Milling System PerformanceMilling System Performance1

PMI Revision 01

The Purpose of a Pulverizer:2

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Must be able to pulverize pconsistently the design quantity of coal suitable for combustioncombustion.

Able to dry wet coal up to the design wetness even at full output condition.

Able to maintain the products within the desired grading within the desired grading

The Pulverizer’s are the HEART of a Pulverized Coal F l d B il !

PMI Revision 01

Fueled Boiler!

PULVERIZER DESIGN CRITERIA3

PULVERIZERS SIZE & TYPE ARE SELECTED TO PROVIDE A GRINDING CAPACITY BASED ON COAL CHARACTERISTICSCHARACTERISTICS

MARGINS IN CAPACITY ARE BASED ON EXPECTED WEAR LIFE AND RANGE OF EXPECTED WEAR LIFE AND RANGE OF COAL TO BE FIRED

OPERATING CAPACITY AFFECTED BY OPERATING CAPACITY AFFECTED BY COAL MOISTURE, INLET AIR TEMPERATURE AND AIR FLOWS

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MILL PERFORMANCE INDICATORS4

MILL OUTLET TEMPERATURE

MILL OUT PUT M/T

MILL FINENESS FRACTIONS

MILL AIR FLOW

MILL PRESSURE DROP (D.P)( )

MILL REJECT RATE

MILL MOTOR AMPERES

WEAR PART LIFE

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CAUSE OF MILL PERFORMANCE DETORIARATION

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QUALITY OF COAL (MOISTUR, HGI ETC.)Q ( , )

UN BALANCED FLOW IN COAL PIPES.

SETTLEMENT OF COAL IN COAL PIPES.

POOR COAL FINENESS

LOW PA TEMPERATURE

FOREIGN MATERIAL INGRESS.

LOSS THROUGH REJECTION

WEAR PART LIFE

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MILL PERFORMANCE - IDENTIFICATION METHODS

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NO LOAD RUN

CLEAN AIR FLOW TEST

DIRTY PITOT TUBE TEST

MILL FINENESS SAMPLING

TRENDING OF COAL PIPE METAL TEMPERATURE

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OBJECTIVE OF TESTING

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CLEAN AIR BALANCING BETWEEN BURNERS

DIRTY AIR BALANCING BETWEEN BURNERS

FUEL BALANCING BETWEEN BURNERS

FINENESS TESTING OF PF BETWEEN BURNERSBURNERS

SETTLEMENT OF COAL PARTICLE IN COAL PIPE

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COAL PIPE

CLEAN AIR FLOW TESTS8

STEPS INVOLVED ARE DETAILED IN THE TEST PROCEDURE

STANDARD “ L” TYPE PITOT IS USED.

CLEAN AIR FLOW BALANCE IS COMPUTED BETWEEN COAL PIPES

DEVIATION OF +\- 2% INDICATES A SATISFACTORY FLOW BALANCE AND FURTHER TRENDING IS REQUIREDFURTHER TRENDING IS REQUIRED

PMI Revision 01

The Pitot Static tube measures the

PITOT TUBE – HOW IT WORKS ?

total pressure (or impact pressure) at the nose of the Pitot tube and the static pressure of the gas stream at side ports The difference of theseside ports. The difference of these pressures, i.e. the dynamic or velocity pressure (Pdynamic) varies with the square of the gas velocity. Thus the gas velocity may be expressed as:

where r is the gas density and C is a correction constant dependent on thecorrection constant dependent on the design of the Pitot Static tube. NOTE: This equation is typically valid for incompressible (constant density)

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Generic pitot tube configurationp ( y)

flow

Pitot Static tube duct surveys

• if average duct velocities, or mass or volumetric flow rates are required, i i f Pi f h d

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it is necessary to perform a Pitot traverse of the duct.

• This involves taking measurements at various positions across the duct.,

• it is necessary to select a suitable location to perform the survey. It is preferable to survey at least 8 duct diameters downstream and 2 duct diameters upstream of the equipment.

Aduct is the duct cross sectional area.n is the number of points (total number of points surveyed).

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Vi is the indicated velocity at each measurement point.

An Example of mechanical tuning

Before mechanical tuning

After mechanical tuningAfter mechanical tuning

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Dirty pitot tube – measuring the two phase mixture of Coal & Air

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Acceptance range14

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

PMI Revision 01

Clean Air Balancing, A Vital First Step to Fuel Line Balancing

Fuel lines balanced to each burner by “Clean Air” test ±2% or better.

