Znd. Eng. Chem. Res. 1995,34, 1404-1411
1404
Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash Hiroaki Tsuchiai,+p*Tomohiro Ishizuka,*Tsutomu Ueno,S Hideshi Hattori,*J and Hideaki Kitat Division of Materials Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan, and Department of Research and Development, Hokkaido Electric Power Co., Znc., 4-9-2-1Utsukushigaoka, Toyohira-ku, Sapporo 004, Japan
The absorbent for SO:! from the flue gas of a coal-fired electric power station was prepared from calcium oxide, calcium sulfate, and coal fly ash and examined for the relation between the desulfurization activity and the structure. The activity is closely related to the progress of the hydration reaction taking place during preparation procedures. The activity increased with the hydration time and reached a maximum activity i n 12 h. The hydration resulted in the formations of ettringite and calcium silicate. By elevation of the temperature for drying the hydration products, the activity markedly increased up to 400 "C, which was caused by the removal of water covering the calcium component in the ettringite. For the efficient removal of S02, the existence of NO in the flue gas is required. NO, plays a catalytic role for oxidation of SO2 to SO3 which reacts with CaO to form Cas04 a s a final product.
Introduction Reduction of emissions of air pollutants is required for industrial operations. In particular, SO:! and NO, are the main targets to be reduced (Livengood, 1991). Among the stationary sources of emissions of air pollutants are coal-fired electric power plants which discharge large amounts of SO:! and NO,. For example, a coal-fired power plant of 600 MW capacity burning coals containing 1.2% sulfur releases 1785 m3 h-l (NTP) SO:! and 318 m3 h-I (NTP) NO,. A typical composition of the flue gas is SO2 350 ppm, NO, 125 ppm, 0 2 5.2%, CO:! 13%,H20 7.8%. For the removal of SO:! from flue gas, a wet process using calcium carbonate as an absorbent is most commonly adopted in commercial plants (Dalton, 1990).The wet process shows a high efficiency but needs a large amount of water. The desulfurization is believed to be initiated by dissolution of SO:! into water followed by reaction with calcium carbonate to form calcium sulfite as a precipitate. The calcium sulfite is oxidized by air t o form calcium sulfate dihydrate as a final product (Dalton, 1990). A dry process using calcium hydroxide as an absorbent is used commercially but is not as common as the wet process (Tischer, 1991). In the dry process, a powdery calcium hydroxide is injected into the duct. The efficiency of this duct injection dry process, however, is not high. A large fraction of calcium hydroxide remains unreacted. The low utilization efficiency of calcium in the dry process is considered to be due t o the formation of calcium sulfate which covers the outer surface of the calcium hydroxide particles (Brown et al., 1991). Jozewicz and Rochelle (1986) reported that calcium hydroxide becomes active for semidry desulfurization by the addition of coal fly ash and claimed that calcium silicate formed by the reaction of calcium hydroxide with a silicone compound eluted from coal fly ash in the preparative procedures is an active material t o absorb SO2 (Jozewicz and Rochelle, 1986). The calcium silicate
* Author t o whom correspondence +
*
should be addressed.
Hokkaido University. Hokkaido Electric Power Co. 0888-5885/95/2634-1404$09.00/0
formed has a large surface area capable of adsorbing a large amount of water. SO2 dissolves into the water to react with calcium ion. The final products are calcium sulfite and calcium sulfate, calcium sulfite being formed predominantly. This ADVACATE (ADVAnced siliCATE) absorbent has been tested in a 10 MW-scale pilot plant, and 89% of SO:! removal and 61% of lime utilization were achieved (Lepovitz et al., 1993). Ueno found that the absorbent prepared from calcium oxide, calcium sulfate, and coal fly ash shows a high calcium utilization efficiency in the dry desulfurization process (Ueno, 1986). He reported that the aging of the slurry containing calcium oxide, calcium sulfate, and coal fly ash at about 100 "C and the successive drying are essential for an active absorbent. The high activity is considered to be due to the formation of microporous structures in the absorbent. This absorbent has actually been used in the dry-type flue gas desulfurization system installed at the Tomato-Atsuma Power Station, Hokkaido Electric Power Co., for the treatment of the flue gas 644 000 m3 h-' (NTP) since 1991. Other than calcium hydroxide based absorbents, alkalized alumina is emerging as one of the candidates for highly efficient SO2 and NO, removal. A proof-ofconcept test was conducted, and SO2 removal of 99% or more and NO, removal of 95% or more were simultaneously achieved during 6500 h of operation in a 5 MWscale pilot plant (Haslbeck et al., 1993). In this process, y-alumina was used as a regenerative adsorbent in a fluidized bed reactor. SO:! is captured in the form of Na2S04. The present paper aims to elucidate the nature of the absorbent practically used at the Tomato-AtsumaPower Station. We wish t o report the structural changes of the absorbent during aging and drying periods in conjunction with the activity changes with preparative conditions.
