Characterisation of the properties of size fractions from ten world coals and their chars produced in a drop-tube furnace

A range of coals from different parts of the world was studied to determine if there were any common relationships that could be determined to gain a clearer understanding of the distribution of coal properties within different particle-sizes. The properties examined were proximate analysis, maceral analysis and %Unreactives from image analysis. Each fraction was also pyrolysed in a drop-tube furnace at 1300°C, 1 vol% oxygen and a residence time of 200 ms and the resulting chars analysed for morphology using image analysis. There were substantial variations between the particle-size distributions of the different coal samples even though they were ground to the same specification for trials on a combustion rig. Ash distributions showed in all cases that the smallest particle size (−38 μm) had either the highest ash level or was very close to it. However, the trends in ash level for increasing particle size showed variations between coals with some coals showing increases in ash towards the larger particles. Fusinite content did not necessarily concentrate in the smallest size fraction, however, liptinite content did increase with particle size. %Unreactives generally increases with particle size and is related to char morphology through an empirical parameter, the ACA [5]. In addition the ACA [5] parameter showed the effect of both particle size and %Unreactives on char morphology and clearly showed the significant influence of particle size on burnout. A parameter such as this could, therefore, be used in burn-out models and further correlated with %Unreactives and particle size.