Page 241 - Geothermal Energy Renewable Energy and The Environment

230 Geothermal Energy: Renewable Energy and the Environment

concentrations of certain metals that can be of environmental concern that, in the case of geother-
mal waters are mainly arsenic (As) and boron (B). For waters that meet regulatory standards, the
primary challenge is finding a disposal location that can handle the volumetric output, once the
water has been sufficiently cooled to prevent an environmental impact.
For mitigation of As, numerous approaches are possible, most of which rely on oxidizing the
3+
+5
reduced form of As (As ) that occurs in geothermal fluids to As by reacting it with some appropriate
aqueous species that will result in its precipitation from solution. An example of such a reaction is
Fe (SO ) + As + 2 H O <=> FeAsO 2 H O(s) + Fe + 2 SO + O (g),
3+
3+
4 3
2
2
2
4
2
2
where (s) indicates a solid precipitate and (g) indicates a gas.
The necessity to remove B from geothermal fluids is less frequent, but nevertheless can be an
issue in some instances. Successful treatment has been accomplished using ion exchange resins
(Kabay et al. 2004), and nanofiltration with reverse osmosis (RO; Dydo et al. 2005).
Geothermal waters with high solute loads are of growing interest because they often contain
high concentrations of valuable resources. Although concentrated geothermal fluids have long been
perceived as analogues for solutions that formed precious and base metal ore deposits (Bird and
Helgeson 1981; Helgeson 1964; McDowell and Elders 1980; Muffler and White 1969; Skinner et al.
1967) it has been relatively recently that attention has focused on extracting commercially valuable
resources from them directly (Bourcier et al. 2006; Entingh and Vimmerstedt 2005; Gallup 1998,
2007; Gallup et al. 2003). This change in interest reflects two rapidly evolving developments. One
of these developments is the growing ability in the materials science field to design nanomaterials
and ion- selective membranes suitable for application with geothermal brines. These materials incor-
porated into RO technologies and other applications have improved the ability to extract useful
resources (e.g., Bourcier et al. 2006). The other factor is the rapidly evolving market for resources
contained in geothermal brines. The electronics and energy storage industries are particularly
important markets for the resources in geothermal brines (Entingh and Vimmerstedt 2005).
Listed in Table 12.1 are concentrations of precious and base metals in some geothermal brines.
The table is selective in that the concentrations shown are among the highest reported and therefore
are not necessarily from the same well nor indicative of what can be expected in all geothermal
systems. They are shown as an indication of the concentrations that have thus far been observed.
Also shown in the Table 12.1 are the estimated annual market values for the resources, based solely
on recovery of the resource from several generating facilities in the general Salton Sea region. The
values are based on the outflow from a 50 MW geothermal generating facility and are 2001 dollars
(Entingh and Vimmerstedt 2005).
Recovery of resources is an attractive complement to power production, since it would provide
an additional revenue stream to a producing facility and would also make use of a resource that is
readily available, thus reducing the need for surface mining, which can have serious environmental
impacts. However, the technology for resource recovery remains a challenge. Most importantly, sep-
aration of a resource into its pure state is difficult because of the chemical complexity of geothermal
brines. A substantial research and development effort is required to obtain a useful technological
approach at each site since the solutions at a particular location tend to have unique characteristics.
As well, separation of resources carried out on site will be a parasitic load on the power generation
since recovery of these materials requires energy input. Nevertheless, successful approaches have
been developed for separation of silica, manganese, and zinc. Separation technologies for other
resources are likely to be available in the near future.
An additional commodity that can be obtained from geothermal brines is water in arid settings
(Gallup 2007). With relatively modest investment, water that has been used in a geothermal applica-
tion can be cleaned to meet a broad range of standards that can satisfy industrial as well as drinking
water needs. In this sense, the water that is the solvent for the metals discussed above also becomes
a resource that can be recovered.