Ind. Eng. Chem. Res. 1996, 35, 851-855
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Removal of Sulfur Dioxide from Flue Gas by the Absorbent Prepared from Coal Ash: Effects of Nitrogen Oxide and Water Vapor 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
Removal of SO2 from flue gas by the absorbent prepared from coal fly ash, calcium oxide, and calcium sulfate was studied under different reaction conditions to elucidate the effects of the reaction temperature, water vapor pressure, and coexistence of NO in a flue gas. The SO2 removal activity increased with an increase in NO concentration up to 500 ppm at 130 °C. The SO2 removal activity increased as the water vapor pressure increased until a monolayer coverage with water molecule is achieved. As the adsorbed water exceeded the monolayer coverage, the SO2 removal activity suddenly decreased and calcium sulfite became the main product instead of calcium sulfate. The NO removal activity increased with an increase in SO2 concentration up to 2000 ppm at 130 °C. The NO removal also increased with an increase in water vapor pressure. The NO removal markedly decreased as the adsorbed water exceeded the monolayer coverage. Introduction For the removal of SO2 from flue gas, a wet process using calcium carbonate as an absorbent is most commonly adopted in commercial plants. The wet process shows a high efficiency but needs a large amount of water. SO2 is fixed in the form of calcium sulfate. In the semidry process, several applications were successful for achieving high SO2 removal and high lime utilization (Brown and Felsvang, 1991; Lepovitz et al., 1993). In these cases, SO2 reacts with absorbents at the reaction temperature of 60-70 °C and a high relative humidity. The reaction chemistry is the same as the wet process; SO2 dissolves into the water adsorbed on the absorbent to react with the calcium ion. The final products are calcium sulfite and calcium sulfate, calcium sulfite being formed predominantly. The dry process using calcium hydroxide as an absorbent is also used commercially but is not so common as the wet process. In a typical dry process, a powdery calcium hydroxide is injected into the duct. A large fraction of calcium hydroxide remains unreacted. SO2 is fixed mostly in the form of calcium sulfite. The low utilization efficiency of calcium in the dry process is considered to be due to the formation of calcium sulfite and/or calcium sulfate which cover the outer surface of the calcium hydroxide particles to prevent SO2 from further permeation into the bulk (Van Houte and Delmon, 1979). However, Ueno (1986) 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. This absorbent has been actually 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. * To whom correspondence should be addressed. † Hokkaido University. ‡ Hokkaido Electric Power Co. § E-mail address:
[email protected].
0888-5885/96/2635-0851$12.00/0
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. The present paper aims to elucidate the dry desulfurization reaction in which this absorbent is used. We report the effects of reaction conditions such as the concentration of NO and SO2, the reaction temperature, and the vapor pressure of water on the absorption of SO2 and NO in the absorbent. It was found that NO and SO2 enhance each other in the SO2 and NO absorption and that the amount of water adsorbed on the surface is critical for determining the final product for SO2 absorption and the activities for SO2 and NO absorption. Experimental Section Preparation of the Absorbent. The absorbent was prepared from calcium oxide, calcium sulfate, and coal fly ash. The calcium oxide used was of industrial grade (Wako Pure Chemical Industries). The calcium sulfate was reagent grade calcium sulfate hemihydrate (Wako). Coal fly ash was supplied by Hokkaido Electric Power Company, Tomato-Atsuma Power Station, and had a composition: SiO2, 59%; Al2O3, 24%; and CaO, 1.4%. For preparation of 200 g absorbent, calcium oxide (45.4 g) was added to 1 L of water at a temperature of 65 °C. The temperature of the slurry increased to about 90 °C on 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. Then, the absorbent slurry was filtered and dried normally at 200 °C for 2 h. The resulting absorbent had a composition: calcium hydroxide, 30%; calcium sulfate, 30%; and coal fly ash, 40% (except H2O and CO2). 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 an absorbent cotton (5 g) and placed in a quartz reactor of 40 mm in diameter. The absorbent © 1996 American Chemical Society
