Study of wildfire and biomass combustion physics using X-ray computed tomography
Keywords: Engineering.
Mechanical Engineering.

Biomass feedstock represents an attractive renewable energy source, either through direct solid combustion or in the form of derived biofuels. The combustion of biomass is also relevant to wildfires, a growing hazard raising environmental challenges. In addressing climate challenges, understanding the fundamental physical processes underlying biomass combustion is therefore increasingly relevant. In particular, accurate experimental measurements are critical to guide our understanding of fuel consumption and the design of efficient chemical conversion systems. However, because of the multiphase nature of biomass combustion, the release of smoke, and the requirement for optical access in traditional diagnostic techniques, acquiring detailed experimental measurements remains challenging. Therefore, it is necessary to establish new diagnostic techniques that can provide quantitative multiphase measurements at high spatio-temporal resolution, limited invasive impact, and without restrictions in optical access. Leveraging the unique capabilities of X-ray computed tomography (CT), this dissertation establishes that this technique can be applied in situ to study biomass combustion. The benefits of this approach are demonstrated using both laboratory and synchrotron CT systems. First, we examine the use of krypton as an inert radiodense tracer to enable 3D measurements of gas-phase temperature in reacting flows. For this purpose, premixed laminar flames are analyzed with a laboratory CT scanner, and the experimental measurements of density and temperature are compared to detailed numerical simulations. To demonstrate the merit of CT for optically inaccessible flames, measurements of complex flame geometries in a tubular confinement are performed. The method used to retrieve temperature from the X-ray attenuation is analyzed, with a particular emphasis on the invasive impact, experimental uncertainties, and statistical convergence of the measurements. Then, an experiment is devised on the same laboratory system to demonstrate how X-ray CT can provide multiphase measurements of biomass combustion. By instrumenting heated samples in flows diluted with krypton, the gas-phase temperature and solid-phase density are simultaneously measured during the in situ combustion of biomass. The X-ray density measurements are validated using a load cell, and the CT images are used to track the propagation of both the pyrolysis and char oxidation fronts at the sub-millimeter scale. Using a second improved setup, a broad campaign of experimental acquisitions is conducted to examine the impact of the type of biomass material, the oxygen concentration, and the fire suppression from chemical retardant. Specifically, the detailed CT measurements are used to identify different combustion regimes, and analyze the heterogeneous processes that occur within the solid fuel. These measurements are complemented by a low-order model to simultaneously evaluate the reaction rates of pyrolysis and char oxidation. The prospects of the method for quantitative measurements in bench-scale fires are investigated. Lastly, to increase the spatial resolution beyond the 300 µm limit of the laboratory system used, micro-scale CT measurements of biomass pyrolysis are acquired at the Advanced Light Source (ALS) synchrotron. The dynamic pyrolysis processes are resolved in situ at a spatial resolution below 10 µm for different materials, and a custom radiant heating system is used to provide heat flux of up to 6 MW/m² at the sample. These measurements provide unique insights on the pore-scale structural changes occurring during the primary pyrolysis near 350°C, and the subsequent char devolatilization at fuel temperatures of nearly 1000°C