Article pubs.acs.org/EF
Effects of Torrefaction on the Pyrolysis Behavior and Bio-Oil Properties of Rice Husk by Using TG-FTIR and Py-GC/MS Dengyu Chen,* Jianbin Zhou, and Qisheng Zhang Materials Science & Engineering College, Nanjing Forestry University, Nanjing 210037, China ABSTRACT: The properties of biomass directly result in the quality of bio-oil. Torrefaction pretreatment is an alternative and promising approach for biomass updating to produce high-quality bio-oil. The effects of torrefaction on the pyrolysis of rice husk were investigated using thermogravimetry−Fourier transform infrared spectroscopy (TG-FTIR), a pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and a fast pyrolysis device. The results show that with increasing torrefaction temperature, the weight loss decreases and the shoulder peaks of torrefied rice husk in DTG curves fade away. The pyrolysis characteristics and kinetics analysis of torrefied rice husk at 290 °C are unique. Three-dimensional FTIR analysis of the evolved gases clearly shows the generation properties of individual volatile components. Fast pyrolysis of torrefied rice husk produces improved bio-oil low moisture content and high heat value. Py-GC/MS analysis shows that the acidic content does not increase, while the content of many highly valued products (e.g., levoglucose) increases greatly.
1. INTRODUCTION Fast pyrolysis of biomass for bio-oil production is one of the most promising technologies for the utilization of biomass resources. However, bio-oils are low-grade liquid fuels with poor fuel properties, including high moisture content, high oxygen content, low heat value, acidity, corrosivity to common metals, poor thermal and chemical stability, and nonmiscibility with fossil fuel.1,2 Thus, primary bio-oil cannot be directly applied in various thermal devices. To improve the quality of bio-oil, much research has focused on the refining of bio-oil by processes such as catalytic hydrogenation, catalytic cracking, catalytic esterification, and emulsification.2−4 Nevertheless, because of the extremely complex composition of bio-oil and the relatively high water content, these refinement methods present some problems such as high costs of catalysts, catalyst deactivation, low conversion ratio of refined bio-oil, and scaleup difficulties. The quality of bio-oil is a direct result of the properties of raw biomass. To improve bio-oil quality, an alternative and effective approach is biomass pretreatment before pyrolysis, rather than refining bio-oil after production.5,6 Torrefaction is a thermochemical process conducted in the temperature range between 200 and 300 °C under an inert atmosphere and is considered an effective method for biomass pretreatment.7−9 Through the moderate processing of torrefaction, the fiber structure of biomass is damaged to a certain degree, and the moisture content and oxygen content are substantially decreased.10−12 Additionally, torrefaction significantly improves the energy density of biomass, enlarges its specific area, and enhances its hydrophobicity.13,14 The biomass also becomes crispy and easy to grind.15,16 After torrefaction, the improved biomass will change the properties of the pyrolysis process and of the pyrolysis products (yield, heat value, moisture content, components, pH value).17,18 Current torrefaction research mainly focuses on the physicochemical properties and pyrolysis characteristics of woody biomass.19−23 However, the effects of torrefaction on the pyrolysis process and pyrolysis products of © 2014 American Chemical Society
rice husk (the most common raw material for biomass pyrolysis) have not yet been reported. Thermogravimetry−Fourier transform infrared spectroscopy (TG-FTIR) has been widely used in biomass pyrolysis research and offers advantages such as smaller sample requirement, high accuracy, high sensitivity, and real-time analysis. It can not only clearly demonstrate the trend of weight loss with time but can also evaluate the functional groups of the volatile matter produced by pyrolysis.24 Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is another effective method for studying the mechanism of pyrolysis.25 Py-GC/MS supports ultrarapid heating of samples and can rapidly and accurately distinguish the components of the pyrolysis products. The complementary advantages of these two technologies can further improve biomass pyrolysis studies. Although many studies have been conducted on biomass pyrolysis, few studies have combined the two approaches of TG-FTIR and Py-GC/ MS. The objectives of this paper were to study the effects of rice husk torrefaction on the pyrolysis behavior, kinetics parameters, and pyrolysis products using TG-FTIR, Py-GC/MS, and a fast pyrolysis device, and to provide basic data for high-quality biooil preparation.
