Doctor of Philosophy
Civil and Environmental Engineering
Environment and Sustainability
Gerhard, Jason I.
Applied smouldering combustion is emerging as a highly energy efficient approach towards waste-to-energy. This thesis aims to better understand this potential by addressing a critical knowledge gap between laboratory scale smouldering research and large-scale applications. Altogether, new understanding in provided on the complex and dynamic interactions between the reaction zone (smouldering front) and the cooling zone (treated inert porous media) by integrating theory, experiments, analytical modelling, and global energy balance calculations. The experiments spanned a range of reactor sizes and smouldering conditions (from weak to robust) with dry (carbon) and wet (sludge) fuels mixed in sand. This work revealed that system energy efficiency increased from 65% to 86% with reactor radius increasing from 8 cm to 30 cm. This system energy efficiency was shown to be highly sensitive to improved insulation and increased radius up to ~10 cm (i.e., laboratory-sized reactors), and exhibit medium sensitivity up to ~40 cm radius. Beyond 40 cm radius, the predicted system energy efficiency was invariably high (~85-95%). Non-uniform air flux across the reactor cross-section, driven by temperature gradients associated with radial heat losses, promoted faster smouldering propagation and cooling near the reactor wall and inhibited propagation and cooling near the reactor centre. In critical cases with very wet sludge, this phenomenon caused air channelling that lowered the mass loss rate, peak temperatures, and level of control over the process. Altogether, this work sheds new light on the complexities of applied smouldering and provides important guidance towards optimizing its use for environmental benefit.
Summary for Lay Audience
Combustion describes chemical reactions that release energy from fuels. For almost a million years, we have harnessed this energy. However, combustion is risky. Today, we are facing a new, unprecedented risk from fossil fuel combustion: climate change. In response, we are now searching for new, more sustainable ways to generate energy. One of these new ideas is “waste-to-energy”. Many “wastes” contain valuable energy that is not currently used, and we are now reimagining these “wastes” as “resources”. By harnessing the energy from these wastes, we can limit our reliance on fossil fuels, and therefore limit the impacts of climate change. These new resources are all around us, like the agricultural waste from our farms or sewage sludge in our cities. However, generating energy from these wastes is challenging. Smouldering combustion has recently been shown to be a new, highly energy efficient approach towards capturing energy from these wastes. But, there is a problem. Nearly all smouldering research has been completed on small experiments in laboratories, and the systems needed to fight climate change must be much larger. Therefore, we do not know how these larger systems will perform. This thesis explores this question by conducting small experiments in a laboratory and larger experiments in an industrial research facility. These experiments have led to surprising results that help us understand how even larger smouldering systems might perform. Altogether, this thesis provides engineers with new information, so they can better design the waste-to-energy systems needed to fight climate change.
Rashwan, Tarek L., "Sustainable Smouldering for Waste-to-Energy: Scale, Heat Losses, and Energy Efficiency" (2020). Electronic Thesis and Dissertation Repository. 7364.
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Available for download on Tuesday, October 31, 2023