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Table 1. Base Case Conditions for Activity Tests SO2 NO O2 CO2 H2O N2 flow rate absorbent volume reaction temperature
2250 ppm 700 ppm 6 mol % 13 mol % 10 mol % balance 60 L‚h-1 50 mL 130 °C
bed packed with the absorbent dispersed on the cotton was 150 mm long. The model flue gas was normally composed of 2250 ppm of SO2, 700 ppm of NO, 6% O2, 13% CO2, 10% H2O and N2 as a balance. Flow rate of the model gas was 1 L/min, and the reaction temperature was normally 130 °C. In some experiments, the concentration of each component was varied in the ranges 0-3000 ppm for SO2, 0-1600 ppm for NO, and 0-18% for H2O, and the reaction temperature was varied from 70 to 130 °C. Base case conditions for activity tests are summarized in Table 1. 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 that 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. A 1 mol amount of SO2 per mol of Ca(OH)2 corresponds to 865 mg/(g of Ca(OH)2), and 1 mol of NO/(mol of Ca(OH)2) corresponds to 405 mg/(g of Ca(OH)2). The moles of SO2 and NO removed were measured by integration of the difference between inlet and outlet concentrations. Water Adsorption. The amount of water molecule adsorbed on the absorbent was measured with a quartz spring balance. The powdery absorbent was pressed into a disk and then crushed into particles. The particles were sieved into 0.8-1.0 mm in diameter. The absorbent (0.1 g) was placed in a quartz basket suspended under the quartz spring. Prior to the measurement, the absorbent was pretreated with a dry argon stream flowing 50 mL/min at 200 °C for 2 h. The argon stream was bubbled into thermostated water at 50 mL/ min to obtain a desired vapor pressure of water ahead of the absorbent. The quartz spring has a sensitivity of 2.1 mg/mm. The vapor pressure of water was varied from 0.03 to 0.2 atm, and the adsorption was measured at 70, 100, and 130 °C. A 1 mol % amount corresponds to 1013 Pa. Physical Analyses. 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. XRD measurement was conducted on a Rigaku RAD-C 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-1. The absorbent slurry after hydration was dried in a vacuum at room temperature for the measurement. Results Effects of NO Concentration on SO2 Removal. The activity for SO2 removal was strongly dependent on the presence of NO in the reaction mixture. This is the characteristic feature of the present absorbent. The
Figure 1. Effect of NO concentration on the activity for SO2 removal at a reaction time of 5 (O), 10 (4), and 15 h (0).
Figure 2. Effect of SO2 concentration on NO removal at a reaction time of 5 (O), 10 (4), and 15 h (0).
SO2 removal activity was measured at different concentrations of NO. The variation of the activity for SO2 removal at a reaction temperature of 130 °C is shown in Figure 1 as a function of the inlet NO concentration. In the absence of NO, the activity was not appreciable. The activity markedly increased with the inlet NO concentration up to 500 ppm and remained constant above 500 ppm. Effects of SO2 Concentration on NO Removal. Like the effects of NO concentration on SO2 removal, the SO2 concentration also affected strongly the NO removal activity. The NO removal activity is plotted against the inlet SO2 concentration in Figure 2. Without SO2, NO removal activity was not appreciable. The NO removal increased with an increase in the SO2 concentration up to 2000 ppm at the reaction time of 5 and 10 h and then gradually decreased as the SO2 concentration increased further. At a reaction time of 15 h, the concentration for maximum NO removal shifted to a low SO2 concentration of 1500 ppm. Effects of Reaction Temperature. The temperature dependences of SO2 and NO removal were not the one normally observed for chemical reactions. The variations of the activities for SO2 and NO removal as a function of the reaction temperature are shown in Figure 3. The variations are similar for SO2 and NO removals. For both reactions, the threshold temperature of 75 °C was observed. Below 75 °C, the removal activities for both SO2 and NO were low. On raising
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Figure 3. Effect of reaction temperature on SO2 and NO removal: (O) SO2 removal and (b) NO removal.
