Lignite Drying Mechanism
Lignite drying mechanism
The nature of lignite (brown coal) is changed from hydrophilic to hydrophobic. That is why its thermal value changed very little even after soaking processed lignite (brown coal) with sea water for over 48 hours. As a result, processed lignite (brown coal) is suitable for long distance transportation. Its mechanism is explained as following:
As the drying process of lignite (brown coal) is counter-current, the inlet flue gas temperature is up to 1000C and lignite (brown coal) is heated up to over 300C. As a result, significant amounts of pore collapse and cross-linking occur under this drying temperature, with a significant hysteresis between desorption and re-adsorption of water. The hysteresis is caused by the limitation of water access to the pore structure of the coal. Cross-linking reactions have sealed pores or limited the ability of collapsed pores to be expanded on water re-adsorption.
Fig 01, Particles model of shrinkage for the drying process of lignite (brown coal)
Attachments
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Nature Of Dried Lignite
DBAGlobal (www.dbaglobal.net) has done a significant amount of test work on the drying of Lignite (Brown Coal).
The story is much more complicated that just removing moisture!
The economics around drying Lignite are dominated by;
a.Coal loss once the Lignite is dried, as it breaks down to a very dusty product.
b.The utilisation of waste heat
c.The risk profile of re-wetting
Economics have shown that drying Lignite at the power station just prior to a blending station shows the best risk profile and value. This is the case despite the cost of effectively transporting/shipping water.
DBAGlobal recently conducted economic analysis of seven different drying technologies. The economic differences are not always obvious and can differ radically for different locations and scenarios. ■
Lignite Processing Technology
TECHNOLOGY
Process description
Wet lignite (brown coal) is fed into dryer (DRYER 13) via elevator (ELEV 17) and feed screw (FS 18), then is dried to less than 5% moisture content. Dried lignite (brown coal) is discharged from dryer (DRYER 13) to cooler (not shown ).
Combustion air is sent to combustor (COM 11) via fan (FAN 10).
In order to control the oxygen concentration inside dryer (DRYER 13) to prevent lignite (brown coal) explosion or fire, combustion air is strictly controlled and oxygen isolation chamber (CHAM 12) is put between dryer (DRYER 13) and combustor (COM 11).
Exhaust is then passed through gas separator (GSEP 14) and particle separator (PSEP 15), then is discharged to atmosphere via exhaust fan (FAN 16). Combustible gas separated from gas separator (GSEP 14) is sent to combustor (COM 11) to supply thermal energy for drying process. Fine particles from particle separator (PSEP 15) is also sent to combustor (COM 11) if combustible gas separated from gas separator (GSEP 14) is not enough for drying process. The rest of fine particles from particle separator (PSEP 15) is then sent to pack.
Four lignite (brown coal) processing lines were installed in 2008 with 25 t/hr dried product per line. Another 125 t/hr dried product lignite (brown coal) processing lines is under construction.
Features:
Features:
1, remove free water and bound water, the first industrialization technology in the world
2, it is safe to operate without any explosion or fire risk as very low oxygen concentration inside drying system
3, low processed cost as its high inlet flue gas temperature and utilization of combustible gas released from drying process
4, thermal value of lignite (brown coal) is increased from 3000 kcal/kg to over 5500 kcal/kg
5, volatile matter is decreased to below 40%
6, ash content is reduced to less than 5%;
7, moisture content is reduced to less than 5%;
8, 60% sulphur is removed;
Attachments
flow chart 01 (DOC)
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