Details

Ash Fusion Temperature

Ash Fusion Temperature (AFT)

Ash Fusion Temperature (AFT) is the point at which fuel ash starts to:

a) Deform
b) Start to melt
c) Flow like a liquid

It is usually tested under standardized lab conditions and includes four key stages:

a) Initial Deformation Temperature – when ash starts to soften
b) Softening Temperature – ash begins to slump or deform noticeably
c) Hemisphere Temperature – ash blob looks like a hemisphere
d) Flow Temperature – ash becomes fully liquid and spreads out

AFT is typically expressed in degrees Celsius (degC), and the higher the value, the better the boiler performance.

When you use a fuel with a low AFT (below 1000 degC):

a) Ash melts inside the furnace
b) It fuses to the grates or boiler tubes
c) Clinkers form, reducing combustion efficiency
d) Frequent shutdowns are required to clean or desludge

Ash fusion depends not only on fuel type but also on the mineral composition of the ash, such as:

a) Silica (SiO2): High silica lowers AFT and increases clinker formation
b) Alumina (Al2O3): Increases AFT and improves performance
c) Iron, Sodium, and Potassium: Lower AFT and increase the risk of slagging