Solids Circulation Rate Research Articles

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.

Numerical analysis of the operating characteristics of a large‐scale CFB coal‐gasification reactor with the QC‐EMMS drag model

AbstractThe operating characteristics of a 60 m high industrial circulating fluidized bed (CFB) coal‐gasification reactor were investigated numerically based on the Eulerian‐Eulerian approach. The QC‐EMMS drag model was used to describe the gas‐solid drag reduction caused by particle clusters. The simulations predicted the overall characteristics of fluidization and gasification inside the CFB reactor riser. The simulated results are in agreement with measured data from the field test. Then, the numerical model was used to analyze the influences of the steam/coal ratio, pulverized coal‐particle size, and operating pressure. The results show that each of the three operating parameters has a great impact on different performance parameters, such as the solid mass circulation rate, the H2/CO ratio, the effective syngas yield, and the average bed temperature. The predicted data were then correlated to provide supports for optimizing the operations of large‐scale CFB coal gasifiers.

Numerical simulation of commercial scale autothermal chemical looping reforming and bi-reforming for syngas production

Autothermal Chemical Looping Reforming (a-CLR) is an emerging technology that facilitates CO2 capture and minimizes energy losses in syngas production. The dual-fluidized bed process uses a bubbling fuel reactor (FR) and a riser air reactor (AR). A 1-D multiscale model was developed that couples fluidized bed hydrodynamics with intrinsic reaction kinetics, including catalyst deactivation by oxidation and coke formation. An a-CLR unit with a capacity equivalent to 50 conventional reformer tubes was considered to demonstrate feasibility. The effects of the catalyst activity, main operation conditions and CO2 co-feeding on the performance were then analyzed. The results confirm a-CLR is feasible with realistic reactors dimensions and solids circulation rate. To avoid the risk of sintering and catalyst deactivation, the oxygen carrier should only be slightly oxidized in the AR. Autothermal operation then requires an oxygen-to-CH4 feed ratio of around 1.2. The high catalyst-to-gas feed ratio in the FR and bubble-emulsion phase mass transfer limitations result in a low sensitivity of the methane conversion to the catalyst activity. A low H2O-to-CH4 feed ratio introduces a risk for coke formation in the bottom of the FR, but part of the feed methane is fully oxidized, producing H2O and CO2 in the emulsion phase and mitigating this risk. Co-feeding CO2 with CH4 and H2O is seen to allow adjusting the H2/CO-ratio of the syngas, but increases the risk of coke formation. Finally, heat recovery from the flue gas and syngas from both reactors by preheating the feed gases further increases the energy efficiency.

Investigation on influences of loose gas on gas-solid flows in a circulating fluidized bed (CFB) reactor using full-loop numerical simulation

Abstract The influences of loose gas on gas-solid flows in a large-scale circulating fluidized bed (CFB) gasification reactor were investigated using full-loop numerical simulation. The two-fluid model was coupled with the QC-energy minimization in multi-scale theory (EMMS) gas-solid drag model to simulate the fluidization in the CFB reactor. Effects of the loose gas flow rate, Q, on the solid mass circulation rate and the cyclone separation efficiency were analyzed. The study found different effects depending on Q: First, the particles in the loop seal and the standpipe tended to become more densely packed with decreasing loose gas flow rate, leading to the reduction in the overall circulation rate. The minimum Q that can affect the solid mass circulation rate is about 2.5% of the fluidized gas flow rate. Second, the sealing gas capability of the particles is enhanced as the loose gas flow rate decreases, which reduces the gas leakage into the cyclones and improves their separation efficiency. The best loose gas flow rates are equal to 2.5% of the fluidized gas flow rate at the various supply positions. In addition, the cyclone separation efficiency is correlated with the gas leakage to predict the separation efficiency during industrial operation.

Computational fluid dynamics of dual fluidized bed gasifiers for syngas production: Cold flow studies

Thermo-chemical conversion of biomass using Dual Fluidized Bed (DFB) gasifier is proven to be a promising way for the production of H2 rich syngas. In the present work, the hydrodynamics in the DFB gasifier is investigated experimentally and with Computation Fluid Dynamics (CFD) at cold flow conditions. Three-dimensional Eulerian-Eulerian multi-phase model combined with the kinetic theory for the granular phase has been applied to investigate unsteady-state behaviours of a DFB gasifier. The developed CFD model is validated with experimentally obtained pressure distributions across the riser. The model prediction shows deviation from experimental results (10-14%), which are mainly due to the Gidaspow drag model over predicting drag force. Simulation results are presented in terms of the pressure profile, solids hold-up and solid circulation rate.

