A new study published in Engineering introduces a flue gas‑driven molten‑salt‑heat‑exchanger (MSHE) designed to enhance the operating flexibility of coal‑fired power plants, supporting grid stability amid growing renewable energy integration. This work marks the first experimental investigation into using furnace flue gas directly to drive a molten‑salt heat storage system, offering an alternative to conventional steam‑driven configurations.

The research addresses the need for improved load‑following capability in coal‑fired units, which traditionally face constraints in ramping speed and operational stability when balancing intermittent wind and solar generation. Unlike steam‑vapor‑driven molten‑salt systems, the flue gas‑driven approach eliminates pinch temperature limitations (PTL) associated with phase‑change heat transfer and simplifies system layout by requiring only a single heat exchanger. The MSHE employs three key design innovations: finned tubes to balance thermal resistance between flue gas and molten salt, a weak inclination angle to enable gravity‑driven drainage of molten salt during shutdown, and a modular structure to promote uniform temperature distribution at the tube bundle outlet.

Researchers designed, fabricated, and tested a 300 kW MSHE prototype coupled with a 10 MW furnace. Experimental results show measured overall heat transfer coefficients agree with model predictions within a deviation of less than 10%, and the unit achieved a thermal power of 320 kW, exceeding the design target. A new heat transfer correlation for molten salt was developed across a wide range of Reynolds numbers, supporting accurate performance prediction for engineering scale‑up. The modular design limited temperature deviations among different tubes to below 4 K, reducing risks of local overheating and molten salt decomposition.

The system supports both heat storage and heat compensation modes, using flue gas to offset heat losses from the storage system to the surroundings and lowering auxiliary electricity consumption during standby periods. Transient tests show stable transitions between operating states within 180 to 600 seconds, supporting responsive control under variable conditions. Long‑term testing over one year revealed no significant degradation in heat transfer or flow performance, confirming operational robustness.

Based on prototype data, a 10 MW MSHE has been designed and integrated into a 350 MWe coal‑fired power plant, helping the unit achieve a load variation rate of 6% Pe/min, comparable to gas turbine levels. Compared with steam extraction schemes, the flue gas route delivers higher round‑trip efficiency and a simpler system structure, reducing investment complexity while maintaining reliable heat storage and release. The technology provides a practical pathway to upgrade conventional coal‑fired units for more flexible grid support in low‑carbon energy systems.

The paper “Developing Flue Gas-Driven Molten-Salt-Heat-Exchanger for Flexible Operation of Coal-Fired Power Plant,” is authored by Jinliang Xu, Hongliang Su, Xinyu Dong, Xiongjiang Yu, Chao Liu, Yan Wang, Jian Xie, Wei Wang, Yupu Yu, Qinghua Wang, Yuguang Niu, Jizhen Liu, Ying Huang, Zhengshun Zhang, Anyou Dong, Yan Pan, Hao Wu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.001. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.