Wan Zhang, Yingjie Li,*, Yuqi Qian, Boyu Li, Jianli Zhao, Zeyan Wang

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

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

Keywords:Sorbents Carbon monoxide CO2 capture NO removal Carbonation stage Calcium looping

ABSTRACT Calcium looping realizes CO2 capture via the cyclic calcination/carbonation of CaO.The combustion of fuel supplies energy for the calciner.It is unavoidable that some unburned char in the calciner flows into the carbonator,generating CO due to the hypoxic atmosphere in the carbonator.CO can reduce NO in the flue gases from coal-fired power plants.In this work, NO removal performance of CO in the carbonation stage of calcium looping for CO2 capture was investigated in a bubbling fluidized bed reactor.The effects of carbonation temperature, CO concentration, CO2 capture, type of CaO, number of CO2 capture cycles and presence of char on NO removal by CO in carbonation stage of calcium looping were discussed.CaO possesses an efficient catalytic effect on NO removal by CO.High temperature and high CO concentration lead to high NO removal efficiency of CO in the presence of CaO.Taking account of better NO removal and CO2 capture,the optimal carbonation temperature is 650°C.The carbonation of CaO reduces the catalytic activity of CaO for NO removal by CO due to the formation of CaCO3.Besides, the catalytic performance of CaO on NO removal by CO gradually decreases with the number of CO2 capture cycles.This is because the sintering of CaO leads to the fusion of CaO grains and blockage of pores in CaO, hindering the diffusion of NO and CO.The high CaO content and porous structure of calcium-based sorbents are beneficial for NO removal by CO.The presence of char promotes NO removal by CO in the carbonator.CO2/NO removal efficiencies can reach above 90%.The efficient simultaneous NO and CO2 removal by CO and CaO in the carbonation step of the calcium looping seems promising.

1.Introduction

Global CO2emissions have increased dramatically over the previous decade,which grew by about 1%per year[1].In 2019,global energy-related CO2emissions flattened at around 33 Gt[2].In particular, CO2emissions mainly emit from fossil fuel burning [3].Therefore,coal-fired power plants have been deemed to be the significant objects that need to be retrofitted by the technologies for mitigating CO2emission [4,5].As one of the most promising technologies for the large-scale CO2capture,calcium looping has a series of advantages such as low cost, high efficiency and abundant resources of sorbents [6,7].Calcium looping realizes CO2capture via the calcination/carbonation cycles of calcium-based sorbents[8,9].Calcium-based materials such as limestone and dolomite are applied in the calcium looping as CO2sorbents.First, the calcium-based sorbent is calcined in a calciner (about 800-950 °C), where CaCO3in the sorbent is decomposed into CaO and CO2.Then CaO is transported into a carbonator (about 600-700°C),where CaO absorbs CO2from the flue gas and forms CaCO3.The generated CaCO3is recycled into the calciner for the next calcination/carbonation cycle.The enriched CO2in the calciner could be directly sealed[10-12].The fluidized bed reactors are utilized in the calcium looping.During the calcium looping,fresh sorbents are complemented into the calciner while the disused sorbents are expelled from the carbonator.

Coal-fired power plants are also the primary sources of anthropogenic NOxemissions, which cause serious environmental pollutions [13].Stringent regulations have been made for controlling NOxemissions, so a number of NOxcontrol technologies such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and hybrid SNCR-SCR had been investigated [14-16].At present, NH3-SCR using NH3as the reducing agent is the major technology applied in the power plants.However, the high-cost catalysts and the leak of ammonium hydroxide are the main problems of NH3-SCR that need to be urgently solved [17].Compared with NH3,CO has some advantages such as the lower cost and less toxicity[18,19].Thus,CO as the reductant for NO reduction is suitable.Many researchers reported the good NO removal abilities of CO with the catalysis of noble mental like Pt [20]and Au [21].However, the noble metals applied in the catalysts increase the cost of NO reduction in the industrial applications.Therefore,more attentions should be focused on the relatively cheap catalysts.Liao et al.[22]tested NO reduction by CO in the presence of CaO in a fluidized bed at 600-1000 °C and found that NO removal efficiency was about 75% with CaO catalysis.Allen et al.[23]also found that NO removal efficiency of CO in the presence of CaO reached approximately 90% at 850 °C.The cheap and abundant calciumbased materials such as limestone and dolomite could provide the sources of CaO.Therefore, CaO as a catalyst for NO removal by CO seems promising.

