Computational Analysis of the Hydrodynamic Behavior for Different Air Distributor Designs of Fluidized Bed Gasifier

Fluidized bed gasification has proven to be an appropriate technique for converting various biomass feedstocks into helpful energy. Air distributor plate design is one of the critical factors affecting the thermochemical conversion performance of fluidized bed gasifiers. The present study is proposed to investigate the mixing pattern and pressure drop across different configurations of air distributors using a two-fluid model (TFM) of finite volume method-based solver ANSYS FLUENT. The pressure drop across the bed and mixing pattern have been investigated through qualitative and quantitative analysis of CFD results using three diverse distributor plate designs: perforated plate, 90° slotted plate, and 45° swirling slotted plate. The pressure drop by employing the perforated distributor plate reveals the highest pressure drop due to the smallest open area ratio. However, the pressure drop in the case of 90° slotted plate is found to be 7% and 4% lesser than perforated and 45° slotted plate respectively due to a smaller velocity head developed through the wider open area of the straight slotted plates. The distributor design configuration having a 45° slotted plate exhibits considerable pressure drop compared to the 90° slotted plate due to the longer path length of the slot. Numerical pressure drop results across the bed with different types of distributor plates prove reasonable agreement with the experimental results available in the literature. Mixing behavior in perforated distributor plates exhibits lower portion solid volume fraction of around 0.58. However, it falls rapidly as go up the riser (7.7% of column height); 90° slotted plate shows bottom region solid volume fraction of around 0.5. In addition, it exhibits an even broader range of sand volume fraction and column height (13.46% of column height). Finally, the 45° distributor plate reveals the highest range of volume fraction through the riser height (17.3% of column height), indicating the better mixing characteristics of the fluidized zone.

Computational and Experimental Study on Pressure Drop in a Fluidised Bed with Different Air Distributor Designs

Fluidised bed combustion (FBC) has been recognised as a suitable technology for converting a wide variety of fuels into energy. In a fluidised bed, the air is passed through a bed of granular solids resting on a distributor plate. Distributor plate plays an essential role as it determines the gas-solid movement and mixing pattern in a fluidised bed. It is believed that the effect of distributor configurations such as variation of free area ratio and air inclination angle through the distributor will affect the operational pressure drop of the fluidised bed. This paper presents an investigation on pressure drop in fluidised bed without the presence of inert materials using different air distributor designs; conventional perforated plate, multi-nozzles, and two newly proposed slotted distributors (45° and 90° inclined slotted distributors). A 3-dimensional Computational Fluid Dynamics (CFD) model is developed and compared with the experimental results. The flow model is based on the incompressible isothermal RNG k-epsilon turbulent model. In the present study, systematic grid-refinement is conducted to make sure that the simulation results are independent of the computational grid size. The non-dimensional wall distance, is examined as a key factor to verify the grid independence by comparing results obtained at different grid resolutions. The multi-nozzles distributor yields higher distributor pressure drop with the averaged maximum value of 749 Pa followed by perforated, 45° and 90° inclined distributors where the maximum pressure drop recorded to be about one-fourth of the value of the multi-nozzles pressure drop. The maximum pressure drop was associated with the higher kinetic head of the inlet air due to the restricted and minimum number of distributor openings and low free area ratio. The results suggested that low-pressure drop operation in a fluidised bed can be achieved with the increase of open area ratio of the distributor.

Development of a Sawdust Fluidized Bed Gasifier: Design and Fabrication

Aims: This research involves the development of a sawdust fired fluidized bed reactor for the production of synthetic gas for domestic cooking. Study Design: A sawdust fired fluidized bed reactor using AutoCAD inventor. Place and Duration of Study: Department of Mechanical Engineering, Federal University of Technology, Akure Ondo state Nigeria. Methodology: The reactor consists of a hopper, rolled mild steel plate frame lined with clay which forms the frame, air distributor plate and five radially spaced tuyeres. The reactor is fed with pelletized sawdust, retain heat within it and maintain a temperature of 50⁰C at the external surface to minimize burns. Results: Air was forced into the plenum, after which the air distributor plate evenly distributed jets of air in the bed resulting in complete and incomplete combustion. Combustible gas was produced after 30 minutes and used to boil water. Conclusion: The sawdust fired fluidized bed reactor is recommended for domestic household use.

Acoustic Measurement in a Rectangular Bubble Column to Determine Bubble Size and Interfacial Area for Aqueous Solutions of Ethylene Glycol

Bubble sizes in bubble column affect the bubble induced mixing of phases, interfacial area and transfer processes. Acoustic technique is used to measure bubble size distribution in a rectangular bubble column of cross section 0.2m x 0.02m for air sparged into water and aqueous solutions of ethylene glycol. Five condenser mikes at intermediate distance of 0.05 m measured above the distributor plate were used to find out the variation of bubble size as the bubbles move up. Sauter-mean bubble diameter and specific interfacial area were estimated from bubble size distribution at several superficial air velocity, static bed height, distance above the distributor plate and ethylene glycol concentration. The BSD exhibited mono-modal distribution and indicated non-uniform homogeneous bubbling regime. Sauter-mean bubble diameter is independent of superficial gas velocity, static bed height and concentration of EG, although, the values were higher than that for air-water system. Sauter-mean bubble diameter decreases as the bubbles move up indicating bubble breakup to take place once the bubbles leave the sparger. The value of interfacial area increases as the static bed height decreases and distance above the distributor plate increases. For air-ethylene glycol solution the values of specific interfacial area are about 200% higher than that observed in case of air-water system. The acoustic technique may be used to measure local values of bubble sizes and specific interfacial area.

