摘要 :
This is a summary of the development of conversion of solid fuels in fluidized bed during the hundred years that follow the first patent of Winkler in September 1922.The Winkler gasifiers and their followers are described first.Ot...
展开
This is a summary of the development of conversion of solid fuels in fluidized bed during the hundred years that follow the first patent of Winkler in September 1922.The Winkler gasifiers and their followers are described first.Other fuel converters,such as boilers,appeared only in the 1960-70s and became of interest because of their expected environmental advantages.Initially,bubbling bed boilers were introduced,followed by circulating fluidized bed(CFB)boilers in the beginning of the 1980s.Now,CFB is the dominant technology.The entire development has not been conditioned by technological breakthroughs,but rather by the surrounding conditions:industrial demand,wars,environmental effects,availability and price of fuels.The recent development of the presently rather mature technology depends very much on the necessity to limit greenhouse gas accumulation in the atmosphere.Although fluidized bed technology of fers solutions to reduce CO2 emissions,so far,no decisive line of application has been established for CO2 reduction,except for the use of biomass and waste.
收起
摘要 :
Although industrial fluidized bed dryers have been used successfully for the drying of wet solid particles for many years,the development of industrial fluidized bed dryers for any particular application is fraught with difficulti...
展开
Although industrial fluidized bed dryers have been used successfully for the drying of wet solid particles for many years,the development of industrial fluidized bed dryers for any particular application is fraught with difficulties such as scaling-up,poor fluidization and non-uniform product quality.Scaling-up is the major problem and there are very few good,reliable theoretical models that can replace the expensive laboratory work and pilot-plant trials.This problem is mainly due to the different behavior of bubbles and mixing regimes in fluidized bed dryers of different size.Simple transformation of laboratory batch drying data to continuous back-mixed dryers using the residence time distribution of the solids is insufficient to account for the complex flow and heat and mass transfer phenomena occurring in the bed.Although time scaling using temperature driving forces and solids mass flux for the same change in moisture content in the batch and continuous dryers has been successful in predicting moisture content profiles in the continuous dryer at the constant rate period,it does not take into account solid mixing.Two-phase Davidson-Harrison models have been used in modeling of the continuous back-mixed dryer with various degrees of success.On the other hand,the three-phase Kunii-Levenspiel model is seldom used in modeling fluidized bed dryers because it is too complex to handle.A combination of multi-phase models and residence time distribution could improve predicting power for back-mixed dryers because this combination takes into account both the bubbles and solid mixing phenomena.Incremental models were widely used to model continuous plug flow fluidized bed dryers,but the cross-flow of drying medium has not been sufficiently modeled except by the author.In some incremental models,axial dispersion is modeled using the Peclet number,Pe.A combination of an incremental model with an axial dispersion and cross-flow model of drying medium would improve predicting power.Poor fluidization of Geldart group C particles could be improved by the assistance of external means such as vibration,agitation,rotation and centrifugation.Both vibrated and agitated fluidized bed dryers have been successfully used in industry,but rotating or centrifugal fluidized bed dryers are still not available for industrial use.
收起
摘要 :
Gas solid fluidized beds are commonly used in process industries due to their high operational efficiency regarding for instance solids mixing and heat and mass transfer. It is well known that hydrodynamics plays a critical role i...
展开
Gas solid fluidized beds are commonly used in process industries due to their high operational efficiency regarding for instance solids mixing and heat and mass transfer. It is well known that hydrodynamics plays a critical role in the dynamic behaviour of fluidized beds. The bubbles impair the operation and diffuse the contact between the particles, which results in lowering the efficiency. Electric fields have various actions on the matter and if it is possible to regulate it to control the fluidized bed operation. An axially applied electric field (co-flow arrangement) has been found to give a stabilized fluidized bed at high gas velocity, greater than minimum fluidization velocity. For the two modes of operation (1) Field first and fluidization last (2) Fluidization first and field last, the first mode stabilizes the bed at high gas velocity.
收起
摘要 :
Due to the good air-particle mixing couple with the high heat and mass transfer rates, fluidized and vibrated fluidized beds of particles have been widely used for many chemical engineering processes involving particulate systems....
展开
Due to the good air-particle mixing couple with the high heat and mass transfer rates, fluidized and vibrated fluidized beds of particles have been widely used for many chemical engineering processes involving particulate systems. On the other hand, in practice, for using such beds in the treatment of heat-sensitive materials (i.e., polymer, food products) the installation of heat-exchange surface within the bed are needed to provide indirect heat as well as prevent thermal degradation. Therefore, this paper presents an investigation to determine the local heat-transfer coefficient in fluidized and vibrated fluidized beds (by expressing Nu_θ vs. Re) operated with glass ballotini particles ranging from 500 to 1100 μm, in diameter. The data show that, at a given air velocity, the local heat-transfer coefficient obtained in the vibrated fluidized beds is significantly higher as compared to those of fluidized beds. In addition, vibrated fluidized beds can achieve higher local heat-transfer coefficients as the particle diameter is reduced from 1100 to 500 μm and the vibration dimensionless (Γ) is increased from 1 to 3.
