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Advanced geothermal steam turbines
Advanced geothermal steam turbines
19
Y. Sakai Fuji Electric Co., Ltd., Kawasaki, Japan
19.1
Introduction
19.1.1 Outline of geothermal power generation Geothermal power generation utilizes the thermal energy of the Earth instead of fossil fuels. The amount of thermal energy in the Earth is so vast that it could be said to be an inexhaustible supply for humankind. However, the energy that can be utilized is limited to the small fraction close to the surface of the Earth, which is particularly referred to as geothermal energy. Geothermal energy is “green,” because it scarcely emits any CO2 which causes global warming. The life cycle CO2 emission—including mining, plant construction, fuel transport, refining, plant operations and maintenance, as well as burning of fuel—of geothermal power generation is estimated at only 1 3% of that by conventional thermal power plants burning fossil fuels such as coal, oil, and natural gas; it is even smaller compared to other renewable energies such as solar power and wind power (Fig. 19.1; [1]).
Figure 19.1 Comparison of life cycle CO2 emissions of power sources. Advances in Steam Turbines for Modern Power Plants. DOI: http://dx.doi.org/10.1016/B978-0-08-100314-5.00019-1 Copyright © 2017 Elsevier Ltd. All rights reserved.
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Figure 19.2 Scheme of a typical geothermal power plant.
Geothermal energy is sustainable, because it is a natural energy and its resources are almost inexhaustible. Compared to other renewable energies, geothermal energy is stable, unaffected by weather or time, and its density is high. In geothermal power generation, geothermal fluid, a mixture of steam and hot water heated by subterranean heat, is extracted through production wells dug deep into the Earth (up to 3 4 km). The thermal energy of the geothermal fluid is then used to generate electricity. The used geothermal fluid is returned to the Earth via a reinjection well. Putting it simply, geothermal power generation is a power generation system that utilizes the Earth instead of a boiler. A scheme of a typical geothermal power plant is shown in Fig. 19.2. Generally, steam turbines play an important role as the prime movers to generate electricity, just like in fossil and nuclear power plants. Geothermal power generation systems are classified as given in the following subsections.
19.1.1.1 Dry steam system When the geothermal fluid from the production well is almost dry, it is directly led to the steam turbine to generate electricity (Fig. 19.3).
19.1.1.2 Flash system When the geothermal fluid from the production well is a mixture of steam and hot water, it is flashed and separated into the steam and water, and the steam is led to a steam turbine. This system is called a single-flash system (Fig. 19.4). When the
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Figure 19.3 Dry steam system.
Figure 19.4 Single-flash system.
separated water still has sufficient thermal energy, it can be flashed again and the separated low-pressure (LP) steam is led into the LP part of the steam turbine to produce additional electric power. This system is called a double-flash system (Fig. 19.5). A triple-flash system with further flashing has also been applied (Fig. 19.6).
19.1.1.3 Back-pressure system This system employs a back-pressure turbine instead of a condensing turbine; the exhaust steam is discharged to the atmosphere (Fig. 19.7). The back-pressure system is used for small capacities, up to approximately 5 MW.
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Figure 19.5 Double-flash system.
Figure 19.6 Triple-flash system.
19.1.1.4 Binary system The binary system is used for geothermal resources at lower temperatures. Geothermal fluid is led to a heat exchanger and used to vaporize a secondary fluid with a low boiling point. The secondary fluid then drives a binary turbine to generate electricity (Fig. 19.8). Organic Rankine cycle or Kalina cycle is generally applied.
19.1.1.5 Total-flow system In this system, geothermal fluid is directly led to a total-flow turbine to generate electricity, without separating steam and hot water.
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Figure 19.7 Back-pressure system.
Figure 19.8 Binary system.
19.1.1.6 Hybrid system The hybrid system is a combination of different power generation systems. An example of the hybrid system is shown in Fig. 19.9. In this case, the double-flash system and the binary system are combined to utilize the geothermal energy more efficiently.
19.1.1.7 Enhanced geothermal system At present, most geothermal power generation utilizes a geothermal fluid obtained from natural geothermal reservoirs distributed in volcanic regions. The enhanced
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Figure 19.9 Hybrid system (example).
Figure 19.10 Enhanced geothermal system (EGS).
geothermal system (EGS) is a new technology to enhance and/or create geothermal resources in hot dry rock by producing fractures in the rock and injecting highpressure (HP) cold water through an injection well into them. The water is heated up by the hot rock and can be utilized as artificial geothermal fluid to generate electricity (Fig. 19.10). EGS power plants are operated in Germany and Australia. EGS may be feasible anywhere in the world, offering the possibility to extend geothermal power generation.
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Figure 19.11 Magma power generation.
19.1.1.8 Magma power generation Magma power generation is a future technology that utilizes the high-temperature energy of magma to generate electricity (Fig. 19.11). Its resources are estimated to be huge. In Iceland, the well dug under the Iceland Deep Drilling Project unexpectedly reached magma at 2.1 km depth, and the world-first tests were successfully carried out from 2010 to 2012 to generate high-temperature steam at B450 C by injecting cold water into the well [2]. The installed capacity of each power system as of 2015 is shown in Fig. 19.12 [3].
19.1.2 Brief history of geothermal power generation The first geothermal power generation was demonstrated by Prince Piero Ginori Conti on July 4th, 1904 at Larderello, Italy. He succeeded in lighting four bulbs by geothermal power. In 1913, the world’s first 250 kW commercial geothermal power plant began operation at Larderello [4]. Geothermal power generation has been extended over the years, and Larderello still remains today as the center of geothermal power generation in Italy. Encouraged by the success at Larderello, geothermal power generation began to be developed in several countries. In the US, private 250-kW geothermal power generation began in 1921 at The Geysers, California [5]. In 1960, the first 12.5-MW commercial geothermal power plant began operation at The Geysers, where the world’s largest geothermal power capacity is installed today. In Japan, an experimental geothermal power generation of 1.12 kW was demonstrated in 1925 at Beppu. In 1960, a private 30-kW power plant began operation at Hakone (Fig. 19.13), and in 1965, the first 20-MW commercial geothermal power plant was put into operation at Matsukawa. In New Zealand, the world’s second 6.5-MW geothermal power plant,
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Figure 19.12 Worldwide installed capacity of geothermal power generation systems as of 2015.
Figure 19.13 30-kW 3000-rpm geothermal steam turbine for Hakone Kowakien.
which employed the world’s first flash system, began operation at Wairakei in 1958. In Mexico, the first geothermal power generation began in 1959. In Russia (the former Soviet Union), the world’s first 5-MW binary system geothermal power plant was demonstrated at Pauzhetka, Kamchatka, in 1966.
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Figure 19.14 Trend in worldwide installed capacity of geothermal power plants.
Figure 19.15 Installed capacity of geothermal power plants by country (as of 2015).
Since then, geothermal power plants have been built in many countries (Fig. 19.14). As of 2015, geothermal power plants are operated in 26 countries with a total installed capacity of 12.6 GW (Fig. 19.15). Geothermal power occupies only 0.2% of the total power-generation capacity in the world. In some countries, however, geothermal power plays an important role in power supply. For example,
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around 30% of the electric power is produced by geothermal power plants in Iceland and Kenya. From the viewpoints of global warming and sustainability, the importance of the geothermal power generation is recognized more and more seriously, urging the construction of new geothermal power plants in many countries.
19.2
Construction of modern geothermal steam turbines
19.2.1 Features of geothermal steam turbines The inlet steam of geothermal steam turbines for dry steam and flash systems is low in pressure and temperature, ranging approximately from 0.5 to 2.5 MPa and from 150 to 250 C, respectively. Therefore, geothermal steam turbines resemble the LP turbines for fossil power plants in the basic construction. Because the available heat drop is relatively small, the capacity of geothermal steam turbines is smaller, compared to steam turbines for fossil power plants. At present, the largest single-casing geothermal steam turbine has a capacity of around 150 MW. Most geothermal steam turbines range between 20 and 60 MW. The capacity of geothermal steam turbines is restricted mainly by the annular area of the last stage and the number of exhaust flows. For smaller capacity, a single-flow steam turbine is used, while double-flow steam turbines are used for larger capacity. If necessary, tandem-compound steam turbines composed of multiple casings are used, in order to increase the capacity, or to match the requirement of the power plant. Because geothermal steam turbines are driven by geothermal fluid containing various impurities and noncondensable gases (NCG), there are technical challenges for the design of geothermal steam turbines, as listed below: G
G
G
G
G
corrosion problems due to corrosive impurities and gases contained in the geothermal fluid; scaling problems due to impurities contained in the geothermal fluid; erosion problems due to wet steam; solid particle erosion and foreign object damage due to solids contained in the geothermal fluid; and performance degradation due to NCG.
