Preparation of Active Absorbent for Dry-Type Flue Gas Desulfurization

Preparation of Active Absorbent for Dry-Type Flue Gas. Desulfurization from Calcium Oxide, Coal Fly Ash, and Gypsum. Tomohiro Ishizuka,† Hiroaki Tsu...
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Preparation of Active Absorbent for Dry-Type Flue Gas Desulfurization from Calcium Oxide, Coal Fly Ash, and Gypsum Tomohiro Ishizuka,† Hiroaki Tsuchiai,‡ Takeshi Murayama,‡ Tsunehiro Tanaka,§ and Hideshi Hattori*,† Center for Advanced Research of Energy Technology, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan, Department of Research and Development, Hokkaido Electric Power Co., Inc., Tsuishikari 2-1, Ebetsu 067-0033, Japan, and Department of Molecular Engineering, Kyoto University, Yoshida-honcho, Sakyo-ku, Kyoto 606-8501, Japan

As compared to a commercial absorbent prepared from Ca(OH)2, gypsum (CaSO4) and coal fly ash, an absorbent prepared by use of CaO in place of Ca(OH)2, exhibited a higher activity for dry-type flue gas desulfurization . The order of the addition of the raw materials in the slaking procedure has much effect on the activity of the resulting absorbent. The activity of the absorbent increased further upon hydrothermal treatment following the kneading procedure. The period of hydrothermal treatment was reduced to 3 h to attain the activity which exceeds 20% of the activity of the commercial absorbent which requires an optimum hydrothermal treatment period of 10 h. The activity enhancement by use of CaO is considered to result from exothermic heat of slaking CaO. At a high temperature, the reaction of CaO with a SiO2 component included in the coal fly ash facilitates the formation of calcium silicate. The formation of calcium silicate was suppressed by the existence of CaSO4 in the slaking procedure. Introduction Boilers are generally equipped with a flue gas desulfurization (FGD) unit. Different types of FGD units are being operated at present. A wet-type FGD unit based on a limestone-gypsum method is most widely used and suitable for large-scale boilers such as those installed in coal- or oil-fired power stations. The wettype FGD process has many advantages and has been continuously improved in the efficiency and reduction of the cost to establish technology. However, because the wet-type FGD processes require a large amount of water and a facility for the wastewater treatment, it has been expected to develop a new type of desulfurization process to be applicable to the power stations in the region where water supply is not sufficient. Although several dry-type FGD processes such as the active carbon method1 and electron beam method2 have been practically used, most of them entail a high cost to achieve the same desulfurization performance as those of the wet processes. Calcium compounds, especially calcium silicate, show desulfurization activity in the dry and semidry FGD processes, and a number of studies have been carried out to prepare a highly active absorbent prepared from calcium hydroxide and coal fly ash.3-10 A new dry FGD process adopting the absorbent prepared from coal fly ash, slaked lime, and gypsum has been commercially operated since 1991.11 The characterization of the absorbent and the reaction mechanisms operating in the desulfurization were reported previously.12-14 The process can achieve a high Ca utilization efficiency of 80%, which is much higher than the * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-726-0731. † Hokkaido University. ‡ Hokkaido Electric Power Co. § Kyoto University.

value of about 50% for the dry-type duct injection process using calcium hydroxide as the SO2 absorbent. One disadvantage with the new dry FGD process is that the preparation of the absorbent requires a rather long period of time. In particular, the hydrothermal treatment of the mixture of coal fly ash, calcium hydroxide, and calcium sulfate takes 10.5 h in the commercial plant. It would save some cost of FGD to a great extent if the period of hydrothermal treatment is reduced. We have found that the use of calcium oxide in place of calcium hydroxide resulted in the formation of an active absorbent in a short period of time with hydrothermal treatment. In the present study, we report how the activity of the absorbent is enhanced by the use of calcium oxide as one of the raw materials for the absorbent. Experimental Section Preparation of the Absorbent. Absorbents were prepared from calcium oxide (industrial grade, Wako Pure Chemical Industries) or calcium hydroxide (industrial grade, Wako Pure Chemical Industries), coal fly ash, and the spent absorbent. Coal fly ash and the spent absorbent were supplied by the Tomato-Atsuma coalfired power station of Hokkaido Electric Power Co., Inc. where a dry FGD unit is being operated commercially. Coal fly ash had the following composition: SiO2 47.4%, Al2O3 17.6%, CaO 3.7%, Fe2O3 6.0%, and several percent of various metal oxides and carbon. The average of the particle size was about 13.2 µm. The spent absorbent was obtained from the commercial plant. The spent absorbent contained about 50% of calcium sulfate and therefore was used as the source for calcium sulfate. In the commercial plant, the spent absorbent is used as a gypsum source. Calcium oxide, whose particle size was adjusted from 5 to 3 mm or below 0.1 mm, was used as a raw material. Standard procedures to prepare the absorbent are as follows. To prepare 500 g of the absorbent excluding

