Tire Derived Fuel (TDF) – Overview
A major use for scrap tires is fuel. Tire-derived-fuel (tdf) is a fuel derived from scrap tires of all kinds. This may include whole tire or tires processed into uniform, flowable pieces that satisfy the specifications of the end-user. Scrap tires are used as fuel either shredded or whole depending on the type of combustion unit.
TDF is the oldest and most developed market for scrap tires in the U.S. Industrial facilities across the country, including cement kilns, pulp and paper mills and electric utilities use tdf as a supplemental fuel to increase boiler efficiency, decrease air emissions and lower costs. More than 52 percent of the 300 million scrap tires generated annually are consumed as tdf in these facilities providing a cleaner and more economical alternative to traditional fuels.
The U.S. Environmental Protection Agency (EPA) described TDF as a high Btu-value fuel with lower emissions, including lower greenhouse gas emissions, than comparable traditional fuels, in a 2009 Advanced Notice of Proposed Rulemaking. In earlier studies, EPA concluded, “With proper emission controls, burning tires for their fuel energy can be an environmentally sound method of disposing a difficult waste.”
Scrap tires make an excellent fuel because of their high heat value. Each tire has energy potential. The heating value of an average size passenger tire is between 13,000 and 15,000 Btu/lb., which compares with about 10,000 to 12,000Btu/lb. for coal. The primary reason for using tire fuels is to save fuel costs. Further, they are compact, have a consistent composition and low moisture content-all benefits to the fuel user. Another major reason for combusting tires as fuel is to decrease the number of scrap tires disposed in landfills or stockpiles.
Nationally scrap tires represent a potential energy source of 1.01 quadrillion Btu per year based on a discard rate of 300 million tires per year each weighing an average 22.5 lbs with 15,000 Btu per pound. This is equivalent to 17 million barrels of crude oil and represents about 0.24 percent of the U.S. energy needs. Given this energy value, it is clear that scrap tires compete with comparable traditional fuels including coal, petroleum coke and wood wastes.
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While some combustion systems, typically cement kilns, can accept tires whole, most combustion systems require the tires to be processed to certain sizes and purity to ensure the material consistently meets the needs of the particular fuel user. Shredding scrap tires to produce tdf uses standard material processing technology which includes shredding and removing dirt or other contaminants.
Processing tires into tdf involves two physical processing steps: chipping/shredding and metal removal. In the first step, tires are either fed into the shredder whole or have the beads removed prior to shredding. The processing equipment is typically high-shear, low-torque shredders. Scrap passenger and truck tires up to 48 inches in outside diameter can be initially reduced in these rotary shear shredders to pieces ranging in size from 1” to 4”, depending on the end-use.
To produce tdf-size shreds and chips, whole tires are reduced to nominal 2 inch pieces using one shredder or a series of shredders, screening equipment and magnetic separation equipment. Magnetic separators are required to remove the steel. A screen in the discharge of the shredder controls the shred/chip size where the two-inch sized material falls through the screen openings while the oversized material is re-circulated back to the shredder. Because a significant amount of rubber is entrained and lost in the wire removal stream, downstream shredding and wire removal can be employed to recover additional rubber, make a cleaner steel product for sale as scrap and to avoid landfilling this wire/rubber material. If smaller-sized tdf (1-inch or crumb rubber) is specified, then more size reduction, metal and fiber separation, classifying, screening and cleaning equipment may be required.
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TDF has the flexibility to be used in variety of industries. These include:
Cement manufacturing companies use whole tires and tdf to supplement their primary fuel for firing cement kilns. Several characteristics make scrap tires-either whole or shredded– an excellent fuel for the cement kiln. The very high temperatures and long fuel residence time in the kiln allow complete combustion of the tires. There is no smoke, odor or visible emissions from the tires. .Because the ash is incorporated into the final product, there is no waste. The metal wire contained in the tdf is captured as a raw material or ingredient in the cement making process. Each passenger car tire contains about 2.5 pounds of high-grade steel. The steel portion of the tire becomes a component of the cement product, replacing some or all of the iron required by the cement manufacturing process.
The Portland Cement Association (PCA) reports that studies have shown that the use of tires as fuel can reduce certain emissions. According to a 2008 PCA study of emission tests from 31 cement plants firing tdf, there were no statistically significant differences in the emission data sets for sulfur dioxide, nitrogen oxides, total hydrocarbons, carbon monoxide, and metals between kilns combusting tdf and non-tdf firing kilns.
Separate studies conducted by governmental agencies and engineering consulting firms have also indicated that tdf combustion either reduces or does not significantly affect emissions of various contaminants from cement kilns. In a 2007 study, the United States Department of Energy estimated that the combustion of tdf produces less carbon dioxide (CO2) per unit of energy than coal. This means that when tdf replaces coal in a Portland cement kiln, less CO2 will be produced.
Long term experience shows that tires are being used successfully in cement kilns and good quality cement products are being made while using scrap tire fuels. Higher production rates, lower fuel costs and improved environmental quality achieved when tire fuels are combusted in cement kilns continue to define scrap tires as a viable fuel choice for cement kilns.
