Geothermal Energy

Which Chemicals are Used in Geothermal Energy Production?

Borax, Hydrochloric acid, Boric acid, Maleic acid, Citric acid, Sodium molybdate.

A geothermal source is defined as hot water and steam that accumulates at various depths in the earth due to its inherent heat, with temperatures continuously above the average atmospheric temperature of the region, and which can contain more diverse minerals, salts, and gases than the surrounding surface and underground waters. Geothermal energy covers all forms of direct or indirect utilization obtained from these sources. In Figure 1, the model of formation is shown, with parameters that constitute a geothermal system: the heat source located deep within the Earth, the fluid that transports the heat, the reservoir rocks that contain the fluid, and the cap rocks that prevent heat loss.

At the center of the Earth, there is a molten mass called magma with a temperature of approximately 4200°C. Magma activities that reach shallow depths or the Earth's surface occur through fractured and weakened zones caused by tectonic movements, forming the heat source of the geothermal system. Meteoric water seeping from the cracked and fractured regions of the Earth warms at depth and accumulates in porous and permeable reservoir rocks. Part of this water rises along fault lines forming geothermal sources. Confined by an impermeable cap rock, the geothermal fluid contained in the reservoir rock, which mostly cannot reach the surface, is brought to the surface by drilling operations. Geothermal sources are used for electricity generation, heating, or drying processes, taking into account their energy potential and the minerals they contain.

Applications

Geothermal energy is used in various areas of industry, primarily for electricity generation, heating and therapeutic purposes, depending on its temperature. Many applications can benefit from integrating high-temperature geothermal fluids. Details are presented in Table 1.

Geothermal Energy in Our Country

Turkey is located in a region with intense tectonic movements. Throughout the country, many geothermal sources exist in natural, varied forms and temperatures. Therefore, geothermal energy is an important renewable energy source for our country. ^[1]

Based on the fluid temperatures, geothermal energy sources are classified into three groups:

• Low enthalpy fields (temperature 20 – 70°C)

• Medium enthalpy fields (temperature 70 – 180°C)

• High enthalpy fields (temperature >180°C)

Studies for locating and using geothermal energy in Turkey have advanced especially in Izmir and other parts of the Aegean Region. In Izmir, nearly 15,000 houses in the Balçova and Narlıdere districts are still heated by geothermal energy. In Seferihisar, Dikili, Bergama, Çeşme, Aliağa, Urla, Güzelbahçe, Bayındır, Menderes, Kemalpaşa, and Kozaklı, it is estimated that using the known geothermal sources within the Izmir Metropolitan Municipality borders could heat 220,000 houses. However, the steps taken (such as Izmir Jeotermal A.Ş.) progress more slowly than natural gas distribution works, and when the regulatory measures of energy policies take effect, deterrents to geothermal infrastructure become increasingly prominent. There is growing insistence from various circles to minimize natural gas dependency by bringing forward local geothermal energy, whose technology and manpower are already available.

There are currently 32 geothermal power plants in operation in Turkey. The cities where the plants are located and the number of plants are as follows: Aydın (20), Denizli (5), Manisa (5), Çanakkale (2). Additionally, geothermal power plant construction studies continue in Aydın (4), Manisa (2), and Denizli (1). With a total installed capacity of 820.86 MW, these plants constitute 1.58% of Turkey's total installed capacity.

Areas in Turkey Suitable for Geothermal Energy Production: ^[5]

Turkey is the 7th largest country in the world in terms of geothermal energy potential. In order to determine the geothermal potential of our country, the General Directorate of Mineral Research and Exploration (MTA) started studies in 1962. Up to now, MTA has discovered 227 geothermal regions and drilled nearly 1,100 geothermal wells (550 for direct use and electricity production combined with others). The theoretical geothermal energy potential of Turkey is estimated between 31,500 and 60,000 MWt, while the technically usable capacity is calculated as 4,809 MWt. Out of the predicted technical capacity, 2,880 MWt has been proven.

In our country, there are 1,500 hot mineral water sources with temperatures ranging from 20 to 242°C. Currently in Turkey:

• 90,000 residential equivalent buildings

• 3,000,000 m² of greenhouse area

• 400 spa facilities

are heated by geothermal energy. ^[1]

Table 2 shows the geothermal power plants located in our country. As indicated, the province receiving the most investments is Aydın. Part of the electricity needed in our country is met by these power plants.

