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This paper analyzes the capabilities of a pilot-scale chemical looping combustion plant firing wood biomass in two stages to efficiently achieve negative carbon dioxide emissions. The utilized in situ gasification-chemical looping combustion (iG-CLC) process isolates the oxygen supply via air from the fuel conversion itself with the help of two separate fluidized bed reactors and an oxygen carrier to supply the necessary oxygen for the combustion. As a result, a relatively pure stream of carbon dioxide and steam is generated. Thus, the process makes capturing carbon emissions more feasible since it eliminates the need for the cost- and energy-intensive separation of the produced gases. A major issue when using biomass in a chemical looping plant is the high amount of the volatiles exiting unconverted. This problem was mitigated by using a two-stage fuel reactor system. Two bubbling fluidized beds were arranged one upon the other. The lower stage, where the fuel is introduced, is used to release the volatiles and partly convert them. The remaining volatiles rise up into the second stage and are further converted to a high degree. A series of experiments were carried out with a 25-kWth pilot plant located at the Hamburg University of Technology. Gas concentrations were continuously measured after both stages of the fuel reactor to see the gradual conversion of the fuel gases. Additionally, carbon slip at the exhaust was measured to show the effectiveness. The experiments with the reactor concept showed promising results since already at a reactor temperature of 850 °C, the total oxygen demand needed to oxidize the combustible component in the exhaust gas was well below 2%. The carbon dioxide (CO2) capture efficiency when using German hardwood slightly decreased to 93–96% compared to 97% for German lignite. In the future, the reactor design must prove that it scales and that the efficiency can be further increased. Nevertheless, firing biomass with a two-stage iG-CLC process might allow a cost-efficient negative carbon dioxide emission while generating heat with relatively high efficiency. Therefore, it might be a sustainable alternative to generate heat in the future.

The conversion of lignite under Chemical Looping Combustion conditions differs sharply from air-fired combustion as high volumetric gas concentrations of CO2 and steam prevail in the reactor and the oxygen for the fuel conversion is provided by a solid oxygen carrier. In this work a lab scale fluidized bed reactor (ID = 53 mm) was used to measure the conversion behavior of German Rhenish lignite under Chemical Looping Combustion conditions. The carrier used was 14 wt-% copper oxide on γ-alumina. Special focus was laid on the particle size of the char during gasification, hence 3–6 different size fractions, in the range of 150–1500 μm were converted during each test series, respectively. The gasification experiments showed that gasification of lignite in a fluidized bed of inert sand particles is slightly faster for steam compared to CO2 gasification. In the inert sand bed, chemical reaction limitation of the kinetics could be seen for all lignite size fractions below 850 μm diameter. Gasification rates were in the range of literature findings for fluidized bed gasification in inert sand beds. The inhibition effects of both CO and H2 were reproduced by applying CO and H2 as fluidization gases during gasification. Gasification experiments in a fluidized bed of a completely oxidized oxygen carrier showed for both CO2 and steam gasification an enhancement of the char conversion, even more for smaller particle sizes of the lignite. Simultaneous gasification with CO2 and steam showed only slight increases in the measured gasification rates compared to steam only gasification. The Shrinking Core Model was generally able to describe the experiments. A resistance approach was used to describe the strong mass transfer limitations seen in the reactive bed for larger char particles particularly at higher temperatures. Kinetic parameters were extracted for the simultaneous steam and CO2 gasification under in-situ Gasification Chemical Looping Combustion conditions.

As a result of the increasing transport cost of biomass and benefiting from the more efficient treatment of a compacted, dustless product, the use of pelletized biomass has gained interest over rec...

In a Chemical Looping Combustion system, the fuel and air reactors are strongly coupled because of chemical reactions in both and the circulation of solid oxygen carrier between them. To capture the effects inside the system, a novel dynamic flowsheet simulation environment for solids processes is applied to Chemical Looping Combustion of methane. Flowsheet simulation is a tool for process analysis and optimization covering multiple process units and flows in a system. An experimental 25 kWth pilot plant is operated, and all of its process units are modeled. The modeling comprises three fluidized bed reactors, two operating in bubbling fluidized bed condition and one as a circulating fluidized bed riser. A cyclone is used for gas-solid separation after the air reactor. The loop seals ensure gas sealing between the reactors. Fluid mechanics inside the systems are modeled with empirical and semi-empirical correlations, to enable fast calculations. This approach becomes handy when long-term dynamic effects like abrasion, start-up, or shut-down procedures as well as load changes are to be modeled. Chemical reactions for a gaseous fuel and their implications on gas flows were implemented. In addition, oxidation and reduction of the solid oxygen carrier in the three reactors were part of the simulation. To validate the simulation results, the pilot plant was operated with methane as fuel. Gas measurements were taken after both stages of the fuel reactor. Additionally, solid samples were drawn from the hot facility to examine the oxidation state of the carrier, when fuel is introduced. A transient simulation of plant operation over a total runtime of 40 min reveals that the solids inventories of the fluidized bed reactors in the system need only 30 s in the present case to reach a new steady state after a load change. If the oxidation and reduction reactions of the oxygen carrier are taken into account, however, this response time extends dramatically to several hundreds of seconds, which can also be seen in the experimental campaigns. The simulation of such a system behavior requires a powerful simulation tool for flowsheeting, which has been found here in the dynamic simulation framework.

The Zhundong coalfield in Xinjiang, China, is the largest integrated coal basin newly found. The present work concentrates on the application of chemical looping combustion (CLC) with a Zhundong lignite, which is characterized by high sodium content. Some experiments in a laboratory scale fluidized bed facility with an active iron ore oxygen carrier, were performed using the lignite as fuel and CO2 as gasifying agent at a temperature of 900 °C, with the objective of investigating its combustion performance and sodium transfer behavior in CLC. Results indicate that the gasification reactivity of the three coals follows the order of German lignite > Zhundong lignite > American bituminous coal in the current experimental conditions. During reducing stage, the unique product of sodium transfer from coal to the fly ash is albite (NaAlSi3O8) due to the reactions between sodium and other coal ash. The sodium deposition on the oxygen carrier particles was not found. 40 reducing-oxidizing cycles were performed, and sodium accumulation in the bed materials with cycles was found due to some ash staying in the bed. However, the growth of bed particles due to the sodium accumulation was not observed by determining the particle size distributions of bed materials. This indicates that burning the high sodium Zhundong coal in the present conditions have no influence on the particle agglomeration. Finally, a literature survey was made and results indicate that the main sodium in the Xinjiang coal basin of China is water soluble with an average value of 64%. The pure salt of NaCl, as one common water soluble sodium phase in Zhundong coals, was introduced to a bed of iron ore particles at 900 °C with regard to investigate the influence of NaCl on fluidization stability. Based on the measurements of pressure drop, bed temperature and SEM-EDS, it was found that NaCl does not react with the iron ore but in fact only acts as glue between iron ore particles. Further, the sodium transfer routes in CLC of Zhundong coal with iron ore based oxygen carrier are given and some discussions are made with regard to practical operation. The corrosion problems on the heating surface in the air reactor can be significantly reduced compared to a conventional Zhundong coal fueled furnace, since most of sodium will release and be converted in the fuel reactor.

A novel flowsheet simulation environment is applied to simulate a system of interconnected fluidized bed reactors as they are used for the process of chemical looping combustion of solid fuels. Dynamic models of the main process equipment are used in order to capture dynamic behavior of hydrodynamics inside an actual system, which delivers the experimental validation data. The process itself is carried out in a pair of strongly coupled fluidized bed reactors. Furthermore, a cyclone is used for gas-solid separation and two loop seal siphons prevent gas leakages from one reactor to another. The experimental plant operated at Hamburg University of Technology, which is modeled here, comprises a circulating fluidized bed air reactor and a two-stage bubbling bed fuel reactor. Operation of the plant is carried out at ambient condition and so are the simulations. All the mentioned process units are connected into a flowsheet via material streams and the whole process is simulated for 750 s of runtime. Within this time, the fluidization velocity of the fuel and the air reactor are changed to mimic effects, which occur during actual operation. The simulation was closely accompanied by experiments at the experimental facility and the results of simulation and experiments were compared. The bed mass, solid circulation and pressure profiles were simulated with good agreement to experimental findings. Dynamic effects were captured by the simulations and the system's response to varying the fluidization speed, that is the change of bed masses, was predicted correctly. Time scales, in which these changes occur, were in the same range for experiments and simulations.

To investigate the influence of the fuel characteristics on the conversion behavior in a chemical-looping combustion facility, lignite coal dust (d90,3=233 μm) and two fractions of bituminous coal with different particle sizes (fine fraction d90,3=163 μm, coarse fraction d50,3=707 μm) were used as solid fuel. To improve the conversion performance, a pilot plant with a rated power of 25 kW was constructed with a two-stage fuel reactor. The influence of the fuel composition, particle size, and the presence/absence of elemental oxygen on the conversion in the fuel reactor are presented. The used oxygen carrier was produced by the impregnation of γ-alumina oxide with copper oxide, which is able to release gaseous oxygen, but loses this ability because of deactivation. The lignite dust shows a very good conversion performance and carbon capture efficiencies over 95 % as well as oxygen demands below 2 %. Both bituminous coal fractions have a good performance with regard to fuel conversion and oxygen demand but they suffer from a high carbon slip. Hence the carbon capture efficiency is around 60 % for the fine fraction and 40 % for the coarse one. The performance improvement as a result of the second stage was investigated separately, and we proved that it enhances the overall conversion. In addition, the oxygen carrier generated a favorable reaction environment by releasing elemental oxygen in the second stage of the fuel reactor.

Silica in the form of clay present in the bauxite ore causes lot of complications in the Bayer hydrometallurgical process for the production of alumina. Therefore, bauxite is processed and washed in trommels to disintegrate and remove the clay before sending the ore as a feed to the Bayer process. Estimation of ore washing potential and the selection of cut size is essential for the establishment of a bauxite washing plant. In the current research study, a simple ultrasonic washing technique has been developed for the removal of clay and to determine the potential of an industrial washing process at various cut sizes. Four feed size fractions of clayey bauxite ore were washed in laboratory for 1, 3, 5 and 6 min using ultrasounds. After 5 min, the steady state washing conditions were observed. It was found that at 36 micron cut size, 70% mass and 77.5% alumina recovers whereas 69% of total silica moved towards the tailings streams. Cut size sensitivity shows that by increasing the cut size from 36 micron to 100 micron, nearly 5% reduction in the recoveries of total mass and alumina was observed whereas the recovery of silica slightly reduced in the concentrate.

Due to the advantages of high energy density and convenient transportation, the use of pelletized biomass as fuel for power supply has gained interest over recent years. Chemical looping gasification (CLG) integrates the process of gasification and hot gas conditioning, with the objective to obtain syngas with a low tar amount. The work presents some experimental results using sawdust pellet with high-volatile and low-ash content as fuel, and a natural manganese-iron ore as oxygen carrier for CLG in a bubbling fluidized bed. Without the addition of external steam, the syngas, tar, and char collected at different temperatures (750–950 ℃) and oxygen carrier to biomass ratios (0.2–1.2) were investigated to determine the gasification performance of sawdust pellet. Three-phase product distributions during CLG process were identified under different operating parameters. The increase of reaction temperature enhances the gas production, with decreasing amount of liquid and solid. Also, it has significant influence on the main gases (CO, CO2, H2 and CH4) distributions in the syngas. Meanwhile, more Polycyclic-Aromatic Hydrocarbons (PAHs) are cracked to Mono-Aromatic Hydrocarbons (MAHs) and further decomposition into syngas, which is confirmed by further Gas Chromatography-Mass Spectrometer (GC–MS) analyses. The Mn/Fe-based oxygen carrier also exhibits a beneficial effect on catalytic cracking of PAHs into MAHs. As for the remaining char, a higher temperature is more sensitive to produce the char with more porous structure and brittle texture.

This review of industrial applications of fluidized-bed reactors is focused on the fluid catalytic cracking (FCC) process and the fluidized-bed combustion. Both processes have a tremendous economic importance. In the FCC process, the main challenges are the increase of runtime, the adaption of the product spectrum to regional requirements of the market, and the adaption to heavier feedstocks. In fluidized-bed combustion the main trends are towards larger units sizes, higher efficiency, and further reduced emissions while keeping the boiler as flexible as possible with regard to the mixture of fuels.

Clay impurities associated with bauxite negatively affect the Bayer process for alumina production. These impurities should be removed as far as possible by a beneficiation technique before the ore is used as feed for the Bayer process. In this current investigation, bauxite washing was conducted in the laboratory. Bauxite washing is a physical process that causes the disintegration and deagglomeration of the clay matrix, and bauxite is liberated from the clay (mainly rich in silica). Subsequently, separation occurs with the assistance of wet screening at a predetermined cut size. Three techniques were investigated in the laboratory: drum washing, water-jet washing, and ultrasonic washing. Various operating parameters were investigated for drum washing and water-jet washing, including materials retention time, drum rotation speed, solid concentration, water-jet spray duration, pressure, and height. We concluded that the retention time of bauxite inside the drum at a solid concentration of 55wt% and a drum rotation speed of 31 r/min is the dominant parameter for the removal of clay from the bauxite surface.

A system of coupled fluidized beds for chemical looping combustion of solid fuels was successfully commissioned. The facility has a rated thermal power of 25 kW and consists of a circulating fluidized bed coupled with a two-stage bubbling fluidized bed. The two-stage bubbling fluidized bed is the fuel reactor and the riser of the circulating fluidized bed is the air reactor. In the experiments Australian ilmenite with a particle size in the range of 100–400 μm was used as the oxygen carrier. The solid fuel was lignite dust with more than 70% of the mass having a particle size smaller than 150 μm. The influence of the operational parameters, i.e. reactor temperature, coal feed rate and composition of the fuel reactor feed gas on the operational behaviour of the system was investigated. The two-stage fuel reactor performed well and CO 2 -concentrations in the dry fuel reactor off-gas of above 90 vol.% were achieved. The reason for the appearance of unconverted combustible gases in the fuel reactor off-gas needs further investigation. Solids circulation rates based on the riser cross-section were determined under hot operating conditions and turned out to be between 56 and 70 kg/m 2 s. The carbon slip to the air reactor was small in all tests: only 1.5–6.5 wt.% of the fixed carbon introduced with the coal were oxidized in the air reactor.

In the current paper a simulation tool based on previous work of Burgschweiger and Tsotsas [1] is implemented within the framework of the flowsheet simulation program SolidSim [4]. The implementation within the flowsheet simulation program allows the simulation of the drying process for many different liquids and solids. It can also simulate the drying of mixtures of different solids. The module computes the vapor loading and temperature of the out flowing gas and the moisture and temperature distribution of the solids at the outlet taking into account their residence time distribution, their particle size and their moisture distribution at the inlet. Furthermore the model has been extended by a simple approach to simulate the drying in an elongated fluidized bed dryer. After validation the model has been used to study the influence of residence time, particle size and inlet moisture distribution in addition to drying of mixture of different types of solid. For the modeling it is assumed, that there is no classification occurring within the bed. This assumption is commonly fulfilled as long as the particle size distribution is not too wide and the fluidization velocity is sufficiently high. In this case the mixing action of the bubbles is dominating the segregation tendencies.

This study investigated the applicability of computational particle fluid dynamics (CPFD) numerical schemes for simulating flows in circulating fluidized bed (CFB) risers. Gas–solid flows were simulated in CFB risers containing Geldart A particles with both low and high solid fluxes as well as in a CFB riser containing Geldart B particles for three flow conditions using CPFD. The results are compared to experimental data and previous two-fluid model (TFM) simulations. The time-averaged axial and radial distributions of the solid concentration show that the bottom-dense, upper-dilute and core-annulus heterogeneous structures were successfully captured by the CPFD calculations, but only qualitatively. The results differ from experimental data for Geldart A particles and high solid fluxes, although they were more accurate than two-fluid simulations with conventional drag models. Two-fluid modeling with the EMMS (energy minimization multi-scale) drag model gave more accurate results than the CPFD simulations. The results indicate that the drag force in the CPFD scheme is still overestimated, although the cumulative method used to compute drag force is more accurate than the proportional method in the two-fluid model. An EMMS drag model which takes into account the intrinsic heterogeneity in the CFB risers is needed for the CPFD scheme. The effect of the realistic particle size distribution was seen in the by CPFD results.

In the present work a simulation module is developed for the description of a bubbling fluidized bed reactor and for the riser reactor of a circulating fluidized bed which is suitable for use in the framework of a flowsheet simulation system. A population balance approach is used to determine the particle size distribution within the reactor in steady-state. The model allows to describe the individual fate of particles in terms of attrition (abrasion and shrinkage) and transport effects. The fluidized bed reactor model is one-dimensional. Models of fluid mechanics are combined with kinetic approaches for the description of the local reaction. The implementation within the framework of a simulation package for solids processes allows for the flexible coupling of process models, e.g. of the fluidized bed reactor with one or more cyclone stages for gas cleaning or with other fluidized beds in fluidized bed systems.

In part I of this paper a new fluidized bed reactor module was presented for the simulation of a bubbling fluidized bed and of the riser of a circulating fluidized bed with the flowsheet simulation tool SolidSim so that several process models can easily be coupled. In the present part II the newly developed fluidized reactor modules are connected to existing cyclone modules to simulate a complex coupled fluidized-bed process, the selective oxidation of n-butane to maleic anhydride in a riser–regenerator system [1] , [2] . The calculation results for a laboratory setup are found to be in good agreement with experimental data from the literature. The subsequent simulation of an industrial scale sized plant shows that a high solids recirculation rate is needed to achieve satisfying maleic anhydride yield. The usefulness of flowsheet simulation is demonstrated by further process simulation calculations with varied solids recovery systems.

Attrition of catalyst represents a significant economic penalty due to the necessity to make-up the losses on a continual basis. Designing fluidized bed systems to minimize attrition may sacrifice gas–solid contact efficiency that requires pressure to distribute gases in process vessels uniformly. In this paper, the attrition performance of DuPont's vanadium pyrophosphate oxide catalyst (VPP) used in their commercial Circulating Fluidized Bed (CFB) process to convert n-butane to maleic anhydride has been examined. The analysis is based on simulating the commercial conditions in the individual vessels combined with the physical characteristics of the catalyst determined in the laboratory. The comparison of the catalyst loss rate measured in the industrial plant and the simulation results indicates that the measured values are below the values predicted by the simulation. However, the relative change in the catalyst loss rate with operating time is well described. The simulation allows the quantification of the attrition sources in the process. The simulation results revealed that in the present case over 60% of the catalyst loss originates from attrition in the fluidized beds, whereas only 16% originates from attrition in the cyclones. The simulation is an appropriate tool to investigate the influence of parameter changes. Using the simulation the design parameters and operating conditions can be optimized to minimize the catalyst loss rate in the industrial plant.

Modern production processes in chemical, pharmaceutical and biological industries are characterized by complex process structures, which consist of different apparatuses and process steps. Modeling the entire process requires simulating all units altogether, while taking into account interconnections between them. Nevertheless, in the area of solids processing, there is nowadays an unfilled gap from the side of computer support of process modeling in allowing effective optimization and prediction of the behavior of the whole plant. This paper presents a tool for flowsheet simulation which allows the simulation of the stationary behavior of complex processes dealing with solids and its extension towards dynamic modeling. Also, a new simulation concept is proposed on the basis of the multiscale approach. On the macroscale, flowsheet simulation is performed with the help of the SolidSim system. Parameters for the macromodels in SolidSim are predicted by microscale simulation. The models for the two scales are then coupled by inter-scale communication laws. Application of the proposed modeling concept is shown by an example of fluidized bed granulation.

In the present work, the steady-state behavior of a Chemical Looping Combustion process of interconnected fluidized bed reactors is simulated. The simulations have been carried out in two different scales, 50 kWth and 100 MWth . Attrition model derived from small scale laboratory experiments has been employed for the prediction of the process behavior in terms of attrition and Oxygen Carrier loss. Information on Oxygen Carrier characteristics and reaction kinetics were taken from literature. Realistic circulation mass flows of Oxygen Carrier particles are obtained and Oxygen Carrier losses are quantified. The large scale process looses significantly more Oxygen Carrier than the small scale process based on the same amount of thermal energy produced. Incomplete conversion in the air reactor could be identified as a critical point. Another issue is the fuel gas bypassing the Oxygen Carrier particles through bubbles in the large scale process which leads to lowered fuel conversions. The simulations indicate that a similar performance of a pilot scale and a large scale process is not guaranteed due to the scale-up effect on fluid dynamics. Furthermore, the simulations allow an assessment of the influence of the quality of the solids recovery system on the Oxygen Carrier loss. The distribution of the losses between possible origins is investigated and different changes in the solids recovery system are discussed regarding their potential to decrease the Oxygen Carrier loss. For example, the addition of a second-stage cyclone after the air reactor of the large scale process reduces the Oxygen Carrier loss significantly.

Nowadays, software tools for the flowsheet simulation of industrial processes are commonly used for design, simulation, balancing, troubleshooting and optimization purposes. Most of the tools are applicable to fluid processes only and cannot be effectively used for processes which involve solids. In this contribution we want to present the conceptual design of a new system applicable for the dynamic flowsheet simulation of complex solids processes. This system is developed as an enhancement to the existing simulation program. The novel software is able to simulate the unsteady behaviour of complex circuits of granulation processes. The transient behaviour during the start-up and changing of the process or material parameters can also be examined. As flowsheet examples, a typical spray granulation process with different schemes consisting of fluidized bed granulators, screens, mill and splitters was used. The mathematical model of the fluidized bed granulator is described by a one-dimensional population balance equation and coupled with heat and mass transfer and simple fluid dynamics. Received simulation results have shown that the proposed concept of the dynamic flowsheet simulation of granulation processes can be used effectively and has the potential to be generalized for other types of solids processes.

A model is presented for the simulation of reactive gas–solids flows in large industrial reactors. Circulating fluidized bed (CFB) combustors with several thousands of cubic meters reaction volume are probably the largest reactors of this type. A semi-empirical modeling approach has been chosen to model the three-dimensional concentration distributions of gas and solids components and temperatures inside the combustion chamber of such boilers. Two industrial CFB boilers are investigated in detail: the 105 MWe Duisburg combustor in Germany and the 235 MWe Turow combustor in Poland. The semi-empirical model approach is described first. Then the model is used to show how the three-dimensional concentration and temperature fields are formed by the interaction of several local phenomena. Good agreement between simulation and measurements has been achieved.

This paper proposes a strategy for the global sensitivity analysis in the flowsheet simulation on solid processes which allows to examine and quantify the influences of given parameters on specific target criteria. The strategy is basically divided into two parts: The first is the effect analysis which provides the range of possible values of a target variable. This permits an estimation of the accuracy of a simulation with uncertain parameters. As the second part the influence analysis builds a hierarchy of the different uncertain parameters according to their impacts on a specific target criterion. This latter information can be used to decrease the efforts for parameter estimation in a given process simulation by focussing the efforts on the most influential parameters. The strategy for global sensitivity analysis suggested here is applied to a soil washing process where an oil-contaminated soil is to be cleaned such that both the amount of the contaminant concentrate to be disposed and the residual oil contamination of the cleaned soil are to be minimized.

The optimization of a novel wet centrifugal fluidized bed classifier is discussed in this paper. Experiments in a pilot-scale apparatus and CFD calculations for the pure liquid flow reveal that the Coriolis force induces a strong tangential flow relative to the rotating vessel. Based on this finding a discharge mechanism for the coarse fraction has been developed which consists of a simple nozzle and thus allows a continuous and stable operation of the classifier. The measured pressure drop characteristic of the coarse particle bed reveals the range of fluidizing velocities which are optimal for the performance of classification. With the optimized classifier sharp classifications with a strong reduction of the misclassification of fines were achieved in different experiments with quartz, limestone and glass beads. Down to cut sizes of 5 μm sharp classifications were achieved.

In the current work, a model of the fluid mechanics in the riser of a circulating fluidized bed (CFB) has been implemented using computational fluid dynamics (CFD). The model developed shall be used in future as the basis of 3D-reactor model for the simulation of large scale CFB combustors. The two-fluid model (TFM) approach is used to represent the fluid mechanics involved in the flow. The computational implementation is accomplished by the commercial software FLUENT. Different closure formulations are tested on a simplified geometry. Two different turbulence formulations, namely the swirl modified RNG k–ɛ model and the Realizable k–ɛ model, are tested in combination with two different approaches to solid phase turbulence, namely the dispersion and per phase approach. One focus of the current work is put on the study of different drag correlations. Besides the drag correlations by Syamlal et al. [Syamlal, M., Rogers, W., & O’Brien, T. J. (1993). MFIX documentation theory guide. Technical Report DOE/METC-94/1004, U.S. Department of Energy (DOE). Morgantown Energy Technology Center: Morgantown, WV] and Gidaspow [Gidaspow, D. (1994). Multiphase flow and fluidization. New York: Academic Press] the EMMS model has been used to determine the momentum exchange between the two phases. The resulting formulation is then used to simulate a 1-m × 0.3-m cold CFB setup and is validated by experimental results [Schlichtharle, P. (2000). Fluid dynamics and mixing of solids and gas in the bottom zone of circulating fluidized beds. Unpublished doctoral dissertation, Technische Universitaet Hamburg-Harburg, Shaker Verlag: Aachen].

The concept of simulation-supported measurement is suggested for the elucidation of processes occurring in the combustion chambers of large-scale circulating fluidized bed combustors where the desired information cannot be obtained by direct measurements. The concept is illustrated with the example of secondary air injection where the way the air is released, the penetration depth and the evenness of air distribution over the cross-sectional area of the combustion chamber are of interest. The measured information consists of lateral profiles of oxygen concentrations measured with gas sampling probes at two ports which were located 5 and 9 m, respectively, above the level of secondary air injection. The simulation is carried out on the basis of a 3D semi-empirical fluid-mechanical model of the circulating fluidized bed which is combined with models of gas and solids mixing, fuel distribution, devolatilization and combustion of char and volatiles. The combination of the simulation with the measurements yields a clear picture of the mechanism of secondary air injection, its penetration into the combustion chamber and its effect on the local combustion processes. The results confirm the usefulness of the concept of simulation-supported measurement for this application.

A dynamic simulation model of the particle population in a circulating fluidized bed combustor with external heat exchanger has been developed. It considers the fluid dynamic processes in the various parts of the system, as well as the particle attrition. To handle multiple solids types simultaneously and to fulfill the mass balances, some of the fluid dynamic sub-models taken from the literature were modified. The model allows to calculate the solids mass flows as well as the corresponding particle size distributions at any point inside the combustion system. The model has been applied to the combustion plant of Stadtwerke Neumunster in Germany, which operates on refuse-derived fuel. The particle balancing model provides new insights into the operating behavior of such a system. In particular, the calculation of the residence time of different particle classes in the system reveals a very broad distribution of size dependent average residence times, ranging from several minutes to a maximum of roughly 40 h. A size fraction exists between 100 and 300 μm with a maximum average residence time of about 40 h. The Preprint submitted to Elsevier Science 15 August 2008 simulation provides a means for examining possibilities to control the particle size distribution in the combustion system. It is shown how a recirculation of a fine ash fraction can be used to control the bed particle size distribution in the combustion chamber.

In the present work a method for the treatment of design specifications in the simulation of complex solids processes has been developed. Adding a design specification to the flowsheet of a given solids process means that the solid product should fulfill a certain requirement. For example, its Sauter mean diameter should be kept within a certain interval or the mass fraction of particles above a certain threshold should be kept below a certain limit. The method developed here is not only able to handle product specifications but also other stream properties of the process. These properties can be either distributed or concentrated. Even dependently distributed properties e.g. a particle size-dependent contamination can be handled. The treatment of design specifications has been implemented as an extension of the flowsheet simulation system SolidSim. Practical applications of the implemented extension are illustrated with three examples: (i) a simple process for the preparation of brown coal, (ii) a process for the treatment of gravel and sand and (iii) a complex process for the mechanical separation and dewatering of harbor sediments contaminated with heavy metals.

Ash management in fluidized bed combustion systems means keeping the particle size distribution of the bed inventory in a given range. A dynamic particle population balancing model was developed for this purpose. It was successfully applied to a refuse-derived fuel fired combustor and a coal-fired circulating fluidized bed combustor. Both were large-scale commercial units. The model uses the concept of the attrited ash particle size distribution which represents the particle size distribution of the attrited ash including the generated fines and replaces the consideration of the particle attrition in the model calculations. The model offers the possibility to gain additional information about the particle size distributions and the solids mass flows at any location of the fluidized bed system. In addition, the model provides information about the dynamic behavior of the plant and about mean residence times of particle size classes in the plant. Uncertainties about the ash formation characteristics of fuels make the management of the bed inventory a very important issue. In this context the population balancing model is used to predict the plant behavior under various operating conditions. The results of the calculations carried out give useful information about the possibilities to manage the ash inventory of such a plant. It could be shown that the recirculation of a fine fraction of the bottom drain solids is a very effective method to manage the particle size distribution of the bed inventory. The calculation results further reveal that the mean residence time of particles is strongly dependent on their size.

By combining the advantages of a high-pressure fluidized bed with the adjuvant properties of a supercritical fluid, namely RESS-process, solid proteins could be encapsulated by paraffin. Two different irregular shaped proteins, a model protein (bovine serum albumin, BSA) and a pharmaceutical protein (insulin) were investigated. Mixing the highly non-spherical protein particles with lactose allows its usage as bed material despite the unfavourable shape properties. The paraffin encapsulation of respective bed mixtures with BSA was successful with paraffin loadings up to 9.1 g/100 g bed and yields up to nearly 100%, depending on process parameters. During the encapsulation process, breakage of the bed material occurred, resulting in mean sauter diameters of even less than 35 μm. Dissolution tests pointed out, that 100% dissolution after more than 180 min could be achieved. Thus, the particle breakage induced by the nozzle jet during the coating process seems to support the encapsulation of fines. A principle application of the process for insulin encapsulation for a selected set of process parameters has been demonstrated.

Solids volume concentration and solids velocity were determined in an 8.5 m high circulating fluidized bed (CFB) riser with two types of bubble-cap distributors by applying a capacitance probe. In the bottom region of CFB, the solids volume concentration in the centre region is low, while solids concentration increases significantly towards the wall with the highest solids concentration at wall approaching the value at the packed bed. Furthermore, solids volume concentration at high-pressure drop of a bubble cap is lower than that at low-pressure drop of a bubble cap at all lateral positions. The pressure drop of the distributor has little influence on the profiles of solids volume concentrations and on the profiles of solids velocities in CFB upper dilute region. It has been found that the pressure drop of the distributor has little influence on the axial apparent solids concentration in upper dilute region and on the external circulation rate.

Many of the models used in the flowsheet simulation of solids processes are more or less empirical. In order the get reliable results from a flowsheet simulation it is often necessary to adjust model parameters to measured data. Because measured values are always erroneous to some degree in the first step a data reconciliation calculation has to be performed. Based on the reconciled data that fulfil the balance equations the adjustment of the model parameter can be done by use of numerical optimization methods. In the present work a method for data reconciliation as well as for the adjustment of model parameters has been developed with respect to solids processes. The developed methods have been implemented as extensions of the flowsheet simulation system SolidSim. The practical application of the developed extensions is illustrated by example of a process for the dressing of brown coal operated by RWE Power AG in Germany.

The flow pattern in the bed and in the freeboard of a liquid–solid fluidized bed operated in the centrifugal field was investigated. Centrifugal accelerations between 20 and 500 times the gravity were acting on the particles. Both the radius-dependent centrifugal acceleration and the Coriolis force were found to strongly influence the fluid dynamics. A CFD simulation of the liquid flow in absence of particles showed high tangential velocities which were two orders of magnitude higher than the radially oriented fluidizing velocity. Measurements with a 1 m diameter rotor confirmed the calculations and revealed that also the liquid–solid fluidized bed is moving in the tangential direction although with lower speed than the pure liquid. The bed expansion under these conditions can be described by a Richardson–Zaki type correlation. On a etudie le profil d'ecoulement dans le lit et dans l'espace libre d'un lit fluidise liquide–solide fonctionnant dans un champ centrifuge. Des accelerations centrifuges de 20 a 500 fois la gravite agissaient sur les particules. On a trouve que l'acceleration centrifuge dependante du rayon et la force de Coriolis ont une forte influence sur la dynamique des fluides. Une simulation CFD de l'ecoulement liquide en l'absence de particules montre des vitesses tangentielles elevees de deux ordres de grandeur superieures a la vitesse de fluidisation dans le sens radial. Des mesures avec un rotor de 1 m de diametre confirment les calculs et revelent egalement que le lit fluidise liquide–solide se deplace dans le sens tangentiel, quoique moins vite qu'avec le liquide pur. L'expansion de lit dans ces conditions peut etre decrite par une correlation de type Richardson–Zaki.

