Dual Circulating Fluidized Bed Research Articles

Computational particle fluid dynamics simulation of gas–solid flow in a 3 MWth dual-circulation fluidized bed for chemical looping process

Regulation of gas–solid flow is crucial for optimizing the operation efficiency of dual-circulating fluidized beds that are considered to be the most appropriate type of chemical-looping reactors. Herein, a computational particle fluid dynamics method was employed to simulate the gas–solid flow in a 3-MWth dual-circulating fluidized bed used for chemical-looping combustion and gasification. The influence of structural difference between units on particle residence time was determined. The multi-parameter control mechanism of pressure, particle circulation, and particle residence time in a whole-loop system was investigated. Results revealed that under stable particle circulation, the particle residence time in the fuel reactor is much longer than that in the air reactor. The axial forces on the particles are reduced upon increasing particle density and size, leading to particle accumulation in the dense-phase zone. When the particle properties are stable, increasing the fluidizing gas flow rates by the same proportion leads to identical pressure drops on the involved two loop seals, which cause symmetrical alterations in the particle circulation rate between the air and fuel reactors. The dual-circulating fluidized bed exhibits certain multi-condition adaptability, which is limited by the stock bin volume. Overall, this study is beneficial for effective and economical optimization of the operation of chemical-looping dual-circulating fluidized beds.

Full-Loop Study of a Pilot-Scale Dual Fluidized Bed Cold Flow System: Hydrodynamic Simulation

Computational particle fluid dynamics (CPFD) simulations of three-dimensional cold-flow gas-solid flow were performed for a pilot-scale dual-circulating fluidized bed (DCFB) using Barracuda Virtual Reactor® 17.4.0. This DCFB system is a fluidized bed gasifier used in a calcium-loop gasification process to produce a hydrogen-rich gas with CO2 capture. The effects of different bubble bed inlet airflow rates and different solid inventories on the full-loop pressure distribution and particle concentration distribution were compared. The increase in the bubble bed inlet airflow rate significantly increases the pressure in the bubble bed and lift pipe, and the increase in the solids inventory significantly increases the pressure where the solids are tightly packed. For long-term operation, the appropriate combination of parameters should be found to maintain a stable particle seal at the loopseals.

Bed inventory balance and stability of dual circulating fluidized bed systems

Experiments and modelling are conducted for general understanding on the imbalance phenomenon and for discussion on the strategies to improve the bed inventory balance and stability of a dual circulating fluidized bed (DCFB) system. The experiments are carried out in a 15.5 m high pilot-scale DCFB cold test system. A fluid-dynamic DCFB model is developed based on a 1.5-dimensional semi-empirical model of an industrial CFB boiler, validated by experimental data. The effects of control methods and operation conditions on the bed inventory balance and on the stability of the pilot- and large-scale DCFB systems are discussed. A “stable-unbalanced” state, where the system reaches a steady state, consisting of unbalanced bed inventories, is observed in both experiments and model simulation. To maintain a balanced state and similar bed inventory in the DCFB systems with similar cross-sectional areas, it is recommended to keep similar total pressure drops or gas velocities in both reactors.

Study of biomass gasification in an industrial-scale dual circulating fluidized bed (DCFB) using the Eulerian-Lagrangian method

