Smoldering ignition of wet combustible materials

Applied smoldering has been demonstrated as an efficient waste-to-energy approach for low heating value/high moisture content combustible waste materials. Therefore, smoldering can be used to extract energy from wastes that are not amenable to traditional thermochemical routes (e.g., using flaming-based incinerators). Nevertheless, understanding the process of smoldering-driven drying and its relationship to ignition and quenching within these smoldering systems is critical to determine the viability and economic feasibility of this approach. These interlinked phenomena are not well-understood. To address this knowledge gap, this study developed new analytical methods with a previous validated numerical model to establish a comprehensive framework to better understand ignition and the associated drying process. These new models accurately resolve the coupling between downward water migration, water phase change, and smoldering propagation in space and time, revealing how drying defines ignition. The relationship between residual water saturation (Sw,r) and drying time to enable ignition (tdry,h) was determined analytically to unveil the fundamental relationships between these variables. Sw,r represents a critical limiting water saturation for smoldering ignition, which was found to be solely dependent on material properties rather than operational conditions (e.g., initial water saturation or packing height). In contrast, tdry,h, is the critical drying time that enables ignition, which was shown to be significantly influenced by system heat losses and operational parameters. Conditions such as a slender reactor design, insufficient thermal insulation, and low heater power can substantially extend the required drying period - and lead to ignition failure at critical conditions. Furthermore, a four-zone ignition region was established and used to characterize the requirements for smoldering ignition. Overall, this study untangles interlinked phenomena and supports researchers and engineers in better understanding drying and its influence on ignition within applied smoldering systems.