Developing a Novel CaO-Based Sorbent for Promoted CO2 Capture

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Developing a novel CaO-based sorbent for promoted CO2 capture and tar reduction Long Han, Yuan Zhang, Kang Lin, Xin Jia, Hao Zhang, Yingjie Zhong, Qinhui Wang, and Zheng Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03409 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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Developing a novel CaO-based sorbent for promoted CO2 capture and tar reduction Long Han*, 1, 2, 3, Yuan Zhang1, Kang Lin1, Xin Jia2, Hao Zhang 1, Yingjie Zhong 1, Qinhui Wang*2, Zheng Li3 1

Institute of Energy and Power Engineering, College of Mechanical Engineering, Zhejiang

University of Technology, Chaowang Rd. 18, Hangzhou 310014, China 2

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,

Zhejiang University, Zheda Rd. 38, Hangzhou 310027, China 3

State Key Laboratory of Control and Simulation of Power System and Generation Equipments,

Department of Thermal Engineering, Tsinghua University, Haidian District, Beijing 100084, China ABSTRACT Biomass gasification based on Calcium looping sorption enhanced reforming (CLSER) has advantages of generating syngas with high purity hydrogen and simultaneously capturing CO2 in process. This study firstly aims to develop a novel synthetic CaO-based sorbent, addressing its tar reduction potential and mechanical strength beside of cyclic carbonation reactivity. The novel sorbent was synthesized by integrating CaO with iron catalyst and an inert support based on a two-step sol-gel method. Comprehensive properties of the novel sorbent including chemical components, effects on biomass pyrolysis and tar reduction, cyclic CO2 capture reactivity, morphology and mechanical strength were examined by using various methods and facilities. The characterized chemical and physical properties were also compared with pure CaO and two referred synthetic sorbents. Results showed that a novel sorbent (Ca-Fe-Al) consisting of CaO, iron oxide (Fe2O3) and mayenite (Ca12Al14O33) was successfully synthesized. Evolutions of tar species and CO2 during wheat-straw pyrolysis were found to be lowered in presence of Ca-Fe-Al 1

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as determined by thermogravimetric Fourier transform infrared (TG-FTIR). The enhanced evolutions of light gases such as CO, CH4, and CO2 at temperatures higher than 580℃ examined by thermogravimetry-mass spectrometry (TG-MS), indicated the catalysis effects of Ca-Fe-Al on biomass tar cracking and char decomposition. Different from CaO and other synthetic sorbents, Ca-Fe-Al sorbent showed increasing carbonation capacity and reactivity with growing cycle numbers. After being hydrated, cyclic carbonation performance of Ca-Fe-Al sorbent became superior to other sorbents. Mechanical strength of Ca-Fe-Al sorbent was also largely promoted than CaO and was more suitable for long time storage. KEY WORDS: Calcium looping, CO2 capture, cyclic carbonation, tar reduction, mechanical strength

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1 INTRODUCTION Biomass gasification based on Calcium looping sorption enhanced reforming (CLSER) has raised much attention during the past two decades due to the continuous concerns on biomass utilization and global warming

1-5

. In this technology, biomass is gasified by pure steam in

presence of abundant CaO sorbents. CaO carbonation (Eq. 1) plays a key role to absorb CO2 from the gasification syngas, which is able to enhance water gas shift reaction (Eq. 2), water gas reaction (Eq. 3) and steam reforming reactions (Eq. 4, where CnHm represents hydrocarbons including tar species) towards hydrogen production. Thus this technology has the potential to produce syngas with very high concentration hydrogen content and realize in process CO2 capture 6, 7

.

CaO + CO2 = CaCO3 (1)

CO + H 2O = CO 2 + H 2 (2) C + H 2O = CO + H 2

(3)

Cn H m + nH 2O = nCO +(n + m / 2) H 2 (4) However, there are obvious challenges on CaO sorbents. Firstly, it is well recognized that CO2 capture reactivity of CaO dramatically declined during cyclic carbonation-calcination reactions 8, 9 . Secondly, although CaO has been identified to be beneficial for biomass tar reduction 10, 11, its effects on tar elimination is limited. Taking an 8 MW pilot biomass gasifier for instance3, 12, under the catalysis of abundant CaO-based sorbents, tar content in the raw syngas could be just reduced to around 1g/Nm3. This implies that much deeper tar reduction is essential before syngas can be directly utilized by downstream facilities such as gas engines or gas turbines. Moreover, as a 3


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by-product of tar thermal decomposition, biomass tar coke can deposit on CaO surface and cover CaO active sites for CO2 carbonation, further reducing sorbents’ CO2 capture capacity 13, 14. Finally, conventional CaO sorbents, generally produced from natural minerals such as limestone or dolomite, are mechanically fragile and suffer from large sorbent loss mainly due to attrition 15, 16. Sorbent attrition will be even severe in case of fluidized bed reactors11, 17. In summary, it is essential to develop a feasible sorbent that could overcome all these drawbacks. Previous studies have implied that integration of mayenite as an inert support is a feasible method to enhance CaO cyclic carbonation performance. Li Z. S. et al. synthesized a mayenite-loaded sorbent by integrating CaO with aluminum nitrate 18, 19. Manovic V. and Anthony E. J. combined limestone directly with commercial calcium aluminate cements and produced a synthetic sorbent, which could retain a residual conversion up to 28% even after 1000 cyclic carbonation-calcination reactions 16. Although mayenite is also identified to be beneficial for promoting sorbent’s mechanical strength 15, 20, much less work has been conducted on this issue 21. Meanwhile, most research mainly focused on post-combustion CO2 capture from flue gases, few have concerned about sorbent’s potential for biomass tar reduction and coke elimination. Catalysis reduction is the most popular method for biomass tar elimination. Comparing with many metal catalysts such as alkaline earth metals, noble metals and nickel, iron based catalyst is cheap, environmentally friendly and not prone to agglomeration. It has also been demonstrated to be active for tar reduction 22, 23. Recently, research interests in integrating iron catalyst with CaO sorbent also emerge. Huang B. S. et al. synthesized a new sorbent consisting of brownmillerite (Fe2Ca2O5) and CaO. They observed that brownmillerite had a catalysis effect for decomposition of polycyclic aromatic hydrocarbons (PAHs) 13. Furthermore, Zamboni I. et al. found that the 4


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active iron component in the combined catalyst varied with calcium precursors, iron precursors and synthesis methods

