CO2 capture by double metal modified CaO-based sorbents from pyrolysis gases

Xiaobin Chen,Yuting Tang,Chuncheng Ke,Chaoyue Zhang,Sichun Ding,Xiaoqian Ma

School of Electric Power,South China University of Technology,Guangzhou 510640,China

Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization,Guangzhou 510640,China

Keywords:Pyrolysis gas CO2 capture Co-precipitation CaO-based sorbents Modified sorbents

ABSTRACTHigh-temperature pyrolysis technology can effectively solve the problem of municipal solid waste pollution.However,the pyrolysis gas contains a large amount of CO2,which would adversely affect the subsequent utilization.To address this problem,a novel method of co-precipitation modification with Ca,Mg and Zr metals was proposed to improve the CO2 capture performance.X-ray diffraction (XRD) patterns and energy dispersive X-ray spectroscopy analysis showed that the two inert supports MgO and CaZrO3 were uniformly distributed in the modified calcium-based sorbents.In addition,the XRD results indicated that CaZrO3 was produced by the reaction of ZrO2 and CaO at high temperatures.The effects of doping ratios,adsorption temperature,calcination temperature,CO2 concentration and calcination atmosphere on the adsorption capacity and cycle stability of the modified calcium-based sorbent were studied.The modified calcium-based sorbent achieved the best CO2 capture performance when the doping ratio was 10:1:1 with carbonation at 700°C under 20%CO2/80%N2 atmosphere and calcination at 900°C under 100%N2 atmosphere.After ten cycles,the average carbonation conversion rate of Ca-10 sorbent was 72%.Finally,the modified calcium-based sorbents successfully reduced the CO2 concentration of the pyrolysis gas from 37% to 5%.

1.Introduction

With the rapid development of economy and the acceleration of urbanization,a large amount of municipal solid wastes (MSW)were produced every day [1].Traditional waste disposal methods such as landfill and incineration not only have low energy efficiency,but also produce toxic gases to the environment such as dioxins and SO2[2].Therefore,it is urgent to find a way that can simultaneously solve the problem of MSW disposal and produce renewable energy.Compared with traditional waste disposal methods,pyrolysis is an environmentally-friendly,energy-saving and low-cost treatment method[3].Specifically,pyrolysis is a thermal decomposition process at high temperatures without oxygen[2],and MSW can be decomposed into CO2,CO,H2,CH4and other gases through rapid temperature-based pyrolysis.Meanwhile,pyrolysis gas can be directly used for combustion [4].However,besides the greenhouse effect [5],the presence of CO2would reduce the calorific value of pyrolysis gas,thus reducing the combustion efficiency.Therefore,it is necessary to get rid of the CO2from the pyrolysis gas before its further utilization [6].

It has been found from previous studies that lithium and calcium-based sorbents with high-temperature resistant offer an effective method to capture CO2[7,8].Among numerous sorbents,calcium oxide is considered as one of the most promising CO2capture materials due to its wide source and low cost [9–11].Capturing CO2through the calcium looping cycle is not only pollutionfree to the environment,but also recyclable.In general,a calcium looping cycle system consists of two fluidized bed reactors,namely carbonator and calcinator.In the carbonator,CO2reacts with CaO at 600–700 °Cto produce CaCO3,which is then transported to the calcinator and decomposed to CaO and CO2at 800–900 °C[12,13].The reversible gas–solid reaction between CaO and CO2at high temperatures is shown in the following formula [14]:

However,as the number of carbonization/calcination cycles increases,the ability of calcium-based sorbents to capture CO2decreases dramatically.The loss of capacity is caused by sintering of CaO,leading to the agglomeration of CaO from small particles into large particles and further resulting in a decrease in the adsorption surface area and clogging of the pores.Subsequently,the carbonation reaction control of sorbents is changed from fast kinetics to slow diffusion.Another difficulty for the calcium looping process is sulfation of the CaO-based sorbents.MSW contains sulfur and chlorine element,so SO2and HCl are present in both the carbonator and calcinator [15].The deactivation of CaO-based sorbents would be exacerbated by the presence of SO2and HCl,because of the formation of irreversibly CaSO4and CaClOH[16,17].Sun et al.[18] proved that the interaction between CaO and chlorine released from waste plastic pipe caused deactivation of CaO sorbents.To maintain a desirable CO2capture efficiency,fresh sorbents need to be added when the spent sorbent is deactivated,thus increasing the operating cost.Park et al.[19] investigated that the CO2absorption capacity of pure CaO dropped by 80%after 10–20 cycles,and Benitez-Guerrero et al.[20]found that CO2capture capacity of conventional CaO could deteriorate up to 92% of its original performance after multiple cycles.Therefore,it is very important and meaningful to improve the performance of calcium-based sorbents for calcium looping cycle [21,22].

