Fast pyrolysis of rice husk: Product yields and compositions

Bioresource Technology 98 (2007) 22–28

Fast pyrolysis of rice husk: Product yields and compositions W.T. Tsai a

a,¤

, M.K. Lee b, Y.M. Chang

a

Department of Environmental Engineering and Science, Chia Nan University of Pharmacy and Science, Tainan 717, Taiwan b Department of Occupational Safety and Health, Chia Nan University of Pharmacy and Science, Tainan 717, Taiwan Received 7 August 2004; received in revised form 29 November 2005; accepted 2 December 2005 Available online 19 January 2006

Abstract A series of pyrolysis oils and chars were prepared from agricultural by-product rice husk by the lab-scale fast pyrolysis system using induction heating. The eVect of process parameters such as pyrolysis temperature, heating rate, holding time, nitrogen gas Xow rate, condensation temperature and particle size on the pyrolysis product yields and their chemical compositions was examined. The maximum oil yield of over 40% was obtained at the proper pyrolysis conditions. The chemical characterization by elemental, caloriWc, spectroscopic and chromatographic studies showed that the pyrolysis oils derived from the fast pyrolysis of rice husk contained considerable amounts of carbonyl groups and/or oxygen content, resulting in low pH and low heating values. © 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass; Rice husk; Fast pyrolysis; Yield; Chemical composition

1. Introduction Since the energy crisis in the 1970s, the energy utilization from biomass resources (called biomass energy) has received much attention. The energy obtained from agricultural wastes or agricultural residues is a form of renewable energy and, in principle, utilizing this energy does not add carbon dioxide, which is a greenhouse gas, to the atmospheric environment, in contrast to fossil fuels (McKendry, 2002a). Due to the lower contents of sulfur and nitrogen in the biomass waste, its energy utilization also creates less environmental pollution and health risk than fossil fuel combustion. Because of the warm climate and wide cultivation, rice has been used as a main food for 100 years in Taiwan. In recent years, the annual production of rice had a decreasing trend; there was still ca. 1.7 millions in 2002 (COA, 2002). It meant that the rice husk, which is a major by-product of the rice-milling industries, was abundantly generated at the annual production of over 300,000 metric tons. However, only small fraction of the agrowaste generated in Taiwan

*

Corresponding author. Tel.: +886 6 2660393; fax: +886 6 2669090. E-mail address: [email protected] (W.T. Tsai).

0960-8524/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.12.005

was used for poultry feed or as Wlter materials. Most of it was arbitrarily dumped into Welds, disposed of landWlls, reused as fuel for household cooking and paving materials in the animal husbandry in response to the environmental regulations of solid waste management (Tsai et al., 2004). Like other biomass wastes, rice husk contains a high amount of organic constituents (i.e., cellulose, hemicellulose and lignin) and possesses a high-energy content (Ebeling and Jenkins, 1985; Mansaray and Ghaly, 1997, 1998). Therefore, it could be recognized as a potential source of renewable energy based on both beneWts of energy recovery and environmental protection. With respect to the conversion technologies for energy use, pyrolysis has been widely used for converting biomasses into liquids and solids among the thermo-chemical technologies (Klass, 1998; Demirbas, 2001; McKendry, 2002b; Gross et al., 2003). Pyrolysis is generally described as the thermal decomposition of the organic components in biomass waste in the absence of oxygen at mediate temperature, to yield tar (pyrolysis oil), char (charcoal) and gaseous fractions (fuel gases). For convenience, there are two approaches for the conversion technology. One, called as conventional or traditional pyrolysis, is to maximum the yield of fuel gas at the preferred conditions of high temperature, low heating rate

