Effects of Steam Addition on the Properties of High Temperature Ceramic Acceptors

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Effects of Steam Addition on the Properties of HighTemperature Ceramic CO2 Acceptors

Esther Ochoa-Fernández1; Tiejun Zhao, Ph.D.2; Magnus Rønning3; and De Chen4

Abstract: The effects of steam addition on the stability and CO2 capture and regeneration properties of Li2ZrO3, K-doped Li2ZrO3,Na2ZrO3, and Li4SiO4 at relevant sorption enhanced steam methane reforming conditions has been investigated. It has been observed thatthe presence of water in the form of steam enhances the capture and regeneration rates. However, a large decay in the capacity is observedwhen compared to the performance of the acceptors in dry conditions. The reasons for the improved kinetics are not clear, but a highermobility of the alkaline ions is suggested. The losses in capacity could be due to sintering under high pressure of steam, vaporization ofalkali metals, and/or phase segregation.

DOI: 10.1061/�ASCE�EE.1943-7870.0000006

CE Database subject headings: Steam; Sorption; Temperature effects; Methane.

Introduction

During the last several years, increasing attention has been givento the separation, recovery, and storage/utilization of CO2 becauseof the growing global warming �Anderson and Newell 2004�. Theuse of fossil fuels accounts for about 75% of the current anthro-pogenic CO2 emissions. Therefore, great efforts are being madeto develop technologies that allow the use of fossil fuels withlower carbon dioxide emissions.

A possible alternative is H2 production by CO2 sorption-enhanced steam methane reforming �SESMR�. In this process, aCO2 acceptor is installed together with the catalyst in the reactorbed for removal of CO2 from the gas phase, thereby pushing theequilibrium limits of the reforming and water–gas shift reactions.Balasubramanian et al. �1999� have shown that SESMR has sev-eral advantages compared to conventional steam methane reform-ing, among them, production of high-purity H2 ��95% dry basis�in a single step. The main challenge is, however, to develop CO2

acceptors with fast kinetics at low partial pressures of CO2, easyregeneration, high stability, and with high carbon dioxide capturecapacity at the working temperatures.

Recently, alkali ceramics, including Li2ZrO3, Na2ZrO3, and

1Ph.D. at the Dept. of Chemical Engineering, Norwegian Univ. ofScience and Technology, Sem Sælands vei 4, NO-7491 Trondheim, Nor-way; presently, StatoilHydro Research Centre, Arkitekt Ebbells veg 10,NO-7005 Trondheim, Norway.

2Dept. of Chemical Engineering, Norwegian Univ. of Science andTechnology, Sem Sælands vei 4, NO-7491 Trondheim, Norway.

3Professor at the Dept. of Chemical Engineering, Norwegian Univ. ofScience and Technology, Sem Sælands vei 4, NO-7491 Trondheim,Norway.

4Professor at the Dept. of Chemical Engineering, Norwegian Univ. ofScience and Technology, Sem Sælands vei 4, NO-7491 Trondheim, Nor-way �corresponding author�. E-mail: [email protected].

Note. This manuscript was submitted on April 30, 2008; approved onOctober 13, 2008; published online on March 23, 2009. Discussion pe-riod open until November 1, 2009; separate discussions must be submit-ted for individual papers. This paper is part of the Journal ofEnvironmental Engineering, Vol. 135, No. 6, June 1, 2009. ©ASCE,
ISSN 0733-9372/2009/6-397–403/$25.00.
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J. Environ. Eng. 2009

Li4SiO4, have been referred to as effective CO2 acceptors due totheir reversible CO2 capture/regeneration properties and favorablethermodynamics �Essaki et al. 2004; Fauth et al. 2005; Ida et al.2004; Kato and Nakagawa 2001; Kato et al. 2002; Kimura et al.2005; Lopez-Ortiz et al. 2004; Nair et al. 2004; Nakagawa andOhashi 1998; Ochoa-Fernández et al. 2006a,b; Xiong et al. 2003;Yi and Eriksen 2006; Zhao et al. 2007�

Li2ZrO3 + CO2 ↔ Li2CO3 + ZrO2, �H298 = − 160 kJ/mol

�1�

Na2ZrO3 + CO2 ↔ Na2CO3 + ZrO2, �H298 = − 149 kJ/mol

�2�

Li4SiO4 + CO2 ↔ Li2CO3 + Li2SiO3, �H298 = − 143 kJ/mol

�3�

Several studies have aimed at a better understanding of theproperties of these materials including capture kinetics, regenera-tion conditions, and stability. However, in most cases these stud-ies were carried out in CO2 atmospheres in the absence of steam.Typical steam contents during SESMR are higher than 30%.Therefore, it is obvious that the effect of steam addition on thecapture and regeneration properties and stability of these materi-als needs to be further investigated.

The purpose of this study is to systematically investigate theeffect of water addition on the stability and CO2 capture andregeneration properties of Li2ZrO3, K-doped Li2ZrO3, Na2ZrO3,and Li4SiO4 at relevant SESMR conditions.

Experimental

Preparation of CO2 Acceptors

Li2ZrO3, Na2ZrO3, and K-doped Li2ZrO3 have been prepared bya novel soft chemistry route. Zirconyl nitrate, lithium acetate,sodium citrate, and potassium nitrate were used as Zr, Li, Na, andK precursors, respectively. In all cases, appropriate amounts of
the precursors were dissolved in deionized water and stirred for
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several hours. The resulting solutions were dried at 373 K in anoil bath under continuous stirring. Finally, Li2ZrO3 and K-dopedLi2ZrO3 were obtained by calcination in air of the solid driedproducts at 873 K for 6 h. A two-step calcination was carried outin the case of Na2ZrO3; first, the sample was heated to 1073 K inAr and then calcined at this temperature for 3 h in air. Moredetails about the preparation and characterization of the ceramicoxides are available elsewhere �Ochoa-Fernández et al. 2006b,2008; Zhao et al. 2007�. Li4SiO4 is a commercial sample providedby Toshiba. A summary of the main physical properties of thestudied acceptors is presented in Table 1.

CO2 Capture Performance Test

CO2 capture and regeneration properties were evaluated usinga tapered element oscillating microbalance �TEOM� �Ochoa-Fernández et al. 2006b�. A scheme of the TEOM reactor setup hasbeen reported previously �Chen et al. 1996�. The tapered elementwas loaded with 20–30 mg of the CO2 acceptor together withquartz particles. The acceptor to quartz ratio was approximately1 /1 �% by weight�. Quartz was found inactive and used as dilu-ent. The samples were heated to 848 K at a heating rate of10 K /min in pure argon and held for 60–100 min. The CO2

capture was initiated by switching from Ar to the reaction mixtureat the same temperature. The different partial pressures of CO2

and steam in the reaction mixture were obtained by adjusting theflow rates of Ar, steam, and CO2 at a constant total flow rate of100 mL /min. After saturation of the acceptor, the temperaturewas increased at 10 K /min to the selected regeneration tempera-ture and the feed was changed from CO2 to Ar, or a mixture of Arand steam to release the CO2. The atmosphere used during theheating was identical to the one used in CO2 capture. The capturereaction was typically carried out at 848 K for all the samples,whereas different temperatures were used for regeneration. A de-tailed description of the testing procedure has been reported else-where �Ochoa-Fernández et al. 2006b�.

