Pulveriser-power plant ppt

7/30/2019 Pulveriser-power plant ppt

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PERFORMANCE ANALYSIS OF COAL MILLS

P M V Subbarao

Associate ProfessorMechanical Engineering Department

IIT Delhi

Correct Size, shape and quantity of Diet For Complete Digestion..


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Combustion Limits on Furnace Design

The lower limit of the furnace volume is dominated by the space

required for burning the fuel completely, or to an extent less than the allowable unburned fuel loss.

To complete the fuel combustion within the furnace space, the fuelinjected into the furnace has to reside there for a time longer than

critical time t

*

r. The fuel residence time can also be estimated by the residence time of

the combustion gas produced in the furnace.

An average residence time tr can be proposed.

eunit volumpergenerationheatofrateAllowableMax.furnacetheofVolume

furnacetheenteringenergyFuel

rt

v

cr

Vq

LHVmt


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gas.ofdensity

generatedgasofmassV

v

g

g

cr

qm

LHVmt

v

c

g

g

r

qm

m

LHVt

v

g

r

qF

A

LHVt

1


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Fuel combustion time is mainly dominated by the combustion

reaction velocity and the rate at which oxygen is supplied into thereaction zone.

The combustion reaction velocity depends on chemical

characteristics of the fuel.

Main technical factors that affect the combustion time are: Combustion characteristics of the fuel.

Mixing characteristics.

Fluid flow characteristics of the furnace.

The combustion velocity of an oil fuel droplet is generally lessthan 0.1 msec.

In the case of coal combustion time is much longer.


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Typical Combustion Reaction Velocity ( Flame Speed) of

Pulverized Coal : Effect of Ash Fraction

30%VM & 5 % Ash

30%VM & 15 % Ash

30%VM & 30 % Ash

30%VM & 40 % Ash

F

lamespeedm

/s

A/F ratio


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Typical Combustion Reaction Velocity ( Flame Speed) of

Pulverized Coal: Effect of VM Fraction

30%VM & 5 % Ash

20%VM & 5 % Ash

15%VM & 5 % Ash

F

lamespeedm

/s

A/F ratio


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Coal Mill : A Controller of Combustion Time

Hot Air

~ 2500C

Coal 10 to 25 mm Size

Roller

Bowl


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Coal pulverizers

Coal pulverizers are essentially volumetric devices, because thedensity of coal is fairly constant, are rated in mass units of tons/hr.

A pulverizer accepts a volume of material to be pulverized whichis dependent on the physical dimensions of the mill and the abilityof coal to pass through the coal pulverizing system.

The furnace volume and mill capacity in a specific power stationmay dictate the need to purchase coals which are reactive and easily

grind. The common measure of mass in tons enables matching of energy

requirements with available coal properties and mill capacity.

Increased combustible loss can occur if the furnace volume or millcapacity is less than desirable for a particular coal.

There are a number of possible remedial actions. Operators can correct some deficiencies in the combustion system :

Biasing the performance of the coal pulverizing for variable coalqualities.

Use the spare mill into service for peak periods to ensure fulloutput.


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Size reduction is energy intensive and generally very inefficient withregard to energy consumption.

In many processes the actual energy used in breakage of particles isaround 5% of the overall energy consumption.

Pulverizing coal is no exception to this.

There are basically four different types of pulverizing mills which aredesigned to reduce coal with a top particle size of about 50 mm to theparticle size range necessary for fairly complete combustion in amodern pulverized coal fired boiler.

Each type has a different grinding mechanism and different operatingcharacteristics.

There are four unit operations going concurrently within the mill body,coal drying, transport, classification and grinding.

For coal pulverizers the capacity of a mill is normally specified astonnes output when grinding coal with a HGI of 50, with a particle sizeof 70% less than 75 micron and 1 % greater than 300 micron and with

a moisture in coal of less than 10%. A few manufacturers specify 55 instead of 50 with respect to HGI.

This standardization enables selection of an appropriate mill for aspecific duty.


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Ball & Tube Mill

The oldest pulverizer design still in

frequent use.

25% to 30% of cylinder volume is

filled with wear resistant balls of 30

60mm.

The cylinder is rotated at a speed of

about 20 rpm.

Specific power consumption 22

kWh per Ton.

Suitable for hard coals. Highly reliable in requires low

maintenance.

Bulky and heavy in construction.


