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Energy & Fuels 2007, 21, 2398-2407
Influence of Temperature on the Formation of Oil from Pyrolyzing Palm Oil Wastes in a Fixed Bed Reactor Jianfen Li,†,‡ Rong Yan,*,‡ Bo Xiao,† Xiaoling Wang,‡ and Haiping Yang‡ School of EnVironmental Science & Engineering, Huazhong UniVersity of Science and Technology, Wuhan, 430074, P. R. China, and Institute of EnVironmental Science and Engineering, Nanyang Technological UniVersity, InnoVation Center, Block 2, Unit 237, 18 Nanyang DriVe, Singapore 637723 ReceiVed NoVember 3, 2006. ReVised Manuscript ReceiVed May 13, 2007
The objective of this study is to summarize the biomass conversion pathway through analyzing the changing trend of oil species and revealing the relationship between the evolution of oil species and the evolved gas composition at varied temperatures. For this purpose, the pyrolysis of palm oil wastes was conducted in a countercurrent fixed bed reactor at different final temperatures 500 to 900 °C, and the pyrolysis oil was thoroughly identified using various approaches including CNHS/O elemental analysis, Fourier transform infrared spectroscopy, column chromatography, and gas chromatography/mass spectrometry analyses. With the temperature increasing from 500 to 900 °C, the yield of oil was decreased while that of the total gas was enhanced greatly. Meanwhile, the variety and proportion of oxygenated compounds inside the oil gradually declined with an increase in the temperature, whereas those of the polycyclic aromatic hydrocarbons (PAHs) showed a marked increase, with a peak observed at 800 °C. The changing trend of the oil main components versus the temperature could be concluded as follows: with increasing temperature, the primary oil, containing mainly oxygenated compounds, was transformed into a secondary oil consisting of dominant phenolics; then, the oil became more aromatic with rising temperature and finally led to the tertiary oil that was dominated by PAHs. Meanwhile, the increasing temperature promoted secondary reactions of oil to result in the evolution of gases such as CO, CO2, H2, CH4, and so forth. In particular, the yield of H2 and CO could be regarded as an indicator for secondary reactions of oil.
1. Introduction The yield and composition of pyrolysis products (gas, liquid oil, and char) are highly dependent on the operating parameters,1-3 especially the process temperature, which is one of the most important operating variants in controlling the biomass pyrolysis performance. So far, various experimental studies have been carried out to investigate the influence of temperature on pyrolysis,4-6 focusing mostly on evaluating the yields of the three products. According to the literature,7 conventional slow pyrolysis has been applied for many years and used mainly for the production of charcoal; rapid pyrolysis has been performed generally at ∼500 °C where the liquid oil is of main interest.1,8 Some other biomass pyrolysis/gasification research has been * Corresponding author. Tel: (65) 67943244. Fax: (65) 67921291. E-mail:
[email protected]. † Huazhong University of Science and Technology. ‡ Nanyang Technological University. (1) Bridgwater, A. V.; Meier, D.; Radlein, D. An Overview of Fast Pyrolysis of Biomass. Org. Geochem. 1999, 30 (12), 1479-1493. (2) Demirbas, A.; Arin, G. An Overview of Biomass Pyrolysis. Energy Sources 2002, 24 (5), 475-482. (3) Chen, G.; Andries, J.; Luo, Z.; Spliethoff, H. Biomass Pyrolysis/ Gasification for Product Gas Production: The Overall Investigation of Parametric Effects. Energy ConVers. Manage. 2003, 44 (11), 1875-1884. (4) Demirbas, A. Effect of Temperature on Pyrolysis Products from Four Nut Shells. J. Anal. Appl. Pyrolysis 2006, 76 (1-2), 285-289. (5) Horne, P. A.; Williams, P. T. Influence of Temperature on the Products from the Flash Pyrolysis of Biomass. Fuel 1996, 75 (9), 10511059. (6) 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 (6), 2216-2224. (7) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of Wood/Biomass for Bio-Oil: A Critical Review. Energy Fuels 2006, 20 (3), 848-889.
conducted at high temperatures above 700 °C to focus on converting biomass into higher-value fuel gas9,10 for bioenergy utilization through electrical power production, wherein oil vapor (i.e., tar) present in fuel gas would be disadvantageous to the subsequent equipment and environment.11 Moreover, certain polycyclic aromatic hydrocarbons (PAHs) in the oil are carcinogenic or mutagenic, and the PAH content of oil could be correlated with the process temperature.12,13 Therefore, analyzing the pyrolysis oil generated, in terms of the detailed compositions and species of oil versus the process temperature, is of great interest in the research of biomass pyrolysis and gasification. So far, only limited data12-14 have been found in the literature for the influence of the pyrolysis temperature on detailed components and species distribution of oil products in a broad (8) Yaman, S. Pyrolysis of Biomass to Produce Fuels and Chemical Feedstocks. Energy ConVers. Manage. 2004, 45 (5), 651-671. (9) Zanzi, R.; Sjostrom, K.; Bjornbom, E. Rapid Pyrolysis of Agricultural Residues at High Temperature. Biomass Bioenergy 2002, 23 (5), 357366. (10) Zanzi, R.; Sjostrom, K.; Bjornbom, E., Rapid High-Temperature Pyrolysis of Biomass in a Free-Fall Reactor. Fuel 1996, 75 (5), 545-550. (11) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A Review of the Primary Measures for Tar Elimination in Biomass Gasification Processes. Biomass Bioenergy 2003, 24 (2), 125-140. (12) Elliott, D. C. Relation of Reaction Time and Temperature to Chemical Composition of Pyrolysis Oils. Presented at ACS Symposium Series 376, Denver, CO, April 1987; Pyrolysis Oils from Biomass; Soltes, E. J., Milne., T. A., Eds.; American Chemical Society: Washington, DC, 1987. (13) Morf, P. O. Secondary Reactions of Tar during Thermochemical Biomass Conversion. Ph.D. Thesis, Swiss Federal Institute of Technology, Zurich, Switzerland, 2001. (14) Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass. Energy Fuels 1987, 1 (2), 123-137.
