In this paper, an intensive characterization of ash deposits collected from different positions of a pulverized-coal (PC) boiler has been conducted to diagnose the ash slagging and fouling issues within this boiler and to clarify the mass balance/flow of individual major elements and their role on ash slagging and fouling. A lab-scale drop-tube furnace has also been employed to elucidate the partitioning of the major metals during coal pyrolysis and char oxidation, to interpret the PC boiler results. The lignite tested is rich in Na and Ca, which are mostly present as organically bound cations and superfine mineral grains. In the air-fired boiler, the refractory minerals of silicates, aluminates, or aluminosilicates preferentially remained in fireside slag and bottom ash, forming low-temperature eutectics via the interaction with CaO and Fe2O3 on the receding char surface. The complex eutectic Ca-Al-Si consists of the liquidus matrix of the dense layer of fireside slag, in which Fe2+-bearing oxide was highly crystallized into a diamond-shape crystal on the water-tube surface. The ash fouling on Feston and superheater tubes was formed with a thinner Fe-rich layer that is followed by the deposition of Na2SO4 liquids. The abundance of Fe2O3 and CaO in the char matrix is crucial, which triggered the formation of around 80% liquids in the fireside slag with a viscosity of approximating 100 poise at 1200 degrees C. On the reheat tube surface, about 60% of the fully oxidized hematite was even reduced by the metallic iron into magnetite. Na2O and MgO in the char matrix preferentially escaped into flue gas as vaporized metallic vapor and fine oxide particles, respectively. The sulfation of Na-bearing vapor and CaO particle in flue gas was controlled by the partial pressure of Na2SO4 vapor and reaction rate, respectively.