Dual circulating fluidized bed (DCFB) has emerged as an efficient reactor for biomass gasification due to its unique feature of high gas-solid contact efficiency and separated reactions in two reactors, yet the understanding of complex in-furnace phenomena is still lacking. In this study, biomass gasification in an industrial-scale DCFB system was numerically studied using a multiphase particle-in-cell (MP-PIC) method featuring thermochemical sub-models (e.g., heat transfer, heterogeneous reactions, and homogeneous reactions) under the Eulerian-Lagrangian framework. After model validation, the hydrodynamics and thermochemical characteristics (i.e., pressure, temperature, and species) in the DCFB are comprehensively investigated. The results show that size-/density-induced segregation makes solid fuels concentrate on the bed surface. Interphase momentum exchange leads to the continuous decrease of the gas pressure axially. In the gasifier and combustor, the lower heating value (LHV) of the gas products is 5.56 MJ/Nm3 and 0.2 MJ/Nm3 and the combustible gas concentration (CGC) is 65.5% and 1.86%, respectively. The temperature in the combustor is about 100 K higher than that in the gasifier. A higher solid concentration results in a smaller value of particle heat transfer coefficient (HTC). The HTCs range from 50 to 150 W/(m2 K) for a solid concentration larger than 0.3 in the combustor while the HTCs range from 100 to 200 W/(m2 K) in the gasifier. The Reynolds number of biomass particles is two orders of magnitude larger than that of the sand particle. The numerical results shed light on the reactor design and process optimization of biomass gasification in DCFBs.

Relevance of oxygen carrier properties on the design of a chemical looping combustion unit with gaseous fuels

AbstractChemical looping combustion (CLC) is a novel technology for the combustion of fuels with inherent CO2 capture. CLC is based on the transference of oxygen from air to fuel by means of an oxygen carrier which is based on a metal oxide. CuO/Al2O3 (14 wt.% CuO) and Fe2O3/Al2O3 (20 wt.% Fe2O3) particles have been used in several CLC units with different results when methane combustion was evaluated. In order to shed light on the main processes affecting methane conversion in CLC, a mathematical model for a dual circulating fluidized bed (DCFB) system was developed to simulate the behavior of CuO/Al2O3 and Fe2O3/Al2O3 in a CLC unit. The model consists of the coupling of individual fuel and air reactor models to simulate steady state of the CLC unit. Individual models consider both the fluid dynamics of the fluidized beds at the high velocity regime and the corresponding kinetics of oxygen carrier reactions, that is, reduction in the fuel reactor and oxidation in the air reactor. The model was validated using results obtained in a 120 kWth CLC unit with CuO/Al2O3 and Fe2O3/Al2O3 particles. The validated model was used to simulate the performance of these materials in the 10 MWth CLC unit. Desing parameters of the fuel and air reactors as well as the suitable particle size of the oxygen carriers were determined in order to achieve the complete combustion of natural gas in the CLC unit as a function of the oxygen carrier properties. © 2022 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Eulerian-Lagrangian simulation of chemical looping combustion with wide particle size distributions

Chemical looping combustion (CLC) is a promising technology efficiently utilizing fossil fuels to reduce CO2 emissions. This work numerically studies the CLC process in a three-dimensional dual circulating fluidized bed (DCFB) using a reactive multi-phase particle-in-cell (MP-PIC) method based on the Eulerian-Lagrangian framework with the discussion of the effects of particle size distribution (PSD) width on bed hydrodynamics and system performance. The PSD is described by a log-normal function. A wider PSD width is produced by adjusting the variation of standard deviation. The results show that the PSD width exerts a significant influence on the physical and thermochemical characteristics, including particle residence time, solid holdup, gas component concentration distribution, chemical reaction rates, oxygen transport efficiency, and fuel conversion rates. A larger PSD width gives rise to a shorter particle residence time in the reactor and enhances the particle exchange and oxygen transport efficiency between the air reactor and fuel reactor. Moreover, the CO2 yield and the fuel conversion rates are promoted under a larger PSD width.

Part-Load Operation of a Novel Calcium Looping System for Flexible CO2 Capture in Coal-Fired Power Plants

