Formation of ash deposit on heat transfer surfaces during Zhundong lignite (ZD) combustion in circulating fluidized bed (CFB) boiler has affected boiler operation and its thermal efficiency. This work therefore charac-terised the morphology, mineralogy and thermal conductivity of the ash deposits in a 220 MW CFB boiler firing ZD for an improved understanding of deposit characteristics. By using XRF, SEM-EDS, XRD, and laser con-ductometer, results revealed that deposits on superheaters and reheaters were as thick as 3.0-4.5 cm. Moreover, the inner layer of the deposits consisted of fine ash particles enriching in Na, Ca and S in the forms of Na2SO4, CaSO4, and Na2Ca(SO4)2, whereas the outer layer comprising CaSO4, Na6Ca2Al6Si6O24(SO4)2, Fe2O3, SiO2 and Mg2SiO4 with more SiO2 and Al2O3 presented. Besides, thermal conductivity of the inner layer was ca. 1.83 W/ (m center dot K), and that of the middle layer with an increased pore size was ca. 1.50 W/(m center dot K), both of them were independent of temperature at 550-900 degrees C and overweighed the out layer being 0.65-0.82 W/(m center dot K). This would lead to an increase in coal consumption of ca. 12.9%, highlighting the severity of deposit build-up in adversely impacting the heat transfers and deteriorating the fuel economics during ZD CFB combustion.