MILLING SYSTEM
NTPC
JAN 2021
1
Topics to be covered
• Milling system layout.
• Different types of Mills (Tube Mills, Bowl Mills,
Ball & Race
• Raw Coal feeders.
• Mill Reject system.
2
BOILER AREA PLAN TYPICAL 500MW
MAIN BOILER
COAL MILLS
FD & PA
FANS
ELECTRO STATIC
PRECIPITATOR
ID FANS
AIR PREHEATERS
CHIMNEY
3
BOILER AREA PLAN
4
BOILER AREA LAYOUT
5
BOILER AREA LAYOUT 500 MW
6
TYPES OF PULVERISERS
• BASED ON PRINCIPLES OF PARTICLE SIZE
REDUCTION
- IMPACT
- ATTRITION
- CRUSHING
• PULVERISERS USE ONE, TWO OR ALL THE
THREE PRINCIPLES.
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Principle of Grinding
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TYPES OF PULVERISERS
SPEED
LOW
10=20 RPM
MEDIUM
30 -70 RPM
TYPE
BALL TUBE
MILLS
BOWL
MILLS, MPS
MILLS,BALL
AND RACE
MILLS
HIGH
900-1000
RPM
IMPACT OR
HAMMER
MILLS,
BEATER
MILLS OR
FAN MILLS
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BBD TUBE MILL
SIZE:BBD4772:4.7MDX7.2ML BALL CHARGE 80-110MT
CAPACITY 99T/HR
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RAYMOND BOWL MILL
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BOWL MILL SIZE
•
•
•
•
583 XRS/ 803 XRP BOWL MILLS
58,80 STANDS FOR BOWL DIAMETER IN INCHES
3 - NUMBER OF ROLLERS THREE NOS.
X - FREQUENCY OF POWER SUPPLY 50 CYCLES. IN
USA ’ X’ MEANS 60 CYCLES.
• R- RAYMOND, NAME OF THE INVENTOR
• S- SUCTION TYPE WITH EXHAUSTER AFTER MILL
• P- PRESSURISED TYPE WITH P.A. FAN BEFORE
MILL.
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MPS MILL
13
HAMMER MILL
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GRINDING PROCESS IN THE MILL
RAW COAL
P.F. TO BOILER
HEAVY
PARTICLES
CLASSIFIER
P.F.+AIR
HOT AIR
GRINDING ZONE
DRYING & GRINDING
REJECTS
15
PARAMETERS WHICH DETERMINE THE
PERFORMANCE OF MILL
• RAW COAL-HGI,TM,SIZE
• PF FINENESS
• R.C.FEEDER CALIBRATION
• PA FLOW
• MILL INTERNALS CONDITION AND TUNNING
• MILL OPERATING PARAMETERS
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EFFECTS OF PARAMETERS
RAW COAL CHARACTERISTICS
MOISTURE
DRYING REQUIREMENTS
MILL OUTLET
VOLATILE
DETERMINE
TEMP
MILL INLET
ASH
GRINDING ELEMENTS WEAR
HGI
GRINDABILITY
CALORIFIC VALUE
COAL QUANTITY
17
INPUT COAL QUALITY:
Calorific Value:
• Calorific value directly indicates the quality of coal and
hence, the amount of coal to be fired. In most of the
stations the calorific value of coal is away from the
design value, which in turn causes an increased loading
on the mills in most of the cases. In some cases,
deterioration in coal quality leads to increased number
of mills run for full load.
• While operating, care should be taken to load the mills
evenly to the extent possible for improved combustion
and mill performance. Mill loading should be kept
within the prescribed minimum and maximum limits.
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INPUT COAL QUALITY:
Volatile Matter
• Volatile Matter in coal is a factor, which aids combustion
and gives combustion stability. Higher the VM, the coal
will burn better and burn nearer to the burner. VM varies
from about 12% to 24% in the coals used at our stations.
• Low VM coal requires a better PF fineness compared to
high VM coal. This aspect is to be kept in mind while
loading the mill and fineness readings are to be taken and
monitored more frequently. When fineness cannot be
increased beyond a limit due to various reasons like mill
design, coal flow rate etc. other parameters like total air
flow, PA flow, secondary air distribution, etc. should be
optimized for better combustion stability.
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Pulverised Coal Burning wrt VM
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INPUT COAL QUALITY:
Moisture Content
• Mill should be able to remove total surface moisture
and up to half of the inherent moisture from the design
range of coal. With increased moisture content, the
mill output reduces. Tube mills are more sensitive to
moisture compared to vertical spindle mills .
