Evaluation of heat treated wood swelling by differential scanning calorimetry in relation with chemical composition

24 pagesInternational audienceRetification® is a heat treatment which decreases the swelling of wood and increases its resistance to fungal attack. In this study, differential scanning calorimetry (DSC) was applied in order to determine the fiber saturation point (FSP) of natural and retified® wood. FSP values were used to determine the total swelling of natural and heat-treated wood. The DSC method was compared to the volumetric shrinkage approach. The influence of the heat treatment temperature and duration on the swelling of wood was investigated. Relationships between chemical changes and the reduction of swelling were analysed thoroughly. The equivalence of the DSC method and the volumetric shrinkage method is shown. FSP in association with anhydrous density is a good indicator for the evaluation of the overall swelling of heat-treated wood. Reduction of wood swelling with increasing temperature and duration of thermal treatment is often attributed to hemicelluloses destruction. This study shows that the reduction of beech wood swelling can not only be attributed to the disappearing of adsorption sites that goes with the hemicelluloses destruction. It is suggested that other phenomena such as structural modifications and chemical changes of lignin also play an important part

Evaluation of heat treated beech by non destructive testing

10 pagesImprovement of dimensional stability and durability is wished for the use of wood as a building material. For the last decade, retification® has been industrially developed. It consists in a stabilization and preservation of wood by heat treatment. The aim of this study is to find simple and fast methods to characterize heat treated beech. Non destructive testing is expected to be relevant to evaluate the level of treatment and the properties for the use of heat treated wood. Six treatments were carried out in a pilot reactor. The parameters of the retification® stage (temperature and time) were studied. For each treatment, the non destructive tests (free oscillations in the fundamental mode, colour and dry weight loss)were performed, and the properties for use (mechanical resistance and volumetric shrinkage) measured. Lightness and dry weight loss seem to be suitable properties to characterize beech retification® when the time parameter is fixed. However, they are not suitable for other wood species, and for retification stages with a variable duration. Moreover, the correlation with the properties for use were plotted, but presented too large dispersion to be relevant. After correction of moisture content, the longitudinal Young's modulus of the material is slightly increased by each of the six treatments, but do not present any variation with changing parameters values. On the contrary, the mechanical resistance decreased with increasing temperature and time. Thus the dynamic Young's modulus is not reliable to evaluate the treatment and to predict the loss of mechanical resistance. The logarithmic decrement was not increased by any of the treatments, which is in opposition with the hypothesis that retification® generates cracks and microcracks in the material. Effects of long time at low temperature have been investigated. From these experiments, properties of treated wood may be improved significantly by choosing appropriate values of the parameters

Thermo-gravimetric analysis as a tool for the optimisation of wood heat treatment parameters

9 pagesRetification is a heat treatment that decreases the swelling of wood and increases its resistance to fungal attack. It consists in a mild pyrolysis of wood (180°C-260°C) that takes place in a non oxidative atmosphere (nitrogen). The industrial development of retification requires optimisation of the treatment temperature and duration. In order to enhance the homogeneity of temperature in the furnace, and to avoid exothermic reaction, low temperatures seem to be preferable to high temperature. On the contrary, duration and temperature of treatment have to be high enough to provide good biological resistance and stabilization to the wood. However, high temperatures lead to a loss of mechanical strength. A question arises from these previous observations: is there any equivalence between a treatment of short time carried out at high temperature and a treatment of longer time at lower temperature? Answering this question can help to optimise rétification temperature and duration. The purpose of this study is to evaluate the relevance of a “time temperature equivalence” (TTE) for wood pyrolysis in the temperature range of retification. The principle of TTE is adapted from the study of wood viscoelastic properties. In this study, it is applied to the rate of anhydrous weight loss during wood pyrolysis. Thermo-gravimetric analysis (TGA) were performed on maritime pine (Pinus pinaster Ait.-) and beech (Fagus sylvatica) wood powder. Isothermal degradations were carried out at different temperatures ranging from 160°C to 260°C. A specific data analyse was carried out on the TGA derivative (DTG) in order to assess the relevance of the TTE in the temperature range of retification. It gave interpretable results for maritime pine, but not for beech. It showed that for maritime pine wood the TTE is confirmed from 200°C to 220°C, and not confirmed for temperatures superior to 230°C. An optimization of the temperature and time of treatment is thus possible