The decomposition of CaCO3in the calciner of the calcium looping is an endothermic reaction,and the combustion of fuel such as the coal and biomass supplies energy for the calciner [24].It is unavoidable that some unburned char in the calciner flows into the carbonator in the calcium looping process, generating CO and CO2due to the char oxidation [25-27].Gao et al.[27]found that CO concentration generated by char oxidation was up to 1.2% as O2concentration in the carbonator was 3%-4%.Shimizu et al.[28]tested NO removal performance of unburned char in the carbonator and they found CO promoted NO removal by char in the carbonator, but the NO removal efficiency was relatively low due to the tiny amount of char and CO.Our previous work [29,30]found that NO removal efficiency of char increased by 16% in the presence of CO and CaO in the carbonator.However, the NO removal performance of CO in the carbonation stage of the calcium looping has been ignored and unclear.In addition, if the amount of unburned char could be controlled within a reasonable range, the amount of CO generated by the unburned char oxidation will be high enough for NO reduction under the catalysis of CaO in the carbonator.Therefore,the simultaneous NO removal and CO2capture probably occur in the carbonator.

In this work,NO removal performance of CO in the carbonation stage of the calcium looping for CO2capture was investigated in a bubbling fluidized bed reactor.The effects of the carbonation temperature, CO concentration, CO2capture, type of CaO, number of CO2capture cycles and presence of char on NO removal by CO in the carbonation stage of the calcium looping were tested.In addition, the effect mechanism of the calcium looping on NO removal by CO was determined.

2.Experimental

2.1.Materials

Two limestones were purchased from limestone mines in Henan and Shandong provinces, China, respectively.The dolomite was sampled from a dolomite mine in Liaoning province, China.They were crushed and screened to a particle size between 0.125-0.180 mm, respectively.An X-ray fluorescence (XRF, ZSX Primus II) was used to analyze these calcined samples, as shown in Table 1.The two calcined limestones were denoted by L1-CaO and L2-CaO, respectively.

The coconut shell sampled from Guangdong province, China,was pyrolyzed for 90 min at 900 °C under 100% N2in a tube furnace.After the pickling for ash removal,the obtained coconut shell char was crushed and screened to a particle size between 0.125-0.180 mm.An Elemental Analyzer (EA, Elementar Vario EL) was used to analyze the elementary of the coconut shell char and the results were shown in Table 2.

Table 1 Chemical components of calcined samples

Table 2 Elementary analysis of coconut shell char on air dry basis (%,by mass)

2.2.NO removal test of CO in carbonation stage of calcium looping

The NO removal performance of CO in the carbonation stage of the calcium looping was examined in a bubbling fluidized bed reactor (BFBR), as shown in Fig.1.A quart tube with the inner diameter of 32 mm and the height of 900 mm was used as the furnace of BFBR.In the tube, there was a perforated distributor plate locating at the bottom of the isothermal region (L = 200 mm) of BFBR.BFBR was heated by the electrical heating.The reaction gases controlled by the mass flowmeters(Flowmethod FL-802)were sent to BFBR by the pipelines heated to about 300 °C by the electric heating bands.The critical fluidization velocities of samples were determined by the descending velocity method in BFBR at 650 °C.The fluidization velocity was 0.018 m·s-1for the carbonation stage.The total flow rate of the reaction gas in the carbonation step was about 0.14 m·s-1to ensure that all samples were under the bubbling fluidization.Before the NO removal experience, the limestone or dolomite was calcined at 850 °C in 100 % N2atmosphere, in which the total flow rate of the reaction gas was about 0.12 m·s-1.A Testo 350 flue gas analyzer was used to record CO2concentration in the exhaust gas from BFBR.When CO2concentration tested by the flue gas analyzer was zero, the temperature of BFBR decreased to room temperature.The calcination of the samples was finished.

Fig.1.A bubbling fluidized bed reactor system.

