bDepartment of Environmental Chemistry, CIDKSIC, Jordi Girona Salgado 18-26, 08034. Barcelona ..... Lopez-Avila, V., Dodhivala, N. S. and Beckert, W. F., J.
PII: SOOl6-2361(98)00031-3
ELSEVIER
Fuel Vol. 77, No. 11, pp. 1133-1139, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016.2361/98 $19.00+0.00
Characterisation of polycyclic aromatic hydrocarbons in liquid products from pyrolysis of Eucalyptus grands by supercritical fluid extraction and GWMS determination A. S. Pimentaa, B. R. Vitala, J. M. Bayonabr* and R. Alzagab aDepartment of Forestry Engineering, Laboratory of Wood Energy and Technology, Federal University of Vicosa, Campus Universitario, 36571-000 ViCosa (MG), Brazil bDepartment of Environmental Chemistry, CIDKSIC, Jordi Girona Salgado 18-26, 08034 Barcelona, Spain (Received 9 December 1997; revised 29 January 1998)
Slow pyrolysis of Eucalyptus grandis wood was performed in an oven laboratory and liquid products were quantitatively collected. Polycyclic aromatic hydrocarbons (PAHs) content was obtained by partitioning the pyrolysis liquid products with supercritical CO* extraction followed by GUMS analysis. The SFE procedures were validated by comparing the PAH content with those obtained by means of adsorption column chromatography. While extracted amounts lower than 100 pg/g could be obtained by using the column chromatography fractionation procedures at 8O”C, the WE procedures were able to extract, at 100 and 371 bar, respectively, 17.2 and 69.3% (w/w) based on the initial mass of the anhydrous sample. In the pressure range from 100 to 371 bar, increasing the pressure increases the extraction yields obtained. However, the PAHs showed a different behaviour in the same pressure range. The lighter PAHs with molecular weight up to 192 were most successfully extracted at 100 bar. On the other hand, the PAHs with molecular weight from 202 to 278 were better recovered by SFE! at 371 bar. 0 1998 Elsevier Science Ltd. All rights reserved (Keywords: wood pyrolysis; polycyclic aromatic hydrocarbons; supercritical fluid extraction)
INTRODUCTION In the slow pyrolysis process, wood lignin and carbohydrates are decomposed, giving as by-products charcoal and a complex condensable gaseous mixture of water, methanol, acetic and formic acids, carbonyl compounds, polycyclic aromatic hydrocarbons (PAHs) and wood tar. Wood tar from the slow wood pyrolysis process is basically constituted of phenolic compounds’. The term tar, as used in this paper, refers to a dark and oily mixture that separates from pyrolysis process liquids as a product after standing or centrifugation. Apart from water, the tar is composed of condensable semivolatile and non-volatile compounds, with molecular weights varying from acetic acid (Mw = 60) to tar pitch (Mw > 500) collected at room temperature. The wood tar composition can provide chemical information on thermal cracking reactions, process parameters and its influence on the resulting pyrolysis products yields and composition’. *Author to whom correspondence should be addressed.
Classical methods used to obtain fractions from wood tar include simple steam and vacuum distillation3. However, as wood tar is a heat-sensitive matrix, by using such fractionation methods until final distillation temperatures in the range of 250-3OO”C, the pitch undergoes intense polymerisation, becoming hard and decreasing its solubility in ethanol and acetone4’5 . General sampling and preparation techniques of wood pyrolysis tars for chromatographic analysis have been reviewed6”. The high complexity of the wood tars limits the possibility of direct analysis using a single technique, and the methods commonly used frequently comprise the application of single or combined techniques that allow us to fraction sam les before chromatographic or spectroscopic analysis 7p. In wood pyrolysis liquids, the PAHs and phenolic compounds are in close mixture with tar pitch, and only traces of PAHs, e.g., are present in the aqueous phase’. Commonly, prior to chromatographic analysis of fractions, fractionation procedures are required to dehydrate samples and separate target analytes from tar pitch. LLE and HPLC techniques are widely used for prefractionation,
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however, they have some disadvantages’: (a) HPLC consumes too large solvent volumes to be useful for a rapid tar analysis; and (b) LLE when applied to wood tar samples leads to the formation of emulsions and problems with the location of the phase boundary. Isolation methods using solid phase extraction (SPE) for determination of phenols, aromatic hydrocarbons’, N-organic compounds, preasphaltenes and asphaltenes” have been reported. Due to the complex composition of pyrolysis oils and tars, isolation and identification of chemical groups are still performed off-line. Identification of wood tar-derived products has been accomplished by several techniques7, e.g. liquid chromatography (HPLC) with UV detection, HRGC with mass spectrometry detection, capillary eletrophoresis, size exclusion chromatography with diode array detection, Fourier transform infrared spectroscopy (FTIR) and Fourier transform nuclear magnetic resonance (FINMR). Although several accurate methods for the characterisation of compounds from wood pyrolysis