Eskom's Majuba: at the peak of its career
21 February 1999The Majuba coal fired power plant in South Africa is coping well in peak load duty, demonstrating the flexibility of its once-through Benson boilers.
Back in 1982 when the first boiler and turbine contracts were placed for Eskom's 4110 MWe six-unit Majuba plant (pictured right) few would have imagined that nearly 20 years later the station would still be under construction, with the final three units due to be handed over in April 1999, April 2000 and April 2001 respectively. Nor would it have been expected that South Africa's most modern large coal-fired plant would be called upon to operate in any other role than baseload.
In fact since the first unit entered commercial operation, in April 1996 (subsequent units entering service at yearly intervals after that) the station has run very successfully as a peaking plant. It is on two-shift operation (ie operating for two of the eight hour shifts in a day). Typically the plant shuts down in the evenings around 10 pm and restarts at 5 am. In 1998 the plant did 171 start-ups. "We weren't designed as a cycling plant, but that's what we are doing," says Ben Steyn, power station manager -- and doing it very well the performance figures would suggest. In 1998 the station recorded a unit capability factor of 99.45 per cent, which makes it one of the world's best performing plants. This stems "from being sold a good product", he says, which is not the kind of unqualified praise one normally expects from power plant operators for their suppliers.
The effect of the cycling has been assessed by a team from Duke Engineering, which concluded that the life of the plant would not be significantly shortened.
An important contributor to this flexibility is the plant's use of once through boilers, of the Benson type. These are particularly suitable for sliding pressure operation, short start-up times and quick response to changing load demands. "Majuba power station makes successful use of all these advantages", says Rainer Cossman of boilermaker Steinmuller (currently in the process of being bought by Deutsche Babcock).
With main steam pressure and temperature of 174 bar and 540°C respectively the boilers at Majuba are not operating in the supercritical regime, which is commonplace with today's state-of-the-art large coal-fired plants and is now required by countries such as China and India for their new large coal fired plants. However supercritical systems require once-through boilers so experience at Majuba is relevant. Also of interest to countries such as China and India is Majuba's successful use of abrasive high ash (around 35 per cent) coal, with a calorific value between 19 and 24 MJ/kg. The ability to use "difficult" coal is another positive feature of the Benson boiler. Eskom prides itself on being able to burn coal in its power stations "for which there is no other use" and "in so doing saves millions of tons of better quality coal used for exporting or for specialized industrial use." It also reports that it has reduced its coal consumption to as little as 0.533kg/kWh sent out, "which is especially noteworthy considering the low grade of the coal burned."
Planning for Majuba, which is located near Amersfoort on the highveld of Mpumalanga province (altitude 1709 m (5607 ft)), goes back to the late 1970s/early 80s when , on the assumption of continued economic growth – orders were placed for three six-unit coal stations, Majuba plus Matimba and Kendal. Construction of Majuba started in September 1983, but was deferred on a couple of occasions because of the effects of sanctions and downturns in the country's economy, at one stage work on units 4 to 6 stopping altogether. Economic growth remains sluggish and there is currently no shortage of capacity in South Africa's generation system, Eskom's installed capacity currently amounting to 38 000 MWe (making it the world's largest coal-burning utility) with additional hydroelectric power available through the recently restored link to the 1800 MWe Cahora Bassa plant in Mozambique.
Nevertheless at times of peak load there is a demand for Majuba's electricity. But the plant must compete in an internal market with other plants on the Eskom system. Each day power stations submit to the "pool" their electricity price bids for each hour of the following day, and this determines whether they operate or not. Majuba's fuel costs tend to be greater than its competitors because the coal has to be brought in by rail, from collieries in the Witbank area. This is why the plant's current role is as a provider of peak load electricity, for which it can get a higher price.
The railing in of coal was not envisaged when the plant was planned because it is sited on what was thought to be a large enough coal mine to meet its needs, the Majuba colliery, which was developed to supply the station. It subsequently turned out that the mine was not able to supply the plant's requirements and it was shut in 1993. At full load (90 per cent MCR) the completed power station would consume about 2055 tons of coal per hour.
Boiler design
Despite the drawn out construction period, the Majuba boilers are "among the most modern in the world", according to Rainer Cossman. As well as being able to work with low grade coal other requirements of the basic boiler design were: avoidance of slagging and fouling in furnace and convective boiler parts; minimization of erosion; high efficiency and availability; and operational flexibility.
An opposed firing system is used, with 30 Steinmuller swirl burners and individual burner adjustment, providing assurance of stable ignition and excess air control. The burners are arranged in a 3-burner staggered configuration, resulting in a low burner belt heat release which safely avoids furnace slagging.
The five burner levels, each supplied by a dedicated tube mill, allow high operational flexibility. The furnace is sized to ensure low flue gas temperatures at the furnace outlet as well as the necessary residence time for good burnout of the coal.
Each burner has its own oil gun to start up and stabilise the pulverised fuel flame at low loads.
For the furnace section of the evaporator the proven helical tube configuration is used, which results in even heat take-up and stable flow characteristics under all load conditions.
Among the advantages of the tower type (single pass) design adopted at Majuba are:
Simplicity. The simple arrangement of wall panels, tube banks and connecting pipes, which allows the boiler body to expand freely in all directions, thus avoiding stress concentrations. No connections between wall components at different temperatures, reducing thermal stresses. Uniform flue gas flow pattern, eliminating the dust concentration/stratification found in two-pass boilers and reducing erosion.At Majuba the height of the boiler furnace is 51.9 m. The height of the gas-tight welded tube walls which surround the furnace and the gas path of the convective heating surfaces above the furnace is about 100m. These walls are suspended from a grid which forms the top of the 119 m high boiler house. This suspension method allows for downward expansion. Superheater and reheater tube banks are suspended horizontally above the furnace zone, while the economiser elements are located at the top of the boiler.
Changing technologies
An unusual feature of the Majuba plant is that its first three units use direct-air-cooled condensers (as at Matimba, see last month's MPS), while units 4-6 will use conventional wet cooling with cooling towers. The direct-air-cooled option was initially selected because of lack of local water sources. Dry-cooled systems consume about 0.4 litres/kWh compared with 2.5 litres/kWh for wet cooling. However wet cooling increases overall plant efficiency, pushing unit capacity from 660 MWe to 715 MWe. Prompted by rising coal prices, it became worthwhile to build a dam at Slang River, some 70 km away, to supply the second three units with cooling water.
The two phases of the Majuba project also provide a vivid illustration of how much power plant instrumentation and control has progressed in recent years. Units 1-3 use Teleperm MEA distributed control technology with touch screen man-machine interface supported by conventional hardwired pushbuttons. But the new Siemens' Teleperm XP system to be used in units 4-6 represents a major leap forward from this earlier technology and these units will be operated by screen and mouse using the OM 650 man-machine interface. Units 4-6 will be controlled from one integrated control room compared with the three individual control rooms used for units 1-3.
The I&C project for units 4-6 is also being implemented with much greater involvement of local people. Siemens says that for these units all its "people resources are local" and "the design, project management and ordering of hardware from Germany are done on site."
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