Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol

Accepted Manuscript Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol Tianhua Yang, Jian Wang, Bingshuo Li, Xingping Kai, Rundong Li PII: DOI: Reference:

S0960-8524(17)30004-4 http://dx.doi.org/10.1016/j.biortech.2016.12.110 BITE 17486

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Bioresource Technology

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3 November 2016 29 December 2016 30 December 2016

Please cite this article as: Yang, T., Wang, J., Li, B., Kai, X., Li, R., Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2016.12.110

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Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol Tianhua Yang1, Jian Wang1, Bingshuo Li2, Xingping Kai1, Rundong Li1,* 1

Key Laboratory of Clean Energy of Liaoning, College of Energy and Environment,

Shenyang Aerospace University, Shenyang 110136, PR China 2

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072,

PR China *

Corresponding author. Tel.: +86 (024) 89728889; fax: +86 (024) 89724558.

E-mail address: [email protected]

1

Abstract: This study investigated the influence of temperature and residence time on liquefaction of rice straw in subcritical CO2–subcritical water (subCO2–subH2O) and in subcritical CO2–supercritical ethanol (subCO2–scEtOH), considering the final reaction temperatures (270‒345 °C) and residence times (15 and 30 min). Residence time was identified as a crucial parameter in the subCO2–subH2O liquefaction, whereas residence time had a marginal influence on subCO2–scEtOH liquefaction. When reaction conditions were 320 °C and 15 min, solvents have weak impact on the quality of bio-oil, HHV 28.57 MJ/kg and 28.62 MJ/k g, respectively. There was an obvious difference between the subCO2–subH2O and subCO2–scEtOH liquefaction mechanisms. In subCO2–subH2O, CO2 promoted the carbonyl reaction. In subCO2–scEtOH, supercritical ethanol have the function of donating hydrogen and promoting the reaction of hydroxyl-alkylation. Keywords: Pretreatment; Subcritical H2O; Supercritical Ethanol; CO2 1. Introduction The energetic and chemical utilization of biomass has become a prominent topic in recent years (Anca-Couce et al., 2016), and rapid development of green chemical technology is represented by supercritical fluids (Yuan., 2014). At the same time, the amount of crop residue, such as straw, formed in agricultural production exceeds 700 million tons. The importance of the methods for bio-oil production from straw by suband supercritical fluids have been increasingly emphasized (Tungal et al., 2014). Many studies have been published in the field of hydrothermal liquefaction in recent years (Tungal et al., 2014; Xu et al., 2008; Isa et al., 2015; Güvenatam et al., 2016; Zhu 2

et al., 2015; Singh et al., 2015; Yin et al., 2010; Durak et al., 2015; Kann et al., 2016 ). The influence of the reaction parameters and their optimization for a high bio-oil yield have been the main targets of these studies. Researchers currently either use water or organic solvent as a medium. The bio-oil yields from subcritical water (subH2O) (Zhu et al., 2015) and supercritical ethanol (scEtOH) (Singh et al., 2015) were 34.9 wt% and 47.52 wt%, respectively, at a temperature of 300 °C and a residence time of 15 min. According to the research report by Li et al. (2015), the liquid yields from subH2O and scEtOH were 72.85 and 63.03%, respectively, under the same conditions (300 °C, 60 min). However, to date, there have only been a few studies on the role of the temperature and residence time in hydrothermal liquefaction of rice straw regarding the product distribution and properties using similar experimental methods. To date, researchers have used chemical and biological pretreatment methods to remove lignin. However, such methods destroy the crystal structure of cellulose and hemicellulose, increase the porosity of the straw, reduce the sub- and supercritical residence times and also decrease the cost of production. Before producing gas in anaerobic fermentation, cellulose and hemicellulose were mostly degraded (Zhang et al., 2015; Ferrer et al., 2014). Therefore, these materials were no longer conducive to the subsequent hydrolysis of sugar. Dilute acid pretreatment (Kumagai et al., 2015) and baking equipment cause corrosion and have a high cost because of subsequent processing. A biochemical method (anaerobic fermentation and base catalysis) can effectively reduce the content of lignin (Li et al., 2015). Hydrothermal pretreatment technology has the advantages of dilute acid pretreatment combined with steam 3

