The hydrodynamics of circulating fluidized beds (CFBs) is a complex phenomenon that can drastically vary depending on operational setup and geometrical configuration. A research of the literature shows that studies for the prediction of key variables in CFB systems operating at high temperature still need to be implemented aiming at applications in energy conversion, such as combustion, gasification, or fast pyrolysis of solid fuels. In this work the computational fluid dynamics (CFD) technique was used for modeling and simulation of the hydrodynamics of a preheating gas-solid flow in a cylindrical bed section. For the CFD simulations, the two-fluid approach was used to represent the gas-solid flow with the k-epsilon turbulence model being applied for the gas phase and the kinetic theory of granular flow (KTGF) for the properties of the dispersed phase. The information obtained from a semiempirical model was used to implement the initial condition of the simulation. The CFD results were in accordance with experimental data obtained from a bench-scale CFB system and from predictions of the semiempirical model. The initial condition applied in this work was shown to be a viable alternative to a more common constant solid mass flux boundary condition. 1. Introduction Circulating fluidized bed reactors are systems in which gas-solid heterogeneous reactions take place in a fast fluidization regime. One of the most successful applications of this technology is the combustion of low-grade fuels, such as biomass, waste, and coal with high ash content. Fluidized beds provide high specific transfer rates, high solids throughput, and thermal uniformity within the reactor [1]. The total particulate material circulating in the system, the solids inventory, is an important parameter for an efficient design of CFB for combustion applications. Together with the gas superficial velocity it drives the bed particles along the loop. The pressure drop and the particle residence time are strongly related to the solid distribution in the fast bed zone (riser). Moreover, the solid distribution also affects the mass and heat transfer rates. For combustion and gasification purposes, quartz sand is commonly used as inert material due to its relatively low cost and excellent performance at high temperatures. In such systems, the amount of solid inert material can reach up to 97% of the total mass in the solids inventory [2]. Several studies have been carried out to describe the hydrodynamic behavior of the gas-solid flow in CFB systems, based on empirical and theoretical analysis
References
[1]
E. Hartge, Y. Li, and J. Werther, “Flow structure in fast fluidized beds,” in Fluidization V, K. Ostergaard and A. Sorensen, Eds., pp. 345–352, Engineering Foundation, New York, NY, USA, 1986.
[2]
P. Basu, Combustion and Gasification in Fluidized Beds, Taylor & Francis, Boca Raton, Fla, USA, 2006.
[3]
T. Knowlton, “Nonmechanical solids feed and recycle devices for circulating fluidized beds,” in Circulating Fluidized Bed Technology II, P. Basu and J. Large, Eds., pp. 31–41, Pergamon Press, Oxford, UK, 1988.
[4]
X. Wang, M. Rhodes, and B. Gibbs, “Solids flux distribution in a CFB riser operating at elevated temperatures,” in Fluidization VIII, F. Large and C. Laguérie, Eds., pp. 236–244, Engineering Foundation, New York, NY, USA, 1996.
[5]
J. F. Davidson, “Circulating fluidised bed hydrodynamics,” Powder Technology, vol. 113, no. 3, pp. 249–260, 2000.
[6]
H. Tong, H. Li, X. Lu, and Q. Zheng, “Hydrodynamic modeling of the L-valve,” Powder Technology, vol. 129, no. 1–3, pp. 8–14, 2003.
[7]
A. Gungor and N. Eskin, “Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs,” Journal of the Chinese Institute of Chemical Engineers, vol. 39, no. 6, pp. 541–556, 2008.
[8]
M. J. Rhodes and P. Laussmann, “Study of the pressure balance around the loop of a circulating fluidized bed,” Canadian Journal of Chemical Engineering, vol. 70, no. 4, pp. 625–630, 1992.
[9]
M. L. Mastellone and U. Aren, “The effect of particle size and density on solids distribution along the riser of a circulating fluidized bed,” Chemical Engineering Science, vol. 54, no. 22, pp. 5383–5391, 1999.