W.T. Tsai et al. / Bioresource Technology 98 (2007) 22–28

and long gas resistance time, or to enhance the char production at the low temperature and low heating rate. Another, called as Xash or fast pyrolysis, is to maximize the yield of liquid product at the processing conditions of (1) very high heating rate (>100 °C/min) and heat transfer rate, (2) Wnely ground biomass feed (<1 mm), (3) carefully controlled temperature (around 500 °C), and (4) rapid cooling of the pyrolysis vapors to give the bio-crude products. Fast pyrolysis of biomass solid-waste is at present considered as an emerging energy technology for liquid oil and solid char production (Maschio et al., 1992; Islam and Ani, 1998). However, the pyrolysis oil from biomass waste was found to be highly oygenated and complex, and chemically unstable. Thus, the liquid products still need to be upgraded by lowering the oxygen content and removing residues. Numerous researchers have investigated the study on pyrolysis of rice hull. Liou et al. (1997) investigated pyrolysis kinetics of acid-leached rice hull with a thermal gravimetric analysis (TGA) at a small rate of 2, 3 or 5 °C/min for purposes of manufacturing silicon materials. Islam and Ani (1998) carried out the conversion of rice husk waste into pyrolysis oil and solid char in a bench-scale Xuidized bed fast-pyrolysis system. The yields of liquid oil and solid char were 40 wt.% and 53 wt.% of the total biomass fed, respectively. Mansaray and Ghaly (1998) conducted thermogravimetric analysis on rice husks in pure nitrogen atmosphere at heating rates of 10–50 °C/min to determine its thermal degradation variations. Teng and Wei (1998) studied the pyrolysis of rice hull by TGA at constant heating rates of 3, 10, 30, 60, and 100 °C/min, showing that the global mass loss during rice hull pyrolysis could be successfully simulated by its major components. Sharma and Rao (1999) performed the pyrolysis kinetics of rice husk in nitrogen and carbon dioxide at heating rates of 5, 10, 25, 50, and 100 °C/min. Williams and Nugranad (2000) pyrolyzed rice husks in a Xuidized-bed reactor at 400–600 °C, and also pyrolyzed it with zeolite ZSM-5 catalyst for upgrading the pyrolysis vapors. They found that oxygenated components in the pyrolysis oils consisted mainly of phenols, cresols, benzenediols and guaiacol and their alkylated derivatives. In this study, the fast pyrolysis of rice husk was investigated in an induction-heating furnace. In particular, the inXuences of pyrolysis temperature, heating rate, holding time at speciWed pyrolysis temperature, particle size, sweep gas (i.e., N2) Xow rate, and cryogenic temperature in the condenser on the product yields were studied. In addition, the pyrolysis products (esp. tar) obtained at the conditions of the maximum product yields were further analyzed, using chemical and thermal techniques, to determine its possibility of being a potential source of renewable fuels. 2. Methods 2.1. Materials

from physical impurities, was ground in a rotary cutting mill and was screened into fractions of 0.420 mm < particle diameter (dp) < 0.50 mm, 0.250 mm < dp < 0.420 mm, 0.177 mm < dp < 0.250 mm, and 0.125 mm < dp < 0.177 mm. It was analyzed for carbon, hydrogen, nitrogen, sulfur and chlorine by an elemental analyzer (model CHN-O-RAPID, Heraeus Co., Germany). For each analysis, the standard sample (i.e., acetanilide) was Wrst analyzed for checking the experimental errors of C/H/N elements within §1%. Other elements including silicon and major metal components in the sample were analyzed by an inductively coupled plasma-atomic emission spectrometer (model OPT 1MA 3000DA, Perkin Elmer Co., USA). Prior to analysis, the samples were Wrst digested in the concentrated nitric acid/hydroXuoric acid solution to form the solution samples. The main characteristics of unscreened rice husk are given in Table 1. 2.2. Pyrolysis The Wxed-bed fast pyrolysis experiments were performed in a horizontally tubular reactor (3.67 cm i.d. and 60 cm long), constructed using 310-stainless steel and heated by high-frequency generator (i.e., induction heating) (Fig. 1). For all experiments, rice hull (»11–21 g, in each set of experiments) was placed into a crucible at the center of the tubular reactor. During the experiments, temperature measurements were taken above the bed, with a K-type thermocouple in the middle of the tubular reactor, in order to control and monitor the reactor temperature. Table 1 Main characteristics of rice husk Characteristics

