TOWARDS SUSTAINABLE ENERGY IN SEAWATER DESALTING IN THE GULF AREA

Transcription

1 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 655 TOWARDS SUSTAINABLE ENERGY IN SEAWATER DESALTING IN THE GULF AREA M. A. Darwish a, N. Al-Najem a, and N. Lior b Mechanical Engineering Departments, a- Kuwait University, Kuwait and b- University of Pennsylvania, USA ABSTRACT Gulf countries, experienced very rapid growth in the last four decades by the evolution of oil production and its price increase. The main source of portable water, about 93%, was secured by desalting seawater in The increase of the daily-consumed fresh water in liters per capita (from 137 in 1973 to almost 500 in 2003) and population increase (from 900,000 in 1983 to more than 2,540,000 in 2003) necessitate the production of large quantities in desalted water with huge amounts of consumed fuel energy. Thermally operated desalting units usually obtain their thermal energy input by steam supply; either extracted from steam turbines or from heat recovery steam generators HRSG combined with gas turbines. Therefore, most desalted water is produced in cogeneration power desalting plants CPDP. The effect of fuel consumed for desalting seawater on environment is underestimated by desalination expertise by thinking that, thermal desalination associated to power generation plant will only be minimally responsible for the discharge of the flue gases to the atmosphere as this impact can be associated to the power generation. However, 21% of the fuel consumed in Kuwait CPDP in 2003 was used for desalting the water. Burning fuel to desalt water increases the environment pollution by producing the carbon dioxide CO 2, the nitrogen oxides NOx, and sulfuric oxides SOx, with quantities directly related with the consumed fuel energy for each desalting process. As the efficiency in both power and desalting water increase, the impact on the environment decreases. The consumed electric power and desalted water are almost doubled every 10 years. If the oil production, (Kuwait main source of income) is 2 million barrels per day, then one tenth of this oil production was consumed by power and water productions in 2003, consequently, 20% and 40% of Kuwait oil production (or total income) will be consumed by water and power productions only in 2013 and 2023 respectively. In about thirty years, the total oil production may not be enough for people to drink and live in air-conditioned space in Kuwait. The aim of this paper is to look for energy efficient ways to desalt water and produce power to save the nation s income, non-renewable fuel resources, environment, and the life itself in Kuwait. It is equally important promote the conservation measures for both the water and power.

2 656 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt The paper first reviews the present and possibly used desalting methods and its way of fuel energy supply to point out the most efficient to apply and the less efficient to avoid. Fourteen cases were considered by using the currently used desalting methods, with and without combination of power plants (using steam or gas or combined gas/steam turbines). Foe each case, the specific fuel energy consumed and the amount of emitted CO 2 per m 3 of desalted water were calculated for each presented case. Finally, it compares the amount of desalted water produced in each case for the same amount of energy consumed for all cases. Keywords: desalination, multi stage flash desalting, multi effect desalting, mechanical vapor compression desalting, thermal vapor compression desalting, reverse osmosis desalting, steam turbine power plants, gas turbines power cycle, combined gas/steam power cycle, specific fuel consumption for desalting, equivalent mechanical energy consumption INTRODUCTION Kuwait, like other Gulf countries, experienced very rapid growth in electric power and desalted water production in the last four decades due to the evolution of oil production and its price increase. One may ask, does this growth is sustainable? The most used definition for sustainability, Bruntland report [1], is the growth that meets the present generation needs without compromising future generations ability to meet their own needs. Herman Daly [2] stated that sustainability requires: 1. The using rate of renewable resources (e.g. ground water) does not exceed the rate of their re-generation. 2. The using rate of non-renewable resources (e.g. fossil fuel, mineral ores) does not exceed the developing rate of sustainable substitutes. 3. The pollutants emission rate does not to exceed the capacity of the environment to absorb, or render them harmless. The renewable annual water resource (ground water) in Kuwait is less than 100 m 3 /capita (annual 1000 m 3 /capita marks the water poverty line), while extraction (consumed) rate is more than 500 m 3 /capita per year. Over-extraction depletes the ground water and deteriorates its quality. The non-renewable fossil fuel oil energy resource is finite, and with no alternative to substitute. Although full sustainability may not seem possible for the time being, efficient power and water production is essential as it allows the present resources to keep the development over longer time period, i.e., to make it more sustainable.

