Preparation and Application of the Sol-Gel Combustion Synthesis-Made CaO/CaZrO3Sorbent for Cyclic CO2Capture Through the Severe Calcination Condition☆

Baowen Wang*,Xiaoyong Song,Zonghua Wang,Chuguang Zheng

Separation Science and Engineering

Preparation and Application of the Sol-Gel Combustion Synthesis-Made CaO/CaZrO3Sorbent for Cyclic CO2Capture Through the Severe Calcination Condition☆

Baowen Wang1,2,*,Xiaoyong Song1,Zonghua Wang3,Chuguang Zheng2

1College of Electric Power,North China University of Water Resources and Electric Power,Zhengzhou 450011,China2State Key Laboratory of Coal Combustion,Huazhong University of Science and Technology,Wuhan 430074,China3Henan Institute of Metallurgy Corporation,Zhengzhou 450053,China

A R T I C L EI N F O

Article history:

CO2capture

Calcium looping cycles

CaO/CaZrO3sorbent

Sol-gel combustion synthesis method

Calciumloopingmethodhasbeenconsideredasoneoftheeff i cientoptionstocaptureCO2inthecombustionf l ue gas.CaO-based sorbent is the basis for application of calcium looping and should be subjected to the severe calcination condition so as to obtain the concentrated CO2stream.In this research,CaO/CaZrO3sorbents were synthesized using the sol-gel combustion synthesis(SGCS)method with urea as fuel.The cyclic reaction performance of the synthesized sorbents was evaluated on a lab-scaled reactor system through calcination at 950°C in a pure CO2atmosphere and carbonation at 650°C in the 15%(by volume)CO2.The mass ratio of CaO to CaZrO3as 8:2(designated as Ca8Zr2)was screened as the best option among all the synthesized CaO sorbents for its high CO2capture capacity and carbonation conversion at the initial cycle.And then a gradual decay in the CO2capture capacity was observed at the following 10 successive cycles,but hereafter stabilized throughout the later cycles.Furthermore,structural evolution of the carbonated Ca8Zr2over the looping cycles was investigated. With increasinglooping cycles,theporepeakand meangrainsizeof thecarbonatedCa8Zr2sorbentshiftedtothe bigger direction but both the surface area(SA)ratio Φ and surface fractal dimension Dsdecreased.Finally, morphological transformation of the carbonated Ca8Zr2was observed.Agglomeration and edge rounding of the newlyformedCaCO3grainswere foundasaggravatedatthecycliccarbonation stage.Asa result,carbonation ofCa8Zr2with CO2wasobservedonlyconf i ned to theexternalactive CaObythefastformation oftheCaCO3shell outside,which occluded thefurthercarbonationofthe unreacted CaOinside.Therefore,enoughattention should be paid to the carbonation stage and more effective activation measures should be explored to ensure the unreacted active CaO fully carbonated over the extended looping cycles.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

It is of great signif i cance to control and stabilize the CO2concentration in the atmosphere in response to global warming and greenhouse effect.Among all the measures to CO2capture,calcium looping method hasbeenrecognizedasoneofthebestoptionsforCO2capturefromcoal combustion and gasif i cation processes[1],which consists of such two processes as the carbonation reaction of CaO with CO2to form CaCO3in the carbonator and the subsequent calcination to regenerate CaO in the calciner over the multiple cycles.

CaO sorbent with good cyclic stability was the basis for the application of calcium looping to the realistic CO2capture system.The main limitation to calcium looping cycle to be applied at the industrial scale is the striking decay of the CO2capture capacity with the cycle number [2].In order to overcome the capacity loss for CO2capture and withstand serious sintering during calcination of the calcium-based sorbent, synthesisofdurableCaOsorbentcombinedwithinertsupportsisagood choice[3].Various materials were attempted to mix with active CaO, such as ZrO2[4],SiO2[5],MgO[6,7],La2O3[6,8],Ca12Al14O33[9,10], CaTiO3[11,12]and MgAl2O4[13,14].Among all these inert materials, ZrO2is considered as one of the satisfactory ones for its high melting temperature(2710°C)and good resistance to sintering[4].

