Page 239 - Geothermal Energy Renewable Energy and The Environment

228 Geothermal Energy: Renewable Energy and the Environment

of any CO emissions from binary power plants, make electricity production from geothermal
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power plants an attractive alternative to fossil fuel-based generating systems. In 2008, 51% of the
U.S. electrical generation came from burning coal and 17% from burning natural gas (Energy
Information Administration 2008). For every MW of power generated by a geothermal power
plant that displaces the equivalent power production by this fossil fuel mix, the CO emissions
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are reduced by more than 99%. In addition, since a geothermal power plant uses the local water,
the actual CO emissions from a geothermal power plant will be controlled by the chemistry of
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the reservoir fluid. As a result, the emissions will reflect the geochemical controls at the site. This
suggests that geothermal power plants will compatibly blend into the natural emissions back-
ground, rather than be an exaggerated point source for CO releases, unlike fossil-fueled power
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plants.
It is important to note that reinjection of cooler fluids with lower CO contents into a geothermal
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reservoir can perturb whatever steady-state or equilibrium conditions may have existed in the res-
ervoir prior to exploitation. As a result, it is possible that mineral equilibria similar to the reaction
listed above involving calcite, prehnite, clinozoisite, and quartz will shift toward replenishment of
CO in the aqueous phase. How this may affect the long-term geochemistry and mineralogy of the
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site must be considered on a site-by-site basis.

hydroGen sulfide
Hydrogen sulfide (H S) occurs naturally in the air and in the subsurface. Under normal conditions it
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is present in the air at ground level at <1 part per billion (1 ppb) to several hundred ppb, depending
upon the local environmental conditions. Hydrogen sulfide is a highly reduced sulfur compound
that forms in conditions where the oxygen partial pressure is very low. Hence, environments such
as swamps, where anaerobic decay is underway, can have relatively high H S levels, and is the
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reason such environments often have a rotten egg smell. In the subsurface, where the abundance
of free oxygen can be very low, sulfur occurs as compounds that are usually quite reduced as well.
Minerals such as pyrite (FeS ) and pyrrhotite (FeS) are commonly reduced sulfur compounds while
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H S is the common gaseous form of sulfur. It is a reflection of these conditions that geothermal
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fluids have some H S present in solution.
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The H S, at concentrations of 500 ppb or higher, can cause unconsciousness and death. At
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levels of 10–50 ppb most people find it to have an offensive rotten egg odor. At levels between
50 ppb and a few hundred ppb H S can quickly deaden olfactory sensors so that the smell is not
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perceived. For these reasons, strict emissions regulations are imposed on it and abatement is
required if concentrations are above regulatory limits. This can be an issue during drilling of a
well (as discussed in the Case Study for Chapter 8), as well as throughout the power generation
complex.
Strategies for abating H S in geothermal emissions must take into account several factors. One is
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that, as a noncondensable gas, it is preferable that it be removed from geothermal steam before the
steam is introduced to the turbines, for the same efficiency reasons that other noncondensable gases
such as CO are removed. However, because of its high volatility, H S will partition itself between
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the separated steam and the noncondensable phase. Thus, H S should be expected to be present in
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both the noncondensable separated phase and in the steam at the cooling and condensing stage, if
it is present at the production wellhead. As a result, it is important to determine H S concentrations
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throughout the steam path during the generation process to establish where and to what extent H S
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must be removed and neutralized to meet regulatory requirements.
In addition, H S concentrations can be highly variable from one well to another within a geo-
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thermal system, owing to the heterogeneous distribution of minerals and gases that control H S
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concentrations. For example, as with the carbonate mineral controls on CO , there are a number
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of reactions involving sulfur-bearing minerals that control the amount of H S in the fluid phase. A
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schematic reaction such as