Solids Circulation Rate Research Articles

Loop-seal flow characteristics of a circulating fluidized bed for 3 MWth scale chemical looping combustion system

This study aimed to provide a design and operational approach for a loop-seal, which is the heart of a circulating fluidized bed (CFB). Thousands of CFB systems use this non-mechanical valve to optimize performance by controlling the solids circulation rate while preventing gas mixing between reactors. Nevertheless, its design and operating guidance remains unclear because of its intricate flow characteristics, causing confusion in design and operation of the entire CFB. This study experimentally investigated the three most important elements while designing and operating loop-seal: conditions for initiating solids flow, the maximum obtainable solids flow rate, and quantitative control of solids flow rate. Experiments were conducted in a CFB, which simulated 3 MWth chemical looping combustion (CLC) system that is currently being developed. The onset of solids flow occurs when the gas drag force in horizontal passage overcomes the resistance force generated by the bed of particles. The maximum solids circulation rate was determined by solids height in the supply chamber and pressure around CFB loop. A correlation for quantitative control of solids flow in loop-seal was proposed by the relationship between the gas drag force and particle velocity. Finally, operational conditions for the loop-seal in 3 MWth CLC were proposed.

Numerical study on flow dynamics in the supercritical water circulating fluidized bed riser

There are a few reports about supercritical water circulating fluidized beds (SCWCFBs), and simulations were conducted via a two-fluid model to investigate flow dynamics in a riser of SCWCFB across different flow velocities, solid circulation rates, pressures, and temperatures. The investigated characteristics include the void fraction distributions, fluid velocities, particle velocities, and drag forces. The results show that the flow characteristics in the SCWCFB riser are similar to those in the traditional gas–solid riser. In the supercritical water fluidized bed riser, the void fractions are mainly in the range of 0.85–0.95. They are low at the bottom but high at the top and low near the wall but high at the center. Particle velocities are mainly distributed between 0 and 1 m s−1. They are upward near the axis of the riser and downward near the walls, showing the annular-core flow structures. Fluid paths are tortuous at low fluid velocities but straight at high fluid velocities. Particle drag forces are mainly around 0.7–0.8 times particle weights. The effects of pressures on flow dynamics in the SCWCFB riser can be ascribed to the changes in density and viscosity of supercritical water, affecting the ability of fluid to carry particles.

Gas/particle flow and pressure variation characteristics of Ende pulverized coal gasifier equipped with ejector

AbstractEnde pulverized coal gasifier (EPCG), as a typical representative of circulating fluidized bed gasifier, has been widely used in gasification field. The return conduit of EPCG is generally not equipped with a feedback control device (FCD), which tends to result in a low solid circulation rate (Gs) and high unburned combustibles in fly ash. To address these issues, a new technique using the ejector as the FCD was designed to provide a stable pressure barrier and improve solid circulation. Preliminary thermal experiments demonstrated the feasibility of improving the EPCG with injector system. Currently, the specific mechanisms that contribute to further optimize this technology have not been systematically studied. Considering the large impact of ejector operation on solid circulation rate and stability as well as syngas production, this work adopts a gas/particle 1:15 cold EPCG bench to investigate the gas/particle flow and pressure variation characteristics before and after ejector operation and under different working medium pressures (Pp). When the ejector operates at the design pressure Ps = 0.187 MPa, the outlet pressure of the return conduit is slightly larger than the furnace bottom pressure, thus preventing the reverse gas flow at the furnace bottom. The particle velocity and concentration distribution in the furnace are basically consistent with that without the ejector, showing a similar central spout region and near‐wall annulus flow region. As Pp increases from 0.04 to 0.24 MPa, the average particle velocity in the return conduit increases from 0.95 to 2.4 m/s, the particle concentration decreases from 0.088 to 0.051, and the solid circulation rate increases from 0.37 to 0.53 kg/h. The ejector outlet pressure gradually increases, the inlet pressure decreases, and the pressure barrier provided by the ejector rises from 1052 to 3640 Pa. When Pp is 0.12 MPa, the ratio of GN2/Gair is 0.91%, which can achieve the purpose of continuous and stable pressurization with a small amount of nitrogen.

