A new lease of life

Environmental and economic pressures are causing power plant operators to think twice when deciding on new capacity construction – a refurbishment can often be a better solution. A recent project at Enstedværket coal fired plant in Denmark, which involved a steam turbine upgrade using Siemens` new 3DS blading technology, proves this.

Siân Green

Only a few years ago, rehabilitation of power plants involved the restoration of parts to their original state to ensure longer plant life. Now, with the market pressures of deregulation, the focus has shifted to bringing state-of-the-art technology to older plant. Efficiency, economic and environmental considerations have become just as important as service life.

Utilities and operators have come to realise the great potential that can be exploited through rehabilitation and upgrade. This potential has been demonstrated at Enstedværket Unit 3 coal fired power plant in Denmark, where a recent rehabilitation has improved heat rate, efficiency and environmental performance, and increased capacity. This was achieved largely through upgrading the steam turbine blading, and has helped prepare the plant`s operator, Sonderjyllands Hoespaendingsværket A/S (SH), for market deregulation and competition.

A new approach

While power sector deregulation ensures that economic factors play a role in plant rehabilitation, recent technical advances have brought efficiency and environmental factors into the equation. Materials advances have allowed new coal fired plant to reach efficiencies of more than 46 per cent, and higher inlet temperatures have increased efficiencies in gas turbine plants.

The improvement of plant environmental performance is often a major objective of upgrading, particularly for coal fired capacity. Legislation first passed in the 1980s has become increasingly stringent, and more recent concerns over global warming have placed plant efficiency as being of paramount importance.

But the bottom line is that the changing market and growth of independent power producers means that old or unprofitable plants run the risk of being unable to compete. While this may encourage utilities to construct new plant, an upgrade is often a quicker and more economic solution.

A new approach to plant life extension has therefore evolved in response to these changes. In late 1996, Siemens KWU, which carried out the upgrade and rehabilitation work at Enstedværket, established a specialised service branch for plant upgrades and life extension. The company states that this way, it can develop a tailor-made solution for a particular plant using a standardized approach in order to meet the objectives of the upgrade.

Siemens has carried out a number of rehabilitation projects around the world, including the 100 MW Neyvelli plant in India, where plant availability was increased from 68 to 90 per cent, and the 2 x 285 MW oil fired Bergen power plant in the USA. More recently, the company completed the rehabilitation at Enstedværket in Denmark, where impending deregulation was the main motivation behind the project.

Changing markets

In response to the EU Electricity Market Directive, the Danish power industry is gradually deregulating. The Danish electricity industry currently operates under a cooperative system. SH is one of six cooperative utilities which are owned by the distribution companies in their respective areas. The six utilities in turn own Elsam, their representative organization.

Elsam is responsible for many of the commercial activities of the six utilities, including fuel purchase, power sales, financing and power and heat dispatch. As a result of deregulation, Elsam`s activities were divided in two: I/S Eltra now acts as the independent transmission network operator for western Denmark; and Elsam continues to handle all other activities on behalf of the six utilities.

The six utilities are also divesting the 150 kV and 60 kV networks in their areas, and are cancelling supply and purchase obligations in preparation for competition. Currently, consumers using more than 100 GWh per year are able to choose their supplier, and it is possible that by the end of the year, Denmark will be fully integrated with the NordPool bourse.

As deregulation proceeds, more consumers will be able to choose their electricity supplier. Improving and maintaining competitiveness is therefore currently an important consideration for SH and the other utilities.

But the Danish government also has other objectives that are likely to impact on the utilities in Denmark. Following Kyoto, Denmark is aiming to reduce its carbon dioxide emissions by 20 per cent on 1998 levels by 2005, and by 50 per cent by 2030. Part of this reduction will be met through an increase in biomass and offshore wind generation, but it is likely that other savings will have to be made in the power sector. Natural gas is likely to play a bigger role in Denmark`s electricity industry in the future.

Enstedværket Unit 3 was constructed as a 630 MW plant and commissioned in 1979. Located near Aabenraa, next to the largest north European coal and oil harbour, the plant is coal fired and uses oil as a back-up fuel. Unit 3 consists of a boiler house, turbine plant and a flue gas purification plant. It also uses steam generated in the adjacent straw and wood chip fired Unit 2 to generate power.

Power produced from the plant is sold to distribution companies via Elsam; 50 per cent of the production is sold to Germany by PreussenElektra, which owns half of Unit 3. Some of the steam generated is used as process steam for Samden, a neighbouring milk condensing factory, and also for district heating to Aabenraa.

The boiler is a Babcock and Wilcox once-through Benson type boiler. It is equipped with 36 burners able to burn coal or oil, or a mixture of both. At full load, the boiler develops 542 kg/s of steam at 200 bar and 535 degreesC, and uses around 58 kg/s of coal (or 37.5 kg/s of oil). The silo building adjacent to the boiler house is equipped with six coal silos, feeders and mills. Each mill feeds six low-NOx burners.

During normal operation, five coal mills and 30 burners are operational.

The turbine hall is equipped with one 3000 r/min Siemens four-cylinder reheat condensing steam turbine generator. The high pressure (HP) section is a 17-stage barrel type single flow cylinder and is throttle controlled. Induction steam is delivered via four combination main stop and control valves.