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water, a mixture of the coal fly ash (0-150 g) and calcium oxide (132 g) was kneaded with 28.6-35.5 wt % of water at 70 °C for 5 min. In this stage, the slaking of calcium oxide occurred. Then, 150 g of spent absorbent which contains 70 g of CaSO4 and the rest coal fly ash (150-0 g) was added to the slurry, and kneading was continued for 10 more min. The kneaded material was extruded to form pellets of 6-mm length and 10mm diameter. The pellets were then subjected to hydrothermal treatment with steam at 100 °C followed by drying for 2 h at 200 °C. The standard composition of the absorbent excluding water was calcium hydroxide 35%, spent absorbent 30-50%, and coal fly ash 15-35%. The second preparative procedures are as follows. A mixture of all the raw materials composed of 175 g of fly ash, 132 g of calcium oxide, and 150 g of spent absorbent was kneaded with water at 70 °C (300 g) for 15 min. The kneaded material was extruded and dried as described above. In some experiments, the temperature of water and the period of hydrothermal treatment were changed in the range of 20-70 °C and 0-10 h, respectively. The absorbent prepared from calcium hydroxide, coal fly ash, and spent absorbent, which are the same as those adopted in the commercial plant, was prepared by the following procedures. A mixture of 175 g of fly ash, 175 g of calcium hydroxide, and 150 g of spent absorbent was kneaded with 200 g of water at 20 °C for 5 min. The kneaded material was extruded, treated hydrothermally, and dried as described above. The hydrothermal treatment was conducted for 0-10 h in the present study, though the hydrothermal treatment period of 10.5 h is taken in the commercial operation. Calcium silicate for XPS measurement was prepared from calcium hydroxide and silicic acid. Calcium hydroxide (70 g) and silicic acid (30 g as SiO2) was hydrated in 500 g of water at 100 °C for 15 h followed by drying for 2 h at 200 °C. Activity Test. A fixed-bed reactor was employed for carrying out desulfurization tests. The absorbent (60 mL) of the particle size in the range 2.36-3.36-mm diameter was placed in a Pyrex glass tubing reactor (30mm o.d.). The reactant gas simulated for flue gas from a coal-fired boiler composed of 2250 ppm SO2, 700 ppm NO, 6% O2, 13% CO2, 10% H2O, and N2 as the balance was contacted with the absorbent at a flowing rate of 1 L/min at a reaction temperature of 130 °C. The components of the effluent gas were analyzed after H2O was removed from the effluent gas by a cold trap. The concentrations of SO2 and NOx were continuously monitored by nondispersive infrared spectroscopy and atmospheric chemical luminescence, respectively. The concentrations of CO2 and O2 were measured by nondispersive infrared spectroscopy and paramagnetic susceptibility, respectively, before desulfurization tests. The concentrations of the components of the effluent gas were recorded on the following analyzers: SO2,VIA-510 Horiba Ltd.; NOx, CLA-510-ss Horiba Ltd. CO2, CGT10-3A Shimadzu Co.; O2, OA-580-PS Thermo Electron Nippon Co. The activity of the absorbent was evaluated by the time needed to maintain the removal of SO2 above 80%.12 The value was normalized to the unit amount of calcium hydroxide included in the absorbent. Chemical and Physical Analysis of the Absorbent. The composition of the absorbent was determined by XRF, system 3080, Rigaku Co. being used. Sulfur and

Figure 1. Variations of the desulfurization activity as a function of the period of hydrothermal treatment for the absorbents prepared by the procedures in which calcium oxide was slaked alone before being mixed with coal fly ash and spent absorbent (b) and by the procedures in which all the raw materials were slaked together (9). The activity of the commercial absorbent prepared from calcium hydroxide is plotted by (2).

