Devolatilization and Combustion Kinetics of Quercus cerris Bark

Thermogravimetric curves in air of bark from Quercus cerris, a common oak extensively diffused in Southern. Europe and Asia Minor, and related char ha...
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Energy & Fuels 2007, 21, 1078-1084

Devolatilization and Combustion Kinetics of Quercus cerris Bark Carmen Branca, Antonella Iannace, and Colomba Di Blasi* Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy ReceiVed October 26, 2006. ReVised Manuscript ReceiVed December 6, 2006

Thermogravimetric curves in air of bark from Quercus cerris, a common oak extensively diffused in Southern Europe and Asia Minor, and related char have been measured for heating rates between 5-20 K/min and interpreted by multistep reaction mechanisms. As in the case of wood char, a model consisting of two parallel reactions provides good agreement between predictions and measurements of char oxidation. However, the most important combustion reaction, accounting for about 80% of volatile mass produced, is associated with a lower activation energy (169 versus 182 kJ/mol). This result testifies a reduced reactivity caused, as indicated by scanning electron microscopy (SEM) images, by a less porous structure where high amounts of calcium and a glaze of polymerized products hinder the access of the oxygen toward the active material. In addition to the two char combustion reactions, three parallel reactions are needed to describe the thermogravimetric behavior of bark. These require relatively low activation energies (70-100 kJ/mol) and represent the release of volatiles from the major components of bark, that is, extractives (51%) and lignin (35%), and the small content of holocellulose (14%).

Introduction The composition and properties of wood are dependent upon the species1 and, within the same species, upon the geographical origin,2 age, and morphological part of the tree.3 Bark commonly forms 10-20% of the cross-sectional area of trees of 50 cm in diameter and about 25% of the stem weight.4 These proportions are again dependent on the position throughout the tree, age of the tree, and species variety. However, the chemical composition of bark is always highly different from that of wood. In particular, it is exceptionally rich in extractives5 which, on average, attain values of about 50%.4,6 They comprise large quantities of polyphenols6 and other compounds such as resins and fats.4 The lignin content is usually in the same range as for wood, so that the amount of holocellulose is much lower. Inorganic material is also generally much higher than in wood and widely variable in composition though calcium salts generally predominate.4 Large volumes of bark residues are produced from the forestry industry, which are in part used as fuel, landfilled, or inciner* Corresponding author. Tel.: 39-081-7682232. Fax: 39-081-2391800. E-mail: [email protected]. (1) Gronli, M. G.; Varhegyi, G.; Di Blasi, C. Thermogravimetric analysis and devolatilization kinetics of wood. Ind. Eng. Chem. Res. 2002, 41, 42014208. (2) Varhegyi, G.; Gronli, M. G.; Di Blasi, C. Effects of sample origin, extraction and hot water washing on the devolatilization kinetics of chestnut wood. Ind. Eng. Chem. Res. 2004, 43, 2356-2367. (3) Orfao, J. J. M.; Antunes, F. J. A.; Figueiredo, J. L. Pyrolysis kinetics of lignocellulosic materials-three independent reactions model. Fuel 1999, 78, 349-358. (4) Hillis, W. E. Wood and biomass ultrastructure. In Fundamentals of Biomass Thermochemical ConVersion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; pp 1-34. (5) Theander, O. Cellulose, hemicellulose and extractives. In Fundamentals of Biomass Thermochemical ConVersion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; pp 35-60. (6) Kofujita, H.; Ettyu, K.; Ota, M. Characterization of the major componets in bark from five Japanese tree species for chemical utilization. Wood Sci.ence Technol. 1999, 33, 223-228.

