versión On-line ISSN 0718-221X

Maderas, Cienc. tecnol. vol.16 no.2 Concepción mayo 2014  Epub 12-Mar-2014

ARTÍCULO

Wood thermodegradation: experimental analysis and modeling of mass loss kinetics

A. Pétrissans1,♠ R. Younsi2, M. Chaouch3, P. Gérardin1, M. Pétrissans1

1LERMAB, Université de Lorraine, BP 70239, F-54506 Vandoeuvre les Nancy, France.
2Département de Génie Mécanique, École Polytechnique de Montréal, Montréal QC H3C 3A7, Canada.
3SEREX, 25, rue Armand-Sinclair, porte 5 Amqui (Québec) G5J 1K3, Canada.

In memoriam of Dr. Manfred SCHWANNINGER

Corresponding author

ABSTRACT

In this study, heat treatment was carried out in a relatively low temperature (230˚C). Mass loss kinetics was studied using equipment, specially conceived to measure sample’s mass during the thermal treatment. Laboratory experiments were performed for heating rates of 1˚C min-1. Mathematical model for kinetics of pyrolysis process was used and validated. During the pyrolysis of dry wood samples under inert atmosphere, measurements of temperature distribution and dynamic weight loss were performed. Five different wood species Fagus sylvatica (Beech), Populus nigra (Poplar), Fraxinus excelsior (Ash), Pinus sylvestris (Pine) and Abies pectinata (Silver Fir) were investigated. The unsteady-state mathematical model equations were solved numerically using the commercial package Femlab 2.0. A detailed discussion of the computational model and the solution algorithm is given. The validity of different model assumptions was analyzed. Experimental results were compared with those calculated by the model. Acceptable agreement was achieved.

Keywords: Heat treatment, modeling, reaction kinetics, thermodegradation, wood.

INTRODUCTION

Wood is commonly used as building and engineering material. Unprotected wood exposed to outdoor conditions undergoes a variety of degradation reactions induced by diverse factors such as light, moisture, heat, oxygen, pollutants (Evans et al. 1992). Moisture contain promotes the fungal attack and leads to sever destruction of wood. In the past, wood preservation has been carried out by chemical treatments, some of them including components that are poisonous for environment and human health. Nowadays, heat treatment of the wood by mild pyrolysis is used as an alternative to chemically impregnated wood materials. It is an effective method to improve biological durability of wood (Finnish Thermowood Association 2003, Momohara et al. 2003, Shi et al. 2007). Wood heat treatment induces chemical modification of main wood constituents. Lignin polymer structure is modified (Zammen et al. 2000, Tjeerdsma and Militz 2005, Nguila et al. 2006, Nguila et al. 2007a, Esteves et al. 2008), the ratio between amorphous and crystalline cellulose is also changed (Fengel and Wegener 1989, Sivonen et al. 2002, Yildiz et al. 2006), hemicelluloses are strongly decomposed (Sivonen et al. 2002, Nuopponen et al. 2004, Gérardin et al. 2007), some products precursors of charcoal appear (Nguila et al. 2007b). These chemical modifications confer new properties: improved dimensional stability (Mouras et al. 2002, Esteves et al. 2007, Esteves et al. 2007b), better fungal resistance (Hakkou et al. 2006, Kamdem and Pizzi 2002, Mazela et al. 2003, Welzbacher et al. 2007), some color changes (Mitsui et al. 2001, Bekhta and Niemz 2003, Mitsui et al. 2003, Ayadi et al. 2003, Mitsui et al. 2004), mechanical properties modification (Santos 2000, Yildiz  2002, Unsal and Ayrilmis 2005), wood acquires a hydrophobic character (Mitsui et al. 2001, Bekhta and Niemz 2003, Mitsui et al. 2004, Hakkou et al. 2003, Pétrissans et al. 2003, Hakkou et al. 2005, Kocaefe et al. 2008). Even if wood heat treatment seems simple, there are numerous parameters that control this process. Different methods for wood thermal treatment have been developed in France, Finland, Netherlands and Germany since the middle of the last century (Finnish Thermowood Association 2003). In these processes, wood heat treatment by mild pyrolysis (temperature range between 160°C and 260°C) is performed under different process conditions (diversity in the process stages, choice of inert atmosphere (nitrogen or hot gases produced from the heat treatment), wet or dry process, use of oil etc.), (Rapp 2001). Chemical reactions involved during mild pyrolysis as well as final properties of the material depend strongly on the treatment temperature and process duration. All through the heat treatment, wood is thermally decomposed at a slow rate (Degroot et al. 1988, Repellin and Guyonnet 2005). The resulting anhydrous mass loss (ML) is representative for physical and chemical wood transformations. Mass loss matches to the advancement of the heat treatment process.