carbon contents in the sample after the reaction were measured by infrared spectroscopy. For the measurement, 50 mg of the 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 high-frequency induction coil to convert sulfur compounds to SO2 for detection by infrared spectroscopy. SEM photographs were taken with S-2300, Hitachi Ltd. with 10 kV of accelerating voltage. The XPS spectra were recorded on a PHI model 5500 using an Mg KR radiation source. The observed binding energy was corrected for the binding energy of C1s (284.6 eV) or Na1s (1071.4 eV). The specific surface area was measured by the BET method for the sample pretreated at 300 °C in the stream of N2:He ) 7:3 for 1 h using a Monosorb, Quantachrome Co. The pore volume of the sample was measured by the BJH method with N2 adsorption in static measurement using Omnisorp 100cx, Coulter Co. Results Figure 1 shows the variations of the desulfurization activity as a function of the period of hydrothermal treatment for the absorbents prepared by different procedures. The activities of the absorbent prepared by the procedures in which calcium oxide alone was slaked before being mixed with coal fly ash and spent absorbent are plotted with solid circles and those prepared by the procedures in which all the raw materials are mixed before kneading are plotted with solid squares. The activity of the commercial absorbent prepared from calcium hydroxide, coal fly ash, and spent absorbent followed by 10.5 h of hydrothermal treatment is also shown, with solid triangles in Figure 1. The activity of the absorbent prepared by slaking calcium oxide alone followed by kneading with coal fly ash and spent absorbent increased monotonically with the period of hydrothermal treatment and exceeded the activity of the commercial absorbent for a period of 5 h of hydrothermal treatment. However, the activity of the absorbent prepared by slaking the raw materials all together increased in the initial period of 5 h and then gradually decreased with the period of hydrothermal treatment. At any hydrothermal treatment periods, the

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Figure 2. Variations of the activity as a function of the amount of coal fly ash (O) and spent absorbent (b) added in the slaking process. The absorbents were subjected to hydrothermal treatment for 5 h.

activities of the absorbent were lower than those of the commercial absorbent. These results suggest that when a calcium oxide is used, a highly active absorbent can be prepared within a short hydrothermal treatment if mixing the raw materials is ordered properly. A further increase in the activity was obtained when coal fly ash was mixed with calcium oxide during the slaking procedure. Figure 2 shows the variations of the activity with the amounts of coal fly ash and spent absorbent which are mixed with calcium oxide during the slaking procedure. In these experiments, the rest of the coal fly ash and spent absorbent were added in the latter part of the slaking procedure, as described for the first preparative procedures. Therefore, all absorbents have the same composition, Ca(OH)2 35 wt %, coal fly ash 35 wt %, and spent absorbent 30 wt %. Each absorbent was hydrothermally treated for 5 h. The addition of 0.1% coal fly ash markedly increased the activity. A maximum activity value of 145 min/g of Ca(OH)2 was attained by the addition of 15% coal fly ash; the activity is about 20% higher than that of the commercial absorbent, 117 min/g of Ca(OH)2. The activity decreased with the content of coal fly ash over 15%. The addition of the spent absorbent during the slaking procedure brought about different effects from those of coal fly ash. Although the addition of the spent absorbent in the range 0.1-3 wt % during the slaking process resulted in the activity exceeding that of the commercial absorbent, the addition of more than 3 wt % spent absorbent resulted in activity lower than that of the commercial absorbent. Because the spent absorbent consists essentially of calcium sulfate and coal fly ash, it is suggested that the addition of calcium sulfate to calcium oxide during the slaking process causes a decrease in the desulfurization activity of the absorbent. The effect of the addition of calcium sulfate to calcium oxide during the slaking process on the activity was examined, together with the effect of the addition of SiO2 and Al2O3. Figure 3 shows the variations of the desulfurization activity as a function of the amounts of calcium sulfate, silica, and alumina added during the slaking process. The activity of the absorbent significantly decreased upon the addition of calcium sulfate, while the addition of the silica and alumina caused an increase in the activity. The difference between the absorbents prepared from calcium oxide and calcium hydroxide is shown in Figure 4. The activity of the absorbent prepared from calcium hydroxide increased linearly with the hydrothermal

Figure 3. Variation of the activity as a function of the amount of SiO2(9), Al2O3(2), and CaSO4(b) added in the slaking process. The activity of the absorbent without additives is represented by (O). All the absorbents were subjected to hydrothermal treatment for 1 h.