Results and Discussion

Effect of Steam on the CO2 Capture Kineticsof Ceramic Acceptors

Monoclinic Na2ZrO3 has been reported to have excellent CO2

capture kinetics in dry conditions �Zhao et al. 2007�, but the be-havior in wet conditions has not been reported. Fig. 1�a� showsthe CO2 uptake profiles over Na2ZrO3 at 848 K, 10% CO2 anddifferent amounts of steam. Saturation of the acceptor is reached

Table 1. Physical Properties of the Ceramic Acceptors

Acceptor

Theoretical CO2

capacitya

�% by weight�

Crystallitesizeb

�nm�

BETsurface areac

�m2 /g�

Li2ZrO3 28.8 13 5

K doped Li2ZrO3d 27.1 17 2

Na2ZrO3 23.4 30 5

Li4SiO4 36.6 37 2aStoichiometric gr CO2 /gr acceptor.bMeasured by x-ray diffraction.cMeasured by N2 adsorption/desorption.dK:Li:Zr 0.2:2:1.

within 100 s at dry conditions. Addition of 10% steam results in

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Fig. 1. �a� Experimental CO2 uptake profiles over Na2ZrO3 at 848 K,10% CO2, and different amounts of steam. The total flow was setto 100 mL /min using Ar as balance gas. �b� Experimental CO2

uptake profiles over Li4SiO4 at 798 K, 10% CO2 in the presence�10%� and absence of steam. The total flow was set to 100 mL /minusing Ar as balance gas. �c� Experimental CO2 uptake profiles overK0.2Li2ZrO3.1 at 848 K, 10% CO2 in the presence �20%� and absenceof steam. The total flow was set to 100 mL /min using Ar as balancegas.

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an improvement of the kinetics and saturation takes place withinless than 30 s. Fig. 1�a� also evidences that adding larger amountsof steam does not result in further improvements in the capturerates, but instead yields lower total capacities. The losses in ca-pacity in presence of steam will be discussed later.

Essaki et al. �2004� also observed a beneficial effect of ad-ding water when absorbing CO2 on Li4SiO4 at room temperature�Essaki et al. 2004�. Fig. 1�b� shows the CO2 uptake profiles overLi4SiO4 at 798 K and 10% of CO2 with and without water addi-tion. The addition of water has a great positive impact on thecapture kinetics. In fact, saturation is reached within 15 min when20% steam is added, whereas more than 100 min are needed forsaturation in absence of steam. The same trend is observed athigher CO2 concentrations �30% CO2 and 60% CO2, results notshown here�.

Fig. 1�c� shows similar results in the case of potassium pro-moted Li2ZrO3 at 10% CO2, with and without steam addition.Also here the same trend has been confirmed at different CO2

concentrations �30% CO2 and 50% CO2, results not shown here�.Yi and Eriksen �2006� recently came to the same conclusion afterstudying the effect of steam on the capture kinetics of Li2ZrO3.They observed that the CO2 uptake rate increased considerablywhen introducing fractions of steam larger than 20%.

The reasons for the beneficial effect of steam addition on theCO2 absorption kinetics of the CO2 ceramic acceptors is not clear.The presence of steam is believed to enhance the Li+, K+, and Na+

mobility, and therefore the rate of the reactions. The alkaline ionstend to segregate to the surface due to their low surface freeenergy �Ochoa-Fernándex et al. 2008�. Simultaneously, the diffu-sion of CO2 through the Li2CO3 layer formed during the carbon-ation reaction might also be favored under steam �Yi and Eriksen2006�. Essaki et al. �2004� suggested that the layer of carbonateformed in the surface during absorption might dissolve in water,thus decreasing the diffusion resistance. However, this theory isnot confirmed by the results of the present investigation.

The effect of steam on the carbonation and calcination ofother types of CO2 acceptors, including CaO, hydrotalcites, andperovskites, has recently been addressed by several authors�Hughes et al. 2004; Manovic et al. 2008; Reijers et al. 2006; Sunet al. 2008; Yan et al. 2008; Zeman 2008�. For example, Zemanstudied the effect of steam hydration on the performance of limeas sorbent for CO2 capture. Zeman �2008� concluded that hydra-tion leads to improved stability of CaO, but their results donot give conclusive evidence on the effect of hydration on thekinetics. Similar results have been obtained by other authors�Hughes et al. 2004; Manovic et al. 2008; Sun et al. 2008; Zeman2008�. Increased pore surface area and pore volume via steamhydration has generally been proposed as the main reason for theimprovement on the conversion of the CaO-based sorbents overmulticle cycles �Hughes et al. 2004�.

