Experimental and Simulation Study of Fluidization Behavior of Palm

Article pubs.acs.org/IECR

Experimental and Simulation Study of Fluidization Behavior of Palm Biomass in a Circulating Fluidized Bed Riser Ahmad Hussain,*,† Iqbal Ahmed,*,‡ Hani Hussain Sait, § Mohamed Ismail Bassyouni,∥,⊥ Abdelkarim Morsy Hegab, § Syed Waheed ul Hasan,∥ and Farid Nasir Ani# †

Department of Nuclear Engineering, King Abdulaziz University, Jeddah 21589, Saudi Arabia Chemical Engineering Department, Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia § Department of Mechanical Engineering, King Abdulaziz University, Rabigh 21911, Saudi Arabia ∥ Department of Chemical and Materials Engineering, King Abdulaziz University, Rabigh 21911 Saudi Arabia ⊥ Department of Chemical Engineering, Higher Technological Institute, Tenth of Ramdan City, Egypt # Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor, Malaysia ‡

ABSTRACT: Circulating fluidized bed combustion (CFBC) technology is being used for few decades to burn biomass wastes while meeting the stringent emission standards; however, much is needed to be understood about the complex flow patterns that are encountered. This work is aimed at understanding the major fluidization characteristics in a CFB loop for palm shell waste powders. Effects of inlet air on the fluidization behavior were experimentally investigated. The voidage of the bed material was found to be a function of axial distance above the distributor. The CFD modeling has also been done to quantify the riser exit effects. The flow was simulated using the “algebraic slip mixture (ASM)” model. In the simulation results the deviation of velocity contours owing to variation in riser exit geometries is discussed. It was found that the influence is found to be significant in the upper region of the riser column and the velocity contours are also swayed by the exit geometry. Principally, Brereton and Grace7−9 have performed very inclusive study to date in superficial solids suspension density in risers with smooth and rapid exits. They determined that rapid exits internally reflect a considerable portion of the incoming core solids down the riser wall, whereas smooth exits let the typical of up flowing solids to exit the riser.8 Brereton and Grace established that the exit geometry can influence pressure and density profiles along a considerable length of the riser. Thus, a large number of investigators have reported apparently differing results concerning the influence of the riser exit.1,9,10 Most of these studies were executed in cold, laboratory-scale units particularly of circular and square cross-section risers.9 Contemporary, Harris and Davidson9 have summarized a very comprehensive report of the previous researchers on the influence of riser exit geometry on the riser axial solids concentration profile in circulating fluidized bed. Therefore, this study is focused on determining the fluidization behavior of palm shell waste which is available in abundance in a number of regions of the world. The high degree of fuel flexibility that characterizes many designs of CFBC often allows a plant operator to select fuels on the basis of what may be currently available at an economic price and, where appropriate, produce a fuel blend that combines several such elements.11 The solids holdup, the length of the acceleration section, hydrodynamic mixing, and transfer phenomena in risers are all influenced heavily by the riser

1. INTRODUCTION A circulating fluidized bed is an advantageous alternative for combustion of solid fuels because of its high fuel flexibility and because it is possible to control the combustion temperature.1 Not only is the performance of a CFB boiler influenced by the mixing of gas and particles but also operating variables for a CFB include both the gas flow rate and the solids flow rate.2 Upright arrangement of the gas and solids flow structures in riser reactors is critical for proper industrial design. Essentially the hydrodynamics study of the riser, inside of which a suspended gas−solid flow takes place, is a key part of a CFB system.1,2 Recently Wang et al.2 studied electrical capacitance volume tomography imaging (ECVT) of three-dimensional flow structures and solids concentration distributions in a riser and a bend of a gas−solid CFB. In their study they discussed the flow structures in the riser and the bend investigated on the basis of quantitative ECVT images. CFB technology is now finding applications in biomass combustion. Compared with the other renewable energy resources, biomass is abundant in annual production, up to 2740 quads (1 quad = 1015 Btu), with geographically widespread distribution in the world.3 Wang et al.4 experimentally investigated the flow structure in a circulating fluidized bed (CFB). They found that the solid concentration decreases with the increase of the superficial gas velocity. Zaabout et al.5 investigated the behavior of the solid phase in the upper zone of a circulating fluidized bed riser. It was concluded that superficial gas velocity has a larger effect on descending particles at the wall. Miao et al.6 used sawdust as the bed materials to investigate its fluidization characteristics in a cold model circulating fluidized bed. They concluded that the shapes of the sawdust affect the operating conditions. © XXXX American Chemical Society

