high temperature co2 sorbents and their application for hydrogen production by sorption enhanced...

Chemical Engineering Journal 283 (2016) 420–444

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Chemical Engineering Journal

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Review

High temperature CO2 sorbents and their application for hydrogenproduction by sorption enhanced steam reforming process

http://dx.doi.org/10.1016/j.cej.2015.06.0601385-8947/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +1 418 656 2204; fax: +1 418 656 5993.E-mail address: [email protected] (M.C. Iliuta).

Marziehossadat Shokrollahi Yancheshmeh, Hamid R. Radfarnia, Maria C. Iliuta ⇑Department of Chemical Engineering, 1065 Av. de la Médecine, Université Laval, Québec, Québec G1V 0A6, Canada

h i g h l i g h t s

� SESR is a forefront technology to produce highly pure H2 in one step.� High-temperature CO2 sorbents are the key element for successful SESR process.� Ca-based and alkaline-based sorbents are the most investigated high-temperature CO2 sorbents.� Capacity decay of Ca-based sorbents and slow kinetics of ceramics are the main challenges for industrial applications.� Development of efficient hybrid catalyst/sorbent materials is an interesting opportunity for research.

a r t i c l e i n f o

Article history:Received 2 April 2015Received in revised form 10 June 2015Accepted 11 June 2015Available online 10 July 2015

Keywords:High-temperature CO2 sorbentsCaO-based materialsAlkaline-based materialsHybrid catalyst–sorbent materialsSorption enhanced steam reformingReview

a b s t r a c t

Among the available techniques for hydrogen production, the sorption enhanced steam reforming (SESR)is an emerging technology consisting in the integration of reforming reaction (H2 production) and selec-tive separation (CO2 sorption) in a single step, to shift thermodynamically the reforming reaction andincrease hydrogen production. It is a forefront technology to produce highly pure hydrogen that has sev-eral advantages against the conventional steam reforming operation. The key element for a successfulSESR process is the selection of suitable high-temperature CO2 sorbents. Due to the weakness of currentCO2 sorbents (capacity decay and/or slow kinetics), the improvement of their performance is crucial tomake the SESR process interesting for industrial applications. This review focuses on the main character-istics and preparation methods of CaO-based and alkaline-based sorbents, their advantages anddrawbacks, the available techniques to improve their behavior in severe operating conditions, as wellas the progress of their application in two important SESR processes, namely sorption enhanced steammethane reforming (SESMR) and sorption enhanced steam glycerol reforming (SESGR).

� 2015 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212. High temperature CO2 sorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

2.1. CaO-based sorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
2.1.1. Application of various sources of calcium to produce CaO sorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4232.1.2. Metal-stabilized CaO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4242.1.3. Additional treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427
2.2. Ceramic CO2 sorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
2.2.1. Lithium zirconate (Li2ZrO3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4312.2.2. Lithium orthosilicate (Li4SiO4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4332.2.3. Sodium zirconate (Na2ZrO3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4342.2.4. Other ceramic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4342.2.5. Kinetic models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
3. Hydrogen production by sorption enhanced steam methane reforming (SESMR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435

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3.1. Application of CaO-based sorbents in SESMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4363.2. Application of ceramic sorbents in SESMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

4. Hydrogen production by sorption enhanced steam glycerol reforming (SESGR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4395. Conclusion and recommendations for future works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441

1. Introduction

Carbon dioxide (CO2) is a greenhouse gas (GHG) naturally pre-sent in the atmosphere. However, the increase of CO2 concentra-tion due to human activities is the main factor that contributesto Earth’s global warming. In May 2015 the CO2 concentrationreached 403 ppm, representing an increase by approximately 38%in comparison with the middle of the 19th century [1]. Mostanthropogenic CO2 emissions come from the use of fossil fuelsfor energy production in power plants, car engines and heatingprocesses. Hence, the continuous increase of global consumptionof energy is largely reflected by the increase of CO2 emissions.

Among diverse possible approaches to lower the CO2 emissions,the reduction of fossil fuel consumption by the increase of pro-cesses efficiency and switch to less carbon-intensive fuels (e.g., nat-ural gas) and/or to free-carbon energy sources (e.g., biomass) arethe most effective and economic methods to meet the goals of envi-ronmental rules [2]. In addition, the development of more efficientand cost-competitive technologies to capture and storage the CO2

(CCS) has recently attracted considerable interest as an option tocontrol CO2 emissions. Post-combustion, pre-combustion andoxy-combustion are the main methods for CO2 capture [3]. Theprinciple of post-combustion is to separate CO2 from combustionexhaust gases that mainly contain CO2, N2 and O2. In the case ofoxy-combustion, pure oxygen is used rather than air for fuel com-bustion to avoid the dilution of CO2 by N2; as the resulting exhaustpractically contains only CO2 and water vapor, the CO2 can be easilyseparated. The pre-combustion capture involves the gasification offossil fuels, which are first converted into a mixture of hydrogenand CO2; CO2 is therefore removed prior to combustion process.

Among several available technologies for CO2 capture likechemical absorption, membranes and solid sorbents, the absorp-tion based on chemical solvents (especially amine-based solutions)is the most commonly used method for post-combustion CO2 cap-ture due to its high CO2 removal efficiency, particularly at low CO2

partial pressure. The use of solid sorbents is the most efficient tech-nique for pre-combustion capture. Most available works in theopen literature are directed on high temperature solid sorbents likelithium zirconate (Li2ZrO3) [4,5], sodium zirconate (Na2ZrO3) [6,7],lithium silicate (Li4SiO4) [8–10] and CaO-based sorbents [11–13].High CO2 sorption capacity and adsorption/desorption kineticsrate, relatively mild regeneration temperature, and multi-cycle sta-bility are the most important parameters to be taken into account.Therefore, there are still lots of challenges in the development ofefficient CO2 sorbents, especially related to long-term cyclic stabil-ity, capacity of sorption, regeneration condition and rate of CO2

capture.Up to now, several research groups have provided review

papers about different strategies used to overcome the decay inCO2 capture capacity of CaO-based sorbents over multiple carbon-ation/calcination cycles, including synthesis of CaO-based sorbentsfrom different precursors, stabilization of sorbents by incorporat-ing CaO in support materials, and reactivation of sorbents withhydration, thermal pretreatment and chemical pretreatment [14–17]. Wang et al. [18] provided a review on the synthesis methods,CO2 adsorption/desorption characteristics and possible sorptionmechanisms for LixZryOz with different Li/Zr ratios, but it did not

discuss other types of ceramic CO2 sorbents such as Na2ZrO3 andLi4SiO4. Although the sorption enhanced steam reforming is oneof the most important applications of high-temperature sorbents,no comprehensive review is available in the open literature. Thisreview paper focuses on the main characteristics and preparationmethods of all appropriate high-temperature CO2 sorbents, theavailable techniques for improvement of their behavior in severeoperating conditions and the progress of their application in theSESR process. It is divided into the following main sections: (i)CaO-based sorbent; (ii) ceramic CO2 sorbents; (iii) sorptionenhanced steam methane reforming (SESMR); and (iv) sorptionenhanced steam glycerol reforming (SESGR).

2. High temperature CO2 sorbents

A highly efficient sorbent for CO2 capture at high temperatureshould possess specific properties such as: thermal stability at highoperating temperatures (450–700 �C); adequate CO2 sorptioncapacity and kinetics; easiness of sorbent regeneration;long-term cyclic stability; and reasonable production cost. Themost promising high-temperature solid sorbents available in theliterature mainly include ceramic alkaline-based and CaO-basedsorbents. Hydrotalcite (HTLc) is another type of sorbent, but itsCO2 uptake capacity is very low in comparison to the other mate-rials [19,20]. The following section will therefore review the prop-erties and preparation methods of the most commonly used solidsorbents, including CaO-based and alkaline-based sorbents.

2.1. CaO-based sorbents

One cannot deny that CaO is the most famous natural CO2 sor-bent that exists in nature in the forms of limestone (CaCO3) anddolomite (CaMg(CO3)2). This sorbent has attracted a lot of atten-tion because of its low raw material cost, high CO2 sorption capac-ity and adequate kinetics of reactions. The sorption/desorptionreaction of CaO is given by:

CaOðsÞ þ CO2ðgÞ $ CaCO3ðsÞ DH�298 ¼ �175:7 kJ=mol ð1Þ

Theoretical (stoichiometric) CO2 capture capacity of CaO is ashigh as 0.786 g of CO2/g of sorbent. Nevertheless, dolomite(CaMg(CO3)2) and huntite (CaMg3(CO3)4) have lower CO2 sorptioncapacity (dolomite: 0.46 g of CO2/g of sorbent and huntite: 0.25 gof CO2/g of sorbent) because MgO does not participate in CO2

adsorption.The endothermic regeneration reaction (reverse reaction in Eq.

(1)) needs a high amount of energy and usually occurs at a temper-ature above 900 �C in CO2 atmosphere [21]. Although CaO hassome benefits as a CO2 sorbent, its industrial application encoun-ters with some critical issues such as the loss of sorption capacityin long-term operation and the loss of reactivity with sulfur con-taining gases to form CaSO4 [11,22–24]. The loss of CO2 sorptioncapacity during cyclic operation is mainly resulted from the sinter-ing phenomenon, which consists of the agglomeration of smallparticles, the change of pore shapes, and the pore shrinkage. As itcan be seen in Fig. 1, the amount of unreacted CaO increases alongwith the cycle number until a rigid interconnected CaO skeleton is

Fig. 1. Sintering phenomenon of CaO; light grey: CaO phase and dark grey: CaCO3 phase [26].

Fig. 2. Typical weight changes vs. time for a repeated number of calcination/carbonation cycles of Piaseck limestone [23].

422 M. Shokrollahi Yancheshmeh et al. / Chemical Engineering Journal 283 (2016) 420–444

formed after 50 cycles. The carbonation takes place therefore onthe external surface of the material. Reactive dynamics simulationswith the reactive force field (ReaxFF) performed in NVE ensemblesshowed that the sintering of CaO particles has occured because oftheir expansion during CO2 adsorption [25]. This expansion, whichleads to rapid sintering, is strongly influenced by temperature andparticle separation distance: the higher the adsorption tempera-ture and the shorter the distance between two sorbent particles,the faster the sintering rate during the adsorption process.Therefore, the capacity decay of CaO sorbents during multiple car-bonation/calcination cycles depends on the experimental temper-ature, the precursor type, and the duration of the recarbonationstep [26].

The CO2 chemisorption on CaO generally consists of tworegimes, fast and slow kinetic steps. Dou et al. [27], Rout et al.[28] and Mohammadi et al. [29] performed kinetic studies on thecarbonation reaction of different Ca-based synthetic sorbents andfound that the carbonation reaction was controlled by both chem-ical reaction at the CaOACaCO3 interface and carbonate layer diffu-sion. The fast kinetic step, which is controlled by chemical reaction,will be continued untill the carbonate layer surrounding the unre-acted CaO core is completed. Then, the slow kinetic step that iscontrolled by gas diffusion starts. At this step, the product layerrestricts the access of CO2 molecules to reactive sites. Alvarezand Abanades [30] reported that the gas diffusion and thus, thesorption rate, can be limited above a critical carbonate layer thick-ness of 50 nm. Mostafavi et al. [31] found that the CaO sorbentderived from limestone represented a higher initial carbonationreaction rate in comparison with that derived from dolomite,because of excessive CaO active sites. On the other hand, the ulti-mate conversion was higher for dolomite derived CaO sorbent at

low temperature (550 �C), whereas this value was higher for lime-stone derived CaO sorbent at high temperatures (600–675 �C). As amatter of fact, at the beginning of the reaction, the growth of CaCO3

layer for limestone was faster than that of dolomite because of thehigher rate of reaction. Since Knudsen diffusion is related to thesquare root of temperature, the diffusion through CaCO3 layerenhanced as the temperature increased above 550 �C. Therefore,limestone showed a higher ultimate conversion at 600 and 675 �C.

Some researchers studied the CO2 sorption behavior of lime-stone in long-term cyclic operation. Grasa and Abanades [23] eval-uated the CO2 capture performance of limestone over 500carbonation/calcination cycles and observed that the CO2 uptakecapacity significantly decreased during the first 20 cycles and thenstabilized along with the cycle number around 0.075–0.08 residualconversion up to 500 cycles. For CaO sorbent, Fig. 2 shows thedecrease of CO2 capture capacity along with the cycle number, aswell as the kinetic steps. In another study, Sun et al. [24] examinedthe CO2 sorption behavior of limestone through more than 1000carbonation/calcination cycles. They reported a calcium conversionbetween 4% and 17% (depending on the carbonation time) after150 cycles.

Some kinds of natural CaO sorbent containing MgO, such asdolomite and huntite, possess better stability during cyclic opera-tions. Silaban et al. [11] observed that after six adsorption/desorp-tion cycles, the capture capacity of limestone was decreased from61% to 35% of its theoretical value. However, dolomite was alreadystable at a sorption capacity of 40%. The behavior of dolomite wasascribed to the presence of MgO in its structure, which provided abetter structural stability. Bandi et al. [32] examined the CO2 cap-ture activity of calcite, dolomite and huntite for 47 adsorption/des-orption cycles. The experimental results showed that huntite could

Table 1Summary of investigations on CaO sorbents synthesized from different precursors.

Precursor Reaction conditions Reactor Numberof cycles

CO2 uptake atlast cycle (g-CO2/g-ads)

Ref.

Ads. Reg.

Calcium acetate 700 �C, 30% CO2, 300 min 700 �C, 100% He, 30 min TGA 27 0.49 [13]Calcium acetate 700 �C, 30% CO2, 10% H2O, 300 min 700 �C, 100% He, 30 min TGA 17 0.5 [13]Calcium D-gluconate 650 �C, 15% CO2, 30 min 900 �C, 100% N2, 10 min TGA 9 0.66 [39]Calcium D-gluconate 650 �C, 15% CO2, 30 min 920 �C, 15% CO2, 2 min TGA 57 0.19 [39]Nano-CaCO3 (�40 nm) 650 �C, 15% CO2, 20 min 850 �C, 100% N2, 10 min TGA 100 0.17 [30]Calcium naphthenate (FSP-made CaO) 700 �C, 30% CO2, 300 min 700 �C, 100% He, 30 min TGA 60 0.39 [45]Calcium naphthenate (FSP-made CaO) 700 �C, 30% CO2, 5 min nonisothermal to 900 �C,

100% He, 40 minTGA 20 0.39 [45]

CaAc2 650 �C, 15.30% CO2 850 �C, 100% N2 FB 9 0.303 [40]CaCO3 650 �C, 15.30% CO2 850 �C, 100% N2 FB 9 0.285 [40]CaCO3 650 �C, 15% CO2, 2 h 950 �C, 100% N2 TGA 11 �0.40 [34]Ca(NO3)2�4H2O 650 �C, 15% CO2, 15 min 850 �C, 100% N2, 10 min TGA 20 0.51 [46]Ca(NO3)2�4H2O 650 �C, 15% CO2, 15 min 950 �C, 100% CO2, 10 min TGA 20 0.2 [46]CaO 750 �C, 100% CO2, 40 min 750 �C, 100% N2, 30 min TGA 9 0.32 [35]CaCO3 650 �C, 100% CO2, 30 min 750 �C, 100% N2, 30 min TGA 9 0.1 [35]Ca(OH)2 650 �C, 100% CO2, 40 min 750 �C, 100% N2, 40 min TGA 9 0.25 [35]


preserve around 84% of its initial capacity, while dolomite and cal-cite were able to maintain 55% and 38% of their initial capacity,respectively. However, huntite possessed less CO2 uptake capacityin comparison to dolomite and calcite because of its higher contentof inactive MgO. According to these studies, the deactivation ofCaO sorbent derived from natural sources is unavoidable.Different strategies have therefore been proposed to improve theCO2 sorption performance of CaO sorbent, such as (i) applicationof various sources of calcium to produce CaO sorbent, (ii) incorpo-ration of stable inert materials into CaO structure, and (iii) reacti-vation and treatment of sorbent.

2.1.1. Application of various sources of calcium to produce CaO sorbentMany research groups have focused on the production of CaO

sorbents from various calcium precursors, including CaCO3 [33–35], Ca(OH)2 [35–37], organometallic [13,38–43] and nano-sizedCaO/CaCO3 [39,42,44–49], with the aim of providing a CaO-basedsorbent with meso- and macroporous structure, high specific sur-face area, large pore volume, and small particle size (Table 1).These specifications are mandatory for a sorbent with high andstable CO2 capture capacity. The performance of CaO sorbentsderived from two most promising groups, organometallic andnano-sized CaO/CaCO3 precursors are further discussed.

2.1.1.1. Organometallic precursors. Up to now, a number of studieshave reported the production of CaO sorbents from variousorganometallic precursors (OMPs) and the relation between thestructural characteristics of OMPs-derived CaO sorbents with theirCO2 uptake performance [13,38–43]. The decomposition of OMPsleads to the formation of meso- and macroporous structures withlarge surface area, which results in the enhancement of CO2 cap-ture performance [13,39]. In an earlier work, Silaban et al. [43]found that CaO synthesized from calcium acetate possessed ahigher CO2 capture capacity than CaO synthesized from calciumcarbonate independent of the calcination temperature. Similarly,Lu et al. [13] examined the CO2 capture activity of the CaO sorbentsderived from calcium acetate monohydrate, calcium carbonate,calcium hydroxide, and calcium nitrate tetrahydrate. Accordingto the experimental results, the CaO sorbent obtained from calciumacetate demonstrated the best performance with a CO2 sorptioncapacity of 0.49 g of CO2/g of sorbent after 27 cycles (carbonationat 700 �C under 30% CO2/He balance and calcination at 700 �Cunder He), because of its large BET surface area and pore volume.The SEM images revealed that this sorbent contained a fluffy struc-ture, which contributed to its high surface area and large pore vol-ume. In a further study, Liu et al. [39] prepared CaO sorbents using

different precursors, including calcium acetate hydrate, calciumcitrate tetrahydrate, calcium D-gluconate monohydrate, calciumformate, calcium L-lactate hydrate, calcium hydroxide, microsizecalcium carbonate, nanosize (<70 nm) calcium carbonate, andnanosize (<160 nm) calcium oxide. Among the developed materi-als, the CaO sorbent derived from calcium D-gluconate monohy-drate exhibited the highest CO2 capture capacity of 0.66 g ofCO2/g of sorbent at the 9th cycle. Yang et al. [40] synthesized fourtypes of Ca-based sorbents from calcium acetate monohydrate, cal-cium carbonate, calcium hydroxide, and calcium oxide precursorsby calcination and hydration reactions. The cyclic CO2 captureexperiments showed that CaO sorbents derived from calcium acet-ate and calcium carbonate presented a higher CO2 sorption capac-ity (0.299 and 0.284 g of CO2/g of sorbent at 650 �C, respectively)compared to CaO sorbents derived from calcium oxide and calciumhydroxide due to the larger pore volume and higher specific sur-face area of the former ones. According to the cyclic carbona-tion/decarbonation experiments, the CO2 sorption capacity ofCaAc2ACaO and CaCO3ACaO sorbents increased in the first cycleand then decreased and reached to 0.303 and 0.285 g of CO2/g ofsorbent after 9 cycles. Grasa et al. [38] studied the cyclic CO2 sorp-tion performance of CaO sorbents prepared from calcium hydrox-ide, calcium acetate, and calcium oxalate under realisticcalcination conditions (regeneration at temperatures around900 �C under CO2 atmosphere). Although these synthetic sorbentsperformed well under mild reaction conditions, they showed a dra-matic decay in CO2 capture capacity under realistic severe calcina-tion conditions, the behavior being similar to natural limestone.The best synthetic sorbent, which was derived from calcium acet-ate, exhibited a final CO2 sorption uptake slightly higher than thatof limestone. This CO2 capture behavior did not justify the highercost of sorbent production from chemical precursors.

