). X-ray diffraction analyses of the partially reacted products of reaction 1 are summarized in Table 11. In these experiments, about 40% of the Cas04 was decomposed. SO2 was detected in the sweeping gas wherein SO2 was not introduced externally. As shown in Table 11, both mono- and di-calcium silicates with small amounts of hydrate have been identified. It is uncertain whether the hydrate was formed during the reaction or while it was exposed to the atmosphere after being cooled to room temperature. However, it is likely that it was formed during the reactions, especially with the minor species Ca,$i~Ol.i(OH)~which contains internal hydroxyl groups. Four other compounds were listed but further work is needed for their identifications. Special attention has been focused on the identification of the binary calcium ferrites, ferrates, and aluminates because their formation is thermodynamically feasible (Lowell and Parsons, 1975; Lea, 1970), but none of such materials appeared in the diffraction patterns. It is interesting to note that in the reductive regeneration of CaO from the sulfated limestone, in which coal was burned to generate the reducing gas CO, the bed material agglomerated a t 1040 "C and x-ray diffraction analysis showed Ca3MgSi208 as the major constituent in the agglomerated material (Vogel et al., 1975). With knowledge of the reaction products, we now proceed to discuss the rate data in Figure 1.The most interesting re466
1
Assignment
Ind. Eng. Chem., Fundam., Vol. 16, No. 4, 1977
10-6
1
65
1
1
7.5
85
I
I
9.5
IO~IT, * K - I
Figure 1. Initial rates vs. temperature: (A) reaction (1)with Cas04 (710-1000-p size) and coal ash (250-425 p ) : (B) reaction (1) with Cas04 (710-1000 p ) and coal ash (less than 75 p ) : and (C) reaction 2 from Yang et al. (1975).
sults in this figure are that of the high-temperature dependency of reaction 1. The temperature dependencies of lines A and B in Figure 1 indicate an activation energy of 80-85 kcal/mol for this reaction, in contrast to an activation energy of 7.3 kcal/mol for reaction 2 (Yang et al., 1975). Reaction 1 is a solid-solid reaction forming primarily calcium silicates. The high activation energy indicates that diffusion in the solid, or through the products, is the rate-controlling step. As shown by numerous data on diffusion of atoms and ions in oxides, the total activation energy includes the energy required for the formation of vacancies in the lattice and the energy necessary for the motion of a vacancy through the lattice (Kingery, 1959). In reaction 1, the diffusion of silicon or silicon and oxygen in the silicates is probably the rate-controlling step due to the lack of the highly mobile alkali elements in the reaction system (Schaeffer, 1975). The total activation energy of such diffusion systems normally ranges between 30 and 100 kcal/ mol. More discussion on the diffusion mechanism of reaction 1would be improper due to the great number of "impurities" involved. It has also been observed in this study that, in the presence of 2.9% water vapor in the gas phase, the rate was not affected by the concentrations of SO2 and 0 2 . This observation is in accordance with the above discussion of the diffusion-controlling mechanism. The great difference in the rates with the two different ash particle sizes is obviously caused by the different degree of contacts of the surfaces through which the reaction proceeds. The results of this study can now be related to the fluidized-bed combustion of coal with lime additives. In fluidized-bed combustion, there is a significant amount of coal ash retained in the beds. For example, in the pressurized combustor a t Argonne National Laboratory, approximately 5% of the bed material is coal ash (at a pressure of 8 atm) (Vogel, 1976). During steady-state combustion, the surface of most of the lime particles is sulfated. Besides the coal ash retained in the bed, finer-sized ash particles are circulated through the bed and finally carried out along with SO2 and 0 2 . I t is suggested here that reaction 1can occur in the bed with the sulfated limestone reacting with the ash in the bed as well as that being circulated through the bed. In a sense, it can be said that
SO2 RETENTION I----)
IIT
Figure 2. Relative rates in fluidized b e d combustion.
