Pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor

Journal Pre-proof Pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor Zixiang Gao (Data curation) (Writing - original draft) (Investigation), Ning Li (Methodology) (Validation) (Investigation), Yiqing Wang (Resources), Weisheng Niu (Formal analysis), Weiming Yi (Funding acquisition) (Supervision) (Project administration)

PII:

S0165-2370(19)30591-1

DOI:

https://doi.org/10.1016/j.jaap.2020.104772

Reference:

JAAP 104772

To appear in:

Journal of Analytical and Applied Pyrolysis

Received Date:

2 August 2019

Revised Date:

3 December 2019

Accepted Date:

6 January 2020

Please cite this article as: Gao Z, Li N, Wang Y, Niu W, Yi W, Pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor, Journal of Analytical and Applied Pyrolysis (2020), doi: https://doi.org/10.1016/j.jaap.2020.104772

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Pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor

Zixiang Gao a, Ning Li a, Yiqing Wang a, Weisheng Niu b, Weiming Yi a,* a

Shandong Research Center of Engineering and Technology for Clean Energy, Shandong University of Technology,

Zibo 255049, China.

b



Corresponding author.

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E-mail address: [email protected] (W. Yi); Tel.: +86 0533-2789077

Highlights

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Xylan, xylose, arabinose and glucose melted and yielded bubbles in pyrolysis. Xylan pyrolysis behavior had a close relationship with its building blocks. Many unidentified didehydrated pentose were produced in xylan pyrolysis. Temperature is the key parameter affecting the xylan conversion. Thick material layer enhanced the xylan bio-oil production.

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    

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College of Engineering, Shenyang Agricultural University, Shenyang 110866, China.

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Abstract: Investigation on the biomass components pyrolysis behaviors in a lab-scale reactor is a necessary step before its industrialization due to the poor transportability of the findings obtained

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from the micro-scale pyrolyzer which often neglects the heat and mass transfer effect. In present study, the pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor was investigated. The residue evolution and bio-oil composition of xylan, xylose, arabinose and glucose show the close relationship of pyrolysis characteristics between xylan and its components. It is noteworthy that the xylan and those mono-sugar units melted during pyrolysis, and many bubbles formed.

Investigation of the effect of temperature, material thickness and carrier gas flow rate on the products distribution and bio-oil composition indicates that high temperature promotes the xylan conversion and increases the yield of bio-oil, however, if the temperature is too high (> 500°C), violent decomposition of xylan occurs and leads to the formation of low molecular weight compounds (such as hydroxyl acetaldehyde, acetic acid and 1-hydroxyl-2-propaone etc.). And interestingly, the bio-oil yield increases monotonically with the material thickness, and thick

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material layer can increase the content of furfural in bio-oil. Carrier gas flow rate has a little

influence on the xylan char yield, and relative high carrier gas flow rate (> 2.55L/min) could

decrease the secondary decomposition of primary didehydrated pentose but reduce the bio-oil

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yield.

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Keywords: xylan; pyrolysis; fixed bed reactor; morphology; material thickness

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1. Introduction

Pyrolysis of lignocellulosic biomass individual components (cellulose, hemicellulose and lignin)

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has received increasing attentions in recent years to produce chemicals and fuels, and to put insights into a better understanding of biomass pyrolysis[1]. Hemicellulose, the second most

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abundant polysaccharide on the earth, different form cellulose which has a homogeneous structure formed by d-glucose through β-1,4 glycosidic linkage, is a heterogeneous group of different

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polysaccharides depending on the biomass species and cell types[2], and its monomer includes pentoses (xylose, arabinose), hexoses (glucose, mannose and galactose) and hexuronic acids (glucuronic acid) [3,4]. The complexity of hemicellulose structure and constituents hinder the indepth comprehension on its pyrolysis. Therefore, the hemicellulose model compounds were used extensively in last decades, that is hemicellulose monosaccharides, commercial hemicellulose,

isolated/extracted hemicellulose and native hemicellulose being a part of biomass[5]. With the development of technology, advanced pyrolysis reactors (such as TGA, Py and wiremesh reactor) and analytic instruments (GC-MS, FTIR, HPLC etc.) do play a great role in the study of biomass pyrolysis characteristics and mechanisms, and extensive researches on the pyrolysis of biomass constituents with the aid of those sophisticated equipment were summarized in references[1,5]. However, there are some non-negligible shortcomings should be noted that

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slow heating rate for TGA and big thermal lag for Py when the temperature is low[6,7]. Different

feedstock input for the micro reactors and large scale reactors (such as fixed-bed and fluidized bed reactor) definitely cause different pyrolysis characteristics and products distribution for them.