2.1.1.2. Nano-sized CaO and CaCO3. For porous CaO with critical par-ticle size less than 44 nm or a large single CaO crystal with criticalcrystal size less than 220 nm, the carbonation reaction completeswithin the fast kinetically-controlled regime and the slowsolid-state diffusion controlled regime does not exist [47,50].Therefore, different research groups studied nano-sized CaO andCaCO3 as calcium precursors [39,42,44–49]. All of these studiesshow that nano-sized sorbents possess much better CO2 captureactivity in comparison to micro-sized CaO. However, there aretwo primary hurdles for using the nano-sized CaO sorbents in prac-tical applications: (i) the nano-sized CaO sorbents synthesizedfrom nano-sized particles are susceptive to sintering because ofhigher surface area [44]; (ii) the methods used for the synthesis

Table 2Summary of investigations on metal oxide stabilized CaO-based sorbents.

Stabilizer Stab.content (wt%)

Reaction conditions Reactor Numberof cycles

CO2 uptake at lastcycle (g-CO2/g-ads)

Ref.

Ads. Reg.

Ca12Al14O33 25 690 �C, 14% CO2, 30 min 850 �C, 100% N2, 10 min TGA 13 0.45 [12]Ca12Al14O33 25 690 �C, 14% CO2, 30 min 950 �C, 20% CO2, 10 min TGA 13 0.333 [12]Ca12Al14O33 25 700 �C, 20% CO2, 30 min 850 �C, 100% N2, 5 min TGA 50 0.41 [65]Ca12Al14O33 25 700 �C, 20% CO2, 30 min 980 �C, 100% CO2, 5 min TGA 56 0.22 [65]Ca12Al14O33 25 690 �C, 15% CO2, 30 min 850 �C, 100% N2, 10 min TGA 45 0.26 [66]Ca12Al14O33 65 650 �C, 33% CO2, 10 min 800 �C, 100% N2 TGA 50 0.19 [67]Ca12Al14O33 15 750 �C, 14% CO2, 8.3 min 750 �C, 100% N2, 8.3 min FB 20 0.26 [68]Ca12Al14O33 7.7 690 �C, 15% CO2, 30 min 850 �C, 100% N2, 5 min TGA 30 �0.37 [76]Ca12Al14O33 7.7 650 �C, 15% CO2, 10 min 900 �C, 15% CO2, 5 min TGA 30 �0.22 [80]Ca12Al14O33 9.2 750 �C, 40% CO2,20 min 750 �C, 100% N2, 20 min TGA 30 �0.55 [79]Ca9Al6O18 20 650 �C, 15% CO2, 30 min 800 �C, 100% N2, 10 min TGA 28 0.51 [63]Ca12Al14O33 �42 700 �C, 30% CO2, 10 min 700 �C, 100% He, 10 min TGA 100 0.4 [69]Ca12Al14O33 �42 850 �C, 100% CO2, 10 min 950 �C, 30% CO2, 10 min TGA 100 0.25 [69]Ca12Al14O33 7.7 650 �C, 15% CO2, 30 min 900 �C, 100% N2, 10 min TGA 30 �0.6 [52]Ca12Al14O33 25 700 �C, 90% CO2 700 �C, 100% He TGA 100 0.4 [74]Ca12Al14O33 14.7 675 �C, 100% CO2, 20 min 850 �C, 100% N2, 10 min TGA 31 0.27 [61]Ca12Al14O33 – 550 �C, 7% CO2, 27% H2O, 12 min 750 �C, 100% N2, 15 min PB (Packed-bed) 10 0.34 [194]Ca12Al14O33 ? 600 �C, 20% CO2, 25 min 1000 �C, 86% CO2, 15 min TGA 150 0.13–0.15 [72]Ca12Al14O33 39 600 �C, 100% CO2, 10 min 700 �C, 100% N2, 8 min TGA 40 �0.4 [83]Al2O3 19 750 �C, 40% CO2, 20 min 750 �C, 100% N2, 20 min TGA 30 0.36 [75]Al2O3 19 700 �C, 40% CO2, 20 min 925 �C, 100% CO2, 20 min TGA 10 0.31 [75]Al2O3 8 710 �C, 100% CO2, 30 min 950 �C, 100% N2, 5 min TGA 15 0.62 [73]Ca9Al6O18 20 650 �C, 15% CO2, 30 min 800 �C, 100% N2, 10 min TGA 35 0.52 [195]Ca9Al6O18 20 650 �C, 15% CO2, 30 min 1000 �C, 80% CO2, 10 min TGA 35 0.2 [195]Ca9Al6O18 20 650 �C, 0.015 MPa CO2, 30 min 800 �C, 100% N2, 10 min TGA 50 0.48 [51]Ca9Al6O18 7.5 650 �C, 100% CO2, 30 min 800 �C, 100% N2, 10 min TGA 31 0.57 [78]Ca9Al6O18 7.5 650 �C, 100% CO2, 30 min 930 �C, 100% CO2, 5 min TGA 31 0.33 [78]Ca9Al6O18 22.1 650 �C, 15% CO2, 30 min 750 �C, 100% Ar, 30 min TGA 25 0.33 [71]Ca3Al2O6 34 690 �C, 15% CO2, 30 min 800 �C, 100% N2, 5 min TGA 45 0.45 [77]Ca3Al2O6 9 650 �C, 20% CO2, 30 min 850 �C, 100% N2, 10 min TGA 100 0.35 [64]Cement 10 850 �C, 100% CO2, 10 min 850 �C, 100% N2, 10 min TGA 30 0.17 [125]MgO 26 758 �C, 100% CO2, 30 min 758 �C, 100% He, 30 min TGA 50 0.53 [86]MgO 25 700 �C, 90% CO2 700 �C, 100% He TGA 60 0.44 [53]MgO 25 750 �C, 25% CO2, 20 min 750 �C, 100% N2, 30 min TGA 1250 0.17 [54]MgO 25 650 �C, 15% CO2, 30 min 900 �C, 100% N2, 10 min TGA 24 0.56 [87]MgO 25 650 �C, 15% CO2, 30 min 900 �C, 100% N2, 10 min TGA 44 0.46 [55]MgO 25 700 �C, 20% CO2, 10 min 730 �C, 100% N2, 10 min TGA 50 0.54 [88]CaZrO3 �58 700 �C, 30% CO2, 30 min 700 �C, 100% He, 30 min TGA 100 0.3 [45]CaZrO3 10 650 �C, 15% CO2 800 �C, 100% air TGA 30 0.37 [92]CaZrO3 30 650 �C, 100% CO2 950 �C, 100% CO2 TGA 30 0.31 [92]CaZrO3 30 650 �C, 100% CO2, 60 min 700 �C, 100% N2, 20 min TGA 20 0.48 [29]CaZrO3 N.a. 800 �C, 50% CO2, 5 min 800 �C, 100% N2, 15 min PB (packed bed) 90 0.34 [93]CaZrO3 30 700 �C, 100% CO2, 30 min 700 �C, 100% He, 30 min TGA 1050 0.30 [91]CaZrO3 26.2 650 �C, 15% CO2, 30 min 750 �C, 100% Ar, 30 min TGA 25 0.29 [71]CaZrO3 58.16 600 �C, 100% CO2, 30 min 750 �C, 100% Ar, 30 min TGA 15 0.15 [90]TiO2 10 600 �C, 20% CO2, 10 min 750 �C, 100% N2, 10 min TGA 40 0.23 [56]TiO2 – 400 �C, 100% CO2, 30 min 400 �C, 100% N2 TGA 1 0.2 [58]Mesoporous silica shell 43 675 �C, 100% CO2, 20 min 850 �C, 100% N2, 10 min TGA 31 0.24 [61]Silica 14.7 675 �C, 100% CO2, 20 min 850 �C, 100% N2, 10 min TGA 31 0.15 [61]Y2O3 20 650 �C, 20% CO2, 30 min 850 �C, 100% N2, 5 min TGA 10 0.57 [62]Y2O3 20 650 �C, 20% CO2, 30 min 950 �C, 100% CO2, 5 min TGA 10 0.49 [62]


of nanoparticles, such as flame spray pyrolysis and sol–gel method,are relatively complex and costly. One of the best nano-sized sor-bents was developed by Luo et al. [46] from calcium nitratetetrahydrate and citric acid monohydrate precursors by the sol–gel method. They observed well-dispersed uniform particles(200 nm) within this new sorbent. This sorbent showed the CO2

adsorption capacity of 0.51 g of CO2/g of sorbent under mild calci-nation conditions and 0.20 g of CO2/g of sorbent under severe cal-cination conditions over 20 cycles, which are significantly higherthan those for CaO sorbents synthesized from commercial micro-and nano-sized CaCO3. In addition, this new sorbent exhibited veryhigh reaction rate during the carbonation (60% calcium conversionratio within 20 s) as well as a better sintering-resistant property.

2.1.2. Metal-stabilized CaOAs mentioned above, the loss of CO2 capture capacity during

cyclic carbonation/calcination operations is a main problem evenwhen the synthetic precursors are used to prepare CaO-based

sorbents. As a result, many research groups have focused to finda new technique to improve the stability of Ca-based sorbents.The incorporation of inert support materials, including aluminumoxide (Al2O3) [51,52], magnesium oxide (MgO) [53–55], zirconiumoxide (ZrO2) [29], titanium oxide (TiO2) [56–58], silica (SiO2) [59–61], yttrium oxide (Y2O3) [62], etc., into the sorbent structure is anefficient technique to improve the stability of CaO-based sorbentsderived from synthetic precursors (Table 2). The support material,which possesses a high Tammann temperature, is dispersed amongthe CaO particles during the synthesis and inhibits the CaO grainsintering during carbonation/calcination cycles. In the followingsections, we provide an overview on the studies concerning themost promising supporting materials: Al2O3, MgO and ZrO2.

2.1.2.1. Al-stabilized CaO. Al-stabilized CaO sorbents are the moststudied metal-stabilized CaO-based sorbents. DifferentAl-stabilizer phases can be produced (Al2O3, Ca12Al14O33,Ca9Al6O18, and Ca3Al2O6), depending on calcium and aluminum


precursors as well as synthesis method [12,63,64]. The inert sup-port materials are distributed between the CaO particles duringthe synthesis procedure and prevent the sintering phenomenonduring multiple carbonation/calcination cycles resulting in theenhancement of CO2 capture performance. One of the earliestreports about the synthesis of Al-stabilized CaO-based sorbentshas been published by Li et al. [12,65]. They developedCa12Al14O33-stabilized CaO sorbent from aluminum nitrate non-ahydrate (Al(NO3)3�9H2O) and calcium oxide precursors via wetmixing method. The synthesized sorbent consisting of 75 wt.%CaO and 25 wt.% Ca12Al14O33 presented the highest adsorptioncapacity of 0.41 g of CO2/g of sorbent at the end of the 50th cycleunder mild calcination conditions (850 �C, 100% N2). Using morerealistic calcination conditions (980 �C, 100% CO2) reduced theadsorption capacity to 0.22 g of CO2/g of sorbent after 56 cycles.However, it is still more than the CO2 capture capacity of dolomite(0.16 g of CO2/g of sorbent).

Encouraged by the promising results reported by Li et al.[12,65], several attempts have been made to incorporate alu-minum compounds to the CaO structure, mainly focusing on theapplication of different calcium and aluminum precursors[63,66,67], optimization of Ca/Al ratio [68–70], and use of differentsynthesis methods [64,71–80].

Zhou et al. [63] employed a wet-mixing technique to synthesizeAl-stabilized CaO sorbents from various calcium and aluminumprecursors. They found that different inert support materials,including Al2O3, Ca12Al14O33 or Ca9Al6O18, could be produceddepending on calcium and aluminum precursors used during thesynthesis process. According to the experimental results, most ofthe Al-stabilized CaO sorbents showed higher CO2 sorption capa-bility and stability during multi-cyclic carbonation/calcinationoperation in comparison to pure CaO, which was attributed tothe bimodal pore size distribution with an adequate number ofsmall pores, the high specific surface area of sorbents, and the uni-form distribution of inert support materials among CaO particles.Among the developed sorbents, CaO/Ca9Al6O18 derived from cal-cium citrate and aluminum nitrate presented the best performancewith the CO2 capture capacity of 0.51 g of CO2/g of sorbent after 28cycles. The authors also proposed a formation mechanism for thesupport materials (Fig. 3). The transition between steps 3 and 4depends on the precursors used during the synthesis step, whichcan limit Ca2+ diffusion into the stabilizer structure for furtherreaction.

Pacciani et al. [68,70] prepared a number of CaO/Ca12Al14O33

synthetic sorbents with different CaO/inert material ratios.Among the studied materials, the sorbent consisting of 85 wt.%CaO and 15 wt.% Ca12Al14O33 showed the highest activity in a flu-idized bed reactor with a CO2 capture capacity of 0.26 g of CO2/gof sorbent after 20 cycles (adsorption under 14% CO2 flow andregeneration under pure N2), which decreased to 0.17 g of CO2/gof sorbent after 110 cycles. In another study, Koirala et al. [69]applied the single nozzle flame spray pyrolysis method to synthe-size Al-stabilized CaO sorbents with different Al/Ca ratios. Theyfound that a higher Al doping into CaO improved the sorbent sta-bility. According to the multiple carbonation/calcination experi-ments, the synthetic sorbent with the Al/Ca molar ratio of 3:10exhibited the best performance with a CO2 uptake capacity of0.40 g of CO2/g of sorbent, which was stable over 100 cycles. Theimproved durability of Al-stabilized CaO sorbents was owing tothe uniform distribution of Ca12Al14O33, which was produced in

2 2 2

(1) (2)3 2 3 2 3,

)4()3(12 14 33 9 6 18

Ca & Al precursors CaCO +Al O CaO+Al O

Ca Al O Ca Al O

CO H O CO

CaO

− − −⎯⎯⎯⎯→ ⎯⎯⎯→

⎯⎯→ ⎯⎯⎯→

Fig. 3. Possible mechanism for Al-stabilizer formation (adapted from [63]).

the synthesis step, among the CaO particles. The sorbent with theAl/Ca molar ratio of 3:10 also performed well under severe calcina-tion conditions, indicating a residual sorption capacity of �0.25 gof CO2/g of sorbent at the 100th cycle. These results revealed thatthe presence of CO2 in the regeneration step led to the acceleratedstructural sintering.

Recently, different research groups have developed various syn-thesis methods with the aim of increasing surface area and porevolume and obtaining high dispersion of inert supports withinthe sorbent structure: wet mixing [12,63,65,66,68,70], limestoneacidification by citric acid followed by two step calcination [71],solid-state reaction [72], ultrasonic spray pyrolysis (USP) [73],combination of precipitation and hydration [74], co-precipitation[75], citrate preparation [64], sol–gel [77,79], citrate-assisted sol–gel technique followed by two-step calcination [78], single nozzleflame spray pyrolysis (FSP) [69], and precipitation [80]. Some ofthese techniques result in Al-stabilized CaO sorbents with a greatperformance in cyclic carbonation/calcination experiments.Sayyah et al. [73] developed a series of Al-stabilized CaO sorbentsusing ultrasonic spray pyrolysis (USP) method. The precursors, cal-cium nitrate tetrahydrate (Ca(NO3)2�4H2O) and aluminum nitratenonahydrate (Al(NO3)3�9H2O), were solved in ethanol. Theobtained solution was nebulized via ultrasound, followed by carry-ing through a furnace tube (set at 600 �C) by argon gas. The synthe-sized sorbents were tested in carbonation (710 �C, 100% CO2) andcalcination (950 �C, 100% N2) cyclic operations. The results showedthat the optimum value for Al/Ca ratio is around 0.08. The CO2

adsorption capacity of this sorbent decreased from 0.65 g ofCO2/g of sorbent to 0.62 g of CO2/g of sorbent over 15 calcination/-carbonation cycles. The high performance of the sorbents showedthe capability of USP method in homogenous dispersion of addi-tives in the CaO matrix and production of materials with high sur-face area and high stability. Zhang et al. [64] applied the citratepreparation route to synthesize a Ca-based sorbent consisting of9 wt.% Al2O3 and 91 wt.% CaO from aluminum nitrate, citric acid,and CaCO3. The developed sorbent was activated by a four-stepactivation procedure instead of the common one-step activationmode that promoted the formation of porous structure becauseof the release of CO2, H2O, and NO2 step by step under the mildactivation conditions. Therefore, the developed CaOAAl2O3 sorbentshowed a larger pore volume (0.85 cm3/g) and higher specific sur-face area (958 m2/g) compared to the untreated CaO (pore volume:0.03 cm3/g, specific surface area: 5 m2/g). Moreover, XRD patternsexhibited the formation of the stabilizer Ca3Al2O6 in the developedCaOAAl2O3 sorbent after 16 carbonation and calcination cycles,which prevented the severe sintering of CaO particles. The devel-oped CaO–Al2O3 and untreated CaO sorbents showed the CO2 cap-ture capacity of �0.35 g of CO2/g of sorbent after 100 cycles and�0.2 g of CO2/g of sorbent after 50 cycles, respectively (carbonationat 650 �C under 20% CO2 and calcination at 850 �C under 100% N2).Angeli et al. [77] developed a CaOACa3Al2O6 sorbent by a modifiedsol–gel method. TEA (complexing agent) was added into a mixtureof Ca(NO3)2�4H2O and Al(NO3)3�9H2O diluted in distilled water atthe temperature of 50 �C to obtain a molar ration of 1:1(TEA/metals). A brown viscous solution was obtained when themost of the water was evaporated. This solution was dried over-night at 185 �C and then calcined at 900 �C for 1.5 h in air. This sor-bent indicated a high CO2 uptake capacity of 0.45 g of CO2/g ofsorbent (corresponding to 84% carbonation conversion), whichwas maintained through 45 cycles under mild conditions (calcina-tion at 800 �C under 100% N2). The high and stable CO2 uptakecapacity was attributed to the synthesis procedure. The TEAdecomposition emitted a large amount of gases, which led to theformation of coral-like structure. In addition, the TEA-ion com-plexes formed during the preparation step resulted in the uniformdispersion of Al and Ca ions. It is worth mentioning that the


proposed sorbent represented higher stability compared to previ-ously reported Ca-based sorbents at severe conditions. It main-tained approximately 40% of its initial CO2 uptake capacity at theend of the 100th cycle under severe conditions (calcination at950 �C under 100% CO2). Radfarnia and Sayari [78] employed acitrate-assisted sol–gel technique followed by two-step calcinationprocedure (under inert and air atmosphere) to develop an efficientAl-stabilized CaO sorbent (92.5 wt.% CaO/7.5 wt.% Al2O3). After 31carbonation/calcination cycles, the CO2 capture capacity of the syn-thetic sorbent was 0.57 and 0.33 g of CO2/g of sorbent under mildand severe calcination conditions (mild condition: 800 to 900 �C,100% N2; severe condition: 930 �C, 100% CO2), respectively. Theyreported no loss of activity during 31 cycles, when calcinationwas performed at 800 �C under pure N2 flow. The high stabilityof the proposed sorbent was on account of the uniform dispersionof Ca9Al6O18 binder throughout the CaO matrix, which controlledthe structural sintering.

