Influence of alkali catalyst on product yield and properties via hydrothermal liquefaction of barley straw

Energy 80 (2015) 284e292

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Influence of alkali catalyst on product yield and properties via hydrothermal liquefaction of barley straw Zhe Zhu a, b, Saqib Sohail Toor b, Lasse Rosendahl b, **, Donghong Yu c, Guanyi Chen a, * a

School of Environmental Science and Technology, State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China Department of Energy Technology, Aalborg University, Aalborg, 9220, Denmark c Department of Chemistry and Bioscience, Aalborg University, Aalborg 9000, Denmark b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 April 2014 Received in revised form 22 September 2014 Accepted 25 November 2014 Available online 30 December 2014

Barley straw was successfully converted to bio-crude by hydrothermal liquefaction at temperature of 280 e400
C using an alkali catalyst (K2CO3) in our previous work, and the maximum bio-crude yield was obtained at 300
C. This paper extends previous work on studying liquefaction behavior of barley straw without and with K2CO3 at 300
C. The effect of alkali catalyst on product distribution was investigated, and a detailed analysis of characteristic properties of bio-crude and solid residue has been performed by an elemental analyzer, FTIR (Fourier Transform infrared spectroscopy), TGA (thermogravimetric analysis) and GC-MS. The addition of K2CO3 increased the bio-crude yield to 34.85 wt%, and inhibited solid residue formation. Moreover, the bio-crude produced in the presence of a catalyst had better properties, in terms of higher heating value and lower O/C. GC-MS analysis showed that the major compounds identified in bio-crude were carboxylic acids, phenolic compounds and ketones, irrespective of whether the catalyst was used. However, the distribution and relative content of these compounds were different. More phenolic compounds and less carboxylic acids were observed in the catalytic run. In addition, the carbon and energy recovery with the addition of K2CO3 were twice as high as that without catalyst, indicating an improvement in energy efficiency. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Barley straw Hydrothermal liquefaction K2CO3 Bio-crude Characterization

1. Introduction In a recent report from BP statistical review of world energy, global primary energy consumption grew by 2.3% in 2013. Conventional fossil fuels account for 86.7% of world primary energy consumption, with oil (32.9%), coal (30.1%) and natural gas (23.7%) as the major fuels [1]. However, it is estimated that the depletion time of oil, coal and gas is to be around 35, 107 and 37 years, respectively [2]. Furthermore, the CO2 concentration has increased by 40% since pre-industrial times, which are largely caused by fossil fuels combustion [3]. Thus, the renewable energy is of increasing concern worldwide in recent years due to they can avoid rapid depletion of fossil fuel reserves, and provide clean sources of energy that have a much lower environmental impacts. According to previous research [4e6], it is physically possible for Denmark to have a 100% renewable energy system in 2050, based on domestic

* Corresponding author. Tel.: þ86 22 87402075. ** Corresponding author. Tel.: þ45 99409263. E-mail addresses: [email protected] (L. Rosendahl), [email protected] (G. Chen). http://dx.doi.org/10.1016/j.energy.2014.11.071 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

resources. Especially, carbon-neutral biomass is an important alternative for generating heat and electricity, producing biofuels and chemicals, with reduced greenhouse gas emissions to meet the increasing demands for bioenergy [7,8]. Barley straw, an agricultural residue, is generated abundantly worldwide. In Denmark, nearly over 2.17 million tons of barley straw is produced in 2013. As a matter of fact, only about 18% is utilized for energy production and 32% is used as fodder [9]. There are still more surplus straw with easy availability. Thus, to find a pathway to take advantage of them for liquid fuels or chemicals (e.g. phenolic derivatives) is important to environment and economy. Different ways of conversion technologies have been performed through biochemical methods (such as ethanol fermentation, anaerobic digestion) [10,11] or thermochemical methods (such as liquefaction, pyrolysis) [12,13]. However, for biochemical treatment of barley straw, it needs to be pretreated first and inhibitory effects caused by the generation of chemical products (e.g. furfural, hydroxymethyl furfural and phenolic compounds) are required to be removed, resulting in low efficiency [14]. HTL (hydrothermal liquefaction) is one of the promising technologies for converting biomass or wastes into biofuels, which is

