Circulating Fluidized Bed Reactors Research Articles

Evaluation of mixing of a secondary solid phase in a circulating fluidized bed riser

Mixing of the secondary solids phase, i.e. fuel or sorbent is crucial in fluidized bed reactors due to its effect on the performance of the reactor. Fluctuations in the gas-particle flow affect mixing, thus modeling this is essential for the analysis of mixing. However, simplified mixing models are used for large circulating fluidized bed (CFB) reactors, which causes uncertainties. A novel averaging method is presented to obtain representative results of transient secondary solid phase mixing in CFBs, acknowledging the effect of fluctuations. A parametric study is performed to investigate the effect of particle size, density, and inlet velocity on mixing and for insight into the mixing process. Additionally, the presented approach is used to compare different steady-state mixing methods to evaluate their applicability for modeling mixing in CFB reactors. Initial momentum and elutriation tendency of fuel were found to affect the mixing behavior, with two distinct routes identified.

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.

Integrated Calcium Looping System with Circulating Fluidized Bed Reactors for Low CO2 Emission Cement Plants

In this work, a Calcium Looping (CaL) configuration for cement plants using two interconnected circulating fluidized bed (CFB) reactors integrated into the clinker production process, is investigated. In the proposed system, the oxy-fired CFB calciner has a dual task: (i) calcining the finer fraction of the raw meal to feed the rotary kiln of the plant and (ii) calcining the coarser fed raw meal fraction, so as to recirculate part of it to the carbonator to capture CO2 from the flue gases of the kiln. Such arrangement also exploits the known tendency of limestone materials to experience intense attrition and fragmentation under the severe calcination conditions in the oxyfired CFB calciner, so that the calciner also performs the task of an auxiliary mill. The techno-economic study conducted in this work has been carried out by analysing different levels of fragmentation of the raw material, consistent with experimental results from the TRL7 la Pereda CaL pilot plant. Two different configurations were analysed: the first, in which limestone with a larger particle size than the correctives is fed, to be used in CFB reactors; the second, in which no separation of limestone from other solid species is possible and all raw meal is used as CO2-sorbent in the CaL system. The benefits of this approach are confirmed by the estimated specific primary energy consumption for CO2 avoided (SPECCA) of 2.8-3.0 MJLHV/kgCO2 when pure limestone is used in the carbonator and of 3.5-4.6 MJLHV/kgCO2 when the raw meal is used as sorbent. The economic analysis indicates the competitiveness of the novel proposed process compared to other CaL systems, resulting in a cost of CO2 avoided (CCA) between 49.2 to 57.9 €/tCO2, excluding costs for CO2 transport and storage.

Sustainable process design for manufacturing acrylic acid from glycerol

Acrylic acid is a high-value monomer produced today only from petrochemical resources. This paper investigates the conceptual design of an eco-efficient process using renewable low-cost glycerol. The key step is glycerol dehydration to acrolein, performed in a fluidized bed reactor employing fast nanosheet zeolite catalyst. The investigation makes use of hydrodynamic modelling and computer simulation in Aspen Plus. The feasibility of four alternatives is studied: conventional bubble (BFB) and turbulent (TFB) fluidized bed, as well as two types of circulating fluidized bed (CFB) reactors. Working with smaller bubble size, in the range 2.5–10cm, a BFB reactor may achieve a productivity of 0.45kg/kg-cath for 99.7% conversion, which is only 20% of the performance of an ideal plug flow reactor. Changing to TFB regime raises the productivity to 0.9kg/kg-cath. Implementing CFB reactors leads to a spectacular improvement. High productivity of 3 and 4kg/kg-cath is achieved with turbulent (CTFB) and high-density (HD-CFB) reactors, respectively. The TFB reactor is preferred when catalyst coking is low or moderate, while CTFB is favoured when continuous regeneration is needed. Employing m-xylene driven azeotropic distillation leads to the lowest energy consumption reported so far by acrylic acid purification. The integration of reaction and separation sections by combined heat and power system leads to a positive energy process. Dynamic simulation demonstrates flexibility and robustness of the separation section faced with disturbances occurring in the reaction system. The sustainability indices show substantial improvement in eco-performance with respect to the propylene-based process.

