OCR Text |
Show studies and lead to the conclusion that the majority of the emissions reduction from the full RDF load baseline is a result of reduced RDF load, not destruction of trace organics in the afterburner zone. The second series of tests examined the effect of natural gas injection location on PCDD/PCDF emissions. These tests were run with 15% natural gas co-firing. Three injection locations were used. The first port was parallel to the RDF feeder, 0.0 seconds flue gas residence time downstream, at the suspended RDF turbulent combustion zone in the CTT. Quickly burning natural gas was injected there to determine if higher temperatures and an influx of combustion radicals to the well-mixed zone would improve RDF combustion and reduce PCDD/PCDF emissions. Injection of the natural gas 0.7 seconds downstream of the feed point examined the effect of an afterburner on PCDD/PCDF emissions. Injection at the third port, 0.3 seconds downstream of the feeder, looked at the effect of afterburner location. The flue gas flow closer to the feeder was less laminar than at the 0.7 seconds injection point. It was speculated that a more turbulent afterburner could improve performance. The results of these tests are shown in Figure 10. The baseline data is from tests run at 85% full RDF load without natural gas co-firing. Results of these tests show slight reduction of PCDD/PCDF emissions from the baseline data with natural gas co-firing at all three locations. Injection location does not appear to affect emissions. Natural gas co-firing was expected to greatly reduce emissions of trace organics from this RDF fired system by creating a high temperature afterburner zone with combustion radicals to produce high rates of destruction of PCDD/PCDF and their precursors. However this phenomena has not been seen as only slight reductions in emissions were observed with natural gas co-firing. A natural gas afterburner is known to be very efficient for the destruction of gas phase species. Combustion radicals in a high temperature zone readily react with gas phase species increasing their combustion rate. RDF is a solid fuel and at sufficient temperature the combustion rate of the non-volatile matter in the particles is limited by the rate of oxygen diffusion into the particle rather than combustion reaction kinetics. PCDD/PCDF and precursors which are volatilized during RDF combustion are likely destroyed in an afterburner zone. However, the destruction rate of PCDD/PCDF and precursors bound to the RDF particle matrix in a form non-volatile during combustion would be the oxygen diffusion limited solids combustion rate. The minimal impact of natural gas co-firing on PCDD/PCDF emissions suggests the majority of PCDD/PCDF and precursors are solid phase species in the furnace. The presence of gas phase species in the sampling trains (Figure 4) suggests some of the PCDD/PCDF will volatilize given sufficient time to diffuse from the fly ash matrix. CONCLUSIONS The results of this test program suggest that PCDD/PCDF formation in the furnace is dominated by mechanism(s) involving solid phase PCDD/PCDF and precursors in the incinerator fly ash. Based on this data an RDF incinerator operating strategy for PCDD/PCDF emissions control has been proposed: 1. Eliminate fly ash hold up in the incinerator system at temperatures from 460F to 660F. The downstream formation mechanism occurring in this temperature range can increase the concentration of PCDD/PCDF in fly ash over an order of magnitude and subsequent volatilization of these species can be the dominant source of emissions. Most important to this strategy is the operation of APCDs, such as baghouses and ESPs, 7 |