Materials Transactions, Vol. 52, No. 12 (211) pp. 2211 to 2215 #211 The Japan Institute of Metals Improvement of CO 2 Absorption Properties of Limestone Ore by the Addition of Reagent Grade-SiO 2 and Natural Diatomite Fei Wang 1, Toshihiro Kuzuya 2; * and Shinji Hirai 2 1 Division of Engineering for Composite Function, Muroran Institute of Technology, Muroran 5-8585, Japan 2 College of Design and Manufacturing Technology, Muroran Institute of Technology, Muroran 5-8585, Japan CO 2 absorption/release properties of limestone mixed with 2:54 mass% reagent grade-sio 2 powders or natural diatomite have been investigated. XRD analysis of calcined diatomite-mixed limestone revealed the formation of Ca 2 SiO 4. However, only CaO and SiO 2 were confirmed in the case of the calcined limestone mixed with reagent grade-sio 2 powders. Addition of reagent grade-sio 2 and natural diatomite led to improvement of CO 2 absorption capacity of limestone ore. In the initial absorption process, limestone mixed with 1 mass% reagent grade-sio 2 showed the highest degree of absorption (67%; 673 K, 21.6 ks). In the case of natural diatomite, CO 2 absorption degree attained maximum (55%) at 5 mass% of natural diatomite. Furthermore, the absorption durability of limestone ore was improved by the addition of reagent grade-sio 2 or natural diatomite. These results indicate that the addition of reagent grade-sio 2 particles or natural diatomite is an efficient way to improve the absorption properties of natural limestone ore. [doi:1.232/matertrans.m211144] (Received May 11, 211; Accepted September 22, 211; Published November 25, 211) Keywords: CO 2, absorbent, diatomite, limestone, Ca 2 SiO 4 1. Introduction Limestone pyrolysis products are considered to be a strong candidate for CO 2 absorbents (eq. (1)). They can be used for the selective absorption of CO 2 from H 2 /CO 2 mixture gases such as water-gas shift reaction system (eq. (2)) and a decomposition of dimethyl ether (DME). CaCO 3 (s) ¼ CaO (s) þ CO 2 (g) ð1þ CO þ H 2 O ¼ CO 2 þ H 2 ð2þ As a CO 2 sorbent, a typical sequestration technology would consist of two circulating fluidized beds, one operated in the temperature range 47 C and acting as a carbonator, and the other in the temperature range 7595 C, acting as a cracker. 1) There are many reports related to the properties of limestone as CO 2 sorbent. Calcium oxide possesses a high theoretical capability for cyclic CO 2 absorption. However, in practice, the absorption capability of calcium oxide for many acid gases, such as CO 2, and SO 2 decreases with CO 2 - absorption/release cycle. Diego Alvarez et al. 2) discussed the effects of pore size on the absorption properties of calcium oxide during absorption and release courses, indicating that both the specific surface area and micropore volume decreased with CO 2 -absorption/release cycle. This was due to the agglomeration of calcium oxide particles at high temperatures. Maruko et al. 3) introduced a hydration method, i.e., the addition of water, into the improvement of CO 2 absorption capacity of calcined limestone ore. On the other hand, various additives such as Al 2 O 3, MgO, SiO 2, and TiO 2 have also been investigated. 4,5) Some additives, such as Al 2 O 3 and TiO 2, can react with CaO and form a composite oxide layer, which can prevent the agglomeration of CaO particles during CO 2 release process. Rong et al. reported that SiO 2 - coated nano-caco 3 with a high absorption durability was *Corresponding author, E-mail: kuzuya@mmm.muroran-it.ac.jp synthesized by a sol-gel method. 6) However, sol-gel method is not suitable for massive production, because of its cost problem. In this study, instead of expensive reagents such as alkoxides, natural limestone ore, reagent grade-sio 2 powder, and natural diatomite ore were used as the starting materais to synthesize novel CO 2 absorbents via a solid state reaction. This leads to a simplification of the synthesis procedure and reduction of production cost. The variation of specific surface areas and degrees of cyclic absorption were investigated by using BET and TG-DTA measurements. The effect of the additives on the absorption/release properties of limestone was also discussed. 