Matimba – the
Tsonga word for “power” is an appropriate name for the dry cooled power station
near Ellisras in the Northern Western Province. Designed to generate 3
990 MW, Matimba has the world’s largest turbo generators using the direct dry
cooled steam condensing system. The adjacent Grootegeluk Colliery has
sufficient coal reserves to guarantee Matimba a minimum lifespan of 35 years,
extending to a possible 50 years, at 3 580 tons of coal per hour.
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 greater 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 obtain the correct
steam pressures and temperatures before starting up the turbo generators, and
also allows the boiler to continue operating at about 45% load after the
turbine has been tripped, while minimizing thermal shock on the piping
systems. Each boiler is equipped with 20 burners arranged in five levels
of four. Each burner has its own oil gun to start up and stabiles the
pulverized fuel flame at low loads. The forced draught fans supply
secondary air to the burner wind boxes, while primary air fans supply air to
the coal mills to carry the air fuel mixture to the burners. Two induced
draught fans draw the combustion gases from the furnace over the surfaces of
the super heaters, re heaters, economizer and air pre heaters, then via the
electrostatic precipitators for discharge to the chimney.

Turbines
Each of
Matimba’s six sets has a high pressure cylinder, and intermediate pressure
cylinder and twin double flow low pressu8re cylinders. The HP cylinder
has a single flow in the reverse-direction, whereas the IP cylinder has a
single flow in the normal direction. Both HP and IP cylinders are constructed
with an internal and external casing to allow fast start up as well as rapid
load variations.
Generators
Each generator
produces 665 MW at full load, with a terminal voltage of 20 kV at 50 Hz.
Generator cooling as achieved in two ways. Hydrogen is circulated through
the rotor at a maximum pressure of 400 kPa by two fans locked to the rotor
shaft. The hydrogen is cooled via water cooled heat exchangers mounted on
the generator. The stator winding is cooled by demineralised water pumped
through the stator winding bars and then treated and cooled in an auxiliary
plant external to the generator. The generators operate at 20 kV and
their output is stepped up to 400 kV for distribution via the national grid.

Ash collection
and disposal
At full load
each boiler produces up to 280 tons of coarse ash and 2 500 tons of fly ash
from the precipitators per day. The fly ash is conditioned to a moist
cake from before being mixed with the coarse ash. The mixed ash is fed to
an overland conveyor system, which transport the ash to the surface dump.
Stackers spread the ash over the dump, which is eventually covered with a 200
mm layer of topsoil and then grassed.
Condensers and
feed water
The condensers
are of the dry cooled direct condensing type. Exhaust steam from each
turbine passes through two interconnected 5 m diameter ducts to finned tube
elements, located 45 m above ground level and adjacent to the turbine
house. Each condenser module consists of eight bundles of cooling
elements. There are 119 finned tube cooling elements in each
bundle. Forced draught fans, 9,1 m in diameter and situated beneath each
module, blow air over the elements to cool the exhaust steam in the finned
tubing. The elements are constructed of mild steel, galvanized on the outside,
with a total air side surface area of 1 200 000 m². The 48 fans rotate at
25r/min and are driven through gearboxes by single speed electric motors
mounted vertically. The condensate is collected in a receiver tank inside
the turbine house. One of two condensate extraction pump drives the
condensate through low pressure feed heaters to the de-aerator and from there
through the high pressure feed heaters. The condensate and feed water are
heated in six stages by steam extracted from the turbine cylinders to a
temperature of 274°C at the boiler inlet. Two variable speed pumps driven
by 9,6 MW electric motors pump the feed water, while a third pump is provided
as standby in the event of failure.
