Tyres are not subject to spontaneous combustion. However, as a tyre fire grows in intensity it generates higher temperatures, allowing the fire to spread and the generation of large plumes of dense smoke and other combustion products. The pile composition affects the rate and direction of fire spread. [International Association of Fire Chiefs (IAFC), Scrap Tyre Management Council (STMC) and National Fire Protection Association (NFPA) (2000)"The Prevention and Management of Scrap Tire Fires".] Fires occurring in piles of whole tyres tend to burn down into the middle of the pile where air pockets allow continued combustion. Fires occurring in piles of chipped or shredded tyres tend to spread over the surface of the pile.
A wide variety of decomposition products are generated during scrap tyre fires. Many of the decomposition products have been characterised in test burns [Ibid.] and include:
- ash (typically containing carbon, zinc oxide, titanium dioxide, silicon dioxides, etc)
- sulphur compounds (carbon disulfide, sulphur dioxide, hydrogen sulphide)
- polynuclear aromatic hydrocarbons (such as benzo(a)pyrene, chrysene, benzo(a)anthracene, etc) are usually detected in oil runoff
- aromatic, naphthenic and paraffinic oils
- oxides of carbon and nitrogen
- various light-end aromatic hydrocarbons (such as toluene, xylene, benzene, etc).
These decomposition products are extensive and varied depending on a variety of factors, [Ibid.] , [Basel Convention Working Group (1999)"Basel Convention Technical Guidelines on the Identification and Management of Used Tyres". Basel Convention on the control of transboundary movements on hazardous wastes and their disposal. Document No. 10.] including:
- tyre type
- burn rate
- pile size
- ambient temperature
Uncontrolled tyre fires usually have major environmental impacts, [Ibid.] which include:
- air pollution: black smoke and other substances such as volatile organic compounds, dioxins and polycyclic aromatic hydrocarbons are released into the atmosphere
- water pollution: the intense heat allows pyrolysis of the rubber to occur, resulting in an oily decomposition product which is manifested as an oil runoff. This runoff can be carried by water, if water is used to put out the fire. Other combustion residues (such as zinc, cadmium and lead) can also be carried by fire water off the site
- soil pollution: residues that remain on the site after the fire can cause two types of pollution; these are immediate pollution by liquid decomposition products penetrating soil, and gradual pollution from leaching of ash and unburned residues following rainfall or other water entry.
A more comprehensive analysis of the environmental impact of uncontrolled tyre fires has been carried out by the United States Environmental Protection Association. [EH Pechan & Associates Inc. (October 1997)"Air Emissions from Scrap Tire Combustion". A report prepared for United States Environmental Protection Agency.]
Specific and general mitigation measures to address the environmental risks associated with tyre fire are outlined in Section 8.8.