Hydrothermal liquefaction of crop straws: Effect of feedstock composition

Fuel 265 (2020) 116946

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Hydrothermal liquefaction of crop straws: Effect of feedstock composition a

a,⁎

b

a

a

Ye Tian , Feng Wang , Jesuis Oraléou Djandja , Sheng-Li Zhang , Yu-Ping Xu , Pei-Gao Duan

a,b,⁎

T

a College of Chemistry and Chemical Engineering, Department of Energy and Chemical Engineering, Henan Polytechnic University, No. 2001, Century Avenue, Jiaozuo, Henan 454003, PR China b Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Crop straw Hydrothermal liquefaction Bio-oil Biochemical composition

Hydrothermal liquefaction of four crop straws, including corn straw (CS), peanut straw (PS), soybean straw (SS), and rice straw (RS), were examined at 320 °C for 60 min to explore the effect of feedstock composition on the products distribution and properties of the bio-oil. SS produced the highest bio-oil yield of 15.8 ± 0.8 wt%, followed by RS (15.1 ± 0.7 wt%), PS (14.6 ± 0.7 wt%), and CS (7.9 ± 0.7 wt%). The significant difference in bio-oil yield was ascribed to the cellulose content and the mass ratio of cellulose to hemicellulose (wcellulose/ whemicellulose) of the feedstock. Elemental analysis showed that the four bio-oils had similar elemental compositions of C, H and O. Ketones and phenolic compounds occupied a large proportion of the four bio-oils, and their relative contents were related to the cellulose content and wcellulose/whemicellulose value of the feedstock. Feedstock with high cellulose content and wcellulose/whemicellulose value would lead to bio-oil with high amount of ketones.

1. Introduction The gradual depletion of fossil fuel resources and the intensification of environmental pollution have increased global interest in alternative energy sources [1]. Biomass, as a renewable and CO2-neutral energy source, is considered to be a complementary resource of fossil fuels [2]. Various types of biomass, mainly consisting of lignocellulosic biomass, algae, and organic wastes, are converted to biofuels through biochemical and thermochemical conversion technologies, which have been extensively examined in previous studies [3–7]. Lignocellulosic biomass is considered the most abundant renewable biomass, with a global production of approximately 1.3 × 1010 tons in 2015 [8,9]. The main components in lignocellulosic biomass are cellulose, hemicellulose, and lignin, which can be utilized as cheap and renewable feedstocks for the production of biofuels and chemicals [10,11]. As a typical lignocellulosic biomass, crop straw is abundant worldwide, especially in agricultural countries. In China, approximately 806.9 million tons of crop straw was produced in 2009 [12]. Crop straw has considerable prospects in biofuel production because of its abundance, low cost, and environmental friendliness. Unfortunately, most crop straw is discarded or directly burned after harvesting, which is bound to cause waste of resources and aggravation of environmental pollution. Therefore, it is essential to seek low-cost and efficient

conversion technology to treat crop straw for biofuel production. In most cases, pyrolysis and hydrothermal liquefaction (HTL) are considered the two leading processes to convert biomass into liquid biofuels [1,13]. Crop straw typically contains a certain amount of moisture after harvesting. Therefore, HTL is an attractive method of converting this type of high-moisture feedstock, obviating the energyintensive pre-drying process required for pyrolysis [14]. In HTL, water is typically adopted as a solvent and reaction medium. In fact, under subcritical and supercritical conditions, water exhibits peculiar properties, including a low dielectric constant, high ionic product, and high solubility with respect to small organic compounds [15,16]. These unique properties enable water to carry out and catalyze depolymerization and repolymerization reactions of cellulose, hemicellulose and lignin that account for a large proportion of lignocellulosic biomass [1]. To date, many studies have been conducted on the HTL of various types of crop straw, mainly rice straw, wheat straw, corn straw, legume straw, cotton straw, and barley straw [1,3,17–20]. The influence of process variables, including the temperature, residence time, catalyst, biomass to water ratio, and ambient atmosphere, on the product distribution and composition of bio-oil has been intensively investigated. In contrast, relatively less attention has been paid to the effect of feedstock composition. It has been recognized that the composition of lignocellulosic biomass, in terms of the amount of cellulose,

⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Department of Energy and Chemical Engineering, Henan Polytechnic University, No. 2001, Century Avenue, Jiaozuo, Henan 454003, PR China (P.-G. Duan). E-mail addresses: [email protected] (F. Wang), [email protected] (P.-G. Duan).

https://doi.org/10.1016/j.fuel.2019.116946 Received 25 October 2019; Received in revised form 11 December 2019; Accepted 22 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 265 (2020) 116946

