Hydrothermal liquefaction of Litsea cubeba seed to produce bio-oils

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Bioresource Technology 149 (2013) 509–515

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Hydrothermal liquefaction of Litsea cubeba seed to produce bio-oils Feng Wang a,⇑, Zhoufan Chang a, Peigao Duan a,⇑, Weihong Yan b, Yuping Xu b, Lei Zhang b, Juan Miao b, Yunchang Fan a a b

College of Physics and Chemistry, Department of Applied Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454003, PR China School of Biology and Chemistry Engineering, Nanyang Institute of Technology, Nanyang, Henan 473004, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Temperature was the most influential

factor affecting the products yield. The highest bio-oil yield of 56.9 wt.%

was achieved. Na2CO3 favored the conversion of the feedstock but suppressed the oil formation. The N content of the bio-oil was far below the bio-oil from the HTL of microalgae. The HHV of the bio-oil was estimated at 40.8 MJ/kg.

a r t i c l e

i n f o

Article history: Received 20 June 2013 Received in revised form 21 September 2013 Accepted 25 September 2013 Available online 2 October 2013 Keywords: Hydrothermal liquefaction Litsea cubeba seed Bio-oil

>250℃,>30min water

Litsea cubeba seed

a b s t r a c t Hydrothermal liquefaction (HTL) of Litsea cubeba seed was conducted over different temperature (250–350 °C), time (30–120 min), reactor loading (0.5–4.5 g) and Na2CO3 loading (0–10 wt.%). Temperature was the most influential factor affecting the yields of product fractions. The highest bio-oil yield of 56.9 wt.% was achieved at 290 °C, 60 min, and reactor loading of 2.5 g. The presence of Na2CO3 favored the conversion of the feedstock but suppressed the production of bio-oil. The higher heating values of the bio-oil were estimated at around 40.8 MJ/kg. The bio-oil, which mainly consisted of toluene, 1-methyl-2-(1-methylethyl)-benzene, fatty acids, fatty acid amides, and fatty acid esters, had a smaller total acid number than that of the oil obtained from the direct extraction of the starting material. It also contained nitrogen that was far below the bio-oil produced from the HTL of microalgae, making it more suitable for the subsequent refining. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, the continuing use of fossil fuels accelerated energy crisis, environmental pollution, and global warming. These problems are driving the worldwide efforts to look for new alternative energy, among which biomass is considered to be one of the most promising candidates due to its renewability, carbon neutrality and high productivity. Biomass can be easily converted into fuels and chemical products by using different conversion ⇑ Corresponding authors. Address: College of Physics and Chemistry, Department of Applied Chemistry, No. 2001, Century Avenue, Jiaozuo, Henan 454003, PR China. Tel.: +86 (0391) 3986820; fax: +86 (0391) 3987811. E-mail addresses: [email protected] (F. Wang), [email protected] (P. Duan). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.09.108

technologies (Kumar and Gupta, 2009), among which hydrothermal liquefaction (HTL) is one of the most promising one because it obviates the pretreatment of feedstock drying as did for other conversion technologies such as pyrolysis (Duan and Savage, 2011a; Xiu and Shahbazi, 2012). HTL, which involves the application of heat and pressure to biomass in an aqueous medium, mimics the conversion of ancient plant material into an oily or tarry fluid after a series of complex reactions (Duan and Savage, 2011b). HTL will ‘‘extract’’ more oil from the high lipid-content biomass than the solvent extraction alone does because part of the protein and carbohydrates in the biomass were also converted into oil. Moreover, the HTL products (oils) can be very easily separated (e.g., by decanting the cool reactor effluent) from water after reaction.


