Energy Sources, Part A: Recovery, Utilization, and

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The Hydrothermal Liquefaction of Rice Husk to Bio-crude Using Metallic Oxide Catalysts a

a

a

a

W. Shi , S. Li , H. Jin , Y. Zhao & W. Yu

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School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University , Shanghai , China b

Instrumental Analysis Center, Shanghai Jiao Tong University , Shanghai , China Published online: 30 Sep 2013.

To cite this article: W. Shi , S. Li , H. Jin , Y. Zhao & W. Yu (2013) The Hydrothermal Liquefaction of Rice Husk to Bio-crude Using Metallic Oxide Catalysts, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 35:22, 2149-2155, DOI: 10.1080/15567036.2012.700996 To link to this article: http://dx.doi.org/10.1080/15567036.2012.700996

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Energy Sources, Part A, 35:2149–2155, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567036.2012.700996

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The Hydrothermal Liquefaction of Rice Husk to Bio-crude Using Metallic Oxide Catalysts W. Shi,1 S. Li,1 H. Jin,1 Y. Zhao,1 and W. Yu2 1

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China 2 Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai, China

Hydrothermal liquefaction of rice husk was investigated in a 10-ml stainless steel micro-reactor. Without catalyst, the bio-crude yields are in the range of 11.8–23.8 wt%, depending on temperature, reaction time, and water/rice husk mass ratio. With catalysts, the bio-crude yields are obviously increased and the highest bio-crude yield of 32.5 wt% is obtained with La2 O3 catalyst at 300ıC for 10 min and a water/rice husk mass ratio of 5. The elemental analysis shows that the oxygen contents of bio-crude are significantly decreased by La2 O3 and Dy2 O3 catalysts, respectively. The highest higher heating value (31.78 MJ/kg) of bio-crude is obtained with La2 O3 catalyst, which is obviously higher than that of the raw material (16.19 MJ/kg). Energy balance results show that La2 O3 and Dy2 O3 catalysts have a beneficial effect on energy recovery and energy consumption ratio. Gas chromatography-mass spectrometry and Fourier transform-infrared analysis show that La2 O3 and Dy2 O3 decrease the contents of phenols and acids and promote the formation of hydrocarbon (alkenes and alkynes) and esters. Keywords: bio-crude, catalysts, deoxygenation, hydrothermal liquefaction, rice husk

1. INTRODUCTION Biomass has been attracting more and more attention due to its sustainability and role of alleviating global warming and energy crisis. Hydrothermal liquefaction is one of the most promising methods for conversion of biomass, because it uses water as a medium instead of corrosive acids and toxic solvents, and can directly convert wet biomass into bio-fuel. Extensive research works have been carried out on the hydrothermal liquefaction of biomass (Akhtar and Amin, 2011). The alkali catalysts are often used to improve the yield of bio-crude. Some metal oxides, such as La2 O3 and Dy2 O3 , also have a certain alkalinity. Compared to conventional alkali catalysts, these metal oxide catalysts have certain advantages, such as avoiding corrosion of equipment and generation of wastewater. They are applied extensively in many other areas and show an outstanding performance. However, to the authors’ knowledge, there has been no study on hydrothermal processing of rice husk for production of bio-crude using La2 O3 or Dy2 O3 as catalysts. Address correspondence to Prof. Yaping Zhao, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China. E-mail: [email protected]

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Furthermore, few papers have been published on hydrothermal liquefaction of rice husk for production of bio-crude. Le et al. (2007) studied catalytic liquefaction of rice husk on Cl /Fe2 O3 catalyst. However, they only studied the influences of catalyst on conversion rate of rice husk, and paid more attention to the catalyst. Karagöz et al. (2005) investigated the hydrothermal treatment of lignin, cellulose, rice husk, and sawdust at 280ı C for 15 min without any catalyst. However, they did not research physic-chemical properties of bio-oil and the effects of operating conditions on the bio-oil yields, and the highest yield of bio-oil reported was only 8.3 wt% for rice husk. The main purpose of this work is to explore the effects of metallic oxide catalysts on the yields, physico-chemical properties, and chemical compositions of bio-crudes produced from hydrothermal liquefaction of rice husk. The elemental and chemical compositions of bio-crudes are analyzed by elemental analyzer, gas chromatography-mass spectrometery (GC-MS) and Fourier transform-infrared (FT-IR), respectively.

