Article pubs.acs.org/EF
Production of Bio-oil from Rice Stalk Supercritical Ethanol Liquefaction Combined with the Torrefaction Process Rundong Li,* Bingshuo Li, Tianhua Yang, Yinghui Xie, and Xingping Kai Key Laboratory of Clean Energy of Liaoning, College of Energy and Environment, Shenyang Aerospace University, Shenyang 110136, People’s Republic of China ABSTRACT: The rice stalk (RS) was pretreated using torrefaction in a fixed-bed reactor at 200, 240, and 280 °C, respectively. The torrefied rice stalk (TRS) was liquefied in a batch autoclave with supercritical ethanol as the medium at 325 °C and 14−15 MPa for a residence time of 60 min to obtain bio-oil. The TRS was analyzed via Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was observed from SEM results that the compact, uniform, and lamellar structure of RS was broken down, whereas the FTIR and chemical composition analysis results showed that the thermal decomposition of hemicellulose was the main reaction that occurred between 200 and 280 °C. Gas chromatography−mass spectrometry (GC−MS) results showed that the bio-oil obtained from TRS at a temperature of 200 °C had the highest ester content of 30.60% and the lowest acid content of 0.35% and also the alcohols of bio-oil reached the maximum value of 20.56% at a temperature of 240 °C. The water content slightly decreased from 2.23 to 1.31% when the torrefaction temperature was increased from 200 to 280 °C, and the heating value reached a maximum value of 32.53 MJ/kg at a temperature of 200 °C; however, the bio-oil yield gradually decreased from 55.03% of non-torrefied RS to 49.80% of TRS at 200 °C to 38.56% of TRS at 280 °C.
1. INTRODUCTION Increasing fossil fuel depletion and environmental pollution, especially those caused by SOx, NOx, and particulate matter, are two important global issues. Therefore, renewable clean energy and advanced conversion technologies are necessary to replace petroleum with biomass for economic and environmental gains. Biomass has many advantages. It emits low amounts of organic compounds containing N and S,1 is renewable, and has a carbon-neutral life cycle.2 The annual product of photosynthesis worldwide reaches as high as 1500 to 2000 billion tons and increases by 117 billion tons per year, which is equivalent to 600 billion tons of petroleum.3 China has an abundant source of agricultural stalks, which produce 700 million tons (in terms of weight of dry materials) per year. If these stalks are converted into liquid fuels, they are equivalent to 70% of the petroleum consumption at present.4 Bio-oil produced from liquefaction has many undesirable properties, such as high water content, high viscosity, low heating value, instability, and high corrosiveness.5 For example, the high water content in bio-oil not only accelerates oil delamination and causes deterioration but also decreases the calorific value and adiabatic temperature as well as increases ignition delaying time.6 The increase in water content results in severe corrosion to thermal equipment or storage materials when exposed to acids.7 To improve the quantity and quality of bio-oil obtained from biomass, three routes are considered. The first method is through hydrothermal liquefaction processing with homogeneous or heterogeneous catalysts.8,9 The second method is the introduction of organic compounds, such as ketones and alcohols, as processing solvents.10,11 Among these solvents, ethanol has gained increased attention as a processing solvent. Zhang et al.12 combined the two routes in a single process with ethanol solvent and Raney-Ni and HZSM-5-type zeolite. © 2014 American Chemical Society
The third method is hydrotreatment or deoxy-liquefaction with reduced gas carbon monoxide or hydrogen.13,14 Recently, a new upgraded technique has been investigated to improve the properties of bio-oil, especially in fast pyrolysis. Zheng et al.15 reported that the use of torrefaction as a treatment method prior to fast pyrolysis can improve bio-oil quality. Dobele et al.16 reported that hardwood drying at 200 °C and pyrolysis at 550 °C result in the improvement of bio-oil quality, including a decrease in water content and an increase in pH and calorific value. Ramsurn et al.17 proposed a further improvement in the method for bio-oil preparation based on a two-step liquefaction technique, and this process consists of an acidic subcritical treatment, followed by an alkaline supercritical water treatment. This method produces a significantly higher amount of biocrude compared to the traditional one-step process by reducing the number of condensation/polymerization reactions. To date, few studies have focused on the effects of torrefaction on the structure of torrefied rice stalk (TRS) and its corresponding liquefaction behavior. In the present work, rice stalk (RS) was pretreated by torrefaction at 200, 240, and 280 °C separately and liquefied in supercritical ethanol. The changes in organic compounds and properties of bio-oil before and after torrefaction were compared and analyzed to elucidate the effect of the upgraded torrefaction process.