Experimental Section Preparation of the Absorbent. Absorbents were prepared from calcium oxide, calcium sulfate, and coal fly ash. The calcium oxide used was of industrial grade 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1406 (Wako Pure Chemical Industries). The calcium sulfate was reagent grade calcium sulfate hemihydrate (Wako Pure Chemical Industries). In the practical desulfurization system, a spent absorbent was used as the source of calcium sulfate, because the spent absorbent contains calcium sulfate formed by the reaction of SO2 removal. Coal fly ash was supplied by Hokkaido Electric Power Company, Tomato-Atsuma Power Station, and had the following composition: Si02 59%,A1203 24%, CaO 1.4%, and the mean particle size was 21 pm determined by a laser diffraction method with dry-type dispersion. For the preparation of 200 g of absorbent, calcium oxide (45.4 g) was added to 1L of water at a temperature of 65 "C. The temperature of the slurry increased t o about 90 "C upon addition of calcium oxide. To the slurry, hemihydrate calcium sulfate (64.0 g) and coal fly ash (80.0 g) were added with stirring. The resulting slurry was heated at 95 "C normally for 15 h with stirring. In some experiments, the hydration period was varied from 3 t o 30 h. After hydration, the absorbent slurry was filtered and dried normally at 200 "C for 2 h. In some experiments, the absorbent slurry was dried in a vacuum or dried in air in the temperature range 70-600 "C. The resulting absorbent had the following composition: calcium hydroxide 30%, calcium sulfate 30%, coal fly ash 40%, neglecting H20 and C02. The calcium hydroxide used as a reference absorbent was prepared similarly by slurrying the calcium oxide and drying a t 200 "C. The resulting calcium hydroxide had a surface area of 18 m2/g and a pore volume of 1.4 cm3/g. Activity Test. A flow reactor was employed for the measurement of the activity of the absorbent for desulfurization. The absorbent (50 mL, 23-26 g) was dispersed on cotton (5 g) and placed in the reactor made of quartz, 40 mm in diameter. The absorbent bed packed with the absorbent dispersed on the cotton was 150 mm long. The model flue gas was composed of SO2 2250 ppm, NO 700 ppm, 0 2 6%, C 0 2 13%, H2O lo%, and N2 as a balance. The composition was selected for simulating the composition of the flue gas of a coal-fired boiler. The flow rate of the model gas was 1IJmin, and the reaction temperature was 130 "C at the center of the absorbent bed. At the outlet of the reactor, water was removed with a cold trap and the flue gas was analyzed by the following methods: nondispersive IR spectroscopy for SO2 and CO2, atmospheric chemical luminescence for NO,, and paramagnetic susceptibility for 02. The activity was expressed as the time to keep the removal of SO2 above 80% divided by the weight of calcium hydroxide contained in the adsorbent. The activity means the capacity of the absorbent rather than the kinetics. Chemical and Physical Analyses. The amount of calcium oxide contained in the absorbent was determined by X-ray fluorescence (XRF). The amount of carbonate ion was measured by neutralization titration and was calculated for the amount of calcium carbonate. The sulfur and carbon contents were measured by nondispersive IR spectroscopy. The sample (50 mg) was placed in a ceramic crucible and covered with small spoonful amounts of tin metal, iron metal, and tungsten metal in turn. The crucible was then heated by a highfrequency induction coil to convert sulfur compounds to SO2 for detection by IR spectroscopy. The specific surface area was measured by nitrogen adsorption based on the Brunauer-Emmett-Teller
(BET) method for the sample dried and degassed at 200 "C. The pore volume was measured by the mercury intrusion method based on the Washburn equation for the sample dried and degassed at 200 "C. The weight loss by drying the absorbent was measured by thermal gravimetric analysis. The absorbent (10 mg) dried in a vacuum was placed in a platinum cup and heated by an IR lamp from ambient temperature t o 130 "C at the speed of 10 "C/min. Then, the temperature was held for 20 min to measure the weight loss at 130 "C. The temperature was then increased similarly by 100 "C increments to 600 "C. XRD patterns were recorded on a Rigaku RAD-C system for the powdered samples less than 44 pm with Cu -Kuradiation in the range of diffraction angle (26) 5"-90" at a sweep rate of 3 deglmin. SEM photographs were taken on a JEOL JSM-35CF system with an accelerating voltage of 25 kV for the samples coated with gold metal by ion spattering.
