Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Jason I. Gerhard (Passed away)

2nd Supervisor

Christopher Power

Co-Supervisor

3rd Supervisor

Marco A.B. Zanoni

Co-Supervisor

Abstract

Smouldering is a flameless form of combustion that is driven by oxygen directly diffusing into the surface of condensed phase fuel (liquid or solid) and involves a complex interplay between heat and mass transfer through porous media, chemical reactions, and multiphase flow. Applied smouldering systems have been gaining popularity for a variety of applications including (i) energy and resource recovery, (ii) waste-to-energy, (iii) pyrolysis and gasification, and (iv) applied smouldering of organic liquids/solids for environmental benefits. In all contexts, smouldering is a complicated oxygen-limited phenomenon where multi-dimensional transfer effects (i.e., non-uniformities in temperature, airflow, and chemical reactions) control system operation limits and performance. Therefore, a robust numerical model can be a highly efficient tool to better understand applied smouldering characteristics and these multi-dimensional transfer effects.

For the first objective, a 2D smouldering model was developed, calibrated, and validated for a quantitative and qualitative investigation of multi-dimensional transfer effects. Numerical results confirmed that radial heat losses extracted approximately 52 and 70% of the energy added into systems for granular activated (i.e., fuel) concentrations of 0.02 and 0.03 kgGAC kg sand-1, respectively. The modelled results closely matched experimental observations and the model elucidated the non-uniformities that could not be measured from experiments, e.g., in air flow. For the second objective, the 2D model was used to explore the effects of oxygen mass flux on smouldering robustness and clarify the role of quasi super-adiabatic effects in applied smouldering systems. By diluting the oxygen mass fraction from 0.230 to 0.115 and 0.057, the smouldering condition changed from robust oxygen-rich to robust fuel-rich and weak fuel-rich, respectively. While the peak temperature remained almost uniform at oxygen fractions of 0.230 and 0.115 (i.e., near 930 °C) due to quasi super-adiabatic effects, it decreased to 700 °C at the lowest oxygen fraction of 0.057. The third objective investigated the effects of fuel mobility on applied smouldering systems using a validated 1D model. Sensitivity analyses confirmed that smouldering robustness increased with decreasing Darcy air flux and increasing fuel viscosity, as higher fuel saturation was deposited in the reaction zone. The numerical results compared well with experimental observations from the literature. Overall, this thesis explored these three important aspects of applied smouldering systems to provide a deeper understanding that enables scientists and engineers to design and optimize future smouldering reactors for a wide range of applications.

Summary for Lay Audience

Flaming combustion involves a system of rapid chemical reactions in the presence of oxygen that results in the generation of heat and light in the form of flames. Smouldering combustion, on the other hand, is non-flaming combustion that occurs within a porous medium. A classic example is a glowing, hot charcoal briquette. Traditionally, smouldering has been associated with fire safety concerns due to its hidden nature, release of toxic gases, re-ignition risks, and resilience to quenching. However, in recent years smouldering has been used in various environmental engineering applications, such as waste-to-energy, due to its high energy and cost efficiencies. For these applications, organic contaminants are combined with a porous medium (e.g., sand) and placed in a system where applied smouldering effectively can be used to burn the contaminants to leave behind a clean porous medium and ash. Smouldering systems are complex, where the presence of energy losses to the environment adds a key layer of complexity, leading to non-uniform temperature, air mass flux, and chemical reactions distribution across the system. Moreover, oxygen mass flux plays a crucial role, as these systems typically operate in oxygen-limited conditions. Last, because smouldering is well-suited to treating hazardous liquid wastes (like crude oil sludge), liquid fuel movement must also be understood alongside smouldering dynamics. Altogether, these compounding effects lead to a highly complex system, and numerical modelling is therefore an appropriate tool to untangle these complexities. In this thesis, a smouldering numerical model was developed to better understand applied smouldering under varying conditions. The findings from this thesis will provide engineers and researchers with valuable insights into applied smouldering systems that can be used to enhance the design and optimization of these systems for a wide range of environmental engineering applications.

Available for download on Monday, April 01, 2024

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