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Experimental investigation of Fe-Ni-Al oxygen carrier derived from hydrotalcite like precursors for the chemical looping gasification of biomass char Guoqiang Wei, Fang He, Weina Zhao, Zhen Huang, Kun Zhao, Zengli Zhao, Anqing Zheng, Xianshuang Wu, and Haibin Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00208 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Experimental investigation of Fe-Ni-Al oxygen carrier derived from hydrotalcite -like precursors for the chemical looping gasification of biomass char Guoqiang Weia,b,c,d, Fang Hea,b,c*, Weina Zhaoe, Zhen Huang a,b,c*, Kun Zhaoa,b,c, Zengli Zhaoa,b,c, Anqing Zhenga,b,c, Xianshuang Wua,b,c, Haibin Lia,b,c a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS),

Guangzhou 510640, China b

CAS Key Laboratory of Renewable Energy

c

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development d

State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi

Province and the Ministry of Science and Technology, Taiyuan University of Technology, China e

Guangdong Mechanical & Electrical College, Guangzhou 510515, China

* Corresponding author, Fang He, Tel.:+86 02087057721. Fax: +86 02087057737. E-mail address: [email protected]; [email protected]

Abstract: Chemical Looping Gasification (CLG) of biomass uses the lattice oxygen of oxygen carrier to convert biomass into syngas with low tar content, high heating value and low price. It is key important to exploit well-dispersed and thermally stable oxygen carriers for the CLG process. In the current work, a series of oxygen carriers with varied Fe and Ni molar ratio were synthesized from hydrotalcite compounds precursors (HTlcs), which made the metallic elements mix at molecular level. Consequently, highly dispersed complex metal oxygen carriers can be achieved after precursor calcinations. CLG of biomass char was carried out in TGA and fixed bed

ACS Paragon Plus Environment

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reactor accompanied by various physical and chemical analysis for the fresh and used oxygen carriers. The result manifested the HTlcs crystalline form was formed in the precursors and produced Fe0.99Ni0.6Al1.1O4 compound after calcination suggesting that high degree dispersion multi-metal oxygen carrier were synthesized. The main H2 uptake and CLG reactivity of the oxygen carriers were related to its higher metal dispersion and synergistic effect between Fe and Ni. Accordingly, there was an optimum Fe / Ni ratio of 4:1 in oxygen carriers at which the oxygen carrier can achieve the better CLG reactivity. Also, the oxygen carrier to biomass char mass ratio of 7:3 provided the maximum weight loss 35.59% and largest mass loss rate 2.46%wt/min suggesting higher lattice oxygen releasing efficiency. CO exhibited a higher generating rate in the CLG reactions owing to its higher reaction activation energy with lattice oxygen [O] while H2 was more prone to be consumed. The morphological analysis of fresh and regenerated samples exhibited the oxygen carrier was reduced to Fe3Ni2 alloy phase after the CLG process and the lattice oxygen can be fully recovered in air atmosphere. Although the BET surface displayed a decrease trend in the regenerated oxygen carriers, serious sintering was not observed in the samples as well as the main metallic crystallized phases were still maintained.

1. Introduction It has been known that biomass displays great application prospect in generating electricity and synthesizing liquid fuels through the way of biomass gasification

1, 2

.

Conventionally, biomass gasification is carried out by using oxygen or steam as the gasifying medium

3, 4

. Whereas, this technology requires much energy and

complicated equipments to supply the pure oxygen or steam5. Also, the syngas is mixed with N2 which reduces the heat value of synthesis gas and makes it unsuitable


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for the chemical synthesis when the gasifying agent is replaced by air or N2-steam mixture. In addition, high content tar is generated from the traditional gasification process of biomass6, 7. The tar may contaminate or block the pipes and reactors in the biomass gasification process due to its condensation and viscosity 8. It has been known that the biomass chemical looping gasification (CLG) process may supply high heat value and low tar content synthesis gas with relatively simple devices at a lower cost9. The CLG of biomass can produce syngas by employing the lattice oxygen from oxygen carrier to replace molecular oxygen, which can avoid pure oxygen or steam preparation and cut down the equipment investment. Also, the caloric value of syngas is higher due to the nitrogen dilution. Besides, tar can be decomposed and reformed in the CLG process due to the catalyst effect of oxygen carrier10. There are two separated stages in the CLG system, which are fuel conversion stage and oxygen carrier regeneration stage. In fuel conversion stage, small molecular gas, tar and char firstly decompose from the biomass and then the mixing products are reformed by oxygen carrier to generate syngas. The reduced oxygen carrier reacts with air to recover lattice oxygen in oxygen carrier regeneration stage. The two processes are carried out repeatedly and alternatively to make the CLG system work effectively and generate synthesis gas continuously 11. For the CLG of biomass, it is of great importance to develop oxygen carriers with high lattice oxygen storage and transport capacity with which convert biomass to desirable synthesis gas12. Also, the mechanical strength and anti-sintering of oxygen carrier need to be concerned13. Up to now, metallic oxides including Cu, Fe, Co, Ni, Mn13-18 as well as nonmetallic oxides such as CaSO4 19, 20 have been widely studied as oxygen and heat carriers in Chemical looping combustion (CLC) or CLG process. Among them, Ni-based oxygen carriers have attracted much attention due to their


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higher redox activity and catalytic effect to decompose tar. Nevertheless, Ni-based oxygen carriers suffer from the toxicity to environment and the coke deposition

21

.

