Article pubs.acs.org/EF
Combination of Light Bio-oil Washing and Torrefaction Pretreatment of Rice Husk: Its Effects on Physicochemical Characteristics and Fast Pyrolysis Behavior Shuping Zhang, Qing Dong, Tao Chen, and Yuanquan Xiong* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ABSTRACT: A combination of light bio-oil washing and torrefaction pretreatment of rice husk samples was proposed in this study, to investigate the impact on physicochemical characteristics and fast pyrolysis behavior. The results indicated that pretreatment removed a significant percentage of metallic species and significantly improved the fuel characteristics of rice husk samples. Fourier transform infrared spectroscopy (FTIR) analysis indicated that some organic groups decomposed by dehydration and decarboxylation reactions during the process of torrefaction pretreatment. The results of fast pyrolysis indicated that the yield and quality of pyrolysis products significantly changed with combined light bio-oil washing and torrefaction pretreatment. Pretreatment resulted in a significant increase of pH and higher heating values (HHVs) along with a reduction of the water content. Meanwhile, it can be obtained from gas chromatography/mass spectrometry (GC/MS) analysis of bio-oil that pretreatment enhanced selective pyrolysis toward production of sugars, especially levoglucosan. In view of the developed pore structure and high silica content of char obtained from fast pyrolysis of rice husk with a combination of light bio-oil washing and torrefaction pretreatment, this char can be used as the resource to produce activated carbons and amorphous silica.
1. INTRODUCTION As a result of the renewability and carbon neutrality of biomass, it has played a significant role as an environmentally friendly energy source around the world. Among various thermochemical processes, fast pyrolysis of biomass is considered as an important technology to convert biomass resources to valueadded products (char, bio-oil, and syngas).1 In addition to the conversion units and operating conditions of fast pyrolysis, the composition and properties of biomass feedstock can influence the fast pyrolysis processes.2 It can be seen that biomass is a low-grade fuel because of its high water content, hydrophilicity, low energy density, and grinding difficulty. Furthermore, inherent metallic species contained in biomass resources also play a significant role in fast pyrolysis behavior. Therefore, using pretreatment processes prior to fast pyrolysis is an alternative and efficient approach to improve the physicochemical characteristics and pyrolysis behavior. Torrefaction pretreatment is a promising thermochemical process under an inert atmosphere in the temperature range between 200 and 300 °C.3,4 It has been known that torrefaction could improve the fuel characteristics and grindability and reduce the hydrophilicity of biomass.5 In addition, the quality of bio-oil could be improved by torrefaction pretreatment, such as less fewer water and acids in bio-oil.6 Although torrefaction has been proven to be a promising technology for upgrading biomass, a large amount of inorganic minerals remained in biomass samples after torrefaction pretreatment, especially for agricultural wastes, which usually contain high ash contents because they take up more nutrients during growth.7 Alkali and alkaline earth metals (AAEMs) in biomass, particularly K and Na, have significant impacts on pyrolytic pathways, resulting in a significant change in the yield and quality of pyrolysis products.8 In addition, slagging, fouling, and high-temperature © 2016 American Chemical Society
corrosion can be induced by high ash contents in biomass resources.9 According to recent reports, it was found that light bio-oil has the potential as a washing or leaching agent for removal of metallic species from biomass and biochar.10−12 As washing or leaching agents, light bio-oil has the advantages of both mineral and organic acids. Light bio-oil is cheap and environmentally friendly, and it has high removal efficiency of metallic species. Furthermore, it also avoids introducing undesirable Cl or S into the processed biomass.13 Remarkable advances in understanding washing or torrefaction pretreatment have been achieved. However, to our knowledge, the effects of the combination of light bio-oil washing and torrefaction pretreatment on physicochemical characteristics and fast pyrolysis behavior of biomass have not yet been investigated. The schematic flowsheet of the combination of light bio-oil washing and torrefaction pretreatment and subsequent fast pyrolysis procedure is shown in Figure 1. The present work