Table 2: Geothermal Power Plants in Turkey

Problems and Solutions in Geothermal Energy Production

S scaling

In Turkey’s geothermal fields, which are predominantly liquid systems, the fluids provide advantages for heat extraction. However, the silica and calcium contained in the fluid in high temperature fields cause scale formation and corrosion.

Scaling is one of the most significant problems encountered in the operation of geothermal fields. In the reservoir conditions, the underground hot water is generally in equilibrium with the dissolved substances. Scaling caused by degassing, evaporation and cooling is mainly due to the relative insolubility of carbonate compounds such as CaCO3. The pH rise caused by the dissociation of CO2 during production, and the subsequent conversion of carbonic acid first to bicarbonate and then to carbonate ions, cause supersaturation in the geothermal fluid and eventually lead to scaling when calcium carbonate crystals grow along the equipment surfaces.

In geothermal applications, three types of scale formations are generally encountered based on their chemical compositions: a) scaling due to silica and silicates, b) carbonate scaling, and c) scaling due to sulfates and sulfites. As in many Turkish geothermal fields, carbonate scaling is the prevailing type in geothermal fields used for electricity production such as Kızıldere, Germencik and Salavatlı. Most hydrothermal systems contain large amounts of dissolved CO2. According to Henry's law, the amount of CO2 dissolved in water is directly proportional to its partial pressure. As long as the CO2 remains dissolved, the pH remains low. Once CO2 starts leaving the liquid phase, the pH increases and according to reactions (3) and (4) the concentration of HCO3- and then CO3 2- increases. The increase in CO3 2- in the environment leads to the formation of calcium carbonate scaling with the Ca++ present within the solution.

Methods to Prevent Scaling

The methods to control carbonate scaling can be summarized as follows:

• Adjusting the carbonate-bicarbonate balance using pH and CO2 partial pressure,

• Periodically cleaning the scale with acid or mechanical drilling,

• Preventing scale formation with a suitable inhibitor,

• Preventing pressure drop and thus scaling by using a submersible pump

Although all methods to prevent or delay scaling are of paramount importance, pH adjustments and control of CO2 partial pressure are technically constraining and mechanical cleaning interrupts production. Therefore, the most appropriate solution is to remove the scaling problem using a technically and economically suitable inhibitor.

• CO2 Partial Pressure Control

Partial pressure control can be achieved in low-temperature water producing wells by using a submersible pump. Through this pump, the pressure drop in the well is prevented and the transition of CO2 from the liquid phase to the gaseous phase, and subsequent CaCO3 precipitation, is significantly reduced. Alternatively, CO2 partial pressure control can be achieved using a system called the “Equilibrium Flash System (EFP)”. In this system, it is sufficient to continuously inject CO2 below the gas breakthrough point in the well. The flash system is not widely used in Turkey. In high-pressure geothermal fields, high wellhead pressures partially prevent scaling, but is not the preferred method due to production limitations.

• pH Adjustment

Adjusting the pH of the geothermal fluid, due to its chemical composition, significantly prevents precipitation. For this, sufficient amounts of HCl must be added to the fluid. Although technically effective, this method is not economical since preventing scale in 1 liter of fluid might require adding 200 ml HCl.

• Injection of a Chemical Inhibitor

Using a chemical inhibitor to prevent calcium carbonate scaling is the most technically and economically viable method. In this method, it is crucial to identify and use the inhibitor that best matches the geochemical properties of the field and inject it to the necessary depths.

