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Slow pyrolysis of pistachio shell
Fuel 86 (2007) 1892–1899 www.fuelfirst.com
Slow pyrolysis of pistachio shell Esin Apaydin-Varol a, Ersan Pu¨tu¨n b
b,*
, Aysße E. Pu¨tu¨n
a
a Department of Chemical Engineering, Anadolu University, 26555, Eskisßehir, Turkey Department of Material Science and Engineering, Anadolu University, 26555 Eskisßehir, Turkey
Received 14 June 2006; received in revised form 29 November 2006; accepted 30 November 2006 Available online 2 January 2007
Abstract In this study, pistachio shell is taken as the biomass sample to investigate the effects of pyrolysis temperature on the product yields and composition when slow pyrolysis is applied in a fixed-bed reactor at atmospheric pressure to the temperatures of 300, 400, 500, 550, 700 °C. The maximum liquid yield was attained at about 500–550 °C with a yield of 20.5%. The liquid product obtained under this optimum temperature and solid products obtained at all temperatures were characterized. As well as proximate and elemental analysis for the products were the basic steps for characterization, column chromatography, FT-IR, GC/MS and SEM were used for further characterization. The results showed that liquid and solid products from pistachio shells show similarities with high value conventional fuels. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Pistachio shell pyrolysis; Bio-oil characterization; Char characterization
1. Introduction The disposal of solid wastes such as biomass, industrial and municipal wastes is one of the main problems of the world and it is necessary to find out new ways to reuse this great potential as raw materials to produce advantageous products. Pyrolysis is one of the primary thermochemical conversion methods to convert biomass into valuable products, namely; solid char, liquid and gas product yields and compositions of which depend on pyrolysis conditions [1– 3]. Solid product, char, can be used as a fuel either directly as briquettes or as char–oil or char–water slurries since it has a high calorific value or it can be used as feedstocks to prepare activated carbons [4,5]. The liquid product, pyrolytic oil, approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons. It is useful as a fuel, may be added to petroleum refinery feedstocks or upgraded by catalysts to produce premium grade refined fuels, or may have a *
Corresponding author. Tel.: +90 222 33350580/6350; fax: +90 222 3239501. E-mail address: [email protected] (E. Pu¨tu¨n). 0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.11.041
potential use as chemical feedstocks. Bio-oils are generally preferred products because of their high calorific values, their ease of transportation and storage, their low nitrogen and sulphur content and their opportunity to be converted into chemicals. The third product gas having a high calorific value may also be used as a fuel [1,4–7]. There are many factors affecting pyrolysis product yields such as pyrolysis atmosphere, final temperature, particle size, heating rate, reactor type and initial amount of the sample. Liquid product yield and quality mostly depends on the pyrolysis temperature. It is known from previous studies that higher final temperatures (600 °C and above) favour gas formation, and relatively lower temperatures (400 °C and below) favour char formation. When bio-oil is the target product from pyrolysis an optimum temperature must be determined [1,7–9]. Mohan et al. reviewed pyrolysis of wood/biomass for bio-oil and stated that bio-oil has several advantages over fossil fuels from the environmental point of view [10]. Heating rate is also an important parameter for product yields. Conventional pyrolysis, in other words slow pyrolysis, is applied for thousands of years when charcoal production is aimed. Generally equal amounts of products are formed when
E. Apaydin-Varol et al. / Fuel 86 (2007) 1892–1899
biomass is pyrolsed slowly. A food industry waste, apricot pulp, was pyrolysed in a fixed bed reactor with a heating ¨ zbay et al. [11]. Bio-oil yield reached rate of 5 °C/min by O a maximum value of 23.2% at pyrolysis temperature of 550 °C when nitrogen gas was used as the sweeping gas. Much faster heating rates are preferred for fast pyrolysis and higher yields of bio-oil is produced. Tsai et al., studied fast pyrolysis of three different biomass wastes including rice straw, sugarcane bagasse and coconut shells under different conditions and they found that pyrolysis temperatures at about 500 °C, heating rates about 200 °C/min and holding times about 2 min gave the highest tar yield of 50% for sugarcane bagasse [12]. In this study, pistachio shell has taken as the biomass sample. Pistachio is cultivated mostly in Asia, The United States, Mediterranean Countries; Iraq, Iran and Middle Asian countries being dominant. South eastern part of Turkey has also a great potential with an increasing yield year by year. For the last few years average cultivation yield was 600,000 tons/year in Turkey [13,14]. The objectives of this study are: (i) determine the effect of pyrolysis temperature on product yields, (ii) characterize liquid product obtained under optimum pyrolysis conditions to detect if it can be used instead of conventional fossil fuels or chemical feedstocks, and (iii) characterize solid products, chars, for their possible use of solid fuels.
2. Method 2.1. Raw material Pistachio shell (PS) samples which have been taken from Southeast Anatolia were used as the raw material for the pyrolysis experiments. Air-dried shells were ground to obtain a uniform material of an average particle size (1.82 mm). The average bulk density of this raw material was found to be 750 kg/m3 [ASTM E873-82]. The proximate analyses of the raw material having average particle size were performed according to ASTM procedures and the moisture [ASTM D2016], ash [ASTM D1102-84] and volatile matter [ASTM E872-82] contents were found to be 7.39, 1.34 and 76.93 wt.%, respectively. The rest is calculated to be the percentage of fixed carbon as 14.34. Cellulose, hemicellulose and lignin are the main components of all biomass materials [1,15,16]. Experiments which were performed according to TSE (Turkish Standards Institute) and ASTM methods showed that pistachio shells contain 60.62% cellulose and 12.80% acid-insoluble lignin [TS 4431, ASTM D1106-84]. Hexane solubles of PS correspond to oil content that was 2.44 wt.%. Ultimate analysis was performed on PS with a Carlo Erba, EA 1108 Elemental Analyser to determine the weight fractions of carbon (49.98%), hydrogen (6.16%) and nitrogen (1.59%), and the weight fraction of oxygen (42.27%) was calculated by the difference. Results showed that PS contains very negligible sulfur.