ated.7 However, these applications may not result in the full realization of their total fuel potential.8 On the other hand, despite the peculiar chemical composition, studies on the thermochemical behavior of bark are few in number. Pyrolysis has been applied to produce char, pyrolytic oil, and gas.9-12 Yields of char higher than that for wood are generally obtained at the expense of oil. Efforts have been made to evaluate the surface and bulk chemistry7 and to investigate the activation by steam13 of the solid product for use as an activated carbon. The oil is reported to contain valuable chemicals, especially phenolic compounds which can be used for making modified phenol-formaldehyde resins.11,12 Furthermore, bark has also raised a significant interest for the production of complex chemicals which have applications in the biological or pharmaceutical industries. Vacuum14 or fast15 pyrolysis has been indicated as a convenient extraction method. (7) Darmstadt, H.; Pantea, D.; Summchen, L.; Roland, U.; Kaliaguine, S.; Roy, C. Surface and bulk chemistry of charcoal obtained by vacuum pyrolysis of bark: influence of feedstock moisture content. J. Anal. Appl. Pyrolysis 2000, 53, 1-17. (8) Ross, R. A.; Fikis, D. V. Gasification reactions of chars and modified produced from jack pine bark. The Canadian Journal of Chemical Engineering 1980, 58, 230-224. (9) Arpiainen, V.; Lappi, M. Products from the flash pyrolysis of peat and pine bark. J. Anal. Appl. Pyrolysis 1989, 16, 355-376. (10) Sensoz, S. Slow pyrolysis of wood barks from Pinus brutia ten. and product compositions. Bioresour. Technol. 2003, 89, 307-311. (11) Murwanashyaka, N. J.; Pakdel. H.; Roy, C. Separation of syringol from beech wood derived vacuum pyrolysis oil. Sep. Purif. Technol. 2001, 24, 155-165. (12) Murwanashyaka, N. J.; Pakdel, H.; Roy, C. Step-wise and one-step vacuum pyrolysis of birch-derived biomass to monotor the evolution of phenols. J. Anal. Appl. Pyrolysis 2001, 60, 219-231. (13) Cao, N.; Darmstadt, H.; Soutric, F.; Roy, C. Thermogravimetric study on the steam activation of charcoals obtained by vacuum and atmospheric pyrolysis of softwood bark residues. Carbon 2002, 40, 471479. (14) Pakdel, H.; Roy, C. Separation and characterization of steroides in biomass vacuum pyrolysis oils. Bioresour. Technol. 1996, 58, 83-88. (15) Piskorz, J.; Borys, A. Fast pyrolysis of birch bark. In Science in Thermal and Chemical Biomass ConVersion; Bridgwater, A. V., Boocock, D. G. B., Eds.; CPL Press: Newbury Berks, UK, 2006; pp 1265-1272.

10.1021/ef060537j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007

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Table 1. Ash Content and Chemical Composition of the Bark and Beech Wood Samples components [% wt] asha extractives acetoneb ethanol/benzeneb 5% NaOHb ligninb holocellulosec c

Quercus cerris

Fagus sylVatica

14

0.2

8.9 3.3 39 34.5 14.3

2 20 78

a Percentage on dry original bark. b Percentage on dry water-washed bark. Percentage by difference.

Results of thermogravimetric analysis of bark decomposition can be found in refs 3, 16, and 17. Compared with wood, it appears that the beginning and the peak of bark decomposition are displaced at lower temperatures and that the latter is much lower. Hot water washing reduces the deviations,17 as a consequence of the partial elimination of inorganic constituents, but the differences in the yields of volatile products and the global rate of decomposition still remain. A three-step mechanism has been proposed in ref 3 for the description of the devolatilization process. The kinetic parameters have been estimated by means of one experiments only (with a heating rate of 5 K/min), so that compensation effects are not avoided.18 No information is currently available on bark devolatilization in air and on the subsequent process of char oxidation. In this study, thermogravimetric analysis is applied to investigate the oxidation characteristics of chars generated from bark and the oxidative decomposition of bark itself. The origin is from Quercus cerris (common name “European Turkey oak”), a variety of oak tree typical of the mediterranean countries and extensively used as firewood. The experiments, which consider different heating rates, are interpreted by multistep reaction mechanisms and the related kinetic parameters are estimated.