Values

Proximate analysisa Moisture Combustible matterb Ash

6.37 wt.% 81.93 wt.% 11.70 wt.%

Ultimate analysisa Carbon Hydrogen Nitrogen Sulfur Chlorine

45.28 wt.% 5.51 wt.% 0.67 wt.% 0.29 wt.% 0.19 wt.%

Heating value analysisc

4012 kcal/kg

ICP-AES analysisa,d Silicon Potassium Calcium Iron Aluminum Titanium Sodium Zinc Magnesium Phosphorus

3.9 wt.% 1630 ppmw 94 ppmw 202 ppmw 233 ppmw 7 ppmw 207 ppmw 24 ppmw 699 ppmw 94 ppmw

a b

The rice husk sample was obtained from a rice mill located in southern Taiwan. Sun dried sample, separated

23

c d

As received. Including volatile matter and Wxed carbon. Higher heating value. Inductively coupled plasma-atomic emission spectrometer.

24

W.T. Tsai et al. / Bioresource Technology 98 (2007) 22–28

Fig. 1. Schematic diagram of the fast pyrolysis system: (1) nitrogen gas cylinder; (2) regulator; (3) molecular sieve column; (4) mass Xow controller; (5) Xexible heating tape; (6) power system (high-frequency generator); (7) tubular reactor (incl. induction coil); (8) temperature controller; (9) Ktype thermocouple; (10) temperature recorder; (11) cryogenic condensation (ethylene glycol/water system); (12) tar collectors.

The sweep gas from a nitrogen cylinder was dried and puriWed by a molecular sieve tube. The constant nitrogen Xow rate was precisely metered to the experimental system using a mass Xow controller. The experimental conditions in the fast pyrolysis system were designed as follows: pyrolysis temperature of 400–800 °C, heating rate of 100– 500 °C/min, holding time (at the speciWed pyrolysis temperature) of 1–8 min, particle size of <0.50 mm, nitrogen Xow rate of 500–1500 cm3/min (at room temperature and atmospheric pressure), and cryogenic temperature in the circulating bath of ¡20 to 0 °C. The resulting products after fast pyrolysis were cooled to room temperature, and then taken from holding crucible and condensable collectors in order to weigh the masses of char and tar, respectively. The yields of the resulting products were thus calculated based on the mass of rice husk fed.

the liquid fraction derived from fast pyrolysis of rice husk was mounted on a potassium bromide (KBr) disc that had been previously scanned as a background. The FTIR spectrum in the signiWcant ranges of 500–2000 cm¡1 was measured and recorded with a 200 scans and 4.0 cm¡1 resolution. The gas chromatography/mass spectroscopy (GC/MS) analysis of the pyrolysis oil was performed with a HewlettPackard HP 5890-Series II gas chromatograph equipped with a Hewlett-Packard HP 5972A mass selective detector (MS), using a 60 m £ 0.25 mm HP-1 capillary column (0.25 m Wlm thickness). The following temperature program was adopted: initial, intermediate and Wnal temperatures were 35, 200, and 350 °C, respectively, times at initial, intermediate and Wnal temperatures were 5, 5 and 2 min, respectively, and heating rates were 5 and 10 °C, respectively. The injector temperature and detector temperature were 250 and 280 °C, respectively. The oil sample (ca. 0.2 l) was injected with a Hamilton syringe. The carrier gas was He of 99.999% purity. The MS operated in scan mode and its mass range was 45–500 a.m.u. On the other hand, a typical gas sample from vent gas was collected with a 10-liter Tedlar bag and further analyzed by using GC/MS for the purpose of examining the remaining residues of the resulting product tar in the gas phase. The following temperature program was adopted: initial and Wnal temperatures were 35 and 200 °C, respectively, times at initial and Wnal temperatures were 10 and 5 min, respectively, and heating rate was 10 °C. The injector temperature was 150 °C and the gas sample was injected with a Hamilton syringe of 1 ml. 3. Results and discussion 3.1. Product yields