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The thermal behaviour of PS to 850 °C was studied using Linseis Thermowaage L 81 thermogravimetric analyzer. FT-IR spectrum of PS was recorded to have further information on the chemical structure of the raw material in the near infrared region 4000–4001/cm by preparing KBr pellet. 2.2. Pyrolysis experiments The pyrolysis experiments were performed in a fixed-bed reactor, details were given in the previous studies [17,18]. After reaching the final pyrolysis temperature the reactor was set to cool to room temperature. Pyrolysis product yields were determined gravimetrically by weighing the three products. The liquid phase was collected in cold traps maintained at about 0 °C using salty ice. The liquid phase consisted of aqueous and oil phases were separated and weighed. Solid product, char, was removed from the reactor and weighed. Flow of gas released was controlled using a soap film for the duration of experiments. The gas yield was calculated by the difference. Pyrolysis of PS was carried out to investigate the effect of pyrolysis temperature on the product yields and quality of the liquid and solid products. For the experiments; 20 g sample of PS was placed into the reactor and the experiments were carried out with a heating rate of 7 °C/min to the final temperatures of 300, 400, 500, 550 and 700 °C and held for either a minimum of 30 min or until no further significant release of gas was observed; i.e. the total time for an experiment held at 550 °C was about 100 min. All the yields are expressed on a dry, ash-free basis and were the average yield of at least three with an experimental measurement error of less than ±0.5%. 2.3. Bio-oil characterization Bio-oil obtained at the pyrolysis final temperature that gave maximum oil yield, i.e. optimum temperature, was characterized. Elemental analysis was carried out with Carlo Erba, EA 1108 and the calorific value of the oil was determined. Chemical class compositions of the oil were determined by liquid column chromatographic fractionation. Bio-oil was separated into two fractions according to its pentane solubility. Silica-gel that was pre-treated at 105 °C for 2 h prior to use was the packing material for the column. Pentane soluble materials, were further separated into aliphatic, aromatic and polar fractions using 200 ml of each pentane, toluene and methanol, respectively. Each fraction was dried and weighed. The FT-IR spectra of the oil and its aliphatic, aromatic and polar subfractions were recorded using a Bruker Tensor 27 FT-IR analyser. GC–MS analysis of the aliphatic subfraction was performed using a Hewlett–Packard 6890 Model gas chromatograph equipped with a 5973 mass selective detector using HP 5 column.
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2.4. Char characterization Proximate analysis was applied to the chars obtained at five different pyrolysis temperatures to figure out the moisture, ash and volatile contents according to ASTM methods [ASTM D1762-84]. The elemental analyses of the chars were performed with a Carlo Erba 1108 elemental analyzer. The FT-IR spectra were recorded in the transmission mode between 4000 and 400 cm1 using a Bruker Tensor 27 Model Fourier Transform Infrared Spectrometer. Dried KBr was used to prepare pellets from chars. Typically 10 mg of sample at a concentration of 1% in KBr was used. Pure KBr disc was used as the background and for each sample multiple spectra were recorded to obtain highest signal-to-noise ratio. The surface characteristics of the chars were analysed using Scanning Electronic Microscope Jeol Camscan. Raw material and obtained chars were mounted on an aluminum stub using carbon film and coated with a thin layer of gold and palladium using Agar Sputter Coater. 3. Results and discussion 3.1. Raw material The results for the proximate analysis of PS showed that it has lower ash content when compared with other biomass samples [17–19]. Biomass samples having lower inorganic compounds are desired because of their abilities to give valuable solid products having less ash and higher fixed carbon contents after pyrolysis. In this manner, PS seems to be a promising raw material for thermochemical conversions. Calorific value of PS was calculated as about 18 MJ/kg with Du-Long’s formula with the known values of elemental composition [15].
The chemistry of biomass is very complicated. But, generally it is assumed that biomass has three major constituents; hemicellulose, cellulose and lignin according to the mass loss curve. It is known from previous studies that thermal decomposition of hemicellulose and cellulose have the first (150–350 °C) and second weight-loss (275–350 °C) steps for lignocellulosic materials after the initial weight loss (30–150 °C) associated with the moisture loss. However, lignin undergoes gradual decomposition over a wide temperature interval (275–500 °C) [7,16,20,21]. The TG and DTG curves for PS recorded from room temperature to 850 °C with a heating rate of 10 °C/min are shown in Fig. 1. The initial slight mass loss is due to the evaporation of moisture from PS. Second weight loss occurred between 200 and 575 °C corresponds to main pyrolysis process, devolatilisation. After this major weight loss, there is essentially no further loss of weight. The final residue corresponds to ash and fixed carbon. DTG data showed that initial mass loss gives its maximum peak at about 100 °C. Second major weight loss starts at about 175 °C, having its maximum point at 277 °C, finishes at about 350 °C. According to these results, it can be said that since most of the weight loss, related to volatilisation of hydrocarbons, happens at temperatures lower than 600 °C, choosing pyrolysis temperature as 550 °C is appropriate when maximum amount of liquid and gaseous products is aimed. To have information about the chemical structure of the raw material FT-IR spectrum is taken and given in Fig. 8. It shows that chemical structure of PS, being a lignocellulosic material, is made up of different atomic groupings and a large number of functional groups. The broad and flat band at about 3350 cm1 is ascribed to OH stretching vibrations in hydroxyl groups which was attributed to moisture content of the sample. Two very strong bands at 2925 and 2855 cm1 are assigned to C–H stretching
Fig. 1. TG and DTG curves for PS.