explosion, and the need for efficient and clean biomass conversion technologies has propelled hydrothermal processing as a promising treatment option for biofuel production (Patel et al., 2016). Supercritical CO2 pretreatment technology for lignocellulose is environmentally friendly and has become one of the major focuses of research in recent years. Supercritical CO2 pretreatment favors hydrolysis of biomass, similar to steam explosion. The equipment used for the pretreatment method demands stringent specifications because of the rapid pressure relief. An easier subsequent hydrolysis treatment has also been found to be advantageous for improving the yield of sugar. However, there are unavoidable defects, such as the high pressure and long required time in the pretreatment method. There is almost no related research regarding how the subcritical CO2–subcritical water (subCO2–subH2O) pretreatment method impacts the performance of bio-oil production from biomass. Therefore, 200 °C and 10 min were chosen as the reaction parameters for pretreatment in this study. This paper presents a method to produce bio-oil from rice straw by a two-step liquefaction process, with subCO2-solvent pretreatment—sub- and supercritical liquefaction, focusing on the effects of the second step using subH2O and scEtOH. The purpose of our work was to understand the effect of subH2O-based liquefaction and scEtOH-based liquefaction on the characteristics of the raw materials pretreated by subCO2-solvent at 200 °C for 10 min. Our results might help to better understand the mechanism of biomass liquefaction, and it is therefore useful for developing efficient biomass utilization processes. 2. Materials and methods 4

2.1. Material and reagents Rice straw (RS) material was collected from the suburb of Shenyang, China, and was sieved to a particle size of 30‒50 mesh (0.282‒0.613 mm), dried at 105 °C for 12 h, and stored in a desiccator at room temperature. The acetone, dichloromethane (CH2Cl2) and ethanol reagents used for the experiments were of analytical grade. The proximate, ultimate, and high heating value (HHV) analysis results of the RS material are given in Table 1. 2.2. Liquefaction process and product separation A 500 mL stainless steel pressure vessel with an extreme condition of 500 °C and 30 MPa was used as the reactor for the liquefaction tests. A detailed description of the apparatus is given in a previous paper (Li et al., 2013). Typically, 15 g of RS and 150 mL of solvent (deionized water or ethanol) were loaded into the autoclave; then, the reactor was purged with CO2 to remove air. The autoclave was heated to the desired temperature (200 °C) over 10 min by an external jacket heater, and the stirring rate was set to 100 rpm. The phase detailed above can be regarded as the pretreatment phase. The second step was hydrothermal liquefaction (HTL), in which the autoclave was heated to the desired temperature (275‒345 °C) by an external jacket heater with experimentally desired residence times (15 or 30 min). Upon completion, the heater was removed and the reactor was cooled to room temperature by a fan. Experimental procedure for the subCO2-solvent pretreatment two-step liquefaction process (Fig.1) During the subCO2-subH2O process, the resulting suspension and liquid fraction were poured into a beaker and then filtered under vacuum to obtain suspensions. The 5

suspensions and fraction remaining in the autoclave were rinsed with the proper amount of acetone to obtain the solid and acetone-soluble fractions. The collected acetone solution was evaporated under reduced pressure at 60 °C using a rotary evaporator. After removal of the acetone, the fraction obtained was called WIO (water-insoluble oil). Briefly, the filtrate from the water experiments was extracted using CH2Cl2, followed by solvent evaporation. The product was designed as WSO (water-soluble oil). The acetone-insoluble fraction was dried overnight at 105 °C and then quantified to obtain solid residue. Bio-oil = WIO + WSO During the subCO2-scEtOH process, the residuals inside the autoclave were washed several times with ethanol and then poured into a beaker. The products in the beaker were filtered to separate the solid and liquid phases (ethanol-soluble). The solid products were rinsed with acetone to obtain the acetone-soluble fraction. The ethanol-soluble fraction combined with the acetone-soluble fraction was designated as bio-oil. Ethanol and acetone were recovered under reduced pressure in a rotary evaporator at 80 °C and 60 °C, respectively. Insoluble acetone was dried overnight at 105 °C to obtain a residual solid. 2.4. Analytical methods The conversion and product yields were calculated using the following equations: Conversion ratio %= Bio-oil yield %=

