Estimation of bed expansions in a freely-bubbling three-dimensional gas-fluidized bed

Knowledge of bed expansions is important in the design and operation of gas–solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0·61×0·61 m². All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 μm raw perlite and 1233 μm sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0·0398 to 0·3176 m, while the excess air velocity from 0·034 m s⁻¹to 0·7453 m s⁻¹. Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas–liquid systems as follows: R=0·5482 d^−0·129_p(U_o−U_mf)^0·111 with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations. © 1998 John Wiley & Sons, Ltd.     

CFD simulation of jet behaviour and voidage profile in a gas–solid fluidized bed

Based on an Eulerian–Eulerian approach, a computational fluid dynamic (CFD) model for gas–solid system has been developed to investigate the hydrodynamics in fluidized beds. With this model, jet penetration height, jet frequency, time‐averaged axial gas velocity profile, and time‐averaged voidage profile have been simulated in a two‐dimensional bed. The computational results indicate that the jet penetration height increases with increasing the jet gas velocity. The jet frequency decreases with increasing the jet gas velocity and decreasing particle diameter. The time‐averaged axial gas velocity profile becomes ‘lower’ and ‘wider’ and the time‐averaged voidage decreases with increasing distance from the jet nozzle. These conclusions appear in good agreement with the experimental and simulated data in the literature. Copyright © 2004 John Wiley & Sons, Ltd.     

循环流化床锅炉床上煤气点火的应用

循环流化床锅炉的点火是锅炉运行的重要环节,如果操作不当容易引起床料的超温结焦或灭火。文章着重介绍循环流化床锅炉床上煤气点火系统启动准备工作及操作。这降低了工人的劳动强度,提高了锅炉设备点火安全的可靠性。     

水平管道对流化床中气泡成长的影响

利用FLUENT模拟了二维流化床内水平管道和气体在不同进口速度下对气泡成长的影响.结果表明,流化床中存在水平管道对气泡的成长有着显著的影响.与未加水平管道相比,床内由于水平管道的阻碍和干扰,气泡的生成和成长速度相对缓慢,气泡的形状在不同时刻也发生较大的变化.而在同一条件下仅气体的进口速度不同时,当床内存在管列,气体进口...     

Analysis of particle motion and heat transfer in circulating fluidized beds

In this study, the forces affecting the motion of particle clusters near the wall of a CFB were theoretically analysed. The motion trajectory and the contact time of clusters were determined from the proposed model for two cases, steel ball having density of 6980 kg m^?3 and sand having density of 2500 kg m^?3. Computational results showed that the construction and operational parameters such as the bed equivalent diameter, the gas velocity and the bed temperature have great influence on the contact time of clusters. Based on analysis of the contact time of clusters, a theoretical model was developed for predicting the particle–gas convection heat transfer coefficient. The results were compared with experiments and were a quite agreement with the measured data in the open literature which suggests that the theoretical analysis conducted in this work can very well describe the convection heat transfer in a CFB. Copyright © 2004 John Wiley & Sons, Ltd.     

Theoretical performance analysis of the multi‐stage gas–solid fluidized bed air preheater

The multi‐stage fluidized bed can be used to preheat the combustion air by recovering the waste heat from the exhaust gas from industrial furnaces. The dilute‐phase fluidized bed may be formed to exclude the excessive pressure drop across the multi‐stage fluidized bed. But, in this case, the solid particles do not reach to the thermal equilibrium due to relatively short residence time in each layer of fluidized bed. In this study, a theoretical analysis on the dilute phase multistage fluidized bed heat exchanger was performed. A parameter related to the degree of thermal equilibrium between gas and solid particles at the dilute‐phase fluidized beds was derived. Using this parameter, a relatively simple expression was obtained for the thermal efficiencies of the multi‐stage fluidized bed heat exchanger and air preheater. Copyright © 2001 John Wiley & Sons, Ltd.     

