Circulating Fluidized Bed Loop Research Articles

CFD simulation of a circulating fluidized bed carbon capture system using a solid-supported amine sorbent

Carbon capture, utilization, and sequestration (CCS) is one of the key technologies to reduce the emission of carbon dioxide, including that from exiting flue gas of fossil fuel-fired power plants. In this study, the two-fluid model approach was used to simulate the hydrodynamics of the entire circulating fluidized bed (CFB) loop of the integrated carbonation and regeneration sections using the mass and momentum equations and constitutive relations for both the gas and solid phases. Then, the verified models of the non-reactive simulation of the CFB loop were combined with species, energy equations, and carbonation regeneration rate of reactions for both phases to simulate the entire circulating fluidized bed carbon capture unit. The predicted extent of CO2 capture in this system showed the potential of continuous carbon capture using amine-based solid sorbents through the circulating fluidized bed CO2 capture unit.

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.

Modeling study on the dynamic characteristics in the full-loop of a 350 MW supercritical CFB boiler under load regulation

To improve the load regulation ability of the circulating fluidized bed (CFB) unit, investigating the hydrodynamic characteristics of the gas-solid flow in the CFB loop during such processes is of vital importance. In this work, based on the mass and pressure balance in the whole loop, a one-dimensional dynamic model on the gas-solid flow in the full-loop of an industrial CFB was established. In particular, the solid mass flow rate at the dust exit was correlated with the total inventory of the cyclone, and the re-circulating rate to the furnace was depicted by the pressure drop in the loop seal. The model was validated by the field test data of a 350 MW supercritical CFB boiler during a load increasing process. Subsequently, the impact of the load change rate and operating bed pressure on the hydrodynamic performance in the whole loop of the CFB boiler during the load reduction process were analyzed. The results show that the bed pressure in the furnace will fluctuate more violently when the load change rate increases, while the operating bed pressure has a weak effect on the load regulation process. These conclusions can provide theoretical guidance to the variable load operation of the CFB boiler.

Effect of the solids inventory and fluidization gas velocity on the hydrodynamics of a circulating fluidized bed

The hydrodynamics behavior of a bench-scale circulating fluidized bed (CFB) system has been analyzed in terms of the axial pressure profile and solids circulation rate. Experimental runs were carried out at room conditions using beds of quartz sand and air as the fluid phase in order to know the effect of the overall solids inventory and the superficial gas velocity used in the riser. Static pressure signals obtained from the gas-particle flow were registered at several positions along the CFB loop and the solids circulation rate was measured by using a diverter valve installed at the standpipe section. A statistical analysis applied on the experimental data show that the solids inventory and the superficial gas velocity in the range here studied lead to significant changes on the hydrodynamics of the circulating fluidized bed system.Keywords: Circulating fluidized bed riser, gas-particle hydrodynamics, static pressure signals, solids circulation rate.

CFD simulation of gas and particle flow and a carbon capture process using a circulating fluidized bed (CFB) reacting loop

Circulating fluidized bed (CFB) loop-based reactors are widely used in the chemical and energy industries. CFB reactors are excellent candidates for chemical looping of solid sorbents for CO2 capture and sorbent regeneration processes because they ensure a continuous carbon dioxide removal process in a relatively compact unit. Computational fluid dynamics (CFD) provides an excellent approach to model the carbonator and regenerator reactors and the entire CFB loop design in a systematic and computationally feasible way. The simulation of individual parts of the system was recently performed and provided a qualitative estimation of the system behavior. In order to use CFD to perform simulations of the CO2 capture regenerative process, a model based on the multiphase flow equations taking into account the sorption/regeneration is needed. Therefore, in this work, a CFD approach was developed and applied to simulate gas and solid flows and CO2 sorption and regeneration in the entire CFB loop. The CFB's geometry used in this study is the same as National Energy Technology Laboratory‘s experimental setup.Our calculations showed that the CFB hydrodynamic behavior reached to a statistical steady state after about 10 s in a three-dimensional domain. The time to reach statistical steady state strongly depends on the initial condition of the simulations, and the geometry and the size of the system. Three-dimensional CFD simulations demand significantly high computational time; thus, we used only the three-dimensional domain for understanding the hydrodynamics of the system and to verify the dynamics of the same system in a two-dimensional CFD domain (no reaction was included in the three-dimensional domain). The two-dimensional domain requires considerably less computational time and allowed us to ultimately add two heterogeneous reactions to the circulating fluidized bed's CFD simulation for carbonation and regeneration of MgO-based solid sorbents. The results of our simulation showed the excellent capability of our CFD approach in describing the CO2 conversion profiles and flow behavior in a reacting CFB system that includes the entire range of solid volume fraction from very dilute (i.e., riser) to very dense (i.e., standpipe) ranges. The effect of CO2 sorption and regeneration at elevated temperature and pressure was also studied.

