A new rheometer for direct measurement of the flow properties of coal ash at high temperatures

A Brief Review of Viscosity Models for Slag in Coal Gasification

“…Specifically, and more importantly, they mention that this ash sample (the Loy Yang coal) should be modeled as viscoplastic shear-thinning fluid with a yield stress. They conclude that the presence of a high yield stress for the slag, in the range of operating temperatures, confirm previous findings (see Tonmukayakul and Nguyen 2002) that a high alkali sulphate ash is more likely to agglomerate in fluidized-bed combustion than a silica rich coal. Song et al, (2010) devised a high-temperature rheometer to study the rheological characteristics of slag, specifically the thixotropy and yield stress at different temperatures ranging from 500 to 1550 °C.…”

“…When necessary they used limestone to lower the liquidus temperature and slag viscosity for optimum operation and slag tapping. Tonmukayakul and Nguyen (2002) state that traditional methods for viscosity measurements and instrumentation might be satisfactory for coal ash slags obtained from bituminous (black) coal or those with high silica content but inaccurate for the cases with lower temperatures, where the coal may be partially molten and generally behaves as a non-Newtonian fluid. To overcome this problem, they suggested using a cone and plate rheometer, in which a volume of molten ash contained between a thin gap and a plate at a small cone angle is sheared.…”

Slag Behavior in Gasifiers. Part II: Constitutive Modeling of Slag

“…Specifically, and more importantly, they mention that this ash sample (the Loy Yang coal) should be modeled as viscoplastic shear-thinning fluid with a yield stress. They conclude that the presence of a high yield stress for the slag, in the range of operating temperatures, confirm previous findings [1] that a high alkali sulphate ash is more likely to agglomerate in fluidized-bed combustion than a silica rich coal. Song et al [65] devised a high-temperature rheometer to study the rheological characteristics of slag, specifically the thixotropy and yield stress at different temperatures ranging from 500 °C to 1,550 °C.…”

“…When necessary they used limestone to lower the liquidus temperature and slag viscosity for optimum operation and slag tapping. Tonmukayakul and Nguyen [1] state that traditional methods for viscosity measurements and instrumentation might be satisfactory for coal ash slags obtained from bituminous (black) coal or those with high silica content but inaccurate for the cases with lower temperatures, where the coal may be partially molten and generally behaves as a non-Newtonian fluid. To overcome this problem, they suggested using a cone and plate rheometer, in which a volume of molten ash contained between a thin gap and a plate at a small cone angle is sheared.…”

“…Two possible mechanisms have been proposed for this unwanted phenomenon: (1) partial melting or reactive liquid sintering, and (2) viscous flow sintering [1]. The first situation occurs with partial melt at 500-700 °C, which is normally lower than the standard operating temperatures of many fluidized beds.…”