2. EXPERIMENTAL The rice husk was provided by a local rice processing company. The rice husk was dried at 105ıC for 2 h and then milled to below 100 mesh. The chemical components, proximate and ultimate analysis of the rice husk are shown in Table 1. Hydrothermal liquefaction of rice husk is performed in a 10-ml micro-reactor (316-stainless steel). In a typical run, weighted rice husks are loaded into the micro-reactor with 5.0-ml of de-ionized water and 0.7 g of catalyst (if needed). The micro-reactor is evacuated to remove the oxygen. The reactor is heated to a preset temperature, maintained constant for a desired time, then cooled down to an ambient temperature rapidly with cooling water. Then, liquid and solid products are separated by vacuum filtration. The filtrate is concentrated at 50ıC in a rotary evaporator to obtain the water soluble product (WSP). The water insoluble fraction and the inner wall of the reactor are washed three times with acetone, respectively, and then they are collected together. The mixes are separated by filtration. The filtrate is concentrated at 30ı C to obtain bio-crude. The solid residue (char) is dried in an oven at 105ıC for 2 h. The elemental compositions of bio-crudes were analyzed by an Elmentar Vario EL III analyzer. The higher heating values (HHVs) of bio-crudes were calculated according to Eq. (1) (Brown et al., 2010): HHV (MJ/kg) D 0:3383C C 1:422.H-O=8/:

(1)

The chemical compositions of bio-crude were analyzed by GC-MS on a Perkin Elmer AutoSystem XL GC/TurboMass MS equipped with a column (DB-5MS; 30 m  0.25 mm  0.25 m). The FT-IR spectra of the bio-crudes were analyzed by a Perkin Elmer Spectrum 100.

TABLE 1 Components, Proximate, and Ultimate Analyses of the Rice Husk Components Analysis, wt% Cellulose 22.6 a On

Hemicellulose

Lignin

Extractives

Moisture

Ash

C

H

Ob

N

HHV (MJ/Kg)c

33.7

19.5

9.0

11.90

11.95

45.22

6.48

46.75

1.55

16.19

an ash-free basis. difference. c HHV is calculated according to Eq. (1). b By

Ultimate, wt%a

Proximate, wt%

RICE HUSK BIO-CRUDE

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3. RESULTS AND DISCUSSION

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3.1. Liquefaction of Rice Husk with and without Catalysts The effects of reaction temperature on the yield of bio-crude without catalysts for 10 min with the mass ratio of 0.2 (rice husk/water) are shown in Figure 1a. The reaction temperatures are selected ranging from 280 to 380ıC. The maximum bio-crude yield is 23.8 wt%, which is obtained at the temperature of 300ıC. The bio-crude yields increase at first with increasing the temperature up to 300ıC and then decrease drastically with further increasing of the temperature. This indicates that the yields of bio-crude depend strongly on the temperatures. Figure 1b shows the effects of reaction time on bio-crude yield at 300ıC and water/rice husk mass ratio of 5. The reaction times are selected for 10–90 min. The bio-crude yields decrease continuously with increasing reaction time from 23.8 wt% for 10 min to 11.8 wt% for 90 min. The longer reaction time may promote the conversion of bio-crude into the gas and char and cause a decrease of the bio-crude yield. Thus, a reaction time of 10 min is favorable for hydrothermal liquefaction of rice husk.

FIGURE 1 Effects of operation conditions on the product yields in hydrothermal processing of the rice husk: (a) temperature, (b) reaction time, and (c) mass ratio of water/rice husk.