2. EXPERIMENTAL SECTION 2.1. Materials. RS was collected from the suburb of Shenyang, China. The RS sample was ground, sieved to a size of 0−0.613 mm, dried at 105 °C for 12 h, and stored in a desiccator at room temperature for future use. The solvents used were of analytical-grade ethanol Received: October 17, 2013 Revised: February 12, 2014 Published: February 13, 2014 1948
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Table 1. Proximate, Ultimate, Chemical Composition, and HHV Analyses of RS proximate analysis (wt %)a moisture volatile matter fixed carbon ash HHV (MJ/kg) a
5.99 71.55 6.62 15.84 17.62
ultimate analysis (wt %)b carbon hydrogen oxygend nitrogen sulfur
39.77 5.53 53.64 0.82 0.24
chemical composition analysis (wt %)c hemicellulose cellulose lignin
22.97 35.88 19.38
As received. bOn an air-dry basis. cOn a dry basis. dBy difference.
and acetone. Table 1 shows the proximate, ultimate, and higher heating value (HHV) analyses of RS. 2.2. Torrefaction System and Procedure. Torrefaction experiments were conducted in a fixed-bed reactor with a diameter and length of 70 mm and 1 m, respectively. Figure 1 presents the schematic of the system. The reaction temperature was set at 200, 240, and 280 °C, and the residence time was maintained 20 min. Nitrogen purge gas was used to maintain an inert atmosphere at a flow rate of 0.25 m3/h. When the thermocouple temperature reached a set point, the system was purged with inert nitrogen gas for approximately 10 min before the sample was pushed into the heating zone. The condensable phase of the torrefaction vapor was then collected using three glass condensers maintained at approximately 0 °C, and the noncondensable phase was collected using a ball. 2.3. Liquefaction System and Procedure. A 500 mL stainlesssteel pressure vessel was used as a reactor for the liquefaction experiments. The system is illustrated in Figure 2. The vessel could be operated at extreme conditions of 500 °C and 30 MPa. A proportional−integral−differential (PID) controller was equipped to log the temperature and pressure during the operation using a K-type thermocouple and a pressure transducer, which are connected to a USB-based hardware system. The temperatures were maintained within 5 °C of set conditions. In a typical liquefaction experiment run, RS or TRS (15 g) and ethanol (150 mL) were loaded into the reactor. The reactor was then heated to the desired temperature (325 °C) for 60 min in a N2 atmosphere. The final pressure approached 14−15 MPa. Agitation was set at 80 rpm throughout all experiments. After the reaction was completed, the heater was removed and the autoclave was cooled by flowing tap water to approximately 30 °C. Figure 3 depicts the details of the procedure for separating liquefaction products. The gas products were then vented through an exhaust valve, followed by a condensing unit. The liquid and solid products were separated via vacuum filtration. Oil 1 was produced after the evaporation of ethanol solution in a rotary evaporator at 80 °C, which contains factions of condensed gas products and filtration, and oil 2 was produced after the evaporation of acetone solution in the rotary evaporator at 60 °C. 2.4. Methods of Data Processing. Triplicate runs were performed for each experimental condition, and a mean value with
Figure 2. Schematic diagram of the liquefaction system: (1) motor, (2) handwheel, (3) lifting motion, (4) magnetic drive, (5) cooling water jacket, (6) H2O entrance, (7) N2 entrance, (8) H2O exit, (9) exhaust valve, (10) safety valve, (11) pressure gauge, (12) thermocouple, (13) autoclave body, (14) heater, and (15) PID temperature controller.
