Modelling the mechanical thermal expression behaviour of lignite

A process known as mechanical thermal expression (MTE) is used to dewater lignite to improve its heating value. A numerical model is developed to investigate the compression dewatering stage of the MTE process at ambient (20 °C) and elevated temperature (150 °C). This model includes details not previously investigated such as the wall friction, the compressive yield stress of the lignite network as a function of void ratio, and the relationship between the yield stress and the absolute permeability. This allows gradients of these properties to develop as dewatering progresses. The results from the model are compared with the void ratio, hydraulic pressure drop and permeability, which are independently determined experimentally. A major simplification in this work is that only primary consolidation is incorporated in the model. The results clearly show that secondary consolidation does occur in lignite, but it is not significant except at low applied pressures, or for compression times which are much longer than would be of industrial interest.