19.2.2 Types of geothermal steam turbines Generally, multistage steam turbines of reaction type (Fig. 19.16) or impulse type (Fig. 19.17; [6]) are used. The reaction-type steam turbine has a drum-type rotor, and stationary blades are either installed in a stationary blade holder, or welded in a stationary blade ring. In the case of single-flow reaction turbines, a balance piston is equipped to cancel the thrust force produced by the pressure drop in the moving blades (Fig. 19.20). The impulse-type steam turbine has a disk-type rotor, to minimize leakage losses of the nozzles (stationary blades) produced by the large pressure drop in the nozzle. Nozzles are welded or cast in a nozzle diaphragm. Single-stage impulse steam turbines are also used for smaller capacity (Fig. 19.18). The moving blades are installed in a disk. The nozzles are installed in a nozzle holder (not shown in Fig. 19.18) or directly in a casing, generally with a partial-arc admission.
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Figure 19.16 110-MW 3000-rpm reaction-type geothermal steam turbine.
Figure 19.17 75-MW impulse-type geothermal steam turbine. Source: Courtesy of Toshiba Corporation.
Condensing steam turbines are used in most geothermal power plants. Backpressure steam turbines are also used for small capacity. For double-flash and triple-flash cycles, mixed-pressure steam turbines are used, and the steam generated in the flasher is mixed into the intermediate-pressure (IP) part and/or LP part of the steam turbine to generate more power. The mixed-pressure steam turbine shown in Fig. 19.19 consists of a single-flow HP part for smaller steam flow and a doubleflow LP part for larger steam flow, in order to optimize flow paths. The exhaust flow from the steam turbine is discharged downwards, upwards, or axially. When the condenser is located under the steam turbine, downward exhaust
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Figure 19.18 100-kW 3600-rpm single-stage geothermal steam turbine.
Figure 19.19 96-MW 3000-rpm mixed-pressure geothermal steam turbine for double-flash system.
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Figure 19.20 30-MW 3600-rpm geothermal steam turbine with upward exhaust.
is employed (Fig. 19.17). When the condenser is located separately, the exhaust flow is discharged upwards and led to the condenser through a duct (Fig. 19.20), or it is discharged axially and directly led to the condenser (Fig. 19.21). The axial exhaust has the advantage that it can reduce pressure loss caused by turning of the exhaust flow. Geothermal steam turbines are directly connected to the generator. For smaller capacity, high-speed steam turbines with reduction gears may be used (Fig. 19.22). Skid-mounted (package type) steam turbines are also used for smaller capacity, in order to shorten the construction period on site (Fig. 19.22).
19.2.3 Components and materials of geothermal steam turbines The appearance of a geothermal steam turbine is shown in Fig. 19.23 [7]. Like a steam turbine for fossil power plants, geothermal steam turbines consist of upper and lower casings, a rotor, stationary blades, moving blades, glands, stop valves, control valves, a control and protection system, lubricating system, etc.
19.2.3.1 Casing Single-shell casing is used, because the steam pressure and temperature are low. Carbon steels are generally used as casing materials. If necessary, stainless steel, overlay welding or coating is applied to the parts such as glands where corrosion
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Figure 19.21 20-MW 3600-rpm geothermal steam turbine with axial exhaust.
Figure 19.22 3.3-MW 7266-rpm geothermal steam turbine with reduction gear.
tends to occur. Borescope holes may be prepared in the casing, in order to check the condition of the blades without disassembly of the casing.
19.2.3.2 Rotor A drum-type rotor is employed for reaction turbines, while a disk-type rotor is employed for impulse turbines. A balance piston is equipped in single-flow reaction
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Figure 19.23 81.3-MW (maximum 100.7 MW) 3000-rpm geothermal steam turbine. Source: Courtesy of Mitsubishi Hitachi Power Systems.
turbines to cancel the thrust force. Because of the low steam temperature, 1 2% Cr steel rotors are generally used. Overlay welding or coating may be applied in order to enhance corrosion resistance. 12% Cr rotors are also applied.
19.2.3.3 Blades Stationary blades are installed or integrated in removable stationary blade holders, diaphragms, or stationary blade rings, in order to facilitate cleaning during maintenance. Moving blades are installed in the rotor or disks. Smaller blades are generally shrouded, while longer blades such as LP blades may be free-standing. Seal fins are installed to reduce leakage flows. Generally, geothermal steam turbines are not equipped with a control stage, because they are normally operated at constant load. The blade materials, 12 13% Cr steel, 17 4PH, Ti-6Al-4V, etc., are used, depending on the environmental and operational conditions.
19.2.3.4 Valves Stop valves and control valves of geothermal steam turbines have large diameters, because of large volumetric steam flow. Therefore, check valves are used as stop valves, while butterfly valves are used as control valves (Fig. 19.24). Recently, the HP control oil system has been applied to improve operability of valve actuators and to make the control equipment compact.
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Figure 19.24 Butterfly control valve.
Figure 19.25 Bird’s-eye view of Nga Awa Purua (NAP) geothermal power plant, New Zealand.
19.2.4 Design characteristics of the latest geothermal turbines The geothermal steam turbine for Nga Awa Purua (NAP) geothermal power plant (Fig. 19.25) in New Zealand that began operation in 2008 is the world’s largest single-casing geothermal steam turbine at present, with a capacity of 139 MW (max. 147 MW). The 798 mm (31.4 inch) last stage blades—the world’s largestclass blades for geothermal steam turbines—are employed. The inlet steam
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Specifications of geothermal steam turbine for Nga Awa Purua
Table 19.1
Plant cycle
Triple-flash system
Output First operation Rotating speed High-pressure steam Intermediate-pressure steam Low-pressure steam Exhaust pressure Last-stage blade length
139 MW (max. 147 MW) 2008 3000 rpm 2.35 MPa/221 C 840 kPa/172 C 23 kPa/125 C 8.5 kPa 798 mm (31.4 in.)
Figure 19.26 139-MW (maximum 147 MW) 3000-rpm geothermal steam turbine for tripleflash system.
pressure is 2.35 MPa, which is higher than conventional. A triple-flash system is employed to utilize the geothermal energy most effectively. The water after separation of the steam is flashed in two stages, and the generated steam is led to the IP and LP parts of the turbine, to maximize the output. The design parameters of NAP geothermal power plant are shown in Table 19.1. A turbine sectional drawing is shown in Fig. 19.26. The steam turbine has five inlets: two for HP steam, two for IP steam and one for LP steam; and they are compactly arranged. The outlook of the steam turbine is shown in Fig. 19.27. Fig. 19.28 shows the steam turbine under construction [8].
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Figure 19.27 Outlook of the world’s largest single-casing geothermal steam turbine for Nga Awa Purua (NAP).
Figure 19.28 Geothermal steam turbine for Nga Awa Purua (NAP) during installation.
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19.3
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Technologies to enhance reliability of geothermal steam turbines
19.3.1 Corrosion problems and solutions 19.3.1.1 Evaluation of corrosion resistance of materials Geothermal steam is generally accompanied by several mass percent of NCG and dozens of parts per million (ppm) of dissolved impurities. Generally, more than 90% of the NCG is CO2, and the rest is composed of hydrogen sulfide (H2S), methane (CH4), ammonia (NH3), etc. The impurities dissolved in the steam contain corrosive substances such as chlorides (Cl2), sulfates (SO422), etc. (Table 19.2). It is therefore essential to evaluate the corrosion resistance of the materials and the applicable stress level when designing geothermal turbines. For this purpose, systematic experimental corrosion tests in the simulated geothermal environment have been carried out to accumulate material data related to the corrosion resistance. Since the quality of the geothermal fluid varies from site to site, corrosion tests exposed to the actual geothermal fluid in the fields are also very important to evaluate the corrosion resistance of the materials. Such corrosion tests have been carried out at various geothermal sites for the reliable design of geothermal steam turbines (Fig. 19.29) [9].
