Effect of Coal Quality and Performance of Coal pulverisers / Mills

IMPACT OF COAL
QUALITY ON
PERFORMANCE OF
BOILERS
By Manohar Tatwawadi
total output power solutions,
Pune, Maharashtra, 411045

UNIT PARAMETERS- AFFECTED
DUE TO COAL QUALITY
VARIATION
• Maximum Capacity Rating
• Minimum load
• Availability
• Heat rate
• Maintenance Cost
• Waste disposal Cost
31-Mar-2021 total output power solutions 2

COAL QUALITY –IMPACT- COMPLEX
• Pulverizer capacity, fineness & wear
• Slagging & fouling in boiler
• Corrosion & erosion in boiler
• Ash characteristics affect efficient
collection in ESP.
• Affects furnace & convective pass heat
absorption & boiler availability .
• Quantification of impact is complex.

IMPACT ON FUEL HANDLING &
PULVERIZATION SYSTEM
• Moisture in coal impacts, mill capacity, heat
rate & unit capacity.
• Grind ability affects mill capacity & power
consumption.
• Fineness can be optimized by classifier
adjustment.
• Power consumption & Reject rate should
be bench marked.
• Coal sampling from individual mill is
important to generate data base.

WHY CHANGE THE COAL
• To meet the compliance limits for
particulate or gaseous emissions.
• An alternate coal may be economically
more advantageous.
• Change of coal supply has become
necessary due to availability problem.
• Operational problems like slagging, erosion
can be eliminated by change of coal quality.

The Purpose of a Pulverizer:
Note: About 70% of the 13 Essentials are fuel preparation
and balancing related.
• To Pulverize Coal to a Consistency
Suitable for Proper Combustion in the
Furnace and ;
• To Dry Incoming Coal
For Pressurized Pulverizers,
Classifier’s purpose is twofold:
Size Classification and
Uniform Fuel Distribution to each
Burner Line
The Pulverizer’s are the HEART of
a Pulverized Coal Fueled Boiler!

MILL PERFORMANCE COAL
QUALITY DEPENDANT
• MILL OUTLET TEMPERATURE
• MILL MOTOR AMPERES
• MILL AIR FLOW
• WEAR PART LIFE
• MILL PRESSURE DROP
• MILL OUT PUT
• MILL FINENESS FRACTIONS
• MILL REJECT RATE

PULVERIZER DESIGN CRITERIA
• PULVERIZERS SIZE & TYPE ARE
SELECTED TO PROVIDE A
GRINDING CAPACITY BASED ON
COAL CHARACTERISTICS
• MARGINS IN CAPACITY ARE BASED
ON EXPECTED WEAR LIFE AND
RANGE OF COAL TO BE FIRED
• OPERATING CAPACITY AFFECTED
BY COAL MOISTURE, INLET AIR
TEMPERATURE AND AIR FLOWS.

PERFORMANCE GUARANTEES
MILLING SYSTEM
PERFORMANCE GUARANTEES COVER
DESIGN MARGINS AND OPERATING
FLEXIBILITY.
• PULVERIZER OUT PUT & POWER
CONSUMPTION AT RATED FINENESS
• WEAR PARTS LIFE
• TURN DOWN RATIO (TESTS AT 3
DIFFERENT LOADS).
• CAPABILITY TO OPERATE WITH
ADJOINING MILLS AT 50 % LOAD

PERFORMANCE
CORRECTION CURVES
TESTS CANNOT BE ALWAYS CONDUCTED
USING DESIGN COAL HENCE CORRECTION
CURVES REQUIRED FOR
• MOISTURE IN COAL
• HGI OF RAW COAL
• MILL FINENESS
SANCTITY OF CORRECTION CURVES NEED
TO BE VERIFIED

GUARANTEES REQUIREMENT
TO CHANGE
• Clean air flow distribution to be with in
+/-2.5%.
• Dirty air flow distribution to be with in
+/- 5.0%.
• PF distribution to be with in +/-10%.
• Guarantees to be established for end
mills with unequal length in fuel piping
• Deferred guarantees.