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F. Wang et al. / Bioresource Technology 149 (2013) 509–515

To date, different types of biomass feedstocks have been examined by using the HTL technology (Roussis et al., 2012; Toor et al., 2011; Qian et al., 2007). However, most of the biomass feedstocks examined usually contain a comparatively lower lipid, thereby leading to a lower product oil yield after the HTL process (Biller and Ross, 2011; Sawayama et al., 1999). It was also suggested that the lipid content in the raw material played a very important role in realizing a higher bio-oil yield. Therefore, in the present study, an oilseed from Litsea cubeba was selected as a representive oilseed biomass with comparatively higher lipid content compared to other biomass examined for the HTL. Litsea cubeba is a kind of deciduous shrub. It is native to China, Indonesia and other parts of southeast Asia. In 2011, the Ministry of Science and Technology of China has announced the development value of 147 kinds of local energy plant. In the oil rate charts, ranking first is the Zhejiang Neolitsea aurata which contains the oil as high as 81 wt.%. The utilization of Litsea cubeba seed and the properties of oil extracted from Litsea cubeba seed have been reported previously. Lee et al. investigated the isolation and characterization of quaternary alkaloids from Litsea cubeba seed (Lee et al., 1993). Jiang et al. studied the toxicity of essential oils from Litsea pungens and Litsea cubeba seeds, respectively (Jiang et al., 2009). Reports on the conversions of Litsea cubeba seed into biofuels are rare, all of which focused on the production of biodiesel from the tranesterification reaction between the oil pressed from Litsea cubeba seed and methanol or ethanol. The disadvantage of this process is that the carbohydrate and protein in the oil seed are discarded, resulting in a lower energy utilization efficiency. In contrast, HTL can waive most of the shortcomings of the traditional methods in converting Litsea cubeba seed into biofuels. However, to the best of the authors’ knowledge, the literature provides no reports on the bio-oil production from the HTL of Litsea cubeba seed. This article provides the first such report. In this study, HTL of Litsea cubeba seed into bio-oil was focused. Effects of reaction temperature (250–330 °C), time (30–120 min), reactor loading (0.5–4.5 g), and Na2CO3 loading (0–10 wt.%) on the yields of product fractions were determined, aiming to maximize the oil production. The bulk properties (e.g. elemental composition and heating value) and physico-chemical characteristics (e.g. molecular constituents and functional group allocation) of the bio-oils were characterized. The experiments and methodology that used in present study was similar to that of published in previous work (Duan et al., 2013a), which just want to compare how the biochemical composition of the starting material affected the yields of product fractions and the properties of bio-oil.

Stainless-steel reactors assembled from a nominal 1-in. Swagelok port connector and caps served as the batch reactors in all experiments. They provided a volume of 25 mL. All the metal reactors were conditioned with supercritical water at 400 °C for 1 h to eliminate or significantly reduce the catalytic effect of the reactor wall. 2.2. Procedure In a typical liquefaction reaction, 2.5 g Litsea cubeba seed power, a desired amount of deionized water and Na2CO3 (if needed) were loaded into the reactor. The loaded reactors were tightened firmly. HTL reactions were performed by placing the loaded and sealed reactors in a molten-salts tank preheated to the desired temperature which was controlled by a temperature controller. After the desired total reaction time had elapsed, the reactors were removed from molten-salts bath and immersed in cold water for 10 min to quench the reaction. The reactors were then depressurized and opened. The production of gaseous products was estimated from the weight difference before and after ventilating the reactor. The contents from the reactor were poured into a beaker and then a total of 30 mL dichloromethane was added into the reactor in three aliquots to completely recover all product fractions. The collected product fractions were first subjected to liquid and solid phase separation by using a funnel. The separated solid residue together with the filter paper was dried at 110 °C for 12 h and weighed. The production of solid residue was calculated by removing the weight of filter paper. The liquid phase was further subjected to water and organic layer separation by using a Buchner funnel. The organic layer was transferred to a round bottom flask, and the dichloromethane was then removed by a rotary evaporator at 40 °C until the vacuum fell below 0.095 MPa. The material remaining in the flask was the bio-oil. The production of water soluble products was calculated by using the weight of staring material loaded into the reactor subtracted the total weight of oil, gas, and solid residue. Of course, this value also included the lost materials during the whole operation procedure. The yield of each product fraction was calculated as its mass divided by the mass of staring material loaded into the reactor. The uncertainties of the experimental results were estimated by conducting the reaction for at least twice under the same reaction conditions. The results reported in this paper were the mean value of those at least two parallel experiments. 2.3. Analytical chemistry