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vibrations in CH2 groups and a weak band at 1453 cm1 due to C–H deformation frequencies confirms the presence of these functional groups. The next strong band (1742 cm1) is ascribed to C@O vibrations probably from esters, ketones or aliphatic acids. At 1615 cm1, the very strong band is assigned to either C@C conjugated, aromatic conjugated or C@N conjugated or cyclic double bond vibrations. Skeletal vibrations from (CH3)3–C–R give a strong band at 1246 cm1. The weaker bands between 880 and 600 cm1 are ascribed to aromatic structures [22–24]. 3.2. Pyrolysis experiments It is known from the literature and previous studies that pyrolysis temperature plays an important role on product distribution [17,19]. Product yields for the pyrolysis of pistachio shells with relation to the final temperature (300– 700 °C) when the heating rate was fixed at 7 °C/min are given in Fig. 2. As observed in Fig. 2, the maximum char yield, 28%, is attained at the lowest temperature studied, 300 °C. The yield of char decreases as the temperature is increased, reaching an approximate yield of 23% at 700 °C. As the temperature increases, devolatilasation reactions are favoured and therefore gaseous product yield increase. Temperatures up to 600 °C maximise the production bio-oils and temperatures above 700 °C maximise gaseous products while minimising char formation And temperatures higher than 600 °C favours gas formation. As it is shown, when the final temperature is 300 °C 25% of biomass fed to the reactor is transformed into gases, but this yield increases with temperature, reaching 30% at 700 °C. Pyrolysis of olive residue, soybean cake and rice straw was studied using the same type of reactor and similar values for the increases in gas yields and decreases in the char yield were observed when pyrolysis
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temperature was increased from 400 to 700 °C [17,18,26]. However, type of the reactor and heating rate are also important parameters for the product yields. Ates et al., have studied fast pyrolysis of sesame stalk within a tubular reactor and have found that the char yield was reduced approximately by 17% when pyrolysis temperature was increased from 400 to 700 °C [27]. In this study the target pyrolysis product was the liquid one. Its yield was 15.5% at the pyrolysis temperature of 300 °C, it appeared to go through a maximum of 20.5% at the final temperature between 500 and 550 °C with 550 °C giving a little higher yield. At 700 °C, the oil yield again decreased to a value of 19%. Previous studies with other biomass samples showed that temperatures between 500 and 550 °C give the highest bio-oil yields [5,17,19]. 3.3. Bio-oil characterization Biomass pyrolysis oils contain a very wide range of complex organic chemicals [1,7,10]. For characterization of biooil that obtained under optimum final temperature is taken and column chromatography is applied to separate it into its subfractions. Chemical class fractionation of the oil showing aliphatic, aromatic and polar fractions is given in Fig. 3. Pyrolysis oil consists of approximately 57% n-pentane insolubles, asphaltenes. For the bio-oil, polar fraction is seen to be the dominant one. The elemental compositions of the oil characterized and its subfractions are listed in Table 1. Bio-oil is characterized with a higher calorific value, 30 MJ/kg, than the original biomass sample which indicates that the energy content of the oil is very close to that of petroleum. Containing more carbon and negligible oxygen and having an
60 50 40
Char yield Bio-oil Yield Gas Yield
(%) 30 20 10
30
0 Pentane insolubles
28 26
Pentane solubles
Aliphatics
Aromatics
Polars
Fig. 3. Results for the column chromatography.
(%)
24 22 Table 1 Elemental composition of bio-oil and subfractions (as received)
20 18 16 14 200
300
400 500 600 Temperature (°C)
700
800
Fig. 2. Pyrolysis of pistachio shells at different temperatures, product yields.
Elements
Weight % Bio-oil
Aliphatics
Aromatics
Polars
Carbon Hydrogen Nitrogen Oxygena
67.44 7.82 0.42 24.32
84.10 12.59 0.000 3.31
77.88 8.74 0.37 13.01
62.23 7.16 0.23 30.38
a
By difference.
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between 2800 and 3000 cm1 and C–H deformation vibrations between 1350 and 1475 cm1 indicate the presence of alkanes. The C@O stretching vibrations with absorbance between 1650 and 1750 cm1 indicate the presence of ketones or aldehydes. The absorbance peaks between 1575 and 1675 cm1 and 875 and 950 cm1 represent C@C stretching vibrations, and are indicative of alkenes. A gas liquid chromatogram of the aliphatic subfraction of pentane soluble of bio-oil is shown in Fig. 5. Three types of compounds were identified in the pentane subfraction as normal alkanes, alkenes, and branched hydrocarbons. The straight chain alkanes and alkenes range from C10–C29. When compared with standard diesel, it is obviously seen that the distribution of hydrocarbons for pentane subfraction shows similarities with diesel. 3.4. Char characterization
Fig. 4. FT-IR spectra of bio-oil and its aliphatic, aromatic and polar subfractions.
acceptable H/C ratio, aliphatic subfraction seems to be the most valuable fraction when it is used as a liquid fuel. FT-IR spectra representing functional groups, of the bio-oil at 550 °C and its subfractions are given in Fig. 4. The O–H stretching vibrations between 3200 and 3400 cm1 indicate the presence of phenols and alcohols. It is not surprising that these vibrations do not exist in aliphatic fraction since this fraction contains no highly oxygenated compounds. The C–H stretching vibrations
Many analytical techniques can be applied to characterize the chars, looking for their possible use, i.e. as solid fuels, activated carbons or etc. [5,25,28,29]. In this study chars were characterized to investigate if they can be used as solid fuels or not. For this purpose proximate analysis was applied to the chars. Fig. 6 gives the results for moisture, ash and volatile matter contents of the chars obtained at five different temperatures. As the temperature is raised, ash content is increased slightly. However, the increase in temperatures decreased the volatile matter content due to gasification reactions occurring at higher pyrolysis temperatures. According to ash and volatile matter contents, fixed carbon percentage is increased in a great amount at higher temperatures when compared with those at lower temperatures and raw material. The atomic ratios H/C and O/C are often used to characterize solid fuels [30,31]. The ratio of H/C versus O/C for chars generated at different temperatures is presented in
Fig. 5. GC–MS chromatogram of aliphatic subfraction of bio-oil.
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Moisture Ash Volatiles
15
(%)
10 5 0 300
400
500
550
700
Temperature (°C)
Atomic H:C ratio
Fig. 6. Proximate analyses of chars at different temperatures.
Fig. 8. FT-IR spectra of pistachio shell and its chars obtained at different temperatures.
Pistachio shell
1.4 1.2 1 0.8 0.6
300 400
0.4 0.2
500 550 700
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Atomic O:C ratio
Fig. 7. Van Krevelen diagram for pistachio shell and its chars obtained at different temperatures (°C).
