Experimental study of a bubbling fluidized bed with a rotating distributor

Esta tesis consiste en la caracterización experimental de la hidrodinámica de un nuevo lecho fluido de distribuidor rotatorio. Existen numerosas referencias en la literatura en las que se analizan los factores que influyen en la calidad de la fluidización en lechos burbujeantes, como son la tasa de mezcla, el tamaño de burbuja y la heterogeneidad en el lecho. Entre estos factores se encuentran la geometría del lecho, el caudal de gas empleado en la fluidización y el tipo de distribuidor. Las heterogeneidades que se producen con frecuencia en lechos industriales han hecho que numerosos investigadores hayan incorporado modificaciones de distinta índole sobre los lechos convencionales, como por ejemplo alterar el sistema de suministro de aire o usar diseños innovadores. El nuevo diseño de distribuidor que se estudia en esta tesis intenta, mediante la introducción del giro del distribuidor, lograr mayores tasas de mezcla del gas y aumentar la dispersión de las partículas, al tiempo que se consigue una fluidización más uniforme. La posibilidad de controlar la velocidad de giro del distribuidor permite operar en un amplio rango de condiciones de operación sin perder la calidad de fluidización. Los experimentos fueron realizados en un lecho constituido por un cilindro transparente de diámetro 192 mm y altura 0.8 m lleno de partículas de arena del tipo B de acuerdo con la clasificación de Geldart. El distribuidor rotatorio es una placa perforada acoplada en su eje al eje de un motor eléctrico. La velocidad de giro se controla mediante un inversor de frecuencia que permite trabajar con un rango de velocidades que en los experimentos se varía de 0 a 100 rpm. La descripción completa de la instalación experimental se encuentra en el Capítulo 2. La caracterización experimental de la hidrodinámica del lecho realizada en la tesis incluye, por un lado, la descripción global del lecho sin giro y con giro en el distribuidor, mediante medidas absolutas de presión (Capítulo 4) y por otro, el estudio de las propiedades de las burbujas que se forman en el lecho usando sondas ópticas específicamente diseñadas y construidas para esta tesis (Capítulo 6). Con el fin de interpretar de manera adecuada las señales de presión que se utilizan para la caracterización del lecho, el Capítulo 3 contiene un estudio de los valores de la desviación estándar de las señales de presión en lechos fluidos burbujeantes. Se ha obtenido una función semi-empírica, que depende de la velocidad del gas, que permite predecir dichas fluctuaciones de presión en lechos con partículas del tipo B. Este modelo permite explicar las diferencias en las medidas cuando se emplean sensores de presión en modo diferencial o absoluto, obteniéndose una buena correspondencia entre los valores teóricos y las medidas experimentales para diferentes tamaños de lechos, posición de los sensores y propiedades de las partículas. En el Capítulo 4 se estudia el efecto del giro del distribuidor en el comportamiento hidrodinámico global del lecho, analizándose el cambio en la mínima velocidad de fluidización y en las fluctuaciones de presión. Se ha observado una disminución en el valor de la mínima velocidad de fluidización a medida que aumenta la velocidad de giro. Además se analizaron los espectros de frecuencia y la desviación estándar de las fluctuaciones de presión. Las medidas se repitieron a distintas alturas iniciales del lecho para ver como afectaba esta altura a la magnitud del efecto provocado por el giro. Se ha comprobado que la rotación del distribuidor permite fluidizar lechos con poca altura, que en ausencia de giro presentan una estructura de chorros y no se consiguen fluidizar. Por otro lado, conforme aumenta la altura inicial del lecho, el efecto de la rotación sobre la velocidad de mínima fluidización tiende a disminuir. Se demuestra por tanto que mediante el ajuste de la velocidad de giro en el distribuidor, se puede cambiar la velocidad del aire necesario para fluidizar el lecho, lo que permite mantener unas condiciones uniformes de fluidización en un rango mayor de caudales. Una vez realizado el análisis global, se estudiaron las características locales del lecho. Para ello, se usaron sensores de presión diferencial y sondas ópticas con las que se midieron las cuerdas de las burbujas que se forman en el lecho y su velocidad. Los resultados obtenidos usando las dos sondas se encuentran en el Capítulo 5. Las funciones de densidad de probabilidad de la cuerda y de la velocidad se calcularon aplicando el Método de la Máxima Entropía. Existe un tamaño mínimo de cuerda que es posible medir usando sondas intrusivas para que el error sea tolerable. Este límite inferior se ha tenido en cuenta en la formulación de las ecuaciones para la obtención de las funciones de distribución de probabilidad. La función de densidad de probabilidad de los diámetros se ha deducido a partir de las medidas experimentales de las cuerdas, aplicando herramientas estadísticas. Los resultados de las sondas de presión y de las sondas ópticas son bastante parecidos, aunque las sondas ópticas proporcionan información más local, y pueden utilizarse en posiciones muy próximas al distribuidor. Se ha comprobado que el método de la Máxima Entropía es un método simple que ofrece varias ventajas frente a otros métodos aplicados hasta la fecha para la obtención de las distribuciones de tamaño en lechos fluidos: no es necesario suponer a priori la forma de la distribución, el número de muestras requeridas es menor que en otros métodos y se evita la transformación inversa, que es un cálculo complejo. Una vez desarrollado y validado el método de transformación de cuerdas en diámetros se estudió el efecto del giro del distribuidor en el tamaño de las burbujas y su frecuencia de paso a distintas posiciones en el lecho. Los resultados se presentan en el capítulo 6. Primero se ha obtenido un modelo simple para analizar la influencia de la aceleración centrifuga que actúa sobre la burbuja en el momento en que se desprende del distribuidor una vez formada. Este análisis indica que el giro hace que el diámetro inicial de la burbuja sea menor que si el distribuidor estuviera parado. Los resultados experimentales muestran que, a igualdad en el exceso de aire, el tamaño de las burbujas es menor cuando el distribuidor gira. Los tamaños medidos de burbuja en distintas posiciones radiales confirman la tendencia puesta de manifiesto por el modelo: para el distribuidor rotatorio el diámetro medio disminuye a