Figure 1. Schematic flowsheet of the combination of light bio-oil washing and torrefaction pretreatment and subsequent fast pyrolysis procedure. Received: December 21, 2015 Revised: March 10, 2016 Published: March 15, 2016 3030
DOI: 10.1021/acs.energyfuels.5b02968 Energy Fuels 2016, 30, 3030−3037
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TBRH280, and TBRH310 represent the rice husk samples after the combination of light bio-oil washing and torrefaction pretreatment at the torrefaction temperatures of 250, 280, and 310 °C, respectively. 2.4. Characterization of Rice Husk Samples. The ultimate analysis was carried out on an elemental analyzer (Vario EL-III, Elementar Analysensysteme GmbH, Germany). The proximate analysis was carried out according to the GB/T 28731-2012 standard. The higher heating value (HHV) was measured by a calorimeter (SDACM3000, China). The absolute contents of metallic species in original and pretreated rice husk samples were measured according to the method described in our previous work using inductively coupled plasma optical emission spectrometry (ICP−OES, Leeman Laboratories, Inc., Hudson, NH).15 In this study, each analysis was repeated 3 times to ensure the accuracy. The mass yield, energy yield, and mass energy density calculations of the pretreatment process are shown as follows:
evaluated and compared different pretreatment processes, such as light bio-oil washing and the combination of light bio-oil washing and torrefaction, by analyzing the fuel characteristics and fast pyrolysis behavior of original and pretreated biomass samples. Rice husk, the major byproduct produced in the rice milling factory, was selected as biomass materials in this work. To investigate the feasibility of the combination of light bio-oil washing and torrefaction pretreatment, rice husk was washed with light bio-oil and then torrefied at the temperatures of 250, 280, and 310 °C. Torrefaction and fast pyrolysis were performed in a vertical drop fixed-bed reactor. In addition, the yield and quality of pyrolysis products from fast pyrolysis of rice husk samples undergoing different pretreatment processes were also investigated.
2. MATERIALS AND METHODS
mass yield =
2.1. Raw Materials. Rice husk samples used in this work were collected from a rice milling factory in Yangzhou, China. Prior to the experiments, original rice husk samples were screened to remove coarse contaminants and then rice husk samples were dried further at 105 °C for 48 h to achieve a constant weight. The dried original rice husk sample is denoted as RH in this study. 2.2. Light Bio-oil Washing. The washing pretreatment method was similar to that described in our previous work.14 Briefly, light biooil (pH 2.8) used in this work was obtained from fast pyrolysis of rice husk in a microwave reactor (700 W for 20 min) as described in our previous works.15,16 Washing pretreatment was performed by leaching 20 g of RH in 200 mL of light bio-oil at 60 °C with stirring for 2 h. Then, the solid sample was filtered to separate into liquid and solid. The solid was washed with deionized water until the pH was neutral. Wet rice husk samples were dried at 105 °C to achieve a constant weight. Light bio-oil washed rice husk samples were denoted as BRH. 2.3. Torrefaction. Torrefaction pretreatment was carried out in a vertical drop fixed-bed reactor system, as shown in Figure 2, which has
mpre
energy yield =
m0
× 100%
mpre HHVpre m0 HHV0
mass energy density =
(1)
× 100%
energy yield mass yield
(2)
(3)
