Devolatilization of Conventional Pyrolysis Oils Generated from

Energy & Fuels 2006, 20, 2253-2261

2253

Devolatilization of Conventional Pyrolysis Oils Generated from Biomass and Cellulose Carmen Branca, Colomba Di Blasi,* and Rosario Elefante Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di “Napoli Federico II” P.le V. Tecchio, 80125 Napoli, Italy ReceiVed March 13, 2006. ReVised Manuscript ReceiVed May 24, 2006

Weight loss curves in the air at a heating rate of 5 K/min up to 600 K are measured for liquids generated from the conventional pyrolysis of wood particles (beech and fir), bark, several agricultural residues (straw, olive husks, and nut shells), and cellulose. An aqueous fraction (fraction A), collected from ice-cooled condensers and corresponding to about 70-76% of the total liquid, and a muddy phase (fraction B), which, except for cellulose, appears as a superficial layer above the water scrubbers, are examined. Fraction A, consisting mainly of water; carbohydrates; furans; and, in the case of wood/biomass samples, phenols, guaiacols, and syringols, produces a small amount of solid residues (about 15-20%, dry liquid basis), but for cellulose, where a yield of about 40% is obtained. The global devolatilization rates are well-predicted by a mechanism comprising three parallel first-order reactions with comparable values of the kinetic parameters. Higher yields of solid residues (26-35%, dry liquid basis) are observed for fraction B, consisting of phenolic compounds and presumably extractives and high-molecular-weight species. Although the same global mechanism can also be applied, the kinetic parameters are significantly different from those estimated for fraction A and more affected by the origin of the samples.

Introduction Updraft gasifiers and fixed-bed pyrolyzers usually establish relatively slow heating rates of the biomass feedstock, so that the decomposition process is indicated as conventional pyrolysis.1-3 In the former case, the presence of condensable organic products in the gaseous stream can cause severe operating problems in internal combustion engines and turbines. Adequate cleaning procedures of the gas are applied,3 which give rise to a liquid waste which has to be further treated, for instance, through gasification or oxidation, to avoid pollution problems and to exploit its energetic content. A biomass conversion process has also been proposed,4 where conventional pyrolysis is applied as a pretreatment to produce a slurry fuel, consisting of the liquid and solid products, which is subsequently gasified. Hence, an improved understanding of the thermal behavior of the liquids generated from the conventional pyrolysis of biomass can be useful for the design and the optimization of the related conversion systems. However, scarce information is available for this purpose. The thermogravimetric analysis carried out by Branca et al.5 for the combustion of liquids generated from the conventional pyrolysis of beech wood shows two main reaction stages. The * Corresponding author. Tel: 39-081-7682232. Fax: 39-081-2391800. E-mail: [email protected]. (1) Di Blasi, C.; Signorelli, G.; Portoricco, G. Fixed-bed countercurrent gasification of biomass at laboratory scale. Ind. Eng. Chem. Res. 1999, 38, 2571-2581. (2) Di Blasi, C.; Signorelli, G.; Di Russo, C.; Rea, G. Product distribution from pyrolysis of wood and agricultural residues. Ind. Eng. Chem. Res. 1999, 38, 2216-224. (3) Buekens, A. G.; Bridgwater, A. V.; Ferrero, G. L.; Maniatis, K. Commercial and marketing aspects of gasifiers. Comm. Eur. Communities, [Rep.] EUR 12736 1990. (4) Henrich, E.; Dinjus, E. Tar-free, high-pressure synthesis gas from biomass. In Pyrolysis and Gasification of Biomass and Waste; Bridgwater, A. V., Ed.; CPL Press: Newbury, U. K., 2003; pp 511-526.

first (temperatures below 600 K) concerns the evaporation, formation, and release of gases and the formation of secondary char (coke). Then, at higher temperatures, the heterogeneous combustion of secondary char takes place. A procedure was also proposed to carry out the first stage (devolatilization) under an assigned temperature using a proportional-integral-derivative (PID) controller and the applied heat flux as the manipulated variable. Because of strong physicochemical transformations (sample swelling and solidification) associated with secondary char formation, it was not possible to avoid ignition during heterogeneous combustion. Therefore, this reaction stage was investigated separately after collection and adequate repreparation of the charred sample. Furthermore, the devolatilization stage was well-predicted by a global mechanism consisting of three parallel first-order reactions. Results for the combustion of fast pyrolysis liquids obtained by means of thermogravimetric systems without6,7 or with8 temperature control are also of interest because the chief qualitative features of the process remain the same. In this case, a detailed experimental analysis is also available on the combustion of single droplets,9-11 exposed to heating conditions which try to reproduce those of practical combustion, so that both chemical kinetics and transport phenomena are important. (5) Branca, C.; Di Blasi, C.; Russo, C. Devolatilization in the temperature range 300-600K of liquids derived from wood pyrolysis and gasification. Fuel 2005, 84, 37-45. (6) Ghetti, P.; Ricca, L.; Angelini, L. Thermal analysis of biomass and corresponding pyrolysis products. Fuel 1996, 75, 565-573. (7) Vitolo, S.; Seggiani, M.; Frediani, P.; Ambrosini, G.; Politi, L. Catalytic upgrading of pyrolytic oils to fuel over different zeolites. Fuel 1999, 78, 1147-1159. (8) Branca, C.; Di Blasi, C.; Elefante, R. Devolatilization and heterogeneous combustion of wood fast pyrolysis oils. Ind. Eng. Chem. Res. 2005, 44, 799-810. (9) Wornat, M. J.; Porter, B. G.; Yang, N. Y. C. Single droplet combustion of biomass pyrolysis oils. Energy Fuels 1994, 8, 1131-1142.

10.1021/ef0601059 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/06/2006

2254 Energy & Fuels, Vol. 20, No. 5, 2006

Branca et al.