In order to solve the aforementioned problems,various methods such as thermal pretreatment,steam hydration and doping inert solid carrier,have attracted wide attention [23,24].Among them,doping inert additives into calcium-based sorbents is an economical and effective method to improve the CO2capture performance,and the most widely used additives are Al2O3,La2O3,and MgO [25–27].Jing et al.[28] claimed that doping inert materials into carbon dioxide sorbents could prevent CaO from sintering and thus improve cycle stability,since inert components acted as a skeleton to maintain their microstructure in multiple carbonization and calcination cycles.Ma et al.[29]doped MgO metal into the CaO sorbent to obtain a better CO2capture capacity than pure CaO sorbent,and the capture capacity maintained 0.42 g CO2/g sorbent after 20 cycles,indicating that MgO could enhance the cycling stability of CaO sorbent.Zhang et al.[25] found that when the molar ratio of Mn/Ca was 0.75:100,the adsorption performance of CO2reached the maximum,which was 4.38 times than that of pure CaO after 10 cycles.Radfarnia et al.[9] found that the addition of Zr inhibited the agglomeration and sintering of CaO particles,and improved the cycling stability and absorption capacity of the modified calcium-based sorbent.To the best of our knowledge,there are few studies on adding two different metals to CaO-based sorbents at the same time.In addition,calcium-based sorbents are widely used to capture CO2in flue gas,but few studies have applied them to capture CO2in pyrolysis gas of MSW.

According to Antzara et al.[30,31],it was found that MgO and ZrO2had different mechanisms to improve the performance of calcium-based sorbents.Under high-temperature conditions,MgO didn’t react with CaO,while ZrO2would react with CaO.Mixed doping may further improve the performance of calcium-based sorbents.In order to efficiently remove CO2from the pyrolysis gas,this study modified CaO with MgO and ZrO2simultaneously to produce a binary doped sorbent using coprecipitation method.The effects of doping ratios,adsorption temperature,calcination temperature,CO2concentration and calcination atmosphere on the adsorption capacity and cycle stability of the modified calcium-based sorbent were studied through fixed bed experiments.After obtaining the optimal doping ratio and experimental conditions of the modified calcium-based sorbents,it was applied to the capture of CO2in the pyrolysis gas,and explored the effect of the modified calcium-based sorbents on the capture of CO2in the pyrolysis gas under the optimal conditions.At the same time,the CO2capture capacity of modified and original CaO during the cycle was compared.In addition,the modification mechanism of the sorbents was analyzed by microcosmic test.

2.Materials and Methods

2.1.Materials

Calcium nitrate tetrahydrate (Guangdong Guangshi Reagent Technology Co.,Ltd.,China)was selected as the calcium precursor.Magnesium nitrate hexahydrate (Guangdong Guangshi Reagent Technology Co.,Ltd.,China) and zirconium nitrate pentahydrate(Shanghai Macklin Biochemical Co.,Ltd.,China) were selected as the inert support precursors.Potassium hydroxide (Shanghai Rich Joint Chemical Reagents Co.,Ltd.,China) was selected as the precipitant.The calcium oxide from calcination of calcium carbonate(Tianjin Fuchen Chemical Reagent Co.,Ltd.,China) was defined as a blank control.The pyrolysis gas was simulated by the mixture of CO,CH4,N2and CO2(Guangzhou Shengying Gas Co.,Ltd.,China).The proportion of simulated pyrolysis gas is obtained according to our previous research related to the MSW pyrolysis [32].All reagents used in the experiment were analytical grade.

