Cold Circulating Fluidized Bed Research Articles

Experimental study of solid circulation rate and pressure distribution characteristics in a cold fluidized bed with an external heat exchanger in an municipal solid waste incinerator

AbstractThe external heat exchanger (EHE) can solve the problem of the insufficient heating surface due to the large‐scale circulating fluidized bed (CFB) boiler effectively; furnace temperature and bed temperature can be regulated quickly and accurately with the adjustment of the solids circulation rate (SCR). The ash differs from coal‐fired boilers in particle size, density, and other physical parameters, so the circulation and heat transfer characteristics are not identical. The results of SCR and pressure distribution in a cold CFB experimental system with an EHE in which the height of the partition (HP) could be adjusted were analyzed. Experiments studied the effect of fluidized air velocity in the riser (Ug), HP of the EHE, and the pressure of the supply air chamber (PSA) in the EHE on SCR using quartz sand with an average size of 0.239 mm. It was noted that SCR increased with Ug and PSA in the range of 7.01–28.37 g/s with the increase in Ug, and the maximum value of SCR was obtained when PSA was 750 Pa and the HP was 80 mm. The experiments also noted that the increase in SCR significantly affected the pressure distribution in the region of high solids content (EHE and the region above 600 mm in the riser). A reference to the above characteristics of SCR and pressure distribution variations can be considered for operating commercial CFB waste incinerators.

Experimental study of exit effect on gas–solid flow and heat transfer inside CFB risers

In two bench-scale cold circulating fluidized bed (CFB) risers with similar dimension but different cross section shape, the effect of different exit geometries on both gas–solid flow and heat transfer characteristics was investigated. The measured axial distribution of solid density and bed-to-probe heat transfer coefficient along the riser were found to have good correlation at certain superficial gas velocity and solid circulating flux. Abrupt exit can bring higher solid concentration and heat transfer coefficient near the exit, even in the whole riser. Due to combining effort of cavity and collision effect, solid concentration will first increase with projected height of the abrupt exit increasing. Then solid concentration will stay constant or even decrease when projected height increases over the maximum height particles can reach.Besides exit types and operating conditions, projected height of the abrupt exit, size and shape of cross section are all important factors affecting exit effect. In this paper, exit effect becomes more significant with a circular cross section than a rectangular one.

The Mechanism of Lateral Solid Transfer in a CFB Riser with Pant-Leg Structure

A bench-scale cold circulating fluidized bed (CFB) was set up according to the hydrodynamic scaling law to study the process of bed inventory overturn and the mechanism of lateral solid transfer in...

CFD-simulation of a circulating fluidized bed riser

In the current work, a model of the fluid mechanics in the riser of a circulating fluidized bed (CFB) has been implemented using computational fluid dynamics (CFD). The model developed shall be used in future as the basis of 3D-reactor model for the simulation of large scale CFB combustors. The two-fluid model (TFM) approach is used to represent the fluid mechanics involved in the flow. The computational implementation is accomplished by the commercial software FLUENT. Different closure formulations are tested on a simplified geometry. Two different turbulence formulations, namely the swirl modified RNG k– ɛ model and the Realizable k– ɛ model, are tested in combination with two different approaches to solid phase turbulence, namely the dispersion and per phase approach. One focus of the current work is put on the study of different drag correlations. Besides the drag correlations by Syamlal et al. [Syamlal, M., Rogers, W., & O’Brien, T. J. (1993). MFIX documentation theory guide. Technical Report DOE/METC-94/1004, U.S. Department of Energy (DOE). Morgantown Energy Technology Center: Morgantown, WV] and Gidaspow [Gidaspow, D. (1994). Multiphase flow and fluidization. New York: Academic Press] the EMMS model has been used to determine the momentum exchange between the two phases. The resulting formulation is then used to simulate a 1-m × 0.3-m cold CFB setup and is validated by experimental results [Schlichthärle, P. (2000). Fluid dynamics and mixing of solids and gas in the bottom zone of circulating fluidized beds. Unpublished doctoral dissertation, Technische Universitaet Hamburg-Harburg, Shaker Verlag: Aachen].

Experimental study on the spatial distribution of particle rotation in the upper dilute zone of a cold CFB riser

Particle rotation plays an important role in gas-solid flows. This paper presents an experimental investigation on the spatial distribution of average rotation speed for glass beads in the upper dilute zone of a cold circulating fluidized bed (CFB) riser. It is shown that in the horizontal direction, the average rotation speed in the near-wall area is larger than that in the center area, while in the vertical direction, it decreases as the height increases. The reason resulting in this distribution is analyzed by considering several factors including particle size, particle shape, particle number density, particle collision behavior, and the surrounding flow field, etc. The effects of CFB operation conditions on the spatial distribution of average rotation speed are also studied. The results show that the increasing superficial gas velocity increases the average rotation speed of particles in the near wall area but takes nearly no effect on that in the center area. The external solids mass flux, however, takes the opposite effect. It is found that the average rotation speeds of particles in both areas are increased as the total amount of bed material increases.

