Enhancement of CO2 capture and microstructure evolution of the spent calcium-based sorbent by the self-reactivation process

Rongyue Sun,Hongliang Zhu,Rui Xiao

1 School of Energy and Power Engineering,Nanjing Institute of Technology,Nanjing 211167,China

2 Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,Southeast University,Nanjing 210096,China

Keywords:CO2 capture Calcium looping Self-reactivation Microstructure evolution

ABSTRACT The effect of self-reactivation on the CO2 capture capacity of the spent calcium based sorbent was investigated in a dual-fixed bed reactor.The sampled sorbents from the dual-fixed bed reactor were sent for XRD,SEM and N2 adsorption analysis to explain the self-reactivation mechanism.The results show that the CaO in the spent sorbent discharged from the calciner absorbs the vapor in the air to form Ca(OH)2 and further Ca(OH)2 ·2H2O under environmental conditions,during which process the CO2 capture capacity of the spent sorbent can be self-reactivated.The microstructure of the spent sorbent is improved by the self-reactivation process,resulting in more porous microstructure,higher BET surface area and pore volume.Compared with the calcined spent sorbent that has experienced 20 cycles,the pore volume and BET surface area are increased by 6.69 times and 56.3% after self-reactivation when φ=170%.The improved microstructure makes it easier for the CO2 diffusion and carbonation reaction in the sorbent.Therefore,the CO2 capture capacity of the spent sorbent is enhanced by self-reactivation process.A self-reactivation process coupled with calcium looping process was proposed to reuse the discharged spent calcium based sorbent from the calciner.Higher average carbonation conversion and CO2 capture efficiency can be achieved when self-reactivated spent sorbent is used as supplementary sorbent in the calciner rather than fresh CaCO3 under the same conditions.

1.Introduction

Calcium looping process uses calcium based sorbent represented by limestone as sorbent to capture the CO2in flue gas in coal-fired plant [1].Besides limestone,some industrial wastes including lime mud and carbide slag were also used as CO2sorbents in calcium looping process [2].Synthetic sorbents with higher CO2capture capacity were prepared to reduce the amount of the sorbent in the reactor [3].Calcium looping process also can integrate with chemical looping combustion(CaL-CLC)to form a new CO2capture technology that eliminates the requirement for pure O2for the regeneration of CaO-based sorbents [4].And also,simultaneous capture of CO2and SO2from a coal-based combustion power plant can be achieved through calcium looping process[5].This process is achieved in a dual circulating fluidized bed reactor system,including a carbonator and a calciner.Limestone is firstly calcined in the calciner at around 900°C and decomposed to CaO and CO2.The formed CaO is then delivered to the carbonator to capture the CO2in flue gas at around 650°C,at which temperature about 90% CO2capture efficiency is achieved.A CO2free flue gas is obtained at the outlet of the carbonator after gas–solid separation.Then the separated CaCO3formed by carbonation in the carbonator is delivered to the calciner to be calcined again.The energy needed in the calciner is provided by oxy-fuel combustion.Therefore,the CO2concentration in the flue gas at the outlet of calciner can be higher than 95%,at which concentration the CO2can be directly sent for storage.Owing to the utilization of the low cost sorbent and the proven CFB technique as reactor,the CO2capture cost by calcium looping process is relatively lower than that of other CO2capture techniques[6].The major limit for this technology is that the CO2capture reactivity of the calcium based sorbent decreases sharply with cycle number [7],not only for the natural sorbent [8]but also for the other sorbents such as carbide slag[9],especially under severe calcination conditions [10].Therefore,a certain amount of fresh sorbent should be supplemented to the system and the same amount of CaO would be discharged from the calciner to keep a relatively high CO2capture efficiency.For a thermal power plant with a net electric power of 350MWe,450 t of fresh limestone were needed to be supplemented to the system per day when operating at full load [11].Accordingly,252 t of spent sorbent per day would be discharged from the calciner.The discharge of such a large amount of spent sorbent will lead to serious environmental problems if cannot be reused effectively.