TESTING TECHNIQUE INADEQUATE
• PERFORMANCE TEST IS RUN USING
ON LINE GRAVIMETRIC FEEDER.
• PF FINENESS SAMPLE COLLECTED IS NOT
REPRESENTATIVE.
• TESTING BASED ON ASSUMPTION THAT
AIR & FUEL DIATRIBUTION BETWEEN
VARIOUS FUEL PIPES IS NORMAL.
• RAW COAL SAMPLING DOES NOT
RESULT IN A REPRESENTATIVE COAL
SAMPLE FOR CORRECTIONS.

OBJECTIVE OF
PERFORMANCE TESTING
• CLEAN AIR BALANCING BETWEEN
BURNERS
• DIRTY AIR BALANCING BETWEEN
BURNERS
• FUEL BALANCING BETWEEN BURNERS
• FINENESS TESTING OF PF BETWEEN
BURNERS
• TO ESTABLISH TRENDS OF
DEGRADATION OF GRINDING CAPACITY
• TO DETERMINE THE ECONOMICAL
RUNNING LIFE OF MILL INTERNALS.

PREREQUISITES FOR
TESTING
• VERTICAL CIRCULAR PIPES AND
ADEQUATE STRAIGHT LENGTH .
• MAXIMUM PARTICLE SIZE TO BE LESS
THAN ONE THIRD DIAMETER OF TIP
• INTERNAL PIPE DIAMETER OF COAL
PIPES TO BE IN THE RANGE 250-700MM
• AIRFUEL RATIO TO BE WITHIN IN THE
NORMAL RANGE OF DIRECT FIRED
PULVERIZED SYSTEM.

PREREQUISITES TO
TESTING
• HOT PA FLOW TO EACH MILL TO BE
CALIBRATED REGULARY.
• STABLE UNIT CONDITIONS TO BE ENSURED,
INORDER TO AVOID LOSS OF VALUABLE
TEST DATA, TWO TESTING KITS MAY BE
USED.
• DETAILS OF ORIFICE IN EACH FUEL PIPE
& ITS WEAR PATTERN TO BE LOGGED.

CLEAN AIR CURVES
• CLEAN AIR CURVES PLOT COVERS
MILL DIFFERENTIAL VERSUS
STATIONARY PITOT DIFFERENTIAL
WITH NO COAL FLOW
• MILL MTC. REQUIREMENT IS BASED ON
INCREASED COAL SPILLAGE RATE AND
RUNNING HOURS
• CLEAN AIR FLOW TESTS ARE BEING
CONDUCTED ON A SHUT UNIT ONLY.

CLEAN AIR FLOW TESTS
• STEPS INVOLVED ARE DETAILED IN
THE TEST PROCEDURE
• STANDARD “ L” TYPE PITOT IS USED.
• WHETHER DIRTY PITOT CAN BE USED?
• CLEAN AIR FLOW BALANCE IS
COMPUTED BETWEEN COAL PIPES
• DEVIATION OF +- 2% INDICATES A
SATISFACTORY FLOW BALANCE AND
FURTHER TRENDING IS REQUIRED.

DIRTY PITOT TESTING
• TESTING PROCEDURE IS SIMILAR TO
CLEAN AIR FLOW TESTING .
• DIRTY PITOT TUBE WITH A SPECIFIC
CALIBRATION CONSTANT IS USED .
• DUSTLESS CONNECTOR IS USED TO
FACILITATE DUST FREE WORKING .
• PITOT TUBE NEED TO BE PURGED WITH
MOISTURE FREE COMPRESSED AIR TO
ENSURE RELIABLE MEASUREMENT.

DIRTY PITOT TESTING
• AVERAGE DIRTY AIR FLOW IS COMPUTED
THROUGH EACH COAL PIPE
• DIRTY AIR FLOW BALANCE IS
EXPRESSED AS A DEVIATION % FROM
THE MEAN AIR FLOW OF ALL THE PIPES
• DEVIATION PERMISSIBLE IS +/- 5.0%
• DRIFIT IN FUEL/ AIR MIXTURE
TEMPERATURE BETWEEN DIFFERENT
FUEL PIPES IS INDICATIVE OF UPSET

ISOKINETIC COAL SAMPLING
• TESTING METHODOLOGY IS DETAILED
IN TEST PROCEDURE
• ISOKINETIC SAMPLING ENABLES TO
DETERMINE FUEL /AIR RATIO IN EACH
OF THE FUEL PIPE
• WITHIN LIMITS OF DIRTY AIR
DISTRIBUTION, FUEL DISTRIBUTION IS
EXPECTED TO RESULT IN COMPARABLE
TRENDS