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Patwardhan et al.[8] compared the products distribution of cellulose pyrolysis in a Py reactor and a

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fluidized bed reactor, and found more serious secondary reaction of primary volatiles in the

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fluidized bed reactor than that in the Py reactor due to the longer residence time. Obviously, such differences obstruct the industrial application of the findings obtained from the micro-scale

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reactors. Some studies on the hemicellulose pyrolysis in fluidized bed reactor or fixed-bed reactor were also performed in the last decades. Shen et al.[9,10] investigated the commercial xyaln

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pyrolysis products distribution in a fluidized bed reactor, and found that high temperature can enhance the CO formation substantially. Branca et al.[11] studied the products distribution and

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kinetics of commercial glucomannan pyrolysis in a fluidized bed reactor and found that the most abundant products were char and water, and a two-stage model including the solid intermediate to produce volatiles and char was more better to describe the glucomannan pyrolysis mechanism than one-stage model. Lv et al.[12] investigated the forming behavior of corn stalk hemicellulose pyrolysis products in a tubular reactor and explored the effect of temperature on the products

property, concluding that high temperature promoted the formation of CO and H2, and the char surface became more smooth and porous as temperature increased. In spite of these researches on hemicellulose in a fixed-bed reactor or fluidized bed reactor, most of them focused on the products distribution with consideration of the temperature effect, yet few attentions have been paid to the hemicellulose residue evolution, bio-oil composition in pyrolysis and the influence of other reactor condition parameters, such as material thickness and carrier gas flow rate, on it. Although

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the morphology and structure of xylan under different temperature were characterized by Smith et at.[13] , rare studies focused on the xylan residue evolution with time in a pyrolysis process. A systematic investigation on the hemicellulose pyrolysis behavior in a fixed-bed reactor or

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fluidized bed reactor is important for its industrialization. For this purpose, commercial xylan was

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pyrolyzed in a fixed-bed reactor, and its residues morphology and structure evolution in pyrolysis

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process were examined, bio-oil composition also was analyzed. And same experiments for xylan building blocks, xylose, arabinose and glucose, were conducted to better understand the xylan

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pyrolysis behavior. Besides, the effects of temperature, material thickness and carrier gas flow rate on the products distribution and bio-oil composition of xylan were investigated.

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2. Materials and methods 2.1. Materials

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Xylan, xylose, arabinose and glucose were purchased from Ruibio-Reagent (Hefei, China), all of them are white powders at room temperature. The purity of xylose, arabinose and glucose is over 99 %, and the purity of xylan is larger than 95 %. The elements content of xylan were measured by an auto elemental analyzer (Elementar Vario EL Cube, Germany), and the proximate analysis was performed according to the standard GBT 28731-2012. The content of alkali and alkaline

earth metals (AAEMs) was analyzed by the ICP-MS (Agilent 7500ce, USA). Besides, to obtain the components and structure information of xylan, a similar method as described in [14] was used that about 25 mg of xylan were dispersed in 12 mL of 1 M H2SO4 and heated in a closed tube at 100 °C for 2.5 h to obtain the non-cellulosic neutral sugars, then the hydrolysate was qualitatively and quantitatively analyzed by a HPLC system (Waters e2695, USA) equipped with a RI detector (Waters 2414). Another xylan-D2O solution was characterized by Bruker AVANCE Ⅲ HD

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400MHz to get the 1H-NMR spectra. All the relevant characterization results were summarized in Table 1.

