The process of pre-drying of fossil fuels - in this case raw lignite with a water content of 35 to 55 w% - leads to an increase of the overall efficiency by 2 to 30% points achieved by a reduction of primary fuel. The overall efficiency can be increased by up to 5% points by integration of a vapor condenser and energy reintegration. The necessary heat for the drying process is taken from the process cycle in the form of low pressure steam. Economic reasons limit the rest water content of the dried coal between 10 and 12 w%. The present paper discusses the use of process simulation programs for the thermodynamic optimization of the performance of complicated energy systems and presents six variants for increasing power plant efficiency by lignite drying.