Reijers et al. �2006� also reported that the presence of steamduring CO2 adsorption on hydrotalcites is very important. Theyobserved that the CO2 capacity is lower at dry conditions, butno clear indication about the effect on the kinetics was given. Yanet al. �2008� studied the adsorption of CO2 on perovskites in theabsence and presence of water. Their results indicate that the pres-ence of steam enhances the CO2 absorption probably due to theformation of bicarbonates. The presence of oxygen vacancies may
play an essential role in the formation of such bicarbonates.
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Effect of Steam on the CO2 Regeneration Kinetics ofCeramic Acceptors

Although the CO2 capture properties of different ceramic CO2

acceptors have been described in the literature, there are not manystudies about their regeneration properties. In this investigationthe regeneration of Li2ZrO3, K-doped Li2ZrO3, Na2ZrO3, andLi4SiO4 have been studied for a broad range of temperatures andpressures of steam. The maximum regeneration temperature usedwas 953 K due to limitations in the experimental setup.

Fig. 2�a� shows the regeneration profiles of Li2ZrO3 at 848 Kand steam pressures from 0 to 80%. The regeneration time de-pends strongly on the amount of steam, ranging from severalhours without steam to 30 min when the amount of steam is in-creased to 80%. The regeneration time is, as expected, alsostrongly dependent on the temperature. Fig. 2�b� shows the regen-eration profiles of Li2ZrO3 in 10% of steam at different tempera-tures, from 773 to 898 K. It is observed that faster regeneration is

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Fig. 2. �a� Experimental regeneration profiles over Li2ZrO3 at 848 Kand different amounts of H2O: �A� 0% H2O; �B� 10% H2O; �C� 20%H2O; �D� 50% H2O; �E� 70% H2O; and �F� 80% H2O. The total flowwas set to 100 mL /min using Ar as balance gas. �b� Experimentalregeneration profiles over Li2ZrO3 with 10% H2O at differenttemperatures: �A� 773 K; �B� 823 K; �C� 848 K; and �D� 898 K. Thetotal flow was set to 100 mL /min using Ar as balance gas.

achieved at higher temperatures. In fact, the addition of steam


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allows regeneration and capture to be carried out at the sametemperature in a reasonable timescale. It should be noticed that848 K was found to be the optimum CO2 capture temperature forLi2ZrO3 �Ochoa-Fernández et al. 2006b�.

The extent of regeneration at a time t has been calculated as

Extent regeneration at time t =g CO2 desorbed at time t

g CO2 adsorbed at saturation

The positive effect of steam on the regeneration kinetics and fa-vored decarbonation at temperatures as low as 848 K have alsobeen confirmed for K-doped Li2ZrO3, as observed in Fig. 3. Simi-lar results have been found for Na2ZrO3. However, higher tem-peratures for decarbonation are needed in this case. Fig. 4 showsthe regeneration profiles of Na2ZrO3 at 953 K in differentamounts of steam. As in the case for the K-doped Li2ZrO3, higher

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Fig. 4. Experimental regeneration profiles over Na2ZrO3 at 953 Kand different amounts of H2O: �A� dry conditions; �B� 30% H2O; �C�50% H2O; and �D� 70% H2O. The total flow was set to 100 mL /minusing Ar as balance gas.



rates are obtained with steam addition. However, regeneration at848 K was too slow, even in the presence of steam. The situationwas different in the case of Li4SiO4, where no significant effectof steam addition was observed on the regeneration time as re-flected in Fig. 5�a�. However, Li4SiO4 has the ability to regeneratefaster than its counterparts at temperatures as low as 798 K.Fig. 5�b� shows the regeneration profile of Li4SiO4 at three dif-ferent temperatures.

