Optimization of flow behavior in biomass gasification reactor
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Dual fluidized bed reactor for steam gasification of biomass is a promising technology and can be used in Combined Heat and Power (CHP) production. The producer gas from the reactor can have a calorific value up to 14 MJ/Nm3. The technology is well known for the comparative high efficiency and is neutral to CO2 emission. Although the dual fluidized bed reactor has gained advantages compared to corresponding reactors, the technology has to be improved to become competitive in the world energy market. The current project is focused on optimization of flow behavior and reaction kinetics in the gasification reactor to improve the reactor performance. The study of fluid dynamics and thermo-chemical behavior is performed using experimental and computational methods. Computational Fluid Dynamic (CFD) and Computational Particle Fluid Dynamic (CPFD) models are used in the study. The dual fluidized bed gasification technology consists of a bubbling fluidized bed reactor and circulating fluidized bed reactor. The experimental and computational studies are carried out for both types of reactors. A CFD model is validated against the experimental measurements in a cold flow model of bubbling fluidized bed reactor. Good agreements were obtained between computational and experimental minimum fluidization velocities and pressure drops. The gasification reactor at high temperature conditions is simulated using the validated CFD model. The CFD model is also used for verification of Glicksman’s full set and simplified set of dimensionless parameters for scaling of biomass gasification reactors. In addition, the model is used to study Glicksman’s viscous limit set of dimensionless scaling parameters. The computational results show that Glicksman’s viscous limit set of dimensionless parameters is applicable for scaling of fluidized beds operating at particle Reynold’s number up to 15. The CPFD model is used to simulate reaction and reaction kinetics in the gasification reactor. The computational results of composition of the producer gas agree well with the measured gas compositions reported from the biomass gasification plant in Güssing, Austria. Circulation rate of bed material, steam to biomass ratio, bed material to biomass ratio and the corresponding temperatures are important for optimization of the gasification reactor. The CPFD model is used to study these parameters. The results show that the optimum bed material circulation rate is about 26 times of the biomass feed, the steam to biomass ratio is 0.2 on mass basis and the optimal reaction temperature is 1173 K. The results make a contribution to meet a challenge of increasing the steam conversion rate. Steam production for the biomass gasification reactor requires significant amount of energy. Various gasification data show the steam conversion rate is lower than 10 vol.% [1, 2]. The rest of the 90 vol. % of steam is used only as fluidizing gas. The reduction of particle size of biomass and bed material significantly reduces the amount of steam required for the fluidization. The computational results based on the CPFD model show that decreasing particle size of bed material and the wood increases the producer gas quantity. Experiments have been performed in a lab-scale cold model of a Circulating Fluidized Bed (CFB) reactor. Pressure data and bed material circulation rates show good agreements with the computational results. The CPFD model is used for optimization of gas feed positon in the CFB reactor in order to obtain maximum bed material circulation rate. The results of the CPFD simulations show that the optimum ratio of the heights of the feed position for the primary and secondary gas to the total height of the reactor are 0.125 and 0.375 respectively. The optimization of the flow in the CFB needs identification of all flow regimes occurring in the reactor. The flow regimes have been identified along with the minimum fluidization, transport and fast fluidization velocities for glass particles with mean particle size of 156 μm. The CPFD model prediction shows that the gas velocity range of 10umf to 35umf should be avoided to maintain constant bed material circulation rate in CFB.
Has partsPaper A: Thapa, R.K. & Halvorsen, B.M.: "Study of Flow Behavior in Bubbling Fluidized Bed Biomass Gasification Reactor Using CFD Simulation". In: "The 14th International Conference on Fluidization – From Fundamentals to Products", J.A.M. Kuipers, R.F. Mudde, J.R. van Ommen & N.G. Deen (Eds), ECI Symposium Series, (2013). http://dc.engconfintl.org/fluidization_xiv/69
Paper B: Thapa, R.K., Pfeifer, C. & Halvorsen, B.M.: "Scaling of biomass gasification reactor using CFD simulation". In: Proceedings of the IPCS; edited by Hermann Hofbauer and Michael Fuchs. pp 237-246. Vienna: Vienna University of Technology, 2013. ISBN 978-3-9502754-8-3.
Paper C: Thapa, R.K. & Halvorsen, B.M.: "Scaling of bubbling fluidized bed reactors with Glicksman’s viscous limit set and CFD simulations". International Journal of Computational Methods and Experimental Measurements 2(2) (2014), 135-144. http://dx.doi.org/10.2495/CMEM-V2-N2-135-144. Reproduced by permission.
Paper D: Thapa, R.K., Pfeifer, C. & Halvorsen, B.M.: "Modeling of reaction kinetics in bubbling fluidized bed biomass gasification reactor". International Journal of Energy and Environment 5(1), (2014), 35-44.
Paper E: Thapa, R.K., Pfeifer, C. & Halvorsen, B.M.: "Influence of Size and Size Distribution of Biomass and Bed Material on Performance of a dual Fluidized Bed Gasification Reactor". In: Jinghai Li, Fei Wei, Xiaojun Bao, Wei Wang (Eds.), Proceedings of the 11th International Conference on Fluidized Bed Technology, CFB-11, 2014, p. 377-382.
Paper F: Thapa, R.K. & Halvorsen, B.M.: "Stepwise analysis of reactions and reacting flow in a dual fluidized bed gasification reactor". WIT Transactions on Engineering Sciences 82 (2014), 37-48. http://dx.doi.org/10.2495/AFM140041. Reproduced by permission.
Paper G: Thapa, R.K. & Halvorsen, B.M.: "Heat transfer optimization in a fluidized bed biomass gasification reactor". WIT Transactions on Engineering Sciences 83 (2014), 169-178. http://dx.doi.org/10.2495/HT140161. Reproduced by permission.
Paper H: Thapa, R.K., Frohner, A., Pfeifer, C. & Halvorsen, B.M.: "Circulating fluidized bed reactor: CPFD model validation and gas feed position optimization". In review for "Computers & Chemical Engineering".
Paper I: Thapa, R.K., Pfeifer, C. & Halvorsen, B.M.: "Flow Regime Identification in the Riser of a Dual Fluidized Bed Gasification Reactor". In review for "Industrial Engineering & Chemistry Research’".