Extended surface fluidized bed heat transfer

Experirnental data and theoretical calculation on the heat transfer performance of extended surface submerged: in shallow air fluidized beds ~ less than 150 mm, are presented. Energy t;ransferrence from the bed material was effected by water cooled tubes passing through the fins. The extended surface tested was either manufactured from square or radial copper fins silver soldered to a circular basic tube or commercially supplied, being of the crimped or extruded helical fin type. Performances are compared, for a wide range of geometric variables, bed configurations and fluidized materials, with plain and oval tubes operating under similar experimental conditions. A statistical analysis of all results, using a regression technique, has shown the relative importance of each significant variable. The bed to surface heat transfer coefficients are higher than those reported in earlier published work using finned tubes in much deeper beds and the heat transfer to the whole of the extended surface is at least as good as that previously reported for un-finned tubes. The improved performance is attributed partly to the absence of large bubbles in shallow beds and it is suggested that the improved circulation of the solids when constrained in the narrow passages between adjacent fins may be a contributory factor. Flow visualisation studies between a perspex extended surface and a fluidized bed using air at ambient temperatures, have demonstrated the effect of too small a fin spacing. Fin material and the bonding to the basic tube are more important in the optimisation of performance than in conventional convective applications because of the very much larger heat fluxes involved. A theoretical model of heat flow for a radial fin surface, provides data concerning the maximum heat transfer and minimum metal required to fulfil a given heat exchange duty. Results plotted in a series of charts aim at assisting the designer of shalJow fluidized beds.

Some studies relating to the design of shallow, fluidized bed boilers

The concept of shallow fluidized bed boilers is defined and a preliminary working design for a gas-fired package boiler has been produced. Those areas of the design requiring further study have been specified. Experimental investigations concerning these areas have been carried out. A two-dimensional, conducting paper analog has been developed for the specific purpose of evaluating sheet fins. The analog has been generalised and is presented as a simple means of simulating the general, two-dimensional Helmholtz equation. By recording the transient response of spherical, calorimetric probes when plunged into heated air-fluidized beds, heat transfer coefficients have been measured at bed temperatures up to 1 100°C. A correlation fitting all the data to within ±10% has been obtained. A model of heat transfer to surfaces immersed in high temperature beds has been proposed. The model solutions are, however, only in qualitative agreement with the experimental data. A simple experimental investigation has revealed that the effective, radial, thermal conductivities of shallow fluidized beds are an order of magnitude lower than the axial conductivities. These must, consequently, be taken into account when considering heat transfer to surfaces immersed within fluidized beds. Preliminary work on pre-mixed gas combustion and some further qualitative experiments have been used as the basis for discussing the feasibility of combusting heavy fuel oils within shallow beds. The use of binary beds, within which the fuel could be both gasified and subsequently burnt, is proposed. Finally, the consequences of the experimental studies on the initial design are considered, and suggestions for further work are made.

Fin spacing interaction in fluidized bed heat exchangers

The work presented in this thesis is concerned with the heat transfer performance of a single horizontal bare tube and a variety of finned tubes immersed in a shallow air fluidized bed. Results of experimental investigations with the bare tube indicate that the tube position in the bed influences its performance narticularly where fine bed materials are used. In some cases the maximum heat transfer is obtained with the tube in the particle cloud just above the dense phase fluidized bed - a phenomenon that has not been previously observed. This was attributed to the unusual particle circulation in shallow beds. The data is also presented in dimensionless correlations which may be useful for design purposes. A close approximation to the bare tube data can be obtained by using thetransient heating of a spherical robe and this provides a valuable way of accumulating a lot of data very rapidly. The experimental data on finned tubes shows that a fin spacing less than twenty times the average particle diameter can cause a significant reduction in heat transfer due to the interaction which takes place between the particles and the surface of the fins. Furthermore, evidence is provided to show that particle shape plays an important part in the interaction with spherical particles being superior to angular particles at low fin spacing/particle diameter ratio. The finned tube data is less sensitive to tube position in the bed than bare tubes and the best performance is when the tube is positioned at the distributor.A reduction in bed depth decreases the thermal performance of the finned tube but in many practical installations the reduction in pressure drop might more than comnensate for the reduced heat flux. Information is also provided on the theoretical uerformance of fins and the effect of the root contact area between the fins and the tube was investigated.