Besides the inert support selected,the synthesis method and preparation condition also strongly inf l uence the cyclic carbonation/ calcination performance and CO2capture capacity of the synthetic sorbent[4].Li et al.[9]synthesized CaO/Ca12Al14O33by the sol-gel method, which received great interest and was further followed by others[10, 15].Similar sorbent was also prepared by Mastin et al.[16]using the solution combustion method,a typical wet-chemistry method,which relied on the self-sustained combustion reaction to rapidly produce the expected product without large energy consumption[17].Combining both the sol-gel method and solution combustion method,using citric acidas the fuel,our group had designed a novel sol-gel combustion synthesis (SGCS)method[8,18,19],which displayed great advantages for its rapidity and simplicity of the synthesis process,low energy requirement and good resistance to sintering.Various CaO-based sorbents were produced using the SGCS method,such as CaO/La2O3[8]and CaO/Ca12Al14O33[18] with satisfactory CO2capture capacity over the multiple cycles.Recently, extended duration over the 80 calcination/carbonation cycles was conducted by Santos et al.[20]and verif i ed that SGCS-made CaO sorbent was greatly reactive and resistant to sintering.But in comparison with citric acid as fuel in SGCS,urea was more competitive and advantageous for its ready availability,high exothermicity and effective complexation ability[19].Therefore,it would be meaningful to synthesize CaO sorbent bymeansofSGCSwithureaasfuelandinvestigateitscyclicperformance.

In addition to the inert support and preparation method above,microstructure characteristics of CaO-based sorbent were very important in determining its CO2capture capacity[21].Most researchers considered that the structural characteristics of the formed CaO were determined by the calcination process and great attention is focused on the transformation of both surface area,pore volume[22-25]and the grain size[26-28]at the calcination stage.Although carbonation was another important process and closely correlated with the CO2capture capacity of the sorbent during the calcium looping utilization,few studies were addressed to the evolution of structural characteristics at the carbonation stage,except those by Alvarez and Abanades[29]and Feng et al.[30].More research on the evolution of the structural characteristics at the carbonation stage is needed.

Finally,in order to obtain the concentrated CO2stream suitable for sequestration,it would be more reasonable to perform the calcination of CaO-based sorbent under the severe calcination condition[31,32], where the CO2concentration was higher than 90%and the calcination temperatureexceeded900°C.Butcurrently,mostresearchonthecalcination of CaO-based sorbent was still carried out under the mild condition,i.e.lower temperature below 900°C and dilute CO2concentration, even pure N2atmosphere[6-12].It was found out that the intensity of sintering was greatly determined by the calcination temperature[24, 33,34]and CO2concentration[22,23,35].Therefore,it would be more reasonable to perform the calcination of the synthetic CaO-based sorbent under the severe condition for a better understanding of its cyclic performance through the calcium looping process.

In this study,CaZrO3was selected as the inert binder,and CaO/CaZrO3sorbentwassynthesizedusingthesol-gelcombustionsynthesis(SGCS)method with urea as fuel.Both the CO2capture capacity and carbonation conversion of the as-synthesized CaO-based sorbent were evaluated over the 18 calcination/carbonation cycles through the severe calcination condition.The morphological and structural evolution of the synthetic CaO-based sorbent over the cycles were further studied to obtain the full information of CaO sorbent through the multiple severe calcination processes.

2.Experimental

2.1.Preparation of CaO/CaZrO3sorbents

Synthesis of CaO-based sorbents with good sintering-resistance and big CO2capture capacity is the basis for the application of calcium looping cycles.Hydrated nitrates of calcium and zirconium were used as the precursors to CaO/CaZrO3sorbent and urea was adopted as the fuel.Allthesechemicalswereanalyticallypure(AR)gradeandobtained from Sinopharm Co.,Shanghai,China.

The procedure of the SGCS to prepare CaO/CaZrO3sorbent was similartoourpreviousresearch[19]andbrief l ydescribedasfollows.Firstly, atthewet gelformation stage,thestoichiometric compositionsofmetal nitrates(including nitrates of calcium and zirconium)and urea were calculated to ensure the mass ratios of CaO to the newly formed inert binder(i.e.CaZrO3)as 9:1,8:2 and 7:3,which were designated as Ca9Zr1,Ca8Zr2and Ca7Zr3,respectively.Meanwhile,pure CaO without CaZrO3support was also synthesized and designated as Ca10Zr0for reference.And then,accurately weighted nitrates and urea were dissolved in deionized(DI)water sequentially.The mixture was stirred on a hot plate in the air atmosphere and aged at 75°C until the viscous colloidal was formed.Thereafter,the wet sol was dried overnight at～135°C in a desiccator to form the dry gel,which was transferred to a ceramic dish and heated in a muff l e furnace at 600°C for 15 min to ensure the organics involved fully burnt out.Finally,the as-burnt product was sintered in the same furnace at 950°C for 2 h.After cooled down,theas-synthesized sorbents were sieved with 150-250 μm particles preserved for later use.