distancias mayores del eje del lecho, donde la aceleración centrífuga es mayor. La rotación del distribuidor también hace que la distribución de burbujas en la sección radial del lecho sea más homogénea. Además, para el distribuidor rotatorio se observa que el aumento del tamaño de las burbujas a medida que aumenta la altura es menos acusado que con ausencia de giro. Esto puede deberse a una disminución de la coalescencia lograda por la ruptura de los caminos preferenciales de ascensión de las burbujas gracias al giro. _________________________________________________This thesis presents the experimental fluid dynamic characterization of a new fluidized bed with a rotating distributor. Many works in the literature analyze the factors that influence the quality of fluidization in bubbling beds, e.g. the rate of solids mixing, the size of the bubbles and the extent of heterogeneity in the bed. These factors include among other, the bed geometry, the gas flow rate and the type of gas distributor. The non heterogenous structures often found in industrial fluidization processes have led many investigators to modify the conventional fluidized bed devices, alter the air supply system or try innovative designs to avoid these heterogeneities. The novel distributor design studied in this thesis tries to solve some of these difficulties; the aim of the distributor rotation being to overcome low radial gas mixing and particle dispersion, and to achieve a more uniform fluidization. The possibility to control and adjust the rotational speed of the distributor plate offers a wide range of operating conditions while maintaining the quality of fluidization. The fluidized bed is a transparent cylinder with 192 mm ID and a height of 0.8 m filled with Geldart B silica particles. The distributor is a perforated plate that is coupled to the shaft of an AC electric motor. It can rotates around the bed axis and the rotational speed can be varied using a frequency inverter. In the experiments this speed was varied between 0 and 100 rpm. A complete description of the experimental set-up can be found in Chapter 2. The experimental fluid dynamic characterization presented in this thesis includes a global description of the bed behavior with and without rotation of the distributor, using pressure measurements (Chapter 4). In addition, the differences in the characteristics of the generated bubbles are studied by means of in house made optical probes (Chapter 6). To better understand the pressure signal recorded for the bed characterization, the behavior of the standard deviation of pressure fluctuations in fluidized beds for group B particles in the bubbling regime is studied in Chapter 3. An empirical-theoretical function, which depends on the gas velocity, is proposed for predicting the pressure signal fluctuations. The differences in the standard deviation of pressure fluctuations obtained from absolute or differential sensors are analyzed and compared to experimental values corresponding to different bed sizes, pressure probe positions and particle properties. In chapter 4 the effect of the rotational speed of the distributor plate on the global hydrodynamic behavior of the bed is studied. Minimum fluidization velocity and pressure fluctuations were first analyzed. A decrease in the minimum fluidization velocity is observed when the rotational speed increases. The standard deviation and the power spectra of the pressure fluctuations are also discussed. Measurements with several initial static bed heights were taken in order to analyze the influence of the initial bed mass inventory over the effect of the distributor rotation on the bed hydrodynamics. The rotation of the distributor allows to fluidize very shallow beds which had a jet structure when the static distributor is used; the effect of the rotation becomes globally less important for deeper beds. These characteristics show that adjusting the rotational speed it is possible to change the gas velocity needed to fluidize the bed, facilitating the fluidization and maintaining a uniform fluidization. Once the global fluid dynamic behavior is known, local characteristics are analyzed. Differential pressure and optical probe measurements were carried out in order to obtain the size and velocity of the bubbles rising in the bed. Results obtained from the two types of probes were compared in Chapter 5. The probability distributions of bubble pierced length and velocity were obtained applying the Maximum Entropy Method. The minimum bubble pierced length that it is possible to measure using intrusive probes, due to their finite size, has been introduced as a constraint in the derivation of the size distribution equations. The probability density function of bubble diameter was inferred applying statistical tools to the pierced length experimental data. Results on bubble size obtained from pressure and optical probes have been found to be very similar, although optical probes provide more local measurements and can be used even at very low heights in the bed, near the distributor. The Maximum Entropy Method has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds: the distribution shape does not have to be pre-established, the number of samples required is lower than in other methods and the backward transformation procedure is avoided. The effect of the distributor rotation on the bubble size, bubble passage frequency and bubbles distribution at different radial and axial positions in the bed was studied with the optical probes and the results are presented in Chapter 6. A simple theoretical expression is obtained in order to analyze the centrifugal acceleration influence on the bubble when it detaches from the distributor. This analysis points out that the centrifugal acceleration imparted by the distributor rotation causes the decrease of the initial bubble radius. The experimental results show that the bubbles are smaller when the rotating distributor is used if the excess gas for the static and rotating configuration is similar. The bubble size radial profile indicates that when the distributor rotates, the diameter of the bubbles close to the bed walls is smaller due to the effect of a higher centrifugal acceleration. The distributor rotation also promotes the more homogenous distribution of the bubbles over the bed surface. At higher axial positions even smaller bubbles are found for the rotating case. This may be due to a lower coalescence rate of the bubbles when the distributor rotates as the rotation may break the channeling in the bed

