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Effect of Calcium Sulfate Addition on the Activity of the Absorbent for Dry Flue Gas Desulfurization Tomohiro Ishizuka,† Takashi Yamamoto,‡ Takeshi Murayama,§ Tsunehiro Tanaka,‡ and Hideshi Hattori*,† 1 Center for Advanced Research of Energy Technology, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan, 2 Department of Molecular Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan, and 3 Department of Research and Development, Hokkaido Electric Power Co., Inc., Tsuishikari 2-1, Ebetsu 067-0033, Japan Received August 16, 2000. Revised Manuscript Received December 30, 2000
The addition of calcium sulfate in the procedures of preparing flue gas desulfurization absorbent from calcium oxide and coal fly ash causes a negative effect in the slaking step and a positive effect in the following hydrothermal treatment step. The role of calcium sulfate added in each step was investigated. For the slaking step, XRD, XAFS, and Raman spectroscopic studies revealed that the addition of calcium sulfate in the step of slaking CaO with coal fly ash resulted in the coverage of calcium hydroxide with calcium sulfate to decrease the reactivity of the calcium hydroxide toward coal fly ash to form calcium silicate. For the hydrothermal treatment step, measurements of XRD, surface area, and Raman spectrum revealed that the addition of calcium sulfate promotes the formation of calcium silicate by suppressing crystal growth of calcium hydroxide to keep the reactivity of the calcium hydroxide toward coal fly ash.
Introduction The utilization of ash from coal-fired thermal power stations has been explored worldwide in many fields.1,2 Preparation of the absorbent for flue gas desulfurization (FGD) is one of the ways of utilizing coal fly ash, and it has been studied rather extensively.3-9 Most of the absrobents reported so far were prepared by the hydrothermal reaction of coal fly ash with calcium hydroxide in the state of slurry at a temperature of ca. 370 K under ambient pressure. It is reported that calcium silicate is formed during the hydrothermal treatment and relevant to SO2 capture. Ueno has found the promotive effect of calcium sulfate in preparing the absorbent.10 The material prepared from coal fly ash, calcium hydroxide, and calcium sulfate shows a high desulfurization activity at about 400 K * Corresponding author. Tel: +81-11-706-7119. Fax: +81-11-7260731. E-mail:
[email protected]. † Hokkaido University. ‡ Kyoto University. § Hokkaido Electric Power Co., Inc. (1) Moriya, T. Proc. 13th Intern. Symp. Use Manage. Coal Combustion Products, ACAA EPRI 1999; paper No. 60. (2) Manz. O. E. Proc. 13th Intern. Symp. Use Manage. Coal Combustion Products, ACAA EPRI 1999; paper No. 64. (3) Jozewicz, W.; Rochelle, G. T. Environ. Prog. 1986, 5, 219-223. (4) Corbitarte, F.; Ortiz, M. I.; Irabien, J. A. Thermochim. Acta. 1992, 207, 255-264. (5) Otaigbe, J. U.; Egiebor, N. O. Termochim. Acta. 1992, 195, 183194. (6) Shawabkeh, A. A.; Matsuda, H.; Hasatani, M. J. Chem. Eng. Jpn. 1995, 28, 53-58. (7) Martinez, J. C.; Izquierdo, J. F.; Cunill, F.; Tejero, J.; Qerol, J. Ind. Eng. Chem. Res. 1991, 30 (9), 2143-2147. (8) Jozewicz, W.; Jorgensen, C.; Chang, J. C. S. JAPCA 1988, 38, 796-805. (9) Chu, P.; Rochelle, G. T. JAPCA 1989, 39, 1175-1179. (10) Ueno, T; U. S. Patent 4629721, 1986.
