Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
1

Initiative for CO2 Emission Reduction by Steam Turbine
Retrofitting with High-efficiency Technologies

TAKASHI NAKANO*1

YUJI AKAISHI*2

TAKEO TOKUMOTO*3KENTARO TANAKA*4

HIDEO YAMAMOTO*4

NAOYA ARIMA*5

Initiatives toward decarbonization are being undertaken worldwide. In the power generation
market, the introduction of renewable energy is also in progress. Meanwhile, as a certain numberof thermal power facilities are expected to remain in operation to ensure stable electricity supply,
it is indispensable to implement CO2 emission control in thermal power generation. The existing
power facilities are no exception in this regard. Steam turbines, which have been incorporated as
the main equipment of power facilities, inevitably degraded after long-term operation. Although
the internal parts are often upgraded as a countermeasure, their simultaneous remodeling byapplying the latest high-efficiency technologies can add more value: less fuel consumption due tohigher efficiency, which in turn leads to the reduction of CO2 emissions. This report presents someexamples of the steam turbine retrofits with the latest high-efficiency technologies and the resulting
effect of CO2 emission reduction.

|1. Introduction
The long-term use of steam turbines leads to degradation due to aging, increasing thesusceptibility to damage. The risk of unexpected damage being caused becomes high, especially inthe second half of the life of a power plant. In many cases, the major parts that have aged arereplaced 25 to 35 years after the start of operation, eliminating the potential risk. Such upgrading
can suppress repair costs that may otherwise be incurred in the future and prevent unexpected
damage from being inflicted by unplanned events or shutdowns. As the efficiency of these aging
steam turbines for upgrade is not so good, their simultaneous retrofitting with the latest
high-efficiency technologies can add more value such as less fuel consumption and CO2 emission
reduction.
For the private power generation systems, the factors involved are not only degradation due
to aging, but also others such as changes in the business environment, which result in alterations in
the operational conditions (e.g., the output and the amount of steam supplied to a factory) plannedat the time of construction. These may often lead to deviation from the optimal operationconditions. It is also possible to optimize the plant efficiency by retrofitting according to the mostrecent operational conditions.

*1

Senior General Manager, Steam Turbine Engineering Department, GTCC Business Division, Energy Systems, Mitsubishi*2

Senior Engineer, Steam Turbine Engineering Department, GTCC Business Division, Energy Systems, Mitsubishi Heavy*3Manager, Steam Turbine Engineering Department, GTCC Business Division, Energy Systems, Mitsubishi Heavy Industries,*4Engineering Manager, Steam Turbine Engineering Department, GTCC Business Division, Energy Systems, Mitsubishi*5

Steam Turbine Engineering Department, GTCC Business Division, Energy Systems, Mitsubishi Heavy Industries, Ltd.

Heavy Industries, Ltd.
Industries, Ltd.
Ltd.
Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
2|2. Examples of steam turbine retrofits for higher efficiency
In order to meet the need to improve the efficiency in steam turbines, Mitsubishi HeavyIndustries, Ltd, (MHI) developed a state-of-the-art high-performance side exhaust steam turbine towhich the latest high-efficiency technologies were applied. Its performance and reliability wereverified at T-Point 2, the power generation demonstration facility of MHI(1). The high-efficiency
technologies employed therein are applicable to any steam turbines with a wide output range ofsmall to large capacity, including for retrofitting of existing units. The number of applied units isincreasing gradually.

2.1

Case of entire steam turbine retrofit

A 500 MW class steam turbine operating in an ultra-supercritical thermal power plant wasretrofitted. It consists of a single high-intermediate pressure (HIP) turbine and two low pressure(LP) turbines. Starting its operation in 1995, the steam turbine had been in operation for more than
25 years before being retrofitted. With the purpose of extending the plant life, improving theefficiency and reducing the maintenance costs, the entire HIP turbine and the internal parts of theLP turbines were replaced as part of the plant renovation. Figure 1 summarizes the turbine retrofit.
The resultant improvement in the performance is expected to reduce CO2 emissions by about
95,000 tonnes per year.

