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Ind. Eng. Chem. Res. 1996, 35, 2322-2326
Study of Flue Gas Desulfurization Absorbent Prepared from Coal Fly Ash: Effects of the Composition of the Absorbent on the Activity Hiroaki Tsuchiai,†,‡ Tomohiro Ishizuka,‡ Hideki Nakamura,‡ Tsutomu Ueno,‡ and Hideshi Hattori*,† Center for Advanced Research of Energy Technology, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060, Japan, and Department of Research and Development, Hokkaido Electric Power Company, Inc., 4-9-2-1 Utsukushigaoka, Toyohira-ku, Sapporo 004, Japan
The absorbents for SO2 and NO from flue gas were prepared from calcium hydroxide, calcium sulfate, silicic acid, and aluminum hydroxide. The effects of the composition of the absorbent are studied on the activity for the absorption of SO2 and NO and on the structure of the absorbent. The activity for the absorption of SO2 and NO markedly increased with the content of silica in the absorbent up to 40%. The formation of calcium silicate is suggested to be predominant in a high concentration of silica, while the formation of ettringite was observed by the XRD only for the absorbent containing silica below 30%. The distribution of the sulfur and nitrogen compounds in the absorbent revealed by XPS suggests that adsorbed nitrogen compounds are gradually replaced by sulfur compounds as the reaction proceeds. Introduction For removal of SO2 from flue gas, a wet process using calcium carbonate as an absorbent is most commonly adopted in the commercial plants. The wet process shows a high efficiency, but needs a large amount of water. A dry process using calcium hydroxide as an absorbent is also used commercially, but is not as common as the wet process. This is due to a low utilization efficiency of calcium hydroxide. Regardless of the types of SO2 removal system, one of the important subjects for efficient SO2 removal is the preparation of a highly active absorbent. Coal fly ash has been recognized to be a key material for preparation of the highly active absorbent. Jozewicz and Rochelle reported that calcium hydroxide becomes active for semidry desulfurization by addition of coal fly ash (Jozewicz and Rochelle, 1986). They claimed that calcium silicate formed in the preparative procedures by the reaction of calcium hydroxide with a silicon compound eluted from coal fly ash is an active material to absorb SO2. 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% SO2 removal and 61% calcium utilization were achieved (Lepovitz et al., 1993). However, the effects of the composition of starting materials on the structure of the absorbent and on the time course of the accumulation of SO2 and NO compounds have not been reported yet. Ueno found that the absorbent prepared from calcium oxide, calcium sulfate, and coal fly ash shows a high calcium utilization efficiency in a dry desulfurization process (Ueno, 1986). He reported that hydrothermal reaction of the slurry containing calcium oxide, calcium sulfate, and coal fly ash at about 100 °C and the successive drying are indispensable for an active absorbent. This absorbent shows a high activity even under a high temperature and a low relative humidity * To whom all correspondence should be addressed. E-mail:
[email protected]. Fax: +81-11-7260731. † Hokkaido University. ‡ Hokkaido Electric Power Co., Inc.
S0888-5885(95)00703-2 CCC: $12.00
and has been practically used in the dry-type flue gas desulfurization system (Ueno et al., 1994). Unlike the other dry processes in which SO2 is fixed in the form of calcium sulfite, this process is characterized by the formation of calcium sulfate as a final product. Concerning the absorbent prepared from coal fly ash, we reported the optimum preparative conditions (Tsuchiai et al., 1995). It was also revealed that the existence of NO in the flue gas is required for the efficient removal of SO2 at a normal flue gas temperature of 130-150 °C. A successive study revealed that NO and SO2 enhance the SO2 and NO absorption, respectively, and that the amount of water adsorbed on the surface is critical to determine the final product for SO2 absorption and the activities for the absorption of SO2 and NO (Tsuchiai et al., 1996). The present paper aims to elucidate the role of coal fly ash in the preparation of the absorbent for SO2 and NO. We report the effects of the composition of starting materials on the structure and the activity of the resulting absorbent. It was found that the content of silica in the absorbent was closely related to the activity. Without silica, the prepared absorbent did not show an appreciable activity for both SO2 and NO absorption. Experimental Section Preparation of the Absorbent. The absorbent was prepared from calcium hydroxide, calcium sulfate hemihydrate, aluminum hydroxide, and hydrated silicic acid. All the chemicals used were of reagent grade (Wako Pure Chemical Industries). In some experiments, aluminum oxide that was of reagent grade (Wako Pure Chemical Industries) was used instead of aluminum hydroxide. For preparation of 200 g of absorbent composed of calcium hydroxide (30%), calcium sulfate (10%), aluminum hydroxide (30%), and silica (30%), a premixed powder composed of calcium hydroxide (60.0 g), hemihydrate calcium sulfate (21.3 g), aluminum hydroxide (60.0 g), and hydrated silicic acid (141 g) was added to 1 L of water at a temperature of 65 °C. The resulting slurry was heated at 95 °C for 12 h with stirring. Then, the absorbent slurry was filtered and dried at 200 °C for 2 h. For the absorbents having different ratios of © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2323 Table 1. Physical Properties of the Prepared Absorbents no.
