Spontaneous combustion (self ignition) Johan de Korte
Overview What is spontaneous combustion? Affected materials Causes 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 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
Cause of spontaneous ignition 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 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: 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 2FeS 2 + 7O 2 + 2H 2 O 2Fe 2+ + 4SO 2-4 + 4H + 2Fe 2+ + ½ O 2 + H + 2Fe 3+ + H 2 O Fe 3+ + 3 H 2 O Fe(OH) 3 + 3H + FeS 2 + 14 Fe 3+ + 8 H 2 O 15 Fe 2+ + 2SO 2-4 + 16H + Source: S Motaung
Acid mine drainage (AMD)
Self-heating and spontaneous combustion Coal oxidises slowly until it reaches a temperature of about 50 o C. 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 140 o C is reached Once the coal reaches a temperature of 140 o C, carbon monoxide and water vapour are given off Carbon dioxide liberation increases rapidly until a temperature of about 230 o C is reached. At this temperature, spontaneous combustion may take place At about 350 o C, the coal ignites and combusts
Spontaneous combustion of coal 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
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 Properties of coal External factors
Properties 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)
External factors - oxygen 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 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 60 80 Time (Days)
External factors particle size Small particles have higher surface area oxidizes fasterbut 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 External or surface moisture will initially inhibit selfheating due to evaporation Once dried out, coal will generate heat upon rewetting Temperature increase due to heat of wetting can be considerable
External factors other 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