Quantification of fusion in ashes from solid fuel combustion

The fusion of ashes produced during solid fuel combustion greatly affects t he tendency of these ashes to cause operational problems in utility boilers . In this paper, a new and quantitative laboratory method for assessing the fusion of ashes based on simultaneous thermal analysis, STA, is described. Using STA, melting is detected as an endothermic reaction involving no cha nge in mass. The measurement signals are transferred into a fusion curve sh owing the melt fraction in the ash as a function of temperature. This is do ne either by a simple comparison of the energies used for melting in differ ent temperature ranges or by accounting for the relevant melting enthalpies . The method repeatability is good, melting onset determinations and comple tions generally within 10 degrees C, and melt fractions at given temperatur es generally within 10% melt. Results are presented for simple binary salt mixtures, for which the agreement with fusion as determined by phase diagra ms is very good, and for straw (salt-rich) and coal (silicate-rich) ashes. Comparing ash fusion curves to index points of current standard ash fusion tests showed initial melting at temperatures typically between 50 degrees a nd 100 degrees C - but in extreme cases as low as 260 degrees C - below the melting onset as found by the standard fusion tests. Characterizing the fu sion by STA provides a more detailed description of the ash fusion as compa red to conventional methods, and the onset of ash fusion is more precisely determined. Furthermore, in combination with, e.g. computer-controlled scan ning electron microscopy, the method enables identification of the chemical species melting in different temperature ranges. Since ash melting has a m ajor impact on the deposit formation tendency, the presented detailed ash f usion determination improves the prediction of problems related to ash depo sition in boilers. (C) 1999 Published by Elsevier Science B,V. All rights r eserved.