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Energy & Fuels 2007, 21, 3264–3269
Investigation on the Application of C12A7 in Flue Gas Desulfurization at Low–Moderate Temperature Qun Chen,*,†,‡ Kazumasa Yoshida,† Hidetoshi Yamamoto,† Masaki Uchida,† and Masayoshi Sadakata† Department of EnVironmental Chemical Engineering, Kogakuin UniVersity, Nakano-machi, Hachioji, Tokyo 192-0015, Japan, and Shougang Research Institute of Technology, Beijing 100041, China ReceiVed May 25, 2007. ReVised Manuscript ReceiVed August 10, 2007
It has been demonstrated that radicals and/or anions can promote chain reactions improving desulfurization efficiency in flue gas. Recent studies have evidently shown that specially synthesized C12A7 is a prominent source of O- which has been found to be one of the most active radicals acting as a potential oxidant in many applications. In the interest of applying C12A7 in flue gas desulfurization for coal combustion facilities, reactions of C12A7 with SO2 at 200–700 °C were implemented in a plug flow reactor. Both SO2 removal efficiency and the apparent reaction rate constants derived from the experiments were compared with those of CaO. The products were analyzed by TG-MS and FTIR. The result showed that under the experimental conditions the reactivity of C12A7 directly reacted with SO2 was lower than CaO samples because of its low surface area accessibility. The product analysis verified that O- in C12A7 was involved in desulfurization reaction as an effective oxidant. On the basis of these results, a feasible application approach for C12A7 in desulfurization process was suggested so as to improve SO2 removal efficiency. A preliminary experiment was further devised to support the feasibility.
Introduction SOx emission from coal combustion facilities can cause severe environmental pollution by the formation of acid rain or the combination with fine particles, leading not only to environment degradation but also to economic damage and human health hazard as well. Among all the state-of-the-art technologies, wet flue gas desulfurization (FGD) is now the most widely commercially applied process. However, compared with dry desulfurization, wet FGD requires relatively high capitals and operation cost and considerably large amount of water consumption in particular.1 This deficiency makes the process inapplicable wherever the water reserves are scant, especially when the water resource scarcity is becoming one of increasingly critical issues worldwide. In such a case, the dry desulfurization would be the favorite alternative. The main drawback of the dry desulfurization is its relatively low removal efficiency compared with the wet FGD. The developments of both highly efficient sorbents2–7 and removal processes are the main continuous research efforts. For instance, * To whom correspondence should be addressed: e-mail chenqun99@ mails.tsinghua.edu.cn; Tel 81-042-628-4531; Fax 81-042-628-4531. † Kogakuin University. ‡ Shougang Research Institute of Technology. (1) Toole-O’Neil, B. Dry Scrubbing Technologies for Flue Gas Desulfurization; Kluwer Academic Publishers: Dordrecht, 1998. (2) Svoboda, K.; Lin, W.; Hannes, J.; Korbee, R.; Bleek, C. M. Lowtemperature flue gas desulfurization by Alumina-CaO regenerable sorbents. Fuel 1994, 73, 1144–1149. (3) Li, Y.; Nishioka, M.; Sadakata, M. High calcium utilization and gypsum formation for dry desulfurization process. Energy Fuels 1999, 13, 1015–1020. (4) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Chain reaction mechanism by NOx in SO2 removal process. Energy Fuels 2002, 16, 155–160. (5) Matsushima, N.; Li, Y.; Nishioka, M.; Sadakata, M.; Qi, H.; Xu, X. Novel dry-desulfurization process using Ca(OH)2/fly ash sorbent in a circulating fluidized bed. EnViron. Sci. Technol. 2004, 38, 6867–6874.
