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This is a summary of the development of conversion of solid fuels in fluidized bed during the hundred years that follow the first patent of Winkler in September 1922.The Winkler gasifiers and their followers are described first.Ot...
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This is a summary of the development of conversion of solid fuels in fluidized bed during the hundred years that follow the first patent of Winkler in September 1922.The Winkler gasifiers and their followers are described first.Other fuel converters,such as boilers,appeared only in the 1960-70s and became of interest because of their expected environmental advantages.Initially,bubbling bed boilers were introduced,followed by circulating fluidized bed(CFB)boilers in the beginning of the 1980s.Now,CFB is the dominant technology.The entire development has not been conditioned by technological breakthroughs,but rather by the surrounding conditions:industrial demand,wars,environmental effects,availability and price of fuels.The recent development of the presently rather mature technology depends very much on the necessity to limit greenhouse gas accumulation in the atmosphere.Although fluidized bed technology of fers solutions to reduce CO2 emissions,so far,no decisive line of application has been established for CO2 reduction,except for the use of biomass and waste.
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The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed(CTFB)were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fl...
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The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed(CTFB)were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fluidized bed(HDCFB)riser.The average solids holdup distributions,fluctuations,and probability density distributions of the instantaneous solids holdup signals indicate that the CTFB either more resembles a TFB if the Gs/Ug ratio is large or more resembles an HDCFB if the Gs/Ug ratio is small.The distributions of the bubble occurrence,equivalent bubble size,and bubble phase fraction were extracted from the optical fiber signals of the local solids holdup using the wavelet analysis method.The CTFB was found to have a greater number of smaller bubbles than the bubbling and turbulent fluidized beds and relatively fewer and larger bubbles than that in an HDCFB.The effect of flow dynamics and bubble behaviors on the reaction performance in different types of fluidized beds are compared by analytical calculation based on the K-L reactor model.A superior reactor performance can be achieved using the CTFB reactor for a higher effective reaction rate,higher mass transfer rates,and a lower degree of solids backmixing.
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The relative influence of smooth and abrupt exit configurations on the axial pressure drop profile in circulating Fluidized Bed (CFB) risers has been investigated. Experiments were performed with sand nad FCC catalyst in two cold...
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The relative influence of smooth and abrupt exit configurations on the axial pressure drop profile in circulating Fluidized Bed (CFB) risers has been investigated. Experiments were performed with sand nad FCC catalyst in two cold model CFBs OF 0.1 metre and 0.2 metre riser diameter. The imfluence of the abrupt exit on the profile was observed to extend all the way to the base of the larger diameter riser for sand. However, the abrupt exit influenced only the final 1.5 to 2 metres of the riser length for the case of sand in the narrow diameter riser.
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Transient flow behaviors in a novel circulating-turbulent fluidized bed (C-TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids-volume concentratio...
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Transient flow behaviors in a novel circulating-turbulent fluidized bed (C-TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids-volume concentration, velocity and flux in gas-solid two-phase suspensions. Microflow behavior distinctions between the gas-solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high-density circulating fluidized bed (HDCFB), and the newly designed C-TFB were also intensively studied. The experimental results show that particle-particle interactions (collisions) dominate the motion of particles in the C-TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas-particle interactions (drag forces). In addition, turbulence intensity and frequency in the C-TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating-turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas-solid flow in the C-TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors.
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Transients in a circulating fluidized bed boiler firing biomass are considered. An attempt is made to describe transients with the use of concepts applied in the automatic control theory. The parameters calculated from an analysis...
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Transients in a circulating fluidized bed boiler firing biomass are considered. An attempt is made to describe transients with the use of concepts applied in the automatic control theory. The parameters calculated from an analysis of unsteady heat balance equations are compared with the experimental data obtained in the 12-MW boiler of the Chalmers University of Technology. It is demonstrated that these equations describe the transient modes of operation with good accuracy. Dependences for calculating the time constants of unsteady processes are obtained.
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Liquid phase residence time distribution (RTD) studies have been performed in conventional solid-liquid fluidized bed (SLFB) and solid-liquid circulating multistage fluidized bed (SLCMFB). The riser column was made up of 50 mm i.d...
