With the development of circulating fluidized beds (CFB) and dense upflow bubbling fluidized beds (UBFB) as chemical reactors, or in the capture and storage of solar or waste heat, the associated downcomer has been proposed as an additional heat transfer system. Whereas fundamental and applied research towards hydrodynamics has been carried out, few results have been reported on heat transfer in downcomers, even though it is an important element in their design and application. The wall-to-suspension heat transfer coefficient (HTC) was measured in the downcomer. The HTC increases linearly with the solids flux, till values of about 150 kg/m2 s. The increasing HTC with increasing solid circulation rate is reflected through a faster surface renewal by the downflow of the particle-gas suspension at the wall. The model predictions and experimental data are in very fair agreement, and the model expression can predict the influence of the dominant parameters of heat transfer geometry, solids circulation flow, and particle characteristics.
Compared to the moving bed and downcomer concepts, introducing relative particle movement, such as in BFB, UBFB, or CFB, improves the heat transfer and gas/solid mixing, but abrasion, erosion, and particle elutriation increase with increased gas velocity. BFB and UBFB, operating with A-type powders at low velocity, provide an excellent balance between high heat transfer rates and particle stability. CFB systems offer the advantage of operating at high solid circulation fluxes (
In solar energy applications, the loop is generally composed of a CFB or UBFB heat receiver, a hot storage silo fed from the discharge of the receiver, a downcomer with wall-mounted heat exchanger, and finally feeding a fluid bed heat exchanger (FBHE), where the particles transmit their energy to submerged tubes inside whom a working fluid (e.g., steam) is generated and further expanded in a turbine: the FBHE is a common device in the electrical power industry (mostly implemented for coal and biomass combustion in fluidized beds). A tentative layout is given in Figure
Layout for various applications.
An additional use of the downcomer for heat transfer is illustrated by Van de Velden et al. [
Velocity profiles in the downcomer, for downwards flow
In the downcomer, there are two hydrodynamic flow sections along the axial direction: the acceleration section below the feeding point of the downcomer and the constant velocity section, as illustrated by Zhu et al. [
The axial solids holdup and heat transfer variations also follow the two-section pattern. The average suspension density decreases along the bed depth and so does the heat transfer coefficient: both profiles of heat transfer and solids holdup decrease sharply in the first acceleration section, and then the trend becomes smooth further down the bed. In the constant velocity section, the heat transfer coefficient and solids holdup become almost constant.
The existence of both zones is illustrated when calculating the air velocity in the downcomer. The Ergun equation [
If, for a given
Relevant literature findings concerning the hydrodynamics and heat transfer in downcomers are summarized in Table
Literature review of downcomer operation and heat transfer.
Van de Velden et al., 2008 and 2010 [ |
The downcomer of a CFB is used to supply the endothermic heat of pyrolysis of biomass (450–550°C), through wall-to-sand preheating |
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Brems et al., 2013 [ |
The downcomer of a CFB pyrolysis reaction of solid (wall-to-bed) plastic waste can be used to supply the endothermic heat of pyrolysis by heating the sand carrier to appropriate temperatures |
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Baumann et al., 2012 and 2014 [ |
The effect of powder properties on their use as heat transfer media in a moving bed heat exchanger (with embedded tubes) was investigated in view of concentrated solar power applications |
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Baird et al., 2008 [ |
Empirical and model fittings of experimental data from the wall to a moving bed of nickel pellets were investigated |
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Zhang et al., 1999 [ |
Characterization of local and overall gas-solid flow structure by measuring the distribution of local solids holdups and pressure gradients along the downcomer |
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Zhang and Zhu, 2000 [ |
Local solids fluxes were also calculated from the local particle velocities and solids holdups |
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Ball and Zhu, 2001 [ |
The effect of gas velocity, solids circulation rate, and axial and radial positions on the local solids flux in a gas-solids downcomer of a fluidized bed |
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Chen and Li, 2004 [ |
Probability density distribution was studied through low and high density downcomer operations and confirmed that solids flux is affecting the solids holdup |
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Lehner and Wirth, 1999 [ |
Experimental investigations concerning the local and cross-sectional solids distribution were conducted under different operating conditions and with different solids |
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Kim et al., 2001 [ |
The effects of operative conditions of subbituminous coal gasification in a downcomer reactor were experimentally determined |
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Ma and Zhu, 1999 [ |
Local heat transfer was investigated in a gas-solid concurrent downflow downcomer of a fluidized bed with FCC particles. HTC is closely related to the hydrodynamics, with bed suspension density being the most influential factor |
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Tamarin and Gorbachev, 1968 [ |
Heat transfer coefficient between a bed of moving slag particles and a vertical surface was experimentally determined in several gas atmospheres |
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Kim et al., 1999 [ |
Bed-to-wall heat transfer coefficient was determined in a downcomer reactor and results showed suspension density, gas convection, and particle size as influential factors. A model was proposed to predict the bed-to-wall HTC |
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Lehner and Wirth, 1999 [ |
The effect on the local and overall solids circulation in a downcomer was studied for various gas/solids distributors |
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Obuskovic, 1988 [ |
HTC was measured for a single vertical tube immersed in a moving packed bed of glass beads, sand, or copper in air at atmospheric pressure in order to obtain a general predictive equation |
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Peters and Dziugys, 2012 [ |
Heat transfer prediction in a fixed and moving packed bed by extended discrete element method |
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Basu et al., 2013 [ |
Investigation of heat transfer to cross and vertical tubes in a standpipe of a circulating fluidized bed boiler. A model provides a fair agreement with experimental results |
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Niegsch et al., 1994 [ |
The heat transfer of a steam heated tube bundle in a moving bed was investigated. Detailed heat transfer phenomena were described and a modelling approach was proposed |
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Meier et al., 2009 [ |
An alternative solution to heat and mass transfer problems was presented, by casting the system of equations into a matrix of the Sturm-Liouville type |
The riser and downcomer are depicted in Figure
Layout of the experimental setup:
The experiments consisted of starting the gas flow to the riser, followed by the flow of the fluidizing gas to the solids
The axial pressure profile was recorded during each experiment in order to make sure that the suspension entering the heated section was in fully developed flow conditions, hence with a constant
From the known exposed surface area,
Average values were determined from triplicate repeats, with maximum deviations of + or −4%.
