PERFORMANCE ANALYSIS OF COAL MILLS

P M V SubbaraoAssociate Professor

Mechanical Engineering DepartmentIIT Delhi

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

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 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 volumper generationheat of rate Allowable Max. furnace theof Volume

furnace theenteringenergy Fuel

rt

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generated gas of massV

vg

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LHVmt

vc

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LHVt

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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 velocity of an oil fuel droplet is generally less than 0.1 msec.

• In the case of coal combustion time is much longer.

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

Fla

me

spee

d m

/s

A/F ratio

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

Fla

me

spee

d m

/s

A/F ratio

Coal Mill : A Controller of Combustion Time

Hot Air~ 2500C

Coal 10 to 25 mm Size

Roller

Bowl

Coal pulverizers• Coal pulverizers are essentially volumetric devices, because the

density of coal is fairly constant, are rated in mass units of tons/hr.• A pulverizer accepts a volume of material to be pulverized which

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

• The furnace volume and mill capacity in a specific power station may 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 mill capacity 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 coal

qualities. • Use the spare mill into service for peak periods to ensure full

output.

• 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 around 5% of the overall energy consumption.

• Pulverizing coal is no exception to this.• There are basically four different types of pulverizing mills which are

designed to reduce coal with a top particle size of about 50 mm to the particle size range necessary for fairly complete combustion in a modern pulverized coal fired boiler.

• Each type has a different grinding mechanism and different operating characteristics.

• 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 tonnes output when grinding coal with a HGI of 50, with a particle size of 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 a

specific duty.

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.

mg+m2R

mg-m2R

mg <m2R : Ball will never fall down

Critical Angular Velocity c

mc2R = mg

c2 = g/R

mg+m2R

m2R

mg

m2R+mg Cos > mg sin

m2R

mg

m2R

mg

m2R+mg Cos < mg sin

Pulverization due to ATTRITION

m2R

mg

m2R-mg Cos = 0

Pulverization due to Impact

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 a speed 30 – 50 rpm.

• Specific power consumption 12 kWh/ton.

Schematic of typical coal pulverized system

A Inlet Duct;

B Bowl Orifice;

C Grinding Mill;

D Transfer Duct to Exhauster;

E Fan Exit Duct.

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

Aerodynamic Lifting of Coal Particles

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.

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-solids concentration ("dense-phase flow").

• In pulverized coal combustion, the ratio of coal to carrying gas is usually 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/m 3, 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 state

operation. • An important aerodynamic characteristic of the particles is their

terminal velocity (the free-fall velocity in stagnant air) which for a spherical 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 transport line, a gas velocity of ~ V = 16 -- 20 m/sec has to be chosen.

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.

Pulverizer Capacity Curves

Moisture content, %

Thr

ough

put,

tons

/hr

Grindability

Coal Mill : A Controller of Combustion Time

Hot Air~ 2500C

Coal 10 to 25 mm Size

Roller

Bowl

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 m),

• 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.

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.

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 outlet temperature 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 than 69 percent fixed carbon, all of the surface moisture and one-third of the equilibrium moisture is driven off in the mills.

Energy Balance across pulverizer is very critical for satisfactory operation of Steam Generator.

Hot air

Coal

Dry pulverized coal +Air + Moisture

Puliverizer frictionaldissipation

Motor Power Input

Heat loss

Suggested Primary air fuel ratio

Mill Energy Balance

Hot air

Coal

Dry pulverized coal +Air + Moisture

Puliverizer frictionaldissipation

Motor Power Input

Heat loss

Tempering Air, Tatm

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.

Heat Losses and Gains in A Mill

• Convection and Radiation Losses from the surface of the mill.

• 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.

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

Mill Heat Balance: Energy for heating of dry Coal

Determine the sensible heat increase of the coal.

coalinoutincoalcoalheat CTTMmQ 100,

Mill Heat Balance: Energy for heating of remaing Moisture

Determine the increase in sensible heat of remaining moisture.

moisturecoalinoutoutcoalmoistureheat CTTMmQ ,,

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

Mill Heat Balance: Energy gained by Tempering Air

Determine the sensible heat increase in tempering air air.

airpintairouttairairtempairpri CTTmQ ,,,

Mill Heat Balance: Energy losses

Calculate the heat lost from the surface of the purlverizer:

100

5 genairprilosses QQQ

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 obtain the fraction

consumedavailablesuspense QQQ

available

suspense

Q

QX

Validation of Design

• For best desingn: X = 0.

• Acceptable designs: X = +/- 0.05.

• If X 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.

Derate Analysis and Operating Concerns

• Pulverizer capacity limitation : A derate is due to the fuel burn rate exceeding predicted pulverizer capacity with all pulverizers in service.

• Feeder capacity limitation : A derate is due to the fuel burn rate is greater than the total actual feeder capacity with all pulverizers in service.

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

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

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 in service (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,

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

motor = motor efficiency.

• Finally, the auxiliary power requirements for each load point are summed to obtain the total auxiliary power requirements for the pulverizer system.

motor

iimillmilliysys

HoursPNP

,,,

K

iiysystotsys PP

1,,,

The Control of Coal Mills

Mill PA /Differential Pressure Control

Closed Loop Control of PA Flow

Parallel Control of Feeder Speed & PA Flow

Control of Suction Mills

Mill Temperature Control

A comprehensive Mill Control System

Steam Temperature control with 2 stage Attemperation

OPERATIONS AND MAINTENANCE CONTROLLABLE FACTORS

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

• These are: superheater temperature, reheater temperature, desuperheating spray water flow to the superheater, and desuperheating 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 of combustion at the furnace exit.

• The residence time of the products of combustion from the burners to the superheater flue gas inlet is about one or two seconds.

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

• Optimized combustion at the superheater inlet can be quantified by use of a water-cooled high velocity thermocouple probe.

• 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 the application 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

Design Data Used

On Going Problems• Erratic performance and severe slagging problems in both the

upper furnace and the lower ash hopper.• It was concluded that the furnace exit gas temperature was

varying due to non-uniform products of combustion entering the superheater gas side.

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

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

• For example, the “peak” furnace exit gas temperatures were above 2472ºF. (the maximum point indicated on the digital thermometer, the true temperature was even higher.)

• Also, the “reducing” ash fluid temperature from is 2400ºF. • The combination of the fuel richness and ash chemistry

together is the root cause of severe slagging.

• 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 accelerates slagging.

– 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.