Figure 1

Summary of entire turbine retrofit

In the HIP turbine, the latest high-efficiency reaction blading design was applied to increase
the blade-row efficiency. The original labyrinth seals were replaced with abradable ones to reducethe decline in the performance resulting from the leakage loss that will increase over time. The
retrofit was carried out considering not only improved efficiency but also degradation prevention,enhanced reliability and better maintainability. The cooling cells, which are activated when theturbine is shut down, were additionally attached to the upper outer casing, thereby reducing thetemperature difference between the upper and lower parts of the casing. In this way, the “cat-back”
distortion of the casing, which may lead to the degradation of seals, can be prevented. The newrotor was manufactured without a center bore; the integral coupling was adopted instead of theoriginal shrink-fit coupling to enhance the reliability. The HP exhaust thermal shield was also
removed, making the maintenance simpler.
With regard to the LP turbines, the last stage blades were changed from 31-inch grouped
blades to 33-inch integral shroud blades (ISBs), in addition to the application of high-performance
reaction blading. Moreover, the shape of the exhaust flow guide was optimized to reduce theexhaust loss. The inner casing design was also modified in such a way that the deformation of thecasing can be minimized.

2.2

Case of HIP turbine retrofit

A 100 MW class steam turbine operating in a steelworks was retrofitted. The start of its
operation was in 1986. The customer’s request to retrofit the HIP turbine was based on the need toextend the life of the steam turbine by replacing the components with a very limited life span and toMitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
3improve the economic efficiency (Figure 2). The resultant improvement in the performance isexpected to reduce CO2 emissions by about 46,000 tonnes per year.
The outer casing of the HIP turbine remained unchanged for continued use, whereas theinternal parts of the turbine were replaced with the ones redesigned by applying high-performancetechnologies. A substantial improvement in the performance was achieved by changing the blade
design from the original impulse blade to the high-efficiency reaction blade, by applying thehigh-performance seals to reduce the leakage loss, and by employing the direct lubrication thrustbearing to reduce the mechanical losses. The redesigned main steam inlet improved the flow ofsteam for cooling the casing, thereby realizing the structure by which the inner surface of the outercasing is prevented from being directly exposed to high-temperature steam. In this way, thereliability of the HIP turbine outer casing, which continued to be used, was enhanced.

Figure 22.3

Summary of HIP turbine retrofit

Case of LP turbine retrofit

A 400 MW class steam turbine operating in a subcritical power plant was retrofitted. Since
its operation commencement in 1984, the steam turbine had been in use for more than 30 years
before being retrofitted. The reason to retrofit the LP turbine for higher efficiency was the need toswitch the fuel from coal to gas because of the new policy implemented by the government in thecountry where the carbon tax is increased (Figure 3). The resultant improvement in theperformance is expected to reduce CO2 emissions by about 43,000 tonnes per year.
The outer casing of the LP turbine remained unchanged for continued use, whereas theinternal parts of the turbine were replaced with the ones redesigned by applying high-performancetechnologies. With regard to the upstream side of the LP turbine, the high-efficiency reaction
blading design was newly adopted (it had been originally impulse blading). The length of thelast-stage blades was extended by employing 36-inch ISBs, instead of 33.5-inch grouped blades.
This reduced the exhaust loss and improved the performance. The shape of the exhaust flow guide
was also redesigned for the enhancement.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
4Figure 3

Summary of LP turbine retrofit|3. Examples of steam turbine retrofits according to alteredoperation conditions
The private power generation systems are often forced into operating under non-optimalconditions. This is because of the factors such as the changes in the business environment resulting
in alterations in the operational conditions (such as the output, and the amount of steam supplied toa plant) planned at the time of construction. If this is the case, the performance of steam turbines
can be improved by retrofitting according to the most recent operational conditions. Specifically, it
may be a retrofit to increase/decrease the capacity, convert a condensing turbine into a backpressure turbine, or change the extraction pressure. Table 1 lists some items on the retrofit menu(2).
Table 1 Example items on menu for steam turbine retrofits according to alteredoperation conditions
Steam condition

Electric power

Larger large output .
Steam demand isincreased.
Need to increase turbine
swallow.

Steam demand is notchanged.

Steam demand isdecreased.

Power generation cost >power purchase price.
Need to reduce theamount of power
generation of thecondensing turbine.

