Thermodynamic analysis of an improved flue gas pre-dried lignite-fired power system integrated with water recovery and drying exhaust gas recirculation

Abstract

Lignite is a domestic strategic reserve of low rank coals in many countries owing to its abundant resource and competitive price. Combustion for power generation is still an imperative approach to lignite utilization. However, the high moisture content invariably results in the low efficiency of lignite-direct-fired power plants. The present work focuses on an improved concept of the flue gas pre-dried lignite-fired power system (FPLPS), which integrates fan mill drying, deep water recovery and drying exhaust gas recirculation. A thermodynamic analysis model was developed to investigate the energy and water saving potentials of the FPLPS with various design parameters. Different configuration alternatives of FPLPS design were provided and compared in terms of energy and water conservation. Results indicated that flue gas pre-drying and waste heat recovery into the boiler contributed to 1.47%-pts plant thermal efficiency improvement (Δηtot) in a 660 MW supercritical power unit. Drying exhaust gas recirculation could be implemented for flame temperature reduction and emission control given effective removal and recovery of the moisture in lignite. The Δηtot in this case was determined as 0.87%-pts. Moreover, the Δηtot could be enhanced to 1.09%-pts with utilization of waste heat from the water recovery process into the steam cycle. The plant water saving rate of the improved FPLPS ranged from 0.22 to 0.32 t·MW h−1. Overall, an environmentally-friendly, energy-efficient and water-saving lignite power generation could be accomplished through the improved FPLPS.