2. Experimental Procedure A well crystallized limestone produced by Yoshizawa Lime Industry CO., Ltd., with an average particle-size diameter of 45 mm, was used as a CO 2 sorbent in this experiment. Chemical components are shown in Table 1. High purity amorphous silicon dioxide powders and natural diatomite ore were supplied by KANTO Chemical CO., INC. and WAKKANAI Green Factory Co. Ltd., respectively. In this study, natural limestone ore was mixed with 2.5 4 mass% reagent grade-sio 2 or natural diatomite, and then the mixtures were milled by a magnetic mortar for 7.2 ks to uniformly disperse the additives among the limestone powder. The properties of CO 2 absorption and release were evaluated by using thermogravimetric instrument (TG: TG- DTA2S, MAC Science CO., Ltd. Kanagawa, Japan). At first, the mixture powder was calcined in an Ar atmosphere at 1273 K in a TG apparatus. The heating rate was 1 K/min, and the flow rate of Ar was 1 ml/min. After calcination, the mixture powder was naturally cooled to room temperature in Ar atmosphere. Then, the samples were heated up to 673 K in an Ar atmosphere with a heating rate of 1 K/min, and were maintained at 673 K for 21.6 ks in a CO 2 atmos-
2212 F. Wang, T. Kuzuya and S. Hirai Table 1 Chemical composition of limestone (mass%). CaO MgO SiO 2 Al 2 O 3 Fe 2 O 3 Others Loss of fusion 53.37 1.92.43.7.2.45 43.74 Table 2 Theoretical absorption capacity of calcined limestone ores mixed with different additional content. Additive (mass%) 2.5 5 7.5 1 12.5 15 2 25 3 4 W s.71.68.65.62.59.57.52.47.42.34 XRD Intensity /cps 18 7 a b SiO2 (cristobelite-high) SiO2 (cristobelite) SiO2 (quartz) SiO2 (quartz-hp) Amorphous SiO2 phere. A flow rate of CO 2 was 3 ml/min. Finally, the release-absorption cycle of each sample was repeated three times or more times under the same conditions. The crystal structure of samples were identified by an X-ray Diffractomer (XRD: Rint-UltIma+, Rigaku Corp., Tokyo, Japan) using monochromatic CuK radiation. The samples were scanned from 2 ¼ 1 to 9 at a scanning speed.4 per second. The specific surface area of the samples was measured by using BET method (N 2 gas, Autosorb-1 Qua2117, Ltd. Yuasa Ionics, Japan). The micropore volumes of the samples were determined by the same instrument. SEM micrographs of the samples were collected by using SEM apparatus (JSM-661LV, Acc. Vol. 2 kv, JEOL). The degree of absorption was determined using the following equation under the assumption that the CO 2 - absorption reaction proceeds according to eq. (1), and the CO 2 -saturated absorption per gram of the calcium-mixture is shown in Table 2, which is calculated by eq. (4). W CO2 ¼ðW CaO =F:W: CaO ÞF:W: CO2 ð3þ W s ¼ W CO2 =ðw Cal. Lim. þ W SiO2 Þ ð4þ Where W CO2, W CaO, W cal. lime. and W SiO2 mean the stoichiomeric absorption weight of CO 2 for pure CaO (g) and the mass weights of CaO, calcined limestone and the additive, F:W: CaO and F:W: CO2 are the molecular weights of the CaO and CO 2 respectively (g/mol), W s is the theoretical maximum degree of CO 2 absorption. The actual absorption degree was calculated by eq. (5) as follows: A N ¼ 1% W=ðW s W N 1 Þ ð5þ where A N is the actual cyclic degree of CO 2 absorption in this experiment (%), N is the number of absorption cycle value, W is the change in weight during the absorption process, and W N 1 is the weight of the samples after the previous calcination (g). 3. Results 3.1 Characterization of reagent grade-sio 2 and natural diatomite XRD patterns of reagent grade-sio 2 and natural diatomite are shown in Fig. 1. Diffraction peaks corresponding to crystal SiO 2 (Quartz and Cristobelite) and amorphous SiO 2 have been observed in XRD pattern of natural diatomite (Fig. 1(a)), while only amorphous SiO 2 peaks were confirmed in reagent grade-sio 2 (Fig. 1(b)). 1 2 3 4 5 6 7 8 9 2θ ( ) Fig. 1 XRD patterns of reagent grade-sio 2 and diatomite (a: Diatomite; b: Reagent grade-sio 2 ). Table 3 Sample BET results of reagent grade-sio 2 and diatomite. Specific surface area (m 2 /g) Micro pore volume (1 2 ml/g) Reagent grade-sio 2 4.85.17 Diatomite 162.96 6.84 The results of the BET measurements on the reagent grade- SiO 2 and natural diatomite are shown in Table 3. This indicated that the specific surface area and micropore volume of natural diatomite are much higher than those of reagent grade-sio 2. 