Received: June 11, 2013 Revised: November 8, 2013 Accepted: November 15, 2013

A

dx.doi.org/10.1021/ie401856b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Table 1. Dimensions of Various Sections of the Test Rig sr. no.

section of the rig

details of dimensions

1

riser section

height, 910 mm; width, 265 mm; length, 72 mm

2

inlet air distributor (bubble cap type)

3 4

inlet air distributor (nozzle type) primary cyclone

no. of caps, 23; height of cap, 70 mm; external/internal diameters, 18 mm/10 mm; area of plate, 170 mm × 510 mm; air holes size, 2 mm/(8 holes/cap) diameter of nozzle, 25 mm; no. of nozzles, 6; area of plate, 170 mm × 510 mm

5

secondary cyclone

inlet diameter, 150 mm; exit diameter, 38 mm; length, 555 mm

6

L-valve

diameter, 66 mm; length, 430 mm

inlet diameter, 230 mm; exit diameter, 66 mm; length, 1090 mm

materials Plexiglas, wood stainless steel stainless steel stainless steel stainless steel flexible pipe

connector. The number of tappings available in the riser section is 7 while in the L-connector, 2. The pressure taps were made airtight by using sealing material. All of the pressure taps fed to multitube manometer tubes. The pressure tap through the riser wall had a copper tube inserted into it. The locations of the pressure measuring taps in the riser section were at the height of 0.3, 0.6, 1.02, 1.52, 1.85, and 2.25 m, which were measured from the base of the first riser section. The pressure profiles are used to calculate axial solids volume fraction profiles, by application of

inlet design. Factors that also affect riser bottom operations include the condition and rate of entering solids, the arrangement of main air and secondary air inlets at the riser bottom, and so on.12 Hu et al.13 experimentally studied the effects of riser height and solids inventory on the gas−solids flows in a CFB. They were able to identify an S-shaped voidage profile. Breault14 did the analysis of clustering flow in a CFB riser using numerical computations. The change in the Gibbs free energy was related to cluster size. A cold CFB test rig which has a rectangular riser made of Plexiglas has been used for the study of fluidization behavior as it enabled the visualization of the flow structure inside the riser. Usually a square or rectangular cross-section is common for industrial CFB combustors. By contrast, basic scientific work on CFBs is usually done on risers of circular cross-section, to simplify the flow pattern. The study investigated the hydrodynamics and wall edges effect in the riser section. This is a novel detailed investigation on the fluidization behavior of palm shell waste powders in a CFB. Computational fluid dynamics (CFD) is used investigate various industrial riser exit designs as it can seriously affect the gross behavior of a CFB.

εs̅ = −

1 p2̇ − p1̇ ρs g z 2 − z1

(1)

In the preceding equation, εs̅ is the cross-section average solids volume fraction, (z2 − z1) is the difference in elevation between two consecutive pressure tappings, and (ṗ2 − ṗ1) is the pressure differential between the two elevations.

3. SIMULATION OF FLUIDIZATION IN A CFB The mathematical complexities of the nonlinearity of the equations and defining the interpenetrating and moving phase boundaries make numerical solutions very difficult. Thus, CFD has emerged as a very promising tool in modeling hydrodynamics in fluidized beds. While it is now a standard tool for single-phase flows, it is at the development stage for multiphase systems, such as fluidized beds.16 Simulations were performed by Jalil et al.17 in a fluidized bed with the presence of air and sand using FLUENT 4.56. The research was carried out at various velocities. The performance of the bed was better at higher gas velocities. Many researchers have simulated a threedimensional two-fluids CFD model of gas particle flow in the CFB using the code CFX-4.3. They have modeled turbulence by the k−ε turbulence model in the gas phase and a fixed particle viscosity model in the solid phase. This CFD model showed good agreement with the experiment.18 Experimental study confirms that a dilute phase region exits at the middle of the riser and dense phase at the top and bottom. The numerical results obtained by a revised drag coefficient model that simulates the axial average pressure drop and the apparent solid volume fraction are better than those of the other models, and its results are in better agreement with the experiment results. A wide range of experimental riser exits have been reported in the literature. The most common geometries used in industrial CFBCs are right angle exit, right angle exit with baffle, and blind T exit. The findings of Weinstein et al.19 and Wu and Alliston20 indicated that riser exits can affect the gross behavior of a CFB. Some riser exits

