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

The non-isothermal thermal degradation of Norway spruce wood (Picea abies (L.) H. Karst.) was investigated by simultaneous thermogravimetry and differential scanning calorimetry under nitrogen and synthetic air atmospheres at heating rates of 10, 25 and 50°C min⁻¹. The aim was to evaluate the effects of atmosphere and heating rate on degradation behavior and apparent kinetic parameters using model-free methods. Under nitrogen, the process proceeded through four degradation regions: moisture release, hemicellulose decomposition, a dominant cellulose-related mass-loss stage and high-temperature char consolidation. The temperature of the main DTG peak increased from 619.5 to 638.0 K with increasing heating rate, while the residual mass at 500°C increased from 21.5 to 27.1%, indicating enhanced char retention under non-equilibrium heating conditions. In synthetic air, two main mass-loss stages were observed, corresponding to oxidative pyrolysis and subsequent char combustion. The main degradation peak occurred at lower temperatures than under nitrogen, whereas the second stage was accompanied by a pronounced exothermic DSC effect above 700 K. Residual mass under air was below 5% at 10 and 25°C min⁻¹, confirming near-complete oxidation of the char residue. The Kissinger method yielded apparent activation energies of 262.1 kJ mol-1 under nitrogen and 190.1 kJ mol-1 under synthetic air. Iso-conversional analysis by the Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose methods showed a nearly constant activation energy profile under nitrogen, with mean values of 341 and 348 kJ mol-1, respectively. Lower and more variable values were obtained under synthetic air because of overlapping pyrolytic and oxidative reactions. The results provide a consistent thermal and kinetic characterization of spruce wood degradation under inert and oxidative conditions and demonstrate the importance of atmosphere and heating rate in the interpretation of wood thermal decomposition.