Synthesis of Calcium Aluminates Granule with TiO2 Binder for High-temperature CO2 Capture

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Energy Procedia 37 ( 2013 ) 1246 – 1253

GHGT-11

Synthesis of Calcium Aluminates Granule with TiO

2

Binder for High-temperature CO

2

Capture

Ching-Tsung Yu

a,*

, Wei-chin Chen

a

, Yau-Pin Chyou

a

, San-Yuan Chen

b

a_{Institute of Nuclear Energy Research, Chemistry Division, Longtan 325, Taiwan}

b_{National Chiao Tung University, Department of Material Science and Engineering, Hsinchu 300, Taiwan}

Abstract

High-temperature CO2 captures using powder and granule is investigated using thermogravimetric analyzer (TGA)

and fixed-bed reactor (FBR). The CO2 sorbent, Ca-Al-CO3 powder was prepared using co-precipitation method of

Ca+2_{, Al}+3_{and CO}

32- under alkaline conditions. The granule was fabricated by mixing Ca-Al-CO3 powder, H2O and

w/ or w/o TiO2 binder, followed by granulation and calcinations at 600°C. In TGA experiment, high CO2 capacity of

45~50 wt% after 100 cycles can be achieved with powder at 750°C, while it was dramatically decayed to 40-50% only after 2-5 cycles in FBR. Alternatively, using Ca/Ti granule exhibited recovery of 80-90% for 15 cycles in TGA and 70-80% for 10 cycles in FBR, respectively. This outcome indicates that calcium aluminates granule with TiO2

binder is competitive CO2 sorbent.

Selection and/or peer-review under responsibility of GHGT

Keywords: Ca-Al-CO3; Granule ; high temperature; CO2 capture; reactor

* Corresponding author. Tel.: +886-2-82317717 ext.5103; fax: +886-3-4910419

E-mail address: ctyu@iner.gov.tw

1. Introduction

Capturing of carbon dioxide at high temperature condition has a direct advantage for abating CO2 from

concentrated sources. In terms of material stability and thermodynamic consideration, adsorption of CO2

by solid sorbent is a specific method and environmental compatibility [1]. Among several common adsorbing materials, in terms of oxides [2], CaO-based sorbent exhibits high CO2 adsorption capacity with

average value of 50 wt% at 600-850°C. The stability of CO2 sorption has been tested for repeat cycles in

TGA, for example using powders of nano-sized CaO [3], CaTiO3/nano-CaO [4], CaO-SiO2 [5] and

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Ca(OH)2/cement [9] etc. In these studies, using pure CaO as CO2 adsorbent, avoiding causing micropore

blocking and loss of activity [10] by CaCO3 is a main target. Therefore, it is concluded that modification

of CaO-based sorbent receives a significant improvement in long-term stability and extended CO2

sorption capability.

Utilization of layered double hydroxides (LDHs) with hydrotalcite-like structure as a template for synthesizing CO2 sorbent has showed a vital breakthrough in multi-cycle performance. High-temperature

CO2 adsorption capability could be notably improved via replacement of Mg+2 by Ca+2 in Ca-Al-LDHs,

which accomplished at least 90% of CaO conversion after 100 cycles [11]. The stability is maintained mainly by forming of aluminum oxides that act as the separating layer over CaO aggregates in calcined Ca-Al-Oxide [12]. However, sintering and mechanical strength are crucial considerations for using of them in a fixed-bed reactor. The sintering is still possible existed for spent sorbent, ascribing to larger molar volume of CaCO3 covered on neighboring Ca-Al-Oxide kernels, resulting to de-activate sorbent.

Due to poor heat transfer developed within nascent CaO structure [13], the heat expansion create asymmetric stress difference in grains, leading to crushed particle. Thus Ca/Al oxides boundary should be isolated and/or bonding with suitable inorganic binder as supported material.