Iron oxide shows some obvious advantages in extensive source, anti-sintering and cost savings for the application of CLC or CLG, but there are also some defects for iron oxygen carrier like poor reactivity and low oxygen migration capacity15,

22

.

Several strategies such as searching for suitable preparation process and combination with metal oxides (NiO-Fe2O3, LaFeO3-CeO2, Fe2O3-MnO2) have been applied to improve the lattice oxygen reactivity and create synergistic effects between different metals

23-27

.Particularly, the mixed-metal oxides Fe2O3-NiO are very attractive28. The

complex metal oxygen carriers usually display better reactivity, anti-sintering, coke resistance and lower toxicity than the corresponding monometallic oxygen carrier. However,

there

are

still

difficulties

in

preparing

highly-dispersed

and

thermo-stabilized compound metal oxygen carriers. A promising approach to achieve ideal mixed-metal oxygen carriers is to synthesize the hydrotalcite-like compounds (HTlcs) precursor followed by calcination at high temperature. HTlcs are layered double hydroxide (LDH) that are formed by interaction of the positively charged main sheets and interlayer anion through the non covalent interaction. HTlcs have a brucite-like structure and divalent cations M2+can be substituted by the trivalent cation M3+. Accordingly, the charge balance of the layger is compensated by anions and interlayer space is occupied by water molecule29, 30

. The generated formula is represented as [(M2+)n(M3+)m(OH)2(n+m)]m+[(Ax-)m/x]·yH2O, in

which the M3+ and M2+ are the layer forming trivalent and divalent cations and Ax− is interlayer anion31. After calcination, the HTcls structure decomposes to generate complex metal oxides which exhibit the advantages in metal dispersion, reactivity and thermal stability. It has been reported that HTcls have been used to prepare catalysts


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for H2 production by catalytic reforming of methane

12, 32

. Moreover, the catalysts

derived from the HTcls are widely investigated for ethanol, biomass tar steam reforming and carbon nanofibers synthesis exhibiting high catalytic performance, anti-coke formation and heat stability31, 33-35. The Mg/Fe/Ni/Al compound catalyst from HTcls is suitable for the steam reforming of biomass tar, phenol and toluene than the Mg/Al/Ni samples. Also, it is better than the Ni/Fe/Al2O3 catalyst synthesized by the ordinary impregnation method 36. It is also found that complex metallic oxides prepared from HTcls have some applications of CO2 sorbents or the CLC process for the oxygen carriers. Qilei Song et al. 12applied the Cu–Al HTcls precursor to prepare oxygen carriers which are suitable for the chemical looping process with the characteristics of high dispersion and high metal loadings. They found that the HTcls precursor make the metallic elements mix homogeneously on a molecular level. Moreover, the Cu-based oxygen prepared by the same method displayed stable release and storage capacity of the gaseous O2 as well as the steady reaction rates in reversible phase changes. However, the other kinds of oxygen carrier from HTcls precursor and the specific chemical looping process (e.g. CLG) has not been involved in this paper. Besides, alloy nanoparticles phases and the synergy between different metals in the oxygen carriers from the HTcls precursor are needed to make further study. In the present study, a series of Fe-Al-Ni HTcls precursor are synthesized by co-precipitation at constant PH with molar ratio of Fe: Ni varied to obtain well-dispersed, high activity and thermally stable oxygen carrier for the CLG process as well as investigating the synergetic effect between Fe and Ni in the oxygen carriers. Various analysis techniques are applied to characterize the structure–activity relationship of the oxygen carriers. In addition, the gas-solid reaction between


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biomass pyrolysis gas and oxygen carrier is prone to occur for its more rapid diffusion and mass transfer rate. Therefore, the solid-solid reactions between the oxygen carrier and residue biomass char become the rate controlling steps of CLG process. Further investigation on the solid-solid reaction contributes to the full reaction of biomass CLG process37. Consequently, the biomass char CLG reaction performance with the oxygen carrier from HTcls precursor was researched in the present study by using a thermal gravimetrical reactor coupled with fixed bed reactor at an argon atmosphere. 2. Experimental 2.1 Oxygen carrier synthesis The Fe-Al-Ni HTcls precursors were synthesized though co-precipitation at constant PH and room temperature. Briefly, the analytically pure of Ni (NO3)2•6H2O, Al (NO3)3•9H2O and Fe (NO3)3•9H2O were mixed together and prepared in to aqueous solutions, which were then fed into a boiling flask-3-neck dropwise under vigorous stirring with the Mixing alkaline solution (NaOH/Na2CO3, 1 mol of each) simultaneously. The PH value of the solution was maintained at 9.6± 0.2. The resulting suspension was aged for 24h at room temperature to obtain precipitate followed by the procedures of filtration, washing with de-ionized water and drying at 65oC in air for 24h next. The resulting precursors were calcined by muffle furnace at 900oC for 6h at air atmosphere to get the oxygen carriers. The (Ni +Fe)/Al molar ratio was constant at 2 and the Fe/Ni molar ratio was 0:1, 1:1, 2:1, 3:1, 4:1 and 1:0 in the oxygen carriers, which were denoted as 1#-6# sample correspondingly. 2.2 Char of biomass The char of biomass was achieved by pyrolysis of pine sawdust collected from


