Steam Catalysis in CaO Carbonation under Low Steam Partial Pressure

CaO was widely used to capture CO2 in direct hydrogen production process, where steam always existed simultaneously. The effect of steam on CaO ...
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Ind. Eng. Chem. Res. 2008, 47, 4043–4048

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Steam Catalysis in CaO Carbonation under Low Steam Partial Pressure Shaojun Yang*,†,‡ and Yunhan Xiao† Key Laboratory of AdVanced Energy and Power, Chinese Academy of Sciences, Institute of Engineering Thermophysics, Beijing, China 100080, and Graduate UniVersity of Chinese Academy of Sciences, Beijing, China 100049

CaO was widely used to capture CO2 in direct hydrogen production process, where steam always existed simultaneously. The effect of steam on CaO carbonation performance under low steam partial pressure was investigated using a pressurized thermogravimetric apparatus. The experimental results revealed that steam improved CaO carbonation performance significantly no matter whether Ca(OH)2 was produced or not. At 823 K and 0.5 MPa of steam partial pressure, effect of steam on CaO carbonation performance could not be attributed mainly to production of Ca(OH)2 because the hydration rate of CaO was very slow. The main reason was steam catalysis in CaO carbonation. Enhancement of steam on CaO carbonation performance without Ca(OH)2 production could not be attributed to improvement of steam on the physical property, but to catalytic effect of steam. Effects of CaO precursors, CO2 partial pressure, steam partial pressure, and temperature with steam addition on CaO carbonation performance were also investigated. 1. Introduction H2 is an important chemical substance and a kind of clean energy. It is expected to be an important energy carrier in the near future.1 CO2 is one of the primary greenhouse gases which result in global climate warming.2,3 Hydrogen production with in situ CO2 capture can be achieved by using CO2 sorbent in hydrogen production from steam gasification of carbonaceous energy such as coal,4–6 biomass,7,8 methane,9 heavy oil,10 CO,11,12 etc. Calcium oxide was an effective sorbent to capture CO2 at high temperatures.13,14 Curran et al.4 and Lin et al.15 proposed a CO2 acceptor process and a novel process named HyPr-RING (hydrogen production by reaction-integrated novel gasification), respectively. One of the differences between the two processes was that the HyPrRING process produced high purity hydrogen and low concentration CO and CO2, while about 50% of the original CO and CO2 remained in the products in CO2 acceptor process.16 Sato et al. attributed to different dominant CO2 sorbent component in the two processes. CaO converted to Ca (OH)2 at several MPa of steam partial pressure in HyPr-RING process. However, CaO was the only component in CO2 acceptor process. Ca (OH)2 was considered behaving better performance than CaO.10 Kuramoto et al.17 found intermediate hydration treatment, regardless of liquid hydration or vapor hydration, could improve the CaO reactivity significantly under both atmospheric and pressurized conditions. They considered the main reason was that the formation of porous agglomerates which generated in Ca(OH)2 carbonation to CaCO3 and H2O. Counterdiffusion of H2O with CO2 caused the formation of porous product layer which decreased the diffusion residence of CO2. However, under high steam partial pressure, Ca(OH)2 production would result in forming CaO/Ca(OH)2/CaCO3 tertiary eutectic compounds. The tertiary eutectic compounds not only * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +86 10 82543101. Fax: +86 10 82543096. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

decreased fluidity of solid in the reactor but also increased deactivation of CaO sorbent resulted from interaction between the sorbents and the coal-derived minerals.18,19 Direct hydrogen production from sorption-enhanced reaction using methane took place generally at 923 K and steam partial pressure less than 0.5 MPa which was less than the equilibrium partial pressure of steam.9,20 CaO/Ca(OH)2/CaCO3 tertiary eutectic compounds will not be produced. CaO was also widely used to capture CO2 from flue gas in which steam was contained.2,21,22 Dobner et al.23 found that steam increased carbonation rate of fully calcined dolomite significantly. The authors considered that steam catalyzed CaO carbonation. However, MgO in fully calcined dolomite not only decreased CaO sintering in steam and CO2 but also possible catalyzed CaO carbonation in steam. Since chemical effect of MgO on CaO carbonation at steam atmosphere remained unclear, it was doubtable that the effect of steam on carbonation of fully calcined dolomite was attributed to steam catalysis in CaO carbonation. It is necessary to investigate the effect of steam on CaO carbonation performance without MgO addition. However, little research had focused on it under pressurized conditions. The purpose of this study was to investigate the effect of steam on CaO carbonation performance under lower than 0.5 MPa of low steam partial pressure using a pressurized thermogravimetric apparatus. 2. Experimental Section 2.1. Materials. In this study, analytical grade hydrated calcium oxalate (g99.5%), calcium hydroxide (g95.0%), and calcium oxide (g98.0%) were used as CaO precursors. All the CaO precursors were heated to 1173 K at a heating rate of 10 K/min and, then, calcined at 1173 K for 3 h. CaO was sieved to 60 ∼ 100 mesh and stored in a desiccator. High purity nitrogen (99.999 vol %) were used as reaction gas, purge gas, and furnace gas.