WBiomass.db -WSolid reside.db wBiomass.db

WBio-oil Biomass.db

Solid reside yield %=

×100%

×100%

WSolid reside.db WBiomass.db

(1) (2)

×100%

(3) 6

Gas yield%=100%-Bio-oil yield%-Solid residue yield%)

(4)

where WBiomass;db and WSolid residue;db are the weight of the initial biomass and solid residue on a dry basis, respectively. The elemental compositions (C, H, O and N) of the rice stalk and bio-oils were determined by a CHNOS elemental analyzer (EA3000 CHNS/O, Italy). The HHV (higher heating value) was calculated from elemental analyses results as described in DIN 51900: HHVMJ⁄kg =(34×C)+(124.3×H)+(6.3×N)+(19.3×S)‒(9.8×O)⁄100

(5)

The pH of the pretreatment solution samples were examined using a pH meter. The microstructures of the subCO2-solvent pretreatment simples were examined by scanning electron microscopy (SEM) (ISPECTF, FEI). A STA 449 F3 Jupiter® thermal analyzer was used for simultaneous TG-DSC measurements. Biomass pyrolysis tests were performed with highly pure nitrogen (purity > 99.999%) as the purge gas at a gas flow velocity of 20 mL/min, and the temperature was increased from 30 to 800 °C at a rate of 20 °C/min. Attenuated Total Reflectance (ATR, Nicolet iS50, Thermo Fisher Nicolet, America) was carried out to determine the functional groups in the bio-oil over a wave number range of 4000–500 cm−1. Bio-oil was analyzed via gas chromatography-mass spectrometry (GC/MS, Agilent 6890N/5973). The carrier gas was He at a flow rate of 20 mL/min, and the split ratio was 1/40. An HP-5 column (30 m × 0.25 mm × 0.25 µm) was used for the separation. An oven isothermal program was set at 50 °C for 3 min, followed by a heating rate of 7

5 °C/min to 280 °C and held for 15 min. The sample-injected volume was 1µL. The mass range scanned was from 20 to 500 amu in electron ionization (70 eV) mode. The identification of the peaks in the chromatogram was based on the comparison with standard spectra of compounds in the NIST08 (National Institute of Standards and Technology08) database library and/or on the retention times of known species injected. Quantification was done by the manual integration of the chosen single ion chromatogram. 3. Results and discussion 3.1. Pretreatment 3.1.1. Characterization of the solution As shown in Fig S1, the color of the CO2-ethanol solution after pretreatment was darker than the color of the CO2-H2O solution because the solubility of ethanol is stronger than the solubility of H2O. When ethanol was the solvent, the pH was 5.94, whereas the pH was 3.50 in the CO2-H2O system because CO2 in water (Eq. 6) provided more ionization H+ than CO2 in ethanol (Eq. 7). H2O+CO2→H2CO3

(6)

CH3CH2OH+CO2 →CH3COOH +HCOH

(7)

3.1.2. SEM analysis Fig. S2 shows SEM images of residues from CO2-H2O and CO2-ethanol pretreatment to investigate the microscopic structure of straw after subCO2-solvent pretreatment. Both solvents affect the surface morphology of rice straw, with significant structural changes occurring. For CO2-H2O (Fig. S2(a)), cellulose constituting the skeleton 8