An overview is given on methods and technologies for limiting the gaseous emissions from waste combustion. With the guideline 2000/76/EC recent European legislation has set stringent limits not only for the mono-combustion of waste in specialized incineration plants but also for co-combustion in coal-fired power plants. With increased awareness of environmental issues and stepwise decrease of emission limits and inclusion of more and more substances into the network of regulations a multitude of emission abatement methods and technologies have been developed over the last decades. The result is the state-of-the-art waste incinerator with a number of specialized process steps for the individual components in the flue gas. The present work highlights some new developments which can be summarized under the common goal of reducing the costs of flue gas treatment by applying systems which combine the treatment of several noxious substances in one reactor or by taking new, simpler routes instead of the previously used complicated ones or - in the case of flue gas desulphurisation - by reducing the amount of limestone consumption. Cost reduction is also the driving force for new processes of conditioning of nonhomogenous waste before combustion. Pyrolysis or gasification is used for chemical conditioning whereas physical conditioning means comminution, classification and sorting processes. Conditioning yields a fuel which can be used in power plants either as a co-fuel or a mono-fuel and which will burn there under much better controlled conditions and therefore with less emissions than the nonhomogeneous waste in a conventional waste incinerator. Also for cost reasons, co-combustion of wastes in coal-fired power stations is strongly pressing into the market. Recent investigations reveal that the co-firing of waste can also have beneficial effects on the operating behavior of the boiler and on the gaseous emissions.

Die losungsmittelfreie Ausfallung wasserloslicher Substanzen und deren nachfolgende Verkapselung erlaubt es, ruckstandsfreie Verbindungen mit definierbarem Auflosungsverhalten herzustellen. Eine mit uberkritischem CO2 betriebene Hochdruckwirbelschicht bietet dabei eine Moglichkeit, auch temperaturempfindliche Substanzen wie Proteine zu behandeln. Im Beitrag wird die Proteinausfallung aus wassriger Losung auf biokompatible Tragerpartikel mit nachgeschalteter Paraffinbeschichtung beschrieben. Es zeigt sich, dass sich in einem Zweistufenprozess nahezu agglomerationsfreies beschichtetes Produkt herstellen lasst.

The performance of a catalytic fluidized-bed reactor is strongly dependent on the properties of the catalyst of which the particle-size distribution is one. The main influences on the particle-size distribution are attrition of the catalyst particles, and the classifying effect of the solids recovery system. In a fluidized-bed reactor a particle will be subjected to attrition due to different mechanisms in different parts of the system, namely attrition by gas jets near the bottom of the fluidized bed, bubble-induced attrition in the fluidized bed itself, and attrition during the passage through a cyclone. All these different attrition mechanisms are considered in this work by different mathematical models. It is known that a fresh catalyst is much more fragile and exhibits a much higher attrition rate at the beginning of exposure to mechanical stress than under steady-state conditions. Depending on the mechanism the particles need different times or in the case of attrition in a cyclone a certain number of passages to reach a constant value of the attrition rate. In the fluidized-bed system a particle will during its aging experience all the different attrition mechanisms. In order to summarize the effect of these stresses on the particle within the different parts of the fluidized-bed system, the concept of the “stress history” has been developed, which allows a uniform treatment of the different attrition mechanisms. This concept has been implemented into an existing population balance model. Experiments with a FCC-catalyst in a fluidized/bed-cyclone circulation system are well described by this model. © 2007 American Institute of Chemical Engineers AIChE J, 2007

In the present work a particle population balance model for a circulating fluidized bed combustor (CFBC) is developed, that allows the description of the fate of individual particles in terms of attrition and transport effects. Operating conditions and the actual particle size distribution (PSD) are considered in the description of the shrinking of particles by attrition. Furthermore, a submodel has been developed, that takes into account the fact, that fresh particles have a higher attrition rate than particles which stayed for some time in the system and which therefore have been subjected to some stress already. The presented population balance model is able to describe the dynamic adjustment of the PSD in a given system. It has been validated with experimental data from a pilot-plant combustor, i.e. a CFBC with an inner diameter of 0.1 m and a height of 15 m. A comparison between experimental and calculated data shows the applicability of the model. In a second step the model has been applied to predict the effect of a load change and a change of the fuel, respectively, on the PSD inside the boiler and thus on the operating characteristics

The existing literature data on the saturation carrying capacity G(S)* for vertical concurrent gas-particle flow are limited to Archimedes numbers Ar below 1000. By experimental measurement and dimensionless analysis the present article extended Ar to 2500 to demonstrate how G(S)* varies with gas velocity under these high Ar values. The obtained result revealed that G(S)* is subject to the same kind of correlation with as velocity irrespective of Ar. The degree that G(S)* depends on gas velocity, however, generally decreases with raising Ar and it tends to be a constant when Ar becomes higher than 530. This shows in fact that the flows with heavy/large particles or dense/viscous gas possess a low increase in particle-carrying capability with increasing gas velocity until a steady specific capability per m/s is reached at Ar of 530. The article also reanalyzed the influence of column diameter on G(S)*, demonstrating that the column diameter influence exists only in small columns and for flows with heavy/large particles. The influence is negligible in commercial conveying columns and differs according to Ar in those small columns employed in laboratory studies. All of these findings resulted in an upgraded correlation for G(S)*, which manifested not only wide applicability to flows in differently sized columns and with Ar of up to 2500 but also better accuracy in the mentioned flow conditions when comparing with several other literature correlations. (c) 2006 Elsevier Ltd. All rights reserved.

