Recycle Chamber Research Articles

An investigation of the aeration proportion effect at the supply and recycle chambers on the optimal operation point of the U-type loop seal and the maximum solid circulation rate in a fluidized bed reactor

A loop seal is used to control a solid circulation rate and loop seal aeration ratios at the supply and recycle chambers are another factor that affects its efficiency and circulating behaviors of solid particles in a fluidized bed reactor. This research aimed to find the proper aeration proportion and operation point of the loop seal that affected the highest solid circulation rate. A riser used in the experiment had a cross-sectional area of 100 × 100 mm2 and 4500 mm height above a distributor plate. A U-type loop seal was constructed by a supply chamber and a recycle chamber with the same cross-sectional area. A horizontal passage of 50 mm length was connected with both chambers. The sand was a bed particle with an average diameter of 320 μm. The loop seal aerations were fed at the bottom of the supply and recycle chambers. It was found that the loop seal could not operate with supply-aeration flow rate percentages (PQS) of 0% and 100%. The PQS of about 73% gave the maximum solid circulation rate and was an optimal operation point at which air supplied at the supply chamber started bypassing into the recycle chamber.

Flow rate of solids through recycle chamber of loop seal in a circulating fluidized bed

This study investigated the flow rate of solids through the recycle chamber of a loop seal in a circulating fluidized bed (CFB) at atmospheric pressure and temperature. Flow characteristics of alumina particles were measured in the downcomer, horizontal passage, recycle chamber, and riser (0.05 m i.d., 2.7 m high).As the gas velocity in the riser increased, the pressure drop in the downcomer decreased, and thus, the aeration rate required at the bottom of the supply chamber to attain the minimum fluidization state for the bed of the downcomer increased. Based on experimental data, relationships that could predict the flow rate of solids through the recycle chamber Gs,r were derived by extending the correlation for the solids flow rate through the bubbling fluidized bed that discharged solids via overflow exit. Gs,r increased in proportion to the gauge pressure at the base of the recycle chamber. When the riser was under the fast bed condition, the back pressure against the flow of solids coming from the recycle chamber to the riser appeared significantly, and Gs,r was inversely proportional to the gauge pressure at the base of the riser. The derived relationships for Gs,r were in good agreement with measured data.

On the comprehension of the gas split in loop seal devices

Gas distribution through the chambers of a loop seal in circulating fluidized bed (CFB) units is still difficult to predict and often misunderstood. Dedicated experiments were carried out in an isolated loop seal to assess how the gas fed into a loop seal is distributed under different aeration modes (through the recycle chamber, the supply chamber and both chambers), aeration flowrates and solids fluxes. Experimental results shed light on the non-ideality of the gas-solids flows occurring in these units and, in combination with theoretical considerations, it is demonstrated why semi-empirical models cannot effectively describe the performance of loop seals operating as non-mechanical valves. Recommendations are given for the optimization of the operation of a loop seal as a non-mechanical valve in a solids circulating loop. A method is proposed to characterize the performance of a loop seal in a CFB unit from measurements of that loop seal in isolation mode. • Gas splitting into the chambers of a loop seal is a key issue difficult to predict. • The gas distribution was experimentally assessed in an isolated loop seal. • Loop seal operation as non-mechanical valves leads to non-ideal gas-solids flows. • Non-ideal gas-solids flow pattern establishes the gas splitting into the chambers. • The study gives insights on how to control the solids circulation with a loop seal.

OPERATIONAL INFLUENCE OF THE MONO-CHAMBER AERATION MODE IN THE LOOP SEAL OF A CIRCULATING FLUIDIZED BED

Fluidization numbers varying from 0.84 to 1.68 were used in the loop seal valve of a bench-scale circulating fluidized bed (CFB) system to analyze the influence of the mono-chamber aeration mode on both the solids circulation rate and the static pressure drop inside the solids recycle device. Runs were carried out using 4 kg of overall solids inventory and particles of 183 µm in Sauter mean diameter, which were kept under fast fluidization regime at superficial gas velocity of 4 m/s. Results showed that the choice of the chamber to be aerated can noticeably affect the gas-solid hydrodynamics. In this sense, the analysis of variance applied on the experimental data indicated that the aeration into the recycle chamber of the loop seal offers lower levels of solids circulation rate but also allows to control it within a wider range of fluidization numbers and with less pressure drop or energy demand.