The exit impact on segregation of binary particles in the CFB system

Segregation exists extensively in the circulating fluidized bed (CFB) system, with solid particles of different sizes and densities. This paper studied the segregation of binary particles in a CFB system. Experimental results showed that segregation in the riser became weaker with increasing fluidizing gas velocity, ug, and solid circulating rate, Gs. In the case of high ug (6.0 m/s) and high Gs, segregation almost disappeared and the binary particles in the whole system became complete mixing. The average jetsam mass fraction along the riser height showed different relations with the initial jetsam mass fraction in the literature. In order to explain the discrepancy in the literature, this paper indicated the exit geometry influenced the segregation in the whole CFB loop, i.e. not only in the riser, but also in the external loop. For the abrupt exit, the internal refluxing of solids always existed in the sharp bend for the inertial separation of particles, and more coarse particles rebounded at the riser exit than the fine particles. Thus more coarse particles stayed in the riser than in the standpipe.

Analysis of Solids Flow Rate through Nonmechanical L-Valve in an Industrial-Scale Circulating Fluidized Bed Using Group B Particles

A series of statistically designed experiments were conducted using two different Geldart Group B particles in a 0.3 m diameter circulating fluidized bed (CFB) cold model to evaluate the nonmechanical L-valve for controlling the solids flow rate. The objective of this study was to investigate the effects of standpipe aeration, L-valve aeration, solids inventory, and superficial gas velocity through the riser on the solids flow rate. A stochastic correlation was developed for calculating the solid flow rate as a function of these variables. It was found that the solid flow rate increased directly proportional to each of these variables. Dimensional analysis was used to generate an expression for the solids turnover ratio. The model developed in the present study is intended to simulate flow in the present CFB loop. It is not intended for scale-up and will probably not apply for alternative CFB loops with different geometries/pressure balance conditions.

Design of large scale oxy-fuel fluidized bed boilers: Constant thermal power and constant furnace size scenarios

This work investigates the design of large scale oxy-fuel circulating fluidized bed (CFB) boilers in two industrial pathways of constant thermal power scenario and constant furnace size scenario using a new version of a model validated with 100 kWth and 4 MWth oxy-fuel CFB boilers. This work suggests that the major approach for elevating the circulating solids flux is to utilize more efficient cyclones with better efficiency which leads to higher share of fines and nanoparticles in the CFB loop while the amount of aerosols is reduced. Maximum heat flux from furnace walls is 2.4 times and CO peak is 2.2 times in constant furnace size scenario compared to constant thermal power scenario respectively, showing more intense combustion in the prior scenario. Based on the results found in this work, if the industry is to aim to operate the large-scale oxy-fuel CFB boilers at high O2 concentrations, the utilization of the constant thermal power scenario is much safer for the industry compared to constant furnace size scenario. This work shows that the aspect ratio of future large-scale oxy-fuel CFB furnaces are larger than the current air-fired ones leading the oxy-CFB furnace geometry to approach the fluid catalytic cracking fluidized beds.

Phase-Shift Method to Estimate Solids Circulation Rate in Circulating Fluidized Beds

While solids circulation rate is a critical design and control parameter in circulating fluidized bed (CFB) reactor systems, there are no available techniques to measure it directly at conditions of industrial interest. Cold flow tests have been conducted at NETL in an industrial scale CFB unit where the solids flow has been the topic of research in order to develop an independent method which could be applied to CFBs operating under the erosive and corrosive high temperatures and pressures of a coal fired boiler or gasifier. The dynamic responses of the CFB loop to modest modulated aeration flows in the return leg or standpipe were imposed to establish a periodic response in the unit without causing upset in the process performance. The resulting periodic behavior could then be analyzed with a dynamic model and the average solids circulation rate could be established. This method was applied to the CFB unit operated under a wide range of operating conditions including fast fluidization, core annular flow...

