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Power Plant Operations report esther

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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.
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