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Integration of MSF desalination plants into large steel works complexes in coastal regions
INTEGRATION OF MSF DESALINATION COMPLEXES
Dr. Gerd Scheffler, Department
PLANTS INTO LARGE STEEL WORKS
IN COASTAL REGIONS Dr. Jijrg Lammers
of Sea Water Desalination
DEMAG Metallgewinnung D-6660 Zweibrilcken, West Germany
This paper examines the water requirements of modern steel works. The energy dissipated from the numerous processes is considered with regard to its possible use as energy supply for a central sea water desalination plant, and an estimate of the water costs of two basic steel works concepts is given.
INTRODUCTION During the last decade there has been the tendency in the industrialized world to set up new steel works complexes close to the sea shore. But also in those countries which are in the process of industrialization as those of North Africa, in the Hiddte East, around the Gulf or in South East Asia almost all new plants which are in operation or in the erection or planning phase are in coastal regions. The reasons for this trend are mainly economic considerations governed by the transport costs of huge amounts of iron ore, coal, and other materials, or the proximity of energy sources. The neces. sity of a reliable supply of water for various cooling, gas scrubbing and other purposes in a plant make it particularly favourable to chose a location at the coast especially if other water resources are scarce or subject to seasonal variations in that country.
CURRENT PRACTICE AND TENDENCIES The shortage of sufficient
suitable fresh water sources has
led to an enormous reduction of the specific water requirements due to the increased installation of closed cooling circuits in steel works. According to the required water quality each of these cooling circuits tends to be equipped with its own water demineralitation unit to make up for the unavoidable losses. The water for semi-closed circuits is treated in a similar fashion and, whilst the pre-treatment of make-up water for open circuits is less extensive, a better water quality here not only protects the equipment but actually helps to reduce the water requirements. The extension of this approach to steel works with a very small or no supply of fresh water, however, is bound to become very costly. It is not only that the investment and staffing costs of a considerable number of such demineralitation units usually of the ion exchange type - are high, but this concept also forgoes the possibility of using the energy dissipated in the
plant.
Since sea water desalination by distillation is relatively insensitive to the fluctuations of the concentrations in the raw water supply and produces water of constant quality with salt contents below 50 ppm, it can meet the requirements of almost all consumers in the plant directly. It is therefore useful to study the advantages of incorporating a desalination plant serving all consumers centrally and thus replacing the various ion exchange units throughout the complex.
A CENTRALIZED
DESALINATION
PLANT
The use of a centralized desalination plant shall be considered here from two main aspects. Desalination plants are able to provide 1) a reliable and adequate water supply for the steel works with advantages over conventional water sources 21 an opportunity which
are
to recover large amounts of energy
nntma\lv
lnct.
Stee_ works with their huge requirements of cooling and process water have aIways been a major consumer of fresh water in industrialized countries. Necessity and awareness of environmental problems have meant that potential water losses have been cut back drastically. The specific water consumption of modern steel works can nowadays be reduced to about 2 mn'/ton. This proves to be a remarkable saving compared with the average water consumption in h'estern Germany of 37 ml/ton of raw steel in 1969.
3ut even these relatively small uater requirements of 2 ml/ton mean that an integrated steel works complex producing 10,000 tons/day would need 20,000 EI' of fresh Hater daily. For production rates of this order of magnitude the multistage flash distillation is the undisputed desalination process because of its proven reliability
and economic performance.
This process has been well established and plants all over the world operate by it. For this reason, only a short description shall be given here.
DESCRIPTION
OF THE SEA WATER DESALINATION
Multi-Stage-Flash cess which is designed
PLANT
(MSF) desalination is a distillation proto recover a considerable part of the
energy required for the evaporation Gne distinguishes. correspondingly,
process during condensation.
which
recovery
is the brine
rejection
heater,
the heat
between
the heat section,
input and
section, the heat
section.
The brine is circulated in the plant in counter-current flow, establishing a stable temperature profil e with the top temperature at the brine heater outlet and the lowest temperature at the outlet of the last stage of the heat rejection section. Across the flash chambers of the plant. a pressure profile is established according to the saturation temperatureEvaporation takes place in tAe flash chamber of each stage. The rising vapor passes through demisters and is condensed at the Cube
bundle
of the same
stage.
Fig. 1 illustrates
lip.1
SEA
W7ER
the principle
CESALINATION
of the MSF process.
?iANT.
Treated sea water enters the condenser
bundles of the heat
rejection section as cooling water. After Leaving the heat rejection section, part of it is pretreated and degassified to maintain a good heat transfer, to prevent scale formation and to minimize corrosion. The remainder is discharged. The treated sea water is mixed with the brine of the last evaporation stage. This brine is fed through the condenser tube bundles of the recovery section where it is gradually heated up by absorbing the heat of condensation
of the water vapor. Finally, the temperature
creased to the top temperature
of the process
is in-
in the brine heater.
