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In the summary, or "a" section of the tables, the "Test Methods" entry may indicate <br /> "Unknown." While the details may be unavailable, all facilities with the reference "Clark, <br /> et al., (1991)," refer to the EPA report Burning Tires for Fuel and Tire Pyrolysis:Air <br /> Implications, and have had their methods procedures evaluated and accepted as creditable <br /> by EPA as a condition of being included in that report. <br /> It is extremely difficult to establish a universal emission factor, or even a range of <br /> emission factors as a function of TDF added, because of the limited amount of emissions <br /> data when compared to all the other variables influencing the emission rate of any <br /> pollutant, such as: <br /> • Baseline fuel type and variability, such as sulfur, nitrogen, ash, metals, <br /> chlorine, moisture content, etc. Furthermore, many sources were tested with <br /> multiple fuels (e.g., coal and wood), making it even more difficult to identify <br /> the impact of TDF. <br /> • Air pollution control device efficiency varies with the type of fuel. For <br /> example, the efficiency of a venturi scrubber typically falls when handling <br /> the smaller particulate common to TDF. Fabric filters and electrostatic <br /> precipitators (ESPs) are preferable for particulate control for TDF exhaust <br /> streams. <br /> • Combustor design. There are several boiler design types; suspension <br /> (fluidized bed and cyclone types) and grate firing (traveling, reciprocating, <br /> and chain stokers; stokers may be either spreader, underfeed, or overfeed). <br /> TDF combustion efficiency varies for each design type. For example, TDF is <br /> typically difficult to burn in suspension (e.g., in fluidized bed and cyclone- <br /> type boilers), because of its size and weight. However, this problem may be <br /> remedied with further research and development. To date, the spreader <br /> stoker is the most successful and widely used boiler configuration with TDF. <br /> However, with consistent and well-controlled processing of TDF (i.e., sizing <br /> and de-wiring), most well-maintained solid fuel combustors can successfully <br /> accommodate TDF as a supplemental fuel. <br /> • The amount and type of processing/sizing that is used to convert a scrap tire <br /> to TDF. Size of TDF (whole tires, chunk, shredded, or crumb rubber) and <br /> type (wire-included or de-wired) influences the rate and type of air <br /> emissions. <br /> 40 <br />