24

. Although the above authors had intended to promote biomass tar

reduction and simultaneously capture CO2 using a synthetic sorbent, they did not examine sorbent’s cyclic carbonation reactivity and mechanical strength. In fact, mayenite can not only promote cyclic carbonation performance as well as mechanical strength of CaO, but also be used as a support for biomass tar catalyst. Li C. S. et al. found that, in the test for toluene reforming reduction, only negligible residue coke deposited on nickel catalyst when it was supported by mayenite 25. Meanwhile, Zamboni I. et al. also demonstrated that tar reforming reactions were enhanced by using a novel catalyst, which combined olivine with CaO using mayenite as the support 26. The main reason why mayenite benefits tar reforming and coke elimination is that it has a special crystal structure where free oxygen species are able to be stored and transported 25. However, to the authors’ knowledge, quite limited studies have used mayenite as the support for both tar catalyst and CaO sorbent. In this study, a novel CaO-based sorbent (Ca-Fe-Al) has been synthesized on the basis of integrating mayenite (as the support) and iron oxide (as tar catalyst) with CaO, using a two-step sol-gel method. The novel sorbent has slightly increasing carbonation reactivity with promoted mechanical strength and is beneficial for biomass pyrolysis and tar reduction. Moreover, three referred CaO-based sorbents are also produced in this study, none of which has simultaneously integrated mayenite and iron catalyst with CaO. Comparisons of chemical and physical properties between novel sorbent and referred sorbents are presented and discussed. 2 EXPERIMENTS AND METHODOLOGY Calcium

acetate

monohydrate

[Ca(CH3COO)2·H2O],

Iron

5


(III)

nitrate

nonahydrate

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[Fe(NO3)3·9H2O] and Aluminum nitrate nonahydrate [Al(NO3)3·9H2O] used in this study were all reagent-grade and purchased from Shanghai Silian Chemical Co. The CaO sorbent used in this study was all produced from Calcium acetate monohydrate, which was heated to 850℃ and held for 2 hours in a muffle furnace. The produced CaO powders were cooled to room temperature and ground into fines of 250-310 µm before being transferred into a sealed reagent bottle. 2.1 Synthesis of novel sorbent Ca-Fe-Al A two-step procedure is adopted to synthesize Ca-Fe-Al sorbent, as shown in Fig. 1. The first step is to integrate iron salt with CaO based on a sol-gel method. 2g CaO and 1.443g Iron (III) nitrate nonahydrate was separately dissolved in an excess of propionic acid. The concentration of both solutions was 0.1mol/L. The mass ratio of iron atoms to original CaO was controlled at 1:10. These two solutions were then mixed (forming a sol), stirred and heated at 120℃ to obtain a gel. After that, the gel sample was dried at 120℃, then it was heated to 750℃ at 2℃/min and held for 4h. The calcined solids were finally ground into powders of 250-310µm. In the second step, samples obtained in the first step were further integrated with aluminum salts by mixing 2.168g Aluminum nitrate enneahydrate. The mixture was dissolved in 9.9ml 2-propanal and 58.2ml deionized water. Then the solution was stirred and heated at 75℃, followed by drying the residual solution at 75℃ in an oven. After that, the dried solid was ground into powders and added with deionized water before being manually shaped into spheres with diameters of 3-4 mm. Finally, the spheres were heated to 500℃ (held for 1h) and further heated to 1000℃ (held for an additional hour) at 5℃/min. Fig. 1 synthesis process of the novel Ca-Fe-Al sorbent

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2.2 Synthesis of referred sorbents Three other referred sorbents (namely Ca, Ca-Fe and Ca-Al) were also synthesized in order to compare with the novel Ca-Fe-Al sorbent. 2.2.1 Preparation of Ca sorbent CaO powders were mixed with deionized water and spheres were manually produced with 3-4 mm diameters. After being dried at 120℃, the spheres were calcined at 900℃ for 1h. 2.2.2 Preparation of Ca-Fe sorbent The synthesis of Ca-Fe sorbent was similar to the first step for Ca-Fe-Al sorbent preparation. In order to investigate mechanical strength of Ca-Fe sorbent, the obtained dry powders as in the first step were mixed with deionized water and directly made into spheres with diameters of 3-4 mm and then processed in the same way as Ca sorbent spheres. 2.2.3 Preparation of Ca-Al sorbent A mixture of 1.31g CaO powders and 1.42g Aluminum nitrate enneahydrate was dissolved in 6.5 ml 2-propanal and 38 ml deionized water. The obtained solution then underwent the same treatments as the solution in the second step for Ca-Fe-Al sorbent preparation, producing Ca-Al spheres with 3-4 mm diameters. 2.3 Cyclic experiments in a dual tube furnace system In order to examine sorbents’ morphologies, porosity (Brunauer-Emmett-Teller, BET) properties and mechanical strength after multiple cycles, a dual tube furnace system was adopted to conduct cyclic carbonation-calcination experiment. All spherical sorbents were firstly carbonated for 50 min and then calcined for 5min within a cycle, continuing for 10 carbonation-calcination cycles. The experimental system consists of two separate tube furnaces 7


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shown in Fig. 2. The first furnace (7 in Fig. 2) was used for carbonation, which was kept at 720℃ and purged by 15% CO2 (balanced by nitrogen) with a total flow rate of 500ml/min (gas flow rate was sufficient to minimize gas diffusion). The second furnace (8 in Fig. 2) was adopted as the calcination reactor, which was held at 850℃ and purged by the same gas stream as carbonator with a total flow rate of 1000ml/min (larger flow rate was adopted to minimize gas diffusion due to shorter residence time). Fig.2 A scheme of the dual tube furnace system for cyclic experiments (1, 2: cylinder groups; 3: pressure valves; 4, 5: mass flow meters; 6: ceramic boats; 7, 8: tube furnace; 9: temperature electronic controllers) The dual tube furnace system was also used for a cyclic reduction-oxidation test for spherical Ca-Fe-Al sorbent. This experiment had been performed to examine the existence of free oxygen species in mayenite after cyclic reactions, simulating the cyclic gasification-combustion process in CLSER technology. The pure gas in the reduction/carbonation furnace (7 in Fig. 2) was 50% hydrogen mixed with 7.5% CO2 and balanced N2, while the pure gas in the oxidization/calcination furnace (8 in Fig. 2) was air. Other experimental conditions such as reaction temperatures, reaction times and gas flow rates were kept the same as above cyclic carbonation-calcination tests. 2.4 Characterization of chemical and physical properties Chemical properties of crushed sorbents were examined by various techniques. X-ray diffraction (XRD, X'Pert Pro) and Raman Spectra (LabRam HR UV) were used to determine sorbents’ chemical components. XRD patterns were collected over the angular range of 10-90° (2θ). Raman spectroscopy was taken by a spectrometer of 532 nm with a scanning range 100-4000 cm-1. A Diamond thermogravimetric analyzer (TGA) was used to examine sorbents’ cyclic 8