have been developed, most of them are complex and time consuming. Analytical scale SFE with pure or modified COZ has been increasingly used to extract and separate organic compounds from several matrices11-‘3. PAHs extraction has been successfully achieved from several matrices, e.g. urban14 and house dust15, coal tar16, soils and fly ash17, coal tar-contaminated soils’*, soils and sediments’9-22, geological samples23, polyurethane foam and glass fibre filters24. SFE is particularly suitable for heat-sensitive materials25. Some SFE methods have been commonly optimised by using only one variable each time in attempts that assume no interaction between variables26. Statistical approaches combining experimental designs with multilinear regression analysis have been used in applications of SEE to several kinds of samples27-30. By combining a reduced number of variables with multilinear regression, optimum extraction conditions can be achieved. The objectives of this work were: (a) to determine the PAHs content in liquids from pyrolysis of Eucalyptus grandis wood by SFE procedures followed by GC/MS analysis; (b) to optimise conditions of pressure and temperature in the SFE extraction of PAHs from pyrolysis liquids; and (c) to validate the SFE procedures through comparison with a conventional adsorption chromatography fractionation in terms of PAHs recoveries using GCYMS. EXPERIMENTAL Debarked seven-year-old Eucalyptus grundis wood was obtained from plantations located at Vicosa (MG, Brazil). Wood chips were pyrolysed in a laboratory oven (FABBE, SHo Paulo, Brazil) with an average heating rate of 56.3”C/h from room temperature reaching a final temperature of 450°C (about 8 h). The liquid pyrolysis liquids were obtained by passing the smoke from the pyrolysis bed through a metal condenser at room temperature. Three replications, with a sample size of about 200 g, were performed and the resulting liquid products were mixed and stored at - 20°C. All solvents and reagents (pesticide analysis grade) used in this work were purchased from Merck (Darmstadt, Germany). Supercritical fluid extraction
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soluble in ethanol, acetone and methylene chloride; and partially soluble in diethyl ether. The pyrolysis liquid samples were previously spiked with 0.5 pg/g of perdeuterated pyrene purchased from CIL (Wobum, MA, USA) as internal standard. Before extraction, tar samples were mixed with Hydromatrix (Varian, Harbor City, CA, USA) in a proportion of 1:5 (w/w). The resulting mixture was milled for 2 s at 20,000 rpm by using a 1095 Tecator Knifetec sample mill (Hogan&, Sweden). The extraction optimisation was carried out in a HP 7680T supercritical fluid extractor (Hewlett Packard, Avondale, PA, USA) with two replicates of each SFE condition. Neat (99.999%) carbon dioxide (Carburos Metalicos, Barcelona, Spain) was used as extraction fluid, and for all combinations of pressure and temperature, 1 min static extraction was used, followed by dynamic extraction at a flow rate of 1.5 ml/mm (as supercritical fluid) for 20 min. Seven-millilitre stainless steel extraction cells were used for extraction, and a trapping system (410 ~1 void volume) packed with stainless steel micro-balls was used to collect the extracts. The trap temperature was always maintained at 35°C. After extraction procedures, the extracts were washed from the trap with 1.0 ml of methanol as eluting solvent. The methanolic solution was evaporated almost until dryness under a gentle stream of nitrogen. After solvent evaporation, the extracts were rediluted to 1.0 ml with ethyl acetate. Aliquots were collected from this solution and stored at - 20°C for further chromatographic analysis. Optimisation
of the SFE
The aim of the SFE optimisation was to study the effects of pressure and temperature on the bulk extraction yields of anhydrous tar. Five pressure levels (76, 100, 203, 301 and 371 bar) and seven temperatures (40,50,60,70, 80,90 and 100°C) were combined by using an entirely randomised experimental design in the incomplete factorial scheme, totalling 24 treatments. Two replicates of each treatment were carried out. Initially, the parametric equation statistically adjusted from the experimental data contains the following terms: R=a+bT+cP+dT2+eP2+pT
where R is the extracted amount, a is the intercept, b, c, d, e and fare the parametric coefficients, P is the pressure (bar) and T is the temperature (“C). In order to minimise the bias effects, the extractions were carried out in a randomised order for each combination of pressure and temperature. Multilinear least square regression was used to calculate the parametric coefficients b, c, d, e and J Significance was determined by a t-test with a confidence level of 0.95 (P < 0.05). The minimum extraction yields were obtained at the pressure range 76- 100 bar and maximum extraction yield at 371 bar, with extraction temperatures in the range 70-80°C. From SFE optimisation results, the following conditions for extracting the PAHs from liquid pyrolysis were selected, 100 and 371 bar with extraction temperature of 80°C performing in this manner two SFE procedures, respectively, SFE 1 and SFE 2. Four replicates of each SFE procedure were carried out without previous dehydration of the samples.