2. MATERIALS AND METHODS 2.1. Raw Materials. Rice husk was selected from a rice processing plant in Fuyang, a city of China. The rice husk was screened into a particle size of 40−60 mesh and then dried for 6 h at 110 °C. RH and DRH denote the raw rice husk (9.5% moisture content on a dry basis) and the dried rice husk (dried for 6 h at 110 °C), respectively. 2.2. Torrefaction Process and Analysis. A lab-scale device for torrefaction is shown in Figure 1. The torrefaction temperature was controlled by a temperature controller. The quartz reactor was heated by the heating furnace with an outer thermal insulation coat. Before Received: May 26, 2014 Revised: August 22, 2014 Published: August 24, 2014 5857
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2.3. TG-FTIR Analysis. Pyrolysis of dried and torrefied rice husk was investigated using a TG-FTIR, which is a thermogravimetric analyzer (TGA Q500, TA Instrument, USA) connected to a Fourier Transform infrared spectrometer (Nicolet 6700, Thermo Scientific, USA). Approximately 15 mg of sample was used for each test. The sample was heated from room temperature to 750 °C with different heating rates of 10, 20, and 30 °C/min. Nitrogen (purity >99.999%) was used as a carrier gas with a flow rate of 70 mL/min. The stainless steel transfer pipe and the gas cell in the FTIR were both heated to 200 °C to minimize condensation of volatile products and secondary reaction products. The IR spectra were collected at a wavelength range from 400 to 4000 cm−1 with a resolution of 1 cm−1. The experimental results of TGA and FTIR were recorded automatically by a computer. After the experiment, nitrogen flow was continued for 30 min to remove the volatile components in the gas cell. 2.4. Pyrolysis Kinetics Analysis. The Coats−Redfern kinetics method is one of the most common models that has been successfully used in determining the kinetic parameters of biomass. The reaction equation of kinetics analysis can be described as
⎛ E ⎞ dα ⎟(1 − a) = A exp⎜ − ⎝ RT ⎠ dt
where A is the pre-exponential factor, E is the activation energy, T is the temperature, and t is the time; the conversion rate of biomass, a, can be calculated by m −m a= 0 m0 − m∞ (3)
Figure 1. Lab-scale device for torrefaction. (1) Feedstock container, (2) thermocouple, (3) flowmeter, (4) nitrogen cylinder, (5) quartz reactor, (6) heating furnace, (7) temperature controller, (8) stainless wires, (9) quartz wool, (10) condenser, (11) liquid nitrogen container.
where m0 is the initial mass of the sample, m is the sample mass at any time t, and m∞ is the final mass after pyrolysis. According to the approximate expression of the Coats−Redfern method, eq 2 can be rearranged and integrated as follows:26
the experiment, rice husk (5 g, particle size of 40−60 mesh) was placed in a feedstock container. When the temperature reached and stabilized to the torrefaction temperature, the samples were fed into the downstream quartz reactor; meanwhile, a thermocouple was inserted in the samples. Quartz wool with stainless steel wires was used to support the samples and enhance the heat transfer effect to make the rice husk rapidly achieve the experimental temperature. The samples were torrefied for a given times minutes with a flow rate of 500 mL/min of nitrogen, and the temperature of the samples was measured by the thermocouple. After the experiment, open the heating furnace and move out the quartz reactor. The reactor was quickly cooled by forced convection using a blower. The rice husk was torrefied at 200, 230, 260, and 290 °C for 30 min. Torrefied rice husk is denoted as TRH-X, with X representing torrefaction temperature (in °C). The solid yield of the torrefied rice husk is calculated from eq 1.