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carbonation performances. For each cyclic TG test, around 10mg sorbent was firstly heated from room temperature to 720℃ and held for 50min, then further heated to 850℃ and held for 5min. After the calcination step, the sample was cooled down to 720℃ and next cycle continued. The purge gas in TGA test was 15% CO2 balanced with nitrogen with a flow rate of 20ml/min, the heating and cooling rate was kept at 15℃/min. In order to verify the influences of synthetic sorbents on biomass tar reduction as well as CO2 capture, thermogravimetric Fourier transform infrared (TG-FTIR) tests were performed to detect evolutions of tar species and gases during wheat-straw (particle size of 0.15-0.21mm, properties are shown in Tab. 1) pyrolysis. A Mettler-Toledo TGA/DSC2 thermo-balance, coupled with Nicolet 6700 FTIR was used. The mass of wheat-straw was accurately controlled at about 5.4 mg for each run. The amounts of added Ca, Ca-Fe, Ca-Al and Ca-Fe-Al sorbents were 10.3, 12.8, 13.8 and 16.1 mg respectively so that the mole ratios of calcium in each sorbent to carbon in wheat-straw could be kept at 1.1. For each TG-FTIR test, all wheat-straw samples were always firstly filled into crucible followed by loading sorbent to form a uniform layer on wheat-straw surface. Nitrogen with purity over 99.99% was used as pyrolysis carrier gas at a flow rate of 30ml/min. The experimental samples (either pure wheat-straw or wheat-straw mixed with different sorbents) were dynamically heated from room temperature to 900 ℃ at 20℃/min. Volatiles released during pyrolysis were transferred to FTIR gas cell via a Teflon tube that was heated to 260℃ to avoid tar condensation. The infrared spectrum scope was located in the range of 400–4000 cm−1 and the resolution factor was 4 cm−1. Specially, thermogravimetry-mass spectrometry (TG-MS, a NETZSCH STA 449C thermogravimetric analyzer coupled to a NETZSCH QMS 403C mass spectrometer) tests were also performed to further examine 9


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evolutions of light gases for wheat-straw pyrolysis in presence and absence of Ca-Fe-Al sorbent. Major ions corresponding to H2, CO, CO2 and CH4 were identified and targeted for MS scanning so as to obtain better resolution. Other pyrolysis conditions of TG-MS tests including mole ratio of calcium to carbon, carrier gas and wheat-straw heating program were consistent with corresponding TG-FTIR tests. Tab.1 Ultimate and proximate analyses and heating value of wheat-straw The physical properties including morphology, porosity properties and mechanical strength of sorbents were also evaluated. Textural features and metal element compositions were examined by using scanning electron micrograph combined with energy-dispersive spectrometer (SEM-EDS, SIRON). Sorbent samples were coated with a thin layer of gold-platinum before SEM tests. Surface area, pore volume and average pore diameter were determined by BET method (Quantachrome, ASAP2010). A gravity-based method was used to test sorbents’ crushing strength, which has been considered as a simple but feasible indicator to examine the mechanical strength of spherical sorbents 15, 16. For each mechanical strength test, four sorbent spheres were located at the four corners of a 4cm×4cm square, respectively. Then a flat round plank was put onto the top of the four spheres, keeping its geometric center coincided with that of the square. Steel weights were then gradually added onto the plank until the four sorbent spheres were crushed simultaneously. For each type of sorbent, mechanical test was performed three times and the average mass was used for gravity calculation. The mechanical strength of sorbent was finally obtained by dividing the total gravity of steel weights (N) by area of the square (m2). 3 RESULTS AND DISCUSSIONS 3.1 Chemical components 10


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Fig. 3 shows the XRD results of different fresh sorbents. XRD patterns of spent sorbents after cyclic carbonation-calcination reactions were same to corresponding fresh sorbents and thus not given. It was obvious that pure CaO was obtained by calcining calcium acetate monohydrate. In Ca-Fe sorbent, the main component was also CaO while active iron chemical was brownmillerite (Ca2Fe2O5). This result was consistent with a previous study where Zamboni I. et al. also reported a synthesized sorbent containing Ca2Fe2O5 by a sol-gel method 24. Differently, the above authors adopted iron (II) acetate anhydrous as the iron precursor instead of iron (III) nitrate nonahydrate used in this study. Thus it seemed that iron precursors would not influence the active iron component when using a sol-gel method. Considerable amount of calcium hydrate [Ca(OH) 2] was also observed in Ca-Fe sorbent. It should be noted that all sorbent powders had been calcined at 500℃ for 2h prior to XRD tests so as to release the moisture absorbed by CaO via Eq. 5. The presence of calcium hydrate in Ca-Fe sorbent indicated that this sorbent was liable to form calcium hydrate even under very short exposure time in atmosphere. Fig. 3 XRD results of different sorbents

CaO + H 2O = Ca(OH ) 2

(5)

Ca-Al sorbents consisted of CaO and mayenite (Ca12Al14O33), which also coincided with the result in literature 19. The aluminum nitrate enneahydrate firstly decomposed to generate Al2O3 via Eq. 6. When temperature further increased to be higher than 900℃, mayenite will be produced via Eq. 7.

4 Al ( NO3 )3 ⋅ 9 H 2O = 2 Al2O3 + 12 NO2 + 3O2 + 36 H 2O (6)

7 Al2O3 + 12CaO = Ca12 Al14O33 (7) Different from Ca-Fe sorbent, the active iron component in Ca-Fe-Al sorbent was found as the 11


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form of iron oxide (Fe2O3). Given that the first step of synthesizing Ca-Fe-Al sorbent was similar to Ca-Fe sorbent, it might be concluded that the integration of aluminum salts in the second step affected the active iron component. Ca2Fe2O5, formed as the active iron component in the first step, had probably been transformed into Fe2O3 via Eq. 8 when it was calcined at high temperature in the second stage. It was encouraging that mayenite was also observed in Ca-Fe-Al sorbent, which should be also generated via Eq. 7.

Ca2 Fe2O5 = 2CaO + Fe2O3 (8) Although both Ca2Fe2O5 and Fe2O3 could play the role as a catalyst for biomass tar reduction, Fe2O3 has unique advantages over Ca2Fe2O5. First of all, it was reported that Fe2O3 was able to convert 90% of biomass tar species 22. Moreover, the presence of Fe2O3 in the gasifier could have additional benefits for hydrogen production and CO2 separation. The reason is that Fe2O3 may be partially reduced in the gasifier to produce Fe3O4 via Eq. 9 27. As a result, water gas shift reaction would be promoted by Fe3O4 via Eq. 10 and Eq. 11

22, 28

. However, Ca2Fe2O5 could not enhance

water gas shift reaction because it would be completely reduced to form principal iron phase in one step while not generating other reduced iron oxide phases such as Fe3O4 14, 24.