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the province of Guangdong (China) in tube furnace reactor. Briefly, the pine was grinded and sieved into sawdust, the size range of which was between 0.3mm and 0.45mm. Next, the sawdust particle was loaded into a crucible which was placed in the cold side of a tube furnace reactor. After that, the argon was passed though the tube reactor to substitute the air atmosphere with the increasing of temperature. The reactant was pushed into high temperature section of the tuber furnace by a connecting rod at the temperature of 800 °C. Next, the sawdust pyrolyzed at argon atmosphere for 10 minutes. Finally, the residual char was promptly put out and quenched at argon atmosphere in the low temperature side of tuber furnace. Ultimate and proximate analysis of the pine char were shown in table 1. Table 1. Ultimate and proximate analysis of biomass char Proximate analysis (wt /%,db)

V

FC

A

Ultimate anlysis (wt/%,db)

C

H

O

N

LHV/（MJ/kg,db）

S 30.24

13.66

81.33

5.01 85.55

2.22

11.7

0.52

0.01

FC: fixed carbon; db: dry basis; V: volatile matter; A: Ash; LHV: lower heating value

2.3 The CLG reactions A TGA reactor (STA409C/PC, NETZSCH) was applied to test the reaction activity of the oxygen carriers with biomass char at N2 atmosphere. The oxygen carrier and biomass char were premixed and ground into powder with an agate mortar. An amount of 15mg of mixed sample was added into the TGA crucible. The temperature range setup was 30 ºC- 1000 ºC with a 20 ºC /min heating rate. The flow


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rate of total shielding gas was 20 ml/min (room temperature), which was adjusted by a mass flowmeter. To further study the reaction activity of the derived oxygen carrier and analysis gas product, a fixed bed experiment was conducted after the TGA test. The system was consisted of a quartz tube, a heating furnace, a biomass hopper with ball valve, a gas clean and dry unit and a gas analysis system with computer, as shown in Fig.1. In the procedure of each round, an amount of oxygen carrier sample was added into the quartz tube in fixed bed reactor and specific weight of biomass char was put into the hopper, which was on the top of quartz tube and sealed with inert balance gas after feeding procedure. The tube furnace was adopted electric heating to control the heating rate of 20 oC /min at the atmosphere of Ar. Once heated up to the operating temperature (850 oC), the ball valve under hopper was turned on to put the biomass char with a balance gas of Ar. The CLG process occurred between the char and oxygen carrier to generate syngas in this tube furnace reactor. The flow rate of inert carrier gas and balance gas were 100 ml/min and 50ml/min respectively (room temperature), which were adjusted by mass flowmeter. The exit gas of tube furnace was analyzed by Agilent gas chromatograph（7890A）after removing the tar and water by gas cleaning and drying unit. The reacted oxygen carrier was oxidized by air to recover lattice oxygen after the CLG reactions. The reduction phase of oxygen carrier was sustained 55 min to achieve higher conversion and the oxidization section was kept for 25 min to prevent the sintering occurrence. In the reduction process, all the syngas was collected by the gas bags of 500ml at specific intervals. The volume of


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generated synthesis gas was calculated on base of N2 balance and the gas generated rate was calculated according to the gas volume, concentration and collected time of each gas bag.

Fig. 1 Schematic diagram of the CLG experimental device

2.4 Characterization of oxygen carriers Various analysis method and technology ware adopted to characterize the crystal phase, microtopography and reactivity of the original and used oxygen carrier samples. The crystal form of the fresh and used samples were analyzed by the X-ray diffraction (XRD, X’Pert PRO MPD) applying Cu Kα (40kv, 40mA) with a scanned rate of 2° min-1 and a step of 0.0167° in the 2θ range of 10°-80°. X-ray fluorescence (AXIOSMAX-PETRO, XRF) was used to detected the element composition of the oxygen carriers and the H2-temperature programmed reduction (TPR) test was investigated on TPR (Quantachrome, CPB-1) with 150mg sample in the condition of