Particle holdup in a liquid–solid circulating fluidized bed

An experiment was conducted to acquire a set of systematic data of particle holdup in risers of a liquid–solid circulating fluidized bed. In the experiment, two kinds of riser were provided, their inner diameter being 24 mm and 36 mm, respectively. Tested particles were of glass and ceramics, having a diameter range from 2.10 to 4.95 mm. Water at ambient conditions was used as the fluidizing liquid. Particle holdup was measured using a shut‐off method. Based on the experimental data, a correlation for predicting the particle holdup was derived, which could reproduce almost all experimental data with an accuracy of ±15%. The effect of the wall was not recognized within the experimental range, i.e., the diameter ratio of particle to riser is less than 0.2. The independent parameters affecting the flow characteristics of liquid–solid circulating fluidized beds were identified. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 184–196, 2008; Published online in Wiley InterScience ( www. interscience.wiley.com ). DOI 10.1002/htj.20194     

Numerical study of different gas–solid flow regimes effects on hydrodynamics and heat transfer performance of a fluidized bed reactor

In this study, a two‐?uid Eulerian–Eulerian model has been carried out applying the kinetic theory of granular flow (KTGF) to study the hydrodynamics and heat transfer behavior of a fluidized bed reactor simultaneously. The effects of different gas–solid flow regimes on the operating conditions and heat transfer rate between the hot air and two types of low and high‐density inert particles are investigated in a fluidized bed dryer. Different gas–solid flow regimes for wood and glass particles of groups A, B, and D of Geldart's classification are simulated to introduce the most optimal flow regime in terms of heat transfer rate and operating costs. The compromise between the heating rate, the height required for the reactor, and the ratio of the final mass to the initial mass of solid particles, which specifies the need for a cyclone separator showed that the bubbling regime of Geldart B powder for low‐density particles and the turbulent regime of Geldart D powder or bubbling regime of Geldart B powder for high‐density particles are the optimal operating conditions and flow regimes. Furthermore, it was concluded that the convective heat transfer is the dominant mechanism, which increases with increasing the air velocity and decreasing the particle diameter in each group.     

Eulerian simulations of bubble behaviour in a two‐dimensional gas–solid bubbling fluidized bed

In the present study, the CFD model is based on a two‐fluid model extended with the kinetic theory of granular flow. The simulation results of bubble diameter and bubble rise velocity are compared to the Darton equation and the Davidson model in a free bubbling fluidized bed. The predicted values are in reasonable agreement with the values from the Darton bubble size equation and the Davidson model for isolated bubbles. It is shown that the break‐up and direct wall interaction effects influence the dynamic bubble behavior in the free bubbling fluidized beds. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of chemical and thermodynamic parameters on the flue gas desulphurization efficiency in a circulating fluidized bed

IneductionEven there are W successful Mods Of flue gasdesulPhuriZation (FGD), people have been searching fOrnew ones, which are more econondcal, with higherefficiency and more reliable. Lap, Thyssen and WullffMasclunen developed the teChnulogy of circulatinfluldized bed flue gas desulPhurization (CFBFGD) inlate 1980s, which is similar with circulating fluidized bedboilers in enhancing chendcal reachvity As flue gas andabsothent are Inixed in theulent bed, SO2 is absothedand changed in…     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Physicochemical properties and gasification reactivity of the ultrafine semi-char derived from a bench-scale fluidized bed gasifier

Zhundong coalfield is the largest intact coalfield worldwide and fluidized bed gasification has been considered as a promising way to achieve its clean and efficient utilization.The purpose of this study is to investigate the physieochemical properties and gasification reactivity of the ultrafine semi-char,derived from a bench-scale fluidized bed gasifier,using Zhundong coal as fuel.The results obtained are as follows.In comparison to the raw coal,the carbon and ash content of the semi-char increase after partial gasification,but the ash fusion temperatures of them show no significant difference.Particularly,76.53％ of the sodium in the feed coal has released to the gas phase after fluidized bed gasification.The chemical compositions of the semi-char are closely related to its particle size,attributable to the distinctly different natures of diverse elements.The semi-char exhibits a higher graphitization degree,higher BET surface area,and richer meso-and macropores,which results in superior gasification reactivity than the coal char.The chemical reactivity of the semi-char is significantly improved by an increased gasification temperature,which suggests the necessity of regasification of the semi-char at a higher temperature.Consequently,it will be considered feasible that these carbons in the semi-char from fluidized bed gasifiers are reclaimed and reused for the gasification process.     