Fluidization of Group B particles with a rotating distributor

A novel rotating distributor fluidized bed is presented. The distributor is a rotating perforated plate, with 1% open-area ratio. This work evaluates the performance of this new design, considering pressure drop, Δp, and quality of fluidization. Bed fluidization was easily achieved with the proposed device, improving the solid mixing and the quality of fluidization. In order to examine the effect of the rotational speed of the distributor plate on the hydrodynamic behavior of the bed, minimum fluidization velocity, Umf, and pressure fluctuations were analyzed. Experiments were conducted in the bubbling free regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart's classification. The pressure drop across the bed and the standard deviation of pressure fluctuations, σp, were used to find the minimum fluidization velocity, Umf. A decrease in Umf is observed when the rotational speed increases and a rise in the measured pressure drop was also found. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities. Measurements with several initial static bed heights were taken, in order to analyze the influence of the initial bed mass inventory, over the effect of the distributor rotation on the bed hydrodynamics.Publicad

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, Uc, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor platesPublicad

Study of ash deposition during coal combustion under oxyfuel conditions

This paper presents a comparative study on ash deposition of two selected coals, Russian coal and lignite, under oxyfuel (O₂/CO₂) and air combustion conditions. The comparison is based on experimental results and subsequent evaluation of the data and observed trends. Deposited as well as remaining filter ash (fine ash) samples were subjected to XRD and ICP analyses in order to study the chemical composition and mineral transformations undergone in the ash under the combustion conditions. The experimental results show higher deposition propensities under oxyfuel conditions; the possible reasons for this are investigated by analyzing the parameters affecting the ash deposition phenomena. Particle size seems to be larger for the Russian coal oxy-fired ash, leading to increased impaction on the deposition surfaces. The chemical and mineralogical compositions do not seem to differ significantly between air and oxyfuel conditions. The differences in the physical properties of the flue gas between air combustion and oxyfuel combustion, e.g. density, viscosity, molar heat capacity, lead to changes in the flow field (velocities, particle trajectory and temperature) that together with the ash particle size shift seem to play a role in the observed ash deposition phenomenaThe work presented was financially supported by the RFCS projects BOFCOM and ECOSCRUB, the Dutch National project CATO2, and the Dutch National program EOS-LT, Consortium Biomass Co-firing. The fine work of Peter Heere in operating the reactor is highly acknowledgedPublicad

Experimental heat transfer coefficients between a surface and fixed and fluidized beds with PCM

This work presents an experimental study to determine the capacity of a phase change material (PCM) in granular form to be used in fixed and bubbling fluidized beds for thermal energy storage. The experimental measurements are focused on determination of the heat transfer coefficient between a heated surface immersed in the bed and the granular PCM. The flow rate is varied to quantify its influence on the heat transfer coefficient. The PCM used is Rubitherm GR50 with a phase change temperature of approximately 50° C. The PCM is available in two different particle sizes, 0.54 mm and 1.64 mm, of which the finer is used in the fluidized bed and the coarser is used in the fixed bed. In addition, the results obtained for the PCM are compared with the heat transfer coefficients measured for sand, a material commonly used for thermal storage. In comparing the heat transfer coefficients for fixed and fluidized beds, the heat transfer coefficients in the fluidized bed with PCM are nearly three times higher than those for the fixed bed at the same gas flow rate. This increase in the heat transfer is a result of two main factors: first, the continuous renewal of PCM particles from the heated surface when they are fluidized, and second, the large quantities of energy in latent form absorbed by the PCM. In the fixed bed there is no renovation of particles, consequently only a small percentage of particles are able to change its phase. Hence, there is no increase in the heat transfer coefficient due to this fact.This work was partially funded by the Spanish Government (Project ENE2010-15403), theRegional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662).Publicad