where m stands for the weight of the sample. The subscripts of “pre” and “0” stand for the pretreated and original samples, respectively. The surface functional groups of original and pretreated rice husk samples were investigated using a Fourier transform infrared spectroscopy (FTIR) analyzer (Bruker Vector 22). The FTIR absorption indices obtained from the ratio of the relative intensity of some absorption peaks were used to investigate the changes in the functional groups during the process of pretreatment. The surface morphology of rice husk samples was imaged using scanning electron microscopy (SEM, LVLEO1530VP). 2.5. Fast Pyrolysis Experiment. The fast pyrolysis experiment was also performed in the vertical drop fixed-bed reactor in this study. The fast pyrolysis process was similar to that of torrefaction pretreatment. Briefly, rice husk samples of 5 g were inserted into the vertical drop fixed-bed reactor by the influence of gravity when the temperature of the reactor rose to the pyrolysis temperature of 550 °C and was then maintained for 10 min. After the reaction completed, pyrolysis products of char, bio-oil, and gas were collected from the reactor, condenser, and gas sampling bags, respectively. 2.6. Characterization of Fast Pyrolysis Products. Gas products mainly consisting of CO, CO2, CH4, H2, and some C2+ gases collected by gas sampling bags were detected by gas chromatography equipped with a thermal conductivity detector (Agilent 6890N). Two columns used were the Plot Q packed column and MS molecular sieve column with high-purity helium (99.999%) as the carrier gas. The water content of bio-oil was measured by Karl Fischer titration. The analysis of main components of bio-oil was carried out by gas chromatography/mass spectrometry (GC/MS, Agilent 7890A/ 5975C). A type of capillary column (Varian Cp-sil 8cb; length, 30 m; inner diameter, 0.25 mm; and film thickness, 0.25 μm) was used. Other parameters for GC/MS operation can be seen in our previous works.14,16 All of the chromatographic peaks were identified by the database of the National Institute of Standards and Technology (NIST) MS library. The fuel characteristics (proximate analysis, ultimate analysis, and HHV) of char were also analyzed using the methods mentioned above. Furthermore, specific surface area (SBET) of char were measured via the nitrogen adsorption−desorption isotherm at −196 °C (77 K) (ASAP 2020, Micromeritics) using the Brunauer−Emmett−Teller (BET) model. Prior to gas adsorption measurements, each sample was vacuum-degassed at 300 °C for 6 h. The chemical compositions of rice husk ashes were determined by X-ray fluorescence (XRF, ARL-9800, ARL).
Figure 2. Schematic diagram of the vertical drop fixed-bed reactor system. been described elsewhere.14 Before each experiment, nitrogen gas with a flow rate of 200 mL/min was fed into the reactor for 15 min to ensure an inert atmosphere. Rice husk samples (5 g) placed in the sample feeder at the beginning of the experiment were fed into the vertical drop fixed-bed reactor by the influence of gravity after the temperature of the reactor was stabilized to the torrefaction temperature (250, 280, and 310 °C). Thereafter, rice husk samples were heated to the predetermined temperature rapidly in a stream of nitrogen with a flow rate of 200 mL/min. After holding for 30 min under the torrefaction conditions, the quartz reactor was moved out from the heating furnace. The solid residue was weighed directly after the rice husk samples were cooled to room temperature. TBRH250, 3031
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3. RESULTS AND DISCUSSION 3.1. Effect of Pretreatment on the Physicochemical Characteristics of Rice Husk Samples. 3.1.1. Surface Morphology. It can be observed from the surface morphology of original and pretreated rice husk samples, as shown in Figure 3, that the color of rice husk samples changed from light yellow
conditions are listed in Table 1. It was found that a tiny amount of organic components was lost after light bio-oil Table 1. Mass Yield, Energy Yield, and Mass Energy Density at Different Pretreatment Conditions sample RH BRH TBRH250 TBRH280 TBRH310
Figure 3. Surface morphology of original and pretreated rice husk samples: (a) RH, (b) BRH, (c) TBRH250, (d) TBRH280, and (e) TBRH310.