19.3.1.2 Measures against stress corrosion cracking and corrosion fatigue In the design of geothermal steam turbines, particular problems arise with the turbine blades root and rotor grooves, which are exposed to high centrifugal force and steam pressure during operation. A technology was developed to improve the corrosion resistance by performing shot-peening on the parts of the blade root and grooves where the stress is concentrated (Fig. 19.30). In the results of comparative testing performed in a simulated geothermal environment, it was verified that the shot-peening greatly improved the strength of the components against stress corrosion cracking (SCC) and corrosion fatigue (CF) (Fig. 19.31).
19.3.1.3 Measures against erosion-corrosion In recent years, progress in exploration techniques and drilling technology for geothermal wells has resulted in the development of geothermal resources at comparatively greater depths. Accordingly, the steam pressure at the inlet of the geothermal turbine has tended to be higher, rising from the conventional value of approximately 1 MPa to a value of approximately 2 MPa and more. With wet steam turbines, increased steam pressure at the inlet results in a greater tendency for erosion-corrosion to occur. Erosion-corrosion is a thinning of material surface due to the combined effects of chemical and mechanical action of the steam flow. Because stationary blade holders and rotors are made of carbon steel or low-alloy steel, their surfaces are prone to erosion-corrosion. As a measure against erosioncorrosion, a rotor with 2% Cr steel was developed, which had greater resistance to
Table 19.2
Chemical analysis of geothermal steam at turbine inlet (examples)
Plant
A
B
C
D
E
F
G
Plant system
SF
DF
DF
SF
SF
Dry steam
DF
pH
5.5
5.0 7.8
5.85
6
5.65
7.4
3 4
1
3.2
3.10
2.8
0.082
1
0.25
0.22 0.07 0.84 0.38
2.8 18 0.4 86
0.1 0.1
0.02
1
0.6
0.07 2.15 1.65
0.03 21 12 2
0.2 96 4
3 97 3
Impurities in steam
NCG
Cl
ppm
Na K Ca SiO2 Fe Mg NH3 SO4 B TDS
ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
Total CO2 H2S H2 CH4 N2 NH3 Others
wt% Composition in wt%
DF, double-flash; NCG, noncondensable gases; SF, single-flash; TDS, total dissolved solids.
1 1
0.6 0.05
0.003 0.163
6.1 6 0.1
5 1.15 95.2 1.09 0.04 0.21 3.16 0.03 0.27
2.2 99.0 0.90 0.01 0.02 0.02 0.02 0.04
0.19
1 89.5 2.8 0.7 5.1 1.2 0.7
0.4 84.6 12.7 1.1 0.2 1.4 0.1
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Figure 19.29 Corrosion test rig on site.
Figure 19.30 Shot-peening applied to blade root grooves.
erosion-corrosion than the 1% chromium steel conventionally used as a rotor material. Technologies have also been developed to coat the surface of rotors with a WC-CoCr material using high-velocity oxy-fuel spraying (Fig. 19.32 [10]).
19.3.2 Measures against water-droplet erosion Erosion caused by the impact of water droplets (i.e., drain attack) is the same phenomenon as in steam turbines for fossil power plants. In geothermal power plants
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Figure 19.31 Stress corrosion cracking test results.
Figure 19.32 High-velocity oxy-fuel coating applied to rotor surface.
employing a flash cycle, all stages are operated in wet steam. Therefore, it is necessary to consider drain attack erosion in the design process. For this purpose, drain pockets are provided to remove the water droplets that cause the erosion. Additionally, protective measures are taken with the brazing of an erosion shield onto the leading edge of the moving blades of the last stages (Fig. 19.33).
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Figure 19.33 Measures against water- droplet erosion.
19.3.3 Measures against scale problems Silica, calcium carbonate, and other substances contained in geothermal steam are deposited on the surface of components such as the blades, casing, and rotor. These impurities accumulate and build scales. The scale that builds up on the surface of the blades narrows the passage for the steam and causes a fall in output. Furthermore, the scale that builds up in the gaps between the rotating parts and the stationary parts may cause abrasion of the components. As a countermeasure against the scale problem, blade-washing technology with water droplets sprayed at the inlet of the turbine may be employed.
19.4
Technologies to enhance performance of geothermal turbines
19.4.1 New-generation low-pressure blades for geothermal steam turbines The LP blades of the last stages are important turbine components that determine the size and efficiency of geothermal steam turbines. Since development of the large LP blades requires enormous time and expense, they are standardized in series, and the optimal LP blade group is selected in accordance with the volumetric exhaust flow of the steam turbine. In order to increase the turbine capacity, longer
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Figure 19.34 Computational fluid dynamics analysis of Mach number distribution in the low-pressure blades.
last-stage LP blades with larger exhaust area are required. Conversely, longer LP blades entail increased centrifugal and steam bending forces, which are restricted by the strength of the material. Utilization of large LP blades for geothermal turbines has been suppressed because of the severely corrosive environments in which the geothermal turbines are operated. The new-generation LP turbine blades for geothermal steam turbines were developed based on the wealth of experience in geothermal turbine operation. As geothermal turbines are operated in a very corrosive environment, special consideration was given in their design to suppress stress levels within the allowable limit for SCC and CF. On the other hand, the new-generation LP blades are loaded higher, in order to enable downsizing of the steam turbine. Stress distribution was optimized by means of finite element method (FEM) analysis. State-of-the-art computational fluid dynamics (CFD) was employed to develop the new-generation LP blades, in order to achieve the highest efficiency by optimizing pressure and Mach number distributions around the blade profile (Fig. 19.34). Rotating vibration tests on the actual LP blades were performed to secure blade reliability during operation (Fig. 19.35).
19.4.2 High-load, high-efficiency reaction blades As the inlet pressure of the geothermal steam turbine tends to increase, the blades are required to work efficiently with higher load, i.e., with larger pressure drop. Generally, the higher the blades are loaded, the more the blade efficiency decreases. However, new reaction blades realizing both high load and high efficiency were successfully developed for the stages other than the LP blade stages, using the latest design techniques to minimize the profile loss and to suppress the secondary flow
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Figure 19.35 Rotating vibration test of the last stage blades.
loss. By adopting the high-load, high-efficiency reaction blades, the stage efficiency was improved by 1 2% while increasing the load per stage. The blades have an integral shroud, which is machined from a single bar material together with the blade airfoil and blade root. The integral shrouds of the adjacent blades contact tightly with each other during operation, giving a damping effect to the blade vibration, securing high operational reliability against the corrosive geothermal steam (Fig. 19.36).
19.4.3 High-performance, compact exhaust casing The exhaust casing decelerates the steam discharged from the last stage blades and forms the flow passage leading to the condenser. The steam discharged from the last stage has a high velocity that cannot be converted to rotational energy of the rotor. Furthermore, the total pressure loss in the exhaust casing reduces the effective heat drop of the turbine stages, resulting in reduced turbine efficiency. However, if the velocity of the discharged steam is effectively reduced in the diffuser composing a part of the exhaust casing, a pressure recovery occurs in the diffuser, which compensates or even surpasses the pressure loss in the exhaust casing. In general, if the exhaust casing is made more compact, then the reduction in the passage cross-section increases the velocity of the steam flow, which is disadvantageous for the performance. In order to develop a more efficient and compact exhaust casing, the shapes of the exhaust casing and the diffuser were optimized by means of three-dimensional viscous flow analysis, resulting in a
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Figure 19.36 High-load, high-efficiency reaction blades: Moving blades (A) and stationary blades (B).
diffuser shape that recovers pressure more effectively and an exhaust casing with less pressure loss (Fig. 19.37).