Figure 3. Separation procedure of liquefaction products.
standard deviations was reported. The product yields were calculated to the dry matter basis as follows: yield of TRS (wt %) =
WTRS × 100% WRS
(1)
Figure 1. Schematic diagram of torrefaction system: (1) nitrogen, (2) pressure-regulating valve, (3) flow controller, (4) thermocouple, (5) PID temperature controller, (6) heating reactor, (7) sample, (8) quartz fiber, (9) heating tap, (10) ice traps, and (11) gas collection bag. 1949
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Table 2. Chemical Composition Analysis of TRS (wt %, Dry Basis) torrefaction temperature (°C) component
200
240
280
hemicellulose cellulose lignin
22.61 36.68 19.86
21.15 37.42 20.93
15.44 41.59 22.37
Figure 4. SEM photographs: (a) RS, (b) TRS of 200 °C, (c) TRS of 240 °C, and (d) TRS of 280 °C.
yield of residue (wt %) =
Wresidue × 100% Wmaterial
yield of bio‐oil (wt %) =
Woil 1 + Woil 2 × 100% Wmaterial
(2) Figure 6. Mass yield of RS at different torrefaction temperatures and influences of torrefaction temperatures on the yields of liquefaction products.
(3)
yield of liquefaction conversion (wt %) = 100 − yield of residue (4)
analyses. SEM in combination with energy-dispersive X-ray (EDX) was used to find out the surface morphology and the elemental composition of residues after liquefaction. The bio-oil was analyzed via gas chromatography−mass spectrometry (GC−MS, Agilent 6890N/5973). The carrier gas was He at a flow rate of 20 mL/min, and the split ratio was 1:40. A HP-5 column (30 m × 0.25 mm × 0.25 μm) was used for the separation. An oven isothermal program was set at 50 °C for 3 min, followed by a heating rate of 5 °C/min to 280 °C for 15 min. The injected volume was 1 μL. The mass range scanned was from 20 to 500 amu in an electron impact (70 eV) mode. The physical properties, such as heating value, water content, and viscosity of bio-oil, were measured using a bomb calorimeter, a Karl Fischer moisture titrator, and a kinematic viscosity tester, respectively.
yield of gas (wt %) = 100 − yield of bio‐oil − yield of residue (5) where WTRS is the mass of the TRS (g), Wresidue is the mass of the residue (g), WRS is the mass of RS fed into the fixed-bed reactor (g), Wmaterial is the mass of RS or TRS fed into the autoclave (g), Woil 1 is the mass of oil 1 obtained from the ethanol solution (g), and Woil 2 is the mass of oil 2 obtained from the acetone solution (g). 2.5. Characterization of Torrefaction and Liquefaction Products. To identify the effect of the temperature on the thermal stability of RS, Nicolet Magna 750 Fourier transform infrared spectrometry (FTIR) with a range of 4000−500 cm−1 and FEI Quanta 600 scanning electron microscopy (SEM) were used for RS and TRS
Figure 5. FTIR spectra of RS and TRS at different temperatures. 1950
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Table 3. Relative Content of Several Organic Compounds in Bio-oil
Table 3. continued area (%)
area (%) compound in bio-oil
none 200 °C 240 °C 280 °C
acids
3.17
butanedioic acid, 2-isopropenyl-2methylpropanoic acid, 3-(2hydroxycyclobutylidene)-2-methylacetic acid 5-(4,5,6,7-tetrahydrobenzofuran-2-yl) pentanoic acid acid anhydride propanoic acid, 2-methyl-, anhydride alcohols