Ymass =
M product M feed
× 100%
(2)
⎡ AR ⎛ ⎡ − ln(1 − a) ⎤ 2RT ⎟⎞⎤ E ln⎢ ⎥− ⎥ = ln⎢ ⎜⎝1 − ⎣ ⎦ ⎣ βE E ⎠⎦ RT T2
(4)
where R is the universal gas constant and β = dT/dt is the heating rate. For most temperature regions of biomass pyrolysis, E/2RT ≫1, (1 − 2RT/E) ≈ 1, so eq 4 can be simplified as
⎡ AR ⎤ ⎡ − ln(1 − a) ⎤ E ln⎢ ⎥ = ln⎢ ⎥ − 2 ⎣ ⎦ ⎣ βE ⎦ RT T
(5)
Thus, a plot of the left side of eq 5 versus 1/T should be a straight line with a slope of −E/R and an intercept of ln(AR/βE), from which E and A can be obtained. 2.5. Fast Pyrolysis of the Samples. The lab-scale device, as shown in Figure 1, also can be used for fast pyrolysis of rice husk. The experimental operation process was similar to that of torrefaction process. Nitrogen gas was fed from the top with a flow rate of 500 mL/ min. After the temperature reached a steady state of 500 °C, the samples were fed into the quartz reactor. Quartz wool with stainless steel wires was also used to support the materials and enhance the heat transfer effect to achieve fast pyrolysis. Liquid products were collected in the condenser, which was cooled by liquid nitrogen. In each experiment, approximately 3 g of sample was used, and the sample was held for 5 min in the reactor. After the experiment, the biochar and bio-oil were collected for further analysis.
(1)
where Ymass is solid yield, and the subscripts “feed” and “product” stand for the dried rice husk and torrefied rice husk, respectively. Ultimate analysis of samples was performed using an elemental analyzer (Vario macro cube, Elementar, Germany), and oxygen was estimated by the difference: O(%) =100% − C(%) − H(%) − N(%) − S(%) − ash(%). Proximate analysis was performed according to the ASTM D3172−07a standard practice. The results are listed in Table 1.
Table 1. Proximate Analysis, Ultimate Analysis, and Solid Yield of Dried and Torrefied Rice Husk proximate analysis (wt %, db) sample DRH TRH-200 TRH-230 TRH-260 TRH-290
volatiles 64.6 64.8 60.5 54.6 40.8
± ± ± ± ±
1.8 1.6 2.1 1.5 1.3
fixed carbon 19.8 19.6 22.8 27.2 37.1
± ± ± ± ±
0.5 0.6 0.8 0.6 0.7
ultimate analysis (wt %, db) ash
15.6 15.6 16.7 18.2 22.1
± ± ± ± ±
C 0.3 0.2 0.4 0.5 0.5
41.9 42.2 44.4 45.8 49.9
± ± ± ± ±
H 0.6 0.6 0.3 0.7 0.8
5858
5.3 5.6 5.1 4.8 4.3
± ± ± ± ±
O 0.4 0.2 0.5 0.3 0.2
36.6 36.4 32.9 29.0 22.0
± ± ± ± ±
N 0.6 0.4 0.5 0.3 0.9
0.6 0.6 0.7 0.7 0.7
± ± ± ± ±
solid yield 0.05 0.04 0.04 0.05 0.06
98.1% 91.7% 83.6% 65.2%
± ± ± ±
0.7% 0.3% 0.6% 0.8%
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Figure 2. Temperature profiles of the samples during torrefaction (a) and fast pyrolysis (b).
Figure 3. Pyrolysis characteristics of dried and torrefied rice husk at the heating rate of 30 °C/min: (a) TG curves, (b) DTD curves. 2.6. Py-GC/MS Analysis. A pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system was used to separate and identify the volatile products of pyrolysis. This system is composed of a pyrolyzer (CDS 5250, Chemical Data Systems, USA) and a gas chromatography/mass spectrometer (Trace DSQ II, Thermo Scientific, USA). Once the volatile components of pyrolysis formed in the pyrolyzer, they were rapidly removed from the high-temperature pyrolysis area by a carrier gas, which prevented secondary pyrolysis at the high temperature. Hence, most of the products detected by PyGC/MS were generated by primary pyrolysis of biomass. This ensures the reliability of our analysis of the pyrolysis mechanism. The volatile products of biomass pyrolysis include noncondensable gases (CO, CO2, CH4, H2, etc.), condensable organic products, and nonvolatile oligomers (oligose and pyrolytic lignin), of which the latter two groups condense into bio-oil. The condensed organic products were measured by GC/MS. The test parameters for pyrolyzer operation are as follows: sample mass, 0.5 mg; carrier gas, helium (99.999%) with a flow rate of 1 mL/ min; pyrolysis temperature, 500 °C; heating rate, 20 °C/ms; holding time, 10 s. The parameters for GC/MS operation are as follows: injector temperature, 300 °C; chromatographic separation, TR-5MS capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness); split ratio, 1:80; oven temperature, from 40 °C (3 min) to 280 °C (3 min), with a heating rate of 4 °C/min; GC/MS interface temperature, 280 °C; mass spectrometer, EI mode at 70 eV; mass spectra, from m/z 20 to 400 with a scan rate of 500 amu/s. Peak identification was carried out according to the NIST MS library and literature.