Pulp and paper companies use tire-derived-fuel as a supplement to wood waste-the primary fuel used in pulp mill boilers. The technology is proven and has been in continuous use in the U.S. since the early 1980s. The heating value of the wood waste fuel ranges from about 7,900 to 9,000 Btu/lb. on a dry basis. Tdf’s higher heat value of 15,000 Btu/lb. facilitates uniform boiler combustion, and helps overcome some of the operating problems caused by fuels with low heat content, variable heat content and high moisture content. The consistent Btu value and low moisture content of tdf and its low cost in comparison to other supplemental fuels make tdf an especially attractive fuel in the pulp and paper industry. In addition, pulp and paper mills have the ability to burn tdf without major equipment modifications offering yet another advantage to the use of tdf.
Pulp and paper mills continue to increase their use of tdf to help decrease fuel costs and improve both emissions and combustion efficiency. In addition, the use of tdf in pulp and paper mill boilers helps the mills improve their public image in their local regions by demonstrating environmental responsibility. High energy costs, improved reliability in the tdf processing industry and the consistent product quality of tdf are primary reasons for ongoing growth in both the number of mills consuming tdf and in the amount of tdf consumed per mill.
Electric power utilities use tire-derived-fuel as a supplemental fuel to produce power in boiler operations. Boilers at electric power plants use fuel to generate power for municipalities and industries .In the electric power industry, tdf is used mainly as an additive to other fuels, primarily coal. For electric power utilities, tdf must be correctly sized to fit in fuel conveyors and must be well mixed to ensure proper combustion. Typically, the tdf must be sized at 1 inch x 1 inch and be almost completely dewired for use in the cyclone boilers commonly used in electric power plants. Some electric utilities use stoker-fired units which, because of the longer residence time, can accommodate 2 inch x 2-inch tdf. Smaller, wire-free tdf -50 mesh to 200 mesh can be used in electric power plants that burn pulverized coal.
In electric power utility applications tdf provides an economic fuel with constant Btu content and low moisture. Electric utilities also found that the quality of emissions actually improves with the increased use of tdf as a supplemental fuel. Because of its higher heating value, lower emissions, competitive cost, and ability to create stable operating conditions in the boiler, tdf remains an attractive fuel for the electric power generating industry.
For most industrial and institutional boiler systems, tdf sized 2 inch x 2 inch or less and 95 percent wire-free is an accepted fuel. In industrial boiler applications, combustion of tire derived fuel generates energy in the form of steam and/or electricity, displacing the need to generate energy from other power generating facilities and from other fuels, usually coal. This displacement not only offsets the use of certain fuels, it also offsets the pollution emitted from other fuels. Tdf combustion in industrial boilers can emit less sulfur dioxide and nitrogen oxide than most types of coal on a net energy output basis. Tdf use in industrial boilers remains steady but faces challenges for increased use due to plant closings and depressed markets and overall economic conditions.
One facility specifically designed to burn whole tires or tire-derived-fuel as its only fuel is fully operational in the U.S. This tires-to-energy plant, a 25-megawatt electric-generating facility, has combusted tires to produce electricity for nearly twenty years and currently consumes nearly 10 million tires annually. Historically, the tires-to-energy plant has achieved emission rates below those at electric generating plants with solid fuel combustors powered by traditional fuels.
The U.S. EPA created a Fact Sheet on tdf through the collaborative efforts of EPA’s Resource Conservation Challenge subcommittee on TDF. In 2005, the agency posted the TDF statement on the EPA website. The EPA statement is as follows:
EPA supports the highest and best practical use of scrap tires in accordance with the waste management hierarchy; in order of preference: reduce, reuse, recycle, waste-to-energy, and disposal in an appropriate facility. Disposal of scrap tires in tire piles is not an acceptable management practice because of the risks posed by tire fires, and because of the use of tire piles as a habitat by disease vectors such as mosquitoes. The use of scrap tires as tire derived fuel (TDF) is one of several viable alternatives to prevent newly generated scrap tires from inappropriate disposal in tire piles, and for reducing or eliminating existing tire stockpiles.
EPA testing has shown that TDF has a higher BTU value than coal. Based on over 15 years of experience with more than 80 individual facilities, EPA recognizes that the use of tire derived fuels is a viable alternative to the use of fossil fuels, and supports the responsible use of TDF in Portland cement kilns and other industrial facilities provided the candidate facilities have developed a TDF storage and handling plan, and have secured a permit for all applicable State and Federal environmental programs and are in compliance with all requirements of this permit.
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ASTM 6700-01 “Standard Practice for Use of Scrap Tire-Derived Fuel”
The American Society for Testing and Materials (ASTM) approved ASTM 6700-01, an International Standard for TDF, in 2006. ASTM Standard D-6700-01 “Standard Practice for Use of Scrap Tire-Derived Fuel” offers end-users and potential end-users an industry-accepted standard against which they can compare all tire chips.
- Provides definitive parameters for size, distribution, sampling and testing
- Provides a single sampling and testing protocol
- Provides guidance for the material recovery of scrap tires for their fuel value
- Explains tdf use when blended and combusted under normal operatingconditions with originally specified fuels
The American Society for Testing and Materials (ASTM) is a not-for-profit organization that provides a forum for the development and publication of voluntary consensus standards for materials, products, systems and services. The ASTM process is open to producers, users and consumers.
According to ASTM, a standard is a document that has been developed and established within the consensus of the Society and meets approval requirements of ASTM procedures and regulations. Generally, standards are incorporated into specs or plans.
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