TOXIC METAL EMISSIONS FROM INCINERATION - MECHANISMS AND CONTROL

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 occ ur and how they are addressed through current and proposed regulations . Although emphasis in this paper is on problems associated with metal s from incineration processes, conventional fossil fuel combustion is also considered, insofar as it pertains to mechanisms governing the fa te of metals during combustion in general. This paper examines the rel ease of metals into the vapor phase, with the particle dynamics of a n ucleating, condensing, and coagulating aerosol that may be subsequentl y for-med, and with the reactive scavenging of metals by sorbents. Met als can be introduced into combustion chambers in many physical and ch emical forms. The subsequent transformations and vaporization of any v olatile metal depend on the combustion environment, the presence of ch lorine and other species (reducing or oxidizing), on the nature of the reactive metallic species formed within the furnace, and on the prese nce of other inorganic species such as alumino-silicates. Some insight into how these factors influence metal release can be gained by consi dering the release of organic sodium during coal char combustion. Once vaporized, a metal vapor cloud will normally pass through its dewpoin t to form tiny nuclei, or condense around existing particles. These ae rosols are then affected by other dynamic processes (including coagula tion) as they evolve with time. This paper shows how current mathemati cal descriptions of aerosol dynamics are very useful in predicting met al aerosol size distributions in combustion systems. These models are applied to two prototype problems, namely: the prediction of the tempo ral evolution of a particle size distribution of a self-coagulating ae rosol initially composed of nuclei; and the scavenging of nuclei by co agulation 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, si nce it allows the high temperatures in incineration processes to be ex ploited to allow the formation of water-unleachable metal-containing c ompounds that can be isolated from the environment. Future research pr oblems are also identified.