mills sieve analysis

1. ‘
I
BY
RAMESH KOMMA
Senior Chemist /TSGENCO
KTPP-CHELPUR
MILLS OR PULVARISERS
IN THERMAL POWER PLANT
TSGENCO

2. NTRODUCTION
The most efficient way of utilizing coal for steam generation is to burn it in pulverized form
Pulverized coal burns like gas, can be easily lightened and controlled
the pulverizer receives raw coal from coal feeder and pulverizes it to fine powder
The four principles involved in pulverization
1) Drying
2) Grinding
3) Circulation
4) Classification
Drying
In pulverizer Inherent and surface moisture of the coal is reduced by the hot from the air heaters
Grinding
Grinding of coal involves
1. Impaction: - coal is impacted by an outside force
2. Crushing: - coal is forced between two fixed objects
3. Attrition: - where the coal is ground by rubbing or friction
IMPACT CRUSHING ATTRITION
Circulation
With the help of primary air circulate through the pulaverizer due to the circulation of coal heavy
particles are removed by centripetal force
Classification
The circulation air is also used to classify the pulverized coal while carrying it to the burners
Classifier located at the top of the mill. It returns the oversize particles back to the pulverizer and
allows the proper sized coal to pass out the mills to the burners
Classifiers provides desired fine coal to the burners

3. TYPE OF PULVERIZERS
BALL MILL BOWL MILL HAMER MILL

4. KTPP BOWL MILL SPECIFICATIONS
500MW 600MW
TYPE OF MILL XRP 1043 BOWL MILL
MILL BASE CAPACITY(T/Hr)* 73.6 (Design Coal
MOTOR SPEED(RPM) 985
TOTAL WEIGHT OF MILL(T)
(including motor) 30
TYPE OF LOAD Moderate Shock
ROTATING WEIGHT OF MILL(T) 22.88
SPRING RATE(Kg/cm) 2730
SPRING PRE LOAD(Kg) 9500
MILLOUTLETTEMPERATURE
RANGE
66ºC -100ºC
AIR FLOW (T/Hr) 111
FINENESS * 70%rough 200 Mesh
The capacity for bituminous/ sub-bituminous coal having hard grove index (HGI) as 52 and moisture
less than 9% and outlet fineness 70% thru’ 200 Mesh and 98% through 50 Mesh.

5. MILL PERFORMANCECRITERIA
1. Fineness
2. Coal grindability
3. Rejects
4. Capacity
FINENESS:
Fineness is the indication of quality of the pulverizer action, fineness of coal is measured by passing
100gr of coal through set of sieves i.e. 50,100,200.
A 70% coal passing through 200 sieve, 90% through 100 sieve and above 98% through 50 sieve indicates
optimum mill performance
If more than 70% coal passes through 200 sieve, power consumption and mill wears are increased
If less than 70% coal passes through 200 sieve, higher the carbon loss and fuel consumption
less than 98% of coal passing through 50 sieve indicates improper internal settings, boiler slagging and
high unburned carbon
Reduced fineness is the indication of problems in classifier vane position, loss of roller tension Roller wear
Classifier vane wear and exceeding mill capacity
COAL GRINDABILITY
The measure of the coal resistance to crushing is called coal grind ability
Grind ability of coal is measured by hard grove index (HGI) test
Moisture and ash content effects the grind ability of coal
HGI is determined through a multi-step procedure:
1. A 50-gram sample of prepared coal that is uniform in size is placed inside a grinding unit
2. The unit undergoes a standard number of revolutions 60 under a specified pressure
3. Steel balls within the unit crush the coal sample
4. The coal fines are sorted and the quantity of coal less than a specified size is recorded and
converted into a Hardgrove Grind ability Index (HGI) value
HGI=13+6.93*W

6. KOMMA RAMESH SENIOR CHEMIST TSGENCO KTPP-CHELPUR Page 6
COAL REJECTS
The foreign material, mixed with the coal, that cannot be g rinded is rejected by the mills and
discharged in a “pyrites box”
Rejects are the mixer of different materials such as pyrites, stones, tramp iron, Iron disulphide etc.
The amount of pulverizer rejects is one indication of mill performance
MILL CAPACITY
The grinding capacity of a mill depends upon the grinding mechanism and the operational conditions