Charring and pyrolysis of coniferous wood: insights into heat transfer, gas micro-permeability and influencing factors

• Gas permeability of conifer was elucidated at high-temperature pyrolysis. • Gas permeability and porosity significantly increase with rising temperature. • Tracheid diameter increases then decreases, and parallel tracheids density rises. • Closed pits are main source of flow resistance; pit destruction reduces resistance. • Pit destruction and increased tracheid density are key to enhancing permeability. Coniferous wood, an important natural porous material, contains tracheids that are crucial for heat and mass transfer. Existing theories and simulations related to heat transfer have not adequately accounted for tracheid gas permeability, hindering the development of high-precision numerical simulations. This study measure the longitudinal gas permeability and porosity in pine, cypress, and Cunninghamia woods at various pyrolysis temperatures. Results show that the gas permeability and porosity increased markedly with temperature. At 350 °C, the gas permeability and porosity increase by 190.8 %∼487.8 % of the values at RT. Micro-computed tomography and scanning electron microscopy reveal that the tracheid diameter firstly increases and then decreases, and the tracheid wall thickness reduced to 77 % of RT value. The number of parallel tracheids per unit area increases to 87 % compared to RT, showing a strong positive correlation with the gas permeability. Closed pits on lap surfaces were identified as the primary source of longitudinal flow resistance, while theoretical calculations indicated that the pit destruction significantly reduced gas flow resistance, aligning with the increase of permeability. The increase in parallel tracheids and pit destruction on lap surfaces were key factors contributing to enhanced gas permeability, with respective contributions of 37 %∼51 % and 49 %-63 % to the overall increase in permeability. As charring progressed, the impact of lap surface pit destruction on gas flow became more pronounced.