Simulation and parameter optimization of fluidized-bed and biomass gasification
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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.
Has partsAnnexure A: Literature review: Compositional analysis of pyrolysis products and reaction kinetics for modeling of Biomass Gasification
Paper A: Bandara, J., Eikeland, M.S. & Moldestad, B.M.E.: Analyzing the Effects of Particle Density, Size and Size Distribution for Minimum Fluidization Velocity with Eulerian-Lagrangian CFD Simulation. Proceedings of the 58th International Conference of Scandinavian Simulation Society, SIMS 2017, p. 60-65, 2017. http://dx.doi.org/10.3384/ecp1713860
Paper B: Bandara, J., Nielsen, H.K., Moldestad, B.M.E. & Eikeland, M.S.: Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized Beds. Proceedings of the 59th International Conference of Scandinavian Simulation Society, SIMS 2018, p. 334-341, 2018. http://dx.doi.org/10.3384/ecp18153334
Paper C: Bandara, J., Thapa, R., Nielsen, H.K., Moldestad, B.M.E. & Eikeland, M.S.: Circulating fluidized bed reactors – part 01: analyzing the effect of particle modelling parameters in computational particle fluid dynamic (CPFD) simulation with experimental validation Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized Beds. Particulate Science and Technology, 39(2), (2021), 223-236. https://doi.org/10.1080/02726351.2019.1697773
Paper D: Bandara, J., Moldestad, B.M.E. & Eikeland, M.S.: Analyzing the Effects of Geometrical and Particle Size Uncertainty in Circulating Fluidized Beds using CPFD Simulation. Proceedings of the 60th International Conference of Scandinavian Simulation Society, SIMS 2019, p. 182-189, 2019. https://doi.org/10.3384/ecp20170182
Paper E: Bandara, J., Jayarathna, C., Thapa, R., Nielsen, H.K., Moldestad, B.M.E. & Eikeland, M.S.: Loop seals in circulating fluidized beds – Review and parametric studies using CPFD simulation. Chemical Engineering Science, 227, (2021), 115917. https://doi.org/10.1016/j.ces.2020.115917
Paper F: Bandara, J., Moldestad, B.M.E. & Eikeland, M.S.: Analysis of the Effect of Steam-to-Biomass Ratio in Fluidized Bed Gasification with Multiphase Particle-in-cell CFD Simulation. Proceedings of the 58th International Conference of Scandinavian Simulation Society, SIMS 2017, p. 54-59, 2017. http://dx.doi.org/10.3384/ecp1713854
Paper G: Bandara, J., Moldestad, B.M.E. & Eikeland, M.S.: Analysing the effect of temperature for steam fluidized-bed gasification of biomass with MP-PIC simulation. International Journal of Energy and Environment, 9(6), (2018), 529-542.
Paper H: Bandara, J., Jaiswal, R., Nielsen, H.K., Moldestad, B.M.E. & Eikeland, M.S.: Air gasification of biomass in bubbling fluidized bed – short review and experimental studies. Manuscript submitted to Chemical Engineering Science.
Paper I: Moradi, A., Samani, N.A., Mojarrad, M., Sharfuddin, M., Bandara, J. & Moldestad, B.M.E.: Experimental and Computational studies of circulating fluidized bed. International Journal of Energy Production and Management, 5(4), (2020), 302-313. https://doi.org/10.2495/EQ-V5-N4-302-313