Simultaneous CO2 capture and thermochemical heat storage by modified carbide slag in coupled calcium looping and CaO/Ca(OH)2 cycles

Chunxiao Zhang ,Yingjie Li, *,Zhiguo Bian ,Wan Zhang ,Zeyan Wang

1 School of Energy and Power Engineering,Shandong University,Jinan 250061,China

2 State Key Laboratory of Crystal Materials,Shandong University,Jinan 250100,China

Keywords:Carbide slag Calcium looping CaO/Ca(OH)2 heat storage Modification By-product of biodiesel CO2 capture

ABSTRACT The simultaneous CO2 capture and heat storage performances of the modified carbide slag with byproduct of biodiesel were investigated in the process coupled calcium looping and CaO/Ca(OH)2 thermochemical heat storage using air as the heat transfer fluid.The modified carbide slag with by-product of biodiesel exhibits superior CO2 capture and heat storage capacities in the coupled calcium looping and heat storage cycles.The hydration conversion and heat storage density of the modified carbide slag after 30 heat storage cycles are 0.65 mol.mol-1 and 1.14 GJ.t-1,respectively,which are 1.6 times as high as those of calcined carbide slag.The negative effect of CO2 in air as the heat storage fluid on the heat storage capacity of the modified carbide slag is overcome by introducing CO2 capture cycles.In addition,the CO2 capture reactivity of the modified carbide slag after the multiple calcium looping cycles is enhanced by the introduction of heat storage cycles.By introducing 10 heat storage cycles after the 10th and 15th CO2 capture cycles,the CO2 capture capacities of the modified carbide slag are subsequently improved by 32%and 43%,respectively.The porous and loose structure of modified carbide slag reduces the diffusion resistances of CO2 and steam in the material in the coupled process.The formed CaCO3 in the modified carbide slag as a result of air as the heat transfer fluid in heat storage cycles decomposes to regenerate CaO in calcium looping cycles,which improves heat storage capacity.Therefore,the modified carbide slag with by-product of biodiesel seems promising in the coupled calcium looping and CaO/Ca(OH)2 heat storage cycles.

1.Introduction

Fossil fuels as dominant fuel resources play the critical roles in utility and industrial utilizations.Nevertheless,the consumption of fossil energy has raised lots of problems including global warming and climate change attributed to greenhouse gas emissions[1,2].It is well known that solar energy is a clean and environmentally friendly energy resource.However,the natural shortcomings of solar irradiation such as intermittence and instability impede the large-scale applications of solar energy.For the sake of continuous and stable power supply,it becomes crucial and essential to incorporate heat storage technology with concentrated solar energy power plants[3].Thermochemical heat storage(THS),which stores and releases heat through the reversible chemical reactions,is superior to sensible and latent heat storage technologies due to its low cost and high efficiency [4].

CaO/Ca(OH)2THS has been investigated to focus on solar energy power application,which is based on the reversible hydration/dehydration reactions of CaO[5].The endothermic dehydration reaction of Ca(OH)2happens in a dehydrator heated by solar energy at high temperature above 500°C as shown by Eq.(1),thus energy is stored in the form of chemical energy of CaO and steam[6].When heat is required,CaO reacts with steam to release heat through the reverse exothermic hydration in a hydrator,as presented by Eq.(2)[7].The hydration reaction of CaO occurs at high temperature in the range of 400–600 °C according to the steam pressure [8].CaO/Ca(OH)2THS has some advantages such as high efficiency and low energy loss,making it potential for solar power plants to supply the power continuously [9].

