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I n d . E n g . C h e m . Res 1992,31, 1130-1135
(2nd); Schulman, J. H., Ed.; Butterworths: London, 1957; pp 3-16. Bailey, A. Ph.D. Dissertation, University of Bristol, 1965. Bailey, A.; Cadenhead, D. A.; Davies, D. H.; Everett, D. H.; Miles, A. J. Low Pressure Hysteresis in the Adsorption of Organic Vapours by Porous Carbons. Trans. Faraday Sac. 1971, 67, 231. Ball, P. C.; Evans, R. Temperature Dependence of Gas Adsorption on a Mesoporous Solid: Capillary Criticality and Hysteresis. Langmuir 1989,5,714. Barton, S. S.; Evans, M. J. B.; Holland, J.; Koresh, J. E. Water and Cyclohexane Vapour Adsorption on Oxidized Porous Carbon. Carbon 1984,22,265. Barton, S. S.; Evans, M. J. B.; MacDonald, J. J. The Adsorption of Water Vapor by Porous Carbon. Carbon 1991,29,1099. Burgess, C. G. Ph.D. Dissertation, University of Bristol, 1971. Burgess, C. G. V.; Everett, D. H. The Lower Closure Point in Adsorption Hysteresis of the Capillary Condensation Type. J . Colloid Interface Sci. 1970,33,611. D'Arcy, R. L.; Watt, I. C. Analysis of Sorption Isotherms on NonHomogeneous Sorbents. Trans. Faraday Sac. 1970,66, 1236. Doong, S.J.; Yang, R. T. Adsorption of Mixtures of Water Vapor and Hydrocarbons by Activated Carbon Beds: Thermodynamic Model for Adsorption Equilibrium and Adsorber Dynamics. AIChE Symp. Ser. 1988,83 (No. 259), 87. Dubinin, M. M. Water Vapor Adsorption and the Microporous Structures of Carbonaceous Adsorbents. Carbon 1980,18,355. Dubinin, M. M.; Serpinsky, V. V. Isotherm Equation for Water VaDor Adsomtion bv MicroDorous Carbonaceous Adsorbents. Carbon 1981,'19, 40i. Dubinin, M. M.; Bering, B. P.; Serpinsky, V. V.; Vasil'ev, B. N. The ProDerties of Substances in the Adsorbed State: Studies of Gas Adsorption over a Wide Temperature and Pressure Range. In Surface Phenomena in Chemistry and Biology; Danielli, J. F., Pankhurst, K. G. A,, Riddiford, A. C., Eds.; Pergamon Press: London, 1958;p 172. Eissmann, R. N. Personal communication, 1991. Evans, M. J. B. The Adsorption of Water by Oxidised Microporous Carbon. Carbon 1987,25,81. Evans, R.; Marconi, U. M. B. Fluids in Narrow Pores: Adsorption, Capillary Condensation, and Critical Pointa. J . Chem. Phys. 1986, 84, 2376. Freeman, G. B.; Reucroft, P. J. Adsorption of HCN and H 2 0 Vapor Mixtures by Activated and Impregnated Carbons. Carbon 1979, 17,313. Grant, R. J.; Joyce, R. S.; Urbanic, J. E. The Effect of Relative Humidity on the Adsorption of Water-Immiscible Organic Vapors on Activated Carbon. In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1983; pp 219-227.
Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. Haasan, N. M.; Ghosh, T. K.; Hines, A. L.; Loyalka, S. K. Adsorption of Water Vapor on BPL Activated Carbon. Carbon 1991,29,681. Heffelfinger, G. S.;van Swol, F.; Gubbins, K. E. Adsorption Hysteresis in Narrow Pores. J . Chem. Phys. 1988,89,5202. Kadlec, 0.;Dubinin, M. M. Comments on the Limits of Applicability of the Mechanism of Capillary Condensation. J . Colloid Interface Sci. 1969,31,479. Kaul, B. K. Modern Version of Volumetric Apparatus for Measuring Gas-Solid Equilibrium Data. Ind. Eng. Chem. Res. 1987,26,928. Mahle, J. J.; Friday, D. K. Water Adsorption Equilibria on Microporous Carbons Correlated Using a Modification to the Sircar Isotherm. Carbon 1989,27,835. Manes, M. Estimation of the Effects of Humidity on the Adsorption onto Activated Carbons of Vapors of Water-Immiscible Organic Liquids. In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1983;pp 335-344. Matsumura, Y.; Yamabe, K.; Takahashi, H. The Effects of Hydrophilic Structures of Active Carbon on the Adsorption of Benzene and Methanol Vapors. Carbon 1985,23,263. Miles, A. J. Ph.D. Dissertation, University of Bristol, 1964. Nuttal, S. Ph.D. Dissertation, University of Bristol, 1974. Okazaki, M.; Tamon, H.; Toei, R. Prediction of Binary Adsorption Equilibria of Solvent and Water Vapor on Activated Carbon. J . Chem. Eng. Jpn. 1978,11,209. Peterson, B. K.; Gubbins, K. E. Phase Transitions in a Cylindrical Pore: Grand Canonical Monte Carlo, Mean Field Theory and the Kelvin Equation. Mol. Phys. 1987,62,215. Peterson, B. K.; Walton, J. P. R. B.; Gubbins, K. E. Phase Transitions in Narrow Pores: Metastable States, Critical Pointa, and Adsorption Hysteresis. In Fundamentals of Adsorption; Liapis, A. I., Ed.; Engineering Foundation: New York, 1986,pp 463-471. Ripperger, S.; Germerdonk, R. Binary Adsorption Equilibria of Organic Compounds and Water on Active Carbon. Ger. Chem. Eng. 1983,6,249. Scamehorn, J. F. Removal of Vinyl Chloride from Gaseous Streams by Adsorption on Activated Carbon. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 210. Tsunoda, R. Adsorption of Water Vapor on Active Carbons: Estimation of Pore Width. J . Colloid Interface Sci. 1990,137,563. Voll, M.; Boehm, H. P. Basische Oberfllchenoxide auf Kohlenstoff-111. Aktiver Wasserstoff und Polare Adsorptionszentren. Carbon 1971,9,473. Walker, P. L.; Janov, J. Hydrophilic Oxygen Complexes on Activated Graphon. J . Colloid Interface Sci. 1968,28,449. Received for review August 1, 1991 Accepted December 23, 1991
Ca(OH),/Fly Ash Sorbents for SO2 Removal Ch'un-Sung Ho and Shin-Min Shih* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.
The reactivity of Ca(OH)2/flyash sorbent with SO2has been studied by using a fiied-bed differential reactor under the conditions simulating the bag filters of the spray-drying flue gas desulfurization. The source of fly ash and the sorbent preparation conditions affect the reactivity of the sorbent. The reactivity of the sorbent was found to be closely related to the content of the calcium silicate hydrate formed in the sorbent preparation. The sorbent has a much higher utilization of Ca(OH), than that of pure Ca(OH), sorbent, and in some range of Ca(OH)2content the sorbent also has a higher SO2 capture capacity per unit weight of sorbent than that of pure lime. The fly ash from the Shin-Da plant of the Taiwan Power Company produced the best sorbent of all fly ashes in this study. The higher ratio of fly ash/Ca(OH),, the higher slurrying temperature, the longer slurrying time, and the smaller particles of fly ash enhance the utilization of Ca(OH)2,but the water/solid ratio has an optimal value. The relative humidity in the reactor has a significant effect on the reactivity of Ca(OH),/fly ash sorbents, but the effect of the sulfation temperature is subtle. Introduction Spray-drying flue gas desulfurization (spray-drying FGD)is one of the effective processes for removing SO2
* Author to whom correspondence should be addressed.