The increasing share of renewable energy supply is forcing fossil-fueled power plants to undergo flexible operation, with large load changes during the day and even periods of complete shutdown. Dual circulating fluidized bed (DCFB) calcium looping (CaL) has rapidly emerged as a viable option for efficient post-combustion CO2 capture in baseload coal-fired power stations. However, DCFBs might be unsuitable for flexible carbon capture at very low power plant capacity factors, beyond which the solid entrainment rate becomes insufficient. This work introduces a CaL system featuring the unique option of decoupling the solid circulation from the carbonator flue gas load with a carbonator bottom interlink. The concept has been successfully demonstrated with high CO2 capture efficiency (Hornberger et al. Fuel Process. Technol., 2020, 106557), but not yet regarding its load flexibility behavior. The carbonator performance has been assessed at partial loads as low as 40%, identifying a minimum active space time of 41 s as a basis to achieve an equilibrium normalized capture efficiency of 90%. In addition, the calciner performance has been evaluated for a variety of oxy-fuel cases (31–55 vol %db) and calcination temperatures (850–940 °C). The obtained results endorse the suggested novel CaL system’s suitability for the flexible decarbonization of load-following power plants.

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 research on steady-state operation characteristics of gas–solid flow in a 15.5 m dual circulating fluidized bed system

Gas-solid hydrodynamic steady-state operation is the operating basis in a chemical looping dual-reactor system. This study reported the experimental results on the steady-state operation characteristics of gas–solid flow in a 15.5 m high dual circulating fluidized bed (CFB) cold test system. The effects of superficial gas velocity, static bed material height and solid returning modes on the steady-state operation characteristics between the two CFBs were investigated. Results suggest that the solid distributions in the dual CFB test system was mainly determined by the superficial gas velocity and larger solid inventory may help to improve the solid distributions. Besides, cross-returning mode coupled with self-returning is good for steady-state running in the dual-reactor test system.

Numerical simulation on gas-solid flow characteristics and NOx formation of a full-scale dual circulating fluidized bed boiler

ABSTRACT The gas-solid flow characteristics and fuel NOx formation mechanisms in a novel full-scale Dual Circulating Fluidized Bed (DCFB) was numerically studied. The DCFB was comprised of a bubbling fluidized bed (BFB) at reducing temperature for the release of fuel-N and a circulating fluidized bed (CFB) for combustion of the residue char. A dense discrete phase model coupled with kinetic theory of granular flow (DDPM-KTGF) model was used to determine the gas-solid flow characteristics of the DCFB, and a one-dimensional chemical reaction network (1D-CRN) with detailed chemical reaction mechanisms was used for understanding its NOx formation mechanisms. Results showed that bed particles in the CFB dense phase of DCFB were well-mixed and showed a typical “ring-core” structure in the dilute phase, proving good fluidization quality of bed materials. Meanwhile, circulation ash in BFB with sizes lager than 0.1 mm would pass through the designed overflow port, allowing mass flow from BFB to CFB. Investigation into NOx formation revealed that reactions such as R398, R1-N-1, R569, and R17 associating with O/O2 and volatile N would promote NOx formation in DCFB, whereas reactions mainly R411, R570, R571, R5, and R6 associating with NH2, soot, and char inhibited NOx formation. The overall NOx emission at the furnace outlet of DCFB was calculated to be 97.29 mg/Nm3, which was 56.66% lower than that in traditional CFB with the same configuration. Consequently, the novel DCFB design that separating fuel-N release and char combustion provides an alternative means for manipulating NOx emission during CFB combustion.

Multi-fluid modelling of hydrodynamics in a dual circulating fluidized bed

In this work, a multi-fluid model based on the Eulerian-Eulerian framework is used to study the gas-solid hydrodynamics, such as solid distribution, particle motion and solid velocity, in a three-dimensional (3D) dual circulating fluidized bed (DCFB). The influence of four different drag force models, including two classic models, i.e. Gidaspow, EMMS drag model and two recent drag models, i.e. Rong and Tang drag model, on hydrodynamics in DCFB are assessed. Numerical results show that the characteristics of solid distribution and velocity in different sections are distinct. For qualitative analysis, all the drag models can predict a reasonable radial solid distribution and pressure distribution, but only the EMMS, Rong and Tang drag model can capture the phenomenon of dense solid concentration in the low part. For quantitative analysis, the solid circulating rate predicted by the EMMS drag model is the closest to the experimental value while the Gidaspow drag model shows the most significant deviation. The overall assessments confirm that the drag model selection has a significant influence on the simulations of gas-solid flow in DCFBs. This study sheds lights on the design and optimization of fluidized bed apparatuses.