Y. Tian, et al.

hemicellulose and lignin, has a significant influence on the yield and quality of bio-oil [21–23]. This is believed to be caused by the huge structural differences among the three components. Cellulose is a linear polysaccharide composed of glucose, and hemicellulose is a branched heteropolymer of various monosaccharides, including xylose and pectinose, while lignin is a phenolic polymer with a high degree of cross linking [24,25]. The pyrolysis process of different lignocellulosic biomass and these three components as model compounds have been studied extensively [25–29]. Most of the studies show that increasing cellulose content leads to an increase in the bio-oil yield, while the lignin content contributes most to the char yield [27,30]. In addition, the interactions between the three components have been further studied by comparing the pyrolysis behavior of individual components with that of their synthetic mixtures [27]. It is also suggested that the pyrolysis behavior of the real lignocellulose biomass cannot be completely represented by that of commercial constituents, which is likely due to the physical association and interactions among them [27,31]. Unfortunately, there are relatively few comparative studies on the behavior of different lignocellulosic biomass in the HTL process compared with the pyrolysis process. Karagöz et al. conducted a comparative study of the HTL process of sawdust, rice husk, lignin and cellulose, and observed that cellulose had the highest conversion rate but the lowest bio-oil yield among the four samples investigated [21]. Furthermore, it was also found that sawdust and rice husk have similar conversion rate and product distribution. In a study on the deoxy-liquefaction of four types of crop straws, i.e., legume straw, corn stalk, cotton stalk and wheat straw, Wang et al. suggested that the produced bio-oils from the four samples contained almost the same compounds except for differences in their relative contents [3]. In a comparative study on the HTL process of eight selected green landscaping wastes, Cao et al. indicated that the product distribution of leaves was quite different from that of branches, while less difference was observed for HTL products within the leaves despite obvious component differences among them [22]. Caprariis et al. investigated the HTL process of three types of lignocellulosic biomass, natural hay, oak wood and walnut shells, and found that the bio-oil yields increased with the lignin content in biomass, which is inconsistent with the observation in most pyrolysis processes [23]. However, the obtained results were quite different from each other, probably due to different experimental conditions and different types of biomass. Furthermore, the limited information obtained from previous studies is not conducive to exploring the correlation between biomass composition (mainly cellulose, hemicellulose and lignin) and the bio-oil yield and quality during the HTL process. Therefore, more information about the HTL process of various types of crop straw is necessary to understand the relationship between the biomass composition and product distribution. In this study, the HTL process of four types of crop straw was carried out under the same reaction conditions. The four types of crop straw used in the experiments were corn straw (CS), peanut straw (PS), soybean straw (SS), and rice straw (RS), with different biochemical composition. A comprehensive comparison among the four crop straws was conducted for the yields and composition of the bio-oils. The objective of this study is to provide more information to explore the influence of lignocellulosic biomass composition, especially the relative contents of cellulose, hemicellulose and lignin, on the yield and quality of bio-oil.

Table 1 Proximate and ultimate analyses (wt%, dry basis) of the four crop straws. CS

PS

RS

SS

Proximate analysis Ash Volatiles Fixed carbon Cellulose Hemicellulose Lignin wcellulose/whemicellulose

7.00 68.50 24.50 30.81 25.52 16.76 1.21

13.05 65.50 21.45 36.56 20.27 18.36 1.80

15.10 64.00 20.91 46.33 31.09 10.17 1.49

4.43 73.10 22.47 42.39 22.05 18.93 1.92

Ultimate analysis C H N O S H/C O/C HHV(MJ/kg)

44.57 5.53 0.93 33.70 0.10 1.49 0.57 16.96

41.42 5.51 1.27 35.21 0.15 1.60 0.64 15.60

41.34 5.33 1.12 34.29 0.14 1.55 0.62 15.48

45.99 6.07 1.38 39.00 0.11 1.58 0.64 17.26

analyses of the four crop straws are listed in Table 1. The contents of cellulose, hemicellulose and lignin in the crop straws were analyzed following the protocol from the NREL Chemical Analysis and Testing Standard Procedures: NREL LAP, TP-510-42618 [22]. The analysis results are also attached to Table 1. Dichloromethane with a purity ≥ 99.8 wt% was obtained commercially and was used as an extraction solvent. Deionized water used in the experiments was freshly prepared in the laboratory. A custom-made high-pressure and corrosion-resistant batch reactor was used to perform the HTL process. The reactor had a total internal volume of 1 L and was heated by an electrical heating sleeve with a maximum power of 3.0 kW. Prior to use, the reactor was loaded with water and then treated at 400 °C for 1 h to ensure that the residual organic materials on the inner wall could be removed by supercritical water.