19.5
Operational experiences and lessons learned
Since geothermal fluid contains a large amount of corrosive impurities such as chlorides, hydrogen sulfides, carbonic acids, etc., problems related to the corrosion—e.g., general corrosion, pitting corrosion, erosion-corrosion, SCC, and CF, etc.—are crucial for both plant operators and equipment manufacturers. The problems experienced and proposed countermeasures are described below.
19.5.1 Erosion Since geothermal turbines are generally operated with wet steam, erosion of the blades is one of the problems that occasionally occurs. Long LP blades of geothermal steam turbines are generally provided with a stellite erosion shield brazed on the leading edge of the blades. At the overhaul after approximately 8 years of operation, it was found that the erosion shields had partly peeled off, and severe erosion had occurred (Fig. 19.38). This took place in a large-capacity geothermal turbine, in which drainage was insufficient.
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Figure 19.37 Computational fluid dynamics analysis of exhaust casing.
Figure 19.38 Erosion shield peeled off after eight years of operation.
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Figure 19.39 Erosion-corrosionobserved at rotor steeples (A), Governor side and generator side (B).
Modern geothermal turbines are provided with drain catchers to efficiently remove the large droplets that cause severe erosion to the moving blades. By employing drain catchers, along with the improved brazing process, the problem of blade erosion has been mitigated.
19.5.2 Erosion-corrosion Erosion-corrosion is a phenomenon whereby the metal surface is continuously washed away by corrosive, high-velocity wet steam due to the interaction of chemical reaction and mechanical erosion. The rate of erosion-corrosion is influenced by various parameters such as steam temperature, wetness, velocity, pH value, corrosive elements included in the steam, geometry of flow passage, etc. Fig. 19.39 shows erosion-corrosion observed at the inlet of the blade attachment area of the rotor after 10 years of operation. In this turbine, the governor-side blades and generator-side blades had been operated under different steam conditions and purities. Consequently, the rates of the erosion-corrosion differed greatly on the governor side and generator side. Erosion-corrosion is more or less inevitable in geothermal turbines. If a sufficient margin for the strength of the eroded part remains, the turbine can be further operated without any remedy. Fig. 19.40 shows erosion-corrosion observed in the stationary blade ring after 10 years of operation. The stationary blades made of stainless steel were welded to the blade ring made of carbon steel. The blade ring was eroded under the weld, while the weld and the stationary blades were not eroded. In this case, it is recommended to use stainless steel for the parts of the blade ring on which the stationary blades are welded.
19.5.3 Stress corrosion cracking and corrosion fatigue SCC is a phenomenon whereby cracks occur and propagate under static (tensile) stress in a corrosive environment, while CF is a phenomenon whereby cracks occur and propagate under alternating stress in a corrosive environment. As a matter of
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Figure 19.40 Erosion-corrosion observed at stationary blade ring.
Figure 19.41 Crack found in blade root.
fact, static and alternating stresses act simultaneously on the turbine parts such as the moving blades and rotor. Therefore, it is not always easy to distinguish clearly between SCC and CF. In most cases, the fracture surface is severely corroded, which makes discrimination of both crack modes more difficult. It is known that SCC and CF tend to occur in the ‘dry wet alternating region’ where steam expands across the Wilson line (i.e., steam condition of 4 5% wetness). According to an accepted opinion, it is because the droplets dissolving impurities repeatedly adhere to and evaporate on the metal surface, in accordance with turbine start-stop and load change, thus leaving condensed impurities on the metal surface. Fig. 19.41 shows a fracture surface of a crack found in a blade root. The fracture surface was covered with a brown deposit. The deposit was analyzed using energy
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Figure 19.42 Various phases of pitting and microcracks observed in blade grooves.
dispersive X-ray spectrometry, and as a result, it was found that S, O, and Fe were major elements of the deposit. Crystal structures of the deposit were investigated using an X-ray diffractometer: it turned out that the deposit consisted mainly of iron sulfide. The rotor steeples of the same stage showed various phases of the pitting or microcracking (Fig. 19.42). As a result of various investigations, it was concluded that the crack was initiated and propagated due to sulfide stress corrosion, and during a long period of operation, CF took over. Geothermal fluid that comes out of the ground contains various impurities. Geothermal steam is separated in the flashers and separators from the hot water, and led into the steam turbine. Normally, most of the impurities are removed together with water in this process. The demisters also serve to reduce the carryover of the corrosive mist into the turbine. If the function of the separators, flashers, or demisters is not sufficient, however, corrosive mists are carried over into the turbine, and may cause SCC or CF. Therefore, the function of the separators, flashers, and demisters is very important to protect the turbine blades from corrosion.
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It should also be noted that frequent start-stop and load change, as well as vacuum braking, may exacerbate the corrosive environment in geothermal turbines. Periodic maintenance also plays a very important role in efficient and stable operation of the geothermal power plant [11].
19.6
Future view of geothermal power generation and challenges
With the extended utilization of geothermal resources in the future, technologies required for geothermal steam turbines will be more and more diversified and challenging. The major development subjects are described below. G
G
G
G
Application to deeper geothermal resources. As deeper geothermal resources are developed, the pressure and temperature of geothermal steam will be increasingly greater, and the impurities contained in the geothermal fluid will diversify in their composition and quantity. Therefore, development of geothermal steam turbines and plant equipment along with materials that can accommodate such conditions will be necessary. Development of larger-capacity geothermal steam turbines. In order to increase the capacity of geothermal steam turbines, it is necessary to develop larger LP blades, taking into account the allowable stress in the corrosive geothermal environment. Inlet valves with larger diameters should also be developed. Effective utilization of geothermal resources. Technologies should be developed to utilize geothermal resources more effectively; e.g., binary power generation that uses low-temperature geothermal resources that have not hitherto been used, and hybrid power generation combining a flash system and a binary system that utilizes the water before returning to the Earth, etc. Further improvement of reliability and efficiency of geothermal steam turbines. As geothermal fluid contains a large amount of impurities, corrosion and scale deposition are the most challenging problems for geothermal steam turbines. The stability of geothermal power generation has increased drastically owing to development until now. Further improvement of the reliability along with the efficiency improvement by continuous development are essential to promote geothermal power generation for sustainable human development.
References [1] Federation of Electric Power Companies of Japan, Graphical Flip-Chart of Nuclear & Energy Related Topics 2014, p. 2-1-9. Available from: ,http://www.fepc.or.jp/english/ library/graphical_flip-chart/., 2014. (accessed on 1.10.15). [2] G.O. Fridleifsson, W.A. Elders, G. Bignall, A plan for a 5km-deep borehole at Reykjanes, Iceland, into the root zone of a black smoker on land, Sci. Drill. 16 (2013) 73 79. [3] R. Bertani, Geothermal power generation in the world 2010 2014 update report, in: Proc. World Geothermal Congress 2015.
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[4] M.H. Dickson, M. Fanelli, What is Geothermal Energy? International Geothermal Association, 2004. Available from: ,http://www.geothermal-energy.org/what_is_ geothermal_energy.html. (accessed on 1.10.15) [5] Calpine Corporation, The Geysers: A Very Special Place, 2015. Available from: ,http://www.geysers.com/history.aspx. (accessed on 1.10.15). [6] J. Tajiri, Latest technologies contributing to realization of large- to small-capacity geothermal power plants, Toshiba Rev. 68 (6) (2013) 24 27. [7] S. Saito, T. Suzuki, J. Ishiguro, T. Suzuki, Development of large capacity singlecylinder geothermal turbine, Geoth. Resour. Counc. Trans. 22 (1998) 399 403. [8] T. Horie, Kawerau and Nga Awa Purua geothermal power station projects, New Zealand, Fuji Electr. Rev. 55 (3) (2009) 80 86. [9] Y. Sakai, M. Yamashita, K. Shiokawa, L.B. Niu, H. Takaku, Corrosion resistance of materials for geothermal steam turbines, Proc. Int. Conf. Power Eng.-03 3 (2003) 297 302. [10] Y. Sakai, Y. Oka, H. Kato, The latest geothermal steam turbines, Fuji Electr. Rev. 55 (3) (2009) 87 92. [11] Y. Sakai, M. Yamashita, M. Sakata, Geothermal steam turbines with high efficiency, high reliability blades, Geoth. Resour. Counc. Trans. 24 (2000) 521 526.
