Advanced Optical Diagnostics in Particle Combustion for Biomass and Metal as Alternative Fuels

Sammanfattning: The energy sector is in urgent need for carbon-free strategies considering environmental pollutions and climate change impact. Two kinds of solid fuels, biomass and metal particles have been studied in this thesis aiming at CO2 emission reduction or zero carbon emission. Biomass is a renewable and carbon-neutral energy source that provides heat and power through gasification and combustion. Biomass fuels usually contain varying amounts of potassium, which will cause severe operational problems, such as slagging and corrosion by the potassium released during thermal conversion processes. The detailed potassium chemistry in biomass thermochemical conversion processes has been investigated in our previous study and presented in Weng’s doctoral dissertation. Based on these quantitative studies, the work presented in this thesis is mainly focused on the quantitative measurement of burning a single biomass pellet and the potassium released from burning pulverized biomass char particles. The motivation for the study of biomass char particles is that over 80% of the potassium can remain in the char particles from the raw biomass. Laser-induced photofragmentation fluorescence (LIPF) imaging was adopted here to measure the potassium release process for biomass char particles, which provides spatially resolved information of the dominant species of KCl and KOH that formed during the char oxidation period. A hot laminar gas flow was used for calibration with gas-phase KOH and KCl provided by a homemade multi-jet burner, in which a homogenous temperatures distribution ranging from 1000 to 2000 K are provided. Newly developed UV-absorption spectroscopy was adopted to monitor the concentrations of KOH and KCl. Based on the calibration, the KOH/KCl distribution surrounding the burning char particles was derived, revealing the potassium release process during the char oxidation period.Metal fuels have been used as sustainable energy carriers due to their zero-carbon emission and high energy density. The iron powder has been proposed as one of the most promising recyclable metal fuels for the future low-carbon society. A comprehensive understanding of the combustion behavior of iron particles is essential for investigating fundamental mechanisms and designing efficient iron powder combustors. A versatile metal powder seeding apparatus has been designed and optimized based on electrostatic dispersion. This dispersion system is well calibrated for powder concentration that can provide a stable flow of particles seeding for nearly one minute. The work presented in this thesis mainly focuses on investigating single iron particle combustion in a well-controlled laminar premixed flame with a modified Mckenna flat-flame burner through advanced optical diagnostic techniques. One of the challenges for the study of metal combustion came from the small size (~20 to 80 µm) of the metal particles and their movement in the hot combustion environment. This work, inspired by clustering algorithms, proposed a new clustering-based particle detection (CBPD) method for digital holography (DH) for particle detection. This data-driven method features automatic recognition of particles, particle edges and background, and accurate separation of overlapping particles. Based on CBPD method, high-speed digital in-line holography (DIH), a three-dimensional (3D) imaging technique, is employed to reconstruct the 3D particle field and simultaneously quantify the size, 3D location and velocity of burning iron particles in a well-controlled CH4/N2/O2 premixed Bunsen flame with a stable metal power seeded.The ignition delay time, combustion time of single micron-sized iron particles are studied by high-speed imaging in different flame conditions. Particle temperatures are measured by an ICCD camera equipped with a stereoscopy, and the results are derived through the two-color pyrometry method. An important phenomenon of nano-sized iron-oxides particles releasing during the iron particles combustion has been identified. Micro-explosion of burning iron particles was observed during the combustion process, which is complex and can affect combustion stability and the formation of product components. The morphology of raw iron particles and the combust products (iron oxides) collected by sampling meshes have been analyzed by scanning electron microscopy (SEM).