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Bio-oil production from pyrolysis and steam pyrolysis of soybean-cake: product yields and composition
Energy 27 (2002) 703–713 www.elsevier.com/locate/energy
Bio-oil production from pyrolysis and steam pyrolysis of soybean-cake: product yields and composition Ays¸e E. Pu¨tu¨n ∗, Esin Apaydin, Ersan Pu¨tu¨n Department of Chemical Engineering, Faculty of Engineering and Architecture, Anadolu University, 26470 Eskisehir, Turkey Received 9 September 2001
Abstract The slow pyrolysis of soybean cake in a fixed-bed reactor was investigated under three different atmospheres: static, for determining the effects of pyrolysis temperature and particle size, nitrogen and steam. The liquid yield of 33.78% was attained at 550 °C pyrolysis temperature and 200 cm3/min sweeping gas flow rate with the soybean oil cake samples having 0.850⬍Dp⬍1.250 mm particle size. And the liquid yield reached a maximum value of 42.79% with a steam velocity of 1.3 cm/s. Column chromatography was used to characterize the liquid product, bio-oil. The aliphatic subfractions of the oils were then analyzed by GC/MS. FTIR and 1H-NMR spectra were used to determine structural analysis of pyrolysis oils and aromatic and polar subfractions. The H/C ratios and the structure analysis of the fractions obtained from the biocrudes show that the fractions are quite similar to currently utilized transport fuels. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction With the depletion of fossil fuels and concerns over CO2, NOx, SOx emissions, renewable energy sources are now being considered as an attractive solution to the energy problem of both industrialized and developing countries [1]. Today, many developing countries are spending up to 40–50% of their exchange earnings on importation of crude petroleum [2]. Thus, both the developing world and industrialized nations require new technologies which efficiently utilize renewable energy sources [3]. According to Bridgewater and Grassi [4], renewable energy sources are capable of providing a significant fraction of Europe’s needs in the 21st century. ∗
Corresponding author. Tel.: +90-222-3223662; fax: +90-222-3239501. E-mail address: [email protected] (A.E. Pu¨tu¨n).
0360-5442/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 2 ) 0 0 0 1 5 - 4
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There are a number of waste and biomass sources being considered as potential sources of fuels and chemical feedstocks. The total bioenergy potential in 1990 was estimated at 225 exajoules, or 5.4 billion tons of oil equivalent. For comparison, the actual use of bioenergy in 1990 was 46 exajoules, or 1.1 billion tons of oil equivalent. This potential has been estimated to grow to between 370 and 450 exajoules by 2050 [5]. The economics for biomass pyrolysis are generally considered to be most favorable for (i) plants which grow abundantly and require little cultivation in arid lands, and (ii) wastes available in relatively large quantities from agricultural plants, like sunflower, cotton, hazelnut and soybean [2]. From this point of view, Turkey has both the natural resources and the conditions necessary for agricultural products which means a great potential of biomass [6]. Nearly 25 megatons of wood and biomass are used domestically or industrially to produce heat energy in an inefficient way, direct combustion [7]. Thermochemical processes are thought to have great promise as a means for efficiently and economically converting biomass into higher value fuels [3]. There are four main thermochemical methods of converting biomass: pyrolysis, liquefaction, gasification and combustion. Each gives a different range of products [4]. Pyrolysis lies at the heart of all thermochemical fuel conversion processes [3]. It is attractive because solid biomass and wastes, which are difficult and costly to manage, can be readily converted to liquid products. These liquids have advantages in transport, storage, combustion, retrofitting and flexibility in production and marketing. Pyrolysis also gives gas and solid (char) products, the relative proportions of which depend very much on the pyrolysis method and process conditions [4]. While interest in the pyrolysis of biomass has increased, new technologies to increase the degree of conversion of solid fuels and biomass into liquid products has been developed. Among them, the method of steam pyrolysis is a special interest with the numerous advantages. Steam can be absorbed on the surface of char and this way inhibit the adsorption of tar vapors on the surface. This also prevents the secondary cracking reactions in the gas phase and helps to maximize the yield of liquid products [8,9]. The liquid, pyrolytic oil, approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons [4]. These pyrolytic oils have a high calorific value and it can be upgraded to obtain light hydrocarbons for transport fuel. The solid product, char, and gas also have potential to be used as fuel [6]. In this study, soybean cake has taken as the biomass. Soybean is cultivated mostly in the southern part of Turkey and is assuming an important role in Turkish agriculture. In 1999, the soybean cultivation yield was 2750 kg/Ha in Turkey, and the soybean crush in the USA was 40.8 million metric tons. 2. Experimental The soybean cake sample investigated in this study has been taken from a soybean oil factory around Ceyhan-Adana located in the Mediterranean region, in the southern part of Turkey. The fixed-bed pyrolysis experiments were performed in three different atmospheres: static, nitrogen and steam in Heinze retort [10]. The 316 stainless steel Heinze retort used in this study has a volume of 350 cm3 and is externally heated by an electric furnace with the temperature being controlled by a thermocouple inside the bed.
