mill performace dr. d....

MILL PERFORMACE

Dr. D. Banerjee

Email: [email protected]

CONTENTS

• The Purpose of Mill

• Mill Performance testing

Clean air flow test

Dirty airflow test

Fuel flow measurement

Burner balancing

Online monitoring system & mill up-gradation

The Purpose of a Pulverizer:

To Pulverize Coal to a Suitable size for Proper Combustion in the FurnaceAnd To Dry Incoming CoalFor Pressurized Pulverizers, The Classifier’s purpose is twofold: Size ClassificationUniform Fuel Distribution to

each Burner LineThe Pulverizer’s are the HEART of a Pulverized Coal Fueled Boiler!

PURPOSE MILL

• To grind Feed Coal( 20-25mm) to 75µ size. As per rated

capacity and delivered to boiler.

• Coal delivered to all coal pipes should be balanced

• Coal flow, dirty air & clean air flow should be balanced within permitted rage of deviation.

Clean air flow: When only air is flowing through the mill & coal pipes

Dirty Air Flow: When coal & air mixture is flowing from mill & coal Pipes to boiler.

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.

MILL PERFORMANCE PARAMETERS

• MILL OUTLET TEMPERATURE

• MILL MOTOR AMPERES

• MILL AIR FLOW

• WEAR PART LIFE

• MILL PRESSURE DROP

• MILL OUT PUT

• MILL FINENESS FRACTIONS

• MILL REJECT RATE

PERFORMANCE CORRECTION CURVES

TEST CANNOT BE ALWAYS CONDUCTED USING DESIGN COAL

• MOISTURE IN COAL

• HGI OF RAW COAL

• MILL FINENESS

• CORRECTION CURVES USED & NEED TO BE VERIFIED

OBJECTIVE OF MILL 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

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

Mill Performance Test Purpose

• For conducting performance checks of Pulverisers

• For identification of pulveriser system deficiencies.

• Also provides scope for iso-kinetic collection of pulverized coal sample (using ASME/Rotary sampler) from mill discharge pipes for checking the fineness fractions & coal flow distribution.

• Provide means for coal flow measurement

Instruments

1.ASE The Dirty Air Velocity Test Kit includes the following items:

• Dirty air velocity probe (calibrated) with thermocouple

• Handheld digital manometer (pressure measurement range ( -5 to +15 inches of WC)

• Handheld digital thermometer

• Probe support bushing (threaded pipe that screws into test port and seals probe to port)

• Tube (10' length, contains pressure and thermocouple lines) also called static probe

L-type Pitot for clean air test

2. ASME PF sampler

3. Rotary Sampler

Dirty Air Pitot•The dirty air pitot probe is utilized to measure air velocity, pressure, and temperature within a flow stream that is heavily laden with particulate. •The design of the dirty air probe reduces the potential for pluggage of the pressure lines, a situation that will cause erroneous data. •The probe is connected to a pressure gauge or manometer to measure DP & Sp. • Probe is pre calibrated in wind tunnel. Calibration is necessary in order to determine the calibration coefficient• calibration coefficient is used to obtain dirty air flow velocity.• An integrated thermocouple is used to measure the flow temperature.

Test Procedure – Checking Dirty air balance using Dirty Pitot kit

Test ports are installed on coal pipes to facilitate clean and dirty air traversing using pitot tubes and dirty air probes. Optimum location for these taps should be between five diameters downstream and ten diameters upstream of the nearest elbow or change of direction. A minimum of two test ports per coal pipe, 90 degree apart, is required for testing.

ii. The numbers of sampling ports are determined based on the available straight lengths of coal pipes, upstream or downstream of the nearest bends. The guidelines for locating testing cross sections are described in ASME PTC 4.2.

iii. Traverse points on the pitot tube are marked on an equal area grid in accordance with ASME Performance test Code 4.2 for traversing circular ducts or pipes.

iv. Two equal lengths Tygon tubing sections are cut to desired length. The tubing is then taped or bound together and one tube is marked on both ends to identify as the ‘high pressure’ line or ‘impact’ line. The second tube is used as the ‘low pressure’ line or ‘static’ line.

v. A 5” to 10” vertically inclined manometer is set up on a level and stable work area. The tygon tubing is connected to the high and low side taps on the manometer as indicated below. A digital manometer is preferred in lieu of the inclined manometer, if available, as shown herein.