An investigation of performance of a conventional U type loop-seal for CFB reactors with side and bottom aerations

Experiments were carried out in a circulating fluidized bed (CFB) system. The riser has a cross sectional area of 100×100mm2, while the supply and recycle chambers of the loop-seal have the same cross-sectional areas. Sand having a Sauter mean diameter of 310μm was used as bed material. Throughout the experiment, the superficial air velocity of the riser and the height of the solid particles in the storage column were kept constant. Four types of loop-seal categorized by their aeration scheme were investigated: aeration at the bottoms of supply and recycle chambers with side aeration at the supply chamber wall (USRQS), aeration at the recycle chamber bottom with side aeration at the supply chamber wall (URQS), aeration at the supply chamber bottom with side aeration at the recycle chamber wall (USQR), and aeration at the bottoms of supply and recycle chambers with side aeration at the recycle chamber wall (USRQR). The relation between the solid circulating rate and the side aeration flow rate was modeled as a saturated function. The results show that the URQS and the USQR types provide the same initial solid circulation rate, which is higher than for the conventional U type loop-seal. For a certain fluidization number, the loop-seals with side aeration provide higher solid circulation rate and require lower total aeration flow rate. The loop-seal with side aeration at the supply chamber wall tends to provide higher solid circulation rate and sensitivity of the circulation rate. For a certain solid particles height in the storage column, the maximum solid circulation rate was restricted by the flow through the horizontal passage.

Heat transfer behavior of the immersed tubes and solid circulation rate of a conventional U type loop-seal with an in-line tube bundle in the recycle chamber with and without side aeration at the walls

A loop-seal having a horizontal immersed tube bundle in the recycle chamber with aeration at the bottom of the supply and return chambers or a conventional loop seal with heat exchanger (LSHE) was used for this study. Heat transfer behavior of the immersed tubes with aerations at the side walls of the recycle chamber and without side aerations was investigated. An experiment was carried out in a circulating fluidized bed system. The riser has a cross sectional area of 100mm × 100mm while the supply and return chambers of the loop seal have the same cross-sectional areas. The tube has a diameter of 12.7mm. Sand having an average diameter of 310μm was used as bed particles. In all cases, heat transfer tends to be highest and lowest at the tube bottom and the tube crest, respectively. The side aeration can improve particle mobility in the stagnant zone at the tube crest, while bottom aeration cannot, even at a high velocity. Without the side aeration, there is a large difference in the local heat transfer profiles between the lower half and upper half of the tube. With the side aeration, the profiles tend to be comparable. As a result, the average heat transfer coefficient around the tube in this case is higher, and thermal stress due to temperature difference in the tube is lower than for the case without side aeration. In addition, the LSHE with side aeration has higher responsivities to thermal load and solid circulation rate variation than that without side h aeration.

Design of experiments and analysis of dual fluidized bed gasifier for syngas production: Cold flow studies

Biomass gasification is a thermo-chemical process widely accepted as a future technology for syngas production. Numerous types of gasification systems have been proposed and studied in the past. Recent developments have shown that Dual Fluidized Bed (DFB) gasifier are commercially more attractive for production of the hydrogen-rich syngas as compared to others. DFB gasification system is very complex in construction and operation. Hence, a detailed understanding of hydrodynamics in such systems is essential for optimum design and scale-up. Hydrodynamics of DFB gasifier mainly depends on the Solid Circulation Rate (SCR). SCR is governed by riser velocity, gasifier velocity, and loop seal velocities. In present work, Central Composite Rotatable Design (CCRD) based Response Surface Method (RSM) was employed to determine the effect of riser velocity, gasifier velocity, recycle chamber velocity, supply chamber velocity, and vertical supply chamber velocity and their interaction on the SCR. Adequacy of regression model developed from RSM was confirmed using ANOVA analysis. The value of coefficient of determination (R2) of the model was 0.9729, which confirms model represents the experimental results satisfactorily. Riser and recycle chamber velocity were found to be most significant parameters, plays an important role in SCR in DFB gasifier.