from the flue gas (Miller, 1986). Compared to other proceases, this process needs less space and is easier to retrofit, and it produces dry solid product. There are many coal-fired power plants that adopt this process to remove
sop
0888-5885/92/2631-ll30$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1131 x
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Table I. Chemical Composition of Fly Ashes from Shin-Da, Da-Lin. and Lin-Ko Power Plants' SiOz, AZO3,FeZOS, KzO, CaO, ign loss, source % I % %% Shin-Da (boiler 3) 59.0 26.7 5.5 2.5 1.6 2.7 Da-Lin (boiler 1) 49.6 24.4 8.4 1.6 2.1 9.8 Lin-KOb 51.7 24.0 11.2 2.5 2.1 6.7
'The data were given by the Taiwan power company. fly ash from all boilers in the station.
c a
3 Da-Lin I 01
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I I 40 60 80 P a r t i c l e dia., p
1 100
i
6
11 "
2 1
120
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Figure 1. Particle size distribution of fly ashes.
Experimental Section Lime used was reagent grade Ca(OH)2(Hayashi Pure Chemical Industries, LTD). Fly ashes were from the Shin-Da, Da-Lin, and Lin-Ko power plants of the Taiwan Power Company. The particle size distributions of the fly ashea were measured by laser diffraction (Microtrac, beds Northrup) and are shown in Figure 1. The chemical compositions of the fly ashes are listed in Table I.
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1 0 1 I
I 1
Spray-dryiig FGD includes two main parts: the spray dryer and the bag filters. Alkaline slurry (typically Ca(0H)J is sprayed into the spray dryer to contact with the flue gas. The water of the slurry droplets evaporates and the alkaline material in the slurry droplets reacts with SO2 in the flue gas simultaneously in this chamber. The dried sorbents along with the fly ash are then collected on the bag filters, where the reaction between the dried sorbents and SO2 continues. In order to improve the sorbent utilization, the recycling of both the particles emerging from the spray dryer and those collected on the bag filters to the system has been proposed. Some pilot-plant (Melia et al., 1983;Palazzola et al., 1983;Parsons et al., 1980)and laboratory (Reed et al., 1984,Jozewia and Rochelle, 1986;Jozewicz et al., 1988) works have already found that loading the coal-fired fly ash into Ca(OH)2slurry can effectively increase the desulfurization efficiency and the utilization of Ca(OH)2. Furthermore, the reactivity of Ca(OH),/fly ash sorbent toward SO2was enhanced by using ground fly ash (Petersen et al., 1988) or by adding NaOH into the slurry of fly ash and Ca(OH)2(Peterson and Rochelle, 1988). Fly ash is mainly composed of Si02,A1203,FezO3, and CaO. Since it contains plenty of amorphous Si02 and A1203, fly ash is considered as a kind of pozzolan. Fine powdered pozzolans can react with the hydroxides of alkali metals and alkaline-earth metals in the presence of water at normal temperatures, and these reactions are called "pozzolanic reactions" (Taylor, 1964). The pozzolanic reaction of fly ash with Ca(OH)2in slurry to form reactive products has been considered to be the reason for higher sorbent utilization in the fly ash recycle process (Jozewicz and Rochelle, 1986). However, the fundamental studies of this process are still scarcely reported in the literatures. In this study, fly ashes from the Taiwan Power Company were slurried with Ca(OH)2to prepare Ca(OH)2/flyash sorbents, and the effects of sorbent preparation conditions and sulfation conditions on the reactivities of these sorbents with SO2were investigated by using a differential reactor, which simulated the conditions in the bag filters of the spray-drying FGD.
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7. 8.
9. 10 * 11. 12. 13. 14. 15. 16.
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vent
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SQ cylinder Furnace Quartz sample pan Reactor Rubber stopper Thermocouple Recorder N1 cylinder Water reservoir Micro-tube pump Temperature controller Relay Voltage regulator Evaporator Needle valve Rotameter Stopcock SO, absorber Hot wind blower
4"
Figure 2. Schematic diagram of experimental apparatus.
