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TOXIC METAL EMISSIONS FROM INCINERATION: MECHANISMS AND CONTROL
Linak*, W P. AND J. L. Wendt. TOXIC METAL EMISSIONS FROM INCINERATION: MECHANISMS AND CONTROL. PROGRESS IN ENERGY AND COMBUSTION SCIENCE 19(2):145-185, (1993).
Toxic metals appear in the effluents of many combustion processes, and their release into the environment has come under regulatory scrutiny. This paper reviews the nature of the problems associated with toxic metals in combustion processes, and describes where these problems occur and how they are addressed through current and proposed regulations. Although emphasis in this paper is on problems associated with metals from incineration processes, conventional fossil fuel combustion is also considered, insofar as it pertains to mechanisms governing the fate of metals during combustion in general. This paper examines the release of metals into the vapor phase, with the particle dynamics of a nucleating, condensing, and coagulating aerosol that may be subsequently formed, and with the reactive scavenging of metals by sorbents. Metals can be introduced into combustion chambers in many physical and chemical forms. The subsequent transformations and vaporization of any volatile metal depend on the combustion environment, the presence of chlorine and other species (reducing or oxidizing), on the nature of the reactive metallic species formed within the furnace, and on the presence of other inorganic species such as alumino-silicates. Some insight into how these factors influence metal release can be gained by considering the release of organic sodium during coal char combustion. Once vaporized, a metal vapor cloud will normally pass through its dewpoint to form tiny nuclei, or condense around existing particles. These aerosols are then affected by other dynamic processes (including coagulation) as they evolve with time. This paper shows how current mathematical descriptions of aerosol dynamics are very useful in predicting metal aerosol size distributions in combustion systems. These models are applied to two prototype problems, namely: the prediction of the temporal evolution of a particle size distribution of a self-coagulating aerosol initially composed of nuclei; and the scavenging of nuclei by coagulation with larger sorbent particles. A metal vapor can also react with certain aluminosilicate sorbents. This process, which will occur at temperatures above the dewpoint, is described, and is important, since it allows the high temperatures in incineration processes to be exploited to allow the formation of water-unleachable metal-containing compounds that can be isolated from the environment. Future research problems are also identified.
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