A set of pyrolysis experiments were conducted on fir in inert atmosphere and different ambient pressures, to investigate the char shrinkage and cracking during combustion. The results show that the grain effect influences the char shrinkage. The shrinkage parallel to grain is only 1/4 of the shrinkage perpendicular to grain. The strong restriction on shrinkage parallel to grain leads to the major cracking perpendicular to the grain direction, whereas the cracking parallel to grain is relatively weak with only shallow and random fissures produced. The shrinkage gradient at cracking time increases with increasing heat flux and decreasing ambient pressure while the surface density behaves oppositely. The number of fissures is strongly affected by these two parameters and an empirical correlation is developed for predicting the number of fissures. Increasing the ambient pressure will enhance shrinkage, leading to early cracking with relatively low shrinkage gradient. Under the current experimental conditions, the occurrence of cracking requires a shrinkage gradient greater than 35 m −1 and a surface density lower than 0.8 of the original density.
- Ambient pressure