Table 1 Ultimate and proximate analysis of xylan and its AAEMs and neutral sugars distribution. Ultimate analysis (wt%)

Ash

Volatiles

FC

C

0.06

93.42

6.52

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Proximate analysis (dry basis, wt%)

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H

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42.36

6.36

O

N

S

51.13a

0.09

0.06

Chemical composition of xylan (wt%) Xylose

12.69

59.75

Arabinose

Glucuronic acids

Acetyl group

8.03 b

8.92 b

2.65 b

Mg

K

Ca

10.96

—c

54.33

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Glucose

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53.71

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AAEMs analysis (mg/kg)

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Oxygen content was calculated by difference;

b The

content of glucuronic acids and acetyl group was calculated based on the integral of 1H-

NMR (the NMR spectra see Fig. 1S in the supplementary material) according to references[15,16];

c

“－” means not detected.

2.2. Pyrolysis set-up and procedure The pyrolysis set-up (Fig. 1) was also used in our previous study on other carbohydrates pyrolysis[17,18]. And it is mainly comprised of carrier-gas unit, horizontal tubular reactor, temperature controller and cooling system. The pyrolysis reactor is a quartz tube (length × diameter × thickness:700 mm × 60 mm × 3 mm), four silicon-carbide rods positioned evenly

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around the tubular reactor were used as heater, and a K-type thermal couple was used to measure the reactor temperature in real time. And the cooling system consists of two cold traps and a

cooling tank with the -10 °C mixture of glycol and water (V:V=1:2) as coolant. Prior to each

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pyrolysis experiment, high purity N2 (99.999%) was used to purge the air inside the tubular reactor

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for about 10 min and also was used as carrier gas in all experiments. A semi-cylinder quartz boat

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(length × diameter × thickness: 100 mm × 50 mm × 2 mm) loaded with 2 g feedstock was pushed into the pyrolysis zone immediately when the temperature of the reactor reached the set value..

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After each experiment, the reactor and quartz boat were calcined at 650 °C in an air atmosphere to eliminate the influence of any residue on the consequent experiments. To study the effect of

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conditional parameters, the xylan tentatively was pyrolyzed in the following conditions: temperature range of 300 ~ 600 °C with an interval of 50 °C, material thickness range of 1 ~ 5 mm

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with an interval of 1 mm, and carrier-gas flow rate range of 2.0 ~ 3.5 L/min with an interval of 0.5 L/min.

The yields of char and bio-oil were calculated by the mass difference of the quartz boat and cold traps, respectively, before and after experiments, and the yield of gases (here the gases refers to all the volatiles that did not condensed) was obtained according to the mass balance, gases yield=

(feedstock mass – bio-oil yield – char yield). And each experiment was run at least twice and

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further run was performed when the derivation exceeds 5%.

Fig. 1. Schematic diagram of the pyrolysis set-up. (1. Nitrogen; 2. Gas flow meter; 3. Funnel; 4. Furnace; 5. Quartz boat; 6. Thermal couple.; 7. Temperature controller; 8. Cold trap; 9. Cooling

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tank).

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To examine the residue evolution during pyrolysis, the xylan, xylose, arabinose and glucose were

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pyrolyzed in the tubular reactor mentioned above at 500 °C with different residence time (10~70 s with a interval of about 10 s). A quartz boat loaded with 0.5 g sample (well distributed) was

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pushed into the pyrolysis zone when the reactor reached the designed temperature, meanwhile the timing began, and at the end of timing, the quartz boat was pulled to the end of the tubular reactor

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immediately, then the instant residue morphology was captured by an iPhone 6S smart phone. And similar procedure was used to obtain the residue for structure characterization, after the residue

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was pulled out the pyrolysis zone, liquid nitrogen was poured into the quartz boat to cool the residue rapidly to avoid further reactions. Finally, the residue was characterized by the FTIR (Thermo Electron, Nicolet 5700) using KBr pellet in the spectra range of 4000 ~ 400 cm-1 with a resolution of 4 cm-1. 2.3. Bio-oil characterization

The chemicals of the bio-oil were analyzed by the GC-MS system (Agilent, 6890-5973N, USA). The bio-oil collected in the cold traps was filtered through a nylon filter (diameter 0.4 μm) prior to the injection. A Agilent 7683 series injector was used and the injection volume was 0.2 μL. In the GC system the inlet temperature was set at 280 °C, the split ratio was 60:1, a DB-1701 capillary column (60 m × 0.25 mm × 0.25 μm) was used to separate the bio-oil chemicals, and high purity He (99.999%) with a flow rate of 1 mL/min was used as carrier gas, the over temperature was

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programed from 40 °C to 240 °C with a ramp of 5 °C/min, and hold at 240 °C for 5 min. In the

MS part a solvent delay of 4.67 min was used to prevent the filament from the damaging of water in bio-oil, and the mass-spectrometer worked on EI mode at 70 eV with a scan range f (m/z) 12 ~

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550 amu.