structure of the cell wall was observed, with epidermis coking. As shown in Fig. S2(b), bubbles could be found in the surface after it was treated at 200 °C for 10 min. An acid solution was favored for hemicellulose hydrolysis and cellulose next (Kumagai et al., 2015 and Li et al., 2015), probably due to the removal of components, such as cellulose and hemicellulose, accompanied by cell structure disruption (Zhu et al., 2015). 3.1.3. TG-DSC analysis To investigate the effect of subCO2-solvent pretreatment on rice straws, the decomposition behavior of subCO2-H2O, subCO2-ethanol pretreated and untreated rice straw samples were determined by TG-DSC from 30–800 °C. TG-DSC/DTG curves are presented in Fig. 2, and summary of the results obtained from TG-DSC analyses is presented in Table S1. The pretreated and untreated samples were markedly different. The TG curves of pretreated samples were similar to samples from the curves of samples pretreated with H2SO4, which suggested that the subCO2-solvent pretreatment had the same characteristics as the dilute acid pretreatment (Kumagai et al., 2015). The results of thermal analysis showed that there were almost three stages in the TG curves. In the first phase, with the evaporation of water (Zhu et al., 2015), heat absorption was detected in the DSC curves and the mass evidently decreases at the same time (Luo et al., 2015). The results showed that the initial degradation temperatures of subCO2-H2O, subCO2-ethanol pretreated and untreated rice straw samples were 142.6, 98.2 and 98.1 °C, respectively. The initial degradation temperature of the subCO2-H2O sample was the highest, which indicated the presence of cellulose and lignin compounds. Chaula et al. (2014) stated that hemicellulose was primarily removed due to steam 9

explosion. The sequence of activities of three components during pyrolysis of lignocelluloses is: hemicellulose > cellulose > lignin. The second-A stage was mainly the degradation of hemicellulose. In this stage, the mass of the samples decreased slowly and the heat flow (exothermal) of decomposition was very low (Vakalis et al., 2016). With the subCO2-H2O pretreated sample, the mass loss was the highest because hemicellulose decomposed into furfural (Kumagai et al., 2015). The second-B stage was mainly cellulose devolatilization (Shen et al., 2015). Fig. 2(a) shows that the curve of subCO2-H2O was located at the left upper side of the curve of subCO2-ethanol because lignin compounds cannot be effectively degraded by subCO2-solvent, as was shown in section 3.1.1. In the last period, the initial degradation temperatures of the samples were high. A possible driving factor could be identified as the exothermicity of pyrolysis, which, in principle, is enhanced by the increased presence of lignin, and remainders (lignin) were slowly decomposed during this stage (Zhu et al., 2015; Shen et al., 2015). In summary, the subCO2-solvent pretreated sample had high heat stability, followed by untreated rice straw. Rice straw pretreated by subCO2-H2O had a relatively higher amount of lignin when the weight of the samples was the same. By pretreating with the subCO2-solvent technology, a two-step liquefaction process for bio-oil production from biomass was established with the aim of improving the bio-oil yield and reducing the total energy consumption. 3.2. Conversion ratio and product yields The effects of the residence time and temperature on rice straw the conversion ratio and bio-oil yields using subH2O and scEtOH are illustrated in Fig. 3. Rice straw 10

samples were heated to final reaction temperatures in the range of 275‒345 °C with two different residence times of 15 and 30 min. When subH2O was used, completely opposite values of the conversion and bio-oil yield were observed at residence times of 15 and 30 min when the temperature was increased from 270 to 320 °C. Figure. 3a suggested that the decomposition of the solid matrix into smaller oligomers and monomers was strongly dependent on the synergy of the temperature and residence time, not the residence time. However, prolonged residence times resulted in increased bio-oil yield. When the temperature was over 320 °C, similar values of the conversion ratio and bio-oil yields were obtained, suggesting that decomposition of the solid matrix was strongly dependent on the temperature and that a prolonged residence time resulted in a decrease in the conversion ratio, with an increase in the bio-oil yield. When the residence time was 15 min, the bio-oil yields were directly proportional to the conversion ratio and both of the temperatures were in a direct ratio. For other residence times, the yield of bio-oil fluctuated according to the temperature due to the dynamic equilibrium (depolymerization reaction) between the gas and liquid phases, suggesting that the depolymerization reaction was strongly dependent on the residence time (Qin et al., 2012; Anastasakis, et al., 2011), not the temperature. In sharp contrast to subH2O-based liquefaction, only a marginal effect of the residence time on the conversion ratio was observed when scEtOH was used as the liquefaction solvent (Fig.3a). When scEtOH was used, a smaller effect of the residence time over the temperature range of 270‒320 °C was observed: the conversion ratios 11

