In the early 1990s, following its acquisition by Volkswagen of Germany, Czech car-maker Skoda automobilova, like many other companies around the world, refocused its attention on the core business. In 1995 Skoda therefore transferred its power generation activities, which supply the large amounts of heat and electricity that a modern car factory needs, as well as water supply and waste treatment responsibilities, to a new entity, SKO-ENERGO. This is a Czech–German consortium consisting of Czech companies Skoda automobilova and Stredoceska Energeticka, together with RWE Energie, Energieversorgung Ostbayern AG and VW Kraftwerk (VWK). VWK was itself set up in 1982 by parent company Volkswagen to take on responsibility for electricity, heat, water and compressed air supply to VW plants around the world.
As well as building facilities for supply of service and cooling water, compressed air, natural gas and for sewage treatment, a priority for SKO-ENERGO on its formation was to develop plans for a major upgrade of the power supply system at Mlada Boleslav in the Czech republic, Skoda’s biggest car making facility. The most complex project was a new hard-coal-fired cogeneration plant to provide heat and power to the car factory as well as district heating for 10 000 apartments in the town of Mlada Boleslav. VWK was responsible for overall planning, construction and commissioning of the new plant.
Improving air quality, while at the same time continuing to use locally mined hard coal, was a key objective of the project. In the 1990s coal replaced high-sulphur (2 per cent) lignite in the old CHP plant serving Mlada Boleslav and the Skoda car plant, but even with this improvement in fuel quality, the old CHP station could not have met emission limits that went into force in the Czech republic as from 1 January 1999. The new plant, which entered commercial operation in 1999, is achieving emissions levels of 200 mg/m3 NOx and 200 mg/m3 SO2. Compared with the old plant and CO2 has been reduced by over 60 per cent. In fact, the new plant is well within stiff German emissions levels.
Plant outline
At the heart of the new cogeneration plant, which achieves an overall fuel utilisation efficiency approaching 80 per cent, are two Alstom-supplied hard-coal-fired circulating fluidised bed boilers supplying steam (140 t/h per boiler, 125 bar, 535°C) to two steam turbines supplied by Siemens, KWU, each rated at 44 MWe (condensing mode) or 35 MWe/70 MWt in CHP mode. The turbines can also be fed, through a common header, by a third, back-up, boiler (60 t/h, 125 bar, 535°C), which burns either natural gas or fuel oil. Natural gas is also used for start-up and support in the fluidised bed boilers.
Steam from the turbines is used to heat water for the district heating network system. In addition, to meet peak heat demands, there are three natural-gas-fired hot water boilers (58 MW each) feeding the district heating network. Two of these were part of the original system, while a third was added within the scope of the power plant project. As well as the steam turbines, the Siemens scope also included components of the feedwater heating system and heat exchangers for the district heating system.
Overall, the new plant has a thermal capacity of 300 MWt.
As it is supplying a car factory, very high availability and reliability were key requirements of the power plant. It must also be able to cope with fast load changes and frequent turbine start-ups and shutdowns, as is often the case with industrial power plant applications.
Particular attention was paid to optimising main steam utilisation in the steam turbine to maximise the ratio of thermal to electrical output in summer and winter. This necessitated minimising the cooling steam flow rate in the low pressure range during heat production in winter and optimising the “cold-end” for high electrical power yield in condensing operation in summer.
Due to extreme space limitations, the turbine generators also needed to be of short, compact design.
The steam turbines selected are of the single-casing reverse flow type with central steam admission. They have relatively low thermal stress on the outer casing shells as well as uniform temperature distribution between the top and bottom shells. This prevents excessive casing distortion and consequently tip clearance losses can be minimised. The extraction pressure can be varied between 0.6 and 2 bar depending on the heat demand. That also makes it possible to operate the turbines at subatmospheric extraction pressures, which is necessary in the case of hot-water forward temperatures of less than 100°C.
A turbine stress control system handles start-up and load change. The system calculates the maximum possible rate of load change based on a given life expenditure and the measured turbine metal temperature gradients.
Maximising local supply
As well as improving efficiency and reducing emissions another important objective of the Mlada Boleslav project, according to H -J Paul, Managing Director of VWK, was to help bring “the participating Czech companies up to the standard of Western power plant technology, working methods and management.”
So from the start of detailed design, in 1995, the aim was to maximise local content. The target was achieved in the end but this proved more difficult than first anticipated.
From the beginning the approach adopted by VWK was to break the project down into its main functional packages and to seek the best prices for each on the world market. This contrasted with the traditional approach in the former Eastern Bloc countries, where entire projects were normally awarded to a single “general contractor.” The Czech market was found not to be fully geared up to accommodate the “package approach”, essentially because the Czech companies were generally only used to acting as component suppliers to the general contractors, for example supplying so many meters of pipe, without experience of responding to customer requirements for particular functional packages and preparing proposals themselves. There was competent manufacturing and detailed engineering capability but a distinct lack of “early stage engineering.” So for example, a standard steam turbine was available in the Czech Republic, which was certainly inexpensive, but the supplier could not customise the steam bleeds to meet the particular requirements of the project.
Problems of this sort had been anticipated but perhaps not to the extent that was encountered. Nevertheless the domestic-content target was achieved by bringing in local companies as subcontractors. Siemens, facing tight cost constraints, contracted out a significant portion of its added value to local companies, with their lower wage costs, and it proved possible to obtain the feedwater-heating system components directly from a Czech manufacturer, according to Stefan ter Haar, Siemens project manager. Where contracts were placed with Western European suppliers, eg for the thermal insulation package and acoustic enclosures, the suppliers generally placed the most wage-intensive portions of their added value with Czech companies.
Learning the lessons
Ulf Nagel, head of power plant planning and environmental protection at VWK, sees a major achievement of the project was “to complete a state-of-the-art plant of this kind under these conditions in three years, from beginning of construction to commercial operation”– particularly when it is taken into account that at the start of building the detailed design was not complete and adjustments had to be made well into the construction phase to respond to altered energy demand projections (mainly due to adoption of energy-saving measures). Added to this, the infrastructure was found to be less good than expected.
Among the lessons he draws from the experience for potential future application in similar projects in Eastern Europe is the need to be more rigorous about checking energy demand figures for a particular project and about assessing conditions at site. “You need to look very carefully at the real state of things like pumps and district heating pipework, as outside appearances can be deceptive.”
He also says that inviting bids on a functional basis can be problematic in some of the former Eastern Bloc countries and recommends “trying to use the existing system not working against it.”
Further projects
Nevertheless, the Mlada Boleslav project has generally lived up to its high expectations and is now seen as something of reference project, both in terms of technology and project organisation.
Very similar technology has indeed been applied at another VWK cogeneration project, in Volkswagen’s home town of Wolfsburg, where two new 68 MWe Siemens steam turbine plants went on-line at the end of December 1999, supplying heat and power both to the massive car plant and to the town – with many parallels to Mlada Boleslav.