2.2. Experimental procedure For the HTL process of the crop straw, a typical run was performed as follows: 150 g of dry crop straw powder and 400 mL of freshly deionized water were placed into the 1 L reactor. The reactor was sealed and purged with helium for 15 min to ensure complete removal of air from the reactor. Then, 0.11 MPa helium, which was used as internal standard to quantify the yield of gaseous products, was added to the reactor. More detailed information on how to use helium to calculate the gas yield would be provided in the following section. After air inflation, the reactor was fixed in an electrical heating sleeve to initialize the reaction. The temperature was isothermally controlled by an Omega temperature controller with a preset temperature of 320 °C. The stirring speed was set at 600 rpm in this study. When the reactor was heated to 320 °C, the reaction began and lasted for 60 min. Then, the reactor was removed from the electrical heating sleeve and was quickly immersed in a cold-water bath to quench the reaction. After cooling, the reactor was removed and dried with a hair dryer. The outlet valve of the reactor was unscrewed, and the gaseous products were collected using a 0.5 L gas sampling bag with aluminum-plastic composite film, preparing for subsequent gas chromatographic (GC) analysis. After gas collection, the reactor was depressurized to atmospheric pressure and then opened. Most of the liquid phase together with some solid residue in the reactor was dumped into a 500 mL beaker. Subsequently, approximately 80 mL of dichloromethane was used to wash the inner wall and pipe line of the reactor. Then, the materials in the reactor were transferred to the 500 mL beaker. This step was repeated twice to ensure that all the product fractions in the reactor were collected. Finally, all the materials in the reactor were

2. Experimental section 2.1. Materials The raw materials of the four crop straws (CS, PS, SS and RS) were collected from a local farm in Jiashan, Hunan Province, East China. Before use, the collected crop straws were pulverized into particles of 100–200 μm size and were then dried in an oven (105 °C, 12 h) to remove the moisture in the samples. The proximate and ultimate 2

Fuel 265 (2020) 116946

Y. Tian, et al.

of approximately 73 min for each sample. Compounds were determined by comparing their mass spectra with the National Institute of Standards and Technology (NIST) mass spectral library. The elemental analysis (C, H, N, S, and O) of the feedstock and biooils was performed on a Thermo Scientific FLASH 2000 autoanalyzer (Thermo Fisher Scientific Inc., USA). The higher-heating values (HHV) of the samples were calculated using the Dulong formula as follows: HHV (MJ/kg) = 0.338C + 1.428(H - O/8) + 0.095Swhere C, H, O, and S represent the wt% of each atom in the samples. At least duplicate analyses were conducted for each sample, and the average values were recorded. Thermogravimetric analysis (TGA) of bio-oils was performed using an SDT Q600 simultaneous DSC-TGA instrument (TA Instruments, USA) in a nitrogen atmosphere. All the samples were heated from 25 to 800 °C at a heating rate of 10 °C/min. The flow rate of nitrogen was set to 20 mL/min.

collected in the 500 mL beaker. The collected mixture in the beaker was separated using a pre-weighed medium-speed qualitative filter paper with a Büchner funnel. After filtration, the separated solid together with the filter paper was dried in an oven at 105 °C for 12 h and then weighed. The weight of the solid was calculated by subtracting the weight of the filter paper. The organic phase was further separated from the filtered liquid using a separating funnel and was collected in a preweighed round-bottomed flask. Then, the organic phase was vaporized using a rotary evaporator at 35 °C until the vacuum reached 0.095 MPa, ensuring that almost no dichloromethane was left. After evaporation, the remaining viscous liquid in the flask was bio-oil. The flask containing the bio-oil was weighed again, and the weight of bio-oil was obtained by subtracting the weight of the flask. The yields of product fractions were calculated as follows:

Weight of bio-oil ⎞ Yield of bio-oil (wt%) = ⎜⎛ ⎟ × 100% Weight of feedstock loaded ⎠ ⎝

3. Results and discussion

Weight of solid ⎞ Yield of solid (wt%) = ⎜⎛ ⎟ × 100% Weight of feedstock loaded ⎠ ⎝

3.1. Effect of feedstock composition on the HTL product distribution

Weight of gaseous product ⎞ Yield of gaseous product (wt%) = ⎜⎛ ⎟ × 100% ⎝ Weight of feedstock loaded ⎠

Fig. 1 illustrates the yields of product fractions obtained from the HTL process of the four different crop straws. As shown in Fig. 1, SS produced the highest bio-oil yield of 15.8 ± 0.8 wt% among the four crop straws, followed closely by RS and PS, whose values were 15.1 ± 0.7 wt% and 14.6 ± 0.7 wt%, respectively. In contrast, the bio-oil produced from CS had the lowest yield of 7.9 ± 0.4 wt%, considerably lower than that of the other crop straws. Considering that all other process variables were fixed in HTL, the difference in bio-oil yield should be attributed to the different composition among these samples. As indicated in Table 1, cellulose, hemicellulose, and lignin accounted for a large proportion (73.09–87.59 wt%) of the feedstock composition, and their relative contents varied. As a typical linear polymer, cellulose is easily and completely decomposed in a relatively low and narrow temperature range [27,32]. Therefore, a relatively higher bio-oil yield is likely to be obtained from biomass with a higher cellulose content. Hemicellulose is generally located within cellulose and between cellulose and lignin [33]. Due to its branched chains and poor structural regularity, hemicellulose is less stable and partially decomposed at lower temperatures [27,34]. However, the existence of hydrogen bonds between cellulose and hemicellulose as well as their coating structure may retard the hydrolysis of cellulose and further decomposition. Therefore, the mass ratio of cellulose to hemicellulose, i.e., wcellulose/whemicellulose, was proposed here, and the values are presented in Table 1. The yield of bio-oil should be related to the cellulose content and the value of wcellulose/whemicellulose in crop straws. In the present study, it appeared that samples with a higher cellulose content and wcellulose/whemicellulose value tended to result in a higher yield of bio-