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ISOKINETIC COAL SAMPLING

The Isokinetic Sampler is used to extractThe Isokinetic Sampler is used to extract pulverized coal from (24) points of each coal pipe (depending on piping size).at as close to actual velocity entering the sampler nozzle, as the coal particles were flowing before collection. This iscalled “Isokinetic” Sampling. The prefix “iso” means same, and “kinetic” means energy or velocity of the particles. Because the coal particles are about a thousand times more dense than airthousand times more dense than air, isokinetic sampling is important to obtain truly representative coal samples for sieving Figure No 7

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sieving.Figure No. 7

ISOKINETIC COAL SAMPLING18

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 WITHIN LIMITS OF DIRTY AIR DISTRIBUTION, FUEL DISTRIBUTION IS EXPECTED TO RESULT IN COMPARABLE EXPECTED TO RESULT IN COMPARABLE TRENDS

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Use of the Isokinetic C l S lCoal Sampler

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PULVERIZING PROPERTIES OF COAL

Grindability is defined as the ease with which the

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y fcoal can be pulverized. This should not be confused with hardness. Coal of the same hardness may have a

f diff t i d bilit i di b f range of different grindability indices because of other constituents, such as moisture.

A standard index has been developed based on use of A standard index has been developed based on use of the Hardgrove Grindability machine and is called the Hardgrove Grindability Indexg y

PMI Revision 01

Hardgrove Grindability Index

The HGI was developed in the 1930s to measure

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p 93empirically the relative difficulty of grinding coal to the particle size necessary for relatively complete

b ticombustion

The HGI test tries to mimic the operation of a ball and track type of industrial coal pulverizerand track type of industrial coal pulverizermanufactured by Babcock. It does this using a batch mode of operation compared with a dynamic p p ysituation in an industrial coal pulverizer.

PMI Revision 01

HGI test procedure

A 50 gm sample of coal, which has been prepared in a specific manner

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p p pand which has a limited particle size range, 1.18 x 0.6 mm, is placed in a stationary grinding bowl in which i h l b ll i i l eight steel balls can run in a circular

path. A loaded ring is placed on top of the set of balls with a gravity load of 284 N The machine is run for 50 of 284 N. The machine is run for 50 revolutions. The top is removed and the ground coal removed. This coal is sized and the quantity less than

75 microns recorded. This is converted to a HGI value using a

calibration graph

PMI Revision 01

Standards for measurement of HGI

AS 1038 Part 20:2002, Hardgrove Grindability of coal

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ASTM D 409-2006, Grindability of coal by the Hardgrove machine method

ASTM D 6 H d G i d bilit f t kASTM D5003-06a, Hardgrove Grindability for petcoke

BS 1016-1981 Part 20, Hardgrove Grindability

DIN 51742-2001 Determination of HGI of hard coalDIN 51742-2001, Determination of HGI of hard coal

ISO 5047 1994, Hard coal-Determination of HGI

GOST 15489-93, Hard coal-Determination of HGI54 9 93,

JIS M8861-1993, Determination of HGI

PMI Revision 01

Coal Preparation Method affects Results

Testing as per different standards may result in

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standards may result in commercially significant differences in the value of HGI for a specific coal. Such as ASTM procedure can be higher than values using AS g gprocedure on the same coal sample. The differences generally lie in the allowed generally lie in the allowed method of preparing the sample before testing.

PMI Revision 01

Characteristics of HGI

Has a relatively low reproducibility and repeatability leads to ambiguity

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in evaluating mill performance and coal properties.

Exhibits a non linear change in difficulty to grinddifficulty to grind.

Coals with low values of HGI are more difficult to grind and high values are much easier to grind.

HGI of a blend is not necessarily the weighted sum of the component coals

When combined with coal reactivity When combined with coal reactivity or volatile matter, to provide guidance on likely combustion/ reaction performance

PMI Revision 01

Mill capacity vs HGI

The mill capacity falls when grinding coal with a lower HGI

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grinding coal with a lower HGI

The curve shows variation in mill capacity with HGI with 100% capacity being nominally 100% capacity being nominally set at HGI of 50.

PMI Revision 01

Fineness and Mill Power

Fineness vs mill capacityFineness vs mill capacity Mill power vs HGI at fi

Mill power vs HGI at fi

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at constant HGIat constant HGI constant finenessconstant fineness

HGI is seen as a good measure of the difficulty of grinding coal for coalsHGI is seen as a good measure of the difficulty of grinding coal for coals with fairly standard properties

PMI Revision 01

HGI limitations

Coals with a moisture content of less than about 10% behave in an d H i i h HGI

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expected manner. However as moisture content increases the HGI becomes less effective as a predictor

There is an envelope of coal properties within which HGI is reliable. Thi l i ll d fi dThis envelope is not well defined.

HGI does not provide direct information on the likely wear rates of critical mill grinding components.

The primary shortcoming of the test is the use of a batch process

The test uses sample feed with a limited range of particle sizes, 1.18 x 0.6 mm, rather than a complete range of sizes.

Other measures of coal grindability: Used for grindability of minerals in general is the Bond Work Index uses a pilot scale wet ball mill. Not precise enough for use with coal

PMI Revision 01

ROLE OF HGI IN PULVERIZER PERFORMANCE

It is normal to define the output of a coal pulverizer in

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p pterms of the rate of coal produced and the particle size distribution of this product.

The accepted standard for defining the output of an pulverizer is output in tonnes/hour with a particle size defined by 70% less than 75 microns and 99 5% size defined by 70% less than 75 microns and 99.5% less than 300 microns when grinding with a HGI of known value. Mostly it is 50. This value has no y 5critical importance other than to provide a value against which mill capacity is defined.