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The experiments performed in the Heinze reactor were carried out in three groups: self pyrolysis, sweeping gas atmosphere and steam atmosphere. The first group was performed to determine the effect of pyrolysis temperature on the yields of the soybean pyrolysis. Ten grams of soybean cake, having average particle size of 0.4250⬍Dp⬍1.250, was placed in the reactor and the temperature was raised at 5 °C/min to a final temperature of either 400, 500, 550, or 700 °C and held for either a minimum of 30 min or until no further significant release of gas was observed. The flow of gas released was measured using a soap film for the duration of experiments. The liquid phase was collected in cold traps maintained at about 0 °C using chill water. The liquid phase consisted of aqueous and oil phases which were separated and weighed. After pyrolysis, the solid char was removed and weighed, then the gas yield was calculated by the difference. In order to establish the effect of particle size on the pyrolysis yields, experiments were conducted using five different particle size ranges (Dp), namely, 0.225⬍Dp⬍0.425, 0.425⬍Dp⬍0.850, 0.850⬍Dp⬍1.250, 1.250⬍Dp⬍1.800 and Dp⬎1.800 at a heating rate of 5 °C/min. The final pyrolysis temperature was 550 °C for all experiments. The second group of experiments were performed in the Heinze reactor, to establish the effect of sweep gas velocity on the pyrolysis yields, under a nitrogen atmosphere. The experiments were conducted with sweep gas flow rates of either 50, 100, 200 or 400 cm3/min. For all the experiments, the heating rate, the final pyrolysis temperature, and the particle size were 5 °C/min, 550 °C and 0.850⬍Dp⬍1.250 mm, respectively, based on the results of the first group of experiments. For the last group of experiments, using the results from the first group of experiments, the effect of steam velocity on the pyrolysis yields was investigated. The experiments were performed with a steam velocity of either 0.6, 1.3 or 2.7 cm/s. 3. Characterization Proximate analyses were carried out on the soybean cake (Table 1). The protein content of the soybean cake was determined by the Kjeldahl method using Labconco Rapid still-2. Elemental analysis of the soybean cake and the bio-oils were carried out at Tubitak Laboratories (CHNS-
Table 1 Proximate analysis of soybean cake Analysis Moisture Volatiles Ash Fixed carbon Raw cellulose Oil Protein Bulk densitya (g/cm3) a
For the average particle size.
Percentage (as received) 8.38 71.60 5.63 14.39 5.00 2.18 44.37 0.68
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932 LECO) [11] and the results are given in Table 2. The oils analyzed in this study have been obtained under the experimental conditions that have given the maximum oil yield. The elemental compositions and, using Dulong’s formula, calorific values of the Heinze retort oil were determined. The IR spectra of the oils were recorded using a Mattson 1000 Infrared spectrophotometer. Chemical class composition of the oils was determined by liquid column chromotografic fractionation. The columns used were packed with silica gel 70–230 mesh, pretreated at 105 °C for 2 h prior to use. The column was eluated successively with pentane, toluene, ether and methanol to produce aliphatic, aromatic, ester and polar fractions, respectively. Each fraction was dried and weighed and then subjected to elemental and spectroscopic analyses. GC analysis with flame ionization detection (FID) was performed using a Hewlett-Packard 6890 model gas chromatograph with a HP-5 capillary column supplied from Hewlett-Packard, USA. 1 H-NMR spectra were recorded using a BRUKER DPX-400, 400 MHz High Performance Digital FT-NMR instrument at Tubitak Laboratories [11]. 4. Results and discussion 4.1. Product yields Fig. 1 shows the product yields for the pyrolysis of soybean cake having an average particle size, 0.425⬍Dp⬍1.250 mm, in relation to the final temperature of pyrolysis at a heating rate of 5 °C/min. The yield of conversion increased from 70.00 to 77.56%, when the pyrolysis temperature was increased from 400 to 700 °C. While the oil yield was 25.35% at a pyrolysis temperature of 400 °C, it appeared to go through a maximum of 30.00% at the final temperature of 550 °C. Then, at the final temperature of 700 °C, the oil yield decreased to 28.14%. Varying the particle size from 0.425⬍Dp⬍0.850 mm to Dp⬎1.800 mm at a pyrolysis temperature of 550 °C with a heating rate of 5 °C/min without any sweep gas atmosphere had no significant effect on the pyrolysis conversion, remaining constant at ~74%. However, increasing particle size from 0.224⬍Dp⬍0.425 mm to 0.850⬍Dp⬍1.250 mm increased the oil yield from 26.74 to 30.23%, and decreasing particle size from Dp⬎1.800 mm to 0.850⬍Dp⬍1.250 mm increased the oil yield from 28.02 to 30.23%. Then the optimum particle size was chosen as 0.850⬍Dp⬍1.250 Table 2 Elemental compositions of soybean cake and bio-oils obtained at 550 °C Component C H N Oa H/C O/C a
By difference.
