Tong Luo,Shaolong Liu,Cong Luo*,Xiaolei Qi,Bowen Lu,Liqi Zhang

State Key Laboratory of Coal Combustion,School of Energy and Power Engineering,Huazhong University of Science and Technology,Wuhan 430074,China

Keywords:CO2 capture Calcium precursors Wet mixing synthesis Organic additives

ABSTRACT CaO based sorbents have great potential for commercial use to capture CO2 of power plants.In the demand of producing sorbents with better cyclic performance,CaO-based sorbents derived from different kinds of calcium precursors,containing calcium carbonate (CC-CaO),calcium gluconate monohydrate(CG-CaO),calcium citrate (CCi-CaO) and calcium acetate monohydrate (CA-CaO),were tested cyclically and compared using simultaneous thermal analyzer (STA).And further study was conducted on the sorbents modified with citric acid monohydrate and 50% gluconic acid solution by wet mixing combustion synthesis.The modified sorbents showed better performance and higher pore parameters as well as porous microstructure with more organic acid added.After 20 cycles of carbonation and calcination,the C2CCi8 (CaO:citric acid=2:8 by mass ratio) and C2G8 (CaO:gluconic acid=2:8 by mass ratio) sorbent possess CO2 capture capacity of 0.45 g.g-1 (g CO2 per g sorbents) and 0.52 g.g-1 respectively.The citric acid was more effective for modification than gluconic acid for extended 50 cycles.Furthermore,good linear relationship between CaO conversion and specific surface area as well as pore volume were determined,of which the specific surface area showed closer correlation with CaO conversion.

1.Introduction

Over the centuries,the massive use of fossil fuels,especially coal,petroleum and natural gas,has given sharp rise to the level of greenhouse gas emissions.Among which,the CO2contributes the most of the greenhouse effect [1–3].According to IEA’s report of energy and CO2status in 2019,the total CO2emission of global energy consumption has reached an all-time high value of 33.1 billion tons.Thereinto,the CO2emission generated from coal consumption of power industry has exceeded 10 billion tons [4].On the purpose of lowering the CO2emission level of power plants and postponing the progress of greenhouse effect,the carbon capture and storage(CCS)technologies have come out and been paid a lot of attention to [5–12].A large variety of materials can be used for CCS process,for example,nitrogen-doped carbons [13–15],metal–organic frameworks [16],porous organic polymers [17]and CaO-based materials.The Ca-looping,which depends on the CaO-based materials,has been recognized as one of the most promising method among the CCS technologies.The main mechanism is based on the reversible reaction of CaO and CO2to form CaCO3(carbonation) as well as the decomposition of CaCO3to regain CaO sorbents and high purity CO2flow (calcination),which are operated typically at～650 °C and～900 °C respectively [18].Numerous superiorities can be found on the use of CaO-based sorbents to separate CO2from emission gas of power plants,such as high reserve abundance of natural precursors,low cost,and high CO2capture capacity [19–21].

However,for CaO-based sorbents,there are still problems needed to be solved before applying to practical industrial use.One is that,the carbonation reaction process can be separated into two stages:the initial reaction-control stage with fast carbonation rate and the following diffusion-control stage with relatively slow reaction rate[22].The reaction-control stage lasts for shorter time but contributes more conversion increase.Due to the increasing diffusion resistance for CO2with the formation of product layer covering the CaO sorbent particles,the slow-rate diffusioncontrol stage becomes dominant,which limits the reaction rate.Another problem is that,a phenomenon of drastic decay in CO2capture capacity can be found of the highly cycled CaO-based sorbents [23].This can be attributed to the sintering effect emerging during the high-temperature calcination process,which causes grains agglomeration,reduction of the specific surface area and total pore volume [24–27].Several methods have been put to use in literatures to solve these problems,which are:(1)choosing various calcium precursors for calcination [25];(2) developing new synthesis methods such as precipitated CaCO3(PCC) [19],Sol-Gel method [28–32] or template method [5,33];(3) thermal pretreatment [34,35];(4) hydration treatment [36–38];and (5) modifying CaO-based sorbents with refractory dopants [29,39–43],alkali salts [44–46],biomass [47] or organics [48–50].