Figure 5. Effect of moisture content of the model flue gas on NO removal at 70 (O), 100 (4), and 130 °C (0). Starting materials: Ca(OH)2, 50%; coal fly ash, 30%; and the absorbent used for 300 h of desulfurization (instead of CaSO4), 20%. Hydration time: 12 h.
Figure 4. Effect of moisture content of the model flue gas on SO2 removal at 70 (O), 100 (4), and 130 °C (0). Starting materials: Ca(OH)2, 50%; coal fly ash, 30%; and the absorbent used for 300 h of desulfurization (instead of CaSO4), 20%. Hydration time: 12 h.
Figure 6. Adsorption isotherms of water on the absorbent at 70 (O), 100 (4), and 130 °C (0).
the temperature to 75 °C, the activities jumped up and stayed constant at higher temperatures. Effects of Water Vapor Pressure. The vapor pressure of the water present in a flue gas greatly influenced the activities for SO2 and NO removals. The variations of the activities as a function of the vapor pressure of the water are shown in Figure 4 for SO2 and Figure 5 for NO at different temperatures, 70, 100, and 130 °C. The variations are strongly dependent on the reaction temperature. In particular, the variations at 70 °C are different from those at 100 and 130 °C for both SO2 and NO removal. The activities for SO2 and NO removal at 100 and 130 °C increased rather monotonically with an increase in the vapor pressure of water. At a reaction temperature of 70 °C, however, the activities increased sharply with an increase in the water vapor pressure up to 0.040.05 atm for both SO2 and NO removal. The activities suddenly decreased as the water vapor pressure exceeded 0.06 atm. For SO2 removal, the removal increased again as the water vapor pressure increased, but, for NO removal, the removal did not increase with the further increase in the water vapor pressure. The products also changed depending on the reaction conditions. At the reaction temperatures of 100 and 130 °C, the products consisted exclusively of CaSO4‚xH2O
independent of the water vapor pressure in the range 0-20%. At a reaction temperature of 70 °C, however, the main products were calcium sulfate determined by the XRD peaks that appeared at 2θ ) 25.3, 29.4, and 31.7° (ICDD file No. 30-279 and 37-1496) in the water vapor pressure range 0-5 mol % and calcium sulfite hemihydrate determined by the peaks that appeared at 2θ ) 16.2, 28.1, and 34.0° (ICDD file No. 39-725) in the range 6-20 mol %, where the activity for NO removal became low. Adsorption Isotherm of Water on the Absorbent. The adsorption isotherms of water on the absorbent at 70, 100, and 130 °C are shown in Figure 6. The amount of water is expressed in the number of monolayers calculated by assuming the cross section of the water molecule of 0.078 nm2 (Klingspor et al., 1984), and the specific surface area of the present absorbent is 55.1 m2‚g-1. Slightly different values are reported for a molecular diameter of water. Moore (1962) reported 0.145 nm and Klingspor et al. (1984) 0.150 nm. If the diameter of the water molecule of 0.145 nm is used instead of 0.150 nm, the number of monolayers decreases by 6%. The amount of adsorbed water exceeded a monolayer at 70 °C and at a pressure higher than 0.06 atm. Under these conditions, the surface is covered completely with
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water molecules. At 100 °C, the amount of adsorbed water exceeded a monolayer coverage above a water vapor pressure of 0.15 atm. At 130 °C, the amounts of adsorbed water were far less than a monolayer coverage; there remain a considerable fraction of the sites uncovered with water molecule. Discussion The presence of SO2 and NO enhanced the absorption of NO and SO2, respectively, as shown in Figures 1 and 2. In our previous paper (Tsuchiai et al., 1995), we reported the enhancement of SO2 absorption in the presence of NO. It was assumed that NO plays a catalytic role in the oxidation of SO2 to SO3 by the following equations.
