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 Fe 2 O 3 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 Fe 2+ -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 Na 2 SO 4 liquids. The abundance of Fe 2 O 3 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 °C. On the reheat tube surface, about 60% of the fully oxidized hematite was even reduced by the metallic iron into magnetite. Na 2 O 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 Na 2 SO 4 vapor and reaction rate, respectively.

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This study aims to clarify the abundance of individual elements, particularly those in trace concentrations in lignites, and their emission dynamics during pyrolysis and char oxidation in both air and oxyfuel combustion modes. For this laboratory-scale study, the emission dynamics was represented by element release from the coal/char particle during thermal treatment in a drop-tube furnace. The main coal sample studied is a Victorian brown coal (VBC), which was compared with a Chinese lignite. Irrespective of elemental type, the VBC is rich in organically bound elements, which partly dissociated during the initial flash pyrolysis step. This dissociation extent varied broadly with elemental type. For element release during char oxidation, As release rates in both N 2 and CO 2 bulk gases were slower than char surface consumption rate, because of internal diffusion limitations and scavenging of a portion of As by Ca/Al/Fe-bearing discrete minerals. In contrast, the release rates of Pb from the char surface were faster than the carbon consumption rate. Releases of the remaining elements were simply in linear proportion to the char consumption rate for the two lignites studied, despite their differences in properties with no observable (element release) difference between air and oxyfuel combustion mode.