Figure 4. Variations of the activity as a function of the period of hydrothermal treatment for the absorbent prepared with CaSO4 from CaO (b) and Ca(OH)2(O), and for the absorbent from CaO without CaSO4 (2).

treatment period and achieved the same value of 117 min/g of Ca(OH)2 for the commercial absorbent in 10 h. However, the activity of the absorbent prepared from calcium oxide slaked with 15 wt % of coal fly ash showed different behavior. The absorbent prepared from calcium oxide without hydrothermal treatment showed high desulfurization activity of 100 min/g of Ca(OH)2, which was almost twice as much as that of the absorbent prepared from calcium hydroxide without hydrothermal treatment. In addition, the activity rapidly increased and achieved the value of 147 min/g of Ca(OH)2 in a period of only 3 h of hydrothermal treatment, 80% breakthrough time for desulfurization being 25% higher that that of the commercial absorbent, 115 min/g of Ca(OH)2. The presence of calcium sulfate caused different effects in the hydrothermal treatment from those in the slaking process, as shown in Figure 4. The activity of the absorbent prepared by slaking a mixture of calcium oxide and coal fly ash followed by kneading with the spent absorbent increased with a period of hydrothermal treatment. In contrast, the activity of the absorbent prepared without the addition of the spent absorbent did not change with the hydrothermal treatment. Therefore, it is indicated that the presence of calcium sulfate in the hydrothermal treatment promotes the formation

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1393 Table 1. Desulfurization Activities, Specific Surface Areas, and Pore Volumes for Two Types of Calcium Hydroxides and the Slaking Product Composed of CaO:Coal Fly Ash ) 5.3:3

commercial Ca(OH)2 Ca(OH)2 from CaO slaking product

desulfurization activity (min g-1 of Ca)

specific surface area (m2 g-1)

pore volume (cm3 g-1)

20 30 69

16 12 24

0.079 0.064 0.095

of the active species for desulfurization. It is to be noted that the effects of calcium sulfate are opposite those in the slaking process and hydrothermal treatment. Calcium sulfate lowered the activity of the absorbent if calcium oxide is slaked in the presence of it, while it enhanced the activity if it is added after the slaking of calcium oxide. To clarify the reason the activity of the absorbent prepared from calcium oxide was enhanced by the addition of the coal fly ash in the slaking process, characterization of the slaked product of calcium oxide with coal fly ash was performed. Table 1 lists desulfurization activities, specific surface areas, and pore volumes of the slaking product of the mixture of calcium oxide and coal fly ash in the ratio of 5.3:3 and two types of calcium hydroxides. Two types of calcium hydroxides were a commercial reagent and one prepared by slaking the calcium oxide. The two types of calcium hydroxides did not significantly differ in activity and physical properties. The addition of coal fly ash in the slaking process greatly increased the activity, surface area, and pore volume. It is indicated that a porous material was formed in the slaking process by the reaction between calcium oxide and coal fly ash and that the material is active for desulfurization. The active species are formed by use of calcium oxide as a raw material but not by use of calcium hydroxide. Actually, the mixture prepared from calcium hydroxide and coal fly ash with water showed the same desulfurization activity as that of the commercial calcium hydroxide. The temperature of the slurry increased in the slaking process when calcium oxide was used, while the temperature was unchanged when calcium hydroxide was used. This suggests that the heat generated by slaking the calcium oxide might affect the formation of the active species. Figure 5 shows the effects of the initial temperature of water added to the mixture of calcium oxide and coal fly ash for slaking upon the desulfurization activities of the slaking products. Although calcium oxide was completely converted to calcium hydroxide by the reaction with H2O in all samples, the desulfurization activity of the slaking product prepared by the use of 20 °C water was apparently low when compared with the activities of the slaking products prepared by the use of hot water, such as 40 and 50 °C. The formation of the active species for desulfurization seems to be suppressed by using cold water for slaking the calcium oxide. In the case that a powdery calcium oxide was used as a raw material, the slaking product showed only about 45 min/g of Ca(OH)2 of desulfurization activity. Even though 50 °C water was used in the slaking process, there was no remarkable increase in the desulfurization activity. SEM micrograghs of the coal fly ash and the sample produced by the slaking of calcium oxide with coal fly ash are shown in Figure 6a-d. Coal fly ash consists

