Page 106 - New Trends In Coal Conversion

Minimization of Hg and trace elements during coal combustion and gasification processes 69

According to Clarke and Sloss (1992) and Diaz-Somoano and Martínez-Tarazona
(2003), TEs show similar partitioning behavior in gasification processes. Just some
TEs which can form volatile chloride or hydrides may remain in gas phase under gasi-
fication conditions.
The most volatile TEs (Hg, Se, As), to which we have often paid more attention,
and halogens, etc., remain mostly in the vapor phase as they pass through heat transfer
sections of a boiler. The percentages of the total in-stack concentrations of these ele-
ments in the vapor phase have been reported (Ratafia-Brown, 1994) to be as follows:
Cl, up to 99% as HCl; F, up to 90% as HF; Br, up to 25%e98% as HBr; Hg, up to 98%
as Hg, HgO, and CH 3 Hg; Se, up to 59% as Se and SeO 2 ; As, up to 0.7%e52% as
As 2 O 3 ; and I, up to 90%e99% as HI. Although mercury (Hg) concentration in coal
is usually extremely low, significant attention is focused on its emission because of
its special concern. Many studies dealing with the behavior of most volatile elements
and their gaseous emission exist in the literature (Pacyna, 1987; Germani and Zoller,
1988; Hall et al., 1991; Galbreath and Zygarlicke, 1996; Laudal et al., 1997; Senior
et al. 2000a,b; Seames and Wendt, 2000).
Some authors have used enrichment factors (EFs) to assess concentration differ-
ences between feed coal and residue outputs. The EF typically normalizes the concen-
tration of a TE in the coal and fly ash to the concentration of a nonvolatile element.
Meij et al. (1990) has used the term “relative enrichment” (RE) to describe the enrich-
ment factor normalized to the ash content in the coal. They used calculated RE factors
to classify the elements into the following three partitioning classes:
Class I elements have RE factors 0.7e1.3. They are defined as those elements with insignif-
icant enrichment or depletion in the fly ash (corresponding to Group 1).
Class II elements condense within the installation. These elements have RE factors for bot-
tom ash <0.7, about 1 for fly ash collected in the electrostatic precipitator (ESP) and large RE
factors >1.3 for the smallest particles, including those in the flue gases downstream of the
ESP (corresponding to Group 2).
Class III elements have RE values of <<1 in the bottom ash and fly ash collected in the ESP.
They are volatilized and not enriched in the residue streams (corresponding to Group 3).
A similar study has been carried out by Danihelka et al. (2003). Emission of the TEs
As, Se, Cd, Co, Cr. Cu, Zn, Hg, Tl, Pb, Ni, Sn, Sb, V, Mn, and Fe from six pulverized
coal combustion plants in the Czech Republic, ranging in size from 49 to 258 MW,
was investigated, and the distribution of the elements in the combustion products
was obtained using the RE factor (Fig. 3.5). Based on the results, the authors
concluded that it is typical that approximately 20% of Fe and Mn, which belong to
Group 1, are retained in bottom ash. Up to 80% of Cd, Sn, Zn, Pb, Sb, and Cu (Group
2) are retained in the fly ash, and about 5%e10% of their amount is retained in the
bottom ash. As for the TEs from Group 3, Hg, Se, and As, a relatively large amount
of these elements is emitted in the air with the flue gas compared with the other
elements. About 15% of Se and 2% of As is retained in the solid particles, and only
5% of the same elements is retained in the bottom ash. Up to 85% of Hg is either
emitted into the atmosphere or captured in solid fly ash particles, and a negligible
amount is retained in the bottom ash (Nalbandian, 2012).