Ceramic CO2 acceptors react chemically with CO2 and there-fore, temperature swing regeneration is usually preferred. How-ever, Figs. 2–5 indicate that the use of steam enhances theregeneration kinetics, hence making the regeneration at isother-mal conditions by a pressure swing process a possible alternative.Pressure swing adsorption �PSA� is based on the preferential ad-sorption at high pressure, and desorption at low pressure; whereasthe temperature swing adsorption �TSA� is based on preferentialadsorption at certain temperatures and regeneration at higher tem-peratures �Siriwardane et al. 2005�. PSA technology is gainingground on TSA because it results in lower energy requirements

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Fig. 5. �a� Experimental regeneration profiles over Li4SiO4 at 848 Kin dry and wet conditions �10% H2O�. The total flow was set to100 mL /min using Ar as balance gas. �b� Experimental regenerationprofiles over Li4SiO4 with 10% H2O at three different temperatures.The total flow was set to 100 mL /min using Ar as balance gas.

and lower investment costs. A major advantage of PSA relative to

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other types of adsorption processes is that the pressure can bechanged much more rapidly than the temperature. This allows foroperation in faster cycles, thus increasing the throughput of thesystem. However, the major limitation is that PSA processes arerestricted to components that are not too strongly adsorbed �Ruth-ven et al. 1994�. The results from this investigation indicate thatpartial regeneration of the studied CO2 ceramic acceptors mightbe possible by using PSA technology. However, Air Products hastested the pressure swing behavior of Li4SiO4 at 873 K. It wasfound that a lot of purge gas was necessary due to the low equi-librium pressure of CO2, and the conclusion was reached thatPSA using purge gas may not be the best solution for regenerationof the ceramic acceptors.

Effect of Steam on the Stability of the CeramicCO2 Acceptors

So far, it has been shown that working on environments withsteam has a beneficial effect on both the carbonation and calcina-tion reactions of ceramic CO2 acceptors. However, the effect ofsteam on the stability of the acceptors is not positive. Fig. 6shows the stability profiles of the acceptors included in this studyin the presence and absence of steam. During dry operation thecarbonation reaction was carried out at 848 K and 50% CO2,whereas the regeneration was carried out in pure Ar at 923 K forLi2ZrO3 and K-doped Li2ZrO3, and at 953 K for Na2ZrO3 andLi4SiO4. At wet conditions the carbonation reaction was carriedout at 848 K, 50% CO2, and 20% H2O, whereas the regenerationwas carried out in a mixture of Ar and H2O �8:2� at 923 K forLi2ZrO3 and K-doped Li2ZrO3, at 953 K for Na2ZrO3, and at848 K for Li4SiO4. Figs. 6�a and b� show that all the acceptorsshow good stability in dry conditions. A decrease in capacity isobserved only for K-doped Li2ZrO3, which is most likely due tothe formation of a molten carbonate between Li2CO3 and K2CO3

�Ochoa-Fernández et al. 2008�. The presence of a liquid phaseduring the capture/regeneration reactions favors crystal and par-ticle growth. This was evidenced in a previous scanning electronmicroscopy investigation over fresh and cycled K-doped Li2ZrO3

�Ochoa-Fernández et al. 2008�.A large deactivation of the acceptors is observed after reacting

with steam during capture and regeneration. Possible explanationsfor this phenomenon include phase segregation, sintering, andevaporization of alkali metals.