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mg+mw2R


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mg-mw

2

R

mg


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mg+mw2R


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mw2R

mg

m(mw2

R+mg Cos a) > mg sin a

a


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mw2R

mg


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mw2R

mg

m(mw2

R+mg Cos a) < mg sin a

a

Pulverization due to ATTRITION


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mw2R

mg

(mw2

R-mg Cos a) = 0

a

Pulverization due to Impact


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Bowl Mill

The most widely used mill for

grinding coal. The raw coal is fed into the

center of the mill.

This is an intermediate speed

pulverizer.

The vertical shaft rotates at aspeed 30 50 rpm.

Specific power consumption 12

kWh/ton.


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Schematic of typical coal pulverized system

A Inlet Duct;

B Bowl Orifice;

C Grinding Mill;

D Transfer Duct to Exhauster;

E Fan Exit Duct.


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The primary airflow measurement by round cross-

sectional area venturis (or flow nozzles) should be

provided to measure and control primary airflow to

improve accuracy


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Aerodynamic Lifting of Coal Particles


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Carrying of Particles by Fluid Drag

In view of the age of the technique it would be presumed that the

subject of concurrent fluid-solid flow would be quite well defined

and understood.

Investigation of the published literature indicates, however, that such

conveying is still an extremely empirical art.


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Pneumatic Carrying of Particles

The major goal of pneumatic conveying of solids is to maximize the

carrying capacity of the installation and carry flows with high-solidsconcentration ("dense-phase flow").

In pulverized coal combustion, the ratio of coal to carrying gas isusually in the range of y = 0.5-0.6 kg/kg.

Assuming a coal density c = 1.5 x 103 kg/m 3, and the density of the

carrying gas as g = 0.9 kg/m3, the volume fraction of the coal can be

shown to be very small, 0.036 % . Dilute Phase Transport

The inter particle effects can therefore be neglected for steady stateoperation.

An important aerodynamic characteristic of the particles is their

terminal velocity (the free-fall velocity in stagnant air) which for aspherical particle of d = 0.1 mm has an approximate value of 0.3m/sec.

Experience shows that due to non-uniformities of flow behind bends,and to avoid settling of solids in horizontal sections of the transportline, a gas velocity of ~ V = 16 -- 20 m/sec has to be chosen.


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Pulverizer Capacity

Mill manufacturers provides a set of data or curves, which enable the capacity of a mill

to be determined with a coal with specific properties. The properties, which are of concern, are specific energy, HGI, moisture, particle size

and reactivity.

Specific energy is necessary to determine the required nominal maximum mill capacity

in tons/hour to ensure sufficient coal is delivered to the boiler.

A curve linking HGI and mill capacity provides information on mill performance with

that coal.

A curve linking moisture content of the coal with mill capacity shows what reduction

in capacity will arise if the moisture is excessive.

This is particularly important with ball mills.

The particle size distribution and top size may be of importance.

For ball mills there is a curve linking mill capacity with the top size of coal fed to the

mill.

The reactivity of the coal, measured in the first instance by volatile matter is needed to

determine if the mill can be set to provide standard 70% less than 75 micron or

a finer or coarser setting is necessary with corresponding alteration to mill capacity.


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Pulverizer Capacity Curves

Moisture content, %

Throughput,tons/hr

Grindability


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Coal Mill : A Controller of Combustion Time

Hot Air

~ 2500C

Coal 10 to 25 mm Size

Roller

Bowl


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Sizing of Pulverizers

Feeder capacity is selected to be1.25 times the pulverizer capacity.

Required fineness, is selected to be 60% through a 200 mesh screen for lignite(75 mm),

65% for sub-bituminous coal,

70-75% for bituminous coal, and

80-85% for anthracite.

Heat input per burner is assumed to be to 75 MW for a low

slagging coal and

40 MW for a severely slagging coal,

With intermediate values for intermediate slagging potentials.

General Capacity of A Coal Mill : 15 25 tons/hour.

Power Consumption: 200 350 kW.


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Performance Calculations

Several performance parameters are calculated for

the pulverizer train.

These include the following:

Effectiveness of Coal drying requirements.

Pulverizer heat balance.

Primary air flow requirements.

Number of pulverizers required as a function of

load. Auxiliary power requirements.


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Prediction of Coal Drying

For predicting the amount of coal drying which is needed

from the pulverizers the following methods were accepted. For very high rank coals (fixed carbon greater than 93

percent), an outlet temperature of 75 to 80 C appeared most

valid.

