Geothermal energy for the production of electricity is a minor but important part of many countries' current and potential energy mixes. It is also known as the most reliable renewable resource, far exceeding the reliability of weather-dependent wind, solar and hydropower. Its production environment is often very harsh. From an engineering point of view, nickel alloys and nickel-containing stainless steel are required to utilize geothermal energy. Geothermal power plants are expensive to build, but once up and running they are relatively cheap to run.
Geothermal energy is obtained by harvesting heat from the Earth's thermocore. Geothermal fluids are naturally occurring, mineral-rich mixtures of pressurized water and steam heated underground to 200–325ºC. Production wells pump steam and hot water from geothermal fields up to a depth of 3 kilometers. The high-pressure hot water in the geothermal field is separated into steam and water in the geothermal power plant, and the dry steam is used to turn the turbine of the generator to produce electricity.
Another way is to harvest geothermal energy from dry hot rocks in the ground, which have heat but no liquid.
Enhanced Geothermal System (EGS) is a set of technologies that are gradually developing and mature, which can drill the deepest hole to the extremely high temperature granite at a depth of more than 3,000 meters underground. The water injected into the borehole is heated by the rock and returned to the surface through the return well by the action of the pump for use.
An important benefit of geothermal energy is that waste fluids from geothermal production are reinjected back into the geothermal field. This process helps replenish the geothermal flow and is reheated underground. Reinjection can also prevent surface water from being polluted by geothermal brine. Geothermal brines are corrosive; they are acidic and often contain metal ions, corrosive chlorides, silicon compounds, and corrosive gases such as carbon dioxide, various sulfur oxides, and hydrogen sulfide. Corrosive compounds and gases come into contact with most geothermal power plant process equipment and piping at high temperatures, so nickel-containing materials play a very important role.
Corrosion damage is a significant problem in older geothermal power plants that use carbon steel piping and process equipment. As operating experience improves, nickel-containing stainless steels such as 304L (UNS S30403), 316L (S31603), 310S (S31008) and 321 (S32100) are increasingly used and are now becoming the workhorse alloys used in many geothermal industries.
However, some stainless steels are highly susceptible to pitting and stress corrosion cracking caused by chlorides or sulfides in geothermal brines. Therefore, corrosion-resistant alloys with higher nickel content are increasingly used.
In environments where corrosion cannot be controlled by corrosion inhibitors, duplex stainless steels 2205 (S32205), 2507 (S32750) and 2707 (S32707) and high-nickel stainless steels such as 904L (N08904) and alloys containing 6% molybdenum (such as S31254 or N08367 ) etc. perform well.
The wide variety of geothermal brines means there is no one-size-fits-all solution. Research is therefore required to determine the best performing materials that best match the treatment fluid. Depending on the operating requirements, nickel alloys such as Alloy 625 (N06625), C-276 (N10276) and higher alloys are required. Other nickel alloys such as 600 (N06600), 601 (N06601) and 825 (N08825) are optionally available for certain highly corrosive geothermal operating conditions.
Geothermal heat is a very useful and increasingly used environment-friendly resource, and it is thanks to the use of nickel alloys that this energy source can be exploited.