Entrainment of coarse particles in a circulating fluidized bed under air-stating conditions

Transcription

1 Entrainment of coarse particles in a circulating fluidized bed under air-stating conditions Tadaaki Shimizu and Shou Sasaki Department of Chemistry and Chemical engineering, Niigata University, Japan

2 Introduction For sewage sludge combustion in CFBC, behaviour of coarse ash played a significant role. 2nd. air Prim. air Ash accumulation NOx emission increased. Ash attrition Decrease in NOx. Partitioning of coarse ash particles in the riser or downcomer is of great influence on both NOx emission and ash attrition.

3 Objective Partitioning of coarse particles in riser and downcomer was measured under air-staging condition. Effect of coarse particle size and density, and bed material hold-up on partitioning of coarse particles was studied. A correlation was obtained for force balance between upward forces and gravity of a coarse particle.

4 CFBC experimental apparatus To analyzers cyclone ΔP Fuel hopper Screw feeder Motor Height 1.9m I.D. 2.2cm Temp. Room temp. Gas Vel. 2.7m/s BM quartz sand (80-150mesh) Secondary air Primary air Lope seal air (Primary air)/(total air) =0.7 Press. drop across riser = kpa

5 Cold model experiments Partitioning of particles between riser and loopseal. Height A batch of coarse 2.0m I.D. particles was 22mm injected ΔP Gas into vel. the 2.7m/s(293K) riser. After Prim. fluidizing air/total solids air for B.M. minutes, Quartz the air feed 2nd. air sand(qs) was stopped. The (at 0.66m) solid (0.10~0.18mm) was removed from the bed, sieved, ΔP then weighed. Prim. air

6 Experimental (coarse particles) Coarse solids with different size and density. Particle density of coarse particles, p [kg/m 3 ] Argent (Silver coated) Black pepper (whole) Porous Alumina (crushed) Nonspherical Morishita "JINTAN"(R) Black pepper (crushed) Spherical Coarse particle size, D p [mm]

7 Results and Discussion Entrainment of coarse particles from the riser of CFB.

8 Results and Discussion Effect of particle size on partitioning ratio of coarse particles between riser and loopseal. Partitioning ratio in riser [-] Coarse particles: Argent (Silver coated) B/A: Normalized partitioning ratio Coarse particles Bed material A B Particle concentration in riser [kg/m 3 ] Coarse B/A=0: Coarse solids resided particles in riser when behaved solid in the concentration same manner was bed low. material. They circulated B/A=1: Coarse in the CFB particles when reside solid concentration only in the riser became higher. (no elutriation).

9 Normalized partitioning ratio B.M. concentration particle size/density combination at which normalized partitioning ratio = 0.5 was obtained. 1 Coarse solids: "Jintan" (R) Normalized partitioning ratio [-] Particle concentration [kg/m 3 ]

10 Effect of particle size on entrainment For crushed alumina and crushed pepper, solid density was fixed. Effect of coarse solid size on carry-over was measured. Normalized partitioning. ratio [-] Crushed alumina Particle size [μm] P across riser 0.53 kpa 1.61 kpa Dp at Ut=Ubottom Dp at Ut=Utop

11 Carry-over size Completely mixed with fine bed material (i.e. normalized partitioning ratio = 0) Coarse particle size whose normalized partitioning ratio = 0.5 is defined as carry-over size. Residing only in the riser (i.e. normalized partitioning ratio = 1)

12 Force balance For carry-over size particle, gravity force is assumed to be equal to the sum of drag forces and buoyancy. Gas drag force Buoyancy Particle drag force P Gas Pressure gradient B.M.

13 Gravity, F Mg F Mg Force balance S,C g D 3 p,c D p,c : coarse particle size; S,C : solid density 6 Buoyancy, F B F B D 3 p,c 6 dp/dz: pressure gradient in riser test section dp dz

14 Gas drag force, F G F C G D Re C f Force balance D 2 Dp,c 4 24 Re UD 0.72 Re Re g p,c D p,c : coarse particle size; U: gas velocity; g : gas density; : gas viscosity g U 2 2

15 Force balance Buoyancy and gas drag force could not give an account of the coarse solid entrainment. Particle drag force should be taken into account. Force balance [-] Buoynancy Gas drag Other (particle drag) Solid type ID

16 Force balance Particle drag force, F BM, is assumed to be given in a similar manner as gas drag force as: F BM C D,P D 4 2 p,c susp U U 2 t,bm U t,bm : terminal velocity of bed material; susp : suspension density = (dp/dz)/g C D,P : Coefficient which varies with condition.

17 Force balance Coefficient for particle drag is assumed to be given in a similar manner as C D for gas drag. C f D U U D,P a D p,c p,c susp U susp U t,bm t,bm b

18 Test sections Which is the controlling part. bottom part or upper part? Bottom part: higher solid density, but ΔP 2nd. air ΔP Prim. air lower gas velocity. Upper part: lower solid density, but higher gas velocity. First, C D,P was determined in the bottom part

19 Force balance in bottom part Solid density in the bottom part that gave normalized partitioning ratio of susp in bottom part [kg/m 3 ] Particle size [m] Black pepper (cruhsed) Black pepper (whole) Porous Alumina JINTAN Argent (Silver)

20 Force balance in bottom part Particle drag coefficient C D,P in bottom part CD,P [-] is determined as follows: C 0.11 D D,P U U p,c susp y = 0.11 x t,bm Porous Alumina Black pepper (cruhsed) JINTAN Black pepper (whole) susp D p,c (U - U t,bm ) [kg/(m s)] Argent (Silver)

21 Application of correlation to upper part The correlation of particle drag coefficient is applied for the force balance in the upper part. ΔP C D,P F BM 2nd. air 0.11 D C U U D,P p,c D 4 susp 2 p,c susp t,bm U U 2 t,bm ΔP Prim. air

22 Force balance in upper part Total sum of upward forces > Gravity. Once carried over from the bottom part, these particles are entrained in the upper part to the cyclone. (Upward force)/(gravity) [-] Porous Alumina, riser upper part F BM F G F B D p,c [mm] (Upward force)/(gravity) [-] Black pepper, riser upper part F BM F G F B D p,c [mm]

23 Conclusion Force balance of coarse particle between gravity and upper forces was analyzed under air-staging CFB conditions. Drag force by the circulating fine bed material played an important role in entrainment of coarse solids. A correlation of drag force by the solids was obtained in the bottom part. The entrainment from the bottom is considered to be decisive influence on carry over of coarse particles.