Results Activity. The absorbent prepared from coal fly ash, lime, and gypsum shows a higher calcium utilization compared with calcium hydroxide. The calcium utilization is defined as the percentage of the amount of calcium reacted with SO2 to the amount of calcium contained in the absorbent. The present absorbent prepared under the normal conditions achieved 84% of the calcium utilization, while calcium hydroxide achieved only 36%. These data were measured on each sample when the SO2 removal percent decreased to 0%. The time with the model gas was 41 h for the present absorbent and 186 h for calcium hydroxide. As shown in Figure 1, the present absorbent maintained 100%removal of SO2 for 52 min per unit weight of calcium hydroxide and the activity decayed slowly beyond the duration of 100% removal. On the other hand, calcium hydroxide maintained 100%removal for a shorter time and showed a drastic decrease in activity aflenvard. It should be noted that the present absorbent shows a higher activity not only for SO2 removal but also for NO, removal, although the duration of the 100% removal for NO, was shorter as compared with that for S02. However, the percent removal decayed more rapidly for NO, than for S02. During the reaction time of 123-205 min, the outlet concentration of NO, exceeded the inlet concentration. This was not seen in the case of calcium hydroxide. The amount of NO, released from the absorbent was 7.7%of the amount of NO, once adsorbed or absorbed. It appears that NO, compounds in the absorbent were replaced by SO2 compounds probably because calcium compounds interact more strongly with SO2 than with NO,. Effect of Hydration Period. The activity of the absorbent for SO2 removal depended on the hydration period in the preparation procedures. The variation of the activity as a function of hydration period is shown in Figure 2. The absorbent showed an activity of 55 midg Ca without hydration, while calcium hydroxide showed an activity of 31 midg Ca. Even for the sample with the hydration time of 0 min, the sample was dried a t 200 "C before use in the reaction. During the drying period, the hydration reaction may proceed to some extent, which may cause a high activity as compared to calcium hydroxide. The activity rapidly increased with hydration period and reached a maximum around 15
1406 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
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Reaction time/ min g-Wa(OH):! Figure 1. Time courses of the SO2 and NOz removal percent for the absorbent and Ca(OH)2. Reaction time is expressed as the time divided by weight of calcium hydroxide. (a) Present absorbent prepared under the normal conditions. (b) Calcium hydroxide. 0: Removal percent for SOz. 0: Removal percent for NO,.
h. Beyond 15 h of hydration, the activity gradually de-
creased. Drying Temperature. Figure 3 shows weight loss during drying the sample initially dried at 130 "C for 2 h. The weight loss is mainly due to the removal of water, which was prominent in the temperature range 130-300 "C. The drying temperature greatly affects the activity of the absorbent. The variation of the activity as a function of the drying temperature is shown in Figure 4 together with the variation of the surface area. The activity of the present absorbent markedly increased with an increase in the drying temperature and reached
a maximum when the absorbent was dried a t 400 "C. Above a drying temperature of 400 "C, the activity decreased. The surface area of the absorbent gradually increased and reached a maximum when the absorbent was dried a t 250 "C. In contrast to the activity, the surface area decreased when dried a t 400 "C and higher temperatures. Similar results were observed for a related material and explained by a sintering process (Borgwardt and Rochelle, 1990). The calcium carbonate content notably increased with an increase in the drying temperature. The calcium carbonate content in the absorbent dried at 200 "C was
Ind. Eng. Chem. Res., Vol. 34, NO. 4, 1995 1407 I
100
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Ca(OH)2 ......................................................................
