Exploring waste heat recovery from applied smouldering systems

Applied smouldering systems are used to treat stockpiles of contaminated soils as well as organic liquid/sludge wastes intentionally mixed within a porous medium (e.g., sand). These systems treat virtually all contaminants/wastes, and leave behind a hot, clean, porous media. Large amounts of excess thermal energy are generated during smouldering and ultimately exhausted to the external environment when the hot bed is convectively cooled after treatment. This wasted heat can be harnessed to offset system energy requirements and enhance this system's sustainability, reduce environmental impact, and improve economic competitiveness. In this study, a novel three-dimensional numerical model was developed to simulate treatment bed cooling after smouldering and explore the waste heat recovery potential. Key system parameters and bed temperature data from a real-world commercial smouldering application were used to construct the model boundary and initial conditions. This model quantified the temporal availability and quality (i.e., exergy) of the exhaust waste heat for the first time. The model showed how the practical cooling period lasted 4.1 days, where 86 % of the energy stored in the bed was exhausted over this time and the energy release rate was governed by the bed's cooling velocity. Moreover, it was found that the cumulative output exergy during the cooling phase was 45 % of the cumulative electrical energy needed to drive equipment during the smouldering phase. Therefore, this waste heat could substantially offset the energy requirements in neighbouring systems. Overall, this study reveals the significant opportunity for waste heat recovery from applied smouldering systems.