2. EXPERIMENTAL SECTION 2.1. Design of Cold CFB Test Rig. In order to study fluidization behavior of a CFB, a laboratory-scale cold CFB test rig was used to fluidize various solids, e.g., sand, palm shell waste, and rice husk, etc. The CFB test rig consists of an air supply device, a distributor of stainless steel, a see-through riser column of Plexiglas, primary and secondary cyclones of stainless steel, and a solid feeding system. The typical arrangement of the CFB test rig and the design features are illustrated elsewhere,15 and dimensions of various sections of the test rig are summarized in Table 1. 2.2. Biomass Materials and Preparation. Oil palm shell wastes are one of the major agricultural wastes in Malaysia. The samples of these oil palm wastes were taken from the palm oil mill of the Federal Land Development Authority (FELDA), at Kulai, Johor. The oil palm shell wastes were found to have an average size of 8 mm while the size of the shell particles varied from 4 to 13 mm. In order to make the particle size suitable for fluidization experiments in a CFB, which can vary from 300 to 850 μm, the shell particles were grinded and sieved to get palm shell powders of 600 μm size for experimental purpose. 2.3. Measurement of Axial Voidage Distribution in CFB. The axial voidage distribution in circulating fluidized beds is an important parameter to be determined. A multitube manometer was used to measure the axial pressure at various pressure tappings provided in the riser section and the LB



Article

as the pressure equation are solved. The coefficient of restitution quantifies the elasticity of particle collisions. It has a value of 1 for fully elastic collisions and 0 for fully inelastic collisions. It is utilized to account for the loss of energy due to the collision of particles, which is not considered in the classical kinetic theory. The restitution coefficient is close to unity. In this study, a particle−particle restitution coefficient of 0.95 and a particle−wall restitution coefficient of 0.9 were used. Various authors22−27 have assumed that the pressure gradient at an axial position is proportional to the amount of solid at that position. Thus, the simulation model accounts for the axial and radial distribution of voidage and velocity of the gas and solid phases, and for the solids volume fraction and particle size distribution of the solid phase. It is important to understand the characteristic of the inlet design because this is the area where strong momentum interaction of gas−solid flow occurs. The behavior of gas and solids in the bed is influenced by the upward airflow which was captured using a digital camera to identify the flow pattern in the riser. The simplest ASM formulation is the so-called drift flux model, in which the acceleration of the particle is given by gravity and/or a centrifugal force and the particulate relaxation time is modified to take into account the presence of other particles. From the continuity equation for the secondary phase one can obtain the volume fraction equation for the secondary phase:

appear to invoke regions near the riser wall where the motion of solids tends to be upward. Some riser exits may invoke cavities or regions where solids are disengaged from the main flow. In the FLUENT21 computer program, the governing equations were discretized using the finite volume technique. The discretized equations, along with the initial and boundary conditions, were solved to obtain a numerical solution. Thus in this study the model used for simulating the gas−solid flow is the algebraic slip mixture (ASM) model. The ASM model solves the continuity equation for the mixture, the momentum equation for the mixture, and the volume fraction equation for the secondary phase, as well as an algebraic expression for the relative velocity. In order to account for the effects of turbulent fluctuations of velocities, the number of terms to be modeled in the momentum equations in multiphase is large, and this makes the modeling of turbulence in multiphase simulations extremely complex. The turbulence model used for the current simulations is based on mixture turbulence model (MTM). The κ and ε equations describing this model are as follows: ⎛ μt,m ⎞ ∂ (ρm κ ) + ∇(ρm vm⃗ κ ) = ∇⎜ ∇κ ⎟ + Gκ ,m − ρm ε ∂t ⎝ σκ ⎠

(2)

∂ (ρ ε) + ∇(ρm vm⃗ ε) ∂t m ⎛ μt,m ⎞ ε = ∇⎜ ∇ε⎟ + (C1εGκ ,m − C2ερm ε) ⎝ σε ⎠ κ

(3)

∂ ∂ ∂ (αpρp ) + (αpρp um, i) = − (αpρp uDp , i) ∂t ∂xi ∂xi

where the mixture density and velocity, ρm and vm⃗ , are computed from

At the inlet, all velocities and volume fractions of both phases are specified. The meshing was done using Gambit. Fine meshing was done for riser inlet and exit sections in order to analyze them in a better way. Underrelaxation factors were tuned to achieve convergence. The main parameters of the flow inside the system are calculated using an iteration calculation procedure performed by FLUENT. An iterative cycle starts with the introduction of the initial data and/or initial guessed values, boundary conditions, physical conditions, and constants. In a second step the program calculates the velocity field from the momentum equation. Then, the mass balance equations as well as the pressure equation are solved. The swirl modified RNG k−ε model and the realizable k−ε model are used in combination with the dispersion and per phase approaches to solid-phase turbulence. The simulation results are in good agreement with the experimental results. The model can predict complex gas−solid hydrodynamics. The model results pertaining to axial voidage profile and radial particle velocity profiles are compared with the experimental results and found to be in agreement within the high-density fast fluidization regime.