Investigation of gas‒solids flow hydrodynamics in a cold model of a dual fluidised bed gasifier using electrical capacitance tomography sensors

The flow characteristics of a dual fluidised bed gasifier (DFBG) are more complex than those of a single fluidised bed gasifier. For stable operation and appropriate control, a cold DFBG test facility with both an upper and a lower U-valve was built, and electrical capacitance tomography (ECT) sensors were installed with pressure transducers to investigate the effects of operating conditions on gas‒solids flow hydrodynamics. The operating parameters included gas velocities in the riser and in the bubbling fluidised bed, aeration velocity in the lower U-valve, bed material inventory, and particle size. This is the first time that ECT was applied in different flow zones of a dual fluidised bed gasifier system. The experimental results indicated that ECT in the recycle chamber could monitor the performance of the lower U-valve under different operating conditions for early detection of gas shortcut from the riser to the bubbling bed. Three main flow regimes in the riser and the differences between the reactors were identified by two sets of ECT sensors with pressure transducers. Finally, the effects of the operating conditions on the pressure drop in different parts of the DFBG was investigated.

Effect of level of overflow solid outlet on pressure drop of a bubbling fluidized-bed

The effect of level of the overflow outlet for continuous flow of solid particles on the pressure drop of a bubbling fluidized-bed that employed an in-bed inlet for solid feed was investigated with changing solid properties, solid feed rate, gas velocity, and level of the overflow outlet. The pressure drop of fluidized-bed (Δpbed,f) decreased with increasing gas velocity, but increased with either solid feed rate or level of the overflow solid outlet (L). The Δpbed,f/L increased with L. Irrespective of particle size and density, bed height converted for minimum fluidization condition (pressure head by bed weight, Hmf,f) decreased with increasing the volume flow rate of bubble but increased with either the solid feed rate or the level of the overflow solid outlet. The nominal vertical height, height between the Hmf,f and the level of the overflow outlet, that bubbles transported particles while drawing the solid particles out of the fluidized-bed increased as either the volume flow rate of bubble or level of the overflow outlet increased. However, it decreased as the solid feed rate increased. It appeared that the power of bubble for lifting solid to be discharged through the overflow outlet was same at the fixed volume flow rate of bubble, solid feed rate, and level of the overflow solid exit. The power of bubble increased with the level of the overflow outlet but not linearly. The correlation proposed for the pressure drop across the bubbling fluidized-bed was useful to predict the pressure drop across the recycle chamber of the loop seal and the external solid circulation rate in the circulating fluidized-bed system.

The Hydrodynamics Studies on Loop Seal in CFBC Boiler Using CFD Analysis

Abstract In a circulating fluidized bed (CFB), the loop seal is an important component which recirculates the solids captured by the cyclone to the bottom of the riser and avoids the direct flow of gas from high-pressure riser to the low-pressure cyclone. Most of the CFBC systems employ a Non-Mechanical valve, and its function has been investigated by many researchers. In this work, the flow of solid particle within the loop seal has been studied elaborately, and various design and operating parameters of the loop seal were analyzed in detail using Computational Fluid Dynamics (CFD). The CFD study has handled a loop-seal of dimension 110 mm × 430 mm × 400 mm high. This analysis has been done with a 200 µm sand particle and it is checked for its flow ability through various sizes and by altering the L/H ratio for three solid mass fluxes and for three L/H ratios of loop seal. The rate of solid mass flux depends on the length of the horizontal passage connecting the recycle cycle compartment of the loop seal with the supply chamber and hence the solid flow rate and the pressure difference per unit length are directly proportional to the length of the chamber or passage. Hence, L/H ratio is taken as a driving variable for the optimum performance of loop seal. The CFD analysis results reveal that the aeration of the solid to be used within the loop-seal should be higher than the Minimum Fluidization velocity. Also, keeping the length of the horizontal passage constant and varying the height of the passage will reduce the pressure drop. The pressure drop across the horizontal passage decreases up to a certain L/H ratio after which it increases. By comparing the three L/H ratios, L/H ratio of 2.62 is having a lesser pressure drop for the three mass fluxes. L/H ratio of 2.62 is having a good fluidization phenomenon and also the flow from the recycle chamber is more in comparison to the other L/H ratios. Experimental data from the literature is in good agreement with the CFD results.

