Minimizing tar formation whilst enhancing syngas production by integrating biomass torrefaction pretreatment with chemical looping gasification

Applied Energy 260 (2020) 114315

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Minimizing tar formation whilst enhancing syngas production by integrating biomass torrefaction pretreatment with chemical looping gasification

T

Yuyang Fana,b,c,d, Nakorn Tippayawonge, Guoqiang Weia,b,c, Zhen Huanga,b,c, Kun Zhaoa,b,c, ⁎ Liqun Jianga,b,c, Anqing Zhenga,b,c, , Zengli Zhaoa,b,c, Haibin Lia,b,c a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China c Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d School of Energy and Environment, Southeast University, Nanjing 210096, PR China e Department of Mechanical Engineering, Chiang Mai University, Chiang Mai 50200, Thailand b

HIGHLIGHTS

torrefaction with chemical looping gasification (CLG) was proposed. • Integrating content was reduced by 88.8% through integrating torrefaction with CLG. • Tar yield was enhanced by 27.5% through integrating torrefaction with CLG. • Gas • Effect of torrefaction on the possible pathways for tar formation was revealed. ARTICLE INFO

ABSTRACT

Keywords: Biomass Torrefaction Chemical looping gasification Tar formation mechanism Syngas

The objective of this study is to investigate the effect of torrefaction pretreatment on the syngas production and tar formation from chemical looping gasification (CLG) of biomass over different oxygen carriers. The torrefaction of eucalyptus wood and subsequent CLG were systematically studied by using the fixed bed reactors coupling with various analytical methods. The experimental results demonstrate that torrefaction played significant impacts on CLG of eucalyptus wood using iron ore as an oxygen carrier. The gas yield and carbon conversion efficiency from CLG of eucalyptus wood were lowered by torrefaction, while the tar content was evidently reduced from 43.6 to 17.6 g/Nm3. These results could be due to the devolatilization, polycondensation, and carbonization of eucalyptus wood during torrefaction, resulting in the formation of fewer tar precursors and more char with lower reactivity during subsequent CLG. The negative impacts of torrefaction on the gas yield and carbon conversion efficiency of CLG can be effectively overcome by the selection of suitable oxygen carriers. Five metallic ferrites were successfully synthesized and used to replace iron ore for CLG of torrefied eucalyptus wood obtained at 280 °C. It is found that NiFe2O4 reduced the tar content by 88.8% and improved the gas yield by 27.5% compared to CLG of untreated eucalyptus wood over iron ore. These results suggest that integrating biomass torrefaction pretreatment with CLG is an efficient strategy for enhancing syngas production whilst minimizing tar formation.

1. Introduction With the growing concerns associated with fossil fuel shortages and climate change, biomass has emerged as a potential candidate to replace fossil fuels for the production of heat, electricity, hydrogen, chemicals and liquid fuels [1–3]. Chemical looping gasification (CLG) of biomass is a novel technology for converting solid biomass into

⁎

syngas by using active lattice oxygen in oxygen carriers instead of molecular oxygen [4–6]. Biomass can be partially oxidized in a fuel reactor for the production of syngas by using metal oxides as oxygen carriers. The reduced metal oxides can be recovered by reacting with steam and air in an air reactor [7–9]. During the reduction-oxidation cycles, oxygen carrier mainly acts as a solid medium for oxygen supply and heat transfer. The typical metal oxides used as oxygen carriers

Corresponding author at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail address: [email protected] (A. Zheng).

https://doi.org/10.1016/j.apenergy.2019.114315 Received 7 September 2019; Received in revised form 27 November 2019; Accepted 1 December 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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include Fe, Cu, Ni, Mn, Co, Ca and so on [10–12]. Compared to the traditional biomass gasification, chemical looping gasification can effectively lower the cost and energy penalties of CO2 capture by eliminating the need for gas separation units to achieve a non-nitrogen diluted syngas [10]. Moreover, similar to traditional biomass gasification, CLG of biomass faces many challenges associated with the physicochemical properties of biomass feedstocks. The unfavored properties of biomass feedstocks, such as high moisture and oxygen content, low bulk and energy density, hydrophilic characteristic, tenacious fibrous nature, heterogeneous composition [13–15], present enormous challenges to biomass logistics and subsequent conversion processes [16–18]. It is especially notable that the high volatile content, the low energy density of biomass and resulting low gasification temperature can lead to the formation of large amounts of tar during CLG of biomass, although the oxygen carrier can serve as a catalyst for in-situ cracking of tar. The undesired tar can cause plugging, fouling, corrosion of downstream pipelines and catalyst deactivation of subsequent catalytic synthesis [19–21]. Torrefaction has been considered as an effective pretreatment approach prior to biomass gasification for overcoming these challenges. Torrefaction is a low-temperature pyrolysis process which is carried out at the temperature range of 200–320 °C under an oxygen-free atmosphere [22–24]. The typical mass and energy balance for torrefaction of biomass is that 70–80% of the initial mass is retained as a solid product (torrefied biomass), containing 85–90% of the initial energy. The other 20–30% of the initial mass is converted into volatile products, which contain 10–15% of the initial energy [25,26]. Ideally, the energy from combustion of volatile products should be equal to the thermal energy requirements for biomass drying and torrefaction. Mani proposed that torrefaction of pine wood at 300 °C can achieve this auto-thermal operation to maximize the system efficiency [27]. Previous studies demonstrated that torrefaction can improve the aforementioned shortcomings of biomass feedstocks, thus providing technical and economic benefits for their collection, transportation, storage, handling and gasification processes. Chen found that the O/C ratio of biomass was lowered by torrefaction, whereas the gross calorific value was enhanced by torrefaction [28]. Mani showed that the grinding energy of biomass was reduced by torrefaction [29]. Prins showed that the overall efficiency of entrained flow gasification of torrefied wood was comparable to that of untreated wood, but more chemical exergy was conserved in the product gas from torrefied wood [30]. Bi and Chen found that the gasification reactivity of biomass decreased and its activation energy increased with the elevating torrefaction temperature [31]. Couhert observed that the torrefied wood yielded 7% more H2 and 20% more CO than untreated wood. And the chars from torrefied wood were less reactive than those from untreated wood [32]. There is still controversy