1.84
0.35
0.55
0.35
9.13 9.13 6.98 12.34 1.52 1.15 1.01 0.88
1.42
2-buten-1-ol, (E)1-dodecanol, 3,7,11-trimethyl3-hexanol, 4,4-dimethyl1-hexanol 1-propanol 2-butanol 4-isopropylidene-cyclohexanol 1,3,3-trimethyl-2-oxabicyclo[2.2.2] octane-6,7-endo,endodiol 3-hexanol, 5-methyl1-butanol, 2-methyl2-buten-1-ol, (Z)3-pentanol, 2,4-dimethyl2-methylbutane-1,4-diol, 3-(1ethoxyethoxy)2-pentanol 1-hepten-4-ol cyclopentanol, 1-(3-acetoxy-1butynyl)benzenemethanol, 4-(1,1dimethylethyl)1-pentanol, 2-methyl2-propen-1-ol, 2-methylbenzenemethanol, α-ethyl-4-methoxy4-penten-1-ol 1-pentanol
0.73 0.64 0.56 0.49
3.30
0.61 0.19 0.20 1.87 1.66 1.12 0.48 0.56 0.37 0.30
0.64 0.23
ethyl 3-(4-hydroxy-3-methoxyphenyl) propionate tetradecanoic acid, ethyl ester pentadecanoic acid, ethyl ester hexadecanoic acid, ethyl ester heptadecanoic acid, ethyl ester ethyl oleate octadecanoic acid, ethyl ester
0.39
0.39
11.46 11.46 20.56
14.68 14.68 15.33
2.20 1.91 3.37
1.97
0.22 1.62 1.18 0.23
1.44 0.15
2.01
0.38 0.73 0.46 0.51
0.47
0.24 5.10 1.83
0.77 1.53 1.14 0.57 1.45 0.72 1.05 0.87 1.58 1.13 0.89 1.08 0.81 1.43 0.43
0.82
0.62
21.36
1.43 0.71 0.66 0.53 0.40 26.35
3-octenoic acid, ethyl ester 2-pentenoic acid, 3,4,4-trimethyl-, ethyl ester oxalic acid, 2-isopropylphenyl pentyl ester ethyl o-methylbenzoate (E)-9-octadecenoic acid ethyl ester pentanoic acid, ethyl ester tetracosanoic acid, methyl ester ketones
0.66 0.65 0.42
2.06 1.46 0.71
1.03 0.58
2.05 1.76
1.46
1.26
1.14 0.83 1.06
none 200 °C 240 °C 280 °C 0.50
2.02
0.24
0.52 2.80 0.43 0.68 1.51 0.91
4.64 1.01
1.26 0.50
0.86 2.66 1.90
0.73 0.31
methyl 19-methyl-eicosanoate 0.47 acetic acid, hydroxy-, ethyl ester butanoic acid, ethyl ester acetic acid, ethoxy-, ethyl ester 4-octenoic acid, ethyl ester, (Z)3-hexenoic acid, ethyl ester, (Z)ethyl 2-hexenoate benzoic acid, ethyl ester 1,3-benzodioxole-5-propanoic acid, ethyl ester 12-oxododecanoic acid, ethyl ester eicosanoic acid, ethyl ester docosanoic acid, ethyl ester ethyl tetracosanoate tetracosanoic acid, 2,9-dimethyl-, methyl ester propanoic acid, 2-hydroxy-, ethyl ester cyclopropanecarboxylic acid, 3-formyl2,2-dimethyl-, ethyl ester succinic acid, ethyl tridec-2-ynyl ester 3-cyclohexene-1-methanol, α,α,4trimethyl-, propanoate benzenepropanoic acid, 4-hydroxy-, methyl ester decanedioic acid, dimethyl ester tetraethyl orthosilicate butanoic acid, 2-ethyl-3-hydroxy-, ethyl ester
4.70
0.26
23.27 30.6 propanoic acid, 2-hydroxy-, ethyl ester butanoic acid, 2-hydroxy-, ethyl ester butyrolactone ethyl trans-2-pentenoate hexanoic acid, ethyl ester 3-hexenoic acid, ethyl ester 2-hexenoic acid, ethyl ester cyclobutanecarboxylic acid, undec-10enyl ester butanedioic acid, diethyl ester diethyl methylsuccinate 2,6-dimethyl-8-oxoocta-2,6-dienoic acid, methyl ester geranyl isovalerate hexanedioic acid, monoethyl ester benzenepropanoic acid, ethyl ester 2,4-hexadienedioic acid, 3,4-diethyl-, dimethyl ester
0.87
0.78
2-furanmethanol 1,2-ethanediol 1-butanol propylene glycol
esters
compound in bio-oil
0.77 0.61 3.93 2.61 4.22