In the simultaneous NO/CO2removal test,16 g calcined samples were added into the hopper.When BFBR reached the carbonation temperature(600-700°C),the sample was added into the furnace.Meanwhile, the atmosphere was switched to the reaction atmosphere containing 0 or 15% CO2, 0.05% NO, 0.05% O2, 0-3% CO and N2balance.The flue gas analyzer recorded concentrations of CO, NO and CO2in the exhaust gas from BFBR.The carbonation duration was 15 min.The first CO2capture cycle was finished according to the abovementioned procedure.Then BFBR was heated to 850 °C under 100 % N2atmosphere to begin the next CO2capture cycle.The calcination was finished until CO2concentration recorded by the flue gas analyzer was zero.The cycled calcination/carbonation was repeated according to the abovementioned procedure.The empty bed experiments were also performed according to the abovementioned procedure except for adding the samples.In order to examine the effect of char on NO removal by CO in the carbonation stage of the calcium looping,the mixture of char and CaO with the mass ratio of char/CaO of 1:16 was added into BFBR containing 15% CO2, 0.05% NO, 0.05%O2,1.5%CO and N2balance at 650°C.The experimental procedure was the same as the abovementioned one.

The NO removal efficiency,CO2capture efficiency and carbonation conversion of CaO were defined by Eqs.(1),(2)and(4),respectively, as follows:

2.3.Characterization

The microstructures of the fresh and cyclic calcined samples were observed by a scanning electron microscopy (SEM, JEOL JSM-7600F).The surface areas and pore volumes of the samples were measured by a nitrogen adsorption analyzer (Micromeritics ASAP 2020-M).The surface areas and pore volumes were computed by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively.

3.Results and Discussion

3.1.Effect of reaction temperature on NO removal by CO in presence of CaO

It is generally accepted that the carbonation temperature of the calcium looping is usually between 600-700 °C [31,32].The effect of the reaction temperature (600-700 °C) on NO removal performance of CO during the carbonation process is presented in Fig.2.As exhibited in Fig.2(a), NO concentration in the exhaust gas from empty bed is 0.05%, which is the same as NO concentration in the original reaction gases.It means that NO is not reduced by CO at 600-700°C in the absence of CaO.In the presence of CaO,NO concentrations in the exhaust gas decrease from 0.05%to about 0.0366%,0.0269%and 0.0010% (t=50 s)at 600,650 and 700°C at first,respectively,but they continually increase as the carbonation proceeds until they reach about 0.05%.NO removal efficiencies remarkably increase from 36% to 97% with increasing the temperature from 600 to 700 °C at first but all of NO removal efficiencies decrease to zero at the end of the carbonation stage.It demonstrates that CaO possesses a good catalytic effect on NO removal by CO and the catalytic activity of CaO on NO removal by CO is enhanced by high temperature.However, the catalytic activity of CaO gradually decreases as the carbonation of CaO proceeds.The carbonation of CaO includes two stages: a chemical reaction controlling stage and a product layer controlling stage.Considering the short residence time of sorbents in the fluidized bed carbonator, the chemical reaction controlling stage of the carbonation is more meaningful for industrial application [9].As shown in Fig.2(b), CO2concentrations in the exhaust gas remain at relatively low values in the chemical reaction controlling stage and then they gradually increase with the reaction time in the product layer controlling stage.CO2concentration in the exhaust gas in the chemical reaction controlling stage of the carbonation increases from about 1.10%to 4.30%with increasing the temperature from 600 to 700°C,that is to say, CO2capture efficiency decreases from 95% to 75%.Limited by the chemical equilibrium,the suitable carbonation temperature should not be higher than 650 °C to achieve a high CO2capture efficiency.Thus, the optimal carbonation temperature is 650 °C for the simultaneous CO2and NO removal.

Fig.2.Effect of reaction temperature on NO removal by CO in presence of CaO:(a)NO concentration,(b)CO2 concentration,(650°C,0.05%O2/1.5%CO/0.05%NO/15%CO2/N2 balance, sample: L1-CaO).