Air was used to convey sieved rounded sand, ranging in average particle size between 128
Powder characteristics.
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Sand 128 | 128 | 2620 | 1446 | 0.016 | 0.48 |
Sand 190 | 190 | 2620 | 1542 | 0.044 | 0.45 |
Sand 260 | 260 | 2600 | 1520 | 0.080 | 0.42 |
Limestone | 200 | 2600 | 1560 | 0.048 | 0.45 |
The absolute particle density (
The solids mass flow rate down the downcomer was determined from weighing the short-sequence discharge of the
Smolders and Baeyens [
Maximum downcomer discharge rate (
Within the rather narrow particle size range investigated, the Rausch and Smolders-Baeyens predictions slightly underestimate and overestimate the experimental findings, respectively. The Carleton and Beverlo predictions are less appropriate. Since the Smolders-Baeyens [
The measured HTC appears to be a function of the solids down-flux, as illustrated in Figure
Downcomer wall-to-bed heat transfer coefficient versus solids flux for 128
The value of HTC appears to level off at high values of
Linear fit of HTC for various particles.
The heat transfer coefficient increases with increasing particle size, although not proportionally. This increasing heat transfer coefficient with increasing particle size is opposite to what is normally measured in bubbling fluidized beds, where the heat transfer coefficient increases with decreasing particle size.
The particle velocity is linked to the solid circulation flux,
Previous research has already indicated that the particle residence time at the wall, which is controlled by the particle velocity, is a major factor affecting the heat transfer coefficient [
Since the particle velocity can be easily determined from
The general expression of the surface renewal model [
In BFB operations, the contact times at the surface are small (0.1–0.5 s), due to the bubble-induced particle mixing. The heat transfer resistance upon contact,
With
With
This equation was also developed by Goossens [
Applications of (
Comparison between model prediction (−) and experimental data (■).
The model prediction consistently underestimates the HTC, although by 10% only at high values of
The model equation however demonstrates that the heat transfer coefficient increases with increasing particle size (all parameters of
The values of the heat transfer coefficients in the downcomer are, while lower, not considerably lower than those in the riser [
The wall-to-suspension heat transfer coefficient was measured in the CFB downcomer. The HTC increases linearly with the solids flux. The increasing HTC with increasing solid circulation rate is reflected in an increased surface renewal at the wall. The model predictions and experimental data are in very fair agreement. The model expression can hence tentatively predict the influence of the dominant parameters of heat transfer geometry, solids circulation flow, and particle characteristics.
Surface area of heat exchanger [m2]
Specific heat of particles [J/kg K]
Diameter of downcomer [m]
Average sieve size and average surface/volume diameter of particle, respectively [m]
Mass flux of solids through the riser or downcomer, respectively [kg m−2 s−1]
Heat transfer coefficient [W m−2 K−1]
Heat transfer coefficient upon contact of particles and wall [W/m2 K]
Thermal conductivity of fluidizing gas [W/m K]
Height of downcomer heat exchanger surface [m]
Total length of downcomer [m]
Heat input to downcomer wall [W]
Superficial gas velocity [m s−1]
Minimum fluidization velocity of powder [m s−1]
Superficial gas velocity of transition to the CFB regime [m s−1]
Superficial gas velocity in the downcomer [m s−1]
Absolute velocity of solids in the downcomer [m s−1]
Relative velocity between solids and gas in the downcomer [m s−1]
Pressure drop of downcomer [Pa m−2]
Temperature difference of the downcomer wall and the bulk of downflow solids [K]
Packing arrangement of particle/gas emulsion film at the downcomer wall [—]
Average surface renewal time of particles at the heat exchanger wall [s]
Voidage and voidage of powder at
Density of gas, particle, and bulk bed, respectively [kg m−3]
Viscosity of gas [kg m−1 s−1].
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was performed in the framework of the CSP2 (Concentrated Solar Power in Particles) project, funded by the European Commission (FP7, Project no. 282 932). The author Huili Zhang gratefully acknowledges the China Scholarship Council for sponsoring her Ph.D. study at KU Leuven in Belgium (File no. 201206880024).