net:
NO + 1/2O2 ) NO2
(1)
NO2 + SO2 ) NO + SO3
(2)
SO2 + 1/2O2 ) SO3
(3)
The above equations should occur on the surface of the absorbent. If NO competes with SO2 for the same adsorption sites, there should appear a maximum for the SO2 absorption activity against the NO concentration. However, the maximum would not appear clearly if NO adsorption is much weaker than SO2 adsorption. This may be the case for Figure 1. Concerning the effects of NO on the SO2 absorption, there have been reports by many researchers. Medellin et al. (1978) mentioned that NOx accelerates the absorption of SO2 by the catalytic action of the alumina supporting sodium oxide. Formed SO3 reacts with the sodium oxide to form sodium sulfate several times faster than SO2. alumina
SO2 + NO + O2 98 SO3 + NO2
(4)
Chu and Rochelle (1989) reported for the SO2 removal by a calcium silicate absorbent that NO does not significantly enhance the reaction of SO2 removal. They did not observe the temperature window for optimum SO2 and NOx removal, while Livengood (1991) did. Livengood (1991) reported that he confirmed there exists a temperature window between 100 and 110 °C for optimum SO2 and NOx removal. It was suggested that SO2 removal takes place before NOx reacts with the absorbent. Chu and Rochelle (1989) reported that the optimum humidity for SO2 removal became higher if NOx is absent. They claimed that NOx had a negative effect on SO2 removal at a high level of NOx removal. The discrepancy of the extent in the effect of NO on SO2 absorption among researchers may be caused by the difference in the type of absorbent and the reaction conditions. Particularly, it should be noted that the moisture content of the absorbent was substantially different. Livengood conducted his bench-scale study with a spray dryer. He reported that increasing temperature at the spray-dryer outlet produced a dry and porous powder with increased surface area, suggesting a considerably larger amount of water was removed compared with the normal conditions for a spray dry FGD process. Chu and Rochelle (1989) conducted their laboratory study with a packed bed reactor where the absorbent dried overnight at 65 °C in air was mounted.
These conditions are comparable with a normal spray dry FGD process. Concerning the effect of SO2 on NO removal, on the other hand, many researchers reported a similar observation. Livengood (1991) suggested the formation of the compounds containing nitrogen and sulfur based upon the strong dependence of NO removal on SO2 concentration. As shown in Figure 2 for the effects of SO2 on NO removal, the maximum NO removal appeared at a SO2 concentration of 1500 ppm when reacted for 15 h, while the maxima appeared at a SO2 concentration of 2000 ppm when reacted for 5 and 10 h. As we reported previously (Tsuchiai et al., 1995), NOx compounds are replaced by SO2 compounds when the calcium utilization of the absorbent reaches a high level. At a long reaction time of 15 h under a high SO2 concentration above 2000 ppm, the absorbent achieved a high calcium utilization, and NOx compounds became replaced by SO2 compounds. Therefore, the shift of the maximum NO removal to a lower SO2 concentration for a reaction time of 15 h is suggested to be due to the substitution of the adsorbed NO for SO2 to a considerable extent. The effects of water vapor are remarkable on the absorption of both SO2 and NO. The effects were markedly observed for the reaction at 70 °C. Sudden decreases in SO2 and NO absorption were observed at a water vapor pressure of 0.06 atm. This point coincides with the point in the adsorption isotherm where monolayer adsorption completes on the absorbent surface. Above this water vapor pressure, NO absorption became small and negligible at a higher water vapor pressure. It is suggested that, above a water vapor pressure of 0.06 atm at 70 °C, the surface of the absorbent is completely covered with water molecules and leaves no space for NO adsorption which requires for SO2 absorption. In a spray dry FGD process, NO is not removed when the reaction is carried out at a low temperature. In the