For low- and medium-volatile bituminous coals, an outlettemperature of 65 - 70 C appeared most valid.

Bituminous coals appear to have good outlet moisture an

outlet temperature of 55 to 60 C is valid.

For low-rank coals, subbituminous through lignite (less than69 percent fixed carbon, all of the surface moisture and one-

third of the equilibrium moisture is driven off in the mills.


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Energy Balance across pulverizer is very critical for satisfactory

operation of Steam Generator.


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Hot air

Coal

Dry pulverized coal +

Air + Moisture

Puliverizer frictional

dissipation

Motor Power Input

Heat loss

Suggested Primary air fuel ratio


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Suggested Primary air fuel ratio


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Mill Energy Balance

Hot air

Coal

Dry pulverized coal +Air + Moisture

Puliverizer frictional

dissipation

Motor Power Input

Heat loss

Tempering Air, Tatm


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Pulverizer Heat Balance

To perform the necessary pulverizer heat and mass balance

calculations, the following parameters are required:

Primary air temperature.

Primary air/fuel ratio.

Fuel burn rate.

Coal inlet temperature. Coal moisture entering the mills.

Coal moisture content at the mill exit.

Mill outlet temperature.

Minimum acceptable mill outlet temperature. Tempering air source temperature.

Tempering air flow.


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Heat Losses and Gains in A Mill

Convection and Radiation Losses from the surface of themill.

Heat losses are generally found to be at 5 percent of total

thermal energy available.

Mills consume an electric energy of 60 kJ/kg. The mill grinding heat dissipation, varies from 20 to 40

kJ/kg of coal.


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Mill Heat Balance: Energy for Drying of Coal

Determine the latent heat per kg of water evaporated.

Calculate the total energy absorbed by evaporating the

required amount of water from the coal.

( ) fgambpoutincoaldry hTcMMmQ 100


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Mill Heat Balance: Energy for heating of dry Coal

Determine the sensible heat increase of the coal.

( ) coalinoutincoalcoalheat CTTMmQ 100,


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Mill Heat Balance: Energy for heating of remaing

Moisture

Determine the increase in sensible heat of remaining

moisture.

( ) moisturecoalinoutoutcoalmoistureheat CTTMmQ ,,


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Mill Heat Balance: Energy available

Determine the sensible heat available in the mill inlet air.

( ) airpoutairinairairpriairpri CTTmQ ,,,

Calculate the the mill grinding heat generation

disspationcoalgen qmQ


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Mill Heat Balance: Energy gained by Tempering Air

Determine the sensible heat increase in tempering air air.

( ) airpintairouttairairtempairpri CTTmQ ,,,


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Mill Heat Balance: Energy losses

Calculate the heat lost from the surface of thepurlverizer:

100

5

genairprilosses QQQ


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Mill Energy Balance: Verification

Total Energy Available:

genairpriavailable QQQ

airtemplossesmoistureheatcoalheatdryconsumed QQQQQQ ,,,

Calculate the difference:

Total Energy Consumed:

Divide the difference by the total available to obtainthe fraction

consumedavailablesuspense QQQ

available

suspense

Q

QX


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Validation of Design

For best desingn:X= 0.

Acceptable designs: X = +/- 0.05.

IfX is not in the limits above, the design and performance

calculations should be repeated.

At any time during Operation above conditions should be

maintained for most efficient and reliable operation of mill.


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Derate Analysis and Operating Concerns

Pulverizer capacity limitation : A derate is due to the fuelburn rate exceeding predicted pulverizer capacity with allpulverizers in service.

Feeder capacity limitation : A derate is due to the fuel burnrate is greater than the total actual feeder capacity with allpulverizers in service.

An exhauster mill limitation: A derate is due to thecalculated airflow required with all pulverizers in service isgreater than the actual exhauster fan flow.

Improper pulverizer outlet temperature: A derate is due tothe heat available in the primary air for drying coal in thepulverizers is less than that required.

Auxiliary Power Requirements


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Auxiliary Power Requirements The pulverizer system annual auxiliary power requirements are

calculated in a multistep process.

The first step is to calculate the fractional load per pulverizer inservice (Milllod) at load point i.

where

FBRi = fuel burn rate, t/h, at load i,

Nmill,i= calculated number of pulverizers in service at load i, and

C mill = calculated capacity, t/h, per pulverizer.