Wood thermal degradation is a complex topic given that the material contains several fractions with different thermal behavior. Authors often distinguish a number of stages (Degroot et al. 1988, Repellin and Guyonnet 2005, Nguila et al. 2009). A phase of elimination of some volatile compounds takes place at temperatures lower than 200°C. For temperature range of 200-280°C, hemicelluloses are converted essentially into gases and acetic acid. This step corresponds to the wood roasting. At the temperature range of 250-300°C, lignin and cellulose are decomposed to give three products: gas, tar and char. Wood thermodegradation leads to an anhydrous mass loss, representative for wood chemical transformation. Properties of heat treated wood depend on the mass loss (Weiland et al. 1998). For this reason, controlling the quality of the heat treated wood means to control precisely the mass loss during the treatment. Thus, the knowledge of the kinetic schemes of wood thermodegradation is required.

The aim of this work is to build up a kinetic model which could be applied to the industrial wood heat treatment processes, and make recommendations concerning the operating temperature and treatment duration in order to obtain a required mass loss. Because of the quantity and the complexity of degradation and polymerization reactions occurring in the wood during the heat treatment (Candelier et al. 2011), the concept is a simplified approximation of real phenomena. Wood thermodegradation is described using a two-stage, semi-global kinetic model. Numerous experiences of heat treatment at 230°C under nitrogen were carried out for five wood species (Beech, Poplar, Ash, Pine and Fir). The instantaneous mass loss and temperature were recorded. Experimental data were used to determine model parameters. Finally, a comparison between experimental data and numerical results was proposed.

MATERIAL AND METHODS

Wood sample

Heat treatment is carried out on wood panels of large sections, dried in a steam room at 105°C until mass stabilization. Panel’s dimensions are 250 x 110 x 25 mm3 respectively in the longitudinal, tangential and radial directions. Two softwood species Pinus sylvestris (pine) and Abies pectinata (Silver fir) and three hardwood species Fagus sylvatica (beech), Populus nigra (poplar), Fraxinus excelsior (ash) were used in this study.

Heat treatment process

Wood species were thermally treated under nitrogen, by conduction between two metallic heating plates. The device was placed on a precision balance allowing recording of dynamic mass loss and instantaneous temperature. Experimental apparatus was more precisely detailed in a previous work (Chaouch et al. 2010). Instantaneous weight and temperatures (at the surface and in the heart of the sample) are recorded. The heat treatment consists of three successive thermal stability areas. The first one is kept at 105°C in order to verify the mass stability (the anhydrous mass). The second one (165°C) is useful for the thermal homogenization of the panel. The wood roasting was performed at 230°C.

