OPTIMUM USE OF COAL AND WATER

 Constant research has enabled Eskom to reduce its coal consumption to as little as 0,533 KG/KwH sent out.  This is especially noteworthy considering the low grade of the coal burned (calorific value between 18 and 22 MJ/kg; ash content ±35%).  Not only does Eskom burn coal for which there is no other use, but in so doing it saves millions of tons of better quality coal for export or for specialised industrial use.  The Grootegeluk Colliery, which provides ISCOR with high grade metallurgical coal for iron and steel production, is this able to send the waster low grade coal to Eskom.  This in effect makes Matimba part of a joint venture which assures maximum beneficiation of the colliery.  Ongoing studies to increase the efficiency of water consumption have reduced this figure to 2,02 litres per kWh sent out.  (Matimba has already achieved 0,16 litres per kWh and less).  The decision to build dry cooling power stations is a further step in the conservation of our country’s limited water supplies.

 DRY COOLING VS WET COOLING

 Although the expense involved in the construction and operation of a coal fired power station with a dry cooling system is greate​r than that of one with a wet cooling system, limited available water resources may override economic considerations in determining the choice between the two technologies.  Dry cooled systems consume less than 0,2litres/kWh. Compared with the 2,0 litres/kWh required by wet cooled systems.  Evaporation losses in wet cooling systems account for approximately 80% of the water requirements of a conventional wet cooled power station.  These losses can amount to 1,5 million litres an hour per 600 MW wet cooling tower on the South African Highveld.  The choice of dry cooled technology for Matimba was heavily influenced by the severe shortage of water in the area.

 DRY COOLING TECHNOLOGY

 These are two basic dry cooling systems.  The indirect dry cooling system adopts a principle similar to that used in the car radiator, where heat is conducted from the water to the metal of the radiator and from there to the air passing through it.  The air remains dry, as it does not come into contact with the hot water.  The water is in a closed circuit and evaporation is this minimized.  In the direct system steam from the low pressure turbine is channeled directly into the radiator type heat exchanger.  The heat is conducted from the steam to the metal of the exchanger.  Air passing through the exchanger removes the heat, thus condensing the steam into water to be pumped back to the boiler.  Cooling in both the direct and indirect systems can be achieved either by natural draught in cooling towers or by forced draught using fans.  Cooling in the Matimba direct dry cooling system, however, is by forced draught, whereas an indirect natural draught system is applied at Kendal power station.  The indirect system makes use either of surface condensers as in conventional wet cooled power stations, or of jet condensers.  In the former, cold water flows through the tubes of the condenser, removing heat from the steam passing over them.  This now heated water flows through heat exchangers arranged inside a natural draught cooling tower.  Air to cool the water is drawn through the tower and across the heat exchangers either by natural convection or by electrically driven fans.  In the alternative jet condenser system, exhaust steam leaving the low pressure turbine is condensed by a spray of cold water.  The resultant hot water collects in a sump and is pumped through the heat exchangers in the cooling towers as in the surface condenser system.  In addition, part of the condensed water is led from the sump of the jet condenser to the boiler.  The main advantage disadvantages of this system are the large volume of highly purified water required and the problem of ensuring that no air can leak into the system.  The performance of a dry cooling system depends on the ambient dry bulb temperature.  Temperatures at Ellisras vary as much as 15 degrees from day to night and in summer will frequently rise to the 40s.  Consequently the rate at which the air draws heat from the heat exchangers will vary considerably, as will the back pressure at the turbine exhaust.  This will affect the efficiency of the turbines in such a way that the rated 665 MW might only be achieved when ambient air temperatures are low.

 PLANNING

 The planning of Matimba as a base load station began in 1978.  The six 665 MW turbo generator units make it the largest direct dry cooled station in the world, followed by the single 365 MW unit at Wyodak in the United States.  Annual set out power is approximately 20 000 GWh, with a weighted annual average turbine back pressure of 19,8 kPa.  Construction started in mid 1981 and the first unit was placed on commercial load in September 1987.

