SEPTEMBER 2014 REPORT ESTHER CHIVUNGA PGT(HARARE) – POWER PLANT OPERATIONS Coal receiving Harare Power Station receives its coal supplies from Hwange Colliery, Makomo Resources and Coal Brick which are all situated in the Hwange area in Matabeleland North where coal is mined. The coal is supplied from Hwange to Harare by the National Railways of Zimbabwe (NRZ) using its fleet of wagons. NRZ uses two types of wagons to ferry coal to Harare and these are: small wagons with a capacity of 30tn large wagons having a capacity of 40tn Inside the power station is a railway system for shunting the coal wagons once the coal is delivered by the National Railways of Zimbabwe. Each wagon will have on it a ticket with the following details: name of supplier name of recipient type of coal date loaded gross weight wagon number Before the supplied coal is offloaded several acceptance tests are carried out by the Quality Control laboratory: a. sieve analysis – particles with size<6mm should not exceed 10% b. ash content(non-combustibles) should be less than 13% The diagram below shows the movement of coal at Harare Power Station. JUNCTION TOWER HARARE STATION 3 BUNKER 489ton HARARE STATION 2 BUNKER 102ton NPD STOCK PILE BELT 3 7 A TOWER BELT 2 TOWER Conveyor 10 TOWER RECLAIM HOPPER TIPPLER HOPPER 1 RECLAIM HOPPER PEAS STOCK PILE Fig 1 Coal transportation at Harare Power Station Coal is offloaded from the wagons into the tippler hoppers by a machine called wagon tippler. On the top of the hopper is a screen for trapping larger materials preventing over size material from falling into the hoppers. The coal is then fed onto conveyor belts through vibrating feeders. These feeders are electromagnetic and they control rate of feeding coal onto the belt. If bunkers are full or coal is not required at the boiler house, received coal is diverted to the stock pile or stack yard by belts 1, 7, 7A and 10. This process is called stacking. If coal is urgently required for steam generation the stacked coal is diverted to the bunkers via reclaim hoppers using frontend loaders. This is called reclaiming cycle. Reclaiming belts are numbered 1, 8 and 9. Bunkers are the coal silos found at the boiler house. The coal being conveyed contains scrap metals that are undesirable to the mills hence must be removed. The drive end of the conveyor belt at the junction tower has a rotating magnetic drum that attracts these scrap metals and are then discarded via a chute as shown below. Hopper screen reclaim hopper vibrator Conveyor belt magnetic separator Scrap metal chute Chute to other belt Fig 2 Coal conveying Stock Pile Management Offloaded coal is conveyed to stockpiles by a system of conveyor belts shown above. The coal plant has a total of 11 conveyor belts. Stockpiles serve as long term storage place for coal. Coal stocks are daily updated at the coal plant office and daily communication with the NRZ is done to keep track of all the coal in transit. Coal will normally undergo spontaneous combustion in open space hence the piles are regularly compacted by a front end loader to close possible air pockets. Time and again combustion products of coal on stockpile (oxides of sulphur) dissolve in rain water forming acidic solutions. This has resulted in worrying corrosion of the rail tracks near these piles. Recommendations Welders should provide some protective boards whenever they are doing work since it is only the welder wearing protective goggles, however other persons in the vicinity do not have the protective regalia. Most defects are not attended to in due time eg boiler igniters transformer malfunctioning. The use of a mutton cloth poses a great risk to the employees and the station at large. All work in processes to be barricaded with a red tape so as to reduce risks of falling when walking especially at night.