Total Pressure

Static Pressure

10” Incline Manometer

Sta

tic

Pre

ssu

re

Clean Air: Air going through the fuel lines, at normal operating temperatures and pressures

Clean air velocities are indicative of the ‘transport’ energy in the PF pipes and should be within +/ 2% ‐of the mean.

Clean Air Tests

Clean Air Test Taps

FLOW

Pitots

• Dirty Air: Coal Air mixture going through the fuel lines with mill in service

• 8 to12 point are traversed for measurement of velocity head ( Vh )

• Dirty air velocities are measured in each fuel line to determine airflow and coal flow in each fuel line

• Dirty air velocities should be within +/‐ 5%of the mean.

Dirty Air Tests

1 ¼” Full Ported Ball Valve

Dustless Connector

Dirty Air Probe

PointerDenotes HP

Side

HP

Co nn ec to r

LP Connector

Coal/Air Flow

Inclined Gauge Oil M anometer

In dirty air test coal pipes of some mill are within permissible range while some are outside range which needs to be set right by checking orifice condition, classifier setting etc.

Calculation•

Density (d)= ( 460+70F)/(460 + t F) X( BP+ Sp/13.9)/29.9” Hg)X .075 lbs/f3

Velocity= 1095(Vh) ½ X K f/m (d ) ½

(Vh)1/2 =(Vh1) 1/2+(Vh2) 1/2+(Vh3) 1/2 ...

No of Traverse

% Deviation = Avg. Velocity – Velocity x 100%

Avg. Velocity

Volumetric Flow (Q)= Velocity x Pipe Cross-section Area

Dirty Air Flow (W) = Q x 60 min/hour x Density

Bp = Barometric Pressure (“Hg); Sp = Static Pressure (“WC)

K – Calibration Coefficient of the Pitot

Example

Mill Out Temp–88C/190.4 F, Bp–28.7 ”Hg, Sp– 1.6 ”WC Pipe Cross-section Area - 0.14522 m2

Density (d) = 530 X (28.7 +1.6/13.6) X 0.075 Lbs./f3

650.4 29.92

= 0.05886 Lbs./f3 * 16.01847 = 0.9429 kg/m3

Say (Vh)1/2 = 1.4576 ”WC

Velocity = 0.93 * 1095 * 1.4576 * 0.00508

(0.05886)1/2

= 31.1 m / s

Dirty Air Flow = 31.1 * 0.14522 * 0.9429 * 3600 / 1000

= 15.33 T/hr

MILL- 6A COR-1 COR-2 COR-3 COR-4

SAMPLER DP (“wc) 2.83 2.82 2.64 2.65TEMP. (deg C)

87.50 78.10 84.40 80.30STATIC Pr. (“wc)

1.80 2.40 1.80 2.00AIR DENSITY (KG/CUM) 0.97 0.99 0.97 0.99DESIGN VELOCITY (M/S) 26-28 26-28 26-28 26-28VELOCITY (M/S)

30.43 29.95 29.28 29.16VELOCITY DEVIATION (%) 2.44 0.82 -1.42 -1.84

A typical summary sheet of dirty pitot test report

Iso-kinetic Coal Sampling

Isokinetic: When the velocity of dirty air entering the collection nozzle is equal to the velocity of the flow in the fuel line

24 m/s

30 m/s 19 m/s24 m/s

24 m/s24 m/s

Super Iso Kinetic ‐ Sampling

The particles arepulled sampler

into theat

ahigher rate than the pipe velocity (more fines)

Sub Iso Kinetic ‐ Sampling

The gas velocity isless than thesample rate allowing fines to escape.

Iso‐Kinetic Sampling

particles

sampler at

Theflow into the

thesame rate as the coal in the pipe

PF sample collection (Iso-Kinetics)

Iso kinetic Sampling (Method):1. ASME method Sampler it comes with dirty pitot Kit. Manual sampling

device. Sampling is done in iso-kinetic condition. Sampling is done from 16 point traverse.

2.The Rotary Pulverized Fuel Sampler (M&W) is a manual-sampling device, designed for representative extraction of pulverized coal samples from mill discharge pipes. The sampling procedure is in accordance with international standard ISO 9931.

Rotary Sampler the multi point sampler extracts one representative sample using 4 sampling tips in 16 different configurations giving a total of 64 sampling points, evenly distributed over the cross-section of the circular pipe.