distilled water at 333 K for 2 h)17 has been carried out, resulting in the removal of about 2% (dry sample basis) of the content of inorganic matter. After drying, the washed sample has been extracted first with acetone and then with ethanol-benzene (1:2 v/v) using a Soxtech apparatus and treated with 5% NaOH in twicedistilled water (363 K) at reflux for 2 h and with a solid/liquid ratio of 1/1221 (after each extraction, the sample has been oven dried at 373 K overnight and weighed). In this way the content of polyphenols has been determined avoiding an overestimation of the lignin content.6 Successively, the alkali insoluble residue has been hydrolyzed with aqueous solutions of H2SO4 (66%) and HCl (0.5%) to determine the Klason lignin.22 The ash content is determined by calcination.23 From Table 1, significant differences appear between bark and wood. In particular, bark presents a very high content of extractives (about 51% versus 2% for wood) and a lower content of holocellulose (about 14% versus 78% for wood). The amount of ash is also significantly higher (14% and 0.2%, respectively). Prior to thermogravimetric tests, carried out in a forced air flow (1 L/min), the bark and char samples have been milled to powder (particle sizes below 80 µm) and oven dried for 10 h at 373 K. The experimental system employed is the same as that used in previous studies for the kinetic analysis of solid fuels, and details are already available.20,24-26 The characteristic size of the process is the thickness of the sample layer. It has been observed that values up to 110 µm allow good temperature control to be achieved, given maximum heating rates of 20 and 15 K/min for bark and char, respectively, and a final temperature of 873 K. These experimental conditions also ensure a negligible spatial gradient of temperature and oxygen inside the samples. Hence, the tests have been made for sample layers about 100 µm thick, corresponding to a initial mass of 4.1 mg for bark and char, distributed over a surface 20 × 4 mm2. Heating rates of 5, 10, and 20 K/min have been applied for bark and of 5, 10, and 15 K/min for char. Each thermogravimetric test has been made in triplicate, showing good repeatability.

Kinetic Mechanism Materials and Methods As anticipated, samples examined in this study are bark, originated from Quercus cerris, and related char. Details about the pyrolysis process for char production are already available elsewhere.19 In brief, bark particles (5 mm thick), after drying, were fed in a fixed-bed pyrolyzer consisting of a cylindrical steel reactor externally heated by a radiative furnace. The system was batch operated with about 180 g of sample instantaneously fed once a temperature of 800 K had been achieved. The total char yields were about 35% of the initial dry mass of bark (for the same experimental conditions, the char yield from beech wood was about 24%.19). The main information on the chemical composition of bark and, for comparison purposes, beech wood (Fagus sylVatica)20 is listed in Table 1. As the usually high amounts of polyphenols in bark interfere with the accurate evaluation of lignin and holocellulose contents when using standard methods developed for wood,6 sequential extractions with various solvents have been applied, as described below. In a preliminary step, hot water washing (2 g of finely ground sample (280-100 µm) in 200 mL of hot, twice(16) Tran, D. Q.; Rai, C. A kinetic model for pyrolysis of Douglas fir bark. Fuel 1978, 57, 293-298. (17) Meszaros, E.; Jakab, E.; Varhegyi, G.; Szepesvary, P.; Marosvolgyi, B. Comparative study of the thermal behavior of wood and bark of young shoots obtained from an energy plantation. J. Anal. Appl. Pyrolysis 2004, 72, 317-328. (18) Branca, C.; Albano, A.; Di Blasi, C. Critical evaluation of wood devolatilization mechanisms. Thermochim. Acta 2005, 429, 133-141. (19) 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. (20) Branca, C.; Di Blasi, C. Global intrinsic kinetics of wood oxidation. Fuel 2004, 83, 81-87.

Lignocellulosic fuel devolatilization has been extensively modeled assuming that volatiles are released according to a set of parallel reactions for lumped components (for instance, see refs 1, 2, and 18). Then, the overall mass loss rate is a linear combination of the single component rates. In air, for both biomass and biomass char, weight loss curves show two main reaction zones attributed to devolatilization of the parent fuel and heterogeneous combustion of the char produced.20,26 Hence, the kinetic mechanism proposed here for bark and bark char combustion consists of N independent parallel reactions as follows:

(21) Vazquez, G.; Gonzalez-Alvarez, J.; Freire, S.; Lopez-Suevos, F.; Antorrena, G. Characteristics of Pinus pinaster bark extracts obtained under various extraction conditions. Holz Roh- Werkst. 2001, 59, 451-456. (22) 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. (23) Di Blasi, C.; Branca, C.; D’Errico, G. Degradation characteristics of straw and washed straw. Thermochim. Acta 2000, 364, 133-142. (24) Di Blasi, C.; Branca, C. Kinetics of primary product formation from wood pyrolysis. Ind. Eng. Chem. Res. 2001, 40, 5547-5556.