It has been reported by many researchers that the CaO sorbents derived from organic precursors possess better CO2capture performance comparing to those derived from CaCO3.This can be explained by the enhanced specific surface area as well as total pore volume during the calcination process after release of H2O and CO2[25].Lu et al.[51]produced CaO sorbents by calcining different calcium precursors.And a high conversion of 97% was obtained for CaO sorbents originated from calcium acetate monohydrate during a 5 h carbonation at 600 °C,far more than those originated from calcium carbonate and calcium hydroxide,of which the conversions are 66% and 63% respectively.Liu et al.[26]conducted thermogravimetric analysis on CaO based sorbents produced by calcining nine different organic calcium precursors.And the result shows that,after nine cycles of carbonation and calcination(650°C,30 min,15%CO2for carbonation;900°C,10 min,100%N2for calcination),the level of carbonation conversion ranks like this (represented by precursors):calcium D-gluconate(83.8%) >nano CaO >calcium citrate >calcium acetate >calcium L-acetate>calcium carbonate>nano calcium carbonate>calcium formate.And sorbent originated from calcium D-gluconate shows even no obvious conversion decay during nine cycles.

Moreover,enhanced performance can be obtained for CaO sorbents by modifying with organics.Li et al.[52]modified limestone with acetic acid solution by acetification and the derived CaO sorbents showed more porous structure,higher surface area and smaller grain size during 20 cycles,which in turn ensure better CO2capture performance and higher resistance to calcination temperature of 920–1120 °C of the modified sorbents.Akgsornpeak et al.[49]synthesized compound CaO sorbents modified with CTAB(cetyl trimethyl ammonium bromide) by sol–gel method.They found that the addition of CTAB could alleviate the agglomeration of particles and give rise to the specific surface area and total pore volume of the sorbents.

Numerous researches have proved that organic materials could improve the performance of CaO based sorbents,whether it is to calcine the organic precursors or to modify with organics.In the previous work of our research group [53],the CaO sorbents modified with citric acid by wet mixing combustion synthesis show enhanced CO2capture capacity,which indicates the addition of organic acid could enhance the performance of CaO sorbents.However,the influence of the variety and the content of organics to pore structure and CO2capture capacity of CaO sorbents still needs to be figured out.Moreover,the comparation of CO2capture performance between the CaO sorbents derived from organic calcium precursors and those modified by adding organic acids is yet to be studied.Therefore,our work presented here studies the influence of different preparation methods,i.e.calcining organic calcium precursors and adding organic acids to CaO sorbents,as well as establishes the relationship between the pore parameters and CaO conversion.

In this paper,we conducted cyclic carbonation/calcination experiments on CaO sorbents derived from calcining several different calcium precursors(both organic and inorganic)as well as CaO based sorbents modified with different ratio of citric acid monohydrate and gluconic acid solution using wet mixing combustion synthesis,which is seldom used to modify CaO sorbents with organic acids.The cyclic CO2capture capacity,BET specific surface area,total pore volume,pore size distribution and microstructure of the modified sorbents were determined.And the correlation between CaO conversion and specific surface area or total pore volume was also determined.

2.Experimental

2.1.Chemicals and preparation methods

Four different precursors were chosen for calcination to produce CaO sorbents,including:calcium carbonate (CaCO3,AR,SCR,abbr.CC),calcium acetate monohydrate(2(C2H3O2)Ca,AR,Aladdin,abbr.CA),calcium citrate (C12H10Ca3O14,AR,Aladdin,abbr.CCi)and calcium gluconate monohydrate (C12H22CaO14,USP,Aladdin,abbr.CG).Certain amounts of precursors were placed uniformly into a square porcelain boat which was put into the muffle furnace later at the temperature gradient of 10°C.min-1to 850°C.And the samples were kept at the temperature of 850 °C for 30 min.The after-calcination samples were grinded to powders for the following tests.