3Fe2O3 + CO = 2 Fe3O4 + CO2 (9) Fe3O4 + CO = 3FeO + CO2 (10) 3FeO + H 2O = Fe3O4 + H 2 (11) Fig. 4 shows the Raman spectra of various sorbents. The spectrum appeared at 1080 cm-1 is corresponding to the characterization peak of free oxygen species O2- 25.This peak was found in Ca-Al and Ca-Fe-Al sorbents (Fig. 4a and 4b), while not in Ca-Fe sorbent (Fig. 4d). This result additionally proved the presence of mayenite in Ca-Al and Ca-Fe-Al sorbents. It was interesting 12


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that the 1080cm-1 peak of spent Ca-Fe-Al sorbent after 5 cyclic reduction-oxidation reactions was as apparent as that of fresh sorbent (Fig. 4c). This result implied that free oxygen species were quite steady even when Ca-Fe-Al sorbent was subject to cyclic redox reactions, being beneficial for sorbent’s cyclic utilization. Fig.4 Raman spectra of various sorbents 3.2 Tar reduction and gases evolutions Fig. 5 presents TG and DTG (differential thermogravimetric analysis) curves for pyrolysis of wheat-straw and that mixed with different sorbents. Wheat-straw pyrolysis showed two mass loss stages within the ranges of 60-120℃ and 250-345℃. Beside of these two stages, pyrolysis of wheat-straw mixed with various sorbents all presented two other separate stages within 400-450℃ and beyond 660℃. The online FTIR results were useful to reveal what had released during each pyrolysis stage. Taking pyrolysis of wheat-straw mixed with Ca-Fe-Al sorbent for example, Fig. 6 shows the FTIR spectra at four peaks (86℃, 327℃, 433℃, and 681℃) of the DTG curve. The main product at 86℃ was H2O (3500-3964cm-1, 1300-1700cm-1), indicating free H2O released within 60-120℃29. At 327℃, which was corresponding to the pyrolysis main stage between 250℃ and 345℃, tar species of toluene (3042 cm-1), phenol (1360 cm-1) and formic acid (1745 cm-1) as well as light gases including CO2 (2357 cm-1, 670 cm-1), CH4 (2870 cm-1), CO (2181 cm-1, 2112 cm-1) and H2O were released 10. Within 400-450℃, H2O was released due to the decomposition of Ca(OH) 2 (Eq. 12) in sorbents or Ca(OH) 2 carbonation via Eq. 13. At the end of pyrolysis beyond 660℃, detected volatiles included CO2, CO and H2O. CaCO3 decomposition (Eq. 14) should be the main contributor of CO2 released within this stage. The FTIR spectra at four DTG peaks of wheat-straw pyrolyis in presence of other sorbents were similar to the case of Ca-Fe-Al sorbent 13


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and thus not shown. Fig. 5 TG and DTG curves of wheat-straw pyrolysis in presence and absence of sorbents (Bold line: TG curves; thin line: DTG curves) Fig. 6 FTIR spectra at peaks of DTG curve for wheat-straw pyrolysis mixed with Ca-Fe-Al

Ca(OH )2 = CaO + H 2O (12)

Ca(OH )2 + CO2 = CaCO3 + H 2O (13) CaCO3 = CaO + CO2 (14) Fig. 7 compares the FTIR spectra at DTG peaks of the mass loss main stage for pyrolysis of pure wheat-straw and that mixed with four different sorbents. It was apparent that the relevant spectra absorbance of CO2 and tar species decreased in presence of all sorbents. Thus it was concluded that CaO-based sorbents were beneficial to absorb CO2 and reduce evolutions of tar species during wheat-straw pyrolysis, which was also the main reason why mass loss during pyrolysis main stage was reduced in presence of CaO-based sorbents (Fig. 4). Fig. 7 FTIR spectra at DTG peaks of mass loss main stage for wheat-straw pyrolysis in absence and presence of different sorbents Fig. 8 shows the online MS results for pyrolysis of wheat-straw and that mixed with Ca-Fe-Al sorbent, specially indicating evolutions of light gases with increasing temperature across the whole process of pyrolysis. For pyrolysis of pure wheat-straw, evolution of H2 (m/e=2) had a maximum at 337℃ while CO2 (m/e=44), CO (m/e=28) and CH4 (m/e=16) approached their peaks at 317℃. All these temperatures were within the range of mass loss main stage in TG and DTG curves (Fig. 5). After Ca-Fe-Al sorbent being added, evolution peak temperatures of all gases were lowered, which implied that the novel sorbent had a catalysis effect on wheat-straw pyrolysis. 14


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Meanwhile, it was also observed that production of CH4 was largely enhanced during the whole process. The strengthened CH4 evolution was different from a previous study where CH4 production was reduced in presence of pure CaO sorbent 10. Comparing with pure CaO, Ca-Fe-Al sorbent seemed to have unique effect on biomass pyrolysis due to the presence of iron oxide and mayenite, which could have promoted tar cracking reactions (Eq. 15, where Cn-xHm-y represents hydrocarbons including tar species with smaller molecule than CnHm) for CH4 production 30. Fig. 8 Online MS results for pyrolysis of wheat-straw and that mixed with Ca-Fe-Al (Bold line: in presence of Ca-Fe-Al; thin line: pure wheat-straw)

Cn H m ↔ Cn− x H m− y + H 2 + CH 4 + C (15) MS results as shown in Fig. 8 are also beneficial to better understand pyrolysis process and influences of Ca-Fe-Al sorbent. Within the mass loss main stage between 300℃ and 345℃, the production of CO2 and CO was reduced and further verified the CO2 absorption by Ca-Fe-Al sorbent, consistent with the FTIR results shown in Fig. 6. Although H2 evolution between 315 ℃and 420 ℃ was reduced in presence of the novel sorbent, the variation of H2 generation across whole pyrolysis was not obvious given that H2 production was enhanced before 315℃. A sudden increase of H2 generation was observed within 420-450℃, which could be attributed to enhanced water gas reaction (Eq. 3) and tar reforming reaction (Eq. 4) as a result of H2O evolution from Ca(OH) 2 decomposition (Eq.12) or carbonation (Eq. 13). Between 580℃ and 680℃, separate evolution peaks for CO2, CO and CH4 were observed for pyrolysis with Ca-Fe-Al sorbent. The occurrence of CH4 evolution within this stage was quite different from the previous study where no such a peak could be found 10. It was speculated that 15