2.2.Methods

Thelab-scaledreactorsystemusedinthisresearchisshowninFig.1, where the furnace has two separate compartments,which consists of the calciner in the left side and carbonator in the right.Both the dimensions of the furnace for the calcination and carbonation section are 40 mm i.d.and 500 mm in length.And the constant temperature zone is set as 200 mm in length and controlled by the separate temperature controller.The sample boat is specially tailored with the size of 40× 25×15 mm(i.e.length×width×height),which could be f l exibly switched back and forth between the two reactors to complete the multiple looping cycles.

During the calcium looping cycles,about 0.3 g of the synthetic CaO/CaZrO3sorbent with the sieved particle size was accurately weighed in the calibrated balance and then evenly distributed in the sample boat.Meanwhile,pretreatment was conducted at 700°C in N2before the performance test so as to remove the impurities involved in the CaO sorbent,e.g.CaCO3formed when exposed to the air during transfer of the as-synthesized CaO sorbent from the sealed container to the sample boat.In order to simulate the CO2capture process[32], the carbonation reaction was performed at 650°C in 15%(by volume) CO2with balance N2,while calcination at 950°C in 100%CO2atmosphere.The gas f l ow rates for carbonation and calcination were stabilized as 1.2 L·min?1and the time was f i xed as 30 and 10 min, respectively.

2.3.Data analysis

The capture capacity of CO2(Cn)over the calcium looping cycles is one of the most important indexes for the synthetic sorbent and calculated in Eq.(1),whereas the carbonation conversion(Xn)denotes the reversibility of the synthetic sorbent over the cycles and is calculated in Eq.(2).

where m0and mnare the initial mass before the f i rst cycle and the carbonated mass after the carbonation cycle,respectively.WCaOand WCO2are the molar masses for CaO and CO2.And f is the content of CaO in the initial calcined sample.

Previousstudiesindicatedthatthecalcination/carbonationreactions of calcium-based sorbents were far from reversibility in practice,especially through the severe calcination[22,23].Eq.(4)below representsthe maximum carbonation conversion of the sorbent Xnas a function of the cycle number(N),which has been conf i rmed to be valid for a wide range of reaction conditions during the calcium looping process[36], and is applied to analyze the carbonation results obtained in this study.

Fig.1.Schematic diagram of the f i xed bed reactor system.

where Xrand k are referred to the residual carbonation conversion and the deactivation constant over the looping cycles,which ref l ect the decay characteristics of the sorbents.And the higher Xrimplies that more residual CO2capture capacity is preserved for the sorbents over themultiplecycles,whilethesmaller kvaluerepresentsthebetterreactionreversibilityandresistancetosintering.Therefore,overtheextended cycles,CaO sorbent with bigger Xrbut smaller k should be preferred to sustain its CO2capture capacity.

2.4.Characterization of the synthesized CaO/CaZrO3sorbent

Phase identif i cation of the as-synthesized CaO/CaZrO3sorbent and carbonated products over the multiple cycles was performed through X-ray diffractor(X'Pert PRO,The Netherlands)with 40 kV and 40 mA Cu Kα radiation at the step-scanned range of 10-90°.Based on both the line broadth of each main phase identif i ed in the XRD spectra and its maximal intensity,its mean grain size was evaluated using the Scherrer formula[37]below.

where D is thegrainsize,λ is the radiation wavelength(0.15406 nm for Cu Kα),θ is the diffraction angle and β is the corrected halfwidth for instrument broadening.

The sample morphologies of the synthetic sorbents before and after different looping cycles were observed with a f i eld emission scanning electron microscope(FSEM,Siron 200,The Netherlands).The magnif ication parameter and accumulated voltage of the FSEM adopted were 10000×and 10 kV,respectively.

Furthermore,structural characterization was carried out on an N2adsorption analyzer(Micrometrics ASAP 2020,USA)at 77 K.The specif i c surface area(SA)of the synthesized sorbent was calculated by the Brunauer-Emmett-Teller(BET)theory[38]using the linear part (0＜P/P0＜0.25)of the adsorption branch.The total BET SA was known to consist of the micropore SA plus external SA of the sorbent [39],which played an important role in the carbonation of CaO-sorbent with CO2.In order to evaluate the evolution of both the micropore SA(i.e.SAMicropore)inside the sorbent particle and its external SA(i.e.SAExternal)over the looping cycles,the SA ratio Φ was def i ned below.