Table 1. Chemical Composition, Particle Size, and Bed Density of the Feedstocks Used in the Pyrolysis Tests (Bark Lignin also Includes Contributions of Soluble Polyphenols and Suberin) biomass

holocellulose [wt %]

lignin [wt %]

extractives [wt %]

ash [wt %]

particle size [mm]

bed density [kg/m3]

beech wood fir wood oak bark cellulose nut shells straw olive husks

75 66 31 100 62 66 61

23 31 51

2 3 8

0.4 0.5 9.3

33 23 28

9 5 9

1.4 6.2 2.5

5 5 5 5 1-2 5 1-3

320 180 340 300 450 370 420

Table 2. Conversion Times, Conversion Temperature, Yields of Products (Expressed as Percent of the Initial Dry Mass), Fractions of Liquid Products Collected from the Ice-Cooled Condensers, and Wet Scrubbing (Expressed as Percent of the Liquid) for the Pyrolysis Experiments

biomass

conversion time [s]

conversion temperature [K]

char [wt %]

gas [wt %]

liquids [wt %]

liquids condensers [wt %]

liquids scrubbers [wt %]

beech wood fir wood oak bark cellulose nut shells straw olive husks

444 582 586 1105 354 499 386

635 627 628 632 638 632 657

24 22 34 17 31 29 30

13 11 15 8 14 15 14

56 60 44 66 50 48 49

76 76 55 70 73 69 73

14 9 14 12 12 8 13

After heating, the formation of bubbles and microexplosions is observed. This stage is followed by polymerization and cracking reactions of the less volatile components of the oil with the formation of the secondary char (cenospheres) and its combustion. As recently reported,8 the secondary char formed from fast pyrolysis oils, at least for moderate thermal conditions, comprises about 48-55% of the oil carbon. This high-energetic content evidences the need to improve the efficiency of both homogeneous and heterogeneous combustion. For this purpose, it is also desirable to understand better the component/reactions responsible for the formation of the secondary char. A model, recently published,12 which can predict the major events of droplet evaporation, represents the oil by means of four fractions, that is, organic acids, aldehydes/ketones, water, and pyrolytic lignin [this is the fraction which is obtained from the oil by water extraction and, for fast pyrolysis oils, varies from about 15 to 30% (of the entire oil), depending on the conversion technology and feedstock’s nature].13 Pyrolysis of the last fraction is assumed to be responsible for secondary char formation. In reality, no information is currently available on the combustion characteristics of oil samples generated from the pyrolysis of biomass macrocomponents (holocellulose and lignin) and their contribution to secondary char formation. In this work, the experimental procedure previously developed5 is applied to investigate the devolatilization behavior of oils obtained from the conventional pyrolysis of wood (beech and fir), bark, biomass (straw, olive husks, and nut shells), and cellulose, also in relation to the liquid fractions collected by condensation and wet scrubbing. Comparison between the devolatilization characteristics of the two fractions and between (10) Shaddix, C. R.; Tennison, P. J. Effects of char content and simple additives on biomass pyrolysis oil droplet combustion. In Twenty-SeVenth (Int.) Symposium on Combustion, 27th Symposium on Combustion, University of Colorado, Boulder, CO, August 2-7, 1998; pp 1131-1142. (11) D’Alessio, J.; Lazzaro, M.; Massoli, P.; Moccia, V. Thermo-optical investigation of burning biomass pyrolysis oil droplets. In Twenty-SeVenth (Int.) Symposium on Combustion, 27th Symposium on Combustion, University of Colorado, Boulder, CO, August 2-7, 1998; The Combustion Institute: Pittsburgh, PA, 1998; pp 1915-1922. (12) Hallett, W. H. L.; Clark, N. A. A model for the evaporation of biomass pyrolysis oil droplets. Fuel 2006, 85, 532-544. (13) Scholze, B.; Meier, D. Characterization of the water-insoluble fraction from pyrolysis oil (pyrolytic lignin). Part I. PY-GC/MS, FTIR and functional groups. J. Anal. Appl. Pyrolysis 2001, 60, 41-54.

the cellulose and the biomass/wood oils, also based on detailed chemical characterization, is used to clarify the origin of the secondary char. Furthermore, the applicability is studied of a simplified, three-step mechanism for the description of the devolatilization kinetics of both fractions of the oil samples, and the related kinetic constants are estimated. Experimental Details Pyrolysis Experiments and Liquid Composition. The liquids under investigation are produced from predried wood particles (beech and fir), oak bark, agricultural wastes (straw pellets, olive husks, and nut shells), and cellulose (a fine powder of Avicel PH 105 is amalgamated with water to form tablets which are subjected to drying and then cut into cubic particles). The chemical composition, useful for understanding the oil properties, is summarized in Table 1 (lignin is determined with the Klason method, extractives with a Soxhtec HT2 apparatus,1 ashes with calcination,14 and holocellulose by difference). Particle size and particle bed density are also listed here. For the pyrolysis experiments, a laboratoryscale fixed-bed reactor, described in ref 15, is used. Nitrogen (8 × 103 cm3/min), fed through a jacket at the reactor top, is heated by an electrical furnace and distributed by a perforated steel plate, which also supports the bed. The lower reactor zone (about 1520 cm) is isothermal at 800 K; then, temperatures become successively lower as the top is approached. The fuel sample (about 180 g) is instantaneously dropped inside the hot reactor. The relatively low temperatures of the reactor, the existence of axial thermal gradients, and the short residence time of the vapor/gases (nominal values, with reference to the isothermal reactor zone, equal to 3.5 s) indicate that liquid products are essentially generated from primary reactions. The chief characteristics of the pyrolysis processes (conversion times, conversion temperature, and yields of products) are summarized in Table 2. The temperature versus time profiles, measured at several distances from the flow distributor, and the composition of the gas, evaluated at selected times, are qualitatively similar to those presented for beech wood particles.15 Pyrolysis takes place under dynamic conditions, and conversion is achieved before the particle bed attains the initial reactor temperature. Conversion times, tconv, are assumed to coincide with the time when 90% of the total (14) Di Blasi, C.; Branca, C.; D’Errico, G. Degradation characteristics of straw and washed straw. Thermochim. Acta 2000, 364, 133-142. (15) Branca, C.; Giudicianni, P.; Di Blasi, C. GC/MS characterization of liquids generated from low-temperature pyrolysis of wood. Ind. Eng. Chem. Res. 2003, 42, 3190-3202.

