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Characteristics of dual purpose MSF desalination plants
DESALINATION Desalination 166 (2004) 287-294
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Characteristics of dual purpose MSF desalination plants Ibrahim S. A1-Mutaz*, Abdullah M. A1-Namlah Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia Tel. +966 (1) 4676870; Fax +966 (1) 4682858; e-maih [email protected] Received 4 March 2004; accepted 12 March 2004
Abstract
All of the Saudi large MSF plants operate within the context of dual-purpose facilities for the simultaneous production of power and water. Such co-generation arrangement uses either backpressure or extraction condensing turbine. Co-generation cycles which were used till 1982 were employing extraction condensing turbines with a power to water ratio ranging between 10.2 to 17.5 MW/migd. From 1983 onwards backpressure turbines were used in all new co-generation plants. Backpressure turbines give lower power to water ratio (high water demand) and they are also characterized by high thermal efficiencies. They make the best use of low-grade heat that would otherwise be rejected by the power generating plant cycle. This paper aims to study the characteristics of the dualpurpose MSF desalination plants with special reference to the Saudi experience in this field. The challenge that faces these plants is to provide an operation that satisfy the diverse operational requirements imposed by power and water production and yet retain the inherent economic advantages of the dual-purpose concept. ' Keywords: Seawater desalination; Multi-stage flash (MSF); Dual purpose; Cogeneration; Performance; Characteristics
I. Introduction
Most of the potable water and electricity in the Arabian Gulf countries are produced by dualpurpose multi-stage flash (MSF) desalination plants. A dual-purpose plant is the one that supplies heat for a thermal desalination unit and produces *Corresponding author.
electricity for distribution to the electrical grid. MSF desalination plants need mainly thermal energy, which is convenient to be supplied by lowor medium-pressure steam (sub-atmospheric or below 3 bar, respectively). Dual-purpose MSF desalination plants are economically attractive options for desalination because tile cost of the plant is allocated to two products (water and electricity).
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi;10.1016/j.desal.2004.06.083
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I.S. Al-Mutaz, A.M. Al-Namlah / Desalination 166 (2004) 287-294
Dual-purpose desalination plant has many economic advantages over single purpose plants, such as lower financial investment due to sharing of facilities, less fuel consumption and less manpower requirement. However it suffers from some disadvantages, e.g. less overall flexibility and slightly lower availability factor. Large-scale power-desalting projects undertaken in recent years were reviewed byAwerbuch [ 1] underlined the importance of the power/water ratio in selecting the appropriate technology. Typical power to water ratios for dual MSF desalination plants are shown in Table 1. The powerto-water ratio is defined as megawatt of power generated per million gallons/day of fresh water produced. The values in the table were estimated with knowledge of the achievable gain output ratio (GOR) in MSF desalination plants and the turbine efficiency in power plants. GOR is the ratio ofkg of water produced per kg of steam used. Values of GOR are often between 8 to 10 with a practical maximum of 12, which corresponds to about 55 kWh/m 3 of thermal energy. As clearly shown in Table 1, a backpressure steam coupling system requires half the power plant capacity compared to an extraction steam coupling system. The characteristics of large Saudi dual-purpose MSF desalination plants will be reviewed. This Table 1 Typical power to water ratios for dual MSF desalination plants Technology Steam turbine BTG Steam turbine EST GT-HRSG Combinedcycle BTG Combinedcycle EST
Power-to-waterratioa 5 10 8 16 19
a Power to water ratio - - MW power generatedper mgd of water produced BTG-- backpressure turbine generator EST-- extraction steam turbine GT - - gas turbine HRSG - - heat recovery steam generator
will exhibit the Saudi experience in the desalination field.