Ca-based sorbents derived from Ca-Al layered double hydrox-ides (LDHs) exhibit high stability during multiple carbonation/cal-cination cycles due to the uniform distribution of calciumaluminates between CaO particles [81–84]. Chang et al. [82]applied a sol–gel method to develop CaAAl LDH-derivedmixed-metal oxides by using Al(OiPr)3 and Ca(NO3)2 as the precur-sors and hexadecyl trimethyl ammonium bromide (CTAB) as thestructure-directing agent. For the developed sorbents with aCa2+/Al3+ ratio of 1:1, only the Ca12Al14O33 phase was identifiedafter calcination at 600 �C. For the sorbents with a higherCa2+/Al3+ ratio, the CaO phase was observed along withCa12Al14O33. The CaAAl mixed-metal oxides (CAMO) displayedhigh specific surface areas of up to 191 m2 g�1 and a pore size dis-tribution in the range of 3–6 nm, allowing rapid diffusion of CO2

throughout the sorbent, inducing relatively rapid CO2 absorptionkinetics and enhancing the sintering-resistant nature through mul-tiple carbonation/calcination cycles for CO2 capture. The CAMOshowed a high CO2 capture capacity of 49 wt.%, as well as a fastCO2 absorption kinetics in considerably short period of 5 min forthe Ca2+/Al3+ = 7 composition. Moreover, this sorbent exhibitedhighly stable CO2 capture capacity at high temperatures with only2–6% capacity decay after 50 multiple carbonation/calcinationcycles. As a conclusion, the CAMO framework is a good insulatorfor inhibiting the aggregation of CaO particles and therefore, it issuitable for long-term cyclic operation at high temperatures.Chang et al. [83] developed CaAAl LDH nanoparticles by a reversemicroemulsion method. The results showed no apparent reductionin CO2 sorption capacity over multi-cycle carbonation/calcinationexperiments because of the formation of Ca12Al14O33 oxide duringnanoparticle synthesis, which could avoid the intimate contactbetween CaO nanoparticles. Yu et al. [84] employed an hydrother-mal method to synthesize calciumAaluminum carbonate(CaAAlACO3) sorbents with the Ca/Al molar ratio of 1:1 to 30:1from calcium acetate and aluminum nitrate. By increasing theCa/Al molar ratio, the CO2 capture capacity increased from 13.4to 74.2 wt.% because more Ca2+ loading in CaAAlACO3 formationresulted in higher concentration of CaO in the synthetic sorbentsafter calcination. However, the stability of the sorbent in cyclicoperation might decline with high Ca/Al ratio. The stability of theCaAAlACO3 sorbent with the Ca/Al molar ratio of 7:1 during 10cycles was 99% in TG analyzer (regeneration at 750 �C under100% N2) and 76% in the reactor (regeneration at 750 �C under40% CO2).

2.1.2.2. Mg-stabilized CaO. MgO is another inert support materialthat is capable of stabilizing the CO2 uptake because of its highTammann temperature (1276 �C). To obtain a very high and stableCO2 capture capacity, it is imperative to mix the Ca and Mg ions ona molecular level. The Mg-stabilized CaO-based sorbents that are

composed of microscopic CaCO3 and MgCO3 crystals show the lossof CO2 capture capacity similar to natural limestone [85]. The mor-phology of the sorbent is influenced by different parameters,including the synthesis method, Ca and Mg precursor, and Ca/Mgratio. Li et al. [86] synthesized several Mg-stabilized CaO sorbentsusing various techniques: co-precipitation, dry physical mixing,wet physical mixing and solution mixing. Multi-cyclic carbona-tion/calcination experiments revealed that the most durable sor-bents with high CO2 sorption capacity were prepared by the twophysical mixing methods. In addition to the synthesis method,the MgO precursor had some effect on the performance of syn-thetic sorbent. However, this effect was not as strong as that ofthe mixing method. CaO-based sorbent doped with MgO nanopar-ticles developed by thermal decomposition of magnesium oxalateexhibited the best performance. The sorbent, which is stabilizedby 26 wt.% MgO and prepared by dry physical mixing method,had a CO2 sorption capacity as high as 0.53 g of CO2/g of sorbentafter 50 isothermal carbonation/calcination cycles at 758 �C. Forthe pure CaO sorbent obtained from the same source, the initialCO2 capture capacity of 66 wt.% decreased to 22.1 wt.% after 50cycles under the same operating conditions. Liu et al. [87] preparedMg-stabilized CaO sorbents via a simple wet-mixing method fromdifferent calcium and magnesium precursors. The best cyclic CO2

capture performance belonged to the sorbents produced from cal-cium and magnesium salts of D-gluconic acid, because of the uni-form distribution of MgO nanoparticles among the CaO particlesin these sorbents. For instance, the sorbent containing 25 wt.%MgO and synthesized from calcium and magnesium D-gluconatehydrate precursors showed the CO2 capture capacity of 0.56 g ofCO2/g of sorbent over 24 cycles, which was very close to its theo-retical capacity (0.59 g of CO2/g of sorbent).

Recently, Lan and Wu [88] developed different samples ofNano-CaO/MgO-based sorbents by employing a synthesis methodconsisting of three steps: (1) preparation of magnesium sol by addi-tion of citric acid solution to MgO slurry, (2) addition of magnesiumsol to nano-CaCO3 slurry, and (3) calcination of mixture.Characterization analyses revealed the improvement of specificsurface area (from 9.9 to 15.3 m2 g�1) and average pore radius(from 16 to 30 nm) of sorbents with the increase of MgO content,due to the emission of a large amount of gases (CO2 and H2O) fromthe decomposition of magnesium sol during precalcination process.In addition, the increase of the precalcination temperature from500 to 900 �C led to the complete decomposition of CaCO3 and porestructure changes while the increase of the precalcination timecaused no significant change in the structure. According to carbon-ation/decarbonation experiments, the best performance belongedto the sorbent with the nano-CaO/nano-MgO weight ratio of 3/1that showed the higher reaction rate (by 30%) and adsorptioncapacity (2-fold) in comparison with the nano-CaO/Al2O3-basedsorbent.

2.1.2.3. Zr-stabilized CaO. ZrO2 is another compound that is able toeffectively stabilize the structure of CaO sorbent. Lu et al. [45] syn-thesized several metal (Si, Ti, Cr, Co, Zr and Ce)-stabilized CaO sor-bents by employing the flame spray pyrolysis (FSP) method.Among all developed sorbents, Zr-stabilized CaO sorbent repre-sented the best CO2 sorption activity under identical operatingconditions. This study showed the superiority of FSP method indeveloping a sorbent with nanosize particles, high surface areaand large pore volume, as well as the role of ZrO2 in improvingthe thermal stability of sorbent. In a further study, the sameresearch group prepared a series of CaO-based sorbents doped bya wide range of ZrO2 loadings via the flame spray pyrolysis (FSP)method [89]. The sorbent with a Zr/Ca molar ratio of 0.5 displayedremarkable stability up to 1200 carbonation/calcination cycles. Thegreat thermal stability of developed sorbent was due to the


formation of well-dispersed CaZrO3 nanoparticles that preventedthe growth of CaO grains. Nevertheless, the complexity of the FSPtechnique is a matter of concern for the easy production of CO2 sor-bents. Radfarnia and Iliuta [90] applied the surfactant template/-sonication technique to develop Zr-stabilized CaO sorbents. Theyfound an optimum Zr/Ca ratio of 0.303 to maximize the stabilityand CO2 capture activity of the proposed sorbents. It was shownthat the formation of calcium zirconate phase during the prepara-tion step helped preventing the structural sintering and thereforeimproved the sorbent durability. The results also showed a betterCO2 capture ability of Zr-stabilized CaO sorbent in comparisonwith pure CaO in severe cyclic operating conditions.

Recently, several studies were performed to investigate theinfluence of different parameters such as synthesis method,Ca/Zr ratio, and calcination conditions on the CO2 uptakeperformance of Zr-stabilized CaO sorbents. Reddy et al. [91]prepared Zr-stabilized CaO sorbents by deposition–precipitation,co-precipitation, flame spray pyrolysis, and sol–gel methods toassess the influence of the synthesis method on the structure andCO2 capture activity. According to the experimental results, thesol–gel-synthesized sorbent showed the best CO2 adsorption cyclicperformance. The sol–gel-synthesized sample had better CO2 cap-ture capacity than flame-spray-pyrolysis-synthesized samplebecause of higher Ca/Zr atomic ratio (3.57 vs. 1.5). Althoughco-precipitation-synthesized and deposition–precipitation-synthesized sorbents possessed very high Ca/Zr atomic ratio (8.9 and9.46, respectively), they represented significantly lower molarconversion compared to the sol–gel-synthesized and flame-spray-pyrolysis-synthesized sorbents due to higher crystallite size.TEM measurements revealed that the sol–gel-synthesized samplehad much smaller particles (10–20 nm) than co-precipitation-synthesized and deposition–precipitation-synthesized sorbents(more than 100 nm). The authors reported that the sorbentsprepared by sol–gel and flame spray pyrolysis had very goodstability until 1200 carbonation/calcination cycles.

Zhao et al. [92] employed a wet chemical method to synthesizeCaZrO3-stabilized CaO sorbents with three composition ratios (10,18, and 30 wt.% CaZrO3). Sorbent with the composition ratio of10 wt.% CaZrO3/90 wt.% CaO represented the best CO2 carbona-tion/decarbonation cyclic performance in the mild conditions (car-bonation at 650 �C under 15% CO2 and calcination at 800 �C underair). Its CO2 capture capacity increased from 0.31 g of CO2/g of sor-bent in cycle 1 to 0.37 g of CO2/g of sorbent in cycle 10 and then,stabilized at this value over 20 cycles. This behavior(self-reactivation) was due to a densely packed microstructure ofthe as-prepared powder, which developed to a more porous struc-ture under the first 10 carbonation/decarbonation cycles. The bestperformance in severe conditions (carbonation at 650 �C under100% CO2 and calcination at 950 �C under 100% CO2) belonged tothe sorbent with the composition ratio of 30 wt.% CaZrO3/70 wt.%CaO, which showed CO2 capture capacity decreasing from 0.36 gof CO2/g of sorbent to 0.31 g of CO2/g of sorbent under 30 cycles.Nanoparticles of CaZrO3 (20–80 nm) dispersed within the6200 nm CaO porous matrix were considered as the main reasonfor enhancing multi-cycle stability. Broda et al. [93] prepared sev-eral ZrO2-stabilized CaO sorbents using a sol–gel method from dif-ferent calcium precursors and zirconium (IV) propoxide andstudied the influence of various synthesis parameters such as cal-cium precursor and Ca2+/Zr4+ ratio on CO2 capture. The resultsshowed that ZrO2-stabilized CaO sorbents derived from calciumhydroxide or calcium acetate precursor exhibited a high surfacearea and pore volume and a great CO2 capture properties.However, the sorbents developed from calcium nitrate precursorpresented a low surface area and pore volume and a very smallCO2 capture capacity for the reason that calcium nitrate meltedduring calcination and therefore, created a very coarsely grained

solid. Moreover, the authors mentioned that decreasing theCa2+/Zr4+ ratio led to the improvement of the thermal stability ofthe synthetic ZrO2-stabilized CaO sorbents, as well as the reductionof CO2 capture capacity owing to the lower amount of active CaO inthe sorbent. Among all developed sorbents, the sorbent derivedfrom calcium hydroxide with the Ca2+/Zr4+ ratio of 95:5 showedthe highest CO2 capture capacity of 0.34 g of CO2/g of sorbent atthe end of the 90th cycle (carbonation at 800 �C under 50% CO2,calcination at 800 �C under 100% N2). This sorbent outperformedthe reference limestone by 160%. The great CO2 capture perfor-mance of the developed sorbent was on account of the uniform dis-persion of calcium zirconate framework with a high Tammanntemperature (1036 �C), which resulted in the minimization of ther-mal sintering.

2.1.3. Additional treatmentsThere are several additional treatments of natural (limestone)

or synthetic CaO that can be applied to improve their CO2 captureactivity in multi-cyclic carbonation/calcination processes (calciumlooping process), including hydration, preheat treatment (thermalpretreatment), recarbonation, and chemical pretreatment(Table 3). In the following section, these methods are discussedin details.

2.1.3.1. Hydration. The reactivation of limestone using a hydrationprocess is a promising approach to improve the performance ofCaO-based sorbents in multi-cyclic carbonation/calcination opera-tions. The hydration treatment can be classified into differentgroups, including hydration treatment during carbonation[27,94–102], hydration treatment during calcination [96,97] andseparate hydration treatment on CaO/CaCO3 [97,103–109], whichare determined by the stage where steam or water is introduced.

Steam addition during the carbonation step increases the car-bonation conversion. This positive effect of steam on carbonationcan typically be explained by two different theories: (i) theenhancement of carbonation conversion in the fastkinetically-controlled stage by the formation of Ca(OH)2 as a tran-sient intermediate, which its carbonation is thermodynamicallymore favorable than that of CaO [99–102] and (ii) the enhance-ment of solid-state diffusion in the calcium carbonate productlayer, which is more pronounced at lower carbonation temperatureand for more sintered sorbents [95,98].

On the other hand, there are contrary results about the effect ofsteam addition during calcination. Champagne et al. [96] assessedthe reactivation of two Canadian limestones (Cadomin andHavelock) by steam injection (up to 40 vol.%) during calcinationfor 15 carbonation/calcination cycles. According to the results,steam injection during calcination increased the carbonation reac-tivity of the sorbent at all concentrations of steam. In fact, steaminjection during calcination led to larger pore diameter and lowerspecific surface area via enhancing particle sintering. Althoughlower specific surface area decreased the sorbent carrying capacity,larger pores lessened the diffusional resistance resulted fromCaCO3 formation at the surface, giving higher carbonation conver-sion. Moreover, steam injection during calcination reduced thepartial pressure of CO2 in the calciner, allowing for lower calcina-tion temperatures. Nevertheless, Rong et al. [97] found that therate of sorbent activity decay was accelerated by hydration treat-ment during calcination, maybe because of more porosity loss inthe presence of steam during calcination. They observed that thepresence of steam during calcination (20 and 40 vol.%) resultedin 2–3% decrease in CaO conversion over 10 carbonation/calcina-tion cycles in comparison to the case without steam reactivation.

Champagne et al. [96] found that steam injection during car-bonation leads to a much larger increase in carbonation reactivityof the sorbent compared to steam injection during calcination.

Table 3Summary of investigations on treated CaO sorbents.

Sorbent Treated by Reaction conditions Reactor Numberof cycles


Ref.

Ads. Reg.

