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Ind. Eng. Chem. Res. 2003, 42, 6046-6052
Recovery of CaO by Reductive Decomposition of Spent Gypsum in a CO-CO2-N2 Atmosphere Satoshi Okumura, Naoto Mihara, Ken Kamiya, Shoji Ozawa, Maurice S. Onyango, Yoshihiro Kojima, and Hitoki Matsuda* Research Center for Advanced Waste and Emission Management, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Kyaw Kyaw, Yoshimi Goto, and Tetsushi Iwashita Yabashi Industries Company, Ltd., Akasaka-cho, Ogaki 503-2213, Japan
Reductive decomposition of spent CaSO4 was studied using a packed-bed reactor to regenerate an alternative CaO sorbent. The reactor was operated at various process conditions including an increasing CO concentration, CO/CO2 concentration ratio (0.067-1), and temperature (11231273 K). In all cases, N2 was used as a balancing gas. The regeneration of CaO from CaSO4 was found to be most effective in the CO-CO2-N2 atmosphere and strongly depended on the CO/ CO2 concentration ratio. At 1273 K, an apparent conversion value of 0.91 for the decomposition of CaSO4 to CaO was obtained in a 2 vol % CO and 30 vol % CO2 atmosphere. On the other hand, in a CO-N2 atmosphere, CaS was predominantly produced. The SO2 absorption capacity of CaO regenerated from CaSO4 was higher than that of limestone-calcined CaO. A larger pore diameter of the regenerated CaO was considered to be responsible for the higher SO2 absorptivity. Introduction In 2000, a large amount of waste gypsum (3.26 × tons), including sulfated absorbent (2.28 × 106 tons) and waste gypsum boards (0.98 × 106 tons), were discarded in Japan. The majority of such waste has been landfilled, thereby giving rise to a serious environmental problem due to the emission of harmful gas such as H2S from waste gypsum from landfill sites. Two methods being investigated for the remediation of waste gypsum are material recycling and chemical recycling. Among the approaches for the former are the conditioning of alkaline soil1 and the solidification of hardened gypsum body.2 Among the approaches for the latter are the reductive decomposition of CaSO4 with CO and H23 and the generation of hydroapatite from waste gypsum.4 To maintain a sustainable environment for society based on recycling of Ca-based wastes, it is preferable to reuse waste gypsum as a substitute for lime in a variety of industrial applications. Investigations on the reductive decomposition of CaSO4 with H2, C, and CO have been undertaken to generate SO2 for the production of sulfuric acid and to generate lime from the sulfated absorbent.3,5-8 Wheelock et al. reported that the ratio of unfavorable product CaS to CaO depended on the reaction temperature as well as CO and CO2 concentrations.6 For the reductive decomposition of CaSO4 in a CO-CO2 atmosphere, Wheelock and Boylan, Gruncharov et al., and Kuusik et al. found that the proportion of CaO in the products increased as the CO/CO2 concentration ratio decreased.6-9 Oh and Wheelock also reported from a thermogravimetric analysis at 1423 K that CaSO4 was completely converted to CaO at low reducing potentials (PCO/PCO2 e 0.10), when CaSO4 was subjected to a N2-balanced 106
* To whom correspondence should be addressed. Tel.: +8152-789-3382. Fax: +82-52-789-5619. E-mail: matsuda@ nuce.nagoya-u.ac.jp.
1-2 vol % CO, 0-20 vol % CO2, and 5-10 vol % SO2 gaseous mixture.10 Most of the studies on CaSO4 decomposition with CO have been carried out at temperatures higher than 1273 K. On the other hand, according to the thermodynamic calculations, the equilibrium temperatures for the reductive decomposition of CaSO4 with H2, C, and CO are 1163, 1108, and 1161 K, respectively. However, it seems that the knowledge on reductive decomposition of CaSO4 at lower temperature is limited. In addition, most researches have been conducted by means of thermogravimetric analysis, which offers insufficient data to investigate the decomposition behavior of CaSO4 to CaO. This is because CaO and CaS generated by CaSO4 decomposition via eqs 1 and 2 are not distinguishable by weight change.