TiO2 is a commercial available material with hydrophilic attraction force with Ca+2 and Al+3 under

alkaline condition, emerging an economically competitive potential for as a binder of Ca/Al LDHs. In addition, a heat-resistance compound of CaTiO3 can be formed under high temperature calcinations,

helping to acquire a satisfied CO2 capture activity. In this work, the solid sorbents of Ca-Al-CO3 with

powder and granule form were prepared using coprecipitation method. The pore property, crystalline and morphology of these materials were examined by BET, XRD and SEM. A fixed-bed reactor was built up for testing CO2 capture performance at 600-850°C, using TGA and fixed-bed adsorption of CO2 on

sorbents made of Ca/Al granule with TiO2 binder. Multi-cycling test method was applied for inspection of

CO2 sorption stability in both reactors.

2. Experimental 2.1. Synthesis of sorbents

The CaO based sorbents were prepared by precipitation of inorganic ions of Ca+2_{, Al}+3_{and, CO} 3

2-under alkaline conditions. Raw materials of Ca(OAc)2·H2O, Al(NO3)3·9H2O, NaOH and Na2CO3 were

purchased from Merck Co. TiO2 powder (Degussa P25) is used as a binder for forming granule.

Fig. 1 depicts the synthesis procedure of Ca-Al-CO3 powder and granule. In this method, cationic

solution of Ca+2_/Al+3_{, with molar ratio of 7:1 of Ca(OAc)}

2 and Al(NO3)3, was prepared by dissolution of

both chemicals in Di-H2O. The precipitate of Ca/Al carbonates was formed after addition of 0.16 mol

NaOH with 0.01 mol Na2CO3 in stirred Ca+2/Al+3 solutions and followed by filtration to obtain layered

double hydroxides (LDHs) powder. These LDHs were further calcined at 600°C for producing Ca-Al-CO3 powder, which is used as raw material for granulation. The granule can be fabricated by either

simply adding of H2O into Ca-Al-CO3 powder or with extra TiO2 binder. In the latter case, granule was

made of different weight ratio x of Ca-Al-CO3 (Ca) powder and TiO2 (Ti) binder, with x from 1 to 6. The

obtained cylindrical granule with an average size of 3 mm (diameter) by 5 mm (length) that was shaped by appropriate apparatus. The Ca/Ti granules were calcined at 600~850°C for identifying the specific patterns. The characteristics of surface area, morphology/particle size and crystalline of prepared sample were determined by BET (Quantchrome), SEM (Hitachi) and PXRD (Bruker), respectively.

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cycles

i

W

V

t

W

sorbent i i

)

44 /

24 .

5 _;

(

)

(

1

Fig.1. Flow chart of preparing Ca-Al-CO3 powder, granule and Ca/Ti granule.

2.2 CO2 sorption experiment

Thermogravimetric analysis is used to investigate CO2 capture activity over sorbents at the same

temperature of 750°C for 1 hour adsorption at 100% CO2 (50 mL/min) and 0.5 hour desorption at 100%

N2 (50 mL/min) conditions, respectively. The capture capacity (wt%, g CO2/g sorbent) in TGA was

recorded by balance and expressed in terms of sorption weight with respect to the sorbent weight.

Referring to Fig.2, illustrating of a fixed-bed reactor (FBR) for CO2 adsorption experiment operated

at 600-850°C and ambient pressure. The testing gas stream is 40% CO2 (N2 balance, 2L/min), which is

fed to a 1-inch quartz column packed with the adsorbent. After CO2 uptake on CaO, the passing gas was

cooling down together with CO2 volume monitored by a Non-dispersive Infrared (NDIR) detector. The

average integral method on the values of CO2 volume obtained at each cycles was applied for the

estimation of absorption capacity [14]. In this approximation, CO2 sorption volume was calculated by

integrating the volume difference of CO2 ( VCO2) with time, which was based on 90% breakthrough over

the outlet of column. CO2 capacity (g) was estimated by molar volume at 25°C via an ideal gas equation.

The CO2 specific absorption W(t) as a function of cycle was then obtained according to the formula:

Ca/Al solution: 7 mol of Ca(OAc)2

1 mol of Al(NO3)3

Alkaline solution: NaOH+Na2CO3

Stirred solution and precipitation of Ca-Al-CO3 Calcined Ca-Al-CO3 powder Ca-Al-CO3 Granule H2O H2O, TiO2 Ca/Ti granule Formula (I)

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Fig.2. Schematic of a fixed-bed reactor (FBR) for CO2 adsorption experiment.