It is known that molten alkali carbonates in the presence ofsteam can produce their respective alkali hydroxides according tothe following hydrolysis reaction �Mohn and Wendt 1995�:

Me2CO3 + H2O ↔ 2MeOH + CO2 �4�

Mohn and Wendt have investigated the evaporation of moltencarbonates in atmospheres containing CO2 and water vapor in thetemperature range from 1,023 to 1 ,143 K. They concluded thatthe evaporation of alkali metal ions increases with the ratio of thepartial pressures of water vapor and carbon dioxide according to�PH2O / PCO2

�1/2, and that this is due to the evaporation of thealkali hydroxides formed according to reaction �4�. They showedthat KOH has a higher vapor pressure than LiOH, which againhas a higher vapor pressure than NaOH. The partial evaporationof the alkaline metals can explain the loss in the capacity in thiswork. This is supported by x-ray diffraction �XRD� analysis ofthe samples after reaction with water. It was observed that thediffraction lines of the mixed oxides tend to disappear, whereasdiffraction lines of ZrO2 are predominant. This can be a sign of
the existence of free ZrO2 due to alkali metal evaporation or due
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to phase segregation. It should be mentioned that no indicationsof crystalline carbonates were found after regeneration. Still,phase segregation cannot be excluded as a reason for the decay incapacity. No such decay was observed for Li4SiO4. In fact, afterXRD analysis, clear signs of Li4SiO4 are still present, togetherwith other unidentified phases. It should be recalled that Li4SiO4

is a commercial sample and details about its exact compositionare not available. The higher stability could also be due to thelower temperature used for regeneration in this case �848 K�.

Another possible reason for capacity decay is sintering. Themorphology of the powders before and after reaction with waterwas examined in a field emission scanning electron microscope.No significant differences in the particle size distribution beforeand after reaction were found in the case of Li2ZrO3, andNa2ZrO3. Significant coarsening was, however, observed in thecase of K-doped Li2ZrO3 and Li4SiO4. As discussed earlier, thepresence of a molten phase during the capture/regeneration reac-tions in K-doped samples favors the particle growth. Fig. 7 showsthat large packed particles are formed after reaction in CO2 and

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Fig. 6. �a� Experimental CO2 capture capacity after saturation ofLi2ZrO3 and K doped Li2ZrO3 for successive cycles in dry and wetconditions �20% H2O�. �b� Experimental CO2 capture capacity aftersaturation of Na2ZrO3 and Li4SiO4 for successive cycles in dry andwet conditions �20% H2O�.

steam. This formation can probably explain the large decay in


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capacity. However, even if sintering was observed in Li4SiO4

after reaction on steam, no large deactivation was experienced,probably due to higher porosity of the formed particles. Fig. 8shows SEM images of Li4SiO4 before and after reaction. It shouldalso be pointed out that a large number of fine powders were alsopresent in addition to the larger particles.

Thus, the decay in capacity observed for ceramic acceptorsafter reaction in steam could be explained by sintering, alkalivaporization, and/or phase segregation. None of these deactiva-tion mechanisms can be excluded by the present results.

The behavior of the ceramic acceptors in steam is very differ-ent from the behavior of CaO-based acceptors. If fact, treatmentof CaO-based acceptors in steam or water has been proved to bebeneficial for their capture stability �Hughes et al. 2004; Manovicet al. 2008; Sun et al. 2008; Zeman 2008�.

Conclusions

It has been shown that the addition of steam has an importantimpact in the capture and regeneration kinetics, and also on the

(b)

Fig. 7. SEM images of potassium promoted Li2ZrO3: �a� freshsample; �b� after reaction for eight cycles under steam, regenerationtemperature 923 K

stability of the acceptors. In general, the addition of steam im-



proves the capture kinetics and also favors the regeneration. It isbelieved that the presence of steam enhances the mobility of thealkaline ions, and therefore promotes the rate of the reactions.However, continuous deactivation is also observed. The decay incapacity could be ascribed to either phase segregation, sinteringand/or evaporization of alkali metals.

Acknowledgments

Financial support from the Research Council of Norway is greatlyacknowledged.

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Documents

Effects of Steam Addition on the Properties of High Temperature Ceramic Acceptors