For wet mixing combustion synthesis,the calcium nitrate tetrahydrate (Ca(NO3)2.4H2O,AR,Aladdin) was chosen as calcium precursor.The gluconic acid solution (C6H12O7,49%–53% (mass),Aladdin) and citric acid monohydrate (C12H10Ca3O14,AR Aladdin)were used as additives for modification.

The schematic of wet mixing combustion synthesis is shown in Fig.1:a given amount of calcium nitrate tetrahydrate and organic acid were dissolved in 50 ml deionized water in a glass beaker.The solution was then stirred under the condition of 80 °C in water bath for 4 h until the transparent viscous gelatin was formed.After that,the gelatin was transferred to crucible for the following calcination in muffle furnace.After calcination at the constant temperature of 500 °C for 10 min,the muffle furnace was heated up to 850°C at the heating rate of 5°C.min-1,and kept the temperature for 30 min.After that,the modified CaO sorbents can be obtained.Sorbents with different mass ratios of CaO/organic acid were prepared:60:40,50:50,40:60,30:70 and 20:80.Sorbents modified with citric acid monohydrate were named as:C6CCi4,C5CCi5,C4CCi6,C3CCi7 and C2CCi8 respectively.And for sorbents modified with gluconic acid solution,the abbreviations are:C6G4,C5G5,C4G6,C3G7 and C2G8,which correspond to the mass fraction of CaO/gluconic acid.

2.2.Cyclic carbonation/calcination test

The cyclic carbonation/calcination test was conducted on simultaneous thermal analyzer (STA 2500 Regulus,Netzsch),of which the maximum working temperature is 1600 °C,and the maximum heating rate is 50 °C.min-1.

Before the cyclic test,3–8 mg samples were placed in STA apparatus,which were first calcined at a ramp of 25 °C.min-1,from 20 °C to 850 °C,in a pure N2atmosphere of 100 ml.min-1,and maintained for 10 min to ensure the full decomposition of CaCO3in samples.Afterwards,the temperature was lowered to 650 °C to initiate the cyclic test.And the condition of the cyclic test was as follows:the carbonation reaction proceeded at 650 °C under a gas flow of 100 ml.min-1composed of 60% CO2and 40% N2for 20 min;and the calcination reaction proceeded at 850 °C under pure N2atmosphere of 100 ml.min-1for 10 min.Besides,the heating/cooling rate during the test was 20 °C.min-1.

Based on the data acquired from cyclic test.The CaO conversion(XN) and CO2capture capacity (CN) for Nth cycle can be defined as follows:

Fig.1.The schematic of wet mixing combustion synthesis.

From Eq.(1),XNis equal to the CaO conversion at the Nth cycle,mNrepresents the mass of the sorbents after the Nth carbonation,mnrepresents the mass of the sorbents after the(N-1)th cycle,and m0is equal to the mass of the sorbents after first calcination.WCaOand WCO2represent the relative molecular mass of CaO and CO2respectively.The Eq.(2)defines the sample’s CO2capture capacity CNat the Nth cycle,the unit of which is g CO2per g sorbents(simplified as g.g-1).

2.3.Other characterization methods

The specific surface area,total pore volume and pore size distribution of the samples were determined by N2isothermal adsorption and desorption method combined with BET/BJH methods using the specific surface and pore size analysis instrument (3H-2000PS,Beishide).

And the samples’ microstructure was observed by field emission scanning electron microscope (GeminiSEM300,ZEISS).