Time scale analysis for fluidized bed melt granulation I: granule-granule and granule-droplet collision rates

Fluidized bed spray granulators (FBMG) are widely used in the process industry for particle size growth; a desirable feature in many products, such as granulated food and medical tablets. In this paper, the first in a series of four discussing the rate of various microscopic events occurring in FBMG, theoretical analysis coupled with CFD simulations have been used to predict granule–granule and droplet–granule collision time scales. The granule–granule collision time scale was derived from principles of kinetic theory of granular flow (KTGF). For the droplet–granule collisions, two limiting models were derived; one is for the case of fast droplet velocity, where the granule velocity is considerable lower than that of the droplet (ballistic model) and another for the case where the droplet is traveling with a velocity similar to the velocity of the granules. The hydrodynamic parameters used in the solution of the above models were obtained from the CFD predictions for a typical spray fluidized bed system. The granule–granule collision rate within an identified spray zone was found to fall approximately within the range of 10-2–10-3 s, while the droplet–granule collision was found to be much faster, however, slowing rapidly (exponentially) when moving away from the spray nozzle tip. Such information, together with the time scale analysis of droplet solidification and spreading, discussed in part II and III of this study, are useful for probability analysis of the various event occurring during a granulation process, which then lead to be better qualitative and, in part IV, quantitative prediction of the aggregation rate.

Mass transfer in fluidized bed drying of moist particulate

Bubbling fluidized bed technology is one of the most effective mean for interaction between solid and gas flow, mainly due to its good mixing and high heat and mass transfer rate. It has been widely used at a commercial scale for drying of grains such as in pharmaceutical, fertilizers and food industries. When applied to drying of non-pours moist solid particles, the water is drawn-off driven by the difference in water concentration between the solid phase and the fluidizing gas. In most cases, the fluidizing gas or drying agent is air. Despite of the simplicity of its operation, the design of a bubbling fluidized bed dryer requires an understanding of the combined complexity in hydrodynamics and the mass transfer mechanism. On the other hand, reliable mass transfer coefficient equations are also required to satisfy the growing interest in mathematical modelling and simulation, for accurate prediction of the process kinetics. This chapter presents an overview of the various mechanisms contributing to particulate drying in a bubbling fluidized bed and the mass transfer coefficient corresponding to each mechanism. In addition, a case study on measuring the overall mass transfer coefficient is discussed. These measurements are then used for the validation of mass transfer coefficient correlations and for assessing the various assumptions used in developing these correlations.

A priori prediction of aggregation efficiency and rate constant for fluidized bed melt granulation

This paper presents a predictive aggregation rate model for spray fluidized bed melt granulation. The aggregation rate constant was derived from probability analysis of particle–droplet contact combined with time scale analysis of droplet solidification and granule–granule collision rates. The latter was obtained using the principles of kinetic theory of granular flow (KTGF). The predicted aggregation rate constants were validated by comparison with reported experimental data for a range of binder spray rate, binder droplet size and operating granulator temperature. The developed model is particularly useful for predicting particle size distributions and growth using population balance equations (PBEs).

Influence of reaction atmosphere (H2O, N2, H2, CO2, CO) on fluidized-bed fast pyrolysis of biomass using detailed tar vapor chemistry in computational fluid dynamics

Secondary pyrolysis in fluidized bed fast pyrolysis of biomass is the focus of this work. A novel computational fluid dynamics (CFD) model coupled with a comprehensive chemistry scheme (134 species and 4169 reactions, in CHEMKIN format) has been developed to investigate this complex phenomenon. Previous results from a transient three-dimensional model of primary pyrolysis were used for the source terms of primary products in this model. A parametric study of reaction atmospheres (H2O, N2, H2, CO2, CO) has been performed. For the N2 and H2O atmosphere, results of the model compared favorably to experimentally obtained yields after the temperature was adjusted to a value higher than that used in experiments. One notable deviation versus experiments is pyrolytic water yield and yield of higher hydrocarbons. The model suggests a not overly strong impact of the reaction atmosphere. However, both chemical and physical effects were observed. Most notably, effects could be seen on the yield of various compounds, temperature profile throughout the reactor system, residence time, radical concentration, and turbulent intensity. At the investigated temperature (873 K), turbulent intensity appeared to have the strongest influence on liquid yield. With the aid of acceleration techniques, most importantly dimension reduction, chemistry agglomeration, and in-situ tabulation, a converged solution could be obtained within a reasonable time (∼30 h). As such, a new potentially useful method has been suggested for numerical analysis of fast pyrolysis.