Thermal energy storage in a fluidized bed of PCM

The objective of the present work was to research the storage behavior of a fluidized bed filled with a granular phase change material (PCM) with a small particle diameter (d(p) = 0.54 mm). The performance of the fluidized bed was compared to that of well-known storage methods such as fluidized beds with sand and packed beds based of sand and PCM. For this purpose, heating experiments were conducted in a cylindrical bed with air as the working fluid. The influence of the bed height and flow rate on the storage and recovery efficiencies of the fluidized bed of PCM was analyzed. Additionally, the stability of the PCM during various charging-discharging cycles was studied. The results indicate that this PCM is an alternative material that can be used in fluidized bed systems to increase the efficiency of storing thermal energy in the form of latent heat. Under the experimental conditions tested in this study, higher charging efficiencies were observed for fixed and fluidized beds based on PCM than those of sand. High gas velocity and low bed height shorten the charging time but also reduce the charging efficiency. The cycling test shows that the PCM is stable under bubbling conditions up to 15 cycles, which corresponds to approximately 75 h of continuous operation.This work was founded partially by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662).Publicad

Study on ash deposition under oxyfuel combustion of coal/biomass blends

Combustion in an O₂/CO₂mixture (oxyfuel) has been recognized as a promising technology for CO₂capture as it produces a high CO₂concentration flue gas. Furthermore, biofuels in general contribute to CO₂reduction in comparison with fossil fuels as they are considered CO₂neutral. Ash formation and deposition (surface fouling) behavior of coal/biomass blends under O₂/CO₂combustion conditions is still not extensively studied. Aim of this work is the comparative study of ash formation and deposition of selected coal/biomass blends under oxyfuel and air conditions in a lab scale pulverized coal combustor (drop tube). The fuels used were Russian and South African coals and their blends with Shea meal (cocoa). A horizontal deposition probe, equipped with thermocouples and heat transfer sensors for on line data acquisition, was placed at a fixed distance from the burner in order to simulate the ash deposition on heat transfer surfaces (e.g. water or steam tubes). Furthermore, a cascade impactor (staged filter) was used to obtain size distributed ash samples including the submicron range at the reactor exit. The deposition ratio and propensity measured for the various experimental conditions were higher in all oxyfuel cases. The SEM/EDS and ICP analyses of the deposit and cascade impactor ash samples indicate K interactions with the alumina silicates and to a smaller extend with Cl, which was all released in the gas phase, in both the oxyfuel and air combustion samples. Sulfur was depleted in both the air or oxyfuel ash deposits. S and K enrichment was detected in the fine ash stages, slightly increased under air combustion conditions. Chemical equilibrium calculations were carried out to facilitate the interpretation of the measured data; the results indicate that temperature dependence and fuels/blends ash composition are the major factors affecting gaseous compounds and ash composition rather than the combustion environment, which seems to affect the fine ash (submicron) ash composition, and the ash deposition mechanismsThe research work reported in this paper was partly carried out with the financial support from the RFCS contract number RFCRCT- 2006-00010. The very fine work done by Peter Heere in carrying out the experiments is highly acknowledgedPublicad