mass yield (%) 100 98.8 90.6 85.1 78.6
± ± ± ±
0.5 0.4 0.4 0.6
energy yield (%) 100 99.4 95.0 93.3 88.1
± ± ± ±
0.4 0.5 0.6 0.5
mass energy density 1 1.01 1.05 1.10 1.12
± ± ± ±
0.01 0.01 0.01 0.01
washing because of its relatively lower pH compared to the hydrolysis conditions (strong acids and higher temperature).10 The results show that mass and energy yields decreased gradually with the increase of the torrefaction temperature from 250 to 310 °C. It is worth noting that the energy yield was always higher than the mass yield under the same conditions of pretreatment. Meanwhile, the mass energy density increased as the torrefaction temperature increased, which indicated that the energy per unit mass improved after pretreatment of rice husk. The fuel characteristics of rice husk samples have changed considerably after pretreatment, as shown in Table 2. It can be seen that light bio-oil washing was observed to reduce the ash content from 16.5 to 15.5%. Meanwhile, the volatile matter increased after light bio-oil washing, and the fixed carbon content decreased. After light bio-oil washing, the washed samples were torrefied at the temperatures of 250, 280, and 310 °C for 30 min. It was found that the volatile matter decreased with the increase of the torrefaction temperature; however, the fixed carbon contents and ash contents increased substantially. The results indicated that elemental composition has changed considerably after pretreatment. The molar ratios of H/C and O/C decreased gradually during the process of torrefaction, indicating that some volatiles, such as H2O and CO2, were released. This was mainly attributed to the dehydration and decarboxylation routes in the process of torrefaction.18,19 Table 3 compares the relative content of typical metallic species in
to light brown after light bio-oil washing pretreatment. Furthermore, with an increasing torrefaction temperature, the color of rice husk samples after the combination of light bio-oil washing and torrefaction pretreatment changed significantly, turning from light brown to dark brown gradually. To obtain a further understanding of the impact of pretreatment on surface morphology of rice husk, representative SEM images of RH, BRH, and TBRH310 are shown in Figure 4. It can be found in panels a and b of Figure 4 that a large amount of exterior impurities adheres to the external epidermis of RH. After light bio-oil washing pretreatment (panels c and d of Figure 4), the external epidermis of BRH is almost free from impurities and presents a regular cone convex structure with a bright and smooth surface. As shown in panels e and f of Figure 4, some openings on the external epidermis were exhibited from the combination of light bio-oil washing and torrefaction pretreatment at the temperature of 310 °C. The broken surface was created by the release of volatile products during the process of torrefaction, which is consistent with a previous report.17 3.1.2. Fuel Characteristics. The results of mass and energy yield and mass energy density at different pretreatment
Figure 4. SEM images of (a and b) RH, (c and d) BRH, and (e and f) TBRH310. 3032
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Energy & Fuels Table 2. Fuel Characteristics (Proximate Analysis, Ultimate Analysis, and HHVs) of Rice Husk Samples proximate analysis (wt %, db)a sample RH BRH TBRH250 TBRH280 TBRH310 a
Ad 16.5 15.5 16.2 17.3 19.5
± ± ± ± ±
Vd 0.4 0.3 0.3 0.4 0.5
70.6 71.5 69.0 65.5 59.0
± ± ± ± ±
ultimate analysis (wt %, db)
FCd 0.5 0.4 0.5 0.6 0.4
12.9 13.0 14.8 17.2 21.5
± ± ± ± ±
C 0.3 0.3 0.4 0.4 0.3
38.2 39.5 42.6 44.2 46.9
± ± ± ± ±
H 0.6 0.6 0.5 0.5 0.6
5.5 5.3 5.0 4.8 4.6
± ± ± ± ±
O 0.2 0.2 0.1 0.2 0.1
39.5 39.4 35.9 33.3 28.6
N
± ± ± ± ±
0.5 0.4 0.6 0.5 0.4
0.3 0.3 0.3 0.4 0.4
± ± ± ± ±
0.05 0.05 0.05 0.05 0.05
O/C
H/C
0.78 0.74 0.63 0.57 0.46
1.73 1.61 1.41 1.30 1.18
HHV (MJ/kg) 16.6 16.7 17.4 18.2 18.6
± ± ± ± ±
0.2 0.2 0.2 0.1 0.2
Ad, ash content (dry basis); Vd, volatile content (dry basis); and FCd, fixed carbon content (dry basis).