The properties of CaO-based heat storage materials including reactivity and cyclic stability play key roles in CaO/Ca(OH)2THS system,which have been widely investigated [10,11].Schmidt et al.[12] demonstrated the possibility of storing and releasing the heat in a CaO/Ca(OH)2THS reactor for about 20 kg Ca(OH)2,and they found that Ca(OH)2showed no obvious degradation in the performance after 10 cycles.Schaube et al.[13] found 60 g Ca(OH)2maintained stable heat storage reactivity in terms of overall conversion after 25 THS cycles,in spite of agglomeration confirmed by the change of mean particle diameter.Lin et al.[14] proposed that the hydration rate decreased with the number of THS cycles.These researches indicated the possible application of CaO/Ca(OH)2THS system.However,one of the main constraints of reversible CaO/Ca(OH)2system for industrial application is the poor mechanical property of CaO-based powder materials,which is due to the large molar density changes associated with the hydration/dehydration reactions[15,16].The heat and mass transfer limitations originating from the poor properties of CaO-based materials during CaO/Ca(OH)2cycles could be mitigated by adding additives such as nanostructured SiO2[17],Al2O3[18],and sodium silicate [19].Criado et al.[19] developed CaO-based composites using sodium silicate and investigated the mechanical properties of CaO/Ca-silicate composites over hundreds of CaO/Ca(OH)2THS cycles.Due to the formation of the calcium silicate framework,the grains of CaO had larger volumes to grow as Ca(OH)2during hydration,resulting in high cycling stability of CaO-based materials [19].

Azpiazu et al.[20]proposed that the cycling hydration/dehydration reactions did not exhibit a complete reversibility because incomplete Ca(OH)2dehydration was observed due to the CaCO3formation by the carbonation reaction of Ca(OH)2and CO2either dissolved in hydration water or present in air (as shown in Eq.(3)).Therefore,the inert gas,e.g.N2is commonly used as the heat transfer fluid in most of researches for CaO/Ca(OH)2THS system[13].However,N2separation adds the operational complexity,energy consumption and cost.Air used as the heat transfer fluid is cheaper than N2.However,Yan et al.[21]investigated the effect of CO2in air on the CaO/Ca(OH)2THS system and found the mole content of formed CaCO3after hydration was approximately 5.1%.Therefore,it is essential to mitigate the negative influence of CO2in air as the heat transfer fluid on CaO/Ca(OH)2THS.

CaO-based materials are not only employed in CaO/Ca(OH)2process for heat storage,but also used in calcium looping process for CO2capture.CO2capture and storage (CCS) is effective to reduce the CO2emissions from fossil fuel-fired power plants [22–25].Calcium looping (CaL) process is a promising postcombustion CO2capture technology for removing CO2from flue gas [26,27].CaL,i.e.the repetitive carbonation/calcination cycles of CaO,is presented in Eq.(4) [28]:

The flue gas containing CO2is sent into a carbonator,where the carbonation reaction between CO2and CaO occurs to form CaCO3at 600–700 °C.Subsequently,the formed CaCO3is sent to a calciner,where CaCO3decomposes into CaO and CO2at high temperature over 850 °C under oxy-fuel combustion [29].Therefore,CO2in fuel gas is captured in the carbonation process and concentrated in the calcination process for storage or further utilization.However,the CO2capture capacity of CaO-based material decreases with increasing the number of CaL cycles as a result of sintering at high calcination temperature [30].The large amount of spent CaO sorbents are exhausted from CaL process,so lots of fresh sorbents should be supplied to remain high CO2capture efficiency,which results in high energy consumption and cost.The different strategies to improve CO2capture reactivity of CaO-based sorbents have been proposed such as hydration[31,32],recarbonation[33],doping [34–36],templating [37],thermal pretreatment [38] and chemical treatments [39].Treatment with organic solutions including ethanol and organic acid,etc,is a potential chemical treatment method to modify CaO-based sorbents,because the creation of porous structure can facilitate CO2diffusion to the internal active parts of the sorbents [40,41].Nawar et al.[42] modified waste marble powder with several various acids as alternative to commercial calcium-carbonate sorbents in CaL cycles and found the acid-modified marble powder exhibited the better CO2uptake capacity and stability than limestone.Wang et al.[43] proposed a multi-acidification process to reactivate spent CaO-based sorbents by using the waste acid from industrial chemical processes.The CO2capture capacity of the spent sorbent could be recovered by multi-acidification to its initial values,attributed to the significantly improved physicochemical characteristics such as the grain size and surface area.Radfarnia et al.[44] investigated the CO2absorption behavior of modified limestone with citric acid and found that it showed the superior CO2absorption capacity due to the porous microstructure.The by-product of biodiesel mainly consisting of glycerin is generated in the transesterification process of triglycerides with alcohol[45].Chi et al.[46]proposed that CaO was modified with the by-product of biodiesel by the combustion and thought that the modified CaO with the by-product of biodiesel exhibited higher CO2capture capacity and cyclic stability than modified CaO with original acid and ethanol solutions,which was attributed to the more porous structure.