obtained from both residence times were close to each other at 41.83‒49.20% and 64.66%‒67.91% (295 °C), reaching 74.77%‒74.64%, because the residence time had a weak impact on the degradation of straw with increasing temperature (Fig. 3d). When the temperature was increased above 320 °C, a significant impact of the residence time was observed: higher conversion ratios of 74.77‒80.16% were obtained with a low residence time of 15 min, whereas the conversion ratio obtained at a high residence time of 30 min stagnated at 79.14%. At 15 min, the bio-oil yield increased significantly from 26.43 to 47.78% wt%, while at 30 min, the bio-oil yield increased slightly from 26.40 to 35.70 wt% when increasing the final temperature from 270 to 320 °C. The bio-oil yields were inverse to the conversion, and a similar result was obtained from Brand et al. (2014). When the temperature was over 320 °C, a low residence time could generate a high conversion ratio and bio-oil yield, and as shown in Fig.3d, different residence times may lead to similar gas yields over the range of 295‒345 °C. Thus, ethanol free radical possesses the function of reforming the constituent products (Singh et al., 2015; Liu et al., 2010; Zheng et al., 2012; Matsakas et al., 2012), dependent on the temperature. A large difference between the conversion and bio-oil yields was caused by the polarity of the substance (Brand et al., 2014; Hu et al., 2015). Water releases active H to promote lignocellulose depolymerization, resulting in an increase in the conversion yield. Ethanol releases ethanol free radicals to keep intermediate products stable, which results in an increase in the bio-oil yield. According to reports in the literatures (Zhu et al., 2015; Zheng et al., 2012; Cheng et 12

al., 2012; Madeno˘glu et al., 2012), as the intermediates repolymerization at 320 °C, cyclization and condensation reactions lead to the formation of polymer solid compounds, resulting in an increase in the solid reside yield. However, as shown in Figure 3c, the results showed that the solid residue yield was reduced in different solvents. SubCO2-solvent acid promoted depolymerization (pyrolysis and hydrolysis) of cellulose and hemicellulose as well as hydrolysis (alcoholysis) of lignin with increasing temperatures from 270 to 320 °C. However, pyrolysis of lignin was the main reaction when the temperature was over 320 °C. As shown in Table 3, compared with subCO2-scEtOH, subCO2-subH2O conversion ratio was increased by 8.13% under the same conditions. A bio-oil yield of 47.78 wt% was obtained from straw by subCO2-scEtOH at 320 °C and a residence time of 15 min, and a bio-oil yield of 23.59 wt% was obtained from hydrothermal liquefaction at 295 °C with a residence time of 30 min. Compared with subCO2-subH2O, subCO2-scEtOH conversion was increased by 24.19%. 3.3. FTIR analysis of the bio-oil Four FTIR spectra of the bio-oils had similar peak assignments (Fig.S3), which indicated that the bio-oils contained the same functional groups. The absorbance strength of the FTIR was mainly related to the contents of substances in the hydrothermal liquefaction bio-oil over 4000‒1500 cm‒1 and suggested that the two types of relative contents were different. The broad band of the ‒OH stretching vibration between 3200 and 3450 cm‒1 indicated the presence of alcohol and phenolic groups in the bio-oil. The ‒OH stretching 13

