Rheological properties of coal ash slag at ultra-high temperatures

The integration gasification combined cycle is one of the leading options for power generation that limit the amount of greenhouse gas through carbon capture and storage. Ash-related problems (i.e., slagging and fouling of ash deposition on the furnace wall and tubes in the conversion section) are often the main reason for entrained-flow gasifiers in the integration gasification combined cycle and boilers in oxy-fuel combustion unscheduled shut downs. Thus, understanding the high-temperature physical properties of molten coal ash slag may enable an accurate description of the condition of ash deposition in the gasifier and boiler. The results of a series of high-temperature experiments undertaken to investigate the rheological characteristics of molten coal ash slag are reported. Firstly, the viscosities of 45 coal ash slag samples at high temperature have been measured under different temperatures and shear rates. The computer thermodynamic software package FactSage has been used to predict liquidus temperatures, volume fractions of crystallized solid particles (φ ), and the compositions of remaining liquid phase for 45 coal ash slag samples. The flow properties of completely liquid and partly crystallized coal ash slag samples have been predicted by three viscosity models. The Urbain formalism has been modified to describe the viscosities of fully liquid slag and homogeneous remaining liquid phase in coal ash slag samples. The modified Einstein equation and Einstein-Roscoe equation have been used to describe the viscosities of heterogeneous coal ash slag samples of φ <10.00 vol.% and φ ≥ 10.00 vol.%, respectively. Then, the evolution of the temperature dependence of rheological behaviors, including the viscosity, flow curves, and yield stress of the coal ash slag from entrained-flow gasifier at decreasing temperatures (1340 °C to 1200 °C), was investigated. The transition of the behavior from Newtonian to non-Newtonian, as well as changes in the shear-thinning and thixotropic behaviors with decreasing temperature, has been described. The results show that the sensitivity of the coal ash slag viscosity to temperature decreases with increasing rotation speed. Above its liquidus temperature calculated by FactSage, the coal ash slag behaves as a Newtonian fluid; below its liquidus temperature, however, the rheological behavior of the coal ash slag becomes non- Newtonian owing to its increased solid-phase content. The coal ash slag containing a number of crystalline particles shows dramatic shear-thinning and thixotropic behavior.