Table 3. Relative Content of Typical Metallic Species in Rice Husk Samples metallic species content (mg/kg, db) sample RH BRH TBRH250 TBRH280 TBRH310
K 3000 45 47 50 54
± ± ± ± ±
Na 80 1 2 1 1
41 2.5 2.5 2.6 2.7
± ± ± ± ±
Mg 1 0.2 0.2 0.1 0.1
245 12 13 13 14
± ± ± ± ±
Ca 10 0.5 0.5 0.5 0.5
856 285 300 310 332
± ± ± ± ±
Fe 40 5 5 4 5
77.5 20.5 22 23 24
± ± ± ± ±
Al 2 0.5 0.5 0.5 0.5
76.5 21 22 22 24
± ± ± ± ±
5 0.5 0.5 0.5 0.5
light bio-oil washed rice husk samples were similar to that of the original rice husk. The results turned out that light bio-oil washing resulted in no significant loss of organic groups, as mentioned above. However, with the increase of the torrefaction temperature, it can be found that the absorption peaks centered at 3440 and 1730 cm−1 decreased noticeably as a result of thermal decomposition of some organic groups (especially in hemicellulose) by dehydration and decarboxylation reactions.21 This result was consistent with the decreased O/C and H/C values of rice husk samples after the combined light bio-oil washing and torrefaction pretreatment. In addition, several FTIR adsorption indices, such as ICC/ IC−H and ICC/ICO ratios, can be used to estimate the changes of typical functional groups during the process of pretreatment.22 The ICC/IC−H ratio was used to reflected the aromaticity, and the ICC/ICO ratio was an indication of the aromatic ring to oxygen functionality abundances in rice husk samples.22 It can be found from Table 4 that the values of
rice husk samples. It can be observed that the light bio-oil washing procedure dramatically reduced the relative content of typical metallic species. The removal efficiencies of K, Na, Mg, Ca, Fe, and Al are up to 98.5, 93.9, 95.1, 66.7, 73.5, and 72.5%, respectively. Therefore, the removal of a significant percentage of AAEMs can explain this increase of volatile matter. In general, the combination of light bio-oil washing and torrefaction pretreatment not only increased the mass energy density of rice husk samples but also removed a significant percentage of catalytic metallic species (particularly AAEMs) from rice husk samples. 3.1.3. Typical Functional Groups. The results of FTIR spectra in Figure 5 showed that some typical functional groups
Table 4. FTIR Absorption Indices Derived from the Ratio of the Relative Intensity of Some Typical Absorption Peaks in Original and Pretreated Rice Husk Samples sample
ICC/IC−H
ICC/ICO
RH BRH TBRH250 TBRH280 TBRH310
1 1.01 1.10 1.18 1.30
1.50 1.48 1.53 1.80 1.89
ICC/IC−H and ICC/ICO ratios increased with the increase of the torrefaction temperature, which was attributed to the decomposition of some organic groups mainly in hemicellulose, and the relative content of lignin in pretreated rice husk samples increased significantly.18 3.2. Effect of Pretreatment on the Fast Pyrolysis Behavior of Rice Husk Samples. 3.2.1. Product Yields. The effect of pretreatment on product yields based on pretreated rice husk samples obtained from fast pyrolysis is presented in Figure 6a. It can be observed that a higher bio-oil yield and lower char yield were obtained by light bio-oil washing
Figure 5. FTIR spectra of original and pretreated rice husk samples.
in rice husk samples have changed during pretreatment. It contains several absorption bands, such as a wide band centered at 3440 cm−1 (O−H), a narrow band centered at 2800−3000 cm−1 (C−H), a band centered at 1730 cm−1 (CO), a band located at 1610 cm−1 (CC), some absorption bands around 1200−1500 cm−1 (C−H and C−H2), a broad band centered at 1096 cm−1 (Si−O−Si), and two significant bands at 809 and 460 cm−1 (Si−O).20 It is worth noting that the FTIR spectra of 3033
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Figure 7. Volume concentrations of gas products obtained from fast pyrolysis.