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a flow of 5% H2/He(120ml•min-1) and heating rate of 10ºC/min. A Hitachi S4800 instruments was used to observe the surface microtopography of the samples and the BET surface area for N2 physisorption was inspected by Micromeritics ASAP 2010 instruments. The oxygen carrier was removed air under the vacuum conditions at temperature of 573K for 3 hours before test. 3. Result and discussion 3.1 XRD analysis The HTlcs (LDH) precursor and the calcined samples were characterized by XRD to indentify the crystal form, as displayed in Fig.2 and Fig.3. The precursor samples showed pure LDH crystalline structure and the oc1 precursor exhibited typical symmetric and sharp diffraction at the (003), (006) and (009) peaks coupled with asymmetric and wide peaks at the (015) and (018) planes. With the increasing of Fe content from oc1 precursor to oc5 precursor, the diffraction patterns of the basal (003), (006) and (009) planes was decreased due to fluorescence effect. However the basic reflections formations were still preserved. While for oc6 precursor without the Ni, the LDH structure characteristic diffractions peaks appeared obvious attenuation. The reason can be explained Fe3+ had a larger ion radius of 0.76 A, in comparison, the Ni2+ and Al3+ ion radius were 0.69 and 0.60 A. This factor as well as the fluorescence effect made the lattice of HTlcs precursor hard to receive the Fe3+ ion 38. The XRD diffractions of calcined productions were shown in Fig.3. It was observed that the complex oxides were well dispersed in the oxygen carriers. As the LDH structure collapsed, NiAl2O4 as well as Al2O3 are the main phase in the pure Ni


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oc1 sample and the Fe0.99Ni0.6Al1.1O4 appeared with the concentration of Fe element increasing. When the Fe concentration was increased further, the formation of Fe2O3 was accompanied by considerable sharpening of diffraction peaks. And the reflections of Fe2O3 and Al2O3 were observed in the HTlcs calcination sample 6#. 003

006

009

015 018

110

OC1 pre OC2 pre

Intensity/a.u.

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OC3 pre OC4 pre OC5 pre OC6 pre 10

20

30

40

50

60

70

80

Degree(2θ)

Fig. 2. XRD diffraction patterns of the HTlcs precursors.


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◊ ∆ ∇

∇

•∇

∇

•∇

•

20

◊ ∆

♦ ∇ ∆ ♦

oc1

∆ ◊ ∆

∆

• • ∇ ∇ • ∇♦ ♦

♦ ♦ • •∇ •

♦

♦ • ♦ ∇ ∇ • ∇• ♦ • ♦ ∇ • • ∇♦ • •

♦ ♦ • • ∇ •

♦ •

• ♦

♦ •

• ♦

••

• oc6

• 10

◊ ∇

♦

♦ ∇

∆

∇ ♦

♦

∇

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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∆ ♦

•

∇ 30

♦

• • ♦ ♦

•

•

• ∇

40

∇ 50

•

oc2 ♦

oc3 ♦

60

oc4 oc5

70

80

2θ

Fig.3 –XRD patterns of Fe-Ni-Al oxygen carrier derived from HTlcs precursors ◇NiAl2O4

△NiO

▽Al2O3

◆Fe0.99Ni0.6Al1.1O4

●Fe2O3

3.2 XRF analysis It has been reported that the HTlcs compounds were synthesized with an M3+/ (M2++M3+) ratio of 0.2 -0.3339. In current research, the ratio of (Fe+ Ni)/Al was constant at 2 and the Fe/Ni molar ratio was 0:1, 1:1, 2:1, 3:1, 4:1 and 1:0 in the oxygen carriers, which was considered as the suitable value for the preparation of HTlcs precursors according to previous experiments. The elemental analysis of Fe, Ni and Al elements in oxygen carriers were shown in Table 2, which manifested the similar trend with the preparation stoichiometric value. Additionally, some trace elements like Mg, Ca, Si and Na were observed as impurities in the preparation process, which we had known that the Si and Al can be used as inert carrier to improve the BET surface area and resist sintering of oxygen carrier40. Also, the alkali


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metal element like K, Na can make positive influences on the oxygen carrier reactivity41. Theoretically, the detected elements in the oxygen carriers were only Fe, Ni and Al elements according to the prepared process. However, there were trace elements appeared in the oxygen carriers due to the impurities from reaction reagent and preparation process. OC1 O Ni Fe Al Cl Mg Ca Si Cr

Table2- Elemental analysis of oxygen carrier OC2 OC3 OC4 OC5

OC6

Conc. %

Conc. %

Conc. %

Conc. %

Conc. %

Conc. %

25.35 62.80 0.01 11.41 0.06 0.03 0.18 0.04

30.71 29.83 24.43 11.62 0.03 0.03 0.18 0.05 0.02

31.29 19.69 34.83 11.19 0.06 0.03 0.08 0.03 0.02

30.29 19.34 34.55 10.86 0.05 0.02 0.10 0.05 0.02

31.21 12.65 32.73 10.43 0.04 0.03 0.09 0.07 0.02

31.94 0.02 51.87 10.81 0.04 0.03 0.29 0.08 0.03

3.3 H2-TPR analysis Fig.4 indicated the temperature programmed reduction curves of the derived oxygen carrier. The TPR experiments were implemented in the temperature range from 0 oC to 1000oC and the ratio of Fe/Ni determined the peaks of hydrogen consumption in a different profile. For the Ni/Al HTlcs derived oxygen carrier oc1, there was a reduction peak at 540oC, which might be ascribed to the presence of Ni2+ species in the NiAl2O4 in the external layers of solid combined with the results by XRD analysis. The main H2 uptake exhibited a successive wide peak from 540 to 960 o