N

ρm =

∑ αiρi i=1

(4)

N

vm⃗ =

∑i = 1 αiρi vi⃗ N

∑i = 1 αiρi

(5)

The turbulent viscosity, μt,m, is computed from

μt,m = ρm Cμ

κ2 ε

(6)

and the production of turbulence kinetic energy, Gk,m, is computed from G k,m = μt,m (∇vm⃗ + (∇vm⃗ )T )∇vm⃗

(8)

(7)

At the inlet, all velocities and volume fractions of both phases are specified. The pressure is not specified at the inlet because of the incompressible gas-phase assumption (relatively low pressure drop system). The meshing was done using Gambit. Fine meshing was done for riser inlet and exit sections in order to analyze them in a better way. Underrelaxation factors were tuned to achieve convergence. The convergence tolerance was set at 0.001. The main parameters of the flow inside the system are calculated using an iteration calculation procedure performed by FLUENT. An iterative cycle starts with the introduction of the initial data and/or initial guessed values, boundary conditions, physical conditions, and constants. In a second step the program calculates the velocity field from the momentum equation. Then, the mass balance equations as well

4. RESULTS AND DISCUSSION The experimental observation has led to the understanding that the riser flow typical of the upward solids motion in the core of the riser and downward motion along the walls is characterized as core/annulus, or C/A flow, as observed by many researchers.7,22,28 It was also observed that there is continuous formation and disintegration of clusters and that clusters were formed in a variety of shapes and size. The number of clusters decreases with increasing elevation, especially near the center of C



Article

Figure 1. Fluidization regimes of palm shell waste powders using bubble cap distributor.

Figure 2. Fluidization regimes of palm shell waste powders using nozzle type distributor.

showed that when fluidizing gas velocity exceeded the transport velocity, an S-shaped voidage profile characterized by fast fluidization was established in the riser. Visual observations of the flow in the riser exit suggest dunes of significant size in the riser exit connector. It appears that solids in the horizontal connector may settle under gravity, which means that the remainder of the suspension is accelerated. Acceleration leads

the cross-section. The cluster size near the wall decreases with increasing elevation. The conducted experimental setup to study effects of riser height and total solids inventory on the gas−solids is an ultratall CFB riser. The influences of total solid inventory and fluidizing gas velocity on the axial voidage profile along the riser and the solid circulation rate were investigated. Experimental results D



Article

airflow rates. One can expect to have higher pressure drops at higher airflow rates because the dense gas−solids phase is wellaerated and can deform easily without appreciable resistance. Figure 3 suggests that the pressure in the riser section increases as the primary airflow rate is increased. Due to the