From here, the recirculating brine passes into the flash chambers. The pressure, which has SO far been higher than the pressure of saturation, is reduced from stage to stage, whereby part of the brine evaporates by repeated flashing and is condensed to distilled water. This distillate is collected in the distillate troughs of every stage located below the condenser bundles and is withdrawn
as product water from the last stage. To avoid an excessive concentration of the brine, part of it is discharged from the last stage as blow-down.
A BRIEF DESCRIPTION
OF THE STEEL KORKS PROCESS
Two basic steel works concepts are discussed
in this paper:
11 an integrated steel works complex 2) an electric steel plant Fig. 2 is a symbolic representation of an integrated steel works complex. Ores and sintered raw materials are charged into the blast furnace and smelted to pig iron. The liquid hot metal is transferred to the converter where it is refined to steel by oxidation of its carbon content. In the example of fig. 2, the steel is then cast into billets or slabs in a continuous casting plant from where it is passed to the rolling mills, where it is rolled to profile sections or to sheet metal.
STEEL WORKS
INTEGRATED
1 meltshoo
-coot:.y
box
1
casting plank 1
/rol!ing
miil
J
ClrcLit
-tkyzreccc2cgcLwit -deUustlrg circuit
tis. 2 -.a
-
FLOW SHEET AND LISTING OF WATER CIRCUITS
The electric steel works illustrated in fig. 3 is small compared steel complex. charge consists mainly of is fed to the arc steel produced again furnace.
passed to the continuous casting plant. The rolling mills have been excluded from this example as very often they are not on the same site as the steel works. The flue gases from blast furnace, converter, and electric arc furnaces have to be cleaned in a dedusting plant. EiECTRlC
STEEL WORKS casting p!ant I L/
e
I it -c!eCusting cx~lmg. pzrt of fumoce ccolinq circwt
11g.3
:
-furnace cmllng circuit -electroCe cccltng ckui?
i_F
2s KJ&
-mcchne cc3wng cifcw: -msuld cozhng ci-cult -sycy coohng c;rcu:t
FLOW SHEET AND LISTING OF WATER CIRCUITS
Apart from illustrating the steel making process, figs. 2 and 3 furthermore list the various water circuits which can be divided into cooling and cleaning circuits, though both activities can be combined as in gas scrubbing.
WATER CIRCUITS AND WATER CONSUMPTION The main water circuits of the individual steel works sections are listed in fig. 2 and fig. 3 respectively. Three types of water circuits are generally distinguished: 11 closed cooling circuits, where the water does not come into contact with the object to be cooled. Re-cooling or condensation of the water occurs in heat exchangers which are cooled either by air or some other awailable medium, e.g. sea water,
2) semi-closed circuits are used where low temperatures of the cooling water are essential. Again there is no direct contact between the object and the cooling water but the re-cooling of the water is achieved by evaporating a fraction in cooling towers. 3) Finally, in direct cooling circuits, where the water is sprayed directly onto the object, it usually gets heavily polluted and will have to be treated before it is recycled. Re-cooling is usually done in cooling towers. The make-up water requirements very much depend on the type of circuit. In a closed circuit only losses due to leakage have to be replaced. It is the consequent introduction of these circuits which lead to the remarkable reduction of the specific water consumption mentioned above. The order of magnitude of these losses is about 0.04 per cent of the circulating water (1). A high water quality is desired for these circuits. The water losses of a semi-closed
circuit consist of the
amount of water which evaporates in the cooling tower and an amount of water which is withdrawn in order to prevent the concentration of dissolved solids from building up beyond an acceptable level. The extent to which water has to be discharged for this purpose depends on the make-up water and increases with sinking quality of the watex supply. Losses in semi-closed circuits tend to be of the order of 1 to 3 per cent. Additional
evaporation
and other losses occur for the
direct cooling of the open circuits. Together with the losses in settling basins, the overall water loss amounts to about J- 5 per cent of the water in circulation. These losses evaporation
can
occurs as
however
be much
higher
when
considerable
in the scrubbing of converter gases.
The water requirements per ton of product for an integrated steel works complex are listed in table 1.
