Improvement of CO2 capture performance ofcalcium-based absorbent modi fied with palygorskite☆

Liyuan Shan ,HuiLi,2,Binglu Meng ,YouhaiYu ,*,Yonggang Min ,2,*

1 College ofMaterialand MineralResource,Xi'an University ofArchitecture and Technology,Xi'an 710055,China

2 ShaanxiTechno-Institute ofRecycling Economy,Xi'an 710055,China

3 Xi'an Institute ofOptics and Precision Mechanics ofChinese Academy ofScience,Xi'an 710119,China

1.Introduction

The increase of global temperatures due to global warming has aroused a vast concern,and the contribution of CO2to the greenhouse effect probably more than 60%[1]because of its high levels and long life cycle in the atmosphere[2,3].In the 21stcentury,the CO2emissions in China have been ranked among the top of the world and the cement industry has contributed 14%of the whole industrial CO2emissions[4].Itis calculated that0.815 tons ofCO2emissions willbe produced directly from the production ofevery ton of cement clinker[5].The issue of CO2capture and storage is currently a topic which receives extensive attention between government and academic research.A lot ofresearch has been carried outto reduce the CO2concentration in the atmosphere,such as pre-combustion,post-combustion,oxygen-fueland chemical looping combustion[6–10],but not every method is fully viable.The well operability,reliability and economics play the decisive role.In this case,CCCR[11–13]technology(a Ca-based cycling calcination/carbonation reaction technology)is believed to be a new and effective method for CO2capture in flue gas ofindustry.Many kinds ofcalciumbased absorbent have been studied including limestone.Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite,which are differentcrystalforms ofcalciumcarbonate(CaCO3)and it can be obtained with very cheap price.In addition,44 g CO2can be captured per 56 g calcium-based absorbent in theory.However,large amount ofstudies have showed thatthe CO2capture and sequestration capability ofcalcium-based absorbentdramatically declined aftermultiple cycles due to the surface “sintering”[14–16].In order to improve and keep its activity,scientists have tried a wide variety of methods to modi fied calcium-based absorbent.

The interface structure of absorbent had caught the attention of a number of researchers.Some effective methods to improve the structure and stabilization of calcium-based absorbent have been tried.Such as utilizing the different Ca-precursors to prepare calcium-based absorbent,such as organic calcium salts[17](calcium acetate,calcium gluconate,calcium propionate,etc.)and inorganic calcium salt[18,19](calcium carbonate,calcium nitrate,calcium hydroxide).The results showed that the CO2capture performance of absorbent prepared by organic calcium salts as Ca-precursors was much better than Caprecursors,especially the calcium propionate,while other researchers had tried to use solution to modi fied CaO for improving its microscopic structures,such as ethylalcohol,acetic acid and steam[20–22].Some other effective ways were to add metaloxide materials ofhigh melting pointsuch as Al2O3,MgO,Ca12Al14O33,etc.,good resistance to high temperatures able to delay the sintering and contribute to improving the CO2capture performance of absorbent.These additive substances cannotcapture CO2in high temperature,butthey play an importantrole as skeleton with high melting point to enhance the cycling stability[23–25].Calcium-based absorbent was dealt with steam or solution also can improve the CO2capture performance effectively[26,27].However,economicalfactors should be considered to keep the cyclic carbonation stability of Ca-based absorbents.

In this paper,the calcium-based absorbent was modi fied with palygorskite which is a naturalmineral with high melting point and one-dimensionalstructure.Palygorskite is available and inexpensive which has been widely used around the world in many application.In this paper we found thatby adding appropriate amountofpalygorskite,the CCS capability and anti-sintering ability ofcalcium-based absorbent were enhanced.

2.Experimental

2.1.Materials and equipment

The limestone produced in Fuping of Shaanxi,and palygorskite was purchased from Jiangsu Golden Stone palygorskite R&D Center.The composition oflimestone and palygorskite was detected by X-ray fluorescence spectrometer-S4 PIONEER(XRF,Bruker company,Germany),as shown in Tables 1,and 2.According to Table 2,it can be observed that the main component ofpalygorskite is SiO2,so the deactivation of calcium-based absorbent modi fied with palygorskite can be used as raw materialfor producing cement.

