Waste tyres have a number of important uses as: fuels, construction materials or as raw materials in the production of rubber shred or crumb. In many of these applications whole tyres have to be chopped into small chips. In some cases chips are ground much more finely and the metal content of the tyres is removed.
Chopping and grinding of tyres produces a low density, porous material through which air may percolate. The total surface area of tyre chips or crumb particles may also be large compared with the volume occupied. The combination of permeability to air-flow and a high exposed surface area means that that a combustible material such as rubber is potentially susceptible to spontaneous combustion.
In practice spontaneous ignition of large stockpiles of tyre shred or deep landfill deposits has occurred on numerous occasions. Practical experience suggests auto-ignition normally occurs in large stockpiles (more than 3 metres deep). Finely shredded tyres are more susceptible because of the increased surface area available for reaction.
Surface symptoms of the onset of spontaneous combustion can be subtle: a slight sulphurous odour, condensation aerosols emerging from vents or evidence of oil contamination of rainwater draining through the tyre shred. The fire may intensify from smouldering to flaming as the combustion wave reaches the surface or if the pile is disturbed – allowing ingress of additional air.
Laboratory experiments show that rubber crumb and tyre shred are more susceptible to self-heating than cellulosic materials (like hay and straw) in conditions of high ambient temperature. Typically a given volume of tyre shred will spontaneously ignite at a lower ambient temperature that an equivalent volume of hay. On the other hand tyre shred is not as prone to biological heating that raises the internal temperature above ambient and may act as a trigger to spontaneous combustion.
Controlled experiments suggest that even piles of clean tyre shred or rubber crumb with a depth greater than around one metre may spontaneously ignite from ambient temperatures, if the level of ventilation is unfavourable. The initiation times are generally long (many weeks).
The risk of ignition in practical circumstances may be raised by contamination of the tyres (which may allow biological heating in damp conditions) or by the rusting of exposed wires (which also generates heat). The stripping of rubber from wire in the tyre chopping process may be associated with blunting of blades in the shredding machine.
Many of the standard methods of protection against spontaneous combustion are applicable to tyre shred:
Obviously localised sources of heat e.g. heating pipes, light bulbs, space heaters, braziers, shrink-wrapping guns equipment etc should be kept away from piles of tyre shred.
Once established, combustion in large piles of tyre shred and rubber crumb is difficult to suppress. Direct application of water or foam in situ does not generally provide effective extinguishment and may hamper control of oily run-off pollution.
The first priority is separation of unburned material from the fire to restrict the extent of spread. It may be possible to remove burning material from the fire and finally extinguish with water or by burial.
Without appropriate controls during fire-fighting there is potential for serious occupational exposure for fire service personnel and heavy equipment operators, from poly-aromatic hydrocarbons, oxides of sulphur and other harmful combustion products.
Civil Engineering Committee (1997), Scrap tyre Management Council, Washington DC.
The following recommendations apply to tyre chip layers less than 3 metres deep and greater than 1 metre deep. Layers deeper than 3 metres are not recommended. Lower standards are accepted for layers thinner than 1 m.