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.

阴燃（smoldering）指多孔燃料的慢速、低温无焰燃烧过程，同时也是一种新兴的能量转化与废弃物处置方法。然而阴燃属于不完全燃烧过程，其排放产生的污染物一直是备受关注的核心问题。本研究针对体积密度为200 kg/m³的阴燃木屑，在不同氧化剂流速（4 mm/s~24 mm/s）与氧浓度（14%~21%）条件下，通过多孔介质探究其排放气体的引燃行为。当在试样顶部引燃后，阴燃锋面首先向下传播（第一阶段，逆流型）至底部，随后向上传播（第二阶段，顺流型）。研究发现，在第一阶段向下阴燃传播过程中，阴燃排放气体可被引燃并形成稳定火焰。维持稳定火焰所需的临界阴燃燃烧速率恒定为10~12 g/m²·s。当氧浓度从21%降至14%时，达到该最低阴燃燃烧速率所需的逆流流速从6 mm/s提升至24 mm/s。本研究提出了简化的传热过程模型，以揭示有焰燃烧与阴燃共存的极限条件。本研究丰富了阴燃排放物的清洁处置策略，同时为生物质废弃物的高效节能、环境友好型处置提供了技术支撑。