Baowen Wang *,Zhongyuan Cai ,Heyu Li ,Yanchen Liang ,Tao Jiang ,Ning Ding ,Haibo Zhao

1 Research Institute for Coal Clean and Efficient Utilization,College of Electric Power,North China University of Water Resources and Electric Power,Zhengzhou 450045,China

2 Hebei Ji-Yan Energy Science and Technology Research Institute,Shijiazhuang 050000,China

3 State Key Laboratory of Coal Combustion,Huazhong University of Science and Technology,Wuhan 430074,China

Keywords:Coal combustion CO2 capture Chemical looping combustion CaSO4 mixed oxygen carrier Template combined synthesis method Sulfur evolution

ABSTRACT Calcium sulfate (CaSO4) has been verified as a promising oxygen carrier (OC) in the chemical looping combustion (CLC) for its high oxygen capacity,abundant reserve and low cost,but its low reactivity and deleterious sulfur species emission from the side reactions of CaSO4 should be well considered for its wide application in CLC.In order to promote the reactivity of CaSO4 and increase its potential to inhibit the gaseous sulfur emission,a CeO2-enhanced CaSO4 OC mixed OC of core-shell structure was prepared using the combined template synthesis method.Reaction characteristics of the prepared CaSO4-CeO2 mixed OC with a typical lignite was first conducted and systematically investigated,and an improved reactivity of the prepared CaSO4-CeO2 mixed OC was demonstrated than its single component CaSO4 or CeO2 due to the fast transfer and exchange of oxygen from the CaSO4 substrate to coal via the doped CeO2.Furthermore,the solid products formed from the mixed CaSO4-CeO2 OC with the selected coal were collected and analyzed.Especially,evolution and redistribution of the sulfur species of different forms were focused.At the latter reaction stage of YN reaction with the CaSO4-CeO2 mixed OC,the SO2 emitted from the side reactions of CaSO4 was greatly diminished and the doped CeO2 was proven effective to directionally fix the SO2 released to turn into different solid sulfur compounds,which were determined as Ce2O2S,Ce2S3 and Ce2(SO4)3·5H2O and formed through the different pathways.In addition,good regeneration of the reduced CaSO4-CeO2 mixed OC could be reached in spite of the unavoidable interaction between the included minerals in coal and the reduced mixed OC.Overall,the combined template method-made CaSO4-CeO2 mixed OC reported herein was not only endowed with enhanced reactivity for coal conversion,but also owned the potential to directionally fix the gaseous sulfur emission,which is quite applicable as OC for simultaneous decarbonatization and desulfurization in the real CLC process.

1.Introduction

Strong consensus has been reached that a large part of the anthropogenic carbon emission is caused by fossil fuel combustion,especially coal,which should be greatly reduced to refrain the increasing trend in the global warming and counteract the sharp rise of the global temperature.Among various technologies for significant reduction of carbon emission,chemical looping combustion (CLC) has attracted great attention as a potential breakthrough technology for its remarkable advantages in simultaneously realizing low-cost CO2capture,energy cascade utilization as well as effective inhibition of NOxemission[1].In a CLC system,reactive oxygen carrier(OC)is shuttled from the air reactor(AR)to the fuel reactor (FR) with sufficient oxygen and heat to sustain combustion of the introduced fuel,while the reduced OC is recycled into the AR for good regeneration to follow the ensuing consecutive cycles.As such,direct contact of coal with air in the general combustion process is avoided.After simple condensation,the high-purity CO2stream enriched in the flue gas could be easily obtained for downstream sequestration.In addition,relative to various gaseous fuels,direct use of coal as fuel in CLC is more significant,especially for China,due to such advantages as abundant reserve of coal,easy availability,low price and being a main contributor to meet the increasing energy demand.

Dual roles of oxygen carrier (OC) are generally played in CLC,including active oxygen donator and energy carrier.In order to ensure full conversion of fuel,such demands of a relevant OC should be satisfied [2],including high oxygen capacity,good reactivity,sufficient mechanical integrity,high resistance to sintering,environmental benignity and low cost,etc.So far,relative to such widely used transition metal oxide based OCs as Fe2O3,CuO,Mn3O4and CoO,CaSO4has gained great attention as a potential OC in CLC for its high oxygen capacity,nearly two times of all these metal OCs available and low cost [3].Especially,the reserve of CaSO4was much abundant,either provided from the natural gypsum ores or industrial by-products of different processes (e.g.wet flue gas desulfurization,chemical or phosphate industry),all of which are of great potential as OC in a real CLC process.

Currently,although CaSO4has been verified feasible to apply in CLC,such two main challenges should be well tackled [4-6].One great limitation of CaSO4to be applied as OC in CLC is its inferior reactivity,especially for coal.Another limitation is the unavoidable initiation of various side reactions of CaSO4during its reaction with fuel,which release a large amount of gaseous sulfur species as described in Eqs.(1)-(5) below.Negative consequences would be incurred,including deterioration of the reactivity of CaSO4for the ensuing reaction cycles and decrease of its oxygen carrying capacity involved.In addition,the gaseous sulfur emission from the FR may degrade the purity of the captured CO2and possibly pollute the atmospheric environment.

