ABSTRACT
Samples of coals, fly ashes, and deposits, collected during boiler trials at the Ratcliffe power station burning two coals (Silverdale and Daw Mill), have shown systematic chemical differences between the chemical compositions of the fly ashes and the deposits. The Silverdale deposits are enriched in iron oxide compared to the parent coal. The Daw Mill deposits show an increase in silica/alumina ratio, and they are also slightly enriched in iron oxide, compared to the parent coal, and the fly ashes are correspondingly depleted. Samples of pulverised fuel and fly ash have been analysed by computer-controlled scanning electron microscopy (CCSEM) to determine the chemical distribution of the pf mineral occurrences and the fly ash particles. The Silverdale pf contains clays and pyrite, with minor amounts of quartz and calcite, and some of the pyrite is intimately mixed with the clays. Most of the ash particles have aluminosilicate compositions derived from the clay minerals, but a large fraction of the ash particles is enriched in iron oxide. These particles will have a lower viscosity than the majority and will be retained preferentially on the boiler walls, explaining the iron oxide enrichment in the deposits and depletion in the fly ashes. The Daw Mill pf contains mostly clays, quartz, with pyrite, calcite and other carbonates. Some of the quartz is intimately mixed with the clays; pyrite and calcite show lesser degrees of inter-mixing. Most ash particles have aluminosilicate compositions and a large fraction of the ash particles is enriched in silica. Because of a minimum in liquidus temperature between the aluminosilicate and the silica compositions, the silica-enriched ash particles will be more sticky and retained preferentially on the boiler walls. This effect, together with a more minor retention of iron-enriched ash particles, has explained the observed chemical partitioning.
