High-temperature Fluidized Bed Research Articles

Effect of leakage of volatile synthesis products on silicon carbide yield in an electrothermal fluidized bed reactor

Effect of leakage of volatile synthesis products on silicon carbide yield in an electrothermal fluidized bed reactor

Inhomogeneous temperature in high temperature fluidized beds

In this work, we observe an inhomogeneous temperature distribution within a fluidized bed affects the mixing dynamics and therefore heat transfer performance. High temperature fluidized bed experiments with an initial temperature gradient are performed. As the superficial velocity increases, the bed becomes lightly fluidized. Despite fluidization, the bed temperatures do not converge, and the thermal gradients remain. Eventually the superficial velocity is sufficiently high to completely mix the bed and collapse the bed temperatures together. CFD-DEM coupled simulations are performed to investigate the mixing dynamics. Initial bed temperature differences of 100, 300, and 500 K are simulated with varying superficial velocities to create a regime map. At relatively low superficial velocities, the high temperature regions of the bed begin to bubble before the lower temperature regions due to gas density changes. To fully mix the particle temperatures in the vertical direction, the relative velocity (U/Umf) must be a minimum of 1.30.

CFD-DEM investigation on the agglomeration behavior of micron-sized combusted iron fines

The occurrence of agglomeration due to particle sintering is one of the most significant technical issues hindering the direct reduction process of iron oxide fines (DRI) in a high-temperature fluidized bed. To get a better understanding of the agglomeration mechanism, the (de-)fluidization behavior of micron-sized iron oxide particles in a 3-D fluidized bed is investigated using Computational Fluid Dynamics coupled with a coarse-grained Discrete Element Method (CFD-cgDEM). The inter-particle cohesive force is considered as a temperature-dependent solid bridge force. The coarse-grained method is first verified by comparing results for different scaling factors. Subsequently, the effects of temperature on the bed pressure drop, void fraction distribution, particle velocity distribution, and agglomerate size are investigated. In conclusion, the developed model shows the capacity for reliable prediction of particle agglomeration behavior, which can shed light on the design of the DRI process in high-temperature fluidized beds.

Investigation on the Cause of the SO2 Generation during Hot Gas Desulfurization (HGD) Process

In the integrated gasification combined cycle (IGCC) process, the sulfur compounds present in coal are converted to hydrogen sulfide (H2S) when the coal is gasified. Due to its harmful effects on sorbent/solvent and environmental regulations, H2S needs to be removed from the product gas stream. To simulate the H2S removal process, desulfurization was carried out using a dry sorbent as a fluidizing material within a bubbling, high-temperature fluidized bed reactor. The ZnO-based sorbent showed not only an excellent capacity of H2S removal but also long-term stability. However, unexpected SO2 gas at a concentration of several hundred ppm was detected during the desulfurization reaction. Thus, we determined that there is an unknown source that supplies oxygen to ZnS, and identified the oxygen supplier through three possibilities: oxygen by reactant (fresh sorbent, ZnO), byproduct (ZnSO4), and product (H2O). From the experiment results, we found that the H2O produced from the reaction reacts with ZnS, resulting in SO2 gas being generated during desulfurization. The unknown oxygen source during desulfurization was deduced to be oxygen from H2O produced during desulfurization. That is, the oxygen from produced H2O reacts with ZnS, leading to SO2 generation at high temperature.

Modeling an automated management system of technological processes in public energetics

The article presents an algorithm for managing the fluidized bed boiler during solid fuel combustion at the enterprises of the fuel and energy complex. The review of technological features of fuel combustion in low-power boilers of high-temperature fluidized bed is performed. The calculated parameters of the furnace processes in transient operating modes are modeled. A program for managing calculated parameters in dynamic modes from the operator’s graphical panel has been developed, which is used as a training model in the educational process of training thermal-power and sanitary engineers.

Transient operating regimes modeling in low-power boilers of a high-temperature fluidized bed

The paper proposes a comprehensive solution to the issues of sustainable and high-quality operation of low-power boilers with a fluidized bed at public utilities. The techniques of mathematical modeling of stationary and transient regimes of operation are used, which is necessary for the correct selection and tuning of the controllers of the automated control system. A mathematical model of the change in temperature and concentration of combustible substances over time when burning solid fuel in a fluidized bed is presented. Dependences of changes in the main parameters of the combustion process are shown. Graphs of transient processes of changing the layer temperature over time during the application of disturbing effects during the combustion of coal and peat are given.

