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- Farha, Munavara, 1995, et al.
(författare)
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Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions
- 2023
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Ingår i: Fuel. - 0016-2361. ; 348
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Tidskriftsartikel (refereegranskat)abstract
- Dual fluidized bed systems are indispensable for future energy systems that require solids cycling between different atmospheres. However, controlling the residence time of solids in the reactor, which is crucial for controlling the heat and mass transfer of the fuel, is a significant challenge. This study investigates four experimental techniques to quantify the horizontal flow of solids fluidized under bubbling conditions: integral mass accumulation; differential mass accumulation; thermal tracing; and magnetic solids tracing. Integral mass accumulation entails collecting bed material using a defluidized box within a given time period. Differential mass accumulation measures the material accumulation rate in a section of the bed that is monitored using pressure measurements. Thermal tracing calculates the solids flow rate by solving the heat balance to match the temperature field captured by a thermographic camera. Magnetic solids tracing involves injecting a batch of magnetic tracer solids into the reactor and then measuring the residence time distribution using impedance coils. The experiments were conducted under down-scaled conditions that resemble large-scale operations with a length scaling factor of 0.12. For this study, three operational parameters were varied: the fixed bed height; the volumetric flow rate of the conveying air; and the fluidization velocity in the bed. The horizontal solids circulation rates achieved ranged from 1.7×10−4 to 10 kg/m·s, corresponding to 1.2×10−3 to 70 kg/m·s on a hot up-scaled basis, which is a relevant range to indirect biomass gasification in an industrial setting. The three selected operational parameters led to increases in the horizontal solids flow. While all four methods replicated the trends, quantitative variations in the measured circulation rates occurred due to the inherent characteristics of the methods. High circulation rates resulted in a continuous decrease in the solids inventory, leading to an underestimation of the circulation rate when using the integral mass accumulation method. The accuracy of the differential mass accumulation method relied on transient pressure measurements, which were less-effective at low solids flow rates. Conversely, the accumulation time required for pressure measurements was reduced at high circulation rates, resulting in uncertainties in the analysis. The accuracy of the thermal tracing method decreased drastically with higher solids circulation, resulting in an overestimation of the circulation rate. Moreover, low circulation rates adversely affected the accuracy of the magnetic solids tracing by producing barely discernible tracer concentration gradients.
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| 2. |
- Farha, Munavara, 1995, et al.
(författare)
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Characterization of the solids crossflow in a bubbling fluidized bed
- 2024
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Ingår i: Powder Technology. - 1873-328X .- 0032-5910. ; 443
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Tidskriftsartikel (refereegranskat)abstract
- The horizontal transport of solids in bubbling fluidized beds that have a solids crossflow is characterized by applying magnetic tracer measurements and modeling techniques to determine: the contributions of convective and dispersive forms of transport of solids; the residence time distributions of the solids; the feasibility of achieving a plug-flow or well-stirred tank behavior of the solids flow; and the overall fluidization quality. The latter is quantified by determining the extent of de-fluidized zones under varying operational conditions. The experiments are conducted in a bubbling fluidized bed with different rates of forced horizontal flow of solids, applying different bed heights and fluidization velocities. The setup is designed and operated in accordance with Glicksman's full set of scaling laws for fluidized beds, allowing scaling-up of the results to hot, large-scale conditions that resembling, for example, indirect gasification. The assessment of horizontal solids flow involves the sampling of a ferromagnetic tracer using impedance measurements at distinct locations within the bed to: i) fit the convection-dispersion transport equation and, thereby, determine the horizontal dispersion coefficient and velocity of the solids; and ii) feed a deconvolution routine for studying reduced-order (simplified) representations of the solids flow through compartment models. The results from the fitting to the convection-dispersion equation show a strong and close-to-linear correlation between the horizontal solids dispersion coefficient and the forced horizontal solids velocity. This strong interdependency may be attributable to increased shear-related mixing at higher bed-wall shear rates, and it implies a greater challenge linked to attaining a convection-controlled (plug flow) pattern for the solids crossflow. The residence time distributions obtained reveal the limitations of the convection-dispersion equation in providing a general description of the solids flow. Compartment model fitting, when applied to the observed residence time distributions, reveals that an increase in solids crossflow or, to a lesser extent, increased bed height leads to improved fluidization quality.
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