3. Results and discussion 3.1 Characterization of sorbents

Microscopic characters of TiO2 and calcined samples were shown in Table 1. The binder, TiO2 has

the smallest value of particle size, the next of Ca-Al-CO3 powder and the last of two granules. BET data

indicated that both of powder and granule exhibited mesoporous character with a pore size value of 14~24 nm and the surface area was around 17~23 m2_{/g [15]. Extruded Ca-Al-CO}

3 granule shows a

slightly smaller surface area than powder. However, pore volume and pore size of Ca/Ti granule were enhanced by incroporating of TiO2. The hydroxyl group on the surface of TiO2 provides a linking force

between Ca+2_/Al+3_{ions and OH}-_{, which promotes a separated layer of TiO}

2 formed on surround of

Ca-Al-CO3 aggregates. The carbonation reaction of CO2 and TiO2 is unavailable in 750°C; therefore, with the

advantage using TiO2 as binder, the higher surface area of granule than powder is beneficial for

increasing adsorption performance of sorbent.

Fig. 3 illustrated the SEM image of calcined Ca/Ti granule with a smooth appearance and compacted formation. At the same experimental conditions, granule made from the absent of TiO2 displayed a rapid

decay on CO2 multi-sorption performance in FBR. Actually, CO2 sorption using Ca-Al-CO3 powder has

Sorbents Particle size Surface Area, _m2_/g Pore Volume, _cc/g Pore Diameter, _nm

TiO2 powder 40 nm 63.8 0.25 3.0 Ca-Al-CO3 powder 17.3 0.06 14.6 Ca-Al-CO3 granule (diameter×length) 3 mm× 5 mm 12.9 0.04 23.9 Granule with x= Ca/Ti; x=4 (diameter×length) 3 mm× 5 mm 23.0 0.06~0.16 15.0

Table 1. Microscopic characteristics of sorbents N2 CO2 Mixer Gas inlet Gas outlet NDIR _PC 1

2

1: Furnace

2: Quartz tube with Sorbent

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an obvious drawback, as quickly breakthrough appearance in FBR. In addition, Ca-Al-CO3 granule

became easily fragile material after repeated high-temperature swimming adsorption process. The mechanical strength needs to be improved by using TiO2 as support. At the same time, this figure

indicated a clear co-existence of Ca-Al-CO3 and TiO2 in the magnification part. The morphology showed

Ca-Al-CO3 was entirely separated by surrounded TiO2 that was suspected to as an effective isolator of in

Ca-Al-O LDO reactor.

As shown in this figure, XRD patterns demonstrated that specific absorption peaks were observed in calcined Ca/Ti granule, including of anatase (A) and rutile phase (R) of TiO2, mainly of CaO and CaCO3

for calcined Ca-Al-CO3, and CaTiO3 for granule. Wherein, a commercial product of TiO2 (Degussa P25)

has 80:20 (w/w) of A/R, which is accordance with observed patterns. The hydrotalcite-like structure is merely existed at larger than 400°C calcinations [10], thus resulting in the structure change from LDH-like formation of nascent Ca/Al carbonates to calcined Ca-Al-CO3 with layered double oxide (LDO)

structure at 600°C-calcination. Besides, by using TiO2 as binder, calcination of TiO2/CaCO3 to form

CaTiO3/CaO composition was found in the XRD patterns of granule.

3.2 Investigation of CO2 sorption capacity

Multi-cycle stability was tested using prepared sorbents in TGA. As shown in Fig.4(A), the result showed that calcium acetate derived Ca-Al-CO3 powder exhibited high CO2 capacity of about 54 wt% at

the first cycle. This material exhibited an improved performance with at least 90% recovery of initial capacity, as shown of approximately 50wt% capacity after 100 cycles (9,000 min). There is a conservative estimation that CO2 sorption weight is maintained as 50 times of used sorbent weight after

150 hours.

