Analyzing the effects of particle density, size, size distribution and shape for minimum fluidization velocity with Eulerian-Lagrangian CFD simulation

Fluidized bed reactor systems are widely used due to excellent heat and mass transfer characteristics followed by uniform temperature distribution throughout the reactor volume. The importance of fluidized beds is further demonstrated in high exothermic reactions such as combustion and gasification where fluidization avoids the hot spot and cold spot generation. A bed material, such as sand or catalyst, is normally involved in fluidized bed combustion and gasification of biomass. Therefore, it is vital to analyze the hydrodynamics of bed material, especially the minimum fluidization velocity, as it governs the fluid flowrate into the reactor system. There are limitations in experimental investigations of fluidized beds such as observing the bed interior hydrodynamics, where CFD simulations has become a meaningful way with the high computer power. However, due to the large differences in scales from the particle to the reactor geometry, complex interface momentum transfer and particle collisions, CFD modeling and simulation of particle systems are rather difficult. Multiphase particle-in-cell method is an efficient version of Eulerian-Lagrangian modeling and Barracuda VR commercial package was used in this work to analyze the minimum fluidization velocity of particles depending on size, density and size distribution. Wen-YU-Ergun drag model was used to model the interface momentum transfer where default equations and constants were used for other models. The effect of the particle size was analyzed using monodispersed Silica particles with diameters from 400 to 800 microns. Minimum fluidization velocity was increased with particle diameter, where it was 0.225 m/s for the 600 microns particles. The density effect was analyzed for 600 microns particles with seven different density values and the minimum fluidization velocity again showed proportionality to the density. The effect of the particle size distribution was analyzed using Silica. Particles with different diameters were mixed together according to pre-determined proportions as the final mixture gives a mean diameter of 600 microns. The 600 microns monodispersed particle bed showed the highest minimum fluidization velocity. However, some particle mixtures were composed with larger particles up to 1000 micron, but with a fraction of smaller particles down to 200 microns at the same time. This shows the effect of strong drag from early fluidizing smaller particles. The only variability for pressure drop during packed bed is the particle size and it was clearly observed in all three cases

Analysis of the effect of steam-to-biomass ratio in fluidized bed gasification with multiphase particle-in-cell CFD Simulation

Biomass has been identified as a key renewable energy source to cope with upcoming environmental challenges. Gasification of biomass is becoming interested in large scale operation, especially in synthesis of liquid fuels. Bubbling and circulating fluidized bed gasification technology has overrun the interest over fixed bed systems. CFD studies of such reactor systems have become realistic and reliable with the modern computer power. Gasifying agent, temperature and steam or air to biomass ratio are the key parameters, which are responsible for the synthesis gas composition. Therefore, multiphase particle-in-cell CFD modeling was used in this study to analyze the steam to biomass, S/B, ratio in fluidized bed gasification. Due to the complexity of the full loop simulation of dual circulating fluidized bed reactor system, only the gasification reactor was considered in this study. Predicted boundary conditions were implemented for the particle flow from the combustion reactor. The fluidization model was validated against experimental data in beforehand where Wen-Yu-Ergun drag model was found to be the best. The effect of the S/B ratio was analyzed at a constant steam temperature of 1073K and a steam velocity of 0.47 m/s. Four different S/B of 0.45, 0.38, 0.28 and 0.20 were analyzed. The biomass was considered to be in complete dry condition where single step pyrolysis reaction kinetics was used. Each gasification simulation was carried out for 100 seconds. 8% reduction of hydrogen content from 57% to 49% and 17% increment of carbon monoxide from 13% to 30% were observed when the S/B was reduced from 0.45 to 0.20. Countable amounts of methane were observed at S/B of 0.28 and 0.20. The lower heating value of the product gas increased from 10.1 MJ/kg to 12.37 MJ/kg and the cold gas efficiency decreased from 73.2% to 64.6% when the S/B was changed from 0.45 to 0.20. The specific gas production rate varied between 1.64 and 1.04 Nm3/kg of biomass

Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized Beds

Fluidized bed technology is broadly applied in industry due to its distinct advantages. CFD simulation of fluidized beds is still challenging compared to single-phase systems and needs extensive validation. Multiphase particle-in-cell is a recently developed lagrangian modeling technique and this work is devoted to analyze the sensitivity of grid size, time step, and model parameters, which are the essences of accurate results. Barracuda VR 17.1.0 commercial CFD package was used in this study. 500µm sand particles and air was used as the bed material and fluidization gas respectively. Five different grids, having 27378, 22176, 16819, 9000 and 6656 computational cells were analysed, where five different time steps of 0.05, 0.01, 0.005, 0.001 and 0.0005 were used for each grid. One velocity step was maintained for 8 seconds. The bed pressure drop at packed bed operation was high for simulations with reduced time steps while equal pressure drops were observed during fluidization for all time steps. Time steps of 0.0005s and 0.001s and 0.005s produced equal result of 0.15 m/s for minimum fluidization velocity, irrespective of the grid size. The results from time steps of 0.05 and 0.01 are converged to the results from time steps of 0.005 and 0.001 by increasing simulation time per one velocity step.Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized BedsacceptedVersionpublishedVersionNivå