And then,the pore size distribution of the synthesized sorbents was derived using the Barrett-Joyner-Halenda(BJH)model[40]by calculationofthedesorptionbranchoftheisotherm.Inaddition,surfacefractal dimension(Ds)is also one of the specialties for various materials because the surfaces of most materials are generally rough for their geometric irregularities and defects[41].And the roughness can be well characterizedbyfractal[42].Therefore,inthisresearch,thesurface fractal dimension Dsfor the SGCS-made sorbent before and after the carbonation looping was further estimated using the adsorption method[42],as shown in Eq.(7)

whereP0andParethesaturation andequilibrium pressures,Kis acharacteristic constant,and V and V0are the gas volumes adsorbed at the pressures P and P0,respectively.

Finally,resistance to fragmentation and abrasion are very important criteria for sorbents applied in the realistic CO2capture system[43-45]. And the two criteria for the SGCS-made CaO/CaZrO3sorbent were further measured using a SHIMPO FGJ-5 crushing strength apparatus [NIDEC-SHIMPO(ZheJiang)Corporation,China]and an ordered abrasion tester(Dalian Smart tester Factory,China),respectively.

3.Results and Discussion

3.1.CO2capture capacity for the SGCS-made CaO/CaZrO3sorbents

CO2capture capacity is one of the most important indexes to apply the synthetic sorbent in the real CO2capture system.According to Eq.(1),the CO2capture capacity Cnof the as-synthesized CaO/CaZrO3sorbents at the different mass ratios is calculated and provided in Fig.2(a).Meanwhile,the CO2capture capacity of various natural or synthetic CaO-based sorbents compiled from literature is provided in Fig.2(b)-(c),respectively,for comparison.

From Fig.2(a),it could be observed that though calcination is performed under the severe calcination condition(i.e.in a pure CO2atmosphere at 950°C),after the f i rst cycle,all the synthetic sorbents display satisfactory CO2capture capacity around 0.6 g CO2·(g sorbent)?1,even for pure CaO without CaZrO3support[i.e.Ca10Zr0in Fig.2(a)],mainly duetotheporous structureof theSGCS-madeCaOsorbent[asreference in Fig.7(a)below].But in the following cycles,the CO2capture capacity deteriorates greatlyfor theCaO/CaZrO3sorbentswith differentmassratios.After the 10 carbonated cycles,though the CO2capture capacity of Ca8Zr2is much higher than that of Ca9Zr1with the increase of CaZrO3content,the further increase of CaZrO3content from Ca8Zr2to Ca7Zr3does not greatly promote the CO2capture capacity mainly because the excessive CaZrO3reduces contact of the active CaO with CO2[46].And then,after 18 cycles,the residual CO2capture capacity for Ca10Zr0is far from satisfaction with only 0.0426 g CO2·(g sorbent)?1left,but the CO2capture capacity for CaZrO3-stabilized CaO sorbents is preserved above 0.15 g CO2·(g sorbent)?1mainly due to the presence ofthe refractory CaZrO3,which restrains the growth of CaO grains and inhibits the serious sintering under the severe calcination condition[4].

Meanwhile,Ca8Zr2is taken as an example sorbent and its CO2capture capacity over the cycles through the reference calcination condition,i.e.at 950°C and in a pure N2atmosphere,is included.From Fig.2(a),it could be observed that under the reference calcination condition,CO2capturecapacity of the synthesized Ca8Zr2sorbentis satisfactory enough and stabilized as around 0.65 g CO2·(g sorbent)?1, but the severe calcination has great detrimental effect on the decay of CO2capture capacity of Ca8Zr2sorbent as described above.Therefore, further research on the CO2capture behavior of the synthetic sorbents through the severe calcination should be made.

