Coal Mill1

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Analysis of Pulverizers

P M V Subbarao

Professor

Mechanical Engineering Department

Multi Task Machines to meet the rate of rapid coal combustion


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1920 : A Need for Break Through for Thermal

Power Generation

The first limit on grate firing is that of scale.

A practical engineering limit seems to be reached when the length

and width of the grate are about 9 m with grate area 80 m2.

At 2 MW/m2, the steam capacity at 85% efficiency would be 150

MW or 270 tons per hour.

In practice stokers have rarely exceeded a capacity of 135

tons/hour.

The limitation is partly grate area and partly firing density.

The limitation on firing density exist due to: The rate of movement of the reaction plane could not match the

opposed rate of fuel flow leading to blow-off.

The experience with grate combustion led to development of

many requirements for further development.


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Parameters for Combustion Requirements

Heat Release Rate Volumetric Combustion Intensity

Area Combustion Intensity

Effective Reactor Height

Coal Firing Density

Area Firing Density Products of combustion Velocity

Air Velocity

Combustion time

Particle Heating Rate

Heat Transfer Fluxes Heat exchange surface area per unit cross sectional area of combustion

chamber, f


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Heat Release Rate : A Capacity Limit

Most common Grate fired furnace ~ 30 MW. Maximum obtained ~ 150 MW.

Maximum Power Generation Capacity ~ 50 MW

Future Projected Requirement ~ 3000 MW


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Firing Densities Limits : An Optimal Choice

Firing densities are expressed in two ways:

A volumetric combustion intensity,Iv. Area Firing Intensity,IA


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A volumetric combustion intensity,Iv.

High value ofIv :

Compact furnace

Low Capital cost

Less time for combustion

Low Values of Iv

Bulky furnace

More time for combustion

Low Running Cost


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Area Firing Intensity,IA

High value ofIA

Sleek furnace

Higher combustion Temperatures

Better Ignition

Low Value ofIA Poor Ignition

Fat furnace

Low Nox

Solid Ash


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Combustion Factor Pre 1920

Status

Future Requirement

A volumetric combustion intensity,Iv (kW/m3) 250750 15002500

Area Combustion Intensity,IA (kW/m2) 3001800 Up to 7500

Coal Firing DensityJf,V( kg/m3.hr) 30100 15003000

Area Firing Density,Jf,A (kg/m2.hr) 40250 Up to 1000

Air Velocity (m/sec) Up to 0.5 Up to 20

Exhaust Gas Velocity (m/sec) Up to 3 Up to 20

Combustion time (sec) Up to 5000 ~ 1

Particle Heating Rate (0C/sec)


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Combustion Time

How to reduce Overall Coal Combustion time.

Combustion of solid fuel is heterogeneous reaction.

Proportional to surface area available for reaction.

Simple geometric solution.

For same mass, lower diameter particles will have more surface are

than larger particles.

Grinding the particles will enhance the area of exposure.


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Pulverized Fuel Combustion

Invented in 1920. An universal choice for power plants till 1990.

Fine particles of coal ~ 75 microns.

Surface area : 150 m2/kg.

Huge heat release per unit area : 25 MW/m2.

Steam generation : 2000 tons/hour.


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Coal Particle Combustion During A Journey


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Duties of A Furnace

Dixons Theory: Generate an environment of excited fuel and oxygen molecules.

All the fuel molecules should be surrounded by oxygen molecules.

Facilitate frequent collisions among excited fuel molecules and excited

fuel molecules.

A successful collision can lead to combustion.

Many possible technologies are available to realize above conditions

with varying levels of success and expenditure.

These are called Three Ts and one S technologies.


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The Three Ts & One S Practice

Technology Time Temperature Turbulence Size

Stoker large Medium Low Big

Pulverized Short High Medium Tiny

Cyclone Short+ V High High Medium

Fluid Bed Medium Low High Medium

Size the Coal and Add the Air !!!