cyclopentanone, 2-ethyl2-cyclopenten-1-one, 3-methyl2-cyclopenten-1-one, 2,3,4-trimethyl 2-cyclopenten-1-one, 2,3-dimethyl2-cyclohexen-1-one, 3,4-dimethyl2-cyclopenten-1-one, 2,3,4,5tetramethyl1,3-cyclopentanedione, 2-ethyl-2methyl2-cyclopenten-1-one 2-cyclopenten-1-one, 2-methylcyclooctanone 2-cyclohexen-1-one, 2-hydroxy-3methyl-6-(1-methylethyl)2-cyclohexen-1-one, 2,4,4-trimethyl-3(3-oxo-1-butenyl)-
0.89
0.70 1.13
1.48
1951
0.36 0.34 0.56 0.68 0.68 1.66 1.20 0.93 0.91 1.46 1.23 0.37 0.12
0.22 0.35
0.63 1.2
0.16 0.54 0.38 0.60 0.77
1.32
0.32 0.19
0.99 0.66
0.17 0.17 0.20
0.66
1.84 0.96 0.23 0.23
0.25 0.53 0.43 1.24 0.50 0.79 0.48 0.23
0.73 5.48
9.17
6.81
0.63 0.71 0.65 1.28 0.55 0.45
0.73
0.93 0.97
2.35
0.24 5.57 0.98
2.32
1.21 0.17 1.39 0.58 1.75
0.25 1.23 0.65
1.54
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(Figure 4a) has a very compact lignocellulosic matrix.18 Figure 4b depicts some bubble particles and obvious cracks on the fibers as a result of hemicellulose degradation. The macromolecular substances in RS started to soften and crack into smaller molecules. As shown in Figure 4c, the compact structure became looser and began to disappear into the amorphous structure. Moreover, the macromolecules were broken into small molecules. The fibers were significantly broken down further, and the surface was cavitated (Figure 4d). The laminated structure of the lignocellulosic matrix, which is a main constituent of RS, was fragmented into small segments. 3.1.2. FTIR Analysis. FTIR was used to realize the effect of torrefaction on the lignocellulosic structure. As shown in Figure 5, the FTIR spectra of TRS at 200, 240, and 280 °C were compared to that of RS. The vibration at 3293 cm−1 is attributed to the O−H stretching in hemicellulose. In comparison to RS, broader vibration and peak shifting to a higher wavenumber of O−H stretching could be observed in TRS because of the dehydration reactions, including dehydroxylation and condensation reactions, that occurred in the hemicellulose.19 The presence of the C−Hn (alkyl and aromatic) stretching vibration between 2970 and 2860 cm−1 indicates that the functional groups and structure became simple with an increasing torrefaction temperature. The absorptions at 1533 and 1723 cm−1 are attributed to the stretching vibration of CO in both hemicellulose and cellulose.20 The increasing torrefaction temperature resulted in weak vibration as a result of decarboxylation, decarbonylation, and CO group fracture, which produced a series of acid, aldehyde, or ether-like tar materials and gases, such as CO and CO2.21 The FTIR spectra at the bands around 1090 cm−1 assign to the C−O stretching vibration. The peak of C−O weakened and shifted with an increasing torrefaction temperature, which could contribute to the pyrolysis that occurred in hemicellulose and cellulose. The aromatic C−H bands between 900 and 700 cm−1 are ascribed to the aromatic structure torsional vibration in lignin. Its decrease was attributed to polymerization and was most significant at 280 °C, which suggested the carbonation of RS.22 3.1.3. Effects of the Torrefaction Temperature on the Chemical Composition of TRS. The hemicellulose was the major decomposed component, while RS was torrefied between 200 and 280 °C, apart from the loss of free and combined water according to the chemical composition analysis of TRS listed in