Decomposition and carbonisation of wood biopolymers - A microstructural study of softwood pyrolysis

Pyrolysis of softwood (spruce and pine) is investigated in the temperature range between 298 K and 1673 K within narrow intervals. A heating rate of 2 K/min and the use of thin wood sections allow one to fully preserve the original cellular wood structure without the formation of cracks. Thermal degradation of the wood biopolymers and evolution of the atomic/molecular and mesoscopic structure of the carbonaceous material is studied by wide-angle X-ray scattering, small-angle X-ray scattering and Raman spectroscopy. An isotropic and almost structureless material marks a clear transition region between 580 K and 620 K, where the crystal structure of the cellulose microfibrils is completely degraded and the scattering contrast based on the density difference between cellulose microfibrils and polyoses/lignin has fully disappeared. After the complete decomposition of the wood nanocomposite structure, the charring process commences with the formation of aromatic structures, and a strongly increasing scattering signal at small angles indicates the formation and growth of nanopores. The developing graphene sheets show a slight preferred orientation with respect to the oriented cellular structure of the material. It is concluded that the unique microfibril orientation of cellulose in the native wood cell wall might affect the carbonaceous material, in agreement with recent predictions in the literature.