Energy, exergy and economic analyses of lignite power plant integrated with flue gas pre-drying and a novel boiler exhaust gas waste heat recovery system

Abstract

Lignite is an important alternative fuel for electricity generation worldwide. However, lignite based power generation systems used to suffer from limited thermal efficiency resulting from the high moisture inherently contained in the fuel. Integration of pre-drying has been a proven upgrading technology for lignite power stations. In the present work, two different systems for heat utilization from lignite pre-drying and boiler exhaust flue gases were proposed, including flue gas pre-drying and waste heat utilization system, and a novel type of boiler exhaust waste heat recovery system. Energy, exergy and economic analyses models were established to explore the energy saving principle of lignite drying and waste heat utilization. Case study in a 660 MW air-cooling unit indicated that the plant thermal efficiency could be improved by 1.04 %-pts and 0.75 %-pts in lignite flue gas pre-drying and boiler exhaust gas waste recovery scenarios, respectively. The adoption of pre-evaporation and waste heat recovery would reduce exergy destructions in combustion process and heat transfer process by 3.53% and 0.68%, respectively. The levelised cost of electricity would be reduced by 0.72 $/MWh and 0.30 $/MWh, respectively. The integration of flue gas pre-drying, indirect firing and heat recovery system was more suitable for medium to high moisture lignite boiler. The proposed boiler exhaust gas waste heat recovery system was also feasible for medium moisture content lignite boiler. Overall, the results indicated that the integration of the proposed systems would make the lignite-fired power plants more efficient and cost-effective.