and fixes SO2 in the form of calcium sulfate. The drytype FGD plant using this absorbent has been commercially operated at Hokkaido Electric Power Co., Inc. since 1991. We studied the mechanisms of the desulfurization and the formation of active species during preparative procedures and reported that the desulfurization occurs by the catalytic action of NOx present in the flue gas and that the main species relevant to SO2 capture is calcium silicate formed in the hydrothermal treatment step.11-13 Recently, we found out that the activity of the absorbent increases when calcium oxide was used in place of calcium hydroxide.14 The preparation procedures consist of two steps: one is the slaking calcium oxide with coal fly ash, and the other is the following hydrothermal treatment step. The desulfurization activity depends on which step calcium sulfate is added in. The addition of calcium sulfate in the slaking step brings about a negative effect on the activity of the resulting absorbent. On the contrary, the addition of calcium sulfate in the hydrothermal treatment step brings about a positive effect. In the present work, the roles of calcium sulfate in the slaking step and the hydrothermal treatment step are investigated. (11) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Ind. Eng. Chem. Res. 1995, 34, 1404-1411. (12) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 2322-2326. (13) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 851-855. (14) Ishizuka, T.; Tsuchiai, H.; Murayama, T.; Tanaka, T.; Hattori, H. Ind. Eng. Chem. Res. 2000, 39, 1390-1396
10.1021/ef000186n CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
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Experimental Section Preparation of Absorbents. To clarify the different roles of calcium sulfate added in the slaking step and the hydrothermal treatment step, we prepared several different absorbents. For the study of the slaking step, two samples were prepared as follows. Calcium oxide (159 g) or a mixture containing calcium oxide (159 g) and calcium sulfate (35 g) was slaked with water (2850 g), and then silicic acid (490 or 455 g) was added after 5 min. The slurries were kneaded at 368 K for 5 min and dried at 433 K for 3 h. The absorbent composed of calcium oxide and silicic acid is named H-1, and that composed of calcium oxide, silicic acid, and calcium sulfate is named H-2. For the study of hydrothermal treatment step, samples were prepared as follows. Slurries containing calcium hydroxide, coal fly ash, calcium sulfate, and water in different ratios were subjected to hydrothermal treatment, followed by drying at 433 K for 3 h. The hydrothermal treatment was carried out by heating the sample at 368 K under saturated water vapor of ambient pressure.14 For Raman spectroscopic study, two types of the samples were prepared. For investigation of the interaction between calcium oxide and calcium sulfate in the slaking step, calcium oxide was slaked with calcium sulfate hemihydrate in different ratios and dried at 323 K for 24 h. For investigation of the interaction between calcium hydroxide and calcium sulfate in the hydrothermal treatment step, slurries containing calcium hydroxide, calcium sulfate hemihydrate, and water in different ratios were subjected to hydrothermal treatment for 15 h, followed by drying at 323 K for 24 h. Coal fly ash was supplied by Hokkaido Electric Power Co., Inc., a Tomato-Atsuma coal-fired power station, and had the following composition: SiO2 66.4 wt %, Al2O3 21.6 wt %, CaO 6.5 wt %, K2O 1.7 wt %; Na2O 0.1 wt %, and small amount of other metal oxides; the mean particle size was 5.4 µm. Calcium oxide (particle size; 2-5 mm) was purchased from Kyodo Sekkai Co. The calcium hydroxide used for the most of the experiments (standard calcium hydroxide) and calcium sulfate (reagent grade) were purchased from Wako Pure Chem. Ind. Another type of calcium hydroxide (fine calcium hydroxide) was prepared by slaking the calcium oxide (Kyodo Sekkai Co.,) in our laboratory. The particle sizes of calcium hydroxide (standard) and calcium hydroxide (fine) were 2.7µm and 2.1µm, respectively. Activity Measurement. A flow reactor with a fixed bed was employed for carrying out desulfurization. The