EVALUATION OF MINERAL MATTER BEHAVIOUR DURING COAL UTILISATION

Integration of data from several different analytical techniques has been used to evaluate the nature, abundance and mode of occurrence of the minerals and other inorganic elements in a range of coal samples, and also in the solid residues (ashes and slags) produced by those coals in different utilisation processes. Quantitative X-ray diffractometry of mineral matter isolated from the coal, and with care also of whole-coal samples, allows the abundance of the different phases to be reconciled against chemical data. Special XRD techniques can also provide quantitative assessment of amorphous material (glass) in ashes and other utilisation products. Real-time XRD analysis of coal mineral matter in heating stage experiments allows tracking of phase changes and interactions at high temperatures, providing a basis for linking mineralogical data from feed coals to the characteristics of the resulting ash materials.

Optical and electron microscopy provide complementary information on modes of mineral occurrence, including the extent to which individual phases may be able to be separated from the organic particles during coal preparation and the extent to which the phases might interact with each other during coal utilisation. Such data can be extended by integration of scanning electron microscopy and image analysis techniques (e.g. QEMScan), providing, inter alia, maps showing the distribution of the different phases within coal and ash samples. As well as providing further information on mineral composition, electron microprobe analysis has also been used to identify and in some cases map the distribution of inorganic elements, such as Ca, Al, Fe and Ti, within different maceral components, especially but not only in lower-rank coal samples. Such components become concentrated in the residue as the organic matter is consumed, and are also released in a more reactive form than similar elements from the coal’s mineral components.

Information from these techniques, considered in a geological context, has been used to evaluate the behaviour of different mineral matter components during coal utilisation, including the opportunities for particle interaction and formation of particular, perhaps troublesome, phases during combustion, coking and gasification.