Measurement on particle rotation speed in gas–solid flow based on identification of particle rotation axis

When a two-dimensional (2D) imaging system is used to visualize particle motion in a 3D gas–solid flow, the particle rotation speed was found extremely difficult to be accurately measured due to the fact that the direction of rotation axis was usually random and hard to be distinguished. The paper presents a method to calculate the particle rotation speed from particle images based on the identification of its rotation axis using two or more characteristic points on its surface. The idea was analyzed and realized in a mathematical way and based on which a calculation program was given. The measurement method was verified with an experiment using a small sphere with known rotation axis and rotation speed. The effects of several factors, including the direction of the particle rotation axis, the particle image resolution, the types and positions of characteristic points, etc., on the measurement error are discussed. The error is found to be acceptable for most cases. The measurement method was finally applied to those small glass beads in a real 3D gas–solid flow inside a cold circulating fluidized bed (CFB) riser, which indicates that the problems of 2D imaging system applying to 3D particulate system could be solved by using this mathematical method.

Particle Holdup and Average Residence Time in the Cyclone of a CFB Boiler

AbstractParticle holdup and the average residence time in the cyclone of a Circulating Fluidized Bed (CFB) boiler are important information for describing events post‐combustion in the cyclone that often lead to a noticeable increase in the temperature of the flue gas. The existing results for the variation of particle average residence time with fluidizing gas velocity are contradictory since they were obtained under different operation conditions. A cold CFB apparatus made of plexiglass was established with a riser of 5 m in height and 0.2 m in diameter and equipped with a standard Lapple cyclone. The particle holdup was directly measured by the mass in the cyclone when the system was shut down. The solid concentration at the cyclone inlet was kept in the range generally used in CFB boilers. The experimental results showed that the particle holdup in the cyclone was equal to ca. 10–40 % of the corresponding bed material in the riser and that it increases monotonously with both the fluidizing gas velocity and the initial static bed height, and approximately linearly with the solid circulation rate. In addition, within the experimental conditions, the cyclone pressure drop increases monotonously with particle holdup. It was found that the average residence time of the particles either increased or decreased linearly with the fluidizing gas velocity, depending on the initial static bed height. Nevertheless, both variation rates were very small. In a view of engineering applications, the average residence time of the particles in the cyclone is insignificantly affected by the fluidizing gas velocity, initial bed inventory and solid circulation rate, within the range of experimental conditions examined.

Effect of the bottom-contracted and edge-sloped vent-pipe on the cyclone separator performance

Abstract The influence of the bottom-contracted and edge-sloped vent-pipe on the separation efficiency and pressure drop of a cyclone separator under different vent-pipe insert depth and different orientation of the sloped edge were studied in a visual cold circulating fluidized bed (CFB) experimental setup according to a commercial 100 MW CFB boiler with a scale of 20:1. And the correlative results were also compared with the traditional linear-pipe-shaped cyclone separator. Results indicate that the cyclone inlet stream velocity has a strong influence on the separation efficiency and pressure drop, and the results are similar to that of the conventional cyclone. Namely, both separation efficiency and pressure drop increase with increasing cyclone inlet stream velocity. The separation efficiency of the modified cyclone separator increases firstly and then decreases with the increasing of the vent-pipe insert depth. However, there is not a very clear rule of the effect of vent-pipe insert depth on the pressure drop for modified cyclone separator. Both the separating efficiency and the pressure drop change with the orientation of the sloped edge, and they have the same rule of change, the maximum at 90° and the minimum at 270°. Due to the configuration of the bottom-contracted and edge-sloped vent-pipe is suitable with the flow field inside cyclone separator, the separation efficiencies of the modified cyclone separator are usually higher than those of traditional cyclone separators.

Production of Gaseous Fuel in Circulating Fluidized Bed Gasification Process Employing Refuse-Derived Fuels

A fuel gas production system involving gasification of refuse-derived fuel (RDF) in a circulating fluidized bed (CFB) was explored. The pyrolysis behavior of three kinds of RDF and the particle motion in a cold model CFB were experimentally studied to contribute to the design of a CFB gasification system and to clarify operation factors for optimum control as the first step of R&D on the proposed system. A rise in pyrolysis temperature improved the yield of the combustible gas components and/or the energy recycle efficiency from RDF, but the highest heating value of pyrolysis gas was obtained at temperatures of 873 to 973 K. The total gas yields could be expressed as a function of temperature and volatile matter content in raw materials regardless of the type of RDF. The gas flow rate in the pneumatic valve of the CFB was found to be an important factor to control the circulation rate and the holdup of particles in the cold model riser. For silica sand whose median diameter was 100 μm, a dense bed was formed at the bottom of the riser in the superficial gas velocity explored in the present work, 1.65-2.12 m/s. A gas velocity of 1.65 m/s increased the holdup significantly in the dense bed and could maintain the residence time of RDF needed for the conversion by gasification of the RDF. The pyrolysis test of RDF in CFB was carried out, and inflammable gas with 23.6 MJ/m3 yielded under the riser temperature 1030 K.