(SFE) procedures
For optimising the SFE conditions, samples of anhydrous wood tar were used. The wood tar had the following properties: density = 1.181 g/cm3, insoluble in n-hexane;
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Adsorption procedures
Before
chromatography
extraction
fractionation
procedures,
(ACF)
the pyrolysis
liquid
Polycyclic
aromatic
hydrocarbons
I
in liquid
products:
Evaporated anddiluted with ethyla&ate
I
A. S Pimenta
et al.
Evaporated andrediluted with ethyl
acetate
Figure 1 Fractionation scheme for isolating PAHs from wood pyrolysis liquids by adsorption column chromatography and supercritical fluid extraction techniques
samples were spiked with 0.5 pglg of perdeuterated pyrene from CIL (Woburn, MA, USA) as internal standard. An ACF procedure for obtaining PAH content in pyrolysis liquid products was performed by using a pair of glass columns. For extracting the PAHs from the first column, two solvents of different polarity were tested: methylene chloride or n-hexane, constituting two different procedures, ACF 1 and ACF 2, respectively. For column chromatography fractionation, initially, the sample spiked with the internal standard was quantitatively transferred in a glass column filled with 6 g anhydrous sodium sulphate (previously activated at 400°C for 12 h). After transferring the sample, the column was eluted with 35 ml methylene chloride or n-hexane (ACF 1 and ACF 2 procedures, respectively). The extract from the first column was evaporated until 1 ml in a rotatory evaporator at 35°C and loaded onto a second glass column filled with 9 g of neutral aluminium oxide (70-230 mesh, previously activated for 12 h at 400°C). The second column was conditioned prior to use by gravity feed with 10 ml n-hexane, and after addition of the extract from the first column, a 15 ml n-hexane fraction was eluted and discarded. A second fraction was eluted with 30 ml of a methylene chloride/n-hexane mixture (95:5 v/v). This fraction was collected and evaporated almost until dryness in a rotatory evaporator. After solvent evaporation, the sample was re-dissolved until 1 ml volume with ethyl acetate. Aliquots were collected from this solution and stored for further chromatographic analysis. Four replications of each ACF procedure to assess the method precision were carried out. Figure I shows a schematic flowchart for the ACF and SFE procedures. Both ACF procedures are similar, with the difference that in the first procedure methylene chloride was used for eluting the sodium sulphate column, whereas in the second procedure, n-hexane was used, since it is a lower
toxicity alternative to the former. The following steps were exactly the same for both procedures. The first column, packed with anhydrous sodium sulphate, was used either to dehydrate the samples or retain part of the tar pitch. A second column packed with neutral aluminium oxide was used to retain the remaining pitch tar and fractionation of the extract obtained from the first column. GC/MS analysis of the extracts The analysis of PAHs was performed by using a GC/MS with a Fisons MD 800 instrument (Milan, Italy) in the electron impact (EI) mode (70 eV) operating in the selected ion recording (SIR) mode (mass range from 50 to 400 daltons). The following PAHs molecular ions were monitored: 166 (fluorene), 168 (dibenzofuran), 178 (phenanthrene and anthracene), 180 (methylfluorenes), 192 (methylphenanthrenes), 202 (fluoranthene and pyrene), 2 12 (perdeuterated pyrene), 2 16 (methylfluoranthenes or pyrenes), 228 [benz[a]anthracene and chrysene + triphenylene], 252 [benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene and perylene], 276 (benzo[g,h,i]perylene) and 278 (dibenz[a,h]anthracene). The injector, transfer line and ion source temperature were held at 280, 280 and 2OO”C, respectively. The analytical column was 30 m long and 0.25 mm internal diameter HP-5 (Avondale, PA, USA) of 0.25 pm film thickness. The column temperature was programmed from 80 to 100°C at lS”C/min, and then increased to 310°C at 4”C/min, holding the final temperature for 10 min. For quantification, the external standard method was used with the 16 polycyclic aromatic hydrocarbons, included in the EPA priority pollutants list, as calibrants. The precision studies were performed with at least four independent replicates. The standard PAHs mixture was purchased from Accu Standard (New Haven, CT, USA).
