Crystallization of coal ash melts

When coal is burnt in pulverized fuel (p. f.). utility boilers the inorganic material inherent to the fuel may form deposits on the heat absorbing surfaces causing significant reduction in thermal efficiency. This study has involved the investigation of the crystallization of coal ash melts and the relationship to the formation and growth of these troublesome deposits. 

Thus it will be of great value to be able to predict to some extent the crystallization behaviour of coal ash melts. For simplicity it is necessary to consider that crystallization will be from a homogeneous melt. The data may be applied to the phenomena of boiler slagging by using an accurate model of the viscosity of ash melts based on chemical composition. The main aim of the study has been to obtain relevant crystallization data of coal ashes and... 

An important aspect to this study was the determination of the system which most adequately describes the crystallization of the coal ash melts. The compositions of the three ashes studied have been normalized to the required number of components and plotted on the appropriate phase diagram. Normalization must be treated with care because, while only the major three or four components are assumed to be present, minor components may have a disproportionate effect on the crystallization. In the case of the eastern type ash the low CaO and MgO content reduced the need for plotting on the corresponding diagram. 

While coal ash is a complex mixture of mineral components the crystallization behaviour of homogeneous ash melts has been shown to be governed by the major oxide components. The equilibrium system CaO-FeO-Al2O2-SiO2 has been shown to be able to model the initial crystallization behaviour of ash melts. Furthermore the system also appears to govern the crystallization of boiler deposits to a significant extent. The applicability of the use of phase equilibria data to the phenomena of boiler deposits has been indicated. 

The crystallization behaviour of compositions in the systems CaO-Al2O2-SiO2r CaO-MgO-A 1202-8109 and CaO-Al202-8102"iron oxide relevant to coal ash compositions has been studied. The observations have been compared with the devitrification behaviour of both eastern and western type coal ashes determined under laboratory conditions. Results show that the crystallization of ash melts is well represented by the systen CaO-FeO-A 1202-8109. The quaternary systen has been constructed from literature data as planes of constant FeO content (5-30 wt% at 5% increments) and used to predict the behaviour of boiler deposits. 

---