10.1021/ef060548c CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
Formation of Oil from Pyrolyzing Palm Oil Wastes
Energy & Fuels, Vol. 21, No. 4, 2007 2399
Table 1. Proximate and Ultimate Analyses of Palm Oil Wastesa proximate analysis (wt %) shell fiber EFB
ultimate analysis (wt %, dry basis)
Mad
Vad
Ad
FCad
C
H
N
S
Ob
molecular formula
5.73 6.56 8.75
73.74 75.99 79.67
2.21 5.33 3.02
18.37 12.39 8.65
53.78 50.27 48.79
7.20 7.07 7.33
0.00 0.42 0.00
0.51 0.63 0.68
36.30 36.28 40.18
CH1.61O0.51 CH1.69O0.54 CH1.80O0.62
a M, moisture content; V, volatile matters; A, ash; FC: fixed carbon; ad, on air-dried basis; d, on dry basis. b The oxygen (O) content was determined by difference.
temperature range (500-900 °C), although some are available for gas products at high temperatures and liquid products at low temperatures. Evans and Milne14 investigated the oil compositions in the vapor phase and its characteristic changes as a function of the temperature and residence time by a technique named molecular-beam mass spectrometry. The oil compositions in the vapor phase and its change pathway were experimentally verified, while the condensed-phase transitions could not be determined in their experimental setup. A series of our previous work15-18 was also conducted with focuses on the gas products distribution from pyrolyzing palm oil wastes (a local representative biomass waste) while bio-oil generation in the course of the studies was ignored. Further research on the transitions of both the condensed phase and gas phase is thus meaningful. In this study, the pyrolysis of palm oil wastes (mostly shell) was investigated at different temperatures in a bench-scale countercurrent fixed bed reactor, and the chemical composition and species of the pyrolytic oils were analyzed using several techniques such as elemental analysis, Fourier transform infrared spectroscopy (FTIR), column chromatography, and gas chromatography/mass spectrometry (GC/MS). The objective of this work is to systematically analyze the detailed components and species distribution of oil products derived from pyrolyzing palm oil wastes in a broad temperature range (500-900 °C), to attempt to summarize and explain the related conversion pathway and changing trend of oil species under varied temperatures, and hopefully to find out the relationship between the oil species and the evolved gas composition. The knowledge obtained would enhance our understanding of the mechanisms of oil formation, conversion, and destruction during biomass pyrolysis or gasification and assist in the design of reactors and the optimization of process parameters in converting palm oil wastes to bioenergy. 2. Materials and Methods 2.1. Samples. The palm oil wastes [shell, fiber, and empty fruit bunches (EFB)] were obtained from Malaysia. They were ground in a laboratory-scale centrifugal mill (Rocklabs, New Zealand) and sieved in a Retsch test sieve with a 1 mm screen (Retsch, Fisher Scientific Company, U.S.A.); that is, the particle size of the sample analyzed was less than 1 mm. The results of proximate and ultimate analyses of palm oil wastes are listed in Table 1, and the details of the analytical methods can be found in our previous work.15 It is shown that palm oil wastes are environmentally friendly energy (15) Yang, H.; Yan, R.; Chin, T.; Liang, D. T.; Chen, H.; Zheng, C. Thermogravimetric Analysis-Fourier Transform Infrared Analysis of Palm Oil Waste Pyrolysis. Energy Fuels 2004, 18 (6), 1814-1821. (16) Yan, R.; Yang, H.; Chin, T.; Liang, D. T.; Chen, H.; Zheng, C. Influence of Temperature on the Distribution of Gaseous Products from Pyrolyzing Palm Oil Wastes. Combust. Flame 2005, 142 (1-2), 24-32. (17) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Liang, D. T.; Zheng, C. Pyrolysis of Palm Oil Wastes for Enhanced Production of Hydrogen Rich Gases. Fuel Process. Technol. 2006, 87 (10), 935-942. (18) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Liang, D. T.; Zheng, C. Mechanism of Palm Oil Waste Pyrolysis in a Packed Bed. Energy Fuels 2006, 20 (3), 1321-1328.
