Fire-Relevant Thermal Degradation, Solid-Residue Retention and Apparent Kinetics of Norway Spruce (Picea abies) Under Inert and Oxidative Atmospheres

Wood exposed to fire undergoes both oxygen-independent thermal degradation and oxidative reactions, making the direct transfer of kinetic parameters obtained under inert conditions to fire applications potentially uncertain. This study investigated the effects of atmosphere and heating rate on the thermal degradation and apparent kinetics of Norway spruce (Picea abies (L.) H. Karst.). Simultaneous thermogravimetry and differential scanning calorimetry were performed under nitrogen and synthetic air at heating rates of 10, 25, and 50 °C min−1. Under nitrogen, the principal DTG peak shifted from 346.3 to 364.9 °C with increasing heating rate, while residual mass at 500 °C increased from 21.5 to 27.1%. Under synthetic air, the principal mass-loss event occurred at lower temperatures and was followed by a distinct high-temperature oxidative event accompanied by a strong exothermic DSC response residual mass at 500 °C was 4.6 ± 6.4% and 4.5 ± 3.5% at 10 and 25 °C min−1, respectively. Kissinger apparent activation energies were 263.6 kJ mol−1 under nitrogen and 189.5 kJ mol−1 under synthetic air. Iso-conversional OFW and KAS analyses showed a more consistent kinetic response under nitrogen, whereas synthetic-air data exhibited greater variability and lower regression linearity. The results demonstrate that atmosphere and heating rate substantially influence the thermal response of Norway spruce and provide material-scale experimental constraints for the development and subsequent calibration of solid-phase fire models incorporating both primary wood degradation and oxidative consumption of the resulting solid residue.