Development of a hydrodynamic model and the corresponding virtual software for dual-loop circulating fluidized beds

Dual-loop circulating fluidized bed (CFB) reactors have been widely applied in industry because of their good heat and mass transfer characteristics and continuous handling ability. However, the design of such reactors is notoriously difficult owing to the poor understanding of the underlying mechanisms, meaning it has been heavily based on empiricism and stepwise experiments. Modeling the gas-solid CFB system requires a quantitative description of the multiscale heterogeneity in the sub-reactors and the strong coupling between them. This article proposed a general method for modeling multiloop CFB systems by utilizing the energy minimization multiscale (EMMS) principle. A full-loop modeling scheme was implemented by using the EMMS model and/or its extension models to compute the hydrodynamic parameters of the sub-reactors, to achieve the mass conservation and pressure balance in each circulation loop. Based on the modularization strategy, corresponding interactive simulation software was further developed to facilitate the flexible creation and fast modeling of a customized multi-loop CFB reactor. This research can be expected to provide quantitative references for the design and scale-up of gas-solid CFB reactors and lay a solid foundation for the realization of virtual process engineering.

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.

Numerical simulations and comparative analysis of two- and three-dimensional circulating fluidized bed reactors for CO2 capture

Carbon dioxide (CO 2 ), the main gas emitted from fossil burning, is the primary contributor to global warming. Circulating fluidized bed reactor (CFBR) is proved as an energy-efficient method for post-combustion CO 2 capture. The numerical simulation by computational fluid dynamics (CFD) is believed as a promising tool to study CO 2 adsorption process in CFBR. Although three-dimensional (3D) simulations were proved to have better predicting performance with the experimental results, two-dimensional (2D) simulations have been widely reported for qualitative and quantitative studies on gas–solid behavior in CFBR for its higher computational efficiency recently. However, the discrepancies between 2D and 3D simulations have rarely been evaluated by detailed study. Considering that the differences between the 2D and 3D simulations will vary substantially with the changes of independent operating conditions, it is beneficial to lower computational costs to clarify the effects of dimensionality on the numerical CO 2 adsorption runs under various operating conditions. In this work, the comparative analysis for CO 2 adsorption in 2D and 3D simulations was conducted to enlighten the effects of dimensionality on the hydrodynamics and reaction behaviors, in which the separation rate, species distribution and hydrodynamic characteristics were comparatively studied for both model frames. With both accuracy and computational costs considered, the viable suggestions were provided in selecting appropriate model frame for the studies on optimization of operating conditions, which directly affect the capture and energy efficiencies of cyclic CO 2 capture process in CFBR.

Energy recovery in China from solid wastes by the moving grate and circulating fluidized bed technologies

In recent years, the Chinese waste-to-energy (WTE) industry is growing at the rate of about thirty new plants each year. The municipal solid waste (MSW) fuel has a low heating value of 4–7 MJ/kg, in comparison to about 11 MJ/kg in U.S. and 8–11 MJ/kg in EU. Combustion of the low heating value fuel on a moving grate (MG), the dominant combustion technology worldwide, is difficult to control and measures have to be taken to remove some moisture prior to combustion. For this and other reasons, an alternative technology, the circulating fluid bed (CFB) has been implemented in China. This paper is a comparative study of the two technologies and was carried out by Columbia University and two senior engineers, representing the MG and CFB technologies of China. Data were derived from industrial operating plants and from the literature. The fuel to MG furnaces is as-received MSW, while the MSW to CFB reactors is pre-shredded using high-torque low-speed shredders. The availability of MG plants, over a 1-year period, is 90% + , while that of CFB facilities is 80% +. Also, the in-plant electricity consumption of MG plants is slightly lower than the consumption of CFB plants. The MG furnace is less compact, than that of a CFB combustion chamber, with a heat flux range from 0.5 to 0.6 MW/m2 of grate surface area, while that of CFB furnace was about 1.7 MW/m2 of furnace cross-section. The bottom ash in a MG process is typically wet-discharged and the recovery of metals is less efficient. A drawback of the CFB process is that the fly ash generated is 5–10% of the weight of MSW combusted, as compared to 1–3% for moving grate plants in China.