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where S is the parent solid fuel (bark or char) which produces the lumped volatile products Vi (i ) 1, ..., N). From the chemical point of view, reactions a1, ..., aM are associated with solid fuel devolatilization, and reactions aM+1, ..., aN are associated with char combustion. The number of reactions M and N, strictly required for an accurate description of the experiments, are included among the model parameters to be determined. The reactions rates present the usual Arrhenius dependence (Ai are the pre-exponential factors, and Ei, the activation energies) on the temperature and a power-law (ni) dependence on the solid mass fraction. This treatment is more general than the first-order linear rates of the devolatilization reactions1,2,18 and, at the same time, effectively takes into account the evolution of the pore surface area during char combustion.20,26 As in previous studies,20,26 for the stage of char oxidation, given that the dependence of the combustion rate on the oxygen concentration is not investigated and the oxygen partial pressure is at the constant value of air, its contribution is incorporated in the pre-exponential factors. Since the sample temperature, T, is a known function of time (T ) T0 + ht, where T0 is the initial temperature and h is the heating rate), the mathematical model consists of N ordinary differential equations for the mass fractions, Yi, of the lumped classes of volatiles generated:

dYi ) -Ai exp(-Ei/RT)Yni i , Yi(0) ) νi, i ) 1, ..., N(b1-bN) dt where νi, indicated in the following as stoichiometric coefficients, are the initial mass fraction values. The kinetic parameters are estimated through the numerical solution (implicit Euler method) of the mass conservation equations and the application of a direct method for the minimization of the objective function, which considers both integral (TG) and differential (DTG) data, following the method already described.25 The simultaneous use of experimental data measured for several heating rates avoids possible compensation effects in the kinetic parameters.27 The parameters to be estimated are the activation energies (E1-EN), the preexponential factors (A1-AN), the stoichiometric coefficients (n1nN), and the number of lumped classes of volatile species, N (a total of 3N + 1 parameters). The optimization procedure has been executed by requiring the same values of the activation energies, pre-exponential factors, and exponents for all the curves. The stoichiometric coefficients have been allowed to vary with the heating rate to take into account the effects of temperature on the yields of volatile products generated from the various fractions of the fuel. From the practical point of view, given the relatively small range of heating rates examined, average values can be used. Moreover, in contrast with the devolatilization kinetics investigated here, pyrolysis mechanisms24 are capable of predicting both the formation rates and the yields of the main classes of products. Results A comparison is made first between the thermogravimetric curves of bark and wood (and related chars), followed by a (25) Branca, C.; Di Blasi, C.; Horacek, H. Analysis of the combustion kinetics and the thermal behavior of an intumescent system. Ind. Eng. Chem. Res. 2002, 41, 2104-2114. (26) Branca, C.; Di Blasi, C. Devolatilization and combustion kinetics of wood chars. Energy Fuels 2003, 17, 1609-1615. (27) Conesa, J. A.; Marcilla, A.; Caballero, J. A.; Font, R. Comments on the validita` and utilita` of the different methods for kinetic analysis of thermogravimetric data. J. Anal. Appl. Pyrolysis 2001, 42, 73-87.

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Figure 1. Solid mass fractions and time derivative of the solid mass fraction as functions of temperature for bark (solid lines) and beech wood (dashed lines) and their chars for a heating rate equal to 5 K/min and a final temperature of 873 K.

