R. Younsi

Improvement of the heat treatment of Jack pine (Pinus banksiana) using ThermoWood technology

Thermal treatment in inert atmosphere is used to preserve wood without utilisation of toxic chemical agents. In addition, this process increases the dimensional stability of the wood matrix and results in attractive dark colour. On the other hand, it can deteriorate the mechanical strength and the flexibility of wood. For this reason, heat treatment parameters (such as maximum temperature, heating rate, the duration of the first plateau at constant temperature (100–120°C) and the second plateau at the maximum treatment temperature, humidity, final cool down rate) must be optimised in order to benefit from advantages of thermal treatment without deteriorating significantly the mechanical properties of wood. Correlation between wood properties and thermal treatment parameters depends not only on the wood species but also the environment (climate, soil) where the given species grow. This paper presents a study on thermal treatment of Canadian Jack pine (Pinus banksiana) using a medium size prototype furnace. The aim of this study was to optimize the set of parameters used during industrial treatments. The possibility of shortening the process time without causing any deterioration in wood quality was also investigated. The results show that increasing the maximum heat-treatment temperature increased the dimensional stability of Jack pine and darkened its colour. This parameter did not affect the modulus of elasticity but it decreased the modulus of rupture of Jack pine. A slight reduction in gas humidity during the initial warming up period permitted to shorten the drying period and at the same time increased the mechanical strength. This improvement helps save energy and increase productivity.ZusammenfassungDie Wärmebehandlung von Holz unter Schutzgasatmosphäre wird zur Verbesserung seiner Dauerhaftigkeit ohne Verwendung giftiger chemischer Mittel angewandt. Zusätzlich wird mit diesem Verfahren die Dimensionsstabilität der Holzmatrix verbessert und eine attraktive Dunkelverfärbung erzielt. Andererseits kann sich dieses Verfahren jedoch negativ auf die mechanische Festigkeit und die Steifigkeit des Holzes auswirken. Deshalb müssen die Wärmebehandlungsparameter (wie zum Beispiel Maximaltemperatur, Aufheizrate, Dauer der ersten Temperaturstufe bei 100–120°C und der zweiten Stufe bei der Maximaltemperatur, Feuchte, Abkühlrate) optimiert werden, um die Vorteile einer Wärmebehandlung nutzen zu können ohne dabei die mechanischen Eigenschaften von Holz signifikant zu verschlechtern. Der Einfluss der Wärmebehandlungsparameter auf die Holzeigenschaften hängt nicht nur von der Holzart ab, sondern auch von den Wuchsbedingungen (Klima, Boden). In dieser Studie wird eine Wärmebehandlung von kanadischem Jack Pine Holz (Pinus banksiana) in einer mittelgroßen Prototypanlage vorgestellt. Ziel dieser Untersuchung war es, die Parameter bei der industriellen Behandlung zu optimieren. Zusätzlich wurde die Möglichkeit untersucht, die Verfahrensdauer zu verkürzen ohne die Holzqualität zu verschlechtern. Die Ergebnisse zeigen, dass eine Erhöhung der maximalen Behandlungstemperatur zu einer Verbesserung der Dimensionsstabilität von Jack Pine Holz sowie zu einer dunkleren Verfärbung führte. Dies hatte keinen Einfluss auf den Elastizitätsmodul, reduzierte jedoch die Biegefestigkeit von Jack Pine Holz. Mit einer geringfügigen Reduktion der Gasfeuchte während der Aufheizphase konnte die Trocknungszeit verkürzt und gleichzeitig die mechanische Festigkeit verbessert werden. Diese Verbesserung hilft, Energie zu sparen und die Produktivität zu steigern.