TABLE 1: Unit
Specific Water Requirements Circuit
Circuit Type
Circulation
Make-Up
0.0096 m'/t 24 ms/t cooling cir- closed cuit furnace walls, tuye'res and tuyQre gates 0.41 mJ/t 9 m'/t dedusting open _____________ piant ________--____-----___________________^_---------0.0005 mJ/t 1.28 mJ/t cooling cir- closed converter cuit for lance etc. 3.24 ma/t 0.35 UP/t dedusting open _____________ plant ____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ continuous cooling cir- closed 17.16 mJ/t 0.0068 mJ/t cuit for casting plant machine and mould (CCP) blast furnace
cooling __ 0Ren 0.239 mSLt___ ______________spray __ _______ __________IrX_mYt ______-_____-_ rolling cooling cir- closed 8.91 mJ/t 0.0035 mJ/t mills cuit for furnaces etc. for aggresemigates etc. closed 8.86 mJ/t 0.124 mS/t intermediate open 26.84 m'/t 0.776 m'/t cooling scale removal. etc. _-__-_______________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ personnel 0.1 ms/t --__________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_~~~~_~ The individual values in this table should be understood as examples and as such, they naturally only represent an order of magnitude. Apart from the spray cooling circuit for the continuous casting plant, the electric steel works example in fig. 3 has closed circuits only, with a total circulation of about 40.67 m'/t. The specific consumption is thus mainly dominated by the continuous casting plant and does not exceed 0.3 mr/t of raw steel produced. A point of special interest here is the high purity requirement for the cooling circuit for the electrodes. The conductivity of the water ought to be below SOO,u-Siemens.
POTENTIAL ENERGY SOURCES The provision of fresh water by desalination
requires a
supply of energy in the order of 70 - 80 kcal per kg of distillate produced for a desalination plant with a performance ratio around 7. For an integrated steel works complex with a daily production of 10,000 tons a day, this would mean that a source of at least 1.60 x 10' kcal/d or 66.7 x lo6 kcal/h has to be found. In the following, the various processes are examined for their potential to supply the necessary energy. The figures quoted in this context will give an idea about the energies dissipated. But as this survey stands at the beginning of a detailed examination of this question, the technical feasibility of the utilization of these energies has yet to be fully explored. The blast furnace During the latter part of a campaign, the heat absorbed by the cooling water of a blast furnace with a capacity of 5,000 tons per day amounts to 50 x 106
kcal per hour. This would be suffi-
cient for about 3 rn' of distillate per ton of pig iron. The required temperature level can be achieved by using either an evaporative cooling system or a pressurized hot water cooling system. Converter Converter gases are cooled down to a temperature of 1000° to llOO°C before being cleaned. The heat which has to be withdrawn is, as fig. ? shows, dependent on the degree of combustion of the gases. A detailed survey of the possibility of energy recovery from the converter process published by Simon (21, and I-I.Klammer (3). discusses the energy recovery from gases in general. Kruger (4) reports an energy recovery of 180,000 kcal per ton of pig iron. This would produce 2.25 mJ distillate per ton of pig iron. A problem Lies in the discontinuous operation of a converter. To secure a constant heat supply to a desalination
plant a thermal energy buffer would be required.
n=l
02
fig.L
w HEAT
17.6
; stoichicmelrcc
0.8
TO SE
7.0
12
u
4BSORBED
combustion
15
3s
20
BY CUJLING
air fcctcr
n
SYSTEM
Rolling mills Study of the energy which can be obtained from the cooling system of furnaces and from their flue gases showed that about 1 ton of steam or 3.67 mJ of distillate can be produced per ton of rolled steel. Electric arc furnace . Fig. 5 shows the energy which can be recovered from a cooling system for the furnace, cover and exhaust duct from a 100 ton arc'furnace. The average energy production rate is 11.5 x lo6 kcal/h as indicated by the upper horizontal line in fig. 5. In order to ensure a constant energy supply at this rate, a thermal energy buffer with a capacity of 4.8 x lO'kca1 would be necessary. If the cooling system used hot water and operated with a temperature difference of 20°C this would mean a tank for about 250 ma of water. The storage of such comparatively small amounts of thermal energy is well practised and does not represent any problems (51. It has been shown above that the water consumption of electric steel works is comparatively small. For this reason it is not necessary to recover all the energy possible from the arc.furnace. If only 8 x lo6 kcal are utilized '(second horizontal line in fig. 51, only 1.5 x lo6 kcal have to be stored. This ktould still be sufficient for the production of 3 III'distillate per ton of raw steel.
443
These estimates of possible distillate production were based on rather conservative choices of performance ratios. The high temperatures of steel works processes, necessitated by the melting point of steel at around 1500°C. ensure at least in theory that a top temperature of the brine within the required range of 90“ to 120°C can be achieved without violation of the 2nd law of thermodynamics. The energy withdrawn by various cooling circuits thus not only suffices to cover the water requirements of the steel works but can contribute munity as well.
to the water suppl y of the surrounding
com-
One of the basic problems of the utilization of waste energy is to find a consumer tihose demand for energy is not subject to strong fluctuations. Desalination plants, with their constant energy requirements are ideal in this respect. Their integration into t&e steel producing process to supply the water needed for these processes further enhances this advantage by forming a close link between energy production and consumption.
