Page 240 - Geothermal Energy Renewable Energy and The Environment

Use of Geothermal Resources: Environmental Considerations 229

FeS + H O <=> FeO + H S
2
2
Pyrrhotite Fe-bearing
mineral

demonstrates one of many possible reaction paths that can influence H S abundance. The multiplic-
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ity of paths results from the fact that sulfur has several oxidation states that can form in natural
settings, each of which is a function of the oxidation state of the system. Since the local oxidation
state can vary by several orders of magnitude over distances of a few meters, and the abundance of
Fe-bearing minerals can also vary significantly, it is not surprising that there is great variability in
H S content in fluids from wells in the same geothermal field.
2
Abatement of H S takes advantage of a variety of possible reactions that result in its oxidation.
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The most commonly used pathways involve various means to produce elemental sulfur or SO . The
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overall reaction for reduction to elemental sulfur is
2 H S + O (air) <=> 2 S° + 2 H O.
2
2
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A similar oxidation reaction can be written for producing SO . Either path results in a more
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environmentally acceptable output from the geothermal plant. The reaction path that is used will
depend upon the form in which the H S exists (e.g., within the liquid condensate stream, as part of
2
a separated noncondensable gas phase, etc.). The primary challenge in engineering the abatement
scheme is to employ reactors at appropriate locations in the facility such that the hydrogen sulfide
is oxidized as economically as possible. The concentration of H S and its form, whether gaseous
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or liquid, will dictate the approach that needs to be used. The operating cost for various schemes
for removing H S range from about 20 cents/kg of H S to more than $30/kg of H S (Nagl 2008).
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2
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Removal of more than 90% of the H S can be readily accomplished.
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mercury
Emissions of gaseous Hg from geothermal power plants are usually well below regulatory standards.
However, instances do exist in which geothermal resources occur in close proximity to geological
environments where Hg occurs at elevated levels. Such sites are usually associated with geological
environments in which cinnabar (an ore for mercury) or other mineral deposits occur. Although
mercury is a metal, it boils at a low temperature and is thus preferentially partitioned into the
gas phase, if it is detectably present. Sites where this has been observed include The Geysers and
Piancastagnaio, Italy (Baldacci and Sabatelli 1998).
Mitigation of mercury emissions is usually accomplished using either a condensation and cooling
method that allows mercury separation, or sorption of mercury on to a mineral substrate, such as
a carbon material or a zeolite that is impregnated with sulfur. Such systems are easily employed
in conjunction with an H S mitigation scheme, thus minimizing engineering and design costs.
2
Recovery of mercury using such systems is usually well beyond 90%.

solUTe load and resoUrce recoVery

As discussed in Chapter 5, geothermal fluids can range in composition from dilute solutions to con-
centrated brines (Table 5.1). Reinjection is usually the preferred means for disposal of geothermal
fluids after they have passed through a generating facility or other application. However, in instances
where economic or other considerations do not support reinjection, strategies must be developed
for handling the spent geothermal water. In such cases it is necessary to establish whether the
water is sufficiently dilute to meet regulatory standards for surface disposal without treatment.
Such standards usually consider, among other things, the total dissolved load, the alkalinity, and