vibration performed high intensity in subCO2-subH2O suggested that large portion of phenolics in the oil from subCO2-subH2O compared to that from subCO2-scEtOH. The absorption peaks in the range of 3000‒2870 cm‒1 and 2870‒2850 cm‒1 were caused by C‒H stretching vibrations in methyl and methylene groups, respectively. Besides, two other bands at 1370 and 1442 cm‒1 were attributed to C‒H deformations in ‒CH2 and ‒CH3 groups. These peaks indicated that the aliphatic and alkyl aromatic compounds were present in bio-oil (Zhu et al., 2015 and Zhu et al., 2015). The subCO2-subH2O was benefit to the formation of alkene and alkyl aromatic compounds, and some obvious differences were observed in the same solvents under various reaction conditions. The bands at 1710 and 1640 cm‒1 related to the C=O stretching suggested the presence of ketones and aldehydes. Both ‒OH stretching and C=O stretching vibrations revealed the presence of carboxylic acids. The aromatic skeletal and heterocyclic vibrations at 1600 and 1450 cm‒1 together with some C‒H bending vibrations from aromatics between 3000 and 2700 cm‒1 indicated the presence of aromatics and their derivatives. When the wavenumber was in the FTIR fingerprint spectral range of 1300‒400 cm‒1, the absorbance strength of the FTIR was mainly related to the species of the compound. The C–O stretching intensity of absorption band at 1044 cm−1 faded away, which could be explained as the deeper deoxygenation of alcohols, phenols, or esters in subCO2-scEtOH. In addition, several bands that occurred between 700 and 900 cm−1 could be attributed to C–H out-of plane bending vibration from aromatics and their 14

derivatives, and their intensity also weakened at higher temperature because of ring opening or hydrogenation. From the above results, the undesirable functional groups (–OH, C–O and C=O) in the subCO2-scEtOH were improved. 3.4. GC/MS analysis of bio-oil Solvents affect not only biomass conversion and product yields but also affect the compound distribution of bio-oils. The values of the peak areas (by the area normalization method) illustrated in Tables S2 and S5 do not represent the actual concentrations but indicate the product distributions associated with the solvents. Fig.4 shows the composition classification of bio-oils based on GC/MS analysis, clearly showing that the liquefaction solvents obviously affected the components of bio-oils. As shown in Table S2 and S3, bio-oil obtained from subCO2-subH2O contained 6 and 39 kinds of compounds respectively, whereas bio-oil of subCO2-scEtOH had 104 (Table S4) and 69 (Table S5) types of compounds respectively under the same reaction condition. Bio-oils from HTL contained >24 compounds (Zhu et al., 2015; Li et al., 2015; Madenoglu et al., 2016), showing that subCO2-subH2O made the product more normalized. From the subCO2-subH2O process, the largest peak was ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)- (Table S2, RT 29.922 min), which accounted for 42.21% of the total area, whereas it was phenol, 2,6-dimethoxy- (RT 13.822 min) with the largest peak of 14.82% in the Table S3. This was owing to acid solutions promotes the occurrence of the de-carbonyl reaction. And in the subCO2-scEtOH system, the largest peak was 2-furanmethanol, tetrahydro-, acetate (Table S4, RT9.188 min), which 15

accounted for 5.74% of the total area, whereas it was butanoic acid, (tetrahydro-2-furanyl) methyl ester (RT 14.057 min) with the largest peak of 13.77% in the Table S5. A major 5-Allyl-5-butylbarbituric acid (Table S2, RT30.304 min, 11.54%) and 3-(4-Hydroxy-3-methoxyphenyl)-2-oxopropanoic acid (Table S3, RT24.776 min, 4.39%) that may be derived from the pyrolysis of lignin of the organic acids were detected respectively. It was reported that the decline of the acid content improved the properties of bio-oil with a low oxygen content (Zhu et al., 2015). The relative amount of acid compounds were 5.31% and 4.93% only detected using ethanol as solvent, which was all the typical representative of 2H-Pyran-5-carboxylic acid, 2-oxo- that were 1.45% (Table S4, RT 10.085) and 2.27% (Table S5, RT 15.769) respectively that may be derived from the pyrolysis of cellulose and hemicelluloses. The phenolic compounds were reported to be primarily derived from the degradation of lignin, which were hydrolyzed firstly and then rehydrated, forming hydrated benzenes or alkylated benzenes (Barbier et al., 2012). When H2O was used as solvent, 2, 6-dimethoxy phenol (Table S2, RT 23.120 min, 15.62% and Table S3, RT 13.882 min, 14.82%) with the largest peak of 15.62 % and 35.98% in term of phenolic respectively, whereas phenols was dominated by methoxyphenol and alkylation of phenol in the subCO2-scEtOH system. The phenolic and cyclic compounds mainly originated from the breakdown of lignin. Sing et al. (2015 and 2014) investigated the highly reactive environment created by supercritical ethanol- and methanol-enhanced lignin decomposition to aromatic ethers, 16