Limestone Acetic acid 650 �C, 15% CO2, 20 min 920 �C, 80% CO2, 15 min DFR 20 0.39 [121]Limestone Propionic acid 700 �C, 15% CO2, 20 min 850 �C, 100% N2, 10 min DFR 100 0.24 [120]Limestone Pyroligneous acid 700 �C, 15% CO2, 30 min 850 �C, 100% N2, 15 min DFR 103 0.26 [122]Limestone Citric acid 700 �C, 15% CO2, 30 min 750 �C, 100% Ar, 30 min TGA 18 0.485 [123]Limestone Ethanol 700 �C, 15% CO2, 20 min 920 �C, 80% CO2, 15 min DFR 15 0.31 [118]Limestone Steam (hydration) 700 �C, 15% CO2, 30 min 850 �C, 100% N2, 10 min TGA 10 0.55 [104]CaCO3 Steam (hydration) 780 �C, 100% CO2 960 �C, 100% CO2 TGA 10 0.47 [105]Limestone Pre-heat 800 �C, 50% CO2, 30 min 800 �C, 100% N2, 10 min TGA 30 0.39 [110]Ca(Ac)2 Ethanol/water solution with the

volume ratio of 3600 �C, 50% CO2, 45 min 700 �C, 100% N2, 20 min TGA 11 0.62 [119]

Limestone Pre-heat (950 �C, 100% air, 12 h) andrecarbonation (850 �C, 90% CO2,3 min)

650 �C, 15% CO2, 5 min 850 �C, 100% air, 5 min TGA 50 0.27 [116]

CaCO3 Steam (hydration) 650 �C, 15% CO2, 25 min 900 �C, 100% N2, 10 min TGA 10 0.40 [97]Limestone Acetic acid 650 �C, 15% CO2, 20 min 920 �C, 100% CO2, 10 min TGA 20 0.09 [126]Limestone Steam 650 �C, 15% CO2, 20 min 925 �C, 60% CO2, 15% H2O, 5 min TGA 15 �0.11 [96]Limestone Steam 650 �C, 15% CO2, 15% H2O, 20 min 925 �C, 60% CO2, 40% H2O, 5 min TGA 15 �0.16 [96]Limestone Acetic acid 650 �C, 15% CO2, 20 min 850 �C, 100% N2, 5 min TGA 20 0.23 [124]Limestone Vinegar 650 �C, 15% CO2, 20 min 850 �C, 100% N2, 5 min TGA 20 0.15 [124]Limestone Formic acid 650 �C, 15% CO2, 20 min 850 �C, 100% N2, 5 min TGA 20 0.22 [124]Limestone Oxalic acid 650 �C, 15% CO2, 20 min 850 �C, 100% N2, 5 min TGA 20 0.25 [124]Limestone 10 wt% Aluminate cement, 10% Starch 700 �C, 15% CO2 in air, 30 min 900 �C, 100% air, 20 min – 10 �0.65 [127]Limestone Steam (hydration) 650 �C, 15% CO2 in air, 15 min 940 �C, 70% CO2 in air, 20 min FB (Fluidized-bed) 5 0.37 [106]Al2O3-stabilized CaO-based Steam 600 �C, 20% CO2/20% H2O/60% N2, 10 min 900 �C, 20% CO2 in N2 TGA 10 0.52 [95]Pelletized Al2O3-stabilized CaO-based Steam 600 �C, 20% CO2/20% H2O/60% N2, 10 min 900 �C, 20% CO2 in N2 TGA 10 0.31 [95]Ca(OH)2 Pre-heat 650 �C, 15% CO2, 5 min 850 �C, 100% dry air, 5 min TGA 50 0.17 [113]Limestone Recarbonation after each carbonation

(800 �C, 100% CO2, 5 min)650 �C, 50 kPa CO2 in air, 5 min 875 �C, air, 5 min TGA 75 �0.13 [115]

Limestone Heat-pretreated 650 �C, 15% CO2, 10 min 900 �C, 70% CO2, 5 min TGA 20 �0.09 [117]Limestone Heat-pretreated 650 �C, 15% CO2, 5 min 950 �C, 70% CO2, 5 min TGA 20 �0.05 [117]Limestone Heat-pretreated & recarbonation at

800 �C in 90% CO2 for 3 min650 �C, 15% CO2, 5 min 950 �C, 70% CO2, 5 min TGA 20 �0.03 [117]

Limestone Ground + Heat-pretreated (850 �C,12 h)

650 �C, 50% CO2, 30 min 850 �C, dry air, 5 min TGA 10 0.37 [114]

Limestone Ground + Heat-pretreated (950 �C,12 h)

650 �C, 50% CO2, 30 min 850 �C, dry air, 5 min TGA 10 0.45 [114]

Limestone Liquid water and steam hydration 780 �C, 100% CO2, 40 min 960 �C, 100% N2, 35 min TGA 100 0.23 [109]

428M

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al./Chemical

EngineeringJournal

283(2016)

420–444

Fig. 4. Schematic representation of pore�skeleton model [110].


They reported that although the presence of steam with the con-centration of 15% for carbonation led to a 15% point increase inconversion after 15 cycles, the presence of steam with the concen-tration of 40% for calcination only led to a 5% point increase after15 cycles. However, Rong et al. [97] mentioned that steam additionduring carbonation (20 and 40 vol.%) resulted in only 2–3%increase in the carbonation conversion of CaO sorbent after 10 car-bonation/calcination cycles by decreasing the diffusion resistancethrough the CaCO3 product layer. The poor enhancement of car-bonation conversion by hydration treatment during carbonationwas justified by the limited increase of carbonation conversion inthe diffusion-control stage. In overall, steam hydration during car-bonation and calcination processes have less impacts on the car-bonation conversion in comparison to the separate hydrationtreatment.

Introducing water or steam to CaO after regeneration is themost important hydration approach. Calcium hydroxide(Ca(OH)2) is produced during this hydration treatment and leadsto the agglomerate breakage because of its expanded molar vol-ume. Therefore, separate hydration treatment after calcinationincreases the specific surface area and pore volume of spent sor-bent, resulting in the sorbent reactivation. This approach is brieflycalled ‘‘steam reactivation’’. In an early work, Manovic andAnthony [104] investigated the steam reactivation of spent lime-stone in a pressurized reactor at 200 �C. According to the cyclic car-bonation/calcination experiments, the reactivated limestoneshowed the average CO2 capture capacity of about 0.55 g ofCO2/g of sorbent over 10 cycles, which was considerably higherthan that of the original sorbent (0.27 to 0.31 g of CO2/g ofsorbent).

Rong et al. [97] showed that different parameters such as steamconcentration, hydration temperature, and hydration frequencyaffected the spent sorbent reactivation. Increasing the steam con-centration during hydration treatment led to a better reactivationperformance by retaining more small pores and therefore, increas-ing the ultimate conversion of the fast kinetically control phase ofcarbonation. However, hydration treatment at high temperaturesresulted in a lower carbonation conversion during carbonation/cal-cination cycles, which was on account of more severe sintering ofCa(OH)2 at a higher hydration temperature. With respect to thehydration frequency, hydration after every 3 cycles and once hydra-tion did not represent satisfying reactivation performance duringcyclic CO2 sorption operation because of more severe sintering ofCaO sorbents derived from Ca(OH)2. The activity of the spent sor-bent was recovered by separate steam hydration after every calci-nation step. Recently, Coppola et al. [106] assessed the effects ofwater hydration on the CO2 capture capacity and attrition tendencyof a limestone-derived CaO sorbent. The results demonstrated thatthe CO2 capture capacity of sorbent increased from 0.04 g of CO2/gof sorbent in the last carbonation before hydration to 0.32–0.37 g ofCO2/g of sorbent in the first carbonation after hydration, because ofparticle swelling and development of active porosity. However, thecapacity quickly decayed along with the cycles due to the severesintering of Ca(OH)2-derived CaO sorbents. In addition, the sorbenthydrated for 60 min showed larger CO2 capture capacity, as well aslimited attrition tendency. In fact, two distinct time scales shouldbe considered for the optimal design of hydration stage: (i) the timerequired for full hydration of the free lime (water uptake) and (ii)the time required for improving the connectivity and mechanicalstability of particles by wet chemical sintering. Although 10 minwas enough for water uptake, wet chemical sintering was notachieved during this time. Moreover, more soaking time (more than60 min) led to a decrease of CO2 uptake capacity without any signif-icant improvement in its attrition resistance.

Hydration treatment of the spent limestone not only recoversits CO2 capture capacity but also significantly increases its attrition

tendency, which restricts the industrial applicability of thismethod. The cost impact related to the production of steam is alsoanother matter of concern for a high level of hydration. The partialhydration of limestone is proposed as an acceptable strategy inorder to decrease both negative impact on the mechanical strengthof reactivated material and steam consumption. In addition, pel-letization can be another alternative to improve the mechanicalstrength of sorbents [95,107,108].

2.1.3.2. Thermal pretreatment and recarbonation. Thermal pretreat-ment of limestone has been proposed as another activationapproach, which results in the stabilization of material structureand self-activation of sorbent during cyclic carbonation/calcinationoperations (increase of sorbent activity along with the cycle num-ber). Manovic and Anthony [110] examined the thermal pretreat-ment of four Canadian limestones. According to the CO2 sorptionexperiments, pretreated sorbents showed better conversions atthe end of cyclic operation in comparison to the correspondingoriginal sorbents (natural limestones). After 30 cycles, a CO2 sorp-tion capacity up to 0.39 g of CO2/g of sorbent was obtained for thestudied limestones. The authors also proposed a pore–skeletonmodel to explain self-activation phenomenon (Fig. 4). Theydemonstrated that the formation of hard skeletons during thermalpretreatment of a sorbent stabilizes its structure. The transforma-tion of the hard skeleton to a soft skeleton during cyclic operationincreases the CO2 sorption activity of sorbent by facilitating themass transfer. In fact, the hard skeleton formed during the thermalpreatreatment of CaO sorbent is less active and more stable incomparison to soft skeleton and therefore, results in a lower CO2

sorption capacity in the initial cycles. The continuous growth ofsoft skeleton and decline of hard structure leads to the enhance-ment of sorbent activity until the stabilization takes place betweenskeleton changeovers. In a further study, Chen et al. [111] alsoobserved the self-activation of Strassburg limestone and Arcticdolomite thermally pretreated at 1000 and 1100 �C, respectively,over 1000 carbonation/calcination cycles. However, as mentionedby arias et al. [112], the self-activation may not be effective undertypical conditions of a circulating fluidized-bed carbonator, wherethe reaction time is restricted to few minutes at low CO2 partialpressures (below 10 kPa) and temperatures around 650 �C.

Recently, it was found that conversion of thermal pretreatedCaO sorbent was based on the balance between the increase of sur-face area because of enhanced solid-state diffusion carbonationand the decrease of surface area of renovated CaO structure dueto sintering during calcination stage [113,114]. Heat pretreatmentled to an extremely sintered CaO structure, which decreased car-bonation conversion in the fast kinetically controlled phase andincreased carbonation conversion in the solid-state diffusion


controlled phase in the 1th cycle. Upon calcination, carbonationconversion in the fast kinetically controlled phase of the 2nd cyclewas increased because of the enhancement of surface area of theregenerated CaO structure. However, the carbonation conversionin both kinetically and solid-state diffusion controlled phaseswas decreased at a small rate after the 2nd cycle because of the sin-tering of the regenerated CaO structure during calcination processat high temperatures [114]. Valverde et al. [113] showed thatharshening calcination conditions (calcination temperatures andtime periods) precluded self-reactivation of thermal pretreatedCaO sorbent by speeding up the renovated soft skeleton sintering,which resulted in a conversion reduction in the fast kineticallycontrolled phase. According to their results, self-reactivation ofthe proposed sorbent completely hindered at the calcination tem-peratures above 900 �C (time period: 5 min) or for calcination timeperiod of 15 min (temperature: 850 �C). Sanchez-Jimenez et al.[114] found that sorbent grinding prior to heat pretreatmentresulted in a slow decay rate of carbonation in the diffusion con-trolled phase and so, a steady increase of conversion in the fastkinetically controlled phase with the cycle number. The experi-mental results obtained for raw and preground limestone (withoutthermal pretreatment) revealed that the carbonation conversion inthe solid-state diffusion controlled phase was significantlyenhanced by pregrinding. This enhancement was attributed tolocal high stresses induced by grinding, which exceeded the cohe-sion forces between the lattice atoms, and therefore resulted incrystal cracking and enhancement of structural defects that helpedCO2 diffuse better through the solid. The authors also reported thatheat pretreatment in a CO2 atmosphere led to the inhibition of car-bonation in the solid-state diffusion controlled phase and a highincrease of the carbonation rate in the fast kinetically-controlledphase, which could be ascribed to annealing of the crystalstructure.

The introduction of a recarbonation step between carbonationand calcination steps is a novel process aimed to increase theCO2 capture capacity of CaO-based sorbents in multicyclic CO2 cap-ture systems. Grasa et al. [115] studied the kinetics of the carbon-ation reaction in recarbonation step under the conditions of hightemperature and CO2 partial pressure. The experimental resultsshowed that the addition of a short recarbonation step (100–200 s) on partially carbonated CaO sorbents stabilized the sorbentcapture capacities at 0.15–0.20 M conversion. The carbonationreaction in recarbonation step, which mainly occurred in the slowdiffusion controlled reaction phase, was strongly favored by highrecarbonation temperatures (750–800 �C), high CO2 partial pres-sures (beyond 60 kPa), and a certain presence of steam.

Valverde et al. [116] indicated that the synergetic combinationof heat pretreatment and recarbonation resulted in a high andstable CaO conversion in the carbonation step of multiple carbonation/recarbonation/calcination cycles. To this end, a natural lime-stone was preheated at 950 �C for 12 h in a dry air atmosphereand then, was subjected to the multi-cyclic carbonation/calcina-tion and carbonation/recarbonation/calcination experiments. Theexperimental results revealed that the heat pretreated sorbentshowed a stable but very small CaO conversion through carbona-tion/calcination multi-cyclic operations. Indeed, carbonation underlow CO2 concentrations and calcination at temperatures above850 �C prevented the reactivation of sorbent by heat pretreatment.However, the proposed sorbent indicated a stable and high CaOconversion from the 2nd cycle in the multi-cyclic carbonation/recarbonation/calcination experiments. In fact, the addition of arecarbonation step between the carbonation and calcination stepsled to a reactive and thermally stable CaO structure after calcina-tion by the significant enhancement of solid-state diffusion duringrecarbonation. Later, the same research group studied the influ-ence of the same reactivation methods (heat pretreatment and

recarbonation) on the multi-cyclic CO2 capture performance oflimestone derived CaO sorbent for the cases with a high CO2 partialpressure in the calciner [117]. The authors claimed that the resultswere in contrast with those reported in their previous work [116]and the influence of recarbonation on the cyclic CO2 sorption wasaffected by the CO2 partial pressure in the calciner. When the CO2

partial pressure during calcination was far enough from the equi-librium pressure, the irreversible desorption of CO2 governeddecarbonation, which resulted in a highly porous CaO structurewith increased surface area for the fast kinetically-controlled car-bonation phase. Addition of a recarbonation stage prior to calcina-tion led to the more enhancement of porosity in the resultant CaOstructure, because it let the following decarbonation happen dee-per in the bulk of solid. It was also mentioned that the heat pre-treatment promoted the desirable effect of recarbonation.However, during calcination at high CO2 partial pressure, decar-bonation was ruled by a dynamic and reversible CO2 adsorp-tion/desorption mechanism, which was precluded by theaddition of a recarbonation stage before calcination. In addition,this mechanism precluded the growth of the CaO crystal structureand therefore, prevented the carbonation in the fastkinetically-controlled phase. On the contrary, the heat pretreat-ment alone showed favorable effects on the cyclic CO2 capture pro-cess under harsh calcination conditions. Heat pretreatment andcalcination under high CO2 partial pressure led to the enhancementof CaO conversion during the diffusion-controlled phase. Heat pre-treatment allowed also decreasing the calcination temperature athigh CO2 partial pressure.

2.1.3.3. Chemical pretreatment. Although the cost impacts associ-ated with the use of chemical solutions can be a matter of concern,treatment by chemical solutions is another promising approach toenhance the CO2 capture activity of CaO sorbent. Li et al. [118]evaluated the CO2 uptake performance of a limestone derivedCaO sorbent treated with 50%, 70% and 90% ethanol/water solu-tions. They found that the CO2 capture capacity of the sorbent trea-ted with ethanol/water solution was higher than that of sorbenthydrated with distilled water and much higher than that of sorbentderived directly from limestone. Higher concentration of ethanol insolution led to higher CO2 sorption capacity and betteranti-sintering performance for treated CaO sorbent. The enhancedCO2 adsorption capacity of modified sorbent was attributed to thefact that the ethanol molecule increases H2O molecule affinity andpenetrability to CaO, resulting in the higher specific surface areaand larger pore volume after calcination. In a similar work, Wanget al. [119] treated CaO sorbents derived from calcium acetate withethanol/water solution at different temperatures. They concludedthat CaO modified by ethanol/water solution (volume ratio of 3)at room temperature had a higher CO2 capacity and better stability.For this adsorbent the authors reported CO2 sorption capacity of74 wt.% in the first cycle and 62.5 wt.% in the eleventh cycle. In fact,the addition of ethanol to water decreased the solute solubility ofmixture and resulted in smaller particle size, larger surface areaand pore volume and, consequently, higher capacity. However,the treatment with pure ethanol or pure water led to poor capacity.

The reaction of limestone with organic acids is known asanother chemical treatment that improves the sintering resistanceof CaO sorbents derived from limestone by altering the porousstructure [120–124]. Li et al. [121] investigated the modificationof limestone with 50% acetic acid solution (the molar ratio of aceticacid to calcium: 1.5:1). The modified limestone exhibited the sorp-tion capacity of 0.39 g of CO2/g of sorbent after 20 cycles, whichwas significantly higher than that corresponding to the naturallimestone (0.12 g of CO2/g of sorbent). The better stability of acid-ified limestone during cyclic operation was attributed to its smallerparticle sizes, higher surface area and pore volume obtained during


acidification. These results were confirmed by Li et al. [122] whoreported an enhancement in cyclic stability of limestone treatedwith pyroligneous acid (PA) (ratio of PA to limestone: 20 mL/g).They also mentioned that the cost of PA is lower and hence, theuse of PA is more economical compared to acetic acid. With thepurpose of reducing the cost of sorbent production, natural lime-stone was also considered by Radfarnia and Iliuta [123] and a novelsynthesis technique (limestone acidification by citric acid followedby two-step calcination in Ar and air atmospheres) was applied inorder to prepare highly porous CaO structure with unique CO2 cap-ture ability. The performance of the proposed sorbent was investi-gated in detail, revealing a much better stability and CO2 sorptionactivity of the developed sorbent compared to natural limestone.The principal of the developed synthesis technique consisted inthe decomposition of calcium citrate (product of limestone acidifi-cation) in controlled atmosphere (Ar) to produce in-situ carbon,which controlled the particle size enlargement. The carbon com-bustion during the secondary calcination step (in air) promotedthe dispersion of particle agglomerates and enhanced the materialporosity. Ridha et al. [124] proposed the treatment of limestone byfour organic acids with the aim of improving the cyclic CO2 captureperformance of limestone. The increase of CO2 capture capacity forthe modified sorbents was explained by the fact that the acidifica-tion by organic acids widened the pores of the modified sorbentsand improved the resistivity of them to sintering phenomenon.However, the authors claimed that the treatment of limestone withorganic acids has little positive effect on CO2 capture capacity ofsorbents, but contributes in great measure to the process cost. Inaddition, the reactivity of modified sorbents towards SO2 wasimproved, resulting in the acceleration of the reduction of theirCO2 capture capacity compared to untreated limestone.

