Coal Ash Fusibility Determination for Utility Boiler Slagging Prediction

1. The Phenomenon of Boiler Slagging & Fouling

When pulverized coal is fired inside modern utility boilers operating above 1300°C, the incombustible mineral constituents (silica, alumina, iron oxides, calcium, and alkalies) melt into molten droplets. If these molten particles strike water-wall boiler tubes before solidifying, they form glassy, insulating slag deposits that drastically reduce heat transfer and can cause tube ruptures.

2. The Four Standard Critical Characteristic Temperatures

Under ASTM D1857 and ISO 540, coal ash is prepared into triangular pyrometric cones or cylinders and heated inside a high-temperature furnace at a controlled rate of 5°C/minute up to 1550°C. The automated IF-2000G tracks four distinctive physical transitions:

• Initial Deformation Temperature (IT): The temperature at which the first rounding of the cone apex occurs.
• Softening / Sphere Temperature (ST): The temperature at which the cone height equals its base width (aspect ratio 1:1).
• Hemispherical Temperature (HT): The cone deforms into a semi-circular hemisphere whose height equals half its base width.
• Fluid / Flow Temperature (FT): The molten ash completely flattens into a puddle with height less than 1.5 mm.

3. Reducing vs. Oxidizing Atmosphere Control

In real pulverized fuel boilers, reducing zones frequently occur near burner mouths where oxygen is depleted. In reducing atmospheres (60% CO + 40% CO2), iron is present as Fe2+ (ferrous iron), acting as a potent flux that lowers ash fusion temperatures by 100°C to 200°C compared to oxidizing atmospheres where iron exists as Fe3+ (ferric iron).