Ignition of EPS foam by a hot moving hollow particle: Threshold, auto-ignition, and fire point

The ignition of building insulation materials by hot moving inert particles from fireworks display and welding processes is responsible for many tragic building fires. In this work, a hot hollow steel particle with various void ratio and diameter is dropped to ignite the low-density (16 kg/m³) expandable polystyrene (EPS) foam with three back boundary conditions. Results show that the minimum ignition temperature of hollow particles is close to solid particles (about 800 °C), and the temperature and size of particle are better measures of the spotting fire hazard than the particle mass and energy. As the void ratio increases, the minimum particle temperature for ignition first slightly decreases because the residence time of moving particle increases. For extremely hollow particles, ignition requires a much higher particle temperature to overcome the fast cooling. Besides the piloted ignition by hot particle, the auto-ignition phenomenon is observed for the first time, which is controlled by time scales of mixing and cooling. Moreover, the semi-open fuel back boundary shows the biggest fire hazard, because both good oxygen supply and long particle residence time promote the ignition to the fire point and fuel burnout. This study deepens the understanding of the complex interaction between hot porous particles and foam materials in the spotting ignition process of the building façade.