Kinetics and mechanism during mechanical/thermal dewatering of lignite☆

In order to increase the efficiency of lignite fired power stations the mechanical/thermal dewatering (German abbreviation: MTE, Mechanisch/Thermische Entwässerung, also used for ‘mechanical/thermal expression’) was developed at the University of Dortmund as an energy efficient process for the reduction of the water content of lignite prior to combustion [1–3], [Patentschrift DE 44 34 447 A1, 1994; Patent EP 0 784 660 B1; WO 96/10064, 1996; VDI-Berichte 1280 (1996) 165]. While a 25 t/h demonstration plant has been constructed at the Niederaußem power station of RWE in Germany and came into operation in 2001 [4], [Proceedings of the VGB/EPRI Conference, 2001] additional detailed research has been done on the process fundamentals at the University of Dortmund. Experiments for three different lignites from Germany, Greece and Australia are presented in this paper and it is shown, that the dewatering kinetics depending on time, temperature and pressure can be described by a new model derived from soil-mechanical fundamentals and rate-process-theory [5], [Mechanismen und Kinetik der Mechanisch/Thermischen Entwässerung von Braunkohle, 2001]. Due to differences in lignite composition the experimental determination of some model parameters for each coal is necessary. From the activation energy which is determined from experiments concerning dewatering kinetics it can be deduced, that even during secondary consolidation (creep) the drainage of water is the dominating process. The experiments also provide a clear distinction between the effect of the so-called ‘thermal dewatering’ due to heating of the lignite and the subsequent mechanical expression.