ASME Iso-kinetic Coal Sampler

Cyclone Separator

Sample Container

Filter Canister Assembly

y

LPConnectio n

HPConnection


Connector

Reinforced Tubing

To Air Suppl

Orifice Assembly

Coal Flow

Dustless Pointer Denotes

Sample Tip Location

0

C C

Test Cart

. The ASME PTC 4.2 Coal Sampling System is

utilized to sample particulate carried within an air stream. The sample is extracted isokinetically to determine the particulate flow rate. The sampling probe features a single, slotted nozzle meeting the geometry specifications of the ASME method. The probe is inserted into the flow stream and positioned at specific locations covering the cross section.Magnahelic pressure gauge for orifice DP instead of fragile oil-filled inclined manometerLight weight cyclone assembly to reduce probe weightAn Excel® spreadsheet template that allows the operator to reduce data instantly afer a test

The sampler as shown here is equipped with four sampling tips through which the sample is simultaneously extracted. By means of an angular gear mechanism, the sampler tips are rotated in concentric circles around the sampler head. A dial with eight equally distributed marks (45⁰ ) indicates the angular position of the sampler tips at every 22.5⁰ . When the dial is rotated twice, it gives one full turn of the sampler tips, thus giving 16 angular positions

Fuel flow Imbalance (Test 1) reduced afer classifier adjustments (Test 2)

• Mill discharge pipes offer different resistance to the flows due to unequal lengths and different geometry layouts.

• Fixed orifices are put in shorter pipes to balance velocities / dirty air flow / coal flows. The sizes of the orifices are specified by equipment supplier.

• Dirty air flow distribution should be within +/- 5.0% & Coal distribution within +/- 10% of the average of fuel pipes

Burner Imbalance

Mills

Control Room

Boiler

1 4

32

Pipe to Pipe Fuel Flows (210 MW)

Use of Iso-kinetic Coal Sampler

Cyclone Separator

Sample Container

Filter Canister Assembly

y

LPConnectio n

HPConnection


Connector

Reinforced Tubing

To Air Suppl

Orifice Assembly

Coal Flow

Dustless Pointer Denotes

Sample Tip Location

64 point Rotary Sampler ‐ is preferred due to ease of use and because it requires less

straight lengths

0

50

100

150

200

250

Corner 1 Corner 2 Corner 3 Corner 4

Sa

mp

le W

eig

ht

(gra

ms

)

Test Cart

Variation of coal flows in the four corners in 210 MW unit

Mill A

Mill

D

Mill F

Mill C

Mill E

Average

COAL PIPE INSTRUMENTS AND MEASUREMENT ACCURACY

Comparison of Sampling Grids

Measurements in Coal Pipes Complicated Because of Roping

Rosin & Rammler Chart

Use of four sieves (50,100,140, 200 mesh) and straight line plot

of fineness fractions on R&R Chart can confirm correctness

of fineness of sample

• Fineness is expressed as the percentage pass through a 200 mesh ‐screen (74µm). Coarseness is expressed as the percentage retained on a 50 mesh screen‐ (297µm).

• Typical recommended value of pulverised fuel fineness through 200 mesh Sieve is 70% and 1% retention on 50 mesh sieve.

Use of the ASME Isokinetic Coal Sampler

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

Impact of coal quality on Mill Selection & Mill Performance

1. Quantity of coal fired

2. Moisture in coal

3. PF fineness required

4. Hard Grove Index

5. Abbrasing Index (Yancy-Gear & Price,YGPI)

Coal Quality & Mill Selection

Change in coal Property Result change in Pulveriser Capacity

Moisture content increase Decrease

HGI increase Increase

PF fineness increase Decrease

EFFECT OF COAL PROPERTY ON PULVERISER(MILL) PERFORMANCE

Mills – Typical Design Curves

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

USE OF METAL TEMPERATURE IN FUEL PIPE TO MONITOR CHOCKING

• FUEL PIPING METAL TEMPERATURE IS BEING MONITORED 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 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


• MILL FIRES• HIGH VOLATILES

• MOISTURE

• LOW COAL\ AIR TEMPERATURE.

• BURNER LINE BALANCE

Duct

Forward Reverse velocity probe

Digital Thermometer

Digital Manometer10” Incline Manometer

Primary Air Duct Traverse

Mill UCB Measured Deviation (%)

A 50 63 +26

B 52 65 +25

C 52 62 +19

D 55 66 +20

E 50 61 +22

F 52 62 +19

Primary air flows are calculated by grid traverse at different flows and air temperatures using a calibrated pitot. The following table summarises the flows at 100% open damper position in a 210 MW unit.