low temperature spray dry FGD process, it is plausible that the absorbent surface is completely covered with water molecules and therefore shows no absorption of NO. One of the characteristic features of the absorbent prepared from coal fly ash is a simultaneous absorption of SO2 and NO. This feature reveals only when the reaction is carried out under such conditions that the surface is not completely covered with water molecules. The conditions of 130 °C and 10% water vapor (0.1 atm) adopted in the commercial plant are within the above conditions. The SO2 absorption once decreased at the critical conditions, but increased gradually as the number of water layers on the surface increased. This behavior is similar to that observed for a spray dry FGD process carried out at about 60-70 °C in which SO2 absorption increases linearly with the relative humidity as reported by Jozewicz and Rochelle (1986). In addition, the main product obtained at a high water vapor pressure at 70 °C in the present study was calcium sulfite. The same product is obtained in the spray dry FGD process carried out at a low temperature. Therefore, it is suggested that the reaction mechanisms change depending on the surface coverage of the water molecules on the absorbent. Above the monolayer adsorption of water molecules on the surface, SO2 molecules dissolve into water and then react with calcium hydroxide to form calcium sulfite as a final product, which are proposed for the
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reaction in a spray dry FGD process at a low temperature. Below the monolayer adsorption of water on the surface, SO2 forms calcium sulfate by the catalytic action of NO adsorbed on the surface. NO is also absorbed in the absorbent, though the mechanisms for NO absorption are not certain at present. Nomenclature BET method ) Brunauer-Emmett-Teller method FGD ) flue gas desulfurization ICDD ) International Centre for Diffraction Data
Literature Cited Brown, B.; Felsvang, K. High SO2 Removal Dry FGD Systems. Proceeding. The 1991 SO2 Control Symposium, Washington, DC, December 1991; Electric Power Research Institute: Palo Alto, CA. Chu, P; Rochelle, G. T. Removal of SO2 and NOx from Stack Gas by Reaction with Calcium Hydroxide Solids. JAPCA 1989, 39, 175-179. Jarvis, J. B.; Nassos, P. A.; Stewart, D. A. A Study of SulfurNitrogen Compounds in Wet Lime/Limestone FGD Systems. Proceeding. EPA/EPRI Symposium on Flue Gas Desulfurization, Cincinnati, OH, June 1985; Electric Power Research Institute: Palo Alto, CA. Jozewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5, 219-223. Klingspor, J.; Stroemberg, A.; Karlsson, H. T.; Bjerle, I. Similarities between Lime and Limestone Wet-Dry Scrubbing. Chem. Eng. Process. 1984, 18, 239-247. 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. Proceeding. The 1993 SO2 Control Symposium, Boston, MA, August 1993; Electric Power Research Institute: Palo Alto, CA.
Livengood, C. D. Combined NOx/SO2 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 NOx from Simulated Flue Gases by alkalized Alumina in a Radial Flow Fixed Bed. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 528-536. Moore, W. J. Physical Chemistry; Longman: London, 1962. Oblath, S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. Kinetics of the Formation of Hydroxylamine Disulfonate by the Reaction of Nitrite with Sulfites. J. Phys. Chem. 1981, 85, 1017-1021. Shen, C. H.; Rochelle, G. T. NO2 Absorption in Limestone Slurry for Flue Gas Desulfurization. Proceeding. The 1995 SO2 Control Symposium. Miami, FL, March 1995; Electric Power Research Institute: Palo Alto, CA. 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. Ueno, T. Process for Preparing Desulfurizing and Denitrating Agents. U. S. Patent No.4629721, Dec 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. Van Houte, G.; Delmon, B. Kinetics of Reaction of CaCO3 with SO2 and O2 below 650 °C. J. Chem. Soc., Faraday Trans. 1. 1979, 75, 1593-1605.
Received for review May 31, 1995 Accepted November 14, 1995X IE950322P
X Abstract published in Advance ACS Abstracts, February 1, 1996.