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3. Results and discussion

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3.1. Residue evolution

Fig. 2 shows the morphology evolution of xylan, xylose, arabinose and glucose at 500 °C with

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different residence time, and it can be clearly seen that all of them changed from white solid powders into liquid molten state, then different size bubbles formed and the material became black

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puffy porous char ultimately. It is noteworthy that the xylan seems much easier to decompose than these mono-sugars (xylose, arabinose and glucose) since more bubbles formed in the earlier

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pyrolysis stage for xylan, while these mono-sugars melted into transparent liquid firstly without obvious bubbles appeared in the roughly same period with xylan. And the scatter distribution of bubbles in the pyrolysis of xylose, arabinose and glucose may result from the non-uniform distribution of material because of the inertia in the fast movement of the quartz boat. The melting xylan also was observed by Giudicianni et al.[19] and Hosoya et al.[20]. Different from the

cellulose morphology change in a fixed-bed reactor (cellulose did not undergo the molten state when temperature below 500 °C) [17,18], the melting for xylan would cause a significant influence on its volatiles evaporation. The FTIR spectra of the residue of xylan and its basic sugar units at 500 °C with different residence time is illustrated in Fig. 3 to examine the structure evolution in pyrolysis. In Fig. 3(a) the transmittance peak at 1164 cm-1 and 896 cm-1 can be ascribed to the glycosidic bond (C─O─

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C) stretching vibration between sugar units and C1─H bending vibration of β-glycosidic bond[17,21], the peak at 987 cm-1 indicates the presence of arabinose attached in the side

branches[22], and the peak at 1728 cm-1 can be attributed to the C=O of acetyl and carboxyl

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groups. All those peaks disappeared with the residence time increased implying decomposition

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reactions occurred in the backbone and sidechain of xylan. Besides, a close relationship in chemical structure between xylan and its building units can be found by comparing the Fig. 3 (a) ~

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(d). The broad transmittance peak around 3400 cm-1 (O─H stretching vibration) and wide peak at

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1042 cm-1 (C─OH stretching vibration) of xylan can be attributed to the abundant hydroxyl groups linked on its sugar units. The transmittance peak corresponding to different kind of C─H

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stretching vibration for both the xylan and its sugar units shifts to a homogeneous peak around 2930 cm-1 with the increase of residence time, and together with the appearance of transmittance

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peak around 1700 cm-1 (C=O stretching vibration) indicates the dehydration reaction occurred between adjacent hydroxyl groups of sugars skeleton. The weak transmittance peak at 2139 cm-1 which belongs to the carbon-carbon triple bond or cumulated double bond appeared in xylan residue under 500 °C with the residence time of 10 s and xylose residue under 500 °C with the residence time of 20 s, and that peak also was detected in the residues obtained in earlier pyrolysis

stage of arabinose and glucose (the FTIR graph see Fig. 2S. (a)), which indicates some dehydrated intermediate compounds were formed under 500 °C for xylan and those mono sugars. Then water solution of those sugar residues was analyzed by GC/MS (the chromatograph see Fig. 2S. (b)). Due to many unknown chemicals exist in the solution, it is difficult to identify the intermediate

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compounds precisely.

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Fig. 2. Residue morphology evolution of (a) xylan, (b) xylose, (c) arabinose and (d) glucose at

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500 °C.

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Fig. 3. FTIR spectra of (a) xylan; (b) xylose, (c) arabinose and (d) glucose residues at 500 °C with different residence time. 3.2. Bio-oil analysis

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The bio-oil complexity originates from the complexity of xylan constituents. Many chemicals in

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xylan bio-oil cannot be identified due to the lack of available commercial standards for them[23].