The difference
between the use of xaste energy in steel
works and the use of steam from power stations is that no loss of efficiency is involved. The direct costs of the thermal energy can thus be taken to be zero.
ESTIMATE OF WATER COSTS A rough estimate of the costs of water shall be given for two exanples 1) an integrated steel works complex with energy recovery from the blast furnaces 2) an electric steel works with energy recovery from the arc furnace and the flue gases The details of these calculations
are listed in table 2.
For an integrated steel works with two or more blast furnaces and with a daily production of 10,000 tons of steel, the minimum water requirement is 20,000 ma per day. The heat dissipated by the blast furnaces.tnwards.the.end of their campaign corresponds to 30,000 mS per day of distillate. As the blast furnaces will not start and end their campaign simultaneously. a heat recovery corresponding to 24.000 ms/d of distillate has been assumed. A desalination plant consisting of 3 units and with a total capacity of 30,000 mJ, then will secure the water supply of the plant all the year round including the maintenance of these units. An additional cooler for the blast furnace cooling system will still be necessary but only as a standby and with only a third of the capacity otherwise required. Under these assumptions and with the normal reservations about such estimates, the water costs have been calculated as about 1.82 DM per rns of distillate for a plant installed in the region of the Gulf. The water costs per ton of sectional steel would thu..samount to approximately 3.70 DM/ton or 0.5 - 0.7 per cent of the sectional steel price in Western Germany.
Fig. 6 shows an example of the Hater distribution
within the
plant.
Most -c alscharce
turmce witMrorn
+
concentration
slag
to prevent
a
bulki up
gmuIotion 7Sm?! a tq 6 WATER
OISTRI0UTIOH:
NTEGRATEO
STEEL
WORKS
The electric steel works with a typical production of about 500.000 tons per year can, as has been shown above, produce about 3 mJ distillate per ton of steel requiring only a relatively small storage tank to smooth the energy supply. This uould correspond to about 4,800 mJ of distillate per day. The necessity of a water supply all the year round suggests that the desalination plant is split into two units. As the capital costs for a plant of this size are relatively high, the estimated water costs amount to 2.13 DM/ms. Fig. 7 suggests a distribution system for the water from a feature of this arrangecentralized desalination plant. A special ment is that the electrode cooling circuit with its circulaticn of 175 m'/h is continuously fed uith distilled water. The other nater circuits are then supplied uith water withdrawn from this circuit. En this way it is ensured that the required low conductivity of the water in the electrode cooling circuit is maintained without having to provide for ion exchange units which might otherwise be necessary.
TABLE 2 Assumption
for Estimated Water Costs
rated steel works complex,
Capital
costs
desalination
of
the
plant
Costs af thermal ~~~r~~ buffer Savings in cooling equipnent Amortization hteFSSt
period
Fate
Consumption of electric exzergy Electric energy coasts CQStS of cfremicals Fersonnel requirements Average hourly wa Maintenance
costs
b, eIecfric stflel plant)
CONCLUSION It is hoped that this survey has demonstrated the potential of desalination plants in steel works as a means of satisfying the water requirements of the works as well as a means of utilizing waste energy. The zero cost of the thermal energy makes these plants a very economic proposition. A further point which has not been considered so far and which is difficult to assess, is the benefit resulting from the purity of the distillates. Reduced corrosion in all circuits and a decrease of the water consumption of open and semi-closed circuits can both be expected, thus adding to the considerable advantages of linking desalination plants and steel works. This paper discussed the use of the energy in cooling water with respect to individual process units. It is of course conceivable to create a centralized cooling system connecting a number of processes to the desalination plant. The resulting high output kould reduce the costs of water even further and enable the steel works to supply conplete communities with water. Many new questions have been raised during the work on this initial study. They are the subject of further investigations on which we hope to report in the near future.
ACKNOWLEDGEMENTS The authors wish to thank Mr. P. Monheim and the other members of the DEMAG and blannesnann departments involved, as well as the VDEh for their assistance in supplying information and data on this topic.
REFERENCES 1. 2. 3. 4. 5.
F. SCHROEDER WAGNER, Symposium on Environnental ConcroL in the Steel Industry, Tokyo 1974 R. SIMON, Berg- und HUttenrtPnnische Efonatshefte, Vol. 120, part 9 H. KLA&!?4ER,"neue" Fachberichte, 1976 W. KRUGER, Stahl und Eisen 83 (1969) A. MARESKE, Btennst.-WZrme-Kraft, Vol. 29. August 1977 A.
