Control of Emissions from Cofiring of Coal and RDF: Final Report

Research has been conducted toward developing technology for co-firing of coal with municipal solid waste (MSW) in order to reduce emissions of chlorinated organic compounds, particularly polychlorinated dibenzo-p-dioxins and furans (PCDDs and PCDFs). Previous benchand pilot-scale research has shown that presence of SO2 can inhibit the PCDD and PCDF formation, and suggested cofiring high-sulfur coal with refuse derived fuel (RDF) to reduce the emissions. The objective of this research is to identify the effect of process and co-firing options in reducing PCDD and PCDF yield from waste combustion, Two types of municipal waste based fuels were used: a "fluff" refuse-derived fuel (simply referred to as RDF) and a densified refuse derived fuel (dRDF). The coal used was bigh-sulfur Illinois No. 6 coal. Experiments were conducted in US EPA's recently constructed Multi-Fuel Combustor (MFC), a state-of-the-art facility with fuel handling and combustion release rates representative of large field units. The MFC was fired, at varying rates, with RDF/dRDF and coal, and sampled for PCDD and PCDF. Tests were conducted over a range of process variables such as lime injection, HCI concentration, flue gas temperature, quench, and residence time so that the results are applicable to a wide variety of waste combustors. The data are used for developing a comprehensive statistical model for PCDD and PCDF formation and control. Phase 1 tests involved the RDF combustion, resulting in significant PCDD and PCDF yields. Results show that improving the combustion quality can lower PCDD and PCDF formation. Coal co-firing with the RDF caused substantial reduction in yields. Also, rfo PCDD or PCDF could be detected from the combustion of the Illinois No. 6 coal alone. Phase I I tests utilized the dRDF. A 21-run statistical test matrix was derived and all the tests were completed. The combustion quality was better than that of the "fluff" RDF, and PCDD and PCDF yields from dRDF combustion, though significant, were lower when compared to the RDF. Majority of the formation took place in the temperature range 600 to 300°C, within about 0.5 sec. Coal co-firing reduced PCDD and PCDF formation substantially. Model results also show that coal addition is effective at both high and low HCI levels in the combustor. Therefore, it appears that co-firing coal with waste-derived fuel is a promising technology for PCDD and PCDF prevention. Besides coal co-firing, high-temperature lime injection also appears to be effective in reducing PCDD and PCDF formation.