2.3. Characterization Elemental analyses were carried out on the pyrolysis products oil and char with a CHN-O-RAPID Element Analyzer (Heraeus Co., Germany). For the analytical calibration, standard samples for elements of C/H/N and O were acetanilide and benzoic acid, respectively. Prior to the measurement, the standard sample was Wrst analyzed for checking the experimental error within §1%. Therefore, a duplicate determination for each sample was found to be suYcient. The caloriWc values of the oil and char were also determined by using a bomb calorimeter (Model: C2000 basic, IKA Co., Germany). In order to evaluate the corrosive property of the oil products, the pH of the tar was measured by a pH meter (Model: SP-701, Suntex Co., Taiwan). Prior to the chromatographic and spectroscopic analyses, the pyrolysis liquids were Wrst decanted and then centrifuged for 15 min at about 2000 rpm, in order to separate an organic phase from aqueous phase and char traces. Functional group analysis of the pyrolysis oil was carried out using Fourier transform infra-red (FTIR) spectrometry (Model: DA8.3, Bomen Co., Canada). A small amount of

The yields of resulting products from the fast pyrolysis of rice husk were studied under the eVect of pyrolysis temperature, heating rate, holding time, sweep gas (i.e., nitrogen) Xow rate, condensation temperature and particle size. First, to determine the eVect of pyrolysis temperature, heating rate and holding time on the pyrolysis product yields, experiments were conducted at particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C and their results were given in Figs. 2–4. Second, to examine the eVect of sweep gas (i.e., nitrogen) Xow rate and condensation temperature on the pyrolysis product yields, experiments were carried out at the optimal pyrolysis conditions determined previously. Finally, to relate the eVect of particle size on the pyrolysis product yields, experiments were also performed at the optimal pyrolysis conditions determined previously. As shown in Fig. 2, the char yield had a decline trend as the Wnal pyrolysis temperature increased from 400 to 800 °C (Putun et al., 1999; Onay et al., 2001; Shinogi and Kanri, 2003; Acikgoz et al., 2004; Ates et al., 2004; Laresgoiti et al., 2004). Clearly, the rate of declination in the ranges

W.T. Tsai et al. / Bioresource Technology 98 (2007) 22–28 100 80

Yield (%)

Oil Char

60 40 20 0 300

400

500

600

Temperature (°C)

700

800

900

Fig. 2. Dependence of yields of resulting products on pyrolysis temperature at a heating rate of 200 °C/min, holding time of 1 min, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C.

50

Yield (%)

45 40 35 30 Oil

25

Char

20 0

100

200

300

400

500

600

Heating rate Fig. 3. Dependence of yields of resulting products on heating rates at a pyrolysis temperature of 500 °C, holding time of 1 min, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C.

50

Yield (%)

45 40 35 Char 200°C/min

30

Oil 200°C/min

25

Char 400°C/min Oil 400°C/min

20 0

2

4

6

8

10

Holding time (min) Fig. 4. Dependence of yields of resulting products on holding time with heating rates of 200 °C/min (denoted by full line) and 400 °C/min (denoted by broken line) under the conditions of pyrolysis temperature of 500 °C, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C.

of 500–800 °C was not fast as that in the ranges of 400– 500 °C (i.e., 47.06–84.22% vs. 30.70–47.06%). On the other hand, the oil yield also increased in the rages of the pyrolysis temperature. As expected, the oil yield signiWcantly increased as the pyrolysis temperature was raised from 400 to 500 °C (i.e., 11.26% vs. 35.92%). It is well known that