monomeric phenols and phenols. Under the same conditions, subCO2-subH2O bio-oil contained better phenolic and cyclic compounds than subCO2-scEtOH. The results indicated that the subCO2-solvent two-step pretreatment liquefaction, especially subCO2-subH2O, had a high degrading efficiency for cellulose and hemicelluloses. A similar result was obtained by Cheng et al (2012). Under the same conditions, subCO2-subH2O bio-oil contained much more alcohol than subCO2-scEtOH because subCO2-subH2O was more capable of reducing the sugar than subCO2-scEtOH. Ketones were primarily derived from monosaccharides generated from the decomposition of hemicellulose, and the ketones may transform between alcohols and acids because they were less stable under hydrothermal conditions (Chen et al., 2014). Because CO2 promoted the occurrence of the carbonyl reaction, the amount of ketone compounds is enhanced: subCO2-subH2O bio-oil contained 42.21% and 24.78% ketone compounds, respectively, whereas scEtOH can inhibit the occurrence of the carbonyl reaction and thus will cause their number to decrease in the subCO2-scEtOH system. The carbonyl group is not stable and leads to a complex chemical reaction (Chang et al., 2011). Therefore, the addition of ethanol was helpful for improving bio-oil stability (Li et al., 2011 and Li et al., 2016). 3.5. Elemental analysis of bio-oil Elemental analysis and heating value were measured and are shown in Table 3. Bio-oil was a product with high oxygen content. When reaction conditions were 320 °C and 15 min, the more hydrogen content of the bio-oil obtained in the scEtOH system might be attributed to hydrodeoxygenation because ethanol can be used as a hydrogen 17

donor in liquefaction. SubCO2-scEtOH bio-oil contained more carbon content under the same reaction conditions. This suggested that subCO2-scEtOH produced less carbon gases, such as CO, CO2 and CH4. This was due to the formation of H2, CO, CH4 and CO2 gases might be attributed to the dehydrogenation of sugars as well as dehydrogenation of acids, aldehydes, and ketones derived from sugars as well as to the water-gas shift reaction (Tungal et al., 2014). In term of heating value, subCO2-subH2O bio-oil was of higher quality than subCO2-scEtOH bio-oil at 295 °C, 30min, and when reaction conditions were 320 °C and 15 min, subCO2-scEtOH bio-oil was higher. 4. Conclusions The two-step liquefaction process provided a new idea regarding the study of bio-oil production from biomass. This study revealed that the residence time was a crucial parameter in the hydrothermal liquefaction, while it had a marginal impact on scEtOH-based liquefaction. Employing CO2 as the atmosphere changed the distribution of chemical compounds. When reaction conditions were 320 °C and 15min, solvents had weak impact on the quality of bio-oil, HHV 28.57 MJ/kg and 28.62 MJ/kg, respectively. Supercritical ethanol acted as a hydrogen donor and promoted the hydroxyl-alkylation reaction and CO2 promoted the carbonyl reaction in the process of hydrothermal liquefaction, which distinguished subCO2-subH2O from subCO2-scEtOH. Acknowledgements The authors acknowledge a research grant support by the National Natural Science Foundation of China (51576135) and the Aeronautical Science Foundation of China (No. 2015ZB54006). 18

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Figure captions: Fig. 1. Experimental procedure for the subCO2-solvent pretreatment two-step continuous liquefaction process Fig. 2. Comparison of TG, DTG and DSC curves for rice straw and subCO2-solvent pretreatment of samples: (a) TG-DSC curves; (b) DTG curves Fig. 3. Influence of the residence time and temperatures on conversion ratio and products distribution in subCO2-solvent Fig. 4. Classification of the bio-oils obtained from subCO2-subH2O and subCO2-scEtOH

24

Table 1 Proximate, ultimate and heating value analysis of the RS material Proximate Analysis (wt%)