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Ca-Fe-Al was beneficial to enhance tar cracking (Eq. 15) and biomass char decomposition reactions and thus contributed to those evolution peaks for CO2, CO and CH4

30

. Beside of tar

cracking and char secondary reactions, CaCO3 decomposition (Eq. 14) and Boudouard reaction (Eq. 16) were also responsible for the production of CO2 and CO within this stage. When pyrolysis proceeded beyond 700℃, it is very interesting to observe a third stage of CO evolution across a broad temperature range of 730-900℃, which was also not found in the previous study10. Probably the CO released in this stage was attributed to partial oxidation of carbon in biomass char by free oxygen species existed in the mayenite structure of Ca-Fe-Al sorbent, which had been determined by XRD and Raman spectrum (Fig. 3 and Fig. 4). Similarly, an additional CO2 evolution stage was also observed within 800-900℃, which could also be attributed to the reaction of biomass char carbon with free oxygen species. It was understandable that the evolution of CO was much more apparent than CO2 at high temperatures, given that free oxygen species in mayenite structure were quite limited. In summary, online MS results indicated that Ca-Fe-Al sorbent had catalysis effects on wheat-straw pyrolysis and was beneficial for tar cracking and char decomposition reactions to produce more CO and CH4. The presence of free oxygen species in Ca-Fe-Al sorbent was also verified.

CO2 + C ↔ 2CO (16) 3.3 Cyclic carbonation performances 3.3.1 CO2 capture capacity Fig. 9 presents the TG results for cyclic carbonation-calcination reactions of synthetic sorbents. The mass of each sorbent has been normalized to 1mg in order to compare with each other. 16


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Fig. 9 TG results for cyclic carbonation-calcination reactions of synthetic sorbents (bold line: sorbent mass; thin line: temperature; carbonation at 720℃ ℃ for 50 min, calcination at 850℃ ℃ for 5 min; carbonation and calcination atmosphere: 15% CO2 balanced with nitrogen; gas flow rates: 20ml/min) The CO2 capture capacity of Ca-Fe sorbent dramatically dropped with increasing cycle numbers. Although this sorbent had a relatively high initial CO2 capture capacity up to 0.578mg/mg, its capacity decreased to a residue value of only 0.114 mg/mg, indicating a 80% drop after 10 carbonation-calcination cycles. Comparing with Ca-Fe sorbent, CO2 capture capacity of Ca-Al sorbent had declined much slower. The capacity drop was relatively obvious in the first three cycles but became smaller afterwards. At the 10th cycle, CO2 capture capacity was still larger than 0.28mg/mg, which was higher than the twice of Ca-Fe sorbent. It was observed that there was a remarkable drop in sorbent mass during the initial stage of the first carbonation reaction. The reason was that Ca(OH)2, probably generated during sorbent storage, decomposed via Eq. 12. Different from Ca-Fe and Ca-Al sorbents, CO2 capture capacity of Ca-Fe-Al sorbent presented a growth with increasing cycles. CO2 capture capacity increased by 44% within 10 cycles, from 0.099mg/mg in the first cycle to 0.143 mg/mg in the 10th cycle. It is interesting that a similar capacity growth with increasing cycles has been observed when mayenite was integrated into CaO based on a precipitation method 31. In comparison with Ca-Fe and Ca-Al sorbents, Ca-Fe-Al sorbent seemed to have much lower CO2 capture capacity, especially in the initial cycles. There were two major reasons for the lower CO2 capture capacity. On the one hand, considering that Ca-Fe-Al sorbent had integrated both iron catalyst and mayenite into CaO, the mass ratio of CaO 17


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in Ca-Fe-Al sorbent is much smaller than other two synthetic sorbents. On the other hand, porosity of Ca-Fe-Al sorbent might differ from other sorbents given that sorbent porosity is a significant factor affects carbonation performance 32. Nonetheless, the examined carbonation capacity of Ca-Fe-Al achieved the same level to a reference31 where the same mass ratio of CaO to mayenite had been adopted as this work. Furthermore, the authors examined the influence of hydration on Ca-Fe-Al sorbent’s cyclic carbonation performance. Fresh Ca-Fe-Al sorbent was firstly hydrated by deionized water and then dried at 120℃ for 2h, followed by calcination at 550℃ for 1h. The hydrated Ca-Fe-Al sorbent showed largely enhanced CO2 capture capacity, presenting an increase from 0.099 mg/mg to 0.356 mg/mg in the first cycle. With increasing cycles, the hydrated sorbent just had minor drop in carbonation capacity and approached 0.28 mg/mg in the 10th cycle, which was even superior to Ca-Al sorbent given that Ca-Fe-Al had a lower CaO content. 3.3.2 CaO conversion In order to compare CaO carbonation reactivity for various sorbents, it is necessary to analyze conversions of CaO for different sorbents, while not the carbonation capacities of whole sorbents. The mass ratio of CaO component in each sorbent was firstly determined. Tab. 2 shows the metal elements and chemical compositions for different synthetic sorbents. EDS tests were used to obtain metal elements mole ratios. Mass ratios of CaO and other chemicals for various synthetic sorbents were calculated by combining EDS and XRD results. Then CaO conversion for each sorbent was calculated via Eq. 17, where XCaO is CaO conversion, ∆m is mass increase of synthetic sorbent in corresponding carbonation reaction, m0 is the initial mass of synthetic sorbent, xCaO is the mass ratio of CaO in synthetic sorbents as shown in Tab. 2. Tab. 2 Metal elements and chemical compositions for different synthetic sorbents 18


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X CaO =

56 × ∆m ×100% 44 × m0 × xCaO

(17)

Fig. 10 shows CaO conversions of synthetic sorbents with carbonation-calcination cycles, comparing with the conversion of pure CaO sorbent induced by Eq. 18. XN is CaO conversion in the Nth cycle, fm and fw are empirical parameters and equal to 0.77 and 0.17, respectively 8.