Measurements of local solids volume-fraction in fluidized bed boilers

An optical probe has been developed for measurements of solids volume-fraction in high-temperature fluidized beds. The probe consists of two optical fibers: one for exposing the bed particles to monochrome laser light, and the other to receive the light reflected by the particles (backscatter principle). Optical filtering and amplitude modulation of the light source reduce the effect of background radiation. The probe was used in an electrically heated bubbling fluidized bed without combustion, in a cold circulating fluidized bed (CFB) and in the transport zone of a CFB boiler. The results from the electrically heated bed showed that the probe was capable of detecting bubbles at temperatures up to 800°C (maximum operating temperature of the unit). Thus, the optical filtering and modulation techniques were sufficient to eliminate the effects of background radiation. The CFB boiler results (850°C) gave a wall-layer thickness in agreement with corresponding results from a momentum probe and previous solids–flux measurements. Statistical analysis of the optical probe signals showed that the downward solids flux at the wall might consist of clusters but their solids volume-fraction is not uniform. Instead, there is a continuous variation in solids volume-fraction with time, and cluster definitions suggested in literature cannot be applied to the conditions investigated.

Investigation on horizontal mixing of particles in dense bed in Circulating Fluidized Bed (CFB)

A two dimensional cold CFB test rig has been established, investigation on horizontal mixing of particles in dense bed has been carried out on this test rig. Mixing model has been used in data reduction, the horizontal mixing coefficients of particles in different experimental conditions and in different structures of dense bed have been obtained and compared. By using dimensional analysis, non-dimensional expression of experimental condition and mixing coefficient have been obtained.

Exit effects in circulating fluidized bed risers

AbstractThe 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 and FCC catalyst in two cold model CFBs of 0.1 metre and 0.2 metre riser diameter. The influence 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. When FCC catalyst was circulated, the effect of the abrupt configuration was seen only in the upper portion of the riser, irrespective of the riser diameter. A simple empirical modification to the model of Pugsley and Bermti (1996) provides a good description of the axial pressure drop profiles for the abrupt exit configurations, with the exception of the case of sand in the 0.2 metre diameter riser. This discrepancy is attributed to the lack of fully developed flow in the larger riser.

Dimension measurements of hydrodynamic attractors in circulating fluidized beds

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 hydrodynamic attractor, whose dimension varied between 1.5 and 2.0 over the range of gas velocities and solids fluxes studied. Differential pressure measurements indicated that the attractor dimension decreased slightly in the lower section of the CFB and at higher solids fluxes. Localized radial γ-ray bed porosity measurements indicate that the attractor dimension did not significantly vary across the bed cross-section but did show a tendency to be slightly lower near the riser wall.

A comparison of laser Doppler and single-fibre reflection probes for the measurement of the velocity of solids in a gas-solid circulating fluidized bed

In this study, an attempt has been made to compare an intrusive solids velocity measurement technique with the laser Doppler anemometer (LDA) which is a typical non-intrusive solids velocity measurement technique. Single-fibre reflection (SFR) probe and LDA measurements were directly compared under identical operating conditions in the upper dilute zone of a circulating fluidized bed (CFB). This experimental unit was a cold model CFB (inner diameter of the riser, 0.4 m; height, 15.6 m). The measurements were taken at a height of 6 m above the distributor. Quartz sand with a mean diameter of 166 µm was fluidized at a superficial gas velocity of 4 m s −1. The cross-sectional average solids volume concentration was 1.2 vol.%. The highest instantaneous local solids concentration was 5 vol.% close to the riser wall. LDA measurements were taken with a miniaturized measuring head (diameter, 14 mm) operating in the backscatter mode and inserted in a probe casing (diameter, 22 mm). The focal point of the laser beams was located at a distance of 19 mm from the probe casing window. This allows the LDA measuring head to be moved through the fluidized bed and valid LDA measurements to be acquired inside relatively dense suspensions. The fibre-optical probe measurements were taken with a two-channel SFR probe (fibre: diameter, 600 µm; probe: diameter, 8 mm; centre-to-centre distance between the two sensing surfaces, 2.3 mm). The measurements at a superficial gas velocity of 4 m s −1 showed mean solids velocities between 10 m s −1 upwards and 3ms −1 downwards. The LDA system faces problems in the regions of higher solids concentration located close to the riser wall where data acquisition rates decrease considerably. The good agreement between the local average solids velocities acquired with the two systems proves that intrusive probe measurements cause negligible distortion of the solids velocity Pattern in a CFB environment.