powder form of the absorbent(25 g) was supported on 5 g of a cotton wool and placed in a Pyrex glass tubing reactor 30 mm in inner diameter. An artificial flue gas simulated for the flue gas from a coal-fired boiler composed of 2250 ppm SO2, 700 ppm NO, 6% O2, 13% CO2, 10% H2O, and N2 as a balance was passed through the absorbent at the flow rate of 1 L/min (STP) at 403 K. The effluent gas was analyzed after H2O was removed by a cold trap. The concentrations of SO2 and NOx were continuously monitored by nondispersive infrared spectroscopy and atmospheric chemical luminescence, respectively. Inlet compositions of CO2 and O2 were measured by nondispersive infrared spectroscopy and paramagnetic susceptibility, respectively. Concentrations of the components of the gas were recorded on the following analyzers: SO2 VIA-510, HORIBA Ltd., NO CLA-510-ss, HORIBA Ltd., CO2 CGT-10-3A, SHIMADZU Co., O2 OA-580-PS, and Thermo Electron Nippon Co. The activity of the absorbent was expressed by the percent of calcium hydroxide converted to calcium sulfate, ηCa, after the desulfurization reaction for 40 h. In 40 h, all absorbents examined completely deactivated. Therefore, ηCa indicates the maximum calcium utilization efficiency of the absorbent. Chemical and Physical Analysis. XRD patterns were recorded on a RIGAKU RAD-C system for the powdered
Figure 1. XRD patterns of calcium hydroxide (a), H-1 (b), and H-2 (c). samples less than 44 µm, with Cu-ΚR radiation in the diffraction angle (2θ) range of 5-90° at a sweep rate of 3° min-1. The specific surface area was measured by BET method for the sample pretreated at 573 K in the stream of N2:He ) 7:3 for 1 h using Monosorb, QUANTACHROME Co. The sulfur contents in the sample before and after desulfurization reaction were measured by infrared spectroscopy. For the measurement, 50 mg of sample was placed in a crucible, and small amounts of tin metal, iron metal, and tungsten metal were added. The sample in the crucible was then heated by a highfrequency induction coil to convert sulfur compounds to SO2 for detection by infrared spectroscopy. Laser Raman spectra were recorded on a JASCO NRS-2000 spectrometer using the 514.5 nm line of an Ar+ laser emission. An incident laser power was 1 mV at the sample position, and the scan time was 30 or 60 s under ambient conditions at room temperature. The Ca K-edge X-ray absorption experiment was carried out at BEB-XAFS station installed on BL01B1 at Spring-8, Japan Synchrotron Radiation Research Institute (JASRI), Hyogo, Japan with a ring energy 8 GeV and a stored current 80-100 mA.15 The measurements were performed in an X-ray transmission mode with a Si(111) two-crystal monochromator set between two Rh-coated mirrors to remove higher harmonics. The upstream mirror was slightly bent to collimate vertically dispersed X-ray beam. Intensities of incident X-ray and transmitted X-ray were determined with ionizing chambers filled with atmospheric N2/He gas flow and N2 gas flow, respectively. The analysis of the spectra was performed with a KABO program at the Kyoto University Data Processing Center.16
Results Role of Calcium Sulfate Added in the Slaking Step. To clarify the effect of the addition of calcium sulfate on the reactivity of calcium hydroxide formed in the slaking step with silicic acid, we measured XRD patterns of calcium hydroxide, the sample H-1 (slaking calcium oxide alone followed by reaction with silicic acid), and the sample H-2 (slaking calcium oxide with calcium sulfate followed by reaction with silicic acid), and they are shown in Figure 1. The peak appearing at (15) Uruga. T.; Tanida, H.;, Yoneda, Y.; Takeshita, K.; Emura, S.; Takahashi, M.; Harada, M.; Nishihata, Y.; Kubozono, Y,; Tanaka, T.; Yamamoto, T.; Maeda, H.; Kamishima, O.; Takabayashi, Y.; Nakata, Y.; Kimura, H.; Goto, S.; Ishikawa, T. J. Synchrotron Radiat. 1999, 6, 143 145. (16) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2987-2999.
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Figure 3. κ3-weighted Ca K-edge spectra of H-1 (a), H-2 (b), CaSiO3 (c), CaSO4 (d), and Ca(OH)2 (e).
Figure 2. Ca K-edge XANES spectra of Ca(OH)2 (a), CaSO4 (b), CaSiO3 (c), H-1 (d), and H-2 (e). Spectrum f was synthesized as found in the text.
