In 2004 the owners of the Farge power plant saw work begin to increase its generated output without the consumption of more fuel. Along with other measures, improvements to the turbines contributed massively to the efficiency drive.

Helmut König, Siemens Power Generation, Germany

The development of the steam turbine has been in progress for over 100 years and while it has reached a high level in terms of efficiency and reliability the challenge to beat previous results is forever ongoing. New requirements pertaining to engineering, economic and environmental aspects again and again provide impetus for further refinements. For example, numeric methods were created using high performance computers that now permit calculation and depiction of complex three dimensional flow conditions. This, coupled with new fabrication methods, has made it possible to manufacture not only turbine blading but also inlet and exhaust casings with innovative geometry that have yielded substantial advances in efficiency. This has not only given turbine engineering new momentum, it has also opened new possibilities for upgrading existing turbine plants.

Even after several decades of continuous service involving more than 170 000 operating hours, steam turbines are not necessarily ready for the scrap heap. For this reason it is definitely worth considering, before the end of the design service life has been reached, whether such robust workhorses justify upgrading coupled with uprating. In addition, operational flexibility usually increases. These factors are of great significance in liberalized power markets, where cost-effectiveness is the dominant parameter. Measures of this type, which Siemens Power Generation (PG) has implemented in numerous power plants, pay off for the power plant operator in only a few years because the fuel required to generate one kilowatt of power can be clearly reduced. Furthermore, the environmental balance is improved, as specific emissions are also reduced.

Figure 1. Turbine efficiency was increased through new blade design, improved blading clearance seals and increased flow cross sections
Click here to enlarge image

The Siemens upgrading concept does not merely focus on restoring the original output capability of steam turbines, instead it is geared to increase efficiency and reliability. The key elements of this approach are new blade design, improved blading clearance seals and increased flow cross sections. Modern twisted blades with integral shrouds which allow for three-dimensional flow characteristics are much more capable of meeting the requirements imposed by the flow of steam than previous simple designs. The geometry of these blades varies from stage to stage as deemed necessary by flow analysis. Three-dimensional flow analysis design techniques also significantly reduce secondary losses at the blade tips and roots. Additionally the degree of reaction of the individual stages can be optimally selected for each stage, increasing overall turbine efficiency further still.

Fit for future

The Farge power plant, situated on the lower Weser River in the northern part of Bremen, is owned by E.On Kraftwerke GmbH. The first coal fired unit was built at the Farge site back in 1921 and was continually expanded and improved over the years that followed. The unit that is still in service was built in 1969 and originally had an output of 300 MW. All earlier turbine and power plant equipment at this site has been removed. Despite its age of more than 35 years, Farge is definitely comparable to modern power plants, because this plant was optimized again and again by implementing technical measures, increasing output to 325 MW prior to performing the upgrading work described in this article.

In October 2002 Siemens was contracted during performance of a major inspection of the turbine plant to make Farge fit for the future. The comprehensive turbine upgrading measures were selected so as to increase efficiency and output without changing fuel consumption. Furthermore, reliability was to be improved and the service life of the turbines considerably extended.

The primary focus of the upgrading performed there last year was the renewal of the intermediate-pressure (IP) and the two low-pressure turbines (LP) as well as the condensers. Specifically, Siemens replaced the turbine rotor, blading, and inner casing including the blade sealing configuration and upgraded the bearing shell linings. The exhaust flow conditions for the LP turbines and the efficiency of condensers were improved by installing new, optimized flow condenser internals with a new tube pattern.

Figure 2. The location of the Farge plant allows for the co-combustion of sludge from the adjacent sewage works with the supply of coal brought in to the plant‘s own dock
Click here to enlarge image

The configuration of turbines built today diverges sharply in certain aspects from previous designs. As an example, the previous mechanical design of the low pressure rotor had nine rows of blades; by contrast the new design has only six rows. The new rotors for the IP and LP turbines are major components with impressive physical dimensions. The LP rotors have a diameter of 3.5 metres; they are some seven metres long and weigh 63 tonnes each. These parts are manufactured by Siemens in Mülheim, Germany.

After completion of upgrading, the Farge plant will chiefly be used between base load and intermediate load, logging over 6500 operating hours per year and generating more than two billion kWh of electricity per year. “Furthermore, one particularly advantageous aspect for the E.ON power plant fleet is the fact that our unit will be able to make rapid load changes over a broad range and offers considerable grid control capability. Internal ranking performed by E.ON indicates that we lead the pack in this regard,” reports Fritz Henken-Mellies, plant manager of Farge.