Ultimate Analysis (wt%) Higher heating Fixed

Moistur-

value/(MJ·kg-1

VolatilAsh

e

carbo-

C

H

O

N

S

e

) n

15.8 5.99

39.7 71.55

4

5.5

53.6

0.8

0.2

6.62

15.24 7

3

25

4

2

4

Table 2 Parameters of the highest conversion ratio and bio-oil yield in water (ethanol) Residence time

Temperatures

Conversion ratio

Bio-oil yield

（min）

（°C）

（wt%）

（wt%）

15

345

88.295

30

295

15

345

15

320

Solvent

Water 23.585 80.162

Ethanol 47.776

26

Table 3 Elemental analysis of the bio-oils obtained from SubCO2-SubH2O and SubCO2-ScEtOH SubCO2-SubH2O

SubCO2-SubH2O

SubCO2-ScEtOH

SubCO2-ScEtOH

(295 °C, 30 min)

(320 °C,15 min)

(295 °C,30 min)

(320 °C,15 min)

62.94

65.98

61.53

62.96

7.70

6.94

7.52

8.00

28.82

26.06

30.29

28.35

0.54

1.02

0.66

0.69

H/C

0.12

0.11

0.12

0.13

O/C

0.46

0.41

0.49

0.45

HHV (MJ/kg)

28.18

28.57

27.34

28.62

Specification

Carbon (wt%) Hydrogen (wt%) Oxygen (wt%) Nitrogen (wt%)

27

Fig. 1

Fig.1 (a) SubCO2-SubH2O process

28

Fig.1 (b) SubCO2-ScEtOH process

29

Fig. 2 TG(CO2-H2O)

TG(CO2-Ethanol) TG(RS)

DSC(CO2-H2O)

DSC(CO2-Ethanol) DSC(RS)

14

(a)

100

12

TG(%)

8 60 6 4

40

2 20 0 0

100

200

300

400

500

600

700

-2 800

Temp (°C) 2

(b) 0

DTG (%)

-2 -4 -6

CO2-H2O

-8

CO2-Ethanol

-10 -12 318°C -14

RS

353°C

100

200

300

330°C 400

500

600

Temp (°C)

30

700

800

DSC(mW/mg)

10

80

Fig. 3 90

70

(a)

SubCO2-subH2O-15min 60

80

(b)

SubCO2-subH2O-30min

70

Bio-oil Yield(%)

Conversion ratio(%)

SubCO2-scEtOH-15min 50

60 SubCO2-subH2O-15min

50

SubCO2-subH2O-30min SubCO2-scEtOH-15min SubCO2-scEtOH-30min

40 260

270

280

290

300

310

320

330

340

SubCO2-scEtOH-30min

40 30 20 10 0 260

350

270

280

(c)

60

80

SubCO2-subH2O-15min SubCO2-scEtOH-15min

310

40

30

270

280

290

300

310

320

330

340

350

(d)

50 40 30 20

SubCO2-subH2O-15min

10

SubCO2-scEtOH-15min

SubCO2-subH2O-30min

20

10 260

320

60

SubCO2-scEtOH-30min

Gas yield(%)

Solid reside yield(%)

300

70

SubCO2-subH2O-30min 50

290

Temperature(°C)

Temperature(°C)

330

340

0 260

350

SubCO2-scEtOH-30min 270

280

290

300

310

320

Temperature(°C)

Temperature(°C)

31

330

340

350

Fig. 4

(a)

(c)

60

SubCO2-SubH2O(295°C,30min)

(b)

SubCO2-SubH2O(320°C,15min)

50

40

30

20

(d)

10

0

10

SubCO2-ScEtOH(295°C,30min) Nitrile Amine Nitrides Cyclic compounds Ketone Aldehyde Phenol Ether Hydrocarbon Acid Alcohol Ester

SubCO2-ScEtOH(320°C,15min)

20

Area(%)

32

30

40

50

60

Research highlights: 1. Subcritical CO2-solvent pretreatment had the characteristics of hydrothermal and dilute acid. 2. Residence time had marginal impact on subcritical CO2-supercritical ethanol liquefaction. 3. CO2 promoted the carbonyl reaction in subcritical CO2- subcritical water.

33