Study of Multi-dimensional Transfer Effects in Applied Smouldering Systems

Seyed Ziaedin Miry Mr, Western University

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 kg_GAC 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.