Researchers have done lots of works about how to utilize the spent sorbent in environmentally friendly ways.Erans et al.[12]used spent calcium based sorbent from calcium looping process as raw material to make cement.The produced cement showed similar characteristics and performance as those of commercial CEM 1 cement.Chi et al.[13]proved that the spent Mg-stabilized carbide slag discharged from calcium looping process after CO2capture cycles appeared promising to remove HCl.He et al.[14]took spent CaO-based sorbents experiencing dozens of carbonation/calcination cycles under severe calcium looping conditions as SO2sorbent and the results showed that the spent sorbent even had a better performance than the fresh CaO.However,these processes for reusing spent calcium based sorbent need to transit the large amount spent sorbent to other factories firstly,which will lead to high-cost.Therefore,developing the technologies that can reuse the spent sorbent in situ is very significant.Some researchers proposed to reactivate the spent calcium based sorbent and send the reactivated sorbent again to the calcium looping process for CO2capture.

Li et al.[15]treated the spent lime mud by prolonged carbonation,which was proved helpful to retain high CO2capture capacity of the lime mud.Su et al.[16]proposed to reactive the spent dolomite by ball milling with H2O and dry ice,resulting with re-mixed of Ca and Mg at the atomic level and notable activity recovery of the spent dolomite.Steam hydration in a bubbling fluidized reactor was used to reactivate spent cement-supported CO2sorbent pellets for recycle [17].The results showed that superheating treatment optimized the microstructure and enhanced the CO2capacity of the spent pellets,with little effect on the strength of spent synthetic pellets.Our previous research proved that the spent sorbent discharged from calcium looping process can be self-reactivated at environmental conditions through absorbing the H2O in air.If the spent sorbent can effectively reactivated and sent back to the calcium looping process,the cost of the sorbent would be decreased while the spent sorbent can also be effectively reused.The detailed self-reactivation of the spent calcium based sorbent under environmental conditions were investigated in this manuscript.

2.Materials and Methods

2.1.Samples

The calcium based sorbent used here was analytically pure CaCO3(>99%).The CaCO3was sieved to <0.125 mm before the experimental test.The calcium based sorbent after multiple calcination/carbonation cycles,with relatively low carbonation conversion,was identified as spent sorbent.Then,the spent sorbent was put under environmental condition to absorb H2O in air to be reactivated.The mass of the sorbent was weighted by an electronic balance during the self-reactivation period to calculate the selfreactivation degree of the spent sorbent.The water absorption rate φ,which denotes the molar ratio of the H2O absorbed to the CaO that in the spent sorbent,was defined to describe the selfreactivation degree of the spent sorbent,as shown in Eq.(1).

where φ is the water absorption rate,%;mr,tis the mass of the calcined spent sorbent after a t time self-reactivation,mg;ms,calis the weight of the calcined spent sorbent,mg;MCaOand MH2Oare the molar masses of CaO and H2O,g·mol-1.

The reactivated spent sorbent was sent back for CO2capture after the water absorption rate reached at the corresponding value.

2.2.Cyclic calcination/carbonation tests

The cyclic calcination/carbonation tests of the sorbent were accomplished in a dual fixed bed reactor (DFR),which was described in detail elsewhere.The internal diameter of the reactor is 30 mm and the constant temperature zone of the reactor is about 300 mm,which can make sure that the samples stay in the constant temperature zone at any operating mode.The N2and CO2feed were controlled by mass flow controllers and introduced into the reactor.The sample was firstly calcined for 10 min at 850°C in pure N2and then was carbonated at 700°C for 10 min.The sample mass after calcination and carbonation were measured by an electronic balance,and the carbonation conversions of the sorbents were calculated according the mass change during the carbonation and calcination stage,as shown in Eq.(2).

where XNis the carbonation conversion of the sample after N cycles.m0is the initial mass of the sample,mg.mNis the mass of the carbonated sample after N cycles,mg.mcalis the mass of the completely calcined sample (the mass of the sample after each calcination is the same),mg.MCaOand MCO2are molar masses of the CaO and CO2respectively,g·mol-1.A is the content of CaO in the initial sample,%.

