Spontaneous combustion (self ignition)
Johan de Korte
Overview
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What is spontaneous combustion?
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Affected materials
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Causes
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Coal
•
Areas where spontaneous combustion occurs
Spontaneous combustion (spontaneous ignition)
A type of combustion which occurs by self heating
(an increase in temperature due to exothermic
internal reactions), followed by thermal runaway and
finally ignition
Affected materials
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Most organic substances with low ignition temperatures
•
•
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Hay
Charcoal
Oil seeds and oil-seed products (eg. Flax, soy beans,
sunflower seeds)
Linseed oil + cotton and linen
Pistachio nuts
Bagasse (sugar cane residue)
Bio-fuels
Coal
•
•
•
•
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Cause of spontaneous ignition
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Organic substances react with oxygen in the air at low
temperatures
•
The reaction may be promoted by moisture and bacterial
action
•
The oxidation reaction is exothermic and heat is generated
•
If the heat is unable to escape the temperature of the
material rises
•
As the temperature increases, the oxidation reaction rate
increases (typically doubling for each 10°C)
•
Eventually the temperature can rise above the ignition point
of the material and combustion can result
Bio-fuel stockpile
Paper yellowed by oxidation
Spontaneous ignition of hay
•
One of the more studied materials in terms of spontaneous
heating
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Hay varies by type of grass and location
•
It is generally accepted that heating will occur in hay that
contains more than 25% moisture
•
Majority of spontaneous hay fires start after 4 to 5 weeks
Spontaneous combustion of hay bales
Spontaneous heating in bales of hay
Fire due to spontaneous ignition of hay
Self-ignition of linen / vegetable oil
Spontaneous ignition of charcoal
•
Freshly prepared charcoal is known to be prone to self
heating and ignition
•
Charcoal that has been exposed to air for more than 8 days
is not considered to be prone to self heating
Oxidation (weathering) of coal
Weathering or oxidation of coal is a degenerative process
and is known to have several negative effects on coal.
The following properties of coal are affected by weathering:
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Heat value (CV)
•
Moisture content
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Volatile matter content
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Size consist
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Surface chemistry
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Coking properties
Weathering of coal
• The oxidation of mineral matter in coal also contributes to the
weathering process
• A significant contribution may be the oxidation of pyrite to iron
sulphates, sulphuric acid and various other compounds.
• Elemental sulphur is not a native constituent of coal but can
often be observed in weathered coal
• A white sulphate (gypsum) deposit may also frequently be
observed in weathered coal
• Coal containing shale will usually deteriorate much faster than
clean coal - especially in terms of size consist
Oxidized coal pile
Oxidized coal
Gypsum?
Pyrite in coal
Pyrite / water reactions
2FeS2 + 7O2 + 2H2O ⇒ 2Fe2+ + 4SO42- + 4H+
2Fe2+ + ½ O2 + H+ ⇒ 2Fe3+ + H2O
Fe3+ + 3 H2O ⇒ Fe(OH)3 + 3H+
FeS2 + 14 Fe3+ + 8 H2O ⇒ 15 Fe2+ + 2SO42- + 16H+
Source: S Motaung
Acid mine drainage (AMD)
Self-heating and spontaneous combustion
• Coal oxidises slowly until it reaches a temperature of about
50oC. Main oxidation product is carbon dioxide
• As the temperature increases, the rate of oxidation increases
and the coal will heat at an accelerated pace until a temperature
of approximately 140oC is reached
• Once the coal reaches a temperature of 140oC, carbon
monoxide and water vapour are given off
• Carbon dioxide liberation increases rapidly until a temperature
of about 230oC is reached. At this temperature, spontaneous
combustion may take place
• At about 350oC, the coal ignites and combusts
Spontaneous combustion of coal
•
During mining
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Raw coal and product stockpiles at mine
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During transport by rail / sea
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Stockpiles at ports
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Stockpiles / silos/ bunkers at end user site
•
Discard dumps
Raw coal stockpile
Hot coal in rail truck
Stockpile on fire at port
Coal fire in hold of ship
Burning bord in old mine workings
Burning coal in opencast mine
Reasons for self-heating of coal
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Properties of coal
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External factors
Properties of coal
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Low rank coals generally more prone to self
heating
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Higher porosity
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Higher oxygen and lower carbon content
•
High hydrogen to carbon ratio (which correlates to
high volatile matter content)
External factors - oxygen
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Available air – oxygen in air necessary for
oxidation to proceed
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The oxidation of coal and carbonaceous shale
produces heat
•
At ambient temperature the reaction is slow but
when heat accumulates the temperature rises
•
At higher temperatures the reaction speeds up
generating more heat and a further rise of
temperature which can ultimately result in a fire
External factors – initial temperature
Initial or starting temperature – rate of oxidation
doubles for each 10° C rise in temperature
Temperature / Time curves
140
Temperature (degrees C)
•
120
100
80
60
40
20
0
0
20
40
Time (Days)
60
80
External factors – particle size
•
Small particles have higher surface area – oxidizes
faster but flow of air is restricted
•
Large particles allow free circulation of air –
oxidizes but heat can dissipate
•
Mixture of sizes – heat generated can not be
dissipated fast enough – results in heating of
stockpile
Segregation of different sizes of coal
External factors - moisture
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External or surface moisture will initially inhibit selfheating due to evaporation
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Once dried out, coal will generate heat upon rewetting
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Temperature increase due to heat of wetting can
be considerable
External factors – other
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Pyrite and shale can disintegrate and generate
new surface area for oxidation
All coals have the potential to
spontaneously combust…….
But no coal has to…….
MCJ van Vuuren