2.2.Sample preparation

The modified calcium-based sorbents were prepared by the coprecipitation method [6],as shown in Fig.1.Calcium nitrate tetrahydrate,magnesium nitrate hexahydrate and zirconium nitrate pentahydrate were mixed and dissolved in 300 ml deionized water.Potassium hydroxide was dissolved in 200 ml deionized water.All the reagents were weighted according to the different metal doping ratios and the amount of the corresponding precipitant.The mixed solution and the potassium hydroxide solution were designated as SA and SB,respectively.SA was put into a three-neck flask and then the mixture was magnetically stirred at 80°C for 20 min with stirring speed of 250 r.min-1.Subsequently,SB was added into the three-neck flask tardily(about a drop every three seconds).After SB was completely added into the three-neck flask,the mixtures of SA and SB were cooled down to ambient temperature and continuously stirred for 4 h.Then,the wet sample was placed in a drying oven at 105 °C until the water was completely evaporated.The evaporated solid residue was ground and sieved to the desired size of 0.075–0.18 mm,and then calcined in muffle furnace at 650 °C under an air atmosphere for 1 h.Finally,the calcium-based sorbents were taken out from the muffle furnace and stored in a desiccator for the cycle experiment.In this study,the molar ratio of MgO and ZrO2doped in each modified calcium-based sorbent was the same.For example,when the molar ratio of CaO:MgO:ZrO2was 4:1:1,this modified calcium-based sorbent was denoted as Ca-4.When the modified calcium-based sorbent was denoted as Ca-4-A or Ca-4-B,A represents before the cycle and B represents after the cycle.For example,the pure CaO before the cycle and after the cycle were denoted as Ca-0-A and Ca-0-B,respectively.

Fig.1.Preparation procedure of modified calcium-based sorbents by co-precipitation method.

2.3.Calcium looping cycle experiments

The calcium-based sorbent cycle experiments were carried out in a dual fixed-bed reactor,consisting of a calciner and a carbonator.The main experimental device is shown in Fig.2.Firstly,the carbonator and calciner were heated from room temperature to 700 and 900°C at the rate of 1°C.s-1,respectively.When exploring the effects of adsorption temperature and calcination temperature on the cycle stability of the sorbents,these two temperatures were variable.The gas flow rates in the carbonator and calciner were both 1 L.min-1.The sample boat which carried 0.1 g sample was quickly sent to the carbonator.Subsequently,the sorbent was carbonated in the carbonator under a gas mixture of 20% CO2/80% N2for 20 min and was calcined in the calciner under pure N2for 15 min.When exploring the effects of CO2concentration on the cycle stability of calcium-based sorbents,carbonation was carried out under the atmosphere of 10%–30% CO2/N2balance.The simulated pyrolysis gas was composed of 40% CO2,30% CO,22% CH4and 8% N2[32].After all the carbonation and calcination process,the sorbent was taken out from the reactor quickly and put into the desiccator for 10 min.Then the sorbent was weighted by electronic balance and was sent to the carbonator or calciner to continue the next cycle.The carbonation/calcination cycle was repeated for 10 cycles to analyze the stability of the CaO-based sorbent.The carbonation conversion rate of the CaO-based sorbent was calculated by following equation:

Fig.2.Schematic diagram of the calcium looping cycle for CO2 capture.

where,XNwas the carbonation conversion rate of the CaObased sorbent after N cycles;N denoted the number of carbonation/calcination cycles;anddenoted the weight of the sorbent after N carbonation and calcination,respectively;α denoted the content of CaO within sorbent;MCaOand MCO2denoted the molar mass of CaO and CO2,respectively.In order to ensure the accuracy,the average value of three experiments under the same condition was taken as the experimental result.

2.4.Analysis of calcium-based sorbents

The microstructure of the sorbents was investigated by a scanning electron microscopy (SEM,Merlin,Zeiss,Germany).The sorbents’ surface was coated with electric conductive gold film for better observation.A N2physisorption (ASAP2020,Micromeritics,America) was used to measure the specific surface areas and pore volumes of the sorbents.The specific surface areas and pore volumes were calculated by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods,respectively.The phase composition of sorbents was determined by X-ray diffraction(XRD,X’pertPowder,PANalytical,Netherlands).Gas chromatography-flame ionization detector (GC-FID,7890B,Agilent,America)was applied to analyze the concentration of components in pyrolysis gas.

