A person can tolerate a maximum concentration of 170 to 330 parts per million for approximately one hour without serious consequences. At 500 parts per million, however, the person loses reasoning and balance and possibly experiences respiratory disturbance. When a person is exposed to 700 to 1,000 parts per million for up to an hour, death may occur within minutes. Emissions In the journal publication, "A Guide to geothermal energy and the environment," Alyssa Kagel, Diana Bates, and Karl Gawell (2007), all of the Geothermal Energy Association, explain that the visible plumes rising from some thermal power plants consists of water vapor emissions (steam), not smoke. Geothermal power plants virtually do not release any air emissions as they do not burn fuel as fossil fuel plants. "A case study of a coal plant updated with scrubbers and other emissions control technologies emits 24 times more carbon dioxide, 10,837 times more sulfur dioxide, and 3,865 times more nitrous oxides per megawatt hour than a geothermal steam plant" (Kagel, Bates & Gawell, p. ii). Table** notes four significant pollutants that geothermal and coal facilities emit.
Table**: Average Geothermal Energy Production, 1990, 2003, and EIA Conversion Information (Energy Information Administration (EIA) cited in Kagel, Bates & Gawell, 2007, p. ii).
Geothermal plants do not directly emit sulfur dioxide, however after hydrogen sulfide is released into the atmosphere as a gas; it ultimately converts into sulfur dioxide and sulfuric acid. Consequently, sulfur dioxide emissions linked to geothermal energy evolved from hydrogen sulfide emissions (Kagel, Bates & Gawell, 2007). Currently, geothermal power plants routinely abate hydrogen sulfide, which results in more than 99.9% of the hydrogen sulfide from geothermal noncondensable gases being converted into elemental sulfur, which may in turn be utilzed as a non-hazardous soil amendment and fertilizer feedstock. "Since 1976, hydrogen sulfide emissions have declined from 1,900 lbs/hr to 200 lbs/hr or less, although geothermal power production has increased from 500 megawatts (MW) to over 2,000 MW" (Kagel, Bates & Gawell, p. ii).
Minerals and Corrisons
Benjamin Valdez, Michael Schorr and Addis Arce (N.d). Institute of Engineering, Area of Chemical Engineering Materials, Minerals and Corrosion Department, Universidad Autonoma de Baja California, report in the journal publication, "The influence of mineral on equipment corrosion in geothermal brines," that industrial minerals, extracted from geothermal brines, significantly contribute to the economy of numerous countries. In countries which own and operate geothermal fields and geothermoelectric plants, these minerals constitute the raw materials "for the chemical, fertilizer, metal, ceramic and building industries" (Valdez, Schorr & Arce, p. 1). Valdez, Schorr and Arce explain:
Corrosion affects the different types of equipment, machinery and structures, made from two basic engineering materials: steel and concrete, used in geothermal plants and installations. Minerals undergo ionic dissociation in the brines, contribute to their salinity, chlorinity, and electrical conductivity; alter their pH and increase their corrosivity. Other corrosive substances are present in the brines such as dissolved gases: Oxygen (02) carbon dioxide~02), ammonia (NH3) and hydrogen sulfide (112S). Some minerals, depending on their chemical nature and solubility, deposit on metallic surfaces as a hard scale and corrosion appears underneath. Corrosion control engineering applies methods and techniques of prevention and protection, avoid the interaction of the equipment and structures with the corrosive constituents of the eotheimal brines.
Valdez, Schorr and Arce (N.d.) purport that geothermal brines contain a high concentration of dissolved; ionized mineral salts mainly chlorides and sulfates, which are aggressive ions in the context of corrosion. Their amount, relative to carbonates and bicarbonates, are of primary importance in any assessment of the corrosion characteristics of the brines.
Table ** depicts the chemical composition of a typical geothermal brine
Table**: Chemical composition of Typical Cerro Prieto geothermal brine (Valdez, Schorr and Arce N.d, p. 1).