Many treatments used to improve the cyclic CO2 capture behav-ior of CaO-based sorbents, such as acidification and hydration, havethe drawback of decreasing the mechanical strength of sorbents. Insuch cases, pelletization is often considered as an option toimprove the mechanical strength of sorbents [61,125]. However,it should be noted that pelletization is an expensive process andtherefore, the application of a pretreated sorbent is considered eco-nomical when it shows a high improvement of CO2 sorption activ-ity [126]. Ridha et al. [126] determined the influence ofacidification and pelletization on CO2 capture behavior ofCaO-based pellets. For this purpose, raw limestone was treatedby acetic acid solution (10 vol.%) and then, both untreated andacidified limestones were pelletized by a calcium aluminatecement binder (10–14 wt.%). According to the experimentalresults, the acidified pellets adsorbed 41% more CO2 thanun-acidified pellets after 20 repetitive cycles of carbonation(650 �C, 15% CO2) and calcination (920 �C, 100% CO2), because ofthe enhanced morphology of acidified pellets. Indeed, the acidifiedpellets possessed large pores of size within 20–200 nm, whichenabled the sorbent to continue hosting more CaCO3. However,the increase in CO2 capture capacity could not justify such treat-ments due to the high price of acetic acid (around $900/ton).Moreover, the acidified pellets also showed an improved reactivitytowards SO2. A pretreatment of flue gases to remove SO2 is there-fore necessary when acidified pellets are used for CO2 capture.

Chemical pretreatment is not limited to treatment with etha-nol/water solution and acidification. Chen et al. [127] studied theeffect of pelletization of sorbent and addition of pore formingagents on the attrition resistance and CO2 capture capacity ofCa-based sorbent. The original limestone, aluminate cement (con-sisting of 58 wt.% Al2O3) and starch were used as precursors. Theexperimental results demonstrated that pelletization with10 wt.% aluminate cement considerably improved the mechanicalproperty of pellets, because of the high mechanical strength of alu-minate. Additionally, the pellets developed with 10 wt.% aluminate

cement and 5–10 wt.% starch indicated a higher and more stableCO2 capture capacity in multi-cyclic carbonation/calcinationexperiments in comparison to the natural limestone. After 10 car-bonation (at 700 �C in 15% CO2/air balance) and calcination (at900 �C in air) cycles, the pellets containing 10 wt.% aluminateand 10 wt.% starch offered a capture capacity of �0.65 g of CO2/gof pellets, which was higher compared to pellets of natural lime-stone (�0.5 g of CO2/g of pellets). The excellent performance ofthe pellets developed with aluminate cement and starch wasattributed to the increase of pore volume of pellets because ofstarch decomposition during calcination and the deceleration ofsintering due to the presence of alumina which has a high meltingpoint.

2.2. Ceramic CO2 sorbents

In addition to CaO-based sorbents, alkaline ceramic materialshave also been proposed as potential candidates for CO2 removalprocesses such as SESR [4,6,128–137]. However, their kinetic limi-tations during the CO2 capture still remain their main hurdle. Theliterature data for lithium zirconate (Li2ZrO3), lithium orthosilicate(Li4SiO4), and sodium zirconate (Na2ZrO3), which are the mostinvestigated ceramic sorbents, is summarized in Table 4. The car-bonation reactions for the most well-known alkaline oxide sor-bents are as follow:

Li2ZrO3ðsÞþCO2ðgÞ$ Li2CO3ðsÞþZrO2ðsÞ DH�298 ¼�160 kJ=mol

ð2Þ

Na2ZrO3ðsÞþCO2ðgÞ$Na2CO3ðsÞþZrO2ðsÞ DH�298¼�149 kJ=mol

ð3Þ

Li4SiO4ðsÞþCO2ðgÞ$ Li2CO3ðsÞþLi2SiO3ðsÞ DH�298¼�143 kJ=mol

ð4Þ

2.2.1. Lithium zirconate (Li2ZrO3)In 1998, Nakagawa and Ohashi [4] proposed Li2ZrO3 as a

promising candidate for CO2 adsorption. The material has a theo-retical uptake capacity of 0.28 g of CO2/g of sorbent in the temper-ature range of 450–600 �C. They employed solid-state reaction ofLi2CO3 and ZrO2 precursors to prepare Li2ZrO3. The synthesizedsorbent showed the CO2 capture capacity of 0.22 g of CO2/g of sor-bent at the temperature of 500 �C under a flow containing 20 vol.%CO2 and 80 vol.% H2. In a further study, the same research group[135] synthesized potassium carbonate doped Li2ZrO3 (K-Li2ZrO3)via solid-state reaction method. The developed sorbent showed ahigher CO2 adsorption rate in comparison to the undopedLi2ZrO3. The significant improvement in kinetic rate was ascribedto the formation of a eutectic molten carbonate layer of Li2CO3

and K2CO3 above 500 �C, which considerably increased the CO2 dif-fusion rate. Later, Ida and Lin [136] proposed a comprehensivedouble-shell model to explain the mechanism of CO2 chemisorp-tion on pure and potassium-doped Li2ZrO3 sorbents. According totheir model, CO2 reacts with Li+ and O2� ions after diffusion tothe surface of Li2ZrO3 to form ZrO2 and Li2CO3. These materialsform a double solid shell around the unreacted Li2ZrO3 (Fig. 5).At this step, Li+ and O2� have to diffuse through the ZrO2 layer toreact with CO2 and also CO2 molecules have to diffuse throughthe Li2CO3 layer for reaction. Therefore, the adsorption rate beginsto decrease. In the case of K-Li2ZrO3, the formation of a eutecticmolten carbonate layer of Li2CO3 and K2CO3 enhances the adsorp-tion kinetic rate by increasing the CO2 diffusion rate through theexternal layer. However, Ochoa-Fernandez et al. [138] showed thatdespite the positive effect of potassium carbonate doping intoLi2ZrO3 on the kinetic rate of CO2 adsorption, the CO2 capture

Table 4Summary of investigations on ceramic-type sorbents.

Sorbent Synthesis technique Reaction conditions Reactor Numberof cycles


Ref.

Ads. Reg.

Li2ZrO3 Citrate sol–gel 500 �C, 50% CO2, 60 min 650 �C, 100% N2, 50 min TGA 3 0.26 [143]Li2ZrO3 Co-precipitation 550 �C, 100% CO2, 20 min 690 �C, 100% N2, 10 min TGA 3 0.23 [134]Li2ZrO3 Liquid-state 575 �C, 100% CO2, 12 min 650 �C, 100% Ar TEOM 100 0.24 [5]Li2.2ZrO3.1 Liquid-state 575 �C, 100% CO2, 20 min 630 �C, 100% N2 TEOM 8 0.24 [138]KALi2ZrO3

a Solid-state 550 �C, 60% CO2, 100 min 800 �C, 100% N2, 30 min TGA 6 0.2 [130]K0.2ALi1.6ZrO2.9 Citrate sol–gel 550 �C, 25% CO2, 60 min 675 �C, 100% N2, 50 min TGA 4 0.23 [144]K0.2ALi1.6ZrO2.9 Liquid-state 575 �C, 100% CO2, 20 min 630 �C, 100% N2 TEOMc 8 0.18 [138]KALi2ZrO3

b Solid-state 500 �C, 100% CO2, 190 min 750 �C, 100% N2, 230 min TGA 5 0.26 [142]Li2ZrO3 Ultrasound-assisted

surfactant-template575 �C, 100% CO2, 30 min 690 �C, 100% Ar, 30 min IGAd 11 0.22 [145]

Li2CO3/K2CO3-doped Li2ZrO3 Solid-state 525 �C, 15% CO2, 60 min 850 �C, 60 min TGA 12 �0.083 [139]KALi4SiO4 – 580 �C, 4% CO2, 60 min 700 �C, 100% N2, 15 min TGA 25 0.15 [150]NaALi4SiO4 – 580 �C, 4% CO2, 60 min 700 �C, 100% N2, 15 min TGA 25 0.07 [150]Li4SiO4 Solid-state 680 �C, 100% CO2, 15 min 800 �C, 100% N2, 10 min TFBe 15 0.3 [149]Li4SiO4 Solid-state 550 �C, 100% CO2, 90 min 550 �C, 100% N2, 90 min TGA 10 �0.044 [147]Ball milled Li4SiO4 Solid-state 550 �C, 100% CO2, 90 min 550 �C, 100% N2, 90 min TGA 10 �0.154 [147]Li4SiO4 Solid-state 700 �C, 50% CO2, 30 min 700 �C, 100% N2, 30 min TGA 16 0.28 [148]Na2ZrO3 Solid-state 600 �C, 100% CO2, 30 min 780 �C, 100% N2, 65 min TGA 2 0.24 [6]Na2ZrO3 Liquid-state 550 �C, 100% CO2, 25 min 800 �C, 100% N2, 55 min TGA 2 0.15 [207]Na2ZrO3 Liquid-state 575 �C, 50% CO2 680 �C, 100% Ar TEOM 8 �0.15 [7]Na2ZrO3 Surfactant template/

ultrasound assisted575 �C, 100% CO2, 10 min 840 �C, 100% Ar, 30 min IGA 4 �0.13 [156]

a Li2CO3:ZrO2:K2CO3 (1.15:1.0:0.2).b 91.3% Li2ZrO3 + 3.4% Y2O3 + 0.2% Al2O3 + 5.1% K2O.c Tapered Element Oscillating Microbalance.d Intelligent Gravimetric Analyzer.e Twin Fixed-Bed Reactor.

Fig. 5. Adsorption mechanism on Li2ZrO3 solid sorbent [130].


capacity and cyclic stability of sorbent was lowered by the additionof potassium.

Alkali zirconates are normally prepared by solid-state reactionof ZrO2 and alkali salts such as alkali carbonate[4,129,130,135,136,139]. The synthesized materials often show aslow kinetic rate and a small capture capacity because of theirlarge particle sizes and poor porosity. Hence, the CO2 capture prop-erties of alkali zirconates can be improved by the reduction of theirparticle sizes, which can be achieved by controlling the startingprecursor sizes. For instance, Xiong et al. [140] studied the prepa-ration of K-doped Li2ZrO3 by the solid-state reaction of ZrO2 (withtwo different precursor sizes of 1 and 45 lm), Li2CO3, and K2CO3.They found that the particle size of developed sorbent wasdecreased by using ZrO2 of smaller particle size. The authors alsoobserved that the sorbent with smaller particle size had higherCO2 adsorption rate. However, it should be noted that the final pro-duct (Li2ZrO3) is subsequently prepared by the solid-state reactionof mixed powders of ZrO2 and Li2CO3 at high temperature, whichresults in material sintering. Therefore, reducing the particle sizeof sorbent with decreasing the particle sizes of starting materialsis a problematical issue.

Some researchers tried to improve the CO2 uptake properties ofLi2ZrO3 by developing different synthesis methods [5,134,141–145]. Ochoa-Fernandez et al. [5] prepared nanocrystalline tetrago-nal and monoclinic phases of Li2ZrO3 by a novel liquid-state

soft-chemistry route from zirconoxy nitrate and lithium acetateas precursors. The nanocrystalline tetragonal Li2ZrO3 developedby the proposed method showed a faster CO2 adsorption andregeneration rate in comparison to the sorbents synthesized bythe solid-state reaction method. This behavior was attributed tothe small crystallite size (about 13 nm) of the developed sorbent.In addition, this sorbent maintained above 90% of its initial CO2

capture capacity after 100 adsorption/regeneration cycles. Yiet al. [134] compared the CO2 capture performance of Li2ZrO3 syn-thesized via a liquid-state precipitation technique and a solid-statereaction technique. The Li2ZrO3 sample developed by the precipita-tion method showed a CO2 sorption rate ten times faster than thatdeveloped by solid-state reaction method. This observation wasattributed to the smaller particle size of the precipitated Li2ZrO3,which led to a higher CO2 diffusion rate. The cyclic adsorption/des-orption experiments showed that the CO2 capture rate and capac-ity were almost unchanged through three cycles. Moreover, theauthors indicated that the addition of more than 20% steamenhanced the CO2 sorption rate considerably. In another study,Iwan et al. [142] put forward a modified solid-state synthesismethod for the production of Li2ZrO3 CO2 sorbents. The main dif-ference between the proposed synthesis method and the tradi-tional solid-state reaction method was the use of zirconiumhydroxide instead of zirconium oxide at the stage of mixing withlithium carbonate. This permitted a much closer mixing of the pre-cursors in the aqueous slurry phase and a lower calcination tem-perature (700–750 �C) compared to that of the traditionalsolid-state method (P900 �C). Therefore, the agglomeration ofthe final product was prevented and a sorbent with higher surfacearea (about 11 m2/g) and reactivity was produced. The CO2 captureexperiments showed that the sorbent with higher surface area,even undoped with promoters such as potassium carbonate, pos-sessed much higher kinetic rates. However, the adsorption rate ofthe sorbent developed by the proposed synthesis method waslower than that of tetragonal Li2ZrO3 prepared by using theliquid-state soft-chemistry route [5].


Xiao et al. [143] proposed a citrate sol–gel method to synthesizeLi2ZrO3 nanocrystals. The nanocrystalline Li2ZrO3 with a tetragonalphase showed a better CO2 capture performance in comparison tothe sorbents fabricated by co-precipitation or liquid-phase meth-ods. It exhibited a faster sorption rate and a higher, nearly stoichio-metric capture capacity (�0.26 g of CO2/g of sorbent). Furthermore,the developed sorbent demonstrated a good stability in cyclicadsorption/desorption process, maintaining its initial capturecapacity (�0.26 g of CO2/g of sorbent) after 3 cycles. Later, thesame research group employed a citrate sol–gel method to prepareK-doped Li2ZrO3 sorbents [144]. The developed K-doped Li2ZrO3

sorbents showed better CO2 capture performance compared tothe Li2ZrO3 synthesized by a similar method, especially at lowCO2 partial pressures. The K-doped Li2ZrO3 with an optimizedK:Li:Zr molar ratio of 0.2:1.6:1 exhibited a CO2 sorption rate of1.5 wt.%/min at 550 �C and CO2 partial pressure of 0.25 bar.Moreover, this sorbent displayed a good stability and maintainedits initial CO2 capture capacity (�0.23 g of CO2/g of sorbent) after4 adsorption/desorption cycles .In another study, Radfarnia andIliuta [145] developed Li2ZrO3 by applying surfactant template/-sonication method and showed an increase of the CO2 capturecapacity of developed sorbent compared to the material preparedby simple surfactant template method (without sonication) or con-ventional wet-mixing route. Li2ZrO3 prepared by surfactant tem-plate/sonication method contained less agglomerated structureand its porous framework facilitated gas and ion diffusions to/fromparticle layers. The sonication time and surfactant concentrationwere found to affect the sorbent properties (crystallite size andBET surface area). Porous Li2ZrO3 prepared by less surfactant andirradiation time could achieve maximum uptake capacity of22 wt.%, which was significantly higher compared to other testedsamples. Despite the good sorption capacity, it was mentioned thatporous Li2ZrO3 still suffered from slow kinetics of sorption at lowCO2 partial pressure (below 0.75 bar), which can limit its applica-tion for SESR operation.

2.2.2. Lithium orthosilicate (Li4SiO4)Lithium orthosilicate (Li4SiO4) was introduced as a novel CO2

sorbent by Kato and Nakagawa [8]. They employed thesolid-state reaction method to synthesize Li4SiO4 from Li2CO3

and SiO2 precursors. The developed sorbent displayed an adequateCO2 sorption capacity up to 0.36 g of CO2/g of sorbent in the tem-perature range of 450–700 �C. Later, Kato et al. [131] reportedabout the fascinating characters of Li4SiO4 as a CO2 sorbent.According to their results, the CO2 sorption capacity and rate ofLi4SiO4 were, respectively, around 50% more and 30 times fasterthan that of Li2ZrO3, even at CO2 concentration as low as 20%. Itwas also found that at 2% CO2 concentration, Li4SiO4 clearlyadsorbed CO2 even though the rate of adsorption was less than thatat 20% CO2 concentration. Nevertheless, Li2ZrO3 showed no obvi-ous CO2 sorption. However, Rodriguez-Mosqueda and Pfeiffer andYi et al. [9,134] observed that Li4SiO4 had slow kinetic rate atlow CO2 partial pressures in comparison to other alkaline ceramicssuch as Li2ZrO3 and Na2ZrO3 developed by liquid–liquid statereaction.

Venegas et al. [146] studied the CO2 capture properties ofLi4SiO4 with different particle sizes. Different synthesis methods,including solid-state reaction, precipitation and sol–gel, wereemployed to synthesize Li4SiO4 samples with different particlesizes. The experimental results revealed that the Li4SiO4 sampleprepared via precipitation method had the highest CO2 adsorptionreactivity. This behavior was attributed to its smaller particle size(about 3 lm), which was associated with the presence of moreactive lithium atoms over the surface of particles. Romero-Ibarraet al. [147] initially synthesized lithium orthosilicate (Li4SiO4) bythe solid-state reaction of LiOH and SiO2 precursors, and then

modified it by the ball milling process. The characterization analy-sis of both Li4SiO4 sample and ball milled Li4SiO4 sample showedthat the crystal size was decreased from >500 Å to 175 Å and thesurface area was increased from 0.4 to 4.9 m2 /g. The modifiedLi4SiO4 sample showed better efficiencies during the CO2 chemisorption–desorption process without further sintering effects.

Shan et al. [148] prepared Li4SiO4-based sorbents from diato-mite (as an inexpensive source of silicon) by the solid-state reac-tion method at 700 �C. They studied the effect of different molarratios of raw materials on CO2 adsorption capacity in a gas mixtureof CO2 and N2 (50 vol.%). They found that the CO2 adsorption capac-ity reached the largest value (30.32 wt.%) at Li2CO3/SiO2 molarratio of 2.6. They also observed that the CO2 adsorption capacitydecreased by only 6.44 wt.% during 16 adsorption–desorptioncycles because of the specific morphologies of Li4SiO4-based sor-bents synthesized from diatomite. Wang et al. [149] developedLi4SiO4-based sorbents from Li2CO3 and different types of silica(citric acid pretreatment rice husk ash (CRHA), nano-structuredAerosil, and crystalline Quartz powders) by the solid-state reactionmethod and studied the effects of the type of silica on themicrostructure and CO2 capture performance of developed sor-bents. Among all developed sorbents, CRHA-Li4SiO4 sorbentshowed the best performance with the highest CO2 capture capac-ity of 30.5 wt.% more rapid adsorption/desorption process and bet-ter regenerability during multicyclic adsorption/desorptionexperiments (only 2.1 wt.% decrease of CO2 capture capacity after15 cycles). The excellent performance of this sorbent was attribu-ted to its lower crystalline of pure Li4SiO4, smaller particle size, andlarger specific surface area, as a result of the favorable microstruc-ture of nanoparticles and the strong sintering-resistant character ofCRHA.