Numerical simulation of fuel reactor for a methane-fueled chemical looping combustion using bubbling fluidized bed with internal particle circulation

Chemical-looping combustion (CLC) is recognized as a promising technique to efficiently and economically capture emitted carbon dioxide in common combustion processes. In this study, the bubbling fluidized bed (BFB) fuel reactor performance of the CLC system was examined through numerical simulation. The reduction reaction performance obtained from conventional BFB fuel reactor and BFB fuel reactor incorporated with internal particle circulation denoted as internal circulation bubbling fluidized bed reactor (ICBFB), were compared under the same fuel flow rate and operating conditions. By using CH₄ as fuel and ilmenite as the oxygen carrier, it was found the reduction reaction can be enhanced by using the ICBFB fuel reactor due to particle circulation. The particle circulation increased the mixing and contact time between fuel and oxygen carrier that produced reduction reaction enhancement. Moreover, the simulation results indicated that higher reduction reaction performance can be achieved by higher reduction reaction temperature and initial oxygen carrier volume fraction.     

Biomass gasification in a 100 kWth steam-oxygen blown circulating fluidized bed gasifier: Effects of operational conditions on product gas distribution and tar formation

Biomass gasification is one of the most promising technologies for converting biomass, a renewable source, into an easily transportable and usable fuel. Two woody biomass fuels Agrol and willow, and one agriculture residue Dry Distiller’s Grains with Solubles (DDGS), have been tested using an atmospheric pressure 100 kWth steam-oxygen blown circulating fluidized bed gasifier (CFB). The effects of operational conditions (e.g. steam to biomass ratio (SBR), oxygen to biomass stoichiometric ratio (ER) and gasification temperature) and bed materials on the composition distribution of the product gas and tar formation from these fuels were investigated. Experimental results show that there is a significant variation in the composition of the product gas produced. Among all the experiments, the averaged concentration of H₂ obtained from Agrol, willow and DDGS over the temperature range from 800 to 820 °C was around 24 vol.%, 28 vol.% and 20 vol.% on a N₂ free basis, respectively. A fairly high amount of H₂S (∼2300 ppmv), COS (∼200 ppmv) and trace amounts of methyl mercaptan (<3 ppmv) on a N₂ free basis were obtained from DDGS. Due to a relatively high content of K and Cl in DDGS fuel, an alkali-getter (e.g. kaolin) was added to avoid agglomeration during gasification. Higher temperatures and SBR values were favorable for increasing the mole ratio of H₂ to CO and the tar decomposition but less advantageous for the formation of CH₄. Meanwhile, higher temperatures and SBR values also led to higher gas yields, whereas a higher SBR caused a lower carbon conversion efficiency (CCE%), cold gas efficiency (CGE%) and heating values of the product gas due to a high steam content in the product gas. From solid phase adsorption (SPA) results, the total tar content obtained from Agrol was the highest at around 12.4 g/Nm³, followed by that from DDGS and willow gasification. The lowest tar content produced from Agrol, willow and DDGS using Austrian olivine (Bed 1) as bed materials was 5.7, 4.4 and 7.3 g/Nm³, values which were obtained at a temperature of 730, 820 and 730 °C, SBR of 1.52, 1.14 and 1.10, and ER of 0.36, 0.39 and 0.37, respectively.     

Experimental assessment of hydrogen separation from H2/CH4 mixtures by the “steam-iron process” in an interconnected circulating fluidized bed reactor

An experimental study devoted to the continuous separation of hydrogen from its mixtures with methane in continuous regime is presented. The study was carried out in a single-vessel reactor divided into two interconnected adjoining chambers, operating as fluidized beds and with a continuous flow of solids between them. The process is based on iron oxide being selectively reduced or oxidized, depending on the redox nature of the gas stream being fed to the respective chamber (steam-iron process or SIP). Conditions to minimize gas leakage between the interconnected beds, avoiding insufficient circulation of solids, have been experimentally determined. The effects of different interconnecting designs, spatial gas velocities and partial pressures of reactants have been tested. Also the effects of several operating parameters (primarily hydrogen content in the inlet gas stream and spatial velocities) on process efficiency have been analyzed. Accordingly, a final configuration has been proposed for stable hydrogen separation and tested for several hours of time on stream.     