Effect of Novel Swirling Perforated Distributor on Fluid Dynamic Characteristics of Circulating Fluidized Bed Riser

The present work is associated with Circulating Fluidized Bed (CFB) technology, related to the energy sector. The applications of CFB technology span across wide range of areas i.e. boiler, gasifier, combustor, dryer, etc. In the present paper, CFD simulations using ANSYS-Fluent 14.5 were performed to study the effect of novel swirling perforated distributor on fluid dynamics characteristics like pressure drop along the riser and distributor, suspension density variations along the riser of the Circulating Fluidized Bed (CFB). The simulation results were also used to compare qualitatively and quantitatively the dead-zone formations in the four corners of riser just above the distributor plate for swirl and normal distributor plates. The riser alongwith distributor was modeled using Pro-E 5.0, and it was meshed in ICEM CFD 14.5. Post processing simulations were performed using Fluent 14.5. 3D CFD simulations were performed on the CFB riser of cross section 0.15 m × 0.15 m and height 2.85 m. RNG k-ε model was used for turbulence modeling. Eulerian model with Syamlal-O’Brien phase interaction scheme was used to simulate the two phase flow (air + sand mixture flow). RNG k-ε model was used for turbulence modeling of the flow inside the riser. The RNG turbulence model has a calculation for effective viscosity. Modeling and simulations were performed for normal perforated distributor plate and results obtained were compared with available experimental data. In this way, after validation of computational results, further CFD simulations were performed for novel geometry of swirl distributor plate. It is observed that suspension density (particles’ concentration) was more in the middle and upper region of the riser in case of swirl distributor plate. However, pressure drop across the distributor plate increased in the case of novel swirl distributor plate. The objective of significant reduction in the dead-zone formation just above the normal distributor plate was achieved through novel swirl distributor, which in-turn is expected to increase particles’ participation in combustion which takes place in oxygen rich middle portion of CFB riser and subsequently increases heat transfer rate in the CFB riser.

Characterizing Jetting in an Acoustic Fluidized Bed Using X-Ray Computed Tomography

Understanding the jetting phenomena near the gas distributor plate in a fluidized bed is important to gas-solids mixing, heat and mass transfer, and erosion on any bed internals, which can all affect the performance of the bed. Moreover, acoustic vibration in a fluidized bed can be used to enhance the fluidization quality of particulate matter. Characterizing the jetting structure using X-ray computed tomography in a 3D fluidized bed, with and without acoustic intervention, is completed in this study. A 10.2 cm ID fluidized bed filled with glass beads, with material density of 2500 kg/m3 and particles sizes ranging between 212–600 μm, is used in these experiments. X-ray computed tomography (CT) imaging is used to determine local time-average gas holdup. From this information, qualitative characteristics of the hydrodynamic structure of the multiphase flow system are determined. Local time-average gas holdup images of the fluidized bed under acoustic intervention at a high superficial gas velocity show that jets produced near the aeration plate merge with other jets at a higher axial position of the bed compared to the no acoustic condition. Acoustic fluidized beds also have a fewer number of active jets than the no acoustic fluidized bed, which allowed for a more homogeneous gas holdup region deep into the bed. Hence, the acoustic presence has a significant effect on the jetting phenomena near the distributor plate of the fluidized bed.

Eulerian-Eulerian Modeling of Multiple Jet Interactions for Different Distributor Plates

The hydrodynamics of fluidized beds involving gas and particle interactions are very complex, and must be carefully considered when modeling such a system using computational fluid dynamics (CFD). One of the issues involved is the interaction of multiple jets that develop above the distributor plate, which impacts the uniformity of fluidization. Using the common approach of a uniform gas velocity inlet boundary condition may not accurately represent distributor plates with nonuniform holes. The numerical approach will use a multi-fluid Eulerian-Eulerian CFD modeling to predict and examine the hydrodynamics of interacting jets. The present work will model a quasi-two-dimensional (2D) fluidized bed to compare with a corresponding experimental setup designed to examine multiple jet interactions for a distributor plate with 9 holes. Two-dimensional and three-dimensional simulations of the quasi-2D bed will be compared with experiments by investigating solid volume fraction distributions and solid flux distributions with agreeable results qualitatively. Use of experimental data in determining the amount of mass fluidizing will also be assessed using CFD. The efficacy of the new approach in capturing the hydrodynamics is demonstrated.