Cross-section of Condenser Mode
Technical Data
|
|
Generating
capacity
|
3 600
MW
|
Fuel
|
|
Mining
Company
|
Kumba
Rescources, Grootgeluk Mine, Lephalale
|
Calorific
value Mj/kg
|
Between 16
and 22
|
Moisture
content -
|
|
Surface
normal
|
9,3 (max)
4,1 (min)
|
Inherent
|
2,6 (max)
1,0 (min)
|
Acceptable
limit
|
10,0
|
Ash content
|
±36%
|
Total
annual production
|
12 million
tons
|
Coal storage capacity
|
|
Strategic
and seasonal
|
1 200 000
t
|
Stockpiles
|
|
Live
stockpile
|
1 200 000
t
|
Unit silos
(6 x 4 650 t)
|
27 900
t
|
Station
boiler bunker capacity
|
25 500
t
|
Coal
consumed at full load
|
1 750
t/h at 90% load factor
|
Milling Plant
|
|
Manufacturer
|
Stein
Industries
|
Type
|
Horizontal
ball tube type mills
|
Number
|
5 per
boiler (30)
|
Speed
|
15,3 r/min
|
Rated max
output
|
100 t/h
|
Boilers
|
|
Manufacturer
|
SIEVA
|
Number
|
6
|
Height
|
119 m
(overall)
|
Maximum
continuous rating
|
570 kg/s
|
Final steam
temperature
|
540ºC
|
Final steam
pressure
|
16,1 MPa
|
Steam
output
|
2 016
t/h
|
Number of
burners
|
40
|
Combustion
chamber volume
|
17 223
m²
|
Total
heating surface
|
225 950
m²
|
Width of furnace
|
|
Dimensions
(plan)
|
17,7 m x
17,7 m
|
Furnace
flame temperature
|
Approximately
1 500ºC
|
Gas
temperature at furnace exit
|
1 201ºC
|
Gas
temperature at ID fan outlet
|
130º
|
Fuel
consumption per year
|
2 000 000
tons maximum
|
Turbines
|
|
Manufacturer
|
M.A.N.(Germany)
|
Type
impulse
|
4-cylinder
tandem
|
Rating
|
665 MW
|
Speed
|
3 000
r/min
|
Steam
pressure, HP inlet
|
16,1 MPa
|
Steam
pressure HP inlet
|
16.1 MPa
|
Steam
pressure HP inlet
|
535ºC
|
Steam
pressure IP inlet
|
3.69 MPa
|
Steam
temperature IP inlet
|
535ºC
|
Exhaust
steam back pressure
|
19.8 kPa
(yearly average)
|
Generators
|
|
Manufacturer
|
Alsthom Atlantique
(France)
|
Rating
|
739 MVA
|
Rated
capacity
|
665 MW
|
Terminal
voltage
|
20 kV 50 Hz
|
Power
factor
|
0.9 lagging
|
Cooling medium:
|
|
Stator
core and rotor
|
Hydrogen at
250 kPa (normal)
400 kPa
(maximum)
|
Stator
winding
|
Demineralised
water
|
Generator transformers
|
|
Manufacturer
|
ASEA
Electric
|
Rated
capacity
|
700 MVA
|
Terminal
voltage
|
Primary 20
kV
Secondary
420 kV
|
Cooling system
|
|
Manufacturer
|
GEA
Air-cooled Systems (Germany)
|
Number of
systems
|
6
|
Type
|
Forced-draught
direct condensing
|
Annual
energy rejection
|
7 500 GWh
|
Plot area
of condenser
|
6 000 m²
|
Air side
surface area
|
120 ha
|
Mass flux
|
33 500
kg/s
|
Overall dimensions of platform
|
|
Width
|
85 m
|
Length
|
72 m
|
Height
above ground
|
45 m
|
Number of
fans
|
48 per unit
|
Fan
diameter
|
9.2 m
|
Diameter of
main Steam ducts
|
Approximately
5 m
|
Chimneys
|
|
Manufacturer
|
CONCOR
Construction (Pty) Ltd
|
Number
|
2
|
Type
|
3 flues
with windshield
|
Height
|
250 m
|
Top
diameter
|
21.1 m
|
Base
diameter
|
22.7 m
|
Main Contractors
|
|
Earthworks
|
LTA
Construction (Pty) Ltd
|
Civil
Engineering
|
CONCOR
Industrial (Pty) Ltd
|
Steelwork
(subcontractor)
|
GENREC
(Pty) Ltd
|
Boilers
|
SIEVA (Pty)
Ltd
|
Turbine
generators
|
M.A.N
(Germany) Altshom (France)
|
Generator
transformers
|
ASEA
Electric (SA) Ltd
|
Chimneys
1&2
|
CONCOR
Industrial (Pty) Ltd
|
Main
station cabling
|
Industrial
Electrical (Pty) Ltd
|
Unit
control and Instrumentation
|
Siemens Ltd
|
Fire
protection system
|
C.I.W (Pty)
Ltd
|
Coal
handling on terrace
|
LTA MITEC
|
Coal
stockyard conveyor system
|
Bateman
Engineering Ltd
|
Ash
conveyor system
|
Babcock
Moxey
|
Dust-conditioning
plant
|
Babcock
Claudius
|
Auxiliary
cooling system
|
Hamon
Sobelco
|
Sewage
plant
|
Aquazur
Reunert (Pty) Ltd
|
Electrostatic
precipitators
|
Simon
Carves Africa
|
Water
treatment plant
|
Simon
Carves Africa
|
Direct
dry-cooling systems
|
GEA
Air-cooled Systems
|
MATIMBA – PROJECT OF POWER
Near
Ellilsras in the north western Transvaal a different kind of pioneer is
breaking ground and establishing new frontiers among the border people. Matimba, Eskom’s giant coal-fired, dry-cooling
power station, further consolidates Eskom’s position as a world leader in
dry-cooling technology for power generation.