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Therefore, comparing the TIC (total ion current) chromatogram of the bio-oil from xylan with that from its main constituents is an effective way to put insights into the chemicals origins in xylan

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pyrolysis. The TIC chromatogram of bio-oil derived from xylan, xylose, arabinose and glucose under 500 °C is shown in Fig. 4. It can be clearly seen that furfural (shorted as FF in the

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chromatogram) is the main product of xylan pyrolysis, and all the xylose, arabinose and glucose contributed the FF formation. It also can be inferred from Fig. 4 that xylose and arabinose tend to

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produce more furfural compared with glucose, while the latter yields more anhyro-sugars during pyrolysis. Acetic acid (AA), another typical chemical in xylan bio-oil, is mainly yielded from the decomposition of the O-acetyl groups[3,10], and it also can be found from Fig. 4 that just a little AA is produced by the xylose, arabinose and glucose degradation. Hydroxyacetaldehyde (HAA), a common low molecular weight compound in xylan bio-oil, can be derived from the C1─C2 and

C4─C5 segments of β-D-glucopyranose and β-D-xylopyranose and the C4─C5 segment of Oacetyl-β-D-xylopyranose[24], and the side arabinose units also can produce HAA through successive decomposition reactions[25]. The 5-hydroxmethyl furfural (5-HMF), levoglucosan (LG) and its furan isomer (AGF) are the typical pyrolysis products of hexose-based carbohydrates, here they can be ascribed to the decomposition of glucose unit in xylan. However, many unidentified chemicals (named DAPs and U1~U3 in the TIC chromatogram) exist in xylan bio-oil,

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and their detailed mass spectrogram were illustrated in the supplementary material Fig. 3S. It can be seen from the TIC chromatogram that U1 and U3 are produced from arabinose and xylose, respectively, and U2 with the molecular weight (MW) of 129 may be ascribed to the

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decarboxylation and demethoxylation of 4-O-methoxy-glucuronic acid (MW: 208). As to the

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DAPs, it can be seen obviously from Fig. 4 that both xylose and arabinose contribute to the

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formation of DAPs in xylan pyrolysis, while glucose rarely yields such chemicals. All the DAPs has a MW of 114, and speculatively can be produced from the pentose (MW: 150) via losing two

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H2O molecular or the decomposition of 4-O-methyl glucuronic acid units in the side branch. The chemicals with a MW of 114 have been reported in many papers about xylan pyrolysis[20,23–27],

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yet still many of them are unidentified. Akio et al.[26] reported the pyrolytic formation of 3hydroxy-2-penteno-1,5-lactone (MW: 114; this compound was proven to be 5-hydroxy-2H-pyran-

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4(3H)-one in reality by Ponder and Richards[28] using 1H-NMR and 13C-NMR) from xylan, xylose-based oligosaccharides, finding the yield of 3-hydroxy-2-penteno-1,5-lactone decreases with the degree of polymerization of xylose decrease, and even no 3-hydroxy-2-penteno-1,5lactone was detected in xylose pyrolysis, which implies the importance of glycosidic in the 3hydroxy-2-penteno-1,5-lactone formation. Zhou et al.[30] adopted a mechanistic model to

investigate the pyrolysis of hemicellulose and found two main dianhydroxylopyranoses (1,2;4,5dianhydroxylopyranose and 1,2;3,4-dianhydroxylopyranose) in enol or ketal form can be produced

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from hemicellulose.

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Fig. 4. Total ion chromatogram of bio-oil derived from xylan, xylose, arabinose and glucose

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pyrolysis at 500 °C. (HAA: hydroxyacetaldehyde; AA: acetic acid; FF: furfural; DAPs and U1~U3 are the unidentified chemicals; 5-HMF: 5-hydroxymethyl furfural; LG: levoglucosan; AGF: 1,6-

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anhuydro-β-d-glucofuranose.)