Comprehensive secondary pyrolysis in fluidized-bed fast pyrolysis of biomass, a fluid dynamics based modelling effort

Homogenous secondary pyrolysis is category of reactions following the primary pyrolysis and presumed important for fast pyrolysis. For the comprehensive chemistry and fluid dynamics, a probability density functional (PDF) approach is used; with a kinetic scheme comprising 134 species and 4169 reactions being implemented. With aid of acceleration techniques, most importantly Dimension Reduction, Chemistry Agglomeration and In-situ Tabulation (ISAT), a solution within reasonable time was obtained. More work is required; however, a solution for levoglucosan (C6H10O5) being fed through the inlet with fluidizing gas at 500 °C, has been obtained. 88.6% of the levoglucosan remained non-decomposed, and 19 different decomposition product species were found above 0.01% by weight. A homogenous secondary pyrolysis scheme proposed can thus be implemented in a CFD environment and acceleration techniques can speed-up the calculation for application in engineering settings.

Geometrical optimization of a fast pyrolysis bubbling fluidized bed reactor using comutational fluid dynamics

This paper analyzes the physical phenomena that take place inside an 1 kg/h bubbling fluidized bed reactor located at Aston University and presents a geometrically modified version of it, in order to improve certain hydrodynamic and gas flow characteristics. The bed uses, in its current operation, 40 L/min of N2 at 520 °C fed through a distributor plate and 15 L/min purge gas stream, i.e., N2 at 20 °C, via the feeding tube. The Eulerian model of FLUENT 6.3 is used for the simulation of the bed hydrodynamics, while the k - ε model accounts for the effect of the turbulence field of one phase on the other. The three-dimensional simulation of the current operation of the reactor showed that a stationary bubble was formed next to the feeding tube. The size of the permanent bubble reaches up to the splash zone of the reactor, without any fluidizaton taking place underneath the feeder. The gas flow dynamics in the freeboard of the reactor is also analyzed. A modified version of the reactor is presented, simulated, and analyzed, together with a discussion on the impact of the flow dynamics on the fast pyrolysis of biomass. © 2010 American Chemical Society.

Heat-Transfer Study of External Superheater of CFB Incinerator

The heat transfer coefficients for horizontally immersed tubes have been studied in model internally circulating fluidized bed (ICFB) and pilot ICFB incinerators. The characteristics in the ICFB were found to be significantly different from those in a bubbling bed. In ICFB, there is a flowing zone with high velocity, a heat exchange zone, and a moving zone with low velocity. The controllable heat transfer coefficients in ICFB strongly depend on the fluidized velocity in the flowing zone, and also the flow condition in the moving zone. The heat exchange process and suitable bed temperature can be well controlled according to this feature. Based on the results of experiments, a formulation for heat transfer coefficient has been developed. These results were applied to an external superheater of a CFB incinerator with a 450 degreesC steam outlet in a waste-to-energy pilot cogeneration plant of 12 MW in Jiaxing City, China.

Characterizing Particle Dispersion by Image Analysis in ICFB

A new particle image technique was developed to analyze the dispersion of tracer particles in an internally circulating fluidized bed (ICFB). The movement course and the concentration distribution of tracer particles in the bed were imaged and the degree of inhomogeneity of tracer particles was analyzed. The lateral and axial dispersion coefficients of particles were calculated for various zones in ICFB. Results indicate that the lateral diffusion coefficient in the fluidized bed with uneven air distribution is significantly higher than that in uniform bubbling beds with even air distribution. The dispersion coefficients are different along bed length and height.

