High-flux Circulating Fluidized Bed Research Articles

Particle clustering (mesoscale structure) of high-flux gas–solid circulating fluidized bed

Particle clustering is an important dynamic phenomenon in circulating-fluidized-bed (CFBs) systems, and has been suggested as a key contributing factor to the non-uniform hydrodynamics of CFBs. Studies show that particle clusters can be affected by solids flux, in terms of frequency, duration, and solids holdup. To understand the characteristics of particle clusters under high-solids-flux conditions, experimental and modeling studies in high-solids-flux gas–solids CFBs were reviewed and summarized. Optical and electrical measurements and imaging methods were used to monitor the particle-clustering phenomenon in CFBs. Particles were found to cluster in high-flux CFBs, and were characterized by a denser cluster-solids holdup and a shorter time fraction, which was different from the behavior in low-flux CFBs. Particle properties affected particle clustering in high-flux CFBs significantly. In modeling work, Eulerian–Eulerian and Eulerian–Lagrangian methods were used to study the particle-cluster characteristics. Good results can be obtained by using the Eulerian–Eulerian method to simulate the CFB system, especially the high-flux CFBs, and by considering the effects of particle clusters. The Eulerian–Lagrangian method is used to obtain detailed cluster characteristics. Because of limits in computing power, no obvious results exist to model particle clusters under high-solids-flux conditions. Because high-solids-flux conditions are used extensively in industrial applications, further experimental and numerical investigations on the clustering behavior in HF/DCFBs are required.

Hydrodynamic Study of AR Coupling Effects on Solid Circulation and Gas Leakages in a High-Flux In Situ Gasification Chemical Looping Combustion System

In situ gasification chemical looping combustion (iG-CLC) is a novel and promising coal combustion technology with inherent separation of CO2. Our previous studies demonstrated the feasibility of performing iG-CLC with a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor (FR) and a counter-flow moving bed (CFMB) as the air reactor (AR). As an extension of that work, this study aims to further investigate the fundamental effects of the AR coupling on the oxygen carrier (OC) circulation and gas leakages with a cold-state experimental device of the proposed iG-CLC system. The system exhibited favorable pressure distribution characteristics and good adaptability of solid circulation flux, demonstrating the positive role of the direct coupling method of the AR in the stabilization and controllability of the whole system. The OC circulation and the gas leakages were mainly determined by the upper and lower pressure gradients of the AR. With the increase in the upper pressure gradient, the OC circulation flux increased initially and later decreased until the circulation collapsed. Besides, the upper pressure gradient exhibited a positive effect on the restraint of gas leakage from the FR to the AR, but a negative effect on the suppression of gas leakage from the AR to the FR. Moreover, the gas leakage of the J-valve to the AR, which is directly related to the solid circulation stability, was exacerbated with the increase of the lower pressure gradient of the AR. In real iG-CLC applications, the pressure gradients should be adjusted flexibly and optimally to guarantee a balanced OC circulation together with an ideal balance of all the gas leakages.

Optimization of in Situ Gasification Chemical Looping Combustion through Experimental Investigations with a Cold Experimental System

In situ gasification chemical looping combustion (iG-CLC) is a promising coal combustion technology for implementing CO2 capture with a low energy penalty. A novel iG-CLC cold experimental system was developed in the authors’ previous work (Ind. Eng. Chem. Res. 2013, 52, 14208). It mainly consists of a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. As an extension of that work, in this study, we further optimized the iG-CLC system by redesignung the air reactor to enhance the carrying capacity of the gas flow and developing a two-stage separation system by adding a second-stage cyclone to the original first-stage inertial separator. Stability in operation and flexibility in adjusting operating parameters were achieved with the improved system. In the riser (fuel reactor), higher solids fluxes and solids holdups were achieved, which should enhance the gas–solid contact and promote the complicated heterogeneous reactions. In the moving bed (air reactor), the carrying capacity of the gas flow was significantly enhanced, which should lead to a great increase in the system power capacity. The confirmation of the ability to control the gas flow directions in the two reactors means that the gas bypassing between the two reactors can be restrained so as to ensure a high CO2 concentration in the exhaust of the fuel reactor. The high global separation efficiency and selective separation efficiency of the new two-stage separation system for fine particles indicate that a high combustion efficiency of coal can be achieved with a hot iG-CLC system.