Fig. 7. It is obvious that the H/C and O/C ratios of the char prepared at the highest temperature are much lower than that for raw material and chars produced from low temperature pyrolysis. Van Krevelen diagram for conventional fossil fuels can be found elsewhere [2,3]. The comparison of the chars with conventional fossil fuels shows that all the chars, especially the ones obtained at higher temperatures, are situated in the region that is for the most valuable fossil fuels, anthracite. Average calorific value of the chars is calculated to be 30 MJ/kg using Du Long’s formula with the known values of elemental composition [15]. For the structural analysis, chars obtained at five different temperatures were conducted to FT-IR. The spectra are roughly very similar for the chars, however they differ from PS indicating the effect of thermal treatment. The bands at about 3450–3350 cm1 for all chars represent O–H vibrations in hydroxyl groups. Very weak bands at about 2850 cm1 for PS300 and PS400 are assigned to C–H stretching vibrations in CH2 groups. With increasing pyrolysis temperature, decreasing intensities of the bands between 2000 and 1500 cm1 are ascribed to double bond vibrations for all types of aromatics. It is clear from Fig. 8 that, chars obtained at higher temperatures contain little or no vibration bands at around 2900 cm1 indicating that their aliphaticity is lower than the low temperature char. Previous studies for different biomass samples indi-
cate that carbonization, especially at higher temperatures, increases the aromatic carbon content while decreasing aliphaticity [22,24,29]. Also, the presence of many weak bands in PS300 spectrum that are very similar to PS is a good proof of uncompleted pyrolysis reactions at lower temperatures. SEM micrographs of the raw material and chars are given in Fig. 9. It obvious from the micrographs that surface morphology of PS changed after pyrolysis. The increase in pyrolysis temperature leads to the formation of waves and pores on the surface. SEM analysis indicates that at relatively lower temperatures (300 °C), there is no significant change on the surface since pyrolysis has not yet completed. When temperature is increased to 400– 500 °C porosity increases and after 500 °C small pits occurred on the surface. But, generally, it can be said that porosity of the chars are very low. 4. Conclusion Conventional pyrolysis is known as the slow pyrolysis that gives three main products, the yields of which depend mostly on the pyrolysis conditions [1–3]. Previous studies on pyrolysis of coal, oil shales or different biomass samples have shown that final temperature is the major effect on product yields [1,5,6,17–19]. In this study pistachio shell samples were taken as the biomass for the pyrolysis experiments performed in a fixed-bed reactor with a heating rate of 7 °C/min to five different final temperatures. The aim was to find out the final temperature that gives the maximum bio-oil yield. It was observed that the bio-oil yield is sensitive to pyrolysis temperature in the range of 300–700 °C. Bio-oil yield has showed an increase till the final temperature of 500– 550 °C, and a decrease between the range 550–700 °C.
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Fig. 9. SEM pictures of pistachio shell and its chars obtained at different temperatures.
The maximum yield was achieved as 20.5% at 550 °C. Relatively low pyrolysis temperatures around 400 °C favours char formation. The char yield decreased with an amount of 21% while increasing the pyrolysis temperature from 300 to 700 °C. The opposite was observed for the gas yield, it increased significantly when temperature was increased. Bio-oil obtained under optimum final temperature was then fractionated into chemical classes by column chromatography. The characterization depending on the structure analysis of the oil and fractions have shown that these fractions are quite similar to currently utilised transport fuels and can be utilised as transport fuels or chemical feedstocks.
Although the quantity of char yield decreased with increasing temperature, further studies have showed that quality of this char is much higher than that of obtained at lower temperatures. Characterization of the solid product, have showed that produced chars, having a high fixed carbon content, little ash and high calorific value, can replace the most valuable conventional fossil fuels. References [1] Bridgewater AV, Grassi G. Biomass pyrolysis liquids upgrading and utilization. England: Elsevier Applied Science; 1991. [2] McKendry P. Energy production from biomass (part 1): conversion technologies. Biores Technol 2002;83:37–46.
E. Apaydin-Varol et al. / Fuel 86 (2007) 1892–1899 [3] McKendry P. Energy production from biomass (part 2): conversion technologies. Biores Technol 2002;83:47–54. [4] Suarez-Garcia F, Martinez-Alonso A, Tascon JMD. Pyrolysis of apple pulp: effect of operation conditions and chemical additives. J Anal Appl Pyrolysis 2002;62:93–109. [5] Encinar JM, Gonzalez JF, Gonzalez J. Fixed-bed pyrolysis of Cynara cardunculus L. product yields and compositions. Fuel Process Technol 2000;68:209–22. [6] Williams PT, Besßler S. The pyrolysis of rice husks in a thermogravimetric analyser and static batch reactor. Fuel 1993;72:151–9. [7] Antal MJ. Biomass pyrolysis: a review of the literature part-1carbohydrate pyrolysis. Adv Solar Energ 1983:61–109. [8] Di Blasi C, Signorelli G, Di Russo C, Rea G. Product distribution from pyrolysis of wood and agricultural residues. Ind Eng Chem Res 1999;38:2216–24. [9] Seebauer V, Petek J, Staudinger G. Effects of particle size, heating rate and pressure on measurement of pyrolysis kinetics by thermogravimetric analysis. Fuel 1997;76(13):1277–82. [10] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energ Fuels 2006;20:848–89. ¨ zbay N, Uzun BB, Apaydın Varol E, Pu¨tu¨n AE. Comparative [11] O analysis of pyrolysis oils and its subfractions under different atmospheric conditions. Fuel Process Technol 2006;87:1013–9. [12] Tsai WT, Lee MK, Chang YM. Fast pyrolysis of rice straw, sugarcane bagasse and coconut shell in an induction-heating reactor. J Anal Appl Pyrolysis 2006;76:230–7. [13] Republic of Turkey Ministry Agriculture and Rural Affairs; / istatistikler/istatistikler.htm&curdir=nuretimnistatistikler&fl=uretim_ istatistikleri/Bitkisel_uretim/turkiye/meyve_turkiye.htm. [14] Kashaninejad M, Mortazavi A, Safekordi A, Tabil LG. Some physical properties of Pistachio (Pistacia vera L.) nut and its kernel. J Food Eng 2006;72:30–8. [15] Harker JH, Backhurst JR. Fuel and energy. London: Academic Press Limited; 1981. [16] Ververis C, Georghiou K, Christodoulakis N, Santas P, Santas R. Fiber dimensions, lignin and cellulose content of various plant materials and their suitability for paper production. Ind Crops Prod 2004;19:245–54. [17] Pu¨tu¨n AE, Apaydın E, Pu¨tu¨n E. Bio-oil production from pyrolysis and steam pyrolysis of soybean-cake: product yields and composition. Energy 2002;27:703–13.