Energy requirement for fine grinding of torrefied wood

International audienceThe purpose of this study is to investigate the influence of torrefaction on wood grinding energy. Wood chips were torrefied at different temperatures and durations. The energy required to obtain fine powder was measured. Particle size analyses were carried out on each powder sample. It is showed that torrefaction decreases both grinding energy and particle size distribution. A criterion to compare grindability of natural and torrefied wood is proposed. It takes into account both grinding energy and particle size distribution. It accounts the energy required for grinding particles to sizes inferior to 200 μm, for given grinding conditions. Torrefaction is characterised by the anhydrous weight loss (AWL) of wood. For AWL inferior to around 8%, grinding energy decreases fast. Over 8%, grinding energy decreases at a slow rate. Particle size distribution decreases linearly as the AWL increases. Both for spruce and beech, the grinding criterion is decreased of 93% when the AWL is around 28%

Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln

International audienceBeech and spruce chips were torrefied in a batch rotating pilot kiln. For each torrefaction the temperature curve of the moving chips bed was recorded. The anhydrous weight loss (AWL) of each torrefaction was measured. Effect of torrefaction temperature and duration on the AWL was studied. In order to optimise short time torrefaction, models that can estimate the AWL from the chips temperature curve are required. Three phenomenological models were successfully applied. They all gave good correlations between experimental and calculated AWL. These three models can be employed to optimise industrial torrefaction. However, the more complex they are, the more difficult it is to understand their physical meaning. It is thus preferable to use simple model for the industrial control of torrefaction

Effect of torrefaction on grinding energy requirement for thin wood particle production

National audienceThe second generation biofuels exploits the lignocellulosic materials. The main advantage is to not compete with food chain. In the case of thermochemical means (gasification in an entrained flow reactor followed by Fischer-Tropsch synthesis), a grinding step is necessary to inject particles into the burner. The targeted particle size is about 200µm to reach a total conversion and to improve gas quality. Due to the plastic behaviour of the biomass, this step is strongly energy-consuming. Biomass torrefaction (thermal treatment lower than 300°C) is a way to decrease the grinding energy and to standardize materials (composition and moisture). Contrary to natural wood, torrefied wood has a brittle behaviour and a less mechanical strength. The aim of this study is to investigate the interest of torrefaction on wood grinding energy diminution. The torrefactions were carried out on beech and spruce, in an airtight rotating batch kiln under nitrogen. The effect of torrefaction temperature (160-300°C) and duration (5-60min), on weight loss, grinding energy and powder particles size were examined. The grinding energy was calculated by integration of the electric power of the grinder, which was measured by the means of a wattmeter. A grindability criterion, which took into account both grinding energy (E) and the volume fraction (X) of particles lower than the targeted size (200µm), was defined. Results showed a strong interest of torrefaction on the decrease in energy required for fine wood particle grinding. The grindability criterion could be reduced by 93% for treatments beyond 260°C. However, the global energy balance becomes less favourable. It is necessary to reach a compromise between the consumed energy by torrefaction and the decrease in grinding energy. According to the wood species, an optimum could be established around 10% of weight loss and around 85% of the grindability criterion diminution

Optimisation des paramètres durée et température d'un traitement thermique du bois. Modifications des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique.