3.Results and Discussion

3.1.XRD and EDS mapping analysis

The results of XRD of calcium-based sorbents were illustrated in Fig.3.As visualized in Fig.3(a),the components of the sample before carbonation were all Ca(OH)2.This was caused by the reaction of CaO with water vapor in the air [13].For the sample after carbonation,its components contained CaCO3and CaO but no Ca(OH)2,indicating that part of the CaO was wrapped by CaCO3and didn’t participate in the carbonation reaction,otherwise it would react to form Ca(OH)2.The compositions of modified calciumbased sorbents with different metal doping molar ratios were shown in Fig.3(b)–(c).As shown in the XRD patterns,Mg and Zr metals were successfully inserted into the calcium-based sorbents.Among them,Mg existed in the form of MgO,and Zr reacted with CaO to form CaZrO3.According to previous studies[8,30,33],Huang indicated that ZrO2didn’t react with CaO at high temperatures,while Antzara and Liu pointed out that they would react with each other.According to Fig.3(b)–(c),CaO and ZrO2would indeed react with each other and CaZrO3would not decompose at high temperatures.The modified calcium-based sorbents did not completely react with water vapor to form Ca(OH)2before carbonation,indicating that doping metal could retard the moisture absorption of CaO.As the mole ratio of doped metal increased,the characteristic peak intensity of Ca(OH)2gradually decreased through analyzing the characteristic curve of the crystal.Additionally,the characteristic peak intensity of CaZrO3increased while that of MgO decreased,because the relative content of CaZrO3elevated and the relative content of MgO reduced.It indicated that the doping of Mg and Zr would affect the crystallization of CaO,thereby affecting the ability of the modified calcium-based sorbent to capture CO2.

Fig.3.XRD patterns of the sorbents:(a) Ca-0;(b) Ca-4;(c) Ca-10.

The role of metal dopants on calcium-based sorbents was to prevent direct contact between CaO and CaCO3particles,thereby retarding sintering of the sorbents.Thermal sintering depended on the low Tammann temperature of CaO and CaCO3[8,34,35].When the carbonation and calcination temperatures were higher than their Tammann temperature,the solid crystal particles melted and combined to form larger crystal particles.The diagram of CaO sintering and modified calcium-based sorbents to retard sintering could be visualized in Fig.4(a).The pure CaO had already agglomerated between the particles during the first carbonation,resulting in some CaO being wrapped by CaCO3.As the number of cycles increased,this phenomenon became severer,and eventually only a small part of CaO participated in the capture of CO2.For the modified calcium-based sorbents,metal dopants prevented CaO and CaCO3particles from agglomerating when they were melted,because the Tammann temperature of MgO and CaZrO3was higher than the carbonation and calcination temperature[36,37].When the CaO and CaCO3particles were melted,the MgO and CaZrO3remained in the solid crystal form.Therefore,the sintering phenomenon between the particles in the modified calcium-based sorbents was not obvious.After ten cycles,only a small part of the CaO was wrapped.

The EDX mapping of elements Ca,Mg,and Zr in the modified calcium-based sorbents synthesized by the co-precipitation method was shown in the Fig.4(b).Three metal elements were evenly distributed in the modified calcium-based sorbents,indicating that these three particles of CaO,MgO and CaZrO3were in contact with each other [38].Therefore,the MgO and CaZrO3doped between the CaO particles could effectively remain the pore structure and retard the sintering of the sorbents to enhance the ability of CO2capture.

Fig.4.(a) Diagram of sintering and modification;(b) EDX mapping of metal elements in modified calcium-based sorbents.

