Лесотехнически Университет

Резюме

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.

Резюме

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.

Резюме

The heat treatment of wood by mild pyrolysis permit to produce a new material called torrefied or retified wood. Heat treated wood possesses new properties like improved decay resistance, higher dimensional stability, hydrophobic character, while strength properties are considerably reduced. The aim of this work was to study the anatomical structure evolution during heat treatment to explain the decreasing of the mechanical properties. The heat treatment has been carried out for a very large temperature range between 180 to 345°C under inert atmosphere on tow European wood species (pine and beech). The anatomical structure has been investigated by Scanning Electron Microscopy before and after heat treatment. On wood block for different treatment temperature mass, volume and shrinkage has been measured. From these results the apparent specific gravity was calculated and the porosity variation estimated. The results of the observations show that they are no evolution of the macroscopic anatomical structure, the vessels and tracheïds diameters remain constant. The volume evolution is probably caused by the diminution of the cellular wall thickness. By combining specific gravity evolution and the microscopic observation it's possible to conclude that the wood porosity increase on a scale lower than the micrometer (meso or nano porosity).

Резюме

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.

Резюме

Within the next ten years, the Douglas-fir will be the main softwood resource harvested in France, largely due to the growing market for wood frame construction in Europe. A large part of this supply will be devoted to EWP production, particularly plywood and LVL. To process these products, bolts must be peeled and therefore require a heat treatment of around 50 °C. Usually, bolts are soaked in hot water for12 to 72 hours, depending on the wood species and bolt diameter. However, Douglas-fir green wood shows two particularities that complicate boiling efficiency: (i) the heartwood has a MC near FSP (30 to 40 %) i.e. there is nearly no free water in the tracheid; and (ii) it is impossible to impregnate this heartwood at atmospheric pressure with water. As a result, wood material being a very efficient insulator, boiling Douglas-fir prior to peeling for veneer production will take a very long time, since free water is the main medium allowing heat transfer into green wood.

This paper includes a review of the anatomical, chemical and physical factors responsible for the very low impregnability of Douglas-fir heartwood. In this analysis, a first set of preliminary tests was performed in order to improve impregnation. It consisted of (i) soaking small samples (20 × 20 × 120 mm³ in RTL basis) in hot water at atmospheric pressure at different temperatures (every 10°C between 50 and 90 °C for different treatment durations (5 to 22 hours)); (ii) putting a tensoactive product into water; and (iii) applying ultrasonic waves (200 kHz / 400 W) in order to provoke micro-cavitation and then rupture the bordered pit torus. The results proved the inefficiency of such treatments on Douglas-fir impregnability, even on small samples. A second set of tests was conducted to quantify the influence of extractives and of drying (heating, vacuum) under the FSP on heartwood permeability. If extractives do not appear to have a real impact, then drying treatment will also not greatly improve permeability. One of the hypotheses is that the drying process may have caused micro cracking in the cell walls.

Other tests are in progress to quantify the speed of absorption of liquid in both longitudinal and transverse directions.