Simulation of sorption-enhanced gasification: H[formula omitted]O staging to a circulating fluidised bed gasifier to tailor the producer gas composition

Sorption-enhanced gasification (SEG) is an advanced gasification technology for biofuel and biochemical synthesis processes. The SEG process allows tailoring of the producer gas composition for different synthesis processes by adjusting the temperature of the gasifier. Conventionally, the solids circulation rate from the combustor to the gasifier has been used to tailor producer gas composition. In this study, H2O staging is investigated in a circulating fluidised bed (CFB) gasifier as an alternative approach. The H2O staging can be used to decrease the riser temperature locally to enhance the main chemical reactions. The H2O staging as gas and liquid phases were considered in this study. Also, reactor cooling was combined with the staging. A validated one-dimensional CFB gasifier model was used to investigate the process. The simulation results indicate that H2O staging and cooling enhance the gasifier’s limestone carbonation and water-gas shift reactions. Thus, allowing to tailor the producer gas composition directly by adjusting the gasifier’s operating parameters. The methods investigated increase flexibility for operating the SEG CFB gasifier and the reactor system. Understanding different operation methods of the process and their impact on the system’s operation enables more flexible use and design of the gasification process.

Synthetic oxygen carrier C28 compared to natural ores for chemical looping combustion with solid fuels in 80 kWth pilot plant experiments

Chemical Looping Combustion (CLC) is a highly efficient CO2 separation technology with no direct contact between combustion air and fuel. A metal oxide is used as oxygen carrier (OC) in a dual fluidized bed to generate clean CO2. The use of solid fuels, especially biomass, is the focus of current research, because of the possibility of “negative” CO2-emissions. The OC is a key component, because it must meet special requirements for solid fuels, which are different to gaseous fuels. Most frequently naturals ores or synthetic materials are used as OC. Synthetic OC are characterised by higher reactivity at the expense of higher costs. For this reason, so far not so many experiments have been conducted on a larger scale with synthetic OC on solid CLC. This work deals with the synthetic perovskite C28 and investigating the suitability as oxygen carrier in an 80 kWth pilot plant for chemical looping combustion with biogenic fuels. The experiments show a significantly increased combustion efficiency of 99.6 % compared to natural ores and a major influence of the solid circulation rate on general performance, whereby carbon capture rates up to 98.3 % were reached. Furthermore, the role of the fuel reactor's counter-current flow column and its impact on better gas conversion was investigated. C28 suffered no deactivation or degradation over the experimental time, but first traces of ash layer formation, phase shifting and attrition of fines could be detected. The focus of further research should lie on long-term stability and reactivity for their high impact on the economic scale up of C28.

Outstanding performance of a Cu-based oxygen carrier impregnated on alumina in chemical looping combustion

Chemical looping combustion (CLC) allows the combustion of a fuel with inherent CO2 capture by using an oxygen carrier to transfer the oxygen from the air to the fuel. Previously to the CLC scale-up, tests should be done in lab-scale CLC units to determine the combustion performance of promising oxygen carriers and, at the same time, to evaluate the durability of these materials. For that, the effect of operating conditions on the oxygen carrier ageing should be considered, which is often disregarded. In this work, tests were done burning a simulated biogas stream in a continuous 500 Wth CLC unit with a promising Cu-based oxygen carrier (Cu14Al_ICB). Operating conditions were carefully selected to maximize the durability of this oxygen carrier while complete CH4 combustion was achieved. These operating conditions were based on limiting both the variation of solids conversion by choosing suitable solids circulation rates and the oxidation degree of the particles in the air reactor by minimizing the air excess. Thus, the Cu14Al_ICB material exhibited an excellent mechanical resistance, estimating a particle lifetime of 20,000 h at 900 °C. In addition, copper migration to the surface of the particles and loss of copper were not relevant. This is the first time that under certain operating conditions, a Cu-based oxygen carrier can exhibit outstanding mechanical properties during continuous CLC operation at 900 °C.