The power station generates 4 000 MW, enough to provide for the
energy needs of six cities the size of Durban.
The dry-cooling technology is especially significant in this area of
South Africa where water resources are scarce and unreliable. Dry-cooled systems consume approximately 0.4
litres/kWh compared to the 2.5 litres/kWh required by wet-cooling systems.
Statistics
Planning
started in 1978
Construction
started in 1981
First
unit completed in 1987
Completion
1991
Six
turbo-generator units will generate 4 000 MW
Six
unit silos each have a 4 650 ton capacity
Five
boiler mill bunkers each have an 850 ton capacity
Each
of the six boilers is 100 m high
Each
boiler weights 3 000 tons
There
is enough space in each boiler to provide parking for 900 cars
Each
boiler consumes 7 680 tons of coal per day
At
full load each boiler produces up to 280 ton of coarse ash and 2 500 tons
of fly ash per day
The dry-cooling
technology
Matimba
is the largest direct dry-cooled station in the world. Matimba uses the direct dry-cooled steam
condensing system. The principle,
operate in much the same way as a car radiator, where heat is conducted from
the water to the metal of the radiator, and from there to the air passing
through it. Exhaust steam from the
turbine is conducted into the dry-cooling elements or heat exchanger. Forced draught fans drive air through the elements
of the exchanger so that the heat is removed from the steam to condense
it. The water which results is then
pumped back to the boiler. Since this
water is in a closed circuit, there is no evaporation – an advantage especially
in areas where water is scarce.
Evaporation losses in wet-cooling system on the Highveld can amount to
as much as 1.5 million litres an hour per 600 MW wet-cooling Tower.
Approximately
80% of Matimba’s equipment will be locally manufactured. This is obviously a boon to South Africa’s
industry – Matimba has in fact provided work for many people.
The
fly ash produced from coal combustion is carried to electrostatic precipitators
where 99% of it is removed from the flue gasses. It is then conditioned to a moist cake form
and mixed with the coarse ash which has been collected at the bottom of the
boiler. The mixed ash is fed onto an
overland conveyor which transports the ash to the surface dump. Stackers spread the ash over the dump which
will eventually be covered with a 200 mm layer of topsoil and then grassed. Trees can also be grown in pockets of
soil. This allows the entire dump to be
rehabilitated, so that the land is returned to its original condition.
WHAT IS DRY-COOLING
AND HOW DOES IT WORK?
CONVENTIONAL COOLING
In a conventional power station the
energy of steam at high temperature and pressure causes the turbines to spin
and generate electricity. Exhaust steam
from the turbines is converted back into water in the condensers, where it is
passed over tubes containing cold water. This condensed steam is pumped back
into the boiler to repeat its cycle. The
water in the condensers must in turn be cooled before re-use, and this is done in the cooling
tower, where the warm water is sprayed into the natural up-draught in the
tower, losing its heat as it falls like rain into the pond at the base of the
tower. During this process about 1.25
million litres an hour is lost in evaporation.
