Modelling of sorption-enhanced steam reforming (SE-SR) process in fluidised bed reactors for low-carbon hydrogen production: A review

Sorption-enhanced steam reforming (SE-SR) offers lower capital costs than conventional steam reforming with carbon capture, which arises from the compact makeup that allows reforming and CO2 capture to occur in a single reactor. However, the technology readiness level (TRL) of SE-SR technology is currently low and large-scale deployment can be expedited by ramping up activities in reactor modelling and validation at pilot scale. This work first explores the concept of SE-SR technology, then the experimental activities and pilot tests performed for this technology, followed by the review of progress made on SE-SR modelling. It was found that the Eulerian-Eulerian two-fluid model is the most popular approach widely adopted for modelling SE-SR in fluidised bed reactors. However, the averaging method used to close equations ignores flow details at particle level and simplifies the particle system. Moreover, while hydrogen purity and yield have been predicted within an acceptable error, larger errors for CO2 gas output relative to experimental data have been reported for this model type. Limitations and future perspectives for reactor designs and the various models and modelling approaches are also analysed, to provide guidance and advance research, modelling and scaleup of SE-SR technology

Particle Attrition in Circulating Fluidised Beds System

Particle attrition plays an important role throughout the cycles of a circulating fluidised bed (CFB) and a fluidised bed (FB) process, gradually depriving the bed inventory of valuable mass and changing the bed particle size distribution. The mass loss has to be compensated by a make-up stream. For economic and design purposes, attrition cannot be neglected. Although the particles may be efficient catalysts (or reactants), if the compensation for the lost material amounts to very high expenses, the whole process may become uneconomical. It is then clear that the choice of the solids material should take into account its attrition propensity. The main sources of attrition in fluidised bed systems are the jet region, the bubbling bed and the cyclone. It is common practice to predict particle attrition in industrial scale fluidised bed systems by the population balance method, but is it possible to link that prediction with the breakage propensity of a single particle? This work aims at developing a predictive tool for particle attrition in fluidised and circulating fluidised beds, by attempting to build a path line from the single particle breakage propensity to the attrition occurring in the process. Here, the reference industrial process is the Chemical Looping Combustion (CLC). The CLC is a circulating fluidised bed process under development and as such, the choice of a solids material is critical. A powder of crushed manganese oxide is a candidate material for the CLC process and is used here as test material, as well as its equilibrium equivalent. For simplicity, the two materials are referred to as F-CLC (fresh CLC particles) and E-CLC (equilibrium CLC particles), respectively. The single particle breakability of F-CLC and E-CLC is assessed by impact tests. The experimental results are then used to correlate the extent of breakage upon impact with the particle size and impact velocity, according to the theoretical model of chipping of Zhang and Ghadiri (2002). Further tests are carried out to unveil the effect of impact angle and number of impacts. The results suggest that E-CLC is highly more inclined to attrition than F-CLC. Moreover, the single particle breakage is found to correlate with the magnitude of the impact velocity and the sin of the angle of impact for both materials. Recalling the modelling approach of Ghadiri and co-workers, the single particle breakage model, as derived, and the model of surface wear of Archard and Charj (1953) are coupled with CFD-DEM (Computational Fluid Dynamic-Discrete Element Method) simulations to compute the attrition of F-CLC particles in a Stairmand cyclone. Moreover, the same cyclone is used to characterise attrition of F-CLC particles experimentally as a function of particle size, gas inlet velocity and solids loading. Remarkably, the outcomes of the two approaches are found to agree well. A correlation is eventually derived which expresses the extent of attrition in a cyclone as a function of the variables mentioned above. The analysis revealed that the main source of attrition in the cyclone is given by the particle-wall collisions at the opposite section of cyclone inlet, at any operating conditions. Particle-particle collisions and particle sliding against the wall become significant contributors of attrition at high and low solids loading, respectively. Attrition in the jet region is evaluated at room temperature as the steady state loss rate, using a semi-pilot scale fluidised bed equipped with a porous distributor and a central orifice of variable size. The results of the tests show that jet attrition of F-CLC and E-CLC can be described by two different correlations. The steady state attrition propensity of E-CLC is found to be higher than F-CLC, confirming the outcomes of the impact tests. The analysis on the fines collected on the filter reveals that they are mainly composed by very small particles of about 1 μm. The correlations of cyclone and jet attrition are implemented in a non-dimensional population balance model (PBM) that simulates attrition in a fluidised bed and a circulating fluidised bed. The latter is composed of a fluidised bed where the recycle of solids is provided by a cyclone. The PBM is validated for the fluidised bed configuration against the experimental PSD (Particle Size Distribution) of F-CLC particles after jet attrition in the fluidised bed. The PBM is eventually used to simulate hypothetical cases of a FB and CFB with low and high single particle breakability as well as low and high superficial velocities to assess the dynamic response of the system in terms of material loss, solids circulation rate, requirements for a make-up and PSD in different regions of the system. The simulations allowed to identify the presence of two subsequent regimes where the loss is firstly dictated by the pre-existing fines of the bed inventory and then by attrition. During the two regimes the mean particle size of the bed inventory increases and decreases, respectively. The PBM reveals that the circulation rate is strongly affected by attrition because of the accumulation of entrained particles which are large enough to be captured by the cyclone and recycled. The loss of material and the need for the make-up stream are found to increase using either larger superficial velocities and/or weaker particles

CFD STUDY OF COMPLEX CIRCULATING FLUIDIZED BED SYSTEMS

Circulating fluidized bed (CFB) has been widely applied to many chemical engineering processes. Although significant developments have been made in understanding the performance using the complex CFB technology during the last decades, the detailed inner information cannot be obtained by experiments because of complicated flow pattern in the system and backward measuring equipment. Numerical simulation has become the primary method to accelerate the development of complex CFB technology, reduce the cost of design and operating time, as well as reduce the technical risks. This thesis aims to provide more detailed in-furnace phenomena of complex CFB systems, including the hydrodynamic behaviours and chemical reactions based on the numerical simulation method. The promising chemical looping combustion (CLC) technology, as an example of complex CFB systems, will be focused on in this thesis. Meanwhile, the non-uniformity phenomenon in complex CFB units is comprehensively investigated in two symmetrical CFB configurations connected in parallel and series. Sequentially, an integrated method to dynamically combine CFD modelling and the process simulation is developed as a solution to improve the CFB performance. Specifically, it covers the following five aspects: 1. The hydrodynamic characteristics in a full-loop dual CFB CLC unit are comprehensively investigated based on the Eulerian multi-fluid model to give more detailed information about the flow behaviours. 2. The hydrodynamic characteristics in a unique counter-current moving bed full-loop CLC unit are comprehensively investigated based on the Eulerian multi-fluid model to study the unique configuration and in-furnace fluidization. 3. The reaction characteristics in the unique counter-current moving bed full-loop CLC unit are firstly attempt based on the hybrid Eulerian- Eulerian-Lagrangian model to study the in-furnace reaction details. 4. The non-uniformity characteristics of the multiphase flow in two complex CFB units connected in parallel and series, respectively, are studied based on the Eulerian multi-fluid model. 5. A novel direct integrated method to dynamically combine CFD modelling and the process simulation is developed. A case study of real-time regulation of boundary and operating conditions of reactors in complex CFBs is realized. These studies contribute to the deep understanding and further optimization of complex CFB systems