Z. Zhu et al. / Energy 80 (2015) 284e292

performed in water with medium-temperature at 280e370
C and high-pressure between 10 and 25 MPa [15]. An obvious advantage of HTL is that raw biomass is not required to be dry. In addition, the ability of using mixed feedstock, high energy efficiency, as well as better quality products make HTL effective for liquid fuel production [16]. Furthermore, recycling the water soluble fraction as liquefaction medium can reduce waste water treatment costs and heat consumption as well. Since water has unusual properties under subcritical condition, such as high ion product and low dielectric constant, it accelerates acid/base-catalyzed reactions in biomass liquefaction process [17]. For instance, some ionic reactions such as decomposition of carbohydrates and alcohols and aldol splitting were favorable in this region [18]. More importantly, water behaves as a weak polar solvent due to its decreased dielectric constant with increasing temperature (e.g. with the value of 20e24 at 300
C at pressures between 25 and 100 MPa), indicating a better miscibility with organic compounds (derived from biomass) under these conditions [19]. A number of investigations of HTL on different biomass have been carried out in the literature. It was reported that catalysts were benefit to improve liquefaction efficiency, and especially alkali catalysts (e.g., NaOH, KOH, K2CO3 and Na2CO3) were of interest in HTL of wood [20,21], bark [22], EPFB (empty palm fruit bunch) [23], switchgrass [24] and algae [25]. According to their results, alkali catalysts promote biomass conversion and bio-crude yield, as well as bio-crude quality in terms of less oxygen and higher hydrogen content. Additionally, their catalytic activities were in the order of K2CO3 > KOH > NaOH, as far as the liquid yield and biomass conversion were concerned [21,23]. What's more, considering the equipment corrosion caused by hydroxides as catalyst is more serious [26], potassium carbonate was selected as catalyst in this paper. Previously, production of bio-crude from wheat and rice straw in the absence of catalyst has been investigated in subcritical water [27,28], the yield of which were both less than 10%. While, a higher bio-crude yield of 21.1 wt% was obtained when rice straw was liquefied in ethanol at 350
C [29]. To our knowledge, although Meier et al. [12] employed HTL of barley straw at 375
C with palladium catalyst, with bio-crude yield as high as 40.3 wt%, there have been no studies of the role of alkali catalyst in HTL of barley straw on product distribution and properties. The reaction temperature of 300
C was chosen for this study, at which the highest bio-crude yield and lowest solid residue yield was observed when HTL of barley straw was performed with K2CO3, according to our previous study [30]. Besides, for most lignocellulosic biomass, carbonization might occur when the temperature was higher than 300
C [31]. In summary, this study extends previous finding by (1) identifying the effect of potassium carbonate as catalyst on liquefaction behavior of barley straw in subcritical water; (2) demonstrating the product distribution and detailed characterization of bio-crude and solid residue obtained with and without catalyst, by elemental analyzer, heating value, FTIR (Fourier Transform infrared spectroscopy), GC (gas chromatograph) equipped with a MS (mass selective detector) and TGA (thermogravimetric analysis); (3) calculating and comparing the carbon recovery and energy balance between non-catalytic run and catalytic run.

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2. Materials and methods 2.1. Materials The barley straw sample used in this study was obtained from Denmark. It was ground to pass through a 0.5 mm screen before use. The elemental analysis and typical chemical compositions of barley straw in Denmark [32] are shown in Table 1. The moisture content was determined according to ASTM 3173-87. The ash content was measured by the weight difference when the barley straw was burned in muffle at 575
C for 4 h. The HHV (higher heating value) of the straw was 17.38 MJ/kg. K2CO3 was purchased from Sigma-Aldrich and used as received. 2.2. Experimental procedure and separation HTL experiments on barley straw were carried out in a 1 L autoclave. The barley straw powder (60 g), distilled water (400 mL) and catalyst (only in the catalytic run) were added to the reactor. The mass ratio of catalyst to the barley straw was fixed at 10% (w/w) in catalytic run. The influence of concentration of catalyst was just beyond the scope of this study. Then the reactor was sealed and purged with N2 for 3 min to remove residual air. Finally, the reactor was heated to 300
C with a stirring rate of 300 rpm by a magnetic drive agitator, and maintained at that temperature for 15 min. The final pressure in non-catalytic run and catalytic run was around 90 bar and 112 bar, respectively. After that, the autoclave was cooled down to room temperature with an electric fan. Fig. S1 shows a typical temperature profile in the autoclave when barley straw was liquefied at 300
C. Once the reactor cooled down, the gas was released. Subsequently the reactor was opened and solideliquid products were collected. Fig. 1 shows a systematic experimental procedure for separation and analysis of reaction products. The solideliquid products were filtered under reduced pressure to obtain aqueous phase. The stirrer and reactor were rinsed with acetone three times to recover the oil phase and solid. After that, the mixture of oil phase and solid was centrifuged at a relative centrifugal force of 2153 (g/g) to separate the acetone soluble oil phase from insoluble fraction. Afterwards, the acetone soluble fraction was transferred into an evaporation flask and evaporated at 60
C under a 556 mbar to remove acetone. The remaining heavy oil phase was weighed and defined as bio-crude. The solid was dried in an oven at 105
C for 24 h and then cooled and weighed, referred to as solid residue. The product yield of bio-crude and solid residue was calculated based on the dry mass of barley straw by the following equations. Since the mass of gas and organics in aqueous phase after liquefaction was not quantified, and their total yield was obtained by difference, defined as others yield.

Bio­crude yield ¼ ðmass of bio­crude=mass of dried barley strawÞ  100% (1)

Table 1 Elemental composition and typical chemical composition of barley straw. Elemental composition (wt%)

Barley straw a

By difference.

Moisture (wt%)

C

H

N

S

Oa

44.66

6.34

0.46

0.57

47.97

6.21

Ash (wt%)

4.26

Chemical composition (wt%) Cellulose

Hemi-cellulose

Lignin

46

23

15

286

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Fig. 1. Schematic description of hydrothermal liquefaction of barley straw.

Solid residue yield ¼ðmass of solid residue=mass of dried barley strawÞ  100% Others yield ¼ 100%  Bio­crude yield  Solid residue yield

(2)

(3)

The carbon and energy recovery in bio-crude were determined by Eqs. (4)e(5):

Carbon recovery ¼

Carbon content in bio­crude Carbon content in dried barley straw  bio­crude yield

Energy recovery ¼

(4)

HHV of bio­crude HHV of dried barley straw  bio­crude yield

(5)

The liquefaction experiments for each condition were repeated three times to ensure the reproducibility of the results. The average value of product yield was calculated, and the error bars showing the corresponding standard deviations are shown in Fig. 2. The experimental errors were mainly caused by losing some products during separation process, especially for the recover process for oil product, during which some lower boiling point compounds may evaporate. As shown in Fig. 2, the standard deviation of the biocrude and solid residue yield were both less than 1%.

