Medupi and Kusile: supercritical giants of South Africa1 April 2009
The 6 x 794 MW (gross) Medupi supercritical coal fired power plant is the biggest fossil power plant ever ordered by Eskom of South Africa, and will be the first baseload power plant to be built in the country in 20 years. Eskom has also awarded contracts for an identical follow-on plant, known as Kusile.
In the face of severe power shortages, in 2007 South Africa’s state-owned utility, Eskom awarded contracts for two identical coal fired plants. Known as Medupi and Kusile (formerly referred to as projects Alpha and Bravo), each plant will have a generating gross capacity of nearly 4800 MW.
Medupi, ordered just a few months ahead of Kusile represents the largest investment in Eskom’s 84 year history and will be the first baseload project built in the country in 20 years. The combined output of the plants represents about 25% of the country’s current power generation capacity.
The turbine island contracts for both plants were awarded to Alstom. Under these contracts, each worth more than r1 billion, Alstom is responsible for the supply of the steam turbines, generators, associated air cooled condensers (subcontracted to GEA for Medupi and to SPX for Kusile), related turbine island auxiliary equipment and feedwater heating plants. Other major suppliers for both stations include, Hitachi (boilers), SPX (pulse jet fabric filters, air preheaters, and pressure parts), and Clyde Bergemann (on-line boiler cleaning).
South Africa has experienced above average GDP growth in recent times (5.4% in 2006 and 5.1% in 2007), translating into electricity demand growth of about 4%. This resulted in a dwindling reserve margin, which fell from about 20% to less than 10%. Just prior to the financial crisis, reserve margin was estimated to be as low as 5-7%.
The financial crisis has caused a slowdown in economic growth, which in the fourth quarter of 2008 fell to 0.2%, while consumer price inflation (CPI) was 13%. GDP growth in 2009 is forecast to be around 1.2%. The slowdown in growth combined with actions to curb demand has meant that while reserve margin remains low, the load shedding that was taking place a year ago is not expected this year.
Nevertheless the reserve margin is not expected to get above the current low level until 2012, when Medupi and Kusile come on line.
Eskom supplies about 90% of all power in the southern African market. Looking to the future, the company has developed an integrated electricity plan to greatly boost the country’s installed capacity, from the current level of around 39 GW, with a comprehensive strategy
to reduce the risk of power shortages over the
period 2009-2012. The programme includes: installation of open cycle gas turbine power plants for meeting peak demand; a return to service of power stations; an increase in generation by improving operating performance; and the addition of new baseload capacity.
Overall the programme would see the addition of around 40 GW to the grid by 2025. This would be sufficient to meet power demands and sustain an economic growth of around 6%.
With abundant coal supplies, new coal-fired capacity is the natural choice for
new capacity. However other fuel sources are also being considered in order to develop a diversified fuel mix.
Coal has traditionally dominated the energy supply sector in South Africa, from as early as 1880 when coal from the Vereeniging area was supplied to the Kimberly diamond fields.
South Africa produces an average of 224 million tonnes of marketable coal annually, making it the fifth largest coal producing country in the world. Some 25% of production is exported, making South Africa the third largest coal exporting country. The remainder of the country’s coal production is used internally, with 53% used for electricity generation.
About 77% of the country’s primary energy needs are provided by coal, a situation that is unlikely to change significantly over the next decade. Coal reserves are estimated at 53 billion tonnes. At the current production rate there are almost 200 years of coal supply.
Coal for the Medupi project will be supplied to Eskom by Exxaro, the country’s biggest supplier of coal. This supply will consist of 14.6 million tonnes of thermal coal per year over the next 40 years.
Exxaro said it would spend Rand 9 billion ($1.10 billion) on an expansion of its Grootegeluk mine in the north of South Africa in order to meet the new demand from Eskom. The coal for Medupi will be delivered by road from the expanded mine, which will start production in the third quarter of 2011 and ramp-up to full production by 2014.
Much of South Africa’s coal is surface-mined poor quality coal, with high ash and sulphur content. The coal will therefore probably need some washing before being burned in the plant.
New technology, new name
Medupi is located next to the Matimba power station, 15 km west of Lephalale, in the Limpopo province of South Africa. The initial investment decision for the project was made in 2005.
The site was formerly a farm that was bought from Kumba Coal (Pty) Ltd – now known as Exxaro Coal (Pty) Ltd. The site measures 2500 ha and was previously used for game and cattle grazing.