Soybean cake (daf) 55.89 6.57 9.29 28.25 1.41 0.38
Bio-oil static 63.09 8.32 7.70 20.89 1.58 0.25
Bio-oil sweeping gas
Bio-oil steam
62.16 8.33 7.47 22.04 1.61 0.27
55.88 7.45 8.08 28.59 1.60 0.38
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Fig. 1. Yields of pyrolysis products at heating rate of 5 °C/min and at average particle size.
mm, which gives the highest oil yield at the final pyrolysis temperature of 550 °C with a heating rate of 5 °C/min (Fig. 2). As reported in the literature, for increasing pyrolysis temperature, carbon conversion to gas was found to increase. It is also known that the sweeping gas removed the products from the hot zone to minimize secondary reactions such as thermal cracking, repolymerization, and recondensation and to maximize the liquid yield [4,12]. The second group of experiments performed in the Heinze retort, to determine the effect of the sweep gas flow rate, were conducted at a nitrogen flow rate of 50, 100, 200, or 400 cm3/min with the final pyrolysis temperature of 550 °C, a heating rate of 5 °C/min and a particle size range of 0.850⬍Dp⬍1.250 mm. The product yields of pyrolysis in relation to rate of sweep gas
Fig. 2. Yields of pyrolysis in relation to particle size at the pyrolysis temperature of 550 °C.
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Fig. 3.
Effect of sweeping gas on the pyrolysis product yields.
are given in Fig. 3. The oil yield was increased only by 11.74% when reaching a nitrogen flow rate of 200 cm3/min, and there was no obvious influence on the yield of oil and char as the rate of nitrogen was increased. The highest oil yield of 33.78% was obtained at a final pyrolysis temperature of 550 °C with a heating rate of 5 °C/min and nitrogen flow rate of 200 cm3/min. Fig. 4 shows the results obtained in the presence of steam flowing at three different velocities: 0.6, 1.3 and 2.7 cm/s. According to Minkova et al. [8], in a flow of steam, the liquid product yield increases dramatically at the expense of gaseous and solid products. The water vapor is not only a vehicle for the volatiles but also a reactive agent, which reacts with the pyrolysis products. Experiments showed that bio-oil yield reached a maximum value of 42.79% at a steam velocity of 1.3 cm/s, while it had a maximum value of 30.00% at the static retorting. The opposite was observed for the solid product: char yield decreased from 25.1 to 15.86% when the steam was used instead of a static atmosphere.
Fig. 4. Effect of steam velocity on the pyrolysis product yields.
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All the yields are expressed on a dry, ash-free basis and were the average yield of at least three attempts with an experimental measurements error of less than ±0.5%. 4.2. Chemical composition The elemental composition of the oils characterized are listed in Table 2, and the calorific values of the soybean oil cake and the oils and pH values of the oils are listed in Table 3. The average chemical composition of the Heinze retort oil analyzed under static atmosphere, nitrogen atmosphere and steam atmosphere are CH1.58N0.10O0.25, CH1.61N0.10O0.27 and CH1.6N0.12O0.38, respectively. Heinze retort product oil is characterized by high oxygen content with a higher H/C ratio then the original cake. Further comparison of H/C ratios with conventional fuels indicates that the H/C ratios of the oils obtained in this study lie between those of light and heavy petroleum products. Also, calorific values indicate that the energy contents of the oils are very close to that of petroleum. The IR spectrum of the Heinze oil under static atmosphere is given in Fig. 5. The O–H stretching vibrations between 3200 and 3400 cm⫺1 indicate the presence of phenols and alcohols. The C–H stretching vibrations between 2800 and 3000 cm⫺1 and C–H deformation vibrations between 1350 and 1475 cm⫺1 indicate the presence of alkanes. The C=H stretching vibrations with absorbance between 1650 and 1750 cm⫺1 indicate the presence of ketones or aldehydes. The absorbance peaks between 1575 and 1675 cm⫺1 representing C=C stretching vibrations are indicative of alkenes and aromatics. The results of the column chromatography fractions showing aliphatic, aromatic, ester and polar subfractions of the bio-oil obtained from the Heinze retort is given in Fig. 6. The polar fractions are dominate and it can be seen that in steam pyrolysis the contents of the polar fractions are somewhat higher, while the formation of aliphatic and aromatic compounds is seem to be favored under inert and static atmospheres. A gas liquid chromatogram of the aliphatic subfraction of pentane soluble of bio-oil obtained under static conditions is shown in Fig. 7. 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 to C29. Distribution of straight chain alkanes exhibit a maximum on the range of C14⫺C18. Table 3 Calorific values and pH values of soybean cake and Heinze retort oils Material
Calorific values (kcal/kg)
pH
Soybean cake Heinze oila Heinze oilb Heinze oilc
5558 7059 6938 7946
– 7.23 6.71 5.34
a b c
Obtained at 550 °C under static atmosphere. Obtained at 550 °C, with a nitrogen flow rate of 200 cm3/min. Obtained at 550 °C, with the steam velocity of 1.3 cm/s.