Figure 1. Schematics of bench-scale reactor.
sources containing trace amounts of N, S, and mineral matter. If only the main elements (C, H, and O) are considered, the molecular formula of the studied samples based on one C atom can be written as CHxOy, as listed in Table 1. 2.2. Experimental Apparatus and Methods. Pyrolysis of biomass materials was conducted in a bench-scale countercurrent fixed bed, shown in Figure 1.17 The system consists essentially of a quartz tube (i.d. 50 mm, o.d. 55 mm, bed height 41 cm, and air freeboard 21 cm) with a continuous feeding system, a gas cleaning section containing a cyclone solid collector and a quartz wool filter, a cooling system for the separation of water and condensable organic vapors (oil), and various gas measurement devices. The quartz tube reactor was heated by a furnace with four independently controlled electric resistance heaters. In this study, the pyrolysis temperature was controlled at 500, 600, 700, 800, and 900 °C, and the operating pressure in the reactor was close to the atmospheric pressure. The feeding rate was kept at ∼1 g/min, and purge gas (N2) was supplied from the bottom of the reactor at a flow rate of 5 L/min holding a constant gas residence time for about 5 s; each experiment lasted for 30 min. The ground palm oil wastes were held in a hopper with a motor feeder. When the desired temperature was reached, a small flow of N2 was used to blow the biomass powder in the entrainment tube into the reactor. To prevent biomass from degrading, the feeding tube was cooled by means of an air-cooled jacket. The feedstock entering into the reactor was heated up, dried, devolatilized, and finally decomposed to generate solid charcoal, liquid oil, water vapor, and gas products (H2, CO, CO2, CH4, etc.). The solid charcoal residue was mostly collected on the screen sieve (gas distributor). The volatiles and fine particles, carried by purging gas, passed through the cyclone and quartz wool filter, and as such, the fine particles were removed. The condensable matter was quenched as the gas passed through a water-cooling tube and two ice-water condensers in series. After every experiment, the residues collected inside the tube and cyclone were combined and recorded as solid charcoal. The cooling tube and condensers were weighed, and the weight difference before and after the experiment was recorded as the liquid yield (including water). The total gas yield (wt %) could thus be calculated by difference on the basis of the mass balance of the fed biomass in a specific time
2400 Energy & Fuels, Vol. 21, No. 4, 2007 period at a constant feeding rate. Those incondensable gases were precleaned through a glass wool filter and dried by silica gels prior to analysis. The condenser and all connection tubes were then washed using acetone, and the liquid was also collected. Water in the collected solution was removed by anhydrous sodium sulfate; the organic phase was further filtered to remove the fine particles; then, the filtrate was dried at 60 °C to remove the acetone, and the residue was mostly the liquid oil which was collected for further analysis. The gas products were analyzed using a dual channel micro-gas chromatograph (Micro-GC, Varian, CP-4900) with a thermal conductive detector. The detailed measuring procedure can be found in our previous publication.17 The mass balance verification of the system has been done in our previous studies17 where gas products were measured by MicroGC, instead of calculation by difference. The sum of solid charcoal, liquid oil and gas products was reported in the range of 88.295.2% over the fed biomass. The rest 4.8-11.8% could be some incondensable liquid oil, moisture and fine particles. There might be certain errors of gas yield calculated by difference, which is, nevertheless, negligible in view of the focus of this study. 2.3. Oil Analysis. 2.3.1. Elemental Analysis. The contents of carbon, hydrogen, nitrogen, and sulfur in the bio-oils from palm oil wastes were determined using a model 2400II Perkin-Elmer CHNS/O analyzer. The oxygen content was found by difference. 2.3.2. Functional Group Composition Analysis. Functional group compositional analysis of the liquid oils after removal of the water was carried out using Fourier transform infrared spectrometry (BioRad Excalibur Series, model FTS 3000) equipped with a deuterated triglycine sulfate detector. To obtain a homogeneous sample, 0.2 mL of the treated and concentrated liquid oil was injected by microsyringe onto a previously dried KBr pellet; then, the sample-laden pellet was dried under a vacuum at 35 °C. Following that, a thin film of liquid oil on the KBr plate was formed, and it was used for scanning. Before each measurement, the FTIR was run to establish a background with KBr added with 0.2 mL of pure acetone and using the same sample preparation procedure. In this study, IR spectra were recorded by coadding 128 scans between 4000 and 650 cm-1 with a resolution of 4 cm-1 and a sensitivity of 2. 2.3.3. Chemical Class Fractionation of Oil. Chemical class fractionation of the oils (aliphatic, aromatic, oxygenated ,and polar chemical fractions) was performed using open tubular liquid chromatography. The four fractions of oil were differentiated on the basis of the different chemical natures of eluents used. The pentane elution was used for aliphatic compounds and lowmolecular-weight aromatic compounds such as single-ring and tworing aromatic compounds; benzene for higher-molecular-weight PAHs; ethyl acetate for oxygenated compounds such as ether, aldehyde, ketone, ester, and so forth; and methanol for polar compounds such as alcohol, carboxylic acid, nitrogenous alkaline compounds, and so on. In reference to the previous works,19-21 the oil separation method was established as follows. A dual-packed (silica gel-alumina gel) column was used to separate the oils into four fractions. The column was a 1.67 cm (o.d.) by 20-cm-long glass tube of 30 mL capacity, fit with a 10 mL eluant reservoir at the top. A stopcock at the bottom of the column was used to regulate the flow rate. The bottom of the column was packed with fully activated (16 h at 400 °C) 50200 mesh neutral alumina gel (8 g) using the wet filled method, and the middle was packed with fully activated (16 h at 265 °C) 28-200 mesh silica gel (4 g); the mass ratio of alumina gel and
Li et al.