The power station name was changed from Project Alpha to Medupi, which denotes “rain that soaks parched lands, giving prosperity”. Before the 1970s, natural or man-made features in the vicinity were used to name power stations, such as Salt River in the Western Cape and Umgeni in Natal. During the 1970s and 80s the naming convention changed to indigenous words related to the generation of electricity. The convention has been refined with Eskom contracting independent researchers to examine the history of the areas in which new projects are planned, and to identify names that reflect the cultural heritage of the area.
The power island of each of Medupi’s six units consists of a supercritical boiler, steam turbine, generator, and balance of plant equipment.
The superheated steam is generated at 241 bar and a temperature of 560°C. Reheat steam conditions are 50.5 bar at a temperature of 570°C. Steam flow at nominal load is 617 kg/s. At an ambient temperature of 40°C and wind speed of 9 m/s, this gives a gross power output of 794.8 MW, with expected net output to the grid of 764 MW.
Under these conditions, the condenser back-pressure is 141 mbar at a temperature of 23.7°C.
Compared with previous plants built in South Africa in the 1970s, the efficiency levels of supercritical plants like Medupi are much higher, with the new plants expected to have an efficiency that is around 20 to 25% higher than the existing plants – reducing CO2 by about 10% per kWh produced and also resulting in lower water use per unit of power generated.
The turbine island scope for which Alstom is responsible, as well as the items listed above, also includes the turnkey supply of the turbine hall, comprising the steel structure and equipment inside, and the turbine control and protection systems. Alstom is also responsible for the turbine hall cranes, valve piping and local instrumentation, ventilation and fire protection.
In addition Alstom has limited electrical scope covering the busbars, generator circuit breaker, generator current transformers and generator transformer protection.
In terms of arrangement, Medupi and Kusile’s optimised design has the steam turbine generator set at low level, with the turbine axis at 10 m and lateral exhaust LP casings.
The once-through, tower type Benson supercritical boilers (editor’s note: to be described in more detail in a future article) are capable of burning a wide range of coals with diverse ash characteristics.
The steam parameters are: SH, 564°C/258 bar (25.8 MPa); and RH, 572°C /53 bar (5.3 MPa). Steam capacity per boiler: 2288 t/h.
The fuel is bituminous coal.
Each boiler has thirty low NOx burners, with staged combustion.
The 3 x 50% boiler feedwater pumps are configured with a fixed speed motor (20 MW power rated each) and a variable speed coupling. There is a condensate feed extraction pump with a 1 x 100% variable frequency drive plus 1 x 100% fixed speed drive.
Both Medupi and Kusile will use Alstom’s STF100 steam turbines. Each turbine consists of an HP section, IP section and LP section.
The HP section is of double shell design with an outer and an inner casing. The inner casing carries the stationary blading. The parting plane of the outer casing is horizontal at the level of the rotor axis. The outer casing is assembled by means of hydraulically tightened expansion bolts. The inner casing is mainly assembled by means of shrink rings. Flange bolts are only used at the inlet section.
The rotor is of the welded type with integral coupling halves.
After passing stop and control valves the steam flows through a prolonged valve diffuser to the inlet scrolls of the inner casing. These scrolls are designed to harmonise the steam flow upstream of the first blading row. In addition a first stationary radial blade-row optimises the steam flow for most efficient expansion. After expansion through the axial blading, the steam is exhausted via a nozzle at the bottom of the turbine casing. A balance piston in front of the blading is used to compensate for the axial thrust caused by the rotor blading.
The IP casing is also of the double shell design with an outer and an inner casing. The inner casing carries the stationary blading. The parting plane of the outer and inner casing is horizontal at the level of the rotor axis. The rotor is also of the welded design with integral couplings and both casings are assembled by means of hydraulically tightened expansion bolts.
The steam passes the main valves and is then fed directly into the blading path. Intermediate pipes connect the valves with the inner casing. After expansion in the double flow axial blading, the steam is exhausted via two nozzles at the turbine outer casing upper part. The inlet valve casings are connected to the IP section via an intermediate pipe on either side of the turbine.
Steam for feedwater heating is extracted at certain points along the blade path. The extracted steam is gathered in pockets, integrated in the inner casing, and exhausted to the preheaters by nozzles at the turbine bottom.
After passing the crossover pipe the steam enters equally into the two LP casings, with a double-flow configuration. At each LP inlet, an inlet scroll is used to distribute the steam smoothly to both LP flows. After expansion the steam is exhausted horizontally to the air cooled condenser.