Virtual experimentation through 3D full-loop simulation of a circulating fluidized bed

Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale (EMMS) model was used to simulate a semi-industry scale circulating fluidized bed (CFB). Three-dimensional (3D), time-dependent simulation of a full-loop CFB revealed that the axial profiles of cross-sectionally averaged solid volume fraction, and the radial profiles of solid axial velocity and solid volume fraction were in reasonable agreement with experimental data. Based on this agreement, database derived from experiments not yet accomplished was replenished with such simulations, and fluid regime diagrams and pressure balance around the CFB loop were derived accordingly. This work presents an integrated viewpoint on CFB and unfolds a fresh paradigm for CFB modeling, which can be expected to help resolve certain issues long in dispute but hard for experiments.

Prediction of Hydrodynamic Properties of Mixed-Particle Systems and Theoretical Analysis of Loop Pressure Profile in a CFB Unit

The hydrodynamic behaviors of mixed system of particles were investigated in a circulating fluidized bed (CFB) unit consisting of fast column (riser) with an inner diameter of 0.1016 m and a height of 5.62 m. Particle mixtures containing a Geldart group-A-like fluid catalytic cracking (FCC) catalyst with group-B-like sand and iron ore with coal were used to study the hydrodynamic features including static pressure, voidage, and loop pressure profile. The mixed system consisting of FCC catalyst and sand contained 20, 50, and 80 mass % sand, and the coal-iron ore mixture contained 80 mass % coal. The superficial air velocity ranged between 2.01 and 4.681 m/s, and the corresponding mass fluxes were 12.5-50 kg/(m{sup 2} s). A comparison of the available experimental values for static pressure profiles at different operating conditions for mixed-particle systems shows good agreement with those predicted from the single-particle systems. Using experimental data on the loop pressure balance, a simplified theoretical analysis was performed to predict the pressure profile in the CFB loop. The deviations between the two sets of values are within reasonable limits of accuracy.

Multi-scale fluidization of ultrafine powders in a fast-bed-riser/conical-dipleg CFB loop

The multi-scale methodology has been used as a conceptual tool in devising an apparatus for continuous processing of ultra-fine particles in the fluidized state. The apparatus combines a fast-bed riser with a conical dipleg into a circulating fluidized bed (CFB) loop. The micro-scale discrete ultra-fine particles gather together into meso-scale agglomerates on account of inter-particle forces, in both the fast-bed riser and the conical dipleg. In particular, in the latter, the fine particles are sufficiently entrapped because of reduced gas velocity in the top region, into the top-to-bottom and center-to-periphery mixing currents of the solids, to form agglomerates before being recycled to the bottom of the fast-bed riser. The fast-bed riser, because of its high and relatively uniform gas velocity, serves to adjust the size, especially the size distribution of the agglomerates, that is, further capture of discrete micro-scale discrete particles for the smaller agglomerates to make them larger, and attrition of over-size agglomerates to make them smaller. Uniform agglomerates of a given average size ensures stable fluidization, minimizing the danger of deposition of macro-scale lumps of agglomerates to lead to blockage as well as defluidization elsewhere in the loop. The above concept is shown experimentally operable using 4 kinds of ultra-fine particles: calcium carbonate, titanium dioxide, silicon dioxide and saponite.

Effect of pressure on loop seal operation for a pressurized circulating fluidized bed

Solids flow through a loop seal in a pressurized circulating fluidized bed (CFB) was studied in an experimental setup in which the effects of system pressure, aeration rates in the two chambers of the loop seal, and particle size could be investigated. The results show that the solids flow rate increases with aeration rate as well as the system pressure. However solids are not recycled until the aeration rate reaches a threshold limit. The solids flow rate also increases with decreasing particle size. Reasonable agreement between predicted and measured values of solids flow is obtained using a simplified analysis of loop seal operation based on a pressure balance around the CFB loop and formulated by the authors.

Spray Pyrolysis in a Circulating Fluidized Bed.

In order to prepare fine metal-oxide particles and circulating fluidized particles coated with metal compounds, experimental investigations were carried out to study spray pyrolysis using the superior heat transfer characteristics of a circulating fluidized bed (CFB). Quartz sand particles of three different size ranges were used as circulating fluidized particles in the CFB loop, and a nickel-acetate solution was injected as droplets into the CFB riser.It was recognized that the size distribution of the prepared particles was considerably affected by the size of circulating fluidized particles. Increasing the circulating fluidized particle size was found to lead to larger nickel-oxide particles. It was shown that the concentration of the injected nickel-acetate solution had no influence on the geometric characteristics of the prepared nickel-oxide particles.