Tidal Current Energy

Geothermal Energy
Table 12.4 . Geothermal and fossil fuel CO 2 emissions in pounds1 CO 2 per
kilowatt hour.
Geothermal Coal Petroleum Natural gas
0.18 2.13 1.56 1.03
( Sources : Data from Refs [14] and [15] )
221
megawatt-hour of geothermal electricity produced in the USA, the average
emission of CO 2 is about 18% of that emitted when natural gas is burned to produce
electricity. A comparison of fossil and geothermal emissions is shown in
Table 12.4 . Binary plants have no emissions, since all of the produced fluid is
injected back into the reservoir.
Hydrogen sulfide can reach moderate concentrations of up to 2% by weight in
the separated steam phase from some geothermal fields. This gas presents a pollution
problem because it is easily detected by humans at concentrations of less
than 1 part per 10 6 in air. H 2 S concentrations are only high enough to require control
at The Geysers, California, Coso, California and Steamboat Springs, Nevada.
Either the Stretford process or an incineration and injection process is used in
geothermal power plants to keep H 2 S emissions below 1 ppb (part per 10 9 ).
Use of the Stretford process in many of the power plants at The Geysers results
in the production and disposal of about 13 600 kg of sulfur per megawatt of electrical
generation per year.
The incineration process burns the gas removed from the steam to convert
H 2 S to SO 2 , the gases are absorbed in water to form SO 3 2 � and SO 4 2 � in solution,
and iron chelate is used to form S 2 O 3 2
�
[16] . The major product from the incin-
eration process is a soluble thiosulfate, which is injected into the reservoir with
the condensed water used for the reservoir pressure-maintenance program.
Sulfur emissions for each megawatt-hour of electricity produced in 1991, as SO 2
by plant type in the USA, was 9.23 kg from coal, 4.95 kg from petroleum and
0.03 kg from geothermal flashed steam [17] . Because the high pressures of combustion
are avoided, geothermal power plants have none of the nitrogen oxide
emissions common from fossil-fuel plants. For each megawatt-hour of electricity
produced in 1991, the average emission of nitrogen oxides by plant type in
the USA was 3.66 kg from coal, 1.75 kg from petroleum, 1.93 kg from natural gas
and zero from geothermal [17] .
9 . The Future
More than 50 years of geothermal exploration has shown that high-quality
hydrothermal systems are generally limited to areas of active tectonism, in particular
the volcanic belts associated with subduction zones. Even when adequate
temperatures are found in the subsurface, the productivity of wells is quite
often insufficient for economic production. As mentioned earlier in this chapter,
1 1 pound � 0.454 kg.