Figure 2. Product yields from shell pyrolysis at different temperatures.
silica gel to the oil sample was about 25:1. To prevent the formation of a solid phase with the addition of the pentane mobile phase and to improve solvent contact with the oil, the oil was intimately mixed with Chromosorb G/AW/DMCS 80-100 mesh inert support, with the mass ratio of Chromosorb G to oil at 3:1, and the mixture was packed into the top of the column above the analytical phase. The column was then sequentially eluted with 20 mL each of pentane, benzene, ethyl acetate, and methanol (polarity relative to Al2O3 is 0.00, 0.32, 0.58, and 0.95, respectively) at an approximate flow rate of 0.5∼1 mL/min, to produce aliphatic, aromatic, oxygenated, and polar chemical class fractions, respectively. Following that, each fraction was evaporated to dryness, weighed, and recorded as the fraction weight; accordingly, the mass percentage in each fraction was calculated. The evaporation of the solvent would inevitably lead to some loss of volatile materials consequently; this step in the analytical procedure was carefully carried out to minimize the losses. 2.3.4. Analysis of Oil Fractions by GC/MS. The components of the oil fraction were further analyzed using GC/MS (GC6890N/ MSD5973, Agilent Technologies, Inc.), equipped with a DB5ms capillary column (30 m × 0.25 mm i.d. × 0.25 µm d.f.). Among the four fractions, the pentane and benzene fractions were mixed and analyzed for aliphatic and aromatic hydrocarbons, while the ethylacetate and methanol fractions were mixed for identifying oxygenated and polar compounds. The operating parameters of GC/ MS were listed as follows: splitless injection of the sample at 1 µL, injection temperature of 275 °C, and interface temperature of 325 °C. Helium was taken as a carrier gas at a 1.3 mL/min constant flow rate. The oven temperature program consisted of 1 min of isothermal conditions at 60 °C and a ramp to 115 °C at 2 °C/min holding for 1 min, followed by a 10 °C/min ramp to 130 °C and then a 25 °C/min ramp to 295 °C holding for 0.25 min; subsequently, there was a ramp to 300 °C at 1 °C/min and was followed by a 30 °C/min ramp to 325 °C; the total program time was 18.9 min. This program was selected according to the suggestions of Agilent Technologies, Inc.22 The mass spectrometer was a Finnigan-MAT bench-top ion trap detector with a mass range from 50 to 550 u and was linked to a computer with a National Institute of Standards and Technology mass spectral library.
3. Results and Discussions (19) Yorgun, S.; Senso¨z, S.; Kockar, O ¨ . M. Flash Pyrolysis of Sunflower Oil Cake for Production of Liquid Fuels. J. Anal. Appl. Pyrolysis 2001, 60 (1), 1-12. (20) Williams, P. T.; Horne, P. A. Characterisation of Oils from the Fluidised Bed Pyrolysis of Biomass with Zeolite Catalyst Upgrading. Biomass Bioenergy 1994, 7 (1-6), 223-236. (21) Hirsch, D. E.; Hopkins, R. L.; Coleman, H. J. Separation of HighBoiling Petroleum Distillates Using Gradient Elution Through Dual-Packed (Silica Gel-Alumina Gel) Adsorption Columns. Anal. Chem. 1972, 44, 915919.
3.1. Influence of Temperature on Product Yields. Figure 2 shows the weight percentage of final products (oil, char, and gas) from the pyrolyzing oil palm shell, over the mass of biomass fed at different pyrolysis temperatures. The derived (22) Prest, H. Solid-Phase Extraction and Retention-Time Locked GC/ MS Analysis of Seleted Polycyclic Aromatic Hydrocarbons; Agilent Technology: Santa Clara, CA, 2002.