Table 2. With the increase of the torrefaction temperature from 200 to 280 °C, the hemicellulose (degraded at 180−290 °C) content of TRS dropped from 22.61 to 15.44% compared to non-torrefied RS of 22.97%. The cellulose (degraded at 240−350 °C) content increased gradually and increased from 36.68% at 200 °C to 41.59% at 280 °C. The content of lignin (degraded at 280−500 °C) changed slightly, which was increased from 19.86% at 200 °C to 22.37% at 280 °C.23 These results were in accordance with the FTIR analysis. 3.2. Influence of the Torrefaction Temperature on the Mass Yield and Bio-oil Yield. Figure 6 shows the mass yield of RS during the torrefaction process and product yields of TRS during the liquefaction process. As shown from Figure 6, the TRS yield significantly decreased from 97.48 to 78.78% when the torrefaction temperature increased from 200 to 280 °C, which implies that the decomposition rate of RS increased with an increasing temperature. These results are consistent with those from FTIR and SEM analyses. The increase in the torrefaction temperature from 200 to 280 °C resulted in
Table 3. continued area (%) compound in bio-oil
none 200 °C 240 °C 280 °C
2-pentadecanone, 6,10,14-trimethylcyclohexanone, 2,6-dimethylspiro[2.4]heptan-4-one 2-cyclopenten-1-one, 3-(1methylethyl)2-cyclohexen-1-one, 4-ethyl-4-methylfurans 2-furanol, tetrahydro-2-methyl2(3H)-benzofuranone, hexahydro-3methylenefuran, 2-butyltetrahydroether
0.66 0.22 1.70 1.01
1.24 0.55 0.69
1.92 0.98 0.94
8.93
8.93 0.66
propane, 2-ethoxybenzene, 1-ethyl-4-methoxyethyl ether 0.66 ethanol, 2-ethoxy2-ethoxypentane phenols 27.69 24.06 phenol 1.93 1.04 phenol, 3-methyl1.33 phenol, 2-methoxy2.57 2.95 phenol, 2,5-dimethyl0.56 phenol, 2,3-dimethyl1.74 phenol, 4-ethoxy1.39 1.90 phenol, 4-ethyl4.89 4.94 phenol, 2-ethyl-4-methyl1.60 phenol, 2-methoxy-4-methyl1.70 phenol, 4-ethyl-3-methyl0.87 1.04 phenol, 4-ethyl-2-methoxy4.33 4.04 1,4-benzenediol, 2,5-dimethyl1.01 thymol 1.24 phenol, 2,6-dimethoxy0.97 phenol, 2-methoxy-4-propyl0.74 1.47 5-tert-butylpyrogallol 0.82 2,5-dimethylhydroquinone 1.36 3,4-diethylphenol 1.75 4-isopropylthiophenol 2.30 benzenethiol, 4-(1,1-dimethylethyl)-21.27 methylphenol, 2-ethylphenol, 2-ethyl-6-methylphenol, 2-ethyl-4,5-dimethylbutylated hydroxytoluene phenol, 2-methylphenol, 3-ethylphenol, 2-ethyl-5-methyl1,3-benzenediol, 4,5-dimethylhydrocarbon 2.82 2.60 cyclohexene, 3-(1-methylethyl)0.79 1.06 ethylidenecyclooctane 0.46 1,5,5-trimethyl-6-methylene0.96 cyclohexene 1-methylcyclooctene 0.61 0.62 2-hexadecene, 3,7,11,15-tetramethyl0.92 benzene, 1-ethyl-3-(phenylmethyl)total 82.92 88.71
1.08 0.69
0.69 1.45 1.45
28.05 2.09 1.20 3.02
1.68 4.71 1.82 0.87 4.10
1.04
1.57
1.57 2.28
1.17 1.11 21.32 1.37 3.33 2.08 1.93
3.07
1.04 0.82 0.61
0.65
0.54
1.88 1.84 1.08 0.64
2.51
1.46 3.23 3.19 1.12 3.41 1.51
1.11
0.54
0.98
1.08 0.28
0.42 93.76
92.05
3. RESULTS AND DISCUSSION 3.1. Characterization of TRS. 3.1.1. SEM Analysis. Figure 4 shows the microfabric features of RS and TRS. The RS structure 1952
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Figure 7. Distribution of (a) chemical family in bio-oil, (b) light oil (C1−C20), and heavy oil (C20−C28) based on different torrefaction temperatures.