Influencing the solid fraction distribution in a circulating fluidized bed system using differently shaped internals

Solids distribution and gas–solid contact gain in importance for new and alternative energy conversion processes realized with fluidized bed systems. The so-called dual circulating fluidized bed (DCFB) system represents a reactor concept proposed for these novel processes. This concept is based on the interconnection of two circulating fluidized beds via two loop seals. In this work, a cold flow model, designed according to the DCFB system, is used to investigate the influence of constrictions placed along the height of the secondary reactor. Experiments were performed with differently shaped constrictions, aperture ratios, varying distance between the constrictions and different length of the horizontal section at the smallest cross section in the constriction. In general, the mean solid fraction in the upper part of the secondary reactor benefits from the constrictions (mean solid fraction up to +1800% compared to a straight reactor without any constriction). The mean solid fraction in the reactor increased with the solid circulation rate between the reactors and the superficial gas velocity in the secondary reactor. Further, investigations showed substantial deviations for the solid fraction in the upper part of the reactor for the investigated geometries of the constrictions. Fluid dynamic limitations restricting the operating range of the cold flow model could be observed. These limitations depend on combinations of solid circulation rate and superficial gas velocity in the secondary reactor and differ for the investigated geometries. Solid fraction distribution in the secondary reactor showed high solid fractions in the constricted sections (up to 60%) while it remained low between the constrictions.

Control of pressure balance and solids circulation characteristics in DCFB reactors

In the paper, a novel dual circulating fluidized bed (DCFB) reactor is introduced, which can be widely applied in catalytic cracking, chemical looping combustion, coal/biomass combustion and gasification, and gas adsorption, etc. The study mainly investigates the effects of specific operating parameters on pressure balance and solid circulation rate, including total solids inventory, riser gas velocity, bubbling fluidized bed gas velocity and pot-seal gas velocity. It is found that changes of those parameters lead to variations in pressure balance and riser axial voidage in DCFB system, and further affect solid concentration and solid circulation rate in the system. To be more specific, total solids inventory, riser gas velocity and bubbling fluidized bed gas velocity have a major impact on solid concentration and solid circulation rate, while pot-seal gas velocity only exerts a minor impact. Besides, the changing of overall pressure drop across the bubbling fluidized bed is consistent with variations in pressure drops in the other parts of the system, with the former equaling △Pr+△Pc+△Pp-t+△Pud1+△Pud2. In this way pressure balance in the system is maintained. Eventually the paper reaches the conclusion that relevant operating parameters can be adjusted to control pressure distribution and solid circulation rate in different parts of the system and further to ensure the safe and stable operation of DCFB reactors.

Fluid dynamic evaluation of a 10 MW scale reactor design for chemical looping combustion of gaseous fuels

The design of an efficient and scale-up ready reactor system is, together with development of a high performance oxygen carrier, one of the most important research topics in chemical looping combustion. The dual circulating fluidized bed (DCFB) concept is a reactor concept consisting of two interconnected circulating fluidized beds, air and fuel reactor, where the oxygen carrier is the bed material. In the present study a so-called cold flow model is used to investigate the fluid dynamic behavior of a next scale design based on the DCFB concept (10 MW fuel power input) in order to optimize the system design.Four different designs of the fuel reactor are investigated with focus on the solid distribution and general operating parameters and their influence on the operating range. For that purpose, several parameters like e.g. the amount of fluidization gas for both reactors, the total solid inventory, and the fuel power, are variated. Pressure profiles are used to get a comprehensive overview of the fluid dynamic behavior of the different reactor designs.Comparison of the fuel reactor designs shows that operation of the air reactor is not affected by changes of the fuel reactor design. In contrast significant changes, especially in the solid distribution, are recognizable for the different fuel reactor designs. These changes concern mainly the adapted sections of the fuel reactor.Two of the proposed fuel reactor designs are limited in their operating range due to unstable operating conditions, caused by fluid dynamic effects. At such unstable operating conditions the solid inventory distribution between both reactors fluctuates. This behavior is caused by the geometry of the fuel reactor. Further, due to variations in the geometry of the fuel reactor effects leading to the unstable operating conditions are identified. However, unstable operating conditions are caused by the shape of the conical form of the bottom part of the fuel reactor.