Seggiani et al. [150] applied the solid-state reaction method tosynthesize Li4SiO4-based sorbents containing 10, 20, 30 wt.% ofalkali carbonates (K2CO3, Na2CO3), binary (K2CO3/Li2CO3,Na2CO3/Li2CO3) and ternary (K2CO3/Na2CO3/Li2CO3) eutectic car-bonate mixtures. The CO2 adsorption characteristics of the devel-oped sorbents were investigated at high temperatures in therange of 500–600 �C and low CO2 partial pressure of 0.04 atm.According to the results, the CO2 sorption rate and capacity of allthe promoted Li4SiO4-based sorbents were obviously improved incomparison to pure Li4SiO4 sorbent. This was attributed to themuch faster diffusion of CO2 through the molten carbonate shell,compared to that through the solid Li2CO3 shell in no-promotedLi4SiO4. Furthermore, the CO2 sorption rate and capacity of the pro-moted sorbents containing alkali carbonates were higher thanthose of the promoted sorbents containing binary or ternary eutec-tic carbonate mixtures. For the promoted sorbents containingalkali carbonates, (K/Li)CO3 and (Na/Li)CO3 eutectic melt wereformed on the Li4SiO4 surface during the CO2 sorption. However,for the promoted sorbents containing binary and ternary eutecticcarbonate mixtures, a eutectic liquid phase was already presenton the surface of Li4SiO4 at the beginning of CO2 sorption andCO2 had to diffuse through this molten layer in order to reachthe surface of Li4SiO4. Therefore, this molten layer acted as an addi-tional diffusional resistance. For all the promoted Li4SiO4-basedsorbents, 580 �C was reported as optimum sorption temperature,because the equilibrium temperature was around 590 �C at CO2

partial pressure of 4.04 atm, and therefore, the desorption processwas activated at temperatures higher than 580 �C. At 580 �C, pro-moted Li4SiO4-based sorbents containing 30 wt.% of K2CO3 orNa2CO3 indicated the best CO2 adsorption characteristics withsorption capacity of 0.23 g of CO2/g of sorbent corresponding to aLi4SiO4 conversion of 80%. However, the promoted sorbent con-taining Na2CO3 indicated a considerable degeneration of CO2 cap-ture capacity during multi-cyclic adsorption/regenerationexperiments due to the structural sintering. On the contrary, the


promoted sorbent containing K2CO3 presented a very good cyclicstability during 25 adsorption/regeneration cycles.

Fig. 6. CO2 sorption on Na2ZrO3 at different temperatures. (A) T 6 550 �C; theNa2CO3AZrO2 external shell is mesoporous, and the CO2 diffusion occurs throughthe mesoporous structure. (B) T > 550 �C; the Na2CO3AZrO2 external shell is notporous [157].

2.2.3. Sodium zirconate (Na2ZrO3)Na2ZrO3 has been proposed as an alternative for CO2 capture

because of its higher adsorption kinetic rate at high temperaturesand lower initial precursor costs in comparison to Li2ZrO3 andLi4SiO4. Lopez et al. [6] examined the CO2 capture properties of dif-ferent sorbents, including Li2ZrO3, Na2ZrO3, Li4SiO4, Na2TiO3 andNa3SbO4, developed by solid-state reaction of precursors.Although Na2ZrO3 had the best adsorption kinetics at high temper-ature compared with the others, its regeneration performance wasunfavorable compared to Li2ZrO3 and Li4SiO4.

In order to overcome the drawbacks of Na2ZrO3 (such as theneed of the high regeneration temperature and the inherent weak-ness of the regeneration kinetics), different research groups tried topromote its CO2 capture performance by the incorporation of var-ious active metals into its structure and/or the production of smal-ler final particle sizes [151–155]. Most of them applied thesolid-state reaction method to synthesize their sorbents.However, the results were not fully satisfactory. Therefore, differ-ent other synthesis methods were further employed for the syn-thesis of Na2ZrO3 to improve its performance. Zhao et al. [7]applied a novel soft-chemical route to synthesize nanocrystallineNa2ZrO3 from zirconoxy nitrate and sodium citrate. During thesynthesis process, an amorphous zirconium complex was formedand then, it was calcined in a controlled atmosphere. The strongreaction between nitrate and citrate during calcination resultedin the in-situ carbon formation. Subsequent carbon burnoff pro-moted the formation of open pore structure. In the proposed syn-thesis method, the crystal phase of Na2ZrO3 was controlled by thecalcination temperature and atmosphere. A two-step calcination at800 �C caused to the formation of pure monoclinic phase, whichwas much more active rather than hexagonal phase of Na2ZrO3.According to the CO2 capture studies, the monoclinic Na2ZrO3

showed much faster CO2 adsorption rates in comparison to itshexagonal counterpart, even at CO2 partial pressure as low as0.025 bar. It also exhibited an excellent stability over 8 adsorp-tion/desorption cycles. Taking into consideration the improvementof Li2ZrO3 sorption properties by the application of surfactant tem-plate/sonication technique, Radfarnia and Iliuta [156] applied thesame method to develop porous Na2ZrO3 because this materialwas supposed to offer better CO2 sorption kinetics compared toLi2ZrO3. The behavior of the proposed Na2ZrO3 sorbent was com-pared with that of samples prepared by surfactant templatemethod (without sonication) and conventional wet-mixing route.The performance of the new developed Na2ZrO3 was unexpected.The samples prepared by surfactant template/sonication techniquewere found to be less active during cyclic operation compared tothe conventional Na2ZrO3. This behavior was interpreted by thelow resistivity of the pore structure at the high temperature treat-ment required during calcination (compared to Li2ZrO3), resultingin the loss of material main porosity and the creation of agglomer-ated particles.

Martínez-dlCruz and Pfeiffer [157] evaluated the microstruc-tural evolution of Na2CO3AZrO2 external shell produced duringthe CO2 adsorption on Na2ZrO3 as a function of sorption tempera-ture. According to the results, the microstructural properties variedwith the sorption temperature. At T 6 550 �C, CO2 adsorption onNa2ZrO3 was not limited because the Na2CO3AZrO2 external shellwas mesoporous. CO2 diffused through the mesopores and thereaction continued. However, the mesoporosity of theNa2CO3AZrO2 external shell disappeared at T > 550 �C due to ther-mal sintering and the CO2 sorption was kinetically controlled bythe CO2 diffusion through the sodium crystal phases (Fig. 6).

2.2.4. Other ceramic materialsApart from Li2ZrO3, Li4SiO4, and Na2ZrO3, which are the most

studied ceramic CO2 sorbents up to now, a quite few studies havebeen performed on CO2 adsorption by other kinds of ceramic mate-rials including penta-lithium aluminate (Li5AlO4) [158,159],lithium cuprate (Li2CuO2) [160,161], lithium ferrite (LiFeO2)[162,163], lithium oxosilicate (Li8SiO6) [164–166], lithium orthoti-tanate (Li4TiO4) [167,168], sodium metatitanate (Na2TiO3) [6,169],and barium ferrite (Ba2Fe2O5) [170,171]. The CO2 chemisorption onthese materials is similar to that observed for Li2ZrO3, Li4SiO4, andNa2ZrO3 and leads to the formation of an alkaline carbonate andthe corresponding residual oxide or secondary alkaline phase.Some of these materials possess different interesting propertiesas possible CO2 sorbents. For instance, Li5AlO4 has the best theoret-ical CO2 capture capacity (0.87 g of CO2/g of sorbent) among thelithium ceramics because of its high Li/Al molar ratio and the factthat aluminum is a lighter atom compared to any other elementincluded in lithium ceramics. Moreover, it is capable of capturingCO2 in a wide range of temperature (200–700 �C) [158].However, Avalos-Rendon et al. [159] reported the loss of CO2 cap-ture capacity during multi-cyclic adsorption/desorption operationsfor Li5AlO4. They evaluated the performance of a- and b-Li5AlO4

phases in cyclic adsorption/desorption experiments (adsorptionat 700 �C under 100% CO2 and desorption at 750 �C under 100%N2). For a-Li5AlO4 phase, the initial CO2 capture capacity of47.7 wt.% decreased to 22.1 wt.% after 20 cycles. Under the sameoperating conditions, the CO2 capture capacity of b-Li5AlO4 phasedecreased from 62.3 wt.% to 8.1 wt.% over 20 cycles. The reductionof CO2 capture capacity during the multicycle process was attribu-ted to the sublimation of Li2O during desorption. Li2CuO2, which isanother lithium-based ceramic, has also shown interesting resultsas a CO2 sorbent. It has a high theoretical CO2 capture capacity of0.402 g of CO2/g of sorbent. It should also be mentioned that it isable to capture CO2 in a wider range of temperatures (120–690 �C) in comparison to other lithium-based ceramic materials.Moreover, it should be noted that copper is lighter and alsocheaper than zirconium. Hence, it may be considered as an optionfor industrial applications. However, Palacios-Romero and Pfeiffer[160] showed that single-phase Li2CuO2 could not be formed bythe co-precipitation method and thus, the prepared sorbentshowed a CO2 uptake capacity much lower than theoretical capturecapacity (0.136 g of CO2/g of sorbent at 650 �C). Matsukura et al.


[161] applied the solid-state reaction to prepare Li2CuO2 fromLi2CO3 and CuO at 680–685 �C. The single-phase Li2CuO2 obtainedby this method exhibited CO2 adsorption capacity of 0.402 g ofCO2/g of sorbent, which was equivalent to the theoretical value.

The CO2 capture behavior of Li8SiO6 was studied byDurán-Muñoz et al. [164] for the first time. They employed asolid-state reaction method to synthesize a Li8SiO6 sample andthen, evaluated the CO2 capture capacity of developed material.The results showed that Li8SiO6 might be considered as a newalternative for CO2 capture at high temperatures because of itswide temperature range (300–700 �C), its high CO2 capture capac-ity (0.521 g of CO2/g of sorbent at 650 �C), and its kinetic behavior(faster adsorption rate in comparison to Li4SiO4). In a further study,Romero-Ibarra et al. [166] showed that the addition of sodium car-bonate, potassium carbonate, or a mixture of both improved thecapture capacity of Li8SiO6 due to the formation of eutectic phases.Similar to Li8SiO6, Li4TiO4 shows excellent properties for CO2

adsorption. Single phase Li4TiO4 is able to adsorb CO2 in the widetemperature range of 300–850 �C, and possesses the high CO2 cap-ture capacity of 0.42 g of CO2/g of sorbent at 856 �C. Moreover, itsCO2 adsorption rate is faster than that of Li4SiO4 [167]. However,the CO2 capture capacity of Li4TiO4 decreases in cyclic operationbecause of the decline in the amount of Li4TiO4, which is resultedfrom the decrease in the reactable surface area and the vaporiza-tion of Li4TiO4, Li2TiO3, and Li2O during multicyclic sorption/des-orption processes [168].

Among the ceramics mentioned, the very low CO2 capturecapacity of LiFeO2, Na2TiO3, and Ba2Fe2O5 limits their practicalapplications. LiFeO2 readily releases CO2 at lower temperatures(around 530 �C) in comparison with other ceramic materials.However, its CO2 capture capacity and rate are not sufficient forpractical use. Kato et al. [163] reported the CO2 uptake capacityof around 0.01 g of CO2/g of sorbent at 500 �C for LiFeO2. In a fur-ther study, Yanase et al. [162] found that the structural phase tran-sition occurring above 425 �C suppressed the CO2 capture ofLiFeO2. Na2TiO3, which is a sodium-based ceramic sorbent, pre-sents lower CO2 capture capacity and slower adsorption/desorp-tion rate in comparison to the most studied sodium-basedceramic (Na2ZrO3). According to TGA experiments at 600 �C,Na2TiO3 showed the capture capacity of 0.08 g of CO2/g of sorbent,the sorption rate of 0.1097 wt.%/min, and the desorption rate of0.043 wt.%/min. However, Na2ZrO3 presented higher adsorptioncapacity (0.26 g of CO2/g of sorbent) and faster adsorption(10.33 wt.%/min) and desorption (1.02 wt.%/min) rates under thesame operating conditions. The low CO2 capture capacity ofNa2TiO3 was ascribed to the presence of high molecular titanatesthat are less reactive to CO2 [6]. Ba2Fe2O5 shows CO2 adsorptionin the temperature range of 500–1000 �C and CO2 desorptionabove 1000 �C under CO2 partial pressure of 1 atm. SinceBa2Fe2O5 can capture CO2 at higher temperatures in comparisonto Li4SiO4, a better kinetic behavior is observed for Ba2Fe2O5 underpractical conditions compared to Li4SiO4. However, Ba2Fe2O5

shows lower CO2 capture capacity (0.094 g of CO2/g of sorbent at1000 �C) than Li4SiO4 [170].

2.2.5. Kinetic modelsUp to date, different models, including double-shell, multiple

exponential, shrinking core, Avrami-Erofeev, and rate law havebeen proposed for analyzing the CO2 adsorption mechanism onalkaline ceramic materials. As it was mentioned in Section 2.2.1,Ida and Lin [136] proposed a double-shell model to describe themechanism of CO2 adsorption on pure and K-doped Li2ZrO3. Katoet al. [131] and Essaki et al. [172] used the same model to explainthe CO2 sorption mechanism on Li4SiO4. Double exponential isanother model that has been used by different research groups todescribe the CO2 sorption mechanism on different alkaline

ceramics including Li4SiO4, Na2ZrO3, Li8SiO6, and Li5AlO4.According to this model, two different processes happen duringCO2 capture by alkaline ceramics: (1) CO2 chemisorption over thesurface of ceramics, which produces an external shell containingalkaline carbonate and a metal oxide or an alkaline secondaryphase; and (2) alkaline element diffusion throughout the externallayer to reach the surface and react with the CO2, which beginsonce the external layer is totally formed [9,146,158,165,173–175]. Qi et al. [174] analyzed the reaction mechanism of CO2

adsorption/desorption on Li4SiO4 by comparing the double expo-nential, shrinking core and Avrami–Erofeev models. The shrinkingcore model, which is commonly used in nonporous materials,assumes that the rate of reaction is controlled by the rate of chem-ical reaction. The Avrami–Erofeev model has been employed forreactants with highly crystalline structures. According to thismodel, the rate of reaction is controlled by the rate of the forma-tion and growth of the reaction product crystals. Fitting the exper-imental data to the double exponential model showed that thelithium diffusion process was the limiting step of the whole CO2

sorption process. This observation was inconsistent with the modelassumption that the alkaline diffusion process takes place once theexternal shell is completely formed. Shrinking core model did notfit the experimental data well in the whole adsorption temperaturerange (550–700 �C). Hence, the sorption process was not controlledonly by the rate of chemical reaction. The Avrami–Erofeev modelcombined with the double-shell model clearly described the mech-anism of the CO2 sorption on Li4SiO4. First, the superficial reactionbetween the CO2 molecules and Li4SiO4 led to the formation ofsolid Li2CO3 and Li2SiO3 nuclei on the surface. This step was veryshort and the rate of CO2 sorption in this step was limited by therate of the formation of the product crystals. Then, the diffusionprocess took place once the double-shell was formed by thegrowth of the Li2SiO3 and Li2CO3 nuclei over the unreactedLi4SiO4. Pfeiffer and co-workers [164,169,175] have published sev-eral papers about the application of the rate law model for analyz-ing the mechanism of CO2 adsorption on different ceramics such asLi8SiO6, Na2TiO3, and Na2ZrO3. The rate law model is used whenseveral processes are involved in the mechanism of CO2

adsorption.

3. Hydrogen production by sorption enhanced steam methanereforming (SESMR)

The burning of fossil fuels is known to represent the major con-tributor to the global warming phenomenon. The fossil fuels suchas oil, coal, and natural gas are today the major sources of energyfor industrial activities (over 80% of total industrial energy needs).However, the drastic reduction of fossil fuels sources and theirharmful influence on the environment and human health justifythe necessity of alternative energy source development with theadvantages of low emission of pollutants and more energy permass. Hydrogen is considered a great candidate for these goals[176,177]. Since the discovery of this gas in 1766, numerous tech-nological progresses have been made in its production and applica-tions. Recently, the scientific efforts have been concentrated on thedevelopment of new technologies for hydrogen production toincrease the efficiency and reduce the production cost [176–179].

Hydrogen does not occur free in nature. It can be generally pro-duced by reforming of fuels or non-reforming processes like waterelectrolysis and biomass gasification. The fossil fuels (especiallythe natural gas) are usually used in industrial applications[176,177,180–182]. Among the three processes for hydrogen pro-duction from methane feedstock, including steam reforming (SR),partial oxidation (POX), and autothermal reforming (ATR), themost economic process is steam methane reforming (SMR) due


to the highest thermal efficiency and the lowest capital investment[176,177,180,182]. The main reactions in the steam methanereforming (SMR) include the endothermic methane reforming (5)and the exothermic water gas shift (WGS) (6):

Reforming : CH4ðgÞþH2OðgÞ$ 3H2ðgÞþCOðgÞ DH�298 ¼ 206 kJ=mol

ð5Þ

WGS : COðgÞ þH2OðgÞ $ H2ðgÞ þ CO2ðgÞ DH�298 ¼ �41 kJ=mol

ð6Þ

After desulfurization, methane is reformed on nickel-aluminacatalysts at 700–900 �C and 14–20 bar in the presence of steam(steam to carbon molar ratio (S/C) of 2.5 to 5) to produce syngas,a mixture of CO and H2. CO can further react with steam to producemore hydrogen via WGS reaction. The exothermic WGS process isusually performed in two separate reactors at high (350–400 �C)and low (�200 �C) temperatures, on chromium iron oxide andcopper-zinc catalysts. The effluent gas typically contains 71–75%H2, 4–7% CH4, 1–4% CO, and 15–20% CO2 (dry basis). For the sepa-ration of H2 from CO2, chemical absorption or multicolumn pres-sure swing adsorption (PSA) are commonly used in the finalstage of the process, depending on the desired purity [21,177,183].