C and the temperature of pure NiO reduction peak was lower than that in HTlcs

structure in the current work. These phenomena can be ascribed to the well mixing of Al2O3 and the NiAl2O4 phase formation34. In addition, the calcined Fe/Al oxygen


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carrier (oc6) exhibited a narrow peak at 530oC due to the reduced process of Fe3+ to Fe2+. There were also a successive wide peak followed by the narrow peak and it can be attributed to the further reduced process of Fe2+ to metal Fe38. The reduced process had not reacted completely even at 965 oC for the reason that Fe is hard to reduce and the mixing Al2O3. When Fe element was introduced in the HTlcs derived oxygen carriers, the reduction of NiO was shifted to lower temperature due to synergistic effect between Fe and Ni. With Fe contents increasing in oxygen carriers, the first reduction peak of NiO was shifted from the 448 oC to 490 oC and it had been reported previously that the Ni and Fe bimetallic system was more helpful to increase the reduced activity compared to Fe and Ni

28

. Obviously, the complex oxides of Fe and Ni usually

indicated better oxygen transfer performance than the corresponding monometallic oxygen carrier in TPR process. Additionally, a successive reduced stage and wide peaks had been detected at temperature range from 650 oC to 960 oC in these mixed oxides. Combined the XRD analysis, Fe0.99Ni0.6Al1.1O4 and Fe2O3 phases were formed as the increasing Fe concentration, which caused the H2 uptake at the temperature range. Also, the broad peak suggested well dispersed oxides derived from HTlcs structure precursors.


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oc1: Fe/Ni =0:1 oc2: Fe/Ni =1:1 oc3: Fe/Ni =2:1

H2 consumpation (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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oc2

oc1

oc3

oc4: Fe/Ni =3:1 oc5: Fe/Ni =4:1

oc6

oc6: Fe/Ni =1:0

oc5 oc4

0

100

200

300

400

500

600

700

800

900 1000

Temperature oC

Fig.4 represents the TPR profiles of the calcined oxygen carriers 3.4 TGA investigation 3.4.1 The different HTlcs derived oxygen carriers CLG process The different oxygen carriers from HTlcs precursor were investigated in a TGA reactor to discuss the CLG reactivity with biomass char at the mass ratio 3:2, which was the optimal ratio of oxygen carrier and biomass char for the CLG process according to our previous research42. The data were illustrated in Fig.5 (a) and (b). It was found that the char CLG process of six oxygen carriers appeared three stages, drying and dehydration (Ⅰ), further pyrolysis (Ⅱ), oxygen carrier reduction(Ⅲ,Ⅳ). In the drying and dehydration stage which was occurring below 140 oC, the free moisture in the oxygen carrier and biomass char mixture was dried with the rising temperature. Next, the volatile matter in the biomass char escaped at the further pyrolysis stage between 220 oC and 300 oC. The reaction between biomass char and oxygen carriers occurred to produce syngas at oxygen carrier reduction stage. Furthermore, in the whole reactions, it was noted the reaction performance of the


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samples were improved with the increase of Fe concentration and the mixtures of oc5 (Fe/Ni, 4:1) had the maximum weight loss 61.25% and largest mass loss rate of 4.89 wt %/min at 765 oC. Additionally, the samples oc3 (Fe/Ni, 3:1) also showed better mass loss rate of 2.99 wt %/min at 766 oC. In comparison, the pure Ni/Al oxygen carrier and pure Fe /Al samples indicated a relatively poor performance. The sample oc1 with the biomass char displayed minimum weight loss 71.88% and the pure Fe /Al sample reduction weight loss peak located at 919 oC, which was the highest reduced temperature among the six samples. The results implied that the reactivity of the oxygen carriers derived from HTlcs was improved by the synergistic effect between Fe and Ni elements and there was an optimum ratio of Fe/Ni in oxygen carriers to achieve the better reactivity performance. Furthermore, the sample oc 5 was an appropriate oxygen carrier candidate for char CLG process.