Utilization of CFBC Fly Ash as a Binder to Produce In-Furnace Desulfurization Sorbent

Circulating fluidized bed combustion (CFBC) power generation technology is known to efficiently reduce the emission of air pollutants, such as SO2 and NO2, from coal combustion. however, CFBC coal ash contains high contents of free CaO, making it difficult to recycle. This research has been conducted to find ways to use the self-hardening property of CFBC coal ash, one of its inherent characteristics. As part of these efforts, the present study intended to investigate the properties and desulfurization efficiency of Ca-based desulfurization sorbents using CFBC fly-ash as a binder. Limestone powder was mixed with CFBC fly-ash and Ca(OH)2 to fabricate desulfurization sorbents, and it generated hydrate of cement, including portlandite, ettringite, and calcium silicate, etc. The compressive strength of the desulfurization absorbent prepared by CFBC fly ash and Ca(OH)2 was 72–92% that of the desulfurized absorbent prepared by using general cement as a binder. These absorbents were then compared in terms of desulfurization efficiency using a high-temperature fluidized bed reactor. It was confirmed that the desulfurization absorbents fabricated using CFBC fly-ash as a binder achieved the best performance in terms of absorption time, which reflects the time taken for them to remove over 90% of high-concentration SO2 gas, and the conversion ratio, which refers to the ratio of CaO turning into CaSO4.

Changes in physicochemical properties of rice in response to high-temperature fluidized bed drying and tempering

ABSTRACTThe effects of high-temperature fluidized bed drying and tempering on pasting, gelatinization, crystalline and microstructural properties of long-grain Vietnamese rice cultivar A10 were examined. The freshly harvest paddy (33% wet basis) was dried at two drying temperatures (80 and 90°C) and two drying durations (2.5 and 3.0 min) using a custom-made lab-scale batch fluidized bed dryer. The dried rice was immediately tempered in sealed containers at 75 and 86°C for various tempering durations (0, 30, 40, and 60 min), followed by thin-layer drying at 35°C to reduce moisture level to 14% wet basis. Characterization of pasting, gelatinization, crystallinity, and microstructure of the dried rice was undertaken using rapid visco analyzer, differential scanning calorimetry, X-ray diffraction (XRD), and environmental scanning electron microscopy (ESEM), respectively. Pasting properties and gelatinization enthalpy decreased with higher drying temperature and longer tempering time while pasting and gelatinization temperatures increased. Degree of crystallinity changed slightly, however, a shift from A-type to A+V-type XRD pattern was observed. ESEM observation demonstrated some degree of collapse of the rice microstructure. The kernel integrity appeared to be enhanced by the gel network formed during partial gelatinization of starch, corroborating with the changes in pasting and gelatinization properties of the fluidized bed dried and tempered rice.

Discrete element method simulation of the mixing process of particles with and without cohesive interparticle forces in a fluidized bed

The discrete element method (DEM) is used to simulate two types of mixing processes (radial mixing and axial mixing) of particles with and without cohesive interparticle forces in a two-dimensional (2D) fluidized bed. By using the Ashton index, the mixing degree is quantified and the differences between the mixing characteristics of particles with and without cohesive interparticle forces are compared. The cohesive interparticle forces, which impair bubble behaviors, can apparently weaken the mixing process and prolong the mixing time. This effect increases with increasing cohesive interparticle force. Radial mixing, which is mainly dependent on the convective mixing caused by bubbles, is relatively sensitive to cohesive interparticle force and shows a long mixing time, while axial mixing, which is mainly induced by the bubble wake, is less sensitive to cohesive interparticle force and presents a fast mixing process. Finally, to weaken the effect of cohesive interparticle force, an uneven air distribution is introduced, which greatly improves the mixing process by changing the bubble movement. This method may be helpful for the reduction of particle agglomeration in high-temperature fluidized beds.

Nickel membranes for in-situ hydrogen separation in high-temperature fluidized bed gasification processes

The Heatpipe Reformer provides an allothermal gasification process for the generation of hydrogen-rich synthesis gas. This synthesis gas can be used for generation of hydrogen for applications like fuel cells, engines, storage or the chemical industries. Conventional processes for hydrogen production involve CO Shift reactors to enhance hydrogen yield followed by hydrogen separation using gas scrubbing technology. The Institute of Energy Process Engineering (FAU-EVT) follows the approach to apply hydrogen permeable membranes as separation step directly in the reformer. This also allows higher hydrogen yields due to an additional shift of the gasification reactions to the product side as one product is continuously removed.This paper gives an overview on membranes in high temperature applications. It focuses on the temperature range from 750 to 900 °C required for biomass gasification processes. Existing approaches to high temperature hydrogen separation like palladium composite membranes or ceramic materials show advantages and disadvantages mainly regarding stability and prices. The presented approach applies commercial nickel capillary tubes as membranes. Vacuum increases the partial pressure difference between permeate and retentate. This remedies the need for a sweep gas, which is needed with all membranes that cannot withstand a physical pressure difference.In the experimental section several commercial nickel capillaries were tested for their hydrogen permeability and the results from a parameter study regarding the influence of synthesis gas components like CO, CH4 and H2O on permeation are shown. The nickel membranes were also tested in hydrogen containing H2S, which can be present in synthesis gas in concentrations of up to 1000 ppm.