Note that the yield of aqueous product was obtained by subtraction. At least duplicate runs were conducted under identical conditions to ensure the repeatability of the results. The values reported herein are average values. The uncertainty values reported are the experimentally determined standard deviations. 2.3. Analytical chemistry Analysis of gaseous products was conducted using a Shanghai Techcomp Scientific Instrument Co., Ltd., model GC-7900 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). A 15-ft × 1/8-in. i.d. stainless steel column, packed with 60 × 80 mesh Carboxen 1,000 (Supleco), was used to separate each component in the gas mixture. Argon (15 mL/min) was used as the carrier gas for the analysis. The column temperature was isothermally held at 70 °C for 120 min. The temperatures of the injector and detector were set at 120 and 150 °C, respectively. The column pressure was 0.18 MPa, and the filament current of the detector was set at 50 mA. Two consecutive analyses of the gas mixture were conducted for each run to ensure the accuracy and reproducibility of the measuring results. Gas standards were purchased from Changzhou Jinghua Industrial Gas Co., Ltd. (Changhong Rd, Wujin, Changzhou, Jiangsu, China). The calibration curves for each component were generated by analyzing these gas standards and were then used to calculate the mole fraction of each component in the gas samples. Helium, which was used as internal standard gas, was certainly included. The molar quantity of helium could be estimated on the premise that the pressure, temperature, and the surplus volume of the loaded reactor were determined before the reaction. The molar quantity of other gases could be calculated by the mole fraction of helium with determined molar quantity. Then, the weight of each component was obtained, and the total weight of the gaseous product was the sum of all these components’ weights. Gas chromatography-mass spectra (GC–MS) analysis of the bio-oils was conducted on a PE Clarus 600 GC/MS equipped with an Agilent J& W DB-5HT capillary column (30 m × 0.25 mm × 0.10 µm) using helium as the carrier gas. All samples were prepared by dissolving the biooil in dichloromethane at 20 wt%. For each sample, a volume of 2 µL was injected with a split ratio of 3:1. The inlet temperature and flow rate of the carrier gas were set to 300 °C and 3 mL/min, respectively. A two-minute solvent delay was set to protect the filament. The column temperature was procedure-controlled from 40 °C (held for 4 min) to 300 °C (held for 4 min) at a rate of 4 °C/min, resulting in a total runtime

50.0

Bio-oil

Solid

Gaseous product

Aqueous product

40.0

Yields (wt%)

40.0

35.5

36.8

34.4 31.1

31.2

34.0

30.0 19.9

20.0 10.0

16.5

19.7

15.8

15.1

14.7

24.5 22.9

7.9

0.0 CS

PS

RS

SS

Fig. 1. Yields of product fractions produced from the HTL process of the four crop straws. 3

Fuel 265 (2020) 116946

Y. Tian, et al.

oil. Based on this hypothesis, the highest yield of bio-oil (15.8 ± 0.8 wt%) from SS can be attributed to its high content of cellulose (42.39 wt%, see Table 1) and especially the highest wcellulose/ whemicellulose value (1.92). The lowest yield of bio-oil (7.9 ± 0.4 wt%) from CS is likely due to its lowest cellulose content (30.81 wt%) and wcellulose/whemicellulose value (1.21). With respect to RS and PS, the highest cellulose content (46.33 wt% for RS) and high value of wcellulose/whemicellulose (1.80 for PS) resulted in their bio-oil yields (15.1 ± 0.7 and 14.6 ± 0.7 wt%, respectively) being roughly comparable to that from SS. Certainly, the assumption needs to be validated by more samples in future work. For all crop straws, the solid yields (24.5–35.5 wt%) were significantly higher than their bio-oil yields. SS produced the lowest solid yield (24.5 ± 1.2 wt%), markedly lower than that of the three other samples. Previous studies have indicated that during HTL, the macromolecules (cellulose, hemicellulose and lignin) in biomass undergo hydrolysis and subsequent decomposition and depolymerization reactions to form low-molecular-weight intermediates [35]. The solid residue is generally formed by the repolymerization of intermediates with high reactivity. It was also reported that hemicellulose and lignin contribute most to the solid yield during pyrolysis [27]. Therefore, the formation of solids is probably due to the incomplete decomposition of components in crop straws. In addition, the ash content of samples may be an important factor that increases the solid yield. In the present study, the lowest solid yield for SS can be ascribed to the highest wcellulose/whemicellulose value (1.92) and the lowest ash content (4.43 wt %, see Table 1) in the feedstock. For the four crop straws, the yields of gaseous products fluctuated in the range of 16.5–22.9 wt%, which mainly originated from the degradation of cellulose and hemicellulose [25]. The HTL of the crop straws also produced considerable yields of aqueous products within the range of 31.1–40.0 wt%. It might include some low molecular weight organic acids, ketones, and phenols [23]. Information obtained on the components and contents of gaseous products are shown in Fig. 2. For all the crop straws, CO2 was the predominant component, accounting for over 90% of gaseous products, followed by CO, with a content range of 4.2–6.2 %. These two components are probably formed due to the cleavage of carbonyl groups from the degradation of cellulose and hemicellulose [25]. The gas also consisted of H2 (1.9–3.1%), CH4 (0.6–1.1%), and a tiny amount of C2H6 and C3H8 (< 0.2%). CH4 formation is likely due to the cracking of methoxyl groups of lignin molecules [36]. Despite its low value, the CH4 concentration seemed to be correlated with the lignin content in the crop straws. As shown in Fig. 2, a higher lignin content in the feedstock tended to result in a higher CH4 concentration, which is in line with a previous study [25].