Again, the LP casing is of double shell design. The welded outer casing consists of a split upper part and a single lower part. The upper part can be removed for inspection or maintenance without cutting the connection to the condenser neck.
The lower part is supported directly on the foundation. The upper and lower casing halves are bolted at the parting plane at the level of the rotor axis.
The inner casing is of cast design. If LP extraction is required, the extraction chambers are integrated in the inner casing. The casing is split at the level of the rotor axis and bolted together using hydraulically pre-stressed bolts.
The steam turbine is directly linked to a generator, which is of the Alstom GIGATOP 2-pole hydrogen and water cooled type.
The generator stator winding is water cooled while the rotor and stator core are directly hydrogen cooled. This cooling system ensures a high level of efficiency from full load to part load. Its two-plate design also allows it to deliver reactive power to the grid for stabilisation in the event of a disturbance.
The generator features a re-tightenable end-winding that simplifies maintenance and increases the generator’s availability. It is axially flexible to allow thermal expansion but is rigid in the radial and tangential directions so that it can withstand high electromagnetic forces.
A triple circuit hydrogen sealing system is used instead of a double circuit system, which keeps the hydrogen at very high purity levels and reduces the consumption of hydrogen. This results in sustained efficiency over the long term and lower operational costs.
The shortage of water in South Africa dictates the use of air-cooled condensers (ACC) at Medupi and Kusile. The ACC condenses exhaust steam from the steam turbine and returns condensate to the boiler. The ACCs at Medupi and Kusile will be the largest in the world, each occupying an area of more than 72 000 m2. The current record is held by the 3600 MW Matimba power station.
Alstom has sub-contracted the design, manufacture, supply and erection of the ACCs for Medupi to GEA Aircooled Systems, based in Germiston, near Johannesburg (see panel, p22). GEA will also supply the entire steel structure including the supporting steel structure and fan deck, fan rings, the wind walls on the fan deck, the steam duct, the condensate, air and steam piping and the electrical controls.
The main components of the ACC include the air-cooled steam condenser modules, which are based on GEA’s A-tube design. This design has already been used at the Matimba and Majuba projects. The condenser modules consist of galvanised air-cooled condenser tubes, tube sheets and the steam and condensate collection headers.
The other main components include the air moving system. This comprises fans, gearboxes, couplings, electric motors, fan support bridges, as well as all relevant auxiliary components such as condensate tanks, drain pumps, steam ejectors, rupture discs, and the bundle cleaning system.
In addition to the ambient temperature, wind speed and direction will have a big impact on the performance of the ACC, which has an effect on the turbine back-pressure and, therefore, power output of the plant. GEA thus carried out computational fluid dynamics (CFD) studies to predict the airflow distribution around and inside the ACC, as well as the impact of the air distribution on its performance.
On the back of the Medupi order, Eskom awarded Alstom a second contract, to build the Kusile power station, a nearly identical project located at Emalahleni, in the Witbank area. This is in Mpumalanga province, 140 km east of Johannesburg and just a few kilometres north of the site of the existing Kendal power station.
The project will also include additional air quality control systems. In particular Kusile will have South Africa’s first flue gas desuphurisation (FGD) system (in the tendering process at the time of writing). FGD is required because the Kusile plant is in the Greater Witbank where, according to Eskom, existing atmospheric pollution is perceived to be a challenge.
The ACC is also of a different design since Alstom has sub-contracted the design, manufacture, supply and erection of the ACCs for Kusile to DB Technologies, an SPX company. The different ACC will slightly affect the layout of Kusile relative to Medupi but the turbine island and overall performance of the plants will be the same, being only affected by different site conditions (mainly altitude and air temperature).
Contracts and milestones
Medupi and Kusile are multi-contract projects, with Eskom awarding the contract in a number of packages. This is a typical Eskom approach, where the company takes on the role of integrating the packages, effectively taking on the role of architect engineer.
For Medupi, Alstom received a first expression of interest in late 2005. It was an open bid where all the key suppliers were asked to submit a company profile and describe their capabilities in supercritical technology. The actual enquiry for the plant came through in 2006.
While the bid was submitted in 6-8 months, due to the size of the project, the adjudication was a lengthy process. With Eskom being a public sector company, it one of South Africa’s largest ever public sector projects. This meant it was subject to extensive external auditing. The adjudication process lasted for most of 2007. The Notice to Proceed was finally given on 1 October 2007 and the contract signed in November.
At the time of writing Medupi is in the early stages of construction, at around month 16 of the 48 month overall schedule for handover of the first unit.