Ca(OH),/fly ash slurries were prepared by considering the source of fly ash, the particle size of fly ash, the fly a ~ h / c a ( O Hweight )~ ratio, the water/solid weight ratio, the slurrying temperature, and the slurrying time. The weight of fly ash was kept at 8 g, and the amounts of Ca(OH)2and water are added by the required ratio. The slurrying was carried out in a 250-mL polypropylene conical beaker which was heated by a hot plate equipped with a magnetic stirrer. The mouth of the beaker was plugged to prevent water loss and the interference of C02. After slurrying the sample was placed into a vacuum oven at 35 "C for about 24 h to evaporate most of the free water and then further heated to 90 "C to dry, which was in about 2-4 days. The weight of the dry sorbent was equal to the s u m of the weights of fly ash and Ca(OH)2used. The dried samples were then subjected to sulfation. The experimental apparatus for sulfation is shown in Figure 2. The core of it is a fixed bed differential reactor where the SO2 in the sweep gas reacts with the Ca(OH),/fly ash sorbent. About 50 mg of powder of Ca(OH),/fly ash sorbent was dispersed into quartz wool. The
%
1132 Ind. Eng. Cham. Ras., Vol. 31, No. 4,1992
wool was then set into the quartz sample pan, 10-mm 0.d. and 15-mm height, which was perforated at the bottom to facilitate the passage of the sweep gas. The sweep gas entered the bottom of the reactor, passed through the 2.5-mm4.d. and 365-mm-length outer tube and went downward through the sample pan and the 10-mm4.d. and 315-mm-length inner tube. The relative humidity of the sweep gas was controlled hy pumping pure water into an evaporator with a microtube pump. The evaporator was heated by a furnace kept at 2W300 O C . Pure N, from a cylinder passed through the evaporator and was humidified. The reactor was heated by a furnace which was controlled by a PID temperature controller. The temperature of the solid reactant was measured hy a K-type thermocouple located just below the sample pan. Prior to each run, the sample was fmt dried to measure the original water adsorption amount and then humidified for 20 min by humid N, with a certain relative humidity at which the reaction was to be performed. The humidification time had been proved long enough for the sample to equilibrate with the gas stream. The pure Nzwas divided into two streams, the major one flowing through the evaporator and the minor one, about 0.3-0.5 L/min, being connected to the SO,stream to shorten the residence time of SOzin the entering tube. After humidification the SOzgas was admitted into the reactor to start the run. The total flow rate of sweep gas was 4 L/min (corresponding to the condition in the reaction chamber). The amount of water adsorbed on the sample was calculated from the difference between the weights of humidified and dried samples. The utilization of Ca(OH), of the Ca(OH),/fly ash sorbent with SO, expresaed as the fraction of Ca(OH), converted to CaSO,J/,H,O was calculated from the weight increase of the sample due to sulfation using the following equation (Ho, 1987):
where AW is the weight change due to sulfation, W, is the initial dry sample weight, R is the weight fraction of Ca(OH), in the sample, M I is the molecular weight of C&O,J/&I,O, and Mz is the molecular weight of Ca(OH),. Sample weights were measured by a Sartorius electronic balance with an accuracy of O.OOOO2 g. The data of Ca(OH), utilization were further confirmed by the S to Ca molar ratio obtained by iodometric and EDTA titrations. Results and Discussion Results of X-ray Diffraction. The unreacted and reacted sorbents using Shin-Da fly ash were subjected to X-ray diffraction to identify the solid species in the 801'bents. The diffraction pattern of Shin-Da fly ash showed a mild hum of semicrystalline portion from d = 2.75 A to d = 5.16 and typical peaks of crystalline mullite and quartz. In the unreacted Shin-Da fly ash/Ca(OH), samples, &um silicate hydratea (CS-H(I)) besides Ca(OH),, mullite, and quartz were found and the semicrystalline portion of fly ash disappeared. The calcium silicate hydrates are represented by the molecular formula xCaO. SiO,.yH,O, where x is 0.91.5 and y is 0.5-2.5 (Taylor, 1964). Since the weight of the sorbent was equal to its original weight of Ca(OH), and fly ash together, as mentioned in ExperimentalSection, the values of x and y for the calcium silicate hydrates obtained in this study were equal to 1. In the sulfated sorbents, only CaSO3.'/,HZO among all of the sulfites and sulfatea could be found. Thus
1
I I I
Figure 3. SEM micrograph of unslurried fly ash from Da-Lin power station.
