Coal Mill Safety

COAL MILL OPERATION SAFETY ASPECTS

Coal Shop Safety

SOLID FUELS  

 

Coal Petroleum coke (sulfur % )  Delayed coke: the most common  Non grindable fluid coke  Shot coke (as impurity in delayed coke)

Lignite Charcoal (moinha) 2 2

Coal Shop Safety

FUEL DRYING- GRINDING SYSTEM Raw Coal

Dust Collector

Feed Bin

Pulverized Coal Silo

Pulverized Coal Hot gases

Grinding - Drying System

Flowsheet of Grinding - Drying System 3

Coal Shop Safety

General Safety Rules Three Conditions  Combustible dust (or gas) in exploding concentration  Oxygen above the critical concentration  Ignition source

4

COAL IGNITION TEMPERATURE Vs VM

Coal Shop Safety

5

Safety in Coal Shops – Current TA

Coal Shop Safety

High Risk Fuels defined as any fuel >36%VM & High Risk Operation defined when grinding finer. Compliance of both TA and specific requirements for High Risk Fuels & High Risk Operation required PRIOR to use

For blended fuels  Weighted average VM of the blend determines the level of risk.  If any single component of the blend ≥ 20%, by weight has a VM higher than the weighted average then, this fuel’s VM is used to determine the level of risk TA: Technical Agenda - Safety in Coal grinding Shops Update FEB 2007

6

Coal Shop Safety

General Safety Rules Ignition Process  Coal absorbs oxygen from atmosphere Heat Produced

Heat Released

 Temperature increases and speeds up oxidation process (Inside the pile, the heat cannot be released)

 First three weeks of storage = maximum oxygen

intake.  Critical Temperature ( 70 - 80 °C) for self-ignition

7

Coal Shop Safety

General Safety Rules Ignition Process For coals stored at a temperature of 20° C, the risk is worth paying attention to when the storage time exceeds: 

4 months for anthracite coals



2 to 3 months for lean, rich and 1/4 rich coals



1 to 2 months for long-flaming coals



1 week to 1 month for lignites

8

Coal Shop Safety

General Safety Rules Ignition Process  Some of the most important factors which heighten the danger of self-ignition of coal are:  High volatile content in the fuel (>20 % over dry)  The more fine-granular the coal, the larger the reactive surface to absorb oxygen (proportion of fines greater than 10%)  Coal containing, besides the fine-granular portion, also coarse material, since air intake into the coal heap is facilitated by this heterogeneous structure

9

Coal Shop Safety

General Safety Rules Ignition Process  Some of the most important factors which heighten the danger of self-ignition of coal are:  Freshly mined coal reacts faster with oxygen than coal stored for a longer period  Coal with an abnormally high moisture content  Foreign components such as pyrite (>4%), iron, phosphorus in the coal ash  Already hot fuel

10

Coal Shop Safety

General Safety Rules Oxygen Concentration  Coal grinding should be done in inert atmosphere  Basic rules for the Oxygen during coal grinding are: < 14% O2 for Petcoke and Anthracite < 10% O2 for coal with volatiles below 30% < 9% O2 for coal with volatile above 30% The interlocked maxmax is 1 % point higher

11

Coal Shop Safety

General Safety Rules Protection Measures  Explosion protection  Fire protection/ fighting  Water spray

 Inerting installation  CO2

12

Coal Shop Safety

CIRCUIT CONTROL 

Mill outlet temperature constant (70-95°C), mill inlet below safe level.



Sweep air volume: Fan power ( KW) constant (damper).



Fuel fineness: Mill sweep / Separator settings / Roller pressure /Ball charge.



Indirect firing : P.F. bin level constant / high bin level. 13

Typical Coal Mill Flow Sheet

Coal Shop Safety

O2/CO analyser Temp/Press sensors

O2/CO analyser Temp/Press sensors

14

Coal Shop Safety

General Safety Rules Safety Control Sensors

Threshold

Grinding mill outlet temp (coke)

115°C

Grinding Mill outlet temp (coal)

95°C

Mill inlet Temperature: HVF

250°C (550F)

Mill inlet temperature: LVFuel

300°C (680F)

Filter outlet O2 (coke)

15%

Filter outlet O2 (coal)

13%

Filter outlet CO

2000 ppm

Fired fuel into the kiln (% H2O)

0.5 - 1.5%

Filter outlet temperature (coke)

105°C

Temperature difference (outlet - inlet)

10°C

Temperature difference variation

10°C

Filter hopper temperature (coke)

85°

“Pulverized” hopper CO

1000 ppm

“Pulverized” hopper temperature

85°

Sources (JP Senechal 1997 memo, S. Fujimoto 1992 training course)

15

Coal Shop Safety

16

BASIC CONSIDERATIONS RAW COAL: • Feed size

EXHAUST GASES: : 0-50 mm

• Moisture content : 10-15% H2O • Grindability

Coal Shop Safety

• Temperature

: 75-90°C

• Dust load

: 500-600g/m3

: 50-70°H

 DRYING  GRINDING

 CLASSIFYING HOT GASES:

PULVERIZED COAL:

• Temperature

: 250-300°C

• O2 content

: 3-7%

• Dew point

: 20-70°C

• Fineness

: 1-15%

R 0.09 mm • Moisture content : <1.0% H2O

Grinding-Drying of Coal: Basic Considerations 17