Prediction of solid circulation rate in an internal circulating fluidized bed: An empirical and ANN approach

Internal Circulating Fluidized beds (ICFBs) are interested in various industries because of their higher thermal efficiency and high reaction rates. However, understanding the complex flow hydrodynamics inside an ICFB is challenging. Also, the experiments needed are expensive and time-consuming. Therefore this study aims to predict the solids circulation rate in an in-house ICFB (0.3 m internal diameter x 3.0 m height) using an empirical model and an Artificial Neural Network (ANN) technique. The solid circulation rates measured at different operating and design conditions using a high-speed video camera are utilized to develop the models mentioned earlier. A dimensionless approach and nonlinear regression models are adopted to derive the empirical model. The Analysis of Variance (ANOVA) technique calculates F-number and their corresponding probabilities (P-values). The ANN model is developed with four input variables: particle size, static bed height, gap height, and gas superficial velocity. Multi-layer Perception model (MLP) with the Feedforward Back Propagation learning rule is employed to build the ANN model. A single hidden layer with nine neurons predicted a close solid circulation rate with minimal error over the empirical model compared to experimental data. Further, the empirical and ANN model's predicting capability is tested against literature data.

Solids inventory and external solids circulation rate in risers of circulating fluidized beds

• Solids flow rate in riser exit F s depended on riser solids amount M s . • F s depended on elutriation rate constant. • F s for circulating fluidized bed is similar with that for batch riser. • F s decreased with increasing riser diameter D. • M s increased with D. This study was conducted to provide improved relationships on the external solids circulation rate and solids inventory in the riser of the circulating fluidized bed (CFB) through increasing the basic understanding on how the solids flow rate in the riser exit establishes. The relation between the solids inventory in the riser and the solids flow rate in the riser exit was investigated in batch elutriation and CFBs, using air as fluidizing gas at atmospheric pressure and temperature. A batchwise riser (0.1 m-i.d., 2.75 m-height) and the riser of a CFB system (0.1 m-i.d., 2.5 m-height) were used to measure the relation for different gas velocities (1.5–2.4 m/s) and groups of solids (0.064–0.201 mm in diameter, 2045–4080 kg/m 3 in apparent density). The batch elutriation indicated that the amount of solids inventory in the riser determined the solids flow rate in the riser exit, increasing with the amount of the solids inventory in the riser. The solids flow rate in the exit of the riser strongly depended on the elutriation rate constant combined with the amount of the solids inventory in the riser. When the riser was same in size, the relationship on the solids flow rate in the riser exit, derived in the batch elutriation, was valid in the CFB. Relationships on the external solids circulation rate and solids inventory in the riser of the CFB were proposed based on the data from this study and the literature. It was found that the external solids circulation rate per cross-sectional area of the riser decreased with increasing the riser diameter, however, the static bed height of solids in the riser (h st,c ) increased with the riser diameter. The dependency of h st,c on riser diameter was greater in the fast bed than in the bed of pneumatic conveying.

Effect of bed material size on gas-solid flow characteristics in a CFB at low solid recirculation rates

The gas–solid flow characteristics greatly influence the heat transfer, combustion and pollutant formation processes in a circulating fluidized bed (CFB) boiler. The effects of particle size on the gas–solid flow characteristics, including flow structure, axial pressure and bed density profiles were studied in a two-dimensional CFB cold apparatus under low solid circulation rates (Gs’s). At the same time, an optical fiber probe was used to assess the particle size effect on the formation of particle clusters and their properties under different operating conditions. Results revealed that when the averaged size of bed materials decreased from ∼200 μm to ∼100 μm, at the same operating condition, the bottom dense bed became remarkably higher, and the upper dilute zone had stronger pressure fluctuation, more even axial and horizontal particle distributions, and higher solid density. Gs was more sensitive to the variation of the superficial gas velocity than the bed inventory. In addition, the standard deviation and power spectrum density of instantaneous pressure decreased in the dense bed and increased in the dilute zone. The particle velocity was slightly lower in the lower section of the riser but noticeably greater in the upper riser section. In addition, the clusters formed more frequently and lasted significantly longer. The study showed that flow characteristics in the bed qualitatively changed from that of Type B particles to that Type A particles in the Geldart’s classification, even at a small Gs, and previous studies with Type B particles could very likely not apply in the CFB boilers operating with fine bed materials.