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The One & Only One, Which Makes it Possible?

The coal is ground and dried in gas-swept mills equipped withaerodynamic gas-solid separators (classifiers).

The pulverized coal is pneumatically transported to burners and

injected in the form of particle-laden jets into the combustion

chamber. Among the most challenging fluid mechanics problems are those

dealing with preparation of the coal prior to combustion.

These problems are made more serious because coal is

commonly dried and pulverized to a finely divided state in thepresence of hot air.

The coal/air mixtures used in this drying and grinding process

are well within established flammability limits, which are very

broad.


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Coal pulverizers

Coal pulverizers are essentially volumetric devices .

As the density of coal is fairly constant, are rated in mass units of

tonnes/hr. A pulverizer accepts a volume of material to be pulverized which is

dependent on the physical dimensions of the mill and the ability ofcoal to pass through the coal pulverizing system.

The common measure of mass in tonnes enables matching of energyrequirements 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.

The furnace volume and mill capacity in a specific power station maydictate the need to purchase coals which are reactive and which can beground easily.

Size reduction is energy intensive and generally very inefficient with

regard to energy consumption. In many processes the actual energy used in breakage of particles is

less than 5% of the overall energy consumption.


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Mills

There are basically four different types of pulverizing mills which aredesigned to reduce coal with a top particle size of about 50 mm to thenecessary particle size range.

Ball&Tube Mill, Ball &Race Mill, Bowl Mill & Impact Mill

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 as tonnesoutput when grinding coal with a HGI of 50, with a particle size of 70%less than 75 micron and 1 or 2 % greater than 300 micron and with amoisture 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 a specificduty.


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Coal Mills


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Match with Burner Parameters

Primary and secondary air velocities should be decided to locate the

point of ignition from the exit plane of burner.

Low (16-20 m/s) are not suitable for high volatile coals.

Low velocities are recommended for low volatile coals.

Slagging and thermal distortion of the burners can result in variations

in velocities.

Large particles throw active combustion region into the wall region.

This increases slagging and increase in Unburned carbon losses.

For bituminous secondary:primary air velocity ratios are 1.4-1.5.

This gives sufficient reserve for load decrease without significantly

impairing the aerodynamics of P.F. Jet.


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Performance of Cyclone

The balance between the centrifugal and drag forces acting on solidparticles, which controls the radial movement of in the main body of

the furnace.

Secondary air flows, boundary layer flow and turbulence levels.

Fuel size distribution.

Moisture content.

HV

Reactivity of fuel

Operating temperatures and ash composition, which govern whether

the cyclone is used in a slagging or non-slagging mode.

The formation of slag layer aids the burn-out of larger particles.

Temperatures should be controlled by controlling the excess air.


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Particle size distribution

Particles less than 60 mm burn beforeentering the cyclone.

Particles 60 mm to 100 mm are likely to be

carried inside the cyclone. The majority of the particles larger than 150

mm will be flung to the wall and will start to

burn there.


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Satisfy Furnace Limits

The lower limit of the furnace volume is dominated by thespace 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 fuel injected into the furnace has to reside there for atime 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.


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Satisfy Combustion Limits

Fuel combustion time is mainly dominated by thecombustion reaction velocity and the rate at which oxygen

is supplied into the reaction 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

less than 0.1 msec.

In the case of coal combustion time is much longer.


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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 fuel

injected into the furnace has to reside there for a certain time longer

than some critical time t*

r. The fuel residence time can 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 the reaction

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 time of an oil fuel droplet is generally less than 0.1

msec.

In the case of coal combustion time is much longer.


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Typical Flame Speed of PC.

30%VM & 5 % Ash

30%VM & 15 % Ash

20%VM & 5 % Ash

30%VM & 30 % Ash

30%VM & 40 % Ash

15%VM & 5 % Ash

F

lamespeedm

/s

A/F ratio

T i l L f P l i Ci i


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Typical Layout of Pulverizer Circuit

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