3. RESULTS AND DISCUSSION 3.1. Temperature Profiles of the Samples during Torrefaction and Pyrolysis. The temperature profiles of the samples during torrefaction and pyrolysis are shown in Figure 2. For rice husk torrefaction, it can be observed that the rice husk temperature rose quickly and then maintained at the torrefaction temperature for 30 min. For fast pyrolysis, the sample temperature also quickly rose to 500 °C, and the sample was decomposed at 500 °C for 5 min approximately. 3.2. The Effect of Torrefaction on Pyrolysis Based on TG-FTIR Analysis. 3.2.1. Thermogravimetric Analysis. The effects of torrefaction temperature on pyrolysis of dried and torrefied rice husk at a heating rate of 30 °C/min are shown in Figure 3. The TG and DTG curves suggest that the pyrolysis process can be divided into three stages. The first stage is from room temperature to 200 °C. Because of the removal of moisture, the sample weight remains unchanged, with almost no drying peaks. The second stage occurs between 200 and 500 °C, during which intense decomposition of hemicellulose, cellulose, and lignin in samples is observed. However, weight loss of torrefied rice husk under different torrefaction conditions varied significantly. With increasing torrefaction temperature, the total weight loss of torrefied rice husk decreases, which may be attributed to the prior release of some volatile components by torrefaction. At about 280 °C in the DTG curves, obvious 5859
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shoulder peaks are observed for DRH, TRH-200, and TRH230. Rice husk still contains some hemicelluloses after torrefaction. However, the shoulder peaks in the DTG curves of TRH-260 and TRH-290 disappear or may be covered by the main pyrolysis peak. This indicates that hemicellulose pyrolysis in TRH-260 and TRH-290 is very weak. With increasing torrefaction temperature, the maximum weight loss rate of torrefied rice husk increases from 0.92%/°C in TRH-200 to 1.15%/°C in TRH-260, indicating that torrefaction can facilitate rice husk pyrolysis and improve the release of volatile components. However, pyrolysis characteristics of TRH-290 are obviously different from that of TRH-200, TRH-230, and TRH-260. For TRH-290, the maximum weight loss rate drops to 0.79%/°C. This phenomenon could be attributed to the decomposition of hemicelluloses in rice husk after torrefaction at 290 °C. The third stage corresponds to the carbonization process above 500 °C. Residues are mainly ash and fixed carbon. For TRH-260, the weight loss percentages at the first, second, and third stages are 0.7%, 61.5%, and 9.3%, respectively. Taking TRH-260 as an example, Figure 4 shows the effects of heating
Table 2. Calculation Results of Coats-Redfern Model for Dried and Torrefied Rice Husk sample DRH
TRH-200
TRH-230
TRH-260
TRH-290
heating rate, °C·min−1
t, °C
E, kJ·mol−1
R2
10 20 30 10 20 30 10 20 30 10 20 30 10 20 30
220−380 225−385 230−390 220−380 225−385 230−390 220−380 225−385 230−390 220−380 225−385 230−390 220−380 225−385 230−390
70.82 71.25 72.81 71.56 73.75 74.07 74.51 74.08 74.13 71.46 70.02 68.21 41.67 47.11 46.20
0.9937 0.9976 0.9968 0.9930 0.9950 0.9955 0.9943 0.9945 0.9955 0.9737 0.9766 0.9649 0.9882 0.9845 0.9799
that of TRH-230 remains unchanged under different heating rates. Conversely, the activation energy of TRH-260 declines with increased heating rate. For TRH-290, there was no apparent relationship between activation energy and heating rate. 3.2.3. Three-Dimensional FTIR analysis. The infrared spectrum is used in distinguishing various inorganic and organic compounds for pyrolysis studies.28 Figure 5 shows
Figure 4. Pyrolysis characteristics of TRH-260 at different heating rates.