3.2.Effect of CO2 capture by CaO on NO removal by CO

The effect of the carbonation of CaO on NO removal by CO in the carbonator is shown in Fig.3.It is found that NO concentration in the exhaust gas gradually increases as the carbonation reaction of CaO proceeds and it finally reaches 0.05% at the end of the chemical reaction controlling stage.NO removal efficiency is about 55% at the beginning of the carbonation (t ≤50 s) but sharply decreases to zero with the reaction time.However, NO removal by CO is stable in the absence of CO2and NO removal efficiency remains at about 71% in 900 s.It means that the carbonation of CaO results in a decline in NO removal efficiency of CO.CaO is converted into CaCO3as a result of the carbonation of CaO.The compact CaCO3product layer covers the surface of CaO, so the catalytic activity of CaO on NO removal by CO decreases.In the realistic application,CaO is continually flowed into the carbonator,so it will guarantee enough active sites in fresh CaO to realize efficient catalysis for NO removal by CO.

Fig.3.Effect of carbonation of CaO on NO removal by CO, (650 °C, 0.05% O2/1.5%CO/0 or 15% CO2/0.05% NO/N2 balance, sample: L1-CaO).

3.3.Effect of CO concentration on NO removal in presence of CaO

Fig.4 presents the effect of CO concentration in the range of 1.5%-3% on NO reduction by CO in the presence of CaO.As presented in Fig.4(a), NO concentration in the exhaust gas decreases from about 0.0269% to 0.0028% (t = 50 s) with increasing CO concentration from 1.5% to 3%.NO removal efficiency sharply increases from about 55% to 95% (t = 50 s).High CO concentration accelerates NO removal by CO under the catalysis of CaO [33,34].When CO concentration is below 2.5%, NO removal efficiency of CO sharply increases with increasing CO concentration in the presence of CaO, while high CO concentration above 2.5% has lower effect on NO removal efficiency.In order to achieve high NO removal efficiency,high CO concentration in the carbonation is still essential.As shown in Fig.4(b),as CO concentration increases from 1.5% to 3%, CO2concentration in the exhaust gas remains stable and CO2capture efficiency retains approximately 90%.The change in CO concentration barely affects CO2capture by CaO.

3.4.Effect of type of CaO on NO removal by CO in carbonation of CaO

The effect of the type of CaO derived from calcium-based sorbents on NO reduction by CO are compared in Fig.5.As plotted in Fig.5(a), NO concentration in the exhaust gas (t = 50 s) under the catalysis of calcined dolomite is about 0.0313%, which is 1.16 and 3.82 times as high as that under the catalysis of L1-CaO and L2-CaO, respectively.NO removal efficiency with the catalysis of calcined dolomite is only 46% (t = 50 s).NO concentration in the exhaust gas with the catalysis of L2-CaO is 70% lower than that under the catalysis of L1-CaO (t = 50 s).NO removal efficiency of CO under the catalysis of L2-CaO is up to 86%(t=50 s).It suggests that the catalytic activity follows the order:L2-CaO ＞L1-CaO ＞cal cined dolomite.As exhibited in Fig.5(b), CO2concentrations using L1-CaO and L2-CaO in the exhaust gas in the chemical reaction controlling stage are 35% and 22% lower than that using calcined dolomite, respectively.The duration of the chemical reaction controlling stage of the carbonation of L2-CaO is 45%longer than that of L1-CaO.It suggests that the CO2capture capacity follows the order: L2-CaO ＞L1-CaO ＞calcined dolomite.The differences in the catalytic performance and CO2capture performance of calcined limestone and dolomite are caused by the differences in content of CaO in samples.As shown in Table 1, CaO content in limestone is much higher than that in the dolomite.Higher CaO content in limestone results in better CO2capture capacity and catalytic activity.The pore size distributions of different types of CaO are exhibited in Fig.6.Three kinds of CaO have the similar pore size distribution trends.The volumes of pores in the diameter range of 20-100 nm in the samples follow the order: calcined dolomite ＞L2-CaO ＞L1-CaO.The surface areas and pore volumes of samples are shown in Table 3.The surface area and pore volume of L2-CaO are 44% and 17% higher than those of L1-CaO, respectively.It suggests the calcium-based sorbent possessing a high CaO content and a porous structure shows a high catalytic activity for NO removal by CO.It is also be speculated that efficient NO removal by a lower concentration of CO is probably realized in the presence of more porous CaO.