Sequential modular simulation of circulating fluidized bed reactors

AbstractBypassing the mathematical complexity of equation‐oriented approaches in predicting the performance of chemical reactors has recently stimulated a significant amount of interest. Among chemical reactors, circulating fluidized bed reactors (CFBRs) have secured an important role in a broad range of applications in energy sectors due to their advantages, including high fluid‐solid contact efficiency, uniform temperature, and enhanced heat and mass transfer rates. Accordingly, modelling and predicting the performance of these reactors is of great importance. In this study, a sequence‐based model was developed to predict the behaviour of CFBRs. Complex phenomena in CFBRs were mimicked by two sub‐models, namely the hydrodynamics module, which addressed the physical changes, and the reaction kinetics module, which described the chemical evolution of species. The performance of the proposed model was validated with a library of catalytic ozone decomposition experimental data in CFBRs. This work introduces a new infrastructure for modelling CFBRs, which may be combined with the current process simulation software, such as Aspen Plus©, for advanced process modelling applications.

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.

Modeling of the maleic anhydride circulating fluidized bed reactor

Abstract Maleic Anhydride (MA) production by selective oxidation of n-butane in a Multi-Tubular Fixed Bed reactor is constrained by the flammability limits. The use of Fluidized Bed circumvents this constraint but suffers from high back mixing. The Circulating Fluidized Bed (CFB) reduces the back mixing of the catalyst and enhances the catalyst activity by reactivating the catalyst. A simple reactor model with suitable hydrodynamic and kinetic models at the operating conditions of both the pilot and commercial MA CFB reactors is proposed. A dispersion model with variable gas density is used for modeling the hydrodynamics of these reactors. The reactors' configurational complexities are also accounted for in the model. The data fitting exercise of the proposed reactor model is performed. The proposed method simultaneously minimizes the total bias based on the individual biases for each component of the multi component reactor system and the total error in a multi-objective framework.

A Modified Gibbs Free Energy Minimisation Model for Fluid Bed Coal Gasification

Abstract A modified approach to equilibrium modelling of coal gasification is presented, based on global thermodynamic analysis of both homogeneous and heterogeneous reactions occurring during a gasification process conducted in a circulating fluid bed reactor. The model is based on large-scale experiments (ca. 200 kg/h) with air used as a gasification agent and introduces empirical modifications governing the quasi-equilibrium state of two reactions: water-gas shift and Boudouard reaction. The model predicts the formation of the eight key gaseous species: CO, CO2, H2O, H2, H2S, N2, COS and CH4, volatile hydrocarbons represented by propane and benzene, tar represented by naphthalene, and char containing the five elements C, H, O, N, S and inorganic matter.

CPFD simulation of solids residence time and back-mixing in CFB risers

A Computational Particle Fluid Dynamics (CPFD) approach was applied to investigate the solids residence time distribution (RTD) and back-mixing behavior in a circulating fluidized bed (CFB) riser. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. It was found that the solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the riser. Even in the dilute phase transport (DPT) regime, the predicted solids RTD curves had the feature of an early peak and an extended tail, indicating that the solids flow deviates from plug flow and exhibits back-mixing. The solids back-mixing mainly occurred in the lower part of the riser, which provided significant implications for the industrial applications of the CFB reactors since most of the chemical reactions and heat/mass transport processes occur at the lower part of the riser. It is important to minimize the solids back-mixing at the lower part of the riser for the industrial CFB applications like the FCC.

