OCR Text |
Show burn units. PCDD/PCDF have been detected in fly ash from refuse incinerators'. Emissions of trace organic materials can be controlled by collecting any materials formed with add-on air pollution control devices (APCDs). However, the material collected become a disposal problem. In addition, APCDs, such as spray dryers, are not easily retrofitted on existing incinerators and they are expensive to operate. A better alternative to add-on pollution equipment is to design and operate combustion systems to minimize organic emissions by destroying them in the furnace. It was postulated that installation of a natural gas afterburner would result in high destruction of the trace chlorinated organics and reduced emissions. The purpose of this test program was to gather information on the formation and emissions of PCDD/PCDF during RDF combustion and to develop combustion systems for emissions control. EXPERIMENTAL Testing was conducted on a pilot scale RDF incinerator shown in Figure 1. The refractory lined facility has an 18 inch 10. It consists of a horizontal barrel followed by an 18 foot, vertical controlled temperature tower (CTT), cooling sections, and a baghouse. At the base of the CTT sits an aerated grate. Underfire air supplied to a manifold chamber below the grate penetrates the multi-orificed plate. The CTT contains two backfired chambers that supply heat through the refractory wall to the duct. The backfired sections are completely isolated from the main duct and are exhausted through side stacks. The tower is equipped with ports for flue gas sampling and injection of natural gas and/or air. The convective cooling coils simulate heat removal in the convective passages, economizers and air heaters of full scale units. Backfired sections and cooling coils allow simulation of time, temperature, and velocity profiles in full scale systems. To ensure consistent fuel properties a surrogate RDF was used. The surrogate RDF composition was based upon characteristics of fuel burned in real RDF systems2 • It had a nominal HHV of 4600 BTU!1b. Main components were shredded cardboard and water. PVC was included as a chlorine source and copper oxide as a source for copper, a suspected catalyst for PCDD/PCDF formation3 • 4 • A schematic of fuel and air flows is presented in Figure 2. The RDF was blown into the furnace with air about 5 feet above the aerated grate. Overfire air was injected into the furnace parallel to the RDF feeder. Injection of the overfire air and RDF transport air produced a turbulent combustion zone. Lighter RDF components were entrained and burned as they passed through the CTT. Heavier pieces fell to the grate. Preheated vitiated air was supplied from a natural gasfired burner in the horizontal barrel to allow adjustment of the furnace residence timetemperature profile. Combustion conditions at the grate were fuel rich with a theoretical stoichiometric ratio (SR) of 0.6 calculated assuming all RDF fell uncombusted to the grate. Injection of the overfire air produced overall fuel lean conditions with a SR of 1.6 or greater. Natural gas could be injected at a number of locations at or downstream of the RDF feed point. Flue gas composition was continuously monitored by instrumentation. Flue gas sampling ports were located downstream of the convective cooling sections, upstream of the baghouse. Flue gas temperatures at this point ranged from 660F to 800F. Sampling for PCDD/PCDF was performed using a Modified Method 5 (MM5) semi- 2 |