The second step is to calculate the power required per pulverizer.

millimill

ii

CN

FBRMilllod

,

( )

jmillimill Milllod

dld

dPRPP 11,


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where

RPmill = Rated Power Consumption of mill, and

dP/dld = slope of pulverizer power Vs fractional pulverizer

load curve based on manufacturer data.

The third step is to calculate the power required (MWh/yr) for the

pulverizer system at load point i.

where

hmotor = motor efficiency.

Finally, the auxiliary power requirements for each load point aresummed to obtain the total auxiliary power requirements for the

pulverizer system.

motor

iimillmill

iysys

HoursPN

Ph

,

,,

K

i

iysystotsys PP

1

,,,


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The Control of Coal Mills


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Mill PA /Differential Pressure Control


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Closed Loop Control of PA Flow

P ll l C l f F d S d & PA Fl


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Parallel Control of Feeder Speed & PA Flow


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Control of Suction Mills

ill l


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Mill Temperature Control

A comprehensive Mill Control System


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A comprehensive Mill Control System

Steam Temperat re control ith 2 stage


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Steam Temperature control with 2 stage

Attemperation

OPERATIONS AND MAINTENANCE CONTROLLABLE FACTORS


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OPERATIONS AND MAINTENANCE CONTROLLABLE FACTORS

Four controllable heat rate factors are directly related withfurnace performance and furnace flue gas uniformity.

These are: superheater temperature, reheater temperature,desuperheating spray water flow to the superheater, anddesuperheating spray water flow to the reheater

Balancing of the fuel and air to each burner has much to do with

furnace combustion efficiency, and the completeness ofcombustion at the furnace exit.

The residence time of the products of combustion from theburners to the superheater flue gas inlet is about one or twoseconds.

Not very long for furnace mixing of fuel rich and air rich lanes ofcombustion products.

Optimized combustion at the superheater inlet can be quantified

by use of a water-cooled high velocity thermocouple probe.

Sl i t th h t fl i l t h b bl i


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Slagging at the superheater flue gas inlet has been a problem in a

number of boilers due to stratified flue gas.

Slagging at the lower furnace results in large boulder sized

clinkers blocking the lower ash hopper. Tube spacing becomes ever closer as the heat transfer changes

from radiant in the furnace, to convective in the back pass.

Recall the typical tube spacing of pendant superheater and

reheater tubes. If lanes in the furnace outlet flue gas approach the ash softening

or even the ash fluid temperature, upper furnace slagging and

blockage can result in a very short time.

Several cases studies should be reviewed to show how theapplication of the Thirteen Essentials will improved slagging,

heat-rate, capacity factor, reliability, NOx and/or flyash carbon

content.

AIRFLOW AND FUEL FLOW OPTIMIZATIONS : A Case Study


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D i D t U d


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Design Data Used


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On Going Problems


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On Going Problems

Erratic performance and severe slagging problems inboth the upper furnace and the lower ash hopper.

It was concluded that the furnace exit gas temperaturewas varying due to non-uniform products of combustionentering the superheater gas side.

Ash fusion temperatures are often lower in a reducingatmosphere than in an oxidizing atmosphere.

Since many of the opportunities for improvement thatwere experienced on this boiler were slagging related,this was a significant factor.

For example, the peak furnace exit gas temperatureswere above 2472F. (the maximum point indicated onthe digital thermometer, the true temperature was evenhigher.)

Also, the reducing ash fluid temperature from is2400F.

The combination of the fuel richness and ash chemistryto ether is the root cause of severe sla in .


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A highly localized temperature is also usually due

to fuel rich and creates a reducing atmosphere.

Therefore, this peak temperature corresponds to a

reducing atmosphere. It is this combined effect of lower ash fusion

temperature in a reducing atmosphere (chemistry

effect) and the poor fuel and air balance

(mechanical effect) that greatly acceleratesslagging.


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The furnace exit, excess oxygen and temperature

stratifications were found to be the result of a non-

homogeneous mixing of the combustion air and fuel in the

burner belt zone.

Zero oxygen points at the furnace exit could be due to either

an abundance of fuel, or a shortage of combustion air.

The Comprehensive Diagnostic Test technique was utilized

to quantify the opportunities for each.

Upon completion of the Comprehensive Diagnostic Tests,the following changes were implemented:

Fuel lines were balanced

Flow nozzles were installed for primary airflow measurement

and control

Pulverizer classifier changes Secondary air duct changes to balance combustion airflows

to each of the four corners.


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Pulveriser-power plant ppt