2. Co-generation and dual MSF desalination plants MSF desalination plants need mainly thermal energy, which is convenient to be supplied by lowor medium-pressure steam (sub-atmospheric or below 3 bar, respectively). They need also secondary and tertiary mechanical energy for pumping and maintaining vacuum, which is normally supplied by electricity (and possibly, for gas pumping, by high- or medium-pressure steam). In efficient MSF plants it is equivalent to about 8.1 kWh (+15%) per m 3 [2]. The additional secondary and tertiary mechanical and electrical energies are equivalent to 2.5-4.5 kWh/m 3 for MSF plants. The steam source for MSF desalination plants can be a dedicated or non-dedicated (co-generation) plant. The former provides energy exclusively for the desalination process and water is the only product out of the complex. The latter provides only part of its energy to the desalination process, and the rest of the energy is used to generate electricity. Co-generation is an economically attractive option for desalination because the cost of the plant is allocated to two product streams. A number of large co-generation power and desalination plants have been built in various parts of the world in recent years. Combined power and water production represents the largest use of the co-generation concept with over 25,000 MW of installed world electrical capacity [3]. A co-generation plant, which produces both electricity and good quality water, is often called a dual-purpose plant. A dual-purpose desalination plant is a process that produces both power and desalted water with the optimum use of thermal energy in producing the two products. It offers a considerable saving in energy usage. For example a single-purpose power plant producing electrical
LS. AI-Mutaz, A.M. Al-Namlah / DesaBnation 166 (2004) 287-294
power of 1000 MW requires 3000 MW of thermal power. The remaining 2000 MW are exhausted as low temperature waste heat. To produce 1135x 103 m3/d (300 mgd) of desalted water in a single-purpose plant requires about 3000 MW, all of which ultimately is discharged as low temperature waste heat. The total sum of the two separate single-purpose plants requires 6000 MW and exhausts 5000 MW to the atmosphere as waste heat. For a dual-purpose plant produces the same two products (1000 MW of electricity and 1135 x 103 m3/d of water) the total thermal requirement is only 4000 MW, and the total waste heat exhausted is 3000 MW. Thus a savings of 2000 MW is realized, and the exhaust heat load is reduced from 5000 MW to 3000 MW [4]. Dual-purpose desalination plant has the following economic advantages over single purpose plants [5]: • Lowerfinancial investment due to sharing o f facilities. Seawater supply and brine outfall,
land requirement and site preparation costs are less, and some ancillary equipment such as pumps, compressors, lighting, transformers as well as administrative buildings can be used in common. For evaporative desalination there is saving in steam generators and final condensers. • Less f u e l consumption. The total fuel consumption is less since some main components are larger in dual-purpose plants, thus having higher efficiencies. Also, energy is saved in shared facilities and common sub-systems as well as by short distance energy transport. • Less manpower requirement. A dual-purpose plant requires fewer staff than the two singlepurpose plants because of joint operation of common facilities. However, a dual-purpose plant also has disadvantages, some of which are as follows: ° Less overallflexibility. Its operation is not as flexible as a single-purpose plant. There is economic pressure to maximise the combined
289
production of water and power. In particular, it is desirable to operate a power plant near base load to be most economic. The same is true in most cases for desalination. However, such design and operation may reduce the plant flexibility and cause some indirect penalties. Certain designs provide, indeed, for variation in the water to electricity ratio, but at the cost of efficiency or extra investment. • Slightly lower availability factor. Any incident interrupting the output of one of the two products may lead to a disturbance or stoppage in the production of the other, thus increasing the cost per unit product. It is possible to improve the plant availability as a whole and reduce this cost-increase by connecting reverse osmosis (RO) units to the grid or adding devices such as bypass steam lines so that steam can be directed to the evaporative desalination plant if the turbo-generator is out of operation. Likewise, addition of an auxiliary condenser enables the power plant to be operated if the desalination plant is shutdown. However, these devices involve extra investment. ° Off-optimum site and timing. The optimal location and/or commissioning date of the desalination plant, from the aspects of water conveyance, distribution and supply, may not coincide with those of the power plant, bearing another penalty. There are two types of co-generation modes: • Parallel co-generation, ° Series co-generation. In parallel co-generation, electricity is produced as co-product along with desalted water by diverting part of the steam to the turbine to produce electricity and part of the steam to the desalination plant. This configuration allows increased flexibility in energy usage. However, the total energy consumption would be the same as if the steam for desalination and electricity had been separately produced.