2. Methods 2.1. Materials Litsea cubeba seed was collected from the mountains in Guizhou Province, South China. Prior to its use, the Litsea cubeba seed was air- and sun-dried and then milled to about100 mesh by using a multi-functional pulverizer (SB-02) for HTL. The proximate and ultimate analyses of the Litsea cubeba seed are respectively listed below: 6 wt.% moisture, 6 wt.% ash, 12 wt.% carbohydrates (calculated by difference),41 wt.% crude fat, 35 wt.% crude protein and 59.6 wt.% C, 9.3 wt.% H, 1.7 wt.% N, 15.4 wt.% O (calculated by difference), and 30.2 MJ/kg higher heating value. Previous publication provided the detailed quantification methods of evaluating the biochemical composition in the Litsea cubeba seed powder (Duan et al., 2013b). Deionized water was prepared in house for use in the experiments. All other chemicals used in this study were obtained commercially in high purity and used as received.

Bio-oil analysis was performed on a Perkin-Elmer-Clarus 600 GC equipped with an Agilent J&W DB-5MS non-polar capillary column. The bio-oil sample was prepared by re-dissolving the bio-oil in dichloromethane at a concentration of 10 wt.%. More detailed information about the analysis is available in the literature (Duan et al., 2013a). Elemental analyses and Fourier transform infrared spectrometry (FT-IR) were performed as previously described. (Duan et al., 2013a) The Dulong formula (HHV (MJ/kg) = 0.338C + 1.428 (H–O/ 8) + 0.095S) was used to estimate the higher heating value (HHV) of the bio-oils based on their elemental compositions. The total acid number (TAN) is a measurement of acidity that is determined by the amount of potassium hydroxide in milligrams that is needed to neutralize the acidic components in one gram of oil. It is an important indicator of the bio-oil quality, which indicates to the crude oil refinery the potential of corrosion problems. The TAN was measured by dissolving 0.05 g of oil in acetone along with phenolphthalein which had been dissolved in methanol (0.5% wt./v.). 0.1 M KOH was added dropwise from a burette until


511


there was a permanent color change in the sample. The volume of KOH added was recorded and then used to calculate the TAN. 3. Results and discussion This section presents results from the HTL of Litsea cubeba seed conducted at different temperature, time, reactor loading, and Na2CO3 loading, aiming to find how these parameters affect the yields of product fractions, and then provide a detailed molecular characterization (e.g. elemental and molecular composition, and FT-IR) of one of the bio-oil from the HTL to give a closer look to the properties of the bio-oil. 3.1. Effects of temperature on the products yield Previous studies suggested that temperature always showed the most significant effect on the yields of product fractions and the properties of bio-oil among all the variables examined during the HTL of biomass (Babich et al., 2011; Anastasakis and Ross, 2011; Westerhof et al., 2010). Therefore, we first studied the temperature effect rather than other variable influences on the products yield, aiming to select the most suitable temperature for the HTL of Litsea cubeba seed in term of the bio-oil yield. All the reactions were performed for 60 min at a reactor loading of 2.5 g with varying the temperature from 250 to 330 °C. Blank experiment was also conducted to determine how much organic material could be extracted from the Litsea cubeba seed at 25 °C by dichloromethane, the solvent used to extract the bio-oil after a HTL experiment. Fig. 1 shows all the experimental results. The ‘‘bio-oil’’ yield from the blank experiment is 40.8 wt.%, which is very close to the crude lipid content (41 wt.%) of the raw Litsea cubeba seed. Therefore, a bio-oil yield larger than 40.8 wt.% suggests that part of the biooil was derived from the liquefaction of non-lipid portion such as carbohydrate and protein. As inferred from Fig. 1, the bio-oil yield continuously increases from 42.7 wt.% to 56.9 wt.% as the temperature increases from 250 to 290 °C. Huge jump in oil yield is observed with increasing the temperature from 250 to 270 °C, suggesting significant decomposition reactions of carbohydrate and protein in the raw material occurred at this temperature range (Zhao et al., 2007). Further increase in temperature will slightly decrease the bio-oil yield due the partly cracking of bio-oil into light ends that are not captured in the oil fraction. Therefore, it is concluded that 290 °C is