morphological analysis of the bark char. Then, results of kinetic modeling are presented including parameter estimation. Thermogravimetric Analysis. Figure 1 reports the solid mass fractions (TG curves) and the time derivative of the solid mass fraction (DTG curves) as functions of temperature for bark and beech wood and their chars for a heating rate equal to 5 K/min (curves on an ash free basis (Y*) (Y - Yash)/(1 - Yash))). The curves of the beech wood and beech wood char are the same as those reported in refs 20 and 26, respectively. The char sample was produced from the pyrolysis of 4 cm thick particles subject to a heating temperature of about 800 K. In agreement with the previous literature, the DTG curve of beech wood shows two main reaction zones corresponding to the devolatilization of the solid fuel, with the attainment of the maximum devolatilization rate and the combustion of the produced char, characterized by lower rates (a temperature of about 625-650 K can be used as a separation boundary). On the contrary, a clear distinction between the two reaction zones is not possible for bark, most likely owing to the slower decomposition rates, over a wider temperature range, of the main components (extractives and lignin) with respect to the main beech wood components (hemicellulose and cellulose). Weight loss (oxidative decomposition) begins around 490 K, and the position of the DTG peak is roughly the same (about 590 K) for both samples. However, as observed in inert atmosphere,3,17 in the case of bark, it is about 2.5 times lower and consequently the corresponding amount of volatile released is smaller (40% and 66% for bark and wood, respectively). Furthermore, the hemicellulose shoulder, well-evident for beech wood at about 547 K, is absent. Finally, the slower decay of the tail region can be interpreted as an indication of the slower reactivity of the bark char. The weight loss dynamics of char in air are qualitatively similar for the two samples, that is, the DTG curves show a low-temperature zone corresponding to devolatilization, followed by the combustion peak. However, as already observed for bark, the bark char presents a lower reactivity. Although the position of the peak rate and the corresponding mass fractions are roughly the same, the peak value in the case of bark is lower by a factor of about 2. Moreover, the tailing region presents higher values and complete conversion is achieved at higher temperatures. As the heating rate is increased, the characteristics of the weight loss curves remain the same but the occurrence of chemical reactions is displaced toward successively higher temperatures. Physical and Morphological Characteristics of Bark Char. Information about the physical structure of the bark char, as produced from the pyrolysis of a packed bed at 800 K, useful

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Energy & Fuels, Vol. 21, No. 2, 2007 1081

Figure 2. SEM images of the lateral (parallel to the trunk fibers) surface of the bark char: views of the internal (A-D) and external (E and F) side of the wood trunk.

to explain the different reactivity with respect to wood char, can be gained from scanning electron microscopy (SEM) images reported in Figures 2A-F (surface parallel to the trunk fibers) and 3A-F (surface perpendicular to the trunk fibers). From the longitudinal section of the internal surface (wood side), an irregular and stratified structure (Figure 2A) can be observed. Magnified views show the presence of cracks and pores (dimension around 30-40 µm), covered by a thin and transparent material (Figure 2B), and large amount of calcium crystals (Figure 2C and D). The external surface shows a smooth and stratified structure (Figure 2E), characterized by the presence of pores completely covered and occluded again by a glaze (Figure 2F). This is probably a consequence of the condensation, at ambient temperature, or polymerization, under the reaction conditions, of primary vapors generated from the decomposition reactions. Images of the cross section show the presence of surface cracks and holes, up to 300 µm large (Figure 3A). From magnified views, a highly irregular cracked structure is evident, where some zones produce no evidence of pores (Figures 3B and C). In reality, at a larger magnification, the pores appear partially occluded by the presence of the calcium crystals or covered by thin layers of condensed material (Figure 3D-F). A comparison with SEM images of beech wood char can be made using the results of ref 28. In this case the longitudinal and the cross surface show that the chief features of the wood structure are preserved. Both macropores (about 25-50 µm) (28) Branca, C.; Di Blasi, C.; Elefante, R. Devolatilization and heterogeneous combustion of wood fast pyrolysis oils. Ind. Eng. Chem. Res. 2005, 44, 799-810.

and micropores (about 5-10 µm) are present over a very regular, honeycomb structure with no evidence of pore occlusions. Although ash elements can exert a catalytic role on the reactivity of char, their presence in the apertures of char can decrease the size aperture to an extent that the active surface area is also highly decreased.29 In the case of bark char, the massive presence of calcium and transparent solidified material over the porous surface and the reduced porosity, with respect to wood char, certainly contribute to diminishing the extension of the active surface area and, through this, the apparent reactivity of the material. Indeed, it is easily understandable that more of the internal structure is hardly accessible to the gaseous reagent for the heterogeneous reactions. Kinetic Modeling of Bark Combustion. The peculiar chemical composition of the bark samples examined in this study makes difficult the selection of the number of reaction steps and related kinetic constants as the current knowledge on wood or biomass cannot be directly utilized. Indeed, as already discussed, holocellulose, which is the major component for these fuels, is present in a very small amount in the case of bark. On the contrary, the extractives, minor components in a large part of biomass and wood species, are dominant in bark. Literature results on the intrinsic reactivity of these components are scarce. A thermogravimetric analysis is available2 for the extractives from different samples of chestnut wood, a variety particularly rich in these components (up to 16%, mainly tannins). It has been observed that the thermal degradation is very slow (rates (29) Raveendran, K.; Ganesh, A. Adsorption characteristics and poredevelopment of biomass pyrolysis char. Fuel 1998, 77, 769-781.