2.3.Microstructure analysis

The main phase compositions of the spent sorbents before and after self-reactivation were determined by X-ray diffraction (XRD,AXS-D8 Advance).The collected samples were also sent for SEM studies by a JEOL JSM-6700F instrument.The pore volume and pore area distributions of the calcined spent sorbents with different water absorption rate were examined by a nitrogen adsorption analyzer(Micromeritics,ASAP 2020-M).The pore volume and pore size distribution of the sample were computed by BJH (Barrett-Joyner-Halenda) model.

3.Results and Discussion

3.1.Phase transition of the spent sorbent during self-reactivation

The spent sorbent after multiple calcination/carbonation cycles was placed in the environment for self-reactivation by absorbing the H2O in the air.The value of φ with self-reactivation time was shown in Fig.1 and the phase transition during this period was shown in Fig.2.

The value of φ increases quickly during the initial stage of the self-reactivation.As can be seen in Fig.2,the CaO in the spent sorbent firstly reacts with the H2O in the air to form Ca(OH)2when the value of φ is less than 100%.The CaO almost absolutely reacts with H2O to form Ca(OH)2when the value of φ reaches 100%,with very little CaO detected in the spent sorbent.The formed Ca(OH)2can go on absorbing H2O in the air to form Ca(OH)2·2H2O when the value of φ is larger than 100%,while the mass of the spent sorbent stops going on changing after the value of φ reaches at 170%.This may be due to the pore blockage of the micro pores in the spent sorbent during the self-reactivation process,which leads that the CaO coated in the center of the sorbent cannot react anymore.The value of 170%seems to be the extremity of the self-reactivation process.

Fig.1.The value of φ of the spent Ca-based sorbent with time during the selfreactivation process.

Fig.2.The phase transition of the sorbent during the self-reactivation process.

3.2.Effect of φ on CO2 capture of spent sorbent

The carbonation conversion of CaCO3after 20 calcination/carbonation cycles is only about 0.25,which can be considered as spent sorbent.Then,the spent sorbent was placed under environmental conditions for self-reactivation.The self-reactivated spent sorbent was sent back for CO2capture in the DFR when the value of φ arrived at the set value.The effect of φ on CO2capture of the spent sorbent was shown in Fig.3.

The CO2capture capacity of the spent sorbent was dramatically increased after self-reactivation process and higher value of φ results in higher carbonation conversion of the spent sorbent.The carbonation conversions for the spent sorbents in the first cycle after selfreactivation process are 0.33,0.54,0.61 and 0.74 when the values of φ are 50%,100%,158% and 170%,which are 49%,129%,160%and 217% higher compared with the spent sorbent with no selfreactivation process (the value of φ=0).The self-reactivation process not only increases the CO2capture capacity of the spent sorbent in the following cycle after self-reactivation,but also all the subsequent cycles following that.The carbonation conversions for the spent sorbents in the 10th cycle after self-reactivation process are still 29%,53%,76% and 97% higher than the spent sorbent with no self-reactivation process(the value of φ is 0).

Fig.3.Effect of φ on CO2 capture capacity of the sorbent after self-reactivation process.

Fig.4.Effect of the self-reactivation rate on the CO2 capture capacity of the sorbent.

In order to investigate the effect of the self-reactivation rate on the CO2capture capacity of the spent sorbent,a humidifier was employed to increase the humidity in the air and accelerate the self-reactivation process.It took 4 h and 9 h for the spent sorbent to achieve 100%and 170%water absorption rate under the accelerated condition.The results were shown in Fig.4.It was shown that the cyclic carbonation conversions of the self-reactivated sorbent under accelerated self-reactivation condition were almost the same with those achieved under air condition.It reveals that the self-reactivation rate has little effect on the CO2capture capacity of the self-reactivated sorbent under environmental conditions.

XN,R10,which denoted the total carbonation conversions of the following 10 cycles after self-reactivation,was defined to describe the relationship between water absorption rate and the CO2capture capacity of the spent sorbent,as is shown in Fig.5.The higher the value of φ,the higher the value of XN,R10was achieved,which means higher CO2capture capacity of the spent sorbent after self-reactivation.The functional relationship between XN,R10and φ was obtained through liner fitting to the data in Fig.5,as is shown in Eq.(3).The value of R2was higher than 0.995,which means that the fitting result was accurate.According to Eq.(3)we can know that the XN,R10increases linearly with the value of the φ of the spent sorbent during the self-reactivation process.