about the effect of torrefaction on the cold gas efficiency and tar formation of traditional biomass gasification. Zhou mentioned that torrefaction can improve the syngas quality and cold gas efficiency of traditional biomass gasification [33]. Dudyński reported that the calorific value of syngas and cold gas efficiency of traditional biomass gasification was reduced by torrefaction [34]. Cheah illustrated that torrefaction of oak at 270 °C decreased the total tar concentration in the product gas by more than 50% [35]. Tsalidis presented that torrefaction of hardwood increased the tar content in the product gas from subsequent gasification [36]. The effect of torrefaction pretreatment on the traditional gasification of biomass has been intensively investigated. Larachi reported chemical looping combustion of the volatiles obtained from torrefaction of biomass using iron oxide as an oxygen carrier [37,38]. However, to date, the effect of torrefaction pretreatment on the CLG of biomass has not been reported. In addition, torrefaction pretreatment may result in the formation of more char with lower reactivity during subsequent CLG of biomass [39]. In order to improve syngas production whilst minimizing tar formation during CLG of torrefied biomass, an ideal oxygen carrier should possess high oxygen-carrying capacity, high reactivity with char as well as high tar cracking ability. The design and screening of such multifunctional oxygen carriers suitable for CLG of torrefied biomass remain challenging. Hence, in the present study, integrating biomass torrefaction with subsequent CLG is first proposed. The effect of torrefaction severity on the carbon conversion efficiency, gas yield, gas LHV, syngas composition and tar formation from CLG of hardwood (eucalyptus wood) is systematically investigated. The possible pathways for the formation of tar precursors during CLG of torrefied eucalyptus wood are thus proposed. Moreover, considering that iron-based oxygen carries possess high oxygen-carrying capacity [40], and metal-based catalysts, such as Ni, Co or Ca, exhibit high tar cracking ability and/or high reactivity with char [20,41], a series of metallic ferrites are thus synthesized and tested for further improving gasification reactivity whilst minimizing tar formation during CLG of torrefied eucalyptus wood. 2. Experimental section 2.1. Torrefaction of eucalyptus wood Eucalyptus wood was obtained from a local wood processing factory in Guangzhou, China. Prior to experiments, the eucalyptus wood was ground and sieved to the desired particle size of 180–250 μm and then dried at 105 °C for 24 h. Torrefaction of eucalyptus wood was conducted in a quartz tubular reactor with 11 cm in length and 5.5 cm in internal diameter. The quartz tubular reactor is illustrated schematically in Fig. 1. The reactor was

Fig. 1. Schematic diagram and photograph of the tubular reactor. 2

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purged with N2 at a flow rate of 200 mL/min. Eucalyptus wood was torrefied in the tubular reactor at varying temperature (240, 280, and 320 °C) with a constant residence time of 40 min. The char was obtained from pyrolysis of torrefied eucalyptus wood at 900 °C with a constant residence time of 15 min in the same reactor. The structural changes of eucalyptus wood before and after torrefaction were analyzed by a Fourier transform infrared spectroscopy (FTIR) (Tensor 27, Bruker, Germany) between 7500 and 4000 cm−1 with a resolution of 4 cm−1 and 32 scans.

at a flow rate of 150 mL/min was used as the carrier gas. Three isopropanol-washing bottles immersed in an ice-water bath were used to collect tar. The exit gas was collected for analysis. The end of the reactor and transfer lines were heated by heating tape to prevent the rapid cooling of tar. The solvents in the three washing bottles were mixed together, filtrated and concentrated under reduced pressure with the aid of a rotary evaporator in a water bath (40 °C). The concentrated liquid was defined as tar. The collected gas was analyzed by a gas chromatography equipped with a flame ionization detector (GCFID) (Agilent 7890A, USA). The tar was analyzed by a GC/MS (Trace 1300, Thermo Fisher Scientific, Italy) to identify its compositions. The gas composition, gas yield, gas LHV, carbon conversion efficiency, and tar yield are shown as follows. The gas composition (φi) is defined as the volume percentage of selected gas (CO, CO2, H2 CH4 or C2Hn) in the total volume of gaseous products except argon (V): Vi i (%) = V × 100%, i = CO, CO2, H2, CH4, C2Hn (n = 2,4 and 6) The gas yield (Gv) is defined as the total volume of gaseous products except argon (V) to the mass of untreated or torrefied eucalyptus wood (m), V is calculated according to the argon balance between the inlet and outlet gas of CLG of eucalyptus wood:

2.2. Preparation and characterization of oxygen carriers Iron ore was obtained from Australia. Five metallic ferrites (LaFeO3, NiFe2O4, CoFe2O4, MnFe2O4, and CaFe2O4) were prepared by the solgel method (SG). Taking the preparation of the CoFe2O4 as an example, the detailed procedures are shown as follows: 0.1 mol Co(NO3)2·6H2O, 0.2 mol Fe(NO3)3·9H2O and 0.3 mol citric acid were dissolved in deionized water under constant stirring for 1 h at room temperature. The molar amounts of citric acid were equal to those of metal ions. The obtained aqueous solution was heated in a water bath (70 °C) under constant stirring, aqueous ammonia was then added to the aqueous solution to adjust its pH to 8.5 (pH for CaFe2O4 was 6). After evaporating water at 70 °C for 10 h, a brown and gel-like substance was obtained as the precursor. The resulting gel or iron ore was dried at 105 °C for 12 h, and then calcinated in a muffle furnace at 1000 °C for 2 h. Finally, the resulting samples were ground to obtain fresh oxygen carriers. The crystalline phases of oxygen carriers were measured by an Xray powder diffractometer (XRD) (Bruker D8 Advance, Bruker AXS, Germany) with Cu Kα radiation (k = 1.54060 Å) at 40 kV and 40 mA in the scanning range of 2θ = 10–80° with a step size of 0.02°.

Gv (Nm3 /kg ) =

V × 100% m

The carbon conversion efficiency (ηc) is defined as the proportion of the carbon converted into gaseous products to the total carbon in the 12 × ( VCO +VCO +VCH +VC H ) ×G v

2 4 2 n × 100%, where C% is the biomass: c (%) = 22.4 × C % carbon content of untreated or torrefied eucalyptus wood. The lower heating value (LHV) of the gaseous products is calculated as:

(

)

LHV(MJ / Nm3) = 12.6VCO + 10.8VH2 + 35.9VCH4 + 64.4VC2 H6 + 59.4VC2 H4 + 56.5VC2 H2 /100

2.3. Thermogravimetric analysis

The tar yield (Ytar) is the ratio of the total mass of tar (mtar) to the total volume of gaseous products except argon (V):

A thermogravimetric analyzer (Netzsch, STA409C, Germany) was used to investigate the gasification reactivity of char from torrefied eucalyptus wood with the oxygen carrier under an argon atmosphere. In each run, the samples (Approximately 20 mg mixtures of char and oxygen carrier with a mass ratio of 1:1.5) were heated from 30 to 1200 °C with a ramp of 20 °C/min. The flow rates of the purge and shielding gases were both kept at 20 mL/min.

Ytar (g /Nm3) =

mtar × 100% V

3. Results and discussion 3.1. Characterization of untreated and torrefied eucalyptus wood

2.4. Tar precursors obtained from fast pyrolysis of untreated and torrefied eucalyptus wood

Torrefaction of eucalyptus wood was conducted in a tubular reactor at varying temperature (240 °C, 280 °C and 320 °C) with a constant residence time of 40 min, respectively corresponding to mild, moderate and severe torrefaction. Fig. 3 shows the images of untreated and torrefied eucalyptus wood. It is obvious that the color of eucalyptus wood became darker with increasing torrefaction temperature, indicating that the carbonization severity of eucalyptus wood was related to its torrefaction temperature. The results were further verified by FTIR analysis. The FTIR spectra of untreated and torrefied eucalyptus wood are also illustrated in Fig. 3. The signal located at 1045 cm−1 is assigned to CeO vibration in hemicellulose and cellulose [42]. Its intensity decreased obviously when torrefaction was applied to eucalyptus wood. Its intensity dropped gradually with increasing torrefaction temperature. These results imply that the hemicellulose and cellulose fractions in eucalyptus wood were seriously decomposed by torrefaction. The signal located at 1240 cm−1 is related to CeO stretch in lignin and hemicellulose [42]. Its intensity first decreased and then increased with elevating torrefaction temperature. These results could be explained by that hemicellulose mainly proceeded decomposition reactions at low torrefaction temperature, whereas it primarily underwent polycondensation reactions at high torrefaction temperature. The signal located at 1615 cm−1 is attributed to aromatic skeletal in lignin and C]O stretch [43]. Its intensity from eucalyptus wood torrefied at 240 °C was evidently lower than that from untreated eucalyptus wood,

A commercial pyroprobe reactor (Pyroprobe 5200, CDS Analytical) was used to investigate the effect of torrefaction on the tar precursors obtained from fast pyrolysis of eucalyptus wood, xylan, cellulose, and lignin. In each run, the samples were rapidly heated from room temperature to 850 °C with a heating rate of 10 °C/ms and held there for 20 s. High purity helium (99.999%) was used as the carrier gas. The product was online analyzed by a gas chromatograph (GC) (Agilent 7890, Agilent Technologies, USA) coupled with a mass-selective detector (MS) (Agilent 5975, Agilent Technologies, USA). The GC column used was J&W DB-1701 60 m × 250 μm, and the film thickness was 0.25 μm. All runs are repeated at least two times to verify the experimental reproducibility. 2.5. Chemical looping gasification of untreated and torrefied eucalyptus wood CLG of untreated and torrefied eucalyptus wood was carried out in a fixed-bed reactor. As shown in Fig. 2, untreated or torrefied eucalyptus wood and oxygen carrier were mixed evenly with a mass ratio of 1:1. About 3.0 g mixtures were continuously dropped into the fixed bed reactor at a feeding rate of about 12 g/h. The operating conditions of CLG were kept at 850 °C with a residence time of 1 h. High purity argon 3

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Fig. 2. The fixed-bed reactor used for chemical looping gasification.