100

0.5 0.0

90

-0.5 -1.0

oc1:char=3:2 oc2:char=3:2 oc3:char=3:2 oc4:char=3:2 oc5:char=3:2 oc6:char=3:2

80 70 60

71.88% DTG %/min

Mass %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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66.53%

-1.5 -2.0

I

-2.5

II

-3.0 -3.5 -4.0

61.25%

oc1:char=3:2 oc2:char=3:2 oc3:char=3:2 oc4:char=3:2 oc5:char=3:2 oc6:char=3:2

IV

III

-4.5

50

-5.0 -5.5

40 200

400

600

Temperature oC

800

1000

200

(a)

400

600

800

1000

o

Temperature C

(b)

Fig.5 The reactivity of the HTlcs derived oxygen carriers with biomass char 3.4.2 The reactivity of the different mass ratio of oxygen carriers and char The reaction performance of different mass ratio of derived oxygen carrier/char (i.e., 7:3, 1:1, 4:6, and 6:4) were investigated in TGA with an inert atmosphere, as


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displayed in Fig.6 (a), (b). The residual mass of the four mixtures were 64.41%, 67.17%, 70.49%, and 73.95 wt % at the terminal temperature 1000℃, respectively. It was apparent that the mixtures with the oxygen carriers to biomass char ratio of 7:3 provided the maximum weight loss to 35.59% and the largest mass loss rate about 2.46 wt % / min at the 912℃ suggesting that the mixtures with the ratio 7:3 had the larger reaction rate and higher lattice oxygen releasing efficiency. According to the reaction kinetics mechanism and Le Chatelier's principle, the plenty lattice oxygen supplying increased the reaction rate and made the reaction equilibrium shift to product generated direction. Therefore, the samples 7:3 displayed the larger reaction rate and higher carbon conversion efficiency. Whereas, as a coin had two sides, the equilibrium shift also made the char reaction process prone to generate H2O and CO2 rather than H2 and CO, which was the target of CLC. Furthermore, the other samples with the ratio of 1:1, 3:2 and 2:3 performed lower weight loss successively. The reasons were viewed as the oxygen carrier can not supply sufficient lattice oxygen to react with the increasing of biomass char.

It was noted that there was a sharp mass

loss peak at the temperature 756℃ for the sample 3:2, which manifested better mass loss rate in the four mixtures at lowest temperature. According to the reaction kinetics mechanism43, the char CLG process was considered a complex solid-solid reaction for that the volatile matter in the biomass had been pyrolysed and released. Fully contact between the oxygen carrier and biomass char was the reaction premise. Low content oxygen carriers or biomass char in the mixtures made the reactants can not achieve plenty contact resulting lower reaction rate and weight loss. Though the sample 3:2


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does not indicate the largest weigh loss due to total quantity restrain of lattice oxygen, it displayed better reaction performance at lower temperature. Moreover, too much lattice oxygen supplied was harmful to the synthesis gas generation. Combined with the XRD analysis results, the reduced peak at 756℃ was assigned to the wustite decomposed from Fe0.99Ni0.6Al1.1O4. Then the metal oxides were successively reduced to metal Fe. Theoretically, the oxygen carrier reduction process can be described as the follows: Fe0.99Ni0.6Al1.1O4→Ni+ Fe1-xO+Al2O3→Ni+ Fe +Al2O3. To investigate the impact of oxygen carrier in char CLG process, the baseline experiments were carried out with oxygen carrier/biomass char ratio 3:2 at the same time. In the baseline experiments, the oxygen carriers were substituted by pure Al2O3 and Fe2O3 impregnated with NiO which was the same stoichiometric number to the derived oxygen carrier. The results manifested there were lower mass loss in the baseline experiments; they were only 10.46% and 21.54% weight loss for the Al2O3 and impregnation oxygen carrier. It was obviously that the oxygen carriers played a key role in the biomass char CLG process and the derived oxygen carrier from HTlcs structure exhibited better reaction performance due to the high dispersion of aluminum, iron and nickel oxides and the strong mutual synergistic effect. The results were agreement with literatures10, 18.


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Page 19 of 29

100

0.5 oc:char=1:1 oc:char=2:3

89.54%

0.0

90

oc:char=3:2

Al2O3

78.46% 73.95%

80

oc:char=1:1 oc:char=2:3

70

Fe2O3 impregnated

-0.5

oc:char=7:3

DTG %/min

Mass %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70.49%

oc:char=3:2 67.17%

Al2O3:char=3:2 Fe2O3 impregnated Ni(NO3)2:char=3:2

60

64.41%

-1.0

oc:char=7:3 oc:char=1:1

-1.5

oc:char=2:3

-2.0

oc:char=3:2 Al2O3:char=3:2

oc:char=7:3

Fe2O3 impregnated Ni(NO3)2:char=3:2

-2.5

200

400

600 Temperature oC

800

1000

200

(a)

400 600 Temperature oC

800

1000

(b)

Fig.6 The reactivity of different mass ratio of oxygen carriers and char 3.5 The CLG experiments in fixed bed The biomass char CLG experiments were implemented in fixed bed reactor to further evaluate the reaction performance of oxygen carriers, as shown in Fig.1. Based on the TG analysis above, the oxygen carrier of oc5 showed the largest weight loss and reaction rate. Therefore, the oc5 sample was further investigated to reveal the evaluation rule of the CLG reaction. The mass ratio of oc5 and char was 1:1 to ensure that the char can take reactions for enough time and the product gas was collected by GC analysis at dozens of seconds. The experiments results were represented in Fig.7. It was found that the lattice oxygen [O] of oxygen carrier reacted with biomass char to generate synthesis gas like H2, CH4, CO and CO2. The reaction rate increased sharply at the initial stage of the test and reached a maximum value at the time 120s after which the reaction tended to be stable. This depended on the lattice oxygen supplied and heat transferred between char /oxygen carriers, which was delayed at the beginning and then achieved the maximum rate. It was evident that the reaction rate tended to decrease and level off with the lattice oxygen depletion. The reaction formula should be expressed by the follows on the ground of the XRD analysis of