Deviations of the results for the properties of a dense hard-sphere gas near the walls of a micro channel using the hybrid molecular dynamics—Monte Carlo simulation method

We study the deviations for the results of the properties of a hard-sphere gas near the walls of a micro/nano channel using the hybrid MD–MC simulation method compared to the pure MD and MC results. Our model for the micro channel consists of two parallel infinite plates situated at distance L apart from each other, and of gas molecules confined between these two walls. We study the dependence of the deviations for higher densities, considering different lengths of the different simulation domains in the hybrid MD–MC method. We find that when density is increased, the deviations in the pure MC results are increasing compared to pure MD results. The deviations in the hybrid simulation results are decreasing and are very small when increasing the width of the solid–gas interface. The deviations of the pure MC simulation results from the pure MD simulation results for the number density are found to be around 0.9%, when the reduced density η=0.1 and the width of the channel L=50λ, where λ is the mean free path. When the hybrid method is used, the deviations are decreasing with a factor from two to three, and are between 0.32%–0.42%. For more dense gas (η=0.2), the deviations of the MC simulation results for the number density are found to be 1.71%, and the deviations of the hybrid MD–MC simulation results between 0.246% and 0.6977%. We discuss how these deviations in case of a dense gas (η=0.2) depend on the width of the interface, and we study it for the case when the MD domain is 10% and MC domain is 90% from the simulation domain, and also for the case when the MD domain is 50% and MC domain 50% from the whole simulation domain. For more dilute gas, the MC, MD and hybrid MC–MD simulations are in very good agreement and the deviations are negligible.     

Investigation on the effectiveness of various methods of information dissemination aiming at a change of occupant behaviour related to thermal comfort and exergy consumption

These days the number of projects trying to urge a change in the occupant's behaviour towards a sustainable one is increasing. However, still less is known about the effect of such measures. This paper describes the findings of two investigations, a field measurement and an Internet-based survey, both including the dissemination of information about strategies for a high level of comfort without much energy usage. The focus was on the ability to quantify the effect of such measures on the heating and cooling behaviour. As a result, those who participated in a workshop were more likely to change their behaviour than those who received an information brochure only; whether this was due to the method employed or the type of participants could not be ascertained. However, the workshop participants reduced their cooling device usage by up to 16%. The concept of exergy was used to show how this reduction affects the exergy consumption of the cooling device, because it enables us to consider the qualitative aspect of energy as a quantity to be calculated. This showed that the exergy consumed by the workshop group was reduced by up to 20% comparing their behaviour before and after the information dissemination.     

Impact of a large penetration of wind generation on the GB gas network

Wind power is expected to be the major element of renewable electricity generation in Great Britain (GB) by 2020 with a capacity of around 30 GW. The potential impact of a large amount of wind generation on the GB gas network was investigated using a combined gas and electricity network model. The varying nature of gas and electric power flows, network support facilities such as gas storage and compressors, and the power ramping characteristics of various power plants were considered. Three case studies were modelled, one case uses the existing network and the other two make use of a hypothesised network in 2020 with two distinct levels of wind generation representing low and high wind periods. The simulation results show that a large penetration of wind generation will influence the electricity generation mix as the wind power varies. Gas-fired generation is used to compensate for wind variability. This will cause increased flows and compressor power consumption on the gas network. Linepack depletion during low wind periods was shown to limit the ability of the gas network to fully supply gas-fired generators.     

Numerical simulation of fluid flow and heat transfer in a rotating cylindrical container with a counter-rotating disk at the fluid surface

Fluid flow and heat transfer in a rotating cylindrical container with a counterrotating disk at the fluid surface are numerically investigated. The effects of disk rotation and of Prandtl numbers on the fluid flow and heat transfer in the container are discussed. Flow and temperature fields are obtained for various rotational Reynolds numbers of the disk and for Prandtl numbers of the fluid. Nusselt numbers on the walls are calculated for the temperature fields and are compared with available experimental data. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 172–182, 1999     

Dynamic modeling of a SOFC/MGT hybrid power system based on modified OIF Elman neural network

Solid oxide fuel cell (SOFC) integrated into micro gas turbine (MGT) cycle is a promising power‐generation technology. This article proposes a modified output–input feedback (OIF) Elman neural network model to describe the nonlinear temperature and power dynamic properties of the SOFC/MGT hybrid system. A physics‐based mathematical model of a 220 kW SOFC/MGT hybrid power system is used to generate the data required for the training and prediction of the modified OIF Elman neural network identification model. Compared with the conventional Elman neural network, the simulation results show that the modified OIF Elman identification model can follow the temperature and power response of the SOFC/MGT hybrid system with higher prediction accuracy and faster convergent speed. Copyright © 2010 John Wiley & Sons, Ltd.