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Figure 1c shows the effects of water/rice husk mass ratio (from 2 to 20) on the bio-crude yields at 300ıC for 10 min. The bio-crude yields significantly increase with the increasing water/rice husk ratio from 16.9 wt% with the ratio of 2 to 26.6 wt% with the ratio of 20. However, when the water/rice husk ratio is over 5, the bio-crude yields level off and remain stable at about 23.8–26.6 wt%. Thus, the ratio of 5 is chosen in the following study. Based on the above experiments, the effects of La2 O3 and Dy2 O3 catalysts on the bio-crude yields are studied under the optimum conditions (300ıC, 10 min, and a mass ratio of 5) and the results are listed in Table 2. The maximum yield of 32.5% is obtained with La2 O3 catalyst. La2 O3 and Dy2 O3 catalysts lead to significant increases of the yields of bio-crude. This may be attributed to the presence of basic sites on the surface of rare earth oxides (Sato et al., 2009), which cannot only break the hydrogen bonds in cellulose, but also prevent the dehydration and promote the hydrolysis and cracking reaction, leading to an increase of bio-crude yield. Besides, the yields of char produced a decrease to 19–21 wt% with La2 O3 and Dy2 O3 catalysts. 3.2. Physico-chemical Properties of Bio-crude Elemental analysis and higher heating values (HHVs) of bio-crudes are shown in Table 2. The biocrudes have a significant higher C content (64–74%) and a lower O content (18–28%) compared to the initial feedstock (45.22% C and 46.75% O). With La2 O3 and Dy2 O3 catalysts, the C contents of bio-crude are increased and the O contents are greatly reduced. The bio-crude produced using La2 O3 catalyst consists of 74.04% C, 7.00% H, 18.14% O, and 0.82% N. The decreased O content results in a higher HHV of 31.78 MJ/kg. The HHVs of bio-crudes are significantly improved by La2 O3 and Dy2 O3 . Moreover, the nitrogen contents of bio-crude are lower using La2 O3 and Dy2 O3 than that using pure water. These results indicate that La2 O3 and Dy2 O3 catalysts may play an important role in the deoxygenation and denitrification of bio-crudes.

3.3. Energy Balance The energy balance of the hydrothermal processing of rice husk is shown in Table 2. Energy recovery (ER) is calculated according to Eq. (2): ER D

HHV bio-crudeYbio-crude  100%: HHV biomass

(2)

With La2 O3 or Dy2 O3 catalysts, the value of energy recovery is 58–64%, which is significantly higher than that with pure water (38.40%). This means that over half of the heat content of

TABLE 2 Yields of Liquefied Products, Energy Balance, and Physico-chemical Properties of Bio-crude Obtained with and without Catalysts Elemental Contents, wt%a

Yields, wt% Catalysts Pure water La2 O3 Dy2 O3 a On b By

Bio-crude

WSP

Gas

Char

C

H

Ob

N

HHVs, MJ/Kg

ER, %

ECR

23.8 32.5 31.2

11.7 16.4 18.6

33.8 31.8 27.9

30.7 19.3 21

64.46 74.04 70.41

6.52 7.00 6.56

27.89 18.14 22.02

1.13 0.82 1.01

26.12 31.78 29.24

38.40 63.80 58.69

0.57 0.34 0.37

an ash-free basis. difference.

RICE HUSK BIO-CRUDE

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initial feedstock remains in the bio-crudes. In addition, energy consumption ratio (ECR) is also calculated according to Eq. (3):

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ECR D

ŒWi Cpb T C .1 Wi /Cpw T .1 Ybio-crudeHHV bio-crudeWi Rc

Rh /

;

(3)