Heat treatment of retification is aimed to increase wood stability and durability. When the treatment duration is as short as possible, high temperatures are necessary to obtain significant improvement of these two properties. This involves two main drawback : firstly, exothermic reactions may happen and can make the monitoring of the treatment difficult ; secondly, a loss of mechanical strength occurs. By another way, heterogeneity of retified wood is large, since it includes heterogeneity of natural wood and heterogeneity due to the heat treatment process. It is thus necessary to find some tools to check quickly retified wood end-use properties.The first part of this study deals with the optimisation of parameters of treatment duration and temperature. On the one hand, it is shown that it is possible to improve the heat treatment monitoring by increasing duration and decreasing temperature. On the other hand, the possibility to improve the end-use properties of retified wood is tested and discussed on massive samples.The second part of this study deals with the means to evaluate quickly the end-use properties of retified wood. This work contributes to find the connection between modifications of physico-chemical and ultrastructural and modifications of wood end-use properties (stability and mechanical resistance).Le traitement thermique de rétification est destiné à stabiliser le bois et à augmenter sa durabilité. Lorsque la durée de traitement est la plus courte possible, des températures élevées sont nécessaire pour obtenir des améliorations significative de ces deux propriétés. Ceci induit deux inconvénients : d'une part le risque d'apparition de réactions exothermiques rendant la conduite du traitement difficile, et d'autre part, la diminution de résistance mécanique. Par ailleurs l'hétérogénéité due au procédé de traitement thermique s'ajoute à l'hétérogénéité du bois naturel. Il est donc nécessaire de mettre au point des moyens de contrôle rapides des propriétés du bois rétifié.La première partie de cette étude concerne l'optimisation des paramètres durée et température de traitement. Dans un premier temps une approche procédé montre qu'il est possible d'améliorer la conduite du traitement en diminuant la température et en augmentant la durée. Les possibilité d'amélioration des propriétés du bois rétifié sont ensuite vérifiées et discutées à l'échelle du matériau massif.Dans la deuxième partie, nous nous sommes attaché à trouver des moyens de contrôle rapides des propriétés d'usage du bois rétifié. Nous avons contribué à mettre en relation les modifications physico-chimiques et ultrastructurales avec les modifications des propriétés d'usage du bois (stabilité et résistance mécanique)

Optimisation des paramètres durée et température d'un traitement thermique du bois. (modifications des propriétés d'usage du bois en relation avec les modifications physico-chimiques et ultrastructurales occasionnées par le traitement thermique)

Le traitement thermique de rétification est destiné à stabiliser le bois et à augmenter sa durabilité. Lorsque la durée de traitement est la plus courte possible, des températures élevées sont nécessaire pour obtenir des améliorations significative de ces deux propriétés. Ceci induit deux inconvénients : d'une part le risque d'apparition de réactions exothermiques rendant la conduite du traitement difficile, et d'autre part, la diminution de résistance mécanique. Par ailleurs l'hétérogénéité due au procédé de traitement thermique s'ajoute à l'hétérogénéité du bois naturel. Il est donc nécessaire de mettre au point des moyens de contrôle rapides des propriétés du bois rétifié.La première partie de cette étude concerne l'optimisation des paramètres durée et température de traitement. Dans un premier temps une approche procédé montre qu'il est possible d'améliorer la conduite du traitement en diminuant la température et en augmentant la durée. Les possibilité d'amélioration des propriétés du bois rétifié sont ensuite vérifiées et discutées à l'échelle du matériau massif.Dans la deuxième partie, nous nous sommes attaché à trouver des moyens de contrôle rapides des propriétés d'usage du bois rétifié. Nous avons contribué à mettre en relation les modifications physico-chimiques et ultrastructurales avec les modifications des propriétés d'usage du bois (stabilité et résistance mécanique).Heat treatment of retification is aimed to increase wood stability and durability. When the treatment duration is as short as possible, high temperatures are necessary to obtain significant improvement of these two properties. This involves two main drawback : firstly, exothermic reactions may happen and can make the monitoring of the treatment difficult ; secondly, a loss of mechanical strength occurs. By another way, heterogeneity of retified wood is large, since it includes heterogeneity of natural wood and heterogeneity due to the heat treatment process. It is thus necessary to find some tools to check quickly retified wood end-use properties.The first part of this study deals with the optimisation of parameters of treatment duration and temperature. On the one hand, it is shown that it is possible to improve the heat treatment monitoring by increasing duration and decreasing temperature. On the other hand, the possibility to improve the end-use properties of retified wood is tested and discussed on massive samples.The second part of this study deals with the means to evaluate quickly the end-use properties of retified wood. This work contributes to find the connection between modifications of physico-chemical and ultrastructural and modifications of wood end-use properties (stability and mechanical resistance).ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF