The formation of deposits and their evolutionary characteristics during pressurized gasification of Zhundong coal char

The characteristics of deposits formed on stainless steel substrate (boiler tube) during pressurized gasification of Zhundong coal char with excessively high Na content were investigated in this work. The occurrences of alkali and alkaline earth metallic species in raw coal and its derived chars were determined by a sequential extraction method. The morphology, mineralogy and chemical compositions of deposits and bottom ash produced during the gasification of Zhundong coal char at 800, 1000 and 1200 °C were analyzed by X-ray diffraction and scanning electron microscopy combining energy dispersive X-ray spectrometer. Results show that water-soluble and ammonium acetate solution soluble sodium and potassium in raw coal tended to be converted to hydrochloric acid solution soluble form during Zhundong coal pyrolysis. Rising pyrolysis temperature increased the content of water-soluble calcium by enhancing the decomposition of CaCO3 and organically bonded calcium. During the gasification of Zhundong coal char, the released inorganic species, especially Na, Ca, Cl and S, could condense and/or agglomerate as sticky fine particles, resulting in fouling and slagging on the probes. Specifically, after Zhundong char gasification at 800 °C, alkali and alkaline earth metallic species-bearing minerals loosely clung on the probes as silicate or aluminosilicate. During char gasification at 1000 °C, a Na-S-O and/or Na-S deposit layer with a thickness of ca. 2 μm was formed on the probe No. 1 at 514 °C, while abundant Ca-Mg-Si-O and few Na2SO4 crystals were accumulated on the probe No. 2 at 771 °C. During Zhundong char gasification at 1200 °C, the minerals deposited at low temperature region (599 °C) were mainly composed of agglomerated and sintered alkali and alkaline earth metallic species-bearing compounds (mainly NaCl, KCl and Ca-Mg-Si-O). However, only few isolated particles were observed on the probe with temperature higher than 924 °C. Residual alkali and alkaline earth metallic species in bottom ash could form low-melting eutectics, such as NaAlSiO4 and Ca2Al2SiO7.