10.1021/ie8000265 CCC: $40.75  2008 American Chemical Society Published on Web 05/16/2008

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Figure 1. Schematic diagram of a pressurized thermogravimetric apparatus (Thermax 500).

2.2. Experimental Apparatus. As shown in Figure 1, a high pressure thermogravimetric apparatus (TherMax 500) manufactured by Thermo Electron Corproation was used to measure the effects of steam on CaO carbonation and hydration performance. The apparatus contains a Cahn D-110 pressure balance of 1 µg sensitivity. The maximal operating temperature was 1373 K under ambient pressure. The maximal operating pressure was 6.89 MPa at 1273 K. Solid mass was recorded at 1 s intervals using a data acquisition system connected to a personal computer running ThermalAcq TherMax software. The pressure of the system was controlled by a backpressure regulator. Four high pressure mass flow controllers were used to control the flow rates of reaction gas (N2 and CO2), purge gas (N2), and furnace gas (N2). Steam vapored from deionized water. The flow rate of deionized water was measured and controlled by a Syltech Model-501 high-pressure liquid chromatography (HPLC) pump. 2.3. Experimental Procedure. About 200 mg CaO sample was dispersed in a quartz sample pan which was placed in the uniform hot zone of the reactor. The temperature of the reactor zone was monitored by a type K thermocouple which was placed 5 mm below the sample pan. After setting the desired pressure of the reactor, nitrogen was introduced to the balance, furnace, and reactor through a purge gas tube, a furnace gas tube, and a reaction gas tube, respectively. After the pressurized thermogravimetric system was pressurized to the predetermined pressure, the furnace temperature increased to the desired temperature at a heating rate of 20 K / min. After the pressure and temperature stabilized, CO2 with purity of 99.95 vol % and steam vapored from deionized water were adjusted to the proper flow rates which were calculated by CO2 partial pressure (PCO2), steam partial pressure (PH2O), total pressure (Pt), and total gas flow rate (Ft). Steam generated and premixed with CO2 and N2 in a steam generator. All the reaction gases were preheated in the tube before entering the inlet of reactor. The following reactions may occur after introducing CO2 and steam: CaO carbonation: CaO + CO2 f CaCO3 CaO hydration: CaO + H2O f Ca(OH)2

The conversions of CaO carbonation and hydration were calculated by means of the following expressions: Xc )

(mt - m0)/44 m0/56

(1)

Xh )

(mt - m0)/18 m0/56

(2)

Where, mt is the total mass of CaO and CaCO3 or Ca(OH)2 at time t, m0 is the mass of reactant of CaO at time t ) 0. The terms 18, 44, and 56 are the molar weights (in grams per mole) of H2O, CO2, and CaO, respectively. 2.4. Characterizations. XRD measurement was performed on a D/Max-RC diffractometer using nickel-filtered Cu KR (λ ) 1.5406 nm, 40 kV, 80 mA) radiation. Scanning was conducted over the range of 2θ ) 10°-70°. 3. Results and Discussion 3.1. Effect of Steam on CaO Carbonation Performance with Ca(OH)2 Production. As shown in Figure 2, CaO conversion increased quickly first and, then, slowed down vs time gradually under different steam partial pressures. The result is consistent with a general characteristic of gas–solid reactions with a higher molar volume solid product.24–27 It can be seen from Figure 2 that steam enhanced markedly CaO carbonation performance. At 823 K, 0.9 SLPM (standard liter per minute) of total gas flow gas, CaO conversions were 40.5% within 15 min and 55.0% within 30 min under PH2O ) 0.3 MPa, and CaO conversions were 42.3% within 15 min and 57.2% within 30 min under PH2O ) 0.5 MPa, while CaO conversions were only 10.3% within 15 min and 13.5% within 30 min without steam addition, respectively. The thermodynamic equilibrium partial pressure of steam of at 823 K is about 0.2 MPa, as shown in Table 1. According to thermodynamics, Ca(OH)2 would be produced at PH2O ) 0.3 MPa and PH2O ) 0.5 MPa since these partial pressures were higher than thermodynamic equilibrium partial pressure of steam at 823 K. It must be pointed out that Ca(OH)2 which was produced from CaO hydration was consumed thoroughly in CaO carbonation. XRD measurement confirmed that Ca(OH)2 did not exist