Carbide slag is a calcium-based industrial waste from ethylene gas production in chlor-alkali plants,which has been researched as a CO2sorbent in CaL process[47,48].Yuan et al.[49]proposed that carbide slag exhausted from CaL cycles for CO2capture was used as heat storage material in CaO/Ca(OH)2THS cycles.They found that deactivated carbide slag exhausted from CaL cycles could be still used in CaO/Ca(OH)2THS,but its heat storage capacity decreased rapidly with the number of THS cycles.It should be noted that the carbide slag experienced CaO/Ca(OH)2THS cycles was not used to capture CO2in Yuan et al.’s work [49].This is a drawback.The interaction effect between CaL process and THS process is still unclear.

Fig.1.Schematic diagram of a simultaneous CO2 capture and heat storage process coupled CaL and CaO/Ca(OH)2 THS.

Fig.2.Schematic diagram of preparation procedure of modified carbide slag.

In this work,we proposed a simultaneous CO2capture and heat storage process coupled CaL and CaO/Ca(OH)2THS using air instead of N2as a heat transfer fluid,as presented in Fig.1.The CaO-based material exhausted from CaL cycles (after the calcination) is fed into CaO/Ca(OH)2THS process using air as the heat transfer fluid.After a series of CaO/Ca(OH)2THS cycles,the CaObased material is again sent to CaL process for CO2capture.Thus,an interaction effect between CaL process and THS process using CaO-based material is produced.It should be noted that the formed CaCO3in the repetitive THS cycles due to air as the heat transfer fluid probably decomposes to regenerate CaO in the calcination stage of the CaL cycles.Thus,the negative effect of CO2in air as the heat transfer fluid on the cyclic heat storage stability of CaO is probably mitigated in the coupled CaL and THS process.

The modified carbide slag with by-product of biodiesel seems promising in efficient CO2capture[46],but its heat storage performance in CaO/Ca(OH)2THS cycles has not been studied.In this work,the modified carbide slag with by-product of biodiesel was employed in the process coupled calcium looping and CaO/Ca(OH)2thermochemical heat storage using air as the heat transfer fluid.The simultaneous CO2capture and heat storage performances of the modified carbide slag were investigated.The heat storage capacity of the modified carbide slag experienced CaL cycles as well as the CO2capture capacity of the modified carbide slag after CaO/Ca(OH)2THS cycles were discussed.The interaction effect between CaL and THS using the modified carbide slag was determined.In addition,the microstructure change of the modified carbide slag in the coupled CaL and CaO/Ca(OH)2THS cycles was also analyzed to confirm the reaction mechanism.

2.Experimental

2.1.Sample preparation

Carbide slag was sampled from a chlor-alkali plant in Shandong Province,China and sieved to size of less than 0.125 mm.The chemical components of carbide slag were analyzed by X-ray fluorescence(XRF)as shown in Table 1.By-product of biodiesel with a glycerol content of above 90% was sampled from the transesterification process of the peanut oil reacted with methanol [46].The preparation procedure of a modified carbide slag with byproduct of biodiesel is presented in Fig.2.Firstly,the carbide slag was calcined at 850 °C under N2atmosphere for 10 min.50 ml of by-product of biodiesel was added into 50 ml distilled water in a beaker with stirring at room temperature.Then 10 g of the calcined carbide slag was added to the beaker.The obtained mixture was stirred at 60 °C and put in a water bath for 60 min.Then the above-mentioned solution was sent to a muffle furnace at 850 °C under air for 60 min.Subsequently,the modified carbide slag with by-product of biodiesel was obtained,which was denoted as MCS.The calcined carbide slag denoted as CS was used to compare with MCS.The particles of MCS and CS are in the size range of 0–0.125 mm.