DeVolatilization of ConVentional Pyrolysis Oils

Energy & Fuels, Vol. 20, No. 5, 2006 2255

Table 3. Chemical Composition of the Liquid Products Collected from Ice-Cooled Condensers (Fraction A, Yields Expressed as Percent of the Liquid) compound [wt % liquid] 2(5H)-furanone water hydroxyacetaldehyde acetic acid 4-propylguaiacol 4-acetonguaiacol propionic acid 1-hydroxy-2-propanone furantetrahydro-2,5-dimethoxy (trans+cis) 3-methyl-2-cyclopentenone 2-methyl-2-cyclopentenone 3-ethyl-2-hydroxy-2-cyclopentenone 1-hydroxy-2-butanone 2-furaldehyde 2-acetylfuran acetoxyacetone phenol 5-methyl-2-furaldehyde o-cresol syringaldehyde m,p-cresol 2-ethylphenol guaiacol 2,5+2,4-dimthylphenol 4-methylguaiacol 3,4-dimethylphenol 4-ethyguaiacol eugenol syringol vanillin isoeugenol (trans+cis) acetosyringone hydroquinone levoglucosan 4-methylsyringol total

Tb [K]

beech wood

fir wood

oak bark

cellulose

nut shells

straw

olive husks

360 373 383 391 399 400 414 419 419 431 431

0.42 34.6 6.8 12.3 0.04 0.09 0.4 2.2 0.08 0.11 0.06 0.18 0.44 0.94 0.08 0.19 0.06 0.08 0.03 0.13 0.05 0.03 0.33 0.02 0.27 0.01 0.13 0.04 0.92 0.07 0.23 0.15 0.10 1.35 0.55 63.4

0.48 30.1 11.3 4.6 0.08 0.20 0.3 3.4 0.04 0.05 0.05 0.14 0.55 0.50 0.06 0.15 0.07 0.08 0.04 0 0.07 0.04 0.61 0.03 1.10 0.01 0.27 0.12 0 0.18 0.05 0 0.07 3.70 0.04 58.5

0.17 57.9 2.7 10.4 0.02 0.10 0.4 1.3 0.04 0.05 0.03 0.08 0.37 0.43 0.03 0.15 0.05 0.05 0.02 0.03 0.30 0.01 0.21 0.01 0.16 0 0.07 0.01 0.31 0.05 0.03 0.05 0.06 1.33 0.17 77.1

0.27 37.6 5.3 1.0 0 0 0.5 1.0 0 0.04 0.02 0.07 0 1.05 0.10 0.05 0 0.23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8.80 0 56.4

0.20 42.2 3.0 14.2 0.04 0.18 0.6 1.5 0.06 0.06 0.05 0.13 0.37 0.48 0.04 0.17 0.08 0.04 0.04 0.01 0.06 0.03 0.69 0.03 0.53 0.01 0.22 0.05 0.18 0.15 0.24 0.01 0.10 0.75 0.08 66.5

0.28 50.1 4.3 11.3 0.01 0.06 0.9 3.1 0.04 0.08 0.07 0.27 0.39 0.33 0.07 0.26 0.10 0.03 0.03 0.02 0.04 0.02 0.24 0.01 0.05 0 0.06 0 0.31 0.06 0.02 0.04 0.14 0.24 0.07 73.1

0.17 46.2 2.0 13.9 0.02 0.12 0.7 1.5 0.05 0.06 0.04 0.17 0.35 0.28 0.04 0.17 0.07 0.02 0.03 0.05 0.05 0.02 0.43 0.02 0.20 0.01 0.11 0.02 0.54 0.06 0.13 0.05 0.12 0.37 0.19 68.2

433 435 446 447 455 460 464 466 475 478 478 484 494 500 508 526 536 539 541 558 659

gas is produced. They are between about 350 and 600 s, except for cellulose, where, probably owing to the higher endothermicity of the conversion process, a value of 1100 s is obtained. The complete (from fuel feeding to conversion) thermal history undergone by the samples is used to define time-averaged temperatures at the different locations along the bed height. A further spatial averaging over the initial bed height (dependent on the specific fuel) is used to introduce the conversion temperature, Tconv. The values, not significantly affected by the feedstock, are low (627-657 K), because the averaging is made over the entire duration of the experiments and not restricted to the actual period of conversion. On the basis of tconv and Tconv, the bed heating rates can be computed, which are between about 0.5 and 1 K/s and thus are typical of conventional pyrolysis. The yields of char, gas, and liquids (expressed as a percent of the initial dry mass) confirm previous findings,1 that is, the predominant role played by chemical composition with respect to the different thermal conditions established during conversion (different physical properties of the bed). The yields of char increase with the lignin content of the fuel, although the presence of ashes, which catalyze the charring process,14 may also be important. As char and gas formations are linked processes, the higher yields of char and gas are associated with lower yields of liquid products. In the pyrolysis experiments, nitrogen and volatile pyrolysis products pass through a condensation train consisting of two watercooled condensers, two ice-cooled catch pots, two wet scrubbers, two cotton wool traps, and a silica gel bed (all connected in series), so that the total amounts of liquid produced take into account the various contributions. The fractions, collected by condensation and wet scrubbing, expressed as percentages of the total liquid, are also reported in Table 2. The highest amount is collected from the condenser train and corresponds to 76% for the wood samples, 70% for cellulose, and 55-73% for the other wastes. This fraction is indicated in the following as fraction A. The amounts of liquid

product collected by wet scrubbing are much lower and are between 8 and 14%. In addition to water-soluble components, which are presumably qualitatively similar to fraction A, a muddy layer of tarry compounds is also observed, except for cellulose. This fraction is indicated in the following as fraction B. Both fractions A and B of the conventional pyrolysis liquids are stored at a temperature of 277 K with no light exposure. The water content of fraction A of the pyrolysis liquids is determined by means of Karl-Fisher titration according to the standard test method ASTM E203-96. Chemical analysis is performed by GC/MS (Focus C, Thermo) with a quadrupole detector and a DB-1701 capillary column, using the same procedure as that detailed by Branca et al. in ref 15. The contributions of the main chemical compounds are specified in Table 3, where they are also grouped according to their boiling temperatures. Carbohydrates are generated from the decomposition of holocellulose, whereas the hydroxy-phenolics, guaiacols, and syringols are derived from the lignin building blocks, as also confirmed by the results obtained for cellulose. Apart from water, the most abundant species are acetic acid, hydroxypropanone, hydroxyacetaldehyde, and levoglucosan. The higher percentages of hydroxyacetaldehyde, acetic acid, and levoglucosan reported here for beech wood, with respect to the results in ref 15, may be attributed to the different reaction conditions (a nitrogen flow higher by a factor of 4, which highly reduces the activity of vapor-phase reactions across the packed bed) and the reduced aging undergone by the sample.16 Although the yields of carbohydrates for the fir and beech oils are roughly the same, in the former case, the higher yields of hydroxyacetaldehyde and levoglucosan are nearly compensated by (16) Diebold, J. A review of chemical and physical mechanisms of the storage stability of fast pyrolysis bio-oils. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; CPL Press: Newbury, U. K., 2002; Vol. 2, pp 243-292.


Branca et al.

