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60 New Trends in Coal Conversion

extent in coal-fired power stations. These species are still relatively poorly understood,
and this is reflected in the confusion over the best approach to control emissions.
Proposed methods to control emissions of specific TEs, notably, mercury, are being
investigated.
This chapter describes TE content and their mode of occurrence in coal, their
behavior during coal conversion processes with special emphasis on mercury, and dis-
cusses the most recent legislation and actions to be taken to minimize their emissions
during coal combustion and gasification processes. As a result, trends in TE emissions
are discussed. This chapter also goes briefly through the technologies, the cobenefit
approaches, and specific technologies currently applied for mercury and other TE con-
trol with emphasis on the methodologies most appropriate for compliance.

3.2 Mercury and trace elements in coal

Coal holds and will keep in the future an important position as a world energy source
because of its relatively abundant reserves in comparison with the decreasing reserves
of both petroleum and natural gas. Moreover, the pollutant emissions from coal utili-
zation may cause serious environmental and health risks, so it becomes an important
issue with respect to environment protection. It is known that coal-fired power plants
release to the environment SO 2 and NO x , as well as CO 2 and N 2 O and particulate
matter. Moreover, mercury (Hg) and other TE pollutants such as cadmium (Cd),
arsenic (As), molybdenum (Mo), or vanadium (V) are released during coal conversion
processes.
Coal may contain as many as 76 of the 92 naturally occurring elements of the
periodic table in different concentrations and association forms. However, most of
those elements usually are present in trace amounts in the order of parts per million
(ppm) (Schweinfurth, 2002). According to their different contents, the elements iden-
tified in coals can be classified into three groups (Fig. 3.1) as follows:
Major elements, which occur at concentrations higher than 1000 ppm. These elements are
carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S),
Minor elements, which include coal mineral matter (Si, Al, Ca, Mg, K, Na, Fe, Mn, Ti) and
halogens (F, Cl, Br, I). These may be present in concentrations ranging from 100 to
1000 ppm, and
Trace elements, which are the constituents with concentrations <100 ppm. Originally, heavy
metal was another common term for these elements in low concentration and having impact
on the ecosystem and human health because at the beginning the concerned elements were all
heavy metals. By now, some light elements have also been found to have great impacts on the
environment (such as B, Be, As). It should be noted that the term TE represents all considered
low-content elements in coal.
During the last years, the topic about TEs in coal and their emissions is drawing
more and more interest from scientists due to the great concern for their toxicological
and environmental effects. These TEs even when present in parts per million levels in
coal can result in emission of several tons of these pollutants in the environment