a sharp decrease in the bio-oil yield of TRS from 49.80 to 38.56% compared to 55.03% of RS and also an increase in the residue yield from 26.61 to 40.31% compared to 21.68%. With the torrefaction temperature increased, the lignin content in TRS was also increased, which was contributed to the formation of the solid residue and low carbohydrate convension.24,25 The decrease in bio-oil and increase in the residue yield were primarily due to hemicellulose decomposition and partially cellulose and lignin decomposition during torrefaction, which strengthened the carbonation of RS. 3.3. GC−MS Analysis of Bio-oil. The composition of biooil was analyzed by GC−MS. Table 3 shows the relative content of several organic compounds. The chemical compounds are mainly alcohols, acid anhydride, esters, and phenols, with relatively few acids, hydrocarbons, ketones, furans, and ethers, on the basis of their structures. The relative content of several typical groups in bio-oil in different cases was compared in Figure 7a. Four groups, including acid anhydride, alcohols, esters, and phenols, accounted for more than 60% of the total compounds at different conditions. The acid content first increased from 0.35% at 200 °C to 0.87% at 240 °C and then decreased to 0.39% at 280 °C. A maximum acid content of 3.17% was achieved in RS, whereas the lowest value of 0.35% was achieved in TRS at 200 °C. The alcohol content increased first and then declined with the torrefaction temperature and reached the maximum value of 20.56% at 240 °C; however, only 6.98% was obtained from RS. The esters, which were largely produced from the esterification between acids and solvent ethanol (alcohols), also reached a maximum value of 30.6% at 200 °C and then decreased to the lowest value of 21.36% at 240 °C. However, the amount of solvent ethanol converted into the bio-oil was lower than 4.50%, in accordance with the ester content in the bio-oil, according to the GC−MS results (Table 3), on the assumption that there was no ethanol produced in the liquefaction process. Chen et al.26 had earlier proposed that the molecular weight (MW) of hexadecanoic acid ethyl ester is 284, and according to the reaction equations between acid and ethanol, each generates an ester molecule; the ethanol contribution to MW of ester is 28, lower than 10%. Through this method, coupled with the fact that the amount of esters that was converted from ethanol appeared in the bio-oil based on GC−MS results, the amount of ethanol converted into the bio-oil obtained from RS, TRS at 200 °C, TRS at 240 °C, and TRS at 280 °C was 2.82, 3.59, 2.84, and 4.27%, respectively. The mass of ethanol participated in the esterification
Figure 8. Elemental analysis of bio-oils.
