The work reported in this dissertation explored characterizing biomass fly ashes produced by pulverized fuel boilers (the most commonly used thermal conversion technology). The most suitable biomass fuel types based on the literature review were used for the work. The impact of changes in operational combustion conditions, e.g., additive use and introduction of low-NOx burners combined with over-fire air, on the formation of fly ash and its quality were studied. Those changes were compared to biomass combustion in conventional air conditions. The FACTSage 7.3 software was used to evaluate its potential in predicting mineral phases in ashes when input parameters are varied. The use of FACTSage was found to be an instrumental approach, especially for biomass ashes, or the method could be used for fuel blends. The fly ashes were characterized by chemical analyses of major and minor elements and leaching behavior (environmental aspect). The particle size distributions of the fly ashes were evaluated. X-ray powder diffraction and micro-Raman spectroscopy were used to qualitatively evaluate the mineral phases. Scanning electron microscope and energy dispersive X-ray were used to observe the morphological characteristics of the fly ash particles. Performance tests were conducted to characterize the behavior of the fly ashes and fly ash qualities were tested for a constituent of a render base coat. One fly ash quality was proven viable for use in external renders.