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Y. Sakai Fuji Electric Co., Ltd., Kawasaki, Japan
19.1
Introduction
19.1.1 Outline of geothermal power generation Geothermal power generation utilizes the thermal energy of the Earth instead of fossil fuels. The amount of thermal energy in the Earth is so vast that it could be said to be an inexhaustible supply for humankind. However, the energy that can be utilized is limited to the small fraction close to the surface of the Earth, which is particularly referred to as geothermal energy. Geothermal energy is “green,” because it scarcely emits any CO2 which causes global warming. The life cycle CO2 emission—including mining, plant construction, fuel transport, refining, plant operations and maintenance, as well as burning of fuel—of geothermal power generation is estimated at only 1 3% of that by conventional thermal power plants burning fossil fuels such as coal, oil, and natural gas; it is even smaller compared to other renewable energies such as solar power and wind power (Fig. 19.1; [1]).
Figure 19.1 Comparison of life cycle CO2 emissions of power sources. Advances in Steam Turbines for Modern Power Plants. DOI: http://dx.doi.org/10.1016/B978-0-08-100314-5.00019-1 Copyright © 2017 Elsevier Ltd. All rights reserved.
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Figure 19.2 Scheme of a typical geothermal power plant.
Geothermal energy is sustainable, because it is a natural energy and its resources are almost inexhaustible. Compared to other renewable energies, geothermal energy is stable, unaffected by weather or time, and its density is high. In geothermal power generation, geothermal fluid, a mixture of steam and hot water heated by subterranean heat, is extracted through production wells dug deep into the Earth (up to 3 4 km). The thermal energy of the geothermal fluid is then used to generate electricity. The used geothermal fluid is returned to the Earth via a reinjection well. Putting it simply, geothermal power generation is a power generation system that utilizes the Earth instead of a boiler. A scheme of a typical geothermal power plant is shown in Fig. 19.2. Generally, steam turbines play an important role as the prime movers to generate electricity, just like in fossil and nuclear power plants. Geothermal power generation systems are classified as given in the following subsections.
19.1.1.1 Dry steam system When the geothermal fluid from the production well is almost dry, it is directly led to the steam turbine to generate electricity (Fig. 19.3).
19.1.1.2 Flash system When the geothermal fluid from the production well is a mixture of steam and hot water, it is flashed and separated into the steam and water, and the steam is led to a steam turbine. This system is called a single-flash system (Fig. 19.4). When the
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Figure 19.3 Dry steam system.
Figure 19.4 Single-flash system.
separated water still has sufficient thermal energy, it can be flashed again and the separated low-pressure (LP) steam is led into the LP part of the steam turbine to produce additional electric power. This system is called a double-flash system (Fig. 19.5). A triple-flash system with further flashing has also been applied (Fig. 19.6).
19.1.1.3 Back-pressure system This system employs a back-pressure turbine instead of a condensing turbine; the exhaust steam is discharged to the atmosphere (Fig. 19.7). The back-pressure system is used for small capacities, up to approximately 5 MW.
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Figure 19.5 Double-flash system.
Figure 19.6 Triple-flash system.
19.1.1.4 Binary system The binary system is used for geothermal resources at lower temperatures. Geothermal fluid is led to a heat exchanger and used to vaporize a secondary fluid with a low boiling point. The secondary fluid then drives a binary turbine to generate electricity (Fig. 19.8). Organic Rankine cycle or Kalina cycle is generally applied.
19.1.1.5 Total-flow system In this system, geothermal fluid is directly led to a total-flow turbine to generate electricity, without separating steam and hot water.
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Figure 19.7 Back-pressure system.
Figure 19.8 Binary system.
19.1.1.6 Hybrid system The hybrid system is a combination of different power generation systems. An example of the hybrid system is shown in Fig. 19.9. In this case, the double-flash system and the binary system are combined to utilize the geothermal energy more efficiently.
19.1.1.7 Enhanced geothermal system At present, most geothermal power generation utilizes a geothermal fluid obtained from natural geothermal reservoirs distributed in volcanic regions. The enhanced
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Figure 19.9 Hybrid system (example).
Figure 19.10 Enhanced geothermal system (EGS).
geothermal system (EGS) is a new technology to enhance and/or create geothermal resources in hot dry rock by producing fractures in the rock and injecting highpressure (HP) cold water through an injection well into them. The water is heated up by the hot rock and can be utilized as artificial geothermal fluid to generate electricity (Fig. 19.10). EGS power plants are operated in Germany and Australia. EGS may be feasible anywhere in the world, offering the possibility to extend geothermal power generation.
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Figure 19.11 Magma power generation.
19.1.1.8 Magma power generation Magma power generation is a future technology that utilizes the high-temperature energy of magma to generate electricity (Fig. 19.11). Its resources are estimated to be huge. In Iceland, the well dug under the Iceland Deep Drilling Project unexpectedly reached magma at 2.1 km depth, and the world-first tests were successfully carried out from 2010 to 2012 to generate high-temperature steam at B450 C by injecting cold water into the well [2]. The installed capacity of each power system as of 2015 is shown in Fig. 19.12 [3].
19.1.2 Brief history of geothermal power generation The first geothermal power generation was demonstrated by Prince Piero Ginori Conti on July 4th, 1904 at Larderello, Italy. He succeeded in lighting four bulbs by geothermal power. In 1913, the world’s first 250 kW commercial geothermal power plant began operation at Larderello [4]. Geothermal power generation has been extended over the years, and Larderello still remains today as the center of geothermal power generation in Italy. Encouraged by the success at Larderello, geothermal power generation began to be developed in several countries. In the US, private 250-kW geothermal power generation began in 1921 at The Geysers, California [5]. In 1960, the first 12.5-MW commercial geothermal power plant began operation at The Geysers, where the world’s largest geothermal power capacity is installed today. In Japan, an experimental geothermal power generation of 1.12 kW was demonstrated in 1925 at Beppu. In 1960, a private 30-kW power plant began operation at Hakone (Fig. 19.13), and in 1965, the first 20-MW commercial geothermal power plant was put into operation at Matsukawa. In New Zealand, the world’s second 6.5-MW geothermal power plant,
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Figure 19.12 Worldwide installed capacity of geothermal power generation systems as of 2015.
Figure 19.13 30-kW 3000-rpm geothermal steam turbine for Hakone Kowakien.
which employed the world’s first flash system, began operation at Wairakei in 1958. In Mexico, the first geothermal power generation began in 1959. In Russia (the former Soviet Union), the world’s first 5-MW binary system geothermal power plant was demonstrated at Pauzhetka, Kamchatka, in 1966.
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Figure 19.14 Trend in worldwide installed capacity of geothermal power plants.
Figure 19.15 Installed capacity of geothermal power plants by country (as of 2015).
Since then, geothermal power plants have been built in many countries (Fig. 19.14). As of 2015, geothermal power plants are operated in 26 countries with a total installed capacity of 12.6 GW (Fig. 19.15). Geothermal power occupies only 0.2% of the total power-generation capacity in the world. In some countries, however, geothermal power plays an important role in power supply. For example,
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around 30% of the electric power is produced by geothermal power plants in Iceland and Kenya. From the viewpoints of global warming and sustainability, the importance of the geothermal power generation is recognized more and more seriously, urging the construction of new geothermal power plants in many countries.
19.2
Construction of modern geothermal steam turbines
19.2.1 Features of geothermal steam turbines The inlet steam of geothermal steam turbines for dry steam and flash systems is low in pressure and temperature, ranging approximately from 0.5 to 2.5 MPa and from 150 to 250 C, respectively. Therefore, geothermal steam turbines resemble the LP turbines for fossil power plants in the basic construction. Because the available heat drop is relatively small, the capacity of geothermal steam turbines is smaller, compared to steam turbines for fossil power plants. At present, the largest single-casing geothermal steam turbine has a capacity of around 150 MW. Most geothermal steam turbines range between 20 and 60 MW. The capacity of geothermal steam turbines is restricted mainly by the annular area of the last stage and the number of exhaust flows. For smaller capacity, a single-flow steam turbine is used, while double-flow steam turbines are used for larger capacity. If necessary, tandem-compound steam turbines composed of multiple casings are used, in order to increase the capacity, or to match the requirement of the power plant. Because geothermal steam turbines are driven by geothermal fluid containing various impurities and noncondensable gases (NCG), there are technical challenges for the design of geothermal steam turbines, as listed below: G
G
G
G
G
corrosion problems due to corrosive impurities and gases contained in the geothermal fluid; scaling problems due to impurities contained in the geothermal fluid; erosion problems due to wet steam; solid particle erosion and foreign object damage due to solids contained in the geothermal fluid; and performance degradation due to NCG.