The double-flow 13-stage intermediate pressure (IP) turbine cylinder is split axially, with the reheated steam supplied from the top and bottom at the centre of the casing. The two double flow seven stage low pressure (LP) cylinders are fed from the IP turbine cylinder via four cross-over pipes.

The straw and wood chip fired Unit 2 boiler operates in parallel with Unit 3. Designed to operate on coal, the original boiler was replaced by a biomass boiler as part of the Danish government`s plans to reduce CO2 emissions, and is no longer equipped with a turbine. It delivers some 120 t/h of steam at 200 bar/542 degrees C to Unit 3, and burns 120 000 t/year of straw and 30 000 t/year of wood chips.

The biomass boiler has saved the consumption of 80 000 t of coal per year, resulting in a CO2 emissions reduction of 190 000 t/year. The steam supplied by Unit 2 represents 6.6 per cent of Unit 3`s power generation.

Steam is cooled in two main condensers, which use a total of 80 000 m3/h of cooling water. The cooling water is taken from the Aabenraa fiord. Some of the slightly heated cooling water is used in two fish farms.

In 1995, SH made the decision to upgrade Enstedværket unit 3. In deciding on the refurbishment, SH took into consideration a number of factors, including:

•long term plans for the plant

•fuel costs and trends in power and heat demand

•financing conditions

•plant outage

•achievable output

•energy policy requirements.

Financing for the DM70 million ($39.5 million) project came from Elsam and PreussenElektra.

Plant down-time for the refurbishment work was minimized to a 65-day period. During this period, the HP, IP and two LP turbines were upgraded, and the turbine hydraulic system and C&I system completed. According to Siemens, the recommissioning phase began after only 46 days. Synchronization with the grid took place as planned on August 24, 1997.

Blade design

The HP and IP sections of the turbine were upgraded with Siemens` new 3DS blades. Their design includes a new shape to the airfoil section which reduces secondary losses made at the blade root and tip.

Secondary flows can contribute significantly to overall aerodynamic losses, particularly in the front stages of the HP and IP turbine sections. The three-dimensional design of 3DS blades helps address these losses, and they are specially designed for use in the front stages of the HP and IP turbines.

Designed using advanced CFD and experimental methods, 3DS blades lower secondary losses by reducing the pressure gradient in the near wall regions. This is achieved through a “compound lean” where both ends of the blades are leaned in a way that an acute angle is formed between the blade surface and the end walls at the hub and shroud.

The overall effect of the 3DS blades is an efficiency increase of as much as one percentage point per turbine cylinder, an uprate in capacity and an improved heat rate. Fuel consumption is reduced and plant life cycle costs considerably lowered.

To reduce exhaust losses of both LP turbine cylinders, the exhaust cross-section was increased from 10 to 12.5 m2, a milestone in the design of 50 Hz turbines. Moving blades made of high-grade 16 per cent chromium steel developed for high loads were used in the last standard stage. A heating system for the hollow last stage stationary blades was also installed, preventing large water droplet formation and thus water droplet erosion on downstream blades. Although this does not increase the efficiency of these stages, their service life has been extended.

In addition, the number of rows of moving blades with integral shrouding was increased from three to six to reduce tip clearance losses. Due to the higher turbine capacity, the generator cooling system was also improved by increasing the hydrogen pressure from 5 to 5.5 bar.

The valve actuators of the HP, IP and LP bypass valves were modified for individual valve control so that each valve can be adjusted automatically. The C&I system was also replaced by a Teleperm XP system. As a result, start-up and shut-down times have been shortened and optimized, and changes in the load behaviour of the turbine are faster and more targeted.

Siemens also made improvements to the vacuum in the condenser through the installation of three stationary level-controlled primary cleaning machines, high performance filters in the cooling water inlet of the condenser chambers, and continuously operational condenser tube cleaning systems. The steam side air suction and condenser vacuum equipment was optimized and the two main cooling water pumps were adapted to meet the new operating conditions.

A complete solution

The entire refurbishment at Enstedværket Unit 3 resulted in an extension of the plant`s service life and a reduction in its life cycle costs. An increase in power output from 630 MW to 660 MW with no corresponding increase in fuel consumption was achieved. Fuel savings of 60 000 t/year have therefore been made resulting in a CO2 emission saving of 125 000 t/year.

The heat rate was improved by 4.6 per cent, based on 100 per cent volumetric steam flow. Noise levels from the turbine-generator were also reduced by 2.5 dB(A) over the entire load range.

In the right market conditions, rehabilitation and upgrade can provide a complete solution for utilities seeking to improve plant performance and increase capacity. In the face of deregulation, environmental restrictions, or both, power plant operators, especially of coal fired plant, may find that this is the way forward.

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Figure 1. Enstedværket power plant underwent a refurbishment which reduced life cycle costs and increased output by 30 MW

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Figure 2. Enstedværket is located in Aabenraa, Denmark

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Figure 2a. Enstedværket is located in Aabenraa, Denmark

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Figure 3. During the upgrade, the final stage of the low pressure turbine cylinder was enlarged to permit an exhaust cross section of 12.5 m2

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Figure 4. The plant outage time for the steam turbine upgrade and refurbishment work was minimized

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Figure 5. The older T2 blading was replaced

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Figure 6. The 3DS turbine blades were installed on the HP and IP sections of the steam turbine