Table1 Chemical components of carbide slag (%,by mass)

2.2.Simultaneous CO2 capture and heat storage test of sample

The simultaneous CO2capture and heat storage performances of the samples in the process coupled CaL and THS cycles were performed in a four fixed-bed reactors system,including a carbonator,a calciner,a hydrator and a dehydrator,as shown in Fig.3.Air containing about 0.038% (vol) CO2was used as the heat transfer fluid in THS cycles.The gas cylinders were used to supply N2,CO2and air,respectively.The mass flowmeters (Flowmethod FL-802) were used to control the mass flow rates of the gases to enter the reactors to 2 L.min-1.The CO2capture tests of samples in CaL cycles were carried out in the carbonator and the calciner.The sample(500 mg) placed in a porcelain boat was sent to the carbonator for carbonation at 700 °C under 15% (vol) CO2/85% (vol) N2atmosphere for 20 min.Then the carbonated sample was transferred into the calciner for calcination at 850 °C under N2for 10 min.Next,the sample was put into the carbonator for a next CaL cycle.After each carbonation and calcination steps,the sample was cooled in an air drier until the room temperature and then weighted by a delicate electronic balance (Mettler Toledo-XS105DU).The carbonation conversion was used to describe CO2capture capacity of the sample during CaL cycles,which was obtained as follow:

where N is the number of CaL cycles;CNdenotes the carbonation conversion of sample after N CaL cycles,mol.mol-1;mcarb,Nis the mass of sample after the Nth carbonation,g;mcal,Nrepresents the mass of sample after the Nth calcination,g;m0is the initial mass of sample,g;A means CaO content in the initial sample,%;WCaOdenotes the molar mass of CaO,g.mol-1;is the molar mass of CO2,g.mol-1.

The heat storage tests of samples in THS cycles were done in the hydrator and the dehydrator.The steam at 250°C was provided by a steam generator,which was mixed with air before entering the hydrator.The total flow rate of the gas mixture was 2 L.min-1.The sample was sent into the hydrator for hydration at 400 °C under 95% (vol) steam/5% (vol) air atmosphere for 5 min.Then the hydrated sample was fed to the dehydrator at 550 °C under air atmosphere for 10 min.The hydration/dehydration reactions of samples were repetitively operated according to the abovementioned procedure.The sample after each hydration and dehydration was cooled in the air drier until room temperature and then weighted by the electronic balance.The hydration conversion and heat storage density were respectively used to evaluate heat storage capacity of the sample during THS cycles,as follows:

Fig.3.Schematic diagram of fixed-bed reactors system.

where i denotes the number of THS cycles;Ximeans the hydration conversion of sample after i THS cycles,mol.mol-1;Direpresents the heat storage density of sample after i THS cycles,GJ.t-1;Δmiis the mass loss of sample experienced the ith dehydration,g;mCaOis the mass of CaO in the initial sample,g;WH2Ois the molar mass of H2O,g.mol-1;ΔH represents the reaction heat of hydration,104.4 kJ.mol-1.

The heat storage test of the sample experienced CaL cycles and the CO2capture test of the cycled sample experienced THS cycles were respectively performed in the four fixed-bed reactors system.The experimental runs were shown in Table 2.

Table2 Experimental runs

2.3.Characterization

The functional groups of samples were examined by an attenuated total reflectance-Fourier transform infrared spectrometer(FTIR,Vertex70).The morphologies of samples experienced the different runs were detected by a scanning electron microscope (SEM,SUPRATM 55).The element distributions on the surfaces of the samples were analyzed by an energy dispersive X-ray(EDX,Oxford INCA sight X).A Micromeritics ASAP 2020-M nitrogen adsorption analyzer was used to measure pore volumes and pore size distributions of the samples after CaL and THS cycles according to Barrett-Joyner-Halenda (BJH) model.

