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COAL MILL OPERATIONSAFETY ASPECTS
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Coal Shop Safety
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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)
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Coal Shop Safety
FUEL DRYING- GRINDING SYSTEM
Flowsheet of Grinding - Drying System
Grinding - Drying System
Pulverized Coal
Pulverized Coal Silo
Hot gases
Dust Collector
Feed Bin
Raw Coal
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Coal Shop Safety
General Safety RulesThree Conditions
Combustible dust (or gas) in exploding concentration
Oxygen above the critical concentration
Ignition source
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Coal Shop Safety
COAL IGNITION TEMPERATURE Vs VM
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Coal Shop SafetySafety in Coal Shops – Current TA
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
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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
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Coal Shop Safety
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
General Safety Rules Ignition Process
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Coal Shop Safety
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
General Safety Rules Ignition Process
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Coal Shop Safety
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
General Safety Rules Ignition Process
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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
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Coal Shop Safety
General Safety Rules Protection Measures
Explosion protection
Fire protection/ fighting
Water spray
Inerting installation
CO2
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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.
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Coal Shop Safety
O2/CO analyser
Temp/Press sensors
O2/CO analyser
Temp/Press sensors
Typical Coal Mill Flow Sheet
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Coal Shop Safety
General Safety Rules Safety Control
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Coal Shop Safety
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Coal Shop SafetyBASIC CONSIDERATIONS
RAW COAL:
• Feed size : 0-50 mm
• Moisture content : 10-15% H2O
• Grindability : 50-70°H
EXHAUST GASES:
• Temperature : 75-90°C
• Dust load : 500-600g/m3
HOT GASES:
• Temperature : 250-300°C
• O2 content : 3-7%
• Dew point : 20-70°C
PULVERIZED COAL:
• Fineness : 1-15%
R 0.09 mm
• Moisture content : <1.0% H2O
DRYING
GRINDING
CLASSIFYING
Grinding-Drying of Coal: Basic Considerations