,
Figure 4 SEM micrograph of unsulfated Shin-Da fly ash/Ca(OH), sorbent. Slurrying conditions: water/solid ratio lO:l, flly ash/Ca(OH), ratio 161, 65 "C, 12 h.
the utilization of Ca(OH), which formed the Ca(OH),/fly ash sorbent was calculated according to the product being CaS03J/zHz0. Results of SEM. Figure 3 is the typical micrograph of unslurried fly ash. The particles of fly ash are spherical, and their surfaces are smooth. Figure 4 is the typical micrograph of the Ca(OH),/fly ash sorbent. The particle surface is very rough due to the precipitation of the solid products which were formed during the sorbent preparation. The flakelike precipitates were identified to be calcium silicate hydrates, C-S-H(I) (Taylor, 1964; Chiu, 1989). There were no observable changes in the appearance of the sorbent particle after the sulfation reaction. Effect of the Source of Fly Ash. The composition of fly ash varies with the coal type and the operating
*
Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 1133
loo
I loo
Figure 5. Effect of aource of fly ash on reactivity of fly ash/Ca(OH), sorbent. Slurrying conditions: fly ash/Ca(OH), ratio 161. water/ solid ratio 101,65OC, 12 h. Sulfationconditions: lo00 ppm SO,, 60 OC, 50% RH, 1 h.
1 2 3 4
lime Shin-Da Lin-Ko Da-Lin
q W 1 o
=
0
40 80 Particle d i a . , p
,
I 120
Figure 6. Particle size distribution of lime and fly ash/Ca(OH), sorbents. Slurrying conditions are the same as those in Figure 5. Lime was used as received.
conditions of the boiler. The three fly ashes used in the present study and their compositions are listed in Table I. The reactivities toward SO2 of the Ca(OH),/fly ash samples prepared from the three flyashes are compared in Figure 5. Their slurrying conditions were fly ash/Ca(OH), weight ratio 161,water/solid ratio 101,65"C, and 12 h, and their reaction conditions were 60 "C, 50% relative humidity (RH), lo00 ppm SO,, and 1 h. The best sorbent is the one using Shin-Da fly ash. This may be due to the SiO, content of Shin-Da fly ash being the highest, as seen from Table I, and the particle size of the sorbent prepared with Shin-Da fly aah being the smallest, as shown in Figure 6,and having the largest reacting surface area among the three. The former reason is in agreement with the findings of Jozewicz and Rochelle (1986)that the addition of SiO, can promote the reactivity of Ca(OH),, and the effect of calcium content reported by Petersen et al. (1988)and Peterson and Rochelle (1988)was not observed in these sorbents with low-calcium fly ashes. The latter reason is also supported by the BET surface area data which are 15.7, 11.7, and 11.1 m2/g for sorbents with Shin-Da, Lin-KO, and Da-Lin fly ash, respectively. Figure 5 also shows that the utilizations of Ca(OH)2of the Ca(OH),/fly ash samples, 53-77%, are much higher than that of as received pure Ca(OH),, 17%, although the pure Ca(OH), has a particle size (Figure 6)smaller than and surface area (10.0 m2/g) comparable to the Ca(OH),/fly ash sorbents. The reason for the higher reactivity of the Ca(OH),/fly ash sorbent is attributed to the presence of calcium silicate hydrates in the sorbent.
0
I 3
I 6 Slurrying time , h
I 9
12
Figure 7. Effects of slurrying time and temperature on Ca(OH)z utilization for Shin-Da fly ash/Ca(OH), sorbent. Slurrying conditions: water/solid ratio 101,fly ash/Ca(OH), ratio 161. Sulfation conditions: lo00 ppm SOz, 60 OC, 60% RH, 1 h.