[10]
S. W. Kim and S. D. Kim, “Effects of particle properties on solids recycle in loop-seal of a circulating fluidized bed,” Powder Technology, vol. 124, no. 1-2, pp. 76–84, 2002.
[11]
X. Qi, J. Zhu, and W. Huang, “Hydrodynamic similarity in circulating fluidized bed risers,” Chemical Engineering Science, vol. 63, no. 23, pp. 5613–5625, 2008.
[12]
Y. Chong, D. O'Dea, L. Leung, and D. Nicklin, “Design of standpipe and non-mechanical V-valve for a circulating fluidized bed,” in Circulating Fluidized Bed Technology II, P. Basu and J. Large, Eds., pp. 493–500, Pergamon Press, Oxford, UK, 1988.
[13]
X. L. Yin, C. Z. Wu, S. P. Zheng, and Y. Chen, “Design and operation of a CFB gasification and power generation system for rice husk,” Biomass and Bioenergy, vol. 23, no. 3, pp. 181–187, 2002.
[14]
R. Dewil, J. Baeyens, and B. Caerts, “CFB cyclones at high temperature: operational results and design assessment,” Particuology, vol. 6, no. 3, pp. 149–156, 2008.
[15]
G. C. Lopes, L. M. Rosa, M. Mori, J. R. Nunhez, and W. P. Martignoni, “Three-dimensional modeling of fluid catalytic cracking industrial riser flow and reactions,” Computers and Chemical Engineering, vol. 35, pp. 2159–2168, 2011.
[16]
L. Yu, J. Lu, X. Zhang, and S. Zhang, “Numerical simulation of the bubbling fluidized bed coal gasification by the kinetic theory of granular flow (KTGF),” Fuel, vol. 86, no. 5-6, pp. 722–734, 2007.
[17]
D. Gidaspow, Multiphase Flow and Fluidization, Academic Press, London, UK, 1994.
[18]
B. G. M. van Wachen, Derivation, implementation and validation of computer simulation models for gas-solid fluidized beds [Ph.D. thesis], Delft University, 1998.
[19]
S. Benyahia, M. Syamlal, and T. J. O'Brien, “Particle Technology and Fluidization. Study of the ability of multiphase continuum models to predict core-annulus flow,” AIChE Journal, vol. 53, no. 10, pp. 2549–2568, 2007.
[20]
J. C. S. C. Bastos, L. M. Rosa, M. Mori, F. Marini, and W. P. Martignoni, “Modelling and simulation of a gas-solids dispersion flow in a high-flux circulating fluidized bed (HFCFB) riser,” Catalysis Today, vol. 130, no. 2–4, pp. 462–470, 2008.
[21]
V. Jiradilok, D. Gidaspow, S. Damronglerd, W. J. Koves, and R. Mostofi, “Kinetic theory based CFD simulation of turbulent fluidization of FCC particles in a riser,” Chemical Engineering Science, vol. 61, no. 17, pp. 5544–5559, 2006.
[22]
X. Lan, C. Xu, G. Wang, L. Wu, and J. Gao, “CFD modeling of gas-solid flow and cracking reaction in two-stage riser FCC reactors,” Chemical Engineering Science, vol. 64, no. 17, pp. 3847–3858, 2009.
[23]
X. Wang, B. Jin, W. Zhong, and R. Xiao, “Modeling on the hydrodynamics of a high-flux circulating fluidized bed with geldart group a particles by kinetic theory of granular flow,” Energy and Fuels, vol. 24, no. 2, pp. 1242–1259, 2010.
[24]
A. Samuelsberg and B. H. Hjertager, “An experimental and numerical study of flow patterns in a circulating fluidized bed reactor,” International Journal of Multiphase Flow, vol. 22, no. 3, pp. 575–591, 1996.
[25]
J. J. Ramirez-Behainne, Assessment of mercury emissions from Brazilian coal combustion in fast fluidized bed [Ph.D. thesis], UNICAMP, Campinas, Brazil, 2007.
[26]
W. C. Yang, “Modification and re-interpretation of Geldart’s classification of powders,” Powder Technology, vol. 171, no. 2, pp. 69–74, 2007.
