Temperature at which ash melts or softens is very relevant for the operation of a combustion unit as this affects the required maintenance work. By designing the plant according to these melting temperatures of the fuel, slagging and fouling can be decreased.
Ash melting behaviour can be estimated by the characteristic temperatures method, described in the draft for a technical specification, prCEN/TS 15370 “Solid biofuel – Method for the determination of ash melting behaviour – Characteristic temperatures method”. prCEN/TS 15370 is based on the international coal standard ISO 540 “Determination of ash fusibility”. In the determination the characteristic temperatures (of a cylindrical test specimen made of prepared 550 oC ash) are determined:
- Deformation temperature (DT), which is the temperature where the first rounding of the edges, due to melting, of the specified test specimen occurs.
- Hemisphere temperature (HT) is the temperature, where the test specimen forms approximately a hemisphere.
- Flow temperature is the temperature when the height of the test specimen is half of the height of the test specimen at the hemisphere temperature; i.e. the temperature when the ash has flown out.