Menu

Benefits

Turbine Upsizing

Amount of electrical powerpurchased down due toincreasing output.
e.g.: 22% increased in output for
40 MW turbine.

Turbine Back
Pressurising fromcondensing.

Fuel cost down due toimproving plant efficiency, etc.
e.g.: 7% reduced in main steam
flow for 25 MW turbine.

Need to increase turbine
efficiency at lowloading.

Turbine Downsizing

Steam supply pressure toPower demand is not
the factory is changed.
changed. Need efficient
operation.

Extraction Pressure
Optimization

Output increased due toimproving turbine efficiency,etc.
e.g.: 6% increased in output for
35 MW turbine
Reduction in extraction
pressure loss
e.g.: Extraction pressure is
reduced by 30% and output isincreased by 2.5% for 50 MW
turbineMitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
53.1

Retrofit to reduce capacity and change extraction pressure

Shown here is an example of retrofitting the extraction condensing turbine for private powergeneration, which started operation in 2001. Specifically, the retrofit includes: the introduction of
GT cogeneration system to reduce CO2 emissions, the optimization of the post-retrofit operation of
the entire plant including the existing boiler and steam turbine, the capacity reduction of theexisting steam turbine (from 86 MW to 47 MW), and the change of the extraction pressure (from3.7 MPa to 2.8 MPa). Figure 4 summarizes the retrofit.

Figure 4

Summary of retrofit to reduce capacity and change extraction pressure

The nozzle box was upgraded along with the replacement of the first-stage nozzle. In order toadapt to the reduced capacity and the altered extraction pressure, the blade row specifications suchas the blade height and the number of blades needed to be changed. The diaphragm, nozzle androtor blades were therefore remodeled. The LP last-stage blades were renewed by replacing the oldblades with the new ones with the same specifications. The outer casing remained unchanged forcontinued use. Given the structure in which the diaphragm is directly attached to the outer casing,
we had to make the revised blade row specifications effective within the constraints.

3.2

Retrofit to convert into back pressure turbine

Shown below is an example of retrofitting the double extraction condensing turbine forprivate power generation, whose operation started in 1995. In the country where the turbine wasinstalled, the sale price of electricity to the power company did not rise as initially expected,
despite the soaring coal price. Thus, power generation by the steam turbine turned out to cost morethan the purchase of electricity. At the request of the customer who needed to further improve theplant efficiency for reduced coal consumption and CO2 emissions, we assessed by balancing theamount of steam supplied to the factory against the amount of electricity generated and, based on
the results, converted the existing condensing turbine into a back pressure turbine.
With this conversion, the amount of electricity generated is reduced, but the plant efficiency
can be improved because of reduction heat loss in condenser and less power used for the auxiliary
equipment in the condensate system. There is also the issue of a plume (white smoke) coming outof the cooling tower. The back pressure turbine has an advantage of negating the need to use thecooling tower completely or partially. Figure 5 summarizes the retrofit.
In converting into a back pressure turbine, all the stages until the steam extraction remained
unchanged, whereas the last four stages on the steam outlet side were removed. For continued useof the outer casing, a partition plate was attached to the exhaust outlet, allowing the exhaust toescape from where steam used to be extracted.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 3 (September 2023)
6Figure 5

Summary of retrofit to convert into back pressure turbine|4. Conclusion
For the existing steam turbines, the renewal of the parts that have aged because of thelong-term operation is effective in negating repair costs that may otherwise be incurred in the futureand eliminating the risk of unplanned shutdown due to unexpected damage. In addition, thesimultaneous retrofitting with the latest high-efficiency technologies can add more value such asless fuel consumption and reduced CO2 emissions. Moreover, turbine remodeling according to thealtered operational conditions can optimize the efficiency of the entire plant.
MHI will continue with the development to realize higher efficiency and provide steam
turbines that can contribute to further reduction of environmental impact.

References(1)(2)

Takashi Nakano et. al, Development of Latest High Efficiency Side Exhaust Steam Turbine, Mitsubishi
Heavy Industries Technical Review Vol.58 No.3 (2021)
K. Ikushima, et al., “Strategic Renovation for Existing Steam Turbine in the Electricity MarketChanging”, ICOPE-2021-0106