Effects of phosphorous-based additive on flow properties of high silicon-aluminum coal ash

Abstract

• Effects of Ca 3 (PO 4 ) 2 addition on the fluidity of Huainan coal ash were revealed. • Phosphorus decreased ash fusion temperatures and temperature of critical viscosity. • Phosphorus contributed to the formation of slag during the ash fusion process. • Polymerization of slag structure was enhanced by phosphorus at high temperatures. • Crystallization in the slag during cooling was restricted by phosphorus. Co-gasification with coal is a promising way to dispose the sewage sludge with high phosphorus content, but the lack of data on the flow properties of phosphorus-containing ashes and slags under reducing atmosphere limits the gasification of this fuel. In this study, Ca 3 (PO 4 ) 2 was chosen as the phosphorus model compound of sewage sludge and was added into one high-silicon-aluminum coal (Huainan, HN). Ash fusion temperatures (AFTs) and viscosity-temperature properties of blends were measured and the mechanism of effects of phosphorus were explored based on the results of X-ray diffraction (XRD), thermodynamic software FactSage, Fourier transformation infrared spectroscopy (FTIR) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS). To eliminate the interference of calcium, a phosphorus-free control group was set up with CaCO 3 and the same tests were conducted. The experimental and modelling results showed that phosphorus had a significant fluxing effect on the fusion of coal ash by providing the initial liquid phase and changing the main minerals. The effects of phosphorus on slag viscosity were rather complex. At the temperature above 1530 °C, phosphorus-bearing slags had slightly higher viscosity values than phosphorus-free slags due to the increased polymerization degree of the network structure. However, at the cooling process, phosphorus restricted the crystallization in the slag and decreased the viscosity values and the temperatures of critical viscosity (T CV ). In this paper, a suitable addition ratio of Ca 3 (PO 4 ) 2 for the slagging gasification of HN coal was concluded.