Modeling of solids segregation in circulating fluidized bed boilers

Abstract

Segregation always occurs in a circulating fluidized bed (CFB) because of the wide distribution of particle size and density of the bed material. Terminal velocity has a significant influence on solids segregation; thus, it is convenient to describe the segregation tendency using single particle terminal velocity u_t. This paper proposes a segregation model in CFB boilers based on the Cell Model. In each cell along the riser, varied-sized particles have different tendencies toward segregation; finer particles are carried out more easily, while coarser ones tend to sink into the cell. It is assumed that the average terminal velocity \overline{u_t}, corresponding to the mean particle size in the cell, has a segregation index of x = 1.0 as the reference point. The segregation index of particles with higher terminal velocity is lower than 1.0, while that for finer particles is larger than 1.0. The empirical formulae of segregation parameters, namely x₀ and k₁, are derived by optimizing experimental data in published literature. The test result of ash size distribution in a 220 t/h CFB boiler validates the reasonableness of the model.

Keywords

segregation / model / terminal velocity / circulating fluidized bed (CFB)

References

[1] Bai D, Nakagawa N, Shibuya E, Kinoshita H, Kato K. Axial distribution of solid holdups in binary solids circulating fluidized beds. Journal of Chemical Engineering of Japan, 1994, 27(3): 271-275

[2] Bodelin P, Delabarre A. Behaviour of single solids and their binary mixtures in a circulating fluidized bed. In: Large J F, Claude L, eds. Fluidization VIII. New York: Engineering Foundation, 1996, 270-279

[3] Cammarota A, Chirone R, Marzocchella A, Salatino, P. Relevance of attrition to bed solids inventory and particle size distribution in circulating fluidized bed coal combustors. In: Grace J R, Zhu J-X, de Lasa H I, eds. Proceeding of 7th International Conference on CFB, Ottawa: Canada Society of Chemical Engineering, 2002, 1661

[4] Nakagawa N, Bai D, Shibuya E, Kinoshita H, Takarada T, Kato K. Segregation of particles in binary solids circulating fluidized beds. Journal of Chemical Engineering of Japan, 1994, 27(2): 194-198

[5] Jiang P, Bi H, Liang S, Fan L. Hydrodynamic behavior of circulating fluidized bed with polymeric particles. AIChE Journal, 1994, 40(2): 193-206

[6] Hirschberg B, Werther J. Factors affecting solids segregation in circulating fluidized bed riser. AIChE Journal, 1998, 44(1): 25-34

[7] Van den Moortel T, Azario E, Santini R, Tadrist L. Experimental analysis of the gas-particle flow in a circulating fluidized bed using a phase doppler particle analyzer. Chemical Engineering Science, 1998, 53(10): 1883-1899

[8] Kunni D, Levenspiel O. Fluidization Engineering. 2nd ed. Boston: Buterworth-Heinemann, 1991, 223-225

[9] Rhodes M J, Geldart D. Model for the circulating fluidized bed. Powder Technology, 1987, 53(3): 155-162

[10] Yang H, Xiao X, Wang X, Lu J. Model research on material balance in a circulating fluidized bed boiler. In: Xu X C, Zhao C S, eds. Proceedings of the 5th International Symposium on Coal Combustion, Nanjing, China, Southeast University Press, 2003, 251-255

Acknowledgements

This work was supported by the Hi-Tech Research and Development Program of China (No. 2009AA05Z302) and the National 11th-Five Year Plan (No. 2006BAD07A14-4).

Nomenclature

A	section area/ m2
dp	particle size/m
\overline{d}p	mean particle size of all materials in the cell/m
{\overline{d}}_{p0}	mean particle size of all materials in the riser/m
d50	cut size/m
d90	critical size/m
Gs	circulating rate/(kg·m-2·s-1)
i	cell number
j	particle size number
k1	segregation ability/(m·s-1)
k2	segregation ability/(m·s-1)
mj	mass of size j particles/kg
m	total mass in each cell
ugas	gas velocity/(m·s-1)
ut	terminal velocity/(m·s-1)
uterm	average terminal velocity corresponding to {\overline{d}}_{p0} in riser/(m·s-1)
\overline{u_t}	average terminal velocity corresponding to {\overline{d}}_{\mathrm{p}} in cell i/(m·s-1)
Wdn	downward flowing rate/(kg·s-1)
Wup	upward flowing rate/(kg·s-1)
ξ	segregation index
ϵ	voidage
ρ	particle density/(kg·m-3)

RIGHTS & PERMISSIONS

2014 Higher Education Press and Springer-Verlag Berlin Heidelberg