In order to relax the limitations of CaSO4as OC in CLC,various attempts have been made so far to improve the reactivity of CaSO4OC as well as to eliminate the gaseous sulfur released from the side reactions of CaSO4,respectively.Besides optimization of the reaction conditions,doping of CaSO4with other active metal oxides or alkaline adsorbent is acknowledged as another alternative worthwhile to be noted.On the one hand,active transition metals,such as Fe2O3[5,7],CuO [8],CoO [9] and Mn3O4[10],had been adopted to modify the reactivity of CaSO4and beneficial outcome was reached as expected for fuel conversion due to the synergistic effect between the CaSO4and the added transition metal oxides.On the other hand,aiming to effectively control the gaseous sulfur emitted as shown above in the reactions of Eqs.(1)-(5),various gas sulfur adsorbents such as CaO [11,12]or Ca2CuO3[13] were introduced into the CLC system to fix the gas sulfur released from CaSO4during its reaction with coal.Though possible improvement in conversion of coal is attained,endeavors in this direction are still needed to explore for simultaneous carbon capture andin situgaseous sulfur removal in CLC.

In addition to the transition oxides as CuO,Fe2O3,CoO and different SO2adsorbents mentioned above,CeO2is reported as one of the promising materials with unique oxygen storage and release feature,which enables rapid formation and elimination of oxygen vacanciesviaCe4+/Ce3+redox cycles in the different energy and environmental related processes [14].Besides three-way catalyst for automotive exhaust emission control and water-gas shift reaction for H2production[15],such applications of interest with CeO2were also made,including catalytic gasification [16] and combustion of coal [17],catalytic oxidation of soot [18],removal of such contamination gases as H2S [19],SO2[20] or NO [21],etc.

Furthermore,considering both the remarkable thermal stability and superior mobility of the oxygen ions involved,CeO2has attracted great interest as a reducible support or reactive component to be included in OC for chemical looping application.On the one hand,in the CLC scenario,Bhavsar and Veser [22] found that CeO2supported NiO enabled fast reduction kinetics and accelerated oxygen utilization rate for complete combustion of CH4.Liuet al.[11]proved the promotional effect of CeO2added to Fe2O3for CO combustion,while Miller and Siriwardane [23] revealed that addition of CeO2to the natural hematite promoted the reactivity of Fe2O3.Especially,CeO2doped Fe2O3OC was applied for high temperaturein situgasification CLC of PRB subbituminous coal and its char[24].On the other hand,alternative to the direct combustion of gaseous fuel or coal in CLC process with CeO2involved as OC as above mentioned,CeO2also instigated increasing interest for syngas or H2synthesisvianovel chemical looping reforming due to its particular oxygen transfer feature.Weiet al.[25] used γ-Al2O3supported CeO2as OC for direct selective partial oxidation of methane to syngas,while Heet al.[26]screened Fe2O3modified CeO2composite as the desired OC for syngas synthesis.In addition,modified CeO2also had the great potential to catalytic conversion of CO2to CO as expected for efficient CO2utilization[27,28].But to our best knowledge,in spite of the unique oxygen exchange rate and good stability over the redox cycles,addition of the CeO2as an extra dopant to the CaSO4has never been attempted before,which would be much meaningful to be applied as OC in CLC for coal conversion.

In this research,the CeO2-enhanced CaSO4mixed OC was first prepared and reported using the combined template synthesis method.And its reaction performance with a typical lignite (YN)was evaluated using thermogravimetric (TGA) coupled with gaseous Fourier transform infrared (FTIR).Furthermore,in order to reveal the enhanced reactivity of the prepared CaSO4-CeO2mixed OC and its desired potential to simultaneous carbon capture and gaseous sulfur directional fixation,morphological characterization and phases identification of the solid products formed from the CaSO4-CeO2mixed OC reaction with YN were conducted using field emission scanning electron microscopy (FESEM) coupled with the energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction(XRD)respectively to illuminate the inherent oxygen transfer pathway of great interest and comprehensively explore the migration and redistribution of the solid sulfur compounds involved.

2.Experimental

2.1.Materials and characterization

The mixed particles of CaSO4enhanced by CeO2with their mass ratio around 6:4 were prepared using the combined template synthesis method as developed in our group before [8] to ensure the fabricated nanomaterials of controllable superiority in structure,morphology and dimension.In this combined preparation method,the natural anhydrite ore was adopted as the template substrate with CaSO4of high purity above 95% (mass),while the doped CeO2encapsulated around the CaSO4substrate was synthesized using the sol-gel combustion synthesis method(SGCS)as designed by our group [29,30],which exhibited high reactivity and strong resistance to sintering for the prepared OC,well tailored for the CLC application.

For synthesis of the CaSO4-CeO2mixed OC with good performance,the basic procedures for the combined template synthesis method were simply introduced as followed.Firstly,the chosen Chinese natural anhydrite ore was crushed and sieved with particles within～100 μm collected for use.Meanwhile,both the Ce(NO3)2·6H2O and urea of the analytical grade(AR)with their purity over 99%were ordered from Sinopharm Chemical Reagent Co.,Ltd.and adopted for CeO2synthesis.The fixed amounts of the processed natural anhydrite ore,the precursor of CeO2and urea,were accurately weighed and mixed with the deionized water in a beaker.The mixture was then mechanically stirred and heated on a hot plate around the constant temperature 75 °C until the viscous gel was formed,which was further dried sequentially at 80°C and 135°C.Subsequent to drying,ignition of the dried gel at 600°C for 15 min and then sintering of the as-ignited product at 950 °C for 2 h were conducted in a preheated muffle furnace.Finally,after grinding and sieving,the CaSO4-CeO2particles of 63-106 μm were collected and well stored for use.

