TPS Coal Mills and Fan Performance

Basics for Coal Mills and Fans
Performance Management
By
Manohar Tatwawadi
Director, TOPS
5/8/2019 1Manohar Tatwawadi

SCOPE
• Various types of Coal mills can be found in the
Power Industry. The type of Coal mill under
discussion within this Presentation is a “Bowl
Mill” (Bowl and Rollers), typically used by the
power industry.
5/8/2019 Manohar Tatwawadi 2

How to Manage Performance
Identification of
Performance
Requirements
Maintenance
Strategy for MSI
Finalise the
Maintenance
Plan
Maintenance
Management
and
Maintenance
Operations Plan
Maintenance
Improvement
Plan

Raymond Bowl Mill

Performance Requirements
• Typical requirements set by the user are as follow:
• PF (pulverised fuel) particle size within the boundaries of the Rosin-Rammler
plot.
1. »» Passing 75μm 63 to 74%
2. »» Passing 106μm 77 to 86%
3. »» Passing 150μm 88 to 94%
4. »» Passing 300μm 98.8 to 99.8%
• Minimum mill outlet temperature = 60°C.
• PF velocity between mill and burners has to be between 18 m/s and 23 m/s.
(At 18 m/s PF tends to settle in PF pipes which could cause blockages and at
velocities above 23 m/s the wear rate due to erosion within these pipes is too
high.)
• Coal mass flow equal to OEM (original equipment manufacturer) specification.
• Mill available for use = 90% (Availability)
• Mill unavailable (unplanned maintenance) = 3%
• Mill unavailable (planned maintenance) = 7%

Grindability Index and Capacity
Mill Capacity – 125ooo Lb/hr
Case -1
HGI Index – 55%
Capacity to grind 70% thro 200
mesh – 133000 Lb/hr
Case – 2
HGI Index – 50%
mesh – 122000 Lb/hr
Case – 3
HGI Index – 40%
mesh – 85000 Lb/hr.

Maintenance Strategy
Maintenance Significant Item Inspection / Execution Task
Coal Measure HGI (hard grove index),
moisture content, abrasiveness and size
of coal fed
to mill.
Pulverised fuel (PF) Measure particle size distribution, PF
velocity and temperature.
Grinding media (Balls and Rings) Measure ball diameter, ring depth profile
and material hardness of balls and rings.
Classifier blades Measure throat area and throat gap.
Throat (Rotating or static) Measure throat area and throat gap.
Loading elements (Gas pressure or
spring tension)
Measure load on grinding elements.
Check condition of components.

Maintenance Strategy
Maintenance Significant Item Inspection / Execution Task
Transport air (mill inlet) Measure air temperature and A/F (air
to fuel) ratio inside mill.
Reject chamber Replace reject brushes if worn and
test reject doors for soundness.
Mill internal structure Check for abnormal wear
Mill dampers Check if all dampers are working.
Mill ducting Check for blockages inside primary air
ducting and PF pipes.

Inside the mill
The profile of
the roll should
be parallel with
the grinding ring
profile.

Classifires
A classifier
recirculates
coarse coal in
the grinding
zone and
balances the
flow of coal to
each burner
line to the
furnace.

Classifiers
• The flow of coal particles through a classifier is
several times the amount of coal flowing to the
burners because of the large amount of coal
recirculated within a pulverizer. For example, if a
pulverizer is operating at 100,000 lb/hr coal feed to
the burners, as much as 300,000 lb/hr or more may
be flowing through the classifier for regrinding. For
this reason the surface smoothness and inverted
cone clearances or discharge doors of the MPS
original design mills are extremely important for
acceptable pulverizer performance and of course,
optimum furnace performance.

Pulverisers
• 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.