In low-temperature wells, applying inhibitor injection is simple; however, in high-temperature wells, its application becomes challenging both in selecting the correct inhibitor and due to mechanical difficulties. In Turkey, the first inhibitor test was performed in the Kızıldere KD-1A well. In geothermal fields such as Kızıldere, Germencik and Salavatlı used for power production, the inhibitor method is applied successfully. The inhibitors used in scaling prevention are derived from five main chemical groups:

1. Aminophosphonates

• Amino alcohol phosphonate ethers

• Sodium polyacrylate polymers

• Policarboxylic acid

• Organophosphates

Corrosion

All types of corrosion can occur in geothermal production and applications since geothermal fluids contain all ions and compounds that can cause corrosion. The main ions and compounds responsible for corrosion in geothermal fluids include dissolved oxygen (O2), carbon dioxide (CO2), hydrogen ions (H+), chloride ions (Cl-), hydrogen sulfide (H2S), ammonium compounds (NH4+), and sulfate ions (SO4 2-). There is no universally corrosion-resistant metal or equipment used in geothermal applications. Stainless steels AISI 304 and AISI 316 are known to resist the various forms of corrosion caused by geothermal fluids. Plastic materials are immune to corrosion; however, they are not always suitable for high temperature and pressure conditions.

Corrosion Prevention Methods

The measures to prevent or reduce corrosion of a metallic system in a particular environment can be grouped under the following three categories:

1. Electrochemical methods

• Cathodic protection
  • Anodic protection

• Chemical methods

• Use of inhibitors
  • Changing the chemical composition of the environment

• Protective coating by paint

The most commonly used electrochemical method is cathodic protection. With this method, the potential of the corroding structure is controlled to render the metal thermodynamically stable. Protective coatings, especially organic paints, are a simple and inexpensive method.

The main chemical substances used as corrosion inhibitors can be briefly summarized as:

• Sodium chromate (Na2CrO4)
• Sodium phosphate (Na3PO4)
• Sodium nitrite (NaNO2)
• Sodium hexametaphosphate (Na6P6O18)
• Sodium molybdate (Na2MoO4)
• Zinc salts of organic phosphonic acid

Formation of Microbial Growth Area

Cooling water systems that include evaporative cooling towers can provide a growth area for microorganisms, including pathogens. The presence of bacteria such as Legionella in cooling water can pose a health and safety risk to personnel exposed to it. The growth of microorganisms may also lead to equipment damage and reduced plant efficiency due to microbially induced corrosion (MIC) or fouling of heat exchange surfaces.

In geothermal cooling water systems containing hydrogen sulfide, the selection of commercially available biocides that can be used successfully is limited by the chemistry of the cooling water and sometimes by the detention time (when using direct-contact condensers), as well as the design of the system and environmental effects of any discharge.

Chemical Raw Material Use

Hydrochloric Acid

Molecular Formula (Hydrochloric Acid): HCl

Molecular Weight: 36.46 g/molChemical Name: Hydrochloric AcidCAS Number: 7647-01-0

Other Names:

• Salt Spirit
  • Muriatic Acid
  • Hydrogen Chloride
  • Aqueous hydrogen chloride
  • Acid
  • Chlorohydric Acid
  • 7647-01-0

One of the methods to prevent scaling in a geothermal field is to adjust the pH. This method significantly reduces precipitation by adding sufficient amounts of HCl. Although technically significant, it is not enough on its own. For example, in a 1 L container, scaling is induced by adding 200 ml of HCl.

Physical and Chemical Properties:

• Hydrochloric acid is a colorless aqueous liquid with a sharp and irritating odor.

• Boiling point is -85.1 °C.

• Melting point is -114.2 °C.

• At 0°C, its solubility in water is 82.3 g/100 g water.

• It is soluble in water, ethanol, methyl alcohol and ether. The solubility of Hydrochloric Acid decreases with increasing temperature.

• Its density is 1.639 g/L.

• When heated, it decomposes and releases chlorine gas.

• It reacts exothermically with bases.

Production:

Industrial production of this chemical is first carried out via the electrolysis of hydrogen chloride. In production, hydrogen and chlorine gases are combined in an aqueous solution. The reaction is exothermic, so a furnace or burner is used in the production facility. The produced hydrogen chloride gas is dissolved in pure water to obtain hydrochloric acid, resulting in the pure product.

In the chemical industry, it is most commonly produced as a by-product in the chlorination process. During the production of sodium hypochlorite, hydrochloric acid is produced as a by-product of the chlorination reaction. After separation from the chlorinating gases, the gas stream containing hydrogen chloride is led to an absorption column where the weak hydrochloric acid solution absorbs the HCl vapors. The HCl-free portions are then removed for further processing. The liquid acid is either sold or used elsewhere in the facility. The final gas stream is passed through a scrubber to remove any remaining HCl before being vented.