19.2.2 Types of geothermal steam turbines Generally, multistage steam turbines of reaction type (Fig. 19.16) or impulse type (Fig. 19.17; [6]) are used. The reaction-type steam turbine has a drum-type rotor, and stationary blades are either installed in a stationary blade holder, or welded in a stationary blade ring. In the case of single-flow reaction turbines, a balance piston is equipped to cancel the thrust force produced by the pressure drop in the moving blades (Fig. 19.20). The impulse-type steam turbine has a disk-type rotor, to minimize leakage losses of the nozzles (stationary blades) produced by the large pressure drop in the nozzle. Nozzles are welded or cast in a nozzle diaphragm. Single-stage impulse steam turbines are also used for smaller capacity (Fig. 19.18). The moving blades are installed in a disk. The nozzles are installed in a nozzle holder (not shown in Fig. 19.18) or directly in a casing, generally with a partial-arc admission.
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Figure 19.16 110-MW 3000-rpm reaction-type geothermal steam turbine.
Figure 19.17 75-MW impulse-type geothermal steam turbine. Source: Courtesy of Toshiba Corporation.
Condensing steam turbines are used in most geothermal power plants. Backpressure steam turbines are also used for small capacity. For double-flash and triple-flash cycles, mixed-pressure steam turbines are used, and the steam generated in the flasher is mixed into the intermediate-pressure (IP) part and/or LP part of the steam turbine to generate more power. The mixed-pressure steam turbine shown in Fig. 19.19 consists of a single-flow HP part for smaller steam flow and a doubleflow LP part for larger steam flow, in order to optimize flow paths. The exhaust flow from the steam turbine is discharged downwards, upwards, or axially. When the condenser is located under the steam turbine, downward exhaust
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Figure 19.18 100-kW 3600-rpm single-stage geothermal steam turbine.
Figure 19.19 96-MW 3000-rpm mixed-pressure geothermal steam turbine for double-flash system.
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Figure 19.20 30-MW 3600-rpm geothermal steam turbine with upward exhaust.
is employed (Fig. 19.17). When the condenser is located separately, the exhaust flow is discharged upwards and led to the condenser through a duct (Fig. 19.20), or it is discharged axially and directly led to the condenser (Fig. 19.21). The axial exhaust has the advantage that it can reduce pressure loss caused by turning of the exhaust flow. Geothermal steam turbines are directly connected to the generator. For smaller capacity, high-speed steam turbines with reduction gears may be used (Fig. 19.22). Skid-mounted (package type) steam turbines are also used for smaller capacity, in order to shorten the construction period on site (Fig. 19.22).
19.2.3 Components and materials of geothermal steam turbines The appearance of a geothermal steam turbine is shown in Fig. 19.23 [7]. Like a steam turbine for fossil power plants, geothermal steam turbines consist of upper and lower casings, a rotor, stationary blades, moving blades, glands, stop valves, control valves, a control and protection system, lubricating system, etc.
19.2.3.1 Casing Single-shell casing is used, because the steam pressure and temperature are low. Carbon steels are generally used as casing materials. If necessary, stainless steel, overlay welding or coating is applied to the parts such as glands where corrosion
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Figure 19.21 20-MW 3600-rpm geothermal steam turbine with axial exhaust.
Figure 19.22 3.3-MW 7266-rpm geothermal steam turbine with reduction gear.
tends to occur. Borescope holes may be prepared in the casing, in order to check the condition of the blades without disassembly of the casing.
19.2.3.2 Rotor A drum-type rotor is employed for reaction turbines, while a disk-type rotor is employed for impulse turbines. A balance piston is equipped in single-flow reaction
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Figure 19.23 81.3-MW (maximum 100.7 MW) 3000-rpm geothermal steam turbine. Source: Courtesy of Mitsubishi Hitachi Power Systems.
turbines to cancel the thrust force. Because of the low steam temperature, 1 2% Cr steel rotors are generally used. Overlay welding or coating may be applied in order to enhance corrosion resistance. 12% Cr rotors are also applied.
19.2.3.3 Blades Stationary blades are installed or integrated in removable stationary blade holders, diaphragms, or stationary blade rings, in order to facilitate cleaning during maintenance. Moving blades are installed in the rotor or disks. Smaller blades are generally shrouded, while longer blades such as LP blades may be free-standing. Seal fins are installed to reduce leakage flows. Generally, geothermal steam turbines are not equipped with a control stage, because they are normally operated at constant load. The blade materials, 12 13% Cr steel, 17 4PH, Ti-6Al-4V, etc., are used, depending on the environmental and operational conditions.
19.2.3.4 Valves Stop valves and control valves of geothermal steam turbines have large diameters, because of large volumetric steam flow. Therefore, check valves are used as stop valves, while butterfly valves are used as control valves (Fig. 19.24). Recently, the HP control oil system has been applied to improve operability of valve actuators and to make the control equipment compact.
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Figure 19.24 Butterfly control valve.
Figure 19.25 Bird’s-eye view of Nga Awa Purua (NAP) geothermal power plant, New Zealand.
19.2.4 Design characteristics of the latest geothermal turbines The geothermal steam turbine for Nga Awa Purua (NAP) geothermal power plant (Fig. 19.25) in New Zealand that began operation in 2008 is the world’s largest single-casing geothermal steam turbine at present, with a capacity of 139 MW (max. 147 MW). The 798 mm (31.4 inch) last stage blades—the world’s largestclass blades for geothermal steam turbines—are employed. The inlet steam
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Specifications of geothermal steam turbine for Nga Awa Purua
Table 19.1
Plant cycle
Triple-flash system
Output First operation Rotating speed High-pressure steam Intermediate-pressure steam Low-pressure steam Exhaust pressure Last-stage blade length
139 MW (max. 147 MW) 2008 3000 rpm 2.35 MPa/221 C 840 kPa/172 C 23 kPa/125 C 8.5 kPa 798 mm (31.4 in.)
Figure 19.26 139-MW (maximum 147 MW) 3000-rpm geothermal steam turbine for tripleflash system.
pressure is 2.35 MPa, which is higher than conventional. A triple-flash system is employed to utilize the geothermal energy most effectively. The water after separation of the steam is flashed in two stages, and the generated steam is led to the IP and LP parts of the turbine, to maximize the output. The design parameters of NAP geothermal power plant are shown in Table 19.1. A turbine sectional drawing is shown in Fig. 19.26. The steam turbine has five inlets: two for HP steam, two for IP steam and one for LP steam; and they are compactly arranged. The outlook of the steam turbine is shown in Fig. 19.27. Fig. 19.28 shows the steam turbine under construction [8].
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Figure 19.27 Outlook of the world’s largest single-casing geothermal steam turbine for Nga Awa Purua (NAP).
Figure 19.28 Geothermal steam turbine for Nga Awa Purua (NAP) during installation.
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19.3
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Technologies to enhance reliability of geothermal steam turbines
19.3.1 Corrosion problems and solutions 19.3.1.1 Evaluation of corrosion resistance of materials Geothermal steam is generally accompanied by several mass percent of NCG and dozens of parts per million (ppm) of dissolved impurities. Generally, more than 90% of the NCG is CO2, and the rest is composed of hydrogen sulfide (H2S), methane (CH4), ammonia (NH3), etc. The impurities dissolved in the steam contain corrosive substances such as chlorides (Cl2), sulfates (SO422), etc. (Table 19.2). It is therefore essential to evaluate the corrosion resistance of the materials and the applicable stress level when designing geothermal turbines. For this purpose, systematic experimental corrosion tests in the simulated geothermal environment have been carried out to accumulate material data related to the corrosion resistance. Since the quality of the geothermal fluid varies from site to site, corrosion tests exposed to the actual geothermal fluid in the fields are also very important to evaluate the corrosion resistance of the materials. Such corrosion tests have been carried out at various geothermal sites for the reliable design of geothermal steam turbines (Fig. 19.29) [9].