Formation of Oil from Pyrolyzing Palm Oil Wastes
Figure 3. Gas species distribution profile at different temperatures.
liquid oil represented a single homogeneous liquid phase low in viscosity and brown in color. With the temperature increasing from 500 to 900 °C, both oil and char yields reduced respectively from 30 to 20 wt % and from 29 to 10 wt %, while the total gas yield increased sharply from 40 to 69 wt %. The increase in gaseous products was attributed to the decomposition of char residue and the secondary reaction of the oil vapor. Particularly, the increase of gas products from 800 to 900 °C came predominantly from oil vapor cracking. In general, the vapor residence time of the rapid pyrolysis is less than 2 s, and the yield of the produced liquid oil is more than 50%, while the gas yield is less than 20%.23-25 In this study, the oil yield at 500 °C was lower than those reported, due to the longer gas residence time of ∼5 s encountered, which might enhance the vapor cracking in the gas phase, and accordingly the gas yield in this work would be higher than those reported. Thus, the pyrolysis conducted here is a kind of slow pyrolysis according to the product yields and gas residence time. Although the vapor residence time could be an important influencing parameter, the effect of temperature was the main focus of this study, and the vapor residence time was thus maintained constant. 3.2. Influence of Temperature on Gas Product Components. Details about the influence of temperature on the distribution of gaseous products from pyrolyzing palm oil wastes can be found in our previous works.16,17 Some results are cited here to facilitate the discussion on the relationship of gas releasing and oil vapor cracking at varying temperatures. The gas species distribution profile from shell pyrolysis at different final temperatures is plotted in Figure 3. It indicated that the main gas products were H2, CO, CO2, CH4, and some C2 hydrocarbons (C2H4 and C2H6). Among them, the H2 content increased steadily from 3.6 to 15.0% as the temperature increased from 500 to 800 °C, and with a further increase of temperature to 900 °C, the H2 content increased significantly to 33.5%. Yields of CH4 also increased from 2.8 to 11.9%, while that of CO2 decreased in general with increasing temperature, particularly at 900 °C. The yield of CO decreased first from 36.8 to 22.1% as the temperature increased to 700 °C; however, it increased again to 41.3% as the temperature continuously increased to 900 °C. The yields of C2H4 and C2H6 were relatively small, and the influence of temperature was insig(23) Acikgoz, C.; Onay, O.; Kockar, O. M. Fast Pyrolysis of Linseed: Product Yields and Compositions. J. Anal. Appl. Pyrolysis 2004, 71 (2), 417-429. (24) Islama, N. M.; Zailani, R.; Ani, N. F. Pyrolytic Oil from Fluidised Bed Pyrolysis of Oil Palm Shell and Its Characterisation. Renewable Energy 1999, 17 (1), 73-84. (25) Meier, D.; Faix, O. State of the art of applied fast pyrolysis of lignocellulosic materials-a review. Biosource Technol. 1999, 68 (1), 7177.
Energy & Fuels, Vol. 21, No. 4, 2007 2401
nificant. At low temperatures (500 °C), CO2 (54.0%) and CO (36.8%) demonstrated the highest yields, while CO (41.3%) and H2 (33.5%) showed the highest yields at high temperatures (900 °C). The variation of gas product distribution observed could be explained by secondary reactions of the oil. According to the literature,26,27 the pyrolysis process of palm oil waste can be divided into two steps: (1) Decomposition of Biomass. This step, also called primary pyrolysis, could be performed at a lower temperature such as 300 °C, and lasted until a temperature of 700 °C or even higher was reached. In this step, the biomass feedstock was loaded into the reactor and pyrolyzed very quickly. The main chemical process could be described as
biomass f char + oil (including H2O) + gas (CO2, CO, H2, CH4, CnHm) (2) Secondary Reactions of Oil Cracking and ConVersion. This second step ought to occur at higher temperatures (>400 °C). The main secondary reactions of oil cracking and shifting include decarboxylation, decarbonylation, dehydrogenation, cyclization, aromatization, and polymerizing reactions, which are given in order of increasing pyrolysis severity (e.g., increasing temperature). The variation trend of each gas component as a function of the temperature is believed to be relative to the change of oil composition at different temperatures, which will be discussed in the subsequent sections. 3.3. Influence of Temperature on Elemental Content of Oil. Table 2 shows the results of an elemental analysis of the liquid oil from shell pyrolysis. Carbon and oxygen were the two major elements occurring in pyrolysis oil, followed by hydrogen and nitrogen. Only a small amount of sulfur (0.10.3 wt %) was found there, indicating the negligible concern of SOx emission from the utilization of pyrolysis oil. The molecular formula of the liquid oil based on one C atom can be written as CHxOyNz if considering only the main elements (C, H, O, and N), as given in the last column of Table 2. The data at 500 °C matched well with those previously reported by Islama et al.24 for the pyrolysis oils derived from the same feedstock but only at low pyrolysis temperatures. With the pyrolysis temperature increasing from 500 to 900 °C, a big increase of carbon and a decrease of oxygen and nitrogen contents were observed, together with a slight variation of hydrogen and sulfur contents. The content of carbon increased from 53.28 to 83.44 wt %, while that of oxygen decreased from 36.10 to 9.28 wt %. Consequently, the molar ratio of oxygen to carbon (O/C) was markedly decreased from 0.508 at 500 °C to 0.083 at 900 °C. The decrease of the molar ratio of O/C between the 700 and 800 °C interval was the most significant, with the maximum decrement (13.15 wt %) of oxygen content and increment (12.91 wt %) of carbon content occurring in the oils, whereas the variation of oxygen and carbon contents for other temperature intervals were less than 8 wt %. Clearly, the deoxygenation, dehydration, decarbonylation, and decarboxylation reactions led to the contents of oxygen in oils being greatly decreased. Pyrolysis products at higher temperatures became considerably more deoxygenated, implying a large number of oxygenated functional groups lost during pyrolysis, especially at 900 °C. (26) Dai, X. W.; Wu, C. Z.; Li, H. B.; Chen, Y. The Fast Pyrolysis of Biomass in CFB Reactor. Energy Fuels 2000, 14 (3), 552-557. (27) Fisher, T.; Hajaligol, M.; Waymack, B.; Kellogg, D. Pyrolysis Behavior and Kinetics of Biomass Derived Materials. J. Anal. Appl. Pyrolysis 2002, 62 (2), 331-349.