Some anhydroxylose and dianhyroxylose structure and formation pathways from hemicellulose

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have been proposed by Patwardhan et al.[23] and Zhou et al.[30], however, to get a deep insight into this problem, the dehydration reaction occurred in xylan should consider the position of side-

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branch units linked (C2 and/or C3) and the position of β-1,4-glycosidic bond breaking (C1─O and/or O─C4'), therefore, some possible reactions to form anhydro- and dianhydroxylopyranoses in xylan pyrolysis were proposed as illustrated in Fig. 5. Due to the close bond energy for C1─O and O─C4'[31], the glycosidic linkage can break at both these two bonds: (1) when the glycosidic bond breaks at C1─O, the unsubstituted xylose tends to form the 1,2-

anhydro-xylopyranose (enol or ketal form) or 1,4-anhydro-xylopyranose, the xylose substituted at C3 tends to form 1,2;3,4-dianhydro-xylopyranose (enol or ketal form), while the xylose substituted at C2 is likely to form C1=C2 without any hydroxyl group linked on C1 and C2, and the xylose substituted both at C2 and C3 would produce two carbon-carbon double bonds (C1=C2 and C3=C4) on the xylose pyran-ring or one carbon-carbon double bond (C2=C3) on 1,4-anhydroxylopyranose; (2) when the glycosidic bond breaks at O─C4', the unsubstituted xylose would

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prefer to form the 3,4-anhydro-xylopyranose (enol or ketal form) or 1,4-anhydro-xylopyranose,

the xylose substituted at C2 tends to form 1,2;3,4-anhydro-xylopyranose (enol or ketal form), the

xylose substituted at C3 would form C3=C4 without any hydroxyl group linked on C3 and C4, the

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xylose substituted both at C2 and C3 would produce similar products as such xylose under

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condition (1). It should be pointed out that the anhydo-xylopyranoses mentioned above have the

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potential to dehydrate furtherly to yield other dianhydo-xylopyranoses. Thus, some conclusions can be arrived that the unsubstituted xylose units in xylan have the potential to form anhydro- and

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dianhydro- xylopyranose, the xylose units with one side-branch attached at C2 or C3 prefer to yield dianhydro- xylopyranose when the glycosidic linkage breaks at the bond whose carbon is not

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adjacent to the substituted carbon, while the xylose units substituted both at C2 and C3 would not

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produce any anhydro- or dianhydro- xylopyranose.

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Fig. 5. Possible anhydro- and dianhydro- xylopyranose formation pathway in xylan pyrolysis. The blue dash line and dash arrow mean the glycosidic linkage breaks at O─C4', the red dash line and

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dash arrow mean the glycosidic linkage breaks at C1─O; the side-branch units R and R' can be

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acetyl, arabinose and 4-O-methyl-glucurinic acid groups.

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3.3. Effects of pyrolysis conditional parameters

Temperature, carrier gas flow rate and material thickness are three crucial parameters affecting the

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biomass heat and mass transfer in pyrolysis, and furtherly influence the products. Therefore, the effects of these three parameters on the xylan products distribution and bio-oil composition are

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discussed in this section. 3.3.1. Temperature

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The products distribution and bio-oil composition at different temperature with a carrier gas flow rate of 2.55 L/min and material thickness of 4 mm are shown in Fig. 6. It can be found from Fig. 6 (a) that the char yield decreases with the increase of temperature, which indicates high temperature can enhance the conversion of xylan to volatiles. The yield of bio-oil increases with the increase of temperature and reaches the maximum at 500 °C, while the yield of non-

condensable gases increases slowly when temperature below 450 °C and then presents a reverse trend with the bio-oil. The products distribution of xylan with temperature is different from the study by Lv et al.[12], which may be ascribed to the different mass-scale of feedstock input in pyrolysis experiments. Hemicellulose, as the most unstable component in biomass, has the lowest initial decomposition temperature around 200 °C[10,32]. By comparing the residue of xylan under 300 °C, 350 °C and 500 °C with same residence time of 20 s (Fig. 4S. (a) ~ (c)), it can be seen that

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the xylan just melted at the early stage in 300 °C, and bubbles formed in the thin border area when

the xylan under 350 °C, while many bubbles formed at the same residence time under 500 °C. The bubbles formed in liquid intermediate phase during pyrolysis were also the source of condensable

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and non-condensable volatiles, while its bursts need the internal energy exceeds the free energy of

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the liquid surface[33,34]. Consequently a possible reason for the low conversion rate at low

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temperature may be that the liquid membrane formed during xylan initial pyrolysis hindered the volatiles rapid evaporation and enhanced the secondary reactions, such as oligosaccharides

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repolymerization and dehydration to form char, while relative high temperature could cause violent decomposition leading to the liquid membrane burst due to enough heat provided.