CaSO4 + CO f CaO + SO2 + CO2
(1)
CaSO4 + 4CO f CaS + 4CO2
(2)
In the present work, an attempt is made to characterize the decomposition behavior of CaSO4 particles using a laboratory-scale packed-bed reactor under varying process conditions including CO concentration, reaction temperature, and CO/CO2 concentration ratio. The degree of regeneration of CaO from CaSO4 is determined by the concentration profile of SO2. An optimum condition for the regeneration of CaO from CaSO4 is discussed in terms of CaO yield. The results are then compared with those obtained by Oh and Wheelock.10 Further, the reactivity of CaO thus regenerated from the spent CaSO4 for the SO2 capture is evaluated. Finally, the SO2 absorption characteristic of the regenerated CaO is compared with that of lime prepared by the calcination of limestone. Reaction Equilibrium of CaSO4 Decomposition. The decomposition of CaSO4 to CaO in a CO-CO2 atmosphere is considered to take place, producing CaS
10.1021/ie0302645 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003
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Figure 2. Schematic diagram of the experimental apparatus.
Figure 1. Equilibrium diagram for the decomposition of CaSO4 with CO.
as the main side product.3,5-12 In addition to eqs 1 and 2, the following reactions contribute to the reductive decomposition of CaSO4 with CO.
CaSO4 f CaO + SO3
(3)
CaS + 3SO3 f CaO + 4SO2
(4)
CaSO4 f CaO + SO2 + 1/2O2
(5)
CaSO4 + 1/3CaS f 4/3CaO + 4/3SO2
(6)
CaSO4 + CO f CaSO3 + CO2
(7)
CaSO3 f CaO + SO2
(8)
Figure 1 illustrates the reaction equilibrium calculated by the use of a commercial thermodynamic calculation software (HSC Chemistry, Outokumpu Research Oy Information Service), for the reactions expressed by eqs 1-8. From the calculations, eqs 3 and 5 are thermodynamically unfavorable for CaSO4 decomposition. Chen and Yang reported that the regeneration of CaO from CaSO4 proceeds in two consecutive steps expressed by eqs 2 and 6.11 The reaction between CaSO4 and CaS expressed by eq 6 was found to be the ratecontrolling step at relatively low temperatures. They went further to suggest that the reaction between CaSO4 and CaS proceeded in a two-step mechanism by consecutive SO3 formation (eq 3) and the subsequent SO3 disappearance (eq 4).11 However, Oh and Wheelock suggested a two-step mechanism for eq 1 as given by eqs 7 and 8, assuming the presence of an intermediate, CaSO3.10 Experimental Section Sample. A sample for CaO regeneration was prepared by grinding commercial reagent-grade gypsum (Wako Chemicals, Japan) to a mean particle diameter of 45-63 µm. For comparison in terms of SO2 absorption capacity, another CaO sample was prepared by calcination of limestone (purity 99.5 wt %; Oogaki, Japan). Experimental Setup and Procedure. Figure 2 shows the schematic diagram of the experimental apparatus employed in the present study. A 1 × 10-3 kg gypsum sample was packed in the inner tube (17 mm i.d. and 200 mm height) of the double-tube quartz
reactor by using quartz wool to ensure a uniform distribution of gypsum particles in the reactor. The temperature of the reactor was fixed at a prescribed reaction temperature (1123-1273 K) by a temperatureadjustable electric furnace. The decomposition of gypsum was started by supplying a gaseous mixture of CO, CO2, and N2 to the reactor at a fixed temperature. The concentrations of the gases were varied in the ranges 2-10 vol % CO, 0-30 vol % CO2, and N2 balance at a total gas flow rate of 3 × 10-4 m3/min (at room temperature). Prior to the experiment, it was confirmed that mass-transfer resistance caused by the gas film around the particles was negligible at a gas flow rate higher than 1 × 10-4 m3/min. The concentration of the emitted SO2 from the reductive decomposition of CaSO4 was measured continuously at the outlet of the reactor with a Fourier transform infrared spectroscopy (FTIR; Shimadzu FT-IR8600). Characteristics of CaSO4 Decomposition. Besides SO2, sulfur-derived gases such as COS, CS2, and S2 are considered to be produced to some extent during CaSO4 decomposition, depending on the reaction conditions. The quantitative analysis of these products was rather difficult by our experimental facilities. Therefore, on the premise that the sulfur element was all converted to SO2 and on the basis of the SO2 concentration profile, we determined an apparent conversion of CaSO4 to CaO, X, using eqs 9 and 10.
