Ignition threshold and emission characteristics of self-sustaining smoldering combustion

Smoldering, as a flameless combustion of porous fuels, is slow, low-temperature, and persistent, so its ignition criteria are fundamentally different from igniting a flame. This work designs a 1D smoldering reactor to investigate the minimum smoldering ignition requirements of biomass fuel (150 kg/m³) and applies a porous plate ignitor to control the heating intensity and duration. For initiating smoldering towards self-sustaining, we found the minimum ignition heat flux approaching 0.5 kW/m² under a long heating duration and the minimum ignition energy (MIE) of 0.06 MJ/m² under short and intensive heating. At a low ignition intensity, a large Darcy airflow through the fuel bed will cool the heating zone and delay the ignition. With a strong ignition source, the cooling effect of Darcy airflow becomes negligible, so the increasing airflow enhances the oxygen supply and accelerates smoldering ignition. Reducing fuel moisture content or improving oxygen supply can further lower the required MIE. During smoldering ignition, CO₂ exhibits a much earlier increase than CO and CH₄, so it could be an optimal indicator for the early detection of smoldering fires. This work helps understand the governing mechanisms of smoldering ignition and is of practical significance in mitigating fire hazards in urban and wildland.