70

Oil

Solid

Gas

Water soluble products

60

sufficient to achieve the highest bio-oil yield from the HTL of Litsea cubeba seed under hydrothermal conditions. Similar yield trend as a function of temperature was observed in the HTL of other biomass (Brown et al., 2010). However, the highest bio-oil yield produced from different biomass was usually achieved at different temperature due to their different biochemical composition. The solid residue yield hints the conversion rate of raw material. The lower the solid residue yield, the higher the conversion rate of the raw material. Fig. 1 shows the solid residue yield continuously decreases with increasing the temperature, suggesting the organic matters in the raw material were gradually converted. Smaller decrease rate in solid residue is observed at the temperatures higher than 290 °C. At a temperature of 330 °C, about 11 wt.% raw material was remained as solid residue. Recall, the raw material has an ash content of 6 wt.%, suggesting most of the organic matters in the raw material were converted during the HTL. The gas yield increases with increasing temperature due to the decomposition of carbohydrate in the raw material (Kumar et al., 2009). It hits the highest of 21.4 wt.% at 310 °C and then levels off. Another possible way contributing to the gas formation might attribute to the further decomposition of the bio-oil intermediates. At higher temperatures, organic matters are expected to be gasified, and thus increases the gas yield. However, much higher pressure would also be accompanied at higher temperatures, which is unfavorable for the gas formation. Possibly, the promotion of gas formation from the temperature increase is smaller than the suppression of gas formation from the pressure increase, and thus results in slightly reduced gas yield at temperatures higher than 310 °C. At lower temperatures, the hydrolysis (e.g. protein and carbohydrate) is the predominant reaction and thus resulted in higher yield of water soluble products (Garcia Alba et al., 2012; Zou et al., 2009). At higher temperatures, repolymerization reactions prevailed and water soluble products would decompose into gases or repolymerize to other product such oil, thereby leading to a lower yield of water soluble products at higher temperatures. The lowest water soluble yield of 9.7 wt.% is achieved at 290 °C. Further increase in temperature increases the water soluble yield. Possibly, more polar molecules were formed at higher temperatures, and thus resulted in higher yield of water soluble products. 3.2. Effects of time on the products yield Fig. 2 shows the effect of reaction time on the products yield from the HTL of Litsea cubeba seed at 290 °C and reactor loading of 2.5 g. As inferred from Fig. 2, the products yield is insensitive to the reaction time, indicating the HTL reaction was a thermalcontrolled process. The bio-oil yield increases from 53.5 wt.% at 30 min to 56.9 wt.% at 60 min. Further increase in reaction time 70

Oil

Solid

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40 50

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Yield(wt.%)

Yyield(wt%)

50

20

40

30

10 20

0 0

50

100

150

200

250

300

350

10

Temperature(℃) 0 20

120

Time(min)

Fig. 1. Effect of temperature on the products yield at 60 min and reactor loading of 2.5 g.

Fig. 2. Effect of time on the products yield at 290 °C and reactor loading of 2.5 g.


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adversely affected the bio-oil yield due to the subsequent reactions (e.g. cracking, polymerization) of bio-oil. The solid residue yield is also insensitive to the reaction time, which decreased from 15.3 wt.% at 30 min to 13.2 wt.% at 60 min and then levels off, suggesting most of the organic matters were converted at a reaction time of 60 min. The gas yield increases with increasing the reaction time. At a reaction time of 120 min, the gas yield is 25.6 wt.%, suggesting no lower than one fourth of the raw material was converted to gas products. Since the water soluble is mainly derived from the hydrolysis of protein or/and carbohydrate at the early reaction stage, its yield decreases with increasing the reaction time. It will be further converted to gas or oil products with increasing reaction time. 3.3. Effects of reactor loading on the products yield Fig. 3 shows the effect of reactor loading on the products yields at 290 °C, 60 min with varying reactor loading from 0.5 to 4.5 g. The bio-oil yield increases from 42.4 to 56.9 wt.% as the reactor loading increases from 0.5 to 2.5 g. Further increase in reactor loading slightly decreases the bio-oil yield. Therefore, the reactor loading must be controlled at a desired level if one expects to achieve a higher bio-oil yield. The solid residue yield increases from 9.6 to 13.8 wt.% as the reaction loading increases from 0.5 to 1.5 g, and then levels off. In the HTL process, the water serves both as a solvent and reactant in the hydrolysis of carbohydrate and protein contained in Litsea cubeba seed. Therefore, larger reactor loading is unfavorable for the conversion of Litsea cubeba seed. The gas yield decreases from 40.0 to 19.6 wt.% as the reactor loading increases from 0.5 to 4.5 g. Increasing the reactor loading increased the pressure inside the reactor which suppressed the gas formation according to the Le Chatelier’s principle. The water soluble yield increases with increasing the reactor loading. As the reactor loading increases, more hydrophilic compounds are formed from the hydrolysis of non-lipid part in the raw material, and thus results in higher water soluble yield at higher reactor loadings. 3.4. Effects of Na2CO3 loading on the products yield Usually, the alkali carbonates were used during the HTL of biomass because they would react with water to form bases and bicarbonates, which could promote the conversion of biomass and suppress the formation of heavier molecules (Zhou et al., 2010). Therefore, Na2CO3 was selected to investigate its effect on the yields of product fractions from the HTL of Litsea cubeba seed in present study. The reactions were conducted at 290 °C, 60 min,