3.2.Effects of metal doping molar ratio on the cycle stability of calcium-based sorbents

In order to explore the effects of doped metal concentration on the CO2capture performance of the modified calcium-based sorbents,several samples were prepared with different metal doping molar ratios.Fig.5 illustrated the cycle stability of various modified calcium-based sorbentswith carbonation at 700 °C for 20 min under the 20% CO2/80% N2atmosphere and calcination at 900 °C for 15 min under the 100% N2atmosphere.As can be seen from the Fig.5,the carbonation conversion rate of pure CaO dropped from 71% to 44% after ten cycles,with the decrease rate of 38%.It shows that without any modification,the carbonation conversion rate of pure CaO decreased sharply after each cycle.However,Mohammadi et al.[39] found that the carbonation conversion rate of the pure CaO sorbent prepared from egg shells dropped from 74%to 17%after ten cycles,even with the extremely mild calcinations at 700°C.The comparison of the two results indicated that the type of calcium precursor and the preparation method of the sorbent also influenced the cycle stability of CaO.When the calcium-based sorbents were modified,the first carbonation conversion rate of Ca-12 sorbent was 72% which was higher than that of pure CaO.But after ten cycles the carbonation conversion rate dropped to 54%,and the total decrease rate was 24%.Obviously,the carbonation conversion rate curve of Ca-12 sorbent had always been higher than pure CaO,confirming that the modification method could effectively improve the CO2capture performance of the sorbent.This improvement in capture performance was due to the fact that MgO and CaZrO3produced by the reaction of CaO and ZrO2at high temperatures could effectively inhibit the growth of CaO,which further retarded sintering-agglomeration phenomenon by separating CaO and CaCO3particles [39].When the metal doping molar ratio was 10:1:1,the carbonation conversion rate of this sorbent dropped from 77%to 67%,with the average carbonation conversion rate of 72% in ten cycles.Moreover,the carbonation conversion rate of Ca-10 sorbent after each cycle was higher than other doping molar ratio sorbents.In addition,the average carbonation conversion rate of Ca-10 was higher than the first conversion rate of other sorbents,indicating that the modified calcium-based sorbent prepared in this metal doping molar ratio had the best CO2capture performance.When the metal doping molar ratio was 8:1:1,the carbonation conversion rate of the sorbent in the first five cycles remains stable,followed by a decline after the sixth cycle.The decrease rate of Ca-8 sorbent after the fifth cycle was 1%and after the tenth cycle was 16%,demonstrating that the sorbent was relatively stable in the first five cycles,but as the number of cycles further increased,the rate of carbonation conversion dropped significantly.On the other hand,with the increment of the number of cycles,the carbonation conversion rate of the Ca-6 and Ca-4 sorbents basically unchanged with the total decrease rate of 5%and 1%,respectively.Although the CO2capture performance of Ca-6 and Ca-4 sorbents was relatively stable,their total carbonation conversion rates after ten cycles were low (60%and 55%,respectively).It can be concluded from the above results that the modification method proposed in our paper could effectively improve the carbonation conversion rate and cycle stability of the calcium-based sorbents.With the increment of the metal doping molar ratio,the cycle stability of the modified calciumbased sorbents became better,but the first carbonation conversion rate increased first followed by a decline.In summary,the first carbonation conversion rate and the average carbonation conversion rate in ten cycles of Ca-10 sorbent were the highest,while the cycle stability of Ca-4 sorbent was the best.

Fig.5.Effects of metal doping molar ratio on carbonation conversion rate of the calcium-based sorbents.

The reason why the first carbonation conversion rate of Ca-12 sorbent was higher than that of pure CaO was that the sample was calcined at 650°C for 1 h before the first cycle,resulting in severer sintering of pure CaO.Furthermore,the Ca-12 sorbent contained MgO and CaZrO3particles which could retard sintering,but the content of metal dopants was insufficient which could not effectively retard particles agglomeration and sintering,leading to unstable CO2capture performance.Among all modified calcium-based sorbents,Ca-4 sorbent had the best cycle stability because the content of metal dopants was very sufficient.However,because a large amount of ZrO2reacted with CaO to form CaZrO3,the CaO content was largely reduced.What’s more,the enrichment of CaZrO3occurring on the surface of CaO limited the surface position of CaO for chemical reactions.Therefore,the carbonation conversion rate of Ca-4 sorbent was very low.From the above analysis,it could be known that the doping of MgO and ZrO2in the calcium-based sorbents improved the CO2capture performance and cycle stability.However,the doping ratio could not be too high or too low,because a large amount of CaO would react with ZrO2which resulted in a significant reduction of CaO,and low doping ratio could not effectively retard sintering.In view of this,the doping ratio of Ca-10 sorbent was the most appropriate.