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Figure 2. Effect of steam on CaO carbonation performance with Ca(OH)2 production (823 K, Pt ) 3.0 MPa, PCO2 ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

Figure 4. CaO hydration conversion vs time at 823 K (Pt ) 3.0 MPa, PCO2 ) 0.5 MPa, PH2O ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

Table 1. Thermodynamic Equilibrium Partial Pressure at Different Temperatures T (K)

PH2O (MPa) a

823

873

923

973

ref

0.198 0.219

0.467 0.525

0.971 1.14

1.80 2.30

Lin et al.27 Fujimoto et al.28,a

Calculated from PH2O ) 9 × 1011 exp(-12531.5/T)Pa.

in the product of CaO carbonation, as shown in Figure 3. Therefore, CaO carbonation conversion can be calculated using formula 1. It seems that effect of steam on CaO carbonation performance is attributed to production of Ca(OH)2. Reaction of CaO and CO2 with Ca(OH)2 production will be distinguished into a consecutive reaction including two reactions: CaO hydration and Ca(OH)2 carbonation. It is necessary to considering the hydration rates of CaO at these conditions because the overall reaction rate of consecutive reaction is determined by the slowest reaction. As shown in Figure 4, the hydration rate of CaO was very slow at 823 K and 0.5 MPa of steam partial pressure. CaO hydration conversions were only 2.3% within 15 min and 3.7% within 35 min, respectively. Even if all the Ca(OH)2 were converted to CaCO3, the overall CaO carbonation conversions were only 2.3% within 15 min and 3.7% within 30 min, respectively. The total conversions were much lower than the practical CaO carbonation conversions under 0.5 MPa of steam

Figure 3. X-ray diffraction spectra of product of CaO carbonation at 823 K (Pt ) 3.0 MPa, PCO2 ) 0.5 MPa, PH2O ) 0.5 MPa, Ft ) 0.9 SLPM).

Figure 5. Effect of steam on CaO carbonation performance without Ca(OH)2 production (Pt ) 3.0 MPa, PCO2 ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

partial pressure. They are even much lower than those without steam addition within the same reaction time. It indicated that effect of steam on CaO carbonation performance with Ca(OH)2 production may not be attributed mainly to production of Ca(OH)2. The main reason may be significant enhancement of steam on CaO performance without Ca(OH)2 production since CaO was the dominant component of Ca-based sorbents. 3.2. Effect of Steam on CaO Carbonation Performance without Ca(OH)2 Production. As shown in Figure 5, steam increased CaO carbonation performance significantly. When steam partial pressure increased from 0 to 0.1 MPa at 823 K, CaO conversions increased from 12.9% to 43.4% within 25 min, and from 16.4% to 50.1% within 50 min, respectively. CaO conversion within 30 min under 0.5 MPa of steam partial pressure was 49.8% higher than that under 0 MPa of steam partial pressure at 923 K. The thermodynamic equilibrium partial pressures of steam at 823 and 923 K were 0.198 and 0.971 MPa, respectively.27 Ca(OH)2 would not be produced at 823 K and 0.1 MPa of steam partial pressure and 923 K and 0.5 MPa of steam partial pressure since the steam partial pressures were lower than the corresponding thermodynamic equilibrium partial pressures of steam at 823 and 923 K, respectively. CaO hydration was also confirmed not to occur since solid mass kept unchanged at 823 K and 0.1 MPa of steam partial pressure and 923 K and 0.5 MPa of steam partial pressure using the pressurized thermo-

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Figure 6. Effect of steam on carbonation performance of CaO obtained from different precursors without Ca(OH)2 production (923 K, Pt ) 3.0 MPa, PCO2 ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from Ca(OH)2 and CaO (AP) calcined at 1173 K for 3 h).

