Clean smoldering biowaste process: Effect of burning direction on smoke purification by self-sustained flame

Smoldering is an attractive biowaste removal technology, but its toxic pollutions are significant concerns. This work explores smoldering biowaste removal with the emissions purified by a self-sustained flame and the effect of smoldering direction. The smoldering direction and intensity are controlled by the ignition position and upward internal airflow velocity (5–24 mm/s). We found that the flame can be sustained above the smoldering fronts to purify the smoldering emissions in different burning directions, and the flame duration and burnt mass fraction are much larger for the opposed smoldering. To sustain a flame above an opposed smoldering front, the minimum airflow velocity is 6 mm/s, and the minimum smoldering burning flux is 10 ± 1 g/m²∙s. Comparatively, sustaining a flame above a forward smoldering requires a larger airflow velocity (~11 mm/s) and smoldering intensity (>16 g/m²∙s). Therefore, the flame is easier to remove more emissions from opposed smoldering which is recommended for the waste removal process. The critical smoldering burning flux at flame extinction is constant at 9 ± 1 g/m²∙s, regardless of the smoldering direction and airflow velocity. This work deepens the understanding of the co-existence of smoldering and flaming and enriches strategies for environmentally friendly smoldering waste removal technologies.