3.Results and Discussion

3.1.Heat storage capacity of MCS in THS cycles

Fig.4 shows the hydration conversions and heat storage densities of MCS and CS during 30 THS cycles.It is observed that Xiand Diof MCS and CS decrease with the number of THS cycles after run 1.X30and D30of MCS after run 1 are 0.65 mol.mol-1and 1.14 GJ.t-1,respectively.As THS cycle number increases from 1 to 30 after run 1,Xiof MCS drops by 35%,while that of CS declines by 59%.The presence of CO2in air as the heat transfer fluid has a negative effect on the heat storage reactivity of CaO-based material due to the formation of CaCO3[20].The compact CaCO3product accumulates with the number of THS cycles,which probably results in the degradation in the heat storage capacities of MCS and CS.A more apparent reduction in Xiand Diof CS is observed with the THS cycle number,compared with those of MCS.It means that modification by by-product of biodiesel enhances the heat storage stability of CS.MCS possesses the higher heat storage capacity and the stabler reactivity than CS during the multiple THS cycles.X30and D30of MCS after run 1 is 1.6 times than those of CS,which is attributed the difference in their pore structures (as discussed in Section 3.5).

Fig.4.Heat storage capacity of MCS during 30 THS cycles after run 1.

Fig.5.FT-IR spectra of MCS after the 1st and 30th hydrations.

To elucidate the impact of CO2in air as the heat transfer fluid on the heat storage reactivity of MCS,the functional groups of MCS after the 1st and 30th hydrations analyzed by the FT-IR spectra are presented in Fig.5.The peak located at 3641 cm-1is associated with the stretching vibration of O—H bond in Ca(OH)2.And the peaks located at 1420 and 875 cm-1are associated with the antisymmetric stretching vibration and out-of-plane bending vibration ofbond,respectively.The FT-IR spectra clearly illustrate that the strong O—H bond peaks exist in the spectrum of MCS after the 1st and 30th hydrations,which is attributed to Ca(OH)2formed through the hydration of CaO.bond peaks only exist in the spectrum of MCS after the 30th hydration,while they are not observed in MCS after the 1st hydration,indicating the negative effect of CO2is not evident in the 1st THS cycle.There is about 0.038% (vol) CO2in air as the heat transfer fluid in THS cycles.The equilibrium temperatures of CaO carbonation reaction under hydration and dehydration conditions are calculated according to Eq.(8) [50]:

Fig.6.CO2 capture capacity of MCS during 20 CaL cycles after run 2.

where Peqdenotes the CO2partial pressure,bar;T is the equilibrium carbonation temperature,K.The maximum carbonation temperatures of CaO in the hydration and the dehydration stages are 448 and 533 °C,respectively.The temperatures for the hydration and the dehydration steps are 400 and 550 °C,respectively.Thus,CaO and Ca(OH)2can react with CO2in air in the hydration stage,while the carbonation reaction does not happen in the dehydration stage.This agrees with the experimental results reported by Yan et al.[21].The formed CaCO3forms a dense layer coated on the surface of the unreacted CaO during the hydration process,which increases the diffusion resistance of steam,resulting in a degradation in the hydration rate of the material.

3.2.Heat storage capacity of MCS experienced CaL cycles

The CO2capture capacities of MCS and CS during 20 CaL cycles are compared in Fig.6.It is noted that MCS exhibits higher CO2capture capacity than CS after run 2.C1of MCS is 17% higher than that of CS.CNof MCS and CS decrease with the number of CaL cycles after run 2 due to the sintering.C20of MCS and CS are 0.59 and 0.26 mol.mol-1,respectively,which are 35% and 67%lower than their C1.C20of MCS is 127% higher than that of CS.These results evaluate the enhancement in CO2capture performance of CS by the modification.Therefore,MCS exhibits higher CO2capture capacity and better cyclic stability than CS.

