A Novel Energy-Efficient Process Utilizing Regenerative Burners for the Detoxification of Fly Ash

Abstract

During the last decade, the debate concerning incineration has focused mainly on potential risks from air emissions. Today, waste incineration will only gain full public acceptance if a high quality of ash residues can be guaranteed, particularly with respect to low levels of organic compounds such as polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/Fs) and the high elution stability of toxic heavy metals. Conventional thermal treatment techniques such as vitrification have been used in the past for detoxification of the ash residues from incinerators, but at a significantly high cost due to the massive energy consumption of the process. This paper addresses the problem of safe disposal of millions of tonnes of contaminated fly ash produced each year by developing an economically viable energy efficient process that can convert this toxic material into a non-toxic material. The decontaminated material can then be safely land-filled or used in construction applications such as in road foundations. This approach is based on the fact that sintering of the ash residue results in destruction of its toxic organic components, and also fixation of its heavy metal content. The novelty of the process includes the integration of the sintering process with the concept of regenerative heating. This is a very energy-efficient concept that results in typical fuel savings of 65% in industrial furnaces. A fully operational ash detoxifying pilot plant with a throughput suitable for continuous treatment of fly ash generated by a typical municipal solid waste incinerator plant was constructed and operated. An extensive series of analytical tests were carried out on both the raw and thermally treated ash samples. The results obtained showed an achievement of up to 96% reduction in the heavy metal leaching potential in the sintered ash. The sintering process de-volatilized metals having a high vapour pressure and effectively trapped other heavy metals in oxide sinters/melts. The reduction in leaching potential can be largely attributed to the formation of these stable oxides. This finding was verified by computational analysis that showed that up to 99.9% of some metals were converted into metal oxides during the sintering process. Analysis of the sintered ash samples/pellets for PCDD/Fs using standardized methods with GS-MS/MS equipment showed a major reduction in PCDD/Fs concentrations to below the detectable limit of 0.1 pg g − 1. These results are exceptionally encouraging since they eliminate the concern over the safety and suitability of the sintered ash material/pellets for re-use. Each cyclone was designed to provide enough residence time for the ash particles to be heated up to the sintering temperature of 850°C. High temperature conditionswere maintained throughout the cyclone thus helping to avoid any re-formation of toxic organic compounds such as PCDD/Fs. Detailed analysis showed that the use of heat regeneration in our sintering process provided significant energy and economic savings of up to 50%.