and C2+ gases, while the volume concentration of CO2 showed a slight decreasing trend. In addition, there were no significant changes in the volume concentrations of CO and H2 with light bio-oil washing. These may be due to the catalytic effect of AAEMs in original rice husk on methane reforming or cracking reactions, and it can also alter degradation pathways of pyrolysis to increase CO2 and H2O formation.27−29 For the combination of light bio-oil washing and torrefaction pretreatment, with the increase of the torrefaction temperature, the gas volume concentrations of CH4 and H2 increased slightly, while the volume concentrations of CO2 decreased, as found previously.13 Interestingly, total volume concentrations of combustible compositions (CO, CH4, H2, and C2+) were up to 74.3% with the combination of light bio-oil washing and torrefaction pretreatment, which was much higher than that of original rice husk (64.1%). 3.2.3. Bio-oil Analysis. Table 5 compares the water content, pH value, and HHV of bio-oil obtained from fast pyrolysis. The
Figure 6. (a) Product yields based on pretreated rice husk samples obtained from fast pyrolysis and (b) char yield based on dry rice husk samples obtained from fast pyrolysis.
pretreatment. As mentioned above, light bio-oil washing has a high removal efficiency of metallic species, especially AAEMs, altering degradation pathways of pyrolysis to increase bio-oil formation and decrease char formation.23,24 For the combination of light bio-oil washing and torrefaction pretreatment, the char yield increased gradually and the bio-oil yield decreased noticeably with the increase of the torrefaction temperature. It has been known that devolatilization of rice husk samples during the process of torrefaction resulted in a reduction of the bio-oil yield and an increase of the char yield.25,26 Figure 6b shows the char yield based on original rice husk samples, which are expressed per gram of original rice husk samples. It is noteworthy that with the increase of the torrefaction temperature, the yield of char based on original rice husk samples increased. It can be seen from the results of Figure 6b that the char yield based on the original rice husk samples increased gradually from 33.16% of BRH to 37.37% of TBRH310. Therefore, in addition to the devolatilization during torrefaction increasing the char yield and decreasing the bio-oil yield, carbonization and cross-linking of cellulose during the process of torrefaction predominantly promoted the increase in the char yield from fast pyrolysis.19 3.2.2. Gas Product Analysis. The gas volume concentrations of major components (CO, CO2, CH4, H2, and some C2+ gases) in gas products obtained from fast pyrolysis are shown in Figure 7. The results indicated that light bio-oil washing resulted in a slight increase in volume concentrations of CH4
Table 5. Water Content, pH, and HHV of Bio-oil Obtained from Fast Pyrolysis sample RH BRH TBRH250 TBRH280 TBRH310
water content (%) 45.6 35.3 34.2 32.3 30.0
± ± ± ± ±
1.0 0.8 0.6 0.5 0.5
pH 2.3 3.1 3.2 3.3 3.3
± ± ± ± ±
0.05 0.06 0.05 0.06 0.05
HHV (MJ/kg) 11.3 13.2 13.5 14.4 15.3
± ± ± ± ±
0.2 0.3 0.3 0.2 0.3
results indicated that the water content of bio-oil decreased significantly from 45.6% obtained from RH to 35.3% obtained from BRH and then gradually decreased to 30.0% obtained from TBRH310. Meanwhile, there were obvious increases in the values of pH and HHV of bio-oil by light bio-oil washing. With the increase of the torrefaction temperature, the pH and HHV of bio-oil increased gradually. Thus, the results indicated that the fuel quality and stability of bio-oil were improved with the combination of light bio-oil washing and torrefaction pretreatment. GC/MS analysis was conducted to detect the chemical components of bio-oil for further investigating the effect of pretreatment on the quality of bio-oil. It can be seen from the total ion chromatograms (TICs) of bio-oil (Figure 8) that light 3034
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graphic area percentage.30 It can be seen that pretreatment significantly changed the distribution of the groups of bio-oil by enhancing the production of sugars and reducing the production of acids, ketones, and furans. Light bio-oil washing alone greatly increased the relative content of sugars from 7.97 to 32.81%, which was further increased to 39.61% when the washed rice husk samples were subjected to torrefaction at the temperature of 310 °C. The significant effect of pretreatment on the relative concentrations of typical chemical components of bio-oil is presented in Figure 9b. It has been reported that high relative contents of levoglucosan in bio-oil can be used for production of bioethanol by fermentation.31 