25

maximum liquid yields could be obtained with high heating rates, at reaction temperature around 500 °C and with short vapor residence times to minimize secondary reactions (Bridgwater et al., 1999; Bridgwater and Peacocke, 2000). To determine the eVect of heating rate on the yields of the pyrolysis products, the experiments were conducted at a heating rate of either 100, 200, 300, 400 or 500 °C/min at the Wnal pyrolysis temperature of 500 °C and its holding time of 1 min with a particle size of <0.50 mm, sweep gas Xow rate of 1000 cm3/min and condensation temperature of ¡10 °C. Fig. 3 showed that small variations of yields in the oil and char were observed (Onay et al., 2001; Ates et al., 2004). It could be seen that the maximum yields (ca. 36%) of pyrolysis products were obtained at the heating rate of 200 °C/ min. Also, the increase in the yields of pyrolysis products employing the higher heating rate seems to be negligible. As shown in Fig. 4, it seemed that the holding time at the Wnal pyrolysis temperature of 500 °C and heating rates of 200 and 400 °C/min could play a less important role in the production of pyrolysis oil. It was seen that, by increasing holding time from 1 min to 2 min, the yield of pyrolysis oil was increased as a result of progressive processing of pyrolysis. However, the values thereafter were observed to decrease slightly at longer holding time, which was possibly attributed to the gasiWcation and/or thermal cracking of the pyrolysis products (Tsai et al., 1997). As mentioned in the relevant literature (Onay et al., 2001; Acikgoz et al., 2004; Ates et al., 2004), the yield of pyrolysis oil could be increased because the sweeping gas removed pyrolysis products from the reaction zone to minimize the secondary reaction such as thermal cracking, repolymerization and recondensation of the char residue. The experiments were performed with diVerent nitrogen Xow rates of 500, 1000 or 1500 cm3/min at the Wnal pyrolysis temperature of 500 °C, heating rate of 200 °C/min, holding time of 1 min, particle size of <0.50 mm and condensation temperature of ¡10 °C. When the nitrogen Xow rate was increased from 500 to 1500 cm3/min, the yield of pyrolysis oil reached maximum amount of 36.0% with a slight increase of 0.23%. There was no obvious inXuence on the yield of pyrolysis products at the higher nitrogen Xow rates. The discussion about the eVect of condensation temperature on the yields of pyrolysis products was very scarce in the literature. The experiments were performed with three diVerent condensation temperatures of 0, ¡10 or ¡20 °C using cryogenic system with ethylene glycol/water circulating bath at the Wnal pyrolysis temperature of 500 °C, heating rate of 200 °C/min, holding time of 1 min, particle size of <0.50 mm and nitrogen Xow rate of 1000 cm3/min. The results found that the condensation temperature smaller than ¡10 °C had no signiWcant eVect on the yield of pyrolysis oil, indicating that the minimum condensation temperature for eVectively collecting gas products in the fast pyrolysis was set at ¡10 °C. Experiments were conducted with particle sizes from 0.125 mm to 0.50 mm at the Wnal pyrolysis temperature of 500 °C, heating rate of 200 °C/min, holding time of 1 min,

