Thermodynamic analysis and comparison of retrofitting pre-drying concepts at existing lignite power plants

Abstract

Lignite is considered as a domestic and abundant energy source for several countries. However, its high ash and moisture content have a negative effect on power plant efficiency, on cost of electricity (COE) and consequently on CO2 emissions. The aim of the present work is the investigation and optimization of existing lignite pre-drying concepts and their improvement in terms of overall plant efficiency and integration. The main process parameters examined are the heat source for drying and the respective drying medium. In the conventional lignite drying process, hot recirculating flue gas is used as a heating medium, while in the current state-of-the art pre-drying concepts, a fluidized bed drying system is considered. Different concepts are also examined including a) the utilization of preheated air as heating medium and b) the optimized integration of a heat pump as a heat source for the drying process. Based on the performed thermal cycle calculations, the plant efficiency increase is evaluated. The results of the study indicate that higher plant efficiency is expected, when focussing on the optimized pre-drying process scheme and its integration with the overall steam.