As to coal,a typical lignite of low contents of sulfur and ash was collected from Indonesia and designated as YN hereafter.After being subjected to similar processing procedures to the CaSO4-CeO2mixed OC,the YN coal particles of 63-106 μm were adopted.And its basic properties,including proximate,ultimate and ash analysis,were characterized as provided in Table 1.Both ash and sulfur contents were found to reach only 4.63% and 0.13%,respectively,quite low and thus applicable to use as a model fuel to prevent the interference of the sulfur and the minerals involved on the CaSO4OC during its reaction with YN.

Table 1Basic properties of YN coal

Finally,as to the prepared CaSO4-CeO2mixed OC and YN coal,both of them were evenly mixed in a lab mortar at the designed mass ratio as described below.In order to ensure sufficient conversion of YN coal,the well designed mass balance method [31]was applied to determine the stoichiometrical amount of the CaSO4-CeO2mixed OC to the introduced coal.Assumed that both components in the CaSO4-CeO2mixed OC were fully reduced to CaS and Ce2O3with the inherent oxygen just to oxidize the YN coal,the mass ratio of CaSO4-CeO2to YN coal at the stoichiometrical state was quantified as 6.21.Similarly,for either the single CaSO4or CeO2component,their mass ratios to YN were theoretically determined as 3.97 or 40.2,respectively for sufficient conversion of YN coal.

2.2.Methods

The reducibility and the possible oxygen transfer mechanism for the prepared CaSO4-CeO2mixed OC was firstly investigated using H2-temperature programmed reduction (TPR) mode in a TGA apparatus (TG 209 F3,Netzsch Corp.,Germany).About 30 mg of the prepared OC sample was introduced to the TGA ceramic pan and then heated from the ambient to the designated 1000 °C at 25 °C·min-1.To ensure the reliability and reproducibility of the experimental results,the gas mixture of 20% (vol) H2diluted by the balance N2was adopted with its flow rate determined as 60 ml·min-1.

Furthermore,reaction characteristics of the CaSO4-CeO2mixed OC with YN lignite were evaluated using the same TGA apparatus as that of H2TPR experiment above.About 15 mg of the mixture of CaSO4-CeO2and YN at the designated mass ratio was heated to the final temperature 900 °C at the heating rate 25 °C·min-1.And another 15 min duration was sustained for sufficient conversion of coal.Pure N2stream was used in this research to be convenient for gaseous FTIR analysis and its flow rate was set at 80 ml·min-1to ensure the reproducibility of the experimental results by overcoming the adverse effects of heat and mass transfer.In addition,as references,pyrolysis of YN alone under the N2atmosphere and its reaction with the single CaSO4and CeO2were performed at the same experimental conditions described above for comparison.

After completion of the reduction stage of the prepared CaSO4-CeO2mixed OC and its two single components by YN as stated above,regeneration of the solid reduced residues was also heated in the same TGA from ambient to the 1000 °C at 25 °C·min-1in the air atmosphere.

Finally,the gaseous products produced from YN reaction with the CaSO4-CeO2mixed OC were led from the TGA apparatus to the FTIR spectrometer (EQUINOX 55,Bruker Corp.,Germany) for in-situ analysis.While the morphology and elemental composition of the prepared OC and the formed solid products were characterized using FSEM (Siron 200,FEI Company,Netherlands) coupled with an EDX (Genesis,EDAX Corp.,USA) at a magnification 5000 and an accumulated voltage of 20 kV.In addition,the crystalline phases involved in solid products formed from both the reduction and oxidation stages were identified by XRD(X’Pert PRO,PANalytical Corp.,Netherlands).

3.Result and Discussion

3.1.Characterization of the prepared CaSO4-CeO2 mixed OC

Firstly,both the main phases involved in the prepared CaSO4-CeO2mixed OC and its morphological characteristics were analyzed as shown in Fig.1 and Fig.2 below.From Fig.1,the main crystalline phase involved in the natural gypsum ore was identified as anhydrite(CaSO4).Meanwhile,gypsum(CaSO4·2H2O)as well as such two impurities as calcite(CaCO3)and quartz(SiO2)was found to exist due to the hygroscopic inclination of the anhydrous CaSO4and good association of the gypsum ore with the latter two impurities.As to the prepared CeO2,it was observed from Fig.1(b) and Fig.2(a) to well develop as separate CeO2grains with porous microstructure,and its grain size was calculated as 73.24 nm using the Scherrer equation.While for the prepared CaSO4-CeO2mixed OC,as shown in Fig.1(c),it was mainly composed of CaSO4and CeO2as desired except for a few identifiable CaSO4hydrates of different types and other impurities as described above.And the grain size of the outer CeO2in the mixed OC was calculated as only 41.0 nm,much lower than that of the single CeO2reference shown in Fig.1(b).While for CaSO4,the grain size of the CaSO4either included in the CaSO4-CeO2mixed OC or involved in the gypsum ore was stabilized around 80 nm,similar to that present in our previous research on the prepared CaSO4-CuO mixed OC[8],well indicating quite applicable to be used as the substrate due to its thermal stability.Furthermore,from Fig.2(b),it could be observedthat the dispersed CeO2was formed and well distributed around the substrate CaSO4to form the special ‘‘core-shell” structure as desired,which was much advantageous to react with coal,as further studied below.

