Limits of sustaining a flame above smoldering woody biomass

Smoldering is slow, low-temperature, and flameless burning of porous fuel, and it is also an emerging method for energy conversion and waste removal. However, smoldering combustion is an incomplete combustion process, so the pollutions resulted from smoldering emissions are significant concerns. This work explores the flaming of emission gases from the smoldering wood chips (200 kg/m³) under different oxidizer flow velocities (4 mm/s-24 mm/s) and oxygen concentrations (14%-21%) through porous media. Once ignited on the top, the smoldering front first propagates downward (1^st stage, opposed) to the bottom and then propagates upward (2^nd stage, forward). We found that during the 1^st-stage downward smoldering propagation, a stable flame of smoldering emissions could be piloted and sustained. The critical smoldering burning rate for maintaining a stable flame remains constant at 10–12 g/m²∙s. To reach such a minimum smoldering burning rate, the required opposed flow velocity increases from 6 to 24 mm/s, as the oxygen concentration decreases from 21 to 14%. A simplified heat transfer process is proposed to reveal the limiting conditions for the co-existence of flaming and smoldering. This work enriches strategies for the clean treatment of smoldering emissions and promotes an energy-efficient and environment-friendly method for biowaste removal.