PMI Revision 01

ROLE OF HGI IN PULVERIZER PERFORMANCE

Coal pulverizer manufacturers provide sets of curves

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linking actual performance of the mill with coal with different properties such as HGI, moisture content % and feed particle top size. These curves enable results of a test feed particle top size. These curves enable results of a test on a particular coal to be referenced back to the standard conditions to monitor the performance of the pulverizer.

f h d i h ill f ’Measurement of the HGI and using the mill manufacturer’s curves will provide a first estimate of the performance of a mill with the coal in question when evaluating pulveriser q g pperformance with new coal.

PMI Revision 01

Coal properties complementary to HGI

Pulverized coal with 70% less than 75 microns and 99.5%

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less than 300 microns was recognized as being appropriate for normal conditions, based upon coal with a medium volatile matter content or reactivity, which results in volatile matter content or reactivity, which results in reasonable burnout of coal particles, 98–99%, in a well designed boiler furnace.

f h l il i i i hi h hIf the volatile matter content or reactivity is higher than normal, then coal particles somewhat larger will burn out successfully in the normal furnace volume. This coarser yproduct requirement allows the mill to provide more output. Alternatively it allows the use of coal with a lower HGI with effective particle burnoutHGI with effective particle burnout.

PMI Revision 01

Fineness

Fineness is an indicator of the quality of the pulverizer action.

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q y pSpecifically, fineness is a measurement of the percentage of a coal sample that passes through a set of test sieves usually designated 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. Values greater than 70% require the mill to perform more work. The mill wear and the power onsumption wear and the power onsumption are increased if the 70% value is exceeded. Values lower than 70% mean higher carbon loss and increased fuel consumption.

PMI Revision 01

Coal Finess

Coal retained on the 50 mesh screen should be in the 1–2%

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screen should be in the 1 2% range. Higher values indicate worn internals or improper settings. Also, the higher g gpercentages can cause boiler slagging and high unburned carbon

The 50 mesh screen is an indication of relative coarseness. The 100 mesh screen indicates an

f l d hunsuccessful test, and the 200 mesh screen indicates relative fineness.

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Relationship between grindability and mill capacity34

PMI Revision 01

Moisture and Grindability Effects on Mill Capacity

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PMI Revision 01

WHY IS MILL PERFORMANCE TESTING IMPORTANT?

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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 RELIABLE FEEDBACK SHOULD FOCUS ON TIMELY MILL OVERHAUL

VERY EFECTIVE CROSS CHECK OF THE VERY EFECTIVE CROSS CHECK OF THE STATION INSTRUMENTS FEEDBACK

PMI Revision 01

USE OF METAL TEMPERATURE IN FUEL PIPE TO MONITOR CHOCKING

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FUEL PIPING METAL TEMPERATURE IS BEING MONITORED IN FARAKA TO MONITOR FUEL PIPE CHOCKING.

RECENT STUDIES SHOWED THAT LOWER THAN 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 BETWEEN COAL AIR TEMPERATURE METAL TEMPERATURE FOR A PIPE WHICH WAS CLEAR.

PMI Revision 01

Pulverizer Troubleshooting-Matrix

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LACK OF CAPACITY OR HIGH POWER

HIGH MOISTUREHIGH POWER CONSUMPTION

LOW GCV

INCREASED RAW COAL SIZECOAL SIZE.

GRINDING TOO FINE

EXCESSIVE BED DEPTH

INSTRUMENT ERROR

PMI Revision 01

Pulverizer Troubleshooting-Matrix

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EXCESSIVE MILL CHANGE IN COAL EXCESSIVE MILL REJECTS

CHANGE IN COAL GRINDABILITY, SULFUR & ASH.

IMPROPER COAL/AIR RATIO

THROAT GAP WEAR THROAT GAP WEAR.

PMI Revision 01

Pulverizer Troubleshooting-Matrix

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COARSE GRIND CHANGE IN COAL COARSE GRIND CHANGE IN COAL GRINDABILITY

HIGH MOISTURE

INCREASED THROUGH PUT.

CLASSIFIER SETTINGCLASSIFIER SETTING

MILL WEAR.

PMI Revision 01

Pulverizer Troubleshooting-Matrix41

LOW COAL \AIR HIGH MOISTURELOW COAL \AIR TEMPERATURE

HIGH MOISTURE

LOW PA INLET TEMPERATURE

PASSING OF COLD AIR.

LOW A.H INLET TEMPERATURETEMPERATURE

NON AVAILABILITY OF SCAPH

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Pulverizer Troubleshooting-Matrix42

CHANGE IN MILL LOW GRINABILITYCHANGE IN MILL DIFFERENTIAL

LOW GRINABILITY

LOW MOSITURE

MILL INTERNALS PROBLEMS.

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Pulverizer Troubleshooting-Matrix43

MILL FIRES HIGH VOLATILESMILL FIRES HIGH VOLATILES

MOISTURE

LOW COAL\ AIR \TEMPERATURE.

BURNER LINE BALANCE

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