Fig. 3. Left: SEM imagines of Ca/Ti granule. Right: XRD patterns of TiO2, calcined Ca-Al-CO3 powder

and calcium aluminates granules with molar ratio of Ca:Al of 1:1, 3:1 and 6:1. TiO2 Ca-Al-CO3 Ca/Ti = 1:1 Ca/Ti = 3:1 2 Theta, degree 10 20 30 40 50 60 70 80 Ca/Ti = 6:1 TiO2 Ca-Al-CO3 (A) (B) (C) (D) (E) A R A A AR A AR A A:anatase; R:rutile C1:CaCO3; C2:Ca(OH)2 C1 C1,C2 C1 T:CaTiO3 T T T C2 R C1 C2,C1

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The mixing of Ca-Al-CO3 (Ca) and TiO2 (Ti) powder is feasible to prepare granule as CO2 sorbents.

In various ratio of x=Ca/Ti, CaO content of sorbent is decreased with increased of TiO2. Thus as in

Fig.4(B), the initial capture weight of these granules was raised with higher Ca content. However, CO2

sorption weight be became gradual saturation at x=4-6. When x=4, this sorbent has 90% recovery of initial CO2 capture weight after 15 cycles, which revealed the better performance than others. These

outcomes obviously display the good performance of Ca-Al-CO3 based oxides as CO2 solid sorbent at

high temperature condition.

Granule prepared by x=Ca/Ti

Cycles 0 2 4 6 8 10 12 14 16 w t% , gCO 2 / g sorbent 0 10 20 30 40 50 Time/min 0 2000 4000 6000 8000 10000 g CO 2 /g sorbent 0 10 20 30 40 50 60 (B) (A) x=3,

Fig. 4. Testing of CO2 sorption stability in TGA using (A) Ca-Al-CO3 powder after 100 cycles and (B) Ca/Ti

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In a fixed-bed reactor, CO2 sorption stability tested by using Ca-Al-CO3 powder and Ca/Ti granule

(x=4) was shown in Fig. 5. The sorption weight was calculated by transform of adsorbing volume into molar volume of ideal gas at 25°C and 1 bar condition. CO2 sorption performance was estimated by ratio

of weight change at nth_{cycles (W}

n) by the first cycle (W1) using Ca-Al-CO3 based sorbents. It was found

that a fast breakthrough appeared in a short period, emerging CO2 sorption recovery using pure

Ca-Al-CO3 (powder and granule) was dramatically decayed to 40-50% only after 2-5 cycles and further

deteriorated to 33% after 10 cycles. Alternatively, the stability was obviously improved to 80% using granule after 10 cycles. It is believed that TiO2 provides a binding force to form a matrix of (Ca-Al-CO3

)-Ti-O, effectively separating the Ca-Al-CO3 segment from local sintering effect on neighboring sorbents in

reactor. Therefore, one of the significant drawbacks such as fast breakthrough can be greatly retarded via incorporating TiO2 into Ca-Al-CO3. These promising results suggest that granule derived from

Ca-Al-CO3 absorbent may provide outstanding performance in carbonation calcination processes for CO2

capture reactor. According to the performance of CO2 sorption experiment, granule sorbent displays more

advantages than powder in FBR.

4. Conclusions

In this study, Ca-Al-CO3 has been used as high-temperature CO2 sorbent, which is fabricated as

powder and Ca/Ti granule. In TGA experiment, relatively high CO2 capacity of approximately 45~50

wt% after 100 cycles can be achieved with Ca-Al-CO3 powder; however, CO2 breakthrough tendency

becomes significant, due possibly to the sintering of powder, and results in poor performance for Ca-Al-CO3 in FBR. The reaction of CO2 with sorbent is better conducted on granule than over powder,

especially at fixed-bed reactor. The results showed that sorption-desorption cyclic test using granule worked successfully at 600-850°C, with 80-90% recovery of initial CO2 capture weight after 15 cycles in

TGA and 70-80% of that after 10 cycles in FBR, respectively. This outcome indicates that CO2 sorbent

developed from calcium aluminates granule with TiO2 binder is competitive material for controlling CO2

emission.

Fig. 5. CO2 sorption stability using Ca-Al-CO3 powder, granule and Ca/Ti granule after 10 cycles tested in FBR.

nth_Cycles 0 2 4 6 8 10 12 Ratio of W n /W 1 20 40 60 80 100 120 Ca-Al-CO3 powder Ca/Ti granule Ca-Al-CO3 Granule

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Acknowledgements

This work was financially supported by the National Science Council of the Republic of China, Taiwan under Contract No. NSC 101-3113-E-009-003-.

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