3.2. Characterization of oxygen carriers The crystal structure of different oxygen carriers was confirmed by XRD analysis. Fig. 4 shows the XRD patterns of the six oxygen carriers. The diffraction peaks at 2θ = 24.3, 33.3, 35.7, 41.0, 49.6, 54.2, 62.5 and 64.1° are well indexed to the reflection from the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) crystal planes of αFe2O3 (JCPDS No. 80–2377), indicating that the iron ore mainly contains Fe2O3. The diffraction peaks at 2θ = 23.1, 32.9, 38.2, 45.2, 55.2, 65.8° are in line with the reflection from the (1 1 1), (2 0 2), (3 1 1), (2 2 2), (4 0 0), (3 3 3), (4 0 4), (4 4 0), (5 3 3) and (6 2 2) crystal planes of MnFe2O4 (JCPDS No.74–2403). The diffraction peaks at 2θ = 22.6, 32.1, 39.7, 46.2, 57.4 and 76.5° are attributed to the reflections from the (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0), (5 3 3) and (4 4 4) crystal planes of LaFeO3 (JCPDS No.75-0541). The diffraction peaks at 2θ = 33.6, 35.5, 49.6 and 61.3° are in good agreement with the corresponding (3 2 0), (2 0 1), (2 4 1) and (2 6 0) crystal planes of CaFe2O4 (JCPDS No. 32-0168). The diffraction peaks at 2θ = 30.3, 35.7, 37.3, 43.4, 53.8, 57.4 and 63.0° match well with (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) crystal planes of NiFe2O4 (JCPDS No. 872338). The diffraction peaks at 2θ = 18.2, 35.4, 43.0, 47.1, 56.9 and 62.5° are similar to those reported for typical CoFe2O4 (JCPDS No.221086). These results demonstrate that the typical structure of MnFe2O4, LaFeO3, CoFe2O4, NiFe2O4, and CaFe2O4 were successfully synthesized.

Fig. 3. The images and FTIR spectra of untreated and torrefied eucalyptus wood obtained at varying torrefaction temperature.

indicating that lignin fraction in eucalyptus wood underwent decomposition reactions at 240 °C. As torrefaction temperature increased from 240 to 320 °C, its intensity first changed slightly and then increased obviously, implying that polycondensation and carbonization of eucalyptus wood became the significant reactions at higher torrefaction temperature. Previous studies showed that the hemicellulose fraction is more prone to polycondensation and carbonization during torrefaction of biomass [44–46]. The signal located at 1738 cm−1 is ascribed to unconjugated C]O in hemicellulose [47]. When the torrefaction temperature reached 280 °C, the signal was shifted toward 1710 cm−1 which is associated with new aromatic structure, and its intensity increased drastically as the torrefaction temperature further increased to 320 °C, suggesting that polycondensation and carbonization of eucalyptus wood started to occur at 280 °C.

3.3. The effect of torrefaction on CLG of eucalyptus wood using iron ore as an oxygen carrier To investigate the effect of torrefaction severity on syngas production and tar formation from CLG of eucalyptus wood, three kinds of torrefied eucalyptus wood respectively obtained at 240, 280 and 320 °C were gasified in a fixed bed reactor using iron ore as oxygen carriers. The mass ratio of iron ore to untreated or torrefied eucalyptus wood was maintained at 1:1. The gas yield, gas LHV, carbon conversion efficiency, tar content from CLG of untreated and torrefied eucalyptus

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A

B

LaFeO3

CaFe2O4 NiFe2O4

MnFe2O4

CoFe2O4

Intensity / a.u.

Intensity / a.u.

Iron Ore

20

30

40

50 60 2θ / degree

70

80 20

30

40

50 60 2 θ / degree

70

80

Fig. 4. The XRD patterns of different oxygen carriers.

Gasification temperature Iron ore/feedstocks Gas yield (Nm3/kg) Gas LHV (MJ/Nm3) Carbon conversion efficiency (%) Tar content (g/Nm3)

