Wp. Linak et Jol. Wendt, TOXIC METAL EMISSIONS FROM INCINERATION - MECHANISMS AND CONTROL, Progress in energy and combustion science, 19(2), 1993, pp. 145-185
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.