内旋流流化床内颗粒运动规律的试验与数值研究

通过冷态试验与数值模拟研究了不同布风下装有∧字型布风板的内旋流流化床的流动特性，并进行了对比．气固两相数值模拟结果表明，固相流场产生了大尺度旋流，而且与试验观察到的现象相似．对固相颗粒速度分析表明非均匀布风使内旋流流化床的底部和顶部产生了气、固两相强烈的横向和纵向运动．相比较来说，对称的非均匀布风扩散能力要比均匀布风的扩散能力强．

Influence of HCl on CO and NO emissions in combustion

The influence of HCl on CO and NO emissions was experimentally investigated in an entrained flow reactor (EFR) and an internally circulating fluidized bed (ICFB). The results in EFR show the addition of HCl inhibits CO oxidation and NO formation at 1073 K and 1123 K. At the lower temperature (1073 K) the inhibition of HCl becomes more obvious. In ICFB, chlorine-containing plastic (PVC) was added to increase the concentration of HCl during the combustion of coal or coke. Results show that HCl is likely to enhance the reduction of NO and N2O. HCl greatly increases CO and CH4 emission in the flue gas. A detailed mechanism of CO/NO/HCl/SO2 system was used to model the effect of HCl in combustion. The results indicate that HCl not only promotes the recombination of radicals O, H, and OH, but also accelerates the chemical equilibration of radicals. The influence of HCl on the radicals mainly occurs at 800-1200 K. (C) 2009 Elsevier Ltd. All rights reserved.

Estudo de pirólise catalítica de biomassa em escala piloto para melhoramento da qualidade do bio-óleo

A pirólise rápida é um processo para conversão térmica de uma biomassa sólida em altos rendimentos de um produto líquido chamado de bio-óleo. Uma das alternativas para geração de um bio-óleo com menor teor de oxigênio é uso de catalisadores nos reatores de pirólise, ao invés de um inerte, num processo chamado de pirólise catalítica. O objetivo deste trabalho foi testar catalisadores comerciais, um ácido e outro básico, em uma unidade piloto de leito fluidizado circulante. O catalisador ácido utilizado foi o Ecat, proveniente de uma unidade industrial de craqueamento catalítico fluido (FCC), e como catalisador básico foi utilizado uma hidrotalcita. Os resultados foram comparados com testes utilizando um material inerte, no caso uma sílica. Uma unidade piloto de FCC do CENPES foi adaptada para realizar os testes de pirólise catalítica. Após fase de modificação e testes de condicionamento, foi comprovada a viabilidade na utilização da unidade piloto adaptada. Contudo, devido a limitações operacionais, maiores tempos de residência tiveram que ser aplicados no reator, configurando o processo como pirólise intermediária. Foram então realizados testes com os três materiais nas temperaturas de 450C e 550C. Os resultados mostraram que o aumento do tempo de residência dos vapores de pirólise teve um impacto significativo nos rendimentos dos produtos quando comparada com o perfil encontrado na literatura para pirólise rápida, pois devido ao incremento das reações secundárias, produziu maiores rendimentos de coque e água, e menores rendimentos de bio-óleo. O Ecat e a hidrotalcita se apresentaram mais efetivos em termos de desoxigenação. O primeiro apresentou maiores taxas de desoxigenação via desidratação e a hidrotalcita apresentou maior capacidade para descarboxilação. Contudo, o uso de Ecat e hidrotalcita não se mostrou adequado para uso em reatores de pirólise intermediária, pois acentuou ainda mais as reações secundárias, gerando um produto com alto teor de água e baixo teor de compostos orgânicos no bio-óleo, além de produzirem mais coque. À temperatura de 450C estes efeitos foram mais pronunciados. Em termos de caracterização química, a condição de pirólise intermediária apontou para a produção de bio-óleos com perfil fenólico, sendo a sílica o que proporcionou os melhores rendimentos, principalmente a temperatura de 550C, sendo superiores aos encontrados na literatura. Analisando as composições dos bio-óleos sob a ótica da produção de biocombustíveis, nenhum dos materiais testados apresentou rendimentos consideráveis em hidrocarbonetos. De maneira geral, a sílica foi o que proporcionou os melhores resultados em termos de rendimento e qualidade do bio-óleo. Sua menor área superficial e sua característica de inerte se mostraram mais adequados para o processo de pirólise intermediária, onde a contribuição das reações secundárias em fase gasosa é elevada em função do tempo de residência no reator