3 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 657 In Kuwait, about 93% of potable water need was secured by desalting seawater in Table 1 gives the installed capacity of desalting units and desalted water consumption in the last four decades in million imperial gallons (MIG) per day or MIGD. One MIG is equal to 4546 m 3. The daily-consumed fresh water in liters per capita increased from 137 in 1973 to almost 500 in 2003, and population increased from 900,000 in 1983 to 2,540,000 in These necessitate the production of large amounts of desalted seawater with huge quantities of consumed fuel, as desalting is fuel-based process. Thermally operated desalting units usually obtain their heat input as steam supply; either extracted from steam turbines or from heat recovery steam generators HRSG combined with gas turbines. Thus, the desalted water is produced generally in cogeneration power desalting plants CPDP. Desalination experts, [3] underestimate the impact of the fuel consumed for desalting seawater on the environment, by considering thermal desalination in CPDP is minimally responsible for flue gases discharged to atmosphere as this can be associated to the power production. However, 22% of the fuel consumed in Kuwaiti CPDP in 2003 was used for desalting as shown later. The fuel consumed for desalting depends on the method of desalting, and the way energy is supplied to desalters. Burning fuel for desalting increases the environment pollution by producing carbon dioxide CO 2, nitrogen oxides NOx, and sulfuric oxides SOx, with quantities directly related to the fuel consumed. So, desalting contributes for sure to environment pollution. Increasing the efficiency of both power and desalted water productions lowers the impact of fuel combustion on the environment. Tables 1 and 2 show that the consumed power and desalted water in Kuwait are almost doubled every 10 years. The oil production, (main source of income) was million barrels per day, (724 million barrels /year) in 2002, and little more than one tenth of this oil production (75.92 million barrels) was used in the CPDP in Consequently, 20% and 40% of Kuwait total oil production (or total income) are to be consumed by the CPDP in 2013 and 2023 respectively. In about thirty years, the total oil production may not be enough, if the present production rate is kept the same, for people to drink (mainly desalted water), and live in air-conditioned space (using more than 75% of electric power consumption) in Kuwait if the present trend prevails. So, it is essential to look for energy efficient ways to produce power and desalted water to save the nation s income, non-renewable fuel resources, environment, and the life itself in Kuwait. It is equally important to promote all conservation measures for both the water and power. The present power plants average efficiency is less than 38%, while it can be improved by using combined gas/steam turbine cycle in the future to 60%. Also, the multi stage flash MSF desalting system consumes specific equivalent mechanical in the range of 20-kWh/m 3, and the future use of reverse osmosis desalting system can reduce this energy to the range of 5-kWh/m 3. Beside, dealing with water as free resource gives no incentive to utilize it efficiently and promotes un-sustainability.

4 658 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt As a first step in looking for efficient use of energy, this paper is devoted to review the present and possibly used desalting methods and the way fuel energy passes through to supply the energy needs (either thermal, mechanical, or both) to desalters. This is to point the most efficient to apply and less efficient to avoid. Fuel consumed by desalting seawater in Kuwait in 2003 In 2003, the fuel energy consumed to produce electric power and desalted water in Kuwait was million GJ (equivalent to million barrels of crude oil based on 5.71 GJ energy content of one barrel). The produced desalted water was 431 million m 3, and electric power was 35,577 million kwh, [4]. If the average power production efficiency is 0.38, (heat rate = 9,473 kj/kwh), the fuel charged to power is 337 million GJ (59 million barrels), and to desalted water is 96.5 million GJ (16.92 million barrels). So, the average fuel energy charged to produce 1 m 3 of water is 224 MJ/m 3, (can be obtained by burning about 5 kg of fuel oil). Table 1: Installed desalting capacity and daily consumption in MIGD, [4] Year Installed capacity, MIGD Daily average consumption MIGD Daily per capita fresh water consumption in l/d (imp. g/d) (18.2) (30.2) (53.8) (88.6) (109.3) Table 2: Electric power production and consumption in Kuwait, [4] Year Population Installed power capacity Million kwh Max. MW Min. MW Annual kwh/capita Max. load/capita kw , , ,537, ,546, The estimated cost of fuel used to desalt seawater in 2003, based on $40/barrel is million dollars. Moreover, the consumed fuel in 2003 for power and water added 33,780 tons of CO 2 to the environment.