Simulation and parameter optimization of fluidized-bed and biomass gasification

This thesis gives an insight to experimental studies and computational particle fluid dynamic (CPFD) simulations of fluidized bed and fluidized bed gasification reactors. CPFD models were validated against experimental data and used in subsequent parametric studies. The deviation of simulation results were discussed with possible uncertainties related to both the experiments and the CPFD model setup. Bubbling fluidized bed cold-rig, circulating fluidized bed cold-rig and bubbling fluidized bed gasification reactor were used for the experimental studies. Barracuda VR® 17.3.0 commercial CFD platform was used for the simulations. Understanding of minimum fluidization velocity (MFV) is the prime importance of any fluidized bed study. Sand particles were sieved in 8 different narrow size ranges from 200µm to 1180µm and the MFVs were calculated by plotting experimentally measured bed pressure drop data against superficial gas velocity. The change of MFV was not exactly liner over tested particle sizes and instead, different size groups showed linear relationships separately. A cold-rig of circulating fluidized bed (CFB) with a riser, cycle and a loopseal was constructed at USN to reinforce the CPFD model validation. Contribution of the standpipe aeration in controlling the rate of particle circulation was slightly over 60%, whereas bottom aeration was necessary to put the loopseal in operation. As the gasification reactor was equipped with electrically heated walls, the experiments were designed at lower equivalence ratios (ER) between 0.1-0.16. At lower ER, char particles accumulated in the reactor and the ER was needed to increase up to 0.16 for a steady char concentration at 800ºC. Gasification of grass pellets was not successful due to clinker formation and low carbon conversion. Wood chips and wood pellets showed reasonable results and the temperature was needed to maintain around 800ºC for an efficient carbon conversion above 70%. CPFD simulation with Wen-Yu-Ergun blended construction, as the fluid drag model, could predict the MFV with a 3.5% error for 200-255µm particles. The calculated bed expansion at minimum fluidization was lower in CPFD simulation than experiments. Optimization of the particle modeling parameters was necessary for CPFD simulation of the CFB cold-reactor to achieve the rate of particle circulation observed during the experiments. The pressure constant of the particle stress model was the most affecting parameter followed by particle-wall momentum retention coefficients. The particle hydrodynamics at the loopseal, especially the bubble formation at the standpipe, and core annulus structure in the riser were illustrated using CPFD simulation graphical data. The optimized model parameters were reviewed with follow up simulations for the CFB geometry at USN. The results confirmed the reproducibility of optimized parameters. The predicted gas composition of H2, CO and CH4 from the CPFD simulation for air-blown gasification of biomass in bubbling fluidized bed showed a close match with the experiments. However, the predicted composition of CO2 was higher than the experiments while lower for N2. Local temperature at the biomass feeding point is, however, possible to drop sharply due to endothermic moisture evaporation and pyrolysis reactions, which will in turn cause fluctuating pyrolysis composition. Therefore, high prediction of CO2 with simultaneous low prediction of N2, could be due to the under-prediction of tar generation during the pyrolysis step

Fever of unknown origin in a male patient with systemic lupus erythematosus

Background: Systemic lupus erythematosus (SLE) is an inflammatory autoimmune disorder which is uncommon in men. It has a wide variety of clinical presentations. Case Report: We report a 21-year-old male presented with one month history of fever, loss of appetite, weight loss and reduced hair growth with an examination revealing an oral ulcer, cervical and axillary lymphadenopathy simulating hematological malignancy. Investigations showed pancytopenia, positive anti-nuclear factor and double-stranded DNA, high erythrocyte sedimentation rate with normal C-reactive protein levels and hypocomplementemia. The diagnosis of systemic lupus erythematosus was made and treatment with oral prednisolone conferred a dramatic clinical and biochemical improvement within one week. Conclusion: In the evaluation of fever of unknown origin, one should be guided by the presenting symptoms and signs of a patient and even though uncommon, SLE is a worthwhile diagnosis to investigate even in a male patient if the clinical picture is suggestive

Analysis of the effect of steam-to-biomass ratio in fluidized bed gasification with multiphase particle-in-cell CFD Simulation

