Fast pyrolysis of rice husk under different reaction conditions

Fast pyrolysis of rice husk under different reaction conditions

Journal of Industrial and Engineering Chemistry 16 (2010) 27–31 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chem...

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Journal of Industrial and Engineering Chemistry 16 (2010) 27–31

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Fast pyrolysis of rice husk under different reaction conditions Hyeon Su Heo a, Hyun Ju Park a, Jong-In Dong a, Sung Hoon Park b,**, Seungdo Kim c, Dong Jin Suh d, Young-Woong Suh d, Seung-Soo Kim e, Young-Kwon Park a,* a

Faculty of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea Department of Environmental Engineering, Sunchon National University, Suncheon 540-742, Republic of Korea c Department of Environmental Sciences and Biotechnology, Hallym University, Chuncheon 200-702, Republic of Korea d Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea e Department of Chemical Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 July 2009 Accepted 27 August 2009

In this work, rice husk, an agricultural waste in Korea, was pyrolyzed under different reaction conditions (temperature, flow rate, feed rate, and fluidizing medium) in a fluidized bed with the influence of reaction conditions upon characteristics of the bio-oil studied. The optimal pyrolysis temperature for bio-oil production was found to be between 400 and 450 8C. Higher flow rates and feeding rates were more effective for its production. The use of the product gas as the fluidizing medium led to the highest bio-oil yield. With the exception of temperature, no single operation variable largely affected the physicochemical properties of the bio-oil. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Keywords: Bio-oil Rice husk Fast pyrolysis Fluidized bed

1. Introduction Since biomass is considered a promising energy resource, many developing countries have been interested in the utilization of agricultural residues such as bagasse [1], rice straw [2], rice husks [3], wheat straw [4], corncobs [5–7], corn stalks [8], and sunflower residue [9]. Rice husk from the agricultural industries is an abundant biomass resource in Korea that has typically been treated using traditional methods such as composting and incineration. However, this is not suitable to process these organic solid wastes as they contain small concentrations of nitrogen for composting and a considerable amount of solid grains that would generate smoke into the environment during incineration [3]. Therefore, more efficient treatment methods satisfying both energy conversion and wastes disposal are required. Amongst the thermochemical conversion processes of biomass (pyrolysis, gasification, and combustion), the pyrolytic process is recognized as the most promising since it can be used either as an independent process for fuels and other valuable chemical products or an initial step to gasification or combustion [10–17]. In this work, the pyrolysis characteristics of rice husk were investigated under different reaction conditions, temperature, flow

* Corresponding author. Tel.: +82 2 2210 5623; fax: +82 2 2244 2245. ** Corresponding author. Tel.: +82 61 750 3816; fax: +82 61 750 3816. E-mail addresses: [email protected] (S.H. Park), [email protected] (Y.-K. Park).

rate, feed rate, and fluidizing medium, and their effect upon product distribution, as well as examination of the characteristics of the bio-oil in a fluidized bed reactor. 2. Experimental 2.1. Rice husk Rice husk used in the experiments was supplied from a rice mill in Jeonnam, Korea. Rice husk was dried in an oven at 110 8C for 24 h to reduce the amount of water in the oil product. After the drying procedure, the water content in rice husk was <1 wt%. The size of rice husk was 8–10 mm long, 2.0–2.5 mm wide and 0.1–0.15 mm thick. Table 1 lists the physicochemical properties of rice husk. Ash content of rice husk was higher than other woody biomass with that within 1 wt%, suggesting that higher ash content can adversely affect the bio-oil production. 2.2. Fluidized bed reactor The fast pyrolysis of rice husk was carried out in a fluidized bed reactor. Fig. 1 shows a schematic diagram of the pyrolysis apparatus. The reactor was made from SUS 306 stainless-steel pipe, and its internal diameter and height were 80 and 300 mm, respectively. The main reactor and feed gas were heated electrically. Electrical heating tape, which could be heated to 450 8C, was used to avoid vapor condensation in the product gas stream tube. The condensable bio-oil was collected in a series of