GAS-SOLID FLOW BEHAVIOR IN A PRESSURIZED HIGH-FLUX CIRCULATING FLUIDIZED BED RISER

Gas-solid flow behavior was investigated in a high-flux circulating fluidized bed (HFCFB), whose riser is 0.068 m ID and 5.2 m high. Experiments were carried out at different pressures (0.1–0.5 MPa), solids mass fluxes (122–1015 kg/m2 · s), and standard state superficial gas velocities (6–40 m/s). Geldart group B particles 137 µm in diameter and 2490 kg/m3 in density were used as bed materials. Information on pressure drop, apparent solids holdup, average slip velocity, and solid-to-air mass flow ratio were systematically tested at elevated pressures. The results showed that increase of the operating pressure led to higher total riser pressure drop and solids holdup, lower slip velocity, more homogeneous pressure drop, and longitudinal solids holdup distribution. As a result, a high-density condition with Geldart group B particles, i.e., solids holdup in the riser over 10%, could be successfully formed in the HFCFB at elevated pressure. Similar to the trend at atmosphere, the dimensionless slip velocity at pressurized condition increased with increasing solids holdup for any given superficial gas velocity, and the total riser pressure drop increased approximately linearly with solids-to-air mass flow ratio at the given operating pressure.

Flow Patterns Identification in a High-flux Circulating Fluidized Bed with Shannon Entropy Increment Rate

Experiments of gas-solid suspension were carried out in a high-flux circulating fluidization bed with the solid mass flux up to 536 kg/(m2·s). Differential pressure fluctuation time series were obtained at different locations in the riser. Shannon entropy increment rate analysis was developed to identify the flow pattern and transformation and characterize the dynamic behavior. Effect of superficial gas velocity on the Shannon entropy increment rate was examined under high solid mass flux. It was demonstrated that a circulating fluidization bed at high solid mass flux was a deterministic chaos system and the sensitivity of the system's chaotic characteristic to operating parameters at different flow regimes was different. Shannon entropies increment rate of different flow regimes were distinct. Shannon entropy increment rate analysis method results in high degree of recognition for flow pattern and transformation.

Solids Holdup of High Flux Circulating Fluidized Bed at Elevated Pressure

AbstractExperimental studies on the solids holdup of a high‐flux circulating fluidized bed (HFCFB) at an operating pressure up to 0.5 MPa were carried out. The effects of operating pressure, solids mass flux and superficial gas velocity on the solids holdup distribution were systematically tested. It was found that the solids holdup at elevated pressure increases with increasing solids mass flux but decreases with increasing superficial gas velocity, which is similar to the trends at atmospheric condition. As a result, the condition of a high‐density circulating fluidized bed (HDCFB), i.e., solids holdup in everywhere of the riser is larger than 10 %) is easier to be achieved at elevated pressure than in a HDCFB operated at atmospheric pressure. In the current work, the condition of a full HDCFB with Geldart group B particles has been achieved successfully at 0.5 MPa.

Modeling on High-Flux Circulating Fluidized Bed with Geldart Group B Particles by Kinetic Theory of Granular Flow

Modeling of the hydrodynamic behaviors of high-flux circulating fluidized beds (HFCFBs) with Geldart group B particles has been performed using a Eulerian multiphase model with the kinetic theory of granular flow (KTGF). The essential models involved are the dispersed k-e turbulence model, the Gidaspow shear viscosity model, and the Syamlal—O'Brien drag model, and the boundary condition is the Johnson and Jackson wall boundary condition. The sensitivities of key model parameters (i.e., particle-particle restitution coefficient (e), particle—wall restitution coefficient (e w ), and specularity coefficient (ϕ)) on the predicted gas velocity, solids velocity, and solids volume fraction were tested. It was found that e has remarkable dependence on the particle diameter. Large-sized particles experience a more sensitive effect of e on predictions. The particle—wall restitution coefficient e w has somewhat of an effect on the simulated values of gas velocity, solids velocity, and solids volume fraction; however, no critical changes in the trends of their radial distributions have been found. The specularity coefficient ϕ has a slight effect on the gas velocity and solids velocity distributions but a pronounced effect on the solids volume fraction distribution. An increase in specularity coefficient results in a reduction in the solids volume fraction near the wall. Based on the comparisons of simulated results with experiments, a group of suitable model parameters for modeling the flow of Geldart group B particles in HFCFB risers by a Eulerian multiphase model with KTGF was determined and verified. Besides, some interesting simulated results that are difficult to measure experimentally were presented under the suggested model parameters.

