Sustainable Smouldering for Waste-to-Energy: Scale, Heat Losses, and Energy Efficiency
Abstract
Applied smouldering combustion is emerging as a highly energy efficient approach towards waste-to-energy. This thesis aims to better understand this potential by addressing a critical knowledge gap between laboratory scale smouldering research and large-scale applications. Altogether, new understanding in provided on the complex and dynamic interactions between the reaction zone (smouldering front) and the cooling zone (treated inert porous media) by integrating theory, experiments, analytical modelling, and global energy balance calculations. The experiments spanned a range of reactor sizes and smouldering conditions (from weak to robust) with dry (carbon) and wet (sludge) fuels mixed in sand. This work revealed that system energy efficiency increased from 65% to 86% with reactor radius increasing from 8 cm to 30 cm. This system energy efficiency was shown to be highly sensitive to improved insulation and increased radius up to ~10 cm (i.e., laboratory-sized reactors), and exhibit medium sensitivity up to ~40 cm radius. Beyond 40 cm radius, the predicted system energy efficiency was invariably high (~85-95%). Non-uniform air flux across the reactor cross-section, driven by temperature gradients associated with radial heat losses, promoted faster smouldering propagation and cooling near the reactor wall and inhibited propagation and cooling near the reactor centre. In critical cases with very wet sludge, this phenomenon caused air channelling that lowered the mass loss rate, peak temperatures, and level of control over the process. Altogether, this work sheds new light on the complexities of applied smouldering and provides important guidance towards optimizing its use for environmental benefit.