The SEM morphology of pure CaO and modified calcium-based sorbents was shown in Fig.6.As illustrated in Fig.6(a),(c)and(e),all fresh sorbents had a porous and loose surface structure.Moreover,the particle size of CaO in the calcium-based sorbents modified by metal doping was smaller than that of pure CaO.The porous structure and small particle size of CaO were beneficial to the gas–solid reaction in the CO2capture process,which increased the carbonation conversion rate of the sorbents.However,it could be clearly seen from Fig.6(b) that the small particles in the pure CaO were completely agglomerated into large particles after ten cycles.In addition,the surface of this sorbent no longer had a loose structure but became tighter,and the pores also became larger,indicating that the sintering phenomenon was very serious.As could be visible in Fig.6(d)and(f),the modified calcium-based sorbents still maintained a loose and porous structure after ten cycles,and the particle size of CaO was still relatively small.Compared with the Ca-4 sorbent,the sintering of the Ca-10 sorbent was severer and the phenomenon of CaO particles agglomeration could be clearly observed from the surface,because the content of metal doped in the Ca-10 sorbent was less.The above phenomenon could explain the change in carbonation conversion rate of sorbents with different metal doping ratios in the previous analysis.In general,the porous structure and loose surface were conducive to the absorption and diffusion of CO2,while sintering could greatly decrease the carbonation conversion rate of the sorbents [23].

The pore properties of fresh and spent sorbents were presented in Table 1.The total specific surface area of the three samples decreased after ten cycles.Among them,the pure CaO had the sharpest drop,followed by Ca-10 and Ca-4 sorbents.However,after ten cycles,the micropore specific surface area of the pure CaO still decreased,while that of Ca-4 and Ca-10 sorbents experienced an upward trend.In terms of total pore volume,the decrease of pure CaO was sharper than that of the other two samples.By analyzing the parameters of the micropores,it can be concluded that the addition of metal dopants could maintain the microstructure of the sorbents during carbonation and calcination [40].Pure CaO without modification had poor resistance to CO2adsorption performance degradation.Conversely,metal doping modification could improve the pore properties of the sorbents after multiple cycles.More importantly,the improvement of the porous structure promoted the carbonation reaction and prolonged the period of rapid carbonation,which finally enhanced the CO2adsorption performance [35].

Table 1 Pore properties of pure CaO and modified calcium-based sorbents

3.3.Effects of adsorption temperature on the cycle stability of calciumbased sorbents

The above analysis identified that the CO2capture performance of Ca-10 sorbent was the best and thereby Ca-10 sorbent was selected to explore the effects of adsorption temperature on the cycle stability of calcium-based sorbents.Fig.7 showed the cycle stability of Ca-10 sorbent with the carbonation at different adsorption temperatures for 20 min under 20% CO2/80% N2atmosphere and the calcination at 900 °C for 15 min under 100% N2atmosphere.As the adsorption temperature rose,the carbonation conversion rate of the Ca-10 sorbent in the first cycle also increased.When the adsorption temperature was higher than that of 700 °C,the carbonation conversion rate of the Ca-10 sorbent in the first cycle showed a downward trend.Obviously,the carbonation conversion curve at 650°C was much lower than that of other adsorption temperatures,and the adsorption effect at 700 °C after the sixth cycle was significantly superior.After ten cycles,with the adsorption temperature elevating from 650 to 750 °C,the total decrease rates of the carbonation conversion rate were 22%,22%,14%,23% and 25%,and the average carbonation conversion rates were 67%,69%,72%,70% and 70%,respectively.No matter from the analysis of carbonation conversion rate or cycle stability,the Ca-10 sorbent had the best CO2capture performance when the adsorption temperature was 700 °C.There were two reasons for the above results.Firstly,the higher adsorption temperature caused severer sintering,resulting in a decrease in the carbonation conversion rate during the cycle.Secondly,the reaction rate between CaO and CO2was boosted as the adsorption temperature rose,which improved the CO2capture performance of the sorbent.However,as the Gibbs free energy of the carbonation reaction increased,the rise of temperature had an opposite effect to the total reaction rate [7].Therefore,the reaction rate could not be boosted by ascending the temperature unboundedly.Anthony et al.[41] investigated the CO2adsorption by Kelly Rock lime at the temperatures range of 350–800°C,demonstrated that reaction rate was a monotone increasing function to temperature.Gupta et al.[42] obtained CaO sorbents by calcining CaCO3,and tested the adsorption rate of CO2at 550,600 and 650°C.They found that the total adsorption rate increased considerately as the temperature increased.Although the reaction rate boosted with the increment of temperature,the influence of Gibbs free energy on the carbonation conversion rate needed to be considered.