Predicting cold gas-solid flow in a pilot-scale dual-circulating fluidized bed: Validation of computational particle fluid dynamics model

This study uses the Eulerian-Lagrangian multiphase particle-in-cell (MP-PIC) approach, adopted in the computational particle fluid dynamics (CPFD) code, Barracuda VR®, to simulate a cold-flow dual-circulating fluidized bed. The hydrodynamics is simulated using a novel segregated approach, in which the experimental solids circulation rate is specified as an input parameter. CPFD predictions of pressures and particle concentration distributions are compared with experimental results for varying bed material circulation rate of 76–153 kg/m2s and superficial gas velocity in the reactor riser from 3.3 m/s to 6.4 m/s. Overall trends in these quantities are well captured by the model. Detailed sensitivity studies of the reactor riser are carried out using a riser-only approach with a fixed bed mass inventory. Apart from the drag model selection, the pressure constant Ps and the exponent β of the MP-PIC particle stress model are found to have the greatest influence on the fluidization behavior and on the predicted solids flux.

Experimental study of the solid circulation rate in a pressurized circulating fluidized bed

The solid circulation rate is essential for design of pressurized circulating fluidized beds (PCFBs). With increasing pressure from atmospheric pressure to a few bars, the gas density linearly increases with the pressure, which affects the gas–solid flow characteristics. In this work, experiments were performed at room temperature in a cold PCFB apparatus with a riser of 3.3m in height and 0.05m in diameter. The solid circulation rate was studied from 20 to 80kg/(m2·s) under various conditions with increasing pressure from 0.1 to 0.6MPa and fluidizing gas velocity from around 1.5 to 8.0m/s for different Geldart B group particles. Most of the conditions were in the flow regimes of core-annulus flow (CAF) only and CAF with a turbulent fluidized bed at the bottom. The trend of the apparent slip factor with the dimensionless slip velocity was similar at different pressures and for different average particle sizes, and it converged to an exponential function. An empirical equation was obtained by fitting the solid circulation rate with the operating parameters (particle transport velocity, particle volume fraction, Archimedes number, and Froude number), which is helpful for design and operation of PCFBs.

Experiment and multiphase CFD simulation of gas-solid flow in a CFB reactor at various operating conditions: Assessing the performance of 2D and 3D simulations

Accurate prediction of gas-solid flow hydrodynamics is key for the design, optimization, and scale-up of a circulating fluidized bed (CFB) reactor. Computational fluid dynamics (CFD) simulation with two-dimensional (2D) domain has been routinely used, considering the computational costs involved in three-dimensional (3D) simulations. This work evaluated the prediction capability of 2D and 3D gas-solid flow simulation in the lab-scale CFB riser section. The difference between 2D and 3D CFD simulation predictions was assessed and discussed in detail, considering several flow variables (superficial gas velocity, solid circulation rate, and secondary air injection). The transient Eulerian-Eulerian multiphase model was used. CFD simulation results were validated through an in-house experiment. The comparison between the experimental data and both computational domains shows that the 3D simulation can accurately predict the axial solid holdup profile. The CFD simulation comparison considering several flow conditions clearly indicated the limitation of the 2D simulation to accurately predict key hydrodynamic features, such as high solid holdup near the riser exit and riser bottom dense region. The accuracy of 2D and 3D simulations was further assessed using root-mean-square error calculation. Results indicated that the 3D simulation predicts flow behavior with higher accuracy than the 2D simulation.

Experimental Study of Attrition in the Spouted Bed and Spout-Fluid Bed with a Draft Tube

The attrition of 300 µm natural zeolite particles was studied in a laboratory scale draft tube spouted bed (DTSB) and spout-fluid bed (DTSFB). It has been shown that the attrition rate decreases with time and reaches to an almost constant value. The results show that the prevailing attrition mechanism under the conditions of this work is the surface abrasion which occurs due to the collisions between particles. It has been found that increasing the cone angle from 30o to 60o in the DTSB, causes a decrease in the extent of attrition. In addition, by increasing the spouting air velocity and the height of the entrainment zone in the DTSB, the extent of attrition increases due to a more energetic collision between particles as well as the increased circulation rate of solids. Increasing the auxiliary air velocity in the DTSFB increases the rate of attrition. A comparison between the attrition in the DTSB and DTSFB has been conducted and has indicated that applying the auxiliary air flow causes up to a 6 % increase in the extent of attrition. An empirical correlation is derived for evaluating the extent of the attrition in the DTSB and DTSFB. This empirical correlation is in good agreement with the experimental data.

