This document discusses factors that affect mill performance in coal power plants. Key factors include input coal quality parameters like calorific value, volatile matter, moisture content, hardness, and silica/quartz content. Other important factors are the condition of grinding elements and operating parameters such as mill outlet temperature, primary air flow, mill differential pressure, and mill current. Maintaining these factors within design limits optimizes mill loading capacity and improves combustion stability and efficiency.

1. MILL PERFORMANCE OPTIMISATION
ROLE OF OPERATION

2. PERFORMANCE INDICATORS:
Mill performance is said to be good when it takes the
design / tested load and delivers the product at the
designated fineness values with minimum current
consumption and reject rate.

3. FACTORS AFFECTING PERFORMANCE:
Mill performance depends on a variety of factors
other than proper maintenance which are listed
below:
• Input coal quality.
• Grinding elements life.
• Operating parameters.

4. 1.0 INPUT COAL QUALITY:
1.1 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 less than the design value, which
in turn causes an increased loading on the mills. 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.

5. 1.2 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 23% 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.

6. 1.3 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 (see curve).
Efforts should be made to minimize the external moisture by
coordinating with 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.
SCAPH can be charged to improve the APH outlet temp. Mill
outlet temp. should not be allowed to drop below 60ºC.

7. EFFECT OF MOISTURE ON MILL CAPACITY
(BOWL MILL)
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0 2 4 6 8 10 12 14 16 18 20 22
MOISTURE IN COAL (%)
MILL
CAPACITY
FACTOR

8. EFFECT OF MOISTURE ON MILL CAPACITY
(TUBE MILL)
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25
MOISTURE IN COAL (%)
CAPACITY
FACTOR

9. 1.4 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.

10. GRINDABILITY INDEX VS MILL OUTPUT
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
30 40 50 60 70 80 90 100
GRINDABILITY INDEX (HGI)
MILL
OUTPUT
X
100%

11. 1.5 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 (see
curve).
Apart from this, sand particles settle at the bottom of the mill and form a
bed. This will increase the mill DP, current and reject rate. 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.

12. EFFECT OF QUARTZ IN COAL ON MILL WEAR
0
0.2
0.4
0.6
0.8
1
1.2
4 5 6 7 8 9 10 11 12
QUARTZ, %
ABRASION
DEPTH,
MM/1000
TONS
OF
COAL

13. Whenever more stones are observed in coal, CHP
should be informed for taking corrective action like
increasing the efficiency of stone picking or taking up
the matter with the mines.
Operating the mills with higher spring compression or
an increased air fuel ratio will grind the sand particles
and send it to the furnace. But this should be avoided
it will cause increased wear in coal pipes, burners and
pressure parts. Moreover, the silica / quartz will get
deposited in the furnace walls. This will considerably
reduce heat transfer and are very difficult to remove.

14. 1.6 Foreign Material:
Increased amount of foreign material in coal causes feeder /
mill breakdowns and frequent reject passage choking. If
abnormal sound or reject passage choking is noticed, the mill
should be stopped at the earliest to reduce the extent of
damage. Prolonged operation of the mill with a blocked reject
passage will increase the damage and downtime. Choking of
reject passage with stones can lead to clinkering in the
scrapper box.
Coal Handling Plant should be regularly appraised about the
presence of foreign material / stones in coal. Proper operation
of MD / MS, crusher screens and proper sizing of bunker grills
should be always ensured.

15. 1.7 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.

16. 2.0 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.

17. 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.

18. 3.0 Operating Parameters:
3.1 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.

19. 3.2 PA 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
choked coal pipe / pipes is not advisable.

20. 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.

21. 3.3 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.

22. 3.4 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
to the same degree.

23. 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.

24. MILL LOADING VS MILL FINENESS CURVE FOR AN
XRP 1003 MILL
68.3
63.3
58
56
58
60
62
64
66
68
70
40 50 60 70
MILL LOAD T/HR
MILL
FINENESS
(-200),
%

25. MILL OUTPUT VS MILL FINENESS CURVE FOR AN
XRP 1003 MILL
0.6
1.3
3.5
0
0.5
1
1.5
2
2.5
3
3.5
4
40 50 60 70
MILL OUTPUT, TPH
FINENESS
(+50),
%

26. SIEVE DISTRIBUTION CHART
60
70
80
90
100
0 50 100 150 200 250
US STANDARD SIEVE
FINENESS,
%
PASSING
THRU

27. Factors affecting PF Fineness:
a) Overloading of the mill.
b) Improper setting of spring tension, classifier vanes etc.
c) Excessively worn out grinding elements
d) Damage to mill internals.
e) Higher PA flow.
f) Improper sampling procedure – non-usage of
aspirating fittings.
g) Improper sample analysis.
Whenever a deterioration in fineness is noticed corrective
action should be taken at the earliest, so that the effect on
combustion, boiler efficiency, coal pipe choking etc. are
minimized.

28. MONITORING:
The effect of the various factors mentioned above underscores the
need for monitoring the operating parameters regularly for taking
corrective actions in time. Useful feedback regarding quality of coal
like wetness, foreign material / stones and the damage caused by
them, input coal size etc should be given to CHP for taking corrective
action.
Similarly, mill-operating parameters should be monitored regularly
by maintenance staff also. It will give an insight into the mill
performance and provide input for scheduling mill maintenance. The
operating parameters should be co-related with other mill parameters
at least once in a week and corrective action should be taken for
deviations.

29. CONCLUSION
By proper understanding of the factors affecting mill
performance and by proper monitoring of those factors, one
should be able to predict the mill performance in normal
cases. Mill operating parameters can be adjusted within the
allowable ranges to suite the changing characteristics of
coal. Mill maintenance personnel should also monitor the
mill parameters, which will give an insight into the
healthiness of the mill and will help in planning the
maintenance.

30. MILL OPERATING PARAMETERS
UNIT: DATE:
Unit load, MW: Total Coal Flow, tph:
Total Air flow, tph: PA header pressure, mmwc:
Seal air header pressure, mmwc:
SL NO. PARAMETER MILL
A B C D E F
01 Coal Flow, tph
02 Mill current, A
03 PA flow, tph
04 Mill DP, mmwc
05 Mill to furnace DP, mmwc
06 Mill outlet temp. º C
07 Inlet PA temp. º C
08 HAD position, %
09 CAD position, %
10 Mill fineness, %: +50
- 200
11 Date of PM jobs (last)
12 Date of grinding elements replacement (last)
13 Coal quality (as on: -------):
GCV, kcal / kg:
HGI:
Moisture, %:
VM, %:
14 Unburnt carbon content (as on: --------):
Bottom ash, %:
Fly ash, %:

31. MILL PERFORMANCE MONITORING SHEET
UNIT: DATE:
Unit load, MW: Total Coal Flow, tph:
Total Air flow, tph: PA header pressure, mmwc:
Seal air header pressure, mmwc:
SL NO. PARAMETER MILL
A B C D E F
01 Coal Flow, tph
02 Mill current, A
03 PA flow, tph
04 Mill DP, mmwc
05 Mill to furnace DP, mmwc
06 Mill outlet temp. º C
07 Mill fineness, %: +50
- 200
08 Classifier position
09 Date of PM jobs (last)
10 Date of grinding elements replacement (last)
11 Running hours since last GE replacement
12 Grinding elements make:
BRS:
Rolls:
13 Roll wear, mm
14 Ring – roll gap, mm
15 Spring Compression, psi
16 Venturi height, inches
17 Throat gap, mm
18 Mill Rejects, %:
CV:

32. THANK YOU