Page 236 - Geothermal Energy Renewable Energy and The Environment

12 Use of Geothermal Resources:

Environmental Considerations

In previous chapters we have discussed many of the environmental benefits and challenges associ-
ated with using geothermal energy. In this chapter we will focus on several issues that are of par-
ticular interest since they are of general concern to the public and regulatory agencies. The intent of
this discussion is to provide a knowledge base from which further discussions can evolve. The areas
of concern that we will consider are emissions to the atmosphere of unwanted gases, introduction
to the environment of unwanted chemicals and compounds, and the potential for resource recovery,
seismicity, ground subsidence, water use, and land use.

emIssIons
As detailed in Chapter 5, geothermal systems are complex geochemical environments in which a
range of chemical processes occur. Conceptually, these processes represent interactions in which
the atmosphere, the geological framework, and subsurface fluids evolve toward their lowest Gibbs
energy state. These processes can be schematically represented as an interacting three-part system

atmosphere <=> minerals <=> water. (12.1)

The <=> symbol indicates interactions that reflect exchange and chemical reactions between
parts of the system. These chemical changes can influence the physical attributes of the system by
changing the rock porosity, fracture apertures, and temperature, among other things. Any perturba-
tion in one part of the system will be reflected in some measurable adjustment in other parts of the
system. If, for example, the temperature of the water increases, minerals with which the water is in
contact will dissolve or precipitate, reflecting the temperature-dependence of mineral solubilities.
Likewise, the temperature-dependence of dissolved gas concentrations will change in response to
temperature perturbations, and this will, in turn, result in gases being liberated to or absorbed from
the coexisting atmosphere. For considerations regarding gaseous emissions from geothermal power
plants, the behavior of three types of gaseous emissions are particularly important: greenhouse gas
emissions (especially CO ), hydrogen sulfide (H S), and toxic metals such as mercury (Hg). We will
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discuss each of these groups in detail below. In the discussion, attention will be focused primarily
on flash geothermal generating plants since binary generating plants have no emissions as they do
not expose the geothermal fluid to the atmosphere. However, their operation can influence subsur-
face processes. These effects will be discussed at appropriate points.

carbon dioxide
The solubility of carbon dioxide in geothermal fluids is controlled by the temperature of the fluid
and the minerals along the flow path. If there are no carbonate minerals (such as calcite or dolomite)
along the flow path, the only sources for CO will be atmospheric gases that are at depth or release of
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gases from cooling magmas. In instances where carbonate minerals are present along the flow path,
CO concentrations will be significantly higher. Figure 12.1 shows measurements of CO concentra-
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tion in geothermal fluids as a function of temperature (Arnórsson 2004). These concentrations show

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