This work focuses on the contribution of fuel particles with diameters smaller than 1 mm on the ash deposition behavior of different kinds of biomass feedstock. Special emphasis lies on their impact on industrial-scale boilers. For this purpose, we conduct lab-scale experiments and numerical simulations of biomass boilers in the megawatt range. First of all, we present experimental results from combustion and deposition experiments derived from a 100 kW(th) lab-scale fluidized bed furnace featuring a vertical deposition probe in the flue gas duct. To assess the slagging propensity of fine particles, different amounts of ground fuels were introduced into the combustion chamber. The experiments show a significantly higher amount of deposits (by a factor of 3.9-5.7) in the presence of fine particles in comparison to pelletized feedstock. Scanning electron microscopy with energy-dispersive X-ray spectroscopy and laser diffraction measurements were carried out, identifying the prevailing deposition mechanisms with respect to the respective feedstock. Subsequently, we draft a. fast and scalable computational-fluid-dynamics-based model for the combustion of these fuel particles and ash deposition in megawatt-range biomass boilers. The main focus lies on collision-induced deposition of molten ash particles carried away from the fuel bed, thus burning in the flue gas channel. This modeling approach is capable of identifying the principal sections suffering from ash deposits in the combustion chamber and can be used to evaluate potential mitigation and avoidance strategies.