Improved Lignite Predrying System Integrated with Heat and Water Recovery from Wet Flue Gas at Lignite-Fired Power Plants: Energy and Water Conservation

Abstract

To improve energy efficiency and reduce water consumption simultaneously in lignite-fired power plants, an improved lignite predrying system integrated with heat and water recovery from wet flue gas is proposed in this study. In the proposed system, the heat source for the lignite predrying system was the latent heat from the flue gas instead of the extracted steam, so the extracted steam could return to the turbine to generate more power. Moreover, both the water from the raw lignite and wet flue gas desulfurization (WFGD) processes were recovered, which minimized the water consumption of lignite-fired power plants. This study evaluated the energy- and water-saving potential of the proposed system at both air-cooling and wet-cooling power plants firing three types of lignite separately. The results showed that the higher the moisture content of lignite, the larger the net efficiency improvement and the make-up water rate reduction. The net efficiency improvement and make-up water rate reduction in power plants firing Yimin lignite reached 2.33% points and 0.370 t/MWh, respectively, at wet-cooling power plants, and 2.18% points and 0.443 t/MWh at air-cooling power plants. This means that zero water consumption can be achieved, and water and electricity can be output at the same time at air-cooling power plants. The technical and economic analysis showed that the net present value in 10-years of power plants firing different lignite types could reach 49.31, 36.19, and 16.44 million USD depending on the predry degree. The discounted payback period was 2.34–3.67 years, which showed good economic viability in all cases.