3.2 XRD analysis of calcined limestone mixed with reagent grade SiO 2 and natural diatomite As shown in Fig. 2(b), the XRD pattern of calcined limestone mixed with 5 mass% diatomite revealed the formation of Ca 2 SiO 4. However, in the case of reagent grade-sio 2,Ca 2 SiO 4 was not observed (Fig. 2(a)). This is due to the large specific surface area of diatomite (Table 3), i.e., the contact area of diatomite with CaO particles is much bigger, which can affect the rate of the reaction. After CO 2 absorption, CaO, SiO 2, CaCO 3 and Ca 2 SiO 4 were present in the limestone mixed with 5 mass% diatomite (Fig. 2(d)). However, in XRD pattern of limestone mixed with reagent grade-sio 2,Ca 2 SiO 4 was not observed (Fig. 2(c)). 3.3 SEM analysis of calcined limestone mixed with reagent grade SiO 2 and natural diatomite Scanning Electron Micrographs of calcined limestone (a), mixed with 5 mass% diatomite (b) and 1 mass% reagent grade-sio 2 (c) are shown in Fig. 3. Obviously, the addition of the diatomite and reagent grade-sio 2 led to roughning of the particle surface. Especially, calcined limestone with 1 mass% reagent grade-sio 2 looked like a conglomerate. SEM-EDS analysis of calcined limestone with 5 mass% diatomite indicated that Si was dispersed uniformly. In the case of the addition of 1 mass% reagent grade-sio 2,Si distribution was somewhat heterogeneous, indicating that the solid state reaction between the calcined limestone and the
Improvement of CO2 Absorption Properties of Limestone Ore by the Addition of Reagent Grade-SiO2 and Natural Diatomite (a) CaO SiO2 Ca2SiO4 CaCO3 2213 (a) XRD Intensity /cps (b) (c) (b) (d) 1 2 3 4 5 2θ ( ) 6 7 8 9 Fig. 2 XRD patterns of calcined limestone ores mixed with (a) 1 mass% reagent grade-sio2, (b) 5 mass% diatomite, (c) 1 mass% reagent gradesio2 after CO2 absorption and (d) 5 mass% diatomite after CO2 absorption. (c) reagent grade-sio2 did not proceed. Therefore, the addition of reagent grade-sio2 merely led to the formation of the CaO-SiO2 coaggulation. However, CaSiO3 composite layer, which was too thin to be detected by XRD, might be formed. 3.4 Initial absorption properties of limestone mixed with reagent grade-sio2 and diatomite TG-DTA curves of decomposition processes of limestone mixed with 5 mass% reagent grade-sio2 or 5 mass% diatomite, are shown in Fig. 4. In TG-curves of limestone mixed with 1 mass% reagent grade-sio2 and diatomite, a remarkable weight losses were observed at 9 11 K. Endothermic peaks of DTA curves correspond with these weight losses, which were attributed to the decomposition of CaCO3. Furthermore, the weight loss of limestone mixed with 5 mass% diatomite was bigger than that of 5 mass% reagent grade-sio2 -mixtures, because water contained in natural diatomite ore was released during the calcination step. Figure 5 indicated that calcined limestone mixed with 1 mass% reagent grade-sio2 exhibited the highest degree of absorption (67%) at 673 K for 21.6 ks. When the content of reagent grade-sio2 exceeded 1 mass%, CO2 absorption degree decreased with an increase in the reagent grade-sio2 content. In Fig. 5, calcined limestone mixed with 5 mass% diatomite had the highest degree of absorption (55%) among diatomite-mixed samples. When the additional content of diatomite was above 5 mass%, the absorption degrees also decreased. Initial absorption capability can be improved by the addition of reagent grade-sio2 and diatomite. In order to understand the effect of the additives, BET measurements have been introduced. The specific surface areas of calcined limestone mixed with (a) 1 mass% reagent grade-sio2, (b) 5 mass% diatomite and (c) no additives, were estimated to be Fig. 3 SEM micrographs of calcined limestone mixtures. ( 1) ((a) calcined limestone, (b) 5 mass% diatomite, (c) 1 mass% reagent gradesio2 ) Inset arrows in Fig. 3(c) indicate the reagent grade-sio2 particles. Fig. 4 TG and DTA curves of limestone mixtures (limestone mixed with 5 mass% reagent grade-sio2 and 5 mass% natural diatomite). 97.17, 86.53 and 6.9 m2 /g, respectively. These values were larger than the estimated values calculated by sum of the surface areas of additive and calcined limestone (Xaddit. Saddit. þ Xcal. Lime. Scal. lime., X: the weight ratio, S:
2214 F. Wang, T. Kuzuya and S. Hirai 7 6 CO 2 absorption degree (%) 6 5 4 3 2 1 5 4 3 2 1 1st 2nd 3rd 4th 1 2 3 4 Additional content (mass%) Fig. 5 Initial absorption degrees of calcined limstone mixtures (limestone mixed with reagent grade-sio 2 and natural diatomite). 1 2 3 4 Additional content of natural diatomite (mass%) Fig. 7 Cycilc absorption degree of calcined limstone mixed with natural diatomite under the absorption conditions at 673 K for 21.6 ks. 12 7 Specific surface area (m 2 /g) 1 8 6 4 2 8 6 4 2 Micropore volume (1-2 ml/g) 6 5 4 3 2 1 1st 2nd 3rd 4th 1 2 3 4 Additional content (mass%) Fig. 6 Specific surface areas and micropore volumes of calcined limstone mixtures after calcination at 1273 K. (limestone mixed with reagent grade-sio 2 and natural diatomite) Calculated specific surface areas (limestone mixed with reagent grade-sio 2 and natural diatomite). specific surface area. The suffixes addit. and cal. lime. represent the additive and calcined limestone.). On the other hand, the micropore volumes of the above samples were 3:641 1 2 for (a), 3:279 1 2 for (b) and 2:214 1 2 ml/g for (c), as shown in Fig. 6. Therefore, the addition of SiO 2 (reagent grade or natural diatomite) led to an increase in the specific surface area of calcined limestone. However, overdose of additive led to the decrease in CO 2 absorption capability. These results were consistent with the facts that the specific surface areas and micropore volumes decreased with an increase in content of additives above 1 mass% for reagent grade-sio 2 and 5 mass% for natural diatomite (Fig. 6). Furthermore, it is found that Ca 2 SiO 4 layer generated on the surface of the CaO particles (natural diatomite) prevented the penetration of CO 2 gas, and decreased the activity of CaO. Limestone mixed with 1 mass% reagent grade-sio 2 or 5 mass% diatomite possessed the best initial absorption capabilities among all the samples. 1 2 3 4 Additional content of reagent grade-sio2 (mass%) Fig. 8 Cycilc absorption degree of limstone mixed with reagent grade- SiO 2 under the absorption conditions at 673 K for 21.6 ks. 3.5 Cyclic absorption properties of limestone mixed with reagent grade-sio 2 and diatomite Figures 7 and 8 showed life cycle tests of limestone mixed with natural diatomite and reagent grade-sio 2 in the range from 2.5 to 4 mass% respectively, under the absorption conditions at 673 K for 21.6 ks in CO 2 atmosphere. The absorption degrees of all the samples decreased with CO 2 - absorption/release cycle. This is because the aggregation of CaO (calcined limestone ore) proceeds with CO 2 -absorption/ release cycle. Long life cycle tests were conducted with calcined limestone, mixed with 5 mass% diatomite, and limestone mixed with 1 mass% reagent grade-sio 2 (Fig. 9). It was observed that the absorption degree of limestone with no additives decreased to about 15% at 3 cycles. However, the absorption degrees of samples (1 mass% reagent grade- SiO 2 ) and (5 mass% diatomite) at 3 cycles, were found to be 4 and 3%, respectively, being higher than that of calcined limestone (no additive). Furthermore, after 2 cycles, the absorption degrees of the samples mixed with 5 mass% natural diatomite or 1 mass% reagent grade-sio 2 main-