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Li et al.
Table 2. Ultimate Analyses of Oil from Shell Pyrolysis at Different Temperatures
a
temp (°C)
C (wt %)
H (wt %)
N (wt %)
S (wt %)
Oa (wt %)
H/Cb
O/Cb
molecular formula
500 600 700 800 900
53.28 56.32 62.77 75.68 83.44
7.16 7.02 6.97 7.84 6.7
3.24 3.28 3.47 2.86 0.46
0.22 0.25 0.34 0.32 0.12
36.10 33.13 26.45 13.30 9.28
1.613 1.496 1.332 1.243 0.964
0.508 0.441 0.316 0.132 0.083
CH1.613O0.508N0.054 CH1.496O0.441N0.051 CH1.332O0.316N0.047 CH1.243O0.132N0.032 CH0.964O0.083N0.005
The oxygen (O) content was determined by difference. b Molar ratio
Meanwhile, the molar ratio of hydrogen to carbon (H/C) in this study was obviously declined from 1.613 at 500 °C to 0.964 at 900 °C. As for organic compounds, the H/C atomic ratio in the molecular formula gradually reduced from above 2 for alkane, to 2 for single alkene, to above 1 for general aliphatic compounds, to 1 for benzene, and to below 1 for PAHs, and the H/C molar ratio would become even smaller with an increase of ring numbers in PAHs. Consequently, the decreasing H/C molar ratio versus the increasing temperature suggested that the cyclization, dehydrogenation, and aromatization reactions occurred in the pyrolysis oils. The increasing temperature would further promote these reactions, leading to the amount of aromatic hydrocarbons in oil being markedly increased. Particularly, the lowest H/C ratio of less than 1 for the pyrolysis oil was observed at 900 °C, implying the pyrolysis oil might contain a high concentration of PAHs at that temperature. 3.4. Influence of Temperature on Functional Group Composition of Oil. The FTIR spectra of liquid oil from shell pyrolysis at different temperatures are illustrated in Figure 4, shown in transmittance percentage. The liquid oil derived from shell pyrolysis demonstrated the presence of a wide range of oxygenated compounds and their derivatives. For example, peaks representing O-H vibrations between 3050 and 3600 cm-1 and O-H in-plane deformations between 950 and 1325 cm-1 all indicated the presence of alcohols and phenols; the presence of CdO stretching vibrations between 1650 and 1800 cm-1 was compatible with the presence of ketones, aldehydes, carboxylic acids, and so forth; C-O stretching vibrations between 1050 and 1275 cm-1 correlated with the presence of ether, phenols, alcohols, and esters. Meanwhile, peaks representing alkyl and aryl groups of hydrocarbons were also present in the oil. For example, the C-H stretching vibrations between 2850 and 2960 cm-1 and C-H deformation vibrations between 1350 and 1465 cm-1 indicated the presence of the chemical functional groups -CH3, -CH2, and -CH, which are characteristic of alkane groups. The absorption peaks of C-H bending vibrations on the aromatic ring (i.e., Ar-H) between 690 and 900 cm-1 and CdC stretching vibrations of the aromatic ring between 1450 and 1600 cm-1 were characteristic of single-ring aromatic compounds and polycyclic aromatic compounds. Likewise, the small peak observed between 1620 and 1680 cm-1 implied the existence of CdC bonds inside alkenes. Therefore, the identification of the above-mentioned functional groups most likely indicated the presence of alkyl groups attached to oxygenated compounds and complex oxygenated compounds with attached aromatic groups. These would be subsequently confirmed by GC/MS analysis. Furthermore, the functional groups observed in the liquid oil from the pyrolysis of palm oil shells at 500 °C are compatible with those obtained from hazelnut shells at the same temperature.28 With temperature increasing from 500 to 900 °C, several changes of peak intensities on the oil functional group were (28) Pu¨tu¨n, A. E.; O ¨ zcan, A.; Pu¨tu¨n, E. Pyrolysis of Hazelnut Shells in a Fixed-Bed Tubular Reactor: Yields and Structural Analysis of Bio-Oil. J. Anal. Appl. Pyrolysis 1999, 52 (1), 33-49.