Fig. 2. Effect of K2CO3 on product yield in HTL of barley straw at 300
C. The error bars were determined by standard deviation.

Z. Zhu et al. / Energy 80 (2015) 284e292

2.3. Product characterization Raw biomass, bio-crude and solid residue were analyzed for elemental compositions (C, H, N, S) using a 2400 Series II CHNS/O element analyzer (PerkinElemer, USA). Atomic ratios were derived from the elemental results. Higher heating values (HHVs) were measured by IKA C2000 Basic bomb Calorimeter. Thermogravimetric analysis (TGA) of raw biomass, bio-crude and solid residue was performed under a nitrogen flow (20 mL/ min) on PerkinElmer STA 6000. The temperature was increased from 50 to 950
C at a heating rate of 10
C/min. Fourier transform infrared (FTIR) was conducted on a 370 Avarta FTIR spectrometer (Thermo Nicolet) with an ATR (attenuated total reflectance) accessory in the range of 400e4000 cm1. The major organic components in raw biomass and products were characterized based on the absorbance peaks of functional groups. The main volatile compounds of bio-crude was analyzed by a Varian 3800 gas chromatograph (GC) equipped with a mass selective detector (MS) equipped with a CP 9036 capillary column (5% phenyl 95 dimethylpolysiloxane, 20 m  0.15 mm  0.15 mm). All the samples were dried at 105
C for 24 h before the test. To confer thermal and chemical stability and enhance volatility of the biocrude, the derivatization was carried out with the addition of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA, Fluka 15238) to samples in the test tube, which was heated to 75
C for 1 h after derivatization. Following this, excessive derivatization reagent was removed by N2 stream and then adding 2.0 mL of hexane into the produced derivatives. After that, the hexane phase was filtered through a 0.45 mm filter to remove any particles. The samples were then ready and analyzed by GC-MS. 1 mL of solution was injected in a split mode with a split ratio of 30. The oven temperature program was initially 75
C (holding for 2 min) / 250
C (holding for 10 min) at a heating rate of 20
C/min. The chemical compositions were identified by comparing mass spectra with the NIST (National Institute of Standards and Technology) database. 3. Results and discussion 3.1. Effect of catalyst on liquefaction yields The effect of alkali catalyst on liquefaction product yield at 300
C is illustrated in Fig. 2. The bio-crude yield was improved significantly from 17.88 wt% without catalyst to 34.85 wt% with addition of K2CO3. In addition, an increase yield of gas and aqueous phase was found when compared to the run without catalyst. The results suggested that K2CO3 significantly affected the conversion of barley straw in subcritical water. It reacts with water to form its hydroxide and bicarbonate, which has been identified as an intermediate and assumed to be secondary promoter in liquefaction process [23].

K2 CO3 þ H2 O/KHCO3 þ KOH

(6)

2KHCO3 /H2 O þ K2 CO3 þ CO2

(7)

Furthermore, the change of reaction pathway may occur as a result of increased alkalinity in the presence of K2CO3. Based on previous studies [16,23,27,33], a series of complex reactions occur during HTL of lignocellulosic biomass, including hydrolysis, decomposition, condensation and other reactions follows. Generally, the macromolecules (cellulose, hemicellulose and lignin) in biomass experience hydrolysis first, forming oligomers and monomers such as glucose, fructose, guaiacol monomers etc. Subsequently, these monomers are converted to lower molecular weight intermediates (e.g. acids, aldehydes, alcohols and phenols)

287

through decomposition and depolymerization reactions including dehydration, decarboxylation and fragmentation. Bio-crude and gases are produced by further decomposition and isomerization of these intermediates, whereas compounds with high reactive activity can easily polymerize to form solid residue. Investigation of effect of alkaline addition during degradation of wood [34] showed that the intermolecular interaction of glucoside bonds in biomass was weakened with alkaline solutions. As a consequence, it was assumed that the depolymerization of barley straw was promoted due to its low thermal stability with the addition of K2CO3, and thus producing more hydrolyzed intermediate products in aqueous phase. In this study, an initial pH values of reaction mixture were 7 and 12, respectively for non-catalytic and alkali catalytic run, while the final values of aqueous phase were less than 7 in both runs, which were about 4 and 6, respectively. A more significant decrease of pH after HTL with K2CO3 was observed, indicating that the degradation of barley straw was more severe and more acids were formed in the aqueous phase through hydrolysis and depolymerization reactions. The more increase of these intermediates would result in more aqueous phase, bio-crude and gaseous products through further reactions, which might be the reason for enhanced bio-crude yield with K2CO3 in this study. Similarly, more water-soluble products were significantly produced when switchgrass was liquefied in subcritical water with addition of a small amount of K2CO3 (0.15 wt%) [24]. Besides, the reaction rate of carbonecarbon scission was improved in the case of alkali [35], which might be favorable to gases formation. As shown in Fig. 2, the solid residue yield was as high as 28.74 wt % in non-catalytic liquefaction, whereas it decreased remarkably to 8.10 wt% when liquefied with K2CO3. The formation of solid residue in HTL of cellulose under neutral conditions was previously studied [36], which showed that the intermediates (e.g. 5-HMF) would be dehydrated and polymerized to solid products. In this study, with the increased pH value in the presence of K2CO3, dehydration of monomers, through which unsaturated compounds were formed may be suppressed [15]. The unsaturates can be easily polymerized to solid residue. Therefore, less solid residue was observed in catalytic run. Also, K2CO3 can decrease the condensation and repolymerization reactions of intermediate products, inhibiting the char formation [37]. Furthermore, some organic acids produced which would accelerate polymerization reaction [38], were neutralized by addition of K2CO3. It was confirmed by higher pH value of aqueous phase and less carboxylic acid in the bio-crude in the catalytic run, shown by the GC-MS results (Table 3). These findings supported the reduced solid residue yield observed under alkaline condition in this study. In conclusion, the addition of K2CO3 can affect the reaction pathway and was favorable for the bio-crude formation. Table 2 Elemental composition of bio-crude from HTL of barley straw at 300
C: effect of catalyst. Properties