Conventional power stations require an average of 2.5 litres of water
for ever kilowatt-hour sent out – approximately 35 000 million litres of
water a year for one 3 600 MW station.
The Principle of Dry Cooling
Dry-cooling adopts a principle similar
to that used in the car radiator, whereby heat is conducted from the cooling
water to the metal of the radiator and from there to the air passing through
it. The air remains dry, since it does
not come into contact with the hot water.
The cooling water is closed and does not come into contact with the
atmosphere – evaporation is thus almost eliminated. Water consumption in a dry-cooled power
station drops to about 0.8 litres per kWh.

Matinba - Direct dry-cooling with forced-draught fans Kendal - Indirecr dry-cooluing with surface condenser
T - turbine T - turbine
G - generator G - generator
P - pump P - pump
F - fan F - fan
K - condensate storage tank D - dry-cooling elements
D - dry cooling elements C - condenser
Exhaust steam flows fron the turbine to the dry-cooling Cold water flows through the condenser tubes, removing
elements or heat exchanger. Air is driven through the heat from the steam which passes over them. The
exchanger by forced-draught fans to remove the heat. resultant hot water is pumped through heat exchangers
The cooled water is pumped bak to the boiler. inside natural-draught cooling towers.
There are two methods of dry-cooling,
direct and indirect and Eskom is leading the field in world technology in
introducing both on hitherto unprecedented scale in two of its newest power
stations, - Matimba (direct dry-cooling).

Direct Dry Cooling
Team from the low-pressure turbine is
channelled directly into the radiator-type heat exchanger. Air passing through the exchanger removes the
heat, thus condensing the steam into water to be pumped back to the boiler.
The flow of cooling air through the
exchangers is assured either by natural draught in cooling towers or by forced
draught using fans.
Indirect Dry-Cooling
The indirect system makes use either
of surface condensers as in conventional wet-cooled power stations, or of jet
condensers. In the firmer, cold water
flows through the tubes of the condenser, removing heat from the steam passing
over the, This now heated water then
flows through heat exchangers arranged inside a natural draught cooling
tower. 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 ump and is pumped through the heat exchangers in the cooling
towers.
Optimum use of Ccoal 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 greater 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.
MATIMBA – PROJECT OF POWER
Near
Ellilsras in the north western Transvaal a different kind of pioneer is
breaking ground and establishing new frontiers among the border people. Matimba, Eskom’s giant coal-fired,
dry-cooling power station, further consolidates Eskom’s position as a world
leader in dry-cooling technology for power generation. The power station generates 4 000 MW,
enough to provide for the energy needs of six cities the size of Durban. The dry-cooling technology is especially
significant in this area of South Africa where water resources are scarce and
unreliable. Dry-cooled systems consume
approximately 0.4 litres/kWh compared to the 2.5 litres/kWh required by
wet-cooling systems.
Statistics
Planning
started in 1978
Construction
started in 1981
First
unit completed in 1987
Completion
1991
Six
turbo-generator units will generate 4 000 MW
Six
unit silos each have a 4 650 ton capacity
Five
boiler mill bunkers each have an 850 ton capacity
Each
of the six boilers is 100 m high
Each
boiler weights 3 000 tons
There
is enough space in each boiler to provide parking for 900 cars
Each
boiler consumes 7 680 tons of coal per day
At
full load each boiler produces up to 280 ton of coarse ash and 2 500 tons
of fly ash per day



%20Matimba%20Power%20Station.jpg)
Matimba circuit diagram

![MatimbaPS1[1].jpg](/sites/heritage/Animated%20Books/MatimbaPS1[1].jpg)
Milestones
Matimba has broken a number of records right from the start:
- In 1996 Matimba broke the world record for six units on load. These ran continuously for 80 days - a record still held by Matimba today.
- In 2000, Matimba was the joint winner of the Jan H Smith Trophy awarded by Eskom for top performances in leadership and management excellence.
- On 8 February 2001. Unit 4 ran on full load for 389 days, 19 hours abd 16 minutes - making it the longest single unit continuous run within Eskom. (This record has since been surpassed by Koeberg Nuclear Power Station).