Untreated

850 1:1 0.80 15.17 76.69% 43.6

Torrefaction temperature / °C 240

280

320

850 1:1 0.80 14.96 72.91% 35.0

850 1:1 0.71 14.43 57.49% 33.9

850 1:1 0.57 10.64 32.99% 17.6

Gas composition / mol.%

Item

50

CO2

CO

H2

CH4

C2Hn

0.80

45

0.75

40

0.70

35

0.65

30

0.60

25

0.55

20 15

0.50

10

0.45

5

wood at 850 °C are tabulated in Table 1. It is obvious that the performance of CLG of eucalyptus wood was significantly altered by torrefaction. The gas yield, gas LHV and carbon conversion efficiency from CLG of untreated eucalyptus wood were 0.80Nm3/kg, 15.17 MJ/Nm3 and 76.69%, respectively. They decreased when applying torrefaction pretreatment. As torrefaction temperature increased from 240 to 320 °C, the gas yield, gas LHV and carbon conversion efficiency dropped obviously from 0.80 Nm3/kg, 14.96 MJ/Nm3 and 76.69% to 0.57 Nm3/kg, 10.64 MJ/Nm3 and 32.99%, respectively. These results could be due to the devolatilization, polycondensation and carbonization of eucalyptus wood during torrefaction, resulting in the formation of more char with lower reactivity in subsequent gasification [39]. The tar content from CLG of untreated eucalyptus wood was 43.6 g/Nm3. The tar content was evidently reduced by torrefaction. It declined continuously with increasing torrefaction severity. The lowest tar content (17.6 g/Nm3) was obtained from CLG of eucalyptus wood torrefied at 320 °C. These results could be attributed to the pre-removal of tar precursors (e.g. phenols and holocellulose-derived oxygenates) during torrefaction of eucalyptus wood. The effect of torrefaction on gas composition and H2/CO molar ratio from CLG of untreated and torrefied eucalyptus wood is plotted in Fig. 5. When torrefaction of 240 °C was applied to eucalyptus wood, the gas composition changed slightly. As the torrefaction temperature elevated from 240 to 320 °C, the contents of CO, CH4, and C2Hn decreased gradually, whereas the contents of CO2 and H2 increased steadily. The reduction in the contents of CO, CH4, and C2Hn could be due to the predecarbonylation, pre-demethylation and pre-removal of light oxygenates during torrefaction. The serious reduction in the contents of CH4, C2Hn, and CO can result in the increase in the relative content of CO2 and H2. The H2/CO molar ratio from CLG of eucalyptus wood was obviously enhanced from 0.44 to 0.78 by torrefaction. The highest H2/ CO molar ratio of 0.78 was obtained from eucalyptus wood torrefied at 320 °C. These results demonstrate that torrefaction is an effective pretreatment method prior to CLG of biomass for enhancing H2 content and H2/CO molar ratio.

0

H2/CO molar ratio

55

Table 1 The performance of CLG of untreated and torrefied eucalyptus wood using iron ore as an oxygen carrier.

0.40

Untreated

o

o

280 C 240 C o Torrefaction temperature / C

o

320 C

Fig. 5. The gas composition and H2/CO molar ratio from CLG of untreated and torrefied eucalyptus wood using iron ore as an oxygen carrier.

The total ion chromatograms resulting from GC/MS analysis of tar are depicted in Fig. 6. The identified compounds in tar included toluene, xylenes, styrene, 5,6-dihydro-2H-pyran-2-one, phenol, benzofuran, 1-propynyl-benzene, phenyl acetate, methyl benzofurans, naphthalene, azulene, methyl naphthalenes, biphenyl, acenaphthylene, dibenzofuran, fluorene, pyrene, and phenanthrene. The most abundant compounds in tar were styrene, indene, and naphthalene. It is evident that the peak intensities of these compounds were reduced by torrefaction. And their peak intensities declined with increasing torrefaction temperature. These results could be attributed to the pre-removal of organic oxygenates and phenols during torrefaction of eucalyptus wood, resulting in the formation of fewer tar precursors in subsequent gasification. During CLG of biomass, biomass first underwent fast pyrolysis reactions to form tar precursors, then the tar precursors were reformed into tar over oxygen carrier. To reveal the effect of torrefaction on the formation of tar precursors from CLG of eucalyptus wood, the total ion chromatograms resulting from fast pyrolysis of untreated and torrefied eucalyptus wood, xylan, cellulose, and lignin at 850 °C are illustrated in Fig. 7. The absolute peak areas of selected compounds per mg feedstocks from fast pyrolysis are tabulated in Table 2. Although the absolute peak area of each compound per mg feedstocks does not represent the actual yield, it can provide insight into the effect of torrefaction severity on the variation tendency of the yield of each compound obtained from fast pyrolysis. It is observed that both untreated eucalyptus wood, cellulose, xylan, and lignin produced large amounts of tar precursors, such as aromatics and phenols. It is inferred that, during fast pyrolysis of untreated and torrefied biomass, the 5

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Fig. 6. The total ion chromatograms of tar obtained from CLG of untreated and torrefied eucalyptus wood using iron ore as an oxygen carrier.

Untreated Cellulose

Abundance / a.u.

Abundance / a.u.

Untreated eucalyptus wood

o

Eucalyptus wood torrefied at 240 C

o

Eucalyptus wood torrefied at 280 C

o

Cellulose torrefied at 280 C o

Eucalyptus wood torrefied at 320 C 5

10

15

20 25 30 Retention time / min

35

40

45

5

10

15 20 25 30 Retention time / min

Abundance / a.u.

Abundance / a.u.

o

Lignin torrefied at 280 C

10

40

Untreated xylan

Untreated Lignin

5

35

15 20 25 30 Retention time / min

35

40

o

Xylan torrefied at 280 C

5

10

15 20 Retention time / min

25

30

Fig. 7. The total ion chromatograms of tar precursors obtained from fast pyrolysis of untreated and torrefied eucalyptus wood, cellulose, lignin and xylan at 850 °C.

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Table 2 The yield of tar precursors from fast pyrolysis of untreated and torrefied eucalyptus wood at 850 °C. Absolute peak area of each compound per mg samples/(108/mg)

Compounds

Eucalyptus wood

Acetic acid 1-Hydroxy-2-propanone Benzene Toluene Xylenes 1-Hydroxy-2-butanone 3-methyl-1,2-Cyclopentanedione Styrene Furfural 5-Methyl-2-furancarboxaldehyde Benzofuran Indene Phenol 2-Methoxy-phenol Naphthalene 2-Methoxy-4-vinylphenol p-Cresol 3-Methylphenol 1-Methylnaphthalene 2-Methylnaphthalene Biphenylene a

Cellulose

Lignin

Xylan

Untreated

240

280

320

Untreated

280

Untreated

280

Untreated

280

8.71 3.68 1.37 1.44 0.61 0.92 1.06 0.40 2.08 0.29 0.29 0.58 1.48 0.61 0.32 0.49 0.41 1.11 0.16 0.17 0.18

8.77 3.77 0.81 0.93 0.53 1.01 1.84 0.26 2.73 0.43 0.28 0.62 1.21 1.56 0.33 1.33 0.32 0.85 0.15 0.23 0.05

3.48 3.19 0.69 0.74 0.41 0.83 1.50 0.21 1.83 0.50 0.25 0.54 1.11 0.96 0.24 0.44 0.30 0.84 0.12 0.18 N.D.