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[18] Pu¨tu¨n AE, Apaydın E, Pu¨tu¨n E. Rice straw as a bio-oil source via pyrolysis and steam pyrolysis. Energy 2004;29:2171–80. ¨ zbay N, Pu¨tu¨n AE, Uzun BB, Pu¨tu¨n E. Biocrude from biomass: [19] O pyrolysis of cottonseed cake. Renew Energ 2001;24:615–25. [20] Teng H, Wei YC. Thermogravimetric studies on the kinetics of rice hull pyrolysis and the influence of water treatment. Ind Eng Chem Res 1998;37:3806–11. [21] Fisher T, Hajaligol M, Waymack B, Kellogg D. Pyrolysis behaviour and kinetics of biomass derived materials. J Anal Appl Pyrolysis 2002;62:331–49. [22] El-Hendawy AA. Variation in the FTIR spectra of a biomass under impregnation, carbonization and oxidation conditions. J Anal Appl Pyrolysis 2006;75:159–66. [23] Bellamy LJ. The infra-red spectra of complex molecules. 3rd ed. London: Chapman and Hall Ltd.; 1975. [24] Duran-Valle CJ, Gomez-Corzo M, Pastor-Villegas J, Gomez-Serrano V. Study of cherry stones as raw material in preparation of carbonaceous adsorbents. J Anal Appl Pyrolysis 2005;73:59–67. [25] Vernersson T, Bonelli PR, Cerrella EG, Cukierman AL. Arundo donax cane as a precursor for activated carbons preparation by phosphoric acid activation. Biores Technol 2002;83:95–104. [26] Pu¨tu¨n AE, Uzun BB, Apaydın E, Pu¨tu¨n E. Bio-oil from olive oil industry wastes: pyrolysis of olive residue under different conditions. Fuel Process Technol 2005;87:25–32. [27] Ates F, Pu¨tu¨n E, Pu¨tu¨n AE. Fast pyrolysis of sesame stalk: yields and structural analysis of bio-oil. J Anal Appl Pyrolysis 2004;71:779–90. [28] Cunliffe AM, Williams PT. Influence of process conditions on the rate of activation of chars derived from pyrolysis of used tires. Energ Fuels 1999;13(1):166–75. [29] Sharma RK, Wooten JB, Baliga VL, Lin X, Chan WG, Hajaligol MR. Characterization of chars from pyrolysis of lignin. Fuel 2004;83:1469–82. [30] Van Krevelen DW. Coals typology-physics-chemistry-constitution. The Netherlands: Elsevier; 1993. [31] Lu L, Sahajwalla V, Harris D. Characteristics of chars prepared from various pulverized coals at different temperatures using drop-tube furnace. Energ Fuels 2000;14(4):869–76.
Slow pyrolysis of pistachio shell Esin Apaydin-Varol a, Ersan Pu¨tu¨n b
b,*
, Aysße E. Pu¨tu¨n
a
a Department of Chemical Engineering, Anadolu University, 26555, Eskisßehir, Turkey Department of Material Science and Engineering, Anadolu University, 26555 Eskisßehir, Turkey
Received 14 June 2006; received in revised form 29 November 2006; accepted 30 November 2006 Available online 2 January 2007
Abstract In this study, pistachio shell is taken as the biomass sample to investigate the effects of pyrolysis temperature on the product yields and composition when slow pyrolysis is applied in a fixed-bed reactor at atmospheric pressure to the temperatures of 300, 400, 500, 550, 700 °C. The maximum liquid yield was attained at about 500–550 °C with a yield of 20.5%. The liquid product obtained under this optimum temperature and solid products obtained at all temperatures were characterized. As well as proximate and elemental analysis for the products were the basic steps for characterization, column chromatography, FT-IR, GC/MS and SEM were used for further characterization. The results showed that liquid and solid products from pistachio shells show similarities with high value conventional fuels. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Pistachio shell pyrolysis; Bio-oil characterization; Char characterization
1. Introduction The disposal of solid wastes such as biomass, industrial and municipal wastes is one of the main problems of the world and it is necessary to find out new ways to reuse this great potential as raw materials to produce advantageous products. Pyrolysis is one of the primary thermochemical conversion methods to convert biomass into valuable products, namely; solid char, liquid and gas product yields and compositions of which depend on pyrolysis conditions [1– 3]. Solid product, char, can be used as a fuel either directly as briquettes or as char–oil or char–water slurries since it has a high calorific value or it can be used as feedstocks to prepare activated carbons [4,5]. The liquid product, pyrolytic oil, approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons. It is useful as a fuel, may be added to petroleum refinery feedstocks or upgraded by catalysts to produce premium grade refined fuels, or may have a *
Corresponding author. Tel.: +90 222 33350580/6350; fax: +90 222 3239501. E-mail address: [email protected] (E. Pu¨tu¨n). 0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.11.041
potential use as chemical feedstocks. Bio-oils are generally preferred products because of their high calorific values, their ease of transportation and storage, their low nitrogen and sulphur content and their opportunity to be converted into chemicals. The third product gas having a high calorific value may also be used as a fuel [1,4–7]. There are many factors affecting pyrolysis product yields such as pyrolysis atmosphere, final temperature, particle size, heating rate, reactor type and initial amount of the sample. Liquid product yield and quality mostly depends on the pyrolysis temperature. It is known from previous studies that higher final temperatures (600 °C and above) favour gas formation, and relatively lower temperatures (400 °C and below) favour char formation. When bio-oil is the target product from pyrolysis an optimum temperature must be determined [1,7–9]. Mohan et al. reviewed pyrolysis of wood/biomass for bio-oil and stated that bio-oil has several advantages over fossil fuels from the environmental point of view [10]. Heating rate is also an important parameter for product yields. Conventional pyrolysis, in other words slow pyrolysis, is applied for thousands of years when charcoal production is aimed. Generally equal amounts of products are formed when
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biomass is pyrolsed slowly. A food industry waste, apricot pulp, was pyrolysed in a fixed bed reactor with a heating ¨ zbay et al. [11]. Bio-oil yield reached rate of 5 °C/min by O a maximum value of 23.2% at pyrolysis temperature of 550 °C when nitrogen gas was used as the sweeping gas. Much faster heating rates are preferred for fast pyrolysis and higher yields of bio-oil is produced. Tsai et al., studied fast pyrolysis of three different biomass wastes including rice straw, sugarcane bagasse and coconut shells under different conditions and they found that pyrolysis temperatures at about 500 °C, heating rates about 200 °C/min and holding times about 2 min gave the highest tar yield of 50% for sugarcane bagasse [12]. In this study, pistachio shell has taken as the biomass sample. Pistachio is cultivated mostly in Asia, The United States, Mediterranean Countries; Iraq, Iran and Middle Asian countries being dominant. South eastern part of Turkey has also a great potential with an increasing yield year by year. For the last few years average cultivation yield was 600,000 tons/year in Turkey [13,14]. The objectives of this study are: (i) determine the effect of pyrolysis temperature on product yields, (ii) characterize liquid product obtained under optimum pyrolysis conditions to detect if it can be used instead of conventional fossil fuels or chemical feedstocks, and (iii) characterize solid products, chars, for their possible use of solid fuels.