Modeling on the Hydrodynamics of a High-Flux Circulating Fluidized Bed with Geldart Group A Particles by Kinetic Theory of Granular Flow

A Eulerian multiphase model with the kinetic theory of granular flow (KTGF) was studied for modeling the hydrodynamic behaviors of high-flux circulating fluidized beds (HFCFBs) with Geldart group A particles. The sensitivities of several key models (i.e., turbulence model, drag model, and granular shear viscosity model) and modeling parameters (particle−particle restitution coefficient, particle−wall restitution coefficient, and specularity coefficient) on the predictions have been tested systematacially. Experimental results of Pärssinen and Zhu [AIChE J. 2001, 47 (10), 2197−2205; Chem. Eng. Sci. 2001, 56, 5295−5303] were used as a numerical benchmark to assess the simulations quantitatively. The results show that particle−particle and particle−wall restitution coefficients are not critical for the holistic distribution trends of solid volume fraction and solid velocity. A small specularity coefficient, such as φ = 0, could give the well predictions. The Syamlal−O’Brien drag model displays better agreement with individual radial distributions of both solid volume fraction and solid velocity, in terms of microscopic features. While the Syamlal shear viscosity model fails to obtain right trends at the upper parts of the riser. For turbulence model, the mixture turbulence model and per-phase turbulence model could not predict reasonable trend of radial solid velocity distribution along the riser. As a result, a group of suitable models and modeling parameters—i.e., dispersed turbulence model, Gidaspow shear viscosity model, Syamlal−O’Brien drag model, a specularity coefficient of φ = 0, a particle−particle restitution coefficient of e = 0.9, and a particle−wall restitution coefficient of ew = 0.99—are proposed for modeling Geldart group A particle flow in HFCFB risers. Finally, further validations have been conducted to confirm this suggestion, by comparing simulated results with experimental data.

Modelling and simulation of a gas–solids dispersion flow in a high-flux circulating fluidized bed (HFCFB) riser

Radial solids velocity profiles were computed on seven axial levels in the riser of a high-flux circulating fluidized bed (HFCFB) using a two-phase 3-D computational fluid dynamics model. The computed solids velocities were compared with experimental data on a riser with an internal diameter of 76 mm and a height of 10 m, at a high solids flux of 300 kg m −2 s −1 and a superficial velocity of 8 m s −1. Several hundreds of experimental and numerical studies on CFBs have been carried out at low fluxes of less than 200 kg m −2 s −1, whereas only a few limited useful studies have dealt with high solids flux. The k– ɛ two-phase turbulence model was used to describe the gas–solids flow in an HFCFB. The model predicts a core–annulus flow in the dilute and developed flow regions similar to that found experimentally, but in the region of highest solids concentration it is somewhat overpredicted at the level close to the inlet.

Analysis of the chaotic dynamics of a high-flux CFB riser using solids concentration measurements

A high-flux circulating fluidized bed (CFB) riser (0.076-m I.D. and 10-m high) was operated in a wide range of operating conditions to study its chaotic dynamics, using FCC catalyst particles ( d p = 67 μm, ρ p = 1500 kg·m −3). Local solids concentration fluctuations measured using a reflective-type fiber optic probe were processed to determine chaotic invariants (Kolmogorov entropy and correlation dimension). Radial and axial profiles of the chaotic invariants at different operating conditions show that the core region exhibits higher values of the chaotic invariants than the wall region. Both invariants vary strongly with local mean solids concentration. The transition section of the riser exhibits more complex dynamics while the bottom and top sections exhibit a more uniform macroscopic and less-complex microscopic flow structure. Increasing gas velocity leads to more complex and less predictable solids concentration fluctuations, while increasing solids flux generally lowers complexity and increases predictability. Very high solids flux, however, was observed to increase the entropy.

Axial and radial solids distribution in a long and high‐flux CFB riser

AbstractRadial solids concentration profiles were determined with a fiber‐optic probe on eight axial levels in a 76‐mm‐ID, 10‐m riser, at solids flux (solids circulation rate) up to 550 kg/m2·s and superficial gas velocity up to 10 m/s. Radial concentration profiles at high solids fluxes of over 300 kg/m2·s are less uniform than lower fluxes of less than 200 kg/m2·s. Under all operating conditions, the flow development in the riser center is nearly instant, with the solids concentration remaining low at the riser center throughout the riser. In the wall region, increasing solids flux significantly slows down the flow development, with the solids concentration near the wall decreasing all the way toward the riser top at high fluxes. In the high‐flux circulating fluidized bed (HFCFB), a middle section with intermediate solids holdups of about 7 to 20% was between the bottom dense section and top dilute section, with its length increasing as the solids circulation rate increases and as the gas velocity decreases. Flow conditions in this section resemble those of the dense suspension upflow under high‐density operating conditions by Grace et al. (1999). When its length extends to the riser top, a high‐density circulating fluidized bed for which an HFCFB is a necessary but not a sufficient condition, forms.