Figure 5. Effects of the temperature of water used for slaking granular CaO (b) and powdery CaO (O) on the desulfurization activity. The slaked materials were composed of CaO and coal fly ash in the ratio 5.3:3.

mainly of the spherical particles of different sizes, ranging from 1 to 10 µm in diameter. The particles before the slaking reaction have smooth surfaces. In the slaking product, on the other hand, the spherical particles were scarcely observable. Particles of irregular shapes were prominent in the slaking product, as seen in Figure 6c,d. This suggests that the spherical coal fly ash reacted so extensively with calcium oxide that not only the surface layers of the spherical particles but also the inside of the particles could not retain their original shapes. For the identification of the slaking product, XPS analysis was performed. XPS spectra of Si2p for the slaking product and coal fly ash are shown in Figure 7. For comparison, an XPS spectrum for calcium silicate is also shown. Coal fly ash gave the peak for Si2p at 103.0 eV. The peak is attributed to the Si in mulite which is the main silicon compound in the coal fly ash.15,16 The rest of the silicon compounds in coal fly ash are quartz or amorphous silica. The slaking product gave peaks for Si2p at 101.5 and 103.0 eV in a ratio of 95:5. The peak at 101.5 eV is attributed to Si2p of calcium silicate because the prepared calcium silicate showed the peak for Si2p at 101.5 eV. Therefore, it is concluded that calcium silicate was formed in the slaking process of calcium oxide with coal fly ash. Discussion It is demonstrated that a highly active absorbent is prepared by using calcium oxide in place of calcium hydroxide together with coal fly ash and spent absorbent. The activity is substantially higher than that of the commercial absorbent which is prepared from calcium hydroxide, coal fly ash, and spent absorbent. In particular, the activity becomes high when calcium oxide is used in a granular form that are 3-5 mm in diameter. In addition, the activity of the absorbent is increased by the successive addition of coal fly ash and spent absorbent to the calcium oxide rather than the simultaneous addition of all the raw materials during the slaking process. Hydrothermal treatment of the slaking product also causes an increase in the activity. The commercial absorbent is subjected to hydrothermal treatment for

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Figure 6. SEM photographs of coal fly ash (a), a dominant part of the product of slaking CaO with coal fly ash (b), and parts of the slaking product where the shapes of coal ash particles are observable, (c) and (d).

10.5 h. The absorbent prepared by the procedures in which calcium oxide is slaked with a part of coal fly ash followed by the addition of the rest of coal fly ash and spent absorbent attains activity 25% higher than that of the commercial absorbent, even in a 3-h period of hydrothermal treatment, as shown in Figure 4. The reduction of the period for hydrothermal treatment by the use of calcium oxide in place of calcium hydroxide is the most advantageous point for the practical process. The active species for desulfurization are formed in both the slaking process and hydrothermal treatment. In the slaking process, the presence of a silicon component enhances the activity, as shown in Figure 3. The silicon component is supplied from coal fly ash when calcium oxide is slaked with coal fly ash. The successive addition of coal fly ash and spent absorbent to the calcium oxide avoids the presence of calcium sulfate in the slaking process. As shown in Figures 2 and 3, the addition of calcium sulfate in the slaking process decreased the activity of the resulting absorbent. The presence of calcium sulfate suppresses the formation of the active species during the slaking process. The suppressive effects of the addition of calcium sulfate in the slaking process on the desulfurization ability of the

resulting absorbent have also been reported previously,17-19 although no definite explanations were given. The presence of calcium sulfate showed quite different results in the hydrothermal treatment process from those in the slaking process. The activity of the absorbent prepared by using calcium oxide slaked alone followed by kneading with coal fly ash and spent absorbent increased with the period of hydrothermal treatment (Figure 1), and the activity of the absorbent without calcium sulfate did not increase (Figure 4). These results suggest that the presence of calcium sulfate in the hydrothermal treatment process promotes the formation of the active species for the desulfurization reaction. These effects of calcium sulfate in the slaking process and the hydrothermal treatment process were reported previously.20 It was explained that the presence of calcium sulfate might increase the rate of leaching silica and alumina from the coal fly ash. In the slaking process, the following exothermic reaction takes place:

CaO + H2O(g) ) Ca(OH)2 + 104.1 kJ‚mol-1

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Figure 7. XPS profiles of Si2p for the slaking product (CaO: coal fly ash ) 5.3:3) (a), coal fly ash (b), and calcium silicate (c).

