Environmental Engineering Reference
In-Depth Information
of the Western world. Incombustibles in coal produce
flue gas-borne fly ash (powdery, mostly spherical, spe-
cific gravity 2.1-3.0, emission rates 2.5-5.0 kg/GJ,
dominant in dry-bottom boilers); bottom ash (about
half of the incombustibles in wet-bottom boilers); and
boiler slag (dominant in cyclone furnaces). In 2000 the
U.S. annual production of these solid wastes reached, re-
spectively, about 57 Mt, 14.4 Mt, and 2.4 Mt (Kalyoncu
2000). Worldwide production of fly ash was about 600
Mt in 2000 (Malhotra 1999), and Europe and Asia gen-
erated about 9 Mt of FGD waste.
SiO 2 ,Al 2 O 3 ,Fe 2 O 3 , and oxides of alkaline elements
(CaO, MgO, K 2 O) are the principal constituents of fly
ash. Trace amounts of Pb, Zn, Cr, Mn, and Ni are also
present. Some coals have relatively high levels of Hg.
U.S. coal-fired stations are the country's largest source
of Hg emissions (about 40%), whose main environmental
consequence is the accumulation of highly toxic methyl-
mercury in fish and shellfish; 55% of fish samples col-
lected between 1999 and 2001 had an Hg level
exceeding EPA's safe limit for women (USPIRG 2004).
Most coals also contain U (about 1 ppm) and Th (about
3 ppm), and their decay isotopes (Ra, Po, Bi, Pb). As a
result, populations living near coal-fired plants receive
higher radiation doses than those in the vicinity of
nuclear stations (McBride et al. 1977). A typical U.S.
1-GW coal-fired plant releases annually about 5 t U and
more than 10 t Th. Widespread use of fabric bag filters
and electrostatic precipitators has eliminated visible PM
in affluent countries. Precipitators capture in excess of
99.5% of PM, and their use reduced global fly ash emis-
sions to about 30 Mt/a by 2000. Captured ash is used in
concrete mixes and structural fills, as feed for cement
clinker, and in pavings.
Until the late 1960s there were no commercial con-
trols for SO 2 emissions from large-scale coal combustion.
The United States was the first country to adopt FGD on
a large scale. By 2005 about 102 GW ei were so
equipped, which is about 32% of the country's coal-fired
generating capacity (EIA 2006). Germany was second,
with FGD at about 50 GW ei (Rubin et al. 2003). FGD
processes remove flue gas sulfur (with efficiencies up to
90%) by reactions of SO 2 with basic compounds (CaO,
MgO, CaCO 3 ) that produce a mixture of CaSO 4 (or
MgSO 4 ), CaSO 3 , fly ash, and unreacted lime or lime-
stone. The conversion of all sulfite to sulfate and the re-
duction of impurities can produce gypsum suitable for
wallboard manufacture. Nearly 15% of U.S. FGD waste
was reused in this way in the early 2000s, but some 80%
was simply landfilled, converting an air pollution problem
into a land and water degradation challenge (Kalyoncu
2000).
The first significant efforts to control NO x emissions
from large stationary sources began only during the
1980s. Germany has been the leader, with controls on
some 30 GW ei , followed by Japan, but the global aggre-
gate for the dominant technique, selective catalytic
reduction (SCR), was just 80 GW ei by the year 2000
(Rubin et al. 2003). SCR is a very expensive technique
that removes 60%-90% NO x by using Ti, V, and W
oxides as catalyzers to reduce the gases by NH 3 . Other
control options reduce NO x emissions less expensively
but less effectively by various combustion modifications
(low-excess-air firing, staged combustion, flue gas
recirculation, use of low-NO x burners). In contrast,
reductions of vehicular NO x have been much more wide-
spread and quite effective. Three-way catalytic con-
verters, mandatory on all passenger cars, have reduced
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