Enstedtværket competes after turbine upgrade

19 October 1998

Life extension and upgrading of the 630 MWe Enstedværket power plant close to the German border at Aabenraa in Denmark has brought increased output to 660 MWe, 4.6 per cent lower heat rate, reduced environmental impact and a saving in coal burn of 60 000 t/a at very low investment and outage costs. This kind of plant upgrade is now the preferred investment of the World Bank.

The largest north European coal/oil harbour for power station fuel is located at Aabenraa in Jutland. The harbour of the Enstedværket power station complex in the Aabenraa Fjord boasts some 35 m depth of water, allowing it to dock and discharge coal vessels of up to 180 000 ton capacity and 150 000 ton oil tankers. The harbour distributes fuel to all of the other coal fired power stations in Jutland as well as most of those on the island of Funen.

Denmark, which has long been a leader in emissions control and power plant efficiency, is now bound by EU energy legislation and international environmental protection commitments which have led to a Government directive that no more coal fired power plants may be built, but instead must be substituted with renewable energy and natural gas. The utility group ELSAM has had to divide off its 400 kV transmission system Eltra in accordance with the EU energy directive, but there are international conventions and bipartite agreements covering export and import of electricity, particularly with Germany, which have to be honoured.

Unit E3 of the Enstedværket power plant is essentially half-owned by the German utility PreussenElektra, which is entitled to take half of the output of the plant. Siemens KWU of Germany, which supplied the original 630 MWe turbine generator unit of the EV3 plant, has now completed and handed over a major upgrade and life extension to the steam turbine and condenser system resulting in a highly economic increase in power output of 30 MWe with an increase in thermal efficiency of 4.6 per cent (based on 100 per cent volumetric steam flow). This substantial gain in output was effectively achieved with no increase in fuel consumption. A huge market for this kind of upgrade is now emerging with increasing competition in electricity power generation business.

The EV3 power plant

The Enstedværket power plant at Aabenraa is owned by the Jutland utility co-operative Sonderjyllands Hojspændings-værk (SH) The members of the co-operative include, as well as ELSAM – the co-operative organisation of power generators on Jutland and Funen – the German utility PreussenElektra, the nearby municipal utility Stadtwerke Flensburg on the German side of the Danish border, and five local regional power distribution companies.

SH was founded on 14 November 1922 in Aabenraa. On 24 October 1924, the first steam turbine-generator was commissioned at Aabenraaværket with a power output of 2.1 MWe. Today, two coal/heavy fuel oil units, EV2 and EV3, plus an additional large biomass boiler added to the EV3 steam, are available to generate power on a commercial basis. The more recent plant EV3 is the largest and highest merit on the site.

Unit 3 became a constituent part of the Enstedværket operation in October 1979. The 29 hectare area on which the station is sited has been reclaimed from the sea by pumping in some 1.5 million m3 of sand to raise the ground above sea level.

Steam generation

Steam is generated in a Babcock & Wilcox once-through Benson-type boiler. The two-drive fuel-feed system permits either 100 per cent coal or 100 per cent oil firing or any blend of these two fuels which may be switched on-load. At full load, the boiler supplies 542 kg/s of steam at 200 bar and 535°C with re-superheating at 535°C. Full load fuel consumption is some 58 kg/s of coal or 37.5 kg/s for oil. Five out of the six available coal mills each feed six low NOx burners. Further NOx removal takes place in a selective catalytic conversion system. Electrostatic precipitators are used to remove particulates, wet scrubbers are used for desulphurization, keeping emissions below internationally approved limits.

An additional straw and wood chip burning steam generator required by government biomass combustion regulations is housed in the adjacent EV2 boiler house. This is large enough to provide the superheating steam for EV3, to which it is connected in parallel.

In the summer of 1993, the electricity utilities and the government entered into an agreement under which the utilities are obliged to exploit 1.2 million t/a of straw and 0.2 million t/a of wood chips by the year 2000. Such fuels pose some technical problems because of the alkaline and chloride content in the straw which results in increased corrosion of boiler tubes, and which can result in chemically active waste products which are not recyclable.

At EV2, trucks deliver some 12 tons of straw to the plant every 15 minutes during normal working hours. The biomass system can be operated independently, and it could provide district heating backup when EV3 is out of action.

The main parameters of the biomass boiler are:

  • Straw consumption – 120 000 t/a

  • Wood chip consumption – 0 to 30 000 t/a

  • Combusted biomass – 95.2 MJ/s

  • Electricity from biomass – 39.7 MW

  • Proportion of EV3 power generation – 6.6 per cent

  • Annual CO2 reduction – 191 900 t/a

    Turbine - generator system

    The single shaft 3000 r/min steam turbine supplied by Siemens is the largest in the ELSAM service area, generating a nominal 630 MWe before the upgrade, with a maximum peak load output of 685 MWe gross for short periods. The single HP turbine, single IP turbine and two LP turbines plus the generator and exciter stretch to a length of 47 m on a single concrete spring mounted foundation.

    It is a four cylinder reheat condensing turbine with reaction blading and a hydrogen cooled generator. The 17 stage HP barrel type, single flow turbine cylinder is throttle controlled. Induction steam is delivered to the HP blading via four combination main stop and control valves.

    The double flow 12/13 stage IP turbine cylinder is split axially. The reheated steam is supplied to the blading from the top and bottom at the centre of the casing.