Table 4. Chemical Composition of the Muddy Layer of Liquid Products Collected at the Surface of the Water Scrubber (Fraction B, Yields Expressed as Percent of the Fraction) compound [wt % liquid] water 4-propylguaiacol 4-acetonguaiacol phenol o-cresol syringaldehyde m,p-cresol 2-ethylphenol guaiacol 2,5+2,4-dimthylphenol 4-methylguaiacol 3,4-dimethylphenol 4-ethyguaiacol eugenol syringol vanillin isoeugenol (trans+cis ) acetosyringone 4-methylsyringol total

Tb [K]

beech wood

fir wood

oak bark

nut shells

straw

olive husks

373 399 400 455 464 466 475 478 478 484 494 500 508 526 536 539 541 -

10 0.38 0.89 0.31 0.31 1.30 0.43 0.05 1.13 0.33 1.39 0.07 1.28 0.68 0.24 0.28 4.61 1.24 4.27 29.0

8 0.87 1.65 0.32 0.44 0.01 0.56 0.06 1.62 0.65 4.48 0.09 2.48 1.58 0.02 0.72 9.46 0.03 0.08 32.7

12 0.25 0.54 0.22 0.17 0.09 0.24 0.05 0.21 0.27 0.59 0.04 0.72 0.39 0.19 0.13 2.91 0.10 0.67 19.9

11 1.04 2.16 0.21 0.26 0.14 0.35 0.06 2.65 0.34 3.25 0.06 3.29 1.24 0.10 0.58 8.57 0.12 0.47 36.0

7.5 0.18 0.76 0.69 0.48 0.21 0.65 0.20 0.80 0.55 0.54 1.08 1.06 0.38 0.52 0.26 2.48 0.22 0.40 19.0

6 0.76 1.71 0.44 0.58 0.74 0.58 0.11 2.45 0.42 2.05 0.09 2.57 1.00 0.17 0.30 7.25 0.42 1.87 29.8

the lower yields of acetic acid. Another difference in the fir oil is the absence of syringols and the higher yields of guaiacols (roughly by a factor of 2). In fact, softwood lignins typically degrade to guaiacyl products, whereas hardwood lignin typically gives rise to both guaiacyl and syringyl products. Quantitative differences between liquids generated from the pyrolysis of wood and wastes are high. In accordance with the higher yields of char, liquids produced from agricultural residues present a higher content of water (42-58% versus 30-35% of wood). The yields of acetic acid are comparable for beech wood and wastes; however, in the latter case, lower yields of hydroxyacetaldehyde and levoglucosan are observed. Hydroxyacetaldehyde and levoglucosan are also the major products of cellulose decomposition. Owing to the limited presence of impurities in the fuel, levoglucosan is produced in much higher amounts than in wood (very high quantities of other anhydrosugars, not quantified here, are also reported17). Indeed, in the absence of ashes, the depolymerization path, leading to anhydrosugar formation, is favored with respect to the fragmentation path, leading to acetol, acetic acid, and hydroxyacetaldehyde formation.18 The compounds of fraction A, quantified by gas chromatography-mass spectrometry (GC-MS) and Karl-Fisher titration, correspond to about 60-75% of the liquid (56% for cellulose). Very light compounds not quantified here (such as formaldehyde, acetaldehyde, propionaldehyde, glycolic acid, glyoxal, acetone, and methanol with boiling temperatures below 350 K) have been found8 to contribute for 4-7% in the composition of fast pyrolysis oils. Other contributions are presumably given by nonvolatile compounds (about 15% of the total liquid in the case of fast pyrolysis), to be characterized by high performance liquid chromatography,19 and pyrolytic lignin. An attempt has also been made to carry out a chemical characterization of fraction B, after dissolution in acetone, by means of GC-MS (analysis limited to compounds with a molecular weight between 30 and 300 amu) and Karl-Fisher titration, again in accordance with ref 15. As shown in Table 4, compounds belonging to the classes of carbohydrates and furans are absent, but a (17) Radlein, D. The production of chemicals from fast pyrolysis biooils. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; CPL Press: Newbury, U. K., 1999; Vol. 1; pp 164-188. (18) Piskorz, J.; Radlein, D.; Scott, D. S.; Czernik, S. Liquid products from the fast pyrolysis of wood and cellulose. In Research in Thermochemical Biomass ConVersion; Bridgwater, A. V., Kuester J. L., Eds.; Elsevier Applied Science: New York, 1988; pp 557-571. (19) Meier, D. Summary of the Analytical Methods Available for Chemical Analysis of Pyrolysis Liquids. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; CPL Press: Newbury, U. K., 2002; Vol. 2, pp 59-68.

significant presence of compounds generated from the decomposition of lignin (especially guaiacols and syringols) is shown. Under rapid heating, the holocellulose component produces volatile species that are predominantly monomeric with a small amount of oligomers. However, lignin depolymerization invariably produces more oligomers than monomers.20 The oligomers, from both holocellulose and lignin, are found in the vapor phase because of their carryover as aerosols.17,20 The analysis carried out here confirms the difficulties encountered in the collection of the pyrolysis liquids, especially those generated from the ligninic fraction, and the great importance of the collection system for the properties of the liquid product. About 19-33% of fraction B has been quantified by means of GC-MS and Karl-Fisher titration. This fraction, as noted above, separates from water. Thus, it can be reasonably assumed that the remaining (unknown) contribution is, for a large part, what is indicated as pyrolytic lignin. Also, highly volatile compounds (again not detected by GC-MS) of extractives may be present. This class of compounds has also been reported21 to separate and to form an upper phase in fast pyrolysis oils. The Thermogravimetric System. The thermogravimetric system, used to investigate the devolatilization behavior of fractions A and B of the conventional pyrolysis oils, is the same as that in refs 5 and 8. It consists of a radiant heating chamber, a quartz reactor, a PID temperature controller, a gas feeding system, an acquisition data set, and a precision balance. The furnace is a radiant chamber, which creates a uniformly heated zone, where a quartz reactor is located while a proper reaction environment is established by a continuous air flow. The liquid is exposed to thermal radiation by means of a semispherical quartz cup. This is supported by NiCr wires wrapped to steel rods and positioned at the center of the uniformly heated zone of the furnace. The steel support is connected to the balance for continuous weight loss measurements. To carry out a process under controlled thermal conditions, the intensity of the applied heat flux is used as the adjustable variable and the sample temperature is monitored. Following the differences between the measured and desired temperatures, the PID controller sends an electrical signal, I, which through a silicon controlled rectifier regulates the potential difference at the two ends of the furnace lamps. The temperature is measured by means of a thin chromelalumel thermocouple. (20) Wang, D.; Czernik, S.; Chornet, E. Production of hydrogen from biomass by catalytic steam reforming of fast pyrolysis oils. Energy Fuels 1998, 12, 19-24. (21) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Fast pyrolysis of forestry residue. 1. Effect of extractives on phase separation of pyrolysis liquids. Energy Fuels 2003, 17, 1-12.



