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.

Key words： segregation; model; terminal velocity; circulating fluidized bed (CFB)

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)

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