1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.01.026

H.S. Heo et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 27–31

28 Table 1 Physicochemical characteristics of rice husk. Elemental analysisa

Proximate analysis

Component

Content (wt%)

Component

C H N Ob S

53.5 7.0 1.4 38.1 –

Moisture

a b

2.4. Product analysis

Content (wt%) 9.3

Combustibles

78.8

Ash

11.9

On dry and ash free basis. Calculated by difference.

glass condensers, cooled to a temperature of 25 8C using a circulator (RW-2025G, JEIO TECH) with ethyl alcohol used as the cooling solvent. The non-condensable vapors passing through the condensation system were recovered in the form of tar in an electrostatic precipitator. The product gas passing through the electrostatic precipitator was sampled in Teflon gas bags at 20 min intervals to analyze their composition. 2.3. Pyrolysis conditions The experiments were carried out at a gas velocity above the minimum-bubbling fluidizing velocity (Umb). The minimum fluidizing velocity was approximately 0.6 cm/s. Emery [Al2O3 (NANKO ABRASIVES, Japan)], 1000 g, with mean particle size of 40 mm was used as the bed material. The reactor system was purged with inert nitrogen gas, which was also used as the fluidizing medium, for approximately 3 h before commencing the experiments. In order to decrease the heat loss during the experiments, the fluidizing gas was preheated to 350 8C, and introduced to the pyrolysis reactor. The temperature of the experimental system was adjusted using a PID temperature controller, and monitored with two K-type thermocouples. The errors in the average reaction temperature were within 5 8C. The input of rice husk to the experiments was approximately 150 g, and rice husk was fed continuously into the bed using a screw feeder at feeding rates ranging from 1.5 to 2.5 g/min. Table 2 lists the different pyrolysis conditions for each experiment.

The bio-oil produced was in a heterogeneous state. However, both chemical and physical analyses of the bio-oil were required in the homogeneous state. After sufficient stirring, two bio-oil samples were taken from the 10–20 vol.% parts of the top and bottom sections, respectively. The mean values of the individual results were taken in triplicate for each sample. Since accurate quantitative analyses of bio-oil are difficult to obtain using the existing gas chromatography equipment, the area % of a GC–MS chromatogram was considered to be good approximation because this indicates the amount of various chemical compounds in the bio-oil [10–17]. In this study, both quantitative and qualitative analyses of the bio-oil were performed using GC–MS (HP 5973 inert) with a HP-5MS (30 m  0.25 mm  0.25 mm) capillary column. Helium was used as the carrier gas. The mass spectra obtained by GC–MS were interpreted based on an automatic library search (Wiley 7n). The solid content of the bio-oil was defined as the acetone-insoluble material retained on a filter (Millipore filtration, 0.1 mm pore size, o.d. 37 mm). The ash content of the bio-oil was calculated by measuring the residue after igniting the bio-oil in a muffle furnace (J-FM2, JISICO) at 850 8C overnight. The higher heating value (Parr, Model 1261) of the biooil was measured using the KS M 2057 method. The water content of the bio-oil was measured using the ASTM E 203 method. A Karl Fischer titrator (Metrohm 787 KF Titrino) was used, and HYDRANAL Composite 5 K (Riedel-de Haen) and HYDRANAL Working Medium K (Riedel-de Haen) were used as the titration reagent and titration solvent, respectively. The accuracy of the above analyses was <1%. The pyrolysis gases were analyzed using GC-TCD and GC-FID (ACME 6000, Young Lin Instrument Co., Ltd.), with Carboxen 1000 (15 ft  1/8 in.) and HP-plot Al2O3/KCl columns, respectively. 3. Results and discussion 3.1. Influence of reaction conditions on product distribution The rice husk was pyrolyzed under different reaction conditions. Pyrolysis temperature is one of the most important

Fig. 1. Schematic diagram of fast pyrolysis apparatus.

H.S. Heo et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 27–31

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Table 2 Pyrolysis conditions.