Experiments on particles flow in a dual circulating fluidized bed with symmetrical structure

The particle flow characteristics in a dual circulating fluidized bed (DCFB) with symmetrical CFB units were experimentally studied. Each CFB unit consists of a riser (Hr=3m, Dr=0.1m), a downcomer (Hd=1.73m, Dd=0.1m), a cyclone separator and a loop seal. The solid material applied in the experiments was quartz sand (d=140μm, ρs=2600kg/m3) with a total solid inventory of 70kg. Four steady flow patterns were formed in each CFB unit, including turbulent flow (TF), cluster flow (CF), annular flow (AF) and suspension flow (SF), which leads to five steady coupled flow patterns in DCFB, i.e. CF-TF, CF-CF, AF-CF, AF-AF and AF-SF. Axial solid holdups were examined at these plow patterns and their transitions. The fluctuation strength and particles flow stability were evaluated with standard deviation (σ) and coefficient of variation (CV) of time-series solid holdups, respectively. It was found that a growth in the superficial gas velocity of Unit B (Ug,B) would lead to a more uniform Unit B axial solid holdup. Meanwhile, the variation trends of Unit A solid holdup differed under different coupled flow pattern. With Ug,B increasing, σB along Unit B riser became more homogeneous while the change in σA was roughly linear with the variation of solid circulation rate. Meanwhile, σ and CV of Unit B solid holdups were found to have a similar variation trend under coupled flow patterns of CF-TF and CF-CF. As for coupled flow patterns of AF-CF, AF-AF and AF-SF, with increasing Ug,B, the CV of Unit B bottom region kept decreasing while constant growths were found in other regions.

Pneumatic jet-control valve for dual circulating fluidized beds

With the rapid development of circulating fluidized bed (CFB) technology in different fields, the disadvantages of conventional non-mechanical valves are becoming more apparent, and they are not suitable to be used in complex CFB systems. In this paper, a novel non-mechanical valve named the jet-control valve is presented which can avoid the fluidization of solid particles. The feasibility and performance characteristics of the new valve are investigated with a cold-model dual CFB. The results show that compared with the conventional non-mechanical valve, the jet-control valve can transfer solid particles steadily over a larger range, prevent artesian flow, and improve the leakage characteristics. The effects of the operating parameters and structural parameters on the minimum aeration velocity, solid flow rate, and maximum solid flow rate are studied. A two-valve model is proposed to explain the transport capacity of the valve for one jet pipe. A semi-theoretical expression is obtained based on the experimental data with a maximum deviation of 30% providing useful guide for scaling-up the design.

Multi-scale simulation of chemical looping combustion in dual circulating fluidized bed

Abstract Chemical looping combustion (CLC) in an interconnected fluidized bed has attracted more and more attention owing to its novel technology with inherent separation of CO2. In recent years, some models have been developed to investigate the gas-particle flow and reactive characteristics during the CLC process. However, multi-scale structures in reactors make it complex to perform a simulation. In the current work, a multi-scale gas–solid flow-reaction coupled model is developed and applied to the simulation of the CLC process in a dual circulating fluidized bed (DCFB) system with consideration of the impact of multi-scale structures on chemical reactions, mass and heat transfer. By comparisons of gas pressure and gas components with experimental data, the present model shows a better prediction. The influence of clusters on the gas compositions and temperature field is analyzed.