Measuring particle dynamics in a fluidized bed using digital in-line holography

The quantification of three-dimensional (3D) particle behavior in a fluidized bed is crucial for improving the fundamental understanding and engineering design of such multiphase systems commonly used in industry. This work reports recent development and application of digital in-line holography (DIH) on the in-situ measurement of (a) solid circulation rate, (b) particle size distribution, (c) 3D location, and (d) velocity distribution in the particle field. A Hough-transform-based hologram processing algorithm was developed and tested over a range of flow conditions. Using a hopper system for precise control of particle feed rate, real-time solid flow rates and particle size distributions were measured using DIH. These measurements were then compared to high-precision mass flow measurements made using an electronic balance and size-distribution measurements using a QIPIC® particle size and shape analyzer. DIH is then employed to measure particulate flows in a small-scale circulating fluidized bed system by imaging the standpipe region with 12.7 mm ID. Solid circulation rates evolution was measured and its mean value was in good agreement with the pressure-based estimations. Moreover, DIH is demonstrated to be capable of resolving fine particles with 10 – 40 µm in size, resulting from attrition in the bed of solids. Different from large particles that is dominated by gravitational settling, the motion of small particles is mainly affected by the flow, which has great potential to be used to investigate flow conditions around the large particles.

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.

Modelling of a combined biomass CLC combustion and renewable-energy-based methane production system for CO2 utilization

In this paper, an innovative process layout to promote the integration among the chemical looping combustion of biomass, solar hydrogen, and carbon methanation is proposed and numerically investigated. The core of the layout consists of a multiple interconnected fluidized bed (MFB) system for the chemical looping combustion of solid fuels. A coupled hydrodynamic and reactive model of the system was developed and applied to evaluate solid circulation rate, solid bed levels in different parts of the system, flue gas composition and flow rate, and power production. The performance of the system was evaluated by considering chemical and physical properties of olive wood, a biomass typical of Mediterranean area, as fuel and of CuO supported on zirconia as oxygen carrier, respectively. A complex reaction scheme comprising both gas–solid heterogeneous and gas-phase homogeneous reactions was considered. Results corresponding to steady state operation of MFB are presented in conjunction with an analysis of the operability of the system under the considered ranges of operating conditions.By considering that only energy coming from renewable sources (such as photovoltaic panels or wind turbines) was fed to the electrolysis cell (EC) array, the potential of the proposed process to be used as an energy storage system was assessed.

Experimental demonstration of pressurized chemical looping combustion in an internally circulating reactor for power production with integrated CO2 capture

This study presents an experimental demonstration of pressurized chemical looping combustion (CLC) in an internally circulating reactor (ICR). The ICR concept is a novel alternative to the conventional interconnected fluidized bed CLC configuration as it eliminates all cyclones, loop seals and solids transport lines, and it can be pressurized in a single pressure shell. Stable operation with high fuel conversion was established for about 40 h of operation at pressures up to 6 bar, achieving reasonable CO2 purity and capture efficiency (up to 97%). The solids circulation rate was found to increase with increasing the operating pressure at a constant fluidization velocity with no effect on CO2 capture and purity. The experimental campaign also examined the effects of solids inventory and fluidization velocities in the air and fuel reactors. The CO2 purity and capture efficiency were most sensitive to the solids inventory, whereas the solids circulation rate was most sensitive to the air reactor fluidization velocity and the solids inventory. A correlation for solids circulation rate was derived from the collected experimental data, thus providing a robust tool for designing an ICR system for pressurized operation. This correlation can assist in further scale-up and demonstration of the ICR concept in commercial scale.

Axial flow structure of solids holdup in an 18-m high-density CFB riser based on pressure measurements

The axial flow structure in a high-density CFB riser having a height of 18m is investigated on the basis of pressure measurements. Solids circulation rates reach 1400kg/(m2s) at superficial gas velocities of 5–9m/s and the apparent solids holdup exceeds 0.2, indicating high-density operations have been achieved. The apparent solids holdup increases with the solids circulation rate increasing and/or superficial gas velocity decreasing. Axial distributions of the apparent solids holdup have exponential shapes with denser regions at the bottom and more dilute regions in the upper part. The apparent slip velocity increases with the increasing solids holdup and reaches 14m/s, showing that there are more opportunities of cluster formation in high-density operation. Furthermore, the apparent slip velocity has a power relation with the apparent solids holdup under a wide range of operating conditions.

A novel autothermal fluidized bed reactor for concentrated solar thermal applications