to a higher solids velocity in the cyclone and improves its efficiency. The distributor designs have a strong influence on the fluidization behavior inside the riser. In this study, we have used the nozzle type distributor and bubble cap type distributor to see the effect of primary air distribution inside the riser. Palm shell powders were used as fluidizing particles. In order to observe the fluidization behavior inside the distributor section, a certain amount of 600 μm palm shell powder was introduced into the riser section. The primary air could be adjusted using the bypass valve for fluidizing air. In the start of the experiment, the primary air from the blower was adjusted to a minimum. For observing the fluidization behavior in different distributor designs, the riser connector was fabricated of Plexiglas which can offer visual observations and digital imaging of the fluidization phenomena. The primary airflow rate was varied from a minimum measure of 0.0076 m3 s−1 (30 cfm) to an excess of 0.0015 m3 s−1 (60 cfm). These variations of primary flow have resulted in fluidization behavior starting from the onset of the bubbling phenomena to fast fluidization regimes. These transitions of regimes have been captured using a digital camera. The fluidization regimes along with their digital images using the bubble cap type distributor are being shown in Figure 1. The fluidization regimes along with their digital images using the nozzle type distributor are being shown in Figure 2. The fluidization behavior suggests that the bubble cap type distributor has shown better and uniform fluidization behavior as compared to the nozzle type distributor. Also Guo and Werther29 studied the influence of a gas maldistribution of the distributor design on the hydrodynamics of a CFB riser. The solids volume concentration and solids velocity were determined in an 8.5 m high circulating fluidized bed riser with two types of bubble cap distributors by applying a capacitance probe. They found that, in the bottom region of the CFB, the solids volume concentration in the center region is low, while the solids concentration increases significantly toward the wall with the highest solids concentration at the wall approaching the value at the packed bed. Furthermore, the solids volume concentration at the highpressure drop of a bubble cap is lower than that at the lowpressure drop of a bubble cap at all lateral positions. It has been found that the pressure drop of the distributor has little influence on the axial apparent solids concentration in the upper dilute region and on the external circulation rate. Superficial gas velocity proved to have a larger effect on descending particles at the wall and on ascending particles in the central region. Transversal particle velocities in both directions (center and wall) are relatively equivalent, with a slight difference observed at the wall and at the center. For the 600 μm palm shell waste powder the calculations for the minimum fluidization velocity were done by determining the Archimedes and Reynolds numbers. Using the experimental data the minimum fluidization velocity for 600 μm palm shell waste is found to be 0.20 m/s. Since the cold CFB was designed to work in fast fluidization regimes, the rotameter was so selected that it could measure flow rates during fast fluidization regimes. This flow meter gives a flow rate between 30 and 230 cfm. The experimental study was conducted to obtain hydrodynamic properties such as fluidizing behavior, pressure drop, voidage, and the hydrodynamic behavior of palm shell waste. The pressure drops along the riser section were measured using the multitube manometer at various primary

Figure 3. Variation of pressure in the riser section with varying primary airflow rate.

increase in airflow rates, the bubbling, turbulent, and fast fluidization regimes are observed. These experiments have also provided an understanding of the bubbles behavior in the distributor section. Bubbles growth was observed as they rose through the bed. Mostly the larger bubbles rose more quickly than the smaller ones. It was observed that they overtook the smaller bubbles and coalesced with them. The higher the fluidizing velocity, the larger and more bubbles it caused, because most of the excess gas flows as bubbles. The rising bubbles drew a streak of particles after them and carry some particles in their wake. This mechanism has led to the solid circulation in the riser section. The circulation of the solid is dependent on the flow rate of the primary and secondary air. The faster these supplied air velocities, the better fluidization becomes. The pressure of fluidization gas tends to drop when it up-flows along the riser section. The pressure variation along the riser height for varying primary airflow rates is shown in Figure 4. On the top of the distributor plate, the air pressure is higher which pushes the solid upward. As the velocity is continuously increased, the fluidized bed changes into the turbulent regime and then to fast fluidization. Bubbles usually grow as they rise through the bed. Mostly it occurs by larger bubbles, rising more quickly than a smaller one. They overtake the smaller bubbles and coalesce with them.

Figure 4. Pressure distribution in the riser section at different primary airflow rates. E



Article

The higher the fluidizing velocity, the more bubbles formation due most of the excess gas flowing as bubbles. The primary airflow was monitored by a rotameter which can measure the flow rate range from 30 to 230 cfm. The average air velocity corresponding to 30 cfm was 0.74 m/s, which is much higher than the minimum fluidization velocity of 0.2 m/s. As the airflow rate was increased, the fast fluidization regime appeared. The successful circulation of solid particles in the riser section suggested that experimental transport velocity was adequate to maintain a fast fluidized bed. The axial voidage distribution in circulating fluidized beds is an important parameter to study. It determines the pressure drop along the CFB and is closely related to the mean solid residence time within the riser. As we have discussed earlier in terms of solid distribution, the CFB can be divided into two zones, a dense zone at the bottom and a dilute zone at the top of the riser. Wang et al.4 performed an experimental study to investigate the flow structure in a CFB. They also introduced an efficient drag force model for simulating the flow structure of the two-phase flow in a riser. They found that the solid concentration decreases with the increase of the superficial gas velocity and increases with the increment of the circulation rate at the same height position. The total pressure drop of the main bed represents a linear relationship with the solid flux rate. In the dense-phase zone, the solid concentration increases linearly with the augmentation of the solid flux; however, the change of the solid concentration is slight, even unchangeable at the up zones. A typical S-shaped distribution profile was proposed earlier by Zhang et al.;28 however, the S-shaped distribution was not observed by other authors.12,30,31 Comparing these papers, it seems that the S-shaped distribution is greatly dependent on the solids size distribution, the solids circulating rate, and the superficial velocity. Figure 5 shows the axial voidage profile in