Furthermore,CO2capture capacity of the SGCS-made CaO/CaZrO3sorbents is compared with other natural sorbents by converting their carbonated conversion reported in the literature into CO2capture capacity with Eq.(3).On the one hand,for natural sorbents shown in Fig.2(b),dolomite should be superior to limestone for CO2capture because its reaction stability is well maintained with inert MgO included [47].Its CO2capture capacity after the 17 cycles is still 0.21 g CO2· (g sorbent)?1,far higher than that of the limestone with the least CO2capture capacity around 0.06 g CO2·(g sorbent)?1after only 14 cycles [48].As compared to these two natural sorbents,although SGCS-made Ca10Zr0(i.e.pure CaO)still could not survive the severe calcination after the 18 cycles,the CO2capture capacity of Ca8Zr2with 20%(by mass)of CaZrO3is greatly improved and exceeded that of limestone and even dolomite,though dolomite contains nearly 50%(by mass) inert MgO after the full calcination,which is not economical to transfer such large contentof inertsupportbetween thecarbonatorand calciner in the realistic CO2capture process.

On the other hand,for various synthetic CaO-based sorbent with around 80%(by mass)of active CaO shown in Fig.2(c),f l ame spray pyrolysis(FSP)-made CaO/CaZrO3sorbent[4]displays the most stable CO2capture capacity around 0.24 g CO2·(g sorbent)?1throughout the whole cycles,but the main disadvantages of FSP method are the expensive ethylhexanoate-based precursor used in its synthesis[46]and the complexity of this preparation method[49].Different from FSP-made CaO sorbent,two SGCS-made CaO sorbents synthesized with different fuels(including citric acid and urea)show similar trend in CO2capture andown higher CO2capture capacityattheinitial cycle,and then rapidly decline before the f i rst ten cycles,but retain its cyclic stability after. Especially for SGCS-made CaOsorbent withurea asfuelin this research, itsCO2capturecapacitymuchapproximatesthoseofCaO/La2O3sorbent preparedwith citric acidasfuel[8]andFSP-made sorbent[4].Considering the ready availability and high exothermicity of urea as fuel as well as the simple preparation process of SGCS,our preparation is more economical without costly precursors involved and also friendly to the environment,and thus is much applicable to use in the realistic CO2capture process.

Fig.2.CO2capture capacity of(a)SGCS-made CaO/CaZrO3sorbent,(b)comparison of SGCS-made CaO with natural sorbents and(c)comparison of SGCS-made CaO with other synthetic CaO sorbents.

Fig.3.Carbonation conversion of the SGCS-made CaO/CaZrO3sorbents.The discrete solid dots representing the experimental data and the solid lines corresponding to the data fi tted by Eq.(4).

3.2.Carbonation conversion of the SGCS-made CaO/CaZrO3sorbent

Carbonation conversion of theCaO-based sorbents is another meaningful index,which ref l ects the reaction reversibility of the synthetic CaO-based sorbent over the multiple looping cycles.According to Eq.(2),the conversion of the synthetic CaO/CaZrO3sorbents at the different mass ratios under the severe calcination condition is provided in Fig.3.Similar to the CO2capture capacity for the SGCS-made CaO/CaZrO3sorbents shown in Fig.2(a),their initial carbonationconversion is high enough over 0.8,but their conversion decays with the cycle number.As compared of the carbonation conversion of Ca10Zr0to that of Ca9Zr1over the eighteen cycles,the CaZrO3support provides the great resistance to sintering.And increase of the CaZrO3support in CaO/CaZrO3sorbent improves its carbonation conversion, especially for Ca8Zr2and Ca7Zr3.

Based on the experimental carbonation data scattered in Fig.3,the carbonation conversion of the SGCS-made CaO sorbent is f i tted using Eq.(4)as a function of the cycle number and represented as the solid lines in Fig.3.It could be found out that the f i tted lines agree well with the experimentaldata except for Ca8Zr2and Ca7Zr3because their carbonationconversionisstabilizedaftertheinitial10 cyclesbyformationofthe macropores in the carbonated product,as characterized below.

Furthermore,based on the f i tted conversions,two characteristic indexes for SGCS-made CaO/CaZrO3sorbents are reached,as plotted in Fig.4.It could be found out that for Ca10Zr0(i.e.pure SGCS-made CaO),its deactivation constant k value is 1.5,higher than that of the reported typical value of limestone of 0.52[36],but its residual carbonation conversion is only 0.056,lower than 0.075 as reported for limestone[36],and thus was less resistant to sintering and not applicable to the realistic CO2capture system.But the deactivation constant k decreases sharply for the synthetic CaO sorbents with CaZrO3included. And even for Ca9Zr1,its k value sharply decreases to 0.36 with 10% (by mass)of CaZrO3added,which again verif i es the important role of the CaZrO3support played to withstand the severe sintering.The function of the deactivation constant k with the different mass ratio Rmof CaO to the binder CaZrO3is found to follow an exponential decay model andcouldbewell f i tted belowwithconf i dence R2＞0.999.

