4.1 When considering the specification of fuels for a boiler, issues to evaluate are the fuel's combustion characteristics, handling and feeding logistics, environmental concerns, and ash residue considerations. A thorough understanding of these issues is required to engineer the combustion unit for power and steam generation; however, TDF has demonstrated compatible characteristics allowing it to serve as a supplemental fuel in existing combustion units based on cumulative experience in many facilities originally designed for traditional fossil fuels, or wood wastes, or both. When used as a supplemental energy resource in existing units, TDF usage is generally limited to blend ratios in the 10-30 % range based on energy input. This limit is due to its high heat release rate and low moisture content, which differ significantly from other solid fuels, such as wood, refuse derived fuel, coal and petroleum coke.

4.2 New combustion units dedicated to the use of TDF (or whole tires) as the sole fuel source are rare. The generation and availability of scrap tires is ultimately determined by market conditions for new tires and the depletion rate of scrap tire inventories (stockpiles). Scrap tires account for approximately 1 % of the municipal solid waste stream. Based on a national scrap tire generation rate, there are roughly 2.5 to 3 million tons (annually available for all uses to include fuel, crumb rubber, engineering projects, and so forth). Some dedicated combustion units have been built, however, competition for the scrap tires as other existing sources begin to use TDF will determine the ultimate viability of these facilities. Although most regions can supply TDF demand as a supplemental fuel, a dedicated boiler in the range of 500,000 lb/h (227,000 kg/h) steaming capacity would require over 66 000 scrap tires/day to meet its fuel demand. Such demand may strain a region's ability to supply and put the fuel supply at risk. Some design projects have incorporated TDF as a supplemental fuel with wood, coal, coke, sludge, or some combination of multiple fuels where demand is consistent with supply availability.

4.3 It is important to understand what objectives may lead to TDF's choice as a supplemental fuel in existing power units. Several model objectives may be as follows:

4.4 Boilers generally are engineered around fuels that will be available through the amortized life of the power unit. Boiler design discussions here are limited as TDF standard size specifications have been developed to assure TDF's performance in existing systems. TDF is mined from the solid waste stream as a whole tire, then engineered via processing techniques to fit a new or existing combustion unit. A major modification or re-engineering of the combustion unit to accommodate TDF normally would make its use uneconomical as a supplemental fuel. TDF's use is economically dependent on the following two issues.

4.5 Once an economic decision has been made to develop TDF as a fuel source for a particular unit, issues of fuel specifications including size, proximate and ultimate analysis, combustion characteristics and environmental concerns must be evaluated properly to determine whether TDF is an appropriate supplemental fuel resource without major system modification.

FIG. 1 Relative Energy Comparison of Fuels (Scale in Btu/ton)

1.1 This practice covers and provides guidance for the material recovery of scrap tires for their fuel value. The conversion of a whole scrap tire into a chipped formed for use as a fuel produces a product called tire-derived fuel (TDF). This recovery practice has moved from a pioneering concept in the early 1980s to a proven and continuous use in the United States with industrial and utility applications.

1.2 Combustion units engineered to use solid fuels, such as coal or wood or both, are fairly numerous throughout the U.S. Many of these units are now using TDF even though they were not specifically designed to burn TDF. It is clear that TDF has combustion characteristics similar to other carbon-based solid fuels. Similarities led to pragmatic testing in existing combustion units. Successful testing led to subsequent acceptance of TDF as a supplemental fuel when blended with conventional fuels in existing combustion devices. Changes required to modify appropriate existing combustion units to accommodate TDF range from none to relatively minor. The issues of proper applications and specifications are critical to successful utilization of this alternative energy resource.

1.3 This practice explains TDF's use when blended and combusted under normal operating conditions with originally specified fuels. Whole tire combustion for energy recovery is not discussed herein since whole tire usage does not require tire processing to a defined fuel specification.

1.4 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.