Standard Guide for Use of Scrap Tires as Tire-Derived Fuel

Importancia y uso:

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 to 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 are 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:

4.3.1 To increase boiler efficiency in a co-fired boiler using wood, sludge, and coal;

4.3.2 To procure a competitively priced fuel;

4.3.3 To supplement limited supplies of an existing fuel;

4.3.4 To use a high-quality fuel;

4.3.5 To achieve environmental benefits by using a fuel with a relatively low sulfur content in comparison to certain coals or petroleum coke, and;

4.3.6 To provide a public and social benefit that solves a regional solid waste problem.

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 ensure 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:

4.4.1 A combustion unit’s existing ability to use the fuel without modification (other than minor operational changes in oxygen grate speed adjustments, and feed/material handling) and,

4.4.2 The ability of a supplier to economically collect, process, and transport TDF to the combustion unit.

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)

Subcomité:

D34.03

Referida por:

D5681-23

Volúmen:

11.04

Número ICS:

83.160.01 (Tyres in general)

Palabras clave:

ash; Btu content; chip size; combustion; conveying; minus; moisture; passenger tire equivalent (PTE); quality control; sulfur; tire-derived fuel (TDF); wire; zinc;