Table 1 Components oflimestone(wt%)

Table 2 Components ofpalygorskite(wt%)

A thermal analyzer-1/1600(TGA/DSC,Mettler Toledo company,Switzerland)was used to characterize the carbonation rate(XN)ofabsorbents.The surface changes of absorbents were measured by Scanning Electron Microscope-Quanta 200(SEM,FEIcompany,Czech),and the average particle size ofabsorbents aftermultiple cycles were obtained by Laser Particle Size Distribution-LS230(LPSA,Beckman Coulter Hong Kong companies,United States).The pore size distribution was tested with MIP Autopore IV9510(MIP,Micromeritics Instrument Co.,America.).The structure of samples after multiple cycles was carried out by Fourier Transform Infrared spectrometer-BRUKER UECIOR22(FTIR,Bruker,Germany)with the accuracy ofcentimeter-level4 cm-1.

2.2.Modi fication mechanism and experimentalschematic

Palygorskite has excellent properties such as unique dispersion and high melting points.Therefore,in this paper,palygorskite was used as an anti-sintering additive to modify the surface oflimestone forimproving the reactivity of calcium-based absorbent.Schematic of calciumbased absorbent modi fied with palygorskite is presented in Fig.1.The one-dimensionalstructure ofpalygorskite could adhere to the surface of the limestone particles and act as skeleton which effectively reduced the mutual contact between the limestone particles,so that the sintering and fusion oflimestone particles can be avoided to some degree during the CCS process.Morphologies and microscopic structures of absorbent modi fied with palygorskite will be kept after multiple cycles due to the improvementofanti-sintering ability,and carbonation rate can be enhanced signi ficantly.

Fig.1.Schematic oflimestone modi fied with palygorskite.

2.3.The preparation ofmodi fied limestone and measurement

Preparation ofabsorbent:(1)raw limestone was ground by ballmill,and particles with diameters ranged from 125–180 μm was used in the experiment.The average particle size of limestone was 153.3μm.(2)Palygorskite was smashed by planetary ballmilland the particle size under 15μm was selected to be used.(3)The compounded absorbentwas made by mixing limestone and palygorskite uniformly in ethanolsolution,the existence ofethanolsolution can make palygorskite with nano-fibrous structure dispersed evenly onto the surface oflimestone.The amount of palygorskite in compounded absorbent was 2 wt%,5 wt%,10 wt%and 15 wt%respectively.

The TGA was used to analyze the cyclic calcination/carbonation performance of natural limestone and limestone modi fied with palygorskite.A platinum crucible was placed inside the furnace in the TGAsystem.About(7.0±0.1)mg absorbentwas loaded in the platinum crucible for each test.Calcination was accomplished at 850°C in an atmosphere of 100%N2.Then,the temperature was decreased at a rate of10 °C·min-1from 850 °C to 700 °C,CO2was mixed into the reaction furnace at 35%of the volume fraction by stimulating the gas ofcement industry which controlled with flowmeter,and the carbonation reaction completed during 850–700 °C.5,10,15 cyclic times calcination/carbonation test were performed ofeach sample,and the temperature,weight ofsample were recorded by computer connected with the furnace.In order to avoid the sample capture CO2in air,every sample was not taken out from the furnace chamber until the temperature reduced to ambienttemperature in an atmosphere of100%N2.The testing system ofTGA is shown in Fig.2.Speci fic experimentconditions for the whole test is presented in Fig.3.The carbonation rate is de fined by the formula(1).

XN(%)is carbonation rate of absorbent,and N is the cycle times,mcar(g)is the mass of absorbent after N times carbonation reaction,mcal(g)represents the mass ofsample after N times calcination reaction,and m0(g)indicates the primary mass ofabsorbent,β(%)shows the percent contents of CaO in initial sample.MCaOand MCO2(g·mol-1)represents the molar mass of CaO and CO2respectively.Therefore,the higher XN,the better.

Fig.2.TGA testing system of carbonation rate.

