Solvent swelling rates of low rank coals and implications regarding their structure

The rates of solvent swelling of a suite of low rank US coals have been measured in various organic solvents at various temperatures. The equilibrium extents of swelling are independent of temperature within the temperature range studied (25-70 degrees C). The rate of the swelling process was generally relaxation-controlled. The activation energies for the kinetics of swelling are consistent with other recently published values, and support the hypothesis that the activation barrier is associated with the breakage of internal electron donor-acceptor (e.g. hydrogen bonding) interactions. Thermal pretreatment of a lignite at various temperatures up to 395 degrees C decreased the activation energy barrier and the rate of swelling. Swelling rates of the same lignite in different solvents suggest that the activation energy can exceed the value for breaking single hydrogen bonds, possibly because the true barrier involves cleaving more than one of those bonds at a time. (C) 1998 Elsevier Science Ltd. All rights reserved.