rate on the pyrolysis characteristics of torrefied rice husk. With increased heating rate, the TG curves move toward the hightemperature region, and the release of volatile components per unit temperature decreases.27 3.2.2. Kinetics Analysis. Results of the kinetics analysis are presented in Table 2. With increased torrefaction temperature, the pyrolysis activation energy of rice husk slightly increases from 71 to 74 kJ/mol (DRH and TRH-230, respectively), and then decreases to 70 kJ/mol (TRH-260). The activation energy for dried and torrefied rice husk (TRH-200, TRH-230, and TRH-260) are similar, in the range of 68−74 kJ/mol. However, the activation energy for TRH-290 is only 41−47 kJ/mol, which is significantly different from that of the other torrefied rice husk samples. The pyrolysis characteristics of TRH-290 are also unique compared to other materials (Figure 3). This difference is probably attributed to the decomposition of hemicelluloses in TRH-290, as the chemical composition and structure determine the biomass pyrolysis characteristics. Heating rate also affects the pyrolysis kinetics of dried and torrefied rice husk. For DRH and TRH-200, the activation energy increases slightly with increased heating rate, whereas
Figure 5. Three-dimensional infrared spectra of gases evolved during DRH pyrolysis at a heating rate of 30 °C/min.
the three-dimensional (3D) FTIR spectra of gases evolved during DRH pyrolysis at a heating rate of 30 °C/min, and include information about infrared absorbance, wavenumber, and temperature. The infrared absorption peaks of evolved gases are very complex, indicating that there are many volatile compounds released in the pyrolysis of DRH. According to chemistry principles and as reported in the literature, a series of typical compounds can be identified by their characteristic absorbance.29,30 For example, the most significant absorbance peak is representative of CO2, which is in the region of 2400− 2250 cm−1. The 3D FTIR component analysis is shown in Table 3. Spectral intensity as a function of time can be obtained in 3D FTIR when the wavenumber is fixed. This information can be used to analyze the generation of specific components. The change of spectral intensity during the pyrolysis process, which 5860
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in torrefied rice husk is slightly stronger than that of dried rice husk. This is due to the alteration of the chemical structure of torrefied rice husk, which contributes to the release of volatile matter. However, the spectral intensity for TRH-260 is slightly lower because the hemicelluloses have been significantly decomposed, leading to decreased volatile products. It is worth noting that the FTIR spectrum of TRH-290 is essentially different from the other FTIR spectra. Not only are the spectral intensity very low, but many absorption peaks also disappear. This suggests that the concentration and types of pyrolysis products are significantly reduced. These results are consistent with the very low mass loss of TRH-290 in TG/ DTG results (Figure 3). The above FTIR spectra analysis indicates that a moderate torrefaction temperature (230−260 °C) is more suitable for rice husk pretreatment. 3.3. Effect of Torrefaction on Pyrolysis Using the LabScale Pyrolysis Device. The effects of torrefaction temperature on the product distribution from fast pyrolysis of torrefied rice husk are shown in Table 4. The maximum yield of bio-oil is
Table 3. Main Products of Rice Husk Identified from TGFTIR wavenumbers, cm−1
chemical bond
vibrations
4000−3400 3100−2800 2400−2250 2250−2000 1800−1650
O−H C−H CO C−O CO
stretching stretching stretching stretching stretching
1700−1450 1450−1000
CC C−O, C−C
stretching stretching
750−650
CO
bending
compounds H2O CH4 CO2 CO aldehydes, ketones, acids aromatics alkanes, alcohols, phenols, ethers, lipids CO2