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In series co-generation, electricity is produced by expanding the steam first through a-turbine and then to the desalination process. This form of cogeneration results in reduced total energy consumption as compared to parallel co-generation. From the thermodynamic point of view, it is useful to convert most of the steam enthalpy to mechanical/electrical energy in the turbo-generator before using it as a heating medium in a thermal desalination plant for producing desalted water. Raising the turbine backpressure increases the temperature of the heat available to the desalination plant but reduces the amount of electricity generated. Therefore, in series co-generation the turbine backpressure must be optimized relative to overall plant economics. The optimum configuration for co-generation of power and water depends greatly on the powerto-water ratio at different periods of electricity and water demand. In winter, when the power demand is low and the water demand is continuously high, selection of a plant arrangement with a high power-to-water ratio will result in a significant amount of idle power. The marginal cost of water would rise significantly if auxiliary boilers or bypass steam turbines have to be used by the pressure reducing station to keep the desalination plant at full capacity. For a plant requiring higher power-to-water, normally the backpressure scheme is recommended. On the other hand, the extraction scheme is preferred for satisfying low power-to-water requirements. In the backpressure scheme all the steam is expanded in the turbine to an elevated turbine backpressure depending on its design. Then low-grade steam exiting the turbine is passed directly to the brine heater where it releases its latent heat of vaporization. The condensate is returned to the heat source through a feed water heater. This scheme requires relatively low investment and has a good efficiency when operated at rated capacity. However, backpressure systems cannot vary their power-to-water ratio. In the case of coupling to MSF, the steam flow must remain
relatively constant and therefore the plant must be preferentially base loaded. In the extraction scheme, the steam is expanded in a high-pressure turbine to a selected pressure depending on the design. It is then distributed between two flows. In the first, it passes to the brine heater and in the second to a low-pressure turbine where steam is expanded to a vacuum condenser. The extraction scheme enables the water plant to be permanently supplied with expanded steam independently of power load. The above two schemes can be considered based on the specific power and water requirements at a particular location in the country. Some other types of schemes may be considered for obtaining steam from power plants for desalination as given below: • Low-pressure steam can be supplied to a desalination plant from an existing low-pressure turbine by operating at higher exhaust pressure, but in general this is limited to around 0.2 bar. The power loss is low in this case resulting in low steam cost. However, the low steam pressure limits the top brine temperature, and thus a high GOR cannot be achieved. • The steam can be extracted from the cross-over pipe to the low pressure turbine. This steam has a relatively high energy content compared to that required for low temperature heating purposes. This results in a higher relative power loss. On the other hand, a high GOR can be achieved in the desalination plant by incorporating a larger number of stages/effects subject to design limitations. • Low-pressure steam at desired pressure could be taken from extraction ports of an extraction/ condensing turbine. In this type of coupling arrangement, the full electrical output could come back on line if the desalination plant is shutdown. The turbine arrangement has a higher flexibility for variable water production to power production ratio. A backpressure turbine and a condensing turbine can be installed in parallel. The back-
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pressure turbine steam exhaust at desired conditions would be coupled to the desalination plant.
3. Large Saudi MSF design features Table 2 shows the major Saudi desalination plants that were on operation or under construction under full supervision of Saline Water Conversion Corporation (SWCC). Nine o f the 23 total operated desalination facilities in Saudi Arabia are dual system plants that generate about 3,600 MW of electricity per day or 33% o f the total power generated in the country. Large MSF plants operate within the context of dual-purpose facilities for the simultaneous production o f power and water. Such co-generation arrangement uses either backpressure or extraction condensing turbine. Co-generation
cycles which were used till 1982 were employing extraction condensing turbines with a power to water ratio ranging between 10.2 to 17.5 MW/migd. From 1983 onwards backpressure turbines were used in all new co-generation plants. Backpressure turbines give lower power to water ratio (high water demand) and they are also characterized by high thermal efficiencies. They make the best use of low-grade heat that would otherwise be rejected by the power generating plant cycle. Saudi MSF distillers are characterized by a wide range of design features and performance characteristics [6]. Distiller production capacity ranges from 2.5 to 10.0 migd. Performance ratios vary between 5.6 and 10.6 kg/2326 kJ (2.39 to 4.57 kg/1000 kJ). Salient features of Saudi MSF plants are shown in Table 3 [6]. All the MSF distiller are operated with brine recirculation