The thermogravimetric program foresees a dynamic section, with heating rates of 5 K/min from ambient conditions up to a temperature of 600 K, followed by an isothermal section for about 60 min. This part permits a complete devolatilization and avoids, at the same time, any significant activity of heterogeneous combustion reactions. The weight loss measurements are carried out using samples consisting of 41 mg of liquid. Each test is made in triplicate, showing good repeatability. Changes in the sample mass (8-50 mg) do not result in any variation in the measured weight loss characteristics, indicating the absence of significant heat and masstransfer limitations. Therefore, the measured weight loss curves can be used for the analysis of the intrinsic kinetics of the process. Thermogravimetric measurements are made for fraction A of the oils produced by all of the feedstocks presented above (beech, fir, bark, straw pellets, olive husks, nut shells, and cellulose). Fraction B is not present in the case of cellulose. Furthermore, that portion collected during the pyrolysis of straw is not examined because it is contaminated by small char particles entrained by the volatile products, following the partial destruction of the pellets.

Kinetic Modeling Given the very high number of chemical compounds of biomass pyrolysis liquids, the devolatilization kinetics can only be based on lumped mechanisms. A previous study5 shows that, for conventional pyrolysis liquids generated from beech wood (more specifically, for fraction A introduced above), a set of three parallel independent reactions provides good quantitative predictions. The same treatment is proposed here for both the A and B fractions: Ci f V i

i ) 1,3

(a1-a3)

where Ci are assumed to be lumped groups of volatile species generated from the evaporation and the cracking reactions undergone by the oil samples. The rate of reactions (a1-a3) is assumed to present the usual Arrhenius dependence on temperature and to be proportional to the mass fraction of lumped groups of species Ci: R1 ) A1 exp(-E1/RT)Y1

(1a)

R2 ) A2 exp(-E2/RT)Y2

(1b)

R3 ) A3 exp(-E3/RT)Y3

(1c)

Because the sample temperature, T, is a known function of time, t: T ) T0 + ht

(T e 600 K, t e 3470 s)

T ) 600 K

(t > 3470 s)

(2a) (2b)

(T0 is the initial temperature and h is the heating rate), the mathematical model can be expressed as dY1 ) -R1 dt

Y1(0) ) R

(3a-3b)

dY2 ) -R2 dt

Y2(0) ) β

(4a-4b)

dY3 ) -R3 dt

Y3(0) ) γ

(5a-5b)

where R, β, and γ are the initial mass fractions (indicated in the following as stoichiometric coefficients). The global mechanism a1-a3 is meant to describe oil devolatilization, which consists of evaporation (at low temperatures) and evaporation combined with decomposition of the larger molecules (at higher temperatures). This highly simplified description does not allow a distinction to be made between physical and chemical

processes. On the other hand, a first-order chemical reaction with an Arrhenius dependence on temperature has already been considered an acceptable treatment for moisture evaporation in wood.22-24 It allows for a finite thickness of the evaporating region and also eliminates the complications in the numerical solution, due to the presence of the unknown evaporation rate with an empirical expression for the vapor pressure, instead of an evolution equation or a simple production term. The kinetic parameters are estimated numerically,25 using a direct method for the minimization of the objective functions, which consider both integral (TG) and differential (DTG) data. The parameters to be estimated are the activation energies (E1, E2, and E3), the pre-exponential factors (A1, A2, and A3), and the stoichiometric coefficients (R and β, because given the final mass fraction of secondary char, Yc, the following equations holds: 1 - Yc) R + β+ γ). It should be noted that a reaction rate with a power-law dependence on the mass of the volatile fraction, as sometimes assumed in modeling biomass devolatilization, would introduce three additional parameters, so increasing the complexity of the kinetic model. The fit between measured and calculated curves is defined in accordance with previous analyses25 as %dev ) S)

xS/N × 100 (Φi)exp,peak

∑ [(Φ )

i exp

- (Φi)sim]2

(5)

(6)

i)1,N

where i represents the experimental (exp) or the simulated (sim) variable (Φ is the solid mass fraction, Y, or the devolatilization rate, -dY/dt) at time t (N is the number of experimental points, and the subscript peak indicates the maximum value).

Results and Discussion The main features are illustrated in the weight loss curves measured for fractions A and B of the conventional pyrolysis oils derived from various feedstocks. A comparison between experiments and predictions is also presented, and the values of the kinetic parameters are provided. Moreover, the devolatilization dynamics of the three classes of lumped compounds introduced by the kinetic model are discussed. Fraction A. Predicted and measured integral and differential curves of weight loss and predicted component dynamics for fraction A are reported in Figure 1A,B (oils from beech and fir wood, bark, and cellulose) and Figure 2A,B (oils from olive husks, nut shells, and straw). The mass fraction of char (expressed on both a total liquid basis, Yc, as needed for kinetic analysis, and a water-free liquid basis, Ycd), the estimated activation energies, pre-exponential factors, and stoichiometric coefficients and the parameters giving the goodness of the fitting are listed in Table 5. As testified by the last values, a threestep kinetic model provides a good description of the measurements. The weight loss curves are qualitatively similar in all cases. However, the fir oil, after the attainment of the maximum devolatilization rate, presents a clearly visible shoulder char(22) Shresta, D.; Cramer, S.; White, R. Time-temperature profile across a lumber section exposed to pyrolytic temperatures. Fire Mater. 1994, 18, 211-220. (23) Bryden, K. M.; Ragland, K. W.; Rutlan, C. J. Modeling thermally thick pyrolysis of wood. Biomass Bioenergy 2002, 22, 41-53. (24) Di Blasi, C.; Branca, C.; Sparano, S.; La Mantia, B. Drying characteristics of wood cylinders for conditions pertinent to fixed-bed countercurrent gasification. Biomass Bioenergy 2003, 25, 45-58. (25) Branca, C.; Di Blasi, C.; Horacek, H. Analysis of the combustion kinetics and the thermal behaviour of an intumescent system. Ind. Eng. Chem. Res. 2002, 41, 2104-2114.


