This study investigates bed material aging in industrial-scale fast pyrolysis of biomass, focusing on the roles of particle morphology, ash accumulation, and alkali release in bio-oil yield and composition. Fresh and used bed materials from a commercial pyrolysis reactor were characterized by particle size analysis and SEM/EDS, revealing fragmentation and the development of AAEM-rich ash layers. Alkali release and migration were assessed using a TGA–Surface Ionization Detector (SID) setup, while lab-scale fluidized-bed pyrolysis experiments tested fresh, aged, and different size fractions of the aged bed. GC/MS analysis showed consistent suppression of oxygenated functions in all used bed samples. A bio-oil yield reduction of 8.3% was observed for the fine fraction ('140 μm), compared with only 1% for the coarse fraction ('220 μm), indicating the combined influence of ash accumulation and particle morphology. These findings underscore the catalytic role of AAEM species and offer guidance for ash-management strategies in fast pyrolysis processes.