Environmental Engineering Reference
In-Depth Information
oil are often higher than 3 kW/m
2
, and those burning
natural gas in stationary gas turbines can be twice as
high. Nuclear plants require no extensive fuel storage
and no air pollution controls, and their land claims
are dominated by exclusion and buffer zones. Their den-
sities of about 1-1.5 kW/m
2
are less than those of
hydrocarbon-fired plants but more than those of many
coal-fired stations.
Perhaps the most promising technique to solve the tri-
ple problem of particulates and sulfur and nitrogen oxide
emissions is fluidized bed combustion (FBC). First
patented for gas generators by Fritz Winkler in 1921
and used extensively by the Germans during WW II to
produce feedstock for gasoline synthesis, the technique
has been used commercially in power plants only since
the early 1980s (Valk 1995). A fluidized bed is a layer of
small noncombustible particles, usually limestone, kept
aloft by air forced through perforations in a base plate.
Pulverized fuel is introduced in quantities of less than
5% of the total load, and superior mixing rates make it
possible to burn the fuel at 760
C-930
C, well below
1370
C, the temperature at which NO
x
begin to form.
In addition, finely ground limestone (CaCO
3
)or
dolomite (CaMg(CO
3
)
2
) could be mixed with combus-
tion gases in order to remove as much as 95% (and com-
monly no less than 70%-90%) of all sulfur present in the
fuel by forming sulfates (Henzel et al. 1982; Lunt and
Cunic 2000). Atmospheric FBC is available in sizes up to
300 MW, able to burn coal competitively compared
to new natural gas-fired combined-cycle systems (Schim-
moller 2000). More than 600 AFBC boilers, with a total
capacity of 30 GW, operated in North America in the
year 2000, and a similar capacity was installed in Europe.
Pressurized FBC produces gas that can drive turbines;
about 1 GW of such capacity existed worldwide by the
calculated per square meter of tower base, but large
towers require considerable spacing in order to prevent
high wind loads induced by the Venturi effect. Wet me-
chanical draft units of the cross-flow type need only one-
half or one-third of the area for the same cooling load
and can easily handle 80-100 kW
e
/m
2
, or as little as
10 m
2
/MW
e
. Switchyards typically take up 50-75
kW
e
/m
2
, but overall densities are determined by the
plant's size, its fuel storage, air pollution control, and wa-
ter cooling facilities. Economies of scale are substantial.
On-site storages of plants supplied by gas pipelines or
fuel oil barges may be quite small, but coal piles required
for 60-90 days of operation are necessarily extensive.
With heights between 5 m and 12 m, they have storage
densities of 25-100 kW
e
/m
2
, similar to those of switch-
yards. Because of large volumes of hot gas, electrostatic
precipitators take up about as much space as boilers, and
FGD can be even more compact (up to 400 kW
e
/m
2
),
but the disposal of captured fly ash and sulfate sludge
requires much land. Combustion of steam coal with 22
GJ/t and 10% of ash will generate annually about 250
t/MW
e
. The ash is deposited in fills or ponds to a depth
of 5-10 m, requiring annually 20-40 m
2
/MW
e
, assum-
ing specific density of 1.3 g/cm
3
. During its lifetime of
35-50 years, a coal-fired power plant will thus need
700-2000 m
2
of ash disposal space per MW
e
. Lifetime
disposal of FGD sludge requires 200-600 m
2
/MW
e
.
Stations with once-through cooling, no fuel storage
(mine-mouth plants), no FGD (burning low-S coal),
and off-site ash disposal or commercial ash sales can have
power densities of 5-10 kW
e
/m
2
. Rates for large plants
with coal storage and cooling towers can be up to 4
kW
e
/m
2
, and with on-site disposal of ash and FGD
sludge they decline to mostly between 1-2.5 kW/m
2
.
In contrast, the power densities of plants burning fuel