Table 2 Elemental compositions and estimated HHVs of the bio-oils obtained from the four crop straws.

C H N O S H/C O/C HHV(MJ/kg)

CS bio-oil

SS bio-oil

RS bio-oil

PS bio-oil

72.43 7.81 0.99 13.29 0.28 1.29 0.14 33.24

73.16 7.38 2.19 12.68 0.22 1.21 0.13 32.98

72.68 7.96 1.69 12.40 0.33 1.31 0.13 33.70

72.28 8.09 2.07 12.26 0.24 1.34 0.13 33.78

3.2. Analysis of the elemental composition of the bio-oils Table 2 lists the elemental compositions and estimated HHVs of the bio-oils obtained from different crop straws. As shown in Table 2, the bio-oils had significantly higher C and H contents (72.28–73.16 wt% and 7.38–8.09 wt%, respectively) and a lower O content (12.26–13.29 wt%) than those of the feedstocks (41.34–45.99 wt%, 5.33–6.07 wt%, and 33.70–39.00 wt%, see Table 1), which resulted in significantly increased HHVs (32.98–33.78 MJ/kg). Moreover, no obvious difference was observed in the content of C, H and O atoms and HHVs of the four bio-oils, even though the cellulose, hemicellulose and lignin contents were considerably different among their corresponding feedstocks. The H/C ratio, which is used for bio-oils as a rough measurement of the unsaturated degree, fluctuated in a range of 1.21–1.34, indicating a high degree of unsaturation for the bio-oils. With respect to the SS bio-oil, its H/C ratio (1.21) was obviously lower than that of the other samples, suggesting the inclusion of more unsaturated compounds, which was confirmed by subsequent GC–MS analysis. As indicated in Table 2, the N and S contents (0.99–2.19 wt% and 0.22–0.33 wt%, respectively) in the four bio-oils were higher than their corresponding feedstocks (0.93–1.38 wt% and 0.10–0.15 wt%, respectively, see Table 1), indicating that the removal of N and S was not as effective as deoxidation during HTL [13,35]. The information obtained from elemental analysis was limited, and more details on the molecular characterization of the bio-oils are provided in the subsequent sections of GC–MS analysis. 3.3. Molecular characterization of bio-oils The molecular composition and relative amounts of specific molecules in bio-oils were analyzed by GC–MS. The tentative identities and area% of major peaks in total ion chromatograms for the bio-oils are presented in Table S1. Each identified compound with a peak area ratio of more than 0.5% of the total ion chromatogram was included. All the

Fig. 2. Components and contents of gaseous products produced from the HTL process of the four crop straws. 4

Fuel 265 (2020) 116946

Y. Tian, et al.

100

Table 3 Molecular composition (peak area%) of the bio-oils. CS bio-oil

RS bio-oil

PS bio-oil

SS bio-oil

0.52 –a – 21.21 53.28 3.10 1.29 4.14 1.38

– 3.81 2.12 34.96 33.87 3.99 1.07 1.26 2.22

– 0.80 3.06 25.90 28.31 6.24 0.69 7.86 1.41

– – 5.56 49.29 28.90 8.39 – 0.97 2.24

Weight loss(Wt.%)

Aromatics Alkanes Alkenes Ketones Phenolic compounds Nitrogenous heterocycles Alcohols Carboxylic acid Aldehyde

RS bio-oil CS bio-oil

80

SS bio-oil 60

PS bio-oil

40 20 0

a: could not detected.