Calcium silicate hydrates are highly reactive toward SO, as reported by Chiu (1989). It also has been reported that the reactivity of calcium silicate toward SO, at a temperature above 700 "C was higher than the reactive CaO (Yang and Shen, 1979). Effect of Slurrying Conditions. When fly ash and Ca(OHI2were slurried together in water, dissolution of solids and reactions between the constituents occurred in the slurry, and the reaction products precipitated on fly ash particle surface. Thus the factors affecting the dissolution of the solids and the formation of the reactive species are the major parameters affecting the reactivity of the Ca(OH),/fly ash sorbent. The effect of slurrying time had been studied for three different slurrying periods, 3,6,and 12 h. Figure 7 shows the conversions of these sorbents reacted with loo0 ppm SO2 at 60 "C and 50% RH for 1 h. The dashed line in Figure 7 representa the conversion of the sorbent with zero slurrying time, which is the same as that of as-received pure lime. However, for the pure lime slurried with a water/solid ratio of 101 at 65 "C for 12 h, the conversion of it raised to 22.4%. This is due to the dissolution-precipitation of some part of the lime to form fine lime crystals. In spite of the slurrying temperature, the longer the slurrying time, the higher the reactivity towared SO2. However, for the samples slurried longer than 6 h, the increase of reactivity is less profound. This indicates that in the beginning of slurrying Si02in fly ash and Ca(OH), dissolve rapidly, and these two dissolved species react with each other to form the reactive material for SO, removal on the surface of fly ash particle (Barret et al., 1977; Glasser and Kataoka, 1981;Grutzeck et al., 1983). However, after the slurry has been agitated for a long period of time, say 6 h, the dissolution rate of Si02slows down (Barret et al., 1977);therefore, the amount of the reactive species increases slowly afterwards. The above explanation was confirmed by the SEM micrographs which showed that the surface precipitates of the samples slurried for 3 h at 65 OC were less than that of the samples slurried for 12 h at 65 OC. Figure 7 also shows that the reactivity of the sorbent increases with the slurrying temperature. There is a great change of reactivity between 55 and 65 "C, compared to that above 65 "C. Although the solubility of Ca(OH), decreases with increasing temperature, the reaction rate for producing reactive species increases with temperature; thus samples with higher slurrying temperatures are more reactive than those with lower slurrying temperatures.
1134 Ind. Eng. Chem. Res., Vol. 31, No. 4,1992
0.24
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-= W
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0.25
1
1
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Flyash/Ca(OHh
1
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r a t i o . g/g
Figure 8. Effect of fly ash/Ca(OH)zratio on Ca(OH)zutilization for Shin-Da fly ash/Ca(OH)z sorvent. Slurrying conditions: water/solid ratio 101,65 OC, 12 h. Sulfation conditions: lo00 ppm SOz, 60 "C, 50% RH, 1 h.
The effect of fly ash/Ca(OH), weight ratio on the reactivity of the sorbent toward SO2is represented in Figure 8. For the ratio between 0.25:l and 4:1, the conversions of the sorbents are at the same level, about 40%. When the ratio is between 4:l and 32:1, the conversions of the sorbents increase logarithmically with the ratio, and when the ratio is higher, the conversion of Ca(OH), reaches completion. That the conversions of some samples of ratio M1 are higher than 100% is due to the experimental error. The trend of the conversion change in the range of ratio between 4 1 and 321 is similar to the results obtained by Jozewicz and Rochelle (1986) for their Ca(OH),/fly ash sorbents. In contrast to the conversion of pure Ca(OH), which is only 17% under the same sulfation conditions, the utilizations of Ca(OH), of the fly ash/Ca(OH), sorbents can reach 100% when the ratio is higher than 32:l; thus we may conclude that for these sorbents almost all of the Ca(OH), added had been converted to calcium silicate hydrates, which were also confirmed by X-ray diffraction analysis. The data in Figure 8 are reexpressed as the relationship between the amount of SO,capture per unit weight of the sorbent and the weight percent of the Ca(OH), added in the sorbent, as shown in Figure 9. In this point of view the most reactive sorbent is 80% Ca(OH), sorbent, since it caught the most SO, per unit weight of the sorbent, 0.28 kg of SO,/kg of sorbent, which is about twice that of as received