26

W.T. Tsai et al. / Bioresource Technology 98 (2007) 22–28

nitrogen Xow rate of 1000 cm3/min and condensation temperatures of ¡10 °C. Clearly, the oil yield had an increasing trend as the particle size increased from 0.42–0.50 mm to 0.125–0.177 mm. Also, the rate of increase in the ranges of 0.125–0.250 mm was not fast as that in the ranges of 0.250– 0.50 mm (i.e., 37.12–38.23% vs. 21.44–37.12%). However, the char yield signiWcantly decreased with the particle size from 0.50 mm to 0.125 mm (i.e., 55.94% vs. 44.48%). Particle size was known to inXuence the yields of pyrolysis products (Onay et al., 2001; Shinogi and Kanri, 2003; Acikgoz et al., 2004; Ates et al., 2004). This result suggested that mass- and heat-transfer restrictions had a profound inXuence at a larger particle size, resulting in minimum oil yield. 3.2. Chemical characterization The liquids obtained in the fast pyrolysis of rice husk, which are usually termed pyrolysis oils, are red-brown-colored products with irritative odor (Bridgwater et al., 1999). The caloriWc values and pH of the pyrolysis oils obtained at a typical pyrolysis conditions were presented in Table 2. The results listed in Table 2 also presented the caloriWc values of the pyrolysis char. It could be seen that the caloriWc values (i.e., 1546–1820 kcal/kg) of the pyrolysis oils were not as high as those of commercial heating oils (6000– 9000 kcal/kg). However, the caloriWc values (i.e., 4531– 5089 kcal/kg) of the pyrolysis char were rather high, even higher than that of rice husk listed in Table 1. It was obvious that considerable amounts of carbon organized in the combustible matters of rice husk still presented in the char, not decomposed to produce condensable components in the fast pyrolysis system. Further, the water contents in the pyrolysis oils resulting from the original moisture in the feedstock and as a product of the dehydration reactions occurring during pyrolysis could be high up to 30% (Czernik and Bridgwater, 2004), thus lowering its heating value. Table 2 also showed that the pH values of the pyrolysis oils were in the ranges of 2.3–2.7, which was in agreement with the results in the literature; that is, pyrolysis oil generally contained substantial amounts of organic acids, mostly acetic acid and formic acid, which resulted in a pH of 2–3 (Bridgwater et al., 1999; Czernik and Bridgwater, 2004). The results described above were further demonstrated with the elemental analyses of pyrolysis oils. For example, the carbon, hydrogen and oxygen contents of the pyrolysis

oil obtained at heating rate of 200 °C/min, pyrolysis temperature of 500 °C, holding time of 8 min, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C were 11.38%, 5.80% and 44.88%, respectively. Obviously, the pyrolysis oil contained a considerable amount of oxygen content with a higher H/C molar ratio (*6.1) than the rice husk (*1.5, Table 1) and conventional fuels (*2.0), resulting in the low energy content of the pyrolysis oil listed in Table 2. The FTIR spectrum of the pyrolysis oil from the fast pyrolysis of rice husk showed that the presence of the signiWcant peaks between 1640 and 1700 cm¡1 could be ascribable to CBO (carbonyl) stretching vibration indicative of the ketones, phenols, carboxylic acids or aldehydes, and/or represent CBC stretching vibrations indicative of alkenes and aromatics (Williams and Williams, 1997; Putun et al., 1999; Scholze and Meier, 2000; Onay et al., 2001; Beis et al., 2002; Acikgoz et al., 2004; Ates et al., 2004). The peak at around 1517 cm¡1 could be the cause of CBC stretching vibrations indicative of alkenes and aromatics (Gomez-Serrano et al., 1996; Beis et al., 2002; Acikgoz et al., 2004). Below 1500 cm¡1 all bands were complex and had their origin in a variety of vibrational modes. C–H stretching and bending vibrations between 1380 and 1465 cm¡1 indicated the presence of alkane groups in pyrolysis oils derived from biomass. Therefore, the band at 1395 cm¡1 was ascribable to bending vibrations for CH3 groups (Putun et al., 1999). Absorptions possibly due to C–O vibrations from carbonyl components (i.e., alcohols, esters, carboxylic acids or ethers) occur between 1300 and 900 cm¡1 (Gomez-Serrano et al., 1996). Notably, the band at 1278 may be connected with –C–O–C– (e.g., ethers) (Gomez-Serrano et al., 1996; Putun et al., 1999). In the study of this spectral region described above, FTIR analysis might be used as a fast screen technique to observe the extent of carbonyl group or oxygen content. Furthermore, some features in the elemental, caloriWc, pH and FTIR analyses could be further demonstrated with the GC/ MS analysis. GC–MS analysis was carried out with typical pyrolysis oil in order to get an idea of the nature and type of compounds in the pyrolysis oils. Due to the rather similar peak patterns of chromatograms of the pyrolysis oils obtained at the other pyrolysis parameters, Table 3 listed the tentative compounds, which were the most probable compounds identiWed by the MS search Wle. Clearly, the pyrolysis oils