3.Results and Discussion

3.1.Effecton carbonation rate of the additive amount ofpalygorskite

The carbonation rate directly represents CO2capture performance of absorbent.In order to investigate the optimal additive amount of palygorskite to enhance CO2capture performance,as displayed in Fig.3,the mass percent of palygorskite was designed to be 2 wt%,5 wt%,10 wt%and 15 wt%respectively.And the cyclic carbonation rate ofnaturallimestone and limestone modi fied with palygorskite is presented in Fig.4.It can be observed thatthe carbonation rate ofnaturallimestone was enhanced afterbeing modi fied with palygorskite,and the improvement degree ofabsorbent modi fied with palygorskite was 5 wt%＞2 wt%＞10 wt%＞15 wt%.This result showed that the relation between the improvement degree of carbonation rate and additive amountofpalygorskite was notlinear,instead,the improvementdegree increases firstly then decreases with the increase ofpalygorskite additive amount,therefore the best modi fication effect cannot be reached with the more orless amountofpuri fied palygorskite unless the optimal additive amount 5 wt%.

One reason for this result was the effect of skeleton is not very obvious with the least palygorskite additive amount so that the improvement degree of anti-sintering performance was much less dramatic;another reason was in the process of CO2capture calciumbased absorbent played a main role,however in same quality ofmodified absorbent,the content of calcium-based absorbent(CaO)reduced with the increase of palygorskite additive amount,excessive amounts of palygorskite resulted in reducing the CO2capture performance.Therefore,the content of palygorskite in modi fied absorbent must be appropriate,otherwise,it has notable in fluence on the CO2capture.In this paper,the best CO2capture performance of modi fied absorbent was 13.11%improvement in ef ficiency with only 5 wt%palygorskite added during the CCS process after 15 cycles compared with natural limestone.Therefore,many analysis as below was carried outwith samples which modi fied with 5 wt%palygorskite.In addition,it is observed in Fig.4 thatthe carbonation rate was improved signi ficantly ofabsorbent modi fied with absorbent from the 1st cycle,and there is little difference from the tendency of carbonation rate curve between natural limestone and limestone modi fied with palygorskite.The reason is that the introduction of palygorskite not only prevents the sintering phenomenon ofcalcium-absorbent,but also has effecton the morphology ofcalcium-absorbent.The surface roughness of limestone particle increased due to the introduction ofpalygorskite,so that speci fic surface area of absorbent increased.Therefore,it is easier to capture CO2for calcium-absorbentmodi fied with palygorskite than pure absorbent at the first cycle,as a result,in this paper the carbonation rate was improved signi ficantly of absorbent modi fied with absorbent from the first cycle.

Fig.3.Experiment conditions for the whole test.

3.2.Effecton morphologies changes ofabsorbentmodi fied with palygorskite

Morphologies indicate the anti-sintering ability of absorbent,and play a quite importantrole in CO2capture performance.Image ofsamples modi fied with optimalratio of5 wt%is shown in Figs.5,6,and 7,respectively.

The experiment was conducted based on the mechanism as mentioned above,and Fig.5 was shown to illustrate the mechanism that has been successfully established in this experiment.It is found that the surface ofnaturallimestone particle was quite smooth with layered structure in Fig.5a instead of the surface of limestone modi fied with palygorskite in Fig.5b was covered evenly by fibrous palygorskite.Surface roughness oflimestone particle increased due to the introduction ofpalygorskite,so that speci fic surface area ofabsorbentincreased.

Fig.5.SEMimage oflimestone before and after modi fied with 5 wt%palygorskite(10000×).

Fig.6.SEMimage of limestone after multiple cycles(10000×)(a:5th cycle;b:15th cycle).

Fig.7.SEMimage oflimestone modi fied with 5 wt%palygorskite after multiple cycles(10000×)(a:5th cycle;b:15th cycle).

It is known that the sintering of absorbent was accelerated rapidly during the CCS process.Fig.6 displays the morphologies changes ofnaturallimestone after 5 and 15 cycles.An analysis result shows in Fig.6a and b indicates that with the increase of cycling times,the surface pore ofnaturallimestone for CO2entering into the absorbentreduced gradually,and became larger or even disappeared,so ithas been invalid pore for CO2capture.At the same time,smallparticles merged with each other,and became the larger one.As a result,the speci fic surface area decreased sharply,and “sintering”phenomenon was so serious that affected severely the ef ficiency ofCO2capture.

