Upgrading and dewatering of low rank coals through solvent treatment at around 350 °C and low temperature oxygen reactivity of the treated coals

We have recently presented a novel method which can not only dewater but upgrade low rank coals under rather mild conditions. The method treats coal in non-polar solvents, such as 1-methylnaphthalene, at temperatures below 350 °C. It was found that the treated coals were almost free from moisture and had the heating values corresponding to subbituminous or bituminous coal. One of the important questions remaining to be considered for the method is if this solvent treatment method can suppress the self-ignition tendency of low rank coals.In this study, low temperature oxygen reactivities at 65 °C, which may be linked to the self-ignition tendencies, were examined for the samples prepared from an Australian brown coal by the proposed treatment. The oxygen reactivities of the extracted fractions were found to be much smaller than the oxygen reactivity of the raw coal. The oxidation reactivity of the solvent treated coal, the mixture of extracted fractions and residue, was smaller than that of the residue, but it was slightly larger than the oxygen reactivity of the raw coal. On the basis of the low temperature oxidation data and detailed analyses of the extract and residue, the mechanism of the low temperature oxidation and the possibility of the proposed solvent treatment as a method to suppress the low temperature oxidation reactivity of low rank coals were examined.