Figure 7. Effect of steam partial pressure on CaO carbonation performance (Pt ) 1.5 MPa at 923 K, or Pt ) 3.0 MPa at 823 K, PCO2 ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

gravimetric apparatus. Thus, it is eventually incorrect to attribute the effect of steam on CaO carbonation performance to production of Ca(OH)2 in these conditions. The effect of steam on carbonation performance of CaO obtained from another two kinds of CaO precursors such as calcium hydroxide and calcium oxide is illustrated in Figure 6. Regardless of CaO precursors, CaO conversions were also improved significantly by steam even if Ca(OH)2 was not produced. The results indicated that it was highly possibly that enhancement of steam on CaO carbonation performance with Ca(OH)2 production was attributed mainly to significant enhancement of steam on CaO carbonation performance without Ca(OH)2 production. 3.3. Effect of Steam Partial Pressure on CaO Carbonation Performance. As shown in Figure 7, when steam partial pressure increased from 0.1 to 0.5 MPa at 823 K and Pt ) 3.0 MPa, CaO conversions increased from 33.5% to 42.3% within 15 min, and from 43.4% to 53.1% within 25 min, respectively. Ca(OH)2 was produced at 823 K and PH2O ) 0.5 MPa, while it was not produced at 823 K and PH2O ) 0.1 MPa. It seems that

Figure 8. Effect of temperature on CaO carbonation performance (Pt ) 1.5 MPa, PCO2 ) 0.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

enhancement of CaO carbonation performance with increasing steam partial pressure was attributed to production of Ca(OH)2. However, CaO carbonation performance also increased with increasing steam partial pressure even if Ca(OH)2 was not produced. When steam partial pressure increased from 0.1 to 0.5 MPa at 923 K and Pt ) 1.5 MPa, CaO conversions increased from 64.2% to 77.8% within 15 min and from 74.5% to 84.7% within 25 min, respectively. Moreover, CaO conversions increased from 10.4% to 33.5% within 15 min and from 12.9% to 43.4% within 25 min, respectively, when steam partial pressure increased from 0 to 0.1 MPa at 823 K and Pt ) 3.0 MPa. The enhancements of CaO conversions when steam partial pressure increased from 0 to 0.1 MPa were much higher than those when steam partial pressure increased from 0.1 to 0.5 MPa. The results confirmed that the effect of steam on CaO carbonation performance with Ca(OH)2 production was not mainly attributed to production of Ca(OH)2, but to significant enhancement of steam on CaO carbonation performance without Ca(OH)2 production. 3.4. Effect of Temperature on CaO Carbonation Performance with Steam Addition. As shown in Figure 8, increasing reaction temperature improved CaO carbonation performance markedly, which is consistent with the effect of temperature on CaO carbonation performance without steam addition in the literature.29–31 CaO conversions within 10 min at 873, 923, and 973 K were 8.7% higher, 20.8% higher, and 37.7% higher than that at 823 K, respectively. It must be indicated that Ca(OH)2 would not be produced in the above conditions according to Table 1. However, CaO conversions at 873 K and 0.1 MPa of steam partial pressure were higher than those at 923 K and 0 MPa of steam partial pressure, which revealed the special effect of steam on CaO carbonation performance. 3.5. Effect of CO2 Partial Pressure on CaO Carbonation Performance with Steam Addition. As shown in Figure 9, increasing CO2 partial pressure improved CaO carbonation performance, which agrees well with the findings of the literature.29,32 CaO conversion within 5 min increased from 21.9% to 53.4% when CO2 partial pressure increased from 0.1 to 0.7 MPa. CaO conversions within 10 min under CO2 partial pressure of 0.3, 0.5, and 0.7 MPa were 23.0%, 26.8%, and 31.4% higher than that under CO2 partial pressure of 0.1 MPa, respectively. If steam did not influence CaO carbonation performance, CaO carbonation conversions under 0.5 MPa of CO2 partial pressure

