MILLS IN A THERMAL POWER STATION PART -I

Since last 91 post i have discussed various aspect of thermal power plant .Mainly i have written

my experience on civil , mechanical , boiler etc. In boiler  I have discussed about structural ,pressure

parts , fans ( id ,fd and pa ) . A few other topics are left behind to discuss. A list now being attached

for ready reference .

1. Mills

2. Ducting

3. insulation

4. piping ( critical and internal)

5. LOS ( Lub oil supply unit)

6. Motorised valves and dampers

7. ESP ( electrostatic precipators)

Control and instrumentation and electrical will be discussed at final stage.

Therefore we are now going to discuss about MILLS . Mill is one of the very essential item

of Boiler. Choice of  MILLS depend on the coal characteristic of the fuel. Number of mills

guide the height of furnace . There are different type of mills , Bowl Mill, Tube mill , Ball

and race mills etc are available in India. There are pressurised Mill and suction type Mill.

I will limit my discussion on BHEL make pressurised Mills . BHEL supplied other mills

also on demand but not on regular basis.


MILL.

Purpose of the Mill is defined below :-

Pulverise the coarse coal into fine powder and transport the same to furnace with the help of PA fan.

Because of pulverisation following advantages are drawn.

1. Better Air - Coal fuel ration.

2. Better flame stability

3. Effective utilisation of carbon.

4. Efficient use of low grade coal in Boiler.

5. Reliable ,safe and flexible operation.

6. All the above will increase the efficiency of the boiler.

GRINDING PROCESS OF MILLS.

Coal are fed from Raw coal feeder ( situated just above the mill on concrete floor) . One end of the

Raw coal feeder is attached with Coal bunker through a coal gate. Raw coal feeder is having a

travelling belt with coal weighing instruments. Coal from travelling belt of raw coal feeder drops

in central feed pipe ( connection between Raw Coal Feeder and Mill). After coal reaches Mill bowl

grinding started and hot air also been given to make the pulverised fuel dried enough to classifier

chamber. The portion of  the coal which cannot be grinded finds its way to mill reject.

Mechanical setting is to be carried out for minimum rejects . Zero rejects also is not good as it

causes excessive wear and tear of the mills. Once the fuel air mixture reaches classifier it will

again classified into two parts one part which heavier part will go back to mill for further grinding

other part will directly go to furnace through four coal pipes and MDV ( Mill discharge valves).

COAL VARIATION IN INDIA.

In my earlier post i have indicated the type coal available in India and other part of the world like

Australia , Indonesia , African countries etc. Now i will detail of Indian coal and variation.

a. Moisture                                                               5 to 30 %

b. Ash                                                                    10 to 55%

c. Volatile matters                                                 12 to 40%

d. Abrasion Index mg/kg                                       60 to 125

e. Hard Groove Index                                            45 to 110  

EFFECTS OF COAL ELEMENT ON MILL CHOICE AND SIZING.

MOISTURE -  More hot air is required to dry the coal particles .

VOLATILE MATTERS- Determine temperature of  Mill outlet and furnace.

ASH              - Grinding element ; coating on rollers etc

HGI               - Grind ability

Calorific Value - Mill sizing.

NOMENCLATURE OF BOWL MILLS FOR  XRP xxxx ( e.g. 1003 /1103)

-  X  stands for Frequency of Power Supply ( 50 Hz  for India )

-  R stands for Raymond ( inventor of Bowl Mill)

-  P stands for Pressurised Mill with Primary Air Fan

- 100/110  stands for Bowl diameter ( inches) ( in case of xrp1003 /xrp 1103)

-  3  stands for Number of grinding Roller.

STANDARD MILL CAPACITY 

MILL SIZE                                        CAPACITY( COAL GRINDING)

XRP 623                                                          18 + TPH

XRP 703                                                          26 + TPH

XRP 763                                                          33.8  TPH

XRP 783                                                          36.5  TPH

XRP 803                                                          39.8  TPH

XRP 883                                                          51 + TPH

XRP 903                                                         54+ TPH

XRP 1003                                                       68 .1 TPH

XRP 1103 HP                                                 91 + TPH


Continued to Part -2

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