Table 2 Major Saudi MSF desalination plants Plant
Start-up year
Installed capacity Water mgd
In operation Al-Wajh II A1-Wajh ext 1 A1-Wajh ext 2 Rabig I Farasan I Farassan ext I Jeddah II Jeddah III Jeddah IV Medinah-Yanbu I Shoaibah I Assir I AIKhafji II A1Khobar II A1-Jubail I A1-Jubail II Under construction: Medinah-Yanbu II AI-Khobar II Shoaibah II
Power m3/d
MW
1979 1986 1989 1982 1979 1984 1978 1979 1982 1980 1989 1989 1986 1983 1982 1983
0.125 0.218 0.273 0.520 0,114 0.284 10.000 20.000 50.000 25.000 48.000 20.000 4.920 51.126 30.653 211.036
473 825 1,032 1,978 430 1,075 37,850 75,700 189,250 95,000 181,800 75,700 18,624 193,536 116,035 798,864
------71 200 500 250 190 45 -500 238 812
----
60.000 74.000 120.000
227,200 280,000 455,000
150 467 515
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I.S. AI-Mutaz, A.M. A1-Namlah / Desalination 166 (2004) 287-294
modes and the majority is with cross flow configuration except Jeddah II and IV, which have long tube configurations. Number of stages varies from 16 in Alkhobar and Jeddah III up to 34 in Jeddah II. A1-Jobail II MSF plant is the world largest desalination plant. It comprises 40 MSF units each producing 23,500 mVd (6.21 mgd). The plant consists o f 10 steam turbine generator units with a total capacity of 1,295 MW. The following is a brief description of the plant [12,13]. The plant was built in three contracts; 20 units, 10 units and 10 units. Thirty of the units have 19 stages in the heat recovery section and 3 stages in the heat rejection section. The remaining 10 units have 17 stages in the heat recovery section and 2 stages in the heat rejection section. The units are arranged in 4 blocks o f 10 desalination plants. The evaporator are cross tube type. Table 4 shows the design details o f the evaporator tubes in different plant sections. Evaporators are
designed for two modes o f operations; low temperature operation (LTO) at 90.6°C and high temperature operation (HTO) at 112.8°C. Table 5 summarizes the major design parameters of these conditions. Seawater is deaerated to get oxygen content of less than 20 ppb. Anti-foam materials are added before the deaerators to overcome foaming in the deaerators. Sodium sulfite is then injected to reduce the dissolved oxygen to zero. An on-line sponge-ball cleaning system is used to remove any perception in the heat recovery section, The average ball circulation rate was 8 balls per tube per day. Polyphosphate or polymers are injected also for better scale control and minimization o f heat transfer fouling. The average polyphosphate dosing rate was 3.9 ppm. The average rate of fouling accumulation in the heat recovery section is about 1.32x10 -8 m 2 K/Wh at LTO. So the LTO operating period is 13,000 h (1.48 y) to accumulate the design recovery fouling resistance of
Table 4 Evaporator tube information in A1-Jobail II MSF plant Item
Brine heater
Recovery section
Rejection section
Tube material Tube wall thickrness, mm Tube OD, mm Tube length, turn
66 Cu/30 Ni/2 Fe/2 Mn 1.245 39.0 14.3
90-10 Cu/Ni 1.245 39.0 19.9
Titanium 0.711 29.0 19.9
Table 5 Operation design parameters ofA1-Jobail II MSF plant Item Max. brine temperature, °C Max. brine concentration, ppm Distillate capacity, m3/hr Performance ratio, kg/mJ Average energy consumption, kW Brine velocity in tube, m/s Recovery fouling, m2 K/W Brine heater fouling, m2 K/W Scale control additive dosing rate, ppm Scale control additive type
LTO
HTO
90.6 64,900 985.0 3.44 3873.3
12.8 61,800 1163.3 4.09 3729.2
1.98
1.58
0.000176 0.000176 5 Polyphosphate
0.000146 0.000176 7 Polymers
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0.000176 m 2K/W based on a constant rate o f fouling with time. This period o f time determines the estimated operating period between acid cleanings of the evaporators. The plant has an average performance ratio of 4.09 kg/kJ (9.5 Ib/1000 Btu) and a concentration ratio of recycle brine o f 1.344. The seawater TDS is about 46,500 ppm on average and the maximum product TDS is 25 ppm. Heat transfer coefficient in the brine heat is in the range 3594 to 4134 W/m 2K. Evaporators has a heat transfer coefficient of 2630 to 3925 W/m s K.
References [1] L. Awerbuch, Nuclear desalination of sea water, Proc. Symposium, Taejon, Republic of Korea, May 26-30, 1997. [2] I.S. A1-Mutaz, Energy conservation in seawater desalination plants, the First Saudi Symposium on Energy, Utilization and Conservation, King Abdulaziz University, Jeddah, Saudi Arabia, March 4-7, 1990.
[3] I.S.Al-Mutaz, Operation of dual MSF desalination plants at water/power peak demand, Symposium on Desalination Processes in Saudi Arabia, College of Engineering, King Sand University, Riyadh, Saudi Arabia, June 4-6,1994. [4] I.S. AI-Mutaz and M.A. Soliman, Optimum operation of steam-power cycle in dual purpose MSF desalination plants, Desalination, 84 (1991) 104. [5] IAEA-TECDOC-942,Thermodynamic and Economic Evaluation of Co-production Plants for Electricity and Potable Water, Vienna, 1997. [6] O.A. Hamed, M.A.K. A1-Sofi, G.M. Mustafa, K. Bamardouf and H. A1Washmi, Overview of design features and performance characteristics of major Saline WaterConversionCorporation(SWCC) MSF plants, WSTA 5th Gulf Water Conference, Doha, Qatar, March 24-28, 2001. [7] A.M.A.AI-Mudaiheemand H. Miyamura, Construction and commissioning of AI-Jobail Phase II desalinationplant, 2nd World Congress on Desalination and Water Reuse, Bermuda, Nov. 17-22, 1985. [8] M.AAI-Sofi, R.D. Peterson, D.L. Moen, Y. Soejim and H. Swad, Thermal performance of 10>(5.2 MIGD MSF plants, A1-Jobail Phase II, 2nd World Congress on Desalination and Water Reuse, Bermuda, Nov. 17-22, 1985.