The combination of light biooil washing and torrefaction, which provided the highest levoglucosan yield at the torrefaction temperature of 310 °C, appeared to increase the relative content of levoglucosan by 22.1% from light bio-oil washed rice husk and over 350% from original rice husk. In addition to levoglucosan, pretreatment has noticeable effects on other typical chemical components: (1) relative contents of acetic acid and 1-hydroxy-2-propanone were significantly reduced through light bio-oil washing, and these were further decreased by torrefaction pretreatment; (2) the relative content of furans declined, especially 2,3dihydrobenzofuran; and (3) the relative content of 2methoxy-4-vinylphenol decreased, while the relative content of 2-methoxy-4-methylphenol increased significantly. These significant impacts of the combination of light bio-oil washing and torrefaction pretreatment on the quality of bio-oil can be explained as follows. First, light bio-oil washing removed a significant percentage of AAEMs. It has been found that AAEMs, particularly K, Na, and Ca, can inhibit the thermal depolymerization of cellulose into sugars and promote homolytic fission and fragmentation within glucose rings.10 Second, torrefaction pretreatment reduced the relative amount of hemicellulose, mitigating the interactions between cellulose and hemicellulose. It has been known that the interactions reduced the formation of sugars and enhanced the formation of furans.32 Furthermore, decomposition of a large amount of hemicellulose by torrefaction pretreatment resulted in producing less acetic acid and 1-hydroxy-2-propanone, which were derived primarily from fast pyrolysis of hemicellulose.33 3.2.4. Char Analysis. Char obtained from fast pyrolysis has also been analyzed to investigate the effect of pretreatment. Pyrolysis char of original and pretreated rice husk is labeled as “RH-char”, “BRH-char”, “TBRH250-char”, “TBRH280-char”, and “TBRH310-char”. It can be seen from physicochemical properties of pyrolysis char, as shown in Table 6, that the ash content in the char decreased from 45.1 to 42.5% by light biooil washing pretreatment. However, taking into account the measurement errors, torrefaction has no obvious impacts on fuel characteristics of char obtained from fast pyrolysis. In addition, pretreatments have significant effects on the pore structure of char. It is worth noting that light bio-oil washing increased the specific surface area, while torrefaction decreased the specific surface area with the increase of the torrefaction temperature. In this study, it can be observed that the ash content in char obtained from rice husk samples (42.5−45.1%) was higher than that in char obtained from other biomass.34 Moreover, the HHVs of pyrolysis char in the range of 17.7−18.8 MJ/kg were low in comparison to other solid fuels. In fact, these results indicated that the combustion of this pyrolysis char is not considered as a desirable option. It can be observed from the
Figure 8. TICs of bio-oil.
bio-oil washing dramatically increased levoglucosan production and torrefaction further enhanced it. The chemical components of bio-oil were complicated and could be divided into seven main groups (acids, ketones, aldehydes, furans, phenols, sugars, and esters) according to the functional groups. Figure 9a shows the relative contents of different groups of bio-oil using the semi-quantitative estimation by calculating the chromato-
Figure 9. (a) Relative contents of different groups of bio-oil and (b) typical chemical components of bio-oil. 3035
DOI: 10.1021/acs.energyfuels.5b02968 Energy Fuels 2016, 30, 3030−3037
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Energy & Fuels Table 6. Physicochemical Properties of Char Obtained from Fast Pyrolysis of Rice Husk Samples proximate analysis (wt %, db) samplea RH-char BRH-char TBRH250-char TBRH280-char TBRH310-char
Ad 45.1 42.5 43.1 42.9 43.6
± ± ± ± ±
Vd 0.4 0.3 0.4 0.2 0.3
14.2 15.2 14.3 14.8 14.4
± ± ± ± ±
ultimate analysis (wt %, db)
FCd 0.5 0.3 0.2 0.3 0.2
40.7 42.3 42.6 42.3 42.0
± ± ± ± ±
C 0.6 0.7 0.5 0.6 0.4
44.7 47.0 46.8 46.1 46.4
± ± ± ± ±
H 0.6 0.6 0.7 0.5 0.6
1.8 1.8 1.9 1.9 1.9
± ± ± ± ±
O 0.2 0.1 0.2 0.1 0.1
7.7 8.1 7.5 8.3 7.3
± ± ± ± ±
N 0.6 0.6 0.5 0.8 0.8
0.7 0.6 0.7 0.8 0.8
± ± ± ± ±
HHV (MJ/kg) 0.05 0.04 0.06 0.05 0.06
17.7 18.8 18.1 18.4 18.2
± ± ± ± ±
0.4 0.3 0.3 0.2 0.4
SBET (m2/g) 117.0 187.5 180.2 169.8 141.9
± ± ± ± ±
8 12 9 8 9
a
RH-char and BRH-char represent pyrolysis char from original rice husk and light bio-oil washed rice husk, respectively. TBRH250-char, TBRH280char, and TBRH310-char mean pyrolysis char from rice husk with combined pretreatment at the torrefaction temperatures of 250, 280, and 310 °C, respectively.