The hydration conversions and heat storage densities of MCS and CS experienced 10 CaL cycles during 30 THS cycles are illustrated in Fig.7.Xiand Diof MCS and CS experienced 10 CaL cycles decline with increasing the number of THS cycles after run 3.The aggregation of CaO and the negative effect of CO2in air as the heat transfer fluid lead to the decay in the heat storage capacities of the two materials.X30and D30of MCS after run 3 are 33% lower than those after run 1,while those of CS after run 3 are 38% lower than those after run 1.After 10 CaL cycles,MCS exhibits higher heat storage capacity than CS.This is because MCS probably has higher sintering resistance than CS after CaL cycles.X30and D30of MCS after 10 CaL cycles are 0.44 mol.mol-1and 0.77 GJ.t-1,respectively,which are 76% higher than those of CS after 10 CaL cycles.Thus,MCS experienced CaL cycles possesses higher cyclic heat storage capacity than CS experienced the same CaL cycles.

Fig.7.Heat storage capacities of MCS after 10 CaL cycles during 30 THS cycles.

Fig.8.Effect of introduction of 1 CaL cycle in THS cycles on heat storage capacity of MCS experienced 10 CaL cycles after run 4.

3.3.Effect of introduction of CaL cycles in THS cycles on heat storage capacity of MCS

The effect of the introduction of 1 CaL cycle after 30 THS cycles on the heat storage capacity of MCS experienced 10 CaL cycles is presented in Fig.8.The heat storage capacities of MCS and CS drop by 53% and 73% after the first 30 THS cycles,respectively.1 CaL cycle is introduced after 30 THS cycles,and the hydration conversions and heat storage densities of the two materials are both improved.X31and D31of MCS after run 4 are 0.93 mol.mol-1and 1.63 GJ.t-1,respectively,which are 2.1 times as high as those of its X30and D30.X31and D31of CS after run 4 are 3.6 times as high as those of its X30and D30.It is worth noting that X31of MCS and CS are almost the same as the corresponding values in the 1st THS cycle,respectively.It indicates that the heat storage capacities of the two materials after 30 THS cycles are restored by just 1 CaL cycle.This is because the formed CaCO3product layers in MCS and CS in the THS cycles decompose to regenerate CaO in the calcination step of CaL process.Therefore,the adverse impact of the presence of CO2in air as the heat transfer fluid on the cyclic heat storage capacity is eliminated.

The effect of the respective introduction of 1 CaL cycle after 10 and 20 THS cycles on the heat storage performance of MCS is presented in Fig.9.By introducing the 1st CaL cycle after 10 THS cycles,X11of MCS after run 5 reaches 0.95 mol.mol-1,which is 7% higher than that after run 1;while X11of CS after run 5 is 39%higher than that after run 1,as plotted in Fig.9(a).By introducing the 2nd CaL cycle after 20 THS cycles,X21of MCS and CS after run 5 are both enhanced again,which are 24%and 92%higher than those after run 1,respectively.These results indicate that the introduction of CaL cycles in THS cycles enhances the hydration reactivities of the two materials.In addition,CS gets a greater improvement in heat storage capacity than MCS by introducing CaL cycles in THS cycles.MCS still shows higher cyclic heat storage capacity than CS in 30 cycles.X30and D30of MCS after run 5 are 0.82 mol.mol-1and 1.44 GJ.t-1,respectively,which are 22% higher than those of CS.The average heat storage density is used to describe the average heat released per mass of sample in each cycle during THS,which is defined as follow:

where Di,avedenotes the average heat storage density of sample during i THS cycles,GJ.t-1.Fig.9(b) shows D30,aveof MCS and CS after runs 1 and 5.The average heat storage densities of the two materials in THS cycles are improved by introducing CaL cycles.D30,aveof MCS and CS after run 5 are 1.59 and 1.38 GJ.t-1,respectively,which are 10%and 30%higher than the corresponding values after run 1.In addition,D30,aveof MCS is 15%higher than that of CS after run 5.

Fig.9.Effect of respective introduction of 1 CaL cycle after 10 and 20 THS cycles on heat storage performance of MCS:(a) heat storage capacity (b) average heat storage density.

Fig.10.Effect of introduction of THS cycles in CaL cycles on CO2 capture performance of MCS:(a) carbonation conversion,(b) average CO2 absorption capacity.

Fig.11.SEM micrographs of MCS and CS experienced different THS cycles:(a)original MCS,(b)MCS after 30 THS cycles(run 1)(c)original CS,(d)CS after 30 THS cycles(run 1).

