Abstract
We investigated the amount of dimension change and the shape variation in densified wood after carbonization. The variables in the study included specimen length, specimen type (Japanese cedar (Cryptomeria japonica D.Don) heartwood or sapwood), compression ratio, temperature and time of pre-heat treatment, and the shape of the exhaust slit on the specimen for generating pyrolysis gas under carbonization. Heartwood specimens with a high compression ratio and/or a long length showed intensified degree of shape variation after carbonization, such as splitting and differences in the coefficient of shrinkage in radial direction between the edge and center positions. These results occurred because the heartwood and highly-densified wood had poor gas permeability into the lumen due to the deposition of heartwood extracts on the pit and airflow narrowing by buckling of the cell wall. When the pyrolysis gas was not discharged outside the wood under carbonization, the high-pressure gas into the lumen affected the degree of shape variation and the finished density of the charcoal. The shape stability of the charcoal can be controlled by using pre-heat treatment and slit processing, because the difference in the coefficient of shrinkage in radial direction between the edge and center positions after carbonization decreased with heavier pre-heat treatment and shorter pitch distance of the exhaust slit.
