Pulverizer Operation Performance and Testing
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Transcript of Pulverizer Operation Performance and Testing
.1
Pulverizer Operation Performance and Testing
Rod Pifer
.2
Coal Recirculation
Final Classification
Gravity Separation
Primary
Classification
Fluidized
Bed
Product
OutputRaw Coal Flow
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Hot Air DuctHot Air ControlDamper
Flow
TemperingAir ControlDamper
Cold AirDuct
Coal‐airTransport Pipes
SwingValve
Operators
Pulverizer
Shut‐offDamper
PitotTubes
PrimaryAir
Damper
Primary Air Control – Common PA Fan
.4
Coal Properties Affecting Pulverizer Selection• Basic rating formula
– Corrected capacity =
Base capacity x Cg x Cf
x Cm
• Grindability
(Cg)
– Effect on capacity– As affected by moisture
• Fineness (Cf)– Effect on capacity– Requirements for fuel rank
• Moisture (Cm)– Effect on capacity– Effects on surface and inherent moisture
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Relative Pulverizer Capacity as a Function of Hardgrove
Grindability
0HGI ASTM D40930
1.6
1.5
1.4
1.3
1.2
1.1
1.0
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020 40 60 80 10010 50 70 90
Pulverizer
Capacity
Correction
Factor, Cg
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B&W Pulverizer Capacity Correction with 200 Mesh Fineness Standard Classifier
Pulverizer
Capacity
Correction
Factor, Cf
Fineness Percent Passing 200 Mesh
50 60 70 80 90
1.2
1.0
.8
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B&W Pulverizer Capacity Correction Due to Moisture
Pulverizer
Capacity
Correction
Factor, Cm
Surface Moisture (% by weight)
4 6 8 10 12
1.05
1.00
0.97
0.95
0.93
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Grindability
Index
INDEX SCALE 0 ‐100 HGI
B&W NOMINAL INDEX 50 HGI
TYPICAL COALS BITUMINOUS = 40‐45 HGI
PRB = 50‐55 HGI
.9
Ways to Improve Fineness
• Air Flow– Operate on design air/coal curve, periodically verified
by proper calibration– Improve air distribution in grinding zone
• Rotating throat• Throat orifice, plugs, inserts
– Lower/eliminate unnecessary control bias– Readjustments for coal changes
• New throat castings (Different vane thickness)
• Increase Classifier Speed
.10
High Side
Pressure Tap
Low Side
Pressure Tap
Equalizing Lines
Pitot
Tubes
Primary Air Duct
Between Fan &
Mill
Detail of the pitot
tubes
Air Flow
Averaging Pitot
Tubes for Primary Air Differential
.11
Possible Pitot
Tube Problems
– New pitot
tubes installed but not calibrated or tested– Plugged pitot
tubes
– Cracks or holes in the pitot
tubes – Pitot
tube holes eroded or enlarged
– Leaks or plugs in the signal tubing between the pitot tubes and the pressure transmitters
– Dams missing or broken– Dams left in when new pitot
tubes were installed
.12
What Could Be Wrong With Your Mill?
– Primary air flow is set too high or too low for proper mill operation
– Primary air flow is set too high or too low for good combustion
– Primary air flow measurement is incorrect
– The coal being burned is not what the mill was calibrated to grind
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Why Proper Mill Calibration Is Important
– Primary air is needed to dry the coal and to transport
the coal
– Only use enough primary air to dry and transport the coal
– Proper primary air flow helps maintain the proper bed of
coal in the mill
– The mill is a ball bearing with the coal bed acting as the grease
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What Happens If We Have Too Much PA?
– Coal fineness is reduced as increased circulation through the classifier
increases the chance for large particles to blow through the classifier
– Higher throat velocity blows coal out of the bed in the rings thus reducing
the lubricating effect in the grinding zone
– Less coal bed in the grinding zone can result in rippling, chipping and
cracking of the rings as well as spalling
and uneven wear on the balls
– Increased velocity at the burner can make the flame unstable and
increase
the LOI and possibly NOx
emissions
.15
What Happens If We Have Too Little PA?
– Wet coal at the burner may not ignite properly resulting in
increased slagging
and increased LOI
– Reduced PA temperature at the burner may cause ignition issues resulting in increased slagging
and increased LOI
– The flame could pull back into the burner
– Could have coal dribble at lower mill loads
.16
Compare Pulverizer PA Requirements to PA Requirements for the Rest of the Combustion System
B&W coordinates the size and performance of the burners and coal piping with pulverizers
Some aftermarket burners require modifications to the PA flow to optimize combustion and emissions
Changing the coal pipe size will affect minimum PA flow requirements
Pulverizers, coal pipes, and burners cannot be seen as separate entities, as they are all interrelated
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Perform Primary Air Calibrations Properly– Prepare pulverizer control curve based on the coal being used– Calibrate the PA, Dp
and Mill Dp
pressure transmitters
– Remove, inspect, clean and leak check pitot
tubes
– Isolate all seal air– Fill raw coal chute with coal and close valve at coal silo– Perform PA Dp
versus Mill Dp
test; resulting curve must be a
straight line passing through the origin– Perform clean air tests (traverses) with PA flows in the range
expected during actual operation– Use manometer to measure PA Dp, Mill Dp, and PA pressure– Compare manometer readings to control room readings– Mark test pitot
tube for equal areas
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Perform Primary Air Calibrations Properly (cont.)