Fig.7 shows the morphology of limestone modi fied with 5 wt%palygorskite after cycles.Itcan be observed thatthe surface ofabsorbent after 5,15 cycle times stillretained a fair amount ofeffective pores for CO2enter into the absorbent[28,29],this excellent microscopic structures would play an important role in CO2capture performance.Compared with Fig.6,it is found that the limestone modi fied with palygorskite were stillable to maintain relatively complete structure such as layered structure,and the surface was covered with fibrous palygorskite,this suggested that palygorskite could act as skeleton with high melting pointto preventthe mutualcontactoflimestone particles.Therefore,the anti-sintering ability of limestone modi fied with palygorskite has been greatly improved compared with the pure one and resulting in the CO2capture performance was enhanced effectively.This explains the reason why absorbent modi fied with palygorskite showed the better CO2capture performance.

3.3.Effecton particle size ofabsorbentmodi fied with palygorskite

The average particle size change ofabsorbents after 5,10,15 cycles is shown in Table 3.It can be observed that,the average particle size of natural limestone and limestone modi fied with 5 wt%palygorskite both increased with the increase of cycle times.However,the average particle size ofmodi fied limestone was smaller.Comparing the average particle size of the absorbents,consequently,the particle size growth speed was signi ficantly smaller ofmodi fied limestone than that ofnaturallimestone.This resultaccords with other research's,itwas believed that after multiple cycles,smallparticles grew larger gradually,and the porosity reduced,thus the CO2capture performance decayed seriously[12,29–31].The introduction ofpalygorskite contribute to keeping the originalpore structure oflimestone.This illustrates that the absorbent after being modi fied has the better anti-sintering ability than natural limestone.

Table 3 The average particle size ofabsorbents

3.4.Microstructure analysis

Microscopic structures ofabsorbents have great impact on the CO2capture performance[30].To achieve the results ofmicroscopic structures,the samples of naturallimestone and limestone modi fied with 5 wt%palygorskite after 5,15 cycle times were tested by MIP Autopore analysis.The pore volumes,pore size distributions and physicalparameters ofabsorbents are displayed in Fig.8 and Table 4 respectively.Itcan be observed in Fig.8 that the pore volumes of naturallimestone and limestone modi fied with 5 wt%palygorskite after multiple cycles were distributed for bimodal.This result is different from the one of Huichao Chen's[32]which distribution ofpore volume was unimodal.It can be seen in Fig.8(a)that the first pore volume mainly located at 0.5–2 nm,and the second pore volume mainly located at 10–30 nm.While,in Fig.8(b)the pore volume mainly located at 0.5–1 nm and 8–20 nm,respectively.Both the pore volume were decreased quickly after the second peak,and even disappeared.The speci fic pore volume of modi fied limestone was much higher than that of limestone after multiple cycles.This illustrates that the pore volume structure oflimestone was enhanced after being modi fied with palygorskite.

Physicalparameters ofdifferentabsorbents such as porosity,intrusion volume,and totalpore area is displayed in Table 4.Comparing the data of naturallimestone and modi fied limestone after multiple cycles,it can be seen thatthe porosity has grown by 6.3957%from 84.1291%to 90.5248%,the pore volume increased from 0.09848cm3·g-1to 0.1233 cm3·g-1,and totalpore area also improved from 7.477m2·g-1to 12.559m2·g-1.Therefore,larger porosity,pore volume and totalpore area can provide a large zone for the CO2contacting with absorbentresulting in CO2capture performance was improved effectively.So the results ofpore volumes,pore size distribution and physical parameters of limestone modi fied with 5 wt%palygorskite explains the better anti-sintering ability and better cycling reactivity contribute to enhancing the performance of CO2capture.

3.5.FTIR analysis ofabsorbents after multiple cycles

FTIR is one of the importantphysicalmethods to study the structure ofmaterial.Therefore,the structure changes ofabsorbentafter multiple cycles was carried out by Fourier transform infrared spectrometer-BRUKER UECIOR22(FTIR,Bruker,Germany).