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be easily partitioned from the pitch tar in the experimental conditions. Temperature used to be more important than pressure for achieving high extraction efficiencies when interactions between extractable analytes and sample matrices are strong31.
RESULTS AND DISCUSSION Table 1 shows the average yields from the pyrolysis of the wood chips as charcoal, liquid and gaseous products (% w/w based on initial mass of dry wood). The non-condensable gases yield was obtained by the difference.
GC/MS analysis The ACF and SFE recoveries were calculated based on the average concentration of perdeuterated pyrene in the extracts after extraction and fractionation steps. For the ACF 1 and ACF 2 procedures, recoveries of 115 and 121% were obtained, with coefficients of variation of 11.3 and 6.8% (four replicates of each procedure), respectively. For the SFE 1 and SFE 2 procedures, recoveries of 90 and 52% were obtained with coefficients of variation of 8.1 and 9.6% (four replicates of each procedure), respectively. Table 3 shows the PAH contents obtained by ACF and SFE procedures. The PAH contents obtained by both CF procedures were quite similar, as can be seen in Table 3. Similar total PAHs contents in the pyrolysis liquids equal to 48.89,47.25,49.44 and 46.21 pg/g were obtained, respectively, by ACF 1, ACF 2, SFE 1 and SFE 2 procedures. Combining the data from Tables 1, and 3, it was estimated that pyrolysing 1 ton of Eucalyptus grandis wood would release -22 g of PAHs to the environment. According to their molecular weight, the PAHs extracted amounts showed a different behaviour compared to bulkextracted amounts in the range of pressure tested in this work. The lighter PAHs, with molecular weight up to 192, were most successfully extracted at 100 bar. At this pressure, the recoveries obtained for fluorene, dibenzofuran, phenanthrene, 2-methylphenanthrenes and methylfluorenes are significantly higher than those obtained at 371 bar and by both ACF procedures, showing that at low pressures such compounds had better solubility. This behaviour is not typical and maybe could be explained by a modifier effect of phenols that are bulk components in the SFE extracts. At 371 bar, the achieved recoveries for lighter PAHs are similar to those obtained by ACF procedures, this could be explained by the decrease in fluid polarity at this pressure, since considerably higher amounts of lower polarity phenols (69.3% w/w) are extracted concomitantly with PAHs, e.g. 2,6-dimethoxyphenol and its derivatives. On the other hand, the heavier PAHs with molecular weight from 202 to 278 were better recovered by SE at 371 bar. Nevertheless, except for benz[a]anthracene, the recoveries for heavier PAHs obtained at 371 bar were lower than those obtained by SE methods, showing that even at higher pressures, the solubility of heavy PAHs is relatively low. This low extraction efficiency of the supercritical CO:! for heavier PAHs was observed in extracting them from other matrices, and this deficiency was remedied by the use of modifiers”. Commonly, at conventional extraction conditions, at the
Optimisation of the SFE Table 2 shows the SFE experimental data expressed by the extracted amount (% based on initial mass of anhydrous tar) obtained by SFE at temperatures from 40 to 100°C and pressures of 76, 100, 203, 301 and 376 bar. First, the complete statistical model containing all variables, quadratic terms and first order interaction (P, T, P2, T2 and PT) was adjusted, and the parametric coefficient for temperature was not significant. Another model, where all the variables are significant, was adjusted for explaining the wood tar fractionation by SFE based on the experimental data. The statistical model is shown below, where R, P and T are, respectively, extracted amount (% w/w), pressure (bar) and temperature (“C): R= - 18.1561+0.510181P-O.O00898717P*