3.Results and Discussion

3.1.Effect of calcium precursors on CaO sorbents

The evolution of the CO2capture capacity during 50 carbonation/calcination cycles of the sorbents derived from different precursors above is presented in Fig.2.The typical CaO sorbent derived from CaCO3precursor shows a drastic decay in capture capacity after 50 cycles from 0.54 g.g-1to 0.18 g.g-1.In comparison,better CO2capture performance can be observed from the sorbents derived from organic precursors.CG-CaO and CCi-CaO sorbents show similar decay trends as CC-CaO does but with greater capture capacity and less serious decay,from 0.75 g.g-1to 0.27 g.g-1for CG-CaO and 0.65 g.g-1to 0.19 g.g-1for CCi-CaO,which can be explained by their better pore structure generated from the release of gases in the process of calcination.The CACaO sorbent shows different evolution trend for CO2capture capacity which is due to the phenomenon known as self-reactivation[25,34,54].The self-reactivation phenomenon means the CaO conversion or CO2capture capacity of the sorbents increase ‘‘a(chǎn)bnormally”with increasing cycle number during initial several cycles.The CO2capture capacity of only 0.10 g.g-1is obtained for CACaO at the first cycle.But it goes through a continuous increase due to the self-reactivation effect,and a value of 0.20 g.g-1is obtained at the 50th cycle,which is a little higher than CC-CaO and CCi-CaO.

Fig.2.Cyclic CO2 capture capacity of CaO sorbents derived from different precursors.

Table 1 lists the specific surface area and total pore volume data of the fresh sorbents.It is generally acknowledged that,better carbon capture capacity is in positive correlation with higher specific surface area and pore volume.The CG-CaO possesses the highest value of specific surface area and total pore volume of 16.4348 m2.g-1and 0.1362 ml.g-1respectively.While CA-CaO has the lowest specific surface area and total pore volume,which are 1.5939 m2.g-1and 0.0113 ml.g-1respectively.By ranking these sorbents on the basis of the value of specific surface area and total pore volume,we can get:CG-CaO >CCi-CaO >CC-CaO >CA-CaO,and the corresponding CO2capture capacity at the first cycle of which are 0.75 g.g-1,0.65 g.g-1,0.54 g.g-1,and 0.10 g.g-1respectively.A good correlation can be observed between specific surface area,total pore volume and CO2capture capacity.

Table1 Specific surface area and total pore volume of fresh sorbents derived from different precursors

3.2.Effect of organic additives on CaO sorbents

From the section above,the CG-CaO and CCi-CaO show good performance during the cycles.Further investigation was conducted on the CaO sorbents modified with citric acid and gluconic acid by wet mixing combustion synthesis for comparation.The preparation process is mentioned above.

Fig.3.Cyclic CO2 capture capacity of CaO sorbents modified with citric acid of different mass ratio.

Fig.4.Cyclic CO2 capture capacity of CaO sorbents modified with gluconic acid of different mass ratio.

The evolution of CO2capture capacity with different cycle numbers of CaO sorbents modified with different mass ratio of citric acid and gluconic acid by wet mixing combustion synthesis is presented in Figs.3 and 4 respectively.The data of CaCO3is also drawn in the figure for reference.

From Fig.3,all the sorbents show a similar trend of decay with increasing cycle numbers.Almost all the modified sorbents show better performance than CaCO3except C6CCi4 and C5CCi5,which means the sorbents modified with citric acid show better performance than CaCO3only when the mass ratio of citric acid/CaO is greater than 1.In addition,with the increasing mass fraction of citric acid,the modified sorbents show better performance with higher CO2capture capacity and better stability.The proportion of reduction of CO2capture capacity of C6CCi4,C5CCi5,C4CCi6,C3CCi7 and C2CCi8 during 20 cycles are 51.35%,55.62%,46.84%,35.63% and 35.87% respectively,showing a downward trend on the whole.The deterioration ratios of all the modified sorbents are lower than that of CaCO3i.e.63.61%,which means all the CaO sorbents modified with citric acid show better stability.The C3CCi7 and C2CCi8 sorbents possess the highest CO2capture capacity of 0.43 g.g-1and 0.45 g.g-1at the 20th cycle,which are 1.95 and 2.05 times higher than that of CaCO3respectively.In conclusion,the performance of the modified CaO sorbents can be enhanced with more citric acid added in modification process to a certain degree,and the optimum mass ratio of CaO to citric acid is 2:8.

