Twenty daily samples of pulverized fuel ash (PFA), obtained from the combustion of sub-bituminous Waikato coals, have been analysed. The contents of MgO, SOS, B203 and total and uncombined CaO in the New Zealand material were high when compared with most PFA produced in Europe and the USA. The PFA was separated into three major components, namely a magnetic fraction, and nonmagnetic acid-insoluble and acid-soluble fractions. These three fractions were related to three major mineral suites found in the coal: respectively hydrated iron carbonates and oxides, quartz and aluminosilicates, and calcium minerals. The variable composition of the PFA arises from differences in the proportions of these three mineral suites in the original coal. The composition was also found to vary with particle size. It is suggested that this was due to differences in the particle sizes of minerals in the coal, and to increased thermal decomposition of minerals in the smaller particles of PFA. A lowdensity fraction was separated and found to be similar in composition to cenospheres found in British and American PFA. The decomposition and fusion of a mineral common to many coals may be the source of these cenospheres.
The combustion of pulverized coal in modern thermal power stations produces a residue of pulverized fuel ash (PFA), also known as fly ash, which can be extracted from the flue gas by means of electrical precipitators, cyclones or collector bags. The coal burns rapidly at a temperature of 1500 + 2OO"C, and as a result many of the minerals originally present in the coal undergo chemical and physical changes. Clearly the composition of the resulting PFA will vary with the assemblage of minerals present in the original coal and their physical relation to one another. The composition will also depend upon the combustion temperature and the residence time at high temperatures, and possibly on the oxidizing or reducing conditions in the immediate environment of each ash particle. PFA is therefore a mixture of particles of complex mineral composition. In many countries PFA is treated as a mineral resource, and considerable efforts have been made to ensure commercial use, thus avoiding both a potential pollution problem and disposal costs. The United Kingdom Central Electricity Generating Board has produced a reference volume' giving details of experience gained in the utilization of PFA. Uses in the USA and other countries are covered in a conference report2. Treatments of PFA to give higher-value products have been patented3.
Most coal-fired power stations burn bituminous coals of relatively high ash and constant composition.
In contrast, in New Zealand the Meremere power station burns low-ash (ca. 5%) sub-bituminous coals of variable ash composition taken from the Waikato coalfields. Consequently European and American studies of PFA were not relevant to the material available here. Previous work4 on PFA from the Meremere power station suggested that it was extremely variable in composition, and that it was high in MgO and in total and uncombined CaO. In view of the increasing supply of PFA expected from a second power station on the Waikato coalfields and an interest in potential commercial utilization of the ash, a detailed study of the chemical composition and variability of Meremere PFA has been made.