However,different from the deactivation constant k,the residual carbonation conversion Xrof the synthetic CaO-based sorbent increases with increasing CaZrO3content and could be linearly f i tted below with conf i dence R2＞0.990.

Finally,it should be noted that although higher content of CaZrO3was benef i cial to avoid serious sintering and owned higher residual conversion Xrfor the synthetic CaO-based sorbent[6,13],overdose of CaZrO3would also incur more heat inputted to decompose the carbonated sorbent and more energy was consumed to transport the CaO-based sorbent between the interconnected carbonator and calciner. Meanwhile,the crushing strength of SGCS-made Ca8Zr2was 1.63 N, abovethecrushingcriterion of 1 N suggested for thepotentialmaterials needed in the realistic f l uidized bed system[50]and the abrasion index waslower than 8%(by mass)losswith theinitial10 min duration atthe rotation 3000 r·min-1,comparative to the sorbent prepared with Ca(OH)2and cement as sorbent precursors by Qin et al.[44].Therefore, Ca8Zr2would be applicable to CO2capture in the realistic f l uidized bed system.

Fig.4.Decay characteristics of the SGCS made-Ca8Zr2.

Fig.5.XRD pattern of Ca8Zr2over the different carbonated cycles,a—fresh CaO/ CaZrO3;b—5th cycle;c—10th cycle;d—15th cycle;e—18th cycle;○:CaO;□:CaZrO3;Δ:CaCO3.

3.3.Phase analysis of SGCS-made CaO sorbent over the cycles by XRD

As discussed above,Ca8Zr2was adopted as the representative SGCS-madesorbentanditsphaseevolutionbeforeandaftertheloopingcycles is provided in Fig.5.The SGCS-made Ca8Zr2is found to consist of CaO and CaZrO3instead of ZrO2,where CaZrO3is easily formed by the interaction of CaO with ZrO2and displayed great superiority in contrast to ZrO2for its high melting temperature,thermal and chemical stability, good thermal shock resistance and low thermal expansion coeff i cient [46,51,52].Besides the binder CaZrO3and the carbonated product CaCO3,a little CaO is left and does not react with CO2.

Furthermore,evolution of the mean grain sizes for the three phases, including CaCO3,CaO and CaZrO3over the 18 looping cycles,is derived using Eq.(5)and provided in Fig.6.It could be observed that the mean grain sizeof CaZrO3is stabilizedaround74nmeitherin thefreshCa8Zr2or in its carbonatedproducts after the differentcycles for itsgood phase stability[51].Different from the stable grain size for CaZrO3,a little increase in the mean grain size of the carbonated product CaCO3is observed from 43.3 nm after the 5th cycle to 45.5 nm after the 18th cycle mainly because the formed CaCO3owns the low resistance to sintering for its lower Tammann temperature of only 533°C,as compared to that of CaO as 1313°C[4]and thus much inferior to sintering, which incurs the agglomeration of the newly formed CaCO3grains and further growth in their grain size[26].However,for the unreacted CaO, its initial grain before the looping cycles is estimated as 173 nm,much higherthannearlyalltheformedCaOgrainsas50nmreportedindifferent literatures[26-28,30,53].And in the following carbonation cycles, the unreacted CaO particles retain their grain sizes stable around 83 nm and are far lower than its initial grain size(187 nm),which also displays the great resistance to sintering for the CaO sorbent preparedwithourSGCSandarefarsuperiortotheotherreferencesorbents in the literature[26-28,30,53].

3.4.Morphological analysis of SGCS-made CaO sorbent over the cycles

The microstructure of sorbent plays an important role in the gassolid reaction.

During the multiple looping cycles,transformation of the microstructure of Ca8Zr2is characterized by FSEM,as shown in Fig.7.