The graphs of CO2capture capacity vs cycles of CaO sorbents modified with different amount of gluconic acid are shown in Fig.4.Similar regularity can be observed comparing to the sorbents modified with citric acid.For example,no better performance can be observed comparing to CaCO3unless the mass ratio of gluconic acid to CaO exceeds 50%.Moreover,with the increasing mass ratio of gluconic acid added,the modified sorbents show better performance during 20 cycles.After 20 cycles,the C3G7 and C2G8 sorbents maintain the CO2capture capacity of 0.46 g.g-1and 0.52 g.g-1,i.e.2.1 and 2.3 times higher than that of CaCO3respectively.The CO2capture capacity of C6G4,C5G5,C4G6,C3G7 and C2G8 during 20 cycles is reduced by 44.42%,55.94%,53.43%,33.14% and 17.76% respectively,which are all lower than that of CaCO3of 63.63%.In other word,CaO sorbents modified with gluconic acid show better stability during the cycles.

It should be noted that,the self-reactivation phenomenon appears during the initial several cycles for C3G7 and C2G8 sorbents,that is,the CO2capture capacity is enhanced during the initial several cycles.And the highest capture capacity is reached for C3G7 and C2G8 sorbents at the 4th cycle of 0.75 g.g-1and 0.73 g.g-1respectively,which correspond to 95.8% and 93.2% in CaO conversion.On the whole,better performance can be reached for modified CaO sorbents with more gluconic acid added to some extent and the optimum mass ratio of CaO to gluconic acid is 2:8.

Fig.5.TG curves of sorbents during 20 cycles.(a) CaCO3 and C2CCi8 (Ca:citric acid=2:8 by mass ratio);(b) CaCO3 and C2G8 (Ca:gluconic acid=2:8 by mass ratio).

TG curves of CaCO3and C2CCi8 as well as C2G8 sorbents during the whole cyclic test are shown in Fig.5.The difference on their CO2capture performance can be observed clearly.

The carbonation behavior of modified sorbents at the 1st and 20th cycle with different mass ration of Ca/organic additives are presented in Figs.6 and 7.As can be seen,the carbonation reaction of all the sorbents can be divided into two stages,i.e.the reaction-control stage with faster reaction rate and the diffusion-control stage with slower reaction rate.And the CO2captured during the reactioncontrol stage take the main proportion of the process.At the first cycle,15%–30% of the mass increase is attributed to the reactioncontrol stage,while this value is reduced to 5% at the 20th cycle.After 20 cycles,the duration of the reaction-control stage is shortened and the reaction rate of the diffusion-control stage is also reduced,thus leading to the reduction of the CO2capture capacity.

Moreover,the influence of the amount of the organic additives can also be observed.The gaps between the TG curves representing the 1st cycle and the 20th cycle are narrowed with more organic acid added,which means the stability of the sorbents is enhanced.It should be noted that,for sorbents modified with gluconic acid,due to the self-reactivation effect,the C3G7 and C2G8 sorbents show higher reaction rate during the reaction-control stage.Better performance can be obtained consequently.

3.3.Characterization and discussion

In order to figure out the relationship between CO2capture performance and inherent characteristics of different modified sorbents,the microstructure,specific surface area,total pore volume and pore size distribution are determined and discussed in this section.

Fig.6.TG curves of CaO sorbents modified with different amount of citric acid during the 1st and the 20th carbonation stage.(a)C6CCi4;(b)C5CCi5;(c)C4CCi6;(d)C3CCi7;(e) C2CCi8.

Fig.7.TG curves of CaO sorbents modified with different amount of gluconic acid during the 1st and the 20th carbonation stage.(a)C6G4;(b)C5G5;(c)C4G6;(d)C3G7;(e)C2G8.