0.98 0.03 0.66 0.60 0.26 N.D. 0.08 0.08 0.24 0.08 0.13 0.17 1.03 0.56 0.20 0.08 0.22 0.64 0.08 0.05 N.D.

2.53 2.32 6.84 3.72 1.02 0.41 1.32 1.07 2.27 0.61 0.63 1.69 1.67 N.D. 1.64 N.D. 0.60 0.62 0.91 0.68 0.39

N.D.a N.D. 4.21 3.68 1.55 N.D. 0.24 0.68 N.D. N.D. 0.40 1.65 0.75 N.D. 2.53 N.D. 0.19 0.75 1.83 0.98 0.96

N.D. N.D. 1.97 1.45 0.43 N.D. N.D. 0.54 N.D. N.D. 0.27 0.59 3.06 0.83 0.64 0.52 0.69 0.74 0.36 0.31 0.19

N.D. N.D. 1.13 0.73 0.16 N.D. N.D. 0.22 N.D. N.D. 0.12 0.29 1.68 0.28 0.30 N.D. 0.34 0.46 0.08 0.07 0.08

1.80 2.86 1.80 1.73 0.47 2.29 1.32 0.53 1.38 N.D. 0.33 1.08 0.56 N.D. 0.72 N.D. 0.51 N.D. 0.34 0.20 0.27

0.61 1.44 5.58 3.60 0.55 N.D. 0.63 1.19 1.24 N.D. 0.52 1.48 1.30 N.D. 1.68 N.D. 0.47 N.D. 0.47 0.37 0.42

N. D.: Not detected.

Fig. 8. The possible pathways for the formation of tar precursor from fast pyrolysis of untreated and torrefied biomass at 850 °C.

aromatics and phenols were formed mainly via two possible pathways. The possible pathways for the formation of tar precursors are illustrated in Fig. 8. First, lignin was decomposed into various phenols, which further underwent deoxygenation reactions to form aromatics. The oxygen-containing groups in phenols mainly contained methoxy and phenolic hydroxyl. For example, guaiacol could proceed demethylation-dehydration or direct demethoxylation to form phenol. Phenol could further undergo tautomerization-decarbonylation or direct dehydration to generate aromatics [48,49]. Second, cellulose and hemicellulose were transformed or cracked into organic oxygenates (e.g.

furans, pyrans, aldehydes, and ketones), which proceeded deoxygenation (e.g. decarbonylation, decarboxylation, and dehydration) and oligomerization reactions to generate aromatics and phenols via hydrocarbon pool mechanism. Phenols might be further formed by the combination of aromatic free radicals and hydroxyl radicals. As shown in Table 2, cellulose produced more aromatics than lignin and xylan, indicating that cellulose fraction in biomass might play the most key role during tar formation, and tar was mainly produced by the second route during gasification of biomass. When torrefaction was applied to eucalyptus wood, the yields of monoaromatics and monophenols, such 7

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MnFe2O4, CoFe2O4, CaFe2O4, and NiFe2O4 reacted with char were 31.48, 13.99, 32.87, 29.52, 26.89 and 29.41%, respectively. The rank order in weight losses of different oxygen carriers reacted with char was MnFe2O4 > iron ore > CoFe2O4 > NiFe2O4 > CaFe2O4 > LaFeO3 > Al2O3, which was distinguished from the rank order in the theoretical oxygen-carrying capacity of different oxygen carriers: > MnFe2O4 iron ore (30.06%) > CaFe2O4 (29.66%) (27.75%) > NiFe2O4 (27.31%) > CoFe2O4 (27.28%) > LaFeO3 (19.76%). The weight loss rates of different oxygen carriers reacted with char are also given in Fig. 9. It is observed that the rank order in the maximum weight loss rates was NiFe2O4 (43.05%/min) > CoFe2O4 (32.94%/min) > CaFe2O4 (3.36%/min) > LaFeO3 (2.83%/ min) > MnFe2O4 (2.51%/min) > iron ore (1.67%/min). It is worth noting that the maximum weight loss rates of NiFe2O4 and CoFe2O4 were much higher than those of the other oxygen carriers. And their corresponding peak temperatures were evidently lower than those of the other oxygen carriers. These results demonstrate that NiFe2O4 and CoFe2O4 are the best candidates for improving the CLG reactivity of char derived from torrefied eucalyptus wood. To further confirm the performance of different oxygen carriers in CLG of torrefied eucalyptus wood, the torrefied eucalyptus wood obtained at 280 °C was gasified over different oxygen carriers in a fixed bed reactor at 850 °C. Table 3 lists the gas yield, LHV, carbon conversion efficiency and tar content from CLG of torrefied eucalyptus wood over different oxygen carriers. The gas yield and carbon conversion efficiency from CLG of torrefied eucalyptus wood over iron ore were 0.71 Nm3/kg and 57.49%, respectively. The gas yield and carbon conversion efficiency can be effectively improved by the selection of proper oxygen carriers. CaFe2O4 and NiFe2O4 exhibited the highest gas yields of 1.08 and 1.02 Nm3/kg, respectively. The carbon conversion efficiency was promoted from 57.5% for iron ore to 68.1% for NiFe2O4 and 73.6% for CaFe2O4. The rank order in the carbon conversion efficiency of different oxygen carriers was iron ore < LaFeO3 < MnFe2O4 = CoFe2O4 < NiFe2O4 < CaFe2O4. These results could be a result of the balance of reactivity and oxygen-carrying capacity of different oxygen carriers. The tar content from CLG of torrefied eucalyptus wood over iron ore was 33.9 g/Nm3. The tar content was evidently reduced when synthetic oxygen carriers were adopted. The six oxygen carriers exhibited very different catalytic activity in tar cracking. The lowest tar contents of 12.1 and 4.9 g/Nm3 were respectively obtained by CaFe2O4 and NiFe2O4, implying that NiFe2O4 and CaFe2O4 had the best ability for in-situ cracking of tar. These results were in line with the fact that Ni and Fe are the most efficient catalysts for tar elimination [50,51]. The metal oxides used as oxygen carriers in CLG of biomass, especially their corresponding reduced metals, can serve as the efficient catalysts for in-situ cracking of tar [52]. It is concluded that, comprehensive considering the carbon conversion efficiency and tar content, NiFe2O4 and CaFe2O4 are the preferred oxygen