From Fig.7(a)in FSEM pattern,the SGCS-made fresh Ca8Zr2sorbent is porous with various grains formed.The formed pores between differentgrainsvarygreatlywithmostporeswithinthemesoporesize,which is recognized as the typical characteristics of the SGCS method.Such a typical porous characteristic resulted from the redox reaction of urea with metal nitrates at the preparation stage,as shown in Eq.(10), where urea acts as fuel and metal nitrates worked as oxidant.When heated at 600°C in the muff l e furnace,the dry gel mixture of calcium nitrate,zirconium nitrate and urea begins to react and releases voluminous gaseous products,including CO2,H2O,NH3and N2O as shown in Eq.(10),And all these released gases which made the as prepared sorbent porous,as shown in Fig.7(a)below[8,18,19].where α and β are correspondingly determined by the different mass ratioofCaOtoZrO2andthepropellantchemistrytheory[54].Asaccompanied by formation of active CaO,the inert CaZrO3is also instantaneously formed by interaction between CaO and ZrO2and further well dispersed among the active CaO grains.Meanwhile,gaseous reaction between the NH3and N2O produced in Eq.(10)could be initiated below in Eq.(11),which not only avoids emission of the detrimental gases(e.g.,NH3and NOx),but also produces a high f l ame temperature to make the formed sorbent with good resistance to sintering[55-57]

Fig.6.Transformation of various grain sizes involved in Ca8Zr2over the 18 cycles.

Fig.7.Morphological transformation of Ca8Zr2over the 18 cycles,(a)fresh Ca8Zr2,(b)5th carbonated Ca8Zr2,(c)10th carbonated Ca8Zr2,(d1)18th calcined Ca8Zr2,and(d2)18th carbonated Ca8Zr2.

Therefore,as shown in Figs.2 and 3 before,both the CO2capture capacity and carbonation conversion of Ca8Zr2at the initial looping cycles are high enough.

But with the cycle number increased as shown in Fig.7(b)in FSEM pattern,the sintering and agglomeration of the CaCO3grains occur on thecarbonated Ca8Zr2surfaceafterthe5thcycle,resultingin a greatdecreaseoftheporesizeinthemesoporethoughsomeseparatenanosized grains still existed.

Furthermore,thesinteringeffect is intensif i ed afterthe10thlooping cycleasshowninFig.7(c).Thenanosizedgrainsareevencontactedand aggregated due to the low resistance to sintering for the newly formed CaCO3productlayeroutside,andthusmostporesareshiftedtoincrease and fell within the macropore range,which acts as the main passages for CO2to penetrate,leading to great decay of the CO2capture capacity of Ca8Zr2.

Finally,almostallthemicroporesinthecarbonatedCa8Zr2areclosed with only macropores presented after the 18th looping cycle,as shown in Fig.7(d2),similar to that of the microstructure of the carbonated Ca8Zr2after the initial 10 cycles above.But,in relative to the carbonated Ca8Zr2shown in Fig.7(d2),the texture of Ca8Zr2after the 18th calcination is still sustained as porous without identif i able sintering indication as shown in Fig.7(d1),which further supports that SGCS-synthesized CaO has enough resistance to sintering under the severe calcination condition.Such morphological transformation behavior as observed above could be well accounted for using the“skeleton model”put forward by Lysikov et al.[58]and Manovic and Anthony [59].As the carbonated cycles increase,the interconnected framework is formed among the internal unreacted CaO grains,which owns stable grain size around 83 nm as shown in Fig.6 before, but the external active CaO undergoes fast formation of the CaCO3product layer,and thus incurs some active CaO sites unreacted with CO2atthecarbonatedstages.Therefore,furtherreactivationmeasures should be explored in the future to make the unreacted CaO inside fully carbonated.

Fig.8.N2adsorption-desorptionisothermsofthefreshCa8Zr2anditscarbonatedproductafter18 cycles,(a)N2isothermsof thefreshSGCS-made Ca8Zr2andits18thcarbonatedproduct, (b)pore area and pore volume distribution of the carbonated Ca8Zr2over the different cycles,(c)pore volume distribution of the carbonated Ca8Zr2over the different looping cycles,and (d)evolution of the surface area ratio Φ of micropore to the external surface area&surface fractal dimension Dswith different looping cycles.

3.5.Structural analysis of SGCS-made CaO sorbent over the cycles

Structural characteristics(including pore shape and pore size distribution,surfacearea)of the SGCS-made CaO sorbentgreatlydetermined its carbonation behavior and CO2capture capacity over the looping cycles.The structural evolution of Ca8Zr2over the eighteen cycles is further investigated using the N2adsorption analyzer,as presented in Fig.8.