and reactor loading of 2.5 g. Fig. 4 shows the experimental results. Clearly, the presence of Na2CO3 is unfavorable for the production of bio-oil. The bio-oil yield is 49.6 wt.% at a Na2CO3 loading of 10 wt.% compared to 56.9 wt.% without Na2CO3. Suppression effects on the production of bio-oil from the HTL of other biomass feedstocks such as microalgae and duckweed were also observed when using Na2CO3 as the catalyst (Anastasakis and Ross, 2011; Xiu et al., 2010). The detrimental effect of Na2CO3 on the oil formation was attributed to the process of a soap formation of the lipids in the biomass. It means that high lipid containing biomass should not be processed by using Na2CO3 as the catalyst. Surprisingly, the solid residue yield is largely insensitive to the Na2CO3 loading, suggesting the presence of Na2CO3 did not affect the conversion rate of the organic matter in Litsea cubeba seed. In contrast, the water soluble yield increases with increasing the Na2CO3 loading. It is suspected that the presence of Na2CO3 restrains the polymerization or condensation of water soluble products, thus decreased the bio-oil yield. Onwudili and Williams, (2010, 2011) have also demonstrated that the presence of base catalyzed the degradation of biomass to form formate and acetate salts of the alkali metal which would suppress the polymerization pathway toward the oil formation. Small amount of catalyst addition had a positive effect on the gas formation but a negative effect was noticed as the Na2CO3 dosage increased. Possibly, some of CO2 derived from the decomposition of carbohydrate reacted with the Na2CO3 to form NaHCO3, and thus resulted in lower gas yield at a higher Na2CO3 loading. 3.5. Characterization of bio-oil 3.5.1. Elemental analysis The bio-oils obtained from the HTL of Litsea cubeba seed were brown–red and viscous. They possessed aromatic and pungent odors. Table 1 shows the elemental composition, calorific value, water content, ash content and total acid number of the raw material, oil from the blank experiment and one representative bio-oil obtained at 290 °C, 60 min, and reactor loading of 2.5 g. For comparison purposes, the characteristics of the bio-oils obtained from the typical pyrolysis of different biomass feedstocks (Rick and Vix, 1991) and HTL of microalgae biomass are also shown in Table 1 (Duan et al., 2013b). As inferred from Table 1, the Litsea cubeba seed has higher carbon and hydrogen content and lower oxygen and ash content than that of microalgae-one of the most promising feedstock for biofuels production (Jena and Das, 2011; López Barreiro et al., 2013; Du Oil

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Fig. 4. Effect of Na2CO3 loading on the products yield at 290 °C, 60 min, and reactor loading of 2.5 g.