Application of CPFD method in the simulation of a circulating fluidized bed with a loop seal, part I—Determination of modeling parameters

The Computational Particle Fluid Dynamics (CPFD) method was applied in the simulation of the gas solid flow in a circulating fluidized bed (CFB) with a loop seal in this work, and the modeling parameters were mainly investigated in this part. The influences of some crucial modeling parameters in CPFD simulation, such as mesh size, particle close pack volume fraction, interphase drag model and particle size distribution (PSD) on the flow behaviors in the whole loop of CFB were carefully analyzed with the help of experimental data conducted on the same CFB test rig. A medium mesh size was proved to be appropriate and used for the CFB modeling in this work. The particle close pack volume fraction largely affected the particle packed condition in the dense loop seal and standpipe. A value of 0.58 was proved to be suitable to describe the particle circulation and pressure distribution characteristics. Different gas–solid drag models showed certain influence on the particle circulation behaviors, and the combination of Wen-Yu and standard Ergun model was determined as the optimum choice. The consideration of the real PSD in CPFD scheme would better predict the overall flow characteristics for CFB system than the mono-size simplifications. The determination of modeling parameters in this work provides the base for simulating the gas–solid flow in the whole loop of CFB reactors by CPFD method, especially for the dense flow behavior in the standpipe and loop seal.

A CFD simulation study to evaluate the flow and catalytic hydrogenation of dimethyl oxalate in a packed bed, a two‐stage fluidized bed, and a circulating fluidized bed

ABSTRACTHydrogenation of dimethyl oxalate (DMO) can be operated in various reactors, such as the packed bed (PB), two‐stage fluidized bed (TSFB), and circulating fluidized bed (CFB) reactors. Fluid dynamic behaviors in these reactors are different and can lead to different transport and reaction behaviors. Recent advances in the computational fluid dynamics (CFD) technique show its great potential in advancing current understanding of fluid dynamics and its interactions with chemical reaction dynamics. In this study, CFD models for the simulations of flow and hydrogenation of Diethyl Oxalate (DEO) in PB, TSFB, and CFB reactors were developed. The CFD model for the PB reactor was validated using experimental data. The models were then used to predict flow and hydrogenation behaviors in the other two reactors. Various critical issues to be considered in choosing a reactor and how CFD modeling can be utilized to reduce uncertainties associated with this are discussed in details. The approach, models, and results presented here will be useful for understanding the complex flow dynamics and its interaction with the reaction dynamics of DMO hydrogenation in various types of reactors. © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.

Catalytic pyrolysis of biomass for transportation fuels

AbstractApplication of heterogeneous catalysis in biomass pyrolysis seems to be one of the most promising methods to improve bio‐oil quality by minimizing its undesirable properties (high viscosity, corrosivity, instability, etc.) and producing renewable crude (bio‐crude). This bio‐crude could finally lead to transportation fuels using existing refinery processes (such as hydrotreating). A plethora of catalytic materials have been investigated in the literature as catalysts for the biomass catalytic pyrolysis process. Among them, microporous (zeolitic) or mesoporous (Al‐MCM‐41) acid materials have been tested, either promoted or not with several transition metals. Lately, basic materials are also suggested. For this process, a circulating fluid bed reactor seems to be the most effective technology, since it offers continuous catalyst regeneration. The research till today shows that catalysts based on ZSM‐ are the most promising. With these catalysts, bio‐oil yield up to about 30 wt% (on dry biomass) with about 21 wt% oxygen can be produced. However, tailoring of catalyst properties such as acidity/basicity and porosity characteristics is still needed to develop an optimized catalyst.This article is categorized under: Bioenergy > Science and Materials Bioenergy > Climate and Environment Energy and Climate > Science and Materials

Capture of CO2 during low temperature biomass combustion in a fluidized bed using CaO. Process description, experimental results and economics

The process discussed in this work is aimed at the capture of CO2 with CaO in a circulating fluidized bed (CFB) combustor-carbonator reactor, where the combustion of biomass with air and the carbonation of CaO are taking place simultaneously. This process exploits the high reactivity displayed by most natural biomass materials during their combustion at low temperatures (around 700 °C) and the capability of CaO to absorb CO2 at these temperatures. This is a niche application for the calcium looping cycles being developed for other post-combustion and precombustion processes. The CaO necessary for the CO2 capture reaction by carbonation is being circulated from an oxyfired circulating fluidized bed calciner, operating at temperatures over 900 °C to permit the decomposition of CaCO3. In this publication, we present new experimental results proving the concept in a 30 kW test facility consisting of two interconnected CFB reactors (combustor-carbonator and combustor-calciner). An application case study is discussed, together with an economic analysis using published data of similar systems.

