Effect of heat and moisture on surface titratability and pore size distribution of Victorian brown coals

• Heating brown coals dry reduces access of water to functional groups.
• Heating brown coals wet does not reduce water access as substantially.
• How brown coals are dried affects the final pore size distribution.

Separating brown coal from its water in an energy-efficient manner is a major challenge, which if successful would substantially improve its combustion efficiency. Many dewatering processes are being investigated, most of which involve heat and pressure to reduce the porosity and number of water-binding surface groups in brown coal. Here we report the results of studies into the effect of temperature and moisture levels on the properties of the brown coal produced. The accessibility of surface functional groups to water was measured using an aqueous titration technique and pore size distribution was measured using mercury porosimetry. We found that heating dry brown coals substantially reduces the accessibility of titratable surface groups on brown coals and that loss of carboxylic acid functional groups on heating is insufficient to explain this effect. Mercury porosimetry indicates that blockage of pores on the size scales reachable by this measurement is unlikely to explain this loss of accessibility, and so the mechanism responsible for this pore blockage is acting on <5 nm pores. This effect of heating is less marked if the coal had been heated in the presence of water. This indicates that this blocking process is strongly influenced by the local environment. The porosity of the dried coal decreased with increasing heat treatment temperature.