Автор: Joël Hamada
- MODELING AND NUMERICAL SIMULATION OF WOOD TORREFACTION
Torrefaction is a thermal treatment step in a relatively low temperature range of 210–240˚C, which aims to improve the dimensional stability and durability of wood. The weight loss kinetics for torrefaction of wood samples was studied using equipment specially conceived to measure mass losses during thermal treatment. Laboratory experiments were performed under nitrogen. A mathematical model for the kinetics of the thermodegradation process was used and validated. Measurements of temperature distribution and weight loss were performed sample of poplar wood during pyrolysis in an inert atmosphere at 230°C. The mathematical formulation describing the simultaneous heat and mass transfers requires coupled nonlinear partial differential equations. These unsteady-state mathematical model equations were solved numerically by the commercial package FEMLAB. A detailed discussion of the computational model and the solution algorithm is given. Once the validity of different assumptions of the model had been analyzed, the experimental results were compared with those calculated by the model. Acceptable agreement was achieved.
Ключови думи: Heat treatment, modelling, reaction kinetics; thermo-degradation, wood
- INFLUENCE OF THE NATURAL WOOD VARIABILITY ON THE HEAT TREATMENT PROCESS: EFFECTS OF THE WOOD INITIAL DENSITY ON THE FINAL QUALITY OF THE HEAT TREATED PRODUCT
This study aims at making a link between the investigations of the wood industry on the control of the quality in forest (production of wood adapted to the industrial requirements) and the studies on the wood heat treatment to improve the wood durability and dimensional stability. Heat treatment was carried out under nitrogen at 220 °C. A set of 14 samples (250x110x25 mm) from 7 European oak (Quercus petraea) trees of various French origins were scanned at dry state by X-ray tomography before and after heat treatment for analyzing the effects of treatment on the variation of wood density within and between the samples. The moisture uptake test also was performed and the equilibrium moisture content rates were calculated. The results showed to the naked eye the evident dark color of samples with differences between samples treated at 230 °C and 220 °C; no color differences within samples of heartwood, but differences between heartwood and sapwood. The density values of all samples decreased after the treatment. Mass loss (ML) ranged from 11,27 % to 13,28 % and was major when density was initially high. The treatment had no significant modification on the homogeneity of the density, but the near portion of sapwood was affected. The effects of wood density on the results of heat treatment were the same within samples of same tree but when comparing different trees the trend was highly variable. In conclusion, studying the effects of wood initial density variability on the heat treatment process needs further analysis such as chemical and biological analysis along with X-ray tomography and needs to be carried out on several species.
Ключови думи: characterisation, control, heat treatment, CT scanning, variability, wood density
- EUROPEAN OAK’S GROWTH RINGS PROPERTIES: DENSITY DISTRIBUTION AND THERMAL BEHAVIOR ANALYSIS OF EARLY – AND LATEWOOD
Last decades wood is promoted as building material. Unprotected wood exposed to outdoor conditions undergoes a variety of degradation induced essentially by fungi attacks. Heat treatment of wood by mild pyrolysis (180°C<T<240°C under inert atmosphere) is a preservation process with a weak environmental impact, and therefore, is viewed as an interesting alternative to the chemical impregnation methods. Nowadays, the main difficulties of industrial processes are to obtain final products with a constant quality (durability, dimensional stability, mechanical properties and colour). These difficulties may be due to the heat transfer or the inter-specific or intra-specific wood heterogeneity. The aim of this study is to better understand the effect of the intrinsic wood properties: density according to the position in wood, and especially early wood versus latewood, on the thermo-degradation process. Heat treatment using thermo-gravimetric analysis (TGA) was performed on small samples of the European oak (Quercus petraea Liebl.), where the earlywood and the latewood constituting the annual rings were studied separately. The relationship between the radial variation of wood density components assessed by microdensitometer and their thermo-degradation sensibility was investigated. The results show that globally, earlywood and latewood behaved differently under thermal conditions, for the two studied trees; earlywood tended to be more sensitive to thermo-degradation than latewood.
Ключови думи: heat treatment, intra rings, micro-tomography, oak, TGA