Thermal analysis in fluidized bed drying of moist particles

This paper deals with thermal modeling of the fluidized bed drying of wet particles to study heat and mass transfer aspects and drying thermal efficiencies. The model is then validated with the literature experimental data obtained for corn. A parametric investigation is undertaken to study the effects of the inlet air temperature, the air velocity and the initial moisture content of the material (i.e. corn) on the process thermal efficiency. The results show that the thermal efficiencies of the fluidized bed drying decrease sharply with decreasing moisture content of corn and hence increasing drying time, and apparently become the lowest at the end of the drying process. This clearly indicates that the moisture transfer from the material depends strongly on the air temperature, air velocity and the moisture content of material. A good agreement is obtained between the model predictions and the available experimental results.     

Exergy analysis of fluidized bed drying of moist particles

Energy and exergy analyses are conducted of the fluidized bed drying of moist materials for optimizing the operating conditions and the quality of the products. In this regard, energy and exergy models are developed to evaluate energy and exergy efficiencies, and are then verified with experimental data (for the product, wheat) taken from the literature. The effects of inlet air temperature, fluidization velocity, and initial moisture content on both energy and exergy efficiencies are studied. Furthermore, the hydrodynamic aspects, e.g., the bed hold up, are also studied. The results show that exergy efficiencies are less than energy efficiencies due to irreversibilities which are not taken into consideration in energy analysis, and that both energy and exergy efficiencies decrease with increasing drying time.     

Multi-phase flow and drying characteristics in a semi-circular impinging stream dryer

A physical model was proposed to describe granular material drying in a semi-circular impinging stream dryer, and the multi-phase flow characteristics as well as the heat and mass transfer were numerically investigated. Specially, the influence of various factors (inlet air temperature, mass flow-rate ratio, initial moisture content etc.) on drying process was inspected. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate curvature radius, flow-rate ratio, air velocity and higher inlet air temperature should be chosen for improving the drying performance, and decreasing the energy consumption and operation cost. The numerical predictions were compared with the available experimental results, and they are in quite good agreement with each other.     

Thermodynamic analysis of fluidized bed drying of carrot cubes

In this study, the energy and exergy analyses of fluidized bed drying of carrot cubes were investigated. Drying experiments were conducted at inlet air temperatures of 50, 60, and 70 °C, BD (bed depths) of 30, 60, and 90 mm and square-cubed carrot dimensions of 4, 7, and 10 mm. The effects of drying variables on energy utilization, energy utilization ratio, exergy loss and exergy efficiency were studied. The energy utilization and energy utilization ratio varied between 0.105–1.949 kJ/s and 0.074–0.486, respectively. The exergy loss and exergy efficiency were found to be in the range of 0.206–1.612 kJ/s and 0.103–0.707, respectively. The results showed that small particles, deep beds and high inlet air temperatures increased energy utilization, energy utilization ratio, and exergy loss due to high value of heat and mass transfer. Also, the exergy efficiency had maximum value when higher drying air temperature, larger CS (cube size) and shorter BD were used for drying experiment.     

生物质流化床热风干燥特性研究

在一个自制的能实时检测质量的流化床干燥试验装置上对一种速生林木材的热风干燥特性进行了深入系统的研究,分析了热风温度、热风速度以及料层高度等对生物质样品干燥热特性的影响。结果表明:干燥速率随温度和风速的增加而增大;随料层厚度的增加而下降;物料的含水率越高,干燥速率越快,随含水率的变化,各条件对干燥速率的影响程度发生改变。通过对干燥过程模拟分析发现,干燥曲线以及脱水速率分别符合薄层Page模型和线性方程模型,且干燥方程中的参数k主要受风温和料层厚度影响,参数n则主要受风速影响。     

Local Heat and Mass Transfer for Gas—Solid Two Phase Flow in CFB

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Numerical simulation of fluid bed drying based on two-fluid model and experimental validation

A mathematical model for batch drying based on the Eulerian “two-fluid models” was developed. The two-dimensional, axis-symmetrical cylindrical equations for both phases were solved numerically. The governing equations were discretized using a finite volume method with local grid refinement near the wall and inlet port. The effects of parameters such as inlet gas velocity and inlet gas temperature on the moisture content, temperature of solid and gas at the outlet are shown. This data from the model was compared with that obtained from experiments with a fluidized bed and found to be in reasonably good agreement.     