C H Oa N S H/C O/C HHV(MJ/kg) a b c

Bio-crude from HTL of barley straw None

K2CO3

62.63 6.42 29.75 0.69 0.51 1.23 0.36 24.87

67.89 7.62 23.18 0.75 0.56 1.35 0.26 27.29

Bio-oil from pyrolysis of barley strawb

Petroleumc

50.78 3.20 44.22 1.37 0.00 0.76 0.65 17.7

84e87 11e14 0.1e0.5 0.1e2 0.5e6 e e e

By difference. The data were taken from Mullen et al. The data were taken from Speight et al.

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3.2. Analysis of bio-crude 3.2.1. Elemental analysis The elemental compositions and heating values of bio-crude are presented in Table 2. It can be seen that the bio-crude contained higher carbon (62.63e67.89 wt%) and less oxygen content (23.18e29.75 wt%), compared with the barley straw as shown in Table 1. Besides, the hydrogen content of bio-crude obtained in catalytic run was 1.2% higher than that in the non-catalytic run. Therefore, it was not surprising that the bio-crude obtained with K2CO3 had higher HHV of 27.89 MJ/kg, compared with 24.78 MJ/kg without catalyst. Also, the H/C and O/C ratios for each bio-crude were calculated. It showed that the bio-crude obtained with the addition of K2CO3 have a higher H/C ratio, indicating a lower aromaticity. At the same time, a decreased O/C ratio was observed which could be attributed to the enhanced deoxygenation reactions during HTL process in the presence of alkali catalyst. Generally, there are three ways in which the oxygen was removed: decarbonylation, decarboxylation and dehydration

Table 3 GC-MS analysis for the bio-crude obtained at 300
C with and without catalyst. Peak no. RT (min) Name of compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

3.842 4.565 4.61 4.641 4.785 5.256 5.395 5.490 5.552 5.593 5.862 6.113 6.262 6.37 6.525 6.914 6.984 7.518 7.538 7.595 7.801 7.909 8.034 8.035 8.191 8.361 8.581 9.273 9.853 10.118

31

10.400

32 33 34 35 36 37

10.477 10.557 10.854 11.105 11.653 11.758

38 12.983 39 13.001 40 13.046 41 13.203 Total area (%)

Glycol 2-Cyclopenten-1-one, 2,3-dimethyl Propanoic acid, 2-hydroxyPhenol Acetic acid, hydroxyButanoic acid, 2-hydroxy o-Cresol p-Cresol Tetrahydrofurfuryl alcohol o-Cresol Naphthalene, 1,2,3,4-tetrahydroNaphthalene Phenol, 2-methoxyBenzaldehyde, 4-hydroxyGlycerol Butanedioic acid Catechol 3,5-Dihydroxytolune 2,3-Dimethoxyphenylacetic acid Benzaldehyde, 4-hydroxy-3-methoxy20 -Hydroxyacetophenone 2-Hydroxyphenylethanol Benzoic acid, 3-hydroxy4-Hydroxy-Benzeneethanol Bicyclo[3.3.0]octan-2-one, 7-benzylideneBenzoic acid, 3-hydroxy1,2,3-Trihydroxybenzene 4-[(1E)-3-hydroxyprop-1-en-1-yl]phenol Cinnamic acid 3-Methoxy-4-hydroxy benzaldehyde o-methyl oxime 2H,8H-Benzo[1,2-b:5,4-b']dipyran-2-one, 5-methoxy-8,8-dimethyl-10-(3-methyl-2butenyl)Tetradecanoic acid 1,2-Dihydroxyanthraquinone 2-Heptadecanone Di(pentamethylphenyl)ketone Hexadecanoic acid 4H-1-Benzopyran-4-one, 5,7dihydroxy-2-(3-hydroxy-4,5dimethoxyphenyl)-6,8-dimethoxy11-cis-Octadecenoic acid 9-Oleic acid 11-cis-Octadecenoic acid Octadecanoic acid

Area (%) None

K2CO3

e e 1.37 1.16 1.94 e 0.66 0.67 0.19 0.36 0.91 0.63 4.10 1.54 0.62 0.44 3.04 e 1.12 3.38 0.82 e e 1.78 2.55 e e e 1.45 2.80

0.28 0.35 2.18 0.44 3.03 0.71 0.51 0.64 1.55 e e e 3.91 1.07 1.14 0.56 3.87 2.05 e 5.83 0.46 0.48 5.14 e 4.31 3.01 2.58 1.48 e e