Its production also occurs during the production of common organics such as Teflon, polyvinyl chloride, and perchloroethylene.

Citric Acid Monohydrate

Molecular Formula (Citric Acid Monohydrate): C6H8O7.H2O

Molecular Weight: 210.1 g/molChemical Name: Citric Acid MonohydrateCAS Number: 5949-29-1

Other Names:

• Citrate
  • Citric Acid Salt
  • Citric Acid Ester
  • 2-hydroxypropane-1,2,3-tricarboxylic acid, hydrate
  • 5949-29-1
  • Citrate
  • Citric Acid Hydrate
  • 1,2,3-Propanetricarboxylic Acid, 2-hydroxy-, monohydrate

Citric Acid is used to clean scaling that forms. It is one of the mechanical cleaning methods that must be applied periodically.

Physical and Chemical Properties:

• What are the physical and chemical properties of Citric Acid Monohydrate?
• The density of the monohydrate is 1.542 g/cm³.
• Flash point is 100 °C.
• Melting point is 135 °C.
• It appears as a white granular substance.
• Citric Acid Monohydrate is highly soluble in water and is a stable chemical. It does not react with bases, strong oxidizing agents, reducing agents or metal nitrates.
• At 25 °C, the pH is around 1-2. It is a non-odorous compound.
• It appears as colorless crystals or as a white crystalline powder with a strongly acidic taste.
• Citric Acid Monohydrate is completely biodegradable. This is one of its most important properties.

Production:

The production of Citric Acid Monohydrate falls under the field of Biochemistry and Biotechnology. Citric Acid is an intermediate organic compound in the tricarboxylic acid cycle and naturally occurs in citrus fruits, pears, and pineapples in crystalline form as calcium citrate. It is an organic carboxylic acid obtained from lemon juice. The production of Citric Acid typically involves fermentation. Many microorganisms such as certain yeasts, molds, and bacteria can produce citric acid by fermentation. Alternatively, Citric Acid Monohydrate can be produced by fermenting glucose with a special strain. There are three different methods used in the production of Citric Acid; two of these methods are widely used. One method uses surface culture and the other uses submerged batch fermentation. The latter method is the most common.

Aminotris (Methylene Phosphonic Acid)

Molecular Formula: C3H12NO19P3

Molecular Weight: 299.05 g/mol

Chemical Name: Aminotris (Methylene Phosphonic Acid)

CAS Number: 6419-19-8

Other Names:

• Aminotris (Methylene Phosphonic Acid) (Atmp)

• Nitrilotri(methylene phosphonic acid)

• Nitrilotri (Methylophosphonic Acid)

• Nitrilotris (Methylenphosphonic Acid)

• Nitrilotris (Methylene)

It is used in inhibitor systems to prevent scaling. It is injected to the necessary depths in the wells. The recommended usage is 38-42%.

Physical and Chemical Properties:

• Melting point is 215 °C.

• Density is 25 g/mL.

• Not compatible with bases.

• It is an effective antiscalant widely used in industrial water treatment and detergents.

• It exhibits good corrosion inhibition properties in the presence of zinc and other phosphates.

• ATMP can also be used as a chelating agent in the textile industry.

Maleic Anhydride

Molecular Formula (Maleic Anhydride): C4H2O3

Molecular Weight: 98.057 g/molChemical Name: Maleic AnhydrideCAS Number: 108-31-6

Other Names:

• 2,5-Furandione
  • Toxic Anhydride
  • Maleic Acid Anhydride

• Poly(maleic anhydride)
  • MA Oligomer
  • MA Polymer
  • Poly (MA)
  • Poly(maleic anhydride)
  • Maleic Acid Anhydride Homopolymer
  • Polymers

Maleic anhydride is classified as an organic chemical substance used as a derivative of maleic acid. It is a versatile chemical compound. Acid anhydrides are functional groups that react easily with water to form maleic acid. For this reason, maleic acid is not typically sold in the market.

In inhibitor injection systems, Maleic Acid is used for scaling prevention. The usage is around 1-5% and it is injected to the necessary depths in the wells.

Physical and Chemical Properties:

• It appears in a needle, flake, lump, rod, briquette or fused mass form, as a colorless crystalline substance.

• It has a piercing and suffocating odor.