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It can be seen from Fig. 5 (b) and (c) that the content of furfural and DAP2 in xylan bio-oil declines with temperature increasing, while the content of hydroxyacetaldehyde, 1-hydroxyl-2-

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propane, and propionic acid increases monotonically with the increase of temperature, which indicates that high temperature promotes the violent decomposition of xylan and the secondary reactions of primary volatiles to form low molecular weight compounds. The content of acetic acid decreases firstly and then increases with the temperature increasing, which can be attributed to the competition between the formation and consumption of acetic acid during pyrolysis. The

acetic acid derived from the breakage of O-acetyl group was active even at low temperature, while the acetic acid formation through decomposition of monosaccharides and/or the glucuronic acids would prevail at high temperature, and it is noteworthy that the decarboxylation of carboxyl compounds also would be enhanced with the temperature increasing[3,35]. The content of U1 and U2 keeps stable in the temperature range of 300 °C ~ 500 °C, and decreases slowly when the temperature exceeds 550 °C, while the content of U3 in xylan bio-oil falls firstly before 450 °C

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and then goes uphill rapidly and reaches the peak at 500 °C. The unknown compound U3 has the molecular weight of 86 which is a ion fragment weight in most unidentified chemicals (U1, U2, DAP1 and DAP2), so it can be speculated that U3 may be produced from the secondary

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decomposition of those chemicals, and obviously high temperature (500 °C ~ 600 °C) favors the

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U3 formation.

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Fig. 6. Distribution of products yield and bio-oil compositions at different temperature (Carrier

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gas flow rate:2.55 L/min; material thickness:4 mm). (a) Products distribution; (b) Identified chemicals; (c) Unidentified compounds.

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3.3.2. Carrier gas flow rate

Carrier gas flow rate influences not only the residence time but also the partial pressure of

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volatiles to condensate. The products distribution and bio-oil composition with different carrier gas flow rate at 400 °C with a material thickness of 4 mm and residence time of 30 min are shown in Fig. 7. It can be seen from Fig. 7 (a) that carrier gas flow rate has a negligible effect on the char yield, while the yield of bio-oil increases firstly and then declines with the increase of carrier gas flow rate, and the yield of gases presents an opposite trend with the bio-oil, which can be ascribed

to that high carrier gas flow rate can remove the volatiles rapidly from the pyrolysis zone reducing the secondary reaction when the carrier gas flow rate lower than 2.55 L/min, however, too high carrier gas flow rate would dilute the volatiles so that the volatiles partial pressure to condensate decreased. Similarly, as shown in Fig. 7 (b) and (c), the content of furfural and in bio-oil declines with the increase of carrier gas flow rate, while the content of DAP2 goes uphill with the carrier gas flow rate, and the content of acetic acid and U3 firstly decreases when carrier gas flow rate

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lower than 2.55 L/min and then increases, the content of 1-hydroxy-2-propane, propionic acid,

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DAP1, U1 and U2 in bio-oil presents a relatively stable trend with the carrier gas flow rate.

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Fig. 7. Distribution of products yield and bio-oil compositions with different carrier gas flow rate (temperature: 400 °C; material thickness: 4 mm; residence time: 30 min). (a) Products distribution; (b) Identified chemicals; (c) Unidentified compounds.

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3.3.3. Material thickness

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Material thickness is a crucial parameter influencing the heat and mass transfer when biomass is

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pyrolyzed in a fixed bed reactor. The products distribution and bio-oil composition for the pyrolysis of xylan with different material thickness at 400 °C with a carrier gas flow rate of 2.55

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L/min and residence time of 30 min are illustrated in Fig. 8. Here, it should be pointed out that the increase of material thickness was achieved by increasing the xylan input, and corresponding mass

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for each thickness was also marked in the graph (up axis). It can be seen from Fig. 8(a) that the char yield increases slowly with the increase of material thickness, while the yield of bio-oil