3.4.Effect of introduction of THS cycles in CaL cycles on CO2 capture capacity of MCS

The effect of the respective introduction of 10 THS cycles after 10 and 15 CaL cycles on the CO2capture performance of MCS is presented in Fig.10.By introducing 10 THS cycles after 10 CaL cycles,the CO2capture capacities of MCS and CS are improved,as shown in Fig.10(a).C11of MCS and CS after run 6 retain 0.91 mol.mol-1and 0.81 mol.mol-1,respectively,which are 32%and 72% higher than those after run 2.C16of MCS and CS after run 6 are both improved again by the introduction of another 10 THS cycles after 15 CaL cycles,which are 43%and 110%higher than those after run 2,respectively.The enhancement in CO2capture capacities of MCS and CS in CaL cycles is probably attributed to the increased surface area and porosity by introducing THS cycles.Manovic et al.[51] demonstrated that steam reactivation of the spent CaO-based sorbent could enhance CO2capture reactivity due to the improved specific surface area after steam hydration.Therefore,CO2capture capacities of MCS and CS in CaL cycles are activated by the introduction of THS cycles.In addition,CNof MCS after run 6 drops more slowly with the CaL cycle number,compared with that of CS.This is attributed to higher sintering resistance of MCS.C20of MCS after run 6 is 0.78 mol.mol-1,which is 37%higher than that of CS.The average CO2absorption capacity is used to describe the average mass of CO2absorbed per mass of the sample in each CaL cycle,which can be defined as follow:

Fig.12.SEM-EDX mapping of MCS in THS cycles:(a) marked region in Fig.11(a);(b) marked region in Fig.11(b).

where SN,aveis the average CO2absorption capacity of the sample in N CaL cycles,g CO2.(g sorbent)-1.S20,aveof MCS and CS after run 6 are 17%and 37%higher than those after run 2,respectively,as illustrated in Fig.10(b).In addition,MCS exhibits the higher average CO2absorption capacity than CS after run 6.S20,aveof MCS is 27%higher than that of CS after run 6.Considering the superior CO2capture and heat storage capacities,MCS appears promising in the process coupled CaL and CaO/Ca(OH)2THS cycles.

3.5.Microstructure analysis

Fig.11 illustrates the microstructural evolution of MCS and CS by SEM analysis during 30 THS cycles after run 1.The welldeveloped pore structure of original MCS is observed,as shown in Fig.11(a).Compared with original CS,original MCS possesses more porous and loose structure as a result of the rapid release of the gas products during the preparation process,as shown in Fig.11(c).This is beneficial for the diffusion of CO2in MCS,resulting in higher CO2capture capacity of MCS.In addition,the highly porous structure of original MCS reduces the diffusion resistance of steam in the material in the hydration stage of THS.The aggregation of the CaO grains and pore blockage of the two materials after 30 THS cycles are observed as presented in Fig.11(b) and(d),which increase the diffusion resistance of steam.Therefore,the heat storage capacities of MCS and CS decrease with the number of THS cycles.After 30 THS cycles,the more severe aggregation of the CaO grains in CS occurs,compared with MCS.The size of CaO grains in MCS grows more slowly during 30 THS cycles,compared with that in CS.After 30 THS cycles,MCS still possesses small and uniform grains as well as porous microstructure.Therefore,MCS exhibits greater heat storage capacity and better cyclic stability than CS in the multiple THS cycles.

The SEM-EDX mappings of MCS in 30 THS cycles are presented in Fig.12.The element distributions of the marked regions in Fig.11(a) and (b) are shown in Fig.12(a) and (b),respectively.Ca and O elements are distributed on the surface of original MCS and the mass ratio of Ca/O is 68.22:31.78,indicating that the main component of original MCS is CaO.As shown in Fig.12(b),Ca,O and C elements are observed on the surface of MCS after 30 THS cycles.The mass fractions of Ca and C elements are 58.31% and 1.21%,respectively.C element in MCS after 30 THS cycles is derived from CaCO3,ascribed to the carbonation reaction of Ca(OH)2or CaO with CO2in air during the hydration stage,which is also confirmed by the FT-IR analysis as shown in Fig.5.The compact CaCO3product layer results in the degradation of heat storage capacity of MCS during THS cycles.