Comparison of energy and exergy efficiencies of an underground solar thermal storage system

In this experimental study, solar energy was stored daily using the volcanic material with the sensible heat technique. The external heat collection unit consisted of 27 m² of south‐facing solar air collectors mounted at a 55° tilt angle. The dimensions of the packed‐bed heat storage unit were 6 × 2 × 0.6 m deep. The packed‐bed heat storage unit was built under the soil. The heat storage unit was filled with 6480 kg of volcanic material. Energy and exergy analyses were applied in order to evaluate the system efficiency. During the charging periods, the average daily rates of thermal energy and exergy stored in the heat storage unit were 1242 and 36.33 W, respectively. Since the rate of exergy depends on the temperature of the heat transfer fluid and surrounding, the rate of exergy increased as the difference between the inlet and outlet temperatures of the heat transfer fluid increased during the charging periods. It was found that the average daily net energy and exergy efficiencies in the charging periods were 39.7 and 2.03%, respectively. The average daily net energy efficiency of the heat storage system remained nearly constant during the charging periods. The maximum energy and exergy efficiencies of the heat storage system were 52.9 and 4.9%, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Mathematical modelling of drying based on a surface evaporation source term for coupled energy and mass transfer

A mathematical model is developed by introducing a source term and using the coupled mass and energy balances based on the Luikov's system of equations. The source term is due to the surface evaporation of many particles in a solid bed. A numerical scheme based on explicit finite difference method is applied. The prediction of bulk temperature profile and moisture content from the model shows, qualitatively, the same trend as the experimental results found in literature. Additionally, parametric studies are performed to investigate the effect of different parameters such as the porosity, the heat and mass transfer coefficients and the velocity on the overall energy efficiency of the drying process. The present model provides better approximation to an actual drying process, especially in the initial and final periods of drying. Copyright © 2007 John Wiley & Sons, Ltd.     

The Effects of Bed Height and Initial Moisture Concentration on Drying Lignite in a Batch Fluidized Bed

In this study, drying of a lignite from Erzurum Ispir-Karahan province, with air in a batch fludized bed, was investigated. In the experiments, effects of bed height and initial moisture concentration of the coal on the drying behavior were studied. It was found that the drying rate increased with a decrease in bed height. Also, coal particles with an initial lower moisture content showed a marginal decrease of drying rate compared with higher moisture content particles. It was also observed that the equilibrium moisture content, and the time taken to reach the equilibrium moisture content, decreased with decreasing initial moisture concentration, while the critical moisture content generally decreased. Also, it fixed with decreasing bed height, while the critical moisture content increased.     

Effect of swirling flow on fluidized bed drying of wheat grains

In this paper, batch drying of wheat grains in a fluidized bed dryer, which had a swirling flow field in its drying medium, was experimentally investigated. In the experiments, a laboratory scaled fluidized bed type dryer was used. The effects of the swirling flow field on the drying performance were investigated by using an axial guide vane type swirl generator. The effects of the mass flow rate and temperature of the air on the drying performance were also investigated. The goal of this work is to present the experimental results on the drying process of wheat and the effects of the swirling flow field on the drying performance.     

酒糟流态化干燥压降的实验研究

通过酒糟在流化床干燥器上的冷态实验，得出全床压降随气流速度、料层厚度、物料初始湿含量及气体分布板开孔率的变化规律，经整理数据得出相关参数之间的准则关联式。     

Thermal analysis of a fluidized bed drying process for crops. Part I: Mathematical modeling

Development of a comprehensive mathematical model to simulate the simultaneous heat and mass transfer processes in a bubbling fluidized bed is described. Although the model is applicable to a wide range of particles, wheat is chosen as an example. In the development of the model, the commonly used two‐phase theory is not used because of its insensitivity to the particle group used in the bed. Instead, a new hydrodynamic model is developed for each specific particle group. The behaviour of bubbles in a bed of group D particles (wheat) is modelled with the consideration that they grow in size as they rise in the bed, but are of the same size at any height in the bed. The voidage of bubbles, particles and interstitial gas is modelled separately. A newly developed expression to determine the minimum fluidization velocity of wet particles is used. The model considers the presence of different phases inside the bed, and their physical variation along the bed. The interstitial gas phase, the bubble phase, and the solid phase are modelled separately. The drying mechanism for the solid phase is considered in two stages: the falling rate, and the constant rate, with appropriate temperature and moisture diffusion coefficients and wall effects. The simultaneous heat and mass transfer processes during the drying process including the internal and external effects are modelled for each phase. A set of coupled nonlinear partial differential equations is employed to accurately model the drying process without using any adjustable parameters. A numerical code is developed to solve the governing partial differential equations using a control volume‐based discretization approach. Piecewise profiles expressing the variation of dependent variables between the grid points are used to evaluate the required integrals. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical and experimental investigation of heat and mass transfer in unsaturated porous media with low convective drying intensity