Impact of Particle Size Polydispersity on Chemical Looping Combustion of Biomass under a Continuous Injection Mode

Particle size polydispersity considerably influences the chemical looping combustion (CLC) of solid fuels. In the present work, a three-dimensional multiphase particle-in-cell (MP-PIC) method was established and applied to study the full-loop CLC of biomass under a continuous injection mode. After model validation, the gas–solid flow characteristics, solid circulation rate (SCR), particle mixing in a fuel reactor (FR), and biomass leakage at different oxygen carrier (OC) particle size distribution (PSD) widths and biomass densities were systematically explored. The results showed that OC particles significantly stratified within the FR and both loop seals. Moreover, the fluctuation amplitude of the SCR decreased as the OC PSD width increased. Small biomass particles were more likely to leak. The number fraction of total leaked biomass particles first decreased and then increased as the OC PSD width increased. In the FR, small OC particles were primarily distributed in the upper part of the bed, and large particles were mainly distributed in the lower part. In addition, within the test range, the magnitude of the density difference between biomass and OC particles positively correlated with the likelihood of biomass leaking. These observations are all beneficial for the design and optimization of the CLC process of biomass.

Numerical study on hydrodynamics of gas–solids circulating fluidized bed with L-valve

L-valve is often used as a non-mechanical valve for the circulation of solids in gas–solids fluidized bed (GSFB) due to its advantages in simple construction and easy control. The information on solids circulation rate as well as the hydrodynamics performance of the CFB with L-valve is of great importance for its better control and design. This paper proposes a Eulerian-Eulerian approach based numerical model integrating the computational fluid dynamics (CFD) with turbulent model, the kinetic theory of granular flow (KTGF) and the drag model, thus the solids circulation rate and the local phase velocity as well as solids volume fraction can be predicted simultaneously. With this model, the hydrodynamics performance of the full loop GSCFB with a L-valve is analyzed in detail. It is found that the drag model affects the simulation significantly and the (energy minimization multiscale) EMMS method shows good performance in the full-loop simulation of GSCFB.

CFD simulation of hydrodynamics and heat transfer characteristics in gas–solid circulating fluidized bed riser under fast pyrolysis flow condition

• Proper heat is needed for Circulating fluidized bed riser for fast pyrolysis. • A 3D EE-TFM model was developed and validated with experiment. • Hydrodynamics and heat transfer characteristics were examined for CFB riser. • Demonstrate the gas–solid heat transfer affected under different hydrodynamics. Pyrolysis process through circulating fluidized bed (CFB) is a promising technology to produce synthetic fuel and other products from biomass feedstocks. Computational fluid dynamics (CFD) computing means provide valuable insights to better understand gas–solid flow hydrodynamics, troubleshoot performance issues and optimize reactor operations. In this study, gas–solid flow hydrodynamics and heat transfer characteristics of a CFB riser for fast pyrolysis are investigated using a three-dimensional (3D) Eulerian-Eulerian CFD model. The main observations are discussed to provide insights on the factors affecting CFB riser performance. The model parameters, specularity and particle–particle restitution coefficients, were considered and tuned to accurately predict of gas–solid flow hydrodynamics and heat distribution with respect to different gas velocities and solid circulation rates. The results have shown that the CFD model predicted well the flow hydrodynamics and both specularity and particle–particle restitution coefficients are critical parameters as they affect particle behavior and temperature distribution fields. The lower specularity coefficient (Φ − 0.00001) was fairly able to predict the axial solid holdup profile into CFB riser. However, the lower value for particle–particle restitution coefficient ( e s s - 0.8) significantly overpredict the bottom dense region. The results shows that the increase of operating velocity promotes the mixing behaviors and heat transfer performance. In this work the suitable gas and solid circulation flow rates are U g = 4.5 m/s and G s = 81.23 kg/m 2 s.