Fig.1.XRD patterns of the prepared CaSO4-CeO2 mixed OC.(a) CaSO4 ore;(b) the SGCS-prepared CeO2;(c) CaSO4-CeO2.

Furthermore,the reducibility of the prepared CaSO4-CeO2mixed OC as well as its latent oxygen transfer characteristics was preliminarily ascertained using H2as the model fuel and shown in Fig.3.Generally,the single nonporous CeO2was reported to present two separate reduction profiles with the peak temperatures around 500 °C and above 800 °C due to the surface absorbed oxygen transfer combined with the following lattice oxygen migration,respectively [32].On the contrary,for the porous CeO2,similar to the morphology of the SGCS-made CeO2shown in Fig.2(a) above,the two reduction peaks was merged around 809.2 °C,consistent with the finding reached by Giordano,etc.[33].While for H2reduction of the CaSO4-CeO2mixed OC,only one distinct H2reduction profile was presented for the formed CeO2in the mixed OC,similar to that of the single porous CeO2above,but its peak temperature was shifted to 657 °C due to the higher reactivity of the CeO2involved in the mixed OC,which evidenced that the SGCS-made CeO2of porous structure and lower grain size promoted the lattice oxygen transfer in the mixed OC and the reducibility of the CaSO4-CeO2was thus strengthened.Therefore,the reducibility of the CaSO4substrate in the mixed OC was improved as well and its reduction peak was shifted to 723.4 °C,much lower than that of the reference gypsum ore shown in 859.7°C,which well evidenced that the doped CeO2facilitated the reducibility of CaSO4.In addition,since CeO2was generally reduced by H2to its counterpart no lower than Ce2O3and its latent oxygen transfer capacity only approached 0.046 g·(g CeO2)-1(2CeO2+H2(g) →Ce2O3+H2O(g)),which was nearly one tenth that of CaSO4as 0.47 g·(g CaSO4)-1.And thus,the maximal mass loss rate for H2reduction of the CaSO4substrate doped with CeO2was diminished to 0.4692 %·°C-1,lower than that of the single reference CaSO4as 0.6060 %·min-1shown in Fig.3.

3.2.TGA investigation of CaSO4-CeO2 reaction with YN coal.

Fig.2.Morphological characteristics of the prepared CaSO4-CeO2 mixed OC.(a) SGCS-prepared CeO2;(b) CaSO4-CeO2 mixed OC.

Fig.3.H2-TPR of the prepared CaSO4-CeO2 mixed OC.

After preliminary investigation of the H2reducibility of the CaSO4-CeO2mixed OC with the possible oxygen transfer mechanism,reaction of the mixed OC with YN was further performed using the same TGA under the N2atmosphere to fully learn the reaction characteristics of the CaSO4-CeO2mixed OC.The mass loss variation and its differential mass loss rate for YN reaction with both the CaSO4-CeO2mixed OC and its single reference CaSO4or CeO2were presented in Fig.4.

Firstly,as to the YN reaction with the two single references CaSO4and CeO2,from Fig.4(a),(b)and(c),it could be observed that their reaction behaviors changed greatly.As to YN reaction with the reference CaSO4shown in Fig.4(a) and (b),after removal of the adsorbed water from CaSO4below 200 °C,three distinct reaction stages were observed to sequentially occur in an ascending temperature order with the peak values residing at 455.0 °C,723.7°C and 895.5°C,respectively,among which the first reaction stage was mainly attributed to pyrolysis of YN alone.And then,as the reaction temperature was further increased,reductive decomposition of CaSO4with YN was started above 670°C with the peak temperatures residing around 723.7°C and 895.5°C at the ensuing second and third reaction stages.And these two peak temperatures were higher than the reported values by Yani and Zhang for an Australian lignite reaction with CaSO4[34],mainly due to the disparities in the coal and CaSO4samples,and the reaction conditions.

While for YN reaction with the reference oxide CeO2,as shown in Fig.4(a)and(c),its reaction behavior was far different than that of YN with the added reference CaSO4.After YN reaction with the reference CeO2,the residual TG left was 98.5% (mass) with only two main reaction stages presented.And the maximal DTG value at the first stage was found to reside around the temperature peak 465.3 °C and reached 0.10064 %·min-1.According to our previous experience [35],assuming that only pyrolysis of YN was initiated without its further reaction with the added CeO2at this stage,the theoretical maximal DTG value for the solid mixture of YN with CeO2should be calculated by direct division of the real maximal DTG value 4.524%·min-1for YN pyrolysis alone shown in our previous research [8] by the mass ratio of CeO2to YN as 40.2,4.524/40.2=0.11254 %·min-1.But,in fact,the calculated maximal DTG value 0.11254%·min-1was still higher than the present real value 0.10064%·min-1shown in Fig.4(c),which verified that only pyrolysis of YN occurred at this stage without further reaction with CeO2.This conclusion reached well agreed with that of H2reduction of the reference CeO2shown in Fig.3 above,where the temperature peak for H2reaction with the CeO2reference was much higher and reached 809.2 °C.Furthermore,with the reaction temperature being increased,another reaction stage for YN with the reference CeO2was initiated,which mainly resulted from YN reaction with CeO2.And the temperature peak was resided around 765.8 °C,much lower than that of YN reaction with CaSO4around 895.5°C,which reflected the higher reactivity of CeO2than that of CaSO4.But because the oxygen available to be transferred from the CeO2to YN was much lower than that of CaSO4,the DTG value for YN reaction with CeO2was only 0.0798%·min-1at the second reaction stage,quite lower than that of YN reaction with the reference CaSO4.

