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Professor Arun S. MujumdarMinerals, Metals and Materials Technology Centre (M3TC), National University of Singapore
Blk EA, #06-15, 9 Engineering Drive 1, Singapore 117576Tel: 65-65164623 Fax: 65-67791459 E-mail: [email protected]
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IntroductionLow rank coal (LRC) such as lignite and sub-
bituminous coal constitute around 48% of the global coal reserve. Benefits of LRC includes low mining cost, high reactivity, and low ash. On the other hand, their high moisture content, low calorific value, high transportation cost, potential safety hazards are some of the limitations of LRC. These limitations can be overcome by removing substantial amount of moisture using either thermal, mechanical, or chemical means.
Fig. 2 SCD performance. (a) Volumetric throughput and hold-up with respect to feed valve speed; (b) average residence time with respect to volumetric feed rate; and (c) average residence time with respect to screw speed.
Development of a cost-effective and energy efficient technique for drying Low Rank Coal (LRC)Arun S. Mujumdar, Sachin V. Jangam, and Hafiiz Osman
Pilot-scale SCD1. Heat transfer via jacket and hollow shaft;2. Replaceable screw for three different pitches.3. Screw: len 2.8 m, dia: 170 mm, pitch: 60, 90,120 mm.4. Vacuum level 20-40 mbar.
Aims and Objectives1. Performance study of indirectly-heated 150 kg/h SCD.2. Compare SCD performance with continuous VFBD.3. Characterization of wet and dry feed.4. Simple mathematical modelling for scale-up.5. Develop a cost-effective drying system.
4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 90
100
200
300
400
500
600
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Throughput (m3/h)
Hold-up (kg)
Feed valve speed (RPM)
Th
rou
ghp
ut
(m3/
h)
Hol
d-u
p (
kg)
300 350 400 450 500 550 6003.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Volumetric feed rate (m3/h)
Res
idan
ce t
ime
(min
)
5 6 7 8 9 10 11 12 13 140
0.5
1
1.5
2
2.5
3
3.5
Screw speed (RPM)
Res
idan
ce t
ime
(min
)
SCD Performance using sawdust particles
a
b c
Steam inDrying gas in
Solid flow
Feed: Pulverized coal (< 50 mm)
Steam out
Gear box Motor
Dried coal taken as is(or fed to next stage) Fines
Drying gas out
Cyclone and filter bag house
Fig. 1 Screw conveyor dryer schematic.
Discrete Element Modeling
Fig. 3 Particle mixing for screw speed of 60rpm, at t = 0, 1, and 2 s. (a) side view, (b) isometric view.
a
b
0 s 0 s
1 s 1 s
2 s 2 s
M3TCMinerals Metals and MaterialsTechnology Centre
ConclusionMixing in screw conveyor was studied
qualitatively using colored dust particles. The dust particles of same size range and density were colored with two different colors. Experimental observation reveal that the dust particles mix very well within the screw length provided at a screw speed of 7.5 rpm and a throughput of 300 m3/h.
EDEM was used to model the flow of granular material inside the SCD. For a screw speed of 60 rpm, excellent mixing was observed as shown in Fig. 3. Future modeling work includes: study of the effect of various operating parameters such as feed flow, screw speed, angle of inclination (0-15 degrees) of the trough, on the residence time distribution. Other parameter effects such as those due to changes in mass flow rates, average particle velocities as well as power consumption are evaluated for above mentioned geometric and operating variables. The objective is to utilize trends obtained from DEM studies for innovative designs of SCD.