clearly observed. First, the absorption peaks of O-H vibrations between 3050 and 3600 cm-1 decreased greatly, indicating the significant decreases of the oxygenated compounds containing hydroxy functional groups in concentration. This was in conformity with the former results of the oxygen content declining with increasing pyrolysis temperature seen in Table 2. Second, the peaks between 1450 and 1600 cm-1 and 690 and 900 cm-1 that are characteristic of aromatic compounds showed a marked increase as the temperature increased from 500 to 800 °C, indicating the significant increase of aromatic compounds’ concentration in the pyrolysis oils. Meanwhile, the peak intensity of C-H stretching vibrations between 2850 and 2960 cm-1 and C-H deformation vibrations between 1350 and 1465 cm-1 also increased markedly as the temperature varied between 500 and 800 °C, indicating the great increase of the hydrocarbons in liquid oil. Third, an obvious decrease in the intensity of the characteristic peak of alkenes (CdC) observed between 1620 and 1680 cm-1 could be seen as the temperature was increased, suggesting that the content of the alkenes in the oils was continuously declining. Finally, the FTIR spectra of the oil generated at 900 °C were obviously simpler; not only the number of peaks but also the intensity of most peaks declined, except for the PAH characteristic peak between 1500 and 1700 cm-1. This suggests a great decrease of functional groups inside the liquid oil at 900 °C and sharp thermal cracking of the organics leading to the formation of PAHs. It was noteworthy that the wide absorption peak of PAHs shown in the 900 °C spectra was probably due to the overlapping of the CdC vibration in alkenes (1620 and 1680 cm-1) and the strong CdC stretching vibrations of PAHs (1450-1600 cm-1) with possibly a slight shift of the latter toward the former, which might be explained by the significant formation of conjugated polycyclic aromatic rings causing peak broadening, as evidenced by a negligible Ar-H peak (690-900 cm-1) that appeared at 900 °C.29-33 Moreover, the baseline of the IR transmittance showed an obvious decline from 100% to 60% as the pyrolysis temperature increased from 500 up to 900 °C, probably due to the increase of aromatics content in the oil during pyrolysis.13,28 Serio and Solomon29-33 studied the relationship between gas evolution and the functional group compositions of tar derived from coal pyrolysis in secondary reactions of tar. The evolution of hydrogen and CO was believed to be an indicator for secondary reactions of tar; that is, the enhancement of hydrogen (29) Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Coal Pyrolysis: Experiments, Kinetic Rates and Mechanisms. Prog. Energy Combust. Sci. 1992, 18 (2), 133-220. (30) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Very Rapid Coal Pyrolysis. Fuel 1986, 65 (2), 182-194. (31) Serio, M. A.; Peters, W. A.; Howard, J. B. Kinetics of Vapor-Phase Secondary Reactions of Prompt Coal Pyrolysis Tars. Ind. Eng. Chem. Res. 1987, 26 (9), 1831-1838. (32) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. CrossLinking Reactions during Coal Conversion. Energy Fuels 1990, 4 (1), 4254. (33) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Kinetics of Volatile Product Evolution in Coal Pyrolysis: Experiment and Theory. Energy Fuels 1987, 1 (2), 138-152.
Formation of Oil from Pyrolyzing Palm Oil Wastes
Energy & Fuels, Vol. 21, No. 4, 2007 2403
Figure 5. Mass percentage of each chemical fraction of oil at different temperatures.
Figure 4. FTIR spectra of oil from shell pyrolysis at different temperatures.
and CO yield suggested the decrease of oxygenated compounds and increase of aromatic compounds and PAHs in tar. In this study dealing with biomass, if the results from Figures 3 and 4 are combined, a similar observation can be found: the continuously increasing H2 yield with rising temperature (Figure 3) was consistent with the increasing amount of aromatic compounds and PAHs in oil (Figure 4). In particular, at above 700 °C, the rapidly rising yield of H2 and CO suggested the marked reduction of oxygenated compounds and significant increasing of aromatic compounds in oil. 3.5. Influence of Temperature on Chemical Class Fractionation of Oil. In this study, the aliphatic hydrocarbons were eluted in a pentane elution fraction; aromatic hydrocarbons were mainly present in both the pentane and benzene elution fractions; oxygenated compounds were primarily found in both the ethyl acetate and methanol elution fractions, but of different polarities. The mass percentage of each chemical fraction of oil versus the final pyrolysis temperature is plotted in Figure 5. Increasing the pyrolysis temperature from 500 to 900 °C resulted in a decrease in the oxygenated and polar fractions (ethyl acetate and methanol elution) from 26.3 to 7.3 wt % and from 56.9 to 3.8 wt %, respectively. Correspondingly, an increase in the hydrocarbons fractions (aliphatics and aromatics) existing in the pentane and benzene fractions was found. It was especially arrestive that the benzene fraction comprised of larger molecules of PAHs sharply increased from 10.6 to 74.7 wt % with an increase in pyrolysis temperature from 500 to 900 °C, while the polar chemical fraction in methanol elutions showed the most marked reduction. Subsequent analysis of the elutions by GC/MS further confirmed that this chemical classification method performed well in separating hydrocarbons and oxygenated compounds. 3.6. Influence of Temperature on Detailed Components of Oil. Table 3 shows the detailed analysis by GC/MS of the
Figure 6. Variation trend of the total and various aromatic hydrocarbons versus temperature.