19.3.1.2 Measures against stress corrosion cracking and corrosion fatigue In the design of geothermal steam turbines, particular problems arise with the turbine blades root and rotor grooves, which are exposed to high centrifugal force and steam pressure during operation. A technology was developed to improve the corrosion resistance by performing shot-peening on the parts of the blade root and grooves where the stress is concentrated (Fig. 19.30). In the results of comparative testing performed in a simulated geothermal environment, it was verified that the shot-peening greatly improved the strength of the components against stress corrosion cracking (SCC) and corrosion fatigue (CF) (Fig. 19.31).
19.3.1.3 Measures against erosion-corrosion In recent years, progress in exploration techniques and drilling technology for geothermal wells has resulted in the development of geothermal resources at comparatively greater depths. Accordingly, the steam pressure at the inlet of the geothermal turbine has tended to be higher, rising from the conventional value of approximately 1 MPa to a value of approximately 2 MPa and more. With wet steam turbines, increased steam pressure at the inlet results in a greater tendency for erosion-corrosion to occur. Erosion-corrosion is a thinning of material surface due to the combined effects of chemical and mechanical action of the steam flow. Because stationary blade holders and rotors are made of carbon steel or low-alloy steel, their surfaces are prone to erosion-corrosion. As a measure against erosioncorrosion, a rotor with 2% Cr steel was developed, which had greater resistance to
Table 19.2
Chemical analysis of geothermal steam at turbine inlet (examples)
Plant
A
B
C
D
E
F
G
Plant system
SF
DF
DF
SF
SF
Dry steam
DF
pH
5.5
5.0 7.8
5.85
6
5.65
7.4
3 4
1
3.2
3.10
2.8
0.082
1
0.25
0.22 0.07 0.84 0.38
2.8 18 0.4 86
0.1 0.1
0.02
1
0.6
0.07 2.15 1.65
0.03 21 12 2
0.2 96 4
3 97 3
Impurities in steam
NCG
Cl
ppm
Na K Ca SiO2 Fe Mg NH3 SO4 B TDS
ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
Total CO2 H2S H2 CH4 N2 NH3 Others
wt% Composition in wt%
DF, double-flash; NCG, noncondensable gases; SF, single-flash; TDS, total dissolved solids.
1 1
0.6 0.05
0.003 0.163
6.1 6 0.1
5 1.15 95.2 1.09 0.04 0.21 3.16 0.03 0.27
2.2 99.0 0.90 0.01 0.02 0.02 0.02 0.04
0.19
1 89.5 2.8 0.7 5.1 1.2 0.7
0.4 84.6 12.7 1.1 0.2 1.4 0.1
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Figure 19.29 Corrosion test rig on site.
Figure 19.30 Shot-peening applied to blade root grooves.
erosion-corrosion than the 1% chromium steel conventionally used as a rotor material. Technologies have also been developed to coat the surface of rotors with a WC-CoCr material using high-velocity oxy-fuel spraying (Fig. 19.32 [10]).
19.3.2 Measures against water-droplet erosion Erosion caused by the impact of water droplets (i.e., drain attack) is the same phenomenon as in steam turbines for fossil power plants. In geothermal power plants
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Figure 19.31 Stress corrosion cracking test results.
Figure 19.32 High-velocity oxy-fuel coating applied to rotor surface.
employing a flash cycle, all stages are operated in wet steam. Therefore, it is necessary to consider drain attack erosion in the design process. For this purpose, drain pockets are provided to remove the water droplets that cause the erosion. Additionally, protective measures are taken with the brazing of an erosion shield onto the leading edge of the moving blades of the last stages (Fig. 19.33).
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Figure 19.33 Measures against water- droplet erosion.
19.3.3 Measures against scale problems Silica, calcium carbonate, and other substances contained in geothermal steam are deposited on the surface of components such as the blades, casing, and rotor. These impurities accumulate and build scales. The scale that builds up on the surface of the blades narrows the passage for the steam and causes a fall in output. Furthermore, the scale that builds up in the gaps between the rotating parts and the stationary parts may cause abrasion of the components. As a countermeasure against the scale problem, blade-washing technology with water droplets sprayed at the inlet of the turbine may be employed.
19.4
Technologies to enhance performance of geothermal turbines
19.4.1 New-generation low-pressure blades for geothermal steam turbines The LP blades of the last stages are important turbine components that determine the size and efficiency of geothermal steam turbines. Since development of the large LP blades requires enormous time and expense, they are standardized in series, and the optimal LP blade group is selected in accordance with the volumetric exhaust flow of the steam turbine. In order to increase the turbine capacity, longer
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Figure 19.34 Computational fluid dynamics analysis of Mach number distribution in the low-pressure blades.
last-stage LP blades with larger exhaust area are required. Conversely, longer LP blades entail increased centrifugal and steam bending forces, which are restricted by the strength of the material. Utilization of large LP blades for geothermal turbines has been suppressed because of the severely corrosive environments in which the geothermal turbines are operated. The new-generation LP turbine blades for geothermal steam turbines were developed based on the wealth of experience in geothermal turbine operation. As geothermal turbines are operated in a very corrosive environment, special consideration was given in their design to suppress stress levels within the allowable limit for SCC and CF. On the other hand, the new-generation LP blades are loaded higher, in order to enable downsizing of the steam turbine. Stress distribution was optimized by means of finite element method (FEM) analysis. State-of-the-art computational fluid dynamics (CFD) was employed to develop the new-generation LP blades, in order to achieve the highest efficiency by optimizing pressure and Mach number distributions around the blade profile (Fig. 19.34). Rotating vibration tests on the actual LP blades were performed to secure blade reliability during operation (Fig. 19.35).
19.4.2 High-load, high-efficiency reaction blades As the inlet pressure of the geothermal steam turbine tends to increase, the blades are required to work efficiently with higher load, i.e., with larger pressure drop. Generally, the higher the blades are loaded, the more the blade efficiency decreases. However, new reaction blades realizing both high load and high efficiency were successfully developed for the stages other than the LP blade stages, using the latest design techniques to minimize the profile loss and to suppress the secondary flow
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Figure 19.35 Rotating vibration test of the last stage blades.
loss. By adopting the high-load, high-efficiency reaction blades, the stage efficiency was improved by 1 2% while increasing the load per stage. The blades have an integral shroud, which is machined from a single bar material together with the blade airfoil and blade root. The integral shrouds of the adjacent blades contact tightly with each other during operation, giving a damping effect to the blade vibration, securing high operational reliability against the corrosive geothermal steam (Fig. 19.36).
19.4.3 High-performance, compact exhaust casing The exhaust casing decelerates the steam discharged from the last stage blades and forms the flow passage leading to the condenser. The steam discharged from the last stage has a high velocity that cannot be converted to rotational energy of the rotor. Furthermore, the total pressure loss in the exhaust casing reduces the effective heat drop of the turbine stages, resulting in reduced turbine efficiency. However, if the velocity of the discharged steam is effectively reduced in the diffuser composing a part of the exhaust casing, a pressure recovery occurs in the diffuser, which compensates or even surpasses the pressure loss in the exhaust casing. In general, if the exhaust casing is made more compact, then the reduction in the passage cross-section increases the velocity of the steam flow, which is disadvantageous for the performance. In order to develop a more efficient and compact exhaust casing, the shapes of the exhaust casing and the diffuser were optimized by means of three-dimensional viscous flow analysis, resulting in a
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Figure 19.36 High-load, high-efficiency reaction blades: Moving blades (A) and stationary blades (B).
diffuser shape that recovers pressure more effectively and an exhaust casing with less pressure loss (Fig. 19.37).