Table 2 CaloriWc values and pH of the pyrolysis products Products codea

Heating rate (°C/min)

Temperature (°C)

Holding time (min)

Pyrolysis oil CaloriWc value (kcal/kg)

pH

CaloriWc value (kcal/kg)

FP-01 FP-02 FP-04 FP-08 FP-12

200 200 200 400 200

400 500 500 500 500

1 1 1 1 8

1546 1632 1765 1820 1767

2.31 2.45 2.53 2.48 2.69

4635 4531 4886 4923 5089

Pyrolysis char

a All pyrolysis products were derived from the same conditions: particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C.

W.T. Tsai et al. / Bioresource Technology 98 (2007) 22–28

27

sis oils previously presented in Tables 2 and 3, that showed high oxygen contents and low heating values. For a preliminary study, a corresponding GC–MS of pyrolysis gas sample obtained at the same conditions as the pyrolysis oil in the GC–MS analysis was also performed in order to investigate the presence of condensable compounds in the vent gas. Concerning the compositions of pyrolysis gas, it was often rich in carbon monoxide (CO), carbon dioxide (CO2) and other C1–C4 components (Williams and Williams, 1997; Williams and Nugranad, 2000). As expected and depicted, the pyrolysis gas produced in this work still contained a comparatively small amount of condensable organic compounds such as aromatic hydrocarbons, which were tentatively presented in Table 4. This result explained

were such an unknown and complex mixture of organic compounds that no calibration of the MS detector was set, mainly due to the lack of an appropriate standard mixture for calibration (Torres et al., 2000; Laresgoiti et al., 2004). The empirical formulae and their molecular weights were also included in Table 3. In view of the results presented in Table 3, it could be seen that, as expected, the pyrolysis oils were a very complex mixture of organic compounds and contained a lot of aromatics and oxygenated compounds such as carboxylic acids, phenols, ketones etc. The presence of these aromatic and oxygenated compounds was attributable to its biopolymer textures such as cellulose and hemicellulose. These results were consistent with the elemental compositions and chemical characterization of the pyrolyTable 3 Tentative GC/MS characterization of the pyrolysis oila Peak no.

tR (min)

Tentative assignment

Empirical formula

Molecular weight

% Area

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

14.4 21.8 23.1 24.5 25.7 27.8 29.7 32.1 32.7 34.3 35.2 35.5 35.9 37.2 38.2 39.2 40.0 41.0 41.7 44.0 46.1 49.1 50.1

Acetic acid Furan, 2,5-dimethyl2-Furanmethanol 2H-Pyran, 3,4-dihydro 2(3H)-Furanone, 5-methylPhenol 1,2-Cyclopentanedione-3-methylPhenol, 2-methoxy Maltol Phenol, 4-ethyl1,2-Benzenediol Phenol, 2-methoxy-4-methylBenzofuran, 2,3-dihydro1,2-Benzenediol, 3-methylPhenol, 4-ethyl-2-methoxy4-Hydroxy-3-methylacetophenone Phenol, 2,6-dimethoxy 1,3-Benzenediol, 4-ethylBenzaldehyde, 3-hydroxy-4-methoxy Phenol, 2-methoxy-4-(1-propenyl)2-propanone, 1-(4-hydroxy-3-methoxypheny)Benzeneacetic acid, 4-hydroxy-3-methoxyPhenol, 2,6-dimethoxy-4-(2-propenyl)-

C2H4O2 C6H8O C5H6O2 C5H8O C5H6O2 C6H6O C6H8O2 C7H8O2 C6H6O3 C7H10O2 C6H6O2 C8H10O2 C8H8O C7H8O2 C9H12O2 C9H10O2 C8H10O3 C8H10O2 C8H8O3 C10H12O2 C10H12O3 C9H10O4 C11H14O3

60 96 98 84 98 94 112 124 126 126 110 138 120 124 152 150 154 138 152 164 180 182 194