ELSEVIER
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Characteristics of dual purpose MSF desalination plants Ibrahim S. A1-Mutaz*, Abdullah M. A1-Namlah Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia Tel. +966 (1) 4676870; Fax +966 (1) 4682858; e-maih [email protected] Received 4 March 2004; accepted 12 March 2004
Abstract
All of the Saudi large MSF plants operate within the context of dual-purpose facilities for the simultaneous production of power and water. Such co-generation arrangement uses either backpressure or extraction condensing turbine. Co-generation cycles which were used till 1982 were employing extraction condensing turbines with a power to water ratio ranging between 10.2 to 17.5 MW/migd. From 1983 onwards backpressure turbines were used in all new co-generation plants. Backpressure turbines give lower power to water ratio (high water demand) and they are also characterized by high thermal efficiencies. They make the best use of low-grade heat that would otherwise be rejected by the power generating plant cycle. This paper aims to study the characteristics of the dualpurpose MSF desalination plants with special reference to the Saudi experience in this field. The challenge that faces these plants is to provide an operation that satisfy the diverse operational requirements imposed by power and water production and yet retain the inherent economic advantages of the dual-purpose concept. ' Keywords: Seawater desalination; Multi-stage flash (MSF); Dual purpose; Cogeneration; Performance; Characteristics
I. Introduction
Most of the potable water and electricity in the Arabian Gulf countries are produced by dualpurpose multi-stage flash (MSF) desalination plants. A dual-purpose plant is the one that supplies heat for a thermal desalination unit and produces *Corresponding author.
electricity for distribution to the electrical grid. MSF desalination plants need mainly thermal energy, which is convenient to be supplied by lowor medium-pressure steam (sub-atmospheric or below 3 bar, respectively). Dual-purpose MSF desalination plants are economically attractive options for desalination because tile cost of the plant is allocated to two products (water and electricity).
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi;10.1016/j.desal.2004.06.083
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I.S. Al-Mutaz, A.M. Al-Namlah / Desalination 166 (2004) 287-294
Dual-purpose desalination plant has many economic advantages over single purpose plants, such as lower financial investment due to sharing of facilities, less fuel consumption and less manpower requirement. However it suffers from some disadvantages, e.g. less overall flexibility and slightly lower availability factor. Large-scale power-desalting projects undertaken in recent years were reviewed byAwerbuch [ 1] underlined the importance of the power/water ratio in selecting the appropriate technology. Typical power to water ratios for dual MSF desalination plants are shown in Table 1. The powerto-water ratio is defined as megawatt of power generated per million gallons/day of fresh water produced. The values in the table were estimated with knowledge of the achievable gain output ratio (GOR) in MSF desalination plants and the turbine efficiency in power plants. GOR is the ratio ofkg of water produced per kg of steam used. Values of GOR are often between 8 to 10 with a practical maximum of 12, which corresponds to about 55 kWh/m 3 of thermal energy. As clearly shown in Table 1, a backpressure steam coupling system requires half the power plant capacity compared to an extraction steam coupling system. The characteristics of large Saudi dual-purpose MSF desalination plants will be reviewed. This Table 1 Typical power to water ratios for dual MSF desalination plants Technology Steam turbine BTG Steam turbine EST GT-HRSG Combinedcycle BTG Combinedcycle EST
Power-to-waterratioa 5 10 8 16 19
a Power to water ratio - - MW power generatedper mgd of water produced BTG-- backpressure turbine generator EST-- extraction steam turbine GT - - gas turbine HRSG - - heat recovery steam generator
will exhibit the Saudi experience in the desalination field.
