Developing Flue Gas-Driven Molten-Salt-Heat-Exchanger for Flexible Operation of Coal-Fired Power Plant

The large-scale utilization of renewable energy challenges the stability and safety of the grid; thus, the flexibility of coal-fired power plants should be increased to balance unstable renewable energies. To achieve this, a heat storage system (HSS) is integrated into a power plant. This is the first study utilizing furnace flue gas to drive a molten-salt-heat-exchanger (MSHE). Compared to steam-vapor-driven MSHE, flue gas-driven technology avoids the pinch temperature limitation (PTL) and simplifies the system configuration. In this study, we demonstrate the concept, design, fabrication, and experiments of the MSHE. The novelties include: ① finned tubes to balance the thermal resistances between the flue gas side and the molten salt side; ② a weak angle design to ensure gravity-driven recession of the molten salt; and ③ a modular design to ensure even temperature distribution at the outlet of the tube bundles. A heat transfer correlation is developed for molten salt, covering a wide range of Reynolds numbers. An experimental setup is constructed to collect data and verify the effectiveness of the MSHE. The measured overall heat transfer coefficients matched the predictions well, with deviations of less than 10%. The measured heat power reached 320 kW, exceeding the 300 kW design target. We demonstrate the heat transfer between the flue gas and molten salt to compensate for the heat release from the HSS to the environment, reducing electricity consumption in the standby stage of the system. The modular design of the MSHE ensures minimal temperature deviations of < 4 K among different tubes, avoiding local overheating-induced decomposition of the molten salt. Based on the 300 kW MSHE results, a 10 MW MSHE is designed, fabricated, and integrated into a 350 megawatt electric (MWe) coal-fired plant to achieve a higher load variation rate of 6% Pe·min⁻¹ for a coal-fired power plant.