The right angle exit accumulated more solids than the long radius bend exit. The blind T exit accumulated more solids than the right angle exit and yielded a higher solids volume fraction in the riser. Also the visual observations of the flow in the riser exit revealed that dunes of significant size were formed in the riser exit connector. It is due to the fact that solids in the horizontal connector have settled under gravity, which means that the remainder of the suspension is accelerated. This also highlights the phenomena of flow separations, being shown in Figure 7. The solids holdup is greater for the exit with the baffle. The blind T exit shows larger solids volume fractions along the entire riser height and an increase of solids volume fraction with elevation in the upper half of the riser. The effect of an increase in He appeared to be small. Turbulent kinetic energy (k) is the turbulence kinetic energy per unit mass defined as

K = 0.5μi μi

(9)

Its unit quantity is turbulent kinetic energy. For multiphase modeling, its value corresponds to a particular phase in the multiphase flow. The turbulence kinetic energy was plotted for various riser exit designs, as shown in Figure 8. The right angle exit, blind T exit, and exits with inlet or outlet baffle cause an upstream exit region of increased solids volume fraction. Larger blind T extension heights may invoke a greater upstream exit region, as long as they remain below a critical extension height. Medium size inlet or outlet baffles may yield greater upstream exit regions than large or small baffles.31 The contours suggest that a particle in the middle of a bend exit, which experiences a radial acceleration (u̅st2/R), is equal to the radial component of the acceleration due to gravity. This condition suggests that radial slip is minimized around FrR = 1/√2.15 A radial acceleration balance suggests that inward/outward movement of solids in a riser exit is minimized around a Froude number FrR = 1/√2. Larger values of FrR yield more movement to the outside of the riser exit and smaller values more movement to the inside of the riser exit. The experimental results of Figure 4 revealed that the average exit velocity in the right angle exit bend is about 10 m/s which results in FrR much above 1/√2. So the predominant movement of the particles is outside of the riser exit. The same trend is also visible for other exits. However, the right angle exit with baffle shows more pronounced movement outside the riser exit. It appears that blind T has little effect on the extension height as compared to the right angle exit. It suggests that the slip is more prominent in the exit bends. The slip distribution in the various exits is different with the right angle exit and with a baffle showing greater slip than the blind T exit. The riser outlet effects are studied experimentally and computationally by Van engelandt et al.32 for L and abrupt T outlets having different extension heights and outlet surface areas. In both of the exit geometries, a recirculation vortex is observed along the wall of the riser facing the outlet and it does not affect the exit flow patterns upstream from the riser outlet. However, recirculation vortex shifts downward when the extension height is reduced in the T outlets. Zhou et al.33 have studied a 50 kW CFB coal combustor which is simulated using the CFD approach to study air−coal two-phase hydrodynamics using an energy-minimization multiscale (EMMS/matrix) model. The model predicted the different aspects of hydrodynamics inside the CFB combustor which are similar to our velocity contours.

Figure 5. Axial voidage distribution along the riser height for varying primary airflow rate.

the riser section for various primary airflow rates. Interestingly, we have found that the axial voidage distribution in our study is not purely S-shaped but very close to this shape. Simulations were performed for the riser exits under similar conditions to visualize the riser exits effects. The contours of velocity and velocity vectors are showed in Figure 6 for various exit geometries. It was found that the inward/outward motion, secondary flows of the first kind, tangential acceleration/ deceleration, and cavity formation near the riser exits are mechanisms that can account for asymmetric flow in the exit region. In the upper half of the riser exit, a strong turbulence was observed for the blind T exit, whereas a decrease is found for the right angle exit with the internal baffle. F



Article

Figure 6. Contours of absolute velocity (m/s) and velocity vectors for various riser exit designs.

5. CONCLUSION The experimental investigation of the fluidization behavior in the cold CFB had shown that the riser flow is typical of the upward solids motion in the core of the riser and downward motion along the walls. The fluidization of 1180 μm palm shell powders have shown the tendency to form clusters. It was also observed that there is continuous formation and disintegration of clusters. The bubble cap type distributor has shown a better and uniform fluidization behavior as compared to the nozzle

type distributor. It was also found that the voidage increased with the height above the distributor. However, this increase is more pronounced in the lower section of the riser. Interestingly, we have found that the axial voidage distribution in our studies is not purely S-shaped but close to this shape. The simulation results suggest that the right angle exit and the right angle exit with internal baffle shows an upstream exit region of reduced solids volume fraction. Also the riser exits G



