OCR Text |
Show inorganic compounds for all tests. Comparison of fly ash carbon and PCDD/PCDF emissions is presented in Figure 5. The data shows a positive correlation between flue gas concentrations of fly ash carbon and PCDD/PCDF. This result is not surprising, poor fly ash carbon burnout would provide larger concentrations of organics species that could be precursors for PCDD/PCDF formation. It points to carbonaceous compounds in the fly ash as PCDD/PCDF precursors. PCDD/PCDF emissions as a function of inorganic fly ash emissions is shown in Figure 6. This data does not show a relationship between inorganic fly ash emissions and PCDD/PCDF emissions. These results suggest that operation of incinerators under conditions that enhance burnout of carbon in the flue gas will reduce emissions of PCDD/PCDF. Foremost among these operating parameters are sufficient residence time at temperature for fly ash carbon burnout, high rates of fuel/flue gas mixing and sufficient excess air for maximum combustion rates. These studies have shown fly ash to be a site for PCDD/PCDF formation in RDF incineration systems. A low temperature downstream formation mechanism in hold up fly ash identified in lab studies has been verified at pilot scale. In-furnace formation was shown to occur in both fly ash fines and large carbonaceous fly ash. PCDD/PCDF emissions were found to correlate to fly ash carbon emissions, suggesting organic compounds in the fly ash are precursors for the dominant in-furnace PCDD/PCDF formation mechanism(s). Further studies focused on the impact of incinerator operating parameters on PCDD/PCDF emissions. Impact of RDF Load. A series of tests were run to determine the effect of RDF load on PCDD/PCDF emissions. Full load for the pilot scale tests was an RDF feed rate of 540 KBTU/HR. Reductions in RDF load of 15 and 20% were tested. The air flow required for an overall stoichiometric ratio (SR) of 1.6 (8% O2 dry) at a firing rate of 540 KBTU/HR was used for all tests. Since fly ash was identified as a site for PCDD/PCDF formation, emissions could be influenced by flue gas velocity and particulate entrainment conditions. Therefore the total air flow remained the same for all tests. This allowed the effect of RDF load on PCDD/PCDF emissions to be studied without the influence of variable particulate entrainment. The air flow resulted in overall SRs of 1.8 to 1.9 for reduced loads. Figure 7 presents the impact of RDF load on PCDD/PCDF emissions. The baseline for these tests is the 100% load data point labeled A. The data show a decrease in PCDD/PCDF emissions with a reduction in RDF feed rate. A 15% load reduction resulted in reductions of PCDD/PCDF emissions ranging from 5 to 35 % from full load operation. The reduction in emissions is likely due in part to the lessening of precursors available for PCDD/PCDF formation as the fuel load is decreased, However, incinerator operation at 20% full load produced a greater than three fold decrease in dioxin emissions. The notable drops in PCDD/PCDF emissions at lower RDF feed rates may be partially attributed to improved mixing of air and RDF, and increased oxygen availability from higher excess air levels resulting in increased combustion efficiency. It is possible that the design RDF feed rate was overloading the combustion system, causing poor air/fuel mixing and insufficient residence time at temperature to completely combust the RDF. In addition to the load reduction studies, a test was run to determine the effect of RDF load fluctuations on PCDD/PCDF emissions. The results of this study are shown by data labeled A and B. Test A was run with a steady RDF feed rate of 540 KBTU/HR while Test B was run with an unsteady RDF feed rate. The baseline feed rate was 460 5 |