Finally,based on the analysis above for YN reaction with the single reference CaSO4and CeO2,reaction of YN coal with the mixed CaSO4-CeO2OC was further investigated and shown in Fig.4(a)and(d).After dehydration below 200°C,three main reaction stages were observed to occur.At the first reaction stage,the peak temperature was resided around 458.6°C,which was mainly attributed to single pyrolysis of YN as discussed above.Afterwards,at the second reaction stage,both the CaSO4and CeO2involved in their mixed OC reaction with YN were initiated and the peak temperature was a little shifted to 720.9 °C than those of YN reaction with the CaSO4or the reference CeO2due to the doped CeO2with superior oxygen exchange rate mentioned above.And then,at the third reaction stage,from Fig.4(d),the reaction temperature peak for YN reaction with the mixed OC was a little increased to 896.3°C for YN reaction with the mixed OC,but its maximal real reaction rate was greatly elevated to 5.341 %·min-1.

Furthermore,in order to better reveal the synergistic effects of the CaSO4and CeO2involved in the mixed CaSO4-CeO2OC during its reaction with YN at the third reaction stage,the real DTG value(Yreal) for YN reaction with the mixed CaSO4-CeO2shown in Fig.4(d)was represented as the direct line.According to the research by Puit-Garmero,et al.[36],the theoretical instant DTG valueYcalfor YN reaction with the mixed CaSO4-CeO2was further calculated using the Eq.(6) below.

whereYCaSO4andYCeO2represented the separate reaction rate of YN with reference CaSO4(YCaSO4)shown in Fig.4(b)and that of YN with the reference CeO2(YCeO2)shown in Fig.4(c),respectively.While thefCaSO4andfCeO2were referred to the respective mass ratios shared in the mixed CaSO4-CeO2OC.As such,the instant theoretical DTG valueYcalfor YN reaction with the CaSO4-CeO2was also calculated and included in Fig.4(d)and represented as the circle plus line symbol.And then,from Fig.4(d),compared with the real maximal DTG value at the third stage for YN reaction with the CaSO4-CeO2mixed OC as 5.341%·min-1,the calculated theoretical valueYcalwas much lower and only reached 4.041 %·min-1,which well proved the improved reactivity of YN reaction with the CeO2-enhanced CaSO4during its reaction with YN coal.Therefore,the CeO2enhanced CaSO4was much applicable as OC in a real CLC system.

3.3.Gaseous products evolution for YN reaction with the CaSO4-CeO2 mixed OC

In order to gain comprehensive insight into YN reaction with the prepared CaSO4-CeO2mixed OC,after the TG investigation above,the gaseous products evolved from YN reaction with both the mixed CaSO4-CeO2OC as well as single pyrolysis of YN and its further with the two reference CaSO4and CeO2,were furtherin situdetected.According to our previous research [9],accompanied by pyrolysis of YN alone,such main light gases as CH4,CO,CO2and steam would be emitted,while after reaction of YN with the CaSO4mixed OC,the main gases left in the flue gas would be changed to the dominant CO2and steam except for a few combustible gases as CO,CH4,etc.Among these gaseous products,high purity of CO2was desired for easy sequestration in the downstream.Meanwhile,gaseous sulfur was also inevitable to evolve from the side reactions of CaSO4during its reaction with YN,which caused great harms to the environment as well as degraded the purity of the captured CO2.Therefore,in this research,only CO2and SO2evolved from single pyrolysis of YN and its further reaction with both the CaSO4-CeO2mixed OC and its two separate components were focused and shown in Fig.5(a) and (b) below.

Fig.4.Reaction characteristics of YN with the CaSO4-CeO2 mixed OC.(a) mass loss;(b) mass loss rate of YN reaction with the reference CaSO4;(c) mass loss rate of YN reaction with the reference CeO2;(d) mass loss rate of YN reaction with the CaSO4-CeO2 mixed OC.

Fig.5.FTIR spectra for the gaseous products evolved from YN reaction with the CaSO4-CeO2 mixed OC.(a) CO2 yield;(b) SO2 yield.

Firstly,as a reference,a little CO2evolved from pyrolysis of YN lignite alone under the N2atmosphere was analyzed and shown in Fig.5(a).It could be observed that in correspondence to the two main reaction stages presented for single pyrolysis of YN,there existed two distinguishable CO2peaks,which were ascribed to disintegration of the carboxylic acid salts or esters and then decomposition of the calcite involved in YN[37].But when the CaSO4-CeO2mixed OC or its two single references was separately added to YN,as shown in Fig.5(a),the CO2emission behaviors changed greatly.As to YN reaction with the CeO2reference,though the oxygen transfer capacity of CeO2was quite low as evidenced in Fig.3 above,the peak around the CO2profile formed from YN reaction with the reference CeO2was observed around 791.6 °C.While for YN reaction with the reference CaSO4,after the small CO2peak around 728.5 °C,the main CO2peak was quite remarkable around 899°C.In relative to YN reaction with the single CaSO4or CeO2references,the main CO2peak formed from YN reaction with the CaSO4-CeO2mixed OC was more pronounced than that of YN reaction with the reference CaSO4,which again verified the beneficial synergistic effect between the CaSO4and the CeO2in their mixed OC during reaction with YN,well consistent with the similar conclusion reached from Fig.4(d) above for YN reaction with the mixed CaSO4-CeO2.

