Smouldering fuel processing, emission flammability, and carbon footprint

Abstract

Smouldering is an emerging method for biowaste removal, which has demonstrated many attractive advantages. However, as smouldering is an in-completed combustion, it tends to release many toxic emissions, like CO, CH4, and volatile organic compounds (VOCs), limiting its further promotion and application. Therefore, this thesis proposes and thoroughly investigates a novel combustion method for biowaste removal that uses a self-sustained flame co-existing with smouldering to clean the toxic smouldering emissions. Firstly, the critical conditions for the co-existence of smouldering and the flame of wood waste are explored. The results show that the critical smouldering flux of wood waste for maintaining a stable flame remains constant at 10-12 g/m2∙s. Then, the effects of fuel types and smouldering directions (forward or opposed) on the critical conditions are investigated. An equivalent critical mass flux of flammable gases required for igniting the smouldering emissions is found to be 0.5 g/m2∙s, regardless of the fuel type. Additionally, it is easier for the flame to remove more emissions from opposed smouldering which is recommended for the proposed biowaste removal process. The efficiency of the pollution mitigation of the applied flame is demonstrated by a significantly lower ∆CO/∆CO2 ratio after purification. Finally, four smouldering-based biowaste processing strategies: (a) full smouldering, (b) partial smouldering, (c) full smouldering with a flame, and (d) partial smouldering with a flame, are proposed and evaluated. Results show that full smouldering achieves the highest removal efficiency but generates significant greenhouse and toxic gases, while partial smouldering effectively generates stable biochar, sequestering over 30 % carbon. This work enriches strategies for the clean treatment of smouldering emissions and promotes an energy-efficient and environmentally friendly method for biowaste removal.