Exergy analysis of the flue gas pre-dried lignite-fired power system based on the boiler with open pulverizing system

• The exergy analysis model of the FPLPS was developed.
• The efficiency improvement potential and essential of FPLPS were revealed.
• The inefficiencies in FPLPS components were identified through exergy analysis.
• The retrofitting of flue gas extracting point gained higher efficiency.
• A parametric study was conducted for FPLPS at off-design conditions.

This paper deals with an exergetic analysis of the flue gas pre-dried lignite-fired power system (FPLPS) based on the boiler with open pulverizing system (OPSB) to explore the energy-saving potential of the FPLPS. A steady-state simulation was performed to obtain thermodynamic properties of process streams in a 600 MW unit. Results indicated that the plant efficiency of the FPLPS was relatively 3.42% higher than that of the conventional lignite-fired power system (CLPS). The improvement benefited from the integration of the flue gas dryer with the OPS, in which the dryer exhaust gas was separated from coal powders and prevented from recycling in the furnace. Consequently, the exergy destruction in the combustion process was reduced by 2.32%-pts for increased flame temperature, and the exergy loss of the boiler exhaust gas was reduced by 0.68%-pts for decreased temperature. Moreover, the dryer exergy efficiency was only 20.20% due to considerable exergy destructions in the moisture evaporation and the mixing of drying agents. The proposed retrofitting option by extracting cold flue gas from the economizer outlet could increase the plant efficiency by 0.33% relatively. Finally, attempts were made to examine the influence of varying pre-drying parameters on system performances.