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oxygen carriers: Fe0.99Ni0.6Al1.1O4 →Fe2O3+NiO+Al2O3

（1）

Fe2O3→FeO+[O]

（2）

FeO→Fe+[O]

（3）

NiO→Ni+[O]

（4）

C+[O] →CO

（5）

CO+[O] →CO2

（6）

H2+[O] →H2O

（7）

C+H2 →CH4

（8）

Since the biomass char is composed mainly of the fixed carbon, which is more than 81% according to the proximate analysis of biomass char. The fixed carbon CLG and carbonization processes were evidently enhanced because of the fully contact between the lattice oxygen [O] and fixed carbon. Accordingly, the CO gas achieved the highest reaction rate and gas volume in the gas production, as shown in Fig.7. Although the intermediate gas H2 was generated in char carbonization process, a portion of the gas was consumed fast via oxidation, methanation, alkylation, aromatization and polycondensation reactions like reaction 7 and 8. On the other hand, since these reactions like H2, CO with lattice oxygen [O]（（5）-（7））are competing reactions, the H2 was more prone to be consumed than CO in that the former reaction had lower reaction activation energy. Similar results that H2 manifested better reactivity than CO in chemical looping process had been reported by the literatures, which caused the lower content of H2 in the gas production44. Therefore, that H2


Page 21 of 29

generation rate exhibited a lower value than CO which was ascribed the above two reasons. By contrast, the reactions (6) and (8) were belonging to secondary reactions with more complex reaction mechanism than the primary carbonization and CLG process. The other correlative reactions like (9), (10) were also restricted these reactions further proceeding. Consequently, the gas CO2 and CH4 manifested a smaller generation rate and volume. （9）

CO2+C→CO

（10）

CH4+O2→H2+CO 6

1200

5

1000

4

volume /ml

Gas generatured rate/ ml/s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CH4 CO2

3

H2 CO

2

800 600 400 200

1

0

0 0

500

1000

1500

2000

2500

3000

3500

CH4

Time / s

CO2

CO Gas production

(a)

H2

(b)

Fig.7 CLG reaction evaluation process in Fixed bed 3.6 XRD analysis for reduced and regenerated oxygen carriers The reduced oxygen carriers were regenerated at the air atmosphere at 850 oC when the CLG reaction had been done and the crystal form of the reduced and regenerated oxygen carriers were characterized though the XRD analysis, as illustrated in the Fig.8. The fresh oxygen carrier was consisted of Fe2O3, Fe0.99Ni0.6Al1.1O4, Al2O3, etc. The oxygen carrier was reduced to Fe3Ni2 (03-065-5131)

cubic crystal alloy phase though the reactions (1) to (4), indicating that the lattice


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oxygen was depleted and fully converted to synthesis gas. Moreover, the Fe0.99Ni0.6Al1.1O4 and Fe2O3 diffraction peaks were easily observed in the regenerated

samples, which manifested that the lattice oxygen of oxygen carrier was totally recovered and the NiO, Fe2O3 were highly dispersed in Al2O3 at molecular level to form new compound phase. The results are in accordance with the literatures34, 42. ♦ • •

♦

∇

• • ♦

•

♦ • ♦ •

Fresh

♦ •

∇

• ♦

Fe Ni 3 2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Reduced Fe Ni 3 2

Fe Ni 3 2

Fe Ni 3 2

♦

Regenerated ♦ • 10

20

30

• ∇

• • ♦

•

40

50

• ♦ ♦ •

♦ •

60

∇ • ♦ 70

80

2θ

Fig.8 XRD patterns for the reduced and regenerated oxygen carriers ◇NiAl2O4

△NiO ▽Al2O3

◆Fe0.99Ni0.6Al1.1O4

●Fe2O3

3.7 SEM and BET analysis The shape and microstructure of fresh and regenerated oxygen carrier particles were observed by SEM, as illustrated in Fig.9. The morphology of fresh oxidizes exhibited irregularly granular structure with an average size below 2µm and the porous structure made the reactant gas easy to diffuse into the inner layer of oxygen carriers increasing the CLG reaction rate. Particles of Al2O3, Fe2O3 and NiO were incorporated into a coherent whole, which can not be distinguished between each other suggesting highly dispersion effect. However, some particles were agglomerated into the massive structure with a size 6-8um after the reaction process. Moreover, the


Page 23 of 29

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phenomena of sintering were observed in some regenerated oxygen carrier particles. Although the particle size of the regenerated samples increased and the surface became tighter, the serious sintering was not observed in the samples. Accordingly, the BET-surface area, total pore volume and average pore diameter of the regenerated oxygen carrier expressed the contrary trend, which decreased from 10.342m3/g, 0.035cc/g, and 13.7nm, to 5.918 m3/g, 0.017 cc/g, and 11.5nm, respectively. However, the main diffractive peaks and the phase of the oxygen carrier had been detected by the XRD patterns suggesting that the oxygen carriers were still suitable to the reaction of CLG.