• Boiling point is 202 °C.

• Melting point is 53 °C.

• Regarding its solubility, it is highly soluble in water; for example, at 25 °C, its solubility is 79 g/100 mL.

• It is also highly soluble in hot solvents. In addition, it dissolves well in the following solvents:
  • Acetone
    • Chloroform
    • Ethyl Acetate
    • Benzene
    • Toluene
    • o-Xylene
    • Carbon Tetrachloride
    • Dioxane

• Its density is 1.48 g/cm³.

• Flash point is 102°C.

Production:

The production of Maleic Anhydride is achieved via dehydration synthesis. When two carboxylic acids combine, a water molecule is released resulting in the formation of Maleic Anhydride. Alternatively, Maleic Acid can be obtained by subliming Maleic Acid with P2O5 under low pressure or produced commercially by the catalytic vapor phase oxidation of benzene in the presence of appropriate hydrocarbons. In this case, the oxidation of benzene is carried out at 400 °C with a V2O5 catalyst using atmospheric oxygen. The by-product from phthalic anhydride is o-xylene.

Sodium Molybdate

Molecular Formula (Sodium Molybdate): Na2MoO4

Molecular Weight: 205.9 g/molChemical Name: Sodium MolybdateCAS Number: 7631-95-0

Other Names:

• Disodium Molybdate
  • 7631-35-0
  • Molybdic Acid
  • Disodium Salt of Molybdic Acid
  • Sodium Molybdenum Oxide
  • Disodium Tetraoxo Molybdate

• Anhydrous Form
  • Dihydrate Form

It is widely used as a corrosion inhibitor in geothermal power plants. It serves to prevent or reduce corrosion in metal systems.

Physical and Chemical Properties:

• It appears as a white powder.

• Melting point is 687 °C.

• Its density is 3.78 g/cm³.

• At 100 °C, its solubility in water is 84 g/100 mL.

• When heated, it emits fumes along with sodium oxide in a toxic form and reacts violently in the presence of halogens. For example, in the presence of Hydrochloric acid and sodium hypochlorite, there is a risk of explosion and fire.

• Under normal conditions, it is a flammable substance and can explode in the presence of magnesium.

Production:

Two different methods are applied in the production of Sodium Molybdate.

1. The first method is the Roasting Method;

In this method, the molybdenum compound is roasted at high temperature to form molybdenum trioxide. Then it is washed with caustic soda resulting in a solution. Through filtration, evaporation, cooling, crystallization, separation, and drying at 70-80 °C, a concentrated product is obtained.

• The second method is the Recrystallization Method;

This method achieves a higher quality raw material. After recrystallization, it is dissolved in water and a precipitation process is initiated by adding nitric acid. The precipitate is then washed, dried at 70-80 °C, cooled, and finally sodium molybdate dihydrate is obtained.

Sodium Chromate

Molecular Formula (Sodium Chromate): Na2CrO4

Molecular Weight: 161.972 g/molChemical Name: Sodium ChromateCAS Number: 7775-11-3

Other Names:

• (IV) Valency
  • Dichromium Oxide
  • Sodic Chromate
  • Neutral Form
  • Anhydrous Form
  • Chrome Sodium Oxide
  • Sodium Monochromate
  • Chromium Sodium

It is widely used as a corrosion inhibitor in geothermal power plants to reduce the corrosion rate in metal systems.

Physical and Chemical Properties:

• It is a yellow crystalline solid. The crystals are yellow ortho-phobic crystals.

• It has a bitter metallic taste.

• Melting point is 762 °C.

• It is very soluble in water; at 25 °C, the solubility is determined as 87.6 g/100 g water. However, it is slightly soluble in methanol and ethanol.

• Its density is 2.7 g/cm³.

• Under standard storage conditions, it is a stable chemical.

• Its aqueous solution has alkaline properties.

• It is an abrasive chemical substance due to its oxidizing effects.

• In its hygroscopic form, it quickly absorbs water and transforms into its hydrated salts.

• It has properties that can cause fire if mixed with flammable materials.

Production:

In its production, chromite ore is melted in a reverberatory furnace with lime and soda in the presence of air. The melt is then dissolved in water with a small amount of sodium carbonate. The solution is then acidified with acetic acid, concentrated and crystallized.