2. Co-generation and dual MSF desalination plants MSF desalination plants need mainly thermal energy, which is convenient to be supplied by lowor medium-pressure steam (sub-atmospheric or below 3 bar, respectively). They need also secondary and tertiary mechanical energy for pumping and maintaining vacuum, which is normally supplied by electricity (and possibly, for gas pumping, by high- or medium-pressure steam). In efficient MSF plants it is equivalent to about 8.1 kWh (+15%) per m 3 [2]. The additional secondary and tertiary mechanical and electrical energies are equivalent to 2.5-4.5 kWh/m 3 for MSF plants. The steam source for MSF desalination plants can be a dedicated or non-dedicated (co-generation) plant. The former provides energy exclusively for the desalination process and water is the only product out of the complex. The latter provides only part of its energy to the desalination process, and the rest of the energy is used to generate electricity. Co-generation is an economically attractive option for desalination because the cost of the plant is allocated to two product streams. A number of large co-generation power and desalination plants have been built in various parts of the world in recent years. Combined power and water production represents the largest use of the co-generation concept with over 25,000 MW of installed world electrical capacity [3]. A co-generation plant, which produces both electricity and good quality water, is often called a dual-purpose plant. A dual-purpose desalination plant is a process that produces both power and desalted water with the optimum use of thermal energy in producing the two products. It offers a considerable saving in energy usage. For example a single-purpose power plant producing electrical
LS. AI-Mutaz, A.M. Al-Namlah / DesaBnation 166 (2004) 287-294
power of 1000 MW requires 3000 MW of thermal power. The remaining 2000 MW are exhausted as low temperature waste heat. To produce 1135x 103 m3/d (300 mgd) of desalted water in a single-purpose plant requires about 3000 MW, all of which ultimately is discharged as low temperature waste heat. The total sum of the two separate single-purpose plants requires 6000 MW and exhausts 5000 MW to the atmosphere as waste heat. For a dual-purpose plant produces the same two products (1000 MW of electricity and 1135 x 103 m3/d of water) the total thermal requirement is only 4000 MW, and the total waste heat exhausted is 3000 MW. Thus a savings of 2000 MW is realized, and the exhaust heat load is reduced from 5000 MW to 3000 MW [4]. Dual-purpose desalination plant has the following economic advantages over single purpose plants [5]: • Lowerfinancial investment due to sharing o f facilities. Seawater supply and brine outfall,
land requirement and site preparation costs are less, and some ancillary equipment such as pumps, compressors, lighting, transformers as well as administrative buildings can be used in common. For evaporative desalination there is saving in steam generators and final condensers. • Less f u e l consumption. The total fuel consumption is less since some main components are larger in dual-purpose plants, thus having higher efficiencies. Also, energy is saved in shared facilities and common sub-systems as well as by short distance energy transport. • Less manpower requirement. A dual-purpose plant requires fewer staff than the two singlepurpose plants because of joint operation of common facilities. However, a dual-purpose plant also has disadvantages, some of which are as follows: ° Less overallflexibility. Its operation is not as flexible as a single-purpose plant. There is economic pressure to maximise the combined
289
production of water and power. In particular, it is desirable to operate a power plant near base load to be most economic. The same is true in most cases for desalination. However, such design and operation may reduce the plant flexibility and cause some indirect penalties. Certain designs provide, indeed, for variation in the water to electricity ratio, but at the cost of efficiency or extra investment. • Slightly lower availability factor. Any incident interrupting the output of one of the two products may lead to a disturbance or stoppage in the production of the other, thus increasing the cost per unit product. It is possible to improve the plant availability as a whole and reduce this cost-increase by connecting reverse osmosis (RO) units to the grid or adding devices such as bypass steam lines so that steam can be directed to the evaporative desalination plant if the turbo-generator is out of operation. Likewise, addition of an auxiliary condenser enables the power plant to be operated if the desalination plant is shutdown. However, these devices involve extra investment. ° Off-optimum site and timing. The optimal location and/or commissioning date of the desalination plant, from the aspects of water conveyance, distribution and supply, may not coincide with those of the power plant, bearing another penalty. There are two types of co-generation modes: • Parallel co-generation, ° Series co-generation. In parallel co-generation, electricity is produced as co-product along with desalted water by diverting part of the steam to the turbine to produce electricity and part of the steam to the desalination plant. This configuration allows increased flexibility in energy usage. However, the total energy consumption would be the same as if the steam for desalination and electricity had been separately produced.