收起
摘要 :
Air dense medium fluidized bed (ADMFB) offers a better alternative approach for dry coal beneficiation. In recent years, there has been a rapid advancement in the understanding of fluidized bed behavior and a great deal of work ha...
展开
Air dense medium fluidized bed (ADMFB) offers a better alternative approach for dry coal beneficiation. In recent years, there has been a rapid advancement in the understanding of fluidized bed behavior and a great deal of work has been done to make this process competitive with conventional wet beneficiation processes. At the same time, much information is available in the general technical literature concerning various aspects of ADMFB; however, this is often uncoordinated information and widely dispersed. So, it is necessary to provide the adequate information systematically, which offers researchers an effective and easy way to obtain specific details. This review critically evaluates the understanding of different operating parameters that affect the cleaning performance of this equipment. Then, following the discussion of different operating parameters, we describe three recent practical developments in ADMFB: the dual-density fluidized bed, vibrated fluidized bed, and magnetically stabilized fluidized bed. Finally, some challenging issues that need special attention are suggested in the conclusion for further improvement of this process.
收起
摘要 :
The fluid dynamics in the furnaces of large-scale circulating fluidized bed (CFB) boilers are surprisingly little known in contrast to the many laboratory studies made on conditions related to chemical reactors. Two areas are surv...
展开
The fluid dynamics in the furnaces of large-scale circulating fluidized bed (CFB) boilers are surprisingly little known in contrast to the many laboratory studies made on conditions related to chemical reactors. Two areas are surveyed in the present work: the bottom bed and the upper dilute zone of a furnace. The bottom bed is considered bubbling, but the general opinion is that either it does not exist, or it is turbulent. The flow in the upper furnace is dilute phase transport, judging from regime maps, showing that the state of the flow is outside of the range of fast fluidization. However, this is also not generally accepted. Usually, the regime of fluidization in CFB boilers is said to be fast fluidization. In one work it is considered fast fluidization even though the authors agree that it is different from the general definition. In another investigation it is called entrained flow. Here, the conclusion is that the diversity of opinions should be resolved by further investigations with the aim of defining the conditions for the fluidized flow in furnaces, including the influence of particle size and density, fluidization velocity, gas properties, and effects from the furnace dimensions, if any. (c) 2021 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
收起
摘要 :
Oxy-fired fluidized bed combustion technique combines the advantages of both the fluidized bed combustion and oxy-fired technology. In oxy-fired condition, a mixture of oxygen with CO2 or recycled flue gas (RFG) is used for the co...
展开
Oxy-fired fluidized bed combustion technique combines the advantages of both the fluidized bed combustion and oxy-fired technology. In oxy-fired condition, a mixture of oxygen with CO2 or recycled flue gas (RFG) is used for the combustion. This paper is divided into two parts; First part covers a technical review of the oxy-fired fluidized bed units, including the studies performed on bubbling fluidized bed, circulating fluidized bed and pressurized fluidized bed. Work presented, identifies and illustrates the trends and challenges related to oxy-fired fluidized bed in the current scenario. It is found that it would take many years to utilize the benefit of this technology fully. The second part explores the possibility of this technology for co-firing cases. Rice husk (RH), plant litter (PL), and coal are co-fired inside a 20 kW lab scale bubbling fluidized bed combustor under O-2/N-2/RFG mode. The experimental results show that the blends of coal-PL and coal-RH have been burnt successfully. Carbon dioxide is increased found as a result of increasing oxygen. The measured percentage of NOx and other gasses are found within permissible limit. (C) 2016 Elsevier Ltd. All rights reserved.
收起
摘要 :
The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed(CTFB)were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fl...
展开
The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed(CTFB)were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fluidized bed(HDCFB)riser.The average solids holdup distributions,fluctuations,and probability density distributions of the instantaneous solids holdup signals indicate that the CTFB either more resembles a TFB if the Gs/Ug ratio is large or more resembles an HDCFB if the Gs/Ug ratio is small.The distributions of the bubble occurrence,equivalent bubble size,and bubble phase fraction were extracted from the optical fiber signals of the local solids holdup using the wavelet analysis method.The CTFB was found to have a greater number of smaller bubbles than the bubbling and turbulent fluidized beds and relatively fewer and larger bubbles than that in an HDCFB.The effect of flow dynamics and bubble behaviors on the reaction performance in different types of fluidized beds are compared by analytical calculation based on the K-L reactor model.A superior reactor performance can be achieved using the CTFB reactor for a higher effective reaction rate,higher mass transfer rates,and a lower degree of solids backmixing.
收起
摘要 :
Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of t...
展开
Experiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semi-correlated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data.
收起
原文获取