- 0.0020407T2 + 0.00110107PT The coefficient of variation for the estimated regression equation was 5.7%. All of the parametric coefficients were significant by t-test with a confidence level of 0.95. The adjusted R2 for the equation above was 95.5%. In a factorial experiment like that described in this work, the value of 95.5% found for the adjusted R2 means that the adjusted equation is able to explain 95.5% of the treatments square sum. Taking into consideration the statistical significance of the parameters, the most important factor influencing the extracted amounts from wood tar is the pressure. As can be seen from Table 2, increasing the pressure, the highest extraction efficiency was achieved at 371 bar and a temperature extraction in the range 70-80°C. The temperature affected only slightly the bulk extraction yields when the pressure was maintained constant, and such behaviour was due to the low interactions between wood tar components and the hydromatrix. Thus, a clean phenolic fraction containing PAHs as contaminants could
Products (% w/w based on initial dry wood weight) obtained from the pyrolysis of Eucalyptusgrandis wood
Table 1
Yields (% w/w)”
Products Charcoal Liquid products Non-condensable gases Total
38.5 45.5 16.0 100.0
“Average from three replicates
Table 2
Bulk extracted amounts (% w/w) from anhydrous tar by using Supercritical
Pressure (bar) 76 100 203 301 376
Extraction temperature (“C) 40 50 Extracted amounts (% w/w)”
56.3 59.2 60.4
53.4 66.8 65.4
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Fluid Extraction
60
70
80
90
100
55.5 60.9 62.9
16.4 56.4 70.5 71.2
11.4 16.8 52.3 66.2 69.3
53.6 70.2 65.5
52.0 62.7 65.4
‘Average from two replicates, % w/w based on initial anhydrous tar mass
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Table 3 PAHs contents of liauid oroducts from wood pyrolysis of Eucalyptus grandis obtained by ACF and SFE procedures followed by GUMS analysis 1
Compound
MW
1. Dibenzofuran 2. Fluorene 3. C-Methylfluorenes 4. Phenanthrene 5. Antbracene 6. 3-Methylpbenantbrene 2-Methylphenanthrene 4-Methylphenanthrene 1-Methylphenanthrene 7. Fluoranthene 8. Pyrene 9. C-Methylfluoranthenes or pyrenes 10. Benz[a]anthracene 11. Chysene + Triphenylene 12. Benzo[b]fluoranthene 13. Benzo[k]fluoranthene 14. Benzo[e]pyrene 15.Benzo[u]pyrene 16. Perylene 17. Indene[l,2,3,cdpyrene 18. Dibenz[a,h]a_nthracene 19. Benzo[g,h,i]perylene Total Coefficient of Variation (%)[n = 41
168 166 180 178 178 192 192 192 192 202 202 216 228 228 252 252 252 252 252 276 278 276
MC-methylene
chloride; ACF-adsorption
-
Extraction procedures cont. &g/g) ACF 1 (MC) ACF 2 (n-hexane)
SFE 1 (100 bar)
4.62 7.26 15.55 3.80 2.01 1.39 1.90 2.16 1.09 1.02 1.13 2.52 0.48 0.45 0.22 0.18 0.19 0.23 0.13 0.30 0.30 0.32 47.25 9.0
4.52 7.03 16.13 3.92 2.10 1.43 1.88 2.33 1.14 1.05 1.21 3.16 0.53 0.51 0.24 0.18 0.19 0.24 0.14 0.32 0.31 0.33 48.89 9.6
chromatography fractionation: SFE-supercritical
SFE 2 (371 bar)
6.28 8.80 18.73 4.63 1.95 1.60 2.15 2.14 1.44 0.38 0.26 0.63 0.08 0.08 0.11 0.01 0.02 0.03 0.01 0.07 0.08 0.04 49.44 9.2
4.14 7.40 15.01 3.64 2.06 1.85 1.90 2.3 1 1.55 1.12 0.87 2.80 0.56 0.22 0.18 0.10 0.03 0.12 0.08 0.10 0.10 0.07 46.21 8.4
fluid extraction
ACF
a
5.0
10.0
15.0
20.0
25.0
30.0 Time
Figure 2 correspond
35 0
40 0
45
0
50.0
55
0
(min)
Reconstructed ion chromatograms of the extracts obtained by SFE 2 and ACF 1 procedures. to the compounds listed in Table 3
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temperature and pressure range 40-80°C and 200-400 atm pure COz, have low efficiency for extracting PAHs from environmental samples, e.g. sediments, soils, diesel soot and air particulate matter at conventional extraction conditions, which demonstrates that CO2 has insufficient ability either to solvate some organics or to interact with the analyte/ matrix complex to 1;5pove the analytes into the bulk ’ . This factor, plus the strong supercritical CO2 interaction forces, allows the need for the addition of some organic modifiers with supercritical CO2 to obtain high yields in PAHs from environmental