Svoboda et al.2 developed a kind of alumina–CaO regenerable sorbent through the sol–gel approach. Li et al.3,4 developed a sorbent of Ca(OH)2/fly ash which had high calcium utilization in the dry desulfurization process. The sorbent was later used in a developed circulating fluidized bed (CFB) dry desulfurization process,5 aiming to achieve high SO2 removal efficiency and main production of CaSO4. Renedo and Fernández8 even extended this kind of fly ash/Ca(OH)2 sorbents to dry FGD at temperatures as low as 57 °C. Still, the effort to develop new sorbents for dry desulfurization remains, for further improvement in removal efficiency and development of relatively lowtemperature dry FGD process are of urgent necessity. It is well known that atomic oxygen radical anions have an important role in catalytic oxidation in various processes for their extremely high oxidative power and reactivity.9,10 Among these radical anions, the O- anion is one of the most active and useful radicals for potential applications such as chemical syntheses. Recently, a compound of C12A7 was found to be a
(6) Jozewicz, W.; Chang, J. C. S.; Sedman, C. B.; Brna, T. G. Silicaenhanced sorbents for dry injection removal of SO2 from flue gas. J. Air Pollut. Control Assoc. 1988, 38, 1027–1038. (7) Jozewicz, W.; Chang, J. C. S.; Sedman, C. B.; Brna, T. G. Characterization of advanced sorbents for dry SO2 control. React. Solids 1988, 6, 243–262. (8) Renedo, M. J.; Fernández, J. Preparation, characterization, and calcium utilization of fly ash/Ca(OH)2 sorbents for dry desulfurization at low temperature. Ind. Eng. Chem. Res. 2002, 41, 2412–2417. (9) Li, Q.; Hosono, H.; Hirano, M.; Hayashi, K.; Nishioka, M.; Kashiwagi, H.; Torimoto, Y.; Sadakata, M. High-intensity atomic oxygen radical anion emission mechanism from 12CaO · 7Al2O3 crystal surface. Surf. Sci. 2003, 527, 100–112. (10) DePuy, C. H.; Grabowski, J. J.; Bierbaum, V. M. Chemical reactions of anions in the gas phase. Science 1982, 218, 955–960.
10.1021/ef700268v CCC: $37.00 2007 American Chemical Society Published on Web 09/18/2007
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Figure 2. SO2 concentrations at the reactor outlet versus reaction time at different temperatures for 1.5 g C12A7: (—) 600, (*) 500, (0) 400, (∆) 300, and (9) 200 °C. Figure 1. Schematic diagram of plug flow reactor system for batch reaction of desulfurization.
good source of O- anions.9,11 The C12A7 crystal has a positive charged lattice framework [Ca24Al28O64]4+ possessing 12 crystallographic cages per unit cell, where the remaining two oxide ions, O2-, were trapped.9 It was found that the anions of Oand O2-, as much as more than 1020/cm3, were generated from the electron-transfer reaction between O2- and O2 during the C12A7 annealing process in dry oxygen stream and trapped in the place of O2- anion.12 Thus, C12A7 was found to have high emission capability of O-, giving it a great potential to be used in many applications. A recent study has shown that radicals and/or anions can promote chain reactions homogeneously in gas phase, which can improve desulfurization efficiency in flue gas.13 In order to investigate the possible application of C12A7 in dry desulfurization, especially in the low–moderate temperature section of flue gas, C12A7 desulfurization reactions in a plug flow reactor at 200–700 °C were implemented. For comparison, the desulfurization reactions with pure CaO at 200–500 °C in the same reactor system were also carried out. From the experiments, the apparent reaction rate constants of both sorbents were derived. The products were analyzed by both thermogravimetry–mass spectrometer (TG-MS) and Fourier transform infrared spectroscopy (FTIR) for the compositions and thus the reaction mechanism determination. Through discussions on the results, a possible application approach of using C12A7 in the desulfurization process was suggested, and an experiment was devised to verify the feasibility of this approach. Experimental Section A series of desulfurization reactions were carried out in the plug flow reactor system depicted schematically in Figure 1. The simulated flue gas was the mixture of pure nitrogen and SO2, in which the concentration of SO2 was 297 ppm. The stream of flue gas was fed into a quartz tube reactor whose inner diameter was about 22 mm at the flow rate of 1.0 L/min controlled by a flow meter calibrated by a film flow meter (SF-1100, STEC). Inside the tube reactor, a 1.5 g sorbent sample of C12A7 or CaO was (11) Li, Q.; Hayashi, K.; Nishioka, M.; Kashiwagi, H.; Hirano, M.; Torimoto, Y.; Hosono, H.; Sadakata, M. Reproducibility of O- negative ion emission from C12A7 crystal surface. Jpn. J. Appl. Phys. 2002, 41, L530–L532. (12) Hayashi, K.; Hirano, M.; Li, Q. X.; Nishioka, M.; Sadakata, M.; Torimoto, Y.; Matsuishi, S.; Hosono, H. Electric field emission of high density O- ions from 12CaO · 7Al2O3 engineered to incorporate oxygen radicals. Electrochem. Solid-State Lett. 2002, 5, J12–J16. (13) Song, Q.; Shibamori, Y.; Sadakata, M.; Koshi, M. Research on homogeneous oxidation of NO and SO2 in flue gas by chain reactions. Energy Fuels 2003, 1549–1553.