pure Ca(OH),, 0.15 kg of S02/kgof sorbent. After the maximum point, the amount of SO2capture decreases as Ca(OH), content decreases. The sorbent with zero Ca(OH),, which means that only the fly ash itself was slurried with water, reacted negligibly with SO,. As can be seen from the plot, the sorbent with Ca(OH), content higher than 44% has a higher SO2 capture capacity per unit weight of sorbent than that of pure lime. Also, as shown in Figure 8, the sorbent in this range of Ca(OH), content has a higher Ca(OH), utilization than that of pure lime; for example, the values for 100%, 80%, and 44% Ca(OH), sorbent are 0.17, 0.41, and 0.40, respectively. Thus the using of the Ca(OH),/fly ash sorbent with Ca(OH), content above 44% can increase the desulfurization efficiency as well as the utilization of Ca(OH),. The water/solid ratio in this experiment was varied from 1O:l to 40:l to study its effect. All of the samples were slurried at fly ash/Ca(OH), ratio 16:l and 65 "C for 1 2 h
0
20
0
60
80
w t . percent,
%
40
CaDHIz
100
Figure 9. SOz capture per unit weight of sorbent for the data in Figure 8. I
0
10
20
30
I
I
40
50
Water/solid ratio, g i g
Figure 10. Effect of water/solid ratio on Ca(OH), utilization for Shin-Da fly ash/Ca(OH)z sorbent. Slurrying conditions: fly ash/ Ca(OH), ratio 161, 65 OC, 12 h. Sulfation conditions: loo0 ppm SOP, 60 OC, 50% RH, 1 h.
and reacted with SO2at 60 "C and 50% RH for 1h. The results are shown in Figure 10. The sample of water/solid ratio of 201 has the highest conversion. The trend of the plot for conversion change versus water/solid ratio is due to the variation of pH, concentrations, and amounts of dissolved solids with the ratio. At low water/solid ratio, where pH and concentrations of dissolved species do not change much with the ratio, increasing the ratio will increase the total amounts of solids dissolved and thus that of the calcium silicate hydrates formed. However, at high water/solid ratio, increasing the ratio will decrease pH and concentrations of dissolved species, which in turn reduce the dissolution rate of SiO, (Glasser and Kataoka, 1981; Grutzeck et al., 1983; Peterson and Rochelle, 1988) and the reaction rate of SiO, with Ca(OH),, and thus the amount of the reactive product decreases. The effect of the particle size of fly ash on the reactivity of the sorbent was studied by using the fly ash from the Da-Lin power plant. From Figure 11,one can see that the smaller the particle size, the higher the conversion of the sorbent. This may be because of the larger surface area and the higher content of SiOz for the smaller fly ash
Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 1135
Acknowledgment This research was supported by the National Science Council of Republic of China under Grant NSC 76-0402E002-15. Registry No. SOz, 7446-09-5; CaOH, 1305-62-0; calcium silicate hydrate, 1344-96-3.
Literature Cited
0
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,
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20
40
60
I 80
100
dp,u
Figure 11. Effect of particle diameter of fly ash on Ca(OH)zutilization. The fly ash from Da-Lin power station was sieved to study such an effect. Slurrying conditions: fly ash/Ca(OH), ratio 161, water/solid ratio 101,65 OC, 12 h. Sulfation conditions: loo0 ppm SO*,60 OC, 50% RH, 1 h. Table 11. Conversions of Ca(OH),/Fly Ash Sorbents (Slurrying Conditions: 90 OC, Fly Ash/Ca(OH), Ratio l k l , Water/Solid Ratio 101, 12 h. Reaction Conditions: 1000 ppm SO2, 1 h) temp, OC re1 humidity, % conv, % 60 50 77 50 76 70 90 50 76 30 51 60 60 70 79
particle. From the results of energy dispersive spectrum, the unsieved fly ash contained 66.01% Si02,and the sieved portions selected, -45 pm, +45/-53 pm, and +53/-88 pm, contained 77 9% ,74 % , and 67 % Si02, respectively. Effect of Reaction Conditions. The sorbents used in the following experiments were prepared by slurrying at 90 O C for 12 h with fly a ~ h / c a ( O H ratio ) ~ 16:l and water/solid ratio 101. The samples reacted with lo00 ppm SO2 at 50% RH and 60, 70, and 90 "C, respectively, for 1h. The results of these three runs are nearly the same, as shown in Table 11. This indicates that the effect of reaction temperature on this reaction is subtle. The same samples as above were used to study the effect of the relative humidity in the reactor on the sulfation of sorbents. They reacted with lo00 ppm SO2 at 60 O C and 30%, 50%, and 70% RH, respectively,for 1h. The results are ale0 shown in Table 11. The conversion of the sorbent increases drastically with relative humidity in the range from 30% to 50% RH and levels off from 50% to 70% RH.