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Liquid phase residence time distribution (RTD) studies have been performed in conventional solid-liquid fluidized bed (SLFB) and solid-liquid circulating multistage fluidized bed (SLCMFB). The riser column was made up of 50 mm i.d. and 2 m long glass pipe while the multistage down comer column (glass) consisted of seven stages of 100 mm i.d. and 100 mm long sections each having perforated plate as a distributor (having 480 holes of 2 mm diameter). RTD experiments for SLFB were carried out in the column having the same diameter as the downcomer of SLCMFB. RTD has been estimated for both the riser column and the multistage column of SLCMFB. Computational fluid dynamic (CFD) simulations of SLFB and riser section of SLCMFB have been performed to predict the RTD. In all the above cases good agreement was found between the CFD predictions and the experimental measurements. Ion exchange resins and glass beads were used as a solid phase and water as a fluidizing medium. The dispersion characteristics of SLFB and SLCMFB have been investigated for resin particles with size range of 0.36-0.72 mm and glass beads with size range of 0.1 -0.7 mm. It was observed that the liquid phase axial dispersion coefficient depends strongly on superficial liquid velocity, particle size and particle density. Based on the experimental data, empirical correlations have been proposed for liquid phase axial dispersion coefficient and have been found to be applicable to all the available data in the published literature.
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A thermal technique has been developed for measurement of solid mixing in fluidized beds. A batch of bed particles is preheated and then injected into the bed. An array of thermistor probes measures the spread of the tracer. A ser...
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A thermal technique has been developed for measurement of solid mixing in fluidized beds. A batch of bed particles is preheated and then injected into the bed. An array of thermistor probes measures the spread of the tracer. A series of experiments has been done in the upper core region of a 7 m tall, 0.20 m diameter test circulating fluidized bed (CFB). The experiments involved one type of particle, having 180 μm diameter and 2350 kg/m~3 density. Models for the spread of tracer particles, transfer of heat from tracer particles to surrounding gas and particles, and for heat transfer to the thermistors were used to make estimates of the radial particle diffusivity in the core region. The diffusivity was on the order of 10 cm~2/s for the range of velocities and solid concentrations tested. A second set of preliminary solid mixing experiments has been done in the bottom of a 1.8 m tall, 0.16 m square CFB using 55 μm FCC (fluid cracking catalyst) particles. The mixing in this legion, although not representative of a strictly diffusional process, appears to be somewhat more vigorous than the core mixing in the upper region of the bed.
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A fractal approach is adopted to describe the dynamic behavior of a circulating fluidized bed. Two time series, differential pressure fluctuations along the riser height and solids momentum fluctuations along the radial direction,...
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A fractal approach is adopted to describe the dynamic behavior of a circulating fluidized bed. Two time series, differential pressure fluctuations along the riser height and solids momentum fluctuations along the radial direction, are measured and analyzed in terms of fractal dimensions. The influences of operating conditions and axial/radial positions on the fractal dimension are discussed. Attempts are also made to interpret the flow structure in the bed in terms of the fractal dimension. It is found that fractal analysis can provide a useful tool for understanding the characteristics of gas-solids flow in circulating fluidized beds.
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Experimental measurements of the correlation dimension, Kolmogorov entropy, and Lyapunov exponent of circulating fluidized bed (CFB) chaotic attractors were obtained by recording differential pressure and γ-ray porosity time seri...
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Experimental measurements of the correlation dimension, Kolmogorov entropy, and Lyapunov exponent of circulating fluidized bed (CFB) chaotic attractors were obtained by recording differential pressure and γ-ray porosity time series along the height of a cold experimental CFB operating with 75 μm diameter fluid catalytic cracking (FCC) catalyst. The attractor dimension did not vary with respect to the type of measurement taken. Both differential pressure and localized γ-ray densitometry measurements showed the existence of a low order hydro- dynamic attractor, whose dimension varied between 1.5 and 2.0 over the range of gas velocities and solids fluxes studied.
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the downcomer for solids recycle is a key component in circulating fluidized beds. In the case of fast fluidization, the solids circulation rate through the downcomer is quite large compared with that in the conventional cyclone d...
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the downcomer for solids recycle is a key component in circulating fluidized beds. In the case of fast fluidization, the solids circulation rate through the downcomer is quite large compared with that in the conventional cyclone dipleg, resulting in a certain amount of gas flow entrained by downflow of solids, which changes the behavior of the gas-solid flow. The entrained gas flow rate in the downcomer incorporating a riser was determined by using a gas tracer technique. The entrained gar rates for different particle properties, solids circulation rates as well as aeration gas rates were measured and a hydrodynamic correlation was established.
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