Abstract : Pyrolysis carried out at relatively low temperatures in the range 200-280C produces a material with good dimensional stability and durability, with minimal effect on the mechanical properties because of its resilience. In spite of the large amount of research work injected into this topic, it is still a difficult task to identify the precise loss of product quality that has been incurred as a result of this process. This fact provides the motivation for a fundamental study that explains the mechanisms of thermal treatment. The possibilities offered by numerical simulation allow us to propose an innovative contribution concerning the problem of homogeneity associated with the treatment of a piece of wood. The first objective of this study is to adapt an existing computational model for simulating coupled heat and mass transfer in a porous medium to include the chemical reactions that arise during pyrolysis. Another objective of the study is to devise experimental strategies that enable the wood internal temperatures and pressures to be measured for different board thicknesses during thermal treatment under an inert atmosphere. Paralleling this work, near infrared spectrometry (NIRS) has been used to characterize large wood samples submitted to different thermal treatments.
The results show that the pyrolysis model, when coupled to the comprehensive heat and mass transfer model, is able to account for many of the observable phenomena that evolve during the experiments, particularly the effect of exothermic reactions on the internal overpressure within the board. The method of spectral analysis is an adaptable technique that quickly reveals what type of thermal treatment a particular sample of wood was submitted to. It also enables the thermal history to be retraced from its thickness. NIRS seems to be a promising technique that we believe permits the profiles of degradation to be used to validate the computational model. Furthermore, it offers an interesting perspective to control the quality of wood treated at high temperature through coupling the physical and mechanical properties of a new material with its chemical constitution.