Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Doctor of Philosophy


Civil and Environmental Engineering


Gerhard, Jason I.

2nd Supervisor

Zanoni, Marco A.B.


3rd Supervisor

Rashwan, Tarek L.


The Open University



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.

Summary for Lay Audience

Due to the ever-increasing population and rapid industrialization, millions of tons of municipal waste are sent to landfill disposal every year. It is reported that the US will face a landfill shortage in just 18 years, making the efficient disposal of solid waste an urgent issue. Such wastes often contain significant water, challenging the traditional thermal disposal methods (e.g., incineration). However, recognizing the untapped energy within these wastes shifts our perspective from viewing them as mere wastes to valuable resources. This waste-to-energy concept leverages the concealed energy, decreasing our reliance on fossil fuels and thus mitigating the impact on climate change. Smoldering combustion has been identified as a promising and energy-efficient method to extract energy from these challenging wastes, even though the high moisture content presents a limitation on the combustion process. Research on smoldering combustion for wet fuels has primarily been experimental and based on oversimplified numerical models, leading to a gap in the fundamental understanding of how smoldering interacts with water dynamics, including its movement, evaporation, and condensation. Consequently, the limits of smoldering combustion for treating these wet wastes are unclear. This thesis investigates these questions through experiments, as well as numerical and analytical models, providing practical insights into the criteria for smoldering treatment across different moisture levels, and offering guidance for optimizing smoldering systems for waste management. This research equips engineers with the knowledge to better design waste-to-energy systems that contribute to combating climate change.

Available for download on Saturday, May 31, 2025