2θ ) 29.22° in the XRD patterns for H-1 and H-2 is ascribed to calcium silicate. In XRD pattern of H-1, the peaks for calcium hydroxide which would appear at 2θ )34.06° and 18.02° were not appreciable, suggesting that calcium hydroxide has completely reacted with silicic acid to form calcium silicate during the mixing step of silicic acid and the drying step at 433 K for 3 h. In XRD pattern for H-2, however, the two major peaks for calcium hydroxide were observed at 2θ ) 34.06° and 18.02°, indicating that calcium hydroxide remained unreacted in H-2. These results suggest that calcium sulfate addition in the slaking step decreases the reactivity of calcium hydroxide toward silicone compound in the following hydrothermal treatment step. The effect of calcium sulfate addition in the slaking step was also examined by XAFS spectroscopy. Figure 2A shows the XANES (X-ray absorption near edge structure) spectra at Ca K-edge of the samples H-1 and H-2 together with those of some reference compounds calcium hydroxide, calcium sulfate, and calcium silicate. At a glance, the spectrum of H-1 is almost identical with that of H-2, and these spectra are similar to that of calcium silicate. We may propose that H-1 and H-2 consist mainly of calcium silicate. Although H-2 was prepared from a mixture containing 5 wt % of calcium sulfate, the peaks characteristic to those for calcium sulfate cannot be identified in the spectrum of H-2. Because all of the reference compounds consist of the Ca cations surrounded by six oxygen atoms, the spectral features are fatally similar to each other, and it is difficult to discriminate the feature of the impurity less than 15%. To examine the spectral difference more in detail, we expanded the pre-edge region was expanded and depicted it in Figure 2B. It is evident that the peaks at about 4040 eV are clearly seen in the spectra of calcium silicate and calcium sulfate. However, the spectrum of
calcium hydroxide exhibits a tiny shoulder peak. This difference in the spectra of calcium silicate and calcium sulfate from that of calcium hydroxide arises from the ligand configuration around the Ca cations. The Ca cations in calcium silicate and calcium sulfate are located at a center of distorted oxygen octahedron, and the Ca cation in calcium hydroxide having CdI2 structure is located at a center of more regular octahedron. The peak around 4040 eV is called the pre-edge peak and is attributed to the 1s f 3d electron transition. This dipole transition is generally forbidden, but the mixing of Ca 4p orbitals with Ca 3d orbitals makes it allowed when the Ca cations are located at the center of a distorted ligand symmetry.17 As shown in Figure 2B (curve c), the sample H-1 exhibits a pre-edge peak identical with that for calcium silicate, and this clearly indicates that all the Ca cations in H-1 exist as calcium silicate. The pre-edge peak for H-2 is somewhat similar to those for calcium silicate and H-1, but the peak is broader than these. The XANES spectrum is a linear combination of several XANES spectra of the involved species if the sample consists of more than one compound. Therefore, the discrepancy of the spectral feature between H-1 and H-2 is attributed to the different composition of the involved compounds. Possible compounds included in H-2 are calcium silicate, calcium hydroxide, and calcium sulfate. Of the three, the spectrum of calcium hydroxide exhibits a tiny shoulder peak, and this component compound contributes to the broadness of the peak width. Therefore, in H-2, unreacted calcium hydroxide still remained. Actually, the spectrum synthesized by summation of the spectra (calcium silicate (72%) + calcium hydroxide (13%) + calcium sulfate (15%)) shown in Figure 2B (curve f) is in good agreement with that of H-2 (Figure 2B, curve (e)).18 Figure 3 shows Ca K-edge EXAFS spectra of H-1, H-2, and related compounds. Again, the EXAFS spectrum of H-1 is identical with that of calcium silicate. In these (17) Roe, A. L.; Schneider, D. J.; Meyer, R. J.; Pyrz, J. W.; Wisdom, J.; L. Que, L., Jr. J. Am. Chem. Soc. 1984, 106, 1676-1681. (18) Tanaka, T.; Yamamoto, T.; Kohno, Y.; Yoshida, T.; Yoshida, S. Jpn. J. Appl. Phys. 1999, 38-1, 30-35.
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Table 1. Properties of Several Samples Prepared from Calcium Hydroxide, Coal Fly Ash, and Dalcium Sulfate
Ca(OH)2 Ca(OH)2 Ca(OH)2 + CFAa (50:50) Ca(OH)2 + CFA1) + CaSO4 (50:25:25) Ca(OH)2 + CaSO4 (50:50) a
time for hydrothermal treatment/h
desulfurization activity/%-η Ca
specific surface area/m2 g-1
fwhm deg
0 15 15
18 8 13
10 5 13
0.33 0.19 0.33
15
47
36
0.33
15
28
22
0.30
CFA represents coal fly ash. Table 2. Properties of the Absorbent Prepared from Two Types of Calcium Hydroxide and Coal Fly Ash
Ca(OH)2 (fine)a + CFAb Ca(OH)2 (standard)a + CFAb a
time for hydrothermal treatment/h
desulfurization activity/%-η Ca
specific surface area/m2 g-1
fwhm deg
0 1 0 1
19 33 18 13
14 18 5 13
0.54 0.32 0.33 0.32
Specific surface areas are in 10 m2 g-1 for the standard Ca(OH)2 and 28 m2 g-1 for fine Ca(OH)2. b CFA represents coal fly ash.