where Wi is the feedstock content, Cpb is the specific heat of rice husk (1.25 kJ/kg/k), Cpw is the specific heat of water (4.18 kJ/kg/k), T is 275 k, Rh is the heat recovery efficiency (0.5), and Rc is the combustion efficiency of bio-crude (0.7). An ECR value less than 1 means that the energy balance is positive. ECR values of bio-crude are listed in Table 2. The results indicate that all bio-crudes have a positive energy balance (0.3–0.6). The ECR of bio-crude using pure water is 0.57, while the bio-crudes using La2 O3 and Dy2 O3 have a lower energy consumption ratio of 0.34 and 0.37 due to their higher yields and HHVs. This is in good agreement with the conclusion of energy recovery. These results show that La2 O3 and Dy2 O3 catalysts have a beneficial effect on the energy balance. 3.4. Analysis of the Bio-crudes The GC-MS analysis was performed to characterize the chemical composition of bio-crudes (see Table 3). The result indicates that phenols, ketones, carboxylic acids, and esters are the major compounds. The phenols are the most abundant component, and account for 35–58% of the total chromatographic area. With the addition of La2 O3 or Dy2 O3 catalysts, the phenols and aliphatic acids contents are decreased while the ketones contents are increased. Some new esters and alkynes (such as hexadecanoic acid methyl ester, 9,12-octadecadienoic methyl ester, 10octadecenoic acid methyl ester, 3-methyl-1-pentyne, 2,2-dimethyl-3-heptyne, 4-decyne, etc.) are observed. Interestingly, most esters are aliphatic acid methyl ester. La2 O3 and Dy2 O3 catalysts may promote the esterification between carboxylic acids and methanol. A proposed formation pathway of aliphatic acid methyl ester is shown in Figure 2. Methanol is formed first through the rupture of C–O bonds from methoxy groups in lignin (Fang et al., 2008). Then, carboxylic acids produced are esterified to aliphatic acid methyl esters with methanol. These new compounds can give lower O contents and higher HHVs. This is in good agreement with the elemental analysis results mentioned above. Figure 3 shows the FT-IR spectra of bio-crudes obtained in the presence and absence of La2 O3 or Dy2 O3 catalysts. Strong absorption peaks at 3,200–3,600 cm 1 suggest the stretching vibration of O–H group, which reveals the presence of a large amount of phenols. The peaks at 2,800– 2,960 cm 1 could be attributed to symmetrical and asymmetrical C–H stretching vibrations of methyl and methylene groups. The peaks at 1,050–1,300 cm 1 are likely attributed to the C–O–C stretching vibration from primary, secondary, and tertiary phenols. These results indicate that biocrudes contain large amounts of phenols whether using catalysts or not. However, when La2 O3 or Dy2 O3 catalysts are added, the stretch peak of CDO of bio-crude shifts from 1,669 cm 1 to 1,684 cm 1 and 1,692 cm 1 , respectively, which indicates the generation of ketones and esters. This is consistent with the analysis results from GC-MS. La2 O3 and Dy2 O3 catalysts change the composition of bio-crudes and promote the generation of low-oxygen content compounds.

4. CONCLUSIONS Rice husk has been successfully liquefied into bio-crude using metallic oxide catalysts. The optimum conditions of producing bio-crude are confirmed without catalysts, which are 300ıC, 10 min and 5 of water/rice husk ratio. The metallic oxide catalysts (La2 O3 and Dy2 O3 ) increase the yield of bio-crude, and La2 O3 leads to a highest bio-crude yield of 32.5 wt% under the optimum conditions.

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TABLE 3 Main Chemical Compositions of the Bio-crudes Obtained from Hydrothermal Liquefaction of Rice Husk

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Area, %

No.

RT, min

Name

Formula

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 31 32 33 34 35 36 37 38

5.039 5.074 6.474 7.875 8.453 9.547 10.157 10.231 10.704 11.451 11.754 11.839 12.095 12.214 13.046 14.178 14.264 14.751 14.969 15.9 16.923 16.929 17.321 17.439 19.44 19.867 21.769 21.913 21.923 23.351 24.019 25.869 27.257 31.512 32.644 33.535 35.635 35.766

2-Pentanone, 4-hydroxy-4-methylButanoic acid,3-oxo-,methyl ester 2-Cyclopenten-1-one, 2-methyl2-Cyclopenten-1-one, 3-methylPhenol 2-Propenoic acid, 2-methyl-, 2-hydroxyethyl ester 1-Pentyne,3-methyl2-Cyclopenten-1-one, 2,3-dimethylPhenol, 2-methylPhenol, 3-methylMequinol Phenol, 4-methyl2-Cyclopenten-1-one, 3-(1-methylethyl)3-Heptyne, 2,2-dimethyl4-Decyne Phenol, 3-ethylPhenol, 4-ethyl2-Cyclopenten-1-one, 3,4,4–trimethylPhenol, 2-methoxy-6-methylPhenol, 4-(1-methylethyl)Phenol, 3-propylO-Methoxy-alpha-methylbenzyl alcohol Phenol, 4-ethyl-2-methoxyPhenol, 4-methyl-2-methoxyPhenol, 2,6-dimethoxyPhenol, 2-methoxy-4-propylEthanone,1-(2,3,4-trihydroxyphenyl)Benzenemethanol,3,5-dimethylPhenol, 4-methoxy-3-(methoxymethyl)Butylated hydroxytoluene 2-Propanone,1-(4-hydroxy-3-methoxyphenyl)Phenol, 2,6-dimethyl-4-nitro1,2-Naphthoquinone, 5-methoxy-7-methyl1,2-Benzenedicarboxylic acid, bis (2-methylpropyl)ester Hexadecanoic acid, methyl ester Hexadecanoic acid 9,12-Octadecadienoic acid, methyl ester 10-Octadecenoic acid, methyl ester