The behaviour of gas-emitting particles in fluidised beds

This dissertation concerns solid spheres, with diameters ~ 6 - 10 mm, densities between ~700 - 1500 kg m-3 and emitting gas at various peripheral velocities, Ud, and their tendency to float or sink when introduced into gas-fluidised beds of Geldart Group B particles. This is relevant, for example, to the fluidised bed combustion of biomass, and the apparent tendency of the fuel to devolatilise predominantly near the upper surface of the bed with the attendant undesirable complications of unconverted volatile matter (VM) entering the freeboard. Inert spheres (viz. where Ud = 0) in a bubbling fluidised bed can sink, even if less dense than the fluidised medium, owing to the additional weight of bed particles which tend to settle on top of them forming a defluidised hood. A 2-D fluidised bed, at room temperature, was used to investigate the structure of the fluidised bed in the vicinity of a cylinder emitting gas, as a mimic of a 3-D system. It was found that if Ud is more than 0.7, then the gas emitted can fluidise the bed particles in the entire defluidised hood. Consequently, it was inferred for a 3-D system that gas emitting spheres are not burdened by a defluidised hood and will rise to the surface more rapidly than inert spheres, which are burdened. The hypothesis that a gas-emitting sphere forms a pocket of high pressure around its underside sufficient to enable it to hover above the surface of the fluidised bed, was investigated, in a mechanism akin to the Leidenfrost effect exhibited by liquid drops on a hot plate. Experimentation showed that this hypothesis could be rejected. In fact, by observing the structure of the bed and measuring the pressure around a gas-emitting cylinder close to the surface of a 2-D fluidised bed, it was found that the emission of gas from a freely-floating sphere decreases the net upthrust of the bed on its underside thereby causing the sphere to sink lower into the bed than buoyancy alone would suggest. However, it was also discovered that the emission of gas from a sphere sunk deep within a fluidised bed caused the net upthrust from the bed to increase, causing the sphere to rise more rapidly to the surface than an inert sphere. This suggests that there exists a stable depth at which gas-emitting spheres reach dynamic equilibrium just beneath the surface of the bed where the bed’s upthrust matches the weight of the sphere. An interesting aside of investigating the Leidenfrost mechanism was that, as far as Geldart Group B solids are concerned, experiments showed the two-phase theory of fluidisation holds exactly. To simulate spheres of devolatilising biomass, spheres of dry ice, sublimating in a hot fluidised bed were used, because dry ice emits a single, readily detectable gas. The spheres of dry ice were, however, much denser than any biomass fuel and so only segregated once the rate of sublimation was very high. The external heat transfer coefficient for the spheres of dry ice was measured at a variety of bed temperatures and bed particle sizes. Unlike inert particles, gas emitted by the dry ice particles caused the heat transfer coefficient to a) decrease as the bed material size was decreased and b) decrease as the bed temperature increased. For the first time, a heat transfer model, which accounted for the change in structure of the bed material near the gas-emitting particle, was developed to predict the rate of gas emitted from the dry ice particles and gave good agreement with the experimental results. A novel method for finding the peripheral velocity of VM, emitted by spheres of biomass during devolatilisation in a fluidised bed, was developed and validated experimentally. The mean molar mass and composition of the VM was measured, with the result that measuring the concentration of the combustion products of the VM alone could be used to find the molar flowrate of the VM. Using this method, values of Ud, for spheres of beech, devolatilising in a hot fluidised bed, were measured and, simultaneously, the depth of the spheres in the bed was determined using X-radiography. The simultaneous measurements of gas velocity and depth allowed the behaviour of freely floating, devolatilising spheres to be compared with the calculations obtained with the 2-D fluidised bed. The spheres of beech remained just beneath the surface of the bed throughout devolatilisation and were less influenced by the mixing motions of the bed than inert spheres, even when the fluidisation velocity was increased. The devolatilising beech behaved much as anticipated by the results of the 2-D bed experiments. Tentatively, a dimensionless plot was made which, brings the variables Ud, the incipient fluidisation velocity Umf, the densities of the gas-emitting particles and the bed material, and the depth at which a particle will neither rise nor sink in the bed, together. The plot shows under what conditions a gas-emitting particle is likely to have a sinking or rising tendency in a fluidised bed. The plot is a tool for predicting if segregation of a particular fuel particle is likely to occur in any bubbling fluidised bed. Overall, this dissertation concludes that the emission of VM from a devolatilising particle of biomass not only draws the particle to the surface of the bed but acts to keep it there, even at low rates of gas emission. To eliminate the segregation of biomass during combustion in a bubbling fluidised bed, the biomass must be denser than the emulsion phase of the fluidised bed and the velocity of VM leaving the biomass must be as low as possible. An impracticable degree of pre-processing of the biomass would be required to achieve these conditions.EPSR

The Investigation of Aspects of Chemical Looping Combustion in Fluidised Beds

Chemical looping combustion (CLC) is a promising fossil fuel combustion technology, which is able to separate CO2 from the flue gases without a large consumption of energy. In this thesis, the study was extended to look at the use of chemical looping materials within traditional fluidised bed combustion and investigation of the interaction between the fuel, the supplied air and the chemical looping agent. Three topics of chemical looping combustion are discussed, including 1) the Sherwood number in the fluidised bed; 2) properties of different oxygen carriers, Fe2O3 and CuO (with supporting materials), were tested in the fluidised bed reactor; 3) the simulation of a steady state and a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carriers. The Sherwood number, which represents the mass transfer rate, is important in the calculation of CLC process. With Sherwood number, the mass transfer rate kg around the acting particle can be calculated using correlation Sh=kg∙d/D, where d is the diameter of acting particle, and D is the diffusivity around the acting particle. Hayhurst and Parmar (Hayhurst and Parmar 2002) calculated the Sherwood number in the fluidised bed by using the CO/CO2 ratio, which was measured by the temperature difference between the carbon particle and the bulk phase (Hayhurst and Parmar 1998). However, the temperature of the particle could be overestimated, so the CO/CO2 ratio could be underestimated. In this thesis, a universal exhaust gas oxygen (UEGO) sensor was employed, which could measure the actual carbon consumption rate in the fluidised bed by oxidizing CO in the sample gas into CO2 and. Fe particles of the same size of the char particle is used to measure the O2 consumption rate, and thus eliminate uncertainty in the Sherwood number. The CO/CO2 ratio was calculated by using the carbon consumption rate and the O2 consumption rate. In contrast to Hayhurst and Parmar (Hayhurst and Parmar 2002) who assumed CO2 was the main product, for this char the actual ratio of CO/CO2 was almost zero. The measurement here is in agreement with Arthur. This more accurate determination of CO/CO2 allows a better estimate of the mass transfer coefficient and leads to a correction of the Hayhurst and Parmar’s (Hayhurst and Parmar 2002) correlation by a factor of ½. Interestingly, very small fluidised beds have mass transfer coefficients which are about twice that expected in a large bed (owing to the very different flow and indeterminate flow pattern). This means the correlation of Hayhurst and Parmar (Hayhurst and Parmar 2002), by fortuitous coincidence works wells for beds with diameters < 30 mm., without the correction factor, should be ignored. In the fluidised bed in a typical CLC process, different fluidising material could have different influence on the reactions. Thus, it is worth discussing different kinds of fluidising materials. The char combustion in the fluidised bed was simulated by using inert (sand) and active (Fe2O3 or CuO) fluidising materials, and air as fluidising gas. The results indicated that 1) CO combustion in the boundary layer leads to smaller carbon consumption rate and larger oxygen consumption rate; 2) Using Fe2O3 particles as fluidising materials slows down the carbon consumption rate, since the diffusivity of CO2 is smaller than CO; 3) CuO particles slow down the carbon consumption rate at large Sherwood number (Sh=2 or 2.5). The influence of using CuO as fluidising material is further discussed experimentally by using low O2 fluidising gas. The results indicated that since the amount of CuO used in the experiment is small, when the O2 concentration in the bulk phase is lower than the equilibrium concentration, the O2 concentration in the bulk phase gradually decreases, and the O2 concentration in the bulk phase has large influence on the char particle combustion. A steady state model of a coal-fired CLC power plant was simulated. The aim of the model was to test the suitable operating conditions of the power plant, such as recycle rate of oxygen carriers, for the power plant design. In the steady state model, the power plant consists of a combustor and a steam cycle. Hambach lignite coal, Polish bituminous coal and natural gas were tested as fuels. The results indicated that: (1) The effect of the fuel is largely due to the amount of oxygen required per GJ released; (2) Preheating is important, but seems to have a minor effect since the most of the heat is released at temperatures well above the pinch point; (3) since the temperatures of heat source in this research is well above the pinch point, all heat are usable for the steam cycle. In this case, the steam cycle and the chemical looping plant could be optimised separately; (4) As long as the preheat temperature of the air flow into the air reactor is higher than the temperature of turbines, in most of cases the power output is unaffected by the choice of variables, leaving the designer free to choose the most convenient. With the conclusions above, a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carrier is then simulated. The aims of this simulation include: 1) explaining the kinetics of Fe2O3 oxygen carriers at high temperature (1223K) in a fluidised bed reactor using Brown’s data (Brown 2010); 2) a 1GWth dynamic power plant was simulated to test different cases including changing power supply and power storage. In the dynamic model, a chemical looping power plant using Hambach lignite char is tested, and the parameters of the system are adjusted so as to simulate the operations of a real chemical looping power plant. The two-phase model is employed for the fluidised bed reactors. Experimental data from Brown (Brown 2010) was simulated using this model first to test its validity. Then the model is scaled up to simulate a 1GWth dynamic power plant. The ideal operation conditions are found, and a char stripper is found helpful for carbon capture.EPSR