Fig.13.(a) BJH pore volumes and (b) pore size distributions of MCS and CS experienced different CaL and THS cycles.

The pore volumes and pore size distributions of MCS and CS after the different CaL and THS cycles are shown in Fig.13.It is found that MCS has larger pore volume than CS under the same conditions,as presented in Fig.13(a).For instance,the pore volume of original MCS is 32% larger than that of original CS.After 30 THS cycles,the pore volumes of MCS and CS both decrease.The pore volumes of MCS and CS after 10 CaL and 30 THS cycles are smaller than those after 30 THS cycles,respectively.Considering the change in the heat storage performance of MCS and CS,it indicates that small pore volume tends to constrict the hydration of CaO.The pore volume of MCS after 10 CaL and 30 THS cycles is improved by 93%after the introduction of 1 CaL cycle.This is because the introduction of CaL cycle leads to the decomposition of the compact CaCO3product layer formed in MCS in THS cycles,which is beneficial for the subsequent heat storage.As shown in Fig.13(b),the volumes of pores in the range of 18–100 nm in diameter for MCS and CS both decrease after 30 THS cycles,while those in the range of 2–18 nm both increase.The heat storage capacities of MCS and CS decline with the number of THS cycles,thus it is inferred that the pores in the range of 18–100 nm are the important areas for the hydration of CaO.The pores in the range of 18–100 nm in diameter are also beneficial for CO2absorption of CaO [52].After 10 CaL and 30 THS cycles,the volume of pores in the range of 18–100 nm in diameter for MCS is larger than that of CS,which probably contributes to the greater heat storage capacity of MCS after run 3.It is worth noting that by the introduction of 1 CaL cycle,the volume of pores of 18–100 nm in diameter for MCS after 10 CaL and 30 THS cycles increases by 163% due to the decomposition of CaCO3product layer.Therefore,the cyclic heat storage capacity of MCS is restored by introducing CaL cycles during THS cycles.

4.Conclusions

The simultaneous CO2capture and heat storage performances of modified carbide slag with by-product of biodiesel were investigated in a coupled CaL and CaO/Ca(OH)2THS process using air as the heat transfer fluid.MCS exhibits the higher heat storage capacity compared with CS.X30and D30of MCS are 0.65 mol.mol-1and 1.14 GJ.t-1,respectively,which are 1.6 times as high as those of CS.The heat storage capacity of MCS experienced CaL cycles is higher than that of CS.The negative effect of CO2in air as the heat storage fluid is overcome by the introduction of CaL cycles in THS cycles,and the heat storage capacity of MCS is restored.By introducing 1 CaL cycle after 10th and 20th THS cycles,X11and X21of MCS are subsequently enhanced by 7% and 24%,respectively.After the respective introduction of 1 CaL cycle after 10th and 20th THS cycles,D30,aveof MCS during 30 THS cycles is 15%higher than that of CS.In addition,CO2capture reactivity of MCS after multiple CaL cycles is enhanced by the introduction of THS cycles.MCS possesses more porous and loose structure and exhibits larger pore volume compared with CS,ascribed to the rapid release of the gas products by glycerin combustion during the preparation process.Less aggregation of CaO grains occurs in MCS after repetitive THS cycles than CS.Therefore,the steam diffusion resistance in MCS during THS cycles is smaller than CS.By introducing CaL cycles in THS process,CaCO3product layer formed during the hydration stage of THS decomposes to regenerate CaO in the calcination stage of CaL,which forms lots of pores in the range of 18–100 nm in diameter,which are the important areas for the hydration of CaO.Therefore,MCS exhibits simultaneous efficient CO2capture and heat storage capacities in the coupled process.The modified carbide slag with by-product of biodiesel is promising in the process coupled CaL and CaO/Ca(OH)2THS using air as the heat transfer fluid.

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 was supported by the National Natural Science Foundation of China (51876105),and the Fundamental Research Funds of Shandong University (2018JC039).