5 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 659 In the Ministry of Energy ME report 1, the efficiency of power production was reported as 42% (high), and the cost of x10 6 GJ (75.92 million barrels) was million dollars. This gives the $20.32 as the barrel cost, (low cost estimation). Present Rating Methods of Desalting Processes The gain ratio GR and the performance ratio are the most two applied methods to rate the thermal driven desalting systems, such as the thermal vapor compression TVC, multi stage flash MSF, and multi effect boiling MEB desalting units. The GR is defined by: GR = desalted water output D per kg of supplied heating steam S, or D/S. The GR does not consider the heat given by each kg of used steam, h (enthalpy difference across the desalter), or the actual consumed heat by desalter, Q d. More logic is to calculate the heat required Q d per unit mass of desalted water D or Q d /D. This was considered by correcting the amount of S used in the definition of GR to Q d per standard h = 2330-kJ/kg, and the corrected PR is called PR. So, the PR is defined as the desalted water output D per kg of supplied steam that has reference h equal 2330 kj/kg, or: PR = D/(Q d /2330) = 2330 D/(S h) = (D/S)(2330/ h) = (2330/ h)gr. In other words the GR is corrected by multiplying its value by (2330/ h), and the corrected value is called the performance ratio, PR. As example if the supplied steam enters as saturated vapor and leaves as saturated liquid, and has latent heat L, then PR = (2330/L) GR. The GR and PR do not take in consideration the quality of used steam (pressure, temperature or availability), or the consumed pumping energy. The real value of steam lies in its ability to produce work, which increases by the increase of both temperature and pressure. The TVC units usually use high pressure (and availability) steam (say at 10 bar) compared to the well known multi stage flash MSF and multi effect ME desalting system, and the GR (or PR) rating overlooks this factor (i.e. value of used steam). The best way to evaluate the performance of any desalting system is to count for the actual fuel energy required (consumed) to desalt unit mass or volume of desalted water (specific fuel energy consumption, say in MJ/m 3, kj/kg, or kg fuel/m 3 of desalted water), or the equivalent work to this fuel energy in kwh/m 3, when this fuel energy is transformed to work, say in power plant of 0.36 standard efficiency. Sustainability experts, [5] used the same logic when the desalination sustainability indicators were formulated. The first indicator is the resource fuel, defined by amount of fuel in kg per m 3 of desalted water. Other environmental indicators include: the amount effluent gases produced due desalting one m 3 of water such as: kg CO 2 /m 3, kg

6 660 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt SO 2 /m 3, and NOx/m 3. In viewing the desalting systems, and their energy supply, the resource fuel, and environment indicator of CO 2 are calculated. Water desalination in the Gulf area The renewable water resources in many Middle East counties, especially the Arab Gulf countries, are not enough to satisfy their water needs. Desalting is a viable used water resource. Table 3 gives the renewable water resource (mostly ground water) and the percentage of its extraction (and thus consumption), total and per capita of installed desalting capacity. The capacity of the MSF and RO desalting units installed in the last 10 years are given for countries, where desalted waters are used extensively. The trend of shifting from completely dependent on the MSF system to the more efficient RO system is clear in the last decade as shown in Table 4. Water consumption inflation The high rate of water consumption, especially in Kuwait, shows that the values of water and fuel used to desalt it are underestimated. The high-consumed fuel rate used to produce power and water can lead to environmental damage; degrade natural resources, and un-sustainable economic development. Water is treated by some people free common resource with almost zero economic value, and this leads to unacceptable high water consumption. Table 3: Renewable water resource and percentage of its withdrawal, and total and per capita of installed desalination units in counties using desalination extensively, [6] Country Annual renewable source in m 3 /capita Annual extraction of renewable water % Total installed desalination capacity Desalination installed capacity/capita l/capita % of the world MSF capacity Bahrain 516, Egypt , Iraq 5, , Israel , Jordan , Kuwait ,181,026 1, Libya , Oman , Qatar 762,932 1,276 Saudi A ,569, UAE 1, ,532,