the partially sulfated lime absorbs SO2 and also other oxides (primarily SiOz). Of course, the results shown in Figure 1 cannot directly represent the rates of the reactions taking place in a fluidized bed; the rates are likely to be higher in a fluidized state because of more contacts between the reactants. In this connection, sulfur retention can be calculated based on the rate as shown by Koppel(l970) and Horio et al. (1975). However, the basic mechanisms of the primary reactions in fluidizedbed combustion should obey that in a fixed bed, i.e., the results shown in Figure 1. It is possible, therefore, to represent the rates of the two reactions in fluidized-bed combustion in a qualitative manner as shown in Figure 2. The relatively high activation energy of reaction 1 explains that, a t high enough temperatures, the rates of reaction 1gradually approach that of reaction 2 and sulfur retention by the bed material decreases. Such a peaking and subsequent reduction (with further increase in temperature) in sulfur retention has been observed in the fluidized-bed combustion of coal (Jonke et al., 1972; National Research Development Corporation, 1974). The peaking temperature is in the range of 800 to 900 "C in atmospheric pressure beds and at higher temperatures in pressurized beds. The report by NRDC (1974) d'iscusses severa1 prevalent hypotheses for explaining the phenomenon. The results of the present study (Figures 1and 2) indicate that reaction 1 could also be contributing to the occurrence of such
a phenomenon in the combustion of coal in a fluidized bed of limestone. The highest temperature studied here was 1220 "C. I t is realized that, above this temperature, more complex reactions similar to those in cement formation would occur. Complex phase changes and transitions would also be involved. Studies in this area are currently underway in our laboratories. As mentioned, the rate of reaction 1does not depend on the partial pressures of SO2 and 0 2 in the presence of 2.9% water vapor in the gas phase. The rate of reaction 2, also in the presence of water vapor, is first order in SO2 and 0.22 order in 0 2 (Yang et al., 1975). At elevated pressures, and hence higher SO2 and 0 2 partial pressures, the rate of reaction 2 is increased while reaction 1is unaffected, as depicted in Figure 2. The net result is the formation of a plateau of the sulfur retention with increasing temperature, but at temperatures higher than that at atmospheric pressure. At still higher temperature, complex and still unknown reactions involved prohibit any further meaningful discussion. Acknowledgment We would like to acknowledge the helpful discussions with Dr. A. Macek of the U S . Energy Research and Development Administration, Drs. P. T. Cunningham, A. A. Jonke, W. M. Swift, and G. J. Vogel of Argonne National Laboratory, and Drs. G. Adler, A. B. Auskern, and R. K. Jordan of Brookhaven. We also wish to thank Mr. W. W. Reams for obtaining x-ray diffraction patterns. L i t e r a t u r e Cited Horio, M., Mori, S.,Wen, C. Y., International Fluidization Conference, Vll-5, Pacific Grove, Calif., 1975. Jonke, A. A,, Vogel, G. J., Carls. E. L.. Ramaswami, D.. Anastasia, L., Jarry, R., Hass, M., AlChf Symp. Ser. 68, 126, 241 (1972). Kingery, W. D.,"Diffusion in Oxides", in "Kinetics of High-Temperature Processes", w. D.Kingery, Ed., MIT Press and Wiley, New York. N.Y., 1959. Koppel, L., in ArgonneNational Lab. Annual Reportby A. A. Jonke et al, ANL/ ES-CEN-1002, June 1970. Lea, F. M., "The Chemistry of Cement and Concrete", 3rd ed, Edward Arnold, England, 1970. Lowell, P. S.,Parsons, T. B., "Identification of Regenerable Metal Oxide SO? Sorbents for Fluidized-Bed Coal Combustion", EPA-650/2-75-065, 1975. National Research Development Corp. (NRDC, London), R and D Rep. 85, Interim No. 1, 1974. Schaeffer, H. A.. "Silicon and Oxygen Diffusion in Oxide Glasses", in "Mass Transport Phenomena in Ceramics", A. R. Cooper and A. H. Heuer Ed., Plenum, New York, N.Y., and London, 1975. Vogel. G. J., Argonne National Lab., Argonne, Ill., private communication, 1976. Yang, R . T., Cunningham, P. T., Wilson, W. I., Johnson, S.A,, Adv. Chem. Ser., No. 139, 149 (1975). Yang, R. T., Steinberg, M., Carbon, 13, 41 1 (1975). Yang, R. T., Steinberg, M., J. Phys. Chem., 80, 965 (1976).
Received f o r review June 18,1976 Accepted June 1,1977 T h i s work was performed under t h e auspices o f t h e US.Energy R e search a n d Development Administration.
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