uniformly dispersed among a pile of 30 g quartz beads as a fixed reaction section. The length of the reaction section was about 60 mm. Therefore, the space velocity for the plug flow reactor was about 0.73 s-1 or 2631 h-1. The heater from outside heated the tube reactor. Reaction temperature was monitored by a thermocouple placed inside the reaction section. In the experiments, for the uneven temperature distribution along the tube axis of the heater, the reactions were controlled to take place at the temperature of 200 ( 1, 300 ( 1, 400 ( 3, 500 ( 4, 600 ( 4, and 700 ( 5 °C. For C12A7, the desulfurization reactions were implemented at 200–700 °C, while for CaO, the reactions were implemented at 200–500 °C. The concentration of SO2 in the flue gas after reactions was analyzed by Horiba PG-250 gas analyzer online at the outlet of the tube. The reproducibility study for the reactor system showed that the error limit was about 7%, mainly due to the beads–sorbent mixture condition and gas analyzer’s error. The C12A7 sample used in the experiments was made in Denki Kagaku Kogyo Kabushiki Kaisha, Japan. Powders of CaO and R-Al2O3 were mixed and ground at the molar ratio of 12:7. The mixture was pelletized and then sintered at 1320 °C for 6 h in the stream of dry oxygen. The sintered product was quenched to room temperature in a dry oxygen atmosphere and then milled back to the powder with particle size less than 10 µm. Electron spin resonance (ESR) spectra showed that the concentration of O- in the C12A7 sample was about 2 × 1020 cm-3. The CaO (Wako) sample used in the experiments was 99.9% in purity with particle size less than 40 µm. The specific surface areas of both C12A7 and CaO samples used in the experiments, 1.63 and 3.69 m2/g, respectively, were determined by single-point BET measurement on a Micromeritics Flowsorb II. The pore size distributions of both samples were also analyzed by mercury porosimetry on a Micromeritics AutoPore III 9420. The product of C12A7 reacted with SO2 was analyzed by TGMS (Mac Science TG-DTA 2000S and Thermo-Onix Prolab). Generally, some 16 mg product sample was heated at 10 K/min up to 1300 °C in a helium atmosphere. The gas composition was analyzed by the mass spectrometer. The information of both mass loss and gas mass spectra was simultaneously recorded. The compositions of CaO samples after reaction with SO2 were analyzed through FTIR (Thermo-Nicolet Nexus 670).
Results and Discussion Desulfurization Reaction. In order to investigate the reaction behaviors of C12A7 with SO2 especially at low–moderate temperature, the SO2 concentration in the outlet gas from the plug flow reactor was recorded versus reaction time from the series of desulfurization reactions at 200–700 °C, as illustrated in Figure 2. Note that the response time of the gas analyzer has been subtracted from the curves, the same hereinafter. As can be seen in Figure 2, the SO2 removal efficiencies by C12A7 powder at the specified temperatures dropped to less
3266 Energy & Fuels, Vol. 21, No. 6, 2007
Figure 3. SO2 concentrations at the reactor outlet versus reaction time at different temperatures for 1.5 g CaO: (*) 500, (0) 400, (∆) 300, and (9) 200 °C.
Figure 4. Comparison of the amounts of SO2 removal at different temperatures by C12A7 and CaO.