samples using SFE methods. Figure 2 shows the chromatograms for the extracts obtained by SFE 2 and ACF 1 procedures. The numbers above the peaks correspond to the compounds listed in Table 3. The composition of the extracts obtained by adsorption chromatography fractionation procedures using either methylene chloride or n-hexane as extractor solvents was quite similar. By using this fractionation technique, together with PAHs, several other compounds with concentrations in the range l-5 pglg were extracted. Among these compounds, some fatty acid esters; 1,2,3trimethoxybenzene and I-(4-hydroxy-3,5dimethoxyphenyl) ethanone were identified by mass spectra. In this work, maintaining the temperature constant, phenol recoveries increased as the pressure increased, in the range of 76-371 bar. In fact, a high amount of phenols could be extracted from the pyrolysis liquids samples by both SFE procedures yielding amounts of 16.80 and 69.30% (% w/w), respectively. In the SFE extracts, the PAHs were present as traces. The SFE extracts were composed by monohydroxylated phenols. Mainly, derivatives of phenol, guayacol (Zmethoxyphenol) and syringol (2,6-dimethoxyphenol), and derivatives methyl, ethyl and propyl 4Csubstituted could be identified by mass spectra. Extraction times longer than 20 min did not significantly increase the bulk extraction yields.
work was sponsored by the CICYT, Spain, AMB 95-0042COl.
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The PAHs group could be efficiently extracted from pyrolysis liquid products, however, maintaining the temperature constant, the PAHs extraction efficiency varied according to their molecular weight. At an extraction temperature of 8O”C, the lighter PAHs, with a molecular weight up to 192, were most successfully extracted at 100 bar. On the other hand, heavier PAHs with a molecular weight from 202 to 278 were better recovered at 371 bar. Nevertheless, except for benz[a]anthracene, the heavier PAHs recoveries at such a pressure level were lower than those obtained by ACF procedures. The addition of an organic modifier to the supercritical CO2 could enhance the PAHs solubility. The SFE procedures were simple and rapid to perform, moreover, both SFE procedures tested in the present work were able to extract target analytes from pyrolysis liquid samples, without extracting polymeric pitch as contaminant. Thus, the extracts could be analysed by GC/ MS and no previous fractionations were required.
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ACKNOWLEDGEMENTS
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CONCLUSIONS
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in
Interlaken, Switzerland, 1991. Otani, C., Pasa, V. M. D. and Carazza, F., The structure and chemical characteristics variation of wood tar pitch during its carbonization. In Proceedings of the International Symposium on Carbon, Tsukuba, Japan, 1990. Pasa, V. M. D., Otani, C. and Carazza, F., Wood tar pitch as phenolic resin precursor. In Proceedings of the 3rd Brazilian Symposium of Lignins and other Wood Components, Vol. 4, Belo Horizonte (MG), Brazil, 1993.
15
A. S. Pimenta would like to thank the CNPq (Brazilian Research Agency) for fellowship funding. We are indebted to Dr J. M. Gricia, J. Bosch, R. Chaler and R. Alonso. This
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Linders, S. H. M. A., Hijman, W. C., Kootstra, P. R. and van der Velde, E. G., Supercritical Fluid Extraction of PAHs in Polyurethane Foam and Glass Fibre Filters. National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. Hochgeschurtz, T., Hutchenson, K. W., Roebers, J. R., Liu, G.-Z, Mullins, J. C. and Thiers, M. C., Production of mesophase pitch by supercritical fluid extraction. In American Chemical Society Symposium Series, Vol. 514, 1993, p. 347. Tong, P. and Imagawa, T., Anal. Chim. Acta, 1995,310,93. Lopez-Avila, V., Dodhivala, N. S. and Beckert, W. F., J. Chromatogr. Sci., 1990, 28,468.
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