Ca(OH)2
1 2 3 4 5 6
30 30 30 30 30 30
compositions of starting materials (wt %) CaSO4‚1/2H2O Al(OH)3 Al2O3 SiO2‚xH2O 10 10 10 10 10 10
60 50 30 10 30
10 30 50 60 30
Si/Al, the compositions of hydrated silicic acid and aluminum hydroxide were varied in the range 0-60% as listed in Table 1. The compositions of calcium hydroxide and hemihydrate calcium sulfate were kept constant. Activity Test. A flow reactor was employed for the measurement of the activity for the absorption of SO2 and NO and also for the preparation of the samples for XPS measurement. The absorbent (50 mL, 15-32 g) was dispersed on an absorbent cotton (5 g) and placed in a quartz reactor of 40 mm in diameter. The absorbent bed packed with the absorbent dispersed on the absorbent cotton was 150 mm long. The model flue gas was composed of SO2 (2250 ppm), NO (700 ppm), O2 (6%), CO2 (13%), H2O (10%), and N2 as a balance. The flow rate of the model gas was 1 L/min, and the reaction temperature was 130 °C. At the outlet of the reactor, water was removed with a cold trap and the flue gas was analyzed by the corresponding methods: nondispersive IR spectroscopy for SO2 and CO2, atmospheric chemical luminescence for NOx, and paramagnetic susceptibility for O2. The concentration of NOx includes those of NO and NO2, but not N2O and NH3. SO2 and NO removals are expressed by the moles of SO2 and NO removed from the model flue gas per mole of calcium hydroxide contained in the absorbent. One mole of SO2/mole of Ca(OH)2 corresponds to 865 mg/g Ca(OH)2, and 1 mol NO/mol Ca(OH)2 corresponds to 405 mg/g Ca(OH)2. The amounts of SO2 and NO removed were calculated by integration of the difference between inlet and outlet concentrations. Chemical and Physical Analyses. The amount of calcium hydroxide contained in the absorbent was determined by X-ray fluorescence (XRF). XRD patterns were recorded on a Rigaku RAD-X system for the powdered samples less than 44 µm with Cu KR radiation in the range of diffraction angle (2θ) 5-90° at a sweep rate of 3 deg/min. The sample was vacuum-dried at room temperature for 48 h before the XRD measurement. X-ray photoelectron spectroscopy (XPS) was measured with a Rigaku XPS 7000 system with Mg KR radiation. A binding energy was calibrated to the binding energy of C1S at 184.5 eV. The depth etched by argon bombardment was estimated by assuming the etching rate of the present absorbent is the same as that of silicon (5.1 Å/s). The peak areas of the XPS peaks in different etched depth were calibrated to the peak area of Si2S of the absorbent before etching, because the silicon is supposed to be distributed uniformly in the bulk of the absorbent. The specific surface area was measured by nitrogen adsorption based on the Brunauer-Emmett-Teller (BET) method for the sample dried and degassed at 100 °C. The pore volume was measured by the mercury intrusion method based on the Washburn equation for the sample dried and degassed at 100 °C.