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increases monotonically in the material thickness range studied in present work (1 ~ 5 mm), which significantly different from the influence of material thickness on cellulose pyrolysis, thick material layer (> 2 mm) would suppress the cellulose bio-oil formation[17]. A similar bio-oil yield variation trend with the sample dimension for glucose pyrolysis was observed by Ansari et al.[36], powder form glucose with larger size produced more bio-oil than the thin-film glucose did. The

phenomenon of the high bio-oil yield with thick material layer for xylan can be attributed to two reasons: (1) more xylan input that was used to increase the material thickness would produce more volatiles, so the volatiles partial pressure to condensate also would increase, consequently just as discussed in section 3.3.2, the bio-oil yield would increase; (2) considering the bubbles are the main source of volatiles (the evaporation of volatiles which formed in the material surface also can condensate into bio-oil), thick material layer have the potential to produce bigger bubbles

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since the larger internal pressure for thick material layer comparing with the thin material layer, due to the small surface area for thick material layer and big surface area for thin material layer when the sample volume was similar. The images (Fig. 4S (c) and (d)) of bubbles for residues

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with different thickness (2 mm and 4 mm) while same mass (1g) under 500 °C shows that the

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thick xylan produces bigger bubbles, while many small bubbles formed randomly for the xylan

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with thin layer, and the products yield distribution of xylan with different thickness and same mass (see Fig. 5S) illustrates that thick material lay can enhance the xylan bio-oil yield to some extent.

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But it is not recommended to increase the xylan sample thickness too large, since too thick material layer definitely would suppress the evaporation of volatiles and cause violent secondary

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reactions, in fact, the overcoming of the molten membrane needs not only the enough high inner pressure by volatiles but also the enough heat or high temperature supply as being discussed in

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section 3.3.1. As can be seen from Fig. 8 (b) and (c), the content of DAP2 in bio-oil declines monotonically when the material thickness larger than 2 mm, while the content of furfural, DAP1 and U3 increases with the increase of material thickness, which indicates that thicker material layer favors the dehydration of xylan to form furfural and DAP1, but hinders the formation of DAP2 or promotes the secondary decomposition of DAP2. The content of hydroxyacetaldehyde in

xylan bio-oil peaks at 3 mm, while the content of acetic acid reaches its valley bottom at the same material thickness, and the content of other low molecular weight compounds, such as 1-hydroxy2-propane and propionic acid, goes downhill linearly with the increase of material thickness. Different content evolution profile of chemicals in bio-oil with material thickness and carrier gas flow rate indicates their competing formation reactions, and it can be extrapolated that DAP2 may

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be the precursor of furfural in xylan pyrolysis.

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Fig. 8. Distribution of products yield and bio-oil compositions with different material thickness (temperature: 400 °C; carrier gas flow rate: 2.55 L/min; residence time: 30 min). (a) Products

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distribution; (b) Identified chemicals; (c) Unidentified compounds. 4. Conclusions

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In present work, the pyrolysis behavior of xylan-based hemicellulose in a fixed bed reactor was

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investigated. Residue morphology evolution indicates that xylan and its mono-sugar units (xylose, arabinose and glucose) molted during pyrolysis and bubbles formed in their residue, and structure

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evolution and bio-oil composition of xylan presents a close relationship with those mono-sugars. Possible formation pathway of the unidentified compounds with a MW of 114 was discussed in

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present work. Investigation on xylan products yield distribution and bio-oil composition indicates

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temperature, carrier gas flow rate and material thickness are three coupled parameters influencing the heat and mass transfer, temperature is the key factor affecting the xylan conversion, while carrier gas flow rate and material thickness have a little influence on the char yield. Thick material layer and enough heat supply (high temperature) are needed for xylan to form big bubbles and overcome the liquid membrane suppression to produce more bio-oil, and proper carrier gas flow rate to reduce the secondary reactions meanwhile increase the volatiles partial pressure to

condensate. The detailed relationship among those three factors and the effects of them on the biooil yield and composition need further modeling study, and it also should be pointed out that many chemicals in xylan bio-oil also need to be identified in future studies.

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Declaration of Interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work (the manuscript entitled “ Pyrolysis behavior of xylanbased hemicellulose in a fixed bed reactor”).

Acknowledgement

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This study was funded by the National Natural Science Foundation of China (51536009 and

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51276103).

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