– Measure PA flow in each coal pipe – Calculate the K‐factor for the pitot
tubes
– Repeat testing at 3‐4 PA flows– All of the K‐factor should be within ±5% of the average– Compare PA flows in each coal pipe; this is considered to
be a good indication of the coal balance between coal pipes
– Prepare final mill control curves also called Characterization Curves or Dirty Air Curves
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What Can Change Your K‐Factor?
– New or modified burners
– New grinding zone in mill
– Significant coal change (changes PA range)
– Modifications to PA fans (changes how PA comes off fan toward pitot
tubes)
– Modifications to the PA Ducts (changes how PA comes off fan toward pitot
tubes)
.20
Effect of PRB on Mill Operations
– Significantly higher PA temperatures required due to high
moisture
in PRB
– Significantly higher volume of coal feed due to lower BTU in
PRB and lower bulk density of PRB
– PRB is Sticky ‐
it plugs chutes and hoppers or anyplace it falls
onto a ledge or ridge
– PRB loves to burn ‐
it burns in the boiler, it burns in the
pulverizers, it burns in the hoppers, it burns in the pile, etc.
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Effect of PRB on Mill Operations (cont.)
– Blends of bituminous coal and PRB have special problems
– Grinding bituminous coal is like trying to break a box full of marbles with a sledge hammer
– Grinding a blend of bituminous coal and PRB is like trying to break the box full of marbles with the same sledge
hammer after someone has mixed in a bunch of tennis balls
– PRB is more friable than bituminous coal, so it will have a small size distribution in the bunkers
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Effect of PRB on Mill Operations (cont.)
– Most bunkers and silos are highly effective particle size segregators
– This means average particle size of the coal will vary over a bunker loading cycle
– Coal bulk density could change depending on how small and large particles fit together in your bunker
– Also moisture content could change as smaller particles with more surface to volume ratio bring in more surface moisture
.23
Current B&W Roll Wheel Pulverizer Operating DataThe following data (provided by the customer) is required by B&W
for use indetermining the optimum rotating throat port area:1. Normal full load coal flow per pulverizer.2. Normal full load air flow per pulverizer.3. Pulverizer inlet/outlet air temperature at above normal full load coal and
air flow.4. Mill differential pressure at normal full load coal flow.5. Mill inlet static pressure at normal full load coal flow
(high side of mill differential pressure.6. Maximum coal flow at which pulverizer is operated.7. Minimum coal flow at which pulverizer is operated.8. Number of coal pipes in service per pulverizer.9. Raw coal Hardgrove
grindability.10. Raw coal total/surface moisture.11. Raw coal ash content.12. Rank of fuel.13. Is there a significant amount of pyrites, rock or tramp iron
in the coal?14. Actual vs. desired fineness.15. What is the major goal of installing rotating throats i.e.; reduce mill pressure
drop,
reduce grinding zone erosion, improve fineness?
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Classifier
Discharge
Baffles
Raw Coa
l
Roll Wheel Pulv. Classifier Assembly
.25
Poor Fineness in a Roll Wheel Pulverizer
Hole in classifier cone
Missing flap gate
Flap gate stuck open
Low spring pressure
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Ways to Improve Roll Wheel Pulverizer Fineness• Classifier louver section
– Outside/inside diameter
– Vane angle– Vane length– Number of vanes
– Discharge cylinder length– Bottom plate (retention/spin)
• Classifier cone– Top I.D.– Cone angle– Cone volume
• Classifier discharge (reduce “Seal”
leakage)– Conical baffles– Sloped discharge doors– Improved doors (hinges fitup
weighted)
.27
B&W Roll Wheel Pulverizer Improvements Update
Rod Pifer
.28
B&W Roll Wheel Pulverizer Upgrades
– Cast Low Pressure Drop Rotating Throats
– Wearesistor®
Tires
– Wearesistor®
Low Profile Tires
– Smooth I.D. grinding segments
– Segmented Yoke Air Seal
– Ceramic‐lined swing valve seats
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Substantial increase in usable wear material compared to original design tiresUse with original design standard profile grinding segmentsMounts on existing wheelNo modifications required
Wearesistor®
(Asymmetric) Tires Standard Tire
Profile
Wearesistor®
Tire
Profile
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Smooth I.D. Grinding Segments for B&W Roll Wheel Pulverizers
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Cut‐Away View of Segmented YAS in Pulverizer
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New Segmented Yoke Air Seal (Split Line Shown)
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New YAS Installed in Pulverizer (Match‐Marked Segments Shown)
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Wear‐resistant Cera‐VAM®
Swing Valve Plate and Seat
for Roll Wheel and EL Pulverizers
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