0

bio-oils obviously had complex compositions. For comparative purposes, the identified compounds in the bio-oils were further classified into aromatics, alkanes, alkenes, ketones, alcohols, aldehyde, carboxylic acids, phenolic compounds, and nitrogenous heterocycles. The results are presented in Table 3. As shown in Table 3, ketones and phenolic compounds were the most abundant compounds present in the four bio-oils, while their relative amounts (21.21–49.29% and 28.31–53.28%, respectively) varied. The SS bio-oil had the highest content of ketones, with values up to 49.29%, followed by RS bio-oil with values of 34.96%. In contrast, the contents of ketones in PS and CS bio-oils (25.90% and 21.21%, respectively) were obviously lower. Cyclic ketones, mainly cyclopentanone, 2-cyclopenten-1-one, and their alkyl derivatives, accounted for a large proportion of the detected ketones in the four bio-oils. Previous studies have reported that ketones are mostly derived from cellulose through hydrolysis, dehydration and cyclization [37]. Therefore, this suggested that bio-oils from crop straws with a high cellulose content appeared to contain a higher amount of ketones. Although the cellulose content of RS was even slightly higher than that of SS, its significantly higher hemicellulose content (31.09 wt%, see Table 1), that is, lower wcellulose/whemicellulose value, resulted in a lower content of ketones in its bio-oil compared with that from SS, which is likely due to the hydrogen bonding between cellulose and hemicellulose and their coating structure. Consequently, the lowest content of cellulose and wcellulose/whemicellulose value (30.81 wt% and 1.21, respectively) in CS probably led to the lowest content of ketones in its bio-oil. Phenolic compounds and their derivatives accounted for a considerable proportion in the four bio-oils, mainly phenol, 2-methylphenol, 2-methoxylphenol, 4-ethylphenol, 2-methoxy-4-methylphenol, and 4-ethyl-2-methoxylphenol. Among the four bio-oils, CS bio-oil had the highest content of phenolic compounds (53.28%), significantly higher than that in the other bio-oils (28.31–33.87%). Phenolic compounds are mainly derived from the degradation of lignin, including hydrolyzation and consequent rehydration [38]. In addition, the condensation/cyclization of intermediate products from the hydrolysis of cellulose and hemicellulose is also a primary source of phenolic compounds [1]. Compared with ketones and phenolic compounds, carboxylic acids accounted for only a small proportion in the four bio-oils. The highest carboxylic acid content of 7.86% was observed in the PS bio-oil, followed by CS bio-oil with a value of 4.14%. Long-chain fatty acids, including n-hexadecenoic acid and octadec-9-enoic acid, accounted for a large proportion of carboxylic acids in both bio-oils. These fatty acids are considered to be produced by the decomposition of biomass extractives [39]. In the SS and RS bio-oils, the content of carboxylic acids was approximately 1%, far below that in other samples. The content of nitrogenous heterocycles in the four bio-oils fluctuated within the range of 3.10–8.39 %. The SS bio-oil presented the highest content of nitrogenous heterocycles, whereas the lowest value was measured in the CS bio-oil. The ranking of the nitrogenous heterocycle content was consistent with that observed for the N content

200

400 Temperature(oC)

600

800

Fig. 3. TG curves of the four bio-oils.

(see Table 2) in bio-oils, which seems to indicate that nitrogenous heterocycles are likely to be the main source of N for the four bio-oils. There were obvious differences in the specific nitrogenous heterocycles between these bio-oils. For instance, pyrazole and its derivatives occupied a large proportion of the identified nitrogenous heterocycles in the CS bio-oil, while the specific nitrogenous heterocycles in the RS biooil were derivatives of imidazole and pyrazine. For PS and SS bio-oils, the identified nitrogenous heterocycles mainly included the derivatives of pyridine and indole. This should be due to the different components in their crop straws. Compared with other bio-oils, the SS bio-oil contained more alkenes (5.56%), as well as more cyclopentenone and its derivatives (see Table S1), resulting in its obviously lower H/C ratio. In addition, other types of compounds, including aromatics, alcohols, and aldehydes, accounted for a tiny proportion of the bio-oils or were not detected. 3.4. TG analysis of bio-oils The TG curves of the four bio-oils are presented in Fig. 3, which may provide approximate information on the boiling point distribution of the bio-oils. As shown in Fig. 3, the curves of the bio-oils except for SS bio-oil showed almost the same trend. An obvious weight loss for the bio-oils was observed within the range from 106 to 350 °C. The CS biooil had the highest weight loss rate of 70.5 wt% at this temperature range, slightly higher than the other samples, indicating that CS bio-oil contained more compounds with low boiling points. With respect to the three other bio-oils, there was no obvious weight loss when the temperature reached 480 °C. It was also observed that a certain amount of residue (9.8–12.5 wt%) remained when heated to 800 °C. With respect to the SS bio-oil, the change of its weight loss with temperature was initially consistent with that of the other bio-oils. However, an obvious difference occurred at approximately 280 °C, at which its curve became less steep than that of the other samples, while the descending trend continued until the end of the measurement. This indicated that more compounds with high boiling points existed in the SS bio-oil. The residue left after evaporation of the SS bio-oil was slightly lower than that of the other samples. 4. Conclusions Biochemical composition of the crop straw significantly affected the product distribution and properties of the bio-oil. Crop straw, which had a higher cellulose content and wcellulose/whemicellulose value, tended to produce a higher bio-oil yield. The elemental compositions of the bio-oil is insensitive to the biochemical composition of the feedstock. The bio-oil mainly consisted of ketones and phenolic compounds whose contents were markedly affected by cellulose content and wcellulose/ whemicellulose value of the feedstock. The boiling point fraction with 5