Firstly,pore shapes of both SGCS-made Ca8Zr2and its carbonated product after the 18th cycle were determined.As shown in Fig.8(a), the adsorption(at standard temperature and pressure)of Ca8Zr2increases slowly below P/P0=0.5,and then sharply ascends to its utmost adsorbed volume at P/P0=1,but still does not reach its stable state.Its desorption branch descends quickly from the utmost point at P/P0=1 and retraces back to overlap the adsorption branch at P/P0=0.5.The hysteresis loop formed from the adsorptiondesorption branchof Ca8Zr2is a typicalH3hysteresis loop,asproposed by International Union of Pure and Applied Chemistry(IUPAC).And thus,SGCS-made Ca8Zr2should be mesoporous sorbent with slitlike pore[60],also as observed in Fig.7(a)above for the FSEM f i gure of Ca8Zr2.

But different from the fresh Ca8Zr2sorbent,its carbonated product after 18 carbonated cycles presents the extreme hysteresis loop of H1 type with nearly vertical adsorption-desorption branch above P/P0=0.9,indicating that spherically agglomerated and compacted pore shapes were formed,as evidenced in Fig.7(d2).

Furthermore,for the BJH pore volume distribution as shown in Fig.8(b)for SGCS-made fresh Ca8Zr2,two pore peaks around 12.6 and 53.1 nm are observed and mainly fell within the mesoporous scale(2-50 nm).Its BET surface area is 10.1 m2·g?1,mainly arising from the contribution of the pores in 2.7-24.0 nm.But as the carbonation cycle increases,the pore volume distribution changes greatly. Although there still existed two pore peaks around 3.5 and 76.3 nm for the carbonated Ca8Zr2after the 5th cycle,the two pore peaks shift towards the bigger pore size around 3.9 and 83 nm for Ca8Zr2after the10thcycle.Thereafter,withthefurtherincreaseofthecyclenumber, thesmall porebelow 10 nmis completelyclosed.And a little increase of themacroporesto89.0nmforthecarbonatedCa8Zr2after15 cyclesand thento95.1nmafter18 cyclesisobserved,asverif i edbyAlvarezetal.in their extended carbonated cycles[29].

Finally,under the severe calcination condition from Fig.8(d),the ratio Φ of the micropore SA to the external SA decreases remarkably with the cycle number.After the 15th carbonated cycles,Φ is below unity and most SA arise from the contribution of the external SA in the carbonated Ca8Zr2mainly due to the closure of the micropores in Ca8Zr2,as observed in Fig.7(c)-(d).Meanwhile,the surface fractal dimension Dsof the carbonated Ca8Zr2is also decreased towards 2 with the cycle number,signifying that more coarsening and smoothing of the carbonated Ca8Zr2surface occur,as observed in Fig.7 for Ca8Zr2, which is also consistent with the f i ndings of Li et al.[61].

4.Conclusions

CaO/CaZrO3sorbent was synthesized using the SGCS method with urea as fuel,and its cyclic characteristics through the multiple severe calcination were further evaluated on a lab-scaled reactor and characterized with such experimental means as XRD,FSEM and nitrogen adsorption-desorption analysis.Relevant conclusions were reached.

(1)Through the severe calcination condition,SGCS-made CaO/CaZrO3sorbents displayed a different cyclic reaction performance. Higher CO2capture capacity and conversion at the initial cycles than other reference sorbents were obtained for the typical porous structure and great resistance to sintering,but then degraded over the following successive 10 cycles and f i nally stabilized throughout the later cycles;

(2)Thecarbonationconversionoverthedifferentloopingcycleswas fi tted based on the related experimental data with two characteristic parameters obtained.The deactivation constant k was found to decrease with the mass ratio of CaO to inert CaZrO3in an exponential mode,while the residual conversion Xrwas decreased linearly;

(3)Finally,morphological transformation of the synthesized Ca8Zr2over the cycles was observed.With the cycle increasing,the agglomeration and edge rounding of the newly formed CaCO3grains aggravated for its lower melting temperature than that of CaO,which caused the carbonation of the synthesized CaO sorbent only conf i ned to the external active CaO,and the fast formation of CaCO3shell enveloped the unreacted CaO inside. As accompanied by the carbonation process,the pore peak and mean grain size of the carbonated sorbents shifted to the bigger direction but the surface area SA ratio Φ and surface fractal dimension Dsdecreased.

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13 March 2014

☆Supported by the National Natural Science Foundation of China(51276210, 50906030,31301586),the Partial Financial Grant of North China University of Water Resources and Electric Power(201012)and the National Basic Research Program of China(2011CB707301).

*Corresponding author.

E-mail address:david-wn@163.com(B.Wang).

http://dx.doi.org/10.1016/j.cjche.2014.06.034

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 16 April 2014

Accepted 12 May 2014

Available online 1 July 2014