Water absorption by coals: effects of pore structure and surface oxygen

The water-holding capacity of various coals from lignite to anthracite was measured and its relation to their oxygen content and pore structure was investigated. Both factors were found to play important roles in determining the water-holding capacity. Pyrolysis of a lignite at 400 °C decreased its water-holding capacity by more than two-thirds, because of the decrease in the number of hydrophilic sites. This was caused by the decomposition of oxygen-containing groups and the decrease in surface area which resulted from the plugging of small pores by volatile matter condensation, making their surfaces inaccessible to water.