Semi-continuous Operation of Chemical Looping Combustion of Coal Using a Low-Cost Composite Oxygen Carrier

To reach the carbon peak by 2030 and achieve carbon neutrality by 2060, the implementation of low-carbon combustion approaches for fossil fuels (especially coal, a high-carbon fuel) is urgent in China. Chemical looping combustion (CLC) has been considered as one of the most promising technologies for low-carbon, efficient, and clean utilization of coal. However, a low-cost and high-performance oxygen carrier (OC) holds the bottleneck that constrains the industrial application of coal-fueled CLC. In this paper, a low-cost composite OC named Cu13.0Red87.0@C (with the mixing mass ratio of copper ore to red mud being 13.0:87.0, except for a 20 wt % cement bonder) was first prepared by the hydroforming method and then systematically evaluated by designing CLC tests in a semi-continuous fluidized bed using lignite as fuel. The results showed that this composite OC exhibited superior combustion performance in comparison to pure red mud. Moreover, it was found that increasing the temperature could significantly improve the average carbon capture efficiency but had a relatively small positive effect on the average CO2 yield. Additionally, with the elevation of the oxygen/fuel ratio, the resulting average carbon capture efficiency and average CO2 yield both tended to increase. Generally, the semi-continuous unit can simulate the continuously operated fuel reactor with an adjustable bed inventory and solid circulation rate, and it is easy to attain smooth operation of ca. 30 min, achieving both carbon capture efficiency and a CO2 yield of ca. 90%.

Studies on the cyclone dipleg flow characteristics in a CFB for designing 3 MWth scale chemical looping combustor

3 MWth scale chemical looping combustion (CLC) system using circulating fluidized bed (CFB) is being developed as a liquidized natural gas power generation technology to reduce CO2 emissions. Dipleg, considered a blood vessel of CFB, conveys solids captured by cyclone to the fluidized bed while preventing gas mixing. This vertical passage plays key role in solids circulation and cyclone performance in thousands of CFB systems, yet only a limited information is available on its design and operation. This study experimentally investigated the flow characteristics in dipleg to verify design and operating conditions for smooth solid flow while predicting the amount of gas mixing. When solid circulation rate increased, both upward and downward gas flowrates in dipleg increased. It was found that the sum of two flowrates were equal to the volumetric flowrate of solids in the dipleg. Unstable operation occurred when 1) the gas-solid relative velocity in the dipleg became higher than the minimum slugging velocity, and 2) bubble diameter in bubbling bed became larger than the dipleg diameter. Finally, models for operation range and the amount of gas mixing in dipleg were proposed, and the design and operation range of the dipleg in 3 MWth scale CLC was suggested.

Preliminary study on a counter-current bubble column near flooding point and an inverse gas-liquid-solid circulating fluidized bed

The hydrodynamics of an inverse gas-liquid-solid circulating fluidized bed (IGLSCFB) with downflow liquid and upflow gas was studied for the first time as a promising candidate bioreactor for biological processes. Experiments were carried out in a 0.076 m ID column with 5.4 m height using low-density particles. The hydrodynamics of the rarely studied high liquid velocity counter-current bubble column is also investigated. In IGLSCFB, both gas and solids holdups are axially uniform under lower gas and liquid velocities, but holdups become axially non-uniform at higher operating velocities. The gas holdup increases with superficial gas velocity and superficial liquid velocity but is not sensitive to solids circulation rate. Solids holdup increases with solids circulation rate and superficial gas velocity but decreases with superficial liquid velocity. In comparison with inverse liquid-solid circulating fluidized bed, solids holdup in IGLSCFB is significantly higher, potentially enhancing liquid-solid phase contact in the fluidized bed.