specific surface area (m2‚g-1)
pore volume (cm3‚g-1)
138.4 175.1 280.0 212.6 174.8 143.4
0.934 1.746 3.026 2.807 2.996 1.791
pH of the absorbent slurry before hydration after hydration 12.4 12.4 12.4 12.4 12.4 12.5
12.0 11.7 9.95 9.41 9.38 9.79
Figure 1. XRD patterns of vacuum-dried absorbents. Table 2. Summary of the Compounds Detected by XRDa sample
ettringite
CaSO4‚2H2O
Al(OH)3
Ca3Al2(OH)12
1 2 3 4 5 6
b O × × × ×
× × b b b b
b b O O × ×
O × × × × ×
a
b, strong; O, weak; ×, not appreciable.
Results Effects of the Composition on the Absorbent Structure. The crystalline structure of the absorbents varied with the content of silica and alumina. XRD patterns for the absorbents having different compositions are shown in Figure 1. The compounds detected by XRD are summarized in Table 2. With sample 1 containing no silica, the formation of ettringite (Ca6Al2(SO4)3(OH)12) was obvious. In addition to aluminum hydroxide, calcium aluminate (K) was identified as an aluminum compound. With sample 2 that contains 10% silica, the formation of ettringite was also observable, though the intensities of the peaks were weak compared to those of sample 1. Concerning the starting materials, calcium sulfate and aluminum hydroxide remained partially unreacted.
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Figure 2. Effect of silica content in the absorbents on the activity for SO2 removal at a reaction time of 5 h (O), 10 h (4), and 15 h (0).
Figure 3. Effect of silica content in the absorbents on the activity for NO removal at a reaction time of 5 h (O), 10 h (4), and 15 h (0).
Unlike in sample 1, calcium aluminate was not appreciable in sample 2. With the samples that contain more than 30% silica (samples 3-6), the XRD peaks ascribed to ettringite were not observed. Among the starting materials, very strong peaks ascribed to calcium sulfate dihydrate were observable in the XRD patterns of samples 3-6. The peaks ascribed to hemihydrate calcium sulfate were not detected. The peaks ascribed to aluminum hydroxide were also found for samples 3 and 4, but not for samples 5 and 6. The surface area and pore volume are given in Table 1. The specific surface area and the pore volume increased with the content of silica and reached a maximum at 30% silica. Above a silica content of 30%, the surface area and pore volume gradually decreased with the content of silica. The variations of surface area and pore volume as a function of the silica content are similar to those of the amounts of SO2 and NO removed as shown in a later section. The specific surface areas of all samples were about twice that of the practical absorbent prepared from coal fly ash with the normal composition. Higher values of the surface area and the pore volume are considered to be due to a higher solubility and reactivity of hydrated silicic acid used for the preparation of the present absorbents compared to the values obtained with glassy silica contained in a coal fly ash. The pH of the absorbent slurry decreased during the hydration, and the degree of the decrease was much extended for the sample with a high silica content. The decrease in pH of the absorbent slurry is considered to be due to the consumption of calcium by the formation of calcium silicate to a greater extent for the high silica sample. Effects of the Composition on SO2 and NO Removal Activity. As shown in Figure 2, the amount of SO2 removed from the model flue gas increased monotonously with the silica content up to 40%, and also with the reaction time, and stayed constant above 40% silica. Above 40% silica, the amounts of SO2 removed from the model gas exceeded the theoretical upper limit for the absorption of SO2 in the experiments with samples 4 and 5 in a reaction time of 15 h on stream. The absorbent containing no silica showed a low activity for SO2 absorption. As shown in Figure 3, the amount of NO removed from the model gas also increased with the content of silica and reached a maximum at 40% silica followed by an appreciable decrease at a higher content of silica. The content of silica that gave a maximum absorption