A Diffusion-based Model for Transient High Temperature Treatment of Wood:

A three-dimensional (3D) mathematical model describing simultaneous unsteady heat and moisture transfer between a gas phase and a solid phase during high temperature treatment of wood has been developed. The model is based on a diffusion equation with variable diffusion coefficients. The governing equations representing the heating process in a 3D rectangular object are discretized using an explicit finite-difference approach, and a computer code is developed to predict the temperature and moisture distributions inside the wood sample. The sample was subjected to high temperature treatment under different operating conditions. The model predictions are compared with experimental results obtained for temperature and average moisture content during high temperature treatment of birch wood. Satisfactory agreement is obtained over a range of heating conditions. A parametric study was also carried out to determine the effects of several parameters such as initial moisture content, heat and mass transfer coefficients, and the sample thickness on the temperature and moisture content distributions within the samples during heat treatment.

Modeling of heat and mass transfer during high temperature treatment of aspen

A three-dimensional and unsteady-state mathematical model, which accounts for simultaneous heat and mass transfer taking place during the high temperature treatment of wood, has been developed. It was validated by comparing the predictions with the experimental data. In the model, the coupled heat and mass transfer equations proposed by Luikov are solved, and the temperature and moisture content profiles within wood are predicted as a function of time for different heating rates. For the model validation, an experimental study was carried out with aspen under different operating conditions. The samples were heated to high temperatures using a thermogravimetric system. The weight loss and the temperature distribution within the sample were monitored and registered during the experiment. The model can use constant or variable thermo-physical properties. The temperature and moisture content of the wood predicted by the model using variable properties were compared with those predicted by the same model using constant properties as well as with the experimental data. The experimental and model results are in good agreement, and it was shown that the accuracy of the model depends on the accuracy of the properties. After the model validation was completed, a parametric study was carried out.

Experimental and Numerical Investigation of Heat and Mass Transfer during High-Temperature Thermal Treatment of Wood

In this article, a three-dimensional mathematical model has been used to analyze the transient heat and moisture transfer during high thermal treatment of Aspen wood. The conservation equations for the wood sample are obtained using diffusion equation with variable diffusion coefficients and the three-dimensional incompressible Reynolds averaged Navier-Stokes equations have been solved for the flow field. Temperature distributions in wood and in gas as well as the moisture content of wood were measured during the experiments. Afterwards, hardness, modulus of elasticity, and modulus of rupture were measured. The experimental results and model predictions showed good agreement. Increasing heating rate is beneficial for the modulus of rupture and the modulus of elasticity heat treatment seems to increase for the range of parameters considered here. However, hardness of Aspen increases in the axial direction but does not change in the radial and tangential directions.

Numerical Simulation of Vacuum Drying by Luikov's Equations

A two-dimensional mathematical model was developed to simulate coupled heat and mass transfer in apple under vacuum drying. Luikovs equations are the governing equations in analyzing heat and mass diffusion problems for capillary-porous bodies. The model considers temperature- and moisture-dependent material properties. The aim of this study is to analyze the influence of some of the most important operating variables, in particular, pressure and temperature of drying air, on the drying of apple. The resulting system of unsteady-state partial differential equations has been solved by a commercial finite element method (FEM) package called FEMLAB (COMSOL AB, Stockholm, Sweden). Simulations, carried out in different drying conditions, showed that temperature is more effective than air pressure in determining the drying rate. A parametric study was also carried out to determine the effects of heat and mass transfer coefficients on temperature and moisture content distributions inside apple during vacuum drying. A comparison between the theoretical predictions and a set of experimental results reported in the literature showed very good agreement, especially during the first 4,200 s, when experimental data and theoretical predictions overlapped and relative errors never exceeded 2%.

A high-temperature thermal treatment of wood using a multiscale computational model: application to wood poles.