4.33

3.39

reactions. More importantly, the first two methods, involving the formation of CO and CO2, were the best way to decrease the oxygen content, since the hydrogen was reserved in bio-crude, leading to a higher H/C ratio and HHV. The removal of terminal aldehyde from biomass at lower temperature can produce CO, as described by Lu et al. [39]. Meantime, the unstable hydroxyl radicals produced from the decomposition of straw can react with CO, producing CO2. Due to the accelerated decomposition of barley straw in the presence of K2CO3, more radicals may be generated, and subsequently producing more CO2. Therefore, the decarboxylation was enhanced with alkali catalyst, which was confirmed by the very weak carbonyl stretch, as illustrated in Fig. 5. As shown in Table 2, the properties of pyrolysis oil from barley straw [13] and petroleum [40] are also included for comparison. Interestingly, the properties of bio-crude in the present result were better than those of pyrolysis oil from barley straw. Lower oxygen content was observed, and as a result, a significantly increased HHV were obtained, compared with the data from fast pyrolysis oil. However, it needs to be pointed out that the oxygen content was still too high when compared with petroleum. As such, upgrading process is of great importance to the biofuel industry. 3.2.2. FTIR analysis The bio-crude was also characterized by FTIR for identification of their functional groups. As shown in Fig. 3, there were no significant differences in chemical components between two biocrude. The interpretation of peak assignments were based upon the literature [41]. The broad band of eOH stretching vibration between 3200 and 3700 cm1 indicated the presence of alcohols and phenolic groups in the bio-crude. The CeH stretching vibrations at 2925 and 2850 cm1 as well as the CeH deformation at 1452 cm1 were attributed to methyl and methylene groups in alkenes and alkyl aromatic compounds. The bands at 1695 and 1677 cm1 related to the C]O stretching suggested the presence of carboxylic acids, ketones and aldehydes. The aromatic skeletal vibrations at 1597 and 1514 cm1, together with some CeH bending vibrations from aromatics between 900 and 650 cm1 indicated the existence of aromatics and their derivatives. The CeO stretching vibrations at 1263, 1201, 1113 and 1032 cm1 and eOH bending at 1356 cm1 demonstrated that the carboxylic acids, phenols and alcohols may be present in bio-crude. Therefore, the bio-crude may mainly consist of phenolic derivatives, ketones, aldehydes,

4.06 2.39 e 2.19 2.18 2.04 1.67 e 11.87 4.74 0.84 e

e 1.58 2.88 e 2.65 e 1.41 0.47 63.42 62.38

Fig. 3. FTIR of bio-crude obtained in non-catalytic run and catalytic run.

Z. Zhu et al. / Energy 80 (2015) 284e292

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carboxylic acids and alcohols. However, this technology was not an effective method for the detection of bio-crude. More information about these compounds can be obtained by GC-MS analysis.

Fig. 4. Mass loss (TG) and derivative mass loss (DTG) curves of bio-crude.

Fig. 5. FTIR of raw biomass and solid residues obtained in non-catalytic run and catalytic run.

3.2.3. GC-MS analysis The total ion chromatograms of GC-MS spectrum of the biocrude obtained with and without the presence of catalyst are given in Supplementary Material. The retention time, possible chemical name and percentage of peak area are presented in Table 3. The area (%) for each compound identified is defined as the percentage of each compound's chromatographic area out of the total peak area. Obviously, both bio-crude had complex compositions, consisting of alcohols, ketones, aldehydes, phenols and their derivatives, carboxylic acids and aromatics, being consistent with FTIR analysis. Comparative analysis of bio-crude in Table 3 showed that the use of alkali catalyst affected their content and composition, of which the carboxylic acids, phenols and their derivatives and ketones were dominant, regardless of whether the alkali catalyst was used or not. In the non-catalytic run, carboxylic acids were the most abundant compounds with content up to 29.19%, followed by ketones (11.55%) and phenolic compounds (10.83%). Four major compounds identified were hexadecanoic acid (11.87%), 2H, 8H-Benzo[1,2b:5,4-b0 ]dipyran-2-one, 5-methoxy-8,8-dimethyl-10-(3-methyl-2butenyl) (4.33%), phenol, 2-methoxy (4.10%) and tetradecanoic acid (4.06%). In the case of catalytic run, carboxylic acids were still the major compounds that accounted for 23.81%, followed by phenolic compounds (15.48%) and ketones (12.74%). Among these compounds, benzoic acid, 3-hydroxy (8.15%), benzaldehyde, 4hydroxy-3-methoxy (5.83%), hexadecanoic acid (4.74%) and bicyclo[3.3.0]octan-2-one, 7-benzylidene (4.31%) were the most abundant. In the present study, the carboxylic acids were classified into short chain carboxylic acids (C2eC6), long chain carboxylic acids (C14eC18) and aromatic acid derivatives. In the non-catalytic run, they accounted for 3.75%, 22.87% and 2.57% of the total peak area, respectively; while in the case of catalytic run, it was observed that total acids decreased, probably related to their reactions with alkali catalyst. In addition, it is possible that some acids may be decomposed to CO2 and water in the presence of catalyst [35], which resulted in a decrease in the content of acids. As a consequence, some undesirable reactions, such as repolymerization was inhibited [38], leading to less solid residue formation in the catalytic run, which was observed in Fig. 2. Interestingly, the reduction of polar molecules such as carboxylic acids in this study favored the biocrude production with lower oxygen content, which was evidenced in Table 2. Another important observation in the case with K2CO3 was that the contents of short chain acids and aromatic acids increased to 6.48% and 8.15%, respectively, whereas the content of long chain acids decreased to 9.18%. Additionally, the compounds such as 2-hydroxy-butanoic acid and 3-hydroxy-benzoic acid were observed in the case with K2CO3, while they were not present in the non-catalytic run. These findings were similar to those observed in HTL of cellulose [36], and it was concluded that the reaction pathways depended on the pH conditions. Under alkaline conditions, short chain carboxylic acids were primarily formed through retro-aldo reaction and dehydration, whereas under neutral conditions, some intermediates (e.g. 5-HMF) could be dehydrated to phenolic compounds or polymerized to solid products. There were a variety of ketones detected in bio-crude in both runs, mainly existed in the form of cyclic ketone, which were mostly derived from cellulose through hydrolysis, dehydration and cyclization [42], such as 2H,8H-Benzo[1,2-b:5,4-b0 ]dipyran-2-one, 5-methoxy-8,8-dimethyl-10-(3-methyl-2-butenyl)-, 2-heptadecanone and bicyclo[3.3.0]octan-2-one, 7-benzylidene.