Furthermore,due to the great harms incurred from the sulfur released from the side reactions of CaSO4,SO2was focused as shown in Fig.5(b).As to pyrolysis of YN alone without the OC added,its SO2evolution behavior was still complex though low sulfur content involved.Two SO2peaks were observed around 107.2 °C and 457.3 °C,respectively,which mainly resulted from decomposition of the labile sulfur and the ferric sulfate formed by oxidation of the pyrite with the rich oxygenated functional groups included in YN [38].Furthermore,with the reaction temperature being elevated above 836 °C,a gradual increase in the emission of SO2was observed during pyrolysis of YN alone,possibly due to decomposition of the inherent CaSO4involved in YN[39].When the two CaSO4or CeO2references were separately added to YN,similar to the formation of CO2shown in Fig.5(a) above,with the inherent oxygen transferred from the mixed OC or its references,SO2was also formed as well.As to the CaSO4reference,the SO2released from its reaction with YN was mainly initiated above 800 °C due to the side reactions of CaSO4as listed in Eqs.(1)-(5) above.While for the reference CeO2,due to its fast oxygen transfer,the sulfur included in YN itself was gradually oxidized to SO2with the reaction temperature increased.

But quite different from the CaSO4and CeO2references,during YN reaction with the CaSO4-CeO2mixed OC,great change was made for the SO2emission.Though the SO2formed above 600 °C was sharply increased,but when the peak value was reached,the formed SO2was fast to diminish and even approached the SO2emitted from the reference CaSO4during its reaction with YN.The decrease of the SO2released from the side reactions of CaSO4was much desired and mainly contributed to fixation of the emitted SO2by the doped CeO2viathe complex reaction pathways,which was to be explored in more detail as follows.

Overall,the SO2emission behavior for YN reaction with the CaSO4-CeO2mixed OC verified that the CeO2included in the mixed OC really owned the great potential to fix the SO2released from the CaSO4.Meanwhile,the enhanced reactivity for conversion of YN was also reached as proven above.Therefore,the developed CaSO4-CeO2mixed OC in our research was quite applicable to be used as OC in the coal CLC process.

3.4.Solid products distribution for YN reaction with the mixed CaSO4-CeO2

After TGA-FTIR investigation of the reaction behavior between YN and the CaSO4-CeO2mixed OC above,the solid products as formed were further analyzed to deeply reveal the inherent oxygen transfer mechanisms and redistribution of the formed solid sulfur compounds.The morphologies of the solid residues from YN reaction with the single reference CeO2and the mixed CaSO4-CeO2OC were characterized using FESEM-EDX and shown in Fig.6(a) and(b),respectively.Meanwhile,the elemental compositions of interest,including carbon,calcium,cerium,etc.,were determined by EDX and listed in Table 2.Furthermore,the XRD patterns of the crystalline phases involved were analyzed and presented in Fig.7 below.

As reference,morphological variation of the solid product formed between YN and the reference CeO2was firstly studied and shown in Fig.6(a).It could be observed that relative to the SGCS-made fresh CeO2shown in Fig.2(a)above,after reaction with YN,the pore structure changed much with various pores of different sizes formed through coalescence of the adjacent reduced CeO2grains,but due to the easy inclination of re-oxidation of the reduced ceria even at the room temperature [40],the reduced CeO2in the solid products of YN reaction with the reference CeO2was found to completely revert to CeO2as shown in Fig.7(b) by XRD analysis.Furthermore,the main elemental compositions after YN reaction with the reference CeO2was analyzed,and from Table 2,the residual carbon contents on the two optionally selected point 1 and point 2 were determined as 34.16% and 24.80%by EDX analysis,only a little lower than those for YN reaction with the reference CaSO4as determined in our previous research[10],which were included in Table 2 for convenient comparison.The small gaps for the residual carbon contents determined for YN reaction with the reference CeO2and CaSO4fully indicated that the reactivity of CeO2was a little higher than that of CaSO4,but its oxygen capacity was quite limited,consistent with our finding reached by TGA-FTIR analysis above.In addition,the difference of the Ce content between the spot 1 and spot 2 verified a little mobility of the Ce by coalescence of the different ceria grains.