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F. Wang et al. / Bioresource Technology 149 (2013) 509–515 Table 1 Properties of oil obtained from Litsea cubeba seed and other biomass-derived oils.

a b c

Experimental condition

C%

H%

O%

N%

H/C ratio

HHV (MJ/kg)

Water%

Ash%

TAN

Raw material Blank Bio-oil Microalgae (Chlorella p.)b Bio-oil from Chlorella p.b Pyrolysis oilc

59.6 74.6 76.2 46.8 75.1 50–67

9.3 12.7 11.9 6.8 9.9 7–8

15.4 12.7 10.4 17.2 7.8 15–25

1.7 0.2 1.6 8.4 7.3 8–10

1.9 2.0 1.9 1.8 1.6 1.7–1.4

30.7 41.1 40.8 22.6 38.1 21.1–24.7

8 9 8 11 8 10–14

6.0 0.2 0.1 9.1 2.6 0.4–10

–a 176 100 – 90 –

not detected or provided. Duan et al., 2013b. Xiu et al., 2010.

Table 2 Tentative identities and area% of major peaks in total ion chromatograms for bio-oils produced from (A) blank and (B) 290 °C, 60 min, and reactor loading of 2.5 g. Retention time in GC (min)

Compound

Oil A Area%

Oil B Area%

2.82 7.76 9.46 11.45 11.61

Toluene a-Pinene b-Pinene 1-Methyl-2-(1-methylethyl)-benzene

16.1 0.5 0.4 – 7.2

2.4 – 3.3 –

19.82 20.99 25.24 30.18 31.97 33.27 36.12 39.16 39.70 40.98 43.52 43.53 45.35 46.31 45.78 47.15 49.48 49.96 56.30 59.23 66.09 Total

(Z)-3,7-Dimethyl-2,6-octadienal (E)-3,7-Dimethyl-2,6-octadienal Caryophyllene Caryophyllene oxide Dodecanoic acid 5-Heptyldihydro-2(3H)-furanone Dodecanamide 2,6,11,15-Tetramethyl-hexadeca-2,6,8,10,14-pentaene (3b,22Z)-Chola-5,22- dien-3-ol 3,7,11-Trimethyl-6,10-dodecadine-1-yn-3-ol (3b,4a)-4-Methyl- cholesta-8,24-dien-3-ol Hexadecanoic acid Oleic acid O-Methyloxime-hexanal Hexadecanamide N,N-Dimethyl-4-methyl-heptanamide 9-Octadecenamide N-Butyl-octadecenamide Octanoic acid 1-ethenyl-1,5-dimethyl-4-hexenyl ester cis-Myrtanol Dodecanoic acid ethyl ester

3.3 4.7 0.4 0.6 – – – 0.5 1.1 0.6 2.3 – – – – – – – 0.8 2.1 – 40.6

– – – – 18.5 1.2 1.2 – – – – 0.9 2.5 3.2 3.3 2.6 1.9 1.2 – – 16.4 58.6

D-Limonene

et al., 2013). Therefore, the Litsea cubeba seed has a higher HHV than that of microalgae. The nitrogen content in the Litsea cubeba seed is significantly lower than that of microalgae. This is desirable because HTL of Litsea cubeba seed will produce bio-oil in lower nitrogen and denitrogenation is the most difficult step for the subsequent crude bio-oil upgrading (Anastasakis and Ross, 2011). Surely, the nitrogen content of the bio-oil from the HTL of Litsea cubeba seed is far below the bio-oil produced from the HTL of microalgae. This kind of liquefied oil contains almost the same moisture as the liquefied algal oil, which is lower than that of the oils from pyrolysis. It also has a larger TAN than that of the liquefied algal oil, indicating more fatty acids existed in the oil from the HTL of Litsea cubeba seed. However, it is almost free of ash. As shown in Table 1, the carbon and hydrogen content of the bio-oils from the HTL of Litsea cubeba seed increase while the oxygen content decreases compared to the raw material. Of course, the increased C and H content and decreased O content lead to the bio-oil having a higher energy density than the starting material. The HHV is 40.8 MJ/kg, which is close to that of petroleum-derived fuels (42 MJ/kg), indicating that this kind of bio-oil can be used as boiler fuel or original feedstock for the refinery. The nitrogen content in the bio-oil is slightly lower than the raw material, which is also significantly lower than that of pyrolysis oils and algal oil. The