Improving head rice yield of glutinous rice by novel parboiling process

ABSTRACTMost commercial parboiled rice is produced from high-amylose content rice. Glutinous rice, which is lacking in amylose content, is generally consumed in Southeast Asian countries. Rare study of parboiling glutinous rice has been observed. In this study, glutinous rice was improved in head rice yield by a novel parboiling process. Two rough glutinous rice, rice department 6 (RD6) and black glutinous rice (BGR) cultivars, were soaked in hot water at 70 ± 5°C for 3 h. The ricer 3moisture content after soaking was 50–52% (d.b.), it was dried with hot air and superheated steam (SHS) at 110, 130, and 150°C in a fluidized bed dryer. The results show that SHS at all drying temperatures can improve the high head rice yield in both parboiled glutinous rice cultivars better than hot air drying. Higher temperature drying caused L* value to decrease but the b* value increases in RD6, whereas in BGR, all color values decreased and ΔE* was increased when the drying temperature increased. Increasing drying temperature presented a softer texture of both glutinous rice cultivars. Upper 130°C, completed gelatinization of both varieties can be obtained and seen by scanning electron microscope and differential scanning calorimeter (DSC). This technique of using high-temperature fluidized bed drying can produce completely parboiled glutinous rice in a single process instead of two conventional processes, steaming and drying, in series.

Preparation and characterization of thin-film Pd–Ag supported membranes for high-temperature applications

This paper reports the preparation, characterization and stability tests of thin-film Pd–Ag supported membranes for high-temperature fluidized bed membrane reactor applications. Various thin-film supported membranes have been prepared by simultaneous Pd–Ag electroless plating and have been initially sealed with a sealing procedure previously validated for Water gas shift (WGS) application (400 °C). The membranes have been characterized for single gas and mixed gas permeation, and for methane steam and autothermal reforming in a fluidized bed membrane reactor at 550–600 °C using a Ru-based catalyst. In addition, the performance of these membranes was compared to commercial membranes from REB Research & Consulting under the same reaction conditions. The applied sealing showed nitrogen leaks at 600 °C and different sealing approaches were tested solving this problem. Finally, also the long-term stability of the thin-film Pd–Ag supported membrane at 600 °C has been investigated.

Possible Synergism of High Temperature Fly Ash and Fluidized Bed Combustion Fly Ash in Cement Composites

One of the possibilities how to activate fly ash in cement composites is to add calcium oxide as a chemical activator. This addition can improve pH of composites. Because fluidized bed combustion (FBC) fly ash contain around 15% of calcium oxide, we decided to add FBC fly ash into binder system of cement composite with classic fly ash.Three mixtures were designed. First one contain in binder system only cement and classic fly ash. In second and third mixture was part (25% resp. 50%) of classic fly ash replaced by FBC fly ash. Consistency, shrinkage, compressive strength and freeze-thaw resistance were tested. Microstructure was detected by XRD and TG analyses.

Reduction Kinetics of Fine Iron Ore Powder in Mixtures of H2-N2 and H2-H2O-N2 of Fluidized Bed

Reduction kinetics of fine iron ore powder in different gas mixtures were investigated in high-temperature fluidized bed at a scale of kilograms. Influence of processing parameters, such as particle size, gas flow velocity, height of charge, temperature, compositions of gas mixture, and percentage of inert components, on reduction kinetics was experimentally determined under the condition of fluidization. The equations for calculating instantaneous and average oxidation rates were deduced. It was found that an increasing H2O percentage in the gas mixture could obviously decrease the reduction rate because the equilibrium partial pressure of H2 decreased with increasing content of H2O in the gas mixture and then the driving force of reduction reaction was reduced. When the H2 content was high, the apparent reaction rate was so rapid when the average size of iron ore fines was less than 1 mm that the reaction temperature can be as low as 750 °C; when the average size of iron ore fines was more than 1 mm, a high reaction temperature of 800 °C was required. In addition, it was also found that the content of H2O should be less than 10% for efficient reduction.

Product Yields and Kinetics of Biomass Fast Devolatilization: Experiments and Modeling

The fast devolatilization of biomass is the first step of thermochemical conversion in fluidized bed boilers or gasifiers. Some advanced devolatilization models applicable to a wide range of biomasses and process conditions have been developed on the basis of a detailed description of the physico-chemical mechanisms and of biomass standard analysis. In this study, the results of a biomass devolatilization model taking into account the main physical and chemical phenomena are reported. Attention is focused on biomass thermal properties derived from its composition and density. The kinetic scheme is based on the lumped kinetic model of (Ranzi et al., 2008) and its further development. Predictions of the complete devolatilization time of biomass thick particles are compared with experimental results obtained in different high-temperature fluidized bed reactors. Experimental results obtained with biomass pellets under image furnace conditions are also used to assess the dynamic release of volatile matter predicted in transient state. The great interest of predictive model able to reduce the number of input parameters for any biomass type is finally underlined.