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 rc = 1.5 x 103 kg/m3, and the density of the
carrying gas as rg = 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, %
Throughput,tons/hr
Grindability

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.5/8/2019 Manohar Tatwawadi 18

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 frictional
dissipation
Motor Power Input
Heat loss

Suggested Primary air fuel ratio

Mill Energy Balance
Hot air
Coal
Dry pulverized coal +
Air + Moisture
Puliverizer frictional
dissipation
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.

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.

Mill Units Design A B C D E F G H
Plant load MW 500 491.67 494.33 464.67 464.17 504.33 519.33 496.80 487.70
Coal flow t/h 51.248 59.56 56.42 54.28 56.42 57.51 56.68 58.40 56.47
A/F ratio 2.03 2.09 2.12 2.08 2.15 2.13 2.16 2.15 2.12
PA temp. oC 243 247.17 244.67 245.00 246.33 247.50 241.50 245.17 248.50
Coal-air mixture
temp
oC 85 78 76 81 72 71 70 75 75
Coal fineness
below 75 micron
% 70 65 68 70 60 61 62 61 62
DP mmWC 437.9 494.00 496.83 485.50 467.83 440.33 430.50 496.50 431.90
PA pressure I/L mmWC 991.5 900.50 927.50 999.83 909.67 922.33 933.50 929.17 958.80
PA flow t/h 104 120.31 123.01 110.52 102.18 104.80 108.66 113.13 120.17
Power kW 406 393.72 404.22 458.29 379.51 382.57 365.17 445.18 428.11
Energy kWh/h - 383.52 395.76 451.92 370.62 374.22 350.94 441.48 433.12
Load factor % 84.58 82.03 84.21 95.48 79.06 79.70 76.08 92.75 89.19
Sp. Energy
Consumption
kWh/t of
coal
7.92 8.44 9.01 8.33 8.57 8.51 8.19 8.56 8.67
Performance results of mills for 500 MW plant.

FAN TYPES
• Fan and blower selection depends on the volume
flow rate, pressure, type of material handled, space
limitations, and efficiency. Fan efficiencies differ
from design to design and also by types.
• Fans fall into two general categories: centrifugal flow
and axial flow.
• In centrifugal flow, airflow changes direction twice -
once when entering and second when leaving
(forward curved, backward curved or inclined,
radial)
• In axial flow, air enters and leaves the fan with no
change in direction (propeller, tubeaxial, vaneaxial)

Centrifugal Fan

Axial Fan

Typical Fan Efficiencies

System Resistance
• This term refers to the static pressure.
• It is the sum of the pressure losses in the system.
• It is a function of the configuration of ducts, pickups,
elbows and the pressure drops across equipment-for
example bagfilter or cyclone.
• The system resistance varies with the square of the
volume of air flowing through the system.
• For a given volume of air, the fan in a system with
narrow ducts and multiple short radius elbows is going
to have to work harder to overcome a greater system
resistance than it would in a system with larger ducts
and a minimum number of long radius turns.

System Resistance
• Thus, the system resistance increases
substantially as the volume of air flowing
through the system increases; square of air
flow.
• Conversely, resistance decreases as flow
decreases. To determine what volume the fan
will produce, it is therefore necessary to know
the system resistance characteristics.

System Characteristic

Fan Characteristics
• The fan curve is a performance curve for the particular
fan under a specific set of conditions.
• The fan curve is a graphical representation of a number
of inter-related parameters.
• Typically a curve will be developed for a given set of
conditions usually including: fan volume, system static
pressure, fan speed, efficiency and brake horsepower
required to drive the fan under the stated conditions.
• In the many curves shown in the Figure, the curve
static pressure (SP) vs. flow is especially important.

System Characteristics and Fan Curves

Fan Laws

Fan Performance Assessment
• Static pressure
• Static pressure is the potential energy put into
the system by the fan. It is given up to friction
in the ducts and at the duct inlet as it is
converted to velocity pressure. At the inlet to
the duct, the static pressure produces an area
of low pressure

Fan Performance…….
• Velocity pressure
• Velocity pressure is the pressure along the line
of the flow that results from the air flowing
through the duct. The velocity pressure is
used to calculate air velocity.