Reaction temperature (8C) Fluidizing mediuma Gas flow rate (L/min) Feed rate (g/min) Input (g) a

Run 1

Run 2

Run 3

Run 4

Run 5

Run 6

Run 7

Run 8

400 N 5 2.5 150

450 N 5 2.5 150

500 N 5 2.5 150

550 N 5 2.5 150

450 N 3 2.5 150

450 N 4 2.5 150

450 N 5 1.5 150

450 P 5 2.5 150

N: nitrogen, P: product gas.

parameters affecting the products. The product distribution at different pyrolysis temperatures is shown in Fig. 2. The bio-oil yield was maximized in the pyrolysis temperature range of 400– 450 8C, but decreased with increasing temperature. The gas yields increased with increasing pyrolysis temperature. As the pyrolysis temperature was raised to 550 8C, the char yields decreased. Conversely, the higher gas yields at higher temperatures are ascribed to the secondary cracking of the pyrolysis vapors and char into gas, depending on the pyrolysis temperature. The product distribution under different flow rates is given in Fig. 3. Under fast pyrolysis, volatile residence time is a very important factor that affects yields of gaseous and liquid products. At lower gas flow rates, fluidization must be less vigorous and the

Fig. 2. Product distribution under different pyrolysis temperatures: a flow rate of 5 L/min, feeding rate of 2.5 g/min and a nitrogen atmosphere.

Fig. 3. Product distribution under different flow rates: a pyrolysis temperature of 450 8C, feeding rate of 2.5 g/min and a nitrogen atmosphere.

residence time of the gas and vapor in the reactor somewhat extended, hence the possibility of secondary reactions such as thermal cracking, repolymerization, and recondensation, that lead to a decrease in bio-oil yield. The above possible phenomena thoroughly explain the increase in bio-oil yield at high gas flow rates, whereby higher fluidizing gas flow minimizes the possibility of the secondary reactions. The product distribution under different feed rates is shown in Fig. 4. A fluidized bed is generally separated into 2 beds, dense and suspension, due to the segregation effect. Since the temperature of the dense bed is higher than that of the suspension bed, a higher feeding rate, which will enhance gas flow, can efficiently prevent pyrolysis vapors from being converted into gas via secondary cracking in the dense bed, resulting in an increased bio-oil yield [10–13,18]. This was in good agreement with the results found in this study. As shown in Fig. 4, the bio-oil yield was found to increase with increasing feed rate. As shown in Fig. 5, the fluidizing medium had a significant effect on the level of bio-oil production with the yield increasing up to 60 wt% when the product gas evolved during pyrolysis was used as the fluidizing medium. This suggests that the pyrolysis system recycling the product gas can be more effective in the production of bio-oil than the fluidizing medium of 100% nitrogen. In this work, the bio-oil yields were relatively lower than those from the other woody biomass. This is most likely due to the catalytic cracking of vapors by the higher ash content of rice husk than other woody biomass. Indeed, ash had a significant effect on the biomass pyrolysis behavior than the degree of crystallinity and polymerization [19]. In addition, some elements in ash can catalyze the pyrolysis behavior and reduce the bio-oil yield [20]. These tendencies are in good agreement with Luo et al.’s [18] results. The introduction of the appropriate pretreatment method for the removal of ash appears to help the bio-oil produce more.

Fig. 4. Product distribution under different feed rates: a pyrolysis temperature of 450 8C, flow rate of 5 L/min and a nitrogen atmosphere.

H.S. Heo et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 27–31

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Table 4 Main compounds identified in bio-oil obtained through the experiment Run 8.

Fig. 5. Product distribution under different fluidizing medium: a pyrolysis temperature of 450 8C, feeding rate of 2.5 g/min and flow rate of 5 L/min.

