The influence of porous media heterogeneity on smouldering remediation

Self-sustained Treatment for Active Remediation (STAR) is a thermal remediation technology that uses smouldering, a flameless form of combustion, for destroying organic contaminants in soil. Injected cold air flowing through the soil to the treatment zone supports the release of sufficient energy to maintain a self-sustained reaction and the propagation of the reaction through the contaminated zone as long as the airflow local to the reaction exceeds a minimum value. However, the distribution and magnitude of air flux vectors can be complex in the heterogeneous environment common at contaminated sites. This research presents the first investigation of smouldering remediation under varying degrees and patterns of permeability heterogeneity. Nine experiments examined smouldering remediation in contaminated layers of varying permeability arranged alone and in contrasting layers in series, in parallel, and in two distinct complex patterns. The results suggest that smouldering can successfully propagate across layer boundaries and through layers in series regardless of their permeability (at least down to 1 x 10⁻¹² m²). However, fine layers were not smouldered for layers in parallel with a permeability ratio ≥ 3:1. Numerical modelling of these cases with a published smouldering model revealed that this occurred due to insufficient airflow in the fine layers in some cases, or conductive heat transfer (thermal coupling) between parallel layers in other cases. The more complex heterogeneity patterns underscored the importance of the connected length of the higher permeability pathway on airflow distribution and therefore on smouldering propagation. Disconnected coarse zones supported smouldering in both coarse and fine zones while connected coarse zones kept smouldering in the coarse pathway while bypassing fine zones. Overall, this research provides unique insights into understanding heterogeneous scenarios to ensure the successful application of smouldering remediation.