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Figure 2. Effect of hydration period on the desulfurization activity of the absorbent. Starting materials: Ca(0Hh 30%, Cas04 30%, coal fly ash 40%.
100
200
300
10
100
200 300 400 500 Drying Temp. / "C
600
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Figure 3. Effect of drying temperature on the weight loss of the absorbent. W: Initial weight of the absorbent dried in air at 130 "C for 2 h. A W Weight change by heating at the target temperature.
14%,while the amount increased to 19% when the absorbent was dried a t 600 "C. The formation of calcium carbonate may result from dehydration of calcium hydroxide to calcium oxide followed by reaction with COz in air. Figure 5 shows the variation of the amount of calcium sulfate as a function of drying temperature. The amount of calcium sulfate is represented by the sum of the XRD peak intensities at 28 = 25.4" and 28 = 14.7". The peak at 28 = 25.4" is ascribed to anhydrous calcium sulfate and the peak a t 28 = 25.4" to hemihydrate calcium sulfate. Above 400 "C, the intensity markedly increased, indicating that calcium sulfate crystallites developed when dried above 400 "C. Structure. XRD patterns of the present absorbent were measured to examine the structural change during drying and desulfurization and are shown in Figure 6. The XRD pattern was measured for the absorbent dried in a vacuum at ambient temperature to study the hydration products, because it was reported that the hydration products are difficult to measure by XRD
1
.
500
400
1
.
0
000
700
Drying temperature/" Figure 4. Effect of drying temperature on the desulfurization activity. Starting materials: Ca(0H)z 30%, Cas04 30%, coal fly ash 40%. Hydration time: 15 h. 0: Activity in the presence of NO. 0: Activity in the absence of NO.
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analysis after drying at a high temperature (Skoblinskaya et al., 1975). In Figure 6, parts a-c are the XRD patterns of the vacuum-dried absorbent, the absorbent prepared under the normal conditions dried at 200 "C,and the absorbent after being used for the activity test for 18 h, respectively. For the vacuum-dried absorbent, the peaks characteristic of ettringite ( C ~ ~ A ~ Z ( S O ~ , S ~ O ~ , C O ~ ) (OH)12.26H~O) appeared a t 28 = 9.14", 15.8", 22.9", and the peaks characteristic of the monosulfate (Cad&SO~,*12Hz0) appeared at 9.93" and 19.9". The peaks for unreacted calcium hydroxide and calcium sulfate were also appreciable. For the absorbent prepared under the normal conditions, the peaks for the ettringite and the monosulfate disappeared, while the peaks for unreacted calcium hydroxide and calcium sulfate remained. For the absorbent used for the activity test, the peaks for anhydrous calcium sulfate markedly increased. The peaks for calcium sulfite, which is one of the possible products in desulfurization, were not found. The intensity of the peaks for calcium hydroxide apparently decreased after use in the activity test. It is suggested that SO2 in the model gas is removed and fixed as anhydrous calcium sulfate.
4
1408 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1
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Figure 6. XRD patterns of (a) vacuum-dried absorbent, (b) absorbent dried at 200 "Cin air, and (c) absorbent used for activity test for 18 h. Starting materials: Ca(OH)2 30%,coal fly ash lo%, and absorbent used for 300 h desulfurization 60% (instead of CaS04). Hydration time: 15 h.
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Figure 7. Variations of specific surface area and pore volume as a function of hydration time.
Figure 8. SEM photographs of (a) coal fly ash, (b) present absorbent without hydration, and (c) present absorbent prepared under the normal conditions.