of NO shifted to a lower side as the reaction time was prolonged. The absorbent containing no silica showed a low activity for the absorption of both SO2 and NO. XPS Study of the Absorbent. Distributions of sulfur and nitrogen compounds in the absorbent of the sample 6 at different reaction stages were examined by XPS. Figure 4 shows the variations of XPS peaks for S2P and N1S with etching for the absorbent used for different reaction periods. Spectra were recorded before etching and after etching (50 and 100 Å). The variations of the peak areas for N1S and S2P of the absorbent as a function of the reaction time are shown in Figure 5. As shown in Figure 4a, a strong peak ascribed to S2P of SO42- at a binding energy of 170 eV was observed before etching. The area of the peak increased with the reaction time as shown in Figure 5. This peak was markedly weakened and broadened as the surface of the absorbent was etched by argon bombardment, as shown in Figure 4b,c. The broad peak for S2P indicates the existence of appreciable amount of SO32- at 168 eV and the physically adsorbed SO2 at 166 eV. As shown in Figure 5, the amount of sulfur found by XPS was larger before etching than after etching at a short reaction time. Even for the etched absorbents the sulfur increased with the reaction time. The peak ascribed to N1S was not observed for the absorbent before etching (Figure 4a), but became appreciable and intensified as etching was continued. The peak top for N1S was observed at 404 eV. Because the peak was broad, the existence of NO3- at 408 eV and NH at 400 eV is also suggested. As shown in Figure 5, the peak intensity of N1S for the absorbent that showed a high activity for SO2 removal markedly increased with the reaction time and reached a maximum at a reaction time of 5 h followed by considerable decrease. Although it is not shown, the peak intensity of N1S was weak for the absorbent of low activity such as sample 1, and the maximum of the N1S peak area was not observed within 15 h for the absorbent. The maximum is expected to appear at a longer reaction time for less reactive absorbent. Discussion The surface area of the absorbent markedly increased with the content of silica up to 30%. It is plausible that the calcium compounds become well-dispersed as the silica content increases. The activity for both SO2 and NO absorption also increased markedly with the content of silica up to 40%. Silica should play an important role in the activities for the absorption of SO2 and NO.
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2325
a b
c
Figure 4. Variations of XPS peaks for S2P and N1S with etching for the absorbent used for different reaction time (0, 5, 10, and 15 h): (a) before etching, (b) after etching (50 Å), and (c) after etching (100 Å). Sample: sample 6. Starting materials: Ca(OH)2 (30%), CaSO4 (10%), Al2O3 (30%), SiO2 (30%).
In our previous paper, it was suggested that ettringite was a key material in the practical absorbent prepared from coal fly ash, calcium hydroxide, and calcium sulfate (Tsuchiai et al., 1995). However, the formation of ettringite was observed only for the absorbents containing less than 30% silica in the present study. For the absorbents containing more than 30% silica, the formation of ettringite was not appreciable by XRD. The ettringite crystallites that were large enough to be detected by XRD were not formed. It is, however, not excluded that the ettringite crystallites were formed as an intermediate in the preparative procedures, but
decomposed at the lower final pHs of the high silica slurries. Although the formation of ettringite was not observed, a maximum SO2 absorption activity was attained for the absorbent containing 60% silica. Contrary to the suggestion in our previous paper, it is concluded on the basis of the present study that the formation of ettringite is not necessarily required for a highly active absorbent. At a high concentration of silicon compound in the slurry of the mixture containing starting materials, it is supposed that calcium hydroxide reacts preferentially with the silicon compound to form calcium silicate
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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
Figure 5. Accumulation of sulfur and nitrogen compounds in the absorbents as a function of the reaction time before etching [sulfur (O) and nitrogen (b)] and in the absorbent etched 100 Å [sulfur (0) and nitrogen (9)]. Sample: sample 6. Starting materials: Ca(OH)2 (30%), CaSO4 (10%), Al2O3 (30%), SiO2 (30%).
