An integrated lignite pre-drying system using steam bleeds and exhaust flue gas in a 600 MW power plant

Highlights: • An improved pre-drying system using steam bleeds and exhaust flue gas is proposed. • Thermodynamic and economic analyses are quantitatively conducted. • Lower work loss and better matched energy level are obtained. • Higher energy efficiency and greater economic benefits are achieved. - Abstract: An improved configuration of lignite pre-drying using both steam bleeds and boiler exhaust flue gas was proposed in this study. Differing from the conventional lignite pre-drying power plant (LPDPP), a two-stage pre-drying system is adopted in this proposed LPDPP, where boiler exhaust and steam bleeds are respectively the heat source of the two stage pre-drying. Thus, a part of waste heat of boiler exhaust can be recovered efficiently and the amount of steam required for drying could reduce by 30–40%, which would significantly reduce the work loss of steam turbine brought by drying steam. Furthermore, through employing the steam ejector, a part of steam at the 6th regenerative heater (RH6) can be entrained and the energy grade of drying steam could get lower, thus the work loss of steam turbine could be further reduced. The results of a detailed thermodynamic analysis showed that, for a typical 600 MW lignite-fired power plant (CLPP), the work loss of steam turbine brought by drying steam could be reduced from 14.5 MW in the conventional LPDPP to 8.5 MW in the proposed design. The net power efficiency could further increase by 0.4% with the heat rate reduction of 82.7 kJ/kW h as compared to the conventional one. The additional net economic benefit of the proposed LPDPP could reach $4.63 M, which is $1.09 M greater than that of the conventional one.