Unit 6 will be the first to come on line followed by units 5, 4, 3, 2 and 1, with a 5-month gap between each unit. The last unit will come on line in 2013.
Alstom has completed a major portion of the equipment design and purchased a major portion of the equipment.
Alstom site mobilisation began in September 2008 and will be significantly ramped up this year. Site activities are under the control of Eskom, the first equipment erected by Alstom on site being the air-cooled condenser.
Construction of the ACC, which involves largest Potain crane ever deployed in Africa, started with the building of the 50 m high columns, on the top of which the platforms, fans, gearboxes and ACC modules will be installed (see picture at beginning of the article).
The next main activity at the Medupi site will be the construction of the steel structure of the turbine hall. Excavations have been completed and foundation works for the turbine hall are ongoing.
The schedule for the Kusile project is the same as Medupi but about one year later. The start of the 48 month overall schedule period for Kusile began in December 2008. Advance engineering was performed between January and December. The first unit will be operational at the end of November 2012, and the sixth unit will be operational by December 2014.
Fitting into a project schedule that involves a number of suppliers providing different packages at different dates, can be a challenge, requiring the accommodation of several interfaces during the design and during the implementation of the works at site. There will be a significant amount of concurrent activities around the turbine island and site activities pose one of the biggest challenges of the project. It will call for careful control of the interfaces, timely assessment of the interfaces and good co-operation between the subcontractors at site.
Although this is not a turnkey project, Alstom has used its Plant Integrator approach to optimise the interface between the turbine and the ACC and at the request of Eskom, Alstom is also acting as technical co-ordinator between the boiler, control and instrumentation, electrical and turbine packages, allowing full application of its Plant Integrator concept to the development of the water/steam cycle parameters and its thorough knowledge of the unit load control philosophy to ensuring smooth future operation of the power plant.
By the time the first Medupi unit is commissioned, it is clearly essential that the necessary transmission infrastructure is in place, namely the Medupi–Marang and the Medupi–Dinaledi lines, which will take about two years to complete.
The Medupi–Marang transmission line will call for the construction of a 400 kV transmission line of some 300 km in length from the Medupi substation near Lephalale to the Marang substation near Rustenburg (North West Province) as well as the construction of a 400 kV transformer bay at the Medupi substation and a 400 kV feeder bay at the Marang substation.
The Medupi–Dinaledi transmission line requires the construction of two 400 kV transmission lines of approximately 350 km between the Medupi substation, the Spitskop substation near Northam (Limpopo province) and the Dinaledi substation near Brits (North West Province) as well as the construction of two 400 kV transformer bays at Medupi substation, four 400 kV feeder bays and two 400 kV lines at Spitskop substation, and two 400 kV feeder bays at Dinaledi substation.
Once completed, the lines will provide supplementary energy to meet the growing need in the Brits and Rustenburg areas.
Helping the community
Alstom has set up a fully-fledged regional execution centre in South Africa capable of engineering, procurement, construction etc and support functions such as human resources and finance. The office has grown since its establishment in September 2007 and now employs around 120 people, with about 95% of staff within the execution centre being locals.
Around 80% of all turbines in operation in South Africa are Alstom-designed and the company currently employs over 5000 people to meet new power project commitments, and aiming to recruit hundreds more. Alstom and its contract partners will create more than 300 new jobs in South Africa for the Medupi project, with similar numbers on the Kusile project.
At its peak, Eskom estimates that the Medupi project alone will employ 9000 people on site, most of whom will be sourced locally. As a supplier to Eskom, Alstom has a contractual obligation to be BEE (Black Economic Empowerment) compliant. Elements of BEE compliance include black ownership, and the employment and development of black staff, particularly women. BEE encourages procurement from black suppliers that are BEE compliant.
Priority will thus be given to previously disadvantaged groups in all jobs created. Local skills will be developed through training people as managers and technicians.
A wide range of mechanical and electrical skills will be transferred to local industry through a period of three to four years of training in such fields as design, building, maintenance and electrical technology.
Local industry will also benefit from the new power projects, which are required to have 50% local content, with a number of major components to be manufactured in South Africa (as well as Europe and China).
This high level of local manufacture will create significant challenges, especially in conjunction with the ongoing infrastructure development related to the 2010 World Cup. Indeed, South Africa is currently experiencing a period of unprecedently high capital investment, with consequent strain on the local supply chains. But so far South Africa’s indigenous industry is coping and seems up to the task of delivering the skills and equipment required to bring these giant projects in on time.