Figure 1. (A) Weight loss (Y) and rate of weight loss (-dY/dt) curves of fraction A of the pyrolysis liquids produced from beech and fir wood, bark, and cellulose as functions of time as measured (symbols) and predicted by the model (lines). (B) Predictions of the devolatilization rates of the three lumped classes of volatile products of fraction A of the pyrolysis liquids produced from beech and fir wood, bark, and cellulose as functions of time.

acterized by devolatilization rates significantly higher than those of the other samples. The yield of the solid residue, indicated as secondary char,5,8 cannot be related to the lignin content of the biomass used to produced the oil. Indeed, contrary to the assumption usually made that secondary char (cenosphere) in the combustion of bio-oil is generated from pyrolytic lignin (see, for instance, ref 12), the maximum value is obtained for the cellulose oil. More precisely, with reference to a water-free oil basis, the values are between 14 and 16% for bark and agricultural residues, 15-20% for wood, and equal to 38.5% in the case of cellulose. Hence, it is clear that the specific classes of compounds generated from the decomposition of cellulose highly contribute to the formation of secondary char. Thermogravimetry, applied26 to evaluate the volatilities and the thermal stability of some products of cellulose pyrolysis (heating rate 50 K/min with a 3 mg sample mass), shows that, while levoglucosan almost completely devolatilizes for temperatures between 550 and 723 K, maltosan and cellobiosan lose weight for temperatures roughly between 575 and 800 K and give rise to the formation of quite high yields of solid residues (about 50 and 60%, respectively). In reality, an extensive experimental investigation27 indicated that the thermal behavior of levoglucosan is highly dependent on the flow and pressure conditions. At high flow rates and pressures below 1 MPa, an endotherm is observed, associated with levoglucosan evaporation. However, (26) Piskorz, J.; Radlein, D.; Majerski, P.; Scott D. S. Pyrolysis of cellulose from oligosaccharides to synthesis gas. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; CPL Press: Newbury, U. K., 2002; Vol. 2, pp 381-389. (27) Mok, W. S. L.; Antal, M. J. Effects of pressure on biomass pyrolysis. II. Heats of reactions of cellulose pyrolysis. Thermochim. Acta 1983, 68, 165-186.

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Figure 2. (A) Weight loss (Y) and rate of weight loss (-dY/dt) curves of fraction A of the pyrolysis liquids produced from nut shells, straw pellets, and olive husks as functions of time as measured (symbols) and predicted by the model (lines). (B) Predictions of the devolatilization rates of the three lumped classes of volatile products of fraction A of the pyrolysis liquids produced from nut shells, straw pellets, and olive husks as functions of time.

at low flow rates and high pressures, the endotherm is replaced by a strong exotherm, attributed to decomposition reactions with the formation of gas and carbonized levoglucosan, which produces char. The important role played by mass transfer limitations in the formation of char from levoglucosan is also confirmed by other results.28 Additional information on char formation from the reactions of vapor-phase products (anhydrosugars) of wood/biomass pyrolysis can be found in the reviews of refs 29 and 30. Also, the decomposition of hydroxyacetaldehyde, another product of cellulose decomposition, occurs with cracking to smaller molecules and self-condensation reactions to form many higher-molecular-weight species,31 where the formation of char is again an important aspect. As for the wood/biomass oils, the yields of secondary char do not reproduce the trends shown by the lignin content (Table 1). However, it should be observed that the maximum (20%, dry basis, Table 5) is obtained for oil produced from fir wood, which presents a rather high lignin content. Probably, the chemistry of the ligninic components could also be important. The predictions of the component dynamics (Figure 1B and Figure 2B) are qualitatively the same for all of the samples. They show a significant overlap between the activities of the (28) Milosavljevic, I.; Oja, V.; Suuberg, E. M. Thermal effects in cellulose pyrolysis: Relationship to char formation. Ind. Eng. Chem. Res. 1996, 35, 653-662. (29) Antal, M. J. Biomass pyrolysis: A review of the literature, Part I - Carbohydrate pyrolysis. In AdVances in Solar Energy; American Solar Energy Society: New York, 1982; pp 61-111. (30) Antal, M. J.; Gronli, M. G. The art, science and technology of charcoal production. Ind. Eng. Chem. Res. 2003, 42, 1619-1640. (31) Wang, D.; Montane, D.; Chornet, E. Catalytic steam reforming of biomass-derived oxygenates: Acetic acid and hydroxyacetaldehyde. Appl. Catal., A 1996, 143, 245-270.



Table 5. Yields of Secondary Char (Expressed on Both Total Liquid Basis, Yc, and Water-Free Liquid Basis, Ycd), Estimated Activation Energies (E1, E2, E3), Pre-Exponential Factors (A1, A2, A3), and Stoichiometric Coefficients (r, β, γ) and Parameters Giving the Goodness of the Fitting for Fraction A of the Oil biomass

beech wood

fir wood

oak bark

cellulose

nut shells

wheat straw

olive husks

Yc Ycd E1 [kJ/mol] A1 [s-1] E2 [kJ/mol] A2 [s-1] E3 [kJ/mol] A3 [s-1] R β γ %devTG %devDTG