Bed pressure drop of a bubbling fluidized-bed with overflow solid discharge

The pressure drop of a bubbling fluidized-bed that employed an in-bed inlet and an overflow outlet for continuous flow of solid particles was investigated with variation in the particle size and density, the solid flow rate and the gas velocity. The bed pressure drop decreased with increasing the gas velocity, but increased with the solid flow rate. The characteristics in lifting the solid particles vertically to the level of the overflow outlet by bubbles appeared different from the ones of particle entrainment and bed expansion. Regardless of size and density of particles, bed height in minimum fluidizing condition (pressure head by solid bed weight, Hmf,f) decreased with increasing the volume flow rate of bubble but increased with the mass flow rate of solid particles. The nominal vertical height from Hmf,f to the level of the overflow outlet that the particles should overcome in the course of discharging out of the fluidized-bed with the aid of bubbles increased as either the volume flow rate of bubble increased or the mass flow rate of solid particles decreased. The power consumed while bubbles lifted particles to be discharged appeared to be same at the fixed volume flow rate of bubble. A correlation was proposed successful even for predicting the bed pressure drop of the recycle chamber of the loop seal and the external solid circulation rate in the circulating fluidized-bed system.

Numerical simulation of loop seal for a circulating fluidized bed based on an improved EMMS bubbling model

A typical loop seal consists of a supply chamber and a recycle chamber as well as a bottom connection slit in between. The supply chamber is generally operated in the minimum fluidization state, while the recycle chamber in the bubbling fluidization regime. The energy-minimization multi-scale (EMMS) model, as generally used to calculate interphase drag coefficient in previous simulations of the loop seal, is still open to discussion since it is based on the concept of particle clusters. So an EMMS bubbling model was developed to model dense gas–solid flow by taking into account the interphase interaction between the emulsion and bubble phases. In this article, a heterogeneity index is calculated from an improved EMMS bubbling model to measure the interphase drag coefficient and further integrated into the two-fluid-model (TFM) approach to simulate the loop seal for a circulating fluidized bed (CFB) system. This index is dependent on superficial gas velocity in bubbling fluidization, so a region-specific drag correction scheme is proposed to allow the application of various heterogeneity index correlations to different zones of the loop seal, since superficial gas velocity may differ much from the recycle to supply chamber. Several typical cases of drag correction are comparatively investigated by applying various gas velocities and/or drag models in the recycle and supply chambers, and the optimal one is proven to be able to reasonably simulate the parametric effects on gas inverse flow in the loop seal.

Impact of loop seal structure on gas solid flow in a CFB system

In a circulating fluidized bed (CFB) boiler, loop seal is one of the most commonly used non-mechanical valves. When the solids are conveyed horizontally from the supply chamber to the recycle chamber through the horizontal section, the length of the horizontal section will cause much flow resistance, which not only influences the fan burden but also the pressure and mass balance between the furnace and the external loop.In this paper, a new structure loop seal with a slope passage connecting the supply and recycle chambers (named N-type) is described. The flow characteristic of quartz sand (Geldart-B particles) in the new loop seal was studied by comparison with the traditional loop seal (named U-type).The experimental data show that the passage design connecting the two chambers of a loop seal has an obvious effect on the gas solid flow not only in the loop seal, but also in other components in the whole loop. For the N-type loop seal, the minimum aeration rate required for the onset of solid flow is equal to the minimum fluidization velocity of the bed materials, much less than with the U-type loop seal. The solid recycling capability of the N-type loop seal is better than of U-type one at low aeration rate, but this status will be reversed at high aeration rate. A small amount of vertical aeration into the bottom of the standpipe can further improve the solid recycling capability of the N-type loop seal, by reducing the inter-particle friction and build up a higher pressure drop gradient in the standpipe. At the same time, less air bypassing upward into the cyclone through the standpipe is found for the N-type loop seal at high level of aeration rate, while, at the same condition the U-type suffers severe gas bypassing with a fluidized state in the standpipe. At last, the pressure drop of the new type loop seal is much less than the pressure drop of the traditional one with the same solid flow rate, which provides an advantage for reducing fan power and sustaining more solids in the boiler furnace.