COAL FINENESS ANALYSIS
• FINENESS SAMPLE ANALYSIS NEEDS TO
BE CARRIED OUT IMMEDIATELY TO
AVOID COAGULATION IN CASE OF HIGH
MOISTURE COALS
• MINIMUM FOUR STANDARD MESH
SCREENS TO BE USED
• TVA DEVELOPED SOFTWARE ENABLES
REVIEW OF MILLS PERFORMANCE
DATA TO FOCUS ON VARIOUS TRENDS

WHY IS MILL PERFORMANCE
TESTING IMPORTANT?
• UNIT CAPABILITY GOVERENED BY MILL
PERFORMANCE
• BOILER AND COMBUSTION SYSTEM
PERFORMACE AFFECTED BY QUALITY
OF PF COAL AND ITS DISTRIBUTIION
• RELIABLE FEEDBACK SHOULD FOCUS
ON TIMELY MILL OVERHAUL
• VERY EFECTIVE CROSS CHECK OF THE
STATION INSTRUMENTS FEEDBACK

MILL PERFORMANCE MONITORING
USING NON ISO KINETIC TECHNIQUE
• DIRTY PITOT TESTING TO BE CARRIED OUT
BEFORE AND AFTER MILL OVERHAUL.
• NON ISO KINETIC SAMPLING TO BE USED FOR
ROUTINE CHECKING OF FINENESS.
• PF FINENESS DATA TO BE TRENDED MILL WISE
• SAMPLE TO BE COLLECTED FROM EACH PIPE
IN CASE OF MAJOR DEGRADATION.
• SAMPLE TO BE COLLECTED UNDER STABLE
CONDITIONS.
• STANDARIZE FORMAT FOR COLLECTING DATA

MILL PERFORMANCE
DEGRADATION CASE STUDIES
• STATION PA FLOW INDICATION WERE
FOUND OUT BY 5 TO 20 T/HR
• INDIVIDUAL FUEL PIPE TEMPERATURE
WERE OUT BY 5 TO 20 C
• DIRTY AIR FLOW DISTRIBUTION WAS
FOUND BEYOND +/- 5.0% IN MANY MILLS
• COAL DISTRIBUTION WOULD BE WITH IN
ACCEPATABLE LIMITS FOR CASES WHERE
IN AIR DISTRIBUTION IS NORMAL

FRONT FIRED BOILER
• BOILER IS SERVICED BY TWO PULVERIZERS,
HAVING TWO BURNERS FOR EACH
ELEVATION.
• IMBALNCE IN COAL COMBUSTION WAS
EVIDENT FROM UNEQUAL AMOUNT OF
UNBURNT CARBON IN FLY ASH .
• DIRTY AIR DISTRIBUTION WAS WITH IN 5%
HOWEVER DIFFERENCE BETWEEN TWO
BURNERS WAS AROUND 10% WHICH IS HIGH.
• MILLS IN FRONT FIRED BOILER OF
RAMAGUNDAM CANNOT BE TESTED DUE TO
NON AVAIABILITY OF STRAIGHT LENGTH.

0
1
2
3
4
1 2 3 4 5 6 7 8 9 10 11 12
DP
-
mmWC
Traverse Points
Mill C Dirty Pitot - Pipe 1
(March 02)
Series1 Series2
0
1
2
3
4
1
3
5
7
9
11
DP
-
mmWC
Traverse Points
(March 02)
Series1 Series2
0
1
2
3
4
1
3
5
7
9
11
DP
-
mmWC
TraversePoints
(MAY 02)
Series1 Series2 Series3
0
1
2
3
4
1
3
5
7
9
11
DP
-
mmWC
TraversePoints
(May 02)
Series1 Series2 Series3
Mill Tests with 2 sampling points
Mill Tests with 3 sampling points

Dirty Air Flow Variation (Mill A)
20
21
22
23
24
1 2
Dirty
Air
Flow
(T/hr)
Air Flow T/hr Mean
Dirty Air Flow Variation (Mill B)
20
21
22
23
24
1 2
Dirty
air
Flow
Air Flow T/hr Mean
Dirty Air Flow Variation (Mill C)
22
23
24
25
26
27
1 2
Discharge Pipe
Dirty
Air
Flow
T/hr
Air Flow T/hr Mean
Pipe to Pipe Flow Variations in Mills A,B,& C