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Meanwhile, more than 10% of phenolic compounds and their derivatives were present in both bio-crude, the contents of which increased with the addition of K2CO3. They primarily originated from the cleavage of aryl ether bonds in lignin [43]. Besides, the intermediate products produced from hydrolysis of cellulose and hemicellulose may undergo condensation/cyclization, forming phenolic compounds as well [35,44]. In this study, the effective promotion of lignin degradation in the presence of catalyst, evidenced by a much lower intensity of lignin peak, may contribute to the enhancement of phenolics in bio-crude. Remarkably, when looking at the structure of phenolic compounds in this study, most of them were mainly in the form of eOH instead of eOCH3, such as phenol, catechol, o-cresol, and p-cresol. This might be explained by the fact that methoxy group in lignin was easily to be broken down, especially in the presence of K2CO3. Nevertheless, the maximum removal of oxygen to alkylate aromatic compounds will be necessary in the following work.

3.2.4. TG analysis The TG and DTG (derivate TG) curves of the bio-crude produced without and with the catalyst are presented in Fig. 4. TGA has been used to provide the evaporation and cracking behavior of the biocrude [45]. It can be seen that the TG curve of bio-crude obtained from catalytic run was similar to that from non-catalytic run. Heating bio-crude in N2 atmosphere to 950
C led to a weight loss of 76.13% and 69.09%, respectively. Comparing their DTG profiles, two weight loss peaks at around 260
C and 365
C were shown in the bio-crude with the addition of K2CO3, indicating the volatilization of lighter and heavier components, respectively. Only one weight loss peak at 340
C was observed in the non-catalytic run, which may be due to a major loss of compounds with high molecular weight or high polarity. It is therefore inferred that low molecular weight or less polar compounds were present in the biocrude when potassium carbonate was employed in HTL of barley straw. This was also in agreement with GC-MS data (Table 3) which showed that the relative content of high molecular weight compounds was reduced in catalytic run.

3.3.2. FTIR analysis The FTIR spectra of solid residue are shown in Fig. 5, and they are compared with that of untreated barley straw. As shown from spectrum of barley straw, the absorption peaks such as eOH stretching vibrations (centered at 3330 cm1), CeH stretching (2848 cm1), CeH bending (1368 cm1), CeOH in-plane bending (1315 cm1), CeOeC asymmetry stretching (1156 cm1) and CeO stretching (1033 cm1) were characteristics of cellulose and hemicellulose in straw. Absorption at 1733 cm1 related to C]O stretching was expected from hemicellulose [46] and lignin [47]. The aromatic skeletal vibrations at 1604, 1510 and 1423 cm1 confirmed the existence of lignin in barley straw. Fig. 5 also showed that solid residue had different FTIR spectra compared with barley straw, indicating that some chemical reactions occurred during HTL process. For instance, the characteristic peaks of cellulose such as b-glycosidic band vibration (CeOeC) and CeOH bending and the feature of hemicellulose (C]O stretching) disappeared, which implied that these components had already been decomposed completely at 300
C through hydrolysis, dehydration and decarboxylation. These results were in line with previous studies [48,49], which revealed that 100% of the hemicellulose was solubilized at 230
C and the cellulose was decomposed completely below 300
C, regardless of whether the sodium carbonate was present or not. However, the lignin characteristic

3.3. Analysis of solid residue 3.3.1. Elemental analysis Elemental compositions, HHVs and element ratios of solid residue obtained from non-catalytic and catalytic runs are given in Table 4. In comparison with barley straw (O/C ¼ 0.81, H/C ¼ 1.7), solid residue had lower ratios of O/C (0.27e0.56) and H/C (0.88e0.95), suggesting that dehydration and decarboxylation reactions took place during the liquefaction process, supported by the FTIR result of solid residue (Fig. 5). Moreover, the carbon content decreased while the oxygen content increased when the catalyst was added, thus producing solid residue with lower heating value. At the same time, a higher H/C ratio was obtained in catalytic run, indicating lower aromatic molecules were present in solid residue.

Table 4 Effect of K2CO3 on the elemental analysis and calorific value of solid residue. Sample

None K2CO3 a

Elemental analysis (wt%)

Elemental ratio

C

H

Oa

N

S

H/C

O/C

68.71 53.92

4.97 4.25

24.89 40.15

0.81 0.98

0.63 0.7

0.88 0.95

0.27 0.56

By difference.

HHV(MJ/kg)

25.94 17.19

Fig. 6. Mass loss (TG) and derivative mass loss (DTG) curves of barley straw and solid residue.