The heat and mass transfer in an unsaturated wet cylindrical bed packed with quartz particles was investigated theoretically and experimentally for relatively low convective drying rates. The medium was dried by blowing dry air over the top of the porous bed which was insulated by impermeable, adiabatic material on the bottom and sides. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi‐phase flow in the unsaturated porous medium using the energy and mass conservation equations for heat and mass transfer during the drying. The drying model included convection and capillary transport of the moisture, and convection and diffusion of the gas. The wet and dry regions were coupled with a dynamic boundary condition at the evaporation front. The numerical results indicated that the drying process could be divided into three periods: the initial temperature rise period, the constant drying rate period, and the reduced drying rate period. The numerical results agreed well with the experimental data, verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. ©2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 290–312, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20205     

玉米秸秆循环流化床热解气化试验

采用循环流化床气化中试装置对玉米秸秆进行了气化试验,分别在常温空气与250℃预热空气条件下,研究了空气当量比（ER）和原料含水率对气化特性的影响规律。结果表明：随着ER的增大,循环流化床气化炉内的反应温度升高,气化燃气中的CO₂含量增加,焦油与CO含量及燃气热值降低,气化效率随ER的增大呈先增大后减小的趋势;随着气化原料含水率的增加,循环流化床气化炉内的平均温度下降,燃气中的CO₂与H₂及焦油含量逐渐升高,CO含量下降,CH₄与C_nH_m含量均为先增加后减少。与常温空气工况相比,预热空气工况下的燃气热值与气化效率均有一定程度的提高。采用预热空气为气化介质,提高气化剂温度,可显著促进玉米秸秆的气化反应,提升气化效率。     

单颗粒海藻在流化床内燃烧试验及灰色关联分析

在流化床内进行了单颗粒海藻(条浒苔)的燃烧试验,研究了床高、床温和流化风速对条浒苔颗粒燃烧后剩余质量和固定碳残留率的影响,并对各因素的影响程度进行了灰色关联分析.结果表明:床温和流化风速对条浒苔颗粒燃烧后剩余质量和固定碳残留率的影响较大,而床高对其的影响较小;各工况因素影响固定碳燃尽的主次依次为:床温>流化数>床高,在研究工况范围内床温是影响燃烧的最主要因素.     

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.     

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

Energy and exergy analysis of biomass gasification at different temperatures

Biomass is usually gasified above the optimal temperature at the carbon-boundary point, due to the use of different types of gasifiers, gasifying media, clinkering/slagging of bed material, tar cracking, etc. This paper is focused on air gasification of biomass with different moisture at different gasification temperatures. A chemical equilibrium model is developed and analyses are carried out at pressures of 1 and 10 bar with the typical biomass feed represented by CH_1.4O_0.59N_0.0017. At the temperature range 900–1373 K, the increase of moisture in biomass leads to the decrease of efficiencies for the examined processes. The moisture content of biomass may be designated as “optimal” only if the gasification temperature is equal to the carbon-boundary temperature for biomass with that specific moisture content. Compared with the efficiencies based on chemical energy and exergy, biomass feedstock drying with the product gas sensible heat is less beneficial for the efficiency based on total exergy. The gasification process at a given gasification temperature can be improved by the use of dry biomass and by the carbon-boundary temperature approaching the required temperature with the change of gasification pressure or with the addition of heat in the process.     

Exergy‐based performance analysis of packed‐bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium‐based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed‐bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed‐bed solar air heater. The rate of heat recovered from the packed‐bed solar air heater varied between 9.3 and 151.5 W m^?2, while the rate of thermal exergy recovered from the packed‐bed solar air heater varied between 0.04 and 8.77 W m^?2 during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed‐bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright © 2004 John Wiley & Sons, Ltd.