This work explores the production of biocarbon from forest biomass through pyrolysis in fluidized bed reactors, emphasizing the relationship between the operating conditions, ash behavior, and physicochemical properties of the resulting solid biocarbon. Fluidized-bed reactors offer distinct advantages for biocarbon production, including efficient thermal transfer, isothermal operation, and scalability. These characteristics make them particularly suitable for integration into existing energy infrastructure. A key strategy investigated in this work is the use of a weakly oxidizing atmosphere composed of recycled flue gases from combustion processes as the fluidization medium. This approach enables heat integration with fluidized bed boilers and reduces the need for external inert gases, thereby lowering the operational costs and improving the overall energy efficiency and circularity of the system. The impact of this atmosphere on biocarbon yield and composition was studied in detail, particularly in terms of its influence on ash-forming element behavior. Special attention is given to the transformation and retention of inorganic elements, such as potassium and phosphorus, which affect the suitability of biocarbon for industrial applications. The experimental and modeling results show that fluidized bed conditions favor the selective removal or redistribution of these elements. Analytical techniques including Inductively Coupled Plasma (ICP), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and thermodynamic equilibrium calculations (TECs) were used to assess the mechanisms of ash transformation. In parallel, the evolution of particle properties such as size, density, porosity, and surface area was evaluated under different conversion regimes. Structural degradation owing to attrition and fragmentation was found to play a significant role in carbon retention and fine generation. The elutriated fines, enriched in inorganic content, were also characterized and presented opportunities for valorization in applications, such as soil amendment. Overall, these findings support the development of integrated and sustainable fluidized bed systems for biocarbon production, offering practical pathways to reduce fossil carbon use and improve the resource efficiency of biomass valorization processes.