The effect of densification with alkali hydroxides on brown coal self-heating behaviour and physico-chemical properties

Victorian Morwell coal's physico-chemical properties were modified through densification with Ca(OH) ₂ , KOH, NH ₄ OH and NaOH, in order to compare the effects of varying alkali metals addition on self-heating behaviour. The wire basket results showed that densification of brown coal with alkali hydroxides increased the critical ignition temperature in the order Ca(OH) ₂ < KOH < NaOH relative to densified sample with NH ₄ OH and without additive. The surface area and pore volume of all coal products densified with alkalis reduced. In addition, results showed that the size and charge of the exchanged cations play an important role in their interaction with coal oxygen functional groups, the extent of cross-linking network and consequently on the reduction of surface area of densified products with alkalis. Thermogravimetric analysis (TGA) results demonstrated a decrease in the proportion of the mass loss occurring at lower temperatures, and a shift to higher temperature in the main oxidation/decomposition stages, consistent with increasing the critical ignition temperature. Fourier transform infrared spectroscopy (FTIR) results suggested a catalytic role for cations (K > NH ₄ > Na > Ca) in the formation of quinones during kneading of brown coal under alkaline conditions. The results revealed that the addition of alkalis to brown coal at basic pH improves the oxidation tolerance of the carboxylic/carboxylate groups and also enhances the polymerization and aromatization reactions during the thermal oxidation of coal, generating aromatic ether and/or ketone substituted polymers with higher thermal stability toward oxidation. Using NaCl as additive, at coal's natural acidic pH, resulted in a very brittle densified product, with higher macro and mesopore volumes than the coal densified prepared without additive. However, the T _cr value increased, probably due to its lower micropore volume.