3.2. Characteristics of products Physicochemical properties of the bio-oil produced from the experiment under the optimal pyrolysis condition are listed in Table 3. These results were similar to the values typically reported in the literature [10–13]. A higher heating value is comparable to oxygenated fuel such as ethanol, methanol, and coal [21,22]. The pyrolysis temperature was the most important parameter affecting the chemical composition of the bio-oil. Water contents in the biooil were significantly affected by the pyrolysis temperature and increased from 25 to 57 wt% with increasing reaction temperature. However, the water content in the bio-oils produced under other experiment conditions were all within 25–28 wt%. Table 4 shows the chemical composition of the bio-oil obtained from experimental Run 8. The major compounds in the bio-oil were phenolics, including phenol, cresols, guaiacols, and benzendiols, as well as acetic acid and ketones. After fractioning, such compounds may be used as a feedstock for the production of useful chemicals. In addition, aromatic hydrocarbons, such as benzene derivatives, were observed to be formed only at 550 8C (Table 5), suggesting that a temperature above 550 8C is favorable in successive aromatization reactions, such as cracking of the light organics, oligomerization, cyclization, hydrogen or hydride transfer, and aromatization. However, except for the pyrolysis temperature, other pyrolysis conditions did not significantly affect the chemical composition of the bio-oil. The ultimate analysis of the char obtained from the experiment under optimal pyrolysis condition is listed in Table 6, which indicated that the char is a carbon-rich fuel.

Compound

Area (%)

Acetic acid Propanoic acid 2-Cyclopenten-1-one 2,5-Dimethyl-furan 2-Methyl-2-cyclopenten-1-one 3-Methyl-2-cyclopenten-1-one 1-(2-Furanyl)-ethanone 2-Hydroxy-3-methyl-2-cyclopenten-1-one 2,3-Dimethyl-2-cyclopenten-1-one (10 -Propenyl)thiophene Levoglucosan Hexadecanoic acid Phenol 2-Methyl-phenol 4-Methyl-phenol 2-Methoxy-phenol 2,6-Dimethyl-phenol 2,4-Dimethyl-phenol 3,5-Dimethyl-phenol 2-Ethyl-phenol 4-Ethyl-phenol 3-Ethyl-phenol 3-Methoxy-2-methylphenol 1,2-Benzenediol 4-Vinylphenol 4-Ethyl-3-methyl-phenol 4-(2-Propenyl)-phenol 4-Methyl-1,2-benzenediol 2-Methyl-6-(2-propenyl)-phenol

3.3 1.7 0.7 2.3 0.0 0.7 0.2 0.9 0.3 0.1 9.4 0.1 3.0 1.4 3.1 0.7 0.6 1.2 0.5 0.4 3.0 0.1 0.2 5.1 5.5 0.7 1.1 1.8 0.8

Table 5 Hydrocarbons identified in bio-oil obtained through the experiment Run 4. Compound

Area (%)

Benzene Toluene Styrene p-Xylene

0.2 0.6 0.2 0.1

Table 6 Ultimate analysis of char obtained through the experiment Run 8. Ultimate analysis (wt%) C H Oa N S a

82.0 3.2 14.1 0.7 –

Calculated by difference.

4. Conclusions Table 3 Physicochemcial properties of bio-oil obtained through the experiment Run 8. Ultimate analysis (wt%)a C H Ob N S

55.1 7.2 37.0 0.7 –

Solid content (wt%) Water content (wt%) HHV (MJ/kg)

0.1 25.2 24.8

a b

On dry basis. Calculated by difference.

The optimal reaction conditions for the production of bio-oil were investigated using a fluidized bed. The influence of the reaction conditions on the chemical characteristics of the obtained bio-oils was also examined. The optimal pyrolysis temperature for bio-oil production was between 400 and 450 8C. Higher gas flows and higher biomass feed rates were more favorable for the production of the bio-oil, but did not significantly affect bio-oil yield. The use of the product gas as the fluidizing medium was most effective for bio-oil production, leading to the highest bio-oil yield of approximately 60 wt%. With the exception of the pyrolysis temperature, the other pyrolysis operating conditions did not significantly affect the chemical characteristics of the bio-oil.

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