Automated mineralogical analysis of coal and ash products – Challenges and requirements

To ensure accurate and reliable automated mineralogical analysis of a sample, it is a common practise to use chemically derived elemental proportions to validate the mineralogical proportions. Automated mineralogical analysis of coal and corresponding ash is a challenge as a significant proportion of the phases/minerals are amorphous with variable elemental proportions. In coal, the organic macerals are the amorphous phases and depending on the rank of the coal have varying carbon, hydrogen, nitrogen, oxygen and inorganic element compositions. In ash, aluminosilicate glasses and partially transformed minerals are the major amorphous phases. The elemental composition of these glasses is controlled by the mineral deportment in the source coal, operating conditions and configuration of the boiler, gasifier or furnace.To compound the problem, the traditional chemical analysis of coal (proximate, ultimate and XRF ash elemental analysis) is based on heating the coal to different temperatures and recording mass losses or analysing the ash derived from the coal. These analyses each have inherent limitations, sampling and analytical errors, which when combined can provide misleading interpretations.If accurate and precise mineralogical data are to be provided to coal practitioners for optimising processing plant, predicting combustion/gasification performance and for resolving coal quality problems, it is necessary to develop a cost effective and reliable automated coal and ash analytical system.With the advancement in X-ray detector and scanning electron microscope technology it is potentially feasible to provide a cost effective and quick mineralogical (minerals and macerals) and elemental analysis of raw coal and ash. The new automated systems have the potential of combining the traditional proximate, ultimate, XRF ash elemental, XRD and petrographic analyses into one analysis, without heating or modifying the existing coal sample.Detailed “coal” and ash particle characterisation is now feasible, enabling the development of coal processing, fly ash formation, slagging prediction and combustion performance models. Coal and ash analysis is now entering a new and exciting era.