David Pallarès

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.

The mixing of larger particles (e.g., fuel particles) in a bubbling fluidized bed is governed by buoyancy and drag, which correspondingly promote segregation and internal circulation. Prior studies have shown that gas released from such particles during drying or devolatilization can generate so-called endogenous bubbles, which exert an effective lift force on the particle. However, the impact of gas release under bubbling conditions and with over-bed particle feeding, remains unexplored.This study investigates how gas release influences the mixing of larger particles in a bubbling fluidized bed. Experiments were conducted in a fluid-dynamically downscaled model simulating biomass pyrolysis at 700°C in a bed of silica sand fluidized with flue gas. Magnetic particle tracking captured particle trajectories, and gas-releasing tracers were used.Different behaviors were observed depending on the fluidization velocity. At low fluidization velocities (uo/umf ≥ 2), buoyant particles sink slightly deeper by locally reducing the concentration of surrounding suspension, whereas heavy particles experience inhibited sinking resulting in preferred positions closer to the bed surface. As fluidization velocity increases beyond uo/umf ≤ 3.5, the effect of gas release on axial mixing diminishes, while the lateral mixing becomes more pronounced, with the dispersion coefficient enhanced by up to 40%.

This work investigates the technical feasibility and costs of carbon capture from industrial emission sources with CO2 concentrations in the range of 0.5–4.0%. Three types of post-combustion capture schemes are considered: monoethanolamine absorption (MEA-ABS); alkali absorption combined with calcium looping (ALK-ABS); and temperature vacuum swing adsorption (TVSA). The latter two systems are commonly investigated for direct air capture applications. The carbon capture and avoidance costs of these technologies are compared for different industrial sectors and contrasted with the price of offsetting the emissions through purchases of carbon dioxide removal credits applying current prices on the voluntary market.The results show that MEA-ABS has the lowest levelized cost of capture (LCOC) among the three post-combustion capture technologies for flue gas streams with larger flow rates (>200 t/h) and CO2 concentrations ≥1%. TVSA presents the lowest LCOC for lower flow rates (especially <100 t/h), while ALK-ABS has the lowest LCOC only for a small window of low CO2 concentration (0.5-1%) and 100-200 t/h. However, taking into account the carbon footprints of the input energy changes this picture, as the carbon avoidance costs yield their lowest cost values with ALK-ABS , unless heat with a low carbon intensity is available for MEA-ABS. Comparing the cost of carbon avoidance with carbon dioxide removal credits on the voluntary market, it can be more economical for industries with flue gases with CO2 concentrations <2.6% to offset their emissions through the purchase of carbon dioxide removal credits than to install any of the post-combustion capture technologies considered here.

This vision article accompanies a Special Issue of Applied Thermal Engineering dedicated to the 14th International Conference on Circulating Fluidized Bed Technology (CFB-14), held in Taiyuan, China from 21 to 24 July 2024. The conference brought together prominent and young researchers, educators and practitioners to share research, achievements and practicing experience on circulating fluidized bed (CFB) technology. The conference featured representatives from 15 countries. A total of 164 articles were presented, of which 30 were keynotes, 109 were oral presentations, and 25 were poster presentations. Following peer review, 19 articles were selected for publication in this Special Issue, which collectively highlight recent advances in gas-solid fluidized beds for novel thermal reaction, energy conversion and storage. In this article, these contributions are reviewed, along with a selection of relevant studies in the literature. Based on this review and a brief identification of the state-of-the-art in CFB technology, the article provides a perspective for scientists, researchers and engineers that outlines key challenges, gaps, and identifies promising directions for future research and innovation. The need for further research is identified as being of importance in key areas of CFB design and process optimization in a variety of applications, ranging from the development of advanced processes with improved performance and efficiency, to the realization of novel decarbonization pathways. Focal areas include the development of high-fidelity computational modelling approaches, high-spatial and high-temporal measurement methods for large-scale CFB reactors, and integrated CFD simulation and process measurement to enables the next-generation of CFB designs for thermal reaction, energy conversion and storage.