Output increase

The new turbine parts alone improve output by 16 MW and thus account for the largest share of the overall optimization in terms of the total 27 MW increase. Condensers were fitted with new modules featuring a modified tube pattern, and this measure increased output by a further 4 MW. Shortly after shutting down the plant, a decision was made to ship the high-pressure turbine to Mülheim, where Siemens operates a turbine manufacturing plant, for a major performance inspection; this brought about another 1 MW increase. Thus all three turbines at the Farge plant were overhauled.

Numerous other minor measures performed in various parts of the power plant contributed to the final 6 MW output increase. Major measures included optimization of flow in the scrubber of the flue gas cleaning plant and in the final superheater stage. Siemens had from mid-July 2004 until the end of September that same year to complete their work and perform the ten-day programme of restart tests and inspections. The result of these modifications is a power plant with a net efficiency of approximately 42.3 per cent. “This figure is definitely comparable to more recently built and larger power plant units,” highlighted Henken-Mellies.

Ruhr district coal, used previously at the Farge plant, has been replaced by imported coal. On average, 2000 tonnes of coal is combusted per day in the steam generator; depending on the market this coal is delivered from coal fields located around the world by waterway to the Nordenham port. The fuel, totalling over 700 000 tonnes per annum, is delivered to the plant’s own ship unloading station and from there is forwarded to a large coal stockpile that has a capacity of up to 50 000 tonnes, thereby ensuring an uninterrupted supply of coal to the power plant.

Co-combustion specialty

A special feature of the Farge plant is the co-combustion of sewage sludge. The Farge power plant first demonstrated over ten years ago that such sewage treatment plant residues can be co-combusted with coal. This process involves pumping sludge from the adjacent sewage treatment plant via underground lines, dewatering and co-combusting in the coal-fired boiler. Compared with utilization in agriculture or disposal in a landfill, this solution is considerably more environmentally friendly. Pollutants contained in the sludge are either entirely combusted or firmly bound in the vitreous ash matrix. Co-combustion annual throughput currently entails some 15 000 tonnes of sewage sludge (90 per cent solids).

Highly effective flue gas cleaning equipment – electrostatic precipitators for dust removal, NOx abatement plant for reduction of NOx emissions and flue gas scrubber for desulphurization – ensure top performance in terms of emissions as well. Comparing the present generation performance with the past, a reduction in CO2 emissions of 100 000 tonnes per year is achieved. Without implementation of the output enhancing measures, generating the same amount of electricity would have required combusting 40 000 tonnes more coal each year. The requisite total investment was about €30 million. “This is a sound investment, because Farge is now equipped for at least ten more years of power production,” said Henken-Mellies.

E.ON Kraftwerke holds a similar view for its entire power plant fleet. The power producer worked with Siemens on projects similar to that at Farge in 2002 and 2003. In 2002 the Unit F HP turbine at the Scholven coal fired power plant was modernized, significantly increasing its efficiency. In 2003, modernizing of the HP, IP, and LP turbines at the Mehrum power plant resulted in an efficiency improvement of 3.8 per cent and an output increase of 30 MW. The manager of the Mehrum plant said: “The results of turbine modernizing exceeded our expectations, extended plant service life, reduced CO2 emissions by approximately 200 000 tonnes per year and made a substantial contribution to our competitiveness.” Implications for the Scholven plant were summed up by plant manager Rainer Telöken: “Previously we ran the plant at a constant output, now we have gained considerable operating flexibility.”

For Siemens PG, extending the life of steam turbines that are more than 20 years old has become a significant field of business. Modernizing a turbine by fitting advanced blade-design makes it possible to achieve efficiency comparable to that of new plants.

For this reason it is generally cost effective and very favourable in terms of interfaces of the turbine outer casing to only replace the rotor and inner casing together with new blading design as demonstrated by the projects described earlier.

The German company has pursued upgrading of steam turbines since the mid-1980s and in the last five years alone has completely modernized 40 conventional design HP turbines and 45 conventional design LP turbines using the approach detailed above. In the same time frame, 45 turbine modernizing projects have been completed in nuclear power plants and numerous minor modifications performed such as replacement of individual rows of blades or vanes and refurbishment of control and valve equipment installed on turbine island.

The turbine modernization process increases a power plant’s ability to produce electricity cost-effectively and reliably for years to come.