X)
∫NSO
2
dt/NCaSO4
NSO2 ) f(t,v,CSO2,T)
(9) (10)
where t is the reaction time, v is the space velocity, CSO2 is the SO2 concentration at the outlet of the reactor, T is the reaction temperature, NSO2 is the amount of SO2 produced, and NCaSO4 is the amount of CaSO4 sample initially placed in the reactor. Upon completion of the experiment, the reacted solid sample was quickly collected and kept in a desiccator to prevent CaO and CaS contained in the sample from reacting with moisture in the air. The reacted solid sample was then subjected to X-ray diffraction (XRD) analysis (Rigaku Rint-2500TTR) to identify the crystalline compounds produced. Further, the Ca, O, and S content of the sample was analyzed by an energydispersive X-ray fluorescence spectrometer (EDS; Nihon Denshi JED-2140). The appearance of the external surface of the CaO produced was observed by a scanning electron microscope (SEM; Nihon Denshi JSM-6330). A mercury porosimeter (Shimadzu Micrometrics Auto Porosimeter AutoporeIII) was employed for the measurement of the pore volume and pore diameter of the product CaO.
6048 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Figure 3. Influence of the reaction temperature on the decomposition of CaSO4 with CO. Figure 5. Effect of the reaction temperature on the decomposition of CaSO4.
Figure 4. Concentration profile of SO2 during the decomposition of CaSO4.
Results and Discussion Thermogravimetric Analysis. As a first step in the present study, the decomposition of CaSO4 was carried out by means of a thermogravimetric analyzer. As seen in Figure 3, the final value of the fractional weight loss, ∆w, was in the range of 0.5-0.6 suggesting a coexistence of CaO (∆w ) 0.588) and CaS (∆w ) 0.471) in the reaction product. The rate of weight loss was proportional to the heating temperature. Compared with the data obtained by Gruncharov et al.8 for the decomposition of phosphogypsum at 1273 and 1373 K in an Arbalanced gaseous mixture of 4 vol % CO and 5 vol % CO2, our rate of weight loss during CaSO4 decomposition in a 2 vol % CO and N2 balance atmosphere is faster. On the other hand, the fractional weight loss, ∆w, obtained by Gruncharov et al.8 approaches a higher value of 0.588 when CO2 is present. This means that the CaO yield increases in the presence of the coexisting gases, CO and CO2. This result is consistent with the presumption that the decomposition of CaSO4 may proceed by complex reactions (eqs 1 and 2), in which the selectivity of CaO is affected by the molar ratio of CO to CO2. However, it is difficult to understand the reaction mechanism of CaSO4 decomposition by thermogravimetric analysis only because the amount of reaction products, CaO and CaS, cannot be determined separately by the weight changes. Profile of the SO2 Concentration. Figure 4 shows a typical concentration profile of SO2 obtained when using a gaseous mixture of 2-10 vol % CO, 0-30 vol % CO2, and N2 balance at 1123-1273 K. It is seen from the figure that the SO2 concentration profile is largely dependent on the reaction temperature and CO and CO2