• Efforts should be made to minimize the external
moisture by proper storage in CHP. Mill loading
should be adjusted so as to maintain the mill temp.
within operating limits. Sometimes more number of
mills may have to be run for achieving this.
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EFFECT OF MOISTURE ON MILL CAPACITY
(BOWL MILL)
1.1
MILL CAPACITY FACTOR
1
0.9
0.8
0.7
0.6
0.5
0.4
0
2
4
6
8
10
12
14
MOISTURE IN COAL (%)
16
18
20
22
22
EFFECT OF MOISTURE ON MILL CAPACITY
(TUBE MILL)
CAPACITY FACTOR
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
MOISTURE IN COAL (%)
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INPUT COAL QUALITY:
Hardness
• Mill loading capacity and PF fineness deteriorate when
harder coals (with low HGI) are used (see curve). If the
mill is properly designed taking into consideration the
realistic HGI of coal, limitations in mill output due to a
change in Hardness can be avoided. Normally mills are
designed to handle harder coal and will give an
increased output with softer coals.
• Though this is an uncontrollable factor, monitoring the
hardness will help in understanding the behaviour of
the mill.
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GRINDABILITY INDEX VS MILL OUTPUT
1.6
MILL OUTPUT X 100%
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
30
40
50
60
70
80
90
100
GRINDABILITY INDEX (HGI)
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INPUT COAL QUALITY
Silica / Quartz Content
• Inherent or external silica / quartz content reduces the
grindability of coal and increases the wear on mill parts.
Hardness of coal is 0.5 to 2.5 MHO whereas hardness of
quartz is as high as 7 MHO. As the abrasiveness of coal is
increased, the bull ring segments and rolls of vertical spindle
bowl mills wear preferentially in zones of high local coal bed
pressure. This leads to increased clearances in these areas
thereby reducing grinding efficiency. Increased abrasiveness
reduces the grinding element life
• Whenever the DP crosses a prescribed limit, the mill should be
purged by cutting of coal and air supply to take out the sand
accumulation. Coal pipes also can be purged with air to
prevent deposition of sand / coal.
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ABRASION DEPTH, MM/1000
TONS OF COAL
EFFECT OF QUARTZ IN COAL ON MILL WEAR
1.2
1
0.8
0.6
0.4
0.2
0
4
5
6
7
8
9
10
11
12
QUARTZ, %
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INPUT COAL QUALITY:
Size of Input Coal:
Size of input coal is a factor, which determines the
mill loading capacity. The size should be maintained
nearer to the design values in co-ordination with CHP.
Sieve analysis is of the crushed coal should be done at
regular intervals to ensure that the size of coal is
within limits.
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INPUT COAL QUALITY:
Grinding Elements Life:
The wear life / condition of grinding elements will have a
considerable effect on the performance of the mill. Each station will
have an average grinding element life for a type of mill, depending
on coal quality, mill setting, operating conditions etc. During this life
time the loading capacity gradually reduces and the current
consumption gradually increases. Mill PM requirements also increase
as the wear increases. This phenomenon is more predominant in bowl
mills. If the mill is loaded beyond its capacity w.r.t. its wear life, the
performance will suffer.
Operating personnel should be aware of the life and wear pattern of
each mill and load the mill accordingly.
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INPUT COAL QUALITY:
Classifier Setting:
Classifier setting of the mill should be adjusted to different
positions during its life cycle. Initially when the grinding
elements are new, the losses across the mill are minimum and
primary classification will be less. This necessitates a lower
classifier opening for proper product fineness. When the
grinding elements grow older the mill DP will increase and
primary classification improves. During this time classifier
vanes can be opened gradually which will improve the loading
at the same fineness.
For tube mills, performance level can be maintained by adding
fresh charge of balls at regular intervals.
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Operating Parameters:
Mill Outlet Temp.:
For good pulveriser and combustion performance, the
temperature of the coal air mixture leaving the mill should be
maintained as high as possible within the safe temperature limit
of the particular coal.
A temperature of 85 to 90ºC is acceptable for coals with normal
VM. An outlet temp. below 60ºC may not dry the coal
sufficiently. Operating the mill with lower than allowable temp.
will cause coal pipe choking also due to the presence of
moisture, as the air may reach its dew point.
