Ash fusion temperatures give an indication of the softening and melting behavior of fuel ash. These temperatures are widely cited in fuel specifications for boilers despite a relatively poor record of correlating with slagging or fouling behavior. Reasons for the poor predictive behavior include:

Fusion temperatures at one time were also quite subjectively measured, but this criticism has been addressed by the development of automated techniques for performing the measurements that require no intervention by the operator. Despite the shortcomings, fusion temperatures are valuable guides to the high-temperature behavior of the fuel inorganic material.

Fusion temperatures typically are measured at four defined points under both reducing and oxidizing conditions. These are defined as follows:

Generally, a temperature under reducing conditions should be equal to or lower than the corresponding temperature under oxidizing conditions.  The difference in these temperatures generally increases with increasing iron content in the ash.  As is apparent from the definitions above, fusion temperatures should also monotonically increase in order of IT, ST, HT, FT.  Ash fusion temperatures from some of the coals investigated in the MFC are indicated below.