Increasing coal plants’ efficiency has been a focus for many years now and turbine retrofits currently being executed by Alstom will make an important contribution to raising performance. By overhauling low-pressure, intermediate-pressure and high-pressure turbines, the specific fuel requirements – and therefore the CO2 emissions – of the 30-years old Jänschwalde lignite plant in Germany, have been reduced significantly.
Roman Baumgaertner, Alstom Power, Germany
Located near the village of the same name in Brandenburg on the German-Polish border, the lignite fired Jänschwalde power plant has an installed capacity of 3000 MW and consists of six 500 MW units. It is the second-largest brown coal power plant in operation in Germany and is owned by Swedish utility Vattenfall.
Originally built to generate 500 MW each, Blocks A and D now produce 535 MW with less steam. In future, despite lower fuel consumption, the six units will supply a total of 3210 MW instead of 3000 MW – an increase equivalent to almost half a power block.
Retrofit projects replace or add equipment to existing power plants to improve their energy efficiency, increase their output and extend their lifespan, whilst lowering emissions. Alstom Power replaces older turbine components including complete high, intermediate (IP) and low-pressure (LP) turbines.
The key components at Jänschwalde have gradually been modernized since the plant entered commercial operation. A look at the timeline confirms this: Block A of the plant went onstream in March 1981, with the final block being completed some seven years later. Just three years after that, in 1991, major conversion measures were implemented.
Their key focus was environmental protection. Within the scope of the comprehensive revision, the boilers in Block A were converted for low nitrous oxide combustion and the electrofilters were modernized. The same measures were subsequently performed on the other blocks. Half a decade later, decommissioning of the three old chimney stacks and commissioning of flue gas desulphurization for all blocks marked a milestone in the history of the power plant. Its emissions were now below the legal thresholds.
In the same decade, one modernization ensued which achieved increased reliability and power. Block E began operation in 1994 as the first of the plants with new control technology and LP turbine sections. The low-pressure sections in the other blocks also underwent systematic modernization.
As a result, not only have LP turbine failures been avoided – their geometry and the medium makes LP final stages particularly susceptible to wear – but also a further important contribution has been made to increasing efficiency.
More power from fewer, new components
The second, fundamental step towards a power increase was taken between 2003 and 2006 when Alstom replaced the high-pressure turbines in the six blocks. Wear was not the main reason for this; the actual deciding factor was the relationship of investment to efficiency gains, which proved to be particularly attractive.
Measuring and precision tasks: mechanical wear, strength and form retention are checked by the service technicians on the LP turbines in intervals of around two years Source: Vattenfall
The power plant is currently midway through a phase in which all intermediate pressure turbines are being replaced, which will complete the modernization of the turbine sets. The original impulse machines were produced by the Leningrad Metal Works (LMZ), St. Petersburg. Within the scope of the forthcoming revisions, these turbines are being upgraded using the latest technology. The results of the implementation in the first two blocks have been encouraging.
New design boosts power by more than 6 MW
Block D was the starting point of the project. It was subjected to a major overhaul that involved the inspection of all turbines. There was little to do on the comparatively new high-pressure turbine, and work was limited to testing and, where necessary, overhauling valves and other auxiliary equipment.
However, on the low-pressure turbines, defects became apparent and investigations resulted in the rotors being transferred to the Alstom plant for reworking. Because the damage was discovered at the start of overhaul work, extension of the outage was kept to a minimum.
The intermediate pressure turbine, which had completed around 70 000 operating hours since its last overhaul, was to be replaced anyway. As the rotors and blade path exhibited advanced wear in the first stage area, the retrofit made sense for this reason alone. Furthermore, it was to lead to an increase in power.
The complete renovation of the inner workings, while retaining the outer casing, proved to be the optimum economic and technical solution. To ensure the increase in power had no impact on other power plant components, all steam parameters – including temperature and mass flow at the three extraction points – were to remain the same.
Alstom Power brought together impulse and reaction steam turbine technologies. This capability allows retrofit solutions to be supplied utilizing reaction technology, impulse technology or a combination of both, irrespective of the original blading technology, to provide an optimum result.
Applying reaction blading design to the existing Jänschwalde units, which originally had an impulse design, proved to be the optimum replacement. To ensure that the new design, which is equipped with more stages, fits into the existing outer casing, the inner casing including the blades were renovated as well as the rotor.
Thanks to the two flows with a total of 19 blade rows (or pressure stages) many options were available for steam extraction to preheat the feedwater and supply the district heating network in the city of Cottbus. Complementing this, the outer casing was reworked, as it had been slightly deformed by decades of thermal stress.
The new IP steam path was designed to achieve maximum performance within the constraints of a given outer shell of an existing turbine. Advanced 3D reaction type blading was applied to improve the aerodynamic efficiency while maintaining enhanced mechanical properties. The rotor and inner casing were designed to allow for optimum stage loading while at the same time matching the steam conditions at the various extraction points to the overall steam cycle requirements.
Besides blade twist to control span wise reaction, the advanced 3D reaction type blading also benefits from small fillet radii at the root and the top area of the blade profile, as well as from thin trailing edges, all of which contributes to high efficiency figures. Castellated ribs machined into the fixed and moving blades’ integral shrouding – in conjunction with sealing strips caulked into the outer surface of the rotor and inside surface of the cylinder respectively – form effective labyrinth type sealing.
Thanks to the new design, improved blade geometry with modern 3D reaction blades, as well as an optimized sealing arrangement for both the stationary as well as the moving blade rows, the IP turbine is now 3.3 per cent, or 6.3 MW, more efficient than before. Whilst this may not seem much, it means that, for the same yield, 47 000 tonnes less fuel is required each year.
That equates to the amount of coal carried by 50 coal trains, each of 16 wagons, from the neighbouring open cast mine to the plant. The reduced coal consumption also means a drop in emissions; the IP retrofit spares the environment around 46 000 tonnes of CO2 annually, illustrating the environmental benefits associated with retrofit.
The modernizations performed over the last two decades have increased the block output of 500 MW by approximately 35 MW and the net efficiency to around 36 per cent. Whilst this value may not sound particularly high in comparison with today’s new plants, it actually represents very significant progress, given that the 1970s design boilers limit the steam parameters and thus the efficiency.
As a result of the significant amount and quality of work performed on Block D in 2009, as well as adherence to the schedule, Vattenfall nominated Alstom Power Service, Berlin, for the VGB PowerTech Quality Award 2010 and, from the many submissions, the project emerged victorious.
The prize honours the work of the 40-strong Alstom team in the major overhaul of the 500 MW turbine set in Block D and the minor overhaul of the set in Block C.
Retrofit work on Block A carried out in 2010 significantly exceeded the contractually agreed efficiency values. The work performed on Block A was essentially the same as that done on Block D the previous year. The team pooled their experience and paid particular attention to the low pressure turbine. The LP rotors were subjected to even more rigorous examination in light of the results of the investigations of Block D in 2009 and were overhauled accordingly.
However, things are still far from returning to normal for the Alstom team in Jänschwalde now that the overhaul and modernization of Blocks D and A is complete. It is planned to retrofit unit C in summer 2011. Over the next few years, the remaining three blocks will also be subjected to major overhauls, over the course of which they will be equipped with new IP turbines. Then, there should then be little to stand in the way of economic and failure-free operation.
The co-author of the article is Marco Rediess, Vattenfall, Germany.
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