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Operating Parameters:
Primary Air Flow
Primary Air Flow should be low enough to avoid ignition
instability and high enough to avoid settling and drifting in the
coal pipes or excessive spillage of coal from the pulveriser
through the tramp iron spout. To avoid blockage of fuel pipes,
a minimum average transport velocity of 20 m / s should be
maintained. The normal operating velocity will be between 24
m / s and 27 m/s.
Increase in the ‘mill to furnace DP’ measurement (if provided)
will indicate coal pipe choking. In some stations coal pipe
temp. or drop in temp across the coal pipe is monitored for
early detection of coal pipe choking. Running the mill with
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choked coal pipe / pipes is not advisable.
Operating Parameters:
Primary Air Flow
A higher PA flow also affects the PF fineness by
reducing primary classification as it lifts the coarser
particles. This factor can be confirmed by taking PF
fineness at different PA flows and plotting a curve.
To meet the above requirements, PA flow has to be
varied w.r.to coal flow as per the ‘Coal Vs PA flow’
curve. On medium speed vertical spindle mills the ratio
is approximately 1:1.7 to 1:2.2 and on tube mills its
about 1:1.2 to 1:1.5.
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Operating Parameters:
Mill DP:
Mill DP is an indication of the losses inside the mill. It
increases with a higher mill loading or with other factors
like wear of grinding elements, accumulation of coal or
sand inside the mill, choking of throat area, blockage of
classifier cone etc. In case of Tube mills, mill DP
indicates the amount of coal inside the mill.
If the DP is allowed to cross the design values, mill
performance will deteriorate. The reason for a high DP
should be investigated and corrective actions like mill
purging, PM jobs etc. should be taken.
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Operating Parameters:
Mill Current:
Mill current is an indication of the mill loading and the
condition of the mill. Different types of mills have
different power consumption per ton of coal ground. Mill
power consumption increases with loading as well with
mill wear, particularly in vertical spindle mills. Care
should be taken not to exceed the allowable maximum
current limit.
Maximum economy of milling plant power is obtained by
operating the optimum number of mills at their maximum
output rather than operating more number of mills at part
load. Where partial mill loading is necessary to meet a
reduced boiler output, it is good practice to load each mill
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to the same degree.
3.5
PF Fineness:
Mill output and fineness are interdependent and an increase in
output can be obtained only at the expense of a deterioration in
fineness, assuming that power input to the mill remain same.
This practice is not recommended unless the coarser
pulverised fuel provides satisfactory combustion (see curve).
To take preventive actions, fineness readings should be taken
as frequently as possible and should be monitored by
Operation and Maintenance staff. PF sample collection and
analysis should be done as per the standard procedure and the
readings should be validated in a Rosin-Rammler chart. If the
readings are not in line with the chart, the sampling procedure,
air flow or the mill internals should be checked.
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MILL LOADING VS MILL FINENESS CURVE FOR AN
XRP 1003 MILL
MILL FINENESS (-200), %
70
68.3
68
66
64
63.3
62
60
58
58
56
40
50
60
70
MILL LOAD T/HR
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MILL CAPACITY
• MILL CAPACITY
WHEN HGI
• MILL CAPACITY
P.F.FINENESS
• MILL CAPACITY
RC TM
38
MILL CAPACITY DEPENDS ON
• MILL CAPACITY
RC SIZE
• MILL CAPACITY
MO TEMP
• MILL CAPACITY
BOWL DP
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RAW COAL FEEDERS
There are two type of feeders
• Volumetric feeders
1. Chain type feeders
2. Belt type feeders
3. Table type rotary feeders
• Gravimetric type feeders
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RAW COAL FEEDERS
Volumetric Feeders
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RAW COAL FEEDERS
Gravimetric Type
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RAW COAL FEEDERS
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RAW COAL FEEDERS
Main components on Raw Coal Fedeers are
• Belt or Chain
• Bunker shut off gates
• Front bearings
• Tail end bearings
• Gear Box
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Mill Reject Handling System
• Dense Phase Pneumatic Conveying system :
One discharge hopper is connected to pyrite
hopper and control systems are designed to
remove effectively rejects and pyrites pieces
etc. The material is conveyed through steel
pipes main hopper and then shifted to
respective place by dumper.
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Mill Reject Handling System
Dense Phase conveying system
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Mill Reject Handling System
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Mill Reject Handling System
• Chain/ Conveyor System : Each pyrite hopper
is connected to chain/conveyor moving below
the hopper. Reject is moved to storage bunker
and then shifted to respective place by
dumper.
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Mill Reject Handling System
Chain Conveyor Syste
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Thank you
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