Limiting oxygen concentration and supply rate of smoldering propagation

Smoldering combustion of porous fuel is a slow, low-temperature, and flameless process that is sustained by heterogenous oxidations. The influence of oxygen supply on smoldering propagation is of vital significance but still not fully understood. This work explores the smoldering propagation on peat soil under an internal oxidizer flow with velocity up to 14.7 mm/s and oxygen concentration of 2 - 21%. After ignition in the middle of the fuel bed, by decreasing the internal airflow velocity, the smoldering propagation changes from bidirectional (forward + opposed) to unidirectional (opposed). Further reducing the airflow velocity to 0.3 mm/s, no propagation occurs with a minimum smoldering temperature of about 300 °C. As the internal flow velocity increases, the limiting oxygen concentration for smoldering decreases and approach to a minimum value that is below 2%. The minimum oxygen supply rate for smoldering approaches a constant of 0.08 ± 0.01 g/m²·s, when the oxygen concentration is larger than 10%. Further reducing the oxygen concentration below 10%, the required minimum rate of oxygen supply increases significantly, because of the enhanced convection cooling by a larger internal flow velocity. This work quantifies the minimum oxygen supply for the smoldering combustion and advances the fundamental understanding of the persistence of smoldering propagation in underground peat layers.