Numerical analysis of the steady and transient operating characteristics of the flue gas molten salt heat storage system

Abstract

The flue gas molten salt heat storage system enhances coal-fired power plant flexibility by recovering thermal energy using molten salt as the medium. This study investigates the system’s steady and transient characteristics using a novel transient model that incorporates temperatures of the heat exchange tubes and shell, while accounting for operating mode switching and deviations from design conditions. Key findings reveal that under steady-state operation, the heat exchange tubes bundle stabilizes at 324.20 °C, with molten salt and flue gas outlet temperatures reaching 390.05 °C and 299.99 °C, respectively. Preheating times for startup are optimized to 8 min (thermal storage, 40%–50% flue gas flow) and 17 min (thermal preservation, 75%–100% flue gas flow). During shutdown, cooling to 200 °C requires 70.6 h (thermal storage) and 97.5 h (thermal preservation), with full cooling to ambient taking around 1 month. Transient analyses demonstrate rapid system responses: a 60 °C increase in flue gas inlet temperature triggers the molten salt outlet alarm threshold (395 °C) within 25 s. To mitigate temperature deviations, coupling adjustments prove effective: reducing flue gas temperature by 60 °C restores molten salt outlet temperature to 390 °C in 603 s, while increasing molten salt mass flow by 34% achieves this in 412 s. These results provide critical operational benchmarks, emphasizing the need for real-time parameter monitoring and adaptive control to ensure system safety and efficiency. The findings offer actionable insights for optimizing design and operational strategies in flue gas molten salt heat storage systems.