Convective Drying of Mixtures of Sewage Sludge and Sawdust in a Fixed Bed

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Caren Nichols
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1 Cnvective Drying f Mixtures f Sewage Sludge and Sawdust in a Fixed Bed J. Li, L. Fraikin, T. Salmn, L. Bennamun, D. Tye, A. Lénard University f Liège, Belgium 1

2 Intrductin Increasing amunt f wastewater sludge Sludge valrizatin Land applicatin, cmpsting, incineratin Prblem: misture cntent is a critical parameter Drying Mass and vlume reductin Stabilisatin Hygienisatin Texture imprvement Increase f calrific value Needs fr ther valrizatin ptins Idea = new renewable fuel by mixing sludge with sawdust Culd be used fr gasificatin/pyrlysis Interest fr pasty sludge, difficult t dry Intrductin 2

3 Gals Study f the drying behaviur f sludge/sawdust mixtures Cnvective drying f fixed bed Raw sludge (befre and after mixing) Raw sludge + increasing sawdust additin Use f tmgraphy t characterize the 3D bed structure Intrductin 3

Sludge and sawdust Sludge - Frm WWTP (Grsses

Bis Vielsalm, IBV, Belgium) - Misture cntent

4 Sludge and sawdust Sludge - Frm WWTP (Grsses Battes, Belgium) - Misture cntent (wet basis) = 85.5% Sawdust - Pine (90% èpicèa and 10% duglas), frm a wd pellet s factry (Industrie du Bis Vielsalm, IBV, Belgium) - Misture cntent (wet basis) = 30% - Diameter=0~0.5 mm Materials and methds 4

5 Samples Original sludge Kitchen machine (KM1000, PROline) Sawdust/sludge mixtures - Mass rati (dry matter) f sawdust/sludge=1/9, 2/8, 3/7 and 4/6 - Mixing time: 30 s - Mixing velcity: 40 rpm Mixed sludge Beater - The same prtcl was used t mix the riginal sludge withut any sawdust additin. Materials and methds 5

6 Cnvective pilt scale dryer Diameter: 160 mm Sample: 500 g Extrusin ( = 12 mm) Fixed bed (crss flw) - Temperature: 50, 80 and 110 ºC - Air velcity: 2 m/s - Humidity(ambient):~0.007 kg/kg Materials and methds 6

X-ray tmgraphy X-ray tmgraph 2D image 3D

7 X-ray tmgraphy X-ray tmgraph 2D image 3D image Nn invasive imaging High energy (420 kv) Large-scale (0.45 m in diameter, 4 m in height) Image pixel size: mm Distance between tw slices: 2.2 mm Vlume Bed prsity Ttal exchange surface 7 Materials and methds

8 Drying rate (g/s) Drying behaviur ºC Original sludge Mixed sludge Mass rati=1/9 Mass rati=2/8 Mass rati=3/7 Mass rati=4/ W (kgwater/kgds) Drying behaviur: A shrt preheating perid + a shrt cnstant rate perid + a lng falling rate perid Original sludge Mixed sludge: Drying rate Sawdust additin (1) Drying rate (2) Frm mass rati f 2/8, the drying rate exceed the drying rate f riginal sludge. Sawdust additin has a psitive impact n the drying prcess. Results 8

9 3D images (a) Original sludge (b) Mass rati=4/6 The vlume after drying increases bviusly with sawdust additin. Results 9

10 Bed vlume wet dry Original sludge Mixed sludge (1) Befre drying: little decrease (2) After drying: little change Sawdust additin (1) Befre drying: vlume (2) After drying: vlume Results 10

11 Shrinkage Original sludge Mixed sludge Vlume shrinkage Sawdust additin Vlume shrinkage Temperature Temperature vlume shrinkage Results 11

12 Ttal exchange surface wet dry Original sludge Mixed sludge (1) Befre drying: surface (2) After drying: little change Sawdust additin (1) Befre drying: surface (2) After drying: surface Results 12

13 Cnstant drying rate perid In cnstant drying rate perid, the drying rate depends n the surface. The drying flux is independent f the sludge nature but nly depends n air temperature, velcity and humidity. Discussin 13

14 Falling drying rate perid (1) First decreasing zne: The evapratin happens at the slid surface External diffusin f liquid is cntrlling Drying rate is due t the surface Mixing surface drying rate Sawdust additin surface drying rate (2) Secnd decreasing zne: The evapratin happens within the slid structure Internal diffusin f liquid is cntrlling Drying rate isn t due t the surface Original sludge and mixed sludge: nly the first decreasing zne Mixtures: first decreasing zne + secnd decreasing zne Discussin 14

15 Shrinkage prcess 80ºC Linear increase regin: almst the same Cnstant regin: sawdust additin reaches earlier The sawdust reinfrces the texture f sludge and enhances the heat and mass transfer. Discussin 15

16 Cnclusins The mixing step has a negative impact n the drying prcess. Nevertheless the sawdust additin is shwn t have a psitive impact n the drying prcess frm mass rati f 2/8. Further wrk will be dne in rder t characterize the pre texture and the pyrlysis behaviur f the sludges and mixtures. 16 Cnclusins

17 Thank yu fr yur attentin! 17

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