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Minimization of Hg and trace elements during coal combustion and gasification processes 61

H He

Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Rb Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Figure 3.1 Periodic table of the elements. The elements found in coal are highlighted by colors
according to their general abundance in coal: blue, major elements; orange, minor elements; and
yellow, trace elements.

because of the huge amount of coal burned. At the end of the 1990s, the US Environ-
mental Protection Agency carried out a study on hazardous air pollutant emissions
from electric utility steam generating units. The results from this study led to establish
a classification of TEs by level of concern based on known adverse health effects or
because of their abundances in coal (CTB, 2011; Nalbandian, 2012) as follows:
Major concern: arsenic (As), boron (B), cadmium (Cd), lead (Pb), mercury (Hg), molybde-
num (Mo), and selenium (Se). Selenium is an essential element but is also toxic above certain
levels. High levels of molybdenum and boron in plants are of concern. Arsenic, cadmium,
and lead are highly toxic to most biological systems at concentrations above critical levels.
Mercury is worldwide recognized as an element of special concern due to its toxicity, high
volatility, and persistence in the environment and bioaccumulation.
Moderate concern: chromium (Cr), vanadium (V), copper (Cu), zinc (Zn), nickel (Ni), and
fluorine (F). These elements are potentially toxic and are present in coal combustion residues
at elevated levels.
Minor concern: barium (Ba), strontium (Sr), sodium (Na), manganese (Mn), cobalt (Co),
antimony (Sb), lithium (Li), chlorine (Cl), bromine (Br), and germanium (Ge). These ele-
ments are of little environmental concern. They were classified mainly based on their pres-
ence in residues.
Elements of concern but with negligible concentrations: beryllium (Br), thallium (Tl), silver
(Ag), tellurium (Te). These elements have known documented relationships to health, but the
low levels present in coals are considered to have negligible impact.
Radioactive elements: uranium (U) and thorium (Th).
The distribution of these TEs differs significantly between coals from different
sources and even between coals from the same seams (Swaine, 1990; Swaine and
Goodarzi, 1995; Vassilev, 1994; Querol et al., 1995). The lateral and vertical