BHEL BOWL MILL XRP 783
• FIVE MILLS REQUIRED TO MEET FULL
LOAD REQUIREMENT
• PA HEADER PRESSURE RUNNING LOW DUE
TO HIGH AIR HEATER LEAKAGE
• MILL FUEL PIPE CHOCKING TENDENCY
DUE TO LOW OPERATING VELOCITIES
• MILL OUTLET TEMPERATURE LESS THAN
OPTIMUM
• PA FLOW CALIBRATION HAS DRIFTED
OVER A PERIOD OF TIME

Dirty Pitot Survey - Summary Data (Mill X)
UCB Measured
Air Flow T/hr 42 52.0
Mill Outlet Temp C 75 62.0
Coal Flow T/hr --- 34.0
 Corner to corner temperature variations indicate unstable test conditions
Description Corner
1 2 3 4 Mean Desired
Velocity m/s 23.9 22.7 24.0 28.3 24.7 > 18 m/sec
Air Flow T/hr 12.9 12.1 12.7 14.8 13.1 ----
Dev. From Mean % -1.7 -7.8 -3.2 12.8 --- < +/- 5%
Mill Out Temp o
C 58.0 60.0 62.0 68.0 62.0 ~ 85o
C
Coal Flow T/hr 8.0 9.4 7.0 9.2 8.4 < +/- 10%
A/F Ratio 2.0 2.1 2.2 2.1 2.1 1.8 to 2.5
% retention on 50 mesh 1.2 1.8 0.9 1.7 1.4 < 1%
% retention on 200 mesh 72.6 72.7 80 74.8 75.0 ~70 %

Description Corner
Velocity m/s 31.4 28.4 30.7 27.2 29.4 > 18 m/sec
Air Flow T/hr 15.9 14.6 15.6 13.9 15.0 ----
Dev. From Mean % 6.0 -2.7 4.0 -7.3 --- < +/- 5%
Mill Out Temp o
C 79.0 76.0 79.0 77.0 77.8 ~ 85o
C
Coal Flow T/hr 4.6 6.2 7.0 5.3 5.8 < +/- 10%
A/F Ratio 2.2 2.2 2.3 2.0 2.2 1.8 to 2.5
%Retention 50 mesh 1.2 3.5 7 2.4 3.5 < 1%
%Pass -200 mesh 80.0 65.9 48.2 72.8 66.7 ~ 70%
UCB Measured
Coal Flow T/hr --- 23.0
 Measured Mill Outlet temperature matches with the Control Room
value but the Mill Operating PA Flow differs by 20 T/hr.
 High +50 mesh retention could be ascribed to the high Primary Air
Flow through the mill.

UCB Measured
Coal Flow T/hr - 26.2
Description Corner
Velocity m/s 24.3 22.9 16.0 17.4 20.2 > 18 m/sec
Air Flow T/hr 13.1 12.4 8.7 9.4 10.9 ----
Dev. From Mean % 20.2 13.8 -20.2 -13.8 --- < +/- 5%
Mill Out Temp o
C 56.0 55.0 53.0 55.0 54.8 ~ 85o
C
Coal Flow T/hr 7.9 5.8 5.6 6.9 6.6 < +/- 10%
A/F Ratio 1.7 2.1 1.0 1.4 1.6 1.8 to 2.5
 Operating PA flow through the mill is lower by almost 13 T/hr than
design
 Mill Outlet temperature is low in all the pipes.
 Low mill outlet temperature coupled with low PA flow could be the
reason for the choking observed in Pipes 3 & 4.