Article

u̅ u̅st Um,i u ⃗m u⃗Dk vm⃗ vq⃗ p

average velocity (m/s) average solid velocity near the top of the riser (m/s) ith phase velocity in mixture (m/s) mass-averaged velocity (m/s) drift velocity (m/s) average velocity of mixture (m/s) relative velocity of the secondary phase (p) relative to the primary-phase (q) (m/s)

Greek Letters

α μ μm ρg ρm

■

Figure 7. Dune formation in the riser exit.

appear to invoke regions near the riser wall where solids motion is upward.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: 0600-12-5614631. E-mail: [email protected] or [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This paper was funded by the Deanship of Scientific Research (DSR). King Abdulaziz University, Jeddah, under Grant No. (5829-D1432). The authors, therefore, acknowledge with thanks DSR technical and financial support.

■

a⃗ dp g n p u

REFERENCES

(1) He, Y.; Deen, N. G.; Van Sint, M. A.; Kuipers, J. A. M. Gas−Solid Turbulent Flow in a Circulating Fluidized Bed Riser: Experimental and Numerical Study of Monodisperse Particle Systems. Ind. Eng. Chem. Res. 2009, 48, 8091−8097. (2) Wang, F.; Marashdeh, Q.; Wang, A.; Fan, L. Electrical capacitance volume tomography imaging of three-dimensional flow structures and solids concentration distributions in a riser and a bend of a gas−solid circulating fluidized bed. Ind. Eng. Chem. Res. 2012, 51, 10968−10976. (3) Chen, G.; Andries, J.; Spliethoff, H. Biomass conversion into fuel gas using circulating fluidised bed technology: The concept improvement and modeling discussions. Renewable Energy 2003, 28, 985−994. (4) Wang, X.; Liao, L.; Fan, B.; Jiang, F.; Xu, X.; Wang, S.; Xiao, Y. Experimental validation of the gas-solid flow in the CFB riser. Fuel Process. Technol. 2010, 91, 927−933. (5) Zaabout, A.; Bournot, H.; Occelli, R.; Draoui, A. Solids behavior in dilute zone of a CFB riser under turbulent conditions. Particuology 2011, 9, 598−605. (6) Miao, Q.; Wang, C.; Wu, C.; Yin, X.; Zhua, J. Fluidization of sawdust in a cold model circulating fluidized bed: Experimental study. Chem. Eng. J. (Amsterdam, Neth.) 2011, 167, 335−341. (7) Brereton, C. M. H.; Grace, J. R. End Effects in Circulating Fluidized Bed Hydrodynamics. In Circulating Fluidized Bed Technology IV; Avidan, A. A. , Ed.; Hidden Valley: Somerset, PA, USA, 1994. (8) Brereton, C. M. H.; Grace, J. R. Microstructural aspects of the behaviour of circulating fluidized beds. Chem. Eng. J. (Amsterdam, Neth.) 1993, 48, 2565−2572. (9) Harris, A. T.; Davidson, J. F. Influence of exit geometry in circulating fluidized bed risers. AIChE J. 2003, 49, 52−64. (10) Yan, A.; Zhu, J. Scale-Up effect of riser reactors (1): Axial and radial solids concentration distribution and flow development. Ind. Eng. Chem. Res. 2004, 43, 5810−5819. (11) Minchener, A. J. Fluidized bed combustion systems for power generation and other industrial applications. J. Power Energy 2003, 217, 9−18. (12) Sterneus, J.; Johnson, F.; Leckner, B. Gas mixing in circulating fluidized-bed risers. Chem. Eng. Sci. 2000, 55, 129−148. (13) Hu, N.; Zhang, H.; Yang, H.; Yang, S.; Yue, G.; Lu, J.; Liu, Q. Effects of riser height and total solids inventory on the gas−solids in an ultra-tall CFB rise. Powder Technol. 2009, 196, 8−13. (14) Breault, R. W. An analysis of clustering flows in a CFB riser. Powder Technol. 2012, 220, 79−87. (15) Hussain, A.; Ani, F. N. A.; Darus, N.; Adnan, M. F.; Mustafa, A. Hydrodynamic and fluidization behavior in a circulating fluidized bed. Int. J. Mech. Mater. Eng. 2006, 1, 18−23. (16) Taghipour, F.; Ellis, N.; Wong, C. CFD Modeling of a TwoDimensional Fluidized-Bed Reactor. The Eleventh Annual Conference of the CFD Society of Canada, Vancouver, 2003. (17) Jalil, R.; Tasirin, S.; Takriff, M. S. Computational Fluid Dynamics (CFD) in Fluidized Bed Column: Effect of Internal Baffles. Proc. Reg. Symp. Chem. Eng., Malaysia 2002.