0.11 0.15 56.0 1.8 × 107 32.4 92.0 25.0 0.7 0.38 0.27 0.24 1.1 2.8

0.14 0.20 54.0 1.2 × 106 32.4 70.0 25.0 0.5 0.22 0.37 0.27 0.9 7.0

0.07 0.17 75.5 1.5 × 109 32.4 94.0 25.0 1.1 0.67 0.12 0.14 0.8 3.3

0.24 0.39 55.0 1.3 × 106 32.4 48.0 25.0 0.3 0.37 0.19 0.20 0.7 6.5

0.09 0.16 65.9 6.53 × 107 32.4 110.0 25.0 1.0 0.51 0.20 0.20 0.4 4.8

0.07 0.15 67.4 9.4 × 107 32.4 112.0 25.0 1.5 0.53 0.21 0.19 1.3 3.5

0.09 0.16 66.1 6.5 × 107 32.4 120.8 25.0 1.1 0.51 0.20 0.20 0.6 0.4

three reactions which begin at very low temperatures, although the maximum rates are highly different (especially between the first and the other two components). In all cases, the rate of weight loss starts to increase very rapidly and attains its peak in correspondence to that of the first lumped group of species (this position corresponds to sample temperatures between 336 and 348 K for the different oils). The amount of volatiles released is about the same for beech and cellulose (38 and 37%, respectively), on one hand, and agricultural residues (51-53%), on the other. The highest value is attained by the bark oil (67%), presumably in consequence of the highest water content (Table 3), and the lowest by the fir oil (22%), owing to the release of volatiles over a wide temperature range, as already noticed above. The similarity in the shape of the rate curves and the amount of volatiles generated also gives rise to activation energies that are roughly the same for the wood and cellulose oils (54-56 kJ/mol), on one hand, and for the agricultural residues (66-67 kJ/mol), on the other. A value of 75 kJ/mol is estimated in the case of the bark oil. It can be observed that the estimated activation energies increase with the amount of volatiles which, for the first reaction, are released over a quite narrow range of temperatures. On the basis of the temperature range of interest, roughly 300-350 K, the chemical composition of the pyrolysis oil, and boiling temperatures,8 it can be reasonably postulated that the first zone describes the evaporation of formaldehyde, acetaldehyde, propionaldehyde, glycolic acid, glyoxal, acetone, and methanol (not quantified with the GC-MS analysis of the oil samples of Table 3). Water evaporation is also active, and given the high percentage in the oil composition, it largely determines the maximum global devolatilization rate. In reality, given the slow heating rates of the sample, the evaporation of light components and water occurs before the boiling temperature is achieved. The second devolatilization reaction is characterized by much lower rates, with peak rates reduced by factors from about 10 (bark) to 1.5 (fir). The presence of a well-evident shoulder in the case of the fir oil is responsible for the highest amount of volatile released (37% versus 12-27% of the other samples). Compared with the first reaction zone, the position of the peak rate appears to be more influenced by the origin of the oil (peak temperature is between 359 and 389 K). Also, the activity of the reaction terminates slightly earlier (lower temperatures) for agricultural residues and bark. However, the estimated activation energy is the same (32.4 kJ/mol) in all cases. Again, using the temperature range of interest, approximately 350-400 K, and

the information on the oil compounds (and related boiling temperatures), it can be understood that this step describes the evaporation of components such as formic acid, acetic acid, and hydroxyacetaldehyde, which are among the major organic components of the oil, and some residual water. For temperatures above 400 K, the global devolatilization rate remains at low values until it finally goes to zero. The amounts of volatiles released by the third step are between 14% (bark) and 27% (fir). Again, the same value of the activation energy (25 kJ/mol) is estimated in all cases. Taking into account the chemical composition, this reaction zone may describe the evaporation of the components with high boiling points, such as furans, carbohydrates, and, in the case of the wood/biomass oil, the products of lignin decomposition, and the release of gas (CO, CO2, and CH4) generated by cracking reactions. The important role played by the oil compounds originated from the decomposition of the holocellulosic component of the biomass is confirmed by the large amount of volatiles produced by the cellulose oil (20%), which is comparable with that of the other samples. It can also be observed that the devolatilization process terminates at about 3000 s (550 K) in the case of agricultural residues and bark, before the beginning of the isothermal section (600 K at about 3500 s), whereas it requires up to 4000 s in the case of the other feedstocks. The lower contents of furans and syringols in the former case (Table 3) may be partly responsible for this behavior. The relatively low values of the activation energies estimated for the first (54-75 kJ/mol), second (32.4 kJ/mol), and third (25 kJ/mol) reactions are a consequence of the use of lumped groups of species and the combination of evaporation and cracking processes. Moreover, it is worth noting that the estimated values become successively lower as the temperature interval of interest for the reaction enlarges. Fraction B. Weight loss measurements for fraction B are shown in Figure 3 in terms of integral and differential data for all of the samples. To facilitate a comparison with fraction A, Figure 4 reports, as examples, the measured global weight loss curves of the two fractions for the oil generated from beech wood and nut shells. For times shorter than 1000-1250 s (temperatures below 380-400 K), the devolatilization rates of fraction B are much lower than those of fraction A. The low content of water and the lack of species with low boiling points, such as carbohydrates (for instance, acetic acid, hydroxyacetaldehyde, etc.), are responsible for such behavior. All of the samples of fraction B, with the exception of beech wood, also present a peak in this region, but the position is anticipated at about 328 K (against 336-348 K of fraction A). This is probably attributable to the evaporation of highly volatile


Figure 3. Measured weight loss (Y) and rate of weight loss (-dY/dt) curves of fraction B of the pyrolysis liquids produced from beech and fir wood and bark, nut shells, and olive husks as functions of time.

Figure 4. Measured weight loss (Y) and rate of weight loss (-dY/dt) curves of fractions A and B of the pyrolysis liquids produced from beech wood and nut shells as functions of time.

compounds of extractives, which may be part of this fraction. The peak is very high and coincides with the maximum devolatilization rate in the case of fir wood and nut shells. For longer times (temperatures above 400-425 K), the devolatilization rates of fraction B are always higher than those of fraction A, and in general, the duration of the process is longer, owing to the large contribution of phenols, guaiacols, syringols, and mainly high-molecular-weight species in the chemical composition. The presence of high devolatilization rates (fir wood and nut shells) or the attainment of the absolute maximum (beech wood) at high temperatures (about 575 K) also confirm the presence of high-molecular-weight species, which, upon heating, essentially undergo thermal cracking reactions. On the other hand, a more rapid decay of the rate curves at high temperatures is observed for the bark and olive husk samples. The rate curves of fraction B are both qualitatively and quantitatively similar for the fir wood and nut shell samples. A close similarity also exists between the curves for beech wood and olive husks, apart from the peaks at the very low (olive husks) and very high (beech wood) temperatures as discussed above. At intermediate times (time intervals of about 15003000 s, corresponding to temperatures of 400-550 K), all of the samples attain high rates, except for bark, that present a wide zone of high (roughly constant) values from the beginning. The yields of the secondary char, between 24% (fir wood) and 33% (olive husks) (water-free oil basis, Table 6), are also always higher than those of fraction A, thus confirming that highmolecular-weight compounds, mainly generated from the decomposition of the lignin component, highly contribute to the formation of the solid residue. A three-step kinetic model has also been applied to describe the weight loss curves of fraction B of the oil samples. The estimated activation energies, pre-exponential factors, and

Branca et al.

Figure 5. Weight loss (Y) and rate of weight loss (-dY/dt) curves of fraction B of the pyrolysis liquids produced from beech wood as functions of time as measured (symbols) and predicted by the model (lines). Predicted component dynamics (rate curves) are also shown.