Energy storage with PCM in fluidized beds: Modeling and experiments

In recent years, the development of phase change materials (PCMs) has introduced new ways to increase the energy storage capacity of a system due to the high latent heat and high storage density of these materials. The aim of this work is to model the charging process of a fluidized bed with PCMs operating as an energy storage device. The temperature in the bed during the charging process of the fluidized bed has been modeled using the two phase theory of fluidization. The dense phase is taken to be perfectly mixed, and the bubble phase is taken to be in plug flow. The numerical model presented takes into account the fact that the phase change process of the bed material occurs over a temperature range and also estimates the energy stored in the wall of the bed and in the distributor plate. The energy equation of the dense phase is numerically solved in enthalpy form, considering the dependence of enthalpy on temperature for phase changes occurring over a range of temperatures. The model's validity is verified against experimental data for two granular materials: sand, a typical material used in fluidized beds, and a granular PCM with a mean particle diameter of 0.54 mm and a phase change temperature of approximately 50 °C. For the sand, the temperature profiles obtained numerically perfectly agree with the values measured experimentally. In the case of the granular PCM, the fitting of the curves is improved when slow and similar heating rates are selected for the experiments and for the DSC measurements used to determine the PCM enthalpy&-temperature curve.This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662).Publicad

Defluidization and agglomeration of a fluidized bed reactor during Cynara cardunculus L. gasification using sepiolite as a bed material

This work studies the defluidization time and the agglomerate generation in a bubbling fluidized bed (BFB) reactor during Cynara cardunculus L. gasification using, separately, two different bed materials, silica sand and sepiolite 〖(MG〗_8 〖Si〗_12 O_30 (OH)_4 〖(OH〗_2)(_4^)8 H_2). The high adsorption capacity and the elemental composition of the sepiolite make it suitable as an alternative bed material in order to reduce agglomeration. Experiments were performed on a stainless steel lab-scale BFB reactor operating with air as a gasifying agent at different air excess ratios (u/umf). A quartz reactor was alternatively used for the visualization of bed material and biomass during gasification, allowing one to observe the agglomerate formation process. Pressure signals were analyzed both in time and frequency domain to determine the defluidization time. Furthermore, the shape and size of the bed material after the experiments were evaluated. Higher defluidization times in the case of sepiolite were measured. Particle sizes were affected by the type of bed material and the air excess and agglomerates of different shapes were formed for sepiolite and silica sand.Publicad

Modeling the heat transfer coefficient between a surface and fixed and fluidized feds with phase change material

The objective of this work is to model the heat transfer coefficient between an immersed surface and fixed and bubbling fluidized beds of granular phase change material (PCM). The model consists of a two-region model with two different voidages in which steady and transient conduction problems are solved for the fixed and fluidized bed cases, respectively. The model is validated with experimental data obtained under fixed and fluidized conditions for sand, a common material used in fixed and fluidized beds for sensible heat storage, and for a granular PCM with a phase change temperature of approximately 50 degrees C. The superficial gas velocity is varied to quantify its influence on the convective heat transfer coefficient for both the materials. The model proposed for the PCM properly predicts the experimental results, except for high flow rates, which cause the contact times between the surface and particles to be very small and lead the model to overpredict the results.This work was partially funded by the Spanish Government (Project No. ENE2010-15403), the regional Government of Castilla-La Mancha (Project No. PPIC10-0055-4054), and Castilla-La Mancha University (Project No. GE20101662)