In the present study a novel directly-irradiated fluidized bed autothermal reactor (DIFBAR) for solar-driven chemical processing is presented. The reactor exploits the very concept of the autothermal reactor applied to multiphase flow of fluidized solids: a gas-solid suspension is preheated in a tubular riser prior to being exposed to radiative flux and further heated up in a concentrated solar receiver. Preheating is accomplished by solids leaving the receiver as they flow countercurrently in an annular dipleg coaxial to the riser. The dipleg may be designed so as to make the receiver section gas leak-tight with respect to the other zones of the reactor. An experimental campaign was performed under non-reactive conditions and at moderate radiative flux, hence moderate temperatures, with the purpose of demonstrating the feasibility and operability of the autothermal reactor, as a preliminary step toward a full proof-of-concept. Solids circulation rates and temperatures were measured as a function of the gas superficial velocity in the riser under given simulated solar irradiance. Infrared thermography was applied to further characterize multiphase flow and thermal patterns in the receiver. Experimental data were worked out to calculate the annulus-to-riser heat transfer coefficients. The reactor demonstrated excellent “autothermal” operation, thanks to very efficient heat transfer across the riser-annulus interface. Collection of solar energy was fairly good, with maximum thermal efficiency close to 70%, despite the design of the receiver was not specifically optimized to minimize radiative and convection losses. A preliminary analysis aimed at the scale-up of the autothermal reactor is presented. Altogether, results obtained are quite promising and stimulate further research on the development of reactors based on the proposed design for solar chemistry applications.

Solids flow characteristics and circulation rate in an internally circulating fluidized bed

Internally circulating fluidized beds (ICFBs) enable effective control of the reactions and heat distribution in reactors. The ICFB contains two or more connected fluidized regions with different gas velocities to promote controlled solid circulation. The control of solid circulation rate (G0) is a critical factor. We recorded single particle trajectories by tracing a fluorescent particle, based on which particle flow behaviors were analyzed in different regions. G0 was obtained for a wide range of operating parameters. An increase in gas velocity in the down- and upflow beds shortened the particle circulation time in both beds and G0 increased significantly. As the static bed height increased, the differential pressure on both sides of the circulation port increased, which resulted in an increase in the solid circulation rate. As the orifice area increased, the flow resistance through the orifice decreased and thus the solid circulation rate increased. G0 increased with the decrease in particle size. The gas velocity in the upflowing bed and orifice area was the most important parameter to control the solid circulation rate. G0 was compared with the experimental measurements in literature and predictions using the correlation based on Bernoulli’s equation, and they agreed well.

On the performance of a spouted bed type device for feeding spent coffee grounds to a circulating fluidized bed reactor

Using solid biomass wastes as feedstocks for industrial generation of renewable energy is appealing to meet company's energy demands with environmental awareness; however, maintain continuous feeding of wastes to reactors remains a challenge in processing. In this study, the performance of a non-mechanical spouted bed (SB) feeder was evaluated for handling dry and moist spent coffee ground (SCG) powders to a circulating fluidized bed reactor. A comprehensive experimental study was carried out to investigate the effects of operating variables and air inlet nozzle configuration on the solids circulation rates and pressure profiles. Using a convergent nozzle at the air inlet improved feeding stability compared to a conventional orifice air inlet. The circulation rates could be varied from 3.0<WS<11.0g/s by adjusting the air flowrate and the entrainment length. Although SB performance was affected by the powder's flowability, the feeder operation was rather stable, with fluctuations less than 20% in WS. Moreover, WS could be accurately predicted from pressures measured at the feeder and riser. The SB showed enhanced flexibility in handling SCGs with different properties compared to a non-mechanical L-valve. It suggests that SBs might be an appealing alternative to feed waste biomass powders into reactors in a continuous mode.

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.

Evaluation of sorption-enhanced reforming over catalyst-sorbent bi-functional particles in an internally circulating fluidized bed

An internally circulating fluidized bed (ICFB) has been applied in various industrial processes owing to its potential in the reduction of heat loss and compact size. In this work, the sorption-enhanced reforming process in the ICFB is investigated. The dense discrete phase model (DDPM) is employed to evaluate the performance of catalyst-sorbent bi-functional particles, considering the particle size distribution. The results demonstrate that the utilization of bi-functional particles can significantly increase hydrogen production. The impacts of operating parameters including solid loading and regenerator velocity on solid circulation rate and gas leakage are examined. It is found that the gas leakage between reactors is increased by 46.6% when the regenerator gas velocity varies from 1.8 m/s to 2.4 m/s so as to weaken the hydrogen yield.

Two-dimensional full-loop simulation of CO2 capture process in a novel dual fluidized bed system

The integrated bubbling-transport fluidized bed can realize both sufficient gas-solid contact time and adjustable solid circulation rate. This paper performed a full-loop simulation of continuous CO2 capture process in a dual fluidized-bed system, using the bubbling-transport bed as the adsorber and K2CO3/γ-Al2O3 as the sorbents. Two-dimensional Eulerian-Eulerian models coupling with reaction kinetics were applied in the simulation. The results show that the adsorption reaction mainly takes place in the bubbling section of the adsorber, but rarely in the central riser and transport section. The increase of the sorbent circulation rate and the water vapor concentration in flue gas benefit the CO2 capture performance, and the latter is more preferable. Together with the sorbents, part of flue gas is entrained from the bubbling section to the central riser, causing an apparent fluctuation of sorbent circulation, as well as a low global CO2 capture efficiency due to the insufficient gas-sorbent contact.