Tire derived fuel
Tire-derived fuel is composed of shredded tires. Tires may be mixed with coal or other fuels such as wood to be burned in concrete kilns, power plants, or paper mills. The use of TDF for heat production is controversial due to the possibility for toxin production.
Tires produce the same energy as petroleum and approximately 25% more energy than coal,http://www.epa.gov/epaoswer/non-hw/muncpl/tires/tdf.htm] Burning tires is lower on the hierarchy of reducing waste than recycling, but it is better than placing the tire waste in a landfill or dump. There, there is a possibility for tire fires or the harboring of disease vectors such as mosquitoes.
Reportedly, dioxins and furans are produced during the combustion process and there is supportive evidence to suggest that this is true under some incineration conditions. Other toxins such as NOx, SOx and heavy metals are also produced, though whether these levels of toxins are higher or lower than conventional coal and oil fired incinerators is not clear. [http://www.energyjustice.net/tires/]
On one hand, it is better to use the energy stored in a tire than to put it in a landfill, in line with the waste hierachy. On the other, it is difficult to justify introducing toxins into the atmosphere.
Tire derived fuel is usually consumed in the form of shredded or chipped material with most of the metal wire from the tire's steel belts removed. The analytical properties of this refined material are published in TDF Produced From Scrap Tires with 96+% Wire Removedhttp://www.p2pays.org/ref/11/10504/html/usa/tdfdata.htm]
This fuel has a very high energy content, with an average heat value of 15,500
BTUs per pound of fuel. This is roughly the same as heavy petroleum fuel oils. Fuel begins to burn (Flash point) at 550 to 650 degrees fahrenheit. Complete combustion is achieved with flame temperatures of 1,202 degrees fahrenheit.
Environmental concerns about this fuel center on the chemical contents of the tire other than hydrocarbons. Tires are constructed with steel belts which give shape and structure to the tread and sometimes to the sidewall. Much of this wire is removed when tires are shredded to make TDF, however certainly not all of it will be removed. Therefore it is no surprise that the ash contains a large mass percentage of iron. Zinc, Chromium, Cadmium and lead make up the bulk of the remaining heavy metals in the ash. The alkaline earth metal calcium is also present in significant quantity.
Fluorine, chlorine, sulfur, and nitrogen make up the bulk of the non-metal content of the ash.
One tire manufacturing process involves a salt bath, which likely explains the high content of calcium and chlorine. Trace heavy metals may be explained by metals added for alloying purposes to the steel wire in the belts. High lead and zinc content could be related to the use of those metals in wheel weights resulting in material transfer to the rubber tire during driving, however there is no evidence to substantiate this.
While environmental controversy surounding use of this fuel is wide and varied, the greatest supported evidence of toxicity comes from the presents of dioxins and furans in the flue gases.
Dioxins and furans are chemical families involving halogens, mostly chlorine. Dioxins are extremely toxic carcinogens. A study of dioxin and furan contenthttp://www.energyjustice.net/tires/#16] of stack gasses at a variety of cement mills, paper mills, boilers, and power plants conducted in the 1990s shows a wide and inconsistant variation in dioxin and furan output when fueled partially by TDF as compared to the same facilities powered by only coal. Some facilities added as little as 4% TDF and experienced as much as a 4,140% increase in dioxin and furan emsissions. Other facilites added as much as 30% TDF and experienced dioxin and furan emissions increases of only as much as 58%. Still other facilities used as much as 8% TDF and experienced a decrease of as much as 83% of dioxin and furan emissions. One facility conducted four tests with two tests resulting in decreased emissions and two resulting in increased emissions. Another facility also conducted four tests and had widely varing increases in emissions.http://www.epa.gov/ttncatc1/dir1/tire_eng.pdf]
The widely varing nature of this data suggests that the only conclusion that can be drawn is there is no consistent, conclusive proof of increased dioxin and furan emissions from TDF, though a link is still clear under some (uncertain) conditions. Clearly some other factors beyond the scope of the study were invoived to explain such widely varing numbers. Additional research is required.
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