Tuo Zhou

The increasing penetration of wind and solar power, together with their intermittency and uncertainty, has increased the demand for flexible regulation in power systems. However, peak shaving and frequency regulation can reduce the efficiency of thermal power units, intensify thermal stress, accelerate equipment degradation, and complicate pollutant emission control. Energy storage integration therefore offers a promising pathway for enhancing thermal power flexibility while alleviating unit-level operational burdens. This review examines energy storage technologies for thermal power flexibility, focusing on electrochemical energy storage, molten salt thermal energy storage, flywheel energy storage, and gravity energy storage. Their operational characteristics, application scenarios, and roles in frequency regulation and peak shaving are compared. The impacts of energy storage integration on unit efficiency and peak-shaving capability are further analyzed by considering the operational characteristics of thermal power units. In addition, different energy storage pathways for peak shaving and frequency regulation are evaluated in terms of environmental and economic impacts. Finally, future research priorities are discussed, including deeper thermodynamic integration, multi-energy and multi-timescale coordination, economic value realization, engineering reliability, and life-cycle sustainability.

With the global energy transition and large-scale renewable integration, thermal power units are shifting from base-load to peak-shaving roles, but deep peak shaving may cause combustion instability and increased emissions. High-temperature solid-particle thermal storage offers a low-cost and wide-temperature-range solution for enhancing unit flexibility. This study proposes a flue-gas-driven solid-particle thermal storage system in which high-temperature flue gas extracted from the furnace directly heats solid particles through a multi-stage suspension preheating process. An industrial-scale suspension preheater was modeled using the Computational Particle Fluid Dynamics (CPFD) method to investigate gas–solid flow and heat transfer for quartz sand with different particle sizes and cement raw meal. The effects of particle size and thermophysical properties on particle motion, temperature evolution, gas–solid heat transfer coefficient, and flow-field characteristics were systematically analyzed. The results show that the outlet flue-gas temperature was approximately 705–725 K under the investigated conditions. For quartz sand, the particle outlet temperature decreased from approximately 715 K to 660 K as the particle size increased from 0.45–0.55 mm to 1.45–1.55 mm. The gas–solid heat transfer coefficient of quartz sand remained relatively stable, mainly ranging from approximately 450 to 900 W/(m²·K), whereas that of fine raw meal was approximately 1750–2600 W/(m²·K). Larger quartz sand particles also exhibited more pronounced wall attachment because of their higher inertia. These results demonstrate the potential applicability of suspension-preheater-based direct flue-gas heating for high-temperature solid-particle thermal energy storage and provide quantitative information for the design and operation of such systems.

The increasing penetration of renewable energy requires coal-fired power plants to operate with greater flexibility, while deep load reduction and frequent load variations challenge combustion stability, efficiency, and equipment reliability. High-temperature solid-particle thermal energy storage (TES) offers a promising approach to decouple boiler heat supply from turbine power demand. This review compares particle heating, discharging, transport, and system-integration strategies and relates reported findings to key design and operational constraints. Solid-particle storage media can operate over a wide temperature range of approximately 200–1200 °C; silica sand can withstand temperatures up to about 1200 °C and alumina approximately 1100–1200 °C, substantially exceeding the typical operating range of nitrate molten salts (<600 °C). This broad temperature tolerance provides favorable thermal matching with high-temperature boiler flue gas. Direct flue-gas heating shortens the heat-transfer pathway and reduces intermediate heat-transfer stages, although its effects on boiler heat distribution and downstream heating surfaces require careful evaluation. Moving-bed discharging can provide continuous thermal output, while gravity-driven high-temperature transport combined with mechanical lifting after particle cooling can reduce the demand for high-temperature moving components. Based on these findings, a conceptual configuration integrating direct flue-gas heating, particle sensible-heat storage, moving-bed discharging, and temperature-segmented circulation is proposed. Future studies should focus on system-level efficiency, dynamic response, and load-following capability. Preliminary calculations for a 600 MW subcritical unit at 30% THA load show that particle-storage integration increases the boiler outlet flue-gas temperature by only about 1 °C, indicating no apparent increase in low-temperature corrosion risk under the investigated condition.

