Theory & Method

Ash Fusion Tester – QualiAFT™ 1200B 

Ash fusion testing is the standard analytical method for characterizing the thermal behavior of mineral ash as it transitions from solid to plastic to molten states at elevated temperatures. The test provides four critical characteristic temperatures — deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT) — that describe the temperature range over which coal or biomass ash changes shape and ultimately flows. These temperatures are directly used by combustion engineers to assess fuel slagging risk, optimize furnace operating temperatures, and evaluate the suitability of solid fuels for specific boiler designs.

The test procedure begins with sample preparation: a small quantity of ash is pressed into a truncated cone or pyramid shape using a standard mold. The shaped ash cones are loaded onto a refractory sample boat and placed into the high-temperature furnace of the QualiAFT™ 1200B. On this instrument, up to 10 sample cones are loaded simultaneously using the automatic sample loader at room temperature — no high-temperature manual loading is required.

Inside the furnace, the sample boat is heated at a controlled, programmable rate (4 °C/min to 20 °C/min) under a defined test atmosphere — either oxidizing (representative of excess air combustion conditions) or weakly reducing (representing sub-stoichiometric combustion zones or reducing smelting conditions), using CO/CO₂ or H₂/CO₂ gas blends as specified by the applicable test standard. The QualiAFT™ 1200B supports both the carbon-sealing method and the ventilation method for atmosphere control, providing full compliance with the atmosphere specification requirements of ASTM D1857 and ISO 540.

Throughout the heating cycle, the CE-certified high-definition camera captures one image per degree Celsius, producing a complete photographic record of the entire softening and melting sequence for all 10 cones simultaneously. The onboard image processing algorithm automatically analyzes each image and identifies the temperature at which each cone reaches the DT, ST, HT, and FT shape criteria — eliminating the visual judgment subjectivity that affects manual observation methods. Manual observation through the same optical port remains available for verification and comparison. Temperature calibration is performed using certified metal standards (nickel, gold, palladium) and dextrin powder, ensuring NIST-traceable temperature accuracy throughout the measurement range.