Maintaining momentum at Majuba
Just the name Majuba, meaning “mountain of the doves”, gives an impression of great size. In April, the fourth unit of the massive Majuba power station will be completed. Completion of this unit marks another milestone in a project that first began in 1982.
Forty per cent of South Africans have no access to electricity – a figure that has reduced from a staggering 75 per cent just five or six years ago. The problem is not a shortage of generating capacity, but the huge task of connecting rural areas to the grid.
Nevertheless, government-owned utility, Eskom is more than halfway through building the 4110 MW Majuba plant near Amersfoort on the highveld of Mpumalanga, southeast of Johannesburg. Planning of the coal fired plant began in the late 1970s and the boiler and turbine contracts were placed in 1982. Construction began in September 1983 but a fall off in demand due to a downturn in the country`s economy caused construction of the plant to be deferred twice.
There is still no real demand for more generating capacity, yet Eskom has continued with the project as it prepares for deregulation and further development of the country`s fledgling power pool and a future Southern Africa pool.
Siemens has overall responsibility for building Majuba – a project which presented an interesting challenge. Power from the station is expensive compared to other Eskom plant because coal has to be brought in by rail from collieries in the Witbank area of Mpumalanga. Because all power plants bid on an hourly basis to sell electricity into the South African power pool, Majuba must be operated in a two-shifting mode to become competitive. This is a big task for a plant originally designed for base-load operation only.
With the first three units up and running, Eskom says the plant is handling such cycling adequately. So far, the plant startup rate is 97 per cent and the first three units have achieved a unit capability factor of 99.8 per cent. Eskom has had Duke Engineering carry out an audit of the plant to make sure this cycling was not causing any serious damage.
Boilers: The boiler design had to satisfy a number of key requirements. It had to: allow the use of a range of coals with high ash contents; avoid slagging and fouling in the furnace and convective boiler parts; minimize erosion; have high efficiency and availability; and be highly flexible with respect to unit operation.
The boilers are once-through Benson type boilers built by L&C Steinmàƒ¼ller. The use of this once-through evaporator design allows sliding pressure operation, short startup times and quick response to changing load demands.
High boiler efficiency and reliable steam generation are achieved by a carefully designed firing system. The boilers use an opposed firing system with 30 swirl burners and individual burner adjustment to ensure stable ignition and excess air control.
The burners are arranged in a three burner staggered configuration. This results in a low burner belt heat release which safely avoids furnace slagging. The five burner planes, each supplied by a dedicated tube mill, allow high operational flexibility.
Each burner has its own oil gun to start up and stabilise the pulverised fuel flame at low loads. The boilers have a turndown ratio of about 40 per cent of full load; oil is used if turndown is below this.
The furnace is sized to ensure low flue gas temperature at the furnace outlet; as well as sufficient residence time for good coal burnout. The furnace walls extend to a height of 52 m. The whole boiler is suspended from a grid that forms the top of the 119 m boiler house. This suspension method allows for downward expansion.
Helical tubing is used in the furnace section of the evaporator to provide even heat take-up and stable evaporation under all load conditions to ensure safe, reliable boiler operation.
The boilers have a single-pass design i.e. all heating surfaces are arranged in the first boiler pass. This has some key advantages over a conventional two-pass boiler from an operational and design standpoint.
The main advantages are:
à‚-A simple arrangement of wall panels, tube banks and connecting pipes, which allow the boiler body to expand in all directions and avoid stress points
à‚-No welded connections of wall panels of different temperatures, resulting in lower thermal stresses
à‚-A uniform flue gas flow pattern which eliminates the dust concentration or stratification common in two-pass boilers. This helps to reduce tube erosion.
Boiler feedwater is heated to a temperature of 248à‚°C at a pressure of 21.6 MPa before being fed to the economiser. From the economiser the water passes to the furnace outlet from the wall tubes. The resulting steam is collected in four steam cyclones which also serve as the point where the steam separates from the water under startup conditions.
Saturated steam is led to the superheaters where its temperature is increased to 540à‚°C at a pressure of 17.1 MPa. The superheated steam is then passed to the high pressure turbine. Exhaust steam from the HP turbine returns to the boiler for reheating to 540à‚°C at about 4 MPa before it is passed to the intermediate pressure turbine.
Turbines: Each of the steam turbines designed and supplied by Alstom has a high-pressure cylinder, a double-flow intermediate pressure cylinder and twin double-flow, low-pressure cylinders. The HP cylinder has a single flow in the reverse direction. Both HP and LP cylinders are constructed with an external casing to allow fast starting and rapid load variations.
Cooling system: Majuba is Eskom`s only power station to use both direct and dry cooling. Units 1 to 3 use a forced draught direct dry-cooling system. Although dry cooling systems are more expensive to build, the limited water resources meant that some dry cooling was necessary. However, the rising cost of coal will see Units 4 to 6 use a conventional wet cooling system. The use of different cooling systems means that units 1 to 3 produce 657 MW and while units 4 to 6 deliver 713 MW at full load.
Fuel system: The coal that is delivered to the station has a nominal calorific value of 21.5 MJ/kg and an ash content of between 30 and 35 per cent. The coal is carried by conveyors from the silos to each of the five 1000 ton capacity boiler mill bunkers. Primary air fans blow the coal into the burner region. Secondary air is also used to aid combustion.
The coal combustion produces coarse ash and fly ash in the ratio of approximately 1:10. The coarse ash falls to the bottom of the boiler and is conveyed away for disposal. The fly ash is carried in the flue gases to a fabric filter that removes more than 99 per cent of the fly ash. Majuba is the first Eskom plant to be fitted with bag filters. Since the coal used at the plant is low sulphur coal (0.7 to 0.8 per cent), no deSOx plant is necessary.
Control and instrumentation: The C&I system is supplied by Siemens under a Rand270 million contract. Units 1 to 3 use the Teleperm MEA distribute control system with a touch screen man-machine interface, supported by hardwired push buttons. Units 4 to 6 will use the newer Teleperm XP control system. These three units, unlike 1-3, will be controlled from a single control room that uses the screen and mouse operated OM 650 man-machine interface.
The first three units at Majuba have operated well so far. In 1997 it achieved an average unit capability factor of 95.9 per cent. In 1998, this reached 99.81 per cent. With unit 4 scheduled to startup in April, Unit 5 one year later and Unit 6 in April 2001; the cost of the plant is running to within one to two per cent of cash flow projections. The completed plant is predicted to cost Rand12 billion and Eskom expects the final price tag to be within one per cent of this.
Figure 1. The massive Majuba power station is now nearly two-thirds complete
Figure 2. The Majuba power station is located near Amersfoort