Fan Performance…..
• Total pressure
• Total pressure is the sum of the static and
velocity pressure. Velocity pressure and static
pressure can change as the air flows though
different size ducts accelerating and de-
accelerating the velocity. The total pressure
stays constant, changing only with friction
losses. The illustration that follows shows how
the total pressure changes in a system.

Total Pressure

Using a Pitot Tube

Air Density calculation
• The first calculation is to determine the
density of the air. To calculate the velocity and
volume from the velocity pressure
measurements it is necessary to know the
density of the air. The density is dependent on
altitude and temperature.
• toC – temperature of gas/air at site condition

Velocity Calculation
• Once the air density and velocity pressure have been
established, the velocity can be determined from the
equation:
• Cp = Pitot tube constant, 0.85 (or) as given by the
manufacturer
• Dp = Average differential pressure measured by pitot
tube by taking measurement at number of points over
the entire cross section of the duct.
• γ = Density of air or gas at test condition,

Volume Calculation
The Volume in Duct can be calculated by
the following equation

Fan Mechanical Efficiency

Energy Saving Opportunities
• Minimizing demand on the fan.
1. Minimising excess air level in combustion systems
to reduce FD fan and ID fan load.
2. Minimising air in-leaks in hot flue gas path to
reduce ID fan load, especially in case of kilns, boiler
plants, furnaces, etc. Cold air in-leaks increase ID
fan load tremendously, due to density increase of
flue gases and in-fact choke up the capacity of fan,
resulting as a bottleneck for boiler / furnace itself.
3. In-leaks / out-leaks in air conditioning systems also
have a major impact on energy efficiency and fan
power consumption and need to be minimized.

Energy Saving…..
• The findings of performance assessment trials will
automatically indicate potential areas for
improvement, which could be one or a more of the
following:
1. Change of impeller by a high efficiency impeller
along with cone.
2. Change of fan assembly as a whole, by a higher
efficiency fan.
3. Impeller derating (by a smaller dia impeller)
4. Change of metallic / Glass reinforced Plastic (GRP)
impeller by the more energy efficient hollow FRP
impeller with aerofoil design, in case of axial flow
fans, where significant savings have been reported

Energy Saving
5. Fan speed reduction by pulley dia modifications
for derating.
6. Option of two speed motors or variable speed
drives for variable duty conditions.
7. Option of energy efficient flat belts, or, cogged
raw edged V belts, in place of conventional V belt
systems, for reducing transmission losses.
8. Adopting inlet guide vanes in place of discharge
damper control
9. Minimizing system resistance and pressure drops
by improvements in duct system

Typical Energy Audit of a FD Fan
S. N. Particular Unit Source / Calculations
Design
Value
Actual
Value
1 Rated power of FD fan kW Obtained from TPS 750 900
2 No. of units running No unit Reading 2 1
3 Fan discharge pressure mmWc Reading 198 122
4 Total FD flow t/h Reading 480 157
5 Suction pressure fan mmWc Reading -30 -30
6 Density of air kg/m3 From air property 1.29 1.29
7 Power output of FD fan kW {(3-5)x[4/(3.6x2)]x(0.01/6)} 117.62 51.39
8 Voltage for FD fan kV Reading 6.86 6.5
9 Total current for FD fan Amps Reading 64.1 37
10 Power factor for FD fan No unit Reading 0.58 0.58
11 Power input to fan motor kW (SQRT(3)x8x9x10)/2 220.87 241.60
12 Overall efficiency % (7/11)x100 53.25 21.27
13 SEC kWh/t (11x2)/4 0.92 1.54
14 Load factor of FD fan % (11/1)x100 29.45 26.84
15 Pressure gain in FD fan mmWc (3-5) 228 1525/8/2019 Manohar Tatwawadi 50

TPS Coal Mills and Fan Performance

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