2. Method 2.1. Raw material Pistachio shell (PS) samples which have been taken from Southeast Anatolia were used as the raw material for the pyrolysis experiments. Air-dried shells were ground to obtain a uniform material of an average particle size (1.82 mm). The average bulk density of this raw material was found to be 750 kg/m3 [ASTM E873-82]. The proximate analyses of the raw material having average particle size were performed according to ASTM procedures and the moisture [ASTM D2016], ash [ASTM D1102-84] and volatile matter [ASTM E872-82] contents were found to be 7.39, 1.34 and 76.93 wt.%, respectively. The rest is calculated to be the percentage of fixed carbon as 14.34. Cellulose, hemicellulose and lignin are the main components of all biomass materials [1,15,16]. Experiments which were performed according to TSE (Turkish Standards Institute) and ASTM methods showed that pistachio shells contain 60.62% cellulose and 12.80% acid-insoluble lignin [TS 4431, ASTM D1106-84]. Hexane solubles of PS correspond to oil content that was 2.44 wt.%. Ultimate analysis was performed on PS with a Carlo Erba, EA 1108 Elemental Analyser to determine the weight fractions of carbon (49.98%), hydrogen (6.16%) and nitrogen (1.59%), and the weight fraction of oxygen (42.27%) was calculated by the difference. Results showed that PS contains very negligible sulfur.
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The thermal behaviour of PS to 850 °C was studied using Linseis Thermowaage L 81 thermogravimetric analyzer. FT-IR spectrum of PS was recorded to have further information on the chemical structure of the raw material in the near infrared region 4000–4001/cm by preparing KBr pellet. 2.2. Pyrolysis experiments The pyrolysis experiments were performed in a fixed-bed reactor, details were given in the previous studies [17,18]. After reaching the final pyrolysis temperature the reactor was set to cool to room temperature. Pyrolysis product yields were determined gravimetrically by weighing the three products. The liquid phase was collected in cold traps maintained at about 0 °C using salty ice. The liquid phase consisted of aqueous and oil phases were separated and weighed. Solid product, char, was removed from the reactor and weighed. Flow of gas released was controlled using a soap film for the duration of experiments. The gas yield was calculated by the difference. Pyrolysis of PS was carried out to investigate the effect of pyrolysis temperature on the product yields and quality of the liquid and solid products. For the experiments; 20 g sample of PS was placed into the reactor and the experiments were carried out with a heating rate of 7 °C/min to the final temperatures of 300, 400, 500, 550 and 700 °C and held for either a minimum of 30 min or until no further significant release of gas was observed; i.e. the total time for an experiment held at 550 °C was about 100 min. All the yields are expressed on a dry, ash-free basis and were the average yield of at least three with an experimental measurement error of less than ±0.5%. 2.3. Bio-oil characterization Bio-oil obtained at the pyrolysis final temperature that gave maximum oil yield, i.e. optimum temperature, was characterized. Elemental analysis was carried out with Carlo Erba, EA 1108 and the calorific value of the oil was determined. Chemical class compositions of the oil were determined by liquid column chromatographic fractionation. Bio-oil was separated into two fractions according to its pentane solubility. Silica-gel that was pre-treated at 105 °C for 2 h prior to use was the packing material for the column. Pentane soluble materials, were further separated into aliphatic, aromatic and polar fractions using 200 ml of each pentane, toluene and methanol, respectively. Each fraction was dried and weighed. The FT-IR spectra of the oil and its aliphatic, aromatic and polar subfractions were recorded using a Bruker Tensor 27 FT-IR analyser. GC–MS analysis of the aliphatic subfraction was performed using a Hewlett–Packard 6890 Model gas chromatograph equipped with a 5973 mass selective detector using HP 5 column.
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2.4. Char characterization Proximate analysis was applied to the chars obtained at five different pyrolysis temperatures to figure out the moisture, ash and volatile contents according to ASTM methods [ASTM D1762-84]. The elemental analyses of the chars were performed with a Carlo Erba 1108 elemental analyzer. The FT-IR spectra were recorded in the transmission mode between 4000 and 400 cm1 using a Bruker Tensor 27 Model Fourier Transform Infrared Spectrometer. Dried KBr was used to prepare pellets from chars. Typically 10 mg of sample at a concentration of 1% in KBr was used. Pure KBr disc was used as the background and for each sample multiple spectra were recorded to obtain highest signal-to-noise ratio. The surface characteristics of the chars were analysed using Scanning Electronic Microscope Jeol Camscan. Raw material and obtained chars were mounted on an aluminum stub using carbon film and coated with a thin layer of gold and palladium using Agar Sputter Coater. 3. Results and discussion 3.1. Raw material The results for the proximate analysis of PS showed that it has lower ash content when compared with other biomass samples [17–19]. Biomass samples having lower inorganic compounds are desired because of their abilities to give valuable solid products having less ash and higher fixed carbon contents after pyrolysis. In this manner, PS seems to be a promising raw material for thermochemical conversions. Calorific value of PS was calculated as about 18 MJ/kg with Du-Long’s formula with the known values of elemental composition [15].