concentrations. At 1123 K, the SO2 concentration is kept low. It is considered that the main reaction product is CaS at such a low temperature. The reaction given by eq 2 is thermodynamically favorable in a lower temperature range (see Figure 1). In the absence of CO2, the maximum SO2 concentration increased as the temperature and CO concentration increased. The amount of SO2 emission also increased with an increase in temperature. On the other hand, when CO2 is added to the CO-N2 gas mixture, the amount of SO2 emission becomes increasingly large. This suggests that a higher CO2 concentration may depress the reaction expressed by eq 2 than that expressed by eq 1 from the viewpoint of stoichiometry because the CO2 concentration has more influence on eq 2. Consequently, the reaction expressed by eq 1 proceeds more favorably to form CaO and SO2 than that expressed by eq 2. In the present work, the CaSO4 decomposition behavior was evaluated, for convenience, by using an apparent conversion value determined by eqs 9 and 10, on the assumption that CaSO4 decomposition might take place via the reaction path expressed by eq 1. Reductive Decomposition of CaSO4 in a CO-N2 Atmosphere Effect of the Reaction Temperature. Figure 5 shows a typical example of a conversion profile of CaSO4 conducted in a CO (2 vol %)-N2 atmosphere at 1123, 1173, 1223, and 1273 K. As seen in Figure 5, the apparent conversion increases with an increase in temperature, but the decomposition of CaSO4 seems to practically stop at around X ) 0.3 under the employed conditions. It appears that the CaSO4 decomposition does not proceed below 1123 K, as was shown in Figure 3 also. On the other hand, the decomposition behavior changes drastically at 1173 K. The reaction rate increases with an increase in temperature, but the conversion-time curve approaches an asymptote of X ) 0.3 in 100 min in the temperature range 1173-1273 K. Wheelock and Boylan and Gruncharov et al. reported that there existed an induction period for the decomposition of CaSO4 with CO and that the induction period decreased with an increase in the reaction temperature.6,8 The trend in the CaSO4 decomposition behavior obtained in the present study is in conformity with those obtained by Wheelock and Boylan6 and Gruncharov et al.8 Reaction Products and Sulfur Content. The decomposition products of CaSO4 were identified by
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 6049
Figure 6. Effect of the concentration of CO on the decomposition of CaSO4 at 1273 K.
Figure 7. Effect of the concentration of CO2 on the decomposition of CaSO4.
means of XRD. The content of sulfur that remained in the reaction products was measured by an EDS. In the 2 vol % CO-N2 atmosphere, we observed the XRD characteristic peaks of the unreacted CaSO4 and CaS at 1123 K. However, when the temperature was raised to 1273 K, the presence of CaO was confirmed in the products, besides CaSO4 and CaS. The conversions calculated on the basis of the sulfur content determined by EDS were Xf ) 0.25 and 0.67 at 1123 and 1273 K, respectively. There still remained a large amount of sulfur within the reaction products in the form of unreacted CaSO4 and CaS, when CO2 was not present. Low apparent conversion values (Xf ) 0.08 at 1123 K and Xf ) 0.30 at 1273 K) determined by the SO2 concentration profile in Figure 5 may support this finding. The difference between the EDS results and the apparent conversion value from this experimental run may be due to the following reasons: (1) the existence of an undetected sulfur element, which remained deeper inside the products; (2) the formation of other sulfurderived gases such as COS, CS2, and sulfur vapor. Sulfur-derived gases such as COS, CS2, and S2 may be formed by the following reactions.