19.5
Operational experiences and lessons learned
Since geothermal fluid contains a large amount of corrosive impurities such as chlorides, hydrogen sulfides, carbonic acids, etc., problems related to the corrosion—e.g., general corrosion, pitting corrosion, erosion-corrosion, SCC, and CF, etc.—are crucial for both plant operators and equipment manufacturers. The problems experienced and proposed countermeasures are described below.
19.5.1 Erosion Since geothermal turbines are generally operated with wet steam, erosion of the blades is one of the problems that occasionally occurs. Long LP blades of geothermal steam turbines are generally provided with a stellite erosion shield brazed on the leading edge of the blades. At the overhaul after approximately 8 years of operation, it was found that the erosion shields had partly peeled off, and severe erosion had occurred (Fig. 19.38). This took place in a large-capacity geothermal turbine, in which drainage was insufficient.
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Figure 19.37 Computational fluid dynamics analysis of exhaust casing.
Figure 19.38 Erosion shield peeled off after eight years of operation.
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Figure 19.39 Erosion-corrosionobserved at rotor steeples (A), Governor side and generator side (B).
Modern geothermal turbines are provided with drain catchers to efficiently remove the large droplets that cause severe erosion to the moving blades. By employing drain catchers, along with the improved brazing process, the problem of blade erosion has been mitigated.
19.5.2 Erosion-corrosion Erosion-corrosion is a phenomenon whereby the metal surface is continuously washed away by corrosive, high-velocity wet steam due to the interaction of chemical reaction and mechanical erosion. The rate of erosion-corrosion is influenced by various parameters such as steam temperature, wetness, velocity, pH value, corrosive elements included in the steam, geometry of flow passage, etc. Fig. 19.39 shows erosion-corrosion observed at the inlet of the blade attachment area of the rotor after 10 years of operation. In this turbine, the governor-side blades and generator-side blades had been operated under different steam conditions and purities. Consequently, the rates of the erosion-corrosion differed greatly on the governor side and generator side. Erosion-corrosion is more or less inevitable in geothermal turbines. If a sufficient margin for the strength of the eroded part remains, the turbine can be further operated without any remedy. Fig. 19.40 shows erosion-corrosion observed in the stationary blade ring after 10 years of operation. The stationary blades made of stainless steel were welded to the blade ring made of carbon steel. The blade ring was eroded under the weld, while the weld and the stationary blades were not eroded. In this case, it is recommended to use stainless steel for the parts of the blade ring on which the stationary blades are welded.
19.5.3 Stress corrosion cracking and corrosion fatigue SCC is a phenomenon whereby cracks occur and propagate under static (tensile) stress in a corrosive environment, while CF is a phenomenon whereby cracks occur and propagate under alternating stress in a corrosive environment. As a matter of
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Figure 19.40 Erosion-corrosion observed at stationary blade ring.
Figure 19.41 Crack found in blade root.
fact, static and alternating stresses act simultaneously on the turbine parts such as the moving blades and rotor. Therefore, it is not always easy to distinguish clearly between SCC and CF. In most cases, the fracture surface is severely corroded, which makes discrimination of both crack modes more difficult. It is known that SCC and CF tend to occur in the ‘dry wet alternating region’ where steam expands across the Wilson line (i.e., steam condition of 4 5% wetness). According to an accepted opinion, it is because the droplets dissolving impurities repeatedly adhere to and evaporate on the metal surface, in accordance with turbine start-stop and load change, thus leaving condensed impurities on the metal surface. Fig. 19.41 shows a fracture surface of a crack found in a blade root. The fracture surface was covered with a brown deposit. The deposit was analyzed using energy
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Figure 19.42 Various phases of pitting and microcracks observed in blade grooves.
dispersive X-ray spectrometry, and as a result, it was found that S, O, and Fe were major elements of the deposit. Crystal structures of the deposit were investigated using an X-ray diffractometer: it turned out that the deposit consisted mainly of iron sulfide. The rotor steeples of the same stage showed various phases of the pitting or microcracking (Fig. 19.42). As a result of various investigations, it was concluded that the crack was initiated and propagated due to sulfide stress corrosion, and during a long period of operation, CF took over. Geothermal fluid that comes out of the ground contains various impurities. Geothermal steam is separated in the flashers and separators from the hot water, and led into the steam turbine. Normally, most of the impurities are removed together with water in this process. The demisters also serve to reduce the carryover of the corrosive mist into the turbine. If the function of the separators, flashers, or demisters is not sufficient, however, corrosive mists are carried over into the turbine, and may cause SCC or CF. Therefore, the function of the separators, flashers, and demisters is very important to protect the turbine blades from corrosion.
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It should also be noted that frequent start-stop and load change, as well as vacuum braking, may exacerbate the corrosive environment in geothermal turbines. Periodic maintenance also plays a very important role in efficient and stable operation of the geothermal power plant [11].
19.6
Future view of geothermal power generation and challenges
With the extended utilization of geothermal resources in the future, technologies required for geothermal steam turbines will be more and more diversified and challenging. The major development subjects are described below. G
G
G
G
Application to deeper geothermal resources. As deeper geothermal resources are developed, the pressure and temperature of geothermal steam will be increasingly greater, and the impurities contained in the geothermal fluid will diversify in their composition and quantity. Therefore, development of geothermal steam turbines and plant equipment along with materials that can accommodate such conditions will be necessary. Development of larger-capacity geothermal steam turbines. In order to increase the capacity of geothermal steam turbines, it is necessary to develop larger LP blades, taking into account the allowable stress in the corrosive geothermal environment. Inlet valves with larger diameters should also be developed. Effective utilization of geothermal resources. Technologies should be developed to utilize geothermal resources more effectively; e.g., binary power generation that uses low-temperature geothermal resources that have not hitherto been used, and hybrid power generation combining a flash system and a binary system that utilizes the water before returning to the Earth, etc. Further improvement of reliability and efficiency of geothermal steam turbines. As geothermal fluid contains a large amount of impurities, corrosion and scale deposition are the most challenging problems for geothermal steam turbines. The stability of geothermal power generation has increased drastically owing to development until now. Further improvement of the reliability along with the efficiency improvement by continuous development are essential to promote geothermal power generation for sustainable human development.
References [1] Federation of Electric Power Companies of Japan, Graphical Flip-Chart of Nuclear & Energy Related Topics 2014, p. 2-1-9. Available from: ,http://www.fepc.or.jp/english/ library/graphical_flip-chart/., 2014. (accessed on 1.10.15). [2] G.O. Fridleifsson, W.A. Elders, G. Bignall, A plan for a 5km-deep borehole at Reykjanes, Iceland, into the root zone of a black smoker on land, Sci. Drill. 16 (2013) 73 79. [3] R. Bertani, Geothermal power generation in the world 2010 2014 update report, in: Proc. World Geothermal Congress 2015.
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[4] M.H. Dickson, M. Fanelli, What is Geothermal Energy? International Geothermal Association, 2004. Available from: ,http://www.geothermal-energy.org/what_is_ geothermal_energy.html. (accessed on 1.10.15) [5] Calpine Corporation, The Geysers: A Very Special Place, 2015. Available from: ,http://www.geysers.com/history.aspx. (accessed on 1.10.15). [6] J. Tajiri, Latest technologies contributing to realization of large- to small-capacity geothermal power plants, Toshiba Rev. 68 (6) (2013) 24 27. [7] S. Saito, T. Suzuki, J. Ishiguro, T. Suzuki, Development of large capacity singlecylinder geothermal turbine, Geoth. Resour. Counc. Trans. 22 (1998) 399 403. [8] T. Horie, Kawerau and Nga Awa Purua geothermal power station projects, New Zealand, Fuji Electr. Rev. 55 (3) (2009) 80 86. [9] Y. Sakai, M. Yamashita, K. Shiokawa, L.B. Niu, H. Takaku, Corrosion resistance of materials for geothermal steam turbines, Proc. Int. Conf. Power Eng.-03 3 (2003) 297 302. [10] Y. Sakai, Y. Oka, H. Kato, The latest geothermal steam turbines, Fuji Electr. Rev. 55 (3) (2009) 87 92. [11] Y. Sakai, M. Yamashita, M. Sakata, Geothermal steam turbines with high efficiency, high reliability blades, Geoth. Resour. Counc. Trans. 24 (2000) 521 526.