The variation of the surface area, pore volume, and median pore diameter are shown in Figure 7 as a function of hydration time. The surface area and the pore volume of the absorbent increased monotonically with the hydration time. Similar results were reported for the effect of hydration time on surface area and pore volume (Kind and Rochelle, 1994). On the other hand, the median pore diameter calculated from the pore volume increased with the hydration time to reach a maximum at the hydration time of 15 h and then decreased for further hydration. SEM photographs were taken to observe the macrostructural change with hydration reaction. In Figure 8, photo a shows the whole view of a coal fly ash discharged from a coal-fired power station. Photo b shows the absorbent without hydration. Photo c shows the absorbent prepared under the normal conditions. In photo a, round particles of coal fly ash are observed. In photo b, in addition to the round particles, jagged particles are observed to be packed in between those round particles. In photo c, the surfaces of the round particles are covered with the materials in the form of hexagonal prisms. The size of the prism is 0.3 pm wide
and 2 pm long on average. The shape of the crystal looks like that of ettringite reported to have a needlelike appearance (Skoblinskaya et al., 1975; Moore and Taylor, 1968; Skoblinskaya and Krasilnikov, 1975;AboEl-Enein et al., 1985). Some of these needle-like structures are united to form a board-like structure. Ettringite crystal is suggested to be formed by the reaction of calcium sulfate and calcium hydroxide with aluminum and silicate ions dissolved from coal fly ash (Khalil and El-Didamonjr, 1980;Yuan, 1979). Although the XRD peaks of the ettringite were not observed, the hexagonal prisms are observed by SEM when the absorbent was dried at 200 "C. This suggests that the basic structure of ettringite is retained with considerable deformation caused by removal of part of the combined water. Effect of NO on the SO2 Removal. The activity for SO2 removal was strongly influenced by the presence of NO in the reaction mixture. In the absence of NO, the activity for SO2 removal was very low as shown in Figure 4 as solid circle. The reaction of SO2 removal is significantly enhanced by the presence of NO.
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1409
Top View
Side View of Column
A1
Ca
OH
Ettringite Figure 9. Crystal structure of ettringite.
Discussion The absorbent prepared from calcium hydroxide, calcium sulfate, and coal fly ash is highly active for removal of SO2. SO2 is absorbed in the absorbent to form anhydrous calcium sulfate as a final product. Absorption of SO2 into calcium hydroxide also produces anhydrous calcium sulfate. The present absorbent is characterized by a high calcium utilization as compared to calcium hydroxide. More than 80%of calcium in the absorbent is utilized for SO2 absorption to form calcium sulfate. A low calcium utilization for calcium hydroxide may be caused primarily by a low surface area. SO2 reacts with calcium hydroxide at the surface layer. The outer surface of the calcium hydroxide particles converts to calcium sulfate, but the inner part of the calcium hydroxide particle is left unchanged. Only the surface layers of calcium hydroxide are utilized for SO2 absorption. For the present absorbent, however, most of the calcium in the absorbent is located in such a position that SO2 molecules are accessible. This location of calcium may occur during preparative procedures and/ or the SO2 absorption period. As shown in Figure 2, the activity for SO2 absorption increased with the aging time of preparative procedures in the initial 12 h. During this period, the surface area and pore volume also increased. During the 12 h aging time, ettringite and monosulfate are formed as evidenced by the XRD pattern. In addition, the formation of ettringite was observed by SEM as a needle-like formula. It is apparent that calcium hydroxide, calcium sulfate, and components eluted from coal fly ash undergo a hydration reaction during the aging period. One of the hydration products is ettringite.