7 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 661 Table 4: Capacities of seawater MSF and RO units installed and became operative since 1994 in some Gulf countries and Libya, [6] Country Capacities in cubic meters per day MSF RO Bahrain 136, Kuwait 336,104 Libya 44,240 Oman 194, Qatar 334,640 Saudi Arabia 1,337, ,516 UAE 2,522,109 2,518,650 The present water production rate in Kuwait can meet water needs for many years to come if it is consumed wisely. The basic domestic water needs do not exceed 150 l/capita per day, while the consumed water became almost 500 l/capita per day in Kuwait in It is vital to supply the basic needs of water to every one, irrespective of his income, free or at very low price. However, it is vital also to charge proper cost to any amount of water consumed beyond the water basic needs, to curb the wasteful use of water, and sustain its existence. This means that subsidies should be focused and limited to average people basic needs. Any gain from efficient water producing can be easily be offset by increase of consumption and waste. Actual data from the ME shows that fresh water consumed in 2003 is m 3, and the ME income from selling that water was $ , or $0.1637/m 3. At the same time, as mention before the fuel energy consumed for desalting is 224 MJ/m 3, its cost $1.57 per m 3 based on $40/barrel ($7/GJ). Case 1: Direct boiler operated thermal vapor compression desalting TVC system There are about 127 units, [6] of thermal vapor compression desalting system (TVC) having almost 200 MIGD installed capacity in the Middle East Arab countries and Iran, see Table A-1 in the Appendix. These units are in general: operated with steam supplied directly from fuel fired boilers, desalting seawater (SW), having low top brine temperature (TBT) in the range of 65 C, and using falling film horizontal tube evaporators (HTE). Most of these units were put in service in the last decade. The only reported units that use steam extracted from turbines are those in Um-Al-Nar and Taweela, [7] units in UAE. Steam supplied to the Taweela units is exhausted from backpressure turbine at 2.8 bar. In winter, when fewer turbines are available to supply steam, these units are directly supplied from the power plant s steam generator through high pressure HP reducing station. Figure 1 shows a schematic diagram of a TVC unit.

8 662 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt The unit consists of multi-evaporators, steam ejector (thermal compressor TC), and end condenser. Motive steam S at pressure P s and enthalpy h s expands in the nozzle of the TC to high velocity and pressure low enough to extract amount of vapor D r (out of vapor D n generated in the last effect n). The mixture S + D r passes through the TC diffuser to increase its pressure to P d (discharge pressure), enters the first effect (evaporator) as heating vapor, and leaves as condensate at enthalpy h e. This condensate is divided to S, which returns to steam transformer to be heated and becomes motive steam again; and D r, which joins the distillate product. The steam transformer (heat exchanger between the boiler and TVC unit) avoids mixing the boiler steam with its harmful chemical content with distillate product. Most of the above-mentioned TVC units use fuel-fired boiler to supply steam to these units. The steam generated is at relatively low- pressure (say at 10 bar), compared to steam generated in power plants. The case of four TVC units Jebel Dehnna, UAE, [6], given in Table A1 and Figure 1, is considered as case 1. In this case: the total desalted water output D = 27,180 m 3 /d (314.6 kg/s or 5.98 MIGD), the gain ratio is 9, the steam supply to the units is saturated at 10 bar, and its return condensate is at 65 C. Assuming no heat loss from the steam transformer, and thus the heat supply by the boiler Q b is equal to the heat gain by the TVC units Q d. So, the heat supply to the units Q d is equal: Q b = S (h s - h e ), where S is the rate of steam supplied to the 4-TVC units, h s is the enthalpy of saturated steam at 10 bar, and h e is the enthalpy of condensate 65 C. The fuel supplied to the boilers is: Q f = Q b /η b, where η b is the boiler efficiency estimated by 0.9. For distillate output D = kg/s, GR = 9, S = D/9 = 34.96, h s = kj/kg, h e = kj/kg, Q d = Q b = MW, Q f = Q b /η b = MW, and specific fuel consumption equal to Q f /D = kj /kg. The estimated pumping energy to the TVC is in the range of 2-kWh/m 3 (7.2-kJ/kg). Then 20-kJ/kg more should be added to specific fuel energy to count for pumping energy if it is produced in a power plant of standard efficiency η pp = So, the specific energy (for both heat and pumping energy added) is kJ/kg, and the total fuel energy input is MW. The kj/kg specific fuel energy would produce work equal to kj/kg, (equivalent to kwh/m 3 ), if supplied to power plant having η pp = The fuel energy cost per m 3 desalted water produced by this TVC, based on $40/barrel (or $7 per GJ) is $2.306/m 3. Based on 40,000-kJ/kg of fuel, and 85% carbon content in fuel, the amount of CO 2 released due to desalting 1 m 3 is kg CO 2 /m 3. The results of case 1 are reported in Table 1a. The amount of fuel energy used in this case 1 (= MW) is used as reference rate of fuel input in the following studied cases. This is to compare how much desalted water can be obtained from the same fuel energy input by the different cases to be studied.

