Spontaneous combustion (self ignition)

Johan de Korte

• What is spontaneous combustion?

• Affected materials

• Causes

• Coal

• Areas where spontaneous combustion occurs

Overview

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)

• 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

• 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

Bio-fuel stockpile

Paper yellowed by oxidation

• 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

Spontaneous combustion of hay bales

Spontaneous heating in bales of hay

Fire due to spontaneous ignition of hay

Self-ignition of linen / vegetable oil

• 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

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

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

• 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

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

• Properties of coal

• External factors

Reasons for self-heating of coal

• 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

• 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

• Initial or starting temperature – rate of oxidation doubles for each 10° C rise in temperature

External factors – initial temperature

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140

0 20 40 60 80

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ture

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Time (Days)

Temperature / Time curves

• 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

Segregation of different sizes of coal

• 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

• Pyrite and shale can disintegrate and generate new surface area for oxidation

External factors – other

All coals have the potential to spontaneously combust…….

But no coal has to…….

MCJ van Vuuren