18.5 3.7 2.8 1.3 1.4 2.7 1.8 3.5 1.0 2.5 4.1 1.0 6.8 2.0 1.9 2.2 1.6 0.8 0.9 1.6 1.2 0.4 0.3

Total

64.0

Obtained at heating rate of 400 °C/min, pyrolysis temperature of 500 °C, holding time of 1 min, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C. a

Table 4 Tentative GC/MS characterization of the pyrolysis gasa Peak no.

tR (min)

Tentative assignment

Empirical formula

Molecular weight

% Area

1 2 3 4 5 6 7 8

10.2 13.4 16.0 17.8 19.8 22.4 22.6 23.0

1,3-Cyclopentadiene Furan, 2-methylBenzene Furan, 2,5-dimethylToluene Ethylbenzene p-Xylene Styrene

C5H6 C5H6O C6H6 C6H8O C7H8 C8H10 C8H10 C8H8

66 82 78 96 92 106 106 104

2.8 10.8 6.2 0.9 3.9 0.3 0.3 0.3

Total

25.5b

a Obtained at heating rate of 200 °C/min, pyrolysis temperature of 500 °C, holding time of 1 min, particle size of <0.50 mm, nitrogen Xow rate of 1000 cm3/min, and condensation temperature of ¡10 °C. b Low percentage of total chromatographic area is due to the dilution solvent (i.e., methyl chloride), which was merged from the Wrst chromatographic peak.

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why at the pyrolysis conditions total yields of pyrolysis oil and char were obtained at 75–95%, since at the pyrolysis conditions, all non-condensable and few condensable products derived from the fast pyrolysis of rice husk were vented, resulting in the diYculty with the capture in the present cryogenic system. Further studies on the GC–MS analyses of the resulting pyrolysis oils and gases would be helpful to elucidate the inXuence of pyrolysis parameters on chemical compositions of pyrolysis products. 4. Conclusion In this study, fast pyrolysis experiments of rice husk were carried out in a Wx-bed induction heating system. The optimal yield (>40%) of pyrolysis oil could be achieved at the pyrolysis temperature of >500 °C, heating rate of >200 °C/ min, holding time of >2 min, condensation temperature of <¡10 °C and particle size of <0.50 mm. Moreover the eVect of sweeping gas (i.e., nitrogen) Xow rate on the yield of pyrolysis oil was not seen. All analytical results of the resulting products including oils and/or chars analyzed by elemental analyzer, pH meter and bomb calorimeter were consistent with FTIR spectroscopy and GC–MS. It was conWrmed that the pyrolysis oils contain complex compounds mostly composed of aromatic and carbonyl structures, resulting in low pH and low heating values. Obviously, the highly oxygenated oils should need to be upgraded in order to raise their heating values when they are used as industrial fuels. Acknowledgement This research was partly supported by NSC (National Science Council), Taiwan, under Contract number NSC 922623-7-041-001-ET. References Acikgoz, C., Onay, O., Kockar, O.M., 2004. Fast pyrolysis of linseed: product yields and compositions. J. Anal. Appl. Pyrol. 71, 417–429. Ates, F., Putun, E., Putun, A.E., 2004. Fast pyrolysis of sesame stalk: yields and structural analysis of bio-oil. J. Anal. Appl. Pyrol. 71, 779–790. Beis, S.H., Onay, O., Kockar, O.M., 2002. Fixed-bed pyrolysis of saZower seed: inXuence of pyrolysis parameters on product yields and compositions. Renew. Energy 26, 21–32. Bridgwater, A.V., Meier, D., Radlein, D., 1999. An overview of fast pyrolysis of biomass. Org. Geochem. 30, 1479–1493. Bridgwater, A.V., Peacocke, G.V.C., 2000. Fast pyrolysis process for biomass. Renew. Sustain. Energy Rev. 4, 1–73. Council of Agriculture (COA), 2002. Agricultural Statistics Yearbook. COA, Taipei, Taiwan.

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