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Figure 3. SEM images of the cross (perpendicular to the trunk fibers) surface of the bark char.

of volatile release about 1 order of magnitude lower than those for wood) and is scarcely affected by the temperature (three peaks barely detectable at about 470-490, 560-570, and 600680 K). Moreover, char yields are very high, as a consequence of the high amount of fixed carbon in the starting material. Although the chemical composition of extractives from chestnut wood and bark may be different, it can be assumed that the main features (slow devolatilization rates over a wide temperature range) are retained in both cases. From the mathematical point of view, this behavior can be simulated by means of reactions with relatively low activation energies and a powerlaw dependence on the solid mass fraction. Lignin is also present in significant amounts in bark composition. This component undergoes slow devolatilization rates over a very wide range of temperatures and produces high yields of char again as a result of its high content of fixed carbon for both hardwoods and softwoods.1,2 For an effective description of these features, a devolatilization rate with a power-law dependence on the solid mass fraction has been used30-32 in contrast with the devolatilization of the holocellulose fractions where linear rates have been generally introduced.1,2,18 Kinetic modeling of the weight loss process and evaluation of model parameters have been made considering first the (30) Caballero, J. A.; Conesa, J. A.; Font, R.; Marcilla, A. Pyrolysis kinetics of almond shells and olive stones considering their organic fractions. J. Anal. Appl. Pyrolysis 1997, 42, 159-175. (31) Manya, J. J.; Velo, E.; Puigjaner, L. Kinetics of biomass pyrolysis: a reformulated three-parallel reaction model. Ind. Eng. Chem. Res. 2003, 42, 434-441. (32) Gomez, C. J.; Manya, J. J.; Velo, E.; Puigjaner, L. Further applications of a revisited summative model for kinetics of biomass pyrolysis. Ind. Eng. Chem. Res. 2005, 43, 901-906.

Table 2. Kinetic Constants (Ai, Ei), Stoichiometric Coefficients (νi), Reaction Order (ni), and Deviations for Integral (TG) and Differential (DTG) Curves of Bark Char Combustion, as Estimated by the Two- and One-Step Models

devolatilization combustion h [K/min] ν1 ν2 devDTG [%] devTG [%]

two-step model

one-step model

E1 ) 105.0 [kJ/mol] A1 ) 5.2×105 [1/s] n1 ) 1 E2 ) 168.6 [kJ/mol] A2 ) 1.85×1010 [1/s] n2 ) 1.99 5 10 15 0.22 0.20 0.18 0.55 0.58 0.59 1.7 3.4 2.6 1.1 0.97 0.62

E2 ) 129.0 [kJ/mol] A2 ) 1.70×107 [1/s] n2 ) 1.85 5 10 15 0 0 0 0.77 0.77 0.77 2.0 2.6 7.8 0.54 1.5 3.3

experiments on char combustion. Then, based on the information about the devolatilization dynamics of the two major components summarized above and the char combustion results, the bark weight loss curves have been evaluated. Following the results already obtained for the combustion kinetics of beech wood char,26 the thermogravimetric tests of bark char combustion have been described by a one-step global model (M ) 0 and N ) 1) and a two-step model (M ) 1 and N ) 2). The initial guesses of the model parameters have been assumed to coincide with those already estimated for wood. The results of the kinetic analysis are summarized in Table 2, whereas Figure 4A and B present a comparison between predicted and measured curves. The results of the two-step model are accurate for both the integral and differential data and all the heating rates. Unlike the wood char, the one-step model also produces good results, especially at the slower heating rates. It is associated with