X N = f mN (1 − f w ) + f w (18) Fig. 10 CaO conversions with carbonation-calcination cycles for different sorbents (carbonation at 720℃ ℃ for 50 min, calcination at 850℃ ℃ for 5 min; carbonation and calcination atmosphere: 15% CO2 balanced with nitrogen; gas flow rates: 20ml/min) CaO conversion tendency of Ca-Fe sorbent was similar to that implied by Eq. 18. Ca-Fe sorbent had a very high CaO conversion up to 93% in the first carbonation. However, its conversion greatly decreased with increasing cycles. In the 10th cycle, only 18% of CaO converted into CaCO3. It was obvious that CaO cyclic reactivity of Ca-Fe sorbent needed improvement for cyclic utilizations. Otherwise, a large amount of fresh CaO sorbent and iron precursors would have to be supplemented due to carbonation reactivity loss 33. Comparing with Ca-Fe sorbent, cyclic carbonation reactivity of CaO in Ca-Al sorbent was greatly promoted. Although CaO conversion was lower than the value of Ca-Fe sorbent in the first carbonation, the conversion in the 10th cycle (48.31%) was much higher than Ca-Fe sorbent (18.4%). The integration of mayenite in Ca-Al sorbent resulted in much slower drop in CaO carbonation reactivity, which was consistent with previous studies

18, 19

. The reason was that

mayenite could be uniformly distributed among CaO crystallites, inhibiting CaO from sintering caused by high temperature calcination. Similar to Ca-Al sorbent, integration of mayenite into Ca-Fe-Al sorbent also stabilized CaO 19


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cyclic conversion. Different from the decrease of CaO conversion in Ca-Fe and Ca-Al sorbents, CaO conversion in Ca-Fe-Al sorbent presented a gentle growth with increasing cycle numbers. After 8 cycles, CaO conversion of Ca-Fe-Al sorbent became competitive and even superior to pure CaO and Fe-Ca sorbent. In the 10th cycle, the conversion increased to 28.8%, which was 1.44 times and 1.57 times of the value for pure CaO (20%) and Ca-Fe sorbent (18.4%). Although original Ca-Fe-Al sorbent showed lower CaO conversion than Ca-Al sorbent, hydrated sorbent became superior to Ca-Al and presented the highest CaO conversion after 3 cycles. At the 10th cycle, CaO conversion of hydrated Ca-Fe-Al sorbent approached 56.6%. Summarily, hydration34, 35

is a useful option to promote Ca-Fe-Al sorbent’s carbonation performance. The novel Ca-Fe-Al

sorbent is quite attractive considering its increasing cyclic carbonation conversion and improved performance by hydration. 3.4 Physical properties 3.4.1 Morphologies and pore properties Fig. 11 shows the textural structures of pure CaO before and after cyclic reactions. The fresh sorbent presented apparent gain structures with an average diameter of about 200-300 nm (Figs. 11a and 11c). There were clear boundaries and pores among sorbent grains (Fig. 11c). After 10 cycles, the grain morphology of sorbent almost disappeared, instead of flat surface with cracks probably caused by thermal stress (Fig. 11b). Sorbent particles tended to be adhered to each other (Fig. 11d) and sorbent porosity was largely reduced due to sintering. Thus the great drop in CaO carbonation reactivity during cycle reactions was mainly due to the decrease in sorbent porosity as a result of sorbent sintering 32. Fig. 11 Textural structures of pure CaO before and after cyclic reactions 20


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(a and c: fresh sorbent; b and d: spent sorbent after 10 cycles) Fig. 12 shows morphology of Ca-Fe sorbent before and after cyclic reactions. Fresh Ca-Fe sorbent presented a textural structure rich in grains with much porosity (Figs. 12a and 12c), which could illustrate why Ca-Fe sorbent had a high initial CaO conversion. After 10 cycles, Ca-Fe sorbent presented much denser surface (Fig. 12b). Higher resolution image revealed that the average diameter of sorbent grain grew larger and agglomerate sorbent grains with diameters up to 1µm were observed (Fig. 12d). It was concluded that porosity of Ca-Fe sorbent had decreased with increasing cycles due to sorbent sintering and grain agglomeration, resulting in the dramatic drop in CaO conversion as shown in Fig. 10. Fig. 12 Textural structures of Ca-Fe sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles) Fig. 13 presents the textural structures of Ca-Al sorbent before and after cyclic reactions. Fresh Ca-Al sorbent appeared overlapping grains with diameters between 2 and 4µm (Fig. 13a). Compared with CaO and Ca-Fe, Ca-Al sorbent grains were rich in small pores (Fig. 13c). After 10 cycles, it seemed that Ca-Al sorbent grains linked with each other with abundant minor cracks on sorbent surface (Fig. 13b). Although separate sorbent grains were not existed any more, there was no apparent sorbent agglomeration as found in Ca-Fe sorbent. Moreover, much mesoporosity was also observed beside of micro pores (Fig. 13d). Previous study implied CaO carbonation reactivity was associated with the porosity evolutions during cyclic reactions 8, 18. Fig. 13 indicated that the integration of mayenite increased sorbent mesoporosity with increasing cycles. Although microporosity structures had been more or less reduced due to sorbent sintering, the total porosity of Ca-Al sorbent was not strongly deteriorated. As a result, the cyclic carbonation conversion of 21


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CaO in Ca-Al sorbent declined much slower. Fig. 13 Textural structures of Ca-Al sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles) Fig. 14 shows morphologies of Ca-Fe-Al sorbent before and after cyclic reactions. Fresh Ca-Fe-Al sorbent presented as blocked solids with quite limited porous structures (Fig. 14a). It could be observed that the non-grain sorbent overlapped with rare cracks (Fig. 14c). Thus the limited porosity had resulted in low CaO conversion in initial carbonation reactions (Fig. 10). It was interesting to observe much leaf-vein-like solids distributing on the sorbent surface (Fig. 14c), which could be the iron oxide component produced from brownmillerite. After 10 cycles, Ca-Fe-Al sorbent transformed into a coral-like morphology (Fig. 14b). Comparing with fresh sorbent, surface area and porosity of the spent sorbent was superior because more porous structures were observed (Fig. 14d). It was concluded that the promoted sorbent porosity of Ca-Fe-Al sorbent after cyclic use had contributed to the increase in CaO conversion with increasing cycles. Fig. 14 Textural structure of Ca-Fe-Al sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles) Tab. 3 further presents BET results of surface area, pore volume, and average pore diameter for various sorbents. Although fresh CaO sorbent had the highest surface area, pore volume and the largest average pore diameter, it also presented the greatest deterioration of porosity properties. Similar to CaO, Ca-Fe sorbent also showed obvious deterioration in porosity. On the contrary, surface area and pore volume of Ca-Al sorbent had only minor decrease after 10 cycles. It was also examined that average pore diameter of Ca-Al sorbent increased after 10 cycles, which was 22


