关键词: Waste treatment, Gasification, Thermodynamics, Ash fusion temperature, Phosphorus, Dissolution of mullite

Abstract: High phosphorus content in sewage sludge (SS) significantly affects the ash fusion temperature during the co-gasification of coal with SS. However, the mechanisms underlying the effects of phosphorus on the ash melting behavior remain unclear. In particular, there is no direct evidences to confirm that refractory mineral dissolution processes are affected by the phosphorus-bearing liquid phase formed during the initial stage of ash melting. In this study, different amount of calcium- and iron-phosphates were added to coal to investigate the effect of phosphorus on the coal ash fusion temperature. The phosphorus and mineral phase transformations, as well as the dissolution processes of refractory mullite particles in the phosphorus-bearing liquid phase, were determined using X-ray photoelectron spectroscopy, X-ray diffraction analysis, thermodynamic equilibrium calculations, and thermal platform microscope systems. The results indicated that the addition of 8% calcium-phosphate and iron-phosphate reduced the ash flow temperature (FT) from above 1500 °C to 1336°C and 1261 °C, respectively. The effect of iron-phosphate on the reduction of ash FT was more pronounced than that of calcium-phosphate. The phosphorus vaporization ratio increased with the temperature. Approximately 50% of phosphorus was retained in the samples to which 8% of calcium-phosphate and iron-phosphate were added at 1400 °C. Elevation of the temperatures promoted the conversion of PO₄^3- to molten slag-dissolved P₂O₅. The mechanism whereby phosphorus reduces the ash FT is primarily governed by the dissolution of refractory minerals in the liquid phase formed during the initial heating stage. This dissolution process was enhanced in the presence Fe compared to that in the presence of Ca.

Key words: Waste treatment, Gasification, Thermodynamics, Ash fusion temperature, Phosphorus, Dissolution of mullite