290
1.S. Al-Mutaz, A.M. AI-Namlah ~Desalination 166 (2004) 287-294
In series co-generation, electricity is produced by expanding the steam first through a-turbine and then to the desalination process. This form of cogeneration results in reduced total energy consumption as compared to parallel co-generation. From the thermodynamic point of view, it is useful to convert most of the steam enthalpy to mechanical/electrical energy in the turbo-generator before using it as a heating medium in a thermal desalination plant for producing desalted water. Raising the turbine backpressure increases the temperature of the heat available to the desalination plant but reduces the amount of electricity generated. Therefore, in series co-generation the turbine backpressure must be optimized relative to overall plant economics. The optimum configuration for co-generation of power and water depends greatly on the powerto-water ratio at different periods of electricity and water demand. In winter, when the power demand is low and the water demand is continuously high, selection of a plant arrangement with a high power-to-water ratio will result in a significant amount of idle power. The marginal cost of water would rise significantly if auxiliary boilers or bypass steam turbines have to be used by the pressure reducing station to keep the desalination plant at full capacity. For a plant requiring higher power-to-water, normally the backpressure scheme is recommended. On the other hand, the extraction scheme is preferred for satisfying low power-to-water requirements. In the backpressure scheme all the steam is expanded in the turbine to an elevated turbine backpressure depending on its design. Then low-grade steam exiting the turbine is passed directly to the brine heater where it releases its latent heat of vaporization. The condensate is returned to the heat source through a feed water heater. This scheme requires relatively low investment and has a good efficiency when operated at rated capacity. However, backpressure systems cannot vary their power-to-water ratio. In the case of coupling to MSF, the steam flow must remain
relatively constant and therefore the plant must be preferentially base loaded. In the extraction scheme, the steam is expanded in a high-pressure turbine to a selected pressure depending on the design. It is then distributed between two flows. In the first, it passes to the brine heater and in the second to a low-pressure turbine where steam is expanded to a vacuum condenser. The extraction scheme enables the water plant to be permanently supplied with expanded steam independently of power load. The above two schemes can be considered based on the specific power and water requirements at a particular location in the country. Some other types of schemes may be considered for obtaining steam from power plants for desalination as given below: • Low-pressure steam can be supplied to a desalination plant from an existing low-pressure turbine by operating at higher exhaust pressure, but in general this is limited to around 0.2 bar. The power loss is low in this case resulting in low steam cost. However, the low steam pressure limits the top brine temperature, and thus a high GOR cannot be achieved. • The steam can be extracted from the cross-over pipe to the low pressure turbine. This steam has a relatively high energy content compared to that required for low temperature heating purposes. This results in a higher relative power loss. On the other hand, a high GOR can be achieved in the desalination plant by incorporating a larger number of stages/effects subject to design limitations. • Low-pressure steam at desired pressure could be taken from extraction ports of an extraction/ condensing turbine. In this type of coupling arrangement, the full electrical output could come back on line if the desalination plant is shutdown. The turbine arrangement has a higher flexibility for variable water production to power production ratio. A backpressure turbine and a condensing turbine can be installed in parallel. The back-
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pressure turbine steam exhaust at desired conditions would be coupled to the desalination plant.
3. Large Saudi MSF design features Table 2 shows the major Saudi desalination plants that were on operation or under construction under full supervision of Saline Water Conversion Corporation (SWCC). Nine o f the 23 total operated desalination facilities in Saudi Arabia are dual system plants that generate about 3,600 MW of electricity per day or 33% o f the total power generated in the country. Large MSF plants operate within the context of dual-purpose facilities for the simultaneous production o f power and water. Such co-generation arrangement uses either backpressure or extraction condensing turbine. Co-generation
cycles which were used till 1982 were employing extraction condensing turbines with a power to water ratio ranging between 10.2 to 17.5 MW/migd. From 1983 onwards backpressure turbines were used in all new co-generation plants. Backpressure turbines give lower power to water ratio (high water demand) and they are also characterized by high thermal efficiencies. They make the best use of low-grade heat that would otherwise be rejected by the power generating plant cycle. Saudi MSF distillers are characterized by a wide range of design features and performance characteristics [6]. Distiller production capacity ranges from 2.5 to 10.0 migd. Performance ratios vary between 5.6 and 10.6 kg/2326 kJ (2.39 to 4.57 kg/1000 kJ). Salient features of Saudi MSF plants are shown in Table 3 [6]. All the MSF distiller are operated with brine recirculation