The reaction is as follows:

4Fe (CrO2)2 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2

Sodium Hexametaphosphate

Molecular Formula (Sodium Hexametaphosphate): Na6O18P6

Molecular Weight: 611.77 g/molChemical Name: Sodium HexametaphosphateCAS Number: 10124-56-8

Other Names:

• Sodium Polyphosphates
  • Sodium Polyphosphate; SHMP
  • Calgon

It is widely used as a corrosion inhibitor in geothermal power plants to reduce the corrosion rate in metal systems.

Physical and Chemical Properties:

• Melting point is 616 °C.

• Boiling point is 1500 ℃.

• Its density is 2.181.

• It dissolves in water, but not in organic solvents.

• It appears as a white solid.

• SHMP is also used as a dispersant and as a food additive under the E number E452i in various industries.

• Sometimes sodium carbonate is added to SHMP to raise the pH to 8.0-8.6, resulting in various SHMP products used in water softening and detergents.

• One important application of sodium hexametaphosphate is as a deflocculant in the production of clay-based ceramic particles. It is also used as a dispersing agent for soil analysis and other earth types.

• It is used as an active ingredient in toothpaste as an anti-stain and tartar preventive agent.

Production

Sodium dihydrogen phosphate: A soda solution is subjected to a neutralization reaction with phosphoric acid at 80-100 ℃ for 2 hours; the resulting sodium dihydrogen phosphate solution is concentrated by evaporation, cooled and crystallized to obtain sodium dihydrogen phosphate dihydrate. It is then heated to 110-230°C to remove the two crystal waters; subsequently, the structural water is removed by further heating, which results in dissolved sodium metaphosphate followed by heating to 620°C for dehydration and finally polymerization to obtain sodium hexametaphosphate. A cooling shock at 650 ℃ to 60-80 ℃ is applied and the hexametaphosphate products are crushed.

Phosphorus pentoxide method: Yellow phosphorus is burned in dry air steam, and the obtained phosphorus pentoxide is mixed with soda in a specific ratio (Na2O: P2O5 = 1~1.1). The mixed powder is placed in a graphite pot and heated indirectly to prevent clumping; the cooled product is then subjected to a cooling shock and ground to produce industrial sodium hexametaphosphate products. The reaction is as follows:

P4 + 5O2 → 2P2O5

P2O5 + Na2CO3 → 2NaPO3 + CO2 ↑ 6NaPO3 [△] → (NaPO3)6

Glutaraldehyde

Molecular Formula (Glutaraldehyde): C5H8O2

Molecular Weight: 100.12 g/molChemical Name: GlutaraldehydeCAS Number: 111-30-8

Other Names:

• Gludesin
  • Glutaral
  • Glutaraldehyde
  • Glutardialdehyde
  • Glutarol
  • Korsolex

In evaporative cooling water systems, including cooling towers, microbial growth including pathogens may occur. It is important to select the correct biocide considering the microorganisms present in the fluid. Another important issue is to ensure the chosen biocide does not cause pH fluctuations. The growth of microorganisms also reduces plant efficiency by causing microbially induced corrosion (MIC) or fouling the heat exchange surfaces.

Physical and Chemical Properties:

• Melting point is -15 °C.

• Boiling point is 100 °C.

• Its density at 20 °C is 1.058 g/mL.

• It is an open yellow liquid with a sharp odor.

• It is miscible with water. It is soluble in ethanol, benzene, and ether.

• Its autoignition temperature is 225 °C.

• Glutaraldehyde is incompatible with strong oxidizers and strong bases.

• It is used for disinfection in medical and dental equipment. It is also utilized as a protective agent in industrial water treatment.

Production

Glutaraldehyde is industrially produced by oxidizing cyclopentene. Alternatively, it can be made from acrolein and vinyl ethers through a Diels–Alder reaction followed by hydrolysis. Like many other dialdehydes (e.g. glyoxal) and simple aldehydes (e.g. formaldehyde), glutaraldehyde transforms into various hydrates in aqueous solution and then into other stabilized species. Monomeric glutaraldehyde polymerizes via an aldol condensation reaction yielding α,β-unsaturated polyglutaraldehyde. This reaction typically occurs at alkaline pH.