2. Grinding Of Coals & Pet Cokes

2. Grinding of coals & pet cokes

Content ƒ Coal, pet-cokes Æ varieties & proprieties ƒ Coal grinding systems ƒ Grinding process ƒ Fine coal storage ƒ Statistics

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Coal and Petcoke types

Coal classification after RANK

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Petcoke types Bulk Density

R 31.5mm

R 16.0mm

R 8.0mm

R 4.0mm

%

%

%

%

Sponge

9

23

42

60

0.7

8.8

<70

Shot

14

79

92

95

0.8

2.5

<45

Fluid

6

17

57

96

0.69

8.1

Type

T/m

3

Humidity %

HGI

Picture

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Coal grinding systems ƒ Tube mill system ƒ Roller mill system ƒ Special features

Air Swept Mill ƒ Features

Separator

 



Drying Chamber







Drying compartment Shoe-bearing at the inlet side Retention time of the coal is longer Can deal with coal of variable proprieties Not so sensitive to WEAR NOT suitable for direct firing system

FLS - Tirax 7

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Typical Arrangement for a Coal ASM

Coal dust Bin

Raw Coal Bin

Filter

Hot Gas

Mill

To Kiln Dosing

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Vertical Mill System (ex: Loesche Mill) Feed Product

High efficiency classifier

Not suited for FLEXI COKE!!

Hot gas

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Typical Arrangement – Coal Grinding in VRM Safety Explosion Vent

Bag Filter

CO2 Inertization

Feed Bin Coal Mill (VRM)

Fine Coal Silo

Safety Shut-Off Gates 10

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Coal Grinding Process ƒ Proportioning (feed) ƒ Drying ƒ Grinding ƒ Separating ƒ Special features ƒ Quality requirements ƒ Instrumentation

Proportioning (feeding) Provide homogeneous raw coal mix Front end-loader mix Æ not suitable! ƒ Raw coal and Petcoke show a

typical cohesive property ƒ The feed bin must be designed

for mass flow ƒ Ideal capacity of a feed bin is

3 Hours of continuous operation! ƒ Same precautions as per raw

material

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Hot Gas Generator

ƒ

Cooler hot gases* (only non-inert)

~… m3/h

ƒ

is NECESSARY!!

Kiln gases

For higher moistures Æ pre-drying

ƒ

System

Coal / pet-coke with 0 – 20 % H20

Heat sources

In Roller Mills Æ higher drying capacity

~…m3/h

Product with < 1% H20 …or hygroscopic equilibrium reached

Drying

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Grinding - Material Circulation Mill Exit /

NO recirculation bucket elevator!!

Fine Coal

Raw Coal Feed

Hot gas

Table rejects

e.g. FLS ATOX Coal Mill Holcim Group Support

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Grinding Æ Inert/Non-inert grinding system Æ

Non - inert

Æ

Æ

Æ Æ Æ

Æ Æ

Inert

Æ

Æ

Æ

Oxygen content >14% Hot gas from cooler or hot gas generator Temperature and CO monitoring required Simple system / layout Higher safety risk Not for fuel with > 26 % volatiles content Oxygen content < 12% Hot gas from preheater Low explosion and fire risk -> safety Solution for various solid fuel types (petcoke, coal, lignite) Temperature, O2, CO monitoring required

For new installations Æ Inert system is MANDATORY! Holcim Group Support

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Separation – fineness Dynamic separator ƒ Efficient separation ƒ Constant fineness ƒ Flexible ƒ Reduced oversized particles

High Efficiency separator is recommended! 16

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Filter for coal grinding systems - General ƒ Explosion doors and vents with explosion-proof limit switches ƒ All external surfaces of the bag filter shall be insulated. ƒ All internal surfaces exposed to the process gases shall be

protected against corrosion ƒ Explosion protection measurement shall include:    

CO and O2 at bag filter inlet, outlet, and hopper Alarm on CO rate increase Temperature sensors in bag filter Inerting and fire suppression injection ports