Dr. Gerd Scheffler, Department
PLANTS INTO LARGE STEEL WORKS
IN COASTAL REGIONS Dr. Jijrg Lammers
of Sea Water Desalination
DEMAG Metallgewinnung D-6660 Zweibrilcken, West Germany
This paper examines the water requirements of modern steel works. The energy dissipated from the numerous processes is considered with regard to its possible use as energy supply for a central sea water desalination plant, and an estimate of the water costs of two basic steel works concepts is given.
INTRODUCTION During the last decade there has been the tendency in the industrialized world to set up new steel works complexes close to the sea shore. But also in those countries which are in the process of industrialization as those of North Africa, in the Hiddte East, around the Gulf or in South East Asia almost all new plants which are in operation or in the erection or planning phase are in coastal regions. The reasons for this trend are mainly economic considerations governed by the transport costs of huge amounts of iron ore, coal, and other materials, or the proximity of energy sources. The neces. sity of a reliable supply of water for various cooling, gas scrubbing and other purposes in a plant make it particularly favourable to chose a location at the coast especially if other water resources are scarce or subject to seasonal variations in that country.
CURRENT PRACTICE AND TENDENCIES The shortage of sufficient
suitable fresh water sources has
led to an enormous reduction of the specific water requirements due to the increased installation of closed cooling circuits in steel works. According to the required water quality each of these cooling circuits tends to be equipped with its own water demineralitation unit to make up for the unavoidable losses. The water for semi-closed circuits is treated in a similar fashion and, whilst the pre-treatment of make-up water for open circuits is less extensive, a better water quality here not only protects the equipment but actually helps to reduce the water requirements. The extension of this approach to steel works with a very small or no supply of fresh water, however, is bound to become very costly. It is not only that the investment and staffing costs of a considerable number of such demineralitation units usually of the ion exchange type - are high, but this concept also forgoes the possibility of using the energy dissipated in the
plant.
Since sea water desalination by distillation is relatively insensitive to the fluctuations of the concentrations in the raw water supply and produces water of constant quality with salt contents below 50 ppm, it can meet the requirements of almost all consumers in the plant directly. It is therefore useful to study the advantages of incorporating a desalination plant serving all consumers centrally and thus replacing the various ion exchange units throughout the complex.
A CENTRALIZED
DESALINATION
PLANT
The use of a centralized desalination plant shall be considered here from two main aspects. Desalination plants are able to provide 1) a reliable and adequate water supply for the steel works with advantages over conventional water sources 21 an opportunity which
are
to recover large amounts of energy
nntma\lv
lnct.
Stee_ works with their huge requirements of cooling and process water have aIways been a major consumer of fresh water in industrialized countries. Necessity and awareness of environmental problems have meant that potential water losses have been cut back drastically. The specific water consumption of modern steel works can nowadays be reduced to about 2 mn'/ton. This proves to be a remarkable saving compared with the average water consumption in h'estern Germany of 37 ml/ton of raw steel in 1969.
3ut even these relatively small uater requirements of 2 ml/ton mean that an integrated steel works complex producing 10,000 tons/day would need 20,000 EI' of fresh Hater daily. For production rates of this order of magnitude the multistage flash distillation is the undisputed desalination process because of its proven reliability
and economic performance.
This process has been well established and plants all over the world operate by it. For this reason, only a short description shall be given here.
DESCRIPTION
OF THE SEA WATER DESALINATION
Multi-Stage-Flash cess which is designed
PLANT
(MSF) desalination is a distillation proto recover a considerable part of the
energy required for the evaporation Gne distinguishes. correspondingly,
process during condensation.
which
recovery
is the brine
rejection
heater,
the heat
between
the heat section,
input and
section, the heat
section.
The brine is circulated in the plant in counter-current flow, establishing a stable temperature profil e with the top temperature at the brine heater outlet and the lowest temperature at the outlet of the last stage of the heat rejection section. Across the flash chambers of the plant. a pressure profile is established according to the saturation temperatureEvaporation takes place in tAe flash chamber of each stage. The rising vapor passes through demisters and is condensed at the Cube
bundle
of the same
stage.
Fig. 1 illustrates
lip.1
SEA
W7ER
the principle
CESALINATION
of the MSF process.
?iANT.
Treated sea water enters the condenser
bundles of the heat
rejection section as cooling water. After Leaving the heat rejection section, part of it is pretreated and degassified to maintain a good heat transfer, to prevent scale formation and to minimize corrosion. The remainder is discharged. The treated sea water is mixed with the brine of the last evaporation stage. This brine is fed through the condenser tube bundles of the recovery section where it is gradually heated up by absorbing the heat of condensation
of the water vapor. Finally, the temperature
creased to the top temperature
of the process
is in-
in the brine heater.
