2. 1.Introduction
Coal continues to play a predominant role in the
production of electricity in the world, A very large
percentage of the total coal is burned in pulverized
form.
Pulverized coal achieved its first commercial success
in the cement industry. At Wisconsin Electric Power
Company, the use of pulverized coal was also shown
to be a viable fuel for electrical power production after
World War I
Pulverized coal burns like gas, can be easily lighted
and controlled.
3. 2.Coal pulverizer/mill system
One of the critical power
plant components that is
relied upon to convert
the energy stored in coal
into electricity is the coal
pulverizer or mill.
The coal flow is
controlled by the feeder,
allowing coal to flow into
the pulverizer mill.
The pulverized coal and
air mixture is then
transport to the boiler
burner panels.
4. 3.Boiler
Boiler drum
Reheater
Final Super
Platen Super heater
heater LTSH
Economizer
Coal bunker
Wind Box
Secondary
PA duct
air duct
Furnace
Flue gas
APH duct Coal feeder
F D Fan P A Fan
Coal
Pulverizer
5. 4.Pulverizer
The pulverizer receives the raw coal and
reduces it to a very fine, specified size
consist, similar to face powder.
There are four primary principals involved
in pulverization:
• Drying
• Grinding
• Circulation
• Classification
6. 4.1.Drying
The coal that is passing through a
pulverizer is entrained by the use of hot air
usually from the air heater.
During the pulverization process the
surface area of the coal particles increases
dramatically exposing fresh coal to the
entrainment air.
The inherent and surface moisture of the
coal is reduced by the exposure to hot air.
7. 4.2.Grinding
There are three basic
types of grinding:
1. Impaction – where the
material to be ground is hit
or impacted by an outside
force
2. Crushing – where
material is forced between
two fixed objects
3. Attrition – where material
is ground by rubbing or
friction
8. 4.3.Circulation
The primary air, is the method of
circulating the coal through the pulverizer.
Circulating air is also important in allowing
for the removal of heavy material such as
pyrites, extraneous metal, etc. by
centripetal force that otherwise might
damage the grinding mechanisms.
9. 4.4.Classification
The circulating air is also
used to classify the
pulverized coal product
prior to carrying it to the
burners.
The classifier, located on
the top of a mill returns the
over-size material back to
the pulverizer but allows
the proper-sized material to
pass out of the mill to the
burners. classifiers are
critical in providing the
desired quality of pulverized
coal with the desired
fineness
10. 5.Types of Pulverizers
Speed Low Medium High
10 to 20 40 to 70 900 to
rpm rpm 1000 rpm
Type Ball tube Bowl Hammer
mill Ball & race mill
mill
Dominating Attrition Crushing Impact
principle
15. 10.Bowl Mill
Bowl mills are employed to pulverize the pre-crushed
raw coal to the required fineness before it is admitted
into the boiler furnace for combustion.
The mill output can be easily varied, as per the
turndown ratio from its minimum to maximum load.
Crushed raw coal at a controlled rate is fed into the
revolving bowl of the Bowl Mill.
Centrifugal force feeds the coal uniformly over the
replaceable grinding ring where independently spun
rolls exert the required grinding pressure. The rolls do
not touch the grinding ring even when the mill is
empty.
20. 13.1.Fineness
Fineness is an indicator of the quality of the pulverizer
action. Specifically, fineness is a measurement of the
percentage of a coal sample that passes through a set of
test sieves usually designated at 50, 100, and 200 mesh
A 70% coal sample passing through a 200 mesh screen
indicates optimum mill performance.
The mill wear and the power consumption are increased if
the 70% value is exceeded.
Values lower than 70% mean higher carbon loss and
increased fuel consumption.
In addition, coal retained on the 50 mesh screen should be
in the 1–2% range. Higher values indicate worn internals or
improper settings. Also, the higher percentages can cause
boiler slagging and high unburned carbon.