The mentioned-above experiments demonstrate that the gas yield and carbon conversion efficiency during CLG of eucalyptus wood using iron ore as an oxygen carrier was obviously lowered by torrefaction. The reaction between oxygen carrier and char has been considered as the rate-limiting step during CLG of biomass. Torrefied eucalyptus wood obtained at a moderate torrefaction condition (280 °C) was selected as feedstock for improving its char reactivity. The char was obtained from pyrolysis of this torrefied eucalyptus wood at 900 °C. Five metallic ferrites were synthesized and used as oxygen carriers. The TG/ DTG profiles of different oxygen carriers reacted with char under an argon atmosphere is drawn in Fig. 9. A control experiment of char mixed with Al2O3 was also considered. It is observed that different oxygen carriers exhibited very distinct weight loss behaviors and weight loss rates. The maximum weight losses of iron ore, LaFeO3, 100 95

1000

80

35

o

25

400

70

0

20

40

60 80 Time / min

100

120

400

15 10

200

65

600

Al2O3

20

75

800

CaFe2O4 NiFe2O4

30

600

Al2O3

1000

MnFe2O4 CoFe2O4

Temperature / C Mass loss rate / (wt.%/min)

Weight loss /wt.%

85

40

800

CaFe2O4 NiFe2O4

1200

Iron ore LaFeO3

45

MnFe2O4 CoFe2O4

90

50

1200

Iron ore LaFeO3

o

3.4. The performance of different oxygen carriers in CLG of torrefied eucalyptus wood obtained at 280 °C

200

5 0

0 140

0

20

40

60 80 Time / min

100

120

Fig. 9. The TG/DTG profiles of different oxygen carriers reacted with char under an argon atmosphere. 8

Temperature / C

as benzene, toluene, xylenes, styrene, phenol, and p-cresol, dropped, whereas the yields of holocellulose-derived organic oxygenates, such as acetic acid, 1-hydroxy-2-propanone, 1-hydroxy-2-butanone, 3-methyl1,2-cyclopentanedione and furfural, increased. As torrefaction temperature increased from 240 to 320 °C, the yields of all compounds, including aromatics, phenols and holocellulose-derived organic oxygenates, gradually declined. The same variation trend was observed for these compounds obtained from fast pyrolysis of lignin. For fast pyrolysis of untreated and torrefied cellulose, the yields of the majority of organic oxygenates and monoaromatics were reduced by torrefaction, whereas the yields of naphthalenes and biphenylene were improved by torrefaction. The reduction in the yields of tar precursors could be due to the pre-removal of lignin-derived phenols and holocellulose-derived organic oxygenates during torrefaction of eucalyptus wood. However, the aromatics and phenols from the fast pyrolysis of xylan were enhanced by torrefaction. It is inferred that the torrefaction of xylan at 280 °C proceeded severe polycondensation and carbonization reactions to form torrefied xylan with aromatic structure [44], thus resulting in the formation of more aromatics and phenols during subsequent fast pyrolysis (as shown in Fig. 8). This is a new pathway to generate aromatics and phenols from torrefied biomass. Considering that the hemicellulose content in biomass was relatively low and most of the hemicellulose fraction was decomposed during torrefaction, the formation of tar precursors was less affected by the new pathway. It is speculated that the tar precursors were produced from torrefied biomass via three possible pathways, and the former two pathways dominated. It is thus concluded the pre-removal of holocellulose-derived organic oxygenates and lignin-derived phenols during torrefaction of eucalyptus wood could lower the formation of tar precursors during subsequent CLG, thus resulting in the reduction in tar content.