a lower asymptote when the CO concentration is increased. This finding suggests that CaSO4 decomposition may proceed via complex reactions expressed by eqs 1 and 2. When the CO concentration is increased, the yield of CaO decreases because the formation of CaS expressed by eq 2 becomes predominant. A similar result of the CaSO4 decomposition behavior at 1373 K conducted by thermogravimetric analysis was obtained by Kuusik et al. in a 2-10 vol % CO-N2 atmosphere.9 Effect of the CO/CO2 Concentration Ratio. Figure 7 demonstrates the conversion-time curve of CaSO4 decomposition at 1273 K in a CO-CO2-N2 atmosphere. In this experiment, the CO2 concentration was varied from 10 to 30 vol % at a constant CO (2 vol %) concentration, to investigate the effect of the CO2 concentration on CaSO4 decomposition. It is seen from the figure that the initial reaction rate decreased as the CO2 concentration was increased. A similar trend was reported by Gruncharov et al.8 for the decomposition of phosphogypsum at 1273-1373 K in an Ar-balanced CO (4 vol %)-CO2 (5-20 vol %) atmosphere. As the concentration of CO2 increased, the apparent conversion at a pseudoequilibrium state increased from Xf ) 0.52 (10 vol % CO2) to Xf ) 0.68 (20 vol % CO2) and Xf ) 0.91 (30 vol % CO2). Kuusik et al. mentioned that an increase in the reaction temperature and the addition of either CO2 or O2 gas into the reaction system would promote the decomposition of CaSO4 to CaO.9 Wheelock and Morris proposed an alternative approach to minimize the conversion of CaSO4 to CaS by a cyclic process of reduction and oxidation.7 From our XRD analysis of the reaction product, at 1273 K in a N2-balanced CO (2 vol %)-CO2 (30 vol %) atmosphere, we did not detect any characteristic XRD peak of CaSO4 or CaS except CaO. The sulfur content of the products obtained in this condition, determined by EDS, was about 3 mol % and was equivalent to the conversion of Xf ) 0.97, which is about 6% higher than that determined by the SO2 concentration profile in Figure 7. This suggests that sulfur-derived gases may be formed via eqs 11-13, as mentioned in the previous section. The apparent conversion values obtained at a pseudoequilibrium state for various combinations of CO and CO2 concentrations at 1273 K are summarized in Figure 8. It is clear that the decomposition of CaSO4 to CaO is dependent on both the CO and CO2 concentrations. It is noticed that the combination of a low CO concentration and a high CO2 concentration favors the decomposition of CaSO4. Oh and Wheelock found at 1423 K that the final product distribution depended on the reducing
SO2 + 2CO f 1/2S2 + 2CO2
(11)
2SO2 + 6CO f CS2 + 5CO2
(12)
SO2 + 3CO f COS + 2CO2
(13)
In fact, a slight amount of CS2 was detected by FTIR when the CO concentration was increased, but the quantitative measurement of CS2 was not possible in the present study. Further, it was confirmed that the reaction tube changed to a light yellow color after several runs of the experiment, which may suggest the formation of S2 (sulfur vapor). However, at 1273 K we did not detect COS under the experimental condition when the ratio of CO to CO2 was less than 0.1. Effect of the CO Concentration. A typical conversion profile of CaSO4 decomposition at 1273 K in a CON2 atmosphere is shown in Figure 6. It is seen from this figure that the initial decomposition reaction rate of CaSO4 increases with an increase in the CO concentration. A short induction period is observed for 2 vol % CO concentration. However, no appreciable induction period is observed when the CO concentration is increased to 5 vol %. On the other hand, the conversiontime curve at a pseudoequilibrium condition approaches
6050 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Figure 8. Effect of the concentration of CO and CO2 on the final conversion of CaSO4 to CaO at 1273 K.
Figure 9. Effect of the reducing potential, PCO/PCO2, on the final conversion of CaSO4 to CaO.
Figure 10. Time-change of conversion of CaSO4 decomposition: (a) CaO calcined from limestone (CaCO3); (b) CaO regenerated after the first run; (c) CaO regenerated after the fourth run of the SO2 absorption/CaSO4 decomposition cycle.
potential (PCO/PCO2) of the gases when CaSO4 was subjected to a CO-CO2-SO2-N2 atmosphere.10 In Figure 9, the apparent conversion at a pseudoequilibrium state, Xf, and the reducing potential, PCO/PCO2, were plotted in accordance with the approach used by Oh and Wheelock.10 The data obtained by Oh and Wheelock10 are also shown in the figure. It is observed that the conversion, Xf, increases when the reducing potential, PCO/PCO2, decreases. At a low reducing potential (PCO/PCO2 e 0.10), CaSO4 was almost completely regenerated to CaO. There are, however, some differences in Xf values between those obtained by Oh and Wheelock10 and our results, in particular when the reducing potential, PCO/PCO2, is larger than 0.2. The difference in Xf may be attributed to the different reaction conditions used. In their case, 1-10 vol % SO2
Figure 11. SEM images of CaO: (a) the surface of CaO calcined from limestone; (b and c) the surface of CaO regenerated after the first run and the fourth run of the SO2 absorption/CaSO4 decomposition cycle, respectively.