One of the standard tasks in modern chemical industry is solid–liquid separation. While numerous equipments are commercially available to the engineer for separation with cut sizes in the range of several tens of micrometers the number of capable devices is significantly reduced with decreasing cut size. For very small cut sizes in the range of 1–10 μm none of the established commercially available devices allows sufficient classification with reasonable throughputs and separation sharpness. The present work presents the simulation and optimization of a previously developed classifier that allows sharp separations of suspensions at industrial-scale throughputs with cut sizes in the micrometer range. The optimization was carried out by experimental screening of the influences of main parameters as well as numerical calculation of the flow profile with the CFD program CFX. With the knowledge of the flow profile it is possible to determine and minimize the sources of misclassified particles. The paper explains the results of numerical simulations and the derived optimization as well as its implementation into the design of the prototype and results of initial tests.

Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Two different routes were examined. First, tri-isobutyl-aluminum (TIBA) vapor was enriched in nitrogen and thermally decomposed inside the fluidized bed to deposit elemental aluminum. On the other hand, liquid injection of TIBA via a two-fluid nozzle directly into the fluidized bed was tested. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. Also, different glass types and pretreatments of the substrate surface were investigated to elucidate the influence of the surface chemistry on the growth and morphology of the layers deposited.

Due to the ray effect, it is not suitable to employ the discrete ordinates method to calculate the radiation field and the image-formation process in radiative problems with isolated radiative sources (such as point and line sources, isolated medium or boundary source). In this paper, a hybrid method, named Monte Carlo–discrete ordinates method (MCDOM) is developed. Firstly, the Monte Carlo method is used to calculate the emission process. Secondly, the discrete ordinates method is employed to calculate the scattering process, correspondingly, an alternative energy partitioning method is proposed to combine the above two conventional methods. Thirdly, the DOS+ISW algorithm (JQSRT, 2003, 78: 437–453) is used to calculate the image-formation process. Finally, the MCDOM is applied to computing the image formation of an endoscope, which was used to study the hydrodynamics of circulating fluidized beds (Powder Technology, 2001;114:71–83).

A comprehensive experimental study has been carried out to investigate the fluidization behavior with supercritical carbon dioxide at pressures up to 30 MPa for various solids which behave as Geldart A and B powders, respectively, under ambient conditions. From pressure drop measurements minimum fluidizing velocities and bed voidages were determined. Capacitance probe measurements yielded suspension voidages, bubble sizes and velocities. It was found that Ergun's equation is also applicable to the flow of a supercritical fluid through a fixed bed and that the Wen and Yu relationship can be used for the determination of the minimum fluidizing velocity. Correlations for homogeneous bed expansion, minimum bubbling velocity and the visible bubble flow were derived.

A three-dimensional model of a circulating fluidized bed gasifier was developed, which uses continuous radial profiles of velocities and solids hold-up with regard to the description of fluid mechanics. A complex reaction network of sewage sludge gasification is included in the model. In the simulation calculations the influence of the axial location and the number of feeding points was examined for gasifiers of different scales. It was found that due to the very fast decomposition of the volatiles and the high volatile content in the sewage sludge, lateral mixing of the gas around the feeding port is not complete, and plumes with high pyrolysis gas concentrations are formed. Better mixing was found to be achieved by a larger number of fuel feeding ports which are recommended in the case of high-volatile fuels and are shown to be necessary for larger bed widths.

Fluidized bed technology using supercritical carbon dioxide both as a fluidizing gas and as a solvent for the coating material makes possible the production of thin, uniform and solvent-free coatings. But operation at low fluidizing velocities, which is favorable to facilitate gas cleaning under the high pressure conditions, may lead to uneven distribution of the coating in the fluidized bed and to unstable operation due to agglomeration. Therefore a model has been developed which describes local fluid dynamics within the high pressure fluidized bed. Based on this model, the coating process is described and the distribution of the coating inside the fluidized bed is calculated. Furthermore a submodel for the calculation of local concentrations of liquid paraffin has been set up, which may be used as a basis for the prediction of agglomeration and thus stability of operation.

In the present work, a particle population balance for a circulating fluidized bed combustor (CFBC) is developed that allows a description of the fate of individual particles in terms of attrition and transport effects. In the modeling of abrasion and shrinkage of particles, we consider besides operating conditions and particle size distribution also the residence time of particles in the system. The model takes account of the fact that fresh particles have a higher attrition rate than particles that have stayed for some time already in the system. The model aims at the description of the dynamic adjustment of the particle size distribution in a given system. The model is validated with experimental data from a pilot plant scale combustor, i.e., a CFBC with an inner diameter of 0.1 m and height of 15 m. A comparison between experimental and calculated data shows the suitability of the model. The model gives furthermore the opportunity for studying the effects of changes in operating conditions on the steady state solids particle size distribution and solids mass fluxes. The results of modeling a load change of the CFBC are presented and discussed in this work.

Flow maldistribution was investigated in an 8.5 m high circulating fluidized bed with a rectangular cross section of 1 × 0.3 m using a 14-bubble-cap distributor and a 33-bubble-cap distributor. Experimental results indicate that the bubble caps in the center region have a large flow rate and the bubble caps at the wall region have a small flow rate. The extent of the flow maldistribution is quantitatively characterized by the ratio of the maximum flow rate through a bubble cap to the average flow rate through all bubble caps (maldistribution number, Vmax/Vav). A correlation is proposed for predicting flow rate distribution, maldistribution number, in the bubble cap distributor by the ratio of distributor pressure drop to riser pressure drop (pressure-drop ratio, ΔPd/ΔPr). © 2005 American Institute of Chemical Engineers AIChE J, 2005