than 20% in nearly 10 min. For the desulfurization reaction at 200 °C, the removal efficiency was even lower. The breakthrough time of SO2 from the reactor inlet to the outlet was less than 30 s, and the removal efficiency dropped rapidly to less than 5% in 10 min at 200 °C. As the reaction temperature increased from 200 to 600 °C, the breakthrough time increased to about 2 min. The concentration curves versus reaction time were so close at the temperature of 500–700 °C that in Figure 2 the data of the reaction at 700 °C were not given. Compared with C12A7 sample under the same experimental condition, the breakthrough times of SO2 with CaO were much longer, as shown in Figure 3. Moreover, the removal efficiency by CaO declined in a tardier manner. The quadrature of the areas between the concentration curves and the ordinate in both Figure 2 and Figure 3 gives out the total amount of SO2 removal in each direct desulfurization reaction by C12A7 and CaO. The derived total amount of SO2 removal was found close to that measured through thermogravity analysis on the product compositions. Figure 4 shows the quantitative comparison on the total removal amounts of SO2 at different reaction temperatures between C12A7 and CaO. As can be seen from Figure 4, under the experimental conditions, the SO2 removal amounts by CaO were much higher than those by C12A7. Note that the amount of CaO component in the 1.5 g C12A7 sample was only 0.73 g. However, even with 3.1 g of C12A7, which contains 1.5 g of CaO, the SO2 removal amounts were still lower than those by CaO. For C12A7, the total amount of SO2 removal increased a little at the reaction temperature above 300 °C, while for CaO, as the reaction temperature increased, the total removal amount increased doubly or more. Comprehensive comparison of Figure 2 versus Figure 3 and the data in Figure 4 showed that C12A7 had lower SO2 removal efficiency than CaO did. Apparently, this result was on account
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Figure 5. Comparison on pore size distributions of CaO (--) and C12A7 (—).
of the fact that C12A7 had less specific surface area (1.63 m2/ g) than CaO (3.69 m2/g) had. As can be seen more clearly from the pore size distributions of both samples from mercury porosimetry in Figure 5, the macropore (>50 nm as classified by IUPAC) surface area of CaO is larger than C12A7. In other words, the accessible surface area (ASA) for SO2 into C12A7 sorbent sample was less than that in CaO. Note that the specific surface area of C12A7 was much less than CaO due to the preparation process of C12A7 at over 1300 °C, which caused pore sintering and greatly decreased the specific surface area. It should be mentioned here that the total specific surface areas for both CaO (80.53 m2/g) and C12A7 (40.92 m2/g) by mercury porosimetry were much larger than those by the singlepoint BET measurement on a Micromeritics Flowsorb II. The excessive surface areas were both contributed by the large amount of meso- and micropores. However, in this contribution, the surface areas measured by the former method fairly equal to the surface areas of macropore by the latter one. In the treatments hereafter, therefore, the results by the Micromeritics Flowsorb II were taken as the total specific surface areas for both C12A7 and CaO. From Figures 2 and 3, the apparent reaction rate constants can be derived for C12A7 and CaO, respectively. In general, the first-order reaction14 was assumed for C12A7 and CaO reacted with SO2. For each isothermal reaction in the plug flow reactor, the governing equations of the processes can be expressed as ∂C ∂C - kdSgF(C - Cs) ) (1) ∂z ∂t 1 ∂m kdSgF(C - Cs) ) kcSgFCs ≈ (2) V ∂t where C and Cs are the concentrations of SO2 in bulk flue gas and at sorbent surface, respectively, kd is mass transfer coefficient in gas phase, kc is the temperature-dependent rate constant, Sg is the specific surface area of sorbent, F is the density of sorbent, V is the sorbent volume, m is the mass of sorbent, u is the flow velocity of flue gas, and z is the axial position along the reactor. As the governing equations lead to a complex analytical solution for the concentration, we just use the second part of eq 2 to derive the rate constant of chemical reaction approximately. When the conversion ratio of sorbents is low, the chemical reaction rate is much lower relative to diffusion in the experiment temperature range. Using the data in Figures 2 and 3, rate constant kc based on unit specific surface -u
(14) Li, Y.; Sadakata, M. Study of gypsum formation for appropriate dry desulfurization process of flue gas. Fuel 1999, 78, 1089–1095.
C12A7 in Flue Gas Desulfurization
Figure 6. Arrhenius plots for the apparent rate constants derived from the data of plug flow reactions of C12A7 (∆, - -) and CaO (0, —) with SO2 (SO2: 297 ppm, N2 balance).