Fig.9(a)is FTIR of raw limestone and limestone after multiple calcination/carbonation cycles,and FTIR of raw limestone and limestone modi fied with 5 wt%palygorskite after cycle as shown in Fig.9(b).The FTIR analysis focused on the absorption band between 650 cm-1-1650 cm-1,because of CO32-stretching concentrated during this band mostly which used for measuring CaCO3.In Fig.9(a)and(b)the FTIR revealthat the absorption peak of CO32-stretching was found at 1410 cm-1and another absorption band at 713 cm-1,876 cm-1and 1050 cm-1respectively[33],the absorbance was decreased gradually with the increase of cycles.While in Fig.9(b),new absorption peak appear with the same absorbance at1089 cm-1which is the Si–O stretching ofpalygorskite in compounded absorbent[34].This result shows that the palygorskite always exists and keeps the one-dimensionalstructure during the period ofexperimentbecause of the performance of high temperature resistance which is conducive to enhance the ability ofanti-sintering.This explains why absorbent modi fied with palygorskite has the excellent surface structure and better anti-sintering ability than naturallimestone.

For a quantitative analysis of the relative contentof CO32-stretching of absorbents,the second calculation-peak area method-was carried outbased on CO32-stretching at 1410 cm-1ofraw limestone as the internalstandard,the resultis displayed in Table 5.Itcan be observed that the main absorption band ofCO32-stretching ofdifferentabsorbents all appeared in 1410 cm-1,the blue shifts(△σ)was found to little degrees which could be almost neglected.This indicates that the crystalstructure of CaCO3had no any change.The absorbance(A is abbreviated for absorbance)ofraw limestone at1410 cm-1was 0.332,and the average A of limestone and limestone modi fied with 5 wt%palygorskite after multiple cycles was 0.188 and 0.223,respectively.Meanwhile,it can be found that A was gradually decreased with the increase of cycles,this is because the absorbents getsintering gradually so thatthe capability of CO2capture declined.However,the A had a higher degree of absorbent modi fied with palygorskite than that ofnaturallimestone.The CO32-absorption cross sections(S is abbreviated for absorption cross sections)at main absorption band ofraw limestone at1410 cm-1was calculated to be 78.48×10-20cm2(S0),and S0as the internalstandard.So the results of Sn/S0are displayed in Table 5.The average Sn/S0of limestone during multiple cycles was 0.57,while the average Sn/S0of limestone modi fied with palygorskite was 0.82.It indicates that absorbent modi fied with palygorskite displayed much better CO2capture ability than limestone doped with nothing under the same condition.But why did the absorbent modi fied with palygorskite show the better CO2capture performance?The reason is thatpalygorskite can keep the one-dimensional structure during the period of experiment and decrease the contact effectively between limestone particles for delaying the sintering process oflimestone.Therefore,absorbent modi fied with palygorskite has the better activity to capture CO2.

Fig.8.Pore size distribution ofdifferent absorbents after multiple cycles.

Table 4 Physicalparameters ofabsorbents

Fig.9.FTIR spectrogram comparison ofabsorbents after multiple cycles.

Table 5 Results of FTIR

4.Conclusions

The carbonation rate(XN)ofabsorbent modi fied with palygorskite was studied in TGA system.The appropriate additive amount of palygorskite greatly improved the CO2capture performance ofabsorbent,and TGA results showed that the optimalamount ofpalygorskite was 5 wt%.Calcium-based absorbentmodi fied with 5 wt%palygorskite showed the best CO2capture performance by 13.11%improvementduring the CCS process after 15 cycles than thatofnaturallimestone.Analysis of morphologies changes,microscopic structures,and FTIR were also carried out.Itis found thatthe ability ofsintering resistance oflimestone was not only improved,but also the excellentmicroscopic structures of limestone modi fied with palygorskite was created,because palygorskite can keep the one-dimensionalstructure during the period ofexperimentand decrease the contactbetween limestone particles for delaying the sintering process ofabsorbent.

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