arises from specific components, can be subdivided into three stages. (1) Below 200 °C (0−6 min in 3D FTIR), almost no gaseous compounds are formed. (2) Between 200 and 600 °C (6−17 min in 3D FTIR), some gaseous compounds, such as H2O, CH4, CO, and CO2 are detected first; then, volatile organic species, including furans, phenols, ketones, and aldehydes, are evolved. The range of 300−320 °C generates the strongest spectral intensity for dried and torrefied rice husk. (3) Above 600 °C (17−25 min in 3D FTIR), the infrared absorbance spectra gradually weaken, and carbonization occurs. These results are consistent with and also more detailed than the DTG results. The strongest FTIR spectra in pyrolysis process of the dried and torrefied rice husk are shown in Figure 6, which shows the effect of torrefaction temperature on gaseous products in TGA at a heating rate of 30 °C/min. As seen in Figure 6, the FTIR spectrograms of DRH, TRH-200, TRH-230, and TRH-260 are similar, so the composition of the pyrolysis products is also similar. Nevertheless, when the torrefaction temperature is below 260 °C, the spectral intensity of the various compounds
Table 4. Effect of Torrefaction Temperature on Product Distribution from Fast Pyrolysis sample RH DRH TRH-200 TRH-230 TRH-260 TRH-290
bio-oil (wt %) 53.8 46.6 46.3 41.9 36.9 33.6
± ± ± ± ± ±
0.8 1.1 0.7 0.9 0.5 0.4
biochar (wt %) 27.5 31.9 33.7 35.8 42.9 54.3
± ± ± ± ± ±
0.2 0.5 0.6 0.3 0.7 0.5
noncondensable gas (wt %) 18.7 21.5 20.0 22.3 20.2 12.1
± ± ± ± ± ±
0.2 0.4 0.2 0.3 0.1 0.1
obtained with raw rice husk. With increased torrefaction temperature, the bio-oil yield gradually decreases from 44.6% to 33.6% (DRH and TRH-290, respectively), while the biochar yield gradually increases from 31.9% to 54.3%. The gas yield changed little when the torrefaction temperature is below 260 °C, but it obviously reduced for TRH-290. After lowtemperature torrefaction (below 260 °C), bio-oil is the main pyrolysis product of torrefied rice husk. However, after hightemperature torrefaction, biochar becomes the main pyrolysis product. Thus, it can be concluded that increased torrefaction temperatures improved the yield of biochar and decreased the yield of bio-oil. These results could be explained by the aspects as following. (i) Before fast pyrolysis of torrefied rice husk, the samples have released some volatile components during torrefaction, leading to reduced condensable gas during fast pyrolysis, and thus biooil yield decreased and biochar yield increased.8 (ii) The changes in the composition of rice husk during torrefaction involve the decomposition of hemicelluloses and the partial depolymerization of cellulose and lignin. Although the lignin in the torrefaction process is also decomposed to some extent, due to the large amount of hemicellulose that is decomposed and some compounds cannot be released during composition analysis, the relative amount of lignin rises markedly. Similar results are reported the literature.8,31 In fast pyrolysis of torrefied rice husk, a less decomposition of lignin occurs comparing with that of hemicellulose and cellulose, leading to more biochar production. (iii) Carbonization of the cellulose during torrefaction is unfavorable for bio-oil production, as the volatile matter released from cellulose is the main source of biooil. Previous study has shown that active cellulose species is formed from cellulose depolymerization during torrefaction,
Figure 6. Strongest FTIR spectra in pyrolysis process of the dried and torrefied rice husk. 5861
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and then the active cellulose undergoes cross-linking; this crosslinking and charring of cellulose predominantly produce char in fast pyrolysis, resulting in a higher char yield at increasing torrefaction temperature.18 (iv) The high ash content in rice husk accelerates the generation of biochar. The inorganic elements found in ash include potassium, calcium, sodium, magnesium, silicon, phosphorus, chlorine, etc. Alkali metals (potassium and sodium) and alkaline earth metals (calcium and magnesium) catalyze the secondary reactions in rich husk pyrolysis, resulting in reducing the productivity of bio-oil.4 Table 5 shows the moisture content, pH value, and heat value of bio-oil. With increasing torrefaction temperature, the Table 5. Effect of Torrefaction Temperature on the Properties of Bio-Oil sample RH DRH TRH-200 TRH-230 TRH-260 TRH-290
water content (wt %) 50.2 45.6 43.3 40.1 36.5 31.7