area can be obtained, and thus both the apparent activation energy E (kJ/mol) and the pre-exponential factor k0 can be derived accordingly, as illustrated in Figure 6. Figure 6 shows that the apparent activation energy of CaO reacted with SO2 was about 12.1 ( 1.1 kJ/mol, which was pretty close to the values obtained in other researches.14–18 This value was much lower than those got from reactions at high temperature.19–21 However, Gopalakrishnan and Seehra16 have demonstrated that diffusion through the product layer is the controlling step for the reaction while the diffusion mechanisms are different from reactions at low temperature to at high temperature. At low temperature which this investigation concerns, the diffusion of SO2 through the product layer controls the reaction.16 Thus, the activation energy obtained in this investigation cannot be compared with those from high temperature. The apparent activation energy of C12A7 was a bit lower than CaO. This result indicated that the reaction of C12A7 with SO2 was less dependent on temperature than CaO in the temperature range 200–500 °C. As is well known, the temperature dependence of most radical reactions stimulated by active oxygen anion is weak. Thus, it could be estimated that O- in C12A7 may be involved in the desulfurization process, reducing the apparent activation energy of overall reaction. This estimation will be further verified by product analysis. On the other hand, however, as the diffusion of SO2 through the product layer was the controlling step, this diffusion resistance made the apparent activation energy of C12A7 less different from that of CaO. The distinct difference in the values of pre-exponential factor clearly implied that the accessible surface area of C12A7 was (15) Sadakata, M. Chemical Engineering for Air Cleaning. (in Japanese); Baifukan: Tokyo, 1999. (16) Gopalakrishnan, R.; Seehra, M. S. Kinetics of the high-temperature reaction of SO2 with CaO particles using gas-phase Fourier transform infrared spectroscopy. Energy Fuels 1990, 4, 226–230. (17) Crnkovic, P. M.; Milioli, F. E.; Pagliuso, J. D. Kinetics study of the SO2 sorption by Brazilian dolomite using thermogravimetry. Thermochim. Acta 2006, 447, 161–166. (18) Li, Y. R.; Qi, H. Y.; You, C. F.; Xu, X. C. Kinetic model of hydrated sorbent used for flue gas desulfurization at Moderate Temperatures. The Sixth Korea-China Joint Workshop on Clean Energy Technology, 2006. (19) Hartman, M.; Trnka, O. Influence of temperature on the reactivity of limestone particles with sulfur dioxide. Chem. Eng. Sci. 1980, 35, 1189– 1194. (20) Borgwardt, R. H.; Bruce, K. R. Effect of specific surface area on the reactivity of CaO with SO2. AIChE J. 1986, 32, 239–246. (21) Bhatia, S. K.; Perlmutter, D. D. The effect of pore structure on fluid-solid reactions: Application to the lime-SO2 reaction. AIChE J. 1981, 27, 226–234.
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Figure 7. Thermogravimetry and mass spectrum (m/z ) 64) curves of the product of C12A7 reacted with SO2 (297 ppm) at 300 °C: (—) TG curve; (--) MS (m/z ) 64) curve.
a bit less than CaO in the reaction with SO2. The scarcity of the accessible surface area was mainly attributed to the making process of C12A7. The C12A7 sample was the product of solid reaction at high temperature of 1320 °C, close to the melting point of the substance. It was then cooled slowly to the ambient temperature. This whole preparation process obviously retarded the flourishing growth of macropore in the C12A7 particles. Product Analysis. To clearly understand the reaction behavior of C12A7 with SO2 so as to develop its application in desulfurization, the product from the reactions in the plug flow reactor was further analyzed. As the total amount of SO2 reacted with C12A7 was small, it was hard to detect the compositions of the product through XRD or FTIR. TG-MS, however, was an effective way to analyze quantitatively. Figure 7 shows the TG-MS curves of C12A7 after reaction with SO2 at 300 °C. In this specific analysis, the heating rate of the sample was 25 K/min. At the end of the heating program where the temperature was close to 1300 °C, the sample was decomposed and the mass spectrum m/z ) 64 was detected in the gas. This indicated that SO2 absorbed by C12A7 formed CaSO4 in the product. As is well known that14 at 300 °C the product of SO2 reacted with CaO would mainly be CaSO3, this found CaSO4 might be formed through oxidization from CaSO3 by the O- in C12A7 or through the reaction of CaO with SO32which was produced from SO2 oxidization by O- without oxygen in the reactant gas, as shown in eq 3: CaO + SO2 + O- f CaSO4 + e-
(3)
The zigzags in the MS curve before 1100 °C were caused by the noise in MS measurement. This peculiar oxidation process would cause the loss of Oin C12A7 for there was no oxygen coming from the reactant gas as supplementary. To verify this loss of O- in C12A7, the balance of O- in C12A7 before and after reaction with SO2 at 300 °C were analyzed through TG measurement at a heating rate of 10 K/min. Figure 8 showed the weight losses of these two samples in the heating process. As shown in Figure 8, for the C12A7 sample before reaction, the weight loss in the temperature range 650–900 °C was about 0.76%, which agreed well with other relative researches12 on the properties of C12A7. This weight loss was mainly due to emission of O- and the release of O2 by the following reaction:12 O- + O2- T O2- + O2
(4)
For the C12A7 sample after reaction, however, the weight loss in the corresponding temperature range was decreased to 0.63%. The reduction in weight loss, 0.13%, implied that a
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Figure 8. Difference between the weight loss curves of C12A7 before/ after reacting with SO2: (—) weight loss curve from C12A7 before reacting with SO2; (--) weight loss curve from C12A7 after reacting with SO2.
