Tire-derived fuel

Tire-derived fuel (TDF) is composed of shredded scrap tires. Tires may be mixed with coal or other fuels, such as wood or chemical wastes, to be burned in concrete kilns, power plants, or paper mills. An EPA test program concluded that, with the exception of zinc emissions, potential emissions from TDF are not expected to be very much different from other conventional fossil fuels, as long as combustion occurs in a well-designed, well-operated and well-maintained combustion device.^[1]

Theory

Historically, there has not been any volume use for scrap tires other than burning that has been able to keep up with the volume of waste generated yearly. Tires produce the same energy as petroleum and approximately 25% more energy than coal. 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, where there is a possibility for uncontrolled tire fires or the harboring of disease vectors such as mosquitoes.^[2] Tire Derived Fuel is an interim solution to the scrap tire waste problem. Advances in tire recycling technology^[3] might one day provide a solution other than burning by reusing tire derived material in high volume applications.

Characteristics

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 Removed^[4]

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 290 to 340 degrees Celsius. Complete combustion is achieved with flame temperatures of 650 degrees Celsius.

Environmental concerns about this fuel focus 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. Trace heavy metals may be explained by metals added for alloying purposes to the steel wire in the belts.

Tires are typically composed of about 1 to 1.5% Zinc oxide,^[5] which is a well known component used in the manufacture of tires and is also toxic to aquatic and plant life. The chlorine content in tires is due primarily to the chlorinated butyl rubber liner that slows the leak rate of air. The Rubber Manufacturers Association (RMA) is a very good source for compositional data and other information on tires. The use of TDF for heat production is controversial due to the possibility for toxin production. Reportedly, polychlorinated dibenzodioxins 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.^[6]

On one hand, some argue that it is better to use the energy stored in a tire than to put it in a landfill, in line with the waste hierarchy. On the other, it is difficult to justify introducing toxins into the atmosphere, and much energy can be saved by recycling the tires so that new ones do not need to be remanufactured from raw materials.

Controversy

While environmental controversy surrounding use of this fuel is wide and varied, the greatest supported evidence of toxicity comes from the presence of dioxins and furans in the flue gases. Zinc has also been found to dissolve into storm water, from shredded rubber, at acutely toxic levels for aquatic life and plants.^[7]

A study of dioxin and furan content^[8] of stack gasses at a variety of cement mills, paper mills, boilers, and power plants conducted in the 1990s shows a wide and inconsistent 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 facilities 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 varying increases in emissions.^[1]

A 2004 study of Tire rubber use in energy generation,^[9] deeply studies the environmental impact on soil, water, and air from combustion of waste rubber (TDF).

Alvarez research shows that huge polyaromatic emissions are generated from combustion of tire rubber, at a minimum, 2 orders of magnitude higher than coal alone.

The study concludes with, "atmospheric contamination dramatically increases when tire rubber is used as the fuel. Other different combustion variables compared to the ones used for coal combustion should be used to avoid atmospheric contamination by toxic, mutagenic, and carcinogenic pollutants, as well as hot- gas cleaning systems and COx capture systems."

References