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Fig. 5. IR spectra of (a) oil, (b) pentane eluate, (c) toluene eluate, (d) ether eluate, and (e) methanol eluate.
Fig. 8 shows the 1H-NMR spectra of bio-oils obtained at optimum conditions under static, nitrogen and steam atmosphere. The hydrogen distribution of 1H-NMR is given in Table 4. 1HNMR spectra of the bio-oils indicates that the aromaticity of the bio-oil of steam pyrolysis was higher than the static and nitrogen atmosphere pyrolysis. CH2 and CH β to an aromatic ring (napthenic) protons are very close to each other in amount for bio-oils under static and nitrogen atmosphere while bio-oil under steam has a lower value. CH3앫CH2 and CH to an aromatic ring protons (centered at 2.65) and CH3γ or further from an aromatic ring protons are in very close agreement in the oils and are higher than the other protons.
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Fig. 6.
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Results of column chromatography of the bio-oils.
Fig. 7. GC chromatogram of the n-pentane eluate.
5. Conclusion An agricultural by-product, soybean cake, is taken as the biomass sample for the pyrolysis experiments performed in a fixed bed reactor under different conditions to obtain the maximum bio-oil yield. The oil yield was obtained as 30.23% from pyrolysis of soybean oil cake at the final pyrolysis temperature of 550 °C, with the samples having particle size of 0.850⬍Dp⬍1.250 mm, and with a heating rate of 5 °C/min. The oil reached a maximum yield of 33.78% under a sweep gas atmosphere (nitrogen flow rate of 200 cm3/min) and reached a maximum yield of 42.79% under a steam atmosphere (steam velocity of 1.3 cm/s), indicating the effect of steam on the pyrolysis of biomass, by acting as a physical factor, influencing the heat transfer and favoring the fast desorption of low molecular products [8]. Furthermore, detailed studies are needed on the role of steam flow rate in order to characterize the bio-oils because of the possibility of increasing the amounts of parafinic and aromatic components.
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Fig. 8. 1H-NMR spectra of bio-oils. (a) Bio-oil from static, (b) bio-oil from nitrogen atmosphere, (c) bio-oil from water vapor.
In contrast to coals and oil shales [13], oil yields from the soybean oil cake were found to be largely independent of particle size (⬍2 mm) and sweep gas velocity. The H/C ratios show that the fractions are quite similar to currently utilized transport fuels.
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Table 4 Results of 1H-NMR for three bio-oils from the fixed-bed pyrolysis of soybean cake Type of hydrogen
Chemical shift (ppm) Bio-oil under static atmosphere
Bio-oil under nitrogen Bio-oil under steam atmosphere
Aromatic Phenolic (OH) or olefinic proton Ring-joinmethylene (Ar–CH2–Ar) CH3앫CH2 and CH to an aromatic ring CH2 and CHβ to an aromatic ring (napthenic) β-CH3, CH2, and CHγ or further from an aromatic ring CH3γ or further from an aromatic ring
6.5–9.0 5.0–6.5
13.06 2.06
14.98 3.87
17.06 1.47
3.3–4.5
1.09
0.69
2.80
2.0–3.3
24.86
22.68
26.77
1.6–2.0
14.19
13.96
8.65
1.0–.6
16.88
18.57
23.22
0.5–1.0
27.86
25.25
20.03
References ¨ zbay N, Koc¸ kar O ¨ M, Pu¨ tu¨ n E. Fixed-bed pyrolysis of cottonseed cake: product yield and compo[1] Pu¨ tu¨ n AE, O sitions. Energy Sources 1996;19:905–15. ¨ zcan A, Pu¨ tu¨ n E. Pyrolysis of hazelnut shells in a fixed-bed tubular reactor: yields an structural [2] Pu¨ tu¨ n AE, O analysis of bio-oil. Journal of Analytical and Applied Pyrolysis 1999;52:33–49. [3] Antal MJ. Biomass pyrolysis: a review of the literature part-1—carbohydrate pyrolysis. Advances in Solar Energy 1983;6:1–109. [4] Bridgewater AV, Grassi G. Biomass pyrolysis liquids upgrading and utilisation. England: Elsevier Applied Science, 1991. [5] Fischer G, Schrattenholzer L. Global bioenergy potentials through 2050. Biomass and Bioenergy 2001;20:151–9. ¨ M, Onay O ¨ , Pu¨ tu¨ n AE, Pu¨ tu¨ n E. Fixed-bed pyrolysis of hazelnut shells: a study on mass transfer [6] Koc¸ kar O limitations on product yields and characterisation of the pyrolysis oil. Energy Sources 2000;22:913–24. [7] Republic of Turkey Ministry for Energy and Natural Sources. I˙no¨ nu¨ Bulvarı, No. 27, Bahc¸ elievler, Ankara, Turkey. Email: [email protected]. Available from: http://www.enerji.gov.tr/enerji/genel.htm. [8] Minkova V, Razvigorova M, Bjornbom E, Zanzi R, Budinova T, Petrov N. Effect of water vapour and biomass nature on the yield and quality of the pyrolysis products from biomass. Fuel Processing Technology 2001;70:53–61. ¨ zbay N, Pu¨ tu¨ n AE, Pu¨ tu¨ n E. Structural analysis of bio-oils from pyrolysis and steam pyrolysis of cottonseed [9] O cake. Journal of Analytical and Applied Pyrolysis 2001;60:89–101. [10] Heinze R. Oel u kohle 1993;39:973. [11] The Scientific Counsil of Turkey. Ankara Test ve Analiz Laboratuarı, Konya Yolu, No. 67, Ankara, Turkey. Available from: http://www.tubitak.gov.tr/eal/. [12] Encinar JM, Gonzalez JF, Gonzalez J. Fixed-bed pyrolysis of Cynara cardunculus L. product yields and compositions. Fuel Processing Technology 2000;68:209–22. [13] Harker JH, Backhurst JR. Fuel and energy. London: Academic Press, 1981.
