Water Dynamics inside Applied Smoldering Processes: Water Phase Change, Water Mobility, and Local Thermal Non-equilibrium
Abstract
Smoldering has proven to be an effective approach towards waste-to-energy. Key to the successful smoldering application is the stability of the reaction front. Excess water in high moisture content wastes is a major energy sink that can lead to reaction quenching and process failure. Understanding water dynamics is essential for designing efficient smoldering systems, yet current approaches rely on trial-and-error methods without a deep investigation into how water dynamics affect smoldering. This thesis addressed the gap by combining wet smoldering experiments and numerical models to explore the interaction between water and smoldering dynamics. The first step was to develop and calibrate a smoldering-water phase change numerical model against experiments that accurately addressed the global energy balance within wet smoldering systems. This defined the crucial structural characteristics of the wet smoldering front, where the pre-heating zone protected the smoldering from being extinguished by water and required a minimum thickness to avoid extinction. Building on the initial model, the study further examined the effects of water mobility on smoldering, demonstrating that water movement could hinder ignition by accumulating near the bottom heater, while simultaneously facilitating smoldering propagation by displacing recondensed water ahead of the smoldering front. These effects were greatly influenced by operational conditions (e.g., reactor size), which engineers can modify to effectively control and optimize smoldering systems. Moreover, a novel analytical analysis also determined the criteria for smoldering ignition of wet materials, offering a valuable framework to better understand the key factors governing complex interactions between water dynamics and smoldering reactions. This new insight can support researchers and engineers in better managing smoldering ignition and improving the economic viability of smoldering treatments. Through the integration of all numerical models and experimental observations, this study systematically uncovered the key mechanisms involved in wet smoldering. A novel and practical methodology was developed to approximate the local thermal non-equilibrium behaviors among water, sand, and gas phases. This new methodology allows for a more detailed explanation of multiphase transport during wet smoldering, thereby enabling improved forecasting without the need for extensive model calibration. Overall, this research illuminates the complexities of wet smoldering and provides valuable insights for optimizing its application for environmental benefits.