The present study is devoted to a numerical study with experimental validation of the high-temperature thermal treatment of three-dimensional wood pole. During the heat treatment process, the heat and mass transfer takes place between the solid and the drying medium, and the moisture evaporation occurs within the solid due to the capillarity action and diffusion. The development of the model equations, taking into account both bulk phases and interfaces of the multiphase system is described, starting from the microscopic scale. Fundamental to this model is the ability to quantify the effects of key material and geometric properties of the pole. The three-dimensional and unsteady-state mathematical model equations are solved numerically by the commercial package FEMLAB for the temperature and moisture content histories under different treatment conditions. A detailed discussion of the computational model and the solution algorithm is given. Heat treatment was applied on the test samples in an oven for three final temperatures (180, 200 and 220 degrees C). A series of experimental tests aimed at determination of heat treatment schedules kinetics curves and the temperature and moisture profiles and there time evolution were carried out. A very good agreement between the experimental and predicted results was obtained, implying that the proposed numerical algorithm can be used as a useful tool in designing high-temperature wood pole treatment processes.

Numerical and experimental validation of the transient heat and mass transfer during heat treatment of pine wood

Abstract In the current work, the three-dimensional Navier-Stokes equations along with the energy and concentration equations for the fluid coupled with the energy and mass conservation equations for the solid (wood) are solved to study the transient heat and mass transfer during the heat treatment of wood. The model for wood is based on Luikovs approach and solves a set of coupled heat and mass transfer equations. The model equations are solved numerically for the temperature and moisture content histories under different treatment conditions. The simulation of the proposed conjugate problem allows the assessment of the effect of the heat and mass transfer within wood on the transfer in the adjacent gas, providing good insight on the complexity of the transfer mechanisms. To generate data for comparison, measurements of temperature and moisture content of wood samples in a thermogravimetric system were conducted under different operating conditions. It is shown that the predicted and measured values compared very favourably, implying that the proposed numerical algorithm can be used as a useful tool in designing high-temperature wood treatment processes.

Mathematical Modeling of the High Temperature Treatment of Birch in a Prototype Furnace

In recent years, various wood modification technologies have been commercialized as alternatives to the traditional chemical treatments for wood preservation. The high temperature heat treatment of wood is one of these commercially viable and environmentally friendly alternative wood modification technologies. During this treatment, wood is heated to temperatures above 200°C by contacting it with hot gas. The chemical structure of wood changes leading to increased dimensional stability and resistance to microorganisms. Wood darkens making it aesthetically more attractive. However, it loses some of its elasticity. Therefore, the high temperature heat treatment has to be optimized for each species and each technology. The mathematical modeling is an important tool for optimization. It can also be used as a powerful tool for furnace modification and design. A reliable and predictive model was developed to simulate numerically the heat treatment process. Heat treatment experiments were carried out in the prototype furnace of the University of Quebec at Chicoutimi. The model was validated by comparing the predictions with the experimental data. In this paper, the results of the model applied to birch heat treatment are presented. The model predictions are in good agreement with the data.

Thermal treatment of electrical poles.

Thermal treatment is used to preserve the wood without any addition of any toxic chemicals. This process increases the dimensional stability and darkens the color of the wood. The improvement of the resistance to decay of wood by thermal treatment is also often suggested in the literature. However, some latest works contested if the durability of heat-treated wood is improved when it is used in contact with ground. The objective of this study was to investigate the possibility of thermally treating electrical poles which are larger compared to the standard wood lumber. One of the applications for thermally treated wood poles could be their use in environmental sensitive areas (along rivers, for example) as a replacement for untreated western red cedar (WRC) poles which are more expensive. Green and pre-dried red pine (Pinus resinosa) and jack pine (Pinus banksiana) poles, both with circular and square cross-sections, were heated to high temperatures under humid and inert atmosphere. Operating parameters such as maximum treatment temperature, maintenance time at this temperature, heating rate and gas humidity were varied in order to find most suitable treatment conditions for the poles. The tests showed that most of the cracks are formed during the drying process while thermal treatment only widened already existing cracks. The circular shape seems to promote crack formation during the drying period since the directional dependence of shrinking creates more stresses in circular poles compared to the square poles. A slight decrease in flexibility of the wood with increasing temperatures was observed. The protecting effect of gas humidity against oxidation of wood and the importance of the application of a moderate heating rate for poles with large cross-sections are also demonstrated in this article. The impact of the heat treatment on the resistance to decay of electrical wood pole will be presented in a future publication.