The heat evolved by the reaction is so large that the reaction is utilized for a chemical heat storage heater in practice. The temperature attained is reported to exceed 400 °C if the reaction is proceeded under a proper water vapor pressure.21 The behavior of the slaking process is dependent on the form of calcium oxide. When powdery calcium oxide is used, the slurry temperature increased quickly. The calcium oxide powder was kept in a wet state during the slaking procedure. The temperature of the calcium oxide powders should be, at most, the boiling point of water. In contrast, when granular calcium oxide is used, we could observe water bumping at the surface of the calcium oxide particle. The bumping state continued for a while in the initial stage, and the calcium oxide particles were in a partially dry state. Although the temperature of the calcium particles could not be measured, it seems that the temperature substantially exceeded the boiling point of water. In addition to the formation of calcium hydroxide, the formation of the active species for desulfurization is facilitated at a high temperature in the slaking process when the granular calcium oxide is used. The size of the calcium oxide particles is crucial for the formation of the active species. In the case of adopting too large particles of calcium oxide such as those over 12-mm diameter, the reaction between calcium oxide and coal fly ash would not progress to a great extent, probably because of the evaporation of too much water during the slaking process. As a practical value, 3-5-mm particle size is appropriate for efficiently forming the active species. The temperature of water used for the slaking process is also important for the formation of the active species. Even though CaO particles of appropriate sizes are

utilized for the raw material, the formation of the active species does not progress efficiently when cold water is used in the slaking process. It is considered that a considerable fraction of the slaking heat is consumed for raising the water temperature. The formation of new species during the slaking process is indicated by the surface area and pore volume measurements. The surface area and pore volume of the slaking product are substantially greater than those of the two types of calcium hydroxides. The active species formed during the slaking process in which a mixture containing calcium oxide granules and coal fly ash was used is suggested to be calcium silicate by XPS. Silicon atoms should originate from coal fly ash. In XPS, the percentage of the silicon atoms converted into calcium silicate during the slaking process exceeded 90%, only less than 10% of the silicon atoms remaining in the original form. Because XPS preferentially detects the atoms located in the surface layers, it is indicated that most of the silicon atoms located in the surface layers react with the calcium compound to form calcium silicate which shows activity for desulfurization.22,23 The extensive reaction on the surface of coal fly ash in the slaking process is also suggested by the SEM photographs where the original spherical shape of coal fly ash is hardly observable after the slaking. In both cases where calcium oxide and calcium hydroxide were used, the desulfurization activities increased further by the successive hydrothermal treatment. The activity increased linearly with the time of hydrothermal treatment when calcium hydroxide was used, while the activity increased rapidly in the initial stage of hydrothermal treatment and reached a constant value when calcium oxide was used. The activity of the absorbent prepared from calcium hydroxide before hydrothermal treatment was equal to that of singlecomponent calcium hydroxide. The calcium hydroxide does not react during the kneading process, but reacts with coal fly ash and calcium sulfate to form calcium silicate during the hydrothermal treatment process. The increase in the activity of the absorbent prepared from calcium oxide by the hydrothermal treatment process may be ascribed to the formation of calcium silicate in which the silicon atoms located in the bulk of coal fly ash are utilized. The reaction seems to be promoted by the existence of calcium sulfate in the hydrothermal treatment process, though the role of calcium sulfate in the hydrothermal treatment is not certain at present. Summary A highly active absorbent for dry-type flue gas desulfurization is prepared from calcium oxide, coal fly ash, and calcium sulfate (spent absorbent). The practically optimum preparation procedures using calcium oxide are as follows. A mixture containing calcium oxide granules 3-5 mm in diameter and a portion of the coal fly ash is slaked with water, and then the rest of the coal fly ash and spent absorbent are added followed by kneading. The slurry is subjected to hydrothermal treatment with water vapor for 3 h. The resulting absorbent exhibits activity more than 20% higher than that of the commercial absorbent which is prepared from calcium hydroxide, coal fly ash, and spent absorbent with hydrothermal treatment for 10.5 h.

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Received for review September 16, 1999 Revised manuscript received February 7, 2000 Accepted February 9, 2000 IE990699L