Bio-oil production from pyrolysis and steam pyrolysis of soybean-cake: product yields and composition Ays¸e E. Pu¨tu¨n ∗, Esin Apaydin, Ersan Pu¨tu¨n Department of Chemical Engineering, Faculty of Engineering and Architecture, Anadolu University, 26470 Eskisehir, Turkey Received 9 September 2001
Abstract The slow pyrolysis of soybean cake in a fixed-bed reactor was investigated under three different atmospheres: static, for determining the effects of pyrolysis temperature and particle size, nitrogen and steam. The liquid yield of 33.78% was attained at 550 °C pyrolysis temperature and 200 cm3/min sweeping gas flow rate with the soybean oil cake samples having 0.850⬍Dp⬍1.250 mm particle size. And the liquid yield reached a maximum value of 42.79% with a steam velocity of 1.3 cm/s. Column chromatography was used to characterize the liquid product, bio-oil. The aliphatic subfractions of the oils were then analyzed by GC/MS. FTIR and 1H-NMR spectra were used to determine structural analysis of pyrolysis oils and aromatic and polar subfractions. The H/C ratios and the structure analysis of the fractions obtained from the biocrudes show that the fractions are quite similar to currently utilized transport fuels. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction With the depletion of fossil fuels and concerns over CO2, NOx, SOx emissions, renewable energy sources are now being considered as an attractive solution to the energy problem of both industrialized and developing countries [1]. Today, many developing countries are spending up to 40–50% of their exchange earnings on importation of crude petroleum [2]. Thus, both the developing world and industrialized nations require new technologies which efficiently utilize renewable energy sources [3]. According to Bridgewater and Grassi [4], renewable energy sources are capable of providing a significant fraction of Europe’s needs in the 21st century. ∗
Corresponding author. Tel.: +90-222-3223662; fax: +90-222-3239501. E-mail address: [email protected] (A.E. Pu¨tu¨n).
0360-5442/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 2 ) 0 0 0 1 5 - 4
704
A.E. Pu¨tu¨n et al. / Energy 27 (2002) 703–713
There are a number of waste and biomass sources being considered as potential sources of fuels and chemical feedstocks. The total bioenergy potential in 1990 was estimated at 225 exajoules, or 5.4 billion tons of oil equivalent. For comparison, the actual use of bioenergy in 1990 was 46 exajoules, or 1.1 billion tons of oil equivalent. This potential has been estimated to grow to between 370 and 450 exajoules by 2050 [5]. The economics for biomass pyrolysis are generally considered to be most favorable for (i) plants which grow abundantly and require little cultivation in arid lands, and (ii) wastes available in relatively large quantities from agricultural plants, like sunflower, cotton, hazelnut and soybean [2]. From this point of view, Turkey has both the natural resources and the conditions necessary for agricultural products which means a great potential of biomass [6]. Nearly 25 megatons of wood and biomass are used domestically or industrially to produce heat energy in an inefficient way, direct combustion [7]. Thermochemical processes are thought to have great promise as a means for efficiently and economically converting biomass into higher value fuels [3]. There are four main thermochemical methods of converting biomass: pyrolysis, liquefaction, gasification and combustion. Each gives a different range of products [4]. Pyrolysis lies at the heart of all thermochemical fuel conversion processes [3]. It is attractive because solid biomass and wastes, which are difficult and costly to manage, can be readily converted to liquid products. These liquids have advantages in transport, storage, combustion, retrofitting and flexibility in production and marketing. Pyrolysis also gives gas and solid (char) products, the relative proportions of which depend very much on the pyrolysis method and process conditions [4]. While interest in the pyrolysis of biomass has increased, new technologies to increase the degree of conversion of solid fuels and biomass into liquid products has been developed. Among them, the method of steam pyrolysis is a special interest with the numerous advantages. Steam can be absorbed on the surface of char and this way inhibit the adsorption of tar vapors on the surface. This also prevents the secondary cracking reactions in the gas phase and helps to maximize the yield of liquid products [8,9]. The liquid, pyrolytic oil, approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons [4]. These pyrolytic oils have a high calorific value and it can be upgraded to obtain light hydrocarbons for transport fuel. The solid product, char, and gas also have potential to be used as fuel [6]. In this study, soybean cake has taken as the biomass. Soybean is cultivated mostly in the southern part of Turkey and is assuming an important role in Turkish agriculture. In 1999, the soybean cultivation yield was 2750 kg/Ha in Turkey, and the soybean crush in the USA was 40.8 million metric tons. 2. Experimental The soybean cake sample investigated in this study has been taken from a soybean oil factory around Ceyhan-Adana located in the Mediterranean region, in the southern part of Turkey. The fixed-bed pyrolysis experiments were performed in three different atmospheres: static, nitrogen and steam in Heinze retort [10]. The 316 stainless steel Heinze retort used in this study has a volume of 350 cm3 and is externally heated by an electric furnace with the temperature being controlled by a thermocouple inside the bed.
