Effect of time-dependent layer formation on the oxygen transport capacity of ilmenite during combustion of ash-rich woody biomass

•Ilmenite interacts with inorganic ash, forming an ash layer on the particle surface.•At initial stage, the bed particle layer is composed of an Fe-rich outer and a Ca-rich inner layer.•The Fe-rich outer layer is found primarily in the convex-shaped regions and is eroded by collisions in the bed.•The oxygen transport capacity of ilmenite is reduced as a result of the migration and erosion of iron from the bed particle.•The catalytic activity of the calcium-enriched outer layer of ilmenite leads to an observed increase in the reaction rate.