Figure 6. Weight loss (Y) and rate of weight loss (-dY/dt) curves of fraction B of the pyrolysis liquids produced from fir wood as functions of time as measured (symbols) and predicted by the model (lines). Predicted component dynamics (rate curves) are also shown. Table 6. Yields of Secondary Char (Expressed on Both Total Liquid Basis, Yc, and Water-Free Liquid Basis, Ycd), Estimated Activation Energies (E1, E2, E3), Pre-Exponential Factors (A1, A2, A3), and Stoichiometric Coefficients (r, β, γ) and Parameters Giving the Goodness of the Fitting for Fraction B of the Oil biomass

beech wood

fir wood

oak bark

nut shells

olive husks

Yc Ycd E1 [kJ/mol] A1 [s-1] E2 [kJ/mol] A2 [s-1] E3 [kJ/mol] A3 [s-1] R β γ %devTG %devDTG

0.29 0.32 30.0 50 36.0 16.0 91.0 3.3 × 105 0.06 0.41 0.24 0.6 7.0

0.24 0.26 60.0 1.9 × 107 31.0 6.5 62.0 1.0 × 103 0.21 0.30 0.25 1.3 12.0

0.25 0.28 29.6 68 35.1 47.0 40.0 19.0 0.15 0.24 0.36 8.3 1.1

0.27 0.30 60.0 1.8 × 107 31.0 7.2 61.0 9.8 × 102 0.18 0.32 0.24 1.1 18.6

0.33 0.35 30.0 50 36.5 29.0 42.0 20.0 0.06 0.22 0.39 1.4 8.0

stoichiometric coefficients are listed, together with the parameters giving the goodness of the fitting, in Table 6. Good agreement is obtained between the measurements and predictions, as testified by the values of the fit parameters and the examples of the integral and differential curves, shown in Figures 5 and 6 for beech and fir woods, respectively. The component dynamics, also shown in Figures 5 and 6 for the rate curve, indicate some differences between the two samples. In the case of fir wood (and nut shells, not shown), the first reaction step essentially describes the first, very high peak, whereas the other two steps describe the remaining portion of the curves. An overlap takes place between the three reaction zones, though it is barely visible for the first and the third zones. In the case of beech wood (and olive husks and bark, not


shown), the parameters of the three reactions are not optimized to describe the details of the first peak rate, given the relatively small height. Moreover, no overlap occurs between the first and the third reaction zones. The similarity in the first component dynamics for the fir and nut shell samples produces the same value of the activation energy (60 kJ/mol) and only a slightly different pre-exponential factor. In accordance with the height of the peak, the amount of volatiles released is higher for the fir oil. The estimated activation energy of the first reaction is also not affected by the origin of the oil for the other group of samples (beech, bark, and olive husks), although the value, 30 kJ/mol, is half of that reported above. Furthermore, as expected, the amounts of volatiles released are much lower (6-15%). The peak rates of this reaction zone are positioned at 328-363 K. The comparable values of the global devolatilization rate for the second reaction zone for all of the samples give rise to about the same value of activation energy (31-36 kJ/mol), although the yields of volatile species produced vary over a quite wide range (22-41%) and peak rates are attained at temperatures of 420-471 K. For the third reaction zone, while roughly the same kinetic parameters are obtained for the fir wood and nut shell samples, significant differences are observed in the other cases owing to variations in the shape of the rate curves. Positions of the peak rates are also rather distant (511-587 K). Finally, owing to the lumped character of the mechanism, the activation energies always show rather low values. Conclusions The devolatilization behavior (dynamic heating to 600 K, followed by an isothermal section) has been investigated for oils produced from the conventional pyrolysis of wood (beech and fir), biomass (bark and agricultural residues), and cellulose. An experimental analysis has been carried out for two liquid fractions. The first (fraction A), collected by ice-cooled condensers, is rich in water and compounds with relatively low molecular weights (carbohydrates; furans, and, in the case of wood/biomass, phenols, guaiacols, and syringols). The second (fraction B), absent for cellulose and collected as a superficial muddy layer above the water scrubbers, together with compounds belonging to phenols, guaiacols, and syringols is presumably constituted by extractives and pyrolytic lignin. The conversion to secondary char of fraction B is significant, thus confirming the link between this product and the pyrolytic lignin content of the oil. On the contrary, a result, not reported by previous literature, is obtained for fraction A, where the oil produced from cellulose gives rise to yields of secondary char about 2 times higher than those obtained from the wood and biomass oils (about 40 versus 15-20% on a dry liquid basis).


In other words, oil compounds, typically generated from the holocellulosic component of the starting fuel, play a dominant role in the formation of secondary char during oil devolatilization. This is an important aspect to be taken into account in bio-oil combustion as it is known that about half of the carbon content of the oil is retained by secondary char at least for the conditions typical of thermal analysis. Devolatilization rates for fraction A are not significantly affected by the origin of the oil, and in all cases, the same shape of the weight loss curves is observed. This is not the case for fraction B, where a similarity exists between the fir and nut shell samples, on one hand, and the beech, bark, and olive husk samples, on the other. Furthermore, the differences between fractions A and B are quantitatively large. For temperatures below 400 K, the former is characterized by very high devolatilization rates (essentially, evaporation of low-boiling point species and water), which cause weight losses between 22 and 67% of the initial mass, whereas the rates of the latter are always lower and correspond to weight losses of 6-20%. Hence, in this case, a large part of the weight loss takes place at higher temperatures and is attributable mainly to thermal cracking. A three-step kinetic mechanism gives good predictions of the thermogravimetric curves of fraction A for all of the samples, with equal or comparable values of the activation energies and pre-exponential factors. On the basis of the oil chemical composition and the boiling temperatures of the compounds, the main species involved with each reaction step have been identified. A three-step kinetic model has also been applied to predict the weight loss characteristics of fraction B. The agreement between the predictions and measurements is again acceptable. The kinetic constants are affected by the origin of the oil to a certain extent, although similarities between samples and reaction zones again give rise to comparable or coincident values. These results, combined with a kinetic control in the execution of the measurements, indicate that the lumped mechanism proposed is representative of physicochemical transformations and not a mere fitting procedure. However, although the mild thermal conditions established during the devolatilization process are needed to eliminate heat and mass transfer effects in the measured rate of weight loss, the actual oil heating rates, in practical combustion systems, are high and may induce variations in the devolatilization mechanism. Hence, further research activities are required to better understand these aspects. Also, detailed mechanisms should be investigated, with separate and more accurate descriptions of evaporation and cracking/polymerization reactions. EF0601059

Devolatilization of Conventional Pyrolysis Oils Generated from

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