Mathematical formulation

Wood thermodegradation is a complex set of degradation and polymerization reactions giving several reaction products (Nguila et al. 2009, Weiland et al. 1998). Knowledge of kinetic schemes describing the thermodegradation process is required for elaboration of predicting tools allowing controlling wood heat treatment. Mathematical modeling of occurring coupled heat and mass transfers are a simplified approximation of real phenomena. Variety of methods based on the thermal analysis of polymer thermodegradation has been developed. An overview of required measurement, kinetic analysis of solid state reaction leading the determination of the reaction order and parameterization of the reaction rate, method’s advantages and drawbacks has been proposed in the literature (Liu and Fan 1999, Brown et al. 2000, Vyazovkin et al. 2011). There could be cited the isothermal and the constant heating rate methods (Freeman and Carroll 1958, Coats and Redfern 1964), the multiple heating rate model (Kissinger 1957). Modeling of polymer thermodegradation is often based on global kinetics, since trying to represent full complexity of the process makes no sense. Global kinetics uses the key steps in the overall mass loss process. There is an ongoing debate concerning the determination of the kinetic parameters of the global reactions of wood pyrolysis (Prins et al. 2006, Di Blasi and Lanzetta 1997, Rousset et al. 2006, Grioui et al. 2006). Models available in the literature usually assume that kinetics of all the thermodegradation reactions obey an Arrhenius low defined by its kinetic constant and activation energy (Koufopanos et al. 1991, Prins et al. 2006, Di Blasi and Lanzetta 1997, Rousset et al. 2006, Grioui et al. 2006).

The following section is devoted to the mathematical formulation of coupled heat and mass transfers in wood porous media introducing heat effects due to the reactions of polymer decomposition, production of volatiles and non-degradable solid products. Simplifying assumptions are used for the mathematical formulation:

-  Wood sample has a rectangular form characterized by its length L, width l and thickness e, respectively in the longitudinal, tangential and radial directions. The sample thickness is very weak compared to the other dimension and modeling can be represented in 1D.
-  Convection transfer occurring because of the formation of volatiles inside the wood material during the heat treatment is neglected, following the recommendations of Vyazovkin et al. 2011.
-  Dimensions of the sample are considered invariable; formation of fissures and cracks is neglected.
-  Wood physical and thermal properties respectively the specific heat Cp and the wood thermal conductivity  l are considered as a linear function of temperature (Koufopanos et al. 1991).

The macroscopic conservation equation governing heat transfer phenomenon is given by:

 (1)
 (2)
 (3)

Where:  is the averaged anhydrous wood density, T is the local temperature; Hp is the global reaction enthalpy. The instantaneous sample weight m(t) is detailed by the Eq. (11). Sample’s volume V is considered invariable.

Mechanism adopted in this work is based on a two-stage, semi-global multi-reaction kinetic model of wood mild pyrolysis. Actually; it is not possible to identify the exact mechanism of simultaneous degradation of wood main polymers (hemicelluloses, lignin, and cellulose). Then, wood is assumed to be subdivided into three pseudo-components A1, A2, A3, as it was previously done by (Grioui et al. 2006). Each pseudo-component is characterized by a specific kinetic law of decomposition and a mass fraction: such as α1 +  α2 +  α3 = 1. are the masses of pseudo constituents A1, A2, A3 in the native wood (before treatment) and m0 is the mass of the native anhydrous wood sample. At a temperature lower than 200°C occurs the elimination of some volatile wood compounds. At the temperature range of 165 to 230 °C, the degradation of A1 prevails giving a gaseous product G1. The thermo-degradation of A2 leads to the production of a solid fraction C2 and a gas product G2. Proposed kinetic scheme is given by Eq. (4-5):

 (4) (5)

Mass fraction of non-degradable solid γ2 depends on the operating temperature. With the assumption that kinetic of all involved reactions is described by a first order law, mass balance equations can be written:

 (6) (7)
 (8)

Where, are the instantaneous mass of the constituents A1, A2 and C2; k1 and k2 are the reaction rate constants obeying an Arrhenius low such as:

 (9) (10)

Where, k01 and k02 are the pre-exponential factors; Ea1 and Ea2 are the activation energies of global thermo degradation reactions of pseudo constituents A1 and A2. Instantaneous total sample mass is equal to:

 (11)

The resolution of the Eq. (3-10) requires the determination of 8 parameters: pre-exponential factors k01, k02, and activation energies Ea1, Ea2, mass fraction fractions α1,  α2 and γ2; reaction enthalpy Hp of the global process. In this work, kinetics parameters have not been derived by classic thermal analysis (Brown et al. 2000, Vyazovkin et al. 2011) but numerically, by minimizing the error function between the experimental data and simulation results Eq. (12):

