Understanding the characteristics and formation process of bed particle layers resulting from interactions between ash-forming matter and bed material during fast pyrolysis is crucial for optimizing fast pyrolysis bio-oil (FPBO) production. However, research on this topic remains limited. In this study, the evolution of the bed particle layers formed on quartz bed particles during fast pyrolysis of wood was investigated across bench-, pilot-, and industrial-scale units. Bed material samples with different exposure times were characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS), and focused ion beam-SEM (FIB-SEM) to assess the morphology, elemental composition, and thickness of the bed particle layers. Overall, the time-dependent formation and characteristics of the quartz bed particle layers were similar to those reported for the fluidized-bed combustion of woody biomass. The key difference, however, was that the layers formed during fast pyrolysis were significantly thinner and contained less Ca. The bed particle layer formation began early in the process through the deposition of Ca-rich ash particles, primarily on convex surfaces, likely followed by solid–solid diffusion of Ca²⁺ into the quartz core, forming a Ca-silicate-rich inner layer. The inner layer developed later and more sparsely in concave regions, resulting in thinner layers. After approximately 1 day, an outer layer developed on the convex surfaces due to continued ash particle deposition, while a K–Si-rich inner-inner layer (likely composed of K-rich silicates and associated with gaseous alkali diffusion) formed primarily in concave regions. Over time, the bed particle layer thickness approached a limiting value of approximately 4 μm, likely due to reduced growth of the inner layer, which may be attributed to diminished inward transfer of Ca²⁺ as the diffusion distance increased.