Figure 9. IR spectrum of the product by CaO reacted with SO2 (297 ppm) at 500 °C.
portion of O- encaged in C12A7 was consumed in the reaction of SO2 removal, roughly consistent with the corresponding SO2 removal amount, as illustrated in Figure 4. From the above analyses on C12A7 samples, it can be found clearly that the O- anion encaged in C12A7 was an effective oxidant in the desulfurization reaction, which made the SO2 absorbed turn to be CaSO4. This readily CaSO4 formation feature of C12A7 was an advantage over CaO as a desulfurization sorbent. However, one of the main obstacles for applying C12A7 directly as a SO2 sorbent was its low accessible surface area, leading to the low reactivity with SO2. As a result, both the SO2 removal efficiency and the sorbent utilization were unacceptably low at the low–moderate temperature. The product of CaO reacted with SO2 at 500 °C was analyzed by FTIR, as shown in Figure 9. From this IR spectrum, CaSO3 was found to be the main component of the product. This was quite different from the previous results,14 which found some CaSO4 existing in the product. Two factors may answer for this difference that CaSO4 was absent in this IR spectrum: the less BET surface area of CaO sample (3.64 m2/g) and the lower concentration of SO2 (297 ppm) in the simulated flue gas. In the previous research,14 these two corresponding quantities were 13.82 m2/g and 1834 ppm, respectively. Application of C12A7 in Desulfurization. As has stated previously, the low accessible surface area caused low reactivity of C12A7 with SO2 while the existence of O-, on the other hand, promoted the reaction product to be CaSO4. As a result, the improvement in its specific surface area and pore structure seemed to be one of the effective approaches to apply C12A7 in dry desulfurization. C12A7 with large accessible surface area may have the potential to be used as a sorbent in the CFB dry desulfurization process5 to improve its utilization. As a promisingly prominent source of the highly active oxidant O-, the other more reasonable approach to apply C12A7 in dry FGD desulfurization is to use it as a source of Oemission into flue gas, followed by other high reactivity sorbents
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to remove SO2. To verify the feasibility of this approach, i.e., whether the emission of O- from C12A7 into flue gas can improve the desulfurization efficiency or not, a preliminary experiment was devised in this investigation. Figure 10 shows schematically the setup and flowchart of the reaction system. Five grams of C12A7 was heated and then kept at 725 °C in the left heater throughout this experiment. As from previous researches,12 C12A7 would release O- when heated to above 650 °C. Therefore, highly pure nitrogen went through the hot C12A7 and served as an O- carrier gas. The flow rate of this nitrogen was controlled to be 415 mL/min. Then the gas carrying O- was mixed with another nitrogen stream (containing 508 ppm of SO2) in the flow rate of 585 mL/min. The mixture was then cooled. Thus, the simulated flue gas at the inlet of the right desulfurization plug flow reactor was 1.0 L/min in the flow rate and about 297 ppm in the calculated SO2 concentration with the existence of O-. Pure CaO was used as SO2 sorbent, and the reaction temperature were set at 200 and 300 °C. PG-250 gas analyzer was used to analyze the gas at the outlet. For comparison, two other kinds of simulated flue gas, one with about 2% oxygen, 297 ppm of SO2 and the other with 2% oxygen, 297 ppm of SO2 and O- in it, were used to react with pure CaO sorbent at 200 and 300 °C in the reactor system illustrated in Figure 1 and Figure 10, respectively. Through trapping O- emitted from C12A7 into pure water and then analyzing the water with high-performance liquid chromatography (HPLC, LC-10AVP, Shimadzu), the concentration of O- anion in flue gas in both these two cases mentioned above was estimated to be 0.1–0.2 ppm.22 Figure 11 shows the comparison of the desulfurization results at 200 and 300 °C with different flue gas compositions. It can be seen that the existence of either O- or O2 can improve the SO2 removal efficiency. Moreover, the improvements in the flue gas with 0.2 ppm O- were almost the same as