pentane and benzene fractions for aliphatic and aromatic hydrocarbons present in the pyrolysis oils, including retention time (tR), molecular formula, compound name, similarities, and peak area at different temperatures. The retention times of the same compound generated at the different pyrolysis temperatures are consilient, and the MS similarities obtained in comparison with the standard spectra are relatively high (>90% for most identified compounds). In GC/MS analysis, the exact same conditions were applied for each individual test, and therefore the peak area of each species, instead of the absolute quantification, was used to represent the amount of the concerned species and to compare its changing trend with respect to the condition (i.e., temperature). On the other hand, as pointed out by Evans and Milne,14 ultimately it is probably not affordable, or desirable, to attempt to routinely quantify the hundreds of minor species known to be involved in pyrolysis. In particular, in this study, the results of most significance are the kinds of species observed and their relative change with pyrolysis conditions, rather than their absolute values. From Table 3, it can be seen that the pyrolysis oils generated at different temperatures consist mostly of aromatic hydrocarbons and a few types of aliphatics. The majority of aromatic hydrocarbons inside oils were naphthalene, biphenyl, acenaphthylene, fluorene, phenanthrene, and their alkylated homologues. At 500 °C, the oil contained relatively less PAHs such as naphthalene, acenaphthylene, fluorene, phenanthrene, and fluo-
2404 Energy & Fuels, Vol. 21, No. 4, 2007
Li et al.
Table 3. Aliphatic and Aromatic Compounds in Pyrolysis Oila peak no.
tR (min)
molecular formula
compounds
similarity
500 °C
600 °C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
2.91 3.86 6.35 7.53 7.67 8.64 8.66 8.79 9.00 9.37 9.63 9.89 10.25 10.69 10.72 10.99 11.31 11.92 12.10 13.97 14.97 15.80 17.68 18.31
C8H16 C9H12 C10H8 C11H10 C11H10 C12H10 C12H10 C12H8 C12H10 C13H12 C13H10 C13H10 C14H12 C14H10 C14H10 C16H12 C15H10 C16H10 C16H10 C19H14 C20H12 C21H14 C22H12 C22H12
2-heplene, 3-methylbenzene, 1,3,5-trimethylnaphthalene naphthalene, 2-methylnaphthalene, 3-methylbiphenyl naphthalene, 2-ethenylacenaphthylene acenaphthene 1,1'-biphenyl, 3-methylfluorene 1H-phenalene phenanthrene, 9,10-dihydrophenanthrene anthracene naphthalene, 1-phenyl6H-cyclobuta[jk]phenanthrene pyrene fluoranthene chrysene, 6-methylBenzo[a]pyrene perylene, 3-methyldibenzo[def,mno]chrysene benzo[ghi]perylene
90.8 81.2 93.3 95.4 94.3 90.1 93.3 97.2 94.6 89.7 91.2 89.3 85.3 94.7 94.9 92.7 92.7 95.1 91.0 87.6 95.6 80.4 91.4 94.2
5.2 96.5 3.7 59.9 73.7 24.2 7.2 -
11.3 1.6 102.1 18.7 8.6 72.6 89.2 50.8 20.8 5.6 8.8 -
a
2.2 -
peak area (×106) 700 °C 800 °C 16.2 4.7 113.0 27.6 19.9 48.8 2.2 83.6 9.2 4.2 107.2 99.1 78.2 42.3 80.3 27.0 14.0 1.2 -
42.0 24.6 323.1 58.2 54.9 64.2 68.2 373.1 42.4 17.2 391.0 65.5 144.7 288.4 110.2 7.5 107.6 318.6 34.5 129.4 71.5 21.2 3.4 1.8
900 °C 13.8 19.9 1.5 1.2 16.7 89.3 3.5 7.7 97.1 56.2 7.4 19.9 115.9 27.7 27.2 17.2 1.6 6.2 4.6
“-” means below the detection limit
ranthene, but no four-ring or even bigger PAHs were found. However, there was a marked increase in both the number of PAHs species and the content (represented by peak areas) of each identified PAH with increasing temperatures, particularly at above 700 °C and for four-ring, five-ring, and six-ring PAHs. Some new species of PAHs at significant higher levels were detected only at higher temperatures, including fluorene, pyrene, chrysene, benzo[a]pyrene, dibenzo[def,mno]chrysene, and benzo[ghi]perylene. PAHs are one of the most toxic types of hazardous pollutants;34 some species of PAHs listed in Table 3 have been confirmed to be carcinogenic and/or mutagenic.20 To further iterate the distribution of PAH species in pyrolysis oil, the amounts of two to six rings of PAHs, carcinogenic PAHs, and total aromatic hydrocarbons in the pyrolysis oils versus the pyrolysis temperature are plotted in Figure 6, by summing up the peak areas of the same types of species. For most of the identified PAHs in Table 3, their contents (represented by peak areas) were slightly enhanced at 500700 °C, while a sharp increase of their amounts was generally found from 700 to 800 °C, followed by a significant decrease with a further increase in temperature to 900 °C. Therefore, the total amount of PAHs versus the temperature demonstrated a peak at 800 °C, due to two reasons: (1) PAHs formed slowly at the lower temperatures (