FSEM images of sorbents’ microstructure.The field emission scanning electron microscope (FSEM) images of fresh sorbents modified with citric acid and gluconic acid are shown in Figs.8 and 9 respectively.The C6CCi4,C5CCi5,C6G4 and C5G5 sorbents,of which the organic acid/CaO mass ratio is no more than 1,show similar microstructure of irregular shape with few pores and large particle size.And serious particle agglomeration phenomenon can be observed.With the increasing mass ratio of additives,the microstructure of the sorbents becomes more porous and flocculent with less degree of agglomeration and smaller particle size,thus providing more active-site for carbonation reaction and reducing the influence of sintering during the repeated carbonation/calcination reactions.

Specific surface area and total pore volume.The specific surface area,pore volume and pore size distribution of fresh and used sorbents (after 10 cycles) modified with different amount of citric acid and gluconic acid were determined by N2isothermal adsorption and desorption method.The N2adsorption and desorption isotherms are presented in Fig.10,and the values of the specific surface area and total pore volume before and after 10 cycles are listed in Tables 2 and 3 respectively.

For sorbents modified with citric acid shown in Table 2,the specific surface area and pore volume of the sorbents increase with the increasing mass ratio of citric acid doped.Thereinto,the pore volume data show approximate linear increase.And the increasing rate of specific surface area decreases a lot when the mass ratio of citric acid doped exceeds 60%.Among these five sorbents,the fresh C2CCi8 sorbent possesses the highest specific surface area and pore volume of 13.9488 m2.g-1and 0.1528 ml.g-1,while the fresh C6CCi4 sorbent has the lowest specific surface area and pore volume of 5.4430 m2.g-1and 0.0691 ml.g-1.Furthermore,all the five sorbents suffer from a slight decrease in specific surface area and pore volume after 10 cycles.The specific surface area of C2CCi8 sorbent is still as high as 12.6400 m2.g-1,which can explain its good CO2capture capacity.

Table2 Specific surface area and total pore volume of fresh and used CaO sorbents modified with different amount of citric acid

Fig.8.FSEM images of fresh CaO sorbents modified with different ratio of citric acid.(a) C6CCi4;(b) C5CCi5;(c) C4CCi6;(d) C3CCi7;(e) C2CCi8.

For sorbents modified with gluconic acid shown in Table 3,a similar increasing trend of specific surface area and pore volume with the increase of gluconic acid can be observed except C2G8 sorbent.The fresh C3G7 sorbent has the highest specific surface area of 16.6556 m2.g-1,and the fresh C4G6 sorbent has the highest pore volume of 0.1637 ml.g-1.While C3G7 sorbent shows better performance comparing to C4G6 at the first cycle,which is probably owing to its higher specific surface area.After 10 cycles,the specific surface area and pore volume decrease of all these sorbents.Considering the proportion of decrease in specific surface area and pore volume,C3G7 and C2G8 sorbents suffer from less decrease in pore volume than specific surface area.It can be explained that the self-reactivation phenomenon probably has greater influence to pore volume.

Table3 Specific surface area and total pore volume of fresh and used CaO sorbents modified with different amount of gluconic acid

Pore size distribution.Fig.11 shows the pore size distribution of the modified sorbents.For sorbents modified with citric acid,in Fig.11(a),all the sorbents mainly consist of mesoporous,and the pore size distribution is bimodal between 2–6 nm and 30–60 nm.Fig.11(b)presents the change of the pore size distribution of C4CCi6 sorbent before and after 10 cycles,from which we can see the pores within 2.5–25 nm decrease and the pores in the vicinity of 40 nm increase a lot.This can be explained by the sintering effect that makes the pores larger,which deteriorates the performance of the sorbents.