Fig.5.The relationship of XN,R10 and φ during the self-reactivation process.

3.3.Microstructure analysis

The calcium based sorbent firstly experienced 20 calcination/-carbonation cycles in the DFR.Then,the spent sorbent with different value of φ after self-reactivation were firstly calcined at 850°C and sent for SEM analysis,as is shown in Fig.6.The samples for SEM were chosen according to the value of φ.A spent sorbent sample (φ=0%),a partial hydrated sample (φ=50%),a total hydrated sample (φ=100%) and a total hydrated sample with partial Ca(OH)2·2H2O (φ=170%) were chosen for SEM analysis.The surface of the sorbent that experienced 20 calcination/carbonation cycles seems dense,with fewer pores distributed due to sintering when calcined at high temperature.The CaO in the spent sorbent reacts with H2O to form Ca(OH)2when the value φ of increases from 0 to 100%.The formed Ca(OH)2will decomposed to CaO and H2O again,during which period new pores will be formed due to the release of H2O,when calcined again.As is shown in Fig.6(a)–(c),more porous microstructures were formed after self-reactivation.The microstructure of the sorbent when the value of φ is 170% seems more porous compared with that when the value of φ is 100%.A certain amount of Ca(OH)2·2H2O will be formed when the value of φ exceeds 100%.The crystal water will be released again when calcined again.It has been approved that the vapor in the calcination atmosphere was helpful to enhance the microstructure of the calcined sorbent [18]and advance the starting point of CaCO3decomposition [19].Therefore,the release of the crystal water in the sorbent after self-reactivation is helpful for the sorbent to form a beneficial microstructure to capture CO2.

The effect of φ on the pore volume and BET surface area of the spent sorbent during the self-reactivation process was analyzed by N2adsorption analysis,as shown in Fig.7.It is obvious that selfreactivation process dramatically improves the pore volume and BET surface area of the spent sorbent.The pore volume and BET surface area of the calcined spent sorbent that has experienced 20 cycles were 0.0166 cm3·g-1and 4.8 m2·g-1.The pore volume and BET surface area were increased by 4.30 times and 29.4%after self-reactivation when φ=100%,and further increased by 6.69 times and 56.3% when φ=170%.The react place afforded for CaO and CO2increased and the CO2diffusion resistance in the sorbent was decreased with increasing the values of pore volume and BET surface area.Also,the pore blockage is less likely to occur during the carbonation stage.Therefore,self-reactivation can effectively enhance the CO2capture capacity of the spent sorbent.

Fig.6.SEM analysis of the calcined sorbent with different φ during the self-reactivation process.(a) φ=0,(b) φ=50%,(c) φ=100%,(d) φ=170%.

Fig.7.Effect of φ on the pore volume of the calcined spent sorbent.

3.4.Self-reactivation coupled with calcium looping process

A self-reactivation process coupled with calcium looping system was proposed to achieve the utilization of the reactivated spent sorbent in calcium looping process,as shown in Fig.8.A self-reactivation field was added into the traditional calcium looping process.Instead of accumulating or landfilling,the spent sorbent discharged from the calciner was sent to the selfreactivation field,where the spent sorbent was reactivated by absorbing the vapor in the air.Here,the exhaust steam in the power plant can be selected and sent to the self-reactivation field to increase the air humidity and enhance the self-reactivation process.There is no exhaust steam under the state of normal operation in the power plant.However,when the safety valve opens or when the unit starts up,there will be some exhaust steam discharged from the boiler or from the turbine side,leading to energy and working fluid loss.These exhaust steam then can be sent to enhance this self-reactivation process.The reactivated sorbent was then sent back to the calcium looping process through the calciner again for CO2capture.The supplementary sorbent for this system can be afforded by the reactivated sorbent,with less limestone needed to compensate the sorbent loss due to abrasion and other reasons.Most cost for the mining and pulverizing of limestone can be saved.

