Transcript Solid Fuels

Solid Fuels
Washing of Coal
• Most of the coals when mined contain impurities
associated with it. Impurities are removed by
washing.
• Fixed impurities:
-Originate from the coal forming plants
- Can not be removed by washing
• Free impurities:
-Adhering to the surface
-Can be removed by washing of coal
Objectives of Coal Washing
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To reduce its ash content
To reduce transportation and storage costs
To increase its heating value
To increase the fusion point of its ash by
removing alkali chlorides
• To reduce its clinkering tendency
• To increase its efficiency in use
• To reduce its sulphur and phosphorous contents
Coal Washing Processes
• Most coal washing processes depend upon differences in specific
gravity between coal and its impurities
• Approx. Sp. Gr. Coal: 1.3, shale: 2.5 pyrite: 4.0
• Liquids of different densities for floating off the coal fractions may be
used.
• Such liquids may be mixtures of Carbon tetrachloride or bromoform
with toluene or solutions of zinc chloride
• Some Types of Coal washing/cleaning plants
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Dense Medium Separators
Jigs
Cyclone Washers
Concentrating tables
Froth Floatation
Typical washability data for Coal with 15.3% ash
S.G. of
Liquid
1.25
1.3
1.35
1.4
1.5
1.6
1.8
Floating material
Sinking Material
%yield
Ash %
%yield
Ash%
5
65
75
79
82
85
87
0.8
2.4
3.1
3.8
4.5
5.6
6.8
95
35
25
21
18
15
13
16
39.3
51.8
58.5
64.1
70.0
72.7
Dense Medium Washers
• Properties of the Medium
• The medium used to float the coal from its impurities
must be of low viscosity
• The medium must have also some stability so that it has
effective uniform density
• High specific gravity of the medium solid (the grains
suspended in water) is also desirable to give effective
medium densities upto or over 2.0
• Mixture of sand and water can be used as washing
medium
Dense Medium Washers
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Many designs have been employed so not
possible to generalize
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Mainly there are two processes under this
heading
(i) Chance Process
(ii) Tromp Process
Chance Process
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Washing medium is a mixture of sand and
water
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A deep bath of sand and water having
intermediate specific gravity between clean
coal and ash is used
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Clean coal floats and dirt sinks and removed
Tromp Process
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A shallow bath instead of deep bath of sand
and water is used
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Clean coal floats and dirt sinks which is
removed by scraper conveyor
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Requires much lesser volume of fluid
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Requires lesser building height
Jig Washers
• A coal bed is maintained on a perforated
plate
• Coal bed is subjected to action of upward
and downward current of water
• Clean coal being the lighter is
concentrated at the top while the dirt sinks
and removed from the bottom of the
washer
Baum Jig
• This is the most common type of Jig Washers
• Baum Jig is U-shaped vessel divided into washing
and air compartments
• Compressed air is used to create the pulsating
movement of water
Advantages of Jig Washers
• Better separation efficiency for finer sizes
of coal (25-200mm)
Cyclone Washer
• The cyclone washer employs the same principle as
cyclone dust collector except that the coal is fed in a
STREAM OF WATER (or dense medium) instead in air
• It is a settling chamber in which settling force is replaced
by centrifugal force
• The clean coal is obtained as an overflow from the
cyclone
• The underflow from the cyclone contains the heavier
material or impurities
Advantages
• Efficient for small sized coal
• High flexibility of operation
with respect to near gravity
material, coal size and feed
size
• Disadvantages :
• Higher power consumption
• Higher maintenance cost
Cyclone Washer
Froth Floatation process
• Froth is made in a floatation cell by bubbling air
through water in presence of frothing agents
• Frothing agents used are normally cresol, pine
oil or alcohols
• When dirty coal powder is added to cell, the pure
coal particles adhere to the bubbles of the froth
• Dirt particles sink in the cell
• The clean coal is recovered by filtration
Froth Floatation process
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Advantages:
It can produce ultra pure coal
It is suitable for fine coals
Disadvantages:
High capital and running cost
Coal must be grinded to less than 0.5 mm
Concentrator tables
• The table is fitted with wood riffles and is
oscillated lengthwise.
• Coal is fed to the top and water flows
transversely
• The heavy refuse is trapped in the riffles
and transported to the end
• The coal passes over the riffle and travels
to the other corner