Ash deposition in pulverized coal-fired boilers can lead to operational issues and inhibit heat transfer. This study conducted combustion experiments using a laboratory-scale drop tube furnace (DTF) to investigate how adding biomass affects fly ash formation. During the experiments, two types of coal (CV and TOP coals) were mixed with black pine pellets (KP) and black pellets (BP) biomass samples at 5%, 15%, and 30% (based on anhydrous higher heating value). The study found that the particle size of fly ash increased as the biomass addition rate increased due to the reaction between coal mineral particles and biomass ash during combustion. An increase in aggregated-type particles was identified through shape classification of the produced ash using deep learning.