C6 H12 O2 C5 H8 O3 C6 H8 O C6 H8 O C6 H6 O C6 H10 O3 C6 H10 C7 H10 O C7 H8 O C7 H8 O C7 H8 O2 C7 H8 O C8 H12 O C9 H16 C10 H18 C8 H10 O C8 H10 O C9 H14 O C8 H10 O2 C9 H12 O C9 H12 O C9 H12 O2 C9 H12 O2 C9 H12 O2 C8 H10 O3 C10 H14 O2 C8 H8 O4 C9 H12 O C9 H12 O3 C15 H24 O C10 H12 O3 C8 H9 O3 N C12 H10 O3 C16 H22 O4 C17 H34 O2 C16 H32 O2 C19 H34 O2 C19 H36 O2

Pure Water

La2 O3

Dy2 O3

1.28

2.52 1.07 1.31

2.17 1.03 5.88

1.98

4.22

3.04

1.41 10.19 3.54 1.21

2.28

3.81 1.33

5.14 2.54 1.94 1.16 1.06 3.02 13.05

9.62 1.54

1.48 1.01

12.17

14.79 1.17 4.24 1.14 1.40

13.76 1.07 3.1 1.35 1.44 6.58 4.15 2.89 1.14

7.13 1.4 3.01

8.63 1.91 3.50 1.07

1.16 7.1 1.65

4.87

1.42 1.18 1.39 1.71 2.16 1.18 2.23 1.59

1.03

1.47 2.86 2.18

The heating values of the bio-crude using metallic oxide catalysts are significantly increased due to reduction of oxygen content. La2 O3 and Dy2 O3 catalysts may play an important role in the deoxygenation of bio-crudes. Besides, La2 O3 and Dy2 O3 catalysts have a beneficial effect on the energy balance. GC-MS and FT-IR analysis show that bio-crude is mainly comprised of phenols, acids, ketons, and esters. Some new alkenes, alkynes, and carboxylic acid’s methyl esters are observed in the presence of La2 O3 and Dy2 O3 catalysts, respectively.

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FIGURE 2 Proposed formation pathway of aliphatic acid methyl ester.

FIGURE 3 The FT-IR spectra of bio-crude obtained in the presence and absence of catalysts: (a) pure water, (b) Dy2 O3 , and (c) La2 O3 .

REFERENCES Akhtar, J., and Amin, N. A. S. 2011. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sustain. Energy Rev. 15:1615–1624. Brown, T. M., Duan, P., and Savage, P. E. 2010. Hydrothermal liquefaction and gasifaction of Nannochloropsis sp. Energy Fuels 24:3639–3646. Fang, Z., Sato, T., Smith, Jr., T. R., Inomata, H., Arai, K., and Kozinski, J. A. 2008. Reaction chemistry and phase behavior of lignin in high-temperature and supercritical water. Bioresour. Technol. 99:3424–3430. Karagöz, S., Bhaskar, T., Muto, A., and Sakata, Y. 2005. Comparative studies of oil compositions produced from sawdust, rice husk, lignin and cellulose by hydrothermal treatment. Fuel 84:875–884. Le, Z., Hong, L., Ruan, R., Deng, S., Yu, F., and Zhang, H. 2007. Catalytic liquefaction of rice husk on Cl /Fe2 O3 . 29th Symposium on Biotechnology for Fuels and Chemicals, Denver, Colorado, April 29–May 2. Sato, S., Takahashi, R., Kobune, M., and Gotoh, H. 2009. Basic properties of rare earth oxides. Appl. Catal. A: Gen. 356:57–63.