With the global energy transition and the increasing use of renewable energy, coal-fired power plants are shifting from baseload generation toward flexible operation for load following and frequency regulation. However, low-load operation can destabilize boiler combustion, accelerate equipment wear, and complicate emissions control. Thermochemical energy storage offers high storage density, low heat loss during storage, and a wide operating temperature range. It may therefore support the flexible operation of coal-fired power plants. This narrative review compares six thermochemical energy storage systems: CaO/CaCO₃, CaO/Ca(OH)₂, ammonia decomposition and synthesis, metal hydrides, metal oxide redox systems, and methane dry reforming. The comparison considers reaction temperature, storage density, material cost, cyclic stability, safety, and compatibility with coal-fired power plants. Based on this qualitative comparison, CaO/CaCO₃ appears to offer relatively favorable compatibility because of its low-cost raw materials, relatively high storage density, good temperature matching with high-temperature flue gas, and potential integration with CO₂ capture. Three integration pathways are further reviewed: solar-driven CSP–CaL, calcium-looping carbon capture with sorbent storage, and flue-gas-driven calcination with carbonation-based flue-gas reheating. The first two pathways have received more extensive modeling and experimental investigation. The flue-gas-driven pathway remains mainly at the conceptual and component-validation stages. Its main constraints include CaO deactivation, flue-gas impurities, reactor scale-up, and coordination with variable boiler operation. Overall, the reviewed evidence suggests that CaO/CaCO₃ may offer a favorable temperature and process match for integration with coal-fired power plants. However, the flue-gas-driven pathway is not yet ready for plant-scale application. Future research should focus on long-term testing under real flue gas, reactor scale-up, dynamic reactor–boiler demonstrations, and consistent techno-economic and life-cycle assessments.

In this study, the reaction characteristics and multi‐step reduction kinetics of calcium ferrite (CF) by CO are investigated using thermogravimetric analysis under isothermal conditions. The results indicate that the three stages of the CF reduction exhibit distinct reaction characteristics, with the reaction rate of the first stage being significantly faster than those of the other two stages. The overlapping phenomenon among the multi‐step reactions is confirmed by the phase composition and relative content at representative reduction degree of different stages. A parallel reaction model based on the JMA equation is established to achieve the decoupling analysis of the overall CF reduction process. The first and second stages proceed almost simultaneously, while the third stage commences only after the completion of the first stage. All three reduction stages are governed by the nucleation and growth mechanism, with the corresponding reaction kinetic models being AE2, AE1.5, and AE4, respectively. The average activation energies of the three stages at different CO concentrations, determined by the JMA model‐fitting method, are 23.72 kJ/mol, 17 kJ/mol, and 19.4 kJ/mol, respectively, with the activation energy of the first stage being relatively more sensitive to CO concentration. This study aims to provide fundamental kinetic data for a deeper understanding of the multi‐step reaction process of CF reduction by CO.

The increasing penetration of renewable energy has significantly intensified the demand for flexible operation of thermal power plants, making dynamic simulation an essential tool for understanding transient behaviors and developing advanced operational strategies for circulating fluidized bed (CFB) power plants. This review critically examines the existing dynamic modeling approaches for industrial-scale CFB power plants, with particular emphasis on their applicability to flexibility studies. Existing CFB flue-gas side models are systematically classified into three categories: 3D physics-based CFD models, behavioral/data-driven models, and semi-empirical mechanistic models. Their characteristics are critically compared in terms of spatial and temporal scales, empirical dependence, model generality, computational and implementation burden, and applicability to CFB flexibility studies. Dynamic modeling of the steam–water cycle is also reviewed, showing that it has reached a relatively mature stage owing to well-established thermo-hydraulic theories and standardized modeling platforms. The current research bottleneck is therefore identified as the dynamic coupling between the flue-gas side and the steam–water cycle for integrated CFB whole-plant simulation. Based on the comparative analysis, semi-empirical mechanistic models are identified as a particularly suitable framework for industrial-scale CFB flexibility studies requiring minute-to-hour transient simulation, physical interpretability, and whole-plant coupling. Finally, future research directions are discussed, highlighting how integrated dynamic models can support CFB flexibility-enhancement technologies and the development of new-generation coal-fired power plants.

Circulating fluidized bed (CFB) power plants are urgently transitioning towards peak-shaving operations. However, the complex gas-solid two-phase flow and the strong dynamic coupling among subsystems in CFB boilers hinder the development of effective tools for investigating their operational flexibility. To address this gap, this study proposes a plant-wide mechanistic dynamic modeling framework named “Omni CFB” explicitly resolving the full-loop hydrodynamics, combustion, heat transfer and water-steam cycle from first principles (applying a 1.5-D discretization in the furnace to account for the core-annulus solids flow pattern). Built within the open-source Modelica standard library environment, the model achieves bidirectional coupling, comprehensively describing the transient interactions among mass, energy, and steam-water side thermodynamic parameters during load variations. The model is validated against operational data across both the furnace and turbine systems, demonstrating that the relative errors of key variables are strictly within ±3.5% under steady-state loads of 79.5% and 98%. In addition, dynamic simulations are shown to accurately track the complex transient evolution of major operating parameters like furnace temperatures and power generation. Crucially, transient simulations of ramp-down reveal that the buffering effect of external loop causes solids redistribution, resulting in spatiotemporal mismatch: the substantial thermal inertia of solids causes furnace temperature response to lag behind hydrodynamics by an average of 3 minutes, thereby fundamentally constraining the load-following capability of CFB. This validated “Omni CFB” provides an efficient digital prototype for furthering CFB flexibility.