spectra, highly frequent oscillation cannot be found, suggesting that no ordered array of Ca cations is included in these compounds. On the other hand, calcium hydroxide which involves ordered arrays of Ca cations exhibits highly frequent oscillations. Although the spectrum of H-2 is very similar to those for H-1 and calcium silicate, the amplitude of the oscillation is a little more intense, and a highly frequent oscillatory component can be seen in the region of 6.5-7.5 Å-1, as indicated by arrows. Taking account that an EXAFS spectrum consists of more than one EXAFS component, the spectral difference of H-2 from H-1 can be attributed to the presence of compounds other than calcium silicate. Intense amplitude in the range of 3-6 Å-1 is due to the presence of calcium hydroxide and calcium sulfate, and the highly frequent oscillatory component may indicate the presence of calcium hydroxide. Hereby, the EXAFS spectra can be well interpreted aided by the results of XANES analysis. From the above results, we conclude that H-1 is identified as calcium silicate that and H-2 is the mixture of calcium sulfate, calcium silicate, and calcium hydroxide. The presence of calcium sulfate suppresses the reaction between calcium hydroxide and silicic acid to form calcium silicate, and thus unreacted calcium hydroxide still remains in H-2 to a considerable extent. Role of Calcium Sulfate Added in the Hydrothermal Treatment Step. Effects of Calcium Sulfate Addition on Activity, Surface Area, and Crystallite Size. In Table 1 are given the desulfurization activities, specific surface areas, and full widths at half-maximum (fwhm) of the XRD peak at 2θ ) 34.06° for calcium hydroxide, the absorbent prepared from calcium hydroxide and coal fly ash, the absorbent prepared from calcium hydroxide, coal fly ash, and calcium sulfate, and the absorbent prepared from calcium hydroxide and calcium sulfate. All samples were hydrothermally treated for 15 h. Data for untreated calcium hydroxide are also included. The coal fly ash alone did not exhibit any activity. The activity of calcium hydroxide was reduced to a half by hydrotreatment for 15 h (ηCa ) 18 to 8%). The
activity was increased by mixing calcium hydroxide with coal fly ash (ηCa ) 8 to 13%). Further increase was observed when calcium hydroxide was mixed with calcium sulfate (ηCa ) 8-28%). The highest activity was obtained for the absorbent prepared from calcium hydroxide, coal fly ash, and calcium sulfate. The changes in specific surface area are in accordance with those of the activity. The fwhm reflects the size of crystallites of calcium hydroxide. It is evident that crystallites of calcium hydroxide grew by hydrothermal treatment when calcium hydroxide alone was treated. However, the crystal growth was suppressed by the addition of coal fly ash and/or calcium sulfate. Effect of Crystallite Size of Calcium Hydroxide on the Activity of the Resulting Absorbent. Two kinds of calcium hydroxide different in the specific surface area and cystallite size were hydrothermally treated with coal fly ash for 1 h. The period of the hydrothermal treatment was set up short to minimize the influence caused by the increase of the crystallite size of calcium hydroxide during the hydrothermal treatment. The results are listed in Table 2. The desulfurization activities of the two kinds of calcium hydroxide were almost the same before hydrothermal treatment, ηCa )18% and 19%, although the specific surface areas were different. A large difference was observed in the activity between two absorbents prepared from the two kinds of calcium hydroxide after hydrothermal treatment for 1 h (ηCa ) 33% vs 13%). The surface areas and size of calcium hydroxide crystallites were not largely different after hydrothermal treatment for the two samples. The increase in the activity is considered to be due to the formation of calcium silicate during hydrothermal treatment. It is suggested that easiness of the reaction of calcium hydroxide with coal fly ash to form calcium silicate depends on the crystallite size and/or surface area of calcium hydroxide. Effect of Ca2+ and SO42- Ions on the Surface Area and Crystallite Size of Calcium Hydroxide. In Table 3 are shown the specific surface areas of calcium hydroxide after the hydrothermal treatment for 15 h with the
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Table 3. Specific Surface Area and fwhm of Samples Prepared from Calcium Hydroxide with Na2SO4 and CaCl2a
Ca(OH)2:Na2SO4 Ca(OH)2:CaCl2 a
composition/ wt %
specific surface area/m2 g-1
fwhm/ deg
99.7:0.3 95:5 99.7:0.3 95:5
4.2 3.4 3.3 5.2
0.19 0.20 0.20 0.20
All samples were hydrothermally treated for 15 h.