The chemistry of biomass is very complicated. But, generally it is assumed that biomass has three major constituents; hemicellulose, cellulose and lignin according to the mass loss curve. It is known from previous studies that thermal decomposition of hemicellulose and cellulose have the first (150–350 °C) and second weight-loss (275–350 °C) steps for lignocellulosic materials after the initial weight loss (30–150 °C) associated with the moisture loss. However, lignin undergoes gradual decomposition over a wide temperature interval (275–500 °C) [7,16,20,21]. The TG and DTG curves for PS recorded from room temperature to 850 °C with a heating rate of 10 °C/min are shown in Fig. 1. The initial slight mass loss is due to the evaporation of moisture from PS. Second weight loss occurred between 200 and 575 °C corresponds to main pyrolysis process, devolatilisation. After this major weight loss, there is essentially no further loss of weight. The final residue corresponds to ash and fixed carbon. DTG data showed that initial mass loss gives its maximum peak at about 100 °C. Second major weight loss starts at about 175 °C, having its maximum point at 277 °C, finishes at about 350 °C. According to these results, it can be said that since most of the weight loss, related to volatilisation of hydrocarbons, happens at temperatures lower than 600 °C, choosing pyrolysis temperature as 550 °C is appropriate when maximum amount of liquid and gaseous products is aimed. To have information about the chemical structure of the raw material FT-IR spectrum is taken and given in Fig. 8. It shows that chemical structure of PS, being a lignocellulosic material, is made up of different atomic groupings and a large number of functional groups. The broad and flat band at about 3350 cm1 is ascribed to OH stretching vibrations in hydroxyl groups which was attributed to moisture content of the sample. Two very strong bands at 2925 and 2855 cm1 are assigned to C–H stretching
Fig. 1. TG and DTG curves for PS.
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vibrations in CH2 groups and a weak band at 1453 cm1 due to C–H deformation frequencies confirms the presence of these functional groups. The next strong band (1742 cm1) is ascribed to C@O vibrations probably from esters, ketones or aliphatic acids. At 1615 cm1, the very strong band is assigned to either C@C conjugated, aromatic conjugated or C@N conjugated or cyclic double bond vibrations. Skeletal vibrations from (CH3)3–C–R give a strong band at 1246 cm1. The weaker bands between 880 and 600 cm1 are ascribed to aromatic structures [22–24]. 3.2. Pyrolysis experiments It is known from the literature and previous studies that pyrolysis temperature plays an important role on product distribution [17,19]. Product yields for the pyrolysis of pistachio shells with relation to the final temperature (300– 700 °C) when the heating rate was fixed at 7 °C/min are given in Fig. 2. As observed in Fig. 2, the maximum char yield, 28%, is attained at the lowest temperature studied, 300 °C. The yield of char decreases as the temperature is increased, reaching an approximate yield of 23% at 700 °C. As the temperature increases, devolatilasation reactions are favoured and therefore gaseous product yield increase. Temperatures up to 600 °C maximise the production bio-oils and temperatures above 700 °C maximise gaseous products while minimising char formation And temperatures higher than 600 °C favours gas formation. As it is shown, when the final temperature is 300 °C 25% of biomass fed to the reactor is transformed into gases, but this yield increases with temperature, reaching 30% at 700 °C. Pyrolysis of olive residue, soybean cake and rice straw was studied using the same type of reactor and similar values for the increases in gas yields and decreases in the char yield were observed when pyrolysis
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temperature was increased from 400 to 700 °C [17,18,26]. However, type of the reactor and heating rate are also important parameters for the product yields. Ates et al., have studied fast pyrolysis of sesame stalk within a tubular reactor and have found that the char yield was reduced approximately by 17% when pyrolysis temperature was increased from 400 to 700 °C [27]. In this study the target pyrolysis product was the liquid one. Its yield was 15.5% at the pyrolysis temperature of 300 °C, it appeared to go through a maximum of 20.5% at the final temperature between 500 and 550 °C with 550 °C giving a little higher yield. At 700 °C, the oil yield again decreased to a value of 19%. Previous studies with other biomass samples showed that temperatures between 500 and 550 °C give the highest bio-oil yields [5,17,19]. 3.3. Bio-oil characterization Biomass pyrolysis oils contain a very wide range of complex organic chemicals [1,7,10]. For characterization of biooil that obtained under optimum final temperature is taken and column chromatography is applied to separate it into its subfractions. Chemical class fractionation of the oil showing aliphatic, aromatic and polar fractions is given in Fig. 3. Pyrolysis oil consists of approximately 57% n-pentane insolubles, asphaltenes. For the bio-oil, polar fraction is seen to be the dominant one. The elemental compositions of the oil characterized and its subfractions are listed in Table 1. Bio-oil is characterized with a higher calorific value, 30 MJ/kg, than the original biomass sample which indicates that the energy content of the oil is very close to that of petroleum. Containing more carbon and negligible oxygen and having an
60 50 40
Char yield Bio-oil Yield Gas Yield
(%) 30 20 10
30
0 Pentane insolubles
28 26
Pentane solubles
Aliphatics
Aromatics
Polars
Fig. 3. Results for the column chromatography.
(%)
24 22 Table 1 Elemental composition of bio-oil and subfractions (as received)
20 18 16 14 200
300
400 500 600 Temperature (°C)
700
800
Fig. 2. Pyrolysis of pistachio shells at different temperatures, product yields.
Elements
Weight % Bio-oil
Aliphatics
Aromatics
Polars
Carbon Hydrogen Nitrogen Oxygena
67.44 7.82 0.42 24.32
84.10 12.59 0.000 3.31
77.88 8.74 0.37 13.01
62.23 7.16 0.23 30.38
a
By difference.