was added to the CO-CO2-N2 atmosphere. From the viewpoint of the reaction equilibrium, the presence of SO2 is unfavorable to the decomposition of CaSO4, regardless of the high reaction temperature of 1423 K. Reactivity of CaSO4-Regenerated CaO SO2 Absorption Capacity. The CaSO4-regenerated CaO was subjected to repeated SO2 absorption/CaSO4 decomposition cycles (CaSO4 T CaO). Prior to this, the CaO sample was prepared in a N2 atmosphere by calcining limestone at 1123 K for 1 h. In the SO2 absorption experiment, CaO particles were contacted with 0.35 vol % SO2, 5 vol % O2, and N2 balance at 1223 K. The SO2 absorption performance of regenerated CaO was evaluated from the conversion of CaO to CaSO4,
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 6051
cycles of SO2 absorption/CaSO4 decomposition, the average pore diameter decreased in size to 165 nm, which might be attributed to the agglomeration of CaO grains due to the repetition of reaction cycles. However, the pore volume of these CaO’s regenerated from CaSO4 does not change appreciably. It is reported that SO2 absorption capacity of CaO depends on the pore diameter rather than on the internal surface area.12 The higher SO2 absorption capacity of the regenerated CaO obtained in this work is consistent with the results reported by Dogu.12 Conclusions Figure 12. Pore-volume distribution curves of CaO: (a) CaO calcined from limestone (CaCO3); (b) CaO regenerated after the first run; (c) CaO regenerated after the fourth run of the SO2 absorption/CaSO4 decomposition cycle.
Xs, on the basis of the SO2 concentration profile at the outlet of the reactor. For the regeneration of CaO from CaSO4 at 1273 K, the reaction condition was changed to 2 vol % CO, 30 vol % CO2, and N2 balance. This operation was conducted until SO2 gas was not detected by FTIR at the outlet of the reactor. A typical example of the conversion curve for SO2 absorption by CaO is shown in Figure 10. As seen in the figure, the initial absorption rate of SO2 for each run is almost the same up to 30 min from the initiation of the reaction. The conversion curve of SO2 absorption by limestone-calcined CaO approaches an asymptote at Xs ) 0.68 in 6 h. On the other hand, for the first run of SO2 absorption by the CaSO4-regenerated CaO previously produced by reacting SO2 with calcined lime, the highest conversion, Xs ) 0.93, was obtained. With an increase in SO2 absorption/CaO regeneration cycles up to the fourth cycle, a small drop in the conversion was observed. However, the conversion obtained at the fourth cycle was still higher than that obtained using limestone-calcined CaO. Structural Change of CaSO4-Regenerated CaO. Figure 11 illustrates SEM photos of the external surface of CaO samples. Figure 11a shows a SEM image of the surface of a virgin CaO calcined from limestone (CaCO3). Figure 11b demonstrates the appearance of the surface of CaO regenerated from CaSO4 in the first run of the SO2 absorption/CaSO4 decomposition cycle. In Figure 11b, it is observed that CaO crystal grains of less than 1 µm in size are agglomerated, forming a threedimensional porous structure. The appearance of the surface of CaO regenerated from CaSO4 in the fourth run of the SO2 absorption/CaSO4 decomposition cycle (Figure 11c) is basically similar to that shown in Figure 11b. However, the size of the crystal grain of CaO became a little larger in a compact agglomerated state. It is considered that small CaO grains underwent sintering with the repetition of SO2 absorption/CaSO4 decomposition cycles. Figure 12 illustrates the pore-volume distribution curves of CaO calcined from limestone (part a), CaO regenerated from CaSO4 after the first run (part b) and the fourth run (part c) of the SO2 absorption/CaSO4 decomposition cycle. The average pore diameter of the CaO calcined from limestone is about 29 nm. On the other hand, the CaO regenerated from CaSO4 after the first run of SO2 absorption/CaSO4 regeneration gives a larger average pore diameter of 201 nm. After four