 THE BASIC CYCLE

 The coal delivered from Grootegeluk has a nominal calorific value of 20,5 MJ/kg and an ash content of ±36%. From the stockpile coal is transported by terrace conveyors to one of six unit silos, each with a capacity of 4650 tons.  Inclined conveyor belts carry the coal from the silos to each of the five 850 ton capacity boiler mill bunkers.  Two control feeders channel the coal into five rotating ball tube pulverising mills which run at 15,3 r/min and have an operating output of 100 tons per hour.  Primary air fans blow the pulverized coal into 20 boiler burners per unit arranged on five levels.  Forced draught fans add secondary air to aid combustion, and the mixture is then blown into the boiler furnace.  The coal combustion produces coarse ash and fly ash in a ratio of approximately 1:10.  The coarse ash drops to the bottom of the boiler and is conveyed away for treatment and disposal. The fly ash is carried in the flue gases to the precipitators, where more than 99% of it is removed electro-statically.  Induced draught fans draw the cleaned flue gas from the precipitators and discharge it through the chimney into the atmosphere.  Heat released by the burning coal is absorbed by the boiler feed water inside the many kilometers of tubing which form the boiler furnace walls.    /the maximum continuous rating of each boiler at the turbine stop valve is 570 kg/s with a super heated steam temperature and pressure of 535°C and 16.1 MPa respectively.  The steam passes through a super-heater to the high pressure turbine where it expands and causes the turbine to spin at a speed governed to 3 000 r/min.  After exhausting some of its energy in the high pressure turbine, the steam returns to be reheated in the boiler re-heater and passed through the intermediate pressure turbine and then to the tow low pressure turbines.  The generator rotor, coupled to the turbine shaft, is a cylindrical electromagnet enclosed in a gas tight housing. Electricity passes from the stator windings to a transformer which raises the voltage from 20 kV to the transmission voltage of 400kV.

 COOLING AND RECIRCULATION

 Steam from the low pressure turbines is condensed in a direct dry cooled condenser.  To compensate for small losses in the steam/condensate cycle, demineralised make up water is added at this point.  The condensate then passes through low pressure heaters to a deaerator and condensate storage tank.  Form there three (one always in reserve) 50% electrically driven boiler feed pumps feed the water through  the high pressure heaters into an economizer, where it absorbs additional hear from the flue gases before re-entering the furnace tube walls to recommence the cycle.  Direct dry cooled condensers are located adjacent to the turbine house.  Steam from the turbines is condensed in the finned tubing which constitutes the condensers.  There is approximately 400 km of finned tubing to each condenser unit.  Each condenser unit comprises eight rows of six modules each, five of which are condenser modules while on is a dephlegmator module.  Where as the purpose of the condenser modules is solely to condense the steam, the dephlegmator modules also provide for the extraction of incondensable gases and air.  Heat from the steam circuit is removed by air blown over the condense tubing by the forty eight 9,1 diameter forced draught fans beneath each module.

 PLANT

 Control

 Each of Matimba’s six boiler turbine sets is operated from separate unit control rooms.  From the until control room the plant process is controlled and monitored by a sophisticated process computer capable of automatically starting up and shutting down the plant through a keyboard in the unit control desk.  Manual control of the plant may be carried out via pushbutton stations.  Any part of the process and plant conditions can be graphically displayed on video screens.

 Fuel handling

 Coal crushed to less than 25 mm in size is conveyed from the Grootegeluk Colliery at a rate of up to 3 600 t/h.  The maximum strategic and seasonal stockpile has a capacity of 1 200 000 tons and the live stockpile of 120 000 tons.  The five mill coal bunkers per boiler have a storage capacity of 850 tons each sufficient for eight hours at full load.

 Mills

Two coal feeders control the input of coal into each double ended ball tube mill, the amount being determined by the boiler steam output requirements.  Inside the mills an approximately 90 ton charge of steel balls with maximum, diameters of 50 mm pulverises the coal.  Primary air fans then blow the combustible pulverised fuel into the boilers at a rate of 80 tons per hour.  Mill reserve capacity, however, amounts to 20 tons per hour from each mill.  Five mills feed coal through 20 corner mounted boiler burners into each of the six boilers.  When the boiler is steaming at full load, there is one spare mill per boiler, which means that full load is achieved by using 16 burners.

 Boilers

 The once through boilers are the coal fired radiant furnace type with superheating and reheating. The boiler furnace walls extend to a height of 99 m and are suspended from a grid which forms the top of the 199 m high boiler house.  This suspension method allows for downward expansion.  Super heater and re-heater tube banks are suspended horizontally above the furnace zone of the boiler, while the economizer elements are located in the flue gas duct before the air heaters.  Boiler feed water is heated to a temperature of 248°C at a pressure of 21,8 MPa before being fed to the economizer.  From the economizer the water passes to the furnace walls, where evaporation is completed before 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 start up conditions.  Saturated steam is then led to the super heaters, where its temperature is increased to 540°C at about 4 MPa (abs) before passing to the intermediate pressure tube.  The main steam pipe work incorporates a high pressure bypass system, which provides protection against excessive boiler pressure.  It has a temperature control system, using spray attemperators to reduce the steam temperature to a value suitable for inlet into the re heaters in the event of the automatic operation of the HP bypass.  The turbine piping also includes a low pressure bypass.  When this system is operating, some of the condensate from the condensate extraction pumps fed to spray attemperators to cool the re heated steam before this enters the air condenser at the low pressure turbine exhaust.  Use of the bypasses makes it possible to obt