A Novel Method for Pre-combustion CO2 Capture in Fluidized Bed

La comunidad internacional está realizando enormes esfuerzos para mitigar los efectos de las emisiones de gases de efecto invernadero (GEI) en el cambio climático. Aproximadamente le 25% de las emisiones globales de GEI (fundamentalmente CO2) son generados por la combustión de combustibles fósiles en el sector eléctrico. La captura y almacenamiento de CO2 se ha propuesto como una alternativa para reducir las emisiones de GEI en centrales térmicas. Numerosas tecnologías para la captura de CO2 se han desarrollado en los últimos años, fundamentalmente en tres líneas tecnológicas: postcombustión, oxicombustión y precombustión. Esta tesis presenta un nuevo método para la captura de CO2 en precombustión, produciendo hidrógeno a partir de carbón, sin emisiones de GEI. El objetivo principal de este trabajo ha sido desarrollar un modelo completo, mediante herramientas de fluido dinámica computacional (CFD), del proceso de reformado de un gas de síntesis con alto contenido en metano combinado con la captura de CO2 mediante adsorción con sorbentes sólidos regenerables. Este proceso es conocido como reformado de metano mejorado por adsorción (o SE-SMR, su acrónimo en inglés). SE-SMR representa una novedosa y eficiente energéticamente ruta para la producción de hidrógeno con captura in situ de CO2. Este proceso ha sido estudiado en un lecho fluido burbujeante, usando sorbentes sólidos de óxido de calcio como captores de CO2. Dos sorbentes sólidos han sido estudiados en laboratorio: uno natural (Dolomita) y uno sintético (CaO- Ca12Al14O33). Además, varios tratamientos han sido desarrollados para mejorar la capacidad de captura de estos sorbentes. Un completo modelo CFD del proceso de SE-SMR ha sido desarrollado. Una aproximación Euleriana-Euleriana ha sido combinada con la Teoría Cinética de Flujos Granulares para simular la fluidodinámica del lecho fluido burbujeante. Los reacciones químicas de reformado y carbonatación han sido implementadas en el modelo CFD. Se ha incluido un modelo detallado de captura de CO2 para simular el comportamiento de los diferentes sorbentes sometidos a diferentes pretratamientos para mejorar su rendimiento. Asimismo, un modelo de arrastre de partículas ha sido desarrollado para reducir el coste computacional de las simulaciones a escala semi-industrial. Se ha llevado a cabo una extensa campaña de simulaciones para validar el modelo a escala de laboratorio y semi-industrial. Las simulaciones CFD han sido combinadas con un Diseño de Experimentos Robusto, con el objetivo predecir y evaluar la sensibilidad del proceso SE-SMR a diversos factores operativos

Hydrodynamic modeling of poly-solid reactive circulating fluidized beds: Application to Chemical Looping Combustion

This work deals with the development, validation and application of a model of Chemical Looping Combustion (CLC) in a circulating fluidized bed system. Chapter 1 is an introduction on Chemical Looping Combustion. It rst presents the most important utilizations of coal in the energy industry. Then, it shows that because of the CO2 capture policy, new technologies have been developed in the frame of post-combustion, pre-combustion and oxy-combustion. Then, the Chemical Looping Combustion technology is presented. It introduces multiple challenges: the choice of the Metal Oxide or the denition of the operating point for the fuel reactor. Finally, it shows that there are two specicities for CFD modeling: the influence of the collisions between particles of different species and the local production of gas in the reactor due to the gasication of coal particles. Chapter 2 outlines the CFD modeling approach: the Eulerian-Eulerian approach extended to flows involving different types of particles and coupled with the chemical reactions. Chapter 3 consists in the validation of the CFD model on mono-solid (monodisperse and poly-disperse) and poly-solid flows with the experimental results coming from an ALSTOM pilot plant based at the Universite Tchnologique de Compiegne (France). The relevance of modeling the polydispersity of a solid phase is shown and the influence of small particles in a CFB of large particles is characterized. This chapter shows that the pilot plant hydrodynamics can be predicted by an Eulerian-Eulerian approach. Chapter 4 consists in the validation of the CFD model on an extreme bi-solid CFB of particles of same density but whith a large particle diameter ratio. Moreover, the terminal settling velocity of the largest particles are twice bigger than the fluidization velocity: the hydrodynamics of the large particles are given by the hydrodynamics of the smallest. An experiment performed by Fabre (1995) showed that large particles can circulate through the bed in those operating conditions. Our simulations predicted a circulation of large particles, but underestimated it. It is shown that it can be due to mesh size eect. Finally, a simulation in a periodic box of this case was dened and allowed us to show the major influence of collisions between species. Chapter 5 presents the simulation of a hot reactive CLC pilot plant under construction in Darmstadt (Germany). The simulations account for the chemical reactions and describe its eect on the hydrodynamics. Different geometries and operating conditions are tested