Figure 5. Raman spectra of (a) the slaking product of calcium oxide and calcium sulfate at the ratio of 50:50, (b) a mixture containing calcium hydroxide and calcium sulfate in a ratio of 50:50 with the hydrothermal treatment for 15 h, (c) CaSO4‚ 2H2O, (d) CaSO4‚0.5H2O, (e) and Ca(OH)2.
Figure 4. Effect of calcium sulfate contents on the surface area of calcium hydroxide in the hydrothermal treatment step. An open circle means the specific surface area of calcium hydroxide without the hydrothermal treatment.
addition of CaCl2 and Na2SO4 at concentrations of 0.3 and 5 wt %. Addition of CaCl2 and Na2SO4 did not affect the surface area and crystallite size of calcium hydroxide. The surface areas and fwhm’s in Table 3 are essentially the same as those of calcium hydroxide hydrothermally treated for 15 h, but much smaller than those of the sample containing calcium hydroxide and calcium sulfate shown in Table 1. The results indicate that the existence of Ca2+ ion or SO42- ion was not effective for suppressing the crystal growth of calcium hydroxide when these ions were separately added in the hydrothermal treatment. Effect of Content of Calcium Sulfate on the Surface Area of Calcium Hydroxide. The surface area of the absorbent prepared from calcium hydroxide and calcium sulfate with hydrothermal treatment for 15 h changed with the composition of the starting material. The specific surface areas are plotted against the content of calcium sulfate in Figure 4. The surface area of the raw calcium hydroxide is shown by open circle for reference. The surface area became triple when only 10% calcium sulfate was added and gradually increased with an increase in the content of calcium sulfate. All the samples containing calcium sulfate had surface areas larger than that of the raw calcium hydroxide. The results indicate that hydrothermal treatment for 15 h decreased the surface area of calcium hydroxide to a half. The decrease in the surface area may result from crystal growth of calcium hydroxide. In the presence of calcium sulfate, the surface area did not decrease. It is considered that presence of calcium sulfate suppress the crystal growth of calcium hydroxide. This is consistent with the conclusion derived from the data in Table 1.
Raman study of the Structure of the Compound Formed by Reaction of Calcium Sulfate with Calcium Oxide and Hydroxide in Slaking and Hydrothermal Treatment Steps. Mixtures containing calcium oxide mixed with calcium sulfate at different ratios were slaked and subjected to Raman spectrum measurement. Slurries containing calcium hydroxide and calcium sulfate at different ratios were hydrothermally treated for 15 h and dried, and the resulting samples were also subjected to Raman spectrum measurement. Raman spectra of these samples were measured to examine if calcium hydroxide-calcium sulfate complex is formed during hydrothermal treatment and if there is any differences in the compositions at surface layer relative to those in the bulk for the samples formed in slaking and hydrothermal treatment steps. Raman spectra for the physical mixtures of calcium hydroxide and calcium sulfate were measured as references. In Figure 5 are shown Raman spectra of selected samples. All peaks could be ascribed to calcium hydroxide, calcium sulfate hemihydrate, or calcium sulfate dihydrate. Any peaks which may indicate the formation of calcium hydroxide-calcium sulfate complex were not appreciable. The only change observable during the slaking step and hydrothermal treatment step was the transformation of calcium sulfate hemihydrate used as a raw material to calcium sulfate dihydrate. It is to be noted that some differences in peak intensity were observed between the slaked samples and hydrothermally treated samples. Figure 6 shows Raman spectra in the region 250-550 cm-1 for the slaked samples and hydrothermally treated samples together with physically mixed samples. The peak at 358 cm-1 is attributed to calcium hydroxide, and the peaks at 415 and 495 cm-1 are attributed to calcium sulfate dihydrate. The peak intensity ratios of calcium hydroxide to calcium sulfate were lower for the slaked samples than for the physically mixed samples, while the ratios were higher for the hydrothermally treated samples than for the physically mixed samples. Since Raman spectroscopy is more sensitive to the species present in
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Figure 6. Raman spectra of (a) the physical mixture of calcium hydroxide and calcium sulfate at a ratio of 50:50, (b) the mixture of panel a at a ratio of 90:10, (c) the slaking product of calcium oxide and calcium sulfate at a ratio of 50: 50, (d) the product of panel c at a ratio of 90:10, followed by drying for 24 h at 323 K, (e) the mixture of calcium hydroxide and calcium sulfate at a ratio of 50:50, and (f) the mixture of panel e at a ratio of 90:10 with the hydrothermal treatment for 15h.