[27]
R. I. Hory, J. J. Ramirez-Behainne, A. A. B. Pécora, and L. Goldstein Jr, “An empirical model to predict the mass flow rate of solids in a high temperature circulating fluidized bed system,” in Proceedings of the 11th Brazilian Congress of Thermal Sciences and Engineering, ENCIT, 2006.
[28]
C. Y. Wen and Y. U. Yu, “A generalized method for predicting the minimum fluidization velocity,” American Institute of Chemical Engineers Journal, vol. 12, pp. 610–612, 1966.
[29]
D. Bai, Y. Jin, and Z. Yu, “Flow regimes in circulating fluidized beds,” Chemical Engineering and Technology, vol. 16, no. 5, pp. 307–313, 1993.
[30]
D. Kunii and O. Levenspiel, “Entrainment of solids from fluidized beds I. Hold-up of solids in the freeboard II. Operation of fast fluidized beds,” Powder Technology, vol. 61, no. 2, pp. 193–206, 1990.
[31]
D. Kunii and O. Levenspiel, “Flow modeling of fast fluidized beds,” in Circulating Fluidized Bed Technology III, P. Basu, M. Hasatani, and M. Horio, Eds., pp. 91–98, Pergamon Press, Oxford, UK, 1991.
[32]
W. C. Yang, “Criteria for choking in vertical pneumatic conveying lines,” Powder Technology, vol. 35, no. 2, pp. 143–150, 1983.
[33]
J. Perales, T. Coll, M. Llop, L. Puigjaner, J. Arnaldos, and J. Casal, “On the transition from bubbling to fast fluidization regimes,” in Circulating Fluidized Bed Technology III, P. Basu, M. Hasatani, and M. Horio, Eds., pp. 73–78, Pergamon Press, Oxford, UK, 1991.
[34]
J. J. Ramirez-Behainne and G. Martins, “A semi-empirical model to calculate the inert solid inventory and the main dimensions of a CFB reactor aiming biomass thermochemical conversion,” in Proceedings of the 20th International Congress of Mechanical Engineering, 2009.
[35]
E. Muschelknautz and V. Greif, “Cyclone and other gas-solid separators,” in Circulating Fluidized Beds, J. Grace, A. Avidan, and T. Knowlton, Eds., pp. 181–213, Blackie Academic & Professional, London, UK, 1997.
[36]
T. Knowlton, “Standpipes and return systems,” in Circulating Fluidized Beds, J. Grace, A. Avidan, and T. Knowlton, Eds., pp. 214–260, Blackie Academic & Professional, London, UK, 1997.
[37]
D. Geldart and P. Jones, “The behaviour of L-valves with granular powders,” Powder Technology, vol. 67, no. 2, pp. 163–174, 1991.
[38]
S. Ogawa, A. Umemura, and N. Oshima, “On the equations of fully fluidized granular materials,” Journal of Applied Mathematics and Physics, vol. 31, no. 4, pp. 483–493, 1980.
[39]
C. K. K. Lun, S. B. Savage, D. J. Jeffrey, and N. Chepurniy, “Kinetic theories for granular flow: inelastic particles in Couette flow and slightly inelastic particles in a general flowfield,” Journal of Fluid Mechanics, vol. 140, pp. 223–256, 1984.
[40]
W. E. Ranz and W. R. Marshall Jr., “Evaporation and drops—part I and II,” Chemical Engineering Prog, vol. 48, pp. 173–180, 1952.
[41]
S. Benyahia, H. Arastoopour, T. M. Knowlton, and H. Massah, “Simulation of particles and gas flow behavior in the riser section of a circulating fluidized bed using the kinetic theory approach for the particulate phase,” Powder Technology, vol. 112, no. 1-2, pp. 24–33, 2000.
[42]
S. B. R. Karri and T. M. Knowlton, “A comparison of annulus solids flow direction and radial solids mass flux profiles at low and high mass fluxes in a riser,” in Proceedings of the 6th International Conference on Circulating Fluidized Beds, pp. 71–76, 1999.