Barret, par P.; Menetrier, D.; Cottin, B. Study of Silica-Lime Solution Reaction. Cem. Concr. Res. 1977, 7,61-68. Chiu, C. S. The Reactivity of Ca(OH),/SiO, Sorbent with SO2. MS. Eng. Thesis, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC, 1989. Glasser, L. S. D.; Kataoka, N. The Chemistry of "Alkali-Aggregate" Reaction. Cem. Concr. Res. 1981,11, 1-9. Grutzeck, M. W.; Atkinson, S.; Roy, D. M. Mechanism of Hydration of Condensed Silica Fume in Calcium Hydroxide Solutions. In Fly Ash, Silica Fume, Slag & Other Mineral By-Products in Concrete; Malhotra, V. M., Ed.; American Concrete Institute: Detroit, 1983; Vol. 11, pp 643-664. Ho, C. S. Reaction of Ca(OH), and Flyash/Ca(OH), Slurry with SO2. MS. Eng. Thesis, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC, 1987. Jozewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5 (4), 219-224. Jozewicz, W.; Chang, J. C. S.; Sedman, C. B.; Brna, T. G.Silica-Enhanced Sorbents for Dry Injection Removal of SOzfrom Flue Gas. AF'CA J. 1988,38 (8), 1027-1034. Melia, M. T.; Mckibben, R. S.; Pelsor, B. W. "Utility FGD Survey July 1982-March 1983"; Project Summary EPRI Contract No. RP982-32, Electric Power Research Institute, 1983. Miller, M. J. Retrofit SOz and NOS Control Technologies for CoalFired Power Plants. Environ. Prog. 1986,5 (3), 171-177. Palazzola, M. A.; Brna, T. G.; Kelly, M. E. Current Status for Dry SOzControl System. Presented at the Eighth EPA/EPRI Symposium on Flue Gas Desulfurization, New Orleans, LA, 1983. Parsons, E. L., Jr.; Hemenway, L. F.; Kragh, 0. T.; Ostop, R. L. SOz Removal by Dry FGD. Proceedings: Symposium on FGD, Houston, TX; 1980; pp 801-852. Peterson, J. R.;Rochelle, G. T. Production of Lime/Fly Ash Absorbents for Flue Gas Desulfurization. Proceedings: First Combined FGD and Dry SO2 Control Symposium, St. Louis, MO; 1988. Peterson, T.; Peterson, J.; Karlsson, H. T.; Bjerle, I. Physical and Chemical Activation of Fly Ash to Produce Reagent for Dry FGD Processes. Proceedings: First Combined FGD and Dry SO2 Control Symposium, St. Louis, MO; 1988. Reed, G. D.; Davis, W. T.; Pudelek, R. E. Analysis of Coal Fly Ash Properties of Importance to Sulfur Dioxide Reactivity Potential. Environ. Sci. Technol. 1984,18 (71, 548-552. Taylor, H. F. W. The Chemistry of Cement; Academic Press: London, 1964. Yang, R. T.; Shen, M. S. Calcium Silicate: A New Class of Highly Regenerative Sorbents for Hot Gas Desulfurization. MChE J. 1979,25 (5), 811-819. Received for review May 3, 1991 Revised manuscript received November 26, 1991 Accepted December 10, 1991