rather than with aluminum hydroxide and calcium sulfate to form ettringite. This may be the case with the absorbent containing more than 30% silica. It is plausible that calcium silicate is the material relevant to SO2 absorption, though it is not certain at present whether ettringite contributes to SO2 absorption or not. For the practical absorbent prepared from coal fly ash, calcium hydroxide, and calcium sulfate, the total silica content is about 25%. Daimon et al. studied the solubility of coal fly ash under different conditions (Daimon et al., 1982). They reported that about 50% fly ash reacted with calcium hydroxide to form a hydration product under conditions similar to those for the present absorbent. The other 50% fly ash remained unreacted. Therefore, the silica content available for the reaction with calcium hydroxide and calcium sulfate is considered to be about 12.5% in the practical absorbent. With the practical absorbent, the amounts of SO2 absorbed in 5, 10, and 15 h were 0.28, 0.56, and 0.79 mol SO2/mol Ca(OH)2, and those of NO absorbed were 0.087, 0.16, and 0.18 mol NO/mol Ca(OH)2, respectively. These data lie between those for the absorbent containing 10% silica and those for the absorbent containing 30% silica. These also coincide with the suggestion that about 50% of silica in fly ash is utilized for the formation of the hydration products. Different distribution of sulfur compounds and nitrogen compounds revealed by XPS suggests that replacement of nitrogen compounds by sulfur compounds takes place as the reaction proceeds. NO is adsorbed weakly on the surface, and therefore, the NO is able to penetrate or diffuse into the inner part of the absorbent particles. The NO is adsorbed mostly in the form of NO2-. Unlike NO, SO2 is adsorbed strongly on the surface of the absorbent. Therefore, SO2 is adsorbed mostly on the surface of the outer part of the absorbent particles and converted to SO42- before penetrating or diffusing into the inner part of the particles. Thereby, SO42- species accumulate only in the outer part of the absorbent particles in the initial stage of the time on
stream. The NO species adsorbed in the outer part are replaced by SO2 or SO42- and, therefore, could be detected in the layer 50-100 Å below the outer surface of the absorbent particles. The layers that contain sulfur compounds gradually increase in thickness with the reaction time, while the layers that retain nitrogen compounds become smaller as a result of replacement by sulfur compounds. In our previous paper, we reported that the concentration of NO at the outlet of the reactor exceeded that at the inlet of the reactor beyond the reaction time 123 min/g Ca(OH)2. It was concluded that a replacement of the nitrogen compounds by sulfur compounds occurred in this system. The distribution of the sulfur compounds and nitrogen compounds observed by XPS coincides with the conclusion. Conclusions The following conclusions are drawn in the present study. (1) The activity for the absorption of both SO2 and NO markedly increases with the content of silica in the absorbent up to 40%. (2) The active material for the absorption of SO2 under a normal flue gas condition is suggested to be calcium silicate that is formed by the reaction of calcium hydroxide with silica in the preparative procedures. (3) The distribution of sulfur compounds and nitrogen compounds in the absorbent revealed by XPS explains that nitrogen compounds adsorbed are gradually replaced by sulfur compounds as the reaction proceeds. Literature Cited Daimon, M.; Yamaguchi, O.; Oosawa, H.; Goto, S. Hydration Reaction of Fly Ash in the Presence of Gypsum (Japanese). Semento Gijyutsu Nenpou 1982, 36, 65-68. Jozewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5, 219-223. Lepovitz, L. R.; Brown, C. A.; Pearson, T. E.; Boyer, J. F.; Burnett, T. A.; Norwood, V. M.; Puschaver, E. J.; Sedman, C. B.; TooleO’Neil, B. 10MW Demonstration of the ADVACATE Flue Gas Desulfurization Process. Proceedings of the 1993 SO2 Control Symposium, Boston, MA, August 1993. Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash. Ind. Eng. Chem. Res. 1995, 34, 1404-1411. Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Removal of Sulfur Dioxide from Flue Gas by the Absorbent Prepared from Coal Ash: Effects of Nitrogen Oxide and Water Vapor. Ind. Eng. Chem. Res. 1996, 35, 851-855. Ueno, T. Process for Preparing Desulfurizing and Denitrating Agents. U.S. Pat. 4629721, December 16, 1986. Ueno, T.; Tsuchiai, H.; Nakamura, H.; Ishizuka, T.; Mori, K. Flue Gas Cleaning Technology Using Fly Ash Derived Absorbent (Japanese). Nippon Kagaku Kaishi 1994, 9, 763-770.
Received for review November 22, 1995 Accepted April 23, 1996X IE9507033
X Abstract published in Advance ACS Abstracts, June 1, 1996.