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Figure 9. Effect of CO2 partial pressure on CaO carbonation performance (923 K, Pt ) 1.5 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

sides minor bicarbonates which were produced by the reaction of CO2 with surface hydroxyl groups at room temperature when without steam addition.34 Both CaO and MgO are basic oxides. Water can be adsorbed and dissociated to OH on Ca or Mg, and H on O.35,36 After introducing steam, more bicarbonates were produced through interaction of CO2 with surface hydroxyl groups on MgO surface.37 Since the basicity of CaO was higher than that of MgO,35 more adsorbed hydroxyl groups and bicarbonates were produced on CaO surface than those were produced on MgO surface. On the basis of the findings of the above literatures, we proposed that adsorbed hydroxyl groups and bicarbonates may be the important intermediates in steam catalysis in CaO carbonation. Compared to without steam pretreatment, much more surface hydroxyl groups were produced after steam pretreatment for 30 min if only CO2 was used as reaction gas. After introducing CO2, surface hydroxyl groups reacted with CO2 to form bicarbonates which were converted to CaCO3. Water adsorption on the CaO surface was reversible. After steam pretreatment for 30 min and removing steam from the reaction system, surface hydroxyl groups were desorbed and less and less surface hydroxyl groups remained. However, surface hydroxyl groups with steam pretreatment for 30 min were still much more than those without steam pretreatment for 30 min. Therefore, CaO carbonation performance with steam pretreatment using only CO2 as reaction gases was better than that without steam pretreatment using only CO2 as reaction gas, but worse than that without steam pretreatment using both H2O and CO2 as reaction gases. Due to water was rapidly adsorbed and dissociated to -OH and -H, steam pretreatment for 30 min did not improve CaO carbonation performance when both CO2 and H2O were used as reaction gases. The detailed kinetics and mechanism will be investigated by Fourier transfrom infrared (FTIR) techniques in the near future. 4. Conclusions

Figure 10. Effect of steam pretreatment on CaO carbonation performance (923 K, Pt ) 1.5 MPa, PCO2 ) 0.5 MPa, PH2O ) 0 or 0.3 MPa, Ft ) 0.9 SLPM, CaO obtained from hydrated calcium oxalate calcined at 1173 K for 3 h).

and 0 MPa of H2O partial pressure should be higher than the corresponding conversions under 0.3 MPa of CO2 partial pressure and 0.3 MPa of steam partial pressure. However, CaO carbonation performance under PCO2 ) 0.5 MPa and PH2O ) 0 was much less than that under PCO2 ) 0.3 MPa and PH2O ) 0.3 MPa, as shown in Figure 9. This result also showed the special effect of steam on CaO carbonation performance. 3.6. Catalysis of Steam on CaO Carbonation. As shown in Figure 10, CaO carbonation conversions with steam pretreatment for 30 min were much higher than those without steam pretreatment when only CO2 was used as reaction gas. However, steam pretreatment did not help to improve CaO carbonation performance when both CO2 and H2O were used as reaction gases. It means that effect of steam on CaO carbonation performance cannot be attributed to steam improving CaO physical properties such as specific surface area and pore structure. On the contrary, Borgwardt found that steam pretreatment would reduce specific surface area and pore volume of CaO.33 The effect of steam on CaO carbonation performance should be attributed to catalysis of steam. Without steam addition, some kinds of surface carbonates such as unidentate carbonate, bidentate carbonate, bicarbonate, and carbonate ion were produced when CO2 was adsorbed on CaO surfaces.34 Unidentate carbonates were predominant be-

The effects of steam on CaO carbonation performance with and without Ca(OH)2 production were investigated using a pressurized thermogravimetric apparatus. The effects of temperature, partial pressures of steam and CO2, and CaO precursors were also investigated. The following conclusions were obtained: (1) Steam improved CaO carbonation performance significantly no matter whether Ca(OH)2 was produced or not. (2) The effect of steam on CaO carbonation performance was not primarily due to Ca(OH)2 production at 823 K and less than 0.5 MPa of steam partial pressure because CaO hydration conversion was much less than CaO carbonation conversion. (3) The effect of steam on CaO carbonation performance should be attributed to steam catalysis when Ca(OH)2 was not produced. (4) Steam increased CaO carbonation performance significantly regardless of CaO precursors, temperature, or CO2 partial pressure. Acknowledgment The authors thank the financial supports by National Hightech Research and Development Program of China under Grant No. 2006AA05A103 and Major State Basic Research and Development Program of China Grant No. 2007CB210101. Literature Cited (1) Midilli, A.; Ay, M.; Dincer, I.; Rosen, M. A. On Hydrogen and Hydrogen Energy Strategies: I: Current Status and Needs. Renewable Sustainable Energy ReV. 2005, 9, 255.

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ReceiVed for reView January 6, 2008 ReVised manuscript receiVed March 7, 2008 Accepted March 12, 2008 IE8000265