A.E. Pu¨ tu¨ n et al. / Energy 27 (2002) 703–713
705
The experiments performed in the Heinze reactor were carried out in three groups: self pyrolysis, sweeping gas atmosphere and steam atmosphere. The first group was performed to determine the effect of pyrolysis temperature on the yields of the soybean pyrolysis. Ten grams of soybean cake, having average particle size of 0.4250⬍Dp⬍1.250, was placed in the reactor and the temperature was raised at 5 °C/min to a final temperature of either 400, 500, 550, or 700 °C and held for either a minimum of 30 min or until no further significant release of gas was observed. The flow of gas released was measured using a soap film for the duration of experiments. The liquid phase was collected in cold traps maintained at about 0 °C using chill water. The liquid phase consisted of aqueous and oil phases which were separated and weighed. After pyrolysis, the solid char was removed and weighed, then the gas yield was calculated by the difference. In order to establish the effect of particle size on the pyrolysis yields, experiments were conducted using five different particle size ranges (Dp), namely, 0.225⬍Dp⬍0.425, 0.425⬍Dp⬍0.850, 0.850⬍Dp⬍1.250, 1.250⬍Dp⬍1.800 and Dp⬎1.800 at a heating rate of 5 °C/min. The final pyrolysis temperature was 550 °C for all experiments. The second group of experiments were performed in the Heinze reactor, to establish the effect of sweep gas velocity on the pyrolysis yields, under a nitrogen atmosphere. The experiments were conducted with sweep gas flow rates of either 50, 100, 200 or 400 cm3/min. For all the experiments, the heating rate, the final pyrolysis temperature, and the particle size were 5 °C/min, 550 °C and 0.850⬍Dp⬍1.250 mm, respectively, based on the results of the first group of experiments. For the last group of experiments, using the results from the first group of experiments, the effect of steam velocity on the pyrolysis yields was investigated. The experiments were performed with a steam velocity of either 0.6, 1.3 or 2.7 cm/s. 3. Characterization Proximate analyses were carried out on the soybean cake (Table 1). The protein content of the soybean cake was determined by the Kjeldahl method using Labconco Rapid still-2. Elemental analysis of the soybean cake and the bio-oils were carried out at Tubitak Laboratories (CHNS-
Table 1 Proximate analysis of soybean cake Analysis Moisture Volatiles Ash Fixed carbon Raw cellulose Oil Protein Bulk densitya (g/cm3) a
For the average particle size.