Fig.4.Effect of CO concentration on NO removal by CO in presence of CaO:(a)NO concentration,(b) CO2 concentration, (650°C,0.05%O2/1.5-3%CO/0.05%NO/N2 balance,sample: L1-CaO).

Fig.5.Effect of CaO type on NO removal by CO with CaO catalysis:(a)NO concentration,(b)CO2 concentration,(carbonation:650°C,0.05%O2/1.5%CO/0.05%NO/15%CO2/N2 balance, calcination: 850 °C, 100 % N2).

Fig.6.Pore size distributions of different types of CaO.

3.5.Effect of number of CO2 capture cycles on NO removal by CO

The effect of the number of CO2capture cycles(i.e.calcination/-carbonation cycles)on NO removal by CO in the carbonation stage is shown in Fig.7.As illustrated in Fig.7(a), NO concentration in the exhaust gas in the 1st cycle maintains below 0.006% for about330 s,and NO removal efficiency is above 90%in 330 s.However,as the number of cycles increases to 5, 10 and 20, NO concentrations in the exhaust gas at 330 s respectively increase to 0.0206%,0.0290% and 0.0366%, so the corresponding NO removal efficiencies are respectively 65%, 49% and 34%.It means that the catalytic activity of CaO on NO removal by CO declines with the number of CO2capture cycles.As presented in Fig.7(b),CO2concentrations in the exhaust gas in the chemical reaction controlling stage of the carbonation remain at the same values, in which CO2capture efficiencies are about 90%, but the durations of the chemical reaction controlling stage decrease from about 550 s to about 190 s as the number of CO2capture cycles increases from 1 to 20.The cyclic carbonation conversions of CaO in 900 s are calculated according to eq (4).As presented in Fig.7(c), as the number of cycles increases from 1 to 20, the carbonation conversion of L1-CaO decreases from 53.6% to about 29.4%.The CO2capture capacities of different limestones were tested by the fluidized bed reactors in Refs.[35-38], and corresponding carbonation conversions are also exhibited in Fig.7(c).The reaction conditions are shown in Table 4.The cyclic CO2capture capacity of L1-CaO is similar to that of the limestone reported in the reference[37]but it is higher than the limestones reported in the references[35,36,38].The carbonation conversion of limestone varies with different CO2capture conditions and particle sizes.The carbonation conversions of all the limestones decrease with the number of cycles due to the increasingly serious sintering of CaO, resulting in the decrease in the porosity of the sorbents [39,40].The diffusion of CO2, CO and NO in CaO becomes difficult, which hinders the catalysis of CaO on NO removal by CO and CO2capture by CaO.For the industrial application, fresh CaO should be continually added into the carbonator to maintain the efficient catalysis of CaO on NO removal by CO and CO2capture, so the high simultaneous NO and CO2removal efficiencies can be achieved.

Table 3 Surface areas and pore volumes of different sorbents

Table 4 Reaction conditions in Refs.[35-38]

Fig.7.Effect of cycle number on NO removal by CO with CaO catalysis:(a)NO concentration,(b)CO2 concentration,(c)carbonation conversion of CaO,(carbonation:650°C,0.05% O2/3% CO/0.05% NO/15% CO2/N2 balance, calcination: 850 °C, 100 % N2, sample: L1-CaO).

3.6.Effect of char on NO removal by CO in carbonation of CaO

The effect of the char on NO removal by CO in the carbonation of CaO is shown in Fig.8.The reaction gas contains 0.05%O2/1.5%CO/15% CO2/0.05% NO/N2balance.As shown in Fig.8(a), when char is absent in BFBR, NO concentration in the exhaust gas remainsbelow 0.026%in 50 s,and NO removal efficiency is above 55%.After 50 s, NO concentration in the exhaust increases with the reaction time.In the presence of char, although NO concentration in the exhaust gas still increases with reaction time, NO concentration in the exhaust gas is lower than that in the absence of char and the duration realizing NO removal efficiency of above 55% is 275 s, which is above 5.5 times as long as that in the absence of char.The addition of char improves the NO removal efficiency in the carbonator.This is because the char not only reduces NO but also supplies active sites for NO removal by CO [41].As presented in Fig.8(b),CO2concentration in the exhaust gas in the presence of char remains at the same value with that in the absence of char,and CO2capture efficiency is about 90%.It means the addition of char in BFBR barely affects CO2capture efficiency of CaO.The unburned char from the calciner not only generates CO for NO removal but also plays a positive effect on NO removal in the carbonator before being totally oxidized.