From here, the recirculating brine passes into the flash chambers. The pressure, which has SO far been higher than the pressure of saturation, is reduced from stage to stage, whereby part of the brine evaporates by repeated flashing and is condensed to distilled water. This distillate is collected in the distillate troughs of every stage located below the condenser bundles and is withdrawn
as product water from the last stage. To avoid an excessive concentration of the brine, part of it is discharged from the last stage as blow-down.
A BRIEF DESCRIPTION
OF THE STEEL KORKS PROCESS
Two basic steel works concepts are discussed
in this paper:
11 an integrated steel works complex 2) an electric steel plant Fig. 2 is a symbolic representation of an integrated steel works complex. Ores and sintered raw materials are charged into the blast furnace and smelted to pig iron. The liquid hot metal is transferred to the converter where it is refined to steel by oxidation of its carbon content. In the example of fig. 2, the steel is then cast into billets or slabs in a continuous casting plant from where it is passed to the rolling mills, where it is rolled to profile sections or to sheet metal.
STEEL WORKS
INTEGRATED
1 meltshoo
-coot:.y
box
1
casting plank 1
/rol!ing
miil
J
ClrcLit
-tkyzreccc2cgcLwit -deUustlrg circuit
tis. 2 -.a
-
FLOW SHEET AND LISTING OF WATER CIRCUITS
The electric steel works illustrated in fig. 3 is small compared steel complex. charge consists mainly of is fed to the arc steel produced again furnace.
passed to the continuous casting plant. The rolling mills have been excluded from this example as very often they are not on the same site as the steel works. The flue gases from blast furnace, converter, and electric arc furnaces have to be cleaned in a dedusting plant. EiECTRlC
STEEL WORKS casting p!ant I L/
e
I it -c!eCusting cx~lmg. pzrt of fumoce ccolinq circwt
11g.3
:
-furnace cmllng circuit -electroCe cccltng ckui?
i_F
2s KJ&
-mcchne cc3wng cifcw: -msuld cozhng ci-cult -sycy coohng c;rcu:t
FLOW SHEET AND LISTING OF WATER CIRCUITS
Apart from illustrating the steel making process, figs. 2 and 3 furthermore list the various water circuits which can be divided into cooling and cleaning circuits, though both activities can be combined as in gas scrubbing.
WATER CIRCUITS AND WATER CONSUMPTION The main water circuits of the individual steel works sections are listed in fig. 2 and fig. 3 respectively. Three types of water circuits are generally distinguished: 11 closed cooling circuits, where the water does not come into contact with the object to be cooled. Re-cooling or condensation of the water occurs in heat exchangers which are cooled either by air or some other awailable medium, e.g. sea water,
2) semi-closed circuits are used where low temperatures of the cooling water are essential. Again there is no direct contact between the object and the cooling water but the re-cooling of the water is achieved by evaporating a fraction in cooling towers. 3) Finally, in direct cooling circuits, where the water is sprayed directly onto the object, it usually gets heavily polluted and will have to be treated before it is recycled. Re-cooling is usually done in cooling towers. The make-up water requirements very much depend on the type of circuit. In a closed circuit only losses due to leakage have to be replaced. It is the consequent introduction of these circuits which lead to the remarkable reduction of the specific water consumption mentioned above. The order of magnitude of these losses is about 0.04 per cent of the circulating water (1). A high water quality is desired for these circuits. The water losses of a semi-closed
circuit consist of the
amount of water which evaporates in the cooling tower and an amount of water which is withdrawn in order to prevent the concentration of dissolved solids from building up beyond an acceptable level. The extent to which water has to be discharged for this purpose depends on the make-up water and increases with sinking quality of the watex supply. Losses in semi-closed circuits tend to be of the order of 1 to 3 per cent. Additional
evaporation
and other losses occur for the
direct cooling of the open circuits. Together with the losses in settling basins, the overall water loss amounts to about J- 5 per cent of the water in circulation. These losses evaporation
can
occurs as
however
be much
higher
when
considerable
in the scrubbing of converter gases.
The water requirements per ton of product for an integrated steel works complex are listed in table 1.