3.4.Effects of calcination temperature on the cycle stability of calciumbased sorbents

Fig.8 showed the cycle stability of Ca-10 sorbents with the carbonation at 700 °C for 20 min under 20% CO2/80% N2atmosphere and the calcination at different calcination temperatures for 15 min under 100% N2atmosphere.The calcination temperatures had little effect on the carbonation conversion rate of the sorbents in the first four cycles.After the fifth cycle,the carbonation conversion rates of 925 and 950 °C dropped remarkably compared with those of 850,875 and 900 °C.In the last three cycles,900 °C showed a higher carbonation conversion rate than the other four calcination temperatures,especially higher than 925 and 950 °C.After ten cycles,when the calcination temperature rose from 850 to 950 °C,the total decrease rates of the carbonation conversion rate were 18%,18%,14%,32%and 36%,and the average carbonation conversion rates were 71%,71%,72%,68% and 66%,respectively.Obviously,Ca-10 sorbent had the best CO2capture performance when it was calcined at 900 °C.

Fig.6.SEM images of pure CaO and modified calcium-based sorbents:(a) Ca-0-A;(b) Ca-0-B;(c) Ca-4-A;(d) Ca-4-B;(e) Ca-10-A;(f) Ca-10-B.

In the first four cycles,the carbonation conversion rate of the sample at five different calcination temperatures was similar,because the sintering phenomenon was not severe.As the number of cycles increased,the agglomeration of sorbent particles became severer at high temperatures,resulting in a significant drop in carbonation conversion rates at 925 and 950°C.The carbonation conversion rate at 900 °C was higher than that of 850 and 875 °C,because the rate of desorption of CaCO3was slower when it was calcined at low temperatures,resulting in poor pore properties of the CaO.Therefore,the calcination temperature should not be too high or too low.

Fig.7.Effects of adsorption temperature on carbonation conversion rate of the calcium-based sorbents.

Fig.8.Effects of calcination temperature on carbonation conversion rate of the calcium-based sorbents.

Fig.9.SEM images of Ca-10 sorbent calcined at different temperatures after 10 cycles:(a) 850 °C;(b) 900 °C;(c) 950 °C.

The apparent morphology of Ca-10 sorbents calcined at different temperatures observed by SEM was shown in Fig.9.After ten cycles,the Ca-10 sorbents clearly appeared particle agglomeration and sintering.When calcined at 850 and 900°C,a porous structure was still observed on the surface although the sorbent was sintered.However,the pores of Ca-10 sorbent calcined at 900°C were richer than those at 850 °C.When the Ca-10 sorbent was calcined at 950 °C,its surface after ten cycles became very compact with only a few pores [43].The above phenomenon could explain the changes in the carbonation conversion rate of the Ca-10 sorbents at different calcination temperatures in the previous analysis.

3.5.Effects of CO2 concentration on the cycle stability of calcium-based sorbents

Fig.10 showed the cycle stability of Ca-10 sorbents with the carbonation at 700 °C for 20 min under different CO2concentrations atmosphere and the calcination at 900 °C for 15 min under 100%N2atmosphere.During the previous cycles,as the concentration of CO2elevated,the carbonation conversion rate of the Ca-10 sorbents firstly increased and then basically unchanged.As the number of cycles increased,the gap of carbonation conversion rate between different CO2concentrations became smaller,indicating that the CO2concentration had little effect on the CO2capture performance of Ca-10 sorbents.The reason could be listed as follows:In the first few cycles,the sintering was relatively slight and there were micropores in the sorbents which was difficult for lowconcentration CO2to enter.Therefore,the carbonation conversion rate of the low CO2concentration in the previous cycles was less than that of the high concentration.As the cycle progressed,the sintering of the Ca-10 sorbents became severer,resulting in almost no micropores in the Ca-10 sorbents.This explains that the CO2concentration had little effect on the carbonation conversion rate of the Ca-10 sorbents.