ƒ The housing and plenum shall be equipped with explosion vents. ƒ The minimum valley angles for hoppers and duct slopes shall be

70° in order to avoid coal dust accumulations. ƒ Hoppers shall have rounded corners. 17

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Filter for coal grinding systems - standard HGRS Recommendation ƒ For a filter load (air to cloth ratio) of max. 1.0 m3/m2 x

min. Æ dust concentration in the cleaned (de-dusted gas) < 30 mg/m3 ƒ Utilization of teflon membranes is NOT recommended ƒ BAGS  

  

Must be treated with silicone PEAC-Needle felt (50% Polyester und 50% Polyacryl-Fazes) with surface spec. weight of 600 g/m2 Shall not exceed 3.6 m in length. Grounding system to prevent static electricity accumulation Wire cage supports Æ corrosion resistant

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Recommended Fineness Coal – Petcoke HGRS Standard Recommendation: ƒ Coal:

% R90μm <= ½ * (% volatiles) Æ % R200μm <= 1% Æ

ƒ Petcoke:

% R90μm <= 5% Æ % R200μm < 0.5% Æ

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Specific Energy Consumption vs Coal Grindabilty ƒ Espec in function of

grindability and coal meal fineness

ƒ Grindability according to

Hardgrove Index (HGI) ƒ Example based on Espec

of ATOX coal mill

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Special features Æ Ducts (Gas-ducts) ƒ Pressure resistant up to 8 bar (check local standard) ƒ Before vessels inlets Æ quick shut-off gates must be installed ƒ Dust-deposit possibilities must be avoided:   

No horizontal ducts Sufficient gas speed (20-25 m/s) For de-dusting – 70° must be respected

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Transport of fine coal (product) ƒ Velocity in the ducts: 28-35 m/s ƒ Duct-bends Æ foreseen wear protection ƒ Curve radius ~2m Example: Wear protection by proper design (Diverter pot)

Example: Wear protection on duct elbow (e.g. Basalt)

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Ideal coal bin design ƒ Designed for mass-flow. ƒ A cone-in-cone design Æ bin discharge

hopper with wall slopes of 65 degree for the external and 77 degree for the internal hopper.

ƒ Material of construction for both

hoppers would be stainless steel plates.

ƒ The minimum diameter of the circular

hopper discharge openings would be at least 1400 mm.

ƒ Material discharge would be activated

by a live bottom feeder.

ƒ The flow rate would be controlled by a

proportioning rotor scale (or a flow meter).

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Coal dust dosing Rotor weigh feeder ƒ Proportions & weighs the coal dust ƒ feeds the coal dust by gravimetric or

pneumatic transport to the process

PFISTER ƒ Rate

0.2t/h - 500t/h

ƒ Accuracy

+/- 0.5%

ƒ Material properties

< 5mm free/moderate flowing, non-caking

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Statistics ƒ Maintenance ƒ Specific energy consumption ƒ Process parameters

Specific Energy Consumption vs. Hardgrove (Loesche)

Typical values

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Mill Capacity vs. Hardgrove Mill capacity vs. Hardgrove 180 160

5 % R 90 um 10 % R 90 um

140

15 % R 90 um 20 % R 90 um

Hardgrove

120 100 80 60 40 20 0 50

75

100

125

150

% Mill capacity 27

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Mill Inlet Temperature vs volatiles content Ignition temp. vs. volatiles ignition temp.

0.5 x ignition temp.

Recommended temperature

700

Mill inlet temp. [deg C]

600 500 400 300 200 100

non-inert operation possible

inert operation

0 0

10

20

30

40

50

60

volatiles [%]

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Conclusion ƒ System Design 

Grinding systems for coal and pet-coke require special design features

ƒ Process  

Safety is the major concern Target: constant product fineness

ƒ Future 

HGRS standard: ONLY inert operation installations

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