Circulating fluidized bed (CFB) power plants are urgently transitioning towards peak-shaving operations. However, the complex gas-solid two-phase flow and the strong dynamic coupling among subsystems in CFB boilers hinder the development of effective tools for investigating their operational flexibility. To address this gap, this study proposes a plant-wide mechanistic dynamic modeling framework named “Omni CFB” explicitly resolving the full-loop hydrodynamics, combustion, heat transfer and water-steam cycle from first principles (applying a 1.5-D discretization in the furnace to account for the core-annulus solids flow pattern). Built within the open-source Modelica standard library environment, the model achieves bidirectional coupling, comprehensively describing the transient interactions among mass, energy, and steam-water side thermodynamic parameters during load variations. A hierarchical spatial discretization strategy is also proposed to optimize the trade-offs among physical fidelity, computational robustness and initialization. The model is validated against operational data across both the furnace and turbine systems, demonstrating that the relative errors of key variables are strictly within ±3.5% under steady-state loads of 79.5% and 98%. In addition, dynamic simulations are shown to accurately track the complex transient evolution of major operating parameters like furnace temperatures and power generation. Crucially, transient simulations of ramp-down reveal that the buffering effect of external loop causes solids redistribution, resulting in spatiotemporal mismatch: the substantial thermal inertia of solids causes furnace temperature response to lag behind hydrodynamics by an average of 3 minutes, thereby fundamentally constraining the load-following capability of CFB. This validated “Omni CFB” provides an efficient digital prototype for furthering CFB flexibility.

In addressing the challenge of low combustion efficiency of gasification fine slag (GFS), this study proposes a low-velocity, fine-particle fast fluidization circulating fluidized bed (LFFF-CFB) technology for the complete combustion of GFS. A thorough investigation was conducted to analyze the GFS combustion and pollutant emission processes within the CFB. This investigation utilized the computational particle fluid dynamics (CPFD) method. The present study investigates the influence of fluidization velocity and temperature on the cyclone outlet fly ash carbon mass fraction and pollutant emissions. Numerical simulation results demonstrate that rapid fluidization can be achieved under the LFFF-CFB technical scheme, while ensuring the required material concentration for heat transfer in the upper furnace. The combined influence of temperature and residence time results in the cyclone outlet fly ash carbon mass fraction of just 0.99%, while the mass concentrations of NO and SO2 are 141.79 mg/Nm3 and 2,094.73 mg/Nm3, respectively. This study provides data references and operational process optimization for the practical application of LFFF-CFB technology. Subsequent research endeavors will prioritize the mitigation of pollutant emissions, thereby facilitating resource utilization of GFS.

To address the challenge of spatio-temporal mismatch in the large-scale utilization of renewable energy, this paper proposes the use of calcium carbide slag (an industrial solid waste) as a medium for thermochemical energy storage. By constructing a high-temperature bubbling fluidized bed experimental system, the study systematically investigated the dynamic energy storage characteristics and evolution patterns of calcium carbide slag under bubbling fluidized conditions. The results indicate that calcium carbide slag contains a mass fraction of Ca(OH)2 as high as 79.3%, demonstrating significant energy storage potential. This study achieved, for the first time, direct high-temperature fluidized heat storage using calcium carbide slag in its raw form (without granulation). By blending a small amount of inexpensive quartz sand, a stable fluidized system was established, reducing the minimum fluidization velocity by approximately 40%. The multi-parameter coupled regulation mechanism indicates that under optimal operating conditions: a storage temperature of 550 °C, a fluidization velocity of 9.5 cm/s, and an initial bed aspect ratio of 2, the reaction rate and energy storage efficiency achieve an optimal balance. This study confirms the feasibility of fluidized-bed energy storage using calcium carbide slag and lays a solid experimental and theoretical foundation for low-cost, large-capacity thermal energy storage applications of industrial solid waste