Smoldering ignition and emission dynamics of wood under low irradiation

Wood is one of the longest-standing and sustainable construction and building materials, and has gained a new renaissance for high-rise buildings to achieve global carbon neutrality. However, wood can sustain both flaming and smoldering fires, and numerous timber structure fires have raised fire safety to be a public concern. This work investigates the smoldering ignition of wood blocks under long-lasting low-intensity irradiation and the robustness of smoldering fire after the removal of irradiation. We found a smoldering ignition map including three regimes, (i) no ignition, (ii) unsustained smoldering, and (iii) self-sustained smoldering. The minimum irradiation for smoldering ignition of wood is about 5.5 kW/m² after heating for hours. Without sufficient and in-depth preheating, smoldering ignition cannot self-sustain without irradiation. The criteria for self-sustained smoldering on thick wood include the minimum surface temperature of 350 ± 20°C, the minimum smoldering front thickness of 30 ± 5 mm, and the minimum mass flux of 3.8 ± 0.4 g/m² s before the irradiation is terminated. The CO/CO₂ ratio of the smoldering wood under low irradiation varies between 0.1 and 0.2. This work helps evaluate the fire risk of wood materials and understand their burning behaviors under real fire scenarios.