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consistent with the SEM results shown in Fig. 13. Finally, BET results revealed that porosity properties of Ca-Fe-Al sorbent including surface area, pore volume and average pore diameter were all promoted after cyclic reactions, which provided good explanations for its increasing carbonation reactivity over multiple cycles. Tab. 3 BET results for various sorbents before and after cyclic carbonation reactions 3.4.2 Mechanical strength Tab. 4 shows the mechanical strength for different sorbents before and after cycle reactions. The mechanical strength of fresh CaO sorbent was just 24kPa. After 10 cycles, sorbent spheres of pure CaO appeared much cracks due to thermal stress under different carbonation and calcination temperatures. As a result, the crush strength of spent CaO was quite weak and could be almost ignored. Comparing with pure CaO, other three synthetic sorbents presented much higher crushed strength. The strength of Ca-Fe, Ca-Al and Ca-Fe-Al sorbent was 76.7kPa, 73.4kPa, and 65.0kPa, respectively. All synthetic sorbents had much better strength than pure CaO. Moreover, it was encouraging that mechanical strength of these synthetic sorbents further increased to 111.8kPa, 92.3kPa, and 72.2kPa after 10 cycles. Tab. 4 mechanical strength for different sorbents before and after cycle reactions It should be noted that although Ca-Fe and Ca-Al sorbents seemed to have better mechanical strength than Ca-Fe-Al sorbent, they were not suitable for long time storage. Fig. 15 shows the changes of fresh synthetic sorbents after several days’ storage. For Ca-Fe sorbent, the sorbent appeared much micro cracks on sphere surfaces after 5 days’ storage (Fig. 15a and d), indicating the failure in mechanical strength. For Ca-Al sorbent, it was liable to absorb moisture in atmosphere. As a result, the surface of sorbent became smoother than fresh sorbent and sorbent 23


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expansion was observed after 7 days’ storage (Fig. 15b and e), which also led to the reduction of mechanical strength. However, Ca-Fe-Al sorbent had no obvious change and kept steady mechanical strength even after 9 days’ storage (Fig. 15 c and f), indicating that the novel sorbent was not prone to fracture or absorb moisture. It was recommended that Ca-Fe-Al sorbent was more suitable for application when considering both mechanical strength and storage properties.

Fig. 15 Synthetic sorbents storage properties (a, b, c: fresh Ca-Fe, Ca-Al, Ca-Fe-Al；；d, e, f: Ca-Fe, Ca-Al, Ca-Fe-Al after 5, 7, 9 days’ storage) 4 CONCLUSIONS (1) A novel CaO-based sorbent (Ca-Fe-Al) consisting of CaO, iron oxide (Fe2O3) and mayenite (Ca12Al14O33) was successfully synthesized based on a two-step sol-gel method. (2) Ca-Fe-Al sorbent was beneficial to reduce tar species and capture CO2 during biomass (wheat-straw in this study) pyrolysis. Evolutions of toluene, phenol, formic acid and CO2 were lowered in the mass loss main stage. At temperatures higher than 580℃, separate evolution peaks of CH4, CO, and CO2 was emerged due to enhanced tar cracking and char decomposition in presence of the novel sorbent. The evolution peaks of CO and CO2 at temperatures up to about 850℃ could be attributed to coke oxidation by free oxygen species in the mayenite structure. After cyclic reduction-oxidation reactions, free oxygen species still existed in Ca-Fe-Al sorbent. (3) Different from CaO and referred synthetic sorbents, cyclic CO2 capture reactivity of Ca-Fe-Al sorbent increased with increasing carbonation-calcination cycles. The reason was that its porosity properties including surface area, pore volume and average pore diameter were all improved after cyclic reactions. Hydrated Ca-Fe-Al sorbent presented superior cyclic carbonation performance to 24


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other sorbents. (4) The mechanical strength of Ca-Fe-Al sorbent was twice higher than original CaO and the strength became even stronger after 10 carbonation-carbonation cycles. Comparing with other synthetic sorbents, Ca-Fe-Al sorbent’s mechanical strength was much steady after long time storage. ■ AUTHOR INFORMATION Corresponding author *Telephone/Fax: +86-571-88320192. Email: [email protected] *Telephone/Fax: +86-571-87952802. Email: [email protected] Notes The authors declare no competing ﬁnancial interest. ■ ACKNOWLEDGEMENTS The authors acknowledged the financial supports from National Natural Science Foundation of China (51506186), Science and Technology Department of Zhejiang Province Public Welfare Project (2016C31103), Project of China-USA Joint Research Center on Clean Energy (2016YFE0102500), State Scholarship Fund of China Scholarship Council for a visiting scholar (201308440124), Open Funds of State Key Laboratory of Clean Energy Utilisation from Zhejiang University (ZJU-CEU2015016) and Open Funds of State Key Laboratory of Control and Simulation of Power System and Generation Equipments from Tsinghua University (SKLD16KZ11) for the present work. ■ REFERENCES (1) Florin, N. H.; Harris, A. T. Hydrogen production from biomass coupled with carbon dioxide 25


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kinetics of iron oxides reduction during chemical looping hydrogen generation in a packed bed reactor. Int. J. Hydrogen Energ. 2015, 40, 12097-12107. (28) Matsuoka, K.; Shimbori, T.; Kuramoto, K.; Hatano, H.; Suzuki, Y. Steam reforming of woody biomass in a fluidized bed of iron oxide-impregnated porous alumina. Energ. Fuel 2006, 20, 2727-2731. (29) Liu, Q.; Wang, S. R.; Zheng, Y.; Luo, Z. Y.; Cen, K. F. Mechanism study of wood lignin pyrolysis by using TG-FTIR analysis. J. Anal. Appl. Pyrol. 2008, 82, 170-177. (30) Meilina, W.; Tamara, L. C.; Nicholas, H. F.; Andrew, T. H. Hydrogen synthesis from biomass pyrolysis with in situ carbon dioxide capture using calcium oxide. Int. J. Hydrogen Energ. 2011, 36, 4800-4813. (31) Christina, S. M.; Angeliki, A. L. Parametric study of the CaO-Ca12Al14O33 synthesis with respect to high CO2 sorption capacity and stability on multicycle operation. Ind. Eng. Chem. Res. 2008, 47, 9537-9543. (32) Fuertes, A. B.; Alvarez, D.; Rubiera, F.; Pis, J. J.; Marban, G.; Palacios, J. M. Surface-area and pore-size changes during sintering of calcium-oxide particles. Chem. Eng. Commun. 1991, 109, 73-88. (33) Di Felice, L.; Courson, C.; Foscolo, P. U.; Kiennemann, A. Iron and nickel doped alkaline-earth catalysts for biomass gasification with simultaneous tar reformation and CO2 capture. Int. J. Hydrogen Energ. 2011, 36, 5296-5310. (34) Manovic, V.; Anthony, E. J. Steam reactivation of spent CaO-based sorbent for multiple CO2 capture cycles. Environ. Sci. Technol. 2007, 41,1420-1425. (35) Rong, N.; Wang, Q. H.; Fang, M. X.; Cheng, L. M.; Luo, Z. Y.; Cen, K. F. Steam hydration 29


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reactivation of CaO-based sorbent in cyclic carbonation/calcination for CO2 capture. Energ. Fuel 2013, 27, 5332-5340.