Different from the reference CeO2reaction with YN above,after YN reaction with the mixed CaSO4-CeO2,from Fig.6(b),its basic morphological feature still resembled that of the prepared CaSO4-CeO2OC presented in Fig.3(b)above.Due to a little higher reactivity of the outer CeO2in the mixed OC and the easy re-oxidation potential of the reduced CeO2,the CeO2encapsulated around the CaSO4substrate was firstly reduced by YN but then eventually reverted to its original state by virtue of its fast oxygen release and exchange rate [41],as evidenced in Fig.7(c) by XRD analysis.Meanwhile,the grain size of the re-oxidized CeO2was estimated as 105.7 nm by the Scherrer equation,much higher than that of 41.0 nm for the original CeO2presented in the prepared CaSO4-CeO2mixed OC,which indicated that the reduced CeO2counterpart was inferior to sintering and caused the fast increase in the grain size of the re-oxidized CeO2.While for the CaSO4substrate included in the mixed OC,from Fig.6(b),after reaction with YN,its morphology was well sustained as the plate-like appearance,similar to that of the CaSO4in the prepared CaSO4-CeO2OC shown in Fig.3(b)above.Though reduction of the CaSO4substrate was initiated with a lot of CaSO4being reduced to CaS as shown in Fig.7(c)by XRD analysis,some unreacted CaSO4was still present due to its inferior reactivity,especially for the insufficient contact of YN coal with the CaSO4substrate due to its impervious,plate-like appearance in our previous research [8].And thus,the inherent oxygen available in the CaSO4substrate for YN conversion was not completely utilized.Correspondingly,from Table 2,the residual carbon was left after YN reaction with the CaSO4-CeO2,but their carbon contents were lower than those of YN reaction with the reference CaSO4or the CeO2,which meant that the fast oxygen transfer of the doped CeO2was really beneficial to transfer the oxygen involved in the unreacted CaSO4to YN for its conversion.Especially,across the eutectic state formed at the interface between the unreacted CaSO4and the reduced CaS [42],the oxygen included in the unreacted CaSO4could be continuously transferred out forin situoxidization of the reduced Ce2O3to CeO2as shown in Eq.(7)below.Meanwhile,oxygen exchange rate from the eutectic unreacted CaSO4to the reduced CeO2was greatly augmented as well.

Table 2EDX analysis of the solid products for YN reaction with the CaSO4-CeO2 mixed OC (%,atom)

Furthermore,the formed CeO2was verified as feasible to decompose to the Ce2O3with the molecular O2emitted as shown in Eq.(8)[43].Thereby,the residual char could be directly burned in the chemical looping with oxygen uncoupling (CLOU) mode[44] as shown in Eq.(9).

Fig.6.Morphological characteristics of the solid reaction products during YN reaction with the CaSO4-CeO2 mixed OC.(a) YN reaction with the reference CeO2;(b) YN reaction of with the CaSO4-CeO2 mixed OC.

Fig.7.XRD patterns of the solid products for YN coal reaction with the CaSO4-CeO2 mixed OC.(a)YN-CaSO4(Red);(b)YN-CeO2(Red);(c)YN-(CaSO4+CeO2)(Red);(d)YN-(CaSO4+CeO2) (Ox).In this figure,1.oldhamite (CaS);2.calcium sulfate(CaSO4);3.iron sulfide (FeS);4.cristobalite (SiO2);5.gehlenite (Ca2Al2SiO7);6.anorthite (CaAl2Si2O8);7.Wollastonite (CaSiO3);8.tricalcium silicate (Ca3SiO5);9.lime (CaO);10.cerium oxide (CeO2);11.aluminate silicate (Al2SiO5);12.cerium oxide sulphide(Ce2O2S);13.cerium(III)sulfate pentahydrate(Ce2(SO4)3.5H2O);14.cerium sulfide (Ce2S3);15.hematite (Fe2O3);16.srebrodolskite (Ca2Fe2O5),17.cerium (IV) sulfate tetrahydrate (Ce(SO4)2·4H2O).

And thus,the reactivity of the CaSO4could be rejuvenated and the real reaction rate for YN reaction with the CaSO4-CeO2mixed OC was much improved,as verified in Fig.4(d) above for higher real reaction rate for YN reaction with the CaSO4-CeO2mixed OC as well as the more pronounced CO2peak at the final reaction stage shown in Fig.5(a) above.

Furthermore,as various potential harms could be incurred by the gaseous sulfur released from the side reactions of CaSO4during YN reaction with the CaSO4-CeO2mixed OC,evolution and redistribution of the solid sulfur compounds was further investigated to effectively control the gaseous sulfur emission and combat its harms.From Table 2 for the YN reaction with the mixed OC,the Ce content on the spot 1 was nearly triple of that on the spot 2,implying the spot 1 was mainly situated at the outer shell of the CeO2area,but the solid sulfur content left on the spot 1 was greatly enriched with its solid sulfur content reaching 6.56%,which proved that the retention of the gaseous sulfur released from the CaSO4substrate by CeO2was really initiated.And thus,as shown in Fig.5 (b) for the SO2profile evolved from YN reaction with the CaSO4-CeO2,the emitted SO2at the latter reaction stage was remarkably diminished and fixed by the CeO2involved.Furthermore,the solid sulfur compounds formed during YN reaction with the mixed OC were identified by XRD.As shown in Fig.7(c),it could be found that the main solid sulfur compounds relevant to CeO2were determined as cerium(III) oxysulfide (Ce2O2S),cerium(III)sulfide(Ce2S3)and the hydrated cerium(III)sulfate(Ce2(SO4)3.5H2-O),respectively.And their formation pathways were worthwhile to be further explored below for effective control of the gaseous sulfur emission.

As to the hydrated Ce2(SO4)3(i.e.Ce2(SO4)3.5H2O),due to its easy hydration preference even in the atmosphere [45],its sulfate counterpart formed during YN reaction with the CaSO4-CeO2mixed OC should be anhydrous Ce2(SO4)3,which was generally formedviareaction of CeO2with SO2in the presence of O2[46],as shown in Eq.(10) below.