Fig.8.Effect of char on NO removal by CO in presence of CaO: (a) NO concentration, (b) CO2 concentration, (650 °C, 0.05% O2/1.5% CO/ 15% CO2/0.05% NO/N2 balance).

3.7.Microstructure and mechanism analysis

The SEM images of fresh CaO and CaO after 10 calcination/carbonation cycles are presented in Fig.9.As shown in Fig.9(a), the surface of fresh CaO seems porous,which is beneficial to the diffusion of CO and NO in CaO,so CaO has a high catalytic activity on NO removal by CO.Compared with fresh CaO, the average size of CaO grains after 10 cycles becomes larger, which is attributed to the sintering of CaO,as illustrated in Fig.9(b).As the number of cycles is raised from 0 to 10,the large pores in CaO increase and the small pores gradually decrease.The growth and the fusion of CaO grains become serious with the number of cycles, which probably increase the diffusion resistance of CO and NO in CaO.This is not conducive to the catalysis of CaO on NO removal by CO.The surface areas and pore volumes of the fresh CaO and CaO after 10 cycles are exhibited in Fig.10.It is found that as the number of cycles is raised from 0 to 10, the surface area and pore volume of CaO drop by 50% and 55%, respectively.The sintering of CaO during the CO2capture cycles decreases the porosity of CaO with the number of cycles.This results in tiny NO and CO diffusion into the inner CaO,so the catalytic activity of CaO on NO removal by CO declines with the number of cycles.

Kadossov et al.[42]calculated the adsorption of CO on the surface of CaO(1 0 0)according to density functional theory,and they found the absolute values of the binding energies were quite small(0.028-0.126 eV), indicating a weak and molecular adsorption of CO on CaO.Shimizu et al.[43,44]found that CaO in the carbonator of the calcium looping absorbed NO from the flue gases.NO is strongly bound to the oxide anions on CaO and generates N2O as the intermediate,which is further decomposed into N2and surface oxygen [42,45].Thus, it means that when CO and NO are simultaneously absorbed by CaO, the molecularly adsorbed CO probably reacts with the intermediates generated by the adsorption of NO on CaO, resulting in the consumptions of CO and NO and the desorption of N2and CO2.The possible catalysis mechanism of NO reduction by CO under the catalysis of CaO is described as follows:

Fig.9.SEM images of fresh CaO and CaO after 10 CO2 capture cycles:(a)fresh CaO,(b)after 10 cycles,(carbonation:650°C,0.05%O2/3%CO/0.05%NO/15%CO2/N2 balance,calcination: 850 °C, 100 % N2, sample: L1-CaO).

Fig.10.Surface areas and pore volumes of fresh CaO and CaO after 10 CO2 capture cycles, (carbonation: 650 °C, 0.05% O2/3% CO/0.05% NO/15% CO2/N2 balance,calcination: 850 °C, 100 % N2, sample: L1-CaO).

Voigts et al.[45]studied the adsorption of CO and CO2on CaO films and found that both of CO and CO2could be adsorbed by CaO, but the process for CO adsorption was about two orders of magnitude slower than that for CO2.Therefore, if the active sites in CaO is limited, CO2hinders the adsorption of CO on CaO, weakening the reactions as shown in Eqs.(6) and (7).However, in the realistic industrial applications,the sorbents in the carbonator supporting active sites for NO removal by CO is pretty abundant.Thus,the competition between NO removal by CO and CO2capture will be significantly relieved, which guarantees efficient simultaneous NO/CO2removal by CO/CaO.