HHV of the bio-oil from the HTL of Litsea cubeba seed has higher energy density than that of pyrolysis oils and oil from microalgae. The H/C molar ratio in the bio-oil is also higher than the pyrolysis oils and algal oil, indicating it will be a promising alternative to the fossil fuels. Table 1 also shows the properties of bio-oil from the blank experiment. The bio-oil from the blank experiment has lower carbon and nitrogen content and higher oxygen and hydrogen content than that of the liquefied oil. However, their HHVs are very close to each other. It should be noted that the blank-oil yield is only 40 wt.% compared to 56 wt.% from the HTL process. Moreover, the blank-oil also has a larger TAN than that of the liquefied oil, indicating more fatty acids existed in oil from the direct extraction process. 3.5.2. GC–MS analysis GC–MS analysis was performed to learn the comparative amounts and identities of the specific molecules in the bio-oil. Two bio-oils obtained from the blank and 290 °C, 60 min, and reactor loading of 2.5 g were analyzed, respectively. More than 50 different peaks (products) were identified in these two oils. It should be noted that some other compounds in the bio-oil were not detected due to the solvent delay and their high-boiling point. Table 2 lists the tentative identities (from computer library match-


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ing and inspection of MS fragmentation patterns) and relative amounts of the major molecular products. As inferred from Table 2, the bio-oil (oil A) obtained from the blank experiment shows much difference from that of oil (oil B) produced in the HTL process. The typical compounds in the oil A mainly include toluene, limonene, lemonal, (3b, 4a)-4-Methyl-cholesta-8, 24-dien-3-ol, and cis-myrtanol. Similar compound profile was observed in previous work (Si et al., 2012; Seo et al., 2009). In contrast, the bio-oil from the HTL mainly consists of toluene, 1-methyl-2-(1-methylethyl)-benzene, fatty acids (such as dodecanoic acid and oleic acid), fatty acid amides (such as hexadecanamide and 9-octadecenamide), and fatty acid esters (such as dodecanoic acid ethyl ester). A likely path way for the formation of 1-methyl-2-(1-methylethyl)-benzene is the hydrogenation and isomerization of limonene. The fatty acids in the bio-oil were might derived from the hydrolysis of triglyceride in the raw material. The fatty acid amides might come from the reaction of fatty acids and ammonia derived from the decomposition of protein (Garcia Alba et al., 2012). The GC–MS analysis suggests that part of the components in the liquefied oil derived from the non-lipid portion such as protein and carbohydrate.

3.5.3. FT-IR analysis Fig. S1 shows the FT-IR spectra for the starting material, solid residue, and bio-oil produced at 290 °C, 60 min, and reactor loading of 2.5 g. We show just one of the bio-oils because they all have similar FT-IR spectra, which suggests that the same types of functional groups exist in each. A broad and strong absorbance at around 3400 cm 1 was displayed for the starting material, indicating a high content of carbohydrates and proteins. The solid residue showed a significantly lower absorbance in this wave number range, suggesting that both carbohydrates and proteins were converted during the hydrothermal reaction. The C-H stretching vibrations in aliphatic methylene groups appeared between 2840 and 3000 cm 1 and two large bands were present in this region in the bio-oil. Weak absorbance in this number range was also observed for solid residue which indicates the presence of heatresistant aliphatic structures in raw material (Muradov et al., 2012). The C–O stretching band in COOH groups appears between 1650–1760 cm 1, and the presence in Fig. S1 of a peak in this region was consistent with the presence of fatty acids in the biooil. The spectrum also shows strong absorbance near 1400 cm 1 where the scissoring band in methylene groups appears.

4. Conclusion A higher bio-oil yield was achieved from hydrothermally processing the Litsea cubeba seed due to the partial conversion of carbohydrate and protein in the Litsea cubeba seed into bio-oil. The bio-oil yield was most dependent on the reaction temperature. The presence of Na2CO3 suppressed the polymerization pathway toward the oil formation and thus decreased the bio-oil yield. The liquefied oil was characterized in higher C, H content and much lower N content compared to the starting material. This study suggested that Litsea cubeba seed was a promising feedstock candidate for the biofuel production.

Acknowledgement We gratefully acknowledge assistance with experiments from Xiaodong Cai and financial support from the Scientific and Technological Research Projects of Henan Province (132102210124), and from the China National Science Foundation (21106034).

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