Spontaneous combustion (self ignition)...Self-heating and spontaneous combustion • Coal oxidises...
Transcript of Spontaneous combustion (self ignition)...Self-heating and spontaneous combustion • Coal oxidises...
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Spontaneous combustion (self ignition)
Johan de Korte
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• What is spontaneous combustion?
• Affected materials
• Causes
• Coal
• Areas where spontaneous combustion occurs
Overview
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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
Spontaneous combustion (spontaneous ignition)
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• Most organic substances with low ignition temperatures
• 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
Affected materials
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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
Cause of spontaneous ignition
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Bio-fuel stockpile
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Paper yellowed by oxidation
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• One of the more studied materials in terms of spontaneous heating
• 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 ignition of hay
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Spontaneous combustion of hay bales
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Spontaneous heating in bales of hay
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Fire due to spontaneous ignition of hay
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Self-ignition of linen / vegetable oil
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• 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
Spontaneous ignition of charcoal
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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:• Heat value (CV)
• Moisture content
• Volatile matter content
• Size consist
• Surface chemistry
• Coking properties
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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
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Oxidized coal pile
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Oxidized coal
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Gypsum?
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Pyrite in coal
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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
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Acid mine drainage (AMD)
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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
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• During mining
• Raw coal and product stockpiles at mine
• During transport by rail / sea
• Stockpiles at ports
• Stockpiles / silos/ bunkers at end user site
• Discard dumps
Spontaneous combustion of coal
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Raw coal stockpile
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Hot coal in rail truck
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Stockpile on fire at port
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Coal fire in hold of ship
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Burning bord in old mine workings
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Burning coal in opencast mine
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• Properties of coal
• External factors
Reasons for self-heating of coal
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• Low rank coals generally more prone to self heating
• Higher porosity
• Higher oxygen and lower carbon content
• High hydrogen to carbon ratio (which correlates to high volatile matter content)
Properties of coal
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• Available air – oxygen in air necessary for oxidation to proceed
• 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 - oxygen
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• Initial or starting temperature – rate of oxidation doubles for each 10° C rise in temperature
External factors – initial temperature
0
20
40
60
80
100
120
140
0 20 40 60 80
Tem
pera
ture
(deg
rees
C)
Time (Days)
Temperature / Time curves
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• 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
External factors – particle size
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Segregation of different sizes of coal
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• External or surface moisture will initially inhibit self-heating due to evaporation
• Once dried out, coal will generate heat upon re-wetting
• Temperature increase due to heat of wetting can be considerable
External factors - moisture
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• Pyrite and shale can disintegrate and generate new surface area for oxidation
External factors – other
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All coals have the potential to spontaneously combust…….
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But no coal has to…….
MCJ van Vuuren