In fact,during YN reaction with the CaSO4-CeO2mixed OC,where the CeO2was densely distributed across the CaSO4substrate,and the side reactions of the CaSO4substrate during its reaction with YN would inevitably emit a large amount of SO2as shown in Eqs.(1)-(5)above.Meanwhile,due to the melting state between the unreacted CaSO4and the reduced CaS was formed at the interface,which was convenient to fast transfer of oxygen from the unreacted CaSO4by the adjacent reduced CeO2to form the local oxygen enriched environment.Therefore,the reaction condition for formation of the Ce2(SO4)3as shown in Eq.(10)above was satisfied during YN reaction with the CaSO4-CeO2mixed OC and the presence of hydrated Ce2(SO4)3was possible.

While for the (Ce2O2S) and Ce2S3,they were reported to easily form during desulfurization of H2S by the CeO2and its reduced derivatives (e.g.Ce2O3) in the reducing environment [37].During YN reaction with the CaSO4-CeO2,besides the inferior reactivity of CaSO4and the low oxygen capacity of CeO2,the reducing atmosphere was really existent as found in our previous research[8,10].Meanwhile,the H2S was formed from the side reaction of CaSO4shown in Eq.(4)above.Therefore,both Ce2O2S and Ce2S3were feasible to form as evidenced in Fig.7(c) above by XRD analysis.But their formation pathways were quite complex,and several formation processes were listed in Eqs.(11)-(14) below for reference[47].

Of course,good regeneration of the reduced OC was desired as well for CLC application.Therefore,the reduced CaSO4-CeO2after reaction with YN coal was further re-oxidized by the simulated air and its solid products were analyzed by XRD.As shown in Fig.7(d),it could be observed that both the CaSO4and CeO2were easily formed as expected.Meanwhile,the Ce(SO4)2·4H2O was found as well.Similar to the Ce2(SO4)3·5H2O formed during the mixed OC reduction by YN coal,after the regeneration,the water bound to the Ce(SO4)2possibly arose from exposure of the regenerated products to the atmosphere due to their easy hydroscopic addiction.As to the anhydrous Ce(SO4)2,it was easily formed in the presence of airviathe pathway in Eq.(15) below.Meanwhile,during regeneration of the reduced CaSO4-CeO2OC in the air stream,SO2mainly resulted from the solid interaction between the CaS and CaSO4as shown in Eq.(5)above.In addition,both Ce2-O2S and Ce2S3were also easily oxidized to CeO2and SO2,which could further interact in the presence of air to form Ce(SO4)2following Eq.(15) below.

As to the Ce(SO4)2as formed in Eq.(15),it was reported to reduce by H2in the temperature programmed process [48] or to convert to Ce2(SO4)3and further decompose with new CeO2formed [49].And thus,the formed Ce(SO4)2was promising to act as an extra oxygen carrier with its lattice oxygen transferred or directly decompose to generate new CeO2for the ensuing reduction cycle.

Finally,besides the sulfur evolution and redistribution,both the interaction between various minerals [50] present in YN coal and their further reaction with the reduced CaSO4-CeO2mixed OC would also occur either in the processes for reduction of the mixed CaSO4-CeO2by YN coal or regeneration of the reduced mixed OC.As shown in Fig.7(c) and 7(d),both gehlenite (Ca2Al2SiO7),anorthite (CaAl2Si2O8) and tricalcium silicate (Ca3SiO5) were found to exist.Meanwhile,due to re-oxidization of the FeS in air to Fe2O3and its further interaction with the CaO formed from the side reactions of CaSO4shown in Eqs.(1)-(5) above,calcium iron oxide(Ca2Fe2O5) was found as well (2CaO+Fe2O3→Ca2Fe2O5).In spite of the low contents of minerals in YN coal,interaction of the mineral with the reduced CaSO4mixed OC would deteriorate the reactivity of the CaSO4and jeopardize its cycle stability over the multiple processes.And the potential consequence incurred on the operation of the realistic CLC would be well addressed in the future.

4.Conclusions

The CeO2enhanced CaSO4mixed OC was first developed and prepared using the developed template combined synthesis method.Characterization of the morphological feature of the prepared CaSO4-CeO2mixed OC and evaluation of its reducibility were conducted.Furthermore,its reaction characteristics with typical lignite were further preformed by the TGA-FTIR,which displayed the enhanced reactivity of this mixed OC relative to its two reference components due to the beneficial oxygen transfer and exchange by the doped CeO2,and thus more sufficient utilization of the oxygen involved in the CaSO4was promoted for conversion of YN coal.Meanwhile,gaseous sulfur evolution from YN reaction with the CaSO4-CeO2mixed OC indicated that the added CeO2was active to desulfurization of various gaseous sulfur released from the side reaction of CaSO4.Finally,comprehensive analysis of the solid products formed from the CaSO4-CeO2mixed OC reaction with YN coal indicated that in-situ directional fixation of the gaseous sulfur released from the CaSO4side reactions was effective for the prepared CaSO4-CeO2mixed OC with such Ce2(SO4)3.5H2O,Ce2S3,Ce2O2S being formed.In addition,good regeneration of the reduced CaSO4-CeO2was reached.Therefore,the CaSO4-CeO2mixed OC as reported has the potential to apply in the realistic CLC process as a promising OC for simultaneous carbon capture and sulfur retention.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China(Nos.51776073,51906083),Key Research&Development program of Henan Province(No.162102210233),North China University of Water Resources and Electric Power Innovative Project (Nos.2019XA014,2019XB058),Scientific Research &Development Project of Ji-Yan Energy Science and Technology Research Institute (NKY2020-05).