Experiences with on line measurement system & Mill Up-gradation

Real Time Primary Air Flow Measurement and Dirty Air Flow Measurement & Balancing System

Two sensors were mounted in each discharge pipe in the vertical portion before the burners (Photo 1) and four sensors were mounted across the venturi in PA duct for measurement of Dirty air velocities (Photo 2). The ‘meControl’ panel was installed in Control room (Photo 3).

Photo 2Photo 1Photo 3

Photo 4

Variable Orifices

Installed four manual operated ceramic lined variable valves, one in each pipe, in addition to existing pipe orifices in horizontal run of the four coal pipes of Mill-1A (Photo 4, 5, 6).

Photo 4 Photo 5 Photo 6

Before Afer

PF Pipe Sensors

Valves in PF Pipes

• Installed in Mill 1A under Indo-Germ Energy program & support of GTZ

• Equipment supplied by M/s Promecon, Germany

• Sensors in discharge pipes of mill and Inlet PA Duct

• Manually Operated Variable Valves in PF discharge pipes

• Imbalance in dirty air velocities amongst the four pipes reduced on throttling of the valves in Corner 1 & 2 pipes

‘Real Time’ Dirty Air Flow Measurement and Balancing System

WORKING PRINCIPLE

•The velocity measurement is performed by a pair of two sensors installed at a fixed distance in the air inlet duct to the mill & in the PF discharge pipes. •The sensors are aligned parallel to the longitudinal axis of the pipe and scan the electrostatic fluctuation of particles in the gas between them. •Stochastic signals created by (dust) particles by the pair of upstream/downstream sensors are cross correlated and referenced by their time shif.• By using the cross sectional area of the duct and the temperature and pressure information the gas STP-volume flow is calculated on line.

Velocity m/s by Dirty Pitot test Velocity m/s - Promecon systemCorner Corner

PA flow T/hr

1 2 3 4 1 2 3 4

70.6 29.4 30.6 26.9 27 28.6 29.4 25.6 25.360.7 24.7 27 23.2 23.2 24.5 25.2 22.6 21.750.3 19.8 21.7 18.4 18.9 19.7 20.7 18.3 18.540.9 15.7 18.1 14.5 15.5 14.5 16.7 13.1 14.7

The real time readings of PROMECON system compare well vis-à-vis the test instrument feedback. There’s some difference in the absolute values of velocities but the variation pattern of velocity in different coal pipes are almost the same as observed in dirty pitot tests.

Comparison of Online values with Dirty Pitot Test results

• Project on mill capacity enhancement successfully carried out with support of USAID for one XRP mill at mill 2D.

• Components replaced for mill capacity increase Rotating Vane wheel assembly (SAS Global Patented) High spin Static Classifier (SAS Global Patented)

• Appreciable increase in mill capacity (10-15%) and reduction in APC(~5 A) observed

• Due to deterioration in coal quality our boilers run with higher than design no. of mills leading to high exit gas temperatures and high APC

• Operation with design number of mills will result in lower APC & increase in efficiency

Mill capacity Enhancement

Basic chemical composition Properties

C Si Mn S P Ni Cr Mo

3-3,6 0,3-0,5 0,3-0,7 MAX0,15 MAX0,3 3,3-4,8 1,5-2,6 MAX0,4 53-61 RC

MAX2,9 0,3-0,5 0,3-0,7 MAX0,15 MAX0,3 3,3-5 1,4-2,4 MAX0,4 52-59 RC

1-1,6 0,4-0,7 0,4-0,7 MAX0,05 MAX0,05 4-4,75 1,4-1,8

2,6-3,2 1,8-2 0,4-0,6 MAX0,1 MAX0,06 5-6,5 8-9 MAX0,4

NI -HARD 1

NI –HARD 2

NI –HARD 3

NI –HARD 4

BOWL MILL GRINDING ROLL MATERIAL

In India power house coals are very abrasive as they get worst coal. and the grinding roll and bull ring life is about 2500 to 4000 hours. Till in recent past Ni-hard was being extremely used as grinding roll material

SINTER CAST 11 GRINDING SINTER CAST GRINDING ELEMENTSELEMENTS

.

BREAKTHROUGH IN GRINDING ELEMENTS TECHNOLOGY / METALLURGY.

C - 2.3-3.4 %, Cr.-18–27 %,Mo-0.5-2.5, Mn –1.5 max.,Si–1 % max

THE COMPOSITE CASTING HAVING HARDNESS OF 60 – 66 RC Running Hours 10000

mill performace dr. d....

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