DeVolatilization and Combustion Kinetics of Bark

Figure 4. Mass fractions, on an ash free basis (A), and time derivative of the mass fraction (B) versus temperature of bark char for heating rates, h, of 5-15 K/min (up to 873 K) as measured (symbols) and predicted (lines).

parameter values comparable with those obtained for wood char.26 For the two-step model, while the activation energy for the devolatilization reaction is about the same (105 and 114 kJ/mol for bark and wood, respectively) and the amount of volatiles released is comparable (16% and 22%), the differences are significant for the combustion reaction. The lower activation energy (169 versus 182 kJ/mol) and the higher exponent (2 versus 0.9) estimated for the bark char are a clear indication of a lower reactivity (smaller reaction rates over a wider temperature range), as already observed from the comparison of the weight loss curves and the morphological structure of char. Given the very small cellulose content, the first and simplest model proposed here of the measured thermogravimetric curves of bark in air does not include any specific reaction for the devolatilization of this component. Moreover, it is assumed that the two reactions taking place at the highest temperatures are essentially associated with char dynamics and approximately present the same kinetic constants already estimated for the twostep model (small variations are allowed only for the preexponential factors and reaction orders). From the procedure of parameters evaluation, it has been found that, in addition to the two reactions for char devolatilization and combustion, at least three reactions are required for bark devolatilization (fivestep model) for accurate predictions of the measured curves. A comparison between predictions and measurements is presented in Figure 5A and B for all the heating rates, whereas an example of component dynamics is shown in Figure 6 (heating rate of 10 K/min). Tables 3 and 4 report the estimated values of the kinetic parameters. At a first glance, good agreement appears in all cases between measurements and predictions. As expected, activation energies for the bark devolatilization reactions are relatively low (72, 85, and 96 kJ/ mol). The first reaction, which can be probably associated with the devolatilization of the most volatile components of the extractives, responsible of a volatile release of about 5%, is linear in the solid mass fraction. The other two reactions occur

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Figure 5. Solid mass fraction versus time (A) and time derivative of the solid mass fraction (B) versus temperature for bark as measured (symbols) and predicted (lines) by the five-step model for heating rates, h, between 5-20 K/min (up to 873 K).

Figure 6. Global and component rates of volatile release for bark for a heating rate of 10 K/min (up to 873 K) as measured (symbols) and predicted (lines) by the five-step model. Table 3. Kinetic Constants (Ai, Ei) and Reaction Order (ni) as Estimated for the Five-Step Model of Bark Devolatilization and Combustion at Heating Rates of 5-20 K/min reaction

E [kJ/mol]

A [1/s]

n

a1 a2 a3 a4 a5

72.0 85.0 96.3 105.0 168.6

7.80 × 104 7.40 × 105 1.30 × 106 4.40 × 105 1.85 × 1010

1.00 1.44 1.68 1.04 1.99

at higher temperatures and produce the release of large amounts of volatiles (17% and 31-36%, respectively). Overlap among the activity of the first three reactions is always significant and increases with the heating rates (not shown). Apart from the first reaction, overlap with the combustion reactions is also relevant. Finally, it can be noted that the last two reactions (char dynamics) cause the release of 12-13% and 18-17% of volatiles, respectively. This result indicates that the fourth reaction, in reality, is not truly representative of char devolatilization processes (about 20% of the mass of produced char, Table 2), but also includes, to a certain extent, the effects of bark decomposition. Moreover, the amount of volatiles generated

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Table 4. Stoichiometric Coefficients (νi) and Deviations for Integral (TG) and Differential (DTG) Curves of Bark Devolatilization and Combustion at Heating Rates of 5-20 K/min, as Estimated by the Five-Step Model h [K/min]

5

10

20

ν1 ν2 ν3 ν4 ν5 devDTG [%] devTG [%]

0.049 0.172 0.313 0.135 0.185 1.0 0.65

0.047 0.171 0.330 0.130 0.176 1.5 0.32

0.045 0.170 0.356 0.120 0.166 2.1 0.44

Table 5. Kinetic Constants (Ai, Ei) and Reaction Order (ni) as Estimated by the Six-Step Model of Bark Devolatilization and Combustion at Heating Rates of 5-20 K/min reaction

E [kJ/mol]