Percentage (as received) 8.38 71.60 5.63 14.39 5.00 2.18 44.37 0.68
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932 LECO) [11] and the results are given in Table 2. The oils analyzed in this study have been obtained under the experimental conditions that have given the maximum oil yield. The elemental compositions and, using Dulong’s formula, calorific values of the Heinze retort oil were determined. The IR spectra of the oils were recorded using a Mattson 1000 Infrared spectrophotometer. Chemical class composition of the oils was determined by liquid column chromotografic fractionation. The columns used were packed with silica gel 70–230 mesh, pretreated at 105 °C for 2 h prior to use. The column was eluated successively with pentane, toluene, ether and methanol to produce aliphatic, aromatic, ester and polar fractions, respectively. Each fraction was dried and weighed and then subjected to elemental and spectroscopic analyses. GC analysis with flame ionization detection (FID) was performed using a Hewlett-Packard 6890 model gas chromatograph with a HP-5 capillary column supplied from Hewlett-Packard, USA. 1 H-NMR spectra were recorded using a BRUKER DPX-400, 400 MHz High Performance Digital FT-NMR instrument at Tubitak Laboratories [11]. 4. Results and discussion 4.1. Product yields Fig. 1 shows the product yields for the pyrolysis of soybean cake having an average particle size, 0.425⬍Dp⬍1.250 mm, in relation to the final temperature of pyrolysis at a heating rate of 5 °C/min. The yield of conversion increased from 70.00 to 77.56%, when the pyrolysis temperature was increased from 400 to 700 °C. While the oil yield was 25.35% at a pyrolysis temperature of 400 °C, it appeared to go through a maximum of 30.00% at the final temperature of 550 °C. Then, at the final temperature of 700 °C, the oil yield decreased to 28.14%. Varying the particle size from 0.425⬍Dp⬍0.850 mm to Dp⬎1.800 mm at a pyrolysis temperature of 550 °C with a heating rate of 5 °C/min without any sweep gas atmosphere had no significant effect on the pyrolysis conversion, remaining constant at ~74%. However, increasing particle size from 0.224⬍Dp⬍0.425 mm to 0.850⬍Dp⬍1.250 mm increased the oil yield from 26.74 to 30.23%, and decreasing particle size from Dp⬎1.800 mm to 0.850⬍Dp⬍1.250 mm increased the oil yield from 28.02 to 30.23%. Then the optimum particle size was chosen as 0.850⬍Dp⬍1.250 Table 2 Elemental compositions of soybean cake and bio-oils obtained at 550 °C Component C H N Oa H/C O/C a
By difference.
Soybean cake (daf) 55.89 6.57 9.29 28.25 1.41 0.38
Bio-oil static 63.09 8.32 7.70 20.89 1.58 0.25
Bio-oil sweeping gas
Bio-oil steam
62.16 8.33 7.47 22.04 1.61 0.27
55.88 7.45 8.08 28.59 1.60 0.38
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Fig. 1. Yields of pyrolysis products at heating rate of 5 °C/min and at average particle size.
mm, which gives the highest oil yield at the final pyrolysis temperature of 550 °C with a heating rate of 5 °C/min (Fig. 2). As reported in the literature, for increasing pyrolysis temperature, carbon conversion to gas was found to increase. It is also known that the sweeping gas removed the products from the hot zone to minimize secondary reactions such as thermal cracking, repolymerization, and recondensation and to maximize the liquid yield [4,12]. The second group of experiments performed in the Heinze retort, to determine the effect of the sweep gas flow rate, were conducted at a nitrogen flow rate of 50, 100, 200, or 400 cm3/min with the final pyrolysis temperature of 550 °C, a heating rate of 5 °C/min and a particle size range of 0.850⬍Dp⬍1.250 mm. The product yields of pyrolysis in relation to rate of sweep gas
Fig. 2. Yields of pyrolysis in relation to particle size at the pyrolysis temperature of 550 °C.
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Fig. 3.
Effect of sweeping gas on the pyrolysis product yields.
are given in Fig. 3. The oil yield was increased only by 11.74% when reaching a nitrogen flow rate of 200 cm3/min, and there was no obvious influence on the yield of oil and char as the rate of nitrogen was increased. The highest oil yield of 33.78% was obtained at a final pyrolysis temperature of 550 °C with a heating rate of 5 °C/min and nitrogen flow rate of 200 cm3/min. Fig. 4 shows the results obtained in the presence of steam flowing at three different velocities: 0.6, 1.3 and 2.7 cm/s. According to Minkova et al. [8], in a flow of steam, the liquid product yield increases dramatically at the expense of gaseous and solid products. The water vapor is not only a vehicle for the volatiles but also a reactive agent, which reacts with the pyrolysis products. Experiments showed that bio-oil yield reached a maximum value of 42.79% at a steam velocity of 1.3 cm/s, while it had a maximum value of 30.00% at the static retorting. The opposite was observed for the solid product: char yield decreased from 25.1 to 15.86% when the steam was used instead of a static atmosphere.
Fig. 4. Effect of steam velocity on the pyrolysis product yields.