While for sorbents modified with gluconic acid,in Fig.11(c),the pore size distribution of C4G6,C5G5 and C6G4 sorbents present bimodal curves near 3 nm and 60 nm,while the C4G6 and C5G5 sorbents show approximate unimodal curves near 3 nm.With the increase of gluconic acid added,the peak near 3 nm is increasing (except C2G8).Fig.11(d) shows the pore size distribution of C2G8 sorbent before and after 10 cycles.The C2G8 sorbent after 10 cycles shows a bimodal curve comparing to fresh C2G8,of which the pore size distribution is unimodal.After 10 cycles,the peak intensity between 2–10 nm reduces and the peak intensity between 10–50 nm increases,which indicates the pores are enlarged due to the sintering effect.However,no obvious decay in the capture capacity of C2G8 during the first 10 cycles owing to the self-reactivation effect.The average pore size of fresh and 10 cycle’s C2G8 sorbents are 25.57 nm and 28.52 nm respectively.This minor increase in pore size could be explained by the micropores generated from the self-reactivation effect,which counteracts the negative influence brought by enlargement of pores.

Fig.9.FSEM images of fresh CaO sorbents modified with different ratio of gluconic acid.(a) C6G4;(b) C5G5;(c) C4G6;(d) C3G7;(e) C2G8.

Fig.10.The N2 adsorption and desorption isotherms of the sorbents modified with different amount of citric acid (a) and gluconic acid (b).

Correlation between CaO conversion and pore parameters.Linear fitting was performed for CaO conversion and pore parameters to determine the correlation of these data.The data include the specific surface area and total pore volume of the fresh sorbents and the used sorbents(after 10 cycles)as well as the corresponding CaO conversion both in chemical-control stage (1 min in carbonation time) and the whole carbonation stage.The results are presented in Figs.12 and 13,which correspond to the data of sorbents modified with citric acid and gluconic acid respectively.In Fig.12,good correlation can be observed between the CaO conversion and specific surface area both in chemical control stage andthe whole carbonation stage,the R2of which are 0.94133 and 0.81618 respectively.The pore volume shows less correlation with CaO conversion of 0.81681 and 0.69933 comparing to specific surface area in chemical control stage and carbonation stage respectively.

Fig.11.Pore size distribution of the sorbents.(a) fresh CaO sorbents modified with citric acid;(b) the fresh and used C4CCi6 sorbent(after 10 cycles,abbr.C4CCi6-10);(c)fresh CaO sorbents modified with gluconic acid;(d) the fresh and used C2G8 sorbent (after 10 cycles,abbr.C2G8-10).

Fig.13 shows the results of sorbents modified with gluconic acid.The solid lines represent the fitted line with the data of C2G8 and C3G7 sorbents,which show self-reactivation phenomenon during the initial several cycles.And the dashed lines are the fitted line which exclude the data of C2G8 and C3G7 sorbents.As can be seen,the correlation is quite weak when the data of C2G8 and C3G7 are taken into consideration.This is probably due to the enhanced specific surface area and total pore volume of the sorbents caused by self-reactivation effect,thus making the data unusual.The comparison between the results of reaction-control stage and the whole carbonation stage indicates that the chemical-control stage is more susceptible to selfreactivation effect.

Fig.12.Correlation between CaO conversion and specific surface area as well as pore volume of sorbents modified with citric acid.(a,c)reaction-control stage;(b,d)whole carbonation stage.

3.4.Comparison of sorbents’ performance for extended cycles

In order to figure out long-term performance of CaO sorbents modified with citric acid and gluconic acid,the C3CCi7,C2CCi8,C3G7 and C2G8 sorbents,which show the best performance during the cyclic tests mentioned above,are chosen for an extended 50 cycles carbonation/calcination test with severe condition.CaCO3in the sorbents was decomposed completely before the test.A severe experimental condition of cyclic test was chosen:the carbonation and calcination process both proceeded at 800 °C for 5 min under the gas flow of 100 ml.min-1and the gas components were 60%CO2and 40%N2for carbonation and 100%N2for calcination.For comparation,CaO sorbents derived from calcium citrate and calcium gluconate were also tested.The results are shown in Fig.14.