Thermal Regime of a Cap-Type Gas Distributor with Active Thermal Insulation

The thermal state of a cap-type gas distributor with active thermal insulation in the form of a ventilated granular bed is modeled. The temperatures of air at exit from the granular bed and of the upper gas-tight plate of the gas distributor have been calculated by the method of successive approximations. The efficiency of active thermal insulation that substantially reduces the gas distributor temperature and simultaneously serves as a preheater for the primary air entering into the high-temperature fluidized bed is shown.

Optimal Operating Conditions to Produce Nutritious Partially Parboiled Brown Rice in a Humidified Hot Air Fluidized Bed Dryer

V-type amylose–lipid complexes present in partially parboiled rice can decrease starch digestibility. Formation of such complexes can be accomplished using high-temperature fluidized bed drying; the degree of the complexes depends on the thermal condition. The effects of drying media (hot air and humidified hot air), operating conditions (drying air temperature and relative humidity [RH]), and the initial moisture content on the degree of V-type crystallinity and subsequent starch digestibility (or glycemic index, GI) and brown rice texture were examined experimentally. The results showed that paddy drying with humidified hot air (HHA) requires a longer time than hot air (HA). Higher drying air temperature, RH, and initial moisture content of paddy yield higher degrees of starch gelatinization and V-type amylose–lipid complexes. The brown rice dried by HA or HHA had lower starch digestibility and a harder texture than the reference sample. Within the range of parameters studied, to obtain the lowest GI for the dried brown rice, paddy at an initial moisture content of 33% (db) should be dried by HHA at 150°C and 6.4% RH.

Continuous high-temperature fluidized bed pyrolysis of coal in complex atmospheres: Product distribution and pyrolysis gas

This study is devoted to investigating the continuous coal pyrolysis in a laboratory fluidized bed reactor that fed coal and discharged char continuously at temperatures of 750–980°C and in N2-base atmospheres containing O2, H2, CO, CH4 and CO2 at varied contents. The results showed that the designed continuous pyrolysis test provided a clear understanding of the coal pyrolysis behavior in various complex atmospheres free of and with O2. The effect of adding H2, CO, CH4 or CO2 into the atmosphere on the tar yield was related to the O2 content in the atmosphere. Without O2 in the atmosphere, adding H2 and CO2 decreased the pyrolysis tar yield, but the tar yield was conversely higher with raising the CO and CH4 contents in the atmosphere. In O2-containing atmospheres, the influence from varying the atmospheric gas composition on the product distribution and pyrolysis gas composition was closely related to the oxidation or gasification reactions occurring to char, tar and the tested gas.

Control of particle cohesion with a polymer coating and temperature adjustment

A new approach is presented that introduces interparticle forces induced by a change of temperature at which polymer coated particles are exposed. The particle cohesive flow behavior is shown for two different applications. The first one considers a dense granular flow that is typically observed for wet powder granulation using a modified spheronizer. The other application shows the possibility of mimicking the particle flow behavior observed in high-temperature fluidized beds with the advantage of being operated at ambient conditions. For the first application, as the temperature increases, significant changes in the particle bed surface morphology were observed and an important reduction of the dynamic density was noticed. For the gas–solid fluidized-bed application, pressure drop measurements revealed that the behavior of the particles transited from Geldart group B to Geldart group A and even Geldart group C as the temperature increased. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Computational fluid dynamics simulations of heat transfer between the dense‐phase of a high‐temperature fluidized bed and an immersed surface

AbstractThe transient process of heat transfer between a high‐temperature emulsion packet and the wall of an immersed surface is simulated using computational fluid dynamics (CFD). From these simulations, the total heat transfer coefficient and its radiant contribution due to the emulsion (dense) phase are evaluated. The results are compared with experimental data (Ozkaynak et al., “An experimental investigation of radiant heat transfer in high temperature fluidized beds,” in Fluidization IV, 1983:371–378) and with predicted values from the generalized heterogeneous model (GHM), (Mazza et al., “Evaluation of overall heat transfer rates between bubbling fluidized beds and immersed surfaces,” Chem Eng Commun., 1997;162:125–149). The CFD simulations are in good agreement with both, experimental data and theoretical GHM predictions and provide a reliable way to quantify the studied heat transfer process. Also, the GHM is validated as a practical tool to this end. © 2011 American Institute of Chemical Engineers AIChE J, 58: 412–426, 2012