0 140

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Table 3 The performance of different oxygen carriers in CLG of torrefied eucalyptus wood obtained at 280 °C. Item

Different oxygen carriers

Gasification temperature Oxygen carriers/feedstocks Gas yield (Nm3/kg) Gas LHV (MJ/Nm3) Carbon conversion efficiency (%) Tar content (g/Nm3)

50 45

CO2

CO

H2

Iron ore

MnFe2O4

LaFeO3

CoFe2O4

NiFe2O4

CaFe2O4

850 1:1 0.71 14.43 57.5% 33.9

850 1:1 0.88 13.42 64.6% 19.7

850 1:1 0.87 14.29 60.3% 14.0

850 1:1 0.95 11.39 64.6% 18.8

850 1:1 1.02 11.72 68.1% 4.9

850 1:1 1.08 13.00 73.6% 12.1

CH4

It is observed that the tar compositions from CLG of torrefied eucalyptus wood over different oxygen carriers varied significantly, indicating that the metal species in the metallic ferrites are the key factors governing their catalytic activity in tar cracking. The major compounds in the tar were styrene, benzofuran, indene, and naphthalene. It is obvious that the peak intensities of these compounds from NiFe2O4 and CoFe2O4 were significantly lower than those from CaFe2O4, MnFe2O4, LaFeO3, and iron ore. These results suggest that NiFe2O4 and CoFe2O4 exhibited higher activity for in-situ cracking of styrene, benzofuran, indene, and naphthalene.

0.85

C2Hm

0.80 0.75

35

H2 / CO molar ratio

Gas composition / mol.%

40

30

0.70

25

0.65

20 15

0.60

4. Conclusion

10 0.55

5 0

Iron ore MnFe2O4

LaFeO3

CoFe2O4

NiFe2O4

CaFe2O4

In this study, the effect of torrefaction pretreatment on the syngas production and tar formation from chemical looping gasification of eucalyptus wood over different oxygen carriers was investigated. The main conclusions are summarized as follows:

0.50

Fig. 10. The gas composition and H2/CO ratio from CLG of torrefied eucalyptus wood over different oxygen carriers.

(1) Torrefaction pretreatment is an effective pretreatment method for reducing tar formation and improving H2/CO molar ratio. The tar content from chemical looping gasification of eucalyptus wood over iron ore was lowered evidently from 43.6 to 17.6 g/Nm3 by torrefaction, whereas the H2/CO molar ratio was obviously enhanced from 0.44 to 0.78. (2) Torrefaction pretreatment can also result in the reduction in the gas yield and carbon conversion efficiency of chemical looping gasification of eucalyptus wood. These results could be due to the devolatilization, polycondensation and carbonization of eucalyptus wood during torrefaction, resulting in the formation of fewer tar precursors and more char with lower reactivity in subsequent chemical looping gasification. (3) The reduction in gas yield and carbon conversion efficiency caused by torrefaction can be effectively overcome by the selection of suitable oxygen carriers. Five metallic ferrites were synthesized and tested in chemical looping gasification of torrefied eucalyptus wood obtained at 280 °C. The gas yield was enhanced from 0.71 Nm3/kg for iron ore to 1.02 Nm3/kg for NiFe2O4 and 1.08 Nm3/kg for CaFe2O4, while the carbon conversion efficiency was promoted from 57.5% for iron ore to 68.1% for NiFe2O4 and 73.6% for CaFe2O4. (4) Compared to chemical looping gasification of untreated eucalyptus wood over iron ore, chemical looping gasification of torrefied eucalyptus wood over NiFe2O4 reduced the tar content by 88.8% and improved the gas yield by 27.5%. These results suggest that integrating biomass torrefaction pretreatment with chemical looping gasification provides an efficient strategy for enhancing syngas production whilst minimizing tar formation.

carriers for improving syngas production whilst minimizing tar formation from the CLG of torrefied biomass. CLG of torrefied eucalyptus wood over NiFe2O4 reduced the tar content by 88.8% and improved the gas yield by 27.5% compared to CLG of untreated eucalyptus wood over iron ore. These results suggest that the negative impacts of torrefaction pretreatment on the gas yield and carbon conversion efficiency from CLG of biomass can be effectively overcome by the selection of suitable oxygen carriers with high reactivity. The gas composition and H2/CO ratio from CLG of torrefied eucalyptus wood over different oxygen carriers are illustrated in Fig. 10. Compared to iron ore, the synthetic oxygen carriers produced higher yield of syngas with more H2 content and fewer CH4 and C2Hn contents. These results could be due to that the catalytic reforming of CH4, C2Hn, and tar with H2O were promoted by these synthetic oxygen carriers to yield more H2. NiFe2O4 exhibited the maximum content of CO (43.43%) and H2 (34.58%). Iron ore displayed the maximum content of CH4 (11.74%) and C2Hn (4.03%). The lowest contents of CO2 were obtained by LaFeO3 (11.28%) and CaFe2O4 (13.05%). La2O3 or CaO decomposed from LaFeO3 and CaFe2O4 can react with CO2 to form La2(CO3)3 and CaCO3, resulting in the reduction in the CO2 contents. The H2/CO ratio from CLG of torrefied eucalyptus wood over iron ore was 0.54. The H2/CO ratio was effectively enhanced when synthetic oxygen carriers were applied to CLG of torrefied eucalyptus wood. The H2/CO ratio increased from 0.54 for iron ore to 0.79 for CaFe2O4, 0.80 for NiFe2O4 and 0.81 for LaFeO3. The total ion chromatograms of tar obtained from CLG of torrefied eucalyptus wood over different oxygen carriers are depicted in Fig. 11.

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Fig. 11. The total ion chromatograms of tar obtained from CLG of torrefied eucalyptus wood over different oxygen carriers.

CRediT authorship contribution statement

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