those with 2% O2 at both 200 and 300 °C. In the cases with O- existence only, O- may not only serve as an oxidant for the product as shown by eq 3 but also improve the density of activated sites on the surface of the sorbent in the desulfurization reaction as well, causing more SO2 to react readily with the sorbent. In the cases that both O2 and O- existed, SO2 removal efficiency was improved greatly. The coexistence of O2 and O- may prolong the life of O- existence or multiply the quantity of O- which enhances the reaction rate of SO2 with CaO. However, the detailed mechanism for the improvement is still under exploration. The desulfurization products from these tests were analyzed by FTIR, as shown in Figure 12. From this figure, one can find that in both products some amount of CaSO4 was formed. As has been indicated by Svoboda et al.,2 CaSO4 in the product can be formed by solid-phase oxidation of CaSO3 under the condition of oxygen present in the flue gas. Thus, compared with the result in Figure 9, it can be stated that, as shown in eq 3, O- served as an oxidant in the desulfurization process under the condition of O- present. From these preliminary results, one can be encouraged to develop the approach to apply C12A7 as a source of Oemission in dry flue gas desulfurization at low–moderate temperature. In this case, the low reactivity of C12A7 itself with SO2 then turned to be a positive feature that can protect C12A7 from being damaged in flue gas. On the other hand, researches11 have demonstrated the reproducibility of O- in C12A7. The consumed O- in C12A7 can be recovered by supplying oxygen and electron on the Au-deposited C12A7 surface through the (22) Omori, G.; Morodzumi, T.; Sadakata, M. Mechanism of pH declination caused by O-. SCEJ 72nd Annual Meeting, Kyoto, 2007.
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Figure 10. Schematic diagram of O- contained simulated flue gas desulfurization in the plug flow reactor system.
Figure 11. Comparison on the outlet SO2 concentration from the desulfurization reactions with O- (···), 2% O2 (--), the existence of both O- and 2% O2 (- · -), and of neither O- nor O2 (—) at 300 °C (a) and at 200 °C (b) (SO2 297 ppm, N2 balance).
Figure 12. IR spectra of products from desulfurization by CaO at 300 °C (SO2 297 ppm): (a) with O-; (b) with 2 % O2.
electrocatalytic reactions. This would make C12A7 as highly utilized as possible in the application. Nevertheless, this investigation was indeed a preliminary one. The flue gas from coal combustion facilities is more complex than in the experiments, in particular, the existence of moisture and carbon dioxide. Yet, the gas phase reaction mechanism of moisture and O- is still to be demonstrated, and the application of C12A7 as an O- source for comparably true flue gas will be systematically investigated. Conclusion The features of C12A7 in dry flue gas desulfurization were investigated through a series of reactions in a plug flow reactor at 200–700 °C. From the experiments, the apparent reaction rate constants were also derived for quantitative description. Both the SO2 removal efficiency and the derived rate constants were compared with those of pure CaO sorbent. The products were analyzed by both TG-MS and FTIR for determining the compositions and reaction mechanism. The results showed that
under the experimental conditions both the reactivity and the SO2 removal efficiency by applying C12A7 directly were much lower than with CaO samples for its less accessible surface area. However, the encaged O- in C12A7 was found to be involved in desulfurization reaction as an effective oxidant. This advantage over CaO offers C12A7 a potential to be applied as a source of O- emission in dry flue gas desulfurization at the low–moderate temperature zone. An experiment was devised to preliminarily verify the feasibility of this application of C12A7. The result of this further experiment showed that C12A7 releasing O- anion into the flue gas improved the desulfurization efficiency of pure CaO sorbent. Acknowledgment. This work was partially supported by a Grantin-Aid for scientific research from JST (Japan Science and Technology Agency) and Japan Ministry of Education, Culture, Sports, Science and Technology. EF700268V
