Spontaneous combustion (self ignition) Johan de Korte

Transcription

1 Spontaneous combustion (self ignition) Johan de Korte

2 Overview What is spontaneous combustion? Affected materials Causes Coal Areas where spontaneous combustion occurs

3 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

4 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

5 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

6 Bio-fuel stockpile

7 Paper yellowed by oxidation

8 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

9 Spontaneous combustion of hay bales

10 Spontaneous heating in bales of hay

11 Fire due to spontaneous ignition of hay

12 Self-ignition of linen / vegetable oil

13 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

14 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

15 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

16 Oxidized coal pile

17 Oxidized coal

18 Gypsum?

19 Pyrite in coal

20 Pyrite / water reactions 2FeS 2 + 7O 2 + 2H 2 O 2Fe SO H + 2Fe 2+ + ½ O 2 + H + 2Fe 3+ + H 2 O Fe H 2 O Fe(OH) 3 + 3H + FeS Fe H 2 O 15 Fe SO H + Source: S Motaung

21 Acid mine drainage (AMD)

22 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

23 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

24 Raw coal stockpile

25 Hot coal in rail truck

26 Stockpile on fire at port

27 Coal fire in hold of ship

28 Burning bord in old mine workings

29 Burning coal in opencast mine

30 Reasons for self-heating of coal Properties of coal External factors

31 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)

32 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

33 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) Time (Days)

34 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

35 Segregation of different sizes of coal

36 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

37 External factors other Pyrite and shale can disintegrate and generate new surface area for oxidation

38 All coals have the potential to spontaneously combust.

39 But no coal has to. MCJ van Vuuren