Chemical looping gasification and sorption enhanced gasification of biomass: A perspective

Chemical looping gasification (CLG) and sorption enhanced gasification (SEG) of biomass are promising technologies to improve the energy efficiency and simplify the processes for producing renewable hydrogen, synthetic fuels, and chemicals. In terms of process intensification, CLG achieves inherent air separation through the use of oxygen carriers, eliminating the need of air separation units. With inherent CO2 separation enabled by CO2 sorbents, SEG can directly generate hydrogen-rich syngas (>70 vol.% H2) without individual water-gas shift and CO2 separation units. The state-of-the-art of the reactors, functional materials, and modeling works for CLG and SEG is reviewed in this paper. The impact of essential process parameters, including temperature, solid circulation rate, and steam-to-biomass ratio, is summarized. The research needs and outlook of CLG and SEG are discussed. Combining the perspectives of both basic research and practical application, this paper is expected to serve as a valuable guide for academic research groups, industry stakeholders, and policy makers seeking to research, invest in, or evaluate biomass CLG and SEG.

Hydrodynamics of an inverse liquid–solid circulating conventional fluidized bed

AbstractAn inverse liquid–solid circulating conventional fluidized bed (I‐CCFB) is realized by injecting particles from the top of a conventional liquid–solid fluidized bed (0.076 m ID and 5.4 m height) that is operated in a newly developed circulating conventional fluidization regime located between the conventional and circulating fluidization regimes. The I‐CCFB can achieve a higher solids holdup compared to both conventional and circulating liquid–solid fluidized beds. A new parameter, the bed intensification factor, is defined to quantify the increased solids holdup observed with external solids circulation. The Richardson–Zaki equation is shown to be applicable to the I‐CCFB regime and can be used to correlate the slip velocity and solids holdup, both of which increase with the solids circulation rate. A new flow regime map is presented, including the I‐CCFB and a variety of other liquid–solid fluidized beds.

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed, named circulating conventional fluidized bed (CCFB) which operates below particle terminal velocity was proposed and experimentally studied. The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D. and 4.5 m in height with five different types of particles. Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity. The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates. The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions. The effect of particle properties and operating conditions on bed behavior was investigated as well. Results show that particles with higher terminal velocity have higher average solids holdup.

Hybrid CFD-neural networks technique to predict circulating fluidized bed reactor riser hydrodynamics

Circulating fluidized bed (CFB)-based co-pyrolysis is a promising technology for producing synthetic fuel and chemical co-products from biomass and waste feedstocks. Computational fluid dynamics (CFD) tools provide valuable insights for understanding gas-solid flow hydrodynamics, troubleshooting performance issues, and optimizing reactor operations. CFD simulations are computationally expensive and time-consuming; therefore, in this study, we developed an artificial neural network (ANN)-based machine learning model from a wide CFD simulation campaign. The CFB riser axial solid holdup profile was predicted using a two-fluid model and validated using the experimental data. Finally, a dataset connecting the input features of the model parameter-based simulations with their axial solid holdup was constructed for training the ANN. The performance of the developed model was later compared with the experimental data. The developed ANN model exhibited low mean square error values of order of 10−3 for both validation datasets to predict axial solid holdup. The ANN model performed well with an interpolated solid circulation rate of 45 kg/m2s and 62 kg/m2s and an extrapolated solid circulation rate of 30 kg/m2s and 80 kg/m2s.

Measuring Technologies for CFB Solid Circulation Rate: A Review and Future Perspectives

Solid circulation rate (Gs) represents the mass flux of circulating particles in circulating fluidized bed (CFB) systems and is a significant parameter for the design and operation of CFB reactors. Many measuring technologies for Gs have been proposed, though few of them can be applied in industrial units. This paper presents a comprehensive study on measuring technologies, and the results indicate that though the accumulation method is most widely applied, it is constrained by the disturbance of normal particle circulation. Some publications have proposed mathematic models based on pressure drop or other parameters to establish Gs measurement models; these necessitate the accurate modeling of complicated gas-solid flows in industrial devices. Methods based on certain measurement devices to specify parameters like velocity require device endurance in the industrial operation environment and stable local gas-solid flow. The Gs measuring technologies are strongly influenced by local gas-solid flow states, and the packed bed flow in standpipes make the bottom of standpipes an ideal place to realize Gs measurement.