Figure 8. Turbulent kinetic energy (m2/s2) for various exit designs in a CFB (where, for example, 8.68e+00 represents 8.68 and 5.88e-01, 5.88 × 10−1, and so on).

■

fractional reactant viscosity of the gas (Pa·s) viscosity of mixture (Pa·s) density of gas (kg/m3) density of gas (kg/m3)

SYMBOLS USED secondary phase acceleration (m2/s) diameter of fluidized particle (m) gravitational acceleration (m/s2) number of phases Pressure at a certain elevation (Pa) velocity of gas (m/s) H



Article

(18) Wang, S.; Yang, X.; Lu, H.; Yu, L.; Wang, S.; Ding, Y. CFD studies on mass transfer of gas-to-particle cluster in a circulating fluidized bed. Comput. Chem. Eng. 2009, 33, 393−401. (19) Weinstein, H.; Feindt, H. J.; Che, L.; Graff, R. In Fluidization VII; Potter, O. E., Nicklin, D. J., Eds.; Engineering Foundation: New York, 1989; pp 305−309. (20) Wu, S.; Alliston, M. Cold model testing of the effects of air proportions and reactor outlet geometry on solids behavior in a CFB. In Proceedings of the 12th International Conference on Fluidized Bed Combustion; Rubow, L. N., Commonwealth, G., Eds.; ASME: New York, 1993; pp 1003−1009. (21) Fluent 6.3 User’s Guide; Fluent Inc.: Lebanon, NH, USA, 2010. (22) Monazam, E. R.; Shadle, L. J. A Transient Method for Characterizing Flow Regimes in a Circulating Fluid Bed. Powder Technol. 2004, 139, 89−97. (23) Kunii, D.; Livenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Boston, MA, USA, 1991. (24) Xu, G.; Shiqiu, G. Necessary parameters for specifying the hydrodynamics of circulating fluidized bed risersA review and reiteration. Powder Technol. 2003, 137, 63−76. (25) Contractor, R.; Dry, R. J.; White, C.; Mao, Q. M.; Konstantinidis, S.; Potter, O. E. Circulating fluidized bedsDiameter, solids hold-up, axial gas-mixing, and contact efficiency. Powder Technol. 2000, 111, 132−144. (26) Van, H.; Prins, W.; Swaaij, V. Shear stresses on the wall of a dense phase riser. AIChE Symp. Ser. 1993, 89, 53−72. (27) Monceaux, L.; Azxi, M.; Molodtsof, Y.; Large, J. F. Overall and local characterization of f low regimes in a circulating fluidized bed; In Circulating Fluidized Bed Technology; Basu, P., Ed.; Pergamon Press: NewYork, 1986. (28) Zhang, N.; Lu, B.; Wang, W.; Li, J. Virtual experimentation through 3D full-loop simulation of a circulating fluidized bed. Particuology 2008, 6, 529−539. (29) Guo, Q.; Werther, J. Influence of a gas maldistribution of distributor design on the hydrodynamics of a CFB riser. Chem. Eng. Process. 2008, 47, 237−244. (30) Chen, L.; Weinstein, H. The Radial Pressure Gradient in a Circulating Fluidized Bed Riser. In Circulating Fluidized Bed Technology IV; Avidan, A. A. , Ed.; Hidden Valley: Somerset, PA, USA, 1993. (31) Ludlow, J. C.; Panday, R.; Shadle, L. J. Phase-shift method to estimate solids circulation rate in circulating fluidized beds. Ind. Eng. Chem. Res. 2013, 52, 1958−1969. (32) Van engelandt, G.; Heynderickx, G. J.; De Wilde, J.; Marin, G. B. Experimental and computational study of T- and L-outlet effects in dilute riser flow. Chem. Eng. Sci. 2011, 66, 5024−5044. (33) Zhou, W.; Zhao, C. S.; Duan, L. B.; Qu, C. R.; Chen, X. P. Twodimensional computational fluid dynamics simulation of coal combustion in a circulating fluidized bed combustor. Chem. Eng. J. (Amsterdam, Neth.) 2011, 166, 306−314.

I


Experimental and Simulation Study of Fluidization Behavior of Palm

Recommend Documents