A hydrodynamic model of loop-seal for a circulating fluidized bed

Loop-seal is one of the most common valves used in circulating fluidized bed (CFB) system. In the present work, a hydrodynamic model is developed for the operation that the particles in the recycle chamber are fluidized by the air, while the particles in the standpipe are in moving bed condition. Through hydrodynamic analysis, a hydrodynamic model including three equations has been established, whose solution results in the actual gas velocity, particle velocity and bed voidage in the recycle chamber. The model also predicts the solids flow rate, gas flow rate and pressure drop in both standpipe and recycle chamber. Experiments were conducted in a circulating fluidized bed with silica gel particles (Geldart group A). The model predictions showed good agreement with experimental data.

CFD simulation with experiments in a dual circulating fluidized bed gasifier

Gas and particles hydrodynamic behaviors were investigated in a pilot-scale cold-mode riser and a bubbling fluidized bed gasifier by means of experiment and computational fluid dynamics (CFD). Six different experimental sets were conducted in the cold-rig dual fluidized bed (DFB) at different gas velocities in both the riser and the recycle chamber aeration. A two-dimensional (2D) multi-fluid Eulerian model incorporating the kinetic theory of granular flows was applied to identify unsteady-state behaviors of the fluidized bed. The CFD model predicts well the solid circulation rate in the cold-rig DFB for all the six experimental runs. A discrepancy between experiment and simulation is observed in the axial solid holdup along the riser. The simulation results demonstrate that the cold-bed simulation can be used to predict the solid circulation rate for the hot-bed operation of the DFB gasifier.

Solid circulation and loop-seal characteristics of a dual circulating fluidized bed: Experiments and CFD simulation

A dual circulating fluidized bed reactor (riser: 0.04 m × 0.11 m × 4.5 m high; bubbling fluidized bed: 0.285 m × 0.11 m × 2.13 m high) was designed and constructed to investigate hydrodynamics on dual fluidized bed gasifier (DFBG) and chemical looping combustion (CLC). Effects of gas velocities to a riser, bubbling fluidized bed, supply chamber, and recycle chamber as well as vertical aeration rate on solid circulation rate and solids flow characteristics in a loop-seal were determined experimentally. Hydrodynamic characteristics were identified by computational fluid dynamics (CFD) simulation where a two-dimensional (2D) multiphase Eulerian model incorporating with the kinetic theory of solid particles was used. The solid circulation rate increases with increasing gas velocity into the loop-seal and the riser. At the same vertical aeration rate, the maximum solid circulation rate was obtained at a loop-seal height/base line ( h/ l) ratio of 2.5. When the supply chamber was connected to a bubbling fluidized bed, the pressure drop of the loop-seal was found to be affected by fluidizing state of the bubbling fluidized bed.

An Investigation Into the Operation of the Twin-Exit Loop-Seal of a Circulating Fluidized Bed Boiler in a Thermal Power Plant and Its Design Implication

Loop-seal is a critical component of a circulating fluidized bed (CFB) boiler, and yet very little information on its working or design is available in published literature. Among the limited available information, none is on twin-exit loop-seal though it is one of the most commonly used loop-seal in large commercial CFB boilers. To circulate larger amounts of solids, a twin-exit loop-seal provides larger solids flow sections. It receives solids from one standpipe but delivers it through two recycle chambers and two delivery pipes. The present research was conducted in a twin-exit loop-seal of a 3.2 MWth CFB boiler operating in a thermal power plant for cofiring purpose. Data obtained in this industrial unit were supplemented with those collected in a single-exit bench-scale loop-seal in the authors’ laboratory from the single-exit loop-seal in a 65 t/h CFB boiler and a scale model of a 30 MW CFB boiler. The effect of recycle chamber’s aeration on the solids circulation rate was studied for several particle sizes. Results suggest that the total solids circulation rate does not increase proportionately with the increase in loop-seal discharge area provided by the twin-exit loop-seals. The linear horizontal velocities of solids and the minimum aeration in the recycle chambers are comparable to those measured in a single-exit loop-seal. The implication of these new findings on the design procedure of loop-seals is discussed.

Experimental Investigation into the Hydrodynamics of Flow of Solids through a Loop Seal Recycle Chamber

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