A KINETIC-STUDY OF THE DEHYDRATION OF NORTHERN-IRELAND LIGNITE

The kinetics of dehydration of lignite have been characterized by isot hermal measurements of the water evolved under vacuum at 273-313 K. Th e rate of water release at low relative pressures is deceleratory and data are well expressed by the contracting volume rate equation. When the water vapour pressure, p, approaches the saturated value, p(o), th e rate becomes controlled by the relative vapour pressure, beta = p/p( o). These controls are combined in the kinetic analysis of the isother mal yield-time data. It is shown that the evaporation of water from li gnite is satisfactorily expressed by the equation, dp/dt = k(1 - alpha )2/3(1 - beta), where alpha is the fractional dehydration at time t. T he activation energy was 35 +/- 5 kJ mol-1 between 273 and 313 K. It w as shown that the dehydration rates of different lignite samples were identical, and that they were the same as the rate of dehydration of a rehydrated reactant sample which had previously been dried. It is con cluded that the low temperature drying of lignite occurs by the evapor ation of liquid water previously immobilized within the continuous coh erent carbonaceous matrix. The drying rate depends on the particle siz e, the rate of migration of the drying interface within the particles and on the ambient pressure of water vapour which tends to rehydrate t he matrix.