Radial gas mixing in a circulating fluidized bed

Secondary air was injected horizontally through a single nozzle Ž0.016 m i.d.. at 2.0 m above the distributor. To determine the effect of secondary air injection types on Dr , the tangential and radial air injection nozzles were employed w9x. The SAR, defined as the ratio of volumetric flow rate of secondary air to that of total air, was varied from 0% to 50%.
Received 1 November 1998; received in revised form 1 August 1999; accepted 7 December 1999

Abstract
The effects of gas velocity, solid circulation rate, the secondary air injection ratio, and secondary injection type on the radial gas-mixing coefficient Ž Dr. in a circulating fluidized bed ŽCFB. Ž0.1 m i.d., 5.3 m high. have been determined. It increases with increasing solid circulating rate and the secondary air ratio ŽSAR., but decreases with increasing gas velocity. Based on the present and previous studies, Dr increases with increasing column diameter. The effect of secondary air injection type on the radial gas-mixing coefficient is more pronounced with the tangential than with the radial air injection. The radial gas-mixing coefficient in terms of Peclet number has been correlated based on the isotropic turbulence theory. q 2000 Elsevier Science S.A. All rights reserved.
and stored in a hopper. Solid particles were fed from the hopper into the riser through a loop-seal to regulate solid circulation rate ŽGs.. The superficial gas velocity ŽUg . and Gs were varied in the range of 2.5–4.5 mrs and 0–53 kgrm2 s, respectively.
2. Experimental
Experiments were carried out in a transparent Plexiglas column Ž0.1 m i.d., 5.3 m high. as shown in Fig. 1. The details of the experimental facilities can be found elsewhere w14,15x. The solid particle used in this study was silica based sand particle having a mean diameter of 125 mm and an apparent density of 3055 kgrm3. The solid particles were supported on a perforated plate distributor situated between the main column section and a wind box Ž0.1 m i.d., 0.2 m high. into which air was fed to the column through a pressure regulator, an oil filter and a calibrated flow meter. The entrained particles from the riser were collected by the primary and secondary cyclones
Powder Technology 113 Ž2000. 23–29
rlocaterpowtec
Radial gas mixing in a circulating fluidized bed
Won Namkung, Sang Done Kim)
Department of Chemical Engineering and Energy and EnÕironment Research Center, Korea AdÕanced Institute of Science and Technology Taejon 305-701, South Korea
Keywords: Circulating fluidized bed; Radial gas-mixing coefficients; Secondary air injection
1. Introduction
The contacting between gas and solid phases in a fluidized bed is mainly governed by the degree of gas mixing. Thus, knowledge of gas mixing in fast-fluidized beds is useful in reactor design w1x. However, studies on gas mixing in circulating fluidized beds ŽCFB. are relatively sparse compared to solid hydrodynamic studies w2,3x. The available literature data on the radial gas-mixing coefficient Ž Dr . are inconsistent to determine the effects of gas velocity ŽUg . and solid circulation rate ŽGs. on Dr in CFBs w2x. Yang et al. w4x found that the magnitude of Dr decreases with increasing Ug and decreasing Gs Whereas, Adams w5x reported that the horizontal dispersion coefficient increases with increasing Ug and decreasing Gs. Also, Bader et al. w6x found no relationship between Dr and Ug or Gs since Ug and Gs were varied simultaneously. The apparent contradiction may come from the different experimental apparatus and operating conditions with the employed models to interpret the experimental data w7x.
In the present study, the effects of Ug , Gs, secondary air injection ratio, and the air injection types on the radial gas mixing characteristics in a CFB have been determined.
0032-5910r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 9 9 . 0 0 2 8 8 - 0
24
W. Namkung, S.D. Kim r Powder Technology 113 (2000) 23–29
Fig. 1. Schematic diagram of the experimental apparatus: Ž1. riser; Ž2. cyclone; Ž3. hopper; Ž4. loop-seal; Ž5. sampling bottle; Ž6. butterfly valve; and Ž7. distributor.
) Corresponding author. Tel.: q82-42-869-3913; fax: q82-42-8693910.
E-mail address: kimsd@cais.kaist.ac.kr ŽS.D. Kim..
coefficient w10x have been determined. It has been found that the lateral gas mixing and fuel dispersion in a CFB combustor are inadequate w11x. Also, it has been known that secondary air injection into a CFB can improve gas mixing w12x. However, the effect of secondary air injection on gas mixing in CFBs has not been determined in detail. Therefore, the effect of secondary air injection Žsecondary air ratio ŽSAR., secondary air injection device, etc.. on the hydrodynamics of the gas–solid suspension w13x has to be determined.