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between 2800 and 3000 cm1 and C–H deformation vibrations between 1350 and 1475 cm1 indicate the presence of alkanes. The C@O stretching vibrations with absorbance between 1650 and 1750 cm1 indicate the presence of ketones or aldehydes. The absorbance peaks between 1575 and 1675 cm1 and 875 and 950 cm1 represent C@C stretching vibrations, and are indicative of alkenes. A gas liquid chromatogram of the aliphatic subfraction of pentane soluble of bio-oil is shown in Fig. 5. Three types of compounds were identified in the pentane subfraction as normal alkanes, alkenes, and branched hydrocarbons. The straight chain alkanes and alkenes range from C10–C29. When compared with standard diesel, it is obviously seen that the distribution of hydrocarbons for pentane subfraction shows similarities with diesel. 3.4. Char characterization
Fig. 4. FT-IR spectra of bio-oil and its aliphatic, aromatic and polar subfractions.
acceptable H/C ratio, aliphatic subfraction seems to be the most valuable fraction when it is used as a liquid fuel. FT-IR spectra representing functional groups, of the bio-oil at 550 °C and its subfractions are given in Fig. 4. The O–H stretching vibrations between 3200 and 3400 cm1 indicate the presence of phenols and alcohols. It is not surprising that these vibrations do not exist in aliphatic fraction since this fraction contains no highly oxygenated compounds. The C–H stretching vibrations
Many analytical techniques can be applied to characterize the chars, looking for their possible use, i.e. as solid fuels, activated carbons or etc. [5,25,28,29]. In this study chars were characterized to investigate if they can be used as solid fuels or not. For this purpose proximate analysis was applied to the chars. Fig. 6 gives the results for moisture, ash and volatile matter contents of the chars obtained at five different temperatures. As the temperature is raised, ash content is increased slightly. However, the increase in temperatures decreased the volatile matter content due to gasification reactions occurring at higher pyrolysis temperatures. According to ash and volatile matter contents, fixed carbon percentage is increased in a great amount at higher temperatures when compared with those at lower temperatures and raw material. The atomic ratios H/C and O/C are often used to characterize solid fuels [30,31]. The ratio of H/C versus O/C for chars generated at different temperatures is presented in
Fig. 5. GC–MS chromatogram of aliphatic subfraction of bio-oil.
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Moisture Ash Volatiles
15
(%)
10 5 0 300
400
500
550
700
Temperature (°C)
Atomic H:C ratio
Fig. 6. Proximate analyses of chars at different temperatures.
Fig. 8. FT-IR spectra of pistachio shell and its chars obtained at different temperatures.
Pistachio shell
1.4 1.2 1 0.8 0.6
300 400
0.4 0.2
500 550 700
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Atomic O:C ratio
Fig. 7. Van Krevelen diagram for pistachio shell and its chars obtained at different temperatures (°C).
Fig. 7. It is obvious that the H/C and O/C ratios of the char prepared at the highest temperature are much lower than that for raw material and chars produced from low temperature pyrolysis. Van Krevelen diagram for conventional fossil fuels can be found elsewhere [2,3]. The comparison of the chars with conventional fossil fuels shows that all the chars, especially the ones obtained at higher temperatures, are situated in the region that is for the most valuable fossil fuels, anthracite. Average calorific value of the chars is calculated to be 30 MJ/kg using Du Long’s formula with the known values of elemental composition [15]. For the structural analysis, chars obtained at five different temperatures were conducted to FT-IR. The spectra are roughly very similar for the chars, however they differ from PS indicating the effect of thermal treatment. The bands at about 3450–3350 cm1 for all chars represent O–H vibrations in hydroxyl groups. Very weak bands at about 2850 cm1 for PS300 and PS400 are assigned to C–H stretching vibrations in CH2 groups. With increasing pyrolysis temperature, decreasing intensities of the bands between 2000 and 1500 cm1 are ascribed to double bond vibrations for all types of aromatics. It is clear from Fig. 8 that, chars obtained at higher temperatures contain little or no vibration bands at around 2900 cm1 indicating that their aliphaticity is lower than the low temperature char. Previous studies for different biomass samples indi-
cate that carbonization, especially at higher temperatures, increases the aromatic carbon content while decreasing aliphaticity [22,24,29]. Also, the presence of many weak bands in PS300 spectrum that are very similar to PS is a good proof of uncompleted pyrolysis reactions at lower temperatures. SEM micrographs of the raw material and chars are given in Fig. 9. It obvious from the micrographs that surface morphology of PS changed after pyrolysis. The increase in pyrolysis temperature leads to the formation of waves and pores on the surface. SEM analysis indicates that at relatively lower temperatures (300 °C), there is no significant change on the surface since pyrolysis has not yet completed. When temperature is increased to 400– 500 °C porosity increases and after 500 °C small pits occurred on the surface. But, generally, it can be said that porosity of the chars are very low. 4. Conclusion Conventional pyrolysis is known as the slow pyrolysis that gives three main products, the yields of which depend mostly on the pyrolysis conditions [1–3]. Previous studies on pyrolysis of coal, oil shales or different biomass samples have shown that final temperature is the major effect on product yields [1,5,6,17–19]. In this study pistachio shell samples were taken as the biomass for the pyrolysis experiments performed in a fixed-bed reactor with a heating rate of 7 °C/min to five different final temperatures. The aim was to find out the final temperature that gives the maximum bio-oil yield. It was observed that the bio-oil yield is sensitive to pyrolysis temperature in the range of 300–700 °C. Bio-oil yield has showed an increase till the final temperature of 500– 550 °C, and a decrease between the range 550–700 °C.
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Fig. 9. SEM pictures of pistachio shell and its chars obtained at different temperatures.
The maximum yield was achieved as 20.5% at 550 °C. Relatively low pyrolysis temperatures around 400 °C favours char formation. The char yield decreased with an amount of 21% while increasing the pyrolysis temperature from 300 to 700 °C. The opposite was observed for the gas yield, it increased significantly when temperature was increased. Bio-oil obtained under optimum final temperature was then fractionated into chemical classes by column chromatography. The characterization depending on the structure analysis of the oil and fractions have shown that these fractions are quite similar to currently utilised transport fuels and can be utilised as transport fuels or chemical feedstocks.
Although the quantity of char yield decreased with increasing temperature, further studies have showed that quality of this char is much higher than that of obtained at lower temperatures. Characterization of the solid product, have showed that produced chars, having a high fixed carbon content, little ash and high calorific value, can replace the most valuable conventional fossil fuels. References [1] Bridgewater AV, Grassi G. Biomass pyrolysis liquids upgrading and utilization. England: Elsevier Applied Science; 1991. [2] McKendry P. Energy production from biomass (part 1): conversion technologies. Biores Technol 2002;83:37–46.
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