The schematic diagram of NO removal by CO under the catalysis of CaO experienced different CO2capture cycles is presented in Fig.11.In the fresh CaO, the small pores between the CaO grains are convenient for the diffusion and adsorptions of CO and NO.NO is strongly adsorbed by CaO grains to form (-CaO2) and(-N2O), guaranteeing the efficient catalytic performance of CaO on NO removal by CO according to Eqs.(5)-(7).However, as the number of CO2capture cycles increases, CaO grains grow up as a result of the sintering,so the small pores disappear and the porosity of CaO decreases.The diffusion of CO and NO in the CaO grains become difficult,resulting in the decrease in NO and CO adsorption capacities of CaO, so the catalytic ability of CaO on NO removal by CO is degraded.This is why the catalytic performance of CaO on NO reduction by CO decays with the number of CO2capture cycles.In order to obtain efficient cyclic simultaneous NO/CO2removal by CO/CaO,the CaO-based sorbent with the porous structure and high sintering resistance should be chosen in the calcium looping process.

Fig.11.Schematic diagram of NO removal by CO under catalysis by CaO during calcium looping process.

4.Conclusions

NO removal performance of CO in the presence of CaO during the carbonation of the calcium looping was investigated in a bubbling fluidized bed reactor.CaO possesses an efficient catalytic ability on NO removal by CO.High temperature is beneficial to NO removal by CO with CaO catalysis but playing a negative role in CO2capture.Considering both higher CO2capture and better NO removal efficiencies,the suitable temperature for simultaneous CO2/NO removal is 650 °C.High CO concentration has a positive effect on NO removal by CO with CaO catalysis, but the benefit is limited at high CO concentration (above 2.5%).The carbonation of CaO has a negative effect on the catalytic performance of CaO on NO removal by CO as a result of the generation of CaCO3.Besides,the increasingly serious sintering of CaO with the number of CO2capture cycles causes the fusion of CaO grains and the blockage in pores,hindering the sufficient adsorptions of CO and NO on CaO.Therefore, the catalytic ability of CaO on NO removal by CO decreases with the number of cycles.In the realistic applications,the abundant amount of CaO in the carbonator and the continual supplement of fresh CaO can solve this problem.The presence of char is beneficial to NO removal by CO in the carbonator.The high CaO content and porous structure of calcium-based sorbents promote NO removal by CO and CO2capture.Finally, the efficiencies of NO removal and CO2capture in the carbonator are up to 90%.Efficient NO removal and CO2capture are realized in the carbonator by CO and CaO, which seems a promising method for simultaneous pollutants removal during the calcium looping process.

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

Financial supports from the National Natural Science Foundation of China(51876105),the Joint Foundation of National Natural Science Foundation of China and Shanxi Province for coal-based low carbon (U1510130), Shandong Provincial Natural Science Foundation (ZR2020ME188), the Fundamental Research Funds of Shandong University (2018JC039) and the program for Outstanding PhD candidate of Shandong University are gratefully appreciated.

Nomenclature

A Content of CaO in initial calcined limestone, % (mass)

H Height of furnace of BFBR, mm

L Length of furnace of BFBR, mm

MCaOMolar mass of CaO, g·mol-1

mCaOMass of calcined limestone added into BFBR, g

N Number of calcination/carbonation cycles

t Reaction time, s

V (t) Total volume of reaction gas after addition of sample in BFBR at t, L·min-1

V0Total volume of reaction gas before addition of sample in BFBR, L·min-1

VN2,0N2volume of reaction gas before addition of sample in BFBR, L·min-1

XNCarbonation conversion of calcined limestone after Nth cycle, %

ηCO2（t） CO2capture efficiency in BFBR at t, %

ηNO(t) NO removal efficiency in BFBR at t, %

φ Inner diameter of furnace of BFBR, mm

φCO(t) CO concentration in exhaust gas from BFBR after addition of sample at t, %

φCO2（t） CO2concentration in exhaust gas from BFBR after addition of sample at t, %

φCO2,0（t） CO2concentration in exhaust gas from BFBR before addition of sample at t, %

φNO(t) NO concentration in exhaust gas from BFBR after addition of sample at t, %

φNO,0(t) NO concentration in exhaust gas from BFBR before addition of sample at t, %