± ± ± ± ± ±
1.2 1.1 0.6 0.9 0.5 0.4
pH 2.8 2.7 2.7 2.9 3.2 3.0
± ± ± ± ± ±
0.02 0.04 0.03 0.03 0.02 0.03
HHV (MJ/kg) 11.0 12.8 13.6 14.3 15.6 16.2
± ± ± ± ± ±
0.2 0.4 0.6 0.4 0.3 0.3
moisture content of bio-oil is significantly reduced, while the heat value of bio-oil increases substantially. The pH of the biooil was measured three times and the average value was used. The values range from 2.7 to 3.2 and have a good repeatability. The pH of bio-oil is mainly affected by organic acids, phenols, and moisture content, increasing slightly with torrefaction temperature. In general, torrefaction pretreatment of rice husk improves bio-oil quality. The increase of bio-oil heat value and the decrease of acidity can facilitate the storage of bio-oil and further utilization. 3.4. Effects of Torrefaction on Pyrolysis Using Py-GC/ MS Analysis. The pyrolysis products are very complex and contain numerous compounds. Although it was not possible to analyze every product, the pyrolysis products can be divided into several main categories according to the functional groups detected by Py-GC/MS: sugar dehydration products, furans, small molecules, aldehydes, acids, ketones, and phenols.18,32 For Py-GC/MS results, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area% is linear with its content.33 Some representative compounds for each category were selected due to their relatively higher peak area %. Figure 7 show the relative contents of these typical compounds, which are mainly produced by lignin and cellulose/hemicelluloses. The increase of torrefaction temperature has different effects on these pyrolysis products: (1) acetic acid (AA) and hydroxyacetaldehyde (HAA) content does not change; (2) furan content declines, especially 2,3-dihydro-benzofuran; (3) 1,2-cyclopentanedione content gradually increases; (4) the amount of levoglucose (LG), an important and highly valuable chemical, increases significantly; (5) the amounts of the two phenols (2methoxy-4-propyl-phenol and 2-methoxy-4-vinylphenol) decrease significantly, while the amounts of the other four phenols increase significantly. Generally, the content of acidic materials does not increase, while the content of many high-value products increases greatly. This is of great significance for the enrichment of high-quality components in bio-oil.
Figure 7. Effect of torrefaction temperature on the pyrolysis products identified from GC/MS.
4. CONCLUSIONS The properties of biomass materials are responsible for the quality of bio-oil. Seen from the results of TG-FTIR, Py-GC/ MS, and a fast pyrolysis device, torrefaction has an obvious effect on pyrolysis process, kinetics parameters, and bio-oil properties of rice husk. With increasing torrefaction temperature, the total weight loss of torrefied rice husk decreases, which is attributed to the prior release of some volatile components by torrefaction. The pyrolysis characteristics and kinetics results of TRH-290 are unique compared to other materials, which is probably due to the decomposition of hemicelluloses, as the chemical composition and structure determine the biomass pyrolysis characteristics. A series of typical compounds are identified by their characteristic absorbance in 3D FTIR spectra, which can be used to analyze the generation of specific components. The change of spectral intensity during the pyrolysis process, which arises from specific components, can be subdivided into three stages. After fast pyrolysis, high-quality bio-oil was obtained from torrefied rice husk. The content of many high-value products increases greatly, which is of great significance for further utilization of bio-oil.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial supports provided by the scientific research funds of high-level talents in Nanjing Forestry University (No. G2014010), the national forestry 5862
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industry research special funds for public welfare projects (No. 201304611), and the Priority Academic Program Development of Jiangsu higher education institutions (PAPD).
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dx.doi.org/10.1021/ef501189p | Energy Fuels 2014, 28, 5857−5863