Postcombustion CO2 capture with CaO. Status of the technology and next steps towards large scale demonstration

Abstract Postcombustion CO 2 capture using CaO requires a large scale circulating fluidized bed (CFB) reactor as CO 2 absorber, operating between 600–700 °C. In addition, a large scale oxy-fired CFBC must be interconnected to this reactor to allow for the decomposition of CaCO 3 formed in the carbonator. This allows for a continuous regeneration of the CaO sorbent and the production of a CO 2 rich stream suitable for final purification and compression. Despite the known limitations associated to this technology (mainly sorbent deactivation, solid attrition, and high energy requirements in the calciner) clear operating windows have been identified at which the process could be implemented in practice using the know-how on CFBC technology. Theoretical studies on the thermal integration of a carbonation-calcination loop in new and existing power plants have shown that the technology has the potential to achieve a substantial reduction (around 30%) in capture cost and energy penalties with respect to stand-alone oxy-fired systems. Since the solid materials and operating conditions in the CFB units are similar to those present in existing large scale CFBCs, the prospects for a rapid scaling up of the technology are very promising. A rapid development is taking place in recent years by demonstrating the key concepts in laboratory scale test rigs of 10 s of kW. However, it is essential to move on to the next phase of pilot testing and validate the results in conditions fully comparable with those expected in large scale units. We describe here the design of a 1 MW th pilot plant to capture 70–95% of the CO 2 contained in the flue gas from a 1/150 side stream emitted by an existing 50 MW e CFB power plant. The pilot is made up of two interconnected CFB reactors of 15 m height. The construction of the pilot has been initiated and is expected to enter into full operation in the first half of 2011, providing the necessary experimental results to decide on the launching of an aggressive development programme that aims to demonstrate the technology at large scale well before 2020.

An advanced EMMS scheme for the prediction of drag coefficient under a 1.2 MW th CFBC isothermal flow—Part I: Numerical formulation

Range and size increment of industrial applications regarding Circulating Fluidized Bed (CFB) technology raise numerous design and operating problems. Further insight into the governing and complex multiphase flow physics regarding CFB operation can be provided by CFD analysis. However, it has been proven during the recent years that multi-scale phenomena occurring in CFB reactors cannot be accurately reproduced by conventional models. Such models are unable to accurately predict the momentum exchange between the co-existing phases (gas and inert material) when calculating the drag coefficient. The main reason is that gas–solid two-phase flow features spatiotemporal multi-scale structures, named clusters, as the heterogeneous flow field is developing. Aiming to evaluate the particles tendency to aggregate in clusters and describe the effects of this complicated mechanism on the main characteristics of the induced multiphase flow field, the advanced Energy Minimization Multi-Scale (EMMS) analysis is applied. The EMMS scheme comprises of a set of equalities, constraints and a minimization energy equation, solved for a number of volume fraction's and slip velocity's values, using the optimization software General Algebraic Modeling System (GAMS).

Capturing CO2 from combustion flue gases with a carbonation calcination loop. Experimental results and process development

Abstract Post-combustion carbonate looping processes are based on the capture of CO 2 from the flue gases of an existing power plant in a circulating fluidized bed (CFB) reactor of CaO (the carbonator) at around 650 ∘ C. The calcination of CaCO 3 in a new oxy-fired experimental results from a small test facility (30 kWt) operated in continuous mode using two interconnected CFB reactors as carbonator and calciner. Capture efficiencies between 70 and 97% have been obtained under realistic flue gas conditions in the carbonator reactor. The similarity between process conditions and those existing in CFBC power plants should allow a rapid scaling up of this technology.