COAL PIPE INSTRUMENTS AND
MEASUREMENT ACCURACY
Comparison of Sampling Grids
Measurements in
Coal Pipes
Complicated Because
of Roping

PF Sample collected from Mill 1E discharge pipes
(15.05.02)
0
40
80
120
160
200
Corner 1 Corner 2 Corner 3 Corner 4
Weight
in
gram
s
Sample 1
Sample 2
PF Sample collected from Mill 2C discharge pipes
(06.06.02)
0
40
80
120
160
200
Weight
in
gram
s
Sample 1
Sample 2
PF Sample collected from Mill 2D discharge pipes
(03.06.02)
0
40
80
120
160
200
Weight
in
gram
s
Sample 1
Sample 2
PF sample collected from Mill 2E discharge pipes
(19.06.02)
0
40
80
120
160
Weight
in
gram
s
Sample 1
Sample 2
PF Sample Collected from Mill 2Adischarge pipes
(19.07.02)
0
50
100
150
200
250
Weight
in
gram
s
Sample 1
Sample 2
PF Sample collected from Mill 2F discharge pipes
(15.07.02)
0
4 0
8 0
1 2 0
1 6 0
2 0 0
Weight
in
gram
s
Sample 1
Sample 2

Variation of coal flows in the four corners in Unit 2
0
50
100
150
200
250
Sam
ple
Weight
(gram
s)
Mill A Mill C
Mill D Mill E
Mill F Average

USE OF METAL TEMPERATURE IN
FUEL PIPE TO MONITOR CHOCKING
• FUEL PIPING METAL TEMPERATURE IS BEING
MONITORED IN FARAKA TO MONITOR FUEL
PIPE CHOCKING.
• RECENT STUDIES SHOWED THAT LOWER THAN
MILL OUT LET TEMPERATURE IS NOT
NECESSARILY AN INDICATION OF PIPE
CHOCKING.
• DIFFERENCE OF 7 TO 8 C WAS OBSERVED
BETWEEN COAL AIR TEMPERATURE METAL
TEMPERATURE FOR A PIPE WHICH WAS
CLEAR.

BENEFITS
• CLEAN AIR FLOW BALANCING TESTS
WOULD CONFIRM THE ADEQUACY OF
FUEL PIPE & MILL ORIFICES.
• DIRTY AIR FLOW TESTS WOULD
CONFIRM AIR IMBALANCE IF ANY.
• REASONS FOR SHORT FALL IN MILL
PERFORMANCE OR ADDITIONAL MILL
REQUIREMENT CAN BE CHECKED.

BENEFITS
• PA FLOW AND COAL FLOW RATES CAN
BE CROSS CHECKED AND CALIBRATION
INTIATIVES CAN BE SCHEDULED.
• FUEL PIPING CAN BE CHECKED FOR
ANY CHOKING.
• ADEQUACY OF PIPING LAYOUT IN
ACHIEVING UNIFORM DISTRIBUTION
OF COAL AND AIR FLOW
DISTRIBUTIONCAN BE CHECKED.
• IMBALANCE IN COAL COMBUSTION IF
IT IS DUE TO UNEQUAL BURNER
LOADING WOULD BE KNOWN.

Pulverizer Troubleshooting-Matrix
• LACK OF
CAPACITY OR
HIGH POWER
CONSUMPTION
• HIGH MOISTURE
• LOW GCV
• INCREASED RAW
COAL SIZE.
• GRINDING TOO
FINE
• EXCESSIVE BED
DEPTH
• INSTRUMENT
ERROR

• EXCESSIVE MILL
REJECTS
• CHANGE IN COAL
GRINDABILITY,
SULFUR & ASH.
• IMPROPER
COAL/AIR RATIO
• THROAT GAP
WEAR.

• COARSE GRIND • CHANGE IN COAL
GRINDABILITY
• HIGH MOISTURE
• INCREASED
THROUGH PUT.
• CLASSIFIER
SETTING
• MILL WEAR.

• LOW COAL AIR
TEMPERATURE
• HIGH MOISTURE
• LOW PA INLET
TEMPERATURE
• PASSING OF COLD
AIR.
• LOW A.H INLET
TEMPERATURE
• NON AVAILABILITY
OF SCAPH

• CHANGE IN MILL
DIFFERENTIAL
• LOW GRINABILITY
• LOW MOSITURE
• MILL INTERNALS
PROBLEMS.

• MILL FIRES • HIGH VOLATILES
• MOISTURE
• LOW COAL AIR
TEMPERATURE.
• BURNER LINE
BALANCE

Based on the tests carried out in 2002 Pulverised Coal Corner and front fired Boilers

Effect of Coal Quality and Performance of Coal pulverisers / Mills

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