Biomass has been identified as a key renewable energy source to cope with upcoming environmental challenges. Gasification of biomass is becoming interested in large scale operation, especially in synthesis of liquid fuels. Bubbling and circulating fluidized bed gasification technology has overrun the interest over fixed bed systems. CFD studies of such reactor systems have become realistic and reliable with the modern computer power. Gasifying agent, temperature and steam or air to biomass ratio are the key parameters, which are responsible for the synthesis gas composition. Therefore, multiphase particle-in-cell CFD modeling was used in this study to analyze the steam to biomass, S/B, ratio in fluidized bed gasification. Due to the complexity of the full loop simulation of dual circulating fluidized bed reactor system, only the gasification reactor was considered in this study. Predicted boundary conditions were implemented for the particle flow from the combustion reactor. The fluidization model was validated against experimental data in beforehand where Wen-Yu-Ergun drag model was found to be the best. The effect of the S/B ratio was analyzed at a constant steam temperature of 1073K and a steam velocity of 0.47 m/s. Four different S/B of 0.45, 0.38, 0.28 and 0.20 were analyzed. The biomass was considered to be in complete dry condition where single step pyrolysis reaction kinetics was used. Each gasification simulation was carried out for 100 seconds. 8% reduction of hydrogen content from 57% to 49% and 17% increment of carbon monoxide from 13% to 30% were observed when the S/B was reduced from 0.45 to 0.20. Countable amounts of methane were observed at S/B of 0.28 and 0.20. The lower heating value of the product gas increased from 10.1 MJ/kg to 12.37 MJ/kg and the cold gas efficiency decreased from 73.2% to 64.6% when the S/B was changed from 0.45 to 0.20. The specific gas production rate varied between 1.64 and 1.04 Nm3/kg of biomass

Analyzing the effects of particle density, size, size distribution and shape for minimum fluidization velocity with Eulerian-Lagrangian CFD simulation

Fluidized bed reactor systems are widely used due to excellent heat and mass transfer characteristics followed by uniform temperature distribution throughout the reactor volume. The importance of fluidized beds is further demonstrated in high exothermic reactions such as combustion and gasification where fluidization avoids the hot spot and cold spot generation. A bed material, such as sand or catalyst, is normally involved in fluidized bed combustion and gasification of biomass. Therefore, it is vital to analyze the hydrodynamics of bed material, especially the minimum fluidization velocity, as it governs the fluid flowrate into the reactor system. There are limitations in experimental investigations of fluidized beds such as observing the bed interior hydrodynamics, where CFD simulations has become a meaningful way with the high computer power. However, due to the large differences in scales from the particle to the reactor geometry, complex interface momentum transfer and particle collisions, CFD modeling and simulation of particle systems are rather difficult. Multiphase particle-in-cell method is an efficient version of Eulerian-Lagrangian modeling and Barracuda VR commercial package was used in this work to analyze the minimum fluidization velocity of particles depending on size, density and size distribution. Wen-YU-Ergun drag model was used to model the interface momentum transfer where default equations and constants were used for other models. The effect of the particle size was analyzed using monodispersed Silica particles with diameters from 400 to 800 microns. Minimum fluidization velocity was increased with particle diameter, where it was 0.225 m/s for the 600 microns particles. The density effect was analyzed for 600 microns particles with seven different density values and the minimum fluidization velocity again showed proportionality to the density. The effect of the particle size distribution was analyzed using Silica. Particles with different diameters were mixed together according to pre-determined proportions as the final mixture gives a mean diameter of 600 microns. The 600 microns monodispersed particle bed showed the highest minimum fluidization velocity. However, some particle mixtures were composed with larger particles up to 1000 micron, but with a fraction of smaller particles down to 200 microns at the same time. This shows the effect of strong drag from early fluidizing smaller particles. The only variability for pressure drop during packed bed is the particle size and it was clearly observed in all three cases

Analysing the effect of temperature for steam fluidized-bed gasification of biomass with MP-PIC simulation

Gasification in fluidized beds is an outstanding technology in biomass to energy conversion. The multiphase particle-in-cell modelling has reduced the computational time related to CFD simulations of dense gas-solid systems like fluidized bed gasification. Barracuda VR commercial CFD package was used to analyse the effect of reactor temperature in steam gasification of biomass. The product gas composition, lower heating value and the cold gas efficiency were compared for steam at 873K, 973K and 1073K. The steam-to-biomass ratio was maintained at a constant value of 0.45. The hydrogen content of the product gas changed from 36% to 57% as the temperature was increased from 873K to 1073K whereas the carbon monoxide content changed from 33% to 13%. The lower heating value and the cold gas efficiency changed from 10.4 MJ/kg to 10.1 MJ/kg and 76.6% to 73.2% respectively within the same temperature range. The formation of tar was not modelled and the gas composition showed high sensitivity towards the reactor temperature