A [1/s]

n

a1 a2 a3 a4 a5 a6

72.0 81.0 171.0 92.0 105.0 168.6

7.80 × 104 3.40 × 105 4.60 × 1012 2.60 × 105 4.40 × 105 1.85 × 1010

1.00 1.80 1.00 1.40 1.04 1.99

Table 6. Stoichiometric Coefficients (νi) and Deviations for Integral (TG) and Differential (DTG) Curves of Bark Devolatilization and Combustion at Heating Rates of 5-20 K/min, as Estimated by the Six-Step Model h [K/min]

5

10

20

ν1 ν2 ν3 ν4 ν5 ν6 devDTG [%] devTG [%]

0.042 0.265 0.050 0.180 0.133 0.184 2.1 0.70

0.042 0.270 0.050 0.190 0.130 0.172 2.4 0.24

0.042 0.275 0.050 0.200 0.120 0.167 4.3 0.69

from the actual reaction of char combustion decreases with the heating rate, as the successively higher reaction temperatures favor devolatilization and the separation among the different stages becomes successively more difficult. Modifications of the five-step model, to include a reaction for the decomposition of cellulose (over the proper temperature range and activation energy between 170-200 kJ/mol),18 have also been studied. So, a six-step model has been developed, again keeping constant the kinetic parameters of the last two reactions (active at the highest temperatures) at the values previously determined from the char curves. An example of the dynamics of the components is shown in Figure 7 (heating rate of 10 K/min) and the kinetic parameters are summarized in Tables 5-6. Although the accuracy of the predictions is good (roughly the same as for the five-step model), in accordance with the chemical composition of bark, the cellulosic component plays only a secondary role in the dynamics of the decomposition process (the amount of volatiles produced is 5%). Therefore, the five-step model can be retained sufficiently accurate and apt for engineering applications. Conclusions Bark is an important part in the morphological structure of the trees and presents chemical and physical properties highly different from wood. Nevertheless, only a very few investigations are currently available on the thermochemical behavior of this material. In this study, the combustion kinetics have been analyzed for samples belonging to a variety of oak, Quercus cerris (European Turkey oak), extensively diffused in mediterranean countries.

Figure 7. Global and component rates of volatile release for bark for a heating rate of 10 K/min (up to 873 K) as measured (symbols) and predicted (lines) by the six-step model.

The chemical analysis has confirmed the peculiar characteristics of this part of the tree, with a very high content of extractives (about 50% versus 1.5-5% for the wood), to the detriment of cellulose and hemicellulose components, and ash (about 14% versus 0.5-1% for the wood). Thermogravimetric measurements, carried out for bark and bark char with different thermal conditions, have indicated that the devolatilization process takes place with significantly lower rates, and the produced char is characterized by a lower reactivity. SEM images of the solid residue have shown a lower porosity with large amounts of calcium and a surface glaze, probably generated from the condensation or polymerization of some pyrolysis products, which partially occlude the pores and makes the internal surface of the solid less accessible to the gaseous reagent. A kinetic model consisting of two parallel reactions provides a good description of the thermogravimetric curves for the bark char. As in the case of wood, the two reactions can be attributed to a devolatilization (concerning about 20% of the material) and combustion. However, as a consequence of lower reaction rates over a wide temperature range, the estimated parameters are considerably different. In particular, the activation energy of the combustion reaction is lower (169 versus 182 kJ/ mol). The weight loss characteristics of bark in air are welldescribed by a kinetic model consisting of three parallel reactions for devolatilization, combined with the two reactions corresponding to char conversion. The former group is characterized by low activation energies with values of about 70100 kJ/mol, owing to both the low rates of volatile release from the major components (extractives and lignin) and the lumped nature of the reaction mechanism. In accordance with bark chemical composition, the addition of a specific reaction for cellulose decomposition in the devolatilization mechanism only takes into account a small amount of volatile generated (5%) and can be neglected. Future investigations could be pursued to evaluate the applicability of the kinetic models here developed for barks of different origin (different age and species of trees and/or different parts of the plant). However, presumably this is not a simple task since, notwithstanding the huge amount of scientific work concerning the thermogravimetric behavior of the wood, a general kinetic model is not available even for simpler fuels, such as the hardwoods and the softwoods. EF060537J