Fresh samples

Regneratered samples

Fig.9 XRD analysis for the reduced and regenerated samples of oc5 3.8 Carbon balance analysis of the CLG process The carbon balance during the CLG process at 850 oC was analyzed, as illustrated in Table 3. The input carbon was evaluated through the biomass char mass. The output carbon mass was calculated with the carbonaceous gas content and residual char. The mass of liquid product was too little to be ignored. The gas yield was evaluated on the base of the Ar balance between the inlet and outlet gas of the


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fixed bed reactor. The gaseous product volume (vi) of each species in the fixed bed rector was calculated as (11): t

vi =

∫ ∫ 0

v Ar xi dt t

0

t

=

xAr dt

∫v 0

∫

t

0

x dt

Ar i

(1 − xCO − xCO 2 − xH 2 − xCH 4 )dt

(11)

Where the xi denotes the volume composition of the gas i and vAr is the volume flow of Ar. Carbon conversion efficiency (ηconversion) in the reaction process was represented as Eq. (12):

ηconversion =

(

12 × Vco2 + Vco + VCH 4

)

22.4 × (303 / 273) × Mc%

×100%

(12)

Where the Mc (%) was the carbon fraction in biomass char, Vco, Vco2, VCH4 indicated the volume of CO2, CH4 and CO in the syngas of production. The proportion of output carbon mass in input carbon mass was defined as the recovery of the CLG process. Also, the separated residual fine char Cresidual was calculated according to the total mass of the reactants before and after reactions, which was shown as follows: Cresidual = ( mre − moc (1 − ϕo )) × Mc%

(13)

Where the moc meant the mass of fresh oxygen carrier and mre represented the residual reactants after reaction. Also, theφo was expressed oxygen fraction in the oxygen carrier. The table3 illustrated that carbon conversion efficiency ηc of the char CLG process was 36.65%, which was relatively lower than that of biomass CLG reaction according our previous experiments11. Due to the lower volatile in the biomass char, it was relatively difficult for the oxygen carrier to react with the char in solid–solid reaction indicating that higher carbon conversion efficiency in char CLG


Page 25 of 29

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process needs more reaction time as well as higher temperature. Also, it was found that the recovery of carbonaceous gas coupled with fine carbon separated from the residues was 91.33%, which suggested that the test process was reliable. Additionally, 8.67% percent of total input carbon was mainly included the liquid products which are neglected and the fine char adhered to surface of the fixed bed reactor. Table 3. Carbon Balance Analysis of the CLG process Input carbon mass

1.627g

Recovery (%)

Output carbon mass (STP) VCH4

VCO2

VCO

ηC

ml

ml

ml

%

residue carbon

37.11 94.59 983.76 36.65

91.33 0.89g

STP: Standard Temperature and Pressure

4. Conclusion In the present study, a series of Fe-Ni-Al compound oxygen carriers were prepared from hydrotalcite precursors by co-precipitation with varied molar ratio of Fe and Ni to obtain high dispersion and reactivity multi-metal oxygen carrier for the char CLG process. XRD, TPR, SEM and BET analysis techniques were used to characterize the structure–property relationship of the oxygen carriers. The reaction activity of the oxygen carriers in CLG of biomass char was investigated by the TGA and fixed bed reactor and the experiments results were summarized as follows: The layer hydrotalcite-like structure of Fe-Ni-Al precursor was generated and it was lost to generate Fe0.99Ni0.6Al1.1O4 crystallize phase when calcined at 900 oC, which suggested that the metal element can be dispersed at molecular level. The main H2 uptake of the oxygen carriers in the TPR curves were related to its higher metal


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Page 26 of 29

dispersion and synergistic effect between Fe and Ni. The oxygen carriers played a key role in the biomass char CLG process and there was an optimum Fe / Ni ratio of 4:1 in oxygen carriers to achieve the better CLG reactivity performance. Also, the mass ratio oxygen carrier and biomass char 7:3 indicates the maximum weight loss 35.59% and largest mass loss rate 2.46%wt/min suggesting higher lattice oxygen releasing efficiency. Additionally, the CO achieved the highest generation rated and gas volume in the CLG process in that H2 was more prone to be consumed for its lower reaction activation energy. The oxygen carrier was reduced to Fe3Ni2 alloy phase after the reduction process and the serious sintering was not observed in the samples.

Acknowledgements This work was financially supported by the Science and Technology Projects of Guangdong (2015A020215023, 2015A010106009), National Natural Science Foundation of China (51406208), Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y709jj1001) and State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology,

Taiyuan

University of

Technology

(MKX201701).

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