TABLE 1: Unit
Specific Water Requirements Circuit
Circuit Type
Circulation
Make-Up
0.0096 m'/t 24 ms/t cooling cir- closed cuit furnace walls, tuye'res and tuyQre gates 0.41 mJ/t 9 m'/t dedusting open _____________ piant ________--____-----___________________^_---------0.0005 mJ/t 1.28 mJ/t cooling cir- closed converter cuit for lance etc. 3.24 ma/t 0.35 UP/t dedusting open _____________ plant ____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ continuous cooling cir- closed 17.16 mJ/t 0.0068 mJ/t cuit for casting plant machine and mould (CCP) blast furnace
cooling __ 0Ren 0.239 mSLt___ ______________spray __ _______ __________IrX_mYt ______-_____-_ rolling cooling cir- closed 8.91 mJ/t 0.0035 mJ/t mills cuit for furnaces etc. for aggresemigates etc. closed 8.86 mJ/t 0.124 mS/t intermediate open 26.84 m'/t 0.776 m'/t cooling scale removal. etc. _-__-_______________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ personnel 0.1 ms/t --__________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_~~~~_~ The individual values in this table should be understood as examples and as such, they naturally only represent an order of magnitude. Apart from the spray cooling circuit for the continuous casting plant, the electric steel works example in fig. 3 has closed circuits only, with a total circulation of about 40.67 m'/t. The specific consumption is thus mainly dominated by the continuous casting plant and does not exceed 0.3 mr/t of raw steel produced. A point of special interest here is the high purity requirement for the cooling circuit for the electrodes. The conductivity of the water ought to be below SOO,u-Siemens.
POTENTIAL ENERGY SOURCES The provision of fresh water by desalination
requires a
supply of energy in the order of 70 - 80 kcal per kg of distillate produced for a desalination plant with a performance ratio around 7. For an integrated steel works complex with a daily production of 10,000 tons a day, this would mean that a source of at least 1.60 x 10' kcal/d or 66.7 x lo6 kcal/h has to be found. In the following, the various processes are examined for their potential to supply the necessary energy. The figures quoted in this context will give an idea about the energies dissipated. But as this survey stands at the beginning of a detailed examination of this question, the technical feasibility of the utilization of these energies has yet to be fully explored. The blast furnace During the latter part of a campaign, the heat absorbed by the cooling water of a blast furnace with a capacity of 5,000 tons per day amounts to 50 x 106
kcal per hour. This would be suffi-
cient for about 3 rn' of distillate per ton of pig iron. The required temperature level can be achieved by using either an evaporative cooling system or a pressurized hot water cooling system. Converter Converter gases are cooled down to a temperature of 1000° to llOO°C before being cleaned. The heat which has to be withdrawn is, as fig. ? shows, dependent on the degree of combustion of the gases. A detailed survey of the possibility of energy recovery from the converter process published by Simon (21, and I-I.Klammer (3). discusses the energy recovery from gases in general. Kruger (4) reports an energy recovery of 180,000 kcal per ton of pig iron. This would produce 2.25 mJ distillate per ton of pig iron. A problem Lies in the discontinuous operation of a converter. To secure a constant heat supply to a desalination
plant a thermal energy buffer would be required.
n=l
02
fig.L
w HEAT
17.6
; stoichicmelrcc
0.8
TO SE
7.0
12
u
4BSORBED
combustion
15
3s
20
BY CUJLING
air fcctcr
n
SYSTEM
Rolling mills Study of the energy which can be obtained from the cooling system of furnaces and from their flue gases showed that about 1 ton of steam or 3.67 mJ of distillate can be produced per ton of rolled steel. Electric arc furnace . Fig. 5 shows the energy which can be recovered from a cooling system for the furnace, cover and exhaust duct from a 100 ton arc'furnace. The average energy production rate is 11.5 x lo6 kcal/h as indicated by the upper horizontal line in fig. 5. In order to ensure a constant energy supply at this rate, a thermal energy buffer with a capacity of 4.8 x lO'kca1 would be necessary. If the cooling system used hot water and operated with a temperature difference of 20°C this would mean a tank for about 250 ma of water. The storage of such comparatively small amounts of thermal energy is well practised and does not represent any problems (51. It has been shown above that the water consumption of electric steel works is comparatively small. For this reason it is not necessary to recover all the energy possible from the arc.furnace. If only 8 x lo6 kcal are utilized '(second horizontal line in fig. 51, only 1.5 x lo6 kcal have to be stored. This ktould still be sufficient for the production of 3 III'distillate per ton of raw steel.
443
These estimates of possible distillate production were based on rather conservative choices of performance ratios. The high temperatures of steel works processes, necessitated by the melting point of steel at around 1500°C. ensure at least in theory that a top temperature of the brine within the required range of 90“ to 120°C can be achieved without violation of the 2nd law of thermodynamics. The energy withdrawn by various cooling circuits thus not only suffices to cover the water requirements of the steel works but can contribute munity as well.
to the water suppl y of the surrounding
com-
One of the basic problems of the utilization of waste energy is to find a consumer tihose demand for energy is not subject to strong fluctuations. Desalination plants, with their constant energy requirements are ideal in this respect. Their integration into t&e steel producing process to supply the water needed for these processes further enhances this advantage by forming a close link between energy production and consumption.