the surface layer than to those in the bulk, the peak intensity reflects the composition of the surface layer rather than that of the bulk. Referring to the physically mixed samples, the slaked samples have calcium sulfate rich in the surface layer, and the hydrothermally treated samples have calcium hydroxide rich in the surface layer. Discussion Negative effects of calcium sulfate addition in the slaking step and positive effects of the addition in the hydrothermal treatment step on the desulfurization activity of the resulting absorbent are interpreted in terms of the reactivity of calcium hydroxide toward silicon species originating from coal fly ash. A highly reactive calcium hydroxide would easily form calcium silicate, which is the main active species for the desulfurization. The calcium hydroxide formed by slaking calcium oxide in the presence of calcium sulfate could not completely react with silicic acid; a considerable part of calcium hydroxide was left unreacted, as evidenced by XRD and XAFS. On the other hand, the calcium hydroxide formed by slaking calcium oxide alone could completely react with silicic acid to form calcium silicate. Once unreactive calcium hydroxide is formed in the slaking step, it will not react easily with the silicon species in the following hydrothermal treatment step.
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Raman spectroscopic study revealed that the calcium hydroxide formed by slaking calcium oxide in the presence of calcium sulfate has calcium sulfate rich in the surface layers, indicating that the calcium hydroxide particles are covered with calcium sulfate to reduce the reactivity. The positive effects of the addition of calcium sulfate in the hydrothermal treatment step may be caused by the retention of the cystallite size of calcium hydroxide small in the presence of calcium sulfate. When calcium hydroxide alone is hydrothermally treated, the crystallite size increased, and as a result, the surface area of the particles decreased considerably. In the presence of calcium sulfate, however, the surface area of the calcium hydroxide increased by hydrothermal treatment, and the crystallite size did not increase. Therefore, the reactivity of the calcium hydroxide toward the silicon species was not suppressed during the hydrothermal treatment step, and the formation of calcium silicate proceeded. The same effect of the addition of calcium sulfate was not realized by the addition of Ca2+ and SO42- separately. As evidenced by Raman spectroscopic study, the particles formed in the hydrothermal treatment step with calcium sulfate have calcium hydroxide rich in the surface layer as compared to in the bulk. Calcium hydroxide may repeat dissolution and crystallization in the hydrothermal treatment step. Calcium sulfate may serve as a center of nucleation for calcium hydroxide. Calcium hydroxide is considerd to undergo Ostwald repening during hydrothermal treatment. Calcium hydroxide dissolved from a small particle recrystallizes to grow crystallite if calcium sulfate is not present but recrystallizes on the particles of calcium sulfate if calcium sulfate is present in abundance. Conclusions The effects of the addition of calcium sulfate in preparation of dry-type flue gas desulfurization from coal fly ash and calcium oxide on the activity of the resulting absorbent were studied. The negative effect of the addition of calcium sulfate in the slaking step is considered to be due to the formation of unreactive calcium hydroxide which is covered with calcium sulfate. The positive effect of the addition of calcium sulfate in the hydrothermal treatment step is considered to be due to the suppression of crystallite growth of calcium hydroxide to retain the reactivity of calcium hydroxide toward silicon species. Therefore, for preparation of a highly active absorbent, a mixture of calcium oxide and coal fly ash should be first slaked without calcium sulfate and then subjected to hydrothermal treatment with calcium sulfate. Acknowledgment. The X-ray absorption experiments have been performed under the approval of the JASRI (Proposal 1999A0083). EF000186N