Table 7. Chemical Compositions of Ashes in Char Obtained from Fast Pyrolysis of Rice Husk Samples constituent
RH-char (wt %)
BRH-char (wt %)
TBRH250-char (wt %)
TBRH280-char (wt %)
TBRH310-char (wt %)
SiO2 K2O CaO SO3 P2O5 MgO Fe2O3 MnO Al2O3 Cl Cr2O3 Na2O ZnO CuO TiO2 NiO others
91.98 3.44 0.90 0.52 0.45 0.32 0.21 0.21 0.076 0.046 0.020 0.067 0.009 0.003 0.002 0.001 1.75
99.33 0.030 0.30 0.096 0.12 0.004 0.041 0.028 0.030 0 0.006 0.005 0.004 0.002 0 0.001 0.001
99.36 0.030 0.30 0.090 0.11 0.004 0.040 0.020 0.030 0 0.005 0.005 0.004 0.002 0 0.001 0.001
99.39 0.025 0.29 0.08 0.10 0.005 0.040 0.026 0.025 0 0.006 0.004 0.003 0.002 0 0.001 0.001
99.45 0.021 0.28 0.068 0.07 0.004 0.042 0.030 0.025 0 0.005 0.004 0.003 0.002 0 0 0.001
torrefaction pretreatment indicated this char can be used as the resource to produce activated carbons and amorphous silica.
results of chemical compositions of ashes in char (Table 7) that the silica content was increased from 91.98 to 99.45 wt % with the combined light bio-oil washing and torrefaction pretreatment. In view of the developed pore structure and high silica content of pyrolysis char with the combination of light bio-oil washing and torrefaction pretreatment, this char can be used as the resource to produce activated carbons and amorphous silica.35−38
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-025-83795053-8001. Fax: +86-02583795053-8004. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
4. CONCLUSION The present work investigated the effect of the combination of light bio-oil washing and torrefaction pretreatment on physicochemical characteristics and fast pyrolysis behavior of rice husk samples. Combined pretreatment removed a significant percentage of AAEMs and improved the fuel characteristics of rice husk samples. The results indicated that the yield of pyrolysis products significantly changed during combined pretreatment. In addition, the gas products obtained from fast pyrolysis were rich in combustible compositions. Combined pretreatment has a significant effect on the fuel quality of bio-oil as a result of the obvious increase of pH and HHVs along with the reduction of the water content of bio-oil. Meanwhile, the results of GC/MS analysis indicated that the combined pretreatment significantly changed the chemical component distribution in bio-oil by enhancing the production of sugars, especially levoglucosan, and reducing the production of acids, ketones, and furans. The developed pore structure and high silica content of pyrolysis char from fast pyrolysis of rice husk with the combination of light bio-oil washing and
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National High-Tech R&D Program of China (2011AA05A201) and the National Natural Science Foundation of China (51376047).
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