The difference
between the use of xaste energy in steel
works and the use of steam from power stations is that no loss of efficiency is involved. The direct costs of the thermal energy can thus be taken to be zero.
ESTIMATE OF WATER COSTS A rough estimate of the costs of water shall be given for two exanples 1) an integrated steel works complex with energy recovery from the blast furnaces 2) an electric steel works with energy recovery from the arc furnace and the flue gases The details of these calculations
are listed in table 2.
For an integrated steel works with two or more blast furnaces and with a daily production of 10,000 tons of steel, the minimum water requirement is 20,000 ma per day. The heat dissipated by the blast furnaces.tnwards.the.end of their campaign corresponds to 30,000 mS per day of distillate. As the blast furnaces will not start and end their campaign simultaneously. a heat recovery corresponding to 24.000 ms/d of distillate has been assumed. A desalination plant consisting of 3 units and with a total capacity of 30,000 mJ, then will secure the water supply of the plant all the year round including the maintenance of these units. An additional cooler for the blast furnace cooling system will still be necessary but only as a standby and with only a third of the capacity otherwise required. Under these assumptions and with the normal reservations about such estimates, the water costs have been calculated as about 1.82 DM per rns of distillate for a plant installed in the region of the Gulf. The water costs per ton of sectional steel would thu..samount to approximately 3.70 DM/ton or 0.5 - 0.7 per cent of the sectional steel price in Western Germany.
Fig. 6 shows an example of the Hater distribution
within the
plant.
Most -c alscharce
turmce witMrorn
+
concentration
slag
to prevent
a
bulki up
gmuIotion 7Sm?! a tq 6 WATER
OISTRI0UTIOH:
NTEGRATEO
STEEL
WORKS
The electric steel works with a typical production of about 500.000 tons per year can, as has been shown above, produce about 3 mJ distillate per ton of steel requiring only a relatively small storage tank to smooth the energy supply. This uould correspond to about 4,800 mJ of distillate per day. The necessity of a water supply all the year round suggests that the desalination plant is split into two units. As the capital costs for a plant of this size are relatively high, the estimated water costs amount to 2.13 DM/ms. Fig. 7 suggests a distribution system for the water from a feature of this arrangecentralized desalination plant. A special ment is that the electrode cooling circuit with its circulaticn of 175 m'/h is continuously fed uith distilled water. The other nater circuits are then supplied uith water withdrawn from this circuit. En this way it is ensured that the required low conductivity of the water in the electrode cooling circuit is maintained without having to provide for ion exchange units which might otherwise be necessary.
TABLE 2 Assumption
for Estimated Water Costs
rated steel works complex,
Capital
costs
desalination
of
the
plant
Costs af thermal ~~~r~~ buffer Savings in cooling equipnent Amortization hteFSSt
period
Fate
Consumption of electric exzergy Electric energy coasts CQStS of cfremicals Fersonnel requirements Average hourly wa Maintenance
costs
b, eIecfric stflel plant)
CONCLUSION It is hoped that this survey has demonstrated the potential of desalination plants in steel works as a means of satisfying the water requirements of the works as well as a means of utilizing waste energy. The zero cost of the thermal energy makes these plants a very economic proposition. A further point which has not been considered so far and which is difficult to assess, is the benefit resulting from the purity of the distillates. Reduced corrosion in all circuits and a decrease of the water consumption of open and semi-closed circuits can both be expected, thus adding to the considerable advantages of linking desalination plants and steel works. This paper discussed the use of the energy in cooling water with respect to individual process units. It is of course conceivable to create a centralized cooling system connecting a number of processes to the desalination plant. The resulting high output kould reduce the costs of water even further and enable the steel works to supply conplete communities with water. Many new questions have been raised during the work on this initial study. They are the subject of further investigations on which we hope to report in the near future.
ACKNOWLEDGEMENTS The authors wish to thank Mr. P. Monheim and the other members of the DEMAG and blannesnann departments involved, as well as the VDEh for their assistance in supplying information and data on this topic.
REFERENCES 1. 2. 3. 4. 5.
F. SCHROEDER WAGNER, Symposium on Environnental ConcroL in the Steel Industry, Tokyo 1974 R. SIMON, Berg- und HUttenrtPnnische Efonatshefte, Vol. 120, part 9 H. KLA&!?4ER,"neue" Fachberichte, 1976 W. KRUGER, Stahl und Eisen 83 (1969) A. MARESKE, Btennst.-WZrme-Kraft, Vol. 29. August 1977 A.