Z. Zhu et al. / Energy 80 (2015) 284e292

peaks still existed in solid residue, although they shifted to lower wavenumbers (between 1510 and 1590 cm1) compared with raw biomass. A new band at 796 cm1 can be assigned to aromatic CeH bending vibrations, indicating aromatics and their derivatives were formed in HTL process. What's more, there were differences in carbonyl and aromatic absorption peaks between the solid residue obtained with and without catalyst, as shown in Fig. 5. Weak bands were recorded in the case of K2CO3, suggesting that the decarboxylation reaction and decomposition of lignin could be enhanced with the addition of catalyst. 3.3.3. TG analysis In order to study the thermal stability of barley straw and solid residue, TG analysis was performed and the TG/DTG curves are shown in Fig. 6. It was apparent that the degradation characteristic of raw biomass was different from those of the solid residue. According to the DTG curve of the barley straw, the decomposition process can be divided into three different regions. In the first stage, a bump in temperature range of 50e128
C appeared, which related to the water removal from raw biomass. Then, a significant weight loss was observed in the second stage (128e400
C), with the maximum weight loss occurring at 310
C, mainly because of the volatilization of both cellulose and hemicellulose. Generally, thermal degradation of hemicellulose and cellulose occurred mainly in the temperature range of 150e350
C and 275e350
C, respectively [50], and the peak of hemicellulose overlapped that of cellulose. The third stage went from 400
C to the final temperature (950
C), where the decomposition of lignin occurred [50], with a maximum weight loss at 475
C. In this third stage, the weight loss was not remarkable as in the second stage. A flat region was observed when the temperature was higher than 610
C, where the remaining heavy components can be decomposed slowly. As for both solid residue, their DTG curves (Fig. 6b) showed only one devolatilization peak at 569
C (no catalyst) and 439
C (with catalyst), respectively. It indicated that cellulose and hemicellulose were completely depolymerized and even partial lignin. Meantime, the final weight loss of barley straw (88.74%) was higher than that of solid residue, which was 70.52% without catalyst and 54.84% with catalyst. This could be explained by the fact that most of the biomass components were decomposed. Furthermore, a larger weight loss of solid residue in non-catalytic run may be attributed to its higher carbon content and weak aliphatic chains. Therefore, it can be concluded that addition of K2CO3 led to the solid contained more stable carbonaceous materials with high thermal stability. 3.4. Carbon and energy balance Since bio-crude could be an alternative to fossil fuel or valuable chemicals, it is important to understand the bio-crude production Table 5 Carbon recovery and energy recovery of bio-crude comparison using different biomass. Biomass

Carbon recovery (%)

Energy recovery (%)

Note

Barley straw Barley straw Cunninghamia lanceolata [51] Cunninghamia lanceolata [37] Wheat straw [28] Wheat straw [52] Spirulina [53] Nannochloropsis salina [53]

25.07 52.98 30.05

25.59 54.72 32.89

No catalyst in this study With K2CO3 in this study 300
C without catalyst

36.41

40.24

300
C with K2CO3

16.27 10.04 50.61 60.34

26.81 11.50 51.76 50.17

350 400 310 310




C C
C
C


without without without without

catalyst catalyst catalyst catalyst

291

efficiency in this liquefaction process. Therefore, carbon and energy recovery as indicators are calculated and presented in Table 5. Meanwhile, they are compared with some results from literature [28,37,51e53] on HTL of other biomass under similar conditions. In the present results, adding K2CO3 to the liquefaction process resulted in higher carbon and energy recovery. As shown in Table 5, only 25.07% of the carbon initially present in straw was retained in the bio-crude in the non-catalytic run. Interestingly, it increased to 52.98% with K2CO3, as a result of higher bio-crude yield and enhanced deoxygenation reactions with the addition of catalyst. Similarly, higher energy recovery (54.72%) was obtained in the case of K2CO3, which was almost twice as much as that in non-catalytic run (25.59%). This is in agreement with previous studies [37,51]. More interestingly, HTL of two types of microalgae was also investigated using similar equipment by Toor et al. [53], who reported higher carbon and energy recovery of bio-crude with more than 50%, even performed without catalyst. However, as for the wheat straw in literature [28,52], it exhibited lower recovery, probably due to the lower bio-crude yield obtained under higher temperature or the absence of catalyst. Thus, a catalyst might be required for lignocellulosic biomass in this process to enhance the liquefaction efficiency. The ECR (energy consumption ratio) was also calculated in this process, according to Eq. (8) by the method employed in Ref. [54], aiming to estimate the energy gain in this process under different conditions.

h ECR ¼

Wi Cpw ðT  20Þ þ ð1  Wi ÞCpb ðT  20Þ ½Y  ðHHVÞð1  Wi ÞRc ½1  Rh 

i (8)