Fig.10.Effects of CO2 concentration on carbonation conversion rate of the calciumbased sorbents.

Fig.11.Effects of calcination atmosphere on carbonation conversion rate of the calcium-based sorbents.

3.6.Effects of calcination atmosphere on the cycle stability of calcium based sorbents

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Fig.11 showed the cycle stability of Ca-10 sorbents with the carbonation at 700 °C for 20 min under 20% CO2/80% N2atmosphere and the calcination at 950 °C for 15 min under different atmosphere.It was obvious that the Ca-10 sorbents calcined in pure CO2had lower carbonation conversion rates than those calcined in pure N2.After ten cycles,the average carbonation conversion rate of Ca-10 sorbents calcined in pure CO2was 54%.This large difference in carbonation conversion rate indicated that calcination under pure CO2atmosphere was not conducive to the cycle stability of calcium-based sorbents [24].The desorption of CaCO3was more difficult in the pure CO2atmosphere,resulting in smaller pores formed in the sorbents.In addition,the calcination temperature would be increased in order to fully decompose CaCO3,which caused more severe sintering of the small pores [22].

3.7.CO2 capture by modified calcium-based sorbents

In order to explore the effects of modified calcium-based sorbent on the removal of CO2from pyrolysis gas,enough Ca-10 sorbents were added into the carbonator and remodeled the carbonator to keep the gas circulating inside,which was to imitate the actual situation in the industrial application.The synthesis pyrolysis gas was circulated in the carbonator at 700 °C for 20 min.Then,the reacted gas was collected by the gas sample bag and analyzed by GC-FID.When the sorbents and reaction time were sufficient,the concentration of CO2in the pyrolysis gas dropped from 37%to 5%and the capture rate was 86%,proving that the modified calcium-based sorbents prepared in this research could be a feasible approach to capture CO2from pyrolysis gas.

4.Research Prospection

Conventional CaO-based sorbents are in the form of fine powder,which is not suitable for practical industrial applications because fine powder is easily elutriated from carbonation and calcination [44].This disadvantage can be overcome by preparing CaO-based sorbent with acceptable attrition resistance and high mechanical strength,which is suitable for the calcium looping processes[45].Pelletization can effectively improve the attrition resistance and mechanical strength of CaO-based sorbent,and commonly used approaches are extrusion,rotation and extrusion-spheronization [46,47].In addition,the cost control of CaO-based sorbent is very important.Limestone and egg shells can be used as calcium precursors for synthetic calcium-based sorbents,so as to reduce the costs of production.In future research,we can consider using cheaper calcium precursors and palletization with the modified method of this paper to explore the application prospects of calcium-based sorbents in practical industrial applications.

5.Conclusions

A modified calcium-based sorbent with excellent CO2capture performance prepared by co-precipitation was introduced.XRD result showed that MgO and CaZrO3were successfully inserted into the sorbents,and CaZrO3was produced by the reaction of ZrO2and CaO at high temperatures.According to the EDS results,MgO and CaZrO3were evenly distributed in the sorbents,which retarded sintering effectively.By exploring the effects of metal doping molar ratio,adsorption temperature and calcination temperature,it could be identified that the Ca-10 sorbent had the best CO2capture performance when the carbonation and the calcination were at 700 and 900°C,respectively.After ten cycles,its average carbonation conversion rate and total decrease rate were 72%and 14%,respectively.In addition,the concentration of CO2had little effect on the cycle stability of the sorbents,but the calcination in pure CO2atmosphere would greatly reduce the carbonation conversion rate of Ca-10 sorbent compared with pure N2atmosphere.Finally,the concentration of CO2in synthesis gas was successfully reduced from 37% to 5% by Ca-10 sorbents.This research could potentially provide theoretical support for the industrial application of efficiently removing CO2from pyrolysis gas through modified calcium-based sorbents.

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

The authors are grateful to the support given by the National Key Research and Development Program of China(2018YFC1901203),Natural Science Foundation of Guangdong Province,China (2021A1515010497),Guangzhou Science and Technology Innovation Development Special Fund,and Fundamental Research Funds for the Central Universities (2019MS017).