Table 2 Major Saudi MSF desalination plants Plant
Start-up year
Installed capacity Water mgd
In operation Al-Wajh II A1-Wajh ext 1 A1-Wajh ext 2 Rabig I Farasan I Farassan ext I Jeddah II Jeddah III Jeddah IV Medinah-Yanbu I Shoaibah I Assir I AIKhafji II A1Khobar II A1-Jubail I A1-Jubail II Under construction: Medinah-Yanbu II AI-Khobar II Shoaibah II
Power m3/d
MW
1979 1986 1989 1982 1979 1984 1978 1979 1982 1980 1989 1989 1986 1983 1982 1983
0.125 0.218 0.273 0.520 0,114 0.284 10.000 20.000 50.000 25.000 48.000 20.000 4.920 51.126 30.653 211.036
473 825 1,032 1,978 430 1,075 37,850 75,700 189,250 95,000 181,800 75,700 18,624 193,536 116,035 798,864
------71 200 500 250 190 45 -500 238 812
----
60.000 74.000 120.000
227,200 280,000 455,000
150 467 515
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modes and the majority is with cross flow configuration except Jeddah II and IV, which have long tube configurations. Number of stages varies from 16 in Alkhobar and Jeddah III up to 34 in Jeddah II. A1-Jobail II MSF plant is the world largest desalination plant. It comprises 40 MSF units each producing 23,500 mVd (6.21 mgd). The plant consists o f 10 steam turbine generator units with a total capacity of 1,295 MW. The following is a brief description of the plant [12,13]. The plant was built in three contracts; 20 units, 10 units and 10 units. Thirty of the units have 19 stages in the heat recovery section and 3 stages in the heat rejection section. The remaining 10 units have 17 stages in the heat recovery section and 2 stages in the heat rejection section. The units are arranged in 4 blocks o f 10 desalination plants. The evaporator are cross tube type. Table 4 shows the design details o f the evaporator tubes in different plant sections. Evaporators are
designed for two modes o f operations; low temperature operation (LTO) at 90.6°C and high temperature operation (HTO) at 112.8°C. Table 5 summarizes the major design parameters of these conditions. Seawater is deaerated to get oxygen content of less than 20 ppb. Anti-foam materials are added before the deaerators to overcome foaming in the deaerators. Sodium sulfite is then injected to reduce the dissolved oxygen to zero. An on-line sponge-ball cleaning system is used to remove any perception in the heat recovery section, The average ball circulation rate was 8 balls per tube per day. Polyphosphate or polymers are injected also for better scale control and minimization o f heat transfer fouling. The average polyphosphate dosing rate was 3.9 ppm. The average rate of fouling accumulation in the heat recovery section is about 1.32x10 -8 m 2 K/Wh at LTO. So the LTO operating period is 13,000 h (1.48 y) to accumulate the design recovery fouling resistance of
Table 4 Evaporator tube information in A1-Jobail II MSF plant Item
Brine heater
Recovery section
Rejection section
Tube material Tube wall thickrness, mm Tube OD, mm Tube length, turn
66 Cu/30 Ni/2 Fe/2 Mn 1.245 39.0 14.3
90-10 Cu/Ni 1.245 39.0 19.9
Titanium 0.711 29.0 19.9
Table 5 Operation design parameters ofA1-Jobail II MSF plant Item Max. brine temperature, °C Max. brine concentration, ppm Distillate capacity, m3/hr Performance ratio, kg/mJ Average energy consumption, kW Brine velocity in tube, m/s Recovery fouling, m2 K/W Brine heater fouling, m2 K/W Scale control additive dosing rate, ppm Scale control additive type
LTO
HTO
90.6 64,900 985.0 3.44 3873.3
12.8 61,800 1163.3 4.09 3729.2
1.98
1.58
0.000176 0.000176 5 Polyphosphate
0.000146 0.000176 7 Polymers
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0.000176 m 2K/W based on a constant rate o f fouling with time. This period o f time determines the estimated operating period between acid cleanings of the evaporators. The plant has an average performance ratio of 4.09 kg/kJ (9.5 Ib/1000 Btu) and a concentration ratio of recycle brine o f 1.344. The seawater TDS is about 46,500 ppm on average and the maximum product TDS is 25 ppm. Heat transfer coefficient in the brine heat is in the range 3594 to 4134 W/m 2K. Evaporators has a heat transfer coefficient of 2630 to 3925 W/m s K.
References [1] L. Awerbuch, Nuclear desalination of sea water, Proc. Symposium, Taejon, Republic of Korea, May 26-30, 1997. [2] I.S. A1-Mutaz, Energy conservation in seawater desalination plants, the First Saudi Symposium on Energy, Utilization and Conservation, King Abdulaziz University, Jeddah, Saudi Arabia, March 4-7, 1990.
[3] I.S.Al-Mutaz, Operation of dual MSF desalination plants at water/power peak demand, Symposium on Desalination Processes in Saudi Arabia, College of Engineering, King Sand University, Riyadh, Saudi Arabia, June 4-6,1994. [4] I.S. AI-Mutaz and M.A. Soliman, Optimum operation of steam-power cycle in dual purpose MSF desalination plants, Desalination, 84 (1991) 104. [5] IAEA-TECDOC-942,Thermodynamic and Economic Evaluation of Co-production Plants for Electricity and Potable Water, Vienna, 1997. [6] O.A. Hamed, M.A.K. A1-Sofi, G.M. Mustafa, K. Bamardouf and H. A1Washmi, Overview of design features and performance characteristics of major Saline WaterConversionCorporation(SWCC) MSF plants, WSTA 5th Gulf Water Conference, Doha, Qatar, March 24-28, 2001. [7] A.M.A.AI-Mudaiheemand H. Miyamura, Construction and commissioning of AI-Jobail Phase II desalinationplant, 2nd World Congress on Desalination and Water Reuse, Bermuda, Nov. 17-22, 1985. [8] M.AAI-Sofi, R.D. Peterson, D.L. Moen, Y. Soejim and H. Swad, Thermal performance of 10>(5.2 MIGD MSF plants, A1-Jobail Phase II, 2nd World Congress on Desalination and Water Reuse, Bermuda, Nov. 17-22, 1985.