Ettringite has the formula Cas(Al(OH)6)2(S04)3*26H20, and its crystal looks like a hexagonal prism consisting of columns and channels parallel with the main axis (Skoblinskaya et al., 1975;Moore and Taylor, 1968).The schematic structures are illustrated in Figure 9. The composition of the column is [Cas(Al(OH)6)2*24H20I6+, and the channels are filled with [(S04)y2H20I6-. Each aluminum atom is linked to six hydroxyl groups, and each calcium atom is linked to two hydroxyl groups and four H20 molecules. Each H20 molecule interacts with only one calcium atom, so that the bonding between H20 molecules and calcium atoms does not participate in the column strength in the longitudinal direction. Nevertheless, these H20 molecules form the surface of the column where the positive charge is distributed (Moore and Taylor, 1970). The negative charge is distributed among sulfate ions in the channel (Moore and Taylor, 1970). Thus, columns and channels are united. Therefore, even though the crystal is dried to lose combined water, the column structure could be maintained, while the pore structure of high surface area is created. It is reported that ettringite crystallites gradually decompose as the hydration period is prolonged (Mosalamy et al., 1984; Daimon et al., 1982). Although the decomposition of the ettringite structure was not clearly observed by XRD following an aging period of more than 12 h, it is plausible that the fraction of ettringite reached a maximum at the aging time of 12 h. This explains the activity maximum appearing at the aging time of 12 h. By drying the hydration product at 200 "C, the XRD pattern for ettringite disappeared. The disappearance of the XRD pattern does not necessarily indicate the decomposition of the ettringite structure. It was reported that the ettringite structure that once disap-
1410 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
peared from the XRD pattern by heating at a high temperature restores its structure when exposed to water vapor to facilitate rehydration (Skoblinskaya et al., 1975; Skoblinskaya and Krasilnikov, 1975). Even though the peaks characteristic to ettringite disappear from the XRD pattern, the ettringite structure is intrinsically retained when dried a t 200 "C. It is to be noted that removal of water by drying at 200 "C should result in the formation of pores. By drying the hydration product above 400 "C, the intensity of the XRD peak ascribed to calcium sulfate increased as shown in Figure 4. Ettringite to form calcium sulfate decomposed irreversibly (Fukuda, 1984). At the same time, the C02 content increased. The decrease in the activity on drying above 400 "C is considered to be due to the decomposition of the ettringite structure and the adsorption of COS to convert calcium hydroxide into calcium carbonate. It is, therefore, plausible that the calcium utilization efficiency becomes high when calcium is included in the ettringite structure, though it cannot be excluded that calcium components other than ettringite are also utilized for absorption of SOZ. An alternative explanation for the role of ettringite in the absorption of SO2 may be as follows. Ettringite itself is not capable of absorbing SOz, but it makes the absorbent porous t o facilitate the access of SO2 to the calcium components present in different forms such as calcium silicate and calcium oxide. Jozewicz and Rochelle (1986) reported that an amorphous compound prepared by slurrying calcium hydroxide with coal fly ash and recycled absorbents in water at elevated temperature showed a high activity for SO2 removal (Rochelle and Jozewicz, 1989). They observed the formation of calcium silicate hydrate in the amorphous compound by XRD analysis and showed the correlations of the activity with both BET surface area and moisture content. Because the calcium silicate hydrate has a high surface area, it can retain much water. Therefore, it was concluded that a calcium silicate hydrate is an active material for SO2 removal (Jozewicz and Rochelle, 1986). Apparently, the recycled absorbent contains an appreciable amount of calcium sulfate. Therefore, it is most likely that ettringite was formed in the hydration period, though Rochelle et al. did not report the formation of ettringite. Besides the calcium silicate hydrate, it seems probable that ettringite is also active for SO2 removal in their absorption system. One of the characteristic features of the present absorbent is that the absorbent exhibits a high activity in the presence of NO. Concerning the enhancement of SO2 absorption by the presence of NO, it was reported previously (Medellin et al., 1978; Chu and Rochelle, 1989). Medellin et al. (1978) mentioned that the reaction proceeds through a gas phase reaction. In the absence of NO, the absorbent showed only a small activity for SO2 absorption as shown in Figure 4. NO should play an important role in the absorption of SO2. Since SO2 is absorbed in the form of calcium sulfate as the final product, SO2 should be oxidized to SO3 before reaction with the component of calcium oxide to form calcium sulfate. The requirement of the presence of NO suggests that NO plays a role in the oxidation of SO2. Considering that NO2 is a strong oxidant, it is plausible that the oxidation of SO2 to SO3 proceeds by the action of NO2 that is formed by oxidation of NO. The oxidation
Scheme 1. Desulfurization Reaction Enhanced by the Presence of NO CaO 0 2 SO3 +Cas04
so2
reaction of NO to NO2 was reported to be thermodynamically feasible (Xue et al., 1993). The proposed scheme for the role of NO and NO2 is illustrated in Scheme 1. In this scheme, NO and NO2 act as catalysts to oxidize SO2 to so3. The catalytic role of NO and NO2 is similar to that observed in the lead chamber method to produce sulfuric acid from S02. The NO and NO2 do not seem to react with SO2 in the gas phase but do on the surface of the absorbent. On this point, more information will be reported shortly.