    The two double flow seven stage LP turbine cylinders with twisted cylindrical blading are fed from the IP turbine cylinder via four cross-over pipes located on both sides at the level of the turbine building floor.

    Steam bleeds are extracted for process steam output to the Samden milk processing factory, district heating steam, a 20 MWe feedwater pump and closed circuit feedwater preheating. After circulating through the two main condensers, the 80 000 m3/h of cooling water is fed back through fish farms.

    Turbine upgrade

    According to Siemens KWU project manager Michael Rindler and SH Head of Power Plant Group Knud E. Nielson, the main motivating factor behind a decision to upgrade is operating economy or remaining profitable life. This is generally determined by the operator's own evaluation criteria, usually by means of :

  • Long term planning for power plant usage including considerations such as fuel costs and trends in electric power and process heat demand

  • The additional output achievable through the upgrade, while maintaining compliance with warranted plant performance figures

  • The plant outage which should be within the duration of the usual major inspection period whenever possible

  • The conditions for financing including tax considerations

  • Energy policy requirements

    At the Enstedværket power plant, the most economically attractive solution was to improve the heat rate by 4.6 per cent (based on 100 per cent volumetric steam flow). Given the same rate of fuel consumption, this raises output by 30 MWe to 660 MWe.

    The upgrade provides an additional advantage in terms of noise level. Measurements performed on the upgraded turbine - generator indicated a reduction in the noise level of about 2.5 dB(A) over the entire load range.

    A major feature of the upgrading was the use of Siemens' very advanced 3DS curved turbine blades. These blades have a radial curvature to permit aerofoil corrections which reduce secondary losses at the blade tip and root.

    A reaction turbine is characterised by a stage reaction of 50 per cent, and the enthalpy drop is equally divided between across stator and rotor rows. The symmetry in enthalpy drop entails a symmetry in flow relative to stator and rotor flows and allows the same profile to be used for both blade rows.

    In this case, flow velocities and flow directions along the blade path are moderate and profile and secondary losses are quite low. Since half of the enthalpy drop occurs in the rotor row, the pressure differential across the rotor blades is rather high and exerts a large axial thrust on the rotor. In order to compensate for this axial thrust, a dummy balance piston is required in single flow designs. The leakage flow across this dummy piston reduces the turbine cylinder efficiency.

    In impulse turbines with zero stage reaction, the total stage enthalpy is converted into kinetic energy in the stator row, while the rotor row merely deflects the steam without further acceleration. In this case different profiles must be used for stator and rotor blades because their flows are asymmetrical. Flow velocities and flow deflection along the blade path are significantly larger than the equivalent reaction stages incurring higher profile and secondary losses. Since the pressure differential across the rotor is much less than that for reaction turbines, a much smaller dummy balance piston with lower losses can be used.

    Bent blades: Three dimensional blades are said to have the potential to be able to exploit the best of both worlds, and Siemens maintain that by the use of computer controlled CAE - CAD -CAM - five axis milling machines, any three dimensional blade that the design engineer can invent can now be manufactured.

    Particularly for the first stages of HP and IP turbines, a special three dimensional blade with compound lean has been developed by Siemens. In these stages, secondary losses are significant due to the low volume flow rates and short blade lengths. Compound lean and twist both serve to reduce the secondary losses at the root and the tip of blade. The reaction of each stage is set individually and may vary between 10 and 60 per cent.

    Extensive measurements were performed on a four stage test turbine to confirm efficiency improvements of up to 2 per cent – a major gain – compared with conventional cylindrical blading.

    Exhaust losses: To reduce the exhaust losses of both LP turbine cylinders, the exhaust cross section was increased from 10 to 12.5 m3, a milestone never achieved before. Moving blades for the longest last stage blades had to be made from a specially developed high grade 16 per cent chromium steel to carry the high loads encountered. A heating system for the hollow stationary blades was also installed to prevent large water droplet formation causing excessive erosion on downstream blades. The number of rows of moving LP blades with integral shrouding was increased from three to six to reduce tip clearance losses.

    Condenser vacuum: High vacuum in the condenser has been achieved through the use of three stationary level controlled primary cleaning machines as well as high performance filters in the cooling water inlet to the condenser water chambers.

    Continuously operational condenser tube cleaning systems using foam rubber balls to improve heat transfer conditions in the condenser now replace the previous back wash damper valve system for condenser cleaning. The two main cooling water pumps were adapted to meet the new operating conditions.

    Control systems: Valve actuators for the HP, IP and LP bypass valves were modified for individual valve control so that each valve can now be adjusted automatically. The existing I&C system was also replaced by a Teleperm XP process control system. These measures resulted in shorter, more optimised start up and loading times.

    Short shut down

    A sixty five day plant shut down was required to:

  • Upgrade the HP and IP turbine cylinders, requiring replacement of the rotor and the inner casing

  • Replacement of the rotor, inner and outer sections of the inner casing and diffusors of both LP turbine cylinders

  • The turbine hydraulic regulator and control systems.

    Fifty eight days were available for the assembly of the turbine sections.

    By employing up to 160 assembly and commissioning personnel working in two and three shifts, contractual commitments were met. The assembly of the four turbine cylinders was done in only 44 days, and the recommissioning began after only 46 days.

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