Fuel 265 (2020) 116946

Y. Tian, et al.

boiling point ≤ 300 °C for all bio-oils is no lower than 60%.

and of woody biomass solids to liquid fuels. J Am Chem Soc 2011;133:14090–7. [12] Jiang D, Zhuang D, Fu J, Huang Y, Wen K. Bioenergy potential from crop residues in China: Availability and distribution. Renew Sust Energy Rev 2012;16:1377–82. [13] Wang F, Tian Y, Zhang C, Xu Y, Duan P. Hydrotreatment of bio-oil distillates produced from pyrolysis and hydrothermal liquefaction of duckweed: A comparison study. Sci Total Environ 2018;636:953–62. [14] Toor SS, Reddy H, Deng SG, Hoffman J, Spangsmark D, Madsen LB, et al. Hydrothermal liquefaction of Spirulina and Nannochloropsis salina under subcritical and supercritical water conditions. Bioresour Technol 2013;131:413–9. [15] Savage PE. Organic chemical reactions in supercritical water. Chem Rev 1999;99:603–22. [16] Kruse A, Dinjus E. Hot compressed water as reaction medium and reactant: properties and synthesis reactions. J Supercrit Fluids 2007;39:362–80. [17] Younas R, Hao S, Zhang L, Zhang S. Hydrothermal liquefaction of rice straw with NiO nanocatalyst for bio-oil production. Renew Energy 2017;113:532–45. [18] Singh R, Chaudhary K, Biswas B, Balagurumurthy B, Bhaskar T. Hydrothermal liquefaction of rice straw: effect of reaction environment. J Supercrit Fluid 2015;104:70–5. [19] Patil PT, Armbruster U, Martin A. Hydrothermal liquefaction of wheat straw in hot compressed water and subcritical water–alcohol mixtures. J Supercrit Fluid 2014;93:121–9. [20] Liu H, Li M, Sun R. Hydrothermal liquefaction of cornstalk: 7-lump distribution and characterization of products. Bioresour Technol 2013;128:58–64. [21] Karagöz S, Bhaskar T, Muto A, Sakata Y. Comparative studies of oil compositions produced from sawdust, rice husk, lignin and cellulose by hydrothermal treatment. Fuel 2005;84:875–84. [22] Cao L, Luo G, Zhang S, Chen J. Bio-oil production from eight selected green landscaping wastes through hydrothermal liquefaction. RSC Adv 2016;6:15260–70. [23] Caprariis B, Filippis P, Petrullo A, Scarsella M. Hydrothermal liquefaction of biomass: Influence of temperature and biomass composition on the bio-oil production. Fuel 2017;208:618–25. [24] Kang S, Li X, Fan J, Chang J. Characterization of hydrochars produced by hydrothermal carbonization of lignin, cellulose, D-xylose, and wood meal. Ind Eng Chem Res 2012;51:9023–31. [25] Pasangulapati V, Ramachandriya KD, Kumar A, Wilkins MR, Jones CL, Huhnke RL. Effects of cellulose, hemicellulose and lignin on thermochemical conversion characteristics of the selected biomass. Bioresour Technol 2012;114:663–9. [26] Yang H, Yan R, Chen H, Zheng C, Lee D, Liang D. In-depth investigation of biomass pyrolysis based on three major components: hemicellulose, cellulose and lignin. Energy Fuel 2006;20:388–93. [27] Yu J, Paterson N, Blamey J, Millan M. Cellulose, xylan and lignin interactions during pyrolysis of lignocellulosic biomass. Fuel 2017;191:140–9. [28] Zhao C, Jiang E, Chen A. Volatile production from pyrolysis of cellulose, hemicellulose and lignin. J Energ Inst 2017;90:902–13. [29] Dai L, Wang Y, Liu Y, Ruan R, Yu Z, Jiang L. Comparative study on characteristics of the bio-oil from microwave-assisted pyrolysis of lignocellulose and triacylglycerol. Sci Total Environ 2019;659:95–100. [30] Yu H, Zhang Z, Li Z, Chen D. Characteristics of tar formation during cellulose, hemicellulose and lignin gasification. Fuel 2014;118:250–6. [31] Haykiri-Acma H, Yaman S, Kucukbayrak S. Comparison of the thermal reactivities of isolated lignin and holocellulose during pyrolysis. Fuel Process Technol 2010;91:759–64. [32] Stefanidis SD, Kalogiannis KG, Iliopoulou EF, Michailof CM, Pilavachi PA, Lappas AA. A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin. J Anal Appl Pyrol 2014;105:143–50. [33] Zhang J, Choi YS, Yoo CG, Kim TH, Brown RC, Shanks BH. Cellulose−hemicellulose and cellulose−lignin interactions during fast pyrolysis. ACS Sustain Chem Eng 2015;3:293–301. [34] Cao L, Zhang C, Chen H, Tsang DCW, Luo G, Zhang S, et al. Hydrothermal liquefaction of agricultural and forestry wastes: state-of-the-art review and future prospects. Bioresour Technol 2017;45:21184–93. [35] Zhu Z, Toor SS, Rosendahl L, Yu D, Chen G. Influence of alkali catalyst on product yield and properties via hydrothermal liquefaction of barley straw. Energy 2015;80:284–92. [36] Yang H, Yan R, Chen H, Lee DH, Zheng C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007;86:1781–8. [37] Zhou D, Zhang L, Zhang S, Fu H, Chen J. Hydrothermal liquefaction of macroalgae enteromorpha prolifera to bio-oil. Energ Fuel 2010;24:4054–61. [38] Barbier J, Charon N, Dupassieux N, Loppinet-Serani A, Mahe L, Ponthus J, et al. Hydrothermal conversion of lignin compounds. A detailed study of fragmentation and condensation reaction pathways. Biomass Bioenerg 2012;46:479–91. [39] Akalın MK, Tekin K, Karagöz S. Hydrothermal liquefaction of cornelian cherry stones for bio-oil production. Bioresour Technol 2012;110:682–7.