Literature Cited Abo-El-Enein, S. A.; Al-Nuaimi, K. Kh.; Marusin, S. L.; El-Hemaly, S. A. S. Hydration Kinetics and Microstructure of Ettringite. TI2 1985,109, 116-118. Borgwardt, R. H.; Rochelle, G. T. Sintering and Sulfation of Calcium Silicate-Calcium Aluminate. Znd. Eng. Chem. Res. 1990,29, 2118-2123. Brown, C. A.; Maibodi, M.; McGuire, L. M. 1.7MW Pilot Results for the Duct Injection FGD Process Using Hydrated Lime Upstream of an ESP. Proceeding. The 1991 SO2 Control Symposium, Washington, DC, December 1991. Chu, P.; Rochelle, G. T. Removal of SO2 and NO, from Stack Gas by Reaction with Calcium Hydroxide Solids. JAPCA 1989,39, 175-179. Daimon, M.; Yamaguchi, 0.; Oosawa, H.; Goto, S. Hydration Reaction of Fly Ash in the Presence of Gypsum (Japanese). Semento Guutsu Nenpo 1982,36,65-68. Dalton, S . M. State-of-the-& of Flue Gas Desulfurization Technologies. Proceeding. Power Gen '90, Orlando, FL, December 1990. Fukuda, M. Ettringite (Japanese). Tsuchi-to-Kiso.1984,32,4950. Haslbeck, J. L.; Woods, M. C.; Ma, W. T.; Harkins, S. M.; Black, J. B. NOXSO SOflO, Flue Gas Treatment Process: Proof-ofConcept Test. Proceeding. The 1993 SO2 Control Symposium. Boston, MA, Augst 1993. Jozewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5, 219-223. Khalil, A. A.; El-Didamony, H. Study of the Hydration Products of the System CaO-Al203-SO3-Si02 with Varying CaO Mole Ratio. Thermochim. Acta 1980,40, 337-347. Kind, K. K.; Rochelle, G. T. Preparation of Calcium Silicate Reagent from Fly Ash and Lime in a Flow Reactor. J . Air Waste Manage. Assoc. 1994,44, 869-876. Lepovitz, L. R.; Brown, C. A,; Pearson, T. E.; Boyer, J. F.; Burnett, T. A.; Nonvood, V. M.; Puschaver, E. J.; Sedman, C. B.; Toole@Neil, B. lOMW Demonstration of the ADVACATE Flue Gas Desulfurization Process. Proceeding. The 1993 SO2 Control Symposium. Boston, MA, August 1993. Livengood, C. D. Combined NOdSOz Removal in Spray Dryer FGD Systems. Proceeding. Acid Rain Retrofit Seminar: The Effective Use of Lime, Philadelphia, PA, National Lime Association, January 1991. Medellin, P. M.; Weger, E.; Dudukovic, M. P. Removal of SO2 and NO, from Simulated Flue Gasses by Alkalized Alumina in a Radial Flow Fixed Bed. Znd. Eng. Chem. Process Des. Dev. 1978,17, 528-536. Moore, A. E.; Taylor, H. F. W. Crystal Structure of Ettringite. Nature (London) 1988,218, 1048-1049. Moore, A. E.; Taylor, H. F. W. Crystal Structure of Ettringite. Acta Cystallogr. 1970, B26, 386-393. Mosalamy, F. H.; Shater, M. A.; El-Didamony, H. Hydration Mechanism of Tricalcium Aluminate with Gypsum a t 1:l Mole
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14,1989. Skoblinskaya, N. N.;Krasilnikov, K. G. Changes in Crystal Structure of Ettringite on Dehydration 1. Cem. Concr. Res.
Xue, E.; Seshan, K.; van Ommen, J . G.; Ross, J. R. H. Catalytic Role of DIesel Engine Particulate Emission: Studies on Model Reactions over a EuroF’t (WSiOz). Appl. Cutal. B: Enuiron.
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Received for review July 5, 1994 Revised manuscript received December 29, 1994 Accepted January 9,1995@
IE9404137
* Abstract published in Advance ACS Abstracts, March 1, 1995.