SMR is a high energy consuming process. Additional steps forhydrogen purification by chemical absorption (high energy con-sumption) or PSA (relatively complex process with around 10% lossof H2) increase the capital investment and reduce the process effi-ciency. The cost of WGS and PSA was estimated to around 30% ofthe total cost of H2 production unit [21,184]. An interesting optionto decrease the capital cost of SMR is an integrated process com-bining the reforming reaction with the in-situ CO2 separation byhigh-temperature solid sorbents. The sorption enhanced steammethane reforming (SESMR) was proposed as a novel efficienttechnology to produce highly pure hydrogen by steam methanereforming [185,186]. The concept of sorption enhanced reactionprocess (SERP) is based on the use of a mixture of reforming cata-lyst and selective regenerable solid sorbent to remove CO2 in-situfrom the reaction zone. The CO2 removal from the gas phase allowsthe production of highly pure hydrogen in a single step. The prin-ciple of SESMR is to shift the equilibrium of the reversible reactions(5) and (6) based on the Le Chatelier’s principle to enhance hydro-gen production through in-situ CO2 removal from the reactionzone, in order to obtain a hydrogen conversion as much as 95%(dry basis) in a single step, compared to maximum 80% (dry basis)achieved in a conventional reformer [21]. The selective in-situ CO2

removal from the reaction media can be performed usinghigh-temperature solid sorbents (MeO represents a metal oxide):

CO2 removal : MeOðsÞ þ CO2ðgÞ $MeCO3ðsÞ DH� < 0 ð7Þ

The simultaneous reactions (5)–(7) therefore lead to the overallreaction (8):

CH4ðgÞ þ 2H2OðgÞ þMeOðsÞ $ 4H2ðgÞ þMeCO3ðsÞ DH� � 0

ð8Þ

The sorbent materials for CO2 separation must be able to resistin severe operating conditions such as the presence of steam dur-ing the adsorption process and high temperature and pressure.Various high temperature sorbents developed for CO2 capture havebeen tested in the SESMR process (the current literature data arepresented in Table 5).

The SESMR process can be performed in dual fixed-bed reactorswith periodic switching between hydrogen production and sorbentregeneration or in parallel circulating fluidized-bed reactors [21].Several important advantages of SESMR process were pointed outin the literature [21,68,183,185–188]:

� highly efficient H2 production with less by-products (CO andCO2);� elimination of the individual reactor for WGS;� elimination of downstream hydrogen purification steps;� achieving high conversion of methane to hydrogen at significant

lower temperature (450–600 �C) compared to traditional SMR(700–900 �C);� replacement of high alloy steels by less expensive materials;� 20% to 25% energy reduction compared to traditional SMR;� minimization of carbon deposition in the reformer;� reduction of CO2 release to the atmosphere; relatively pure CO2

can be captured and further sequestrated or used in severalprocesses;� reduction of the excess steam used in conventional SMR.

3.1. Application of CaO-based sorbents in SESMR

CaO-based sorbents are the most well-known CO2 sorbents usedfor the H2 production by SESMR process so far [187,189–191]. In anearly study, Balasubramanian et al. [192] used CaO sorbent obtainedfrom the calcination of high purity CaCO3. At 650 �C and S/C ratio of 4the authors reported the production of H2 with a molar fractionhigher than 95% (dry basis). The influence of several operatingparameters (temperature, steam to carbon ratio, and feedstock com-position) was investigated in order to find the optimum workingconditions. However, as it was mentioned before, the CO2 capturecapacity of CaO-based sorbents decreases in cyclic operation, partic-ularly under severe regeneration conditions (the presence of CO2 inthe regeneration atmosphere) because of the sintering of the CaOparticles and thus, frequent shut down is needed for regeneratingthe CO2 sorbent. Therefore, many attempts have been made todevelop thermal-stable CaO-based sorbents for application inSESMR process. Li et al. [193] studied the application ofCaO/Ca12Al14O33 (75%/25%) sorbent developed previously [12,65]in a continuous SESMR process for a period of 400 min. The processwas cyclically operated in two parallel fixed-bed reactors: (i) hydro-gen production and CO2 sorption at 630 �C, 1 atm, and S/C ratio of 5and (ii) sorbent regeneration under argon at 850 �C and 1 atm. Theauthors reported that hydrogen with a purity of 95% could be contin-uously produced. It was concluded that the switchover time(pre-breakthrough period) between the two reactors was a keyparameter for H2 efficiency. Broda et al. [194] evaluated the perfor-mance of a mixture containing 5.7 g of Ni-hydrotalcite-derived cat-alyst and 1.26 g of Ca-based sorbents in SESMR process for 10successive cycles. The Ni-hydrotalcite (Ni-Htlc)-derived catalystcontaining 47 wt.% of Ni was developed by co-precipitation methodfrom Ni(NO3)2�6H2O, Mg(NO3)2�6H2O, and Al(NO3)3�9H2O precur-sors. Al2O3-stabilized Ca-based sorbents with the Ca2+/Al3+ ratio of90:10 or 80:20 were prepared via sol–gel technique from aluminumisopropoxide and calcium hydroxide precursors. Under SESMR con-ditions (550 �C, S/C of 4), H2 was produced with a purity of 99% (drybasis). The H2 production rate declined by 1.9% per each cycle whenthe mixture of Ni-Htlc and Ca:Al 80:20 was used, representing adecrease of 275% in comparison with Ni-Htlc/limestone mixture.The good activity of the Al-stabilized CaO-based sorbents underSESMR conditions was on account of the homogeneously dispersedhigh Tammann temperature support (Ca12Al14O33), which hinderedthe structural sintering and pore pluggage and stabilized its nanos-tructured morphology. Xu et al. [195] developed a series ofCa9Al6O18-CaO sorbents from different calcium precursors, includ-ing calcium acetate, calcium citrate, calcium lactate, and calciumgluconate by a sol–gel method. Among all developed sorbents, thecalcium lactate-derived sorbent showed the best CO2 uptake perfor-mance in comparison with the other sorbents tested because of itslargest surface area and pore volume. A mixture of 3 g of calciumlactate-derived sorbent containing 90 wt.% CaO and 1 g of

Table 5Summary of investigations on SESMR.

Solid (sorbent, catalyst) Patterna NiO loading(wt.%)

Reaction conditions Reactorb Numberof cycles

H2

concentration(molar %)

Ref.

Reac. Reg.

Ni commercial cat./com. CaO M 22.0 450–750 �C, 15 atm., S/C: 3–5 N.A. FB 1 >95 [192]Ni commercial cat./dolomite M 18 650 �C; 15 atm., S/C: 4 800–950 �C, 100% N2,

4% O2/N2, 100% CO2

FB 25 >95 [190]

Ni commercial cat./CaO:Ca12Al14O33 M 20 630 �C; 1 atm., S/C: 5 850 �C, 100% Ar FB 12 >90 [193]Ni commercial cat./dolomite M (a) 600 �C, 1 atm., S/C: 3 850 �C, 100% N2 FB 4 >98 [187]Ni commercial cat./CaO:Ca12Al14O33 M 18 650 �C, 1 atm, S/C: 3.4 850 �C, 100% He FB 13 >92 [191]NiO/CaO/Ca12Al14O33 H 20 650 �C, 1 atm, S/C: 3.4 850 �C, 100% He FB 1 90 [201]NiO/CaO H (b) 600 �C, 1 atm, S/C: 3 N.a. FB 1 80 [199]La2O3/NiO/CaO/Al2O3 H N.a 600 �C, 1 atm, S/C: 4 800 �C, 100% N2 FB 30 >92 [202]ZrO2/NiO/CaO/Al2O3 H (c) 600 �C, 1 atm, S/C: 4 800 �C, 100% N2 FB 20 >90 [56]NiO/CaO/HTlc (Al-Mg) H (d) 550 �C, 1 atm, S/C: 4 750 �C, 100% N2 FB 10 99 [203]NiACaOACa12Al14O33 H 7 630 �C, S/C: 3 780 �C, 100% N2 FB 4 95 [74]NiOAHTlc (AlAMg)/Al-stabilized CaO M (e) 550 �C, 1 atm, S/C: 4 750 �C, 100% N2 FB 10 99 [196]NiOAHTlc (AlAMg)/CaO:Ca9Al6O18 M (g) 550 �C, 1 atm, S/C: 4.2 800 �C, 20% H2/80% N2 FB 1 97 [200]Ni commercial cat./Li2ZrO3 M N.a. 505 �C, 1 atm, S/C: 4 N.a. FB 1 85 [207]Ni commercial cat./Na2ZrO3 M N.a. 600 �C, 1 atm, S/C: 4 N.a. FB 1 97 [207]NiOAHTlc (AlAMg)/Li2ZrO3 M (f) 575 �C, 5 atm, S/C: 5 650 �C, 100% Ar FB 1 68 [128]NiOAHTlc (AlAMg)/Na2ZrO3 M (f) 575 �C, 5 atm, S/C: 5 750 �C, 100% Ar FB 1 97 [128]NiO-c-Al2O3/CaOACa9Al6O18 M 5 650 �C, 5 atm, S/C: 2.85 850 �C, 100% N2 FB 1 89.1 [206]NiAMgAlO/CaOACa9Al6O18 M N.a. 600 �C, 1 atm, S/C: 4 800 �C, 20% H2/80% N2 FB 35 98 [195]NiAHtlc/CaOACa12All4O33 M 47 wt.% in

catalyst (for Ni)550 �C, S/C: 4 750 �C, 100% N2 PB 10 99 [194]

NiOACaO/Ca9Al6O18 H 25 650 �C, 1 bar, S/C: 4 800 �C, 11.1% H2 (in Ar) FB 30 >95 [204]CaOAZr/NiO H 20.5 650 �C, 1 bar, S/C: 4 800 �C, 11.1% H2 (in Ar) FB 10 >95 [205]

(a) Ni loading: >12 wt%; (b) Ni loading: 12.5 wt%; (C) Ni loading: 15 wt%; (d) Ni loading: 45 wt%; (e) Ni loading: 47 wt%; (f) Ni loading: 40 wt%; (g) Ni:Mg:Al (atomic ratio inHTlc catalyst) = 0.5:2.5:1

a Mixture (M) or hybrid (H).b Fixed-bed (FB) or Packed-bed (PB).


Ni-based catalyst was tested in the SESMR process at 600 �C and S/Cof 4. The H2 concentration during the prebreakthrough stage was98%. During 10 successive reforming-regeneration cycles, themethane conversion and different species concentrations remainedalmost constant, demonstrating the good performance of theCaO-based sorbent developed by sol–gel technique. Broda et al.[196] used a mixture of Ni-HTlc-derived catalyst (47 wt.% Ni, pre-pared by co-precipitation) and a synthetic Ca-based sorbent (CaOsupported on calcium aluminate). The sorbent pellets were synthe-sized from limestone and commercial calcium aluminate (CA-14:71% Al2O3 and 28% CaO). At 550 �C and S/C of 4, 99 vol.% H2 (dryand N2-free basis) was obtained in the reforming process. After 10sorption/regeneration cycles, the CO2 capture capacity was 0.41 gof CO2/g of sorbent, in comparison with limestone (0.22 g of CO2/gof sorbent). The appropriate thermal stability of CO2 sorbent wasattributed to the uniform dispersion of Ca12Al14O33 among CaOparticles.

For large-scale hydrogen production units where mass transferlimitations can significantly affect the process efficiency, the cata-lyst–sorbent mixing configuration is an important parameter tobe considered. Besides a physical admixture of catalyst with sor-bent, hybrid catalyst–sorbent patterns which integrate the catalyticreaction and the CO2 sorption in a single pellet can present someadvantages such as the elimination of mass diffusional limitationand the reduction of reactor volume [197,198]. Several studies havebeen performed on the development of hybrid catalyst–sorbentmaterials for SESMR process [199–206]. A first attempt to combineCaO-based sorbent and catalyst in a single pellet was performed byMartavaltzi and Lemonidou [201] who developed a hybridNiACa12Al14O33ACaO catalyst–sorbent. The optimum NiO loadingof 20 wt.% was shown to lead to a H2 concentration of 90% at650 �C and S/C of 3.4, as well as CO2 and CO effluent concentrationsof 2.8% and 2%, respectively. The process was only limited to onecycle. Wu and Wang [56] further investigated the application ofZrO2-stabilized NiOACaO/Al2O3 hybrid catalyst–sorbent in theSESMR during 20 cycles. The authors reported a H2 concentration

of more than 90%. The favorable stability and activity of the pro-posed hybrid material during cyclic operation was found to be onaccount of avoiding the formation of NiAl2O4 phase by incorporat-ing ZrO2 particles. Feng et al. [202] developed a La2O3-stabilizedNiO-CaO/Al2O3 hybrid catalyst–sorbent prepared by two-stepimpregnation of lanthanum and nickel precursors. At 600 �C andS/C of 4, H2 concentration of more than 92% was achieved during30 SESMR cycles. The incorporation of La2O3 could improve bothstability and nickel grain dispersion over the substrate.

Besides the suitability of CO2 sorbent, appropriate Ni dispersionand surface area of catalyst are also important parameters in H2

production. A HTlc-based hybrid catalyst–sorbent was synthesizedby Broda et al. [203] via co-precipitation technique. The hybridmaterial contained both Ni reforming catalyst and Ca-based CO2

sorbent dispersed in the HTlc structure containing Mg and Al(Fig. 7). The appropriate surface area (54 m2/g) and the high disper-sion of Ni and Ca in the HTlc structure resulted in a high H2 produc-tion efficiency of 99 vol.% (a dry basis composition is alwaysmentioned, if otherwise specified) and adequate thermal stabilityover cyclic SESMR operation. The proposed hybrid material pro-duced a better H2 purity compared to a mixture of limestone andNi–SiO2 or nickel HTlc-derived catalyst. However, the loading ofCaO in the hybrid structure was only 21 wt.%, requiring the use ofa high amount of hybrid material in the reaction to obtain an ade-quate CO2 adsorption capacity. As stated by the authors, the CO2

uptake capacity of the hybrid material averaged only 0.074 g ofCO2/g of sorbent over 10 cycles. Kim et al. [74] studied the synthesisof hybrid CaOACa12Al14O33ANi composite from calcium nitratetetrahydrate, aluminum nitrate nonahydrate, and Ni precursor bycombination of precipitation and hydration methods, and its appli-cation in the SESMR (S/C of 3, 630 �C). Ca12Al14O33 made spaciouspathway available for CO2 diffusion via forming porous structure,thus providing an excellent cyclic stability for Ca-based sorbent.The SESMR experiments using hybrid CaOACa12Al14O33ANi com-posites with different loading of Ni precursor (3, 5, 7, and10 wt.%) revealed that 7 wt.% of Ni loading led to the best

Fig. 7. Schematic diagram of hybrid catalyst-CO2 sorbent arrangement [203].


performance (CH4 conversion and H2 production). The H2 concen-tration was maintained at 94–95% for 70 min (prebreakthrough)during SESMR. It was also found that the presence of a high Ni load-ing (10 wt.%) caused Ni self-agglomeration. Radfarnia and Iliuta[204] developed NiOACaO/Ca9Al6O18 hybrid catalyst–sorbentmaterials with various nickel loading (12, 18, and 25 wt.% NiO)via a simple wet-mixing method (limestone acidification coupledwith two-step calcination technique) and investigated their appli-cation in SESMR operation (S/C of 4, 650 �C and 1 bar). Accordingto the experimental results, the best performance belonged to thehybrid material containing 25 wt.% NiO, which showed an averageH2 production efficiency of 96.1% during 10 cycles (180 min reac-tion per cycle). Moreover, the long-term application (30 cycles,45 min reaction per cycle) led to an average H2 production effi-ciency of 97.3%, proving its high efficiency in the SESMR process.The excellent performance of this catalyst–sorbent hybrid materialwas attributed to the easy access of Ni sites resulted from higher Niloading and proper distribution of Ni on the substrate. The sameauthors also developed a CaOAZr/Ni (13, 18, 20.5 wt.% NiO) sor-bent–catalyst material by wet-mixing/sonication technique andits application in the SESMR showed that the one with 20.5 wt.%NiO loading presented the most suitable activity (H2 yield of 91%at the end of the 10th cycle) [205].

In a recent study, Barelli et al. [206] employed wet mixing meth-ods to prepare three different catalyst–sorbent materials forSESMR: NiOACaO/Ca9Al6O18 composite (M1), NiOACaO/Ca12Al14O33 composite (M2), and a physical mixture ofCaO/Ca9Al6O18 and NiO/cAAl2O3 (M3). M3 showed the best perfor-mance in cyclic carbonation/decarbonation experiments, where theinitial CO2 capture capacity of 0.55 g of CO2/g of sorbent decreasedto 0.495 g of CO2/g of sorbent after 14 cycles (reduction of 17%). Forcomparison, the CO2 capture capacity of standard pure CaOdecreased from 0.49 g of CO2/g of sorbent to 0.288 g of CO2/g of sor-bent over 14 cycles (reduction of 57.5%). XRD, SEM and TEM analy-sis of the used materials justified the performance of all threematerials during cyclic CO2 adsorption/desorption process. Whileno change was observed in the porous structure and morphologyof M3 sorbent, agglomeration of CaCO3 on the surface ofCa9Al6O18 and partial loss of roughness and porosity of used M1led to much lower cyclic CO2 adsorption/desorption performancein comparison with M3. In addition, the formation of NiAl2O4 andthe deterioration of the porous structure during the cyclic CO2

adsorption/desorption process caused the poorest performance ofM2. The material with the best performance (M3) was furtherapplied in SESMR process at different temperatures between 500and 650 �C. Although the molar fraction of H2 at the reactor outletreached 89.1% during the pre-breakthrough period at 650 �C (S/Cof 2.85), it was still much lower than that at equilibrium. This couldbe due to difficult contact between gas and solid because of the par-ticle size of the catalyst, low catalyst load and residence time.

