Abstract: Combustion of high-sodium Zhundong coal in circulating fluidized bed (CFB) leads to severe bed particle agglomeration and ash deposition on heat transfer surface. To reveal the effect of high-calcium additives on ash deposition and agglomeration, this work presents an experimental investigation focused on the combustion of Zhundong coal in a lab-scale circulating fluidized bed, using limestone as additives and quartz as bed material. Systematical analyses including SEM-EDS, XRD and ICP-OES are performed to characterize the morphology, mineralogy and Na/Ca contents of the bed materials, and ash deposit and FactSage is also used to calculate the chemical compositions of products in equilibrium state. Results show that the addition of limestone would accelerate the formation of refractory coating layer enriching in Ca, inhibit the formation of Na/Ca silicates of low melting-points, and mitigate bed particle agglomeration. Meanwhile, ash deposit in the outer layer is found to be enriched in CaSO₄, CaO and calcium silicates of high melting-point, with no tendency of sintering or melting. FactSage calculation confirms that the addition of limestone contributes to the formation of Ca-based compounds of high melting-points and minimizes the generation of Na-based compounds and liquid-phase slag, which is the main reason for adding limestone to mitigate ash deposition and bed particle agglomeration during Zhundong combustion in CFB. This work verifies the feasibility of limestone addition in mitigating bed particle agglomeration and ash deposition, which provides a theoretical guidance for large-scale and high-proportion combustion of Zhundong coal in CFB.