Table 4. Physical Properties of Bio-oil bio-oil from TRS properties
bio-oil from RS
200 °C
240 °C
280 °C
appearance viscosity (cSt at 40 °C) heating value (MJ/kg) water content (wt %)
dark 5.63 28.95 2.23
dark 8.68 32.53 1.80
dark 14.33 31.57 1.64
dark 14.59 32.10 1.31
reaction was 0.38, 0.44, 0.33, and 0.41 g, respectively. The amount of 2-methylpropanoic acid anhydride (RT 12.75) increased from 9.13 to 14.68%. This result was not observed at a torrefaction temperature of 200 °C and was likely attributed to the formation of esters. The phenols also reached a maximum value of 28.05% at 240 °C and then decreased to the lowest value of 21.32% at 280 °C. Figure 7b shows the distribution of organic compounds in the carbon chains. Light oils, whose carbon chains were in the range of 1−20, accounted for a clear majority of 96−99%. With regard to the heavy oil yield with carbon chain numbers above 20, a rising trend was initially observed and then there was a decline in the yield. The amount of heavy oils reached a maximum value of 8.77% at 200 °C compared to 4.45% of RS and then decreased to 1.44% at 280 °C. 3.4. Elemental Analysis and Physical Properties of Bio-oil. Figure 8 compares the oxygen/carbon molar ratio and 1953
dx.doi.org/10.1021/ef402075e | Energy Fuels 2014, 28, 1948−1955
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content of 30.60% and the lowest acid content of 0.35%; moreover, acid anhydrides were simultaneously removed. The alcohol content of bio-oil reached the maximum value of 20.56% at a temperature of 240 °C. The amount of heavy oils reached a maximum value of 8.77% at 200 °C compared to 4.45% of RS and then decreased to 1.44% at 280 °C. However, the increase in the torrefaction temperature from 200 to 280 °C resulted in a sharp decrease in the bio-oil yield of TRS from 49.80 to 38.56%, whereas a maximum value of 55.03% was achieved with RS. Liquefaction bio-oils produced from TRS reduced oxygen/carbon ratios and water content. The water content slightly decreased from 2.23 to 1.31%, and the viscosity of the bio-oil increased from 5.63 to 14.59 cSt, when the torrefaction temperature was increased from 200 to 280 °C. The heating value reached a maximum value of 32.53 MJ/kg at a temperature of 200 °C.
the hydrogen/carbon molar ratio of bio-oils liquefied from nontorrefied RS to those of TRS. It was found that the H/C molar ratio varied from 1.61 to 1.66, the O/C molar ratio varied from 0.24 to 0.18, and the bio-oil obtained from TRS at a temperature of 200 °C had the highest H/C molar ratio and lowest O/C molar ratio. The reason for this reduction of the oxygen/ carbon ratio might be due to the dehydration, decarboxylation, and decarbonylation reactions that occurred during the torrefaction process according to the FTIR analysis. Table 4 shows the physical properties of bio-oil. The kinematic viscosity of bio-oil increased with an increasing torrefaction temperature, possibly because of the decrease in the water content. HHV varied from 28.95 to 32.53 MJ/kg, and under the conditions of TRS at 200 °C, the value was the highest up to 32.53 MJ/kg. The water content decreased from 2.23 to 1.31% when the torrefaction temperature was increased. The probable cause of this result is due to drying and dehydration during torrefaction. 3.5. SEM−EDX Analysis of the Residue. SEM analysis was carried out to determine the surface morphology of the residue from liquefaction. Figure 9 shows the SEM photographs
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge a research grant support by the National Natural Science Foundation of China (51176130), the National Basic Research Program of China (2011CB201500), and the Joint Funds of the Natural Science Foundation of Liaoning Province, China (2013024019).
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Figure 9. SEM photographs of residues from liquefaction: (a) residue of RS, (b) residue of TRS at 200 °C, (c) residue of TRS at 240 °C, and (d) residue of TRS at 280 °C.
of residues from RS and TRS. The images were taken at magnifications of 1000×, 3000×, and 5000× and showed a rough texture on their surfaces with a heterogeneous distribution of pores. The average pore diameters were 3.32, 4.32, 1.93, and 2.30 μm in the residues of RS, TRS at 200 °C, TRS at 240 °C, and TRS at 280 °C, respectively. EDX analysis results showed the presence of Na, Mg, Si, Cl, K, and Ca in the residues.
4. CONCLUSION RS was separately torrefied at 200, 240, and 280 °C before liquefaction in ethanol at supercritical conditions. In comparison to bio-oil from RS in the same conditions, torrefaction pretreatment was effective in enhancing the bio-oil properties, despite the decrease in the bio-oil yield. The bio-oil obtained from TRS at a temperature of 200 °C had the highest ester 1954
dx.doi.org/10.1021/ef402075e | Energy Fuels 2014, 28, 1948−1955
Energy & Fuels
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dx.doi.org/10.1021/ef402075e | Energy Fuels 2014, 28, 1948−1955