21. 13.2.Sieve distribution chart & Fineness testing
screen
- Plot of coal fineness on Fineness testing screen
Rosin and Rammler
22. 13.3.Mill capacity vs Grindability
Figure shows the
effect of coal
grindability (HGI)
and desired
fineness (percent
passing a 200
mesh screen) on
the mill capacity.
23. 13.4.Moisture and grindability
effects on mill capacity
Figure demonstrates how
moisture and grindability
affect pulverizer capacity.
The dashed line estimates
the increase in mill capacity
in going from a high-volatile
B bituminous coal with a 55
HGI, 12% moisture and a
desired mill output of 70%
through a 200 mesh screen
to a highvolatile B bituminous
coal with an HGI value of 60,
a moisture content of 14%
and the same fineness.
24. 13.5.Mill Rejects
The amount of pulverizer
rejects is one indication
of mill performance.
Pyrites are the common
mineral iron disulfide
(FeS2) that has a pale
brass-yellow color and
metallic luster.
However, it is not
economical to attempt to
grind and burn pyrites & Pyrite Scraper and Guard Assembly
rock.
26. 13.7.Logic tree – Mill Fineness
Decrease
in mill
fineness
SIEVE TEST RESULTS
70% THRU 200 MESH
99.2% THRU 50 MESH
Classifier Loss of Ring or Classifier Exceeding
Vane Roller Roller Vane Mill
position tension wear wear capacity
27. 14.Abrasion in pulverizer
In coal pulverization,
5–20% of the material
being crushed is abrasive
mineral. Coal is not
abrasive by itself. The
minerals in coal that are the
most abrasive are quartz
and pyrite, which cause a
abrasion or severe wear.
carbides in the metal part,
High chromium cast iron
materials are used for
improved abrasion
resistance.
Worn Journal Roll
28. 14.1.CHEMICAL COMPOSITION OF THE
INSERTS GRINDING ROLLS
This specification of high chrome white cast iron
inserts for making Insert Grinding Rolls.
CHEMICAL COMPOSITION OF THE INSERTS:
C = 2.6 – 3.0 %
Cr = 15.0 – 22.0 %
Mo = 1.0 – 2.0 %
W = 1.0 – 2.0%
Mn = 1.0 – 2.0 %
Si = 0.5 – 1.0 %
S = 0.1 % max
P = 0.1 % max
29. 15.Erosion in Pulverizer
Erosion by mineral
particles picked up in the
air stream carrying
pulverized coal through
the mill, classifier,
exhauster, and transport
pipe is a recognized
problem.
Erosion can produce
holes in steel liners and
deep depressions in large
section cast parts.
There has been success
in the industry using
ceramic materials.
Classifier cone with ceramic
30. 16.Startup Procedures
The startup procedures, in addition
to the controls and interlocks,
should follow the requirements of
NFPA 85.
31. 17.Condition Monitoring
The main technologies used in
condition monitoring are:
Vibration analysis ,
Lubricant Oil analysis
Particle count
Viscosity
Total acid measure
Condition of oil additives
Sediment in Lubricating Oils
Any other test recommended by OEM.
32. 18.Inspection
Following inspection parameters are critical for bowl
mill performance:
• Classifier internal condition
• Deflector ring length
• Inverted cone clearance
• Journal assembly condition
• Grinding roll-to-bowl clearance
• Spring pressure for rolls
• Pyrite scraper clearance
• Pyrite rejects chute and/or damper condition
• Feeder settings
• Air in-leakage sources
33. 19.Test code of Pulverizer:
:ASME PTC 4.2
The purpose of this code is to establish
procedures for conducting performance
tests to determine:
Capacity, Fineness of product, Raw coal feed,
Grindability, Moisture, Sizing, Power consumption and
Effect of changes in raw coal Characteristics on
product fineness, pulverizer capacity, and power
consumption.
Effect of changes in pulverizer component settings on
product fineness, pulverizer capacity, and power
consumption.
35. Logic tree – Mill Fineness
Decrease
in mill
fineness
SIEVE TEST RESULTS
Classifier Loss of Ring or Classifier Exceeding
Vane Roller Roller Vane Mill
position tension wear wear capacity