3.2. Application of ceramic sorbents in SESMR

Compared to CaO-based materials, only few experimental dataconcerning the application of ceramic sorbents in the SESMR areavailable in the literature. The application of differentalkaline-containing sorbents was studied by Yi et al. [207] in afixed-bed reactor. The use of Na2ZrO3 prepared by liquid–liquidsynthesis method resulted in a H2 concentration of 96.8% at600 �C, 1 bar and S/C of 4. However, data concerning the cyclicadsorption/regeneration were not reported. It was found that dur-ing regeneration at high temperatures Na migrated and coveredthe active Ni-sites, which resulted in the catalyst deactivation.The application of Li2ZrO3 prepared by the same synthesis methodrevealed the inadequacy of this sorbent due to the very low CO2

capture kinetics in the operating conditions (low partial pressureof CO2). An initial H2 concentration of less than 85% (tendingtowards the SMR equilibrium conversion of �73%) indicated thatCO2 could not be efficiently removed from the reaction zone whileit was generated in the reforming process. Another research groupfrom the Norwegian University of Science and Technology (NTNU)also showed interest in the synthesis and application ofalkaline-based sorbents in the SESMR. Ochoa-Fernandez et al.[137] performed simulations of SESMR using CO2 sorption dataobtained experimentally on different kinds of materials likeLi2ZrO3, K-Li2ZrO3, Na2ZrO3, Li4SiO4 and CaO. The favorable ther-modynamic behavior of sorbents at low CO2 partial pressure wasfound to play a key role in the success of the SESMR process.Although the use of CaO resulted in the highest H2 yield (above98%) at 575 �C, 10 bar and S/C ratio of 5, its significant uptakecapacity reduction would be a serious problem in cyclic operation.For comparison, the simulation results obtained for the other con-sidered sorbents were: K-Li2ZrO3 (93%), Na2ZrO3 (90%), Li2ZrO3

(89%), and Li4SiO4 (82%). Based on CO2 sorption data, it was con-cluded that Na2ZrO3 might be an alternative to CaO due to the goodkinetic behavior and the appropriate stability during sorption/des-orption cycles. However, the same group concluded later that thestability of Na2ZrO3 was significantly reduced during the operationat high steam pressure [208]; this was however not in agreementwith data reported by Yi et al. [134]. More thorough investigationson the effect of the steam on ceramic sorbents during cyclic SESMRoperation would be necessary to clarify their behavior. The appli-cation of Na2ZrO3 and Li2ZrO3 sorbents prepared by liquid–liquidsynthesis was also studied by Ochoa-Fernandez et al. [128,137].The use of Na2ZrO3 at 575 �C, 5 bar, and S/C of 5, with sorbentregeneration at 750 �C under Ar, resulted in a H2 yield above97%. For Li2ZrO3, the H2 yield was very close to the thermodynamicequilibrium of traditional SMR process at the reaction conditionsinvestigated (67.9%), due to the kinetic limitation during the CO2

sorption, in agreement with the results of Yi et al. [134]. It isimportant to note that a drastic decrease of H2 yield was observed

Table 6Side reactions during steam glycerol reforming.

Entry Reaction DH�298 (kJ/mol)

1 COþ 3H2 $ CH4 þH2O �206


during the second cycle of operation for both Na2ZrO3 and Li2ZrO3.Even if the reason was not very clear, this behavior seemed to bedue to catalyst poisoning in the presence of impurities present inthe sorbents.

2 CO2 þ 4H2 $ CH4 þ 2H2 �1653 H4 þ CO2 $ 2COþ 2H2 2474 2CO$ CO2 þ Cs �1725 CH4 $ 2H2 þ CðsÞ 756 COþH2 $ H2Oþ CðsÞ �1317 CO2 þ 2H2 $ 2H2Oþ CðsÞ 306

4. Hydrogen production by sorption enhanced steam glycerolreforming (SESGR)

As mentioned in the previous section, methane reforming iscommonly used in industrial applications for hydrogen production[176,177,180–182]. The steam methane reforming provides about48% of worldwide produced hydrogen [209]. However, hydrogenproduction from methane deals with serious difficulties such asenvironmental problems and drastic reduction of fossil fuelsources. Therefore, extensive works were directed on the develop-ment of alternative renewable sources such as water and biomassor biomass-derived oxygenates. Glycerol, a by-product of the trans-esterification of renewable biological sources (i.e., vegetable oilsand animal fat oils), may be considered an interesting renewablesource of hydrogen [210–214]. The production of 10 kg of biodieselgenerates around 1 kg of crude glycerol [210]. Hence, the rapidgrowth in the production of biodiesel from 2000 year has resultedin great increase of crude glycerol. It is anticipated that the annualproduction of glycerol will reach to about 3 megatons in 2020,whereas the industries consume only 500 kilotons glycerol eachyear [214,215]. The increase of the availability of this industrialwaste, as well as the renewability and the low cost of glycerol, makeit very attractive as an alternative source of hydrogen [211].

Pyrolysis, partial oxidation, steam reforming, autothermalreforming, and aqueous-phase reforming are some promisingmethods for converting glycerol into hydrogen. By now, steamreforming is the most common method for converting glycerol intohydrogen [213,215,216]. This global process consists of complexreactions, which lead to the formation of several by-products andtherefore, affect the final purity of H2 adversely [213]. Glycerolpyrolysis (9) and WGS (6) are the main reactions in this process:

C3H8O3 $ 4H2 þ 3CO DHo298 ¼ 251 kJ=mol ð9Þ

CO þ H2O$ H2 þ CO2 DH�298 ¼ �41 kJ=mol ð6Þ

Consequently, the overall reaction of glycerol reforming is rep-resented by Eq. (10), where one mole of glycerol can theoreticallyproduce 7 mol of hydrogen:

C3H8O3 þ 3H2O$ 7H2 þ 3CO2 DH�298 ¼ 128 kJ=mol ð10Þ

The side reactions that can occur in the steam reforming of glyc-erol are presented in Table 6. The steam glycerol reforming processis carried out at temperatures of 500–900 �C, 1 atm andwater/glycerol molar ratio of 6–9, in the presence of Ni, Co andnoble metals (such as Pt, Pd and Rh) based catalysts. H2, CO, CO2,and CH4 are the main gaseous products [213–216].

One of the obstacles to the utilization of hydrogen obtained fromsteam glycerol reforming for energy production is the high CO andCO2 content. In particular, the presence of large amount of CO2 sig-nificantly drops the efficiency of fuel cells, while CO strongly poi-sons the catalyst of proton-exchange membrane fuel cells(PEMEC) [217]. Furthermore, the cost of hydrogen separation froma H2-rich gas containing impurities causes major cost penalties.Therefore, a better system is needed to achieve high purity hydro-gen. The application of the sorption enhanced process is thereforean interesting option to produce high purity hydrogen in a singlestep [210,211,214]. Compared to the traditional steam reforming,the in-situ CO2 removal increases glycerol and steam conversionsas well as hydrogen purity [214,215]. Moreover, the in-situ CO2 cap-ture during the steam glycerol reforming decreases the risk of coke

formation and the reforming reactions can be carried out at rela-tively low steam/carbon ratios [212]. Dou et al. [214] provided anoverview on some issues and challenges of SESGR process such asselecting suitable sorbents, extending operation time, and findinga way for continuous reaction/regeneration in order to achievehigh-efficiency hydrogen production from SESGR process.

To date, much fewer studies have been performed on thesorption enhanced steam glycerol reforming (SESGR) compared toSESMR. In the presence of CaO as the CO2 sorbent, it was found thatthe in-situ CO2 removal resulted in a significant enhancement of H2

production and thermal efficiency as well as an important reduc-tion of CO concentration [210,218–220]. Chen et al. [221] per-formed thermodynamic analyses on the SESGR process based onthe principle of Gibbs free energy minimization for chemical reac-tions. They evaluated the effect of temperature (327–727 �C), pres-sure (1–4 bar), S/C ratio (1–4), percentage of CO2 removal throughadsorption (0–99%), and carrier gas to feed reactants molar ratio(1–5) on the reforming reactions and carbon formation. The resultsdemonstrated that the in-situ CO2 removal led to the enhancementof glycerol conversion to hydrogen. The maximum hydrogen yieldwas increased from 6 to 7 moles/mole of glycerol by the in-situ CO2

removal via adsorption. The analyses proposed that the mostfavorable conditions for the SESGR process were: temperaturerange around 527–577 �C at atmospheric pressure and S/C ofaround 3.0. The most favorable temperature for the SESGR processwas approximately 100 �C lower than that for the traditional steamglycerol reforming without in-situ CO2 removal. As carbon forma-tion can occur at low S/C ratios, the in-situ CO2 removal can consid-erably reduce the lower limit of the S/C ratio to limit the carbonformation [221]. He et al. [211] studied the SESR process of pureglycerol by using the CoANi/HTls as reforming catalysts and cal-cined dolomite as CO2 sorbent, at 500–650 �C and S/G of 3, 4, and9. The CoANi/HTls catalysts (25%Co–15%Ni/HTls and 30%Co–10%Ni/HTls) were developed by co-precipitation fromNi(NO3)2�6H2O, Co(NO3)3, Mg(NO3)3�6H2O and Al(NO3)3�9H2O.According to the experimental results, the hydrogen purity reachedapproximately the theoretical value at temperatures beyond575 �C or S/G no less than 4 (for example, both hydrogen purityand yield reached around 99% for a S/G of 9). Such a high H2 yieldobtained in the SESGR process was attributed to the enhancementof methane steam reforming and water–gas shift reactions and theelimination of non-catalytic reactions in the SESR process. Below550 �C and a low S/G of 3, hydrogen production was compromisedbecause of the high tendency of pyrolysis and the low efficiency ofsteam reforming. Dou et al. [222] studied the SESGR process in afixed-bed reactor using a mixture of commercial Ni-based catalyst(18 wt.% NiO/82 wt.% Al2O3) and CaO-based sorbent derived fromdolomite (weight ratio of catalyst to sorbent of 1:1), at tempera-tures in the range of 400–700 �C, 1 atm, and S/C of 3. The experi-mental results demonstrated that the SESGR is an efficientprocess for obtaining hydrogen purity of more than 90%. 500 �Cwas shown to be the optimum temperature, with the highest H2

purity of 97% and the longest CO2 breakthrough period.One of the most important parameters that affect the economic

benefits of H2 production by SESGR is the price of glycerol. Up to


date, the market price of crude glycerol is between 25 and 46 $/ton,which is significantly lower than that of purified glycerol (55–78 $/ton) [212]. The SESR of crude glycerol becomes thereforemore attractive for a low price H2 production. Fermoso et al.[223] and Dou et al. [224] showed the great potential of crude glyc-erol for producing H2 with high yield and purity. Fermoso et al.[223] evaluated the SESR of crude glycerol in a fixed-bed reactorover a mixture of Ni-Co/HTlc catalyst and calcined dolomite asCO2 sorbent (sorbent/catalyst = 5 g/g), and at temperaturesbetween 550 and 600 �C, 1 atm, and S/C of 3. The experimentalresults revealed that the in-situ CO2 removal through adsorptionsignificantly enhanced the reforming and water–gas shift reac-tions, allowing high H2 yield (88%) and purity (99.7 vol%). In a sim-ilar work, Dou et al. [224] evaluated the steam reforming of crudeglycerol in a fixed-bed reactor over a commercial Ni-based catalyst(18 wt.% NiO/82 wt.% Al2O3) at a temperature range of 400–700 �C,1 atm, and S/C of 3, with and without in-situ CO2 removal.CaO-based sorbent derived from dolomite was used for theSESGR. In the absence of CO2 removal, the H2 purity and the crudeglycerol and steam conversions were 68%, 100% and 11% at 600 �C,respectively. The in-situ CO2 removal during the steam reformingled to a hydrogen purity beyond 88% in pre-breakthrough condi-tions. It is important to mention that, under the same SESGR con-ditions, the hydrogen yield and purity obtained from crude glycerolwere lower compared to pure glycerol [211,222]. This was attribu-ted to the increased coke formation resulted from the high cokingpotential of heavy components present in the crude glycerol (suchas fatty acid methyl esters). The production of almost pure H2 fromcrude glycerol is therefore a great challenge because of the com-plexity of crude glycerol composition.

The previous works have investigated H2 production by SESGRin a fixed-bed reactor using a mixture of catalyst and CO2 sorbent.The operating time for high purity H2 production is quite shortbecause of the limited capacity of the CO2 sorbent. In a continuousSESGR process, which is based on the concept of a continuous flowof catalyst and sorbent for both reaction and regeneration using twomoving bed reactors, the high purity H2 production is expected tobe extended for a longer period of time. Dou et al. [225] investigatedthe continuous SESGR process using a mixture of Ni-based catalyst(NiO/NiAl2O4) and CaO-based sorbent (catalyst to sorbent weightratio: 1:1). The Ni-based catalyst containing 42.1 wt.% NiO wasdeveloped by co-precipitation from Ni(NO3)2�6H2O andAl(NO3)3�9H2O precursors. The tests were carried out at 500 and600 �C, with S/C of 3. The simultaneous regeneration of catalystand sorbent was performed at 900 �C with the gas mixture of N2

and steam. In the SESGR process performed in a fixed-bed reactor,in the pre-breakthrough time that lasted for only 10 min theamounts of CO2, CO and CH4 decreased and H2 concentration wasconsiderably increased in comparison to conventional SGR.However, in the continuous SESGR, the mixture of catalyst and sor-bent was moved constantly between the reforming and regenera-tion moving-bed reactors and the H2 production was notinterrupted for the regeneration of both catalyst and sorbent.During the operating time of 60 min, hydrogen purity was 93.9%and 96.1% at 500 and 600 �C, respectively. However, the maximumconversion of CaO sorbent was only 15.5% due to the very short res-idence time of the sorbent in the reformer. The continuous carbon-ation and calcination of CaO at this low-level conversion did notexhibit a significant drop of the sorbent reactivity. In a furtherstudy, Dou et al. [226] proposed a new continuous system, theenhanced sorption chemical looping steam reforming process. Themixture of catalyst and sorbent (weight ratio of 1:1) were movedcontinuously between two moving-bed rectors. In the reformingreactor, oxidation, steam reforming, water–gas shift and in-situCO2 removal were combined and carried out at 500–600 �C andS/C of 1.5–3.0. NiO/NiAl2O4 was used as catalyst and limestone as

sorbent. In the air reactor, sorbent regeneration, catalyst oxidiza-tion and coke combustion were carried out at 900 �C in an air atmo-sphere. The NiO/NiAl2O4 catalyst containing 42.1 wt.% NiO wasdeveloped by co-precipitation from Ni(NO3)2�6H2O andAl(NO3)3�9H2O precursors. The experimental results showed thatthe increase of temperature and S/C value enhanced the H2 purity.The best results (hydrogen purity higher than 90%) were obtainedat temperatures of 500–600 �C and S/C of 1.5–3.0.

5. Conclusion and recommendations for future works

The sorption enhanced steam reforming is an integrated processcombining reforming, water gas shift and CO2 capture in order toproduce highly pure H2 in a single step. These reactions occursimultaneously over a mixture of reforming catalyst and CO2 sor-bent. This hybrid process has several important advantages incomparison with the traditional steam reforming, especially theenergy efficiency improvement and the reduction in capital cost.Hydrogen can be obtained with a purity of 95–98% (dry basis) withvery low carbon oxides content, compared to maximum 80% (drybasis) for traditional steam reforming. Further purification is usu-ally not required in most applications. Moreover, H2 productionis accompanied by the generation of a rather pure CO2 stream suit-able for storage and further use.

Several CO2 sorbents have been studied as potential candidates,including Ca-based oxides and mixed alkaline oxides of Li and Na.Ca-based sorbents derived from natural precursors are especiallyadvantageous due to the low cost and availability. Despite the highCO2 capacity and good kinetics over a wide range of temperaturesand pressures for Ca-based sorbents, they suffer from the importantdrawback of high instability in long-term operation due to the sin-tering phenomena at high temperatures. In this context, a lot ofworks have been directed on the improvement of their durabilityin cyclic operation by using various calcium precursors, incorpora-tion of stable inert materials into CaO structure, and using differentkinds of treatments (like steam hydration, thermal pretreatment,recarbonation and treatment with chemical solutions). However,the review of available data showed some contradictory resultsabout the effect of treatments such as steam hydration during cal-cination. Further systematic studies are therefore highly needed toconfirm or infirm specific behaviors. Moreover, as most investiga-tions related to this kind of sorbents were performed in operatingconditions far from industrial applications (limited reaction timeand low CO2 partial pressures), extended works have to be doneon wider range of operating parameters.

Alkaline ceramics represent another group of high-temperatureCO2 sorbents that have been attracted interest especially becauseof their superior stability in cyclic operation compared to CaO.Li2ZrO3, Li4SiO4, and Na2ZrO3 are the most studied ceramics mate-rials. However, most of them suffer from slow kinetics especially atlow CO2 partial pressures. Compared to Ca-based sorbents, very lit-tle information is available concerning their application in thesorption enhanced processes. More thorough investigations onthe effect of the steam on ceramic sorbents during cyclic operationwould especially be necessary to clarify their behavior.

Based on the available works related to the use of CO2 sorbentsin the SESMR process, it has been demonstrated that nearly pureH2 product can be potentially obtained. Simple mixing patternsof sorbent and catalyst are used in most investigations. However,in order to eliminate the mass transfer limitations, hybrid cata-lyst–sorbent structures have recently appeared as an interestingoption with the benefits of decreasing both reactor volume andcost of operation. The development of this kind of materials is stillin embryonic stage. Exhaustive studies are highly needed in thedevelopment of efficient hybrid catalyst–sorbent materials and


the evaluation of their efficiency and stability under severe opera-tion conditions and cyclic operation.

In terms of the application of CO2 sorbents in the SESGR process,few works have been done by using CaO-based sorbents and vari-ous catalysts. In all these works, the catalyst and sorbent weremixed physically and none of them considered using hybrid cata-lyst–sorbent arrangement up to now. In this context, systematicstudies on the development of efficient hybrid catalyst–sorbentmaterials and the evaluation of their stability under severe operat-ing conditions and cyclic operation represent interesting opportu-nities for research. Although the SESR of crude glycerol is moreattractive compared to pure glycerol to obtain H2 with a lowerprice, the SESR of crude glycerol has been very scarcely investi-gated. The main challenge in the SESR of crude glycerol is the car-bon formation because of the presence of heavy compounds, suchas fatty acid methyl esters. Therefore, the production of pure H2

from crude glycerol is still a challenge due to the composition com-plexity of crude glycerol.

Acknowledgments

Financial support from Natural Sciences and EngineeringResearch Council of Canada (NSERC), FRQNT Centre in GreenChemistry and Catalysis (CGCC) and Centre de Recherche enCatalyse et Chimie Verte (C3V, Laval University) is gratefullyacknowledged.

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