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Figure Captions Fig. 1 synthesis process of the novel Ca-Fe-Al sorbent Fig. 2 A scheme of the dual tube furnace system for cyclic experiments Fig. 3 XRD results of different sorbents Fig. 4 Raman spectra of various sorbents Fig. 5 TG and DTG curves of wheat-straw pyrolysis in presence and absence of sorbents Fig. 6 FTIR spectra at peaks of DTG curve for wheat-straw pyrolysis mixed with Ca-Fe-Al Fig. 7 FTIR spectra at DTG peaks of mass loss main stage for wheat-straw pyrolysis in absence and presence of different sorbents Fig. 8 Online MS results for pyrolysis of wheat-straw and that mixed with Ca-Fe-Al Fig. 9 TG results for cyclic carbonation-calcination reactions of synthetic sorbents Fig. 10 CaO conversions with carbonation-calcination cycles for different sorbents Fig. 11 Textural structures of pure CaO before and after cyclic reactions Fig. 12 Textural structures of Ca-Fe sorbent before and after cyclic reactions Fig. 13 Textural structures of Ca-Al sorbent before and after cyclic reactions Fig. 14 Textural structure of Ca-Fe-Al sorbent before and after cyclic reactions Fig. 15 Synthetic sorbents storage properties Table Captions Tab.1 Ultimate and proximate analyses and heating value of wheat-straw Tab. 2 Metal elements and chemical compositions for different synthetic sorbents Tab. 3 BET results for various sorbents before and after cyclic carbonation reactions Tab. 4 Mechanical strength for different sorbents before and after cycle reactions 31


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Fig. 1 synthesis process of the novel Ca-Fe-Al sorbent

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Fig.2 A scheme of the dual tube furnace system for cyclic experiments (1, 2: cylinder groups; 3: pressure valves; 4, 5: mass flow meters; 6: ceramic boats; 7, 8: tube furnace; 9: temperature electronic controllers)

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Fig. 3 XRD results of different sorbents

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Fig.4 Raman spectra of various sorbents

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Fig. 5 TG and DTG curves of wheat-straw pyrolysis in presence and absence of sorbents (Bold line: TG curves; thin line: DTG curves)

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Fig. 6 FTIR spectra at peaks of DTG curve for wheat-straw pyrolysis mixed with Ca-Fe-Al

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Fig. 7 FTIR spectra at DTG peaks of mass loss main stage for wheat-straw pyrolysis in absence and presence of different sorbents

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Fig. 8 Online MS results for pyrolysis of wheat-straw and that mixed with Ca-Fe-Al (Bold line: in presence of Ca-Fe-Al; thin line: pure wheat-straw)

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Fig. 9 TG results for cyclic carbonation-calcination reactions of synthetic sorbents (Bold line: sorbent mass; thin line: temperature; carbonation at 720℃ ℃ for 50 min, calcination at 850℃ ℃ for 5 min; carbonation and calcination atmosphere: 15% CO2 balanced with nitrogen; gas flow rates: 20ml/min)

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Fig. 10 CaO conversions with carbonation-calcination cycles for different sorbents (carbonation at 720℃ ℃ for 50 min, calcination at 850℃ ℃ for 5 min; carbonation and calcination atmosphere: 15% CO2 balanced with nitrogen; gas flow rates: 20ml/min)

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Fig. 11 Textural structures of pure CaO before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles)

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Fig. 12 Textural structures of Ca-Fe sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles)

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Fig. 13 Textural structures of Ca-Al sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles)

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Fig. 14 Textural structure of Ca-Fe-Al sorbent before and after cyclic reactions (a and c: fresh sorbent; b and d: spent sorbent after 10 cycles)

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Fig. 15 Synthetic sorbents storage properties (a, b, c: fresh Ca-Fe, Ca-Al, Ca-Fe-Al；；d, e, f: Ca-Fe, Ca-Al, Ca-Fe-Al after 5, 7, 9 days’ storage)

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Tab.1 Ultimate and proximate analyses and heating value of wheat-straw Analyses

Wheat-straw

Proximate analysis [air dried basis, %w] Moisture Volatiles Fixed carbon Ash Ultimate analysis [air dried basis, %w] Carbon Hydrogen Nitrogen Sulfur Oxygen (by difference) High heating value QHHV [kJ/kg]

10.43 57.99 15.69 15.89 36.62 5.29 0.89 0.34 30.54 14,977

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Tab. 2 Metal elements and chemical compositions for different synthetic sorbents Metal elements mole ratios (%)

Mass ratios of chemicals (%)

Name Ca

Fe

Al

CaO

Ca2Fe2O5

Fe2O3

Ca12Al14O33

Ca-Fe

90.90

9.10

0

78.79

21.21

0

0

Ca-Al

85.30

0

14.70

73.65

0

0

26.35

Ca-Fe-Al

76.80

9.00

14.20

62.96

0

12.55

24.49

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Tab. 3 BET results for various sorbents before and after cyclic carbonation reactions Sorbents

Surface area

Pore volume

Average pore diameters

(m2/g)

(cm3/g)

(nm)

Ca

12.5

0.054

18.3

Ca-Fe

4.38

0.013

12.0

Ca-Al

12.8

0.031

8.48

Ca-Fe-Al

5.56

0.014

11.1

Ca*

3.0

0.004

6.88

Ca-Fe*

3.08

0.006

7.80

Ca-Al*

8.19

0.026

17.2

Ca-Fe-Al*

6.15

0.017

13.2

(*sorbent after cyclic carbonation reactions)

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Tab. 4 Mechanical strength for different sorbents before and after cycle reactions Sorbents

Strength before cyclic tests

Strength after 10 cycles

CaO

24.6 KPa

—

Ca-Fe

76.7 KPa

111.8 KPa

Ca-Al

73.4 KPa

92.3 KPa

Ca-Fe-Al

65.0 KPa

72.2 KPa

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Developing a Novel CaO-Based Sorbent for Promoted CO2 Capture

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