In the present work, the ash chemistry and deposition behavior during straw dust-firing were studied by performing experiments in an entrained flow reactor. The effect of using spent bleaching earth (SBE) as an additive in straw combustion was also investigated by comparing with kaolinite. During dust-firing of straw, the large (>2.5 μm) fly ash particles generated were primarily molten or partially molten spherical particles rich in K, Si, and Ca, supplemented by Si-rich flake-shaped particles. The smaller fly ash particles (<2.5 μm) were predominantly formed from the nucleation, condensation, and coagulation of the vaporized K, Cl, S, and P species. Approximately 70% of K in the fly ash from straw combustion was water-soluble, and KCl was estimated to contribute to more than 40% of the water-soluble K. With the addition of SBE to straw dust-firing, the Cl retention in ash was reduced, the SO2 emission was increased, and the formation of water-soluble alkali species was decreased. Compared to kaolinite, the inhibiting effect of SBE on alkali chloride formation was slightly less pronounced at a similar K/(Al + Si) molar ratio of the fuel mixture. The addition of SBE to straw dust-firing significantly decreased the Cl content of the deposits collected on a probe, both due to a dilution effect and chemical reactions. Compared to pure straw combustion, the deposition rate was slightly increased during the SBE addition, in spite of the considerably decreased deposition propensity. The results from the present work suggest that SBE could be a promising additive to be used in straw dust-firing.