 (12)

where n1 is the number of data points of the temperature, Texp(i) is the experimental temperature, Tcalculated(i) is the calculated temperature on the i-th grid point, n2 is the number of data points of the mass loss MLexp(j) and MLcalculated(j) are respectively experimental and the calculated mass loss due to the thermo-degradation on the j-th grid point. The anhydrous mass lost is defined as follows:

 (13)

Experiences are carried out in a way, allowing the determination of model parameters over two stages. At the second isothermal area at 165 °C, the reaction giving by the Eq. (5) can be neglected because of its very slow rate. It has been also considered that reaction enthalpy of this reaction is negligible. Three parameters relative to the first reaction (Eq. 4) are then determined:α1, k01 and Ea1. The third isothermal area at 230°C, corresponding to the heat treatment process, allows the determination of the five other parameters: α2,γ2, k02, Ea2 and Hp. Governing equations are solved numerically with the commercial package Femlab 2.0. This is designed to simulate systems of coupled non-linear and time dependent partial differential equations (PDE) in one-, two- or three- dimensions. The mesh convergence was verified with refined mesh sizes.

Time step of 360 s, 1866 nodes and 8080 elements mesh size were considered to be appropriate.

The kinetic parameters obtained with this method are given in table 1

 Table 1.  The kinetic parameters used in the numerical simulation for the five species

RESULTS AND DISCUSSION

In order to verify the final weight loss predicted by the model, experiments with a reaction time of 20 hours were carried out (including a drying period at 105°C, a stabilization phase at 165°C and the heat treatment at 230°C). The comparison between experimental data and simulation results was performed for five species. Results are shown in figure (1-5). Careful consideration of the experimental weight curves reveals that they do not become completely horizontal; the weight continues to decrease very slowly. Hardwoods (Figure 3-5) were shown to be more sensitive to thermodegradation than softwoods (Figure 1-2). Softwoods degradation products appear at slightly higher temperature. These physical phenomena were reproduced by the simulation; a good agreement was obtained.

 (a) (b) (c) Figure 1. Fir -average mass loss (a), surface temperature (b), center temperature (c).

 (a) (b) (c) Figure 2. Pine - average mass loss (a), surface temperature (b), center temperature (c).

 (a) (b) (c) Figure 3. Poplar - average mass loss (a), surface temperature (b), center temperature (c).

 (a) (b) (c) Figure 4. Beech - average mass loss (a), surface temperature (b), center temperature (c)

 (a) (b) (c) Figure 5. Ash - average mass loss (a), surface temperature (b), center temperature (c)

Concerning the observation of temperature evolution recorded data, it is important to pay attention on the existence of an exothermic peak. This temperature rising is more pronounced in the sample heart, it could be probably due to the heat produced by the reaction of wood thermodegradation which activates the exothermic degradation reactions, obeying themselves to the Arrhenius low. The temperature control of the heating plates attenuates this effect on the sample surface. The coupled consideration of integrated in the model heat and mass transfers allows a good representation of this exothermic phenomenon.

CONCLUSIONS

The kinetics of wood pyrolysis in a temperature of 230°C can be described accurately by the proposed two-step reaction mechanism. The model incorporates the reaction pathway that was developed by Grioui et al. (2006) describing wood transformation during the mild pyrolysis. The model allows the computation of temperature profiles and average weight loss. Femlab 2.0 software was used to solve the system of partial differential equations. This paper includes a validation section that shows the ability of the model to accurately predict temperature profiles and mass loss in the experimental configuration. Comparison between simulated results and experimentally measured values showed a good agreement.

This model requires a preliminary determination of the reaction kinetics parameter for each softwood or hardwood species. However, the model can be used as an accurate tool in the design of industrial pyrolysis installations.

ACKNOWLEDGMENTS

LERMaB is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-12- LABXARBRE-01), the authors gratefully acknowledge this aid.

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Corresponding author: Mathieu.Petrissans@univ-lorraine.fr

Received: 04.03. 2013 Accepted: 21.06. 2013.

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