Computational study of solid circulation in chemical-looping combustion reactor model

A three-dimensional computational fluid dynamic (CFD) model has been developed for simulating full-loop solid circulation in a dual circulating fluidized bed (DCFB) chemical loop combustion (CLC) reactor model. The standard k-ε turbulence model and kinetic theory of granular theory based Eulerian multiphase model were used to describe the gas and solid motions, respectively. The simulations focused on the investigation of solid circulation in a CLC reactor model which consists of an air reactor (AR, height of 1.36m, diameter of 0.05m) and a fuel reactor (FR, height of 0.97m, diameter of 0.054m). Key gas–solid flow behaviors related to gas–solid circulation behaviors, e.g. the transient flow regime, solid velocity, solid distribution and circulation characteristics, were numerically investigated. It was found that a core–annulus flow structure appeared in FR. The influences of FR and LLS fluidization flow rate on global and internal circulation rate were studied separately. With FR fluidization flow rate increasing, the time-averaged axial solid volume fraction in the top region of FR first increased and then decreased. The influences of FR and LLS fluidization rate on circulation dynamic characteristics, including circulation fluctuation strength and circulation stability, were also numerically investigated in this work.

Effects of temperature and composite alumina on pyrolysis of sewage sludge

An interactive dual-circulating fluidized bed system has been proposed in which the pyrolysis of sewage sludge (SS) and incineration of biomass proceed simultaneously, and alumina is used as the bed material and heat carrier. The alumina coated with biomass ash would mix with sewage sludge in the pyrolysis reactor of this device. It is important to know the influence of composite alumina (CA) on the pyrolysis progress. Sewage sludge was pyrolyzed in a fixed bed reactor from 400 to 600°C using CA as catalyst. The effects of temperature and CA additive ratio on the products were investigated. The product yields and component distribution of non-condensable gas were more sensitive to the change of temperature, and the maximum liquid yield of 48.44wt.% and maximum Useable Energy of Liquid of 3871kJ/kg sludge were observed at 500°C with 1/5 CA/SS (mass ratio). The gas chromatography–mass spectrometry results showed that the increase of temperature enhanced devolatilization of organic matter and promoted cyclization and aromatization of aliphatics. The presence of CA could strengthen secondary cracking and interaction among primary products from different organic compounds, such as acid–amine condensation, and reduce the content of oxygenated compounds. When the CA additive amount exceeded a certain proportion, the aromatization was clearly strengthened. The effects of CA on decomposition of fatty acids and formation of aromatics were similar to that of temperature. This means that the reaction temperature could be lowered by introducing CA, which has a positive effect on reducing energy consumption.

Optimization of the Loop Seal in the Counter-Current Reactor of the Dual Circulating Fluidized Bed System for Chemical Looping Processes

The dual circulating fluidized bed (DCFB) system for chemical looping processes includes an internal solids loop on the fuel reactor (FR) side. The particles elutriated from the fuel reactor are separated from the gas stream in a cyclone and directed back into the fuel reactor through a fluidized J-type loop seal, the so-called internal loop seal (ILS). The present work seeks to improve operation stability of this internal solids loop by modification of the design. The study is carried out in a cold flow model specially built for the fluid-dynamic characterization of the 120 kW chemical looping pilot plant unit at the Vienna University of Technology. The cold flow rig is provided with a system for determination of solids residence time distribution using ferromagnetic tracer particles. Pressure profiles and residence time distribution curves are used for the evaluation of the proposed geometry changes. The increase in the loop seal size has avoided the seal failure at low internal solids circulation rates. The back-flow of particles from the bottom of the reactor to the loop seal via the return leg was corrected. The slugging in the downcomer of the ILS has been eliminated by using a larger downcomer diameter. By shifting the ILS upward high pressures at the discharge side are avoided and a smaller amount of particles need to be accumulated in the downcomer and are available for circulation. Overloading of bed material at the bottom of the FR seems to be avoided and particles concentration grows in the upper part of the reactor.