Improvement of CO 2 Absorption Properties of Limestone Ore by the Addition of Reagent Grade-SiO 2 and Natural Diatomite 2215 7 6 5 4 3 2 1 tained at about 2%. Therefore, it is concluded that the additions of reagent grade-sio 2 or natural diatomite could improve the cyclic absorption durability of limestone ore. 4. Discussion 5 Reagent grade-sio2 (1 mass%) Natural diatomite (5 mass%) No additives 1 Cyclic numbers Fig. 9 Cycilc absorption degree of limstone mixtures under the absorption conditions at 673 K for 21.6 ks. The addition of SiO 2 such as diatomite and reagent grade- SiO 2 enhanced the CO 2 absorption activity of limestone. This is considered to be attributed to the formation of the Ca 2 SiO 4 phase and the increase in the specific surface area. Scanning electron micrographs indicated the morphological change of the surface of the calcined limestone mixed with the diatomite and reagent grade-sio 2. The hardness of SiO 2 is larger than that of CaCO 3. Therefore, CaCO 3 may be pulverized by SiO 2 powders in mixing process. However, when the additional content exceeded 5 1 mass%, the specific surface area decreased with the additional content (see Fig. 6). Assuming that the annealing effect and the reaction between CaO and SiO 2 can be ignored, the specific surface area of CaO and SiO 2 mixture is represented as ð1 x=1þ S CaO þðx=1þ S SiO2, where x, S CaO and S SiO2 are additional content of SiO 2, the specific surface areas of pure CaO and SiO 2 powders. In the case of reagent grade- SiO 2, the specific surface area of calcined sample decreases with the increase in additional content. However, in the case of the addition of the diatomite, the decrease in the specific surface could not be explained by above mentioned mechanism. The decomposition of CaCO 3 can provide micropores, which are thermally unstable. The addition of SiO 2 led to the formation of Ca 2 SiO 4 on the surface or boundary layer of CaO particle, which prevent the coagulation of CaO particle and the disappearance of micropore. 7) Indeed, in the both 15 2 cases of diatomite and reagent grade-sio 2, Si atoms were dispersed homogeneously in CaO particles. On the other side, the activity of Ca 2 SiO 4 is predicted by thermo-dynamical dates to be lower than that of CaO. Therefore, the excessive formation of Ca 2 SiO 4 led to the depressant effect for CO 2 absorption. 5. Conclusion In this study, the effect of the addition of reagent grade- SiO 2 and natural diatomite have been investigated. After calcication at 1273 K in an Ar atmosphere, the formation of Ca 2 SiO 4 was confirmed by XRD analysis of natural diatomite-mixed limestone. However, only CaO and SiO 2 were observed in the case of the calcined limestone samples mixed with reagent grade-sio 2. Through TG-DTA results, limestone mixed with 1 mass% reagent grade-sio 2, or 5 mass% natural diatomite, possessed a high initial absorption degree of 67% and 55%, respectively, which was consistent with the fact that their specific surface areas (97 m 2 /g and 86.53 m 2 /g) and micropore volumes (3:641 1 2 ml/g and 3:279 1 2 ml/g) were higher than other samples. However, the generation of Ca 2 SiO 4 caused a decrease in the amount of reactive CaO particles. Compared with the initial absorption degree of limestone (4%) under the same absorption conditions at 673 K for 21.6 ks, the addition of reagent grade-sio 2 or natural diatomite ore could improve the initial absorption capability of natural limestone ore. On the other hand, the absorption degrees of limestone mixed with 1 mass% reagent grade-sio 2 or 5 mass% diatomite, after repeating the process three times, were estimated to be 4 and 3%, respectively, being higher than that of pure limestone (15%). Furthermore, after repeating the process twenty times, the absorption degrees of limestone mixed with 5 mass% natural diatomite or 1 mass% reagent grade-sio 2, showed a level 2%, which was higher than that of limestone. Therefore, the addition of reagent grade-sio 2 or natural diatomite ore can improve the cyclic absorption durability of limestone. REFERENCES 1) B. R. Stanmore and P. Gilot: Fuel Process. Technol. 86 (25) 177 1743. 2) D. Alvarez and J. Carlos Abanades: Energy Fuels 19 (25) 27 278. 3) S. Maruko, S. Satoh, K. Yamashita and H. Tanaka: Inorg. Mater. 4 (1997) 148 151. 4) K. O. Albrecht, K. S. Wagenbach, J. A. Satrio, B. H. Shanks and T. D. Wheelock: Ind. Eng. Chem. Res. 47 (28) 7841 7848. 5) S. F. Wu and Y. Q. Zhu: Ind. Eng. Chem. Res. 49 (21) 271 276. 6) R. Wu and S. F. Wu: J. Chem. Ind. Eng. 57 (26) 1722 1726. 7) M. Muraoka, E. Fujii and K. Kawabata: Bunseki Bunkakai Annual Meeting (21) http://www.nmij.jp/~collab/bb kai/nen-kai/h22/ genkou/1-muraoka.pdf