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All the yields are expressed on a dry, ash-free basis and were the average yield of at least three attempts with an experimental measurements error of less than ±0.5%. 4.2. Chemical composition The elemental composition of the oils characterized are listed in Table 2, and the calorific values of the soybean oil cake and the oils and pH values of the oils are listed in Table 3. The average chemical composition of the Heinze retort oil analyzed under static atmosphere, nitrogen atmosphere and steam atmosphere are CH1.58N0.10O0.25, CH1.61N0.10O0.27 and CH1.6N0.12O0.38, respectively. Heinze retort product oil is characterized by high oxygen content with a higher H/C ratio then the original cake. Further comparison of H/C ratios with conventional fuels indicates that the H/C ratios of the oils obtained in this study lie between those of light and heavy petroleum products. Also, calorific values indicate that the energy contents of the oils are very close to that of petroleum. The IR spectrum of the Heinze oil under static atmosphere is given in Fig. 5. The O–H stretching vibrations between 3200 and 3400 cm⫺1 indicate the presence of phenols and alcohols. The C–H stretching vibrations between 2800 and 3000 cm⫺1 and C–H deformation vibrations between 1350 and 1475 cm⫺1 indicate the presence of alkanes. The C=H stretching vibrations with absorbance between 1650 and 1750 cm⫺1 indicate the presence of ketones or aldehydes. The absorbance peaks between 1575 and 1675 cm⫺1 representing C=C stretching vibrations are indicative of alkenes and aromatics. The results of the column chromatography fractions showing aliphatic, aromatic, ester and polar subfractions of the bio-oil obtained from the Heinze retort is given in Fig. 6. The polar fractions are dominate and it can be seen that in steam pyrolysis the contents of the polar fractions are somewhat higher, while the formation of aliphatic and aromatic compounds is seem to be favored under inert and static atmospheres. A gas liquid chromatogram of the aliphatic subfraction of pentane soluble of bio-oil obtained under static conditions is shown in Fig. 7. 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 to C29. Distribution of straight chain alkanes exhibit a maximum on the range of C14⫺C18. Table 3 Calorific values and pH values of soybean cake and Heinze retort oils Material
Calorific values (kcal/kg)
pH
Soybean cake Heinze oila Heinze oilb Heinze oilc
5558 7059 6938 7946
– 7.23 6.71 5.34
a b c
Obtained at 550 °C under static atmosphere. Obtained at 550 °C, with a nitrogen flow rate of 200 cm3/min. Obtained at 550 °C, with the steam velocity of 1.3 cm/s.
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Fig. 5. IR spectra of (a) oil, (b) pentane eluate, (c) toluene eluate, (d) ether eluate, and (e) methanol eluate.
Fig. 8 shows the 1H-NMR spectra of bio-oils obtained at optimum conditions under static, nitrogen and steam atmosphere. The hydrogen distribution of 1H-NMR is given in Table 4. 1HNMR spectra of the bio-oils indicates that the aromaticity of the bio-oil of steam pyrolysis was higher than the static and nitrogen atmosphere pyrolysis. CH2 and CH β to an aromatic ring (napthenic) protons are very close to each other in amount for bio-oils under static and nitrogen atmosphere while bio-oil under steam has a lower value. CH3앫CH2 and CH to an aromatic ring protons (centered at 2.65) and CH3γ or further from an aromatic ring protons are in very close agreement in the oils and are higher than the other protons.
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Fig. 6.
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Results of column chromatography of the bio-oils.
Fig. 7. GC chromatogram of the n-pentane eluate.
5. Conclusion An agricultural by-product, soybean cake, is taken as the biomass sample for the pyrolysis experiments performed in a fixed bed reactor under different conditions to obtain the maximum bio-oil yield. The oil yield was obtained as 30.23% from pyrolysis of soybean oil cake at the final pyrolysis temperature of 550 °C, with the samples having particle size of 0.850⬍Dp⬍1.250 mm, and with a heating rate of 5 °C/min. The oil reached a maximum yield of 33.78% under a sweep gas atmosphere (nitrogen flow rate of 200 cm3/min) and reached a maximum yield of 42.79% under a steam atmosphere (steam velocity of 1.3 cm/s), indicating the effect of steam on the pyrolysis of biomass, by acting as a physical factor, influencing the heat transfer and favoring the fast desorption of low molecular products [8]. Furthermore, detailed studies are needed on the role of steam flow rate in order to characterize the bio-oils because of the possibility of increasing the amounts of parafinic and aromatic components.
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Fig. 8. 1H-NMR spectra of bio-oils. (a) Bio-oil from static, (b) bio-oil from nitrogen atmosphere, (c) bio-oil from water vapor.
In contrast to coals and oil shales [13], oil yields from the soybean oil cake were found to be largely independent of particle size (⬍2 mm) and sweep gas velocity. The H/C ratios show that the fractions are quite similar to currently utilized transport fuels.
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Table 4 Results of 1H-NMR for three bio-oils from the fixed-bed pyrolysis of soybean cake Type of hydrogen
Chemical shift (ppm) Bio-oil under static atmosphere
Bio-oil under nitrogen Bio-oil under steam atmosphere
Aromatic Phenolic (OH) or olefinic proton Ring-joinmethylene (Ar–CH2–Ar) CH3앫CH2 and CH to an aromatic ring CH2 and CHβ to an aromatic ring (napthenic) β-CH3, CH2, and CHγ or further from an aromatic ring CH3γ or further from an aromatic ring
6.5–9.0 5.0–6.5
13.06 2.06
14.98 3.87
17.06 1.47
3.3–4.5
1.09
0.69
2.80
2.0–3.3
24.86
22.68
26.77
1.6–2.0
14.19
13.96
8.65
1.0–.6
16.88
18.57
23.22
0.5–1.0
27.86
25.25
20.03
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