CRediT authorship contribution statement Ye Tian: Data curation, Writing - original draft, Visualization, Investigation. Feng Wang: Conceptualization, Methodology, Supervision. Jesuis Oraléou Djandja: Software, Validation, Writing review & editing. Sheng-Li Zhang: Data curation, Writing - original draft. Yu-Ping Xu: Writing - review & editing. Pei-Gao Duan: Conceptualization, Methodology, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We gratefully acknowledge the financial support of the National Natural Science Foundation of China (21776063; U1704127), the Scientific and Technological Innovation Team of the University of Henan Province (18IRTSTHN010), the Outstanding Youth Foundation for Scientific and Technological Innovation in Henan Province (184100510013), the Key Scientific Research Projects in Colleges and Universities of Henan Province (18A480003), and the Scientific and Technological Research Projects of Henan Province (172102210026). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116946. References [1] Zhu Z, Rosendahl L, Toor SS, Yu D, Chen G. Hydrothermal liquefaction of barley straw to bio-crude oil: effects of reaction temperature and aqueous phase recirculation. Appl Energy 2015;137:183–92. [2] Liu C, Zhao Q, Lin Y, Hu Y, Wang H, Zhang G. Characterization of aqueous products obtained from hydrothermal liquefaction of rice straw: focus on product comparison via microwave-assisted and conventional heating. Energy Fuel 2018;32:510–6. [3] Wang C, Pan J, Li J, Yang Z. Comparative studies of products produced from four different biomass samples via deoxy-liquefaction. Bioresour Technol 2008;99:2778–86. [4] Saxena RC, Adhikari DK, Goyal HB. Biomass-based energy fuel through biochemical routes: a review. Renew Sustain Energy Rev 2009;13:167–78. [5] Sindhu R, Binod P, Pandey A. Biological pretreatment of lignocellulosic biomass An overview. Bioresour Technol 2016;199:76–82. [6] Williams CL, Westover TL, Emerson RM, Tumuluru JS, Li C. Sources of biomass feedstock variability and the potential impact on biofuels production. Bioenergy Res 2016;9:1–14. [7] Kumar G, Shobana S, Chen W, Bach Q, Kim S, Atbani AE, et al. A review of thermochemical conversion of microalgal biomass for biofuels: chemistry and processes. Green Chem 2017;19:44–67. [8] Cao L, Zhang C, Luo G, Zhang S, Chen J. Effect of glycerol as co-solvent on yields of bio-oil from rice straw through hydrothermal liquefaction. Bioresour Technol 2016;220:471–8. [9] Zhang X, Lei H, Zhu L, Qian M, Zhu X, Wu J, et al. Enhancement of jet fuel range alkanes from co-feeding of lignocellulosic biomass with plastics via tandem catalytic conversions. Appl Energy 2016;173:418–30. [10] Binder JB, Raines RT. Simple chemical transformation of lignocellulosic biomass into furans for fuels and chemicals. J Am Chem Soc 2009;131:1979–85. [11] Matson TD, Barta K, Iretskii AV, Ford PC. One-Pot catalytic conversion of cellulose

6