where Wi is the moisture content of barley straw, Cpw and Cpb denotes the specific heat of water and barley straw, with the value of 4.18 and 1.63 (kJ$kg1$K1) respectively, T is the final reaction temperature, Rc is the combustion efficiency of bio-crude assumed to be 0.6, Rh is the heat recovery efficiency assumed to be 0.5. From Eq. (8), the ECR had values of 0.37 with catalyst, and 0.79 without catalyst, which was calculated based on the heating input of the HTL process and output. Both of the values were less than 1, which revealed that there was a net energy gain. Additionally, the run with K2CO3 had a remarkably good ECR, due to its high yield and HHV. Therefore, it seems that barley straw is a promising lignocellulosic biomass for biofuel production through this process e at least comparable to other feedstock with similar availability on a global scale. What's more, coprocessing of different biomass, such as combination of lignocellulosic biomass and microalgae in HTL is worth studying for further studies. 4. Conclusions Hydrothermal liquefaction of barley straw in subcritical water was performed in a batch reactor to investigate the effect of alkali catalyst (K2CO3) on product distribution and properties in this study. The results showed that K2CO3 can effectively promote the bio-crude yield. At the same time, better quality of bio-crude, such as higher carbon content, lower oxygen content and hence the increased HHV can be obtained from catalytic run. It showed that both bio-crude mainly consist of carboxylic acids, phenolic compounds and their derivatives as well as ketones through GC-MS and FTIR. However, employing K2CO3 as catalyst changed the distribution of chemical compounds, where more phenolic compounds and less carboxylic acids were observed. FTIR analysis of solid residue suggested that decarboxylation was enhanced when barley straw was liquefied in the presence of K2CO3. What's more, the carbon and energy recovery was almost twice as much as that without

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catalyst. In the future, additional work is needed to be carried out to investigate and evaluate the effect of different reaction conditions (e.g. different catalysts, reaction time, heat-up and cool-down process) on HTL of barley straw, in order to achieve high yield of bio-crude with better properties and enhance the process efficiency. Acknowledgment This research was supported by the Danish Agency for Science, Technology and Innovation (grant no. ENMI 10-094552) and National Natural Science Foundation of China (grant no. 51036006). Zhe Zhu thanks the China Scholarship Council for the financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2014.11.071 References [1] BP. Statistical review of world energy. 2014. [2] Shafiee S, Topal E. When will fossil fuel reserves be diminished? Energy Policy 2009;37:181e9. [3] IPCC. Climate change 2013: the physical science basis. In: Contribution of working group I to the fifth assessment report of the intergovernmental panel on climate change. Cambridge: Cambridge University Press; 2013. [4] Lund H, Mathiesen BV. Energy system analysis of 100% renewable energy systems e the case of Denmark in years 2030 and 2050. Energy 2009;34: 524e31. [5] Kwon PS, Østergaard PA. Priority order in using biomass resources e energy systems analyses of future scenarios for Denmark. Energy 2013;63:86e94. [6] Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, et al. The path forward for biofuels and biomaterials. Science 2006;311:484e9. [7] Sarkar S, Kumar A, Sultana A. Biofuels and biochemicals production from forest biomass in Western Canada. Energy 2011;36:6251e62. [8] Bentsen NS, Jack MW, Felby C, Thorsen BJ. Allocation of biomass resources for minimising energy system greenhouse gas emissions. Energy 2014;69: 506e15. [9] Denmark. Straw yield and use by region, crop, unit and use. Statistics Denmark; 2014. [10] Han M, Kang KE, Kim Y, Choi G-W. High efficiency bioethanol production from barley straw using a continuous pretreatment reactor. Process Biochem 2013;48:488e95. [11] Neves L, Ribeiro R, Oliveira R, Alves MM. Enhancement of methane production from barley waste. Biomass Bioenergy 2006;30:599e603. [12] Meier D, Larimer DR, Faix O. Direct thermochemical liquefaction of plant biomass using hydrogenating conditions. In: Palz W, Coombs J, Hall DO, editors. Energy from biomass: 3rd EC Conference. London: Elsevier Applied Science Publishers; 1985. p. 929e32. [13] Mullen CA, Boateng AA, Hicks KB, Goldberg NM, Moreau RA. Analysis and comparison of bio-oil produced by fast pyrolysis from three barley biomass/ byproduct streams. Energy Fuel 2009;24:699e706. [14] Qureshi N, Saha BC, Dien B, Hector RE, Cotta MA. Production of butanol (a biofuel) from agricultural residues: part I e use of barley straw hydrolysate. Biomass Bioenergy 2010;34:559e65. [15] Toor SS, Rosendahl L, Rudolf A. Hydrothermal liquefaction of biomass: a review of subcritical water technologies. Energy 2011;36:2328e42. [16] Peterson AA, Vogel F, Lachance RP, Froling M, Antal JMJ, Tester JW. Thermochemical biofuel production in hydrothermal media: a review of sube and supercritical water technologies. Energy & Environ Sci 2008;1:32e65. [17] Kruse A, Dinjus E. Hot compressed water as reaction medium and reactant: 2. Degradation reactions. J Supercrit Fluids 2007;41:361e79. [18] Akiya N, Savage PE. Roles of water for chemical reactions in high-temperature water. Chem Rev 2002;102:2725e50. [19] Akizuki M, Fujii T, Hayashi R, Oshima Y. Effects of water on reactions for waste treatment, organic synthesis, and bio-refinery in sub- and supercritical water. J Biosci Bioeng 2014;117:10e8. [20] Sun P, Heng M, Sun S, Chen J. Direct liquefaction of paulownia in hot compressed water: influence of catalysts. Energy 2010;35:5421e9. € z S, Bhaskar T, Muto A, Sakata Y, Oshiki T, Kishimoto T. Low-temper[21] Karago ature catalytic hydrothermal treatment of wood biomass: analysis of liquid products. Chem Eng J 2005;108:127e37. [22] Feng S, Yuan Z, Leitch M, Xu CC. Hydrothermal liquefaction of barks into biocrude e effects of species and ash content/composition. Fuel 2014;116: 214e20.

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