The decomposition behavior of CaSO4 was studied in a CO-N2 atmosphere and a CO-CO2-N2 atmosphere for a CO/CO2 concentration ratio of 0.067-1, in the temperature range of 1123-1273 K. It was found that unfavorable product, CaS, was predominantly produced, instead of CaO when CO2 was not present, even at a higher temperature of 1273 K. On the other hand, the regeneration of CaO from CaSO4 proceeded more effectively in a CO-CO2-N2 atmosphere and strongly depended on the CO/CO2 concentration ratio. At 1273 K, an apparent conversion value of 0.91 for the decomposition of CaSO4 to CaO was obtained in a 2 vol % CO and 30 vol % CO2 atmosphere. The SO2 absorption capacity of CaSO4-regenerated CaO was higher than that of limestone-calcined CaO. A larger pore diameter of the regenerated CaO, compared with CaO calcined from limestone, was considered to be responsible for a higher SO2 absorption capacity. Nomenclature K ) equilibrium constant PCO/PCO2 ) reducing potential t ) reaction time, min T ) reaction temperature, K v ) space velocity, s-1 ∆w ) [(winitial - wfinal)/winitial] ) weight loss ratio of samples X ) apparent conversion of CaSO4 to CaO Xf ) apparent conversion of CaSO4 to CaO at a pseudoequilibrium state Xs ) conversion of CaO to CaSO4 Vp ) total pore volume of samples, mL g-1
Literature Cited (1) Suganuma, H. Alkali Soil Amendment by Desulfurization Gypsum and Crop Production in China. Ryusan to Kogyo 1999, Nov., 141. (2) Kojima, Y.; Yasue, T.; Arai, Y. Preparation of Hardened Gypsum Body from Waste Gypsum Board. Inorg. Mater. 1997, 4, 415. (3) Diaz-Bossio, L. M.; Squier, S. E.; Pulsifer, A. H. Reductive Decomposition of Calcium Sulfate Utilizing Carbon Monoxide and Hydrogen. Chem. Eng. Sci. 1985, 40 (3), 319. (4) Huruta, S.; Katsuki, H. Settukouhaizai wo Riyoushita Suisan Apataitotakoutai no Gousei. Phosphorus Lett. 1999, 36, 4. (5) Hull, W. Q.; Schon, F.; Zirngibl, H. Sulfuric Acid from Anhydrite. Ind. Eng. Chem. 1957, 49 (8), 1204. (6) Wheelock, T. D.; Boylan, D. R. Reductive Decomposition of Gypsum by Carbon Monoxide. Ind. Eng. Chem. 1960, 52, 215. (7) Wheelock, T. D.; Morris, C. E. Recovery of Sulfur Dioxide and Lime from Waste Gypsum. TIZ 1986, 1, 37. (8) Gruncharov, Iv.; Kirilov, Pl.; Pelovski, Y.; Dambalov, Iv. Isothermal Gravimetrical Kinetic Study of the Decomposition of Phosphogypsum under CO-CO2-Ar Atmosphere. Thermochim. Acta 1985, 92, 173.
6052 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 (9) Kuusik, R.; Saikkonen, P.; Niinisto, L. Thermal Decomposition of Calcium Sulfate in Carbon Monoxide. J. Therm. Anal. 1985, 30, 187. (10) Oh, J. S.; Wheelock, T. D. Reductive Decomposition of Calcium Sulfate with Carbon Monoxide: Reaction Mechanism. Ind. Eng. Chem. Res. 1990, 29, 544. (11) Chen, J. M.; Yang, R. T. Fluidized-Bed Combustion of Coal with Lime Additives. Kinetics and Mechanism of Regeneration of the Lime Sorbent. Ind. Eng. Chem. Fundam. 1979, 18, 134.
(12) Dogu, T. The Importance of Pore Structure and Diffusion in the Kinetics of Gas-Solid Non-catalytic Reactions: Reaction of Calcined Limestone with SO2. Chem. Eng. J. 1981, 21, 213.
Resubmitted for review July 14, 2003 Revised manuscript received September 29, 2003 Accepted October 1, 2003 IE0302645
