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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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The photocatalytic decolorisation of CI Reactive Black 5 using titanium dioxide nanopowder as a catalyst was studied and the results obtained are discussed in terms of its decolorisation efficiency. All experiments were performed using a double‐walled quartz immersion well batch reactor in which the slurry form of the reactants was at its natural pH of 5.1. The performance of titanium dioxide nanopowder (size <25 nm; surface area 200–220 m2/g) was compared with that of reference titanium dioxide powder (size ca. 230 nm; surface area 11 m2/g); in both cases, the titanium dioxide samples were anatase. It was found that the photocatalytic decolorisation efficiencies obtained using titanium dioxide nanopowder were higher than those of the reference titanium dioxide powder, with the latter taking approximately 8 min longer to achieve almost complete decolorisation of 10 mg/l CI Reactive Black 5. The photocatalytic decolorisation rate of CI Reactive Black 5 using both titanium dioxide photocatalysts typically followed a first‐order reaction and the decolorisation kinetics were successfully fitted to a simplified Langmuir–Hinshelwood kinetic model. In addition, the effects of light type and intensity, catalyst loading and initial CI Reactive Black 5 concentration were investigated using titanium dioxide nanopowder as the photocatalyst in the decolorisation of the dye. This study shows that the recommended parameters for treating 10 mg/l CI Reactive Black 5 based on the experimental set‐up and operating conditions are an ultraviolet light power of 125 W (39.3 mW/cm2) and a 0.3‐g/l catalyst loading.

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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.

We use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr(3+)2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600-1200 °C in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr(6+) formation is 800 °C for the coexistence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 °C with a remarkable reduction in the fraction of Cr(6+). Beyond 1000 °C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr(6+) in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 °C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr(3+) is oxidized, thereby completing the oxidation of Cr(3+) to Cr(6+) within 10 min.