Component
Na
K
Mg
Ca
Cl
SO4
SiO2
HCO
3
ppmn
18.6
11735
Valdez, Schorr and Arce (N.d.) further note that the industrial equipment, structures and installations of geothermal fields are built of two basic materials: Steel and reinforced concrete, the latter with a surface of low porosity to avoid the penetration of the brine dissolved minerals and future corrosion. Other plastic and modern composite materials, with high corrosion resistance are replacing metallic materials. An abridged list of equipment for geothermal wells and brines is given in table 2 (Valdez, Schorr and Arce N.d, p. 2).
Table ** depicts equipment used in geothermal wells and brines.
Table **: Geothermal Equipment (Valdez, Schorr and Arce N.d, p. 2).
Equipment
Materials of Construction
Pipes, tubes and ducts
Steel, reinforced concrete
Pumps, vertical and centrifugal
Steel, brass, bronze
Valves, diverse types
Steel
Fittings and flanges
Steel
Silencers
Reinforced concrete
Brine canals
Reinforced concrete
Geotextiles, sedimentations ponds
Plastic, rubber
Monitoring and safety instrumentation
Metallic, plastic
Valdez, Schorr and Arce (N.d.) stress that this equipment suffers from different forms of wear: erosion, abrasion, fatigue, disintegration, stress, aging, and particular wet corrosion. Several geothermal power plants in the Imperial Valley, CA use stainless steel, titanium alloy and cement-lined carbon steel tubes to prevent and/or minimize corrosion by acidic components and scaling by silica (Si02) in the casings of their geothermal wells. Silica is utilized as an additive for road pavimentation and roofing tiles materials. Calcite and aragonite scaling are frequently encountered in other countries geothermal well fluids. It is worthwhile to mention in the context of this work, the peculiar corrosion behavior of two salty water bodies: the Salton Sea, CA, and the Dead Sea ( called the Salt Sea in the Bible) Israel and Jordan. They contain an high concentration of mineral salts: 45g/1 and 280 g/l respectively. These massive desert seas, without a natural outlet, located at 60m and 400m below sea level, continually evaporate rising their salinity. As a result of this salt content, DO reach condition of hypoxia: 2 to 4 mg/l in the Salton Sea or anoxia: 0.1 mg/l in the Dead Sea. Therefore, the harvest of the solid Na, K and Mg salts in the evaporation ponds for the production of chemicals, fertilizers and Mg metal in Dead Sea Works plants is carried out by unprotected steel-made barges, pumps and pipelines without any practical corrosion (Valdez, Schorr and Arce N.d, p. 2).
Valdez, Schorr and Arce (N.d.) further explain that corrosion, scaling and fouling phenomena often appear simultaneously in equipment and installations handling geothermal wells and brines. Minerals scales and deposits, associated with brines composition and circulation, have a marked effect on corrosion. They occur in the brines depending on their physicochemical interaction with the equipment surface, the operational conditions such as pH (4 to 8), DO content (4 to 6 mg/l) flow -- regime and temperature (30 to 250 C). The mineral salt concentration affects the corrosion rate of carbon steel (Figure 1). The rate increases to a maximum at the concentration of seawater (3.5%) and then decreases nearing cero at the saturation concentration (25%) because DO content reaches a minimum value near zero (Valdez, Schorr and Arce N.d, p. 3).
Valdez, Schorr and Arce (N.d.) conclude that the cost of the aging infrastructure maintenance and repair are considerable and increasing. A recent NACE report estimated that 20 to 30% of this cost could be saved by application of corrosion control technologies. The principal means of corrosion control in the geothermal industry are correct selection of materials of constructions for equipment and structures, use of special paints, coatings and linings resistant to concentrated brines and cathodic protection by impressed current and/or sacrificial magnesium or aluminum anodes. Today, the main and fastest source of information on corrosion control of industrial equipment, plants and facilities is the internet Data bases and computer -- based expert systems, dealing with selection of materials, their properties and corrosion control for many environments and industries are listed in Roberge's Handbook (Valdez, Schorr and Arce N.d, p. 4).
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