As can be seen,all the sorbents show a similar trend of decay with increasing cycle numbers.The sorbents originated from organic acid calcium precursors show worse performance comparing to those modified with organic acid after 4 cycles.The CCi-CaO sorbent has almost the same CO2capture capacity with C3CCi7 and C2CCi8 sorbents at the first cycle but goes through severe deterioration during 50 cycles with 0.19 g.g-1at the 50th cycle.While this value is as high as 0.36 g.g-1and 0.38 g.g-1for C3CCi7 and C2CCi8 sorbents separately,which indicates the CO2capture capacity and cyclic stability of the sorbents can be enhanced by adding citric acid for modification.The CG-CaO possesses higher initial CO2capture capacity than C3G7 and C2G8 sorbents only in the first cycle.Fast decay in capture capacity can be observed for CG-CaO during 50 cycles and a value of 0.27 g.g-1was obtained at the 50th cycle.The sorbents modified with gluconic acid show enhancement in capture capacity and stability to a certain degree,and value of 0.29 g.g-1and 0.35 g.g-1was obtained for C3G7 and C2G8 sorbents at the 50th cycle.

Under the condition in this section,only the C2G8 sorbent shows self-reactivation phenomenon during the initial cycles with the highest capture capacity of 0.66 g.g-1at the 5th cycle.The C3G7 sorbent shows the highest capture capacity than other sorbents during the initial cycles but also shows faster decay during the following cycles.The sorbents modified with citric acid(C3CCi7 and C2CCi8) show no higher capture capacity than those modified with gluconic acid (C3G7 and C2G8) during the initial several cycles.However,better stability is shown during the remaining cycles.In conclusion,the performance of all the sorbents here can be ranked by the following order:C2CCi8>C3CCi7>C2G8>C3G7>CG-CaO>CCi-CaO.We can draw a conclusion that,the addition of citric acid and gluconic acid can enhance the cyclic performance of the CaO sorbents both in capture capacity and cyclic stability.Better performance can be obtained for sorbents modified with organic acid by wet mixing combustion synthesis comparing to the sorbents derived from calcining homogeneous organics.In addition,the sorbents modified with citric acid show slightly better performance than that with gluconic acid.

Fig.13.Correlation between CaO conversion and specific surface area as well as pore volume of sorbents modified with gluconic acid.(a,c) reaction-control stage;(b,d)whole carbonation stage.

Fig.14.CO2 capture capacity of different sorbents for extended cycles.

4.Conclusions

In this work,CaO-based sorbents modified with different amount of citric acid and gluconic acid by wet mixing combustion synthesis were prepared and the sorbents produced by calcining different calcium precursors were also compared.The result of cyclic test shows that better CO2capture performacne(CO2capture capacity and cyclic stability)of the sorbents can be obtained with more cirtic acid or gluconic acid added,and the optimum mass ration of organic acid/CaO is 4.No better performance is observed comparing to CaCO3when the mass ratio of organic acid/CaO is lower than 1.With more organic acid added,the sorbents’specific surface area and total pore volume show incerasing trend,and the microstructure becomes more porous and flocculent.For extended mulit-cycle test,the sorbents modified with citric acid show slightly betterperformacne than those modified with gluconic acid and they all show better performacne than sorbents derived from calcining organic calcium percursors.Good correlation between CaO conversion(both in chemical-control stage and whole carbonation stage)and specific surface area or total pore volume are determined,of which the specific surface area shows closer linear dependence with CaO conversion.

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 National Natural Science Foundation of China (51606076),and Analytical and Testing Center of HUST for FSEM measurements.

Nomenclature

CNCO2capture capacity,g.g-1

mNmass of the sorbents after the Nth carbonation,g

mnmass of the sorbents after the (N-1)th cycle,g

m0mass of the sorbents after the first calcination,g

WCaOrelative molecular mass of CaO

WCO2relative molecular mass of CO2

XNCaO conversion at the Nth cycle