A new structural parameters model based on drag coefficient for simulation of circulating fluidized beds

The goal of this research is to determine how spectral analysis of pressure fluctuations can be used as a diagnostic and design tool for both bubbling fluidized beds (BFB) and circulating fluidized beds (CFB). Static pressure fluctuations are measured from two BFB models, two CFB models, and two Pyropower CFB boilers located at the ISU power plant/cogeneration facility. Analyzing the fluctuations using spectral analysis shows that the structure of pressure fluctuations is governed by multiple phenomena. Similarities and differences between the nature of fluctuations in bubbling, turbulent, and fast fluidization regimes are discussed. The Bode plots of pressure fluctuations under bubbling/turbulent fluidization exhibit the characteristics of an oscillatory second order system. This system is governed by a combination of voidage oscillations and surface eruptions. A theoretical model is developed to predict the frequency of oscillation in fluidized systems, and to explain the second order behavior. A comparison of pressure fluctuation behavior in two geometrically similar BFBs is conducted, which validates previously derived fluidized bed similitude parameters under most operating conditions. Multiple second order phenomena acting concurrently also govern CFB pressure fluctuations. Phenomena similar to that proposed for BFBs governs pressure fluctuations in the lower dense regions of the CFB. A siuface wave phenomena is hypothesized to govem CFB fluctuations above the lower dense bed. Similar pressure dynamics and axial voidage profiles are not observed when currently proposed similitude parameters are matched m two CFBs. By replacing the dimensionless soUds flux with a dimensionless riser loading parameter, this modified set of similitude parameters can be used to establish similar hydrodynamics in CFBs . Primarily due to the periodic operation of the coal feed system in the industrial scale CFB Boiler, pressure fluctuation analysis did not result in any definitive hydrodynamic information.

Modelling the axial voidage profile and flow structure in risers of circulating fluidized beds

Modeling of axial bed-to-wall heat transfer in a CFB combustor with abrupt riser exit geometry

Experimental study and numerical simulation of baffled bubbling fluidized beds with Geldart A particles in three dimensions

Hydrodynamics plays a crucial role in defining the performance of gas-solid circulating fluidized beds (CFB). A two dimensional model was developed considering the hydrodynamic behavior of CFB gasifiers. In the modeling, the CFB riser was divided into two regions: a dense region at the bottom and a dilute region at the top of the riser. Radial distributions of bed voidage were taken into account in the upper zone by using (1991)’s correlation. For model validation purposes, a cold model CFB was employed, in which sawdust was transported with air as fluidizing agent. The column was 10m in height and 280 mm in diameter, and was equipped with pressure transducers to measure axial pressure profile and with a reflective optical fiber probe to measure local solids holdup. A satisfactory agreement between the model predictions and experimental data was found.

3D CFD simulation of a circulating fluidized bed with on-line adjustment of mechanical valve

Choosing structure-dependent drag coefficient in modeling gas-solid two-phase flow

Solid mass fluxes in circulating fluidized beds

A pressure balance model for a circulating fluidized bed (CFB) unit that incorporates a downer has been proposed. A comparison of the operation stability between a CFB downer and a CFB riser showed that, although only one critical operating gas velocity exists in the CFB riser for a given riser solids flux, there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used. Parameters that affect the solids concentration in the CFB downer have been studied using the model and have been validated with experimental data obtained from a special CFB unit that incorporates a downer. Measures to increase the solids concentration in the downer have been proposed.

Oxy-fuel circulating fluidized bed (CFB) combustion technology is in the stage of initial development for carbon capture and storage (CCS). Numerical simulation is helpful to better understanding the combustion process and will be significant for CFB scale-up. In this paper, a computational fluid dynamics (CFD) model was employed to simulate the hydrodynamics of gas-solid flow in a CFB riser based on the Eulerian-Granular multiphase model. The cold model predicted the main features of the complex gas-solid flow, including the cluster formation of the solid phase along the walls, the flow structure of up-flow in the core and downward flow in the annular region. Furthermore, coal devolatilization, char combustion and heat transfer were considered by coupling semi-empirical sub-models with CFD model to establish a comprehensive model. The gas compositions and temperature profiles were predicted and the outflow gas fractions are validated with the experimental data in air combustion. With the experimentally validated model being applied, the concentration and temperature distributions in O2/CO2 combustion were predicted. The model is useful for the further development of a comprehensive model including more sub-models, such as pollutant emissions, and better understanding the combustion process in furnace.

A two‐dimensional reactor model for circulating fluidized beds (CFB) was studied based on the assumption that at every location within the riser, a descending dense phase and a rising lean phase coexist. Fluid mechanical variables may be calculated from one measured radial solids flux profile (upward and downward). The internal mass‐transfer behavior is described on the basis of tracer gas experiments. The CFB reactor model was tested against data from ozone decomposition experiments in a CFB cold flow model (15.6‐m height, 0.4‐m ID) operated in the ranges 2.5–4.5 m/s and 9–45 kg/(m2·s) of superficial gas velocity and solids mass flux, respectively. Based on effective reaction rate constants determined from the ozone exit concentration, the model was used to predict the spatial reactant distribution within the reactor. Model predictions agreed well with measurements.

The so-called core-annulus structure, that is, lower solids concentration in the center region and higher solids concentration in the near wall region along the circulating fluidized beds (CFB) riser, usually has a negative effect on the performance of CFB risers since it may greatly decrease the reactor efficiency and controllability. In order to improve the performance of industrial CFB risers, two potential approaches are proposed in this study: (1) the use of the center-sparse side-dense air jets arrangement for the gas distributor and the side-covered arrangement for the solids distributor and (2) the use of the air jet from the riser circumference. The results show that the patterns of the inlet gas distributor and solids distributor have significant effect on the flow structure in both the entrance region and the fully-developed region in the riser. The gas distributor with center-sparse side-dense air jet arrangement improves the uniformity of the radial solids distribution, while the center-dense side-sparse air jet arrangement steepens the non-uniformity of the solids radial profile. The core-annulus structure can also be greatly flattened by applying a side-covered solids distributor, while it can be heavily steepened by employing the center-covered solids distributor. The results also show that by employing the circumferential air jet approach under appropriate jet velocities, the maximum solids concentration in the near wall region can be greatly reduced, the entrance region can be shortened, and the uniformity of the flow structure can be greatly improved.

Dimension measurements of hydrodynamic attractors in circulating fluidized beds

ECVT study of 3D flow characteristics at L-shaped exit of CFB riser

Application of CPFD method in the simulation of a circulating fluidized bed with a loop seal, part I—Determination of modeling parameters