Fig.8.Flow sketch for self-reactivation process coupled with calcium looping.

Fig.9.Long term carbonation performance of reactivated sorbent with φ=170%.

The long-term carbonation performance of the reactivated sorbent and fresh CaCO3was shown in Fig.9.The reactivated sorbent with φ=170% shows similar carbonation performance compared with fresh CaCO3.The reactivated spent sorbent even shows better long-term carbonation performance.An equation was proposed by Li et al.[20]to fit the carbonation conversion curve for calciumbased sorbent as follows:

where b,fmand fware the fitting constants.

Eq.(3) was employed to fit the experimental results of CaCO3and reactivated spent sorbent with φ=170%,and the obtained fitting constants were shown in Table 1.

The CO2capture efficiencyis defined as follows [21]:

where F0is makeup flow rate of fresh sorbent,kmol·s-1;FRis flow rate of recycled sorbent excluding fresh makeup,kmol·s-1;is flow rate of CO2produced by coal combustion entering the carbonator,kmol·s-1;Xaveis average carbonation conversion.

The average carbonation conversion is defined as follows:

where rkis mass fraction of CaO derived from fresh calcium-based sorbent enter in the carbonator in F0+FR(kmol·s-1) after k cycle;Xkis carbonation conversion after k cycles.

Table 1 Fitting constants for reactivated sorbent and CaCO3 according to Eq.(4)

Fig.10.Xave and ECO2 of reactivated spent sorbent and CaCO3 with different F0 /FCO2 and FR /FCO2 .(a) Xave ,(b) ECO2 .

The average carbonation conversion of the calcium-based sorbent can be described by Eq.(7),and the CO2capture efficiency can be calculated by Eq.(8)

The calculated results of Xaveandfor reactivated spent sorbent with φ=170%and CaCO3under different values ofandwere shown in Fig.10.It is obvious that the values of Xaveandfor reactivated spent sorbent were all a little higher than those for CaCO3with the same values ofwhich means better CO2capture performance can be achieved in calcium looping process if fresh limestone was replaced by reactivated spent sorbent.

4.Conclusions

A self-reactivation process coupled with calcium looping process was proposed to reuse the discharged spent calcium based sorbent from the calciner.When placed in the environment,the CaO in the spent sorbent after multiple calcination/carbonation cycles absorbs the vapor in the air to form Ca(OH)2and further Ca(OH)2·2H2O.The self-reactivation process enhances the microstructure of the spent sorbent,forming more pores on the surface of the sorbent grains.The pore volume and BET surface area of the calcined sorbents were increased by self-reactivation process,making it easier for CO2diffusion and carbonation reaction in the sorbent.Therefore,the CO2capture capacity of the spent sorbent was regenerated after self-reactivation process.The CO2capture capacity of the reactivated spent sorbent increases linearly with the value of φ during the self-reactivation process.The value of 170% is the extremity of φ in the self-reactivation process.The self-reactivated spent sorbent with φ=170% even shows higher long-term CO2capture capacity compared with the fresh CaCO3.Higher Xaveandcan be achieved when self-reactivated spent sorbent with φ=170%is used as supplementary sorbent in the calciner rather than fresh CaCO3under the same conditions.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51706094).

Nomenclature

A content of CaO in the initial sample,%

b fitting constants

F0makeup flow rate of fresh sorbent,kmol·s-1

FRflow rate of recycled sorbent excluding fresh makeup,kmol·s-1

fmfitting constants

fwfitting constants

MCaOmolar masses of CaO,g·mol-1

m0initial mass of the sample,mg

mcalmass of the completely calcined sample,mg

mNmass of the carbonated sample after N cycles,mg

mr,tthe mass of the calcined spent sorbent after a t time selfreactivation,mg

ms,calthe weight of the calcined spent sorbent,mg

rkmass fraction of CaO in F0+FR(kmol·s-1) after k cycle in the carbonator

Xaveaverage carbonation conversion

XNcarbonation conversion of the sample after N cycles

XN,R10total carbonation conversions of the following 10 cycles after self-reactivation

φ water absorption rate,%