Viscosity is the most important physical property for slag and ash deposits, describing a material’s resistance to flow and develop deposit strength.
Viscosity of slags can be measured in either oxidizing or reducing environments using the crucible method illustrated below. T250 is the temperature at a viscosity of 250 poise, at which slag begins to flow (and develop strength). To determine the viscosity of a material, MTI places a sample of ash or slag in a furnace and heats it at a steady rate. We then monitor the test through the stages of melting. Tests are captured on video, and image analysis is used to determine the T250.
MTI tested the T250 method using a National Institute for Standards and Testing (NIST) glass standard as shown below where the results are also compared to rotating bob measurements. The results are within 5% of the certified value. Physical viscosity measurements are used to support viscosity models. MTI can predict the viscosity of a slag or ash for various ranges of compositions. However, in some cases, actual physical measurements are necessary if the composition is outside the ranges where predictions are less accurate. An example of the use of additives to modify slag flow behavior is shown below.