9 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 663 Case 2 Direct boiler operated multi-stage flash MSF desalting system The multi stage flash MSF is the main desalting method used in the Gulf area. Most of the operating MSF units are supplied with steam extracted from steam turbines. When no operating turbines (or no turbines at all) exit, steam is supplied directly from boiler through reducing pressure station. In Shuwaik desalting plant in Kuwait, there are 3 MSF units operated with steam supplied directly from three fuel-fired boilers, and this is considered here as case 2, Figure 2. Each MSF unit has 6.5 MIGD (342 kg/s) capacity, GR = 9, and supplied with steam S = 38 kg/s. Each boiler output Q b = heat input to one MSF unit Q d = S(h s - h e ) Q b = 38( ) = 83.7 MW, and supplied with fuel energy Q f = Q b /η b = 93 MW. This gives Q f /D = kj/kg. The MSF units consume a typical 4 kwh/m 3 (14.4 kj/kg) pumping energy. The fuel energy required to produce this pumping energy is 40 kj/kg (if produced in power plant of typical η pp = 0.36). This 40 kj/kg should be added to Q f /D to count for pumping energy. Then Q f /D = counted for heat and pumping energy added, and the consumed fuel energy for each unit is MW. Again, the specific fuel energy kj/kg can produce kj/kg work ( kwh/m 3 ) in power plant of η pp = The CO 2 added to the environment is 24.3 kg CO 2 /m 3 desalted water. When the reference rate of fuel energy input (= MW of case 1) is applied here, the desalted water becomes kg/s as reported in Table 1a. Comparing case 1, and 2 indicates that: 1. Although the GR = 9 for both cases Q f /D counted for heat input only is kj/kg for case 1, and for case 2. As a result PR is in case 1, and 9.52 in case 2. The shows that GR does not really indicate the heat consumed by the desalters, and considering GR and PR as the same is not right. 2. The specific pumping energy in case 2 is 100% more than case 1, and this was not counted for in rating by PR or GR. 3. Supplying steam to the MSF at about 3 bar, or to TVC at bar directly from fuel fired boiler is in-efficient way of using fuel, as indicated in next case 3.

10 664 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt Cases (steam supply method) Table 5a: Data of the considered cases 1, 2, 3, and 4 Case 1 (Boiler operated) Case 2 (Boiler operated) Case 3 (EC/ST extracted) Case 4 Desalting system TVC MSF MSF MVC+ MED D, kg/s S, kg/s, (GR) Enthalpy, hi kj/kg Enthalpy, he kj/kg Q d (heat to to desalter), MW Qf (for main energy), MW W (pump) kwh/m Qf (by heat + pump), MW Qf (total)/d, kj/kg We, (kj/kg), kwh/m 3 ( ) ( ) (71.535) D (for Qf = ) kg/s and (MIGD) (5.98) (6.314) (9.91) (16.14) Kg CO 2 /m Case 3 Cogeneration power desalting plant with steam extracted from turbine to MSF unit(s) The use of high availability fuel to generate low availability steam (as required in cases 1 and 2) is wasteful process from thermodynamics viewpoint. It is always better to generate steam at high temperature and pressure (as required for modern steam power plants), expands it in steam turbines to produce work before its supply to the desalting plants. A plant, where steam produces both power (work) and desalted water is known as Co-generation Power Desalting Plants, CPDP. The main merit of the CPDP is saving energy compared to two separate plants: separate desalting plant SDP driven by boiler as in cases 1 or 2; and one separate power plant SPP. The CPDP using steam turbines and MSF desalting units are extensively used in the Gulf area. The steam turbines are either extraction-condensing EC/ST or backpressure BP/ST type. Each turbine is combined with one or more MSF unit. The steam supply to the MSF unit(s) is extracted from the EC/ST, Figure 3; or exhausted from BP/ST, Figure 4. Table 2A in Appendix gives examples of some CPDP using steam turbines and MSF units in the Gulf area. A typical plant in Kuwait plants is used here as case 3. Steam is generated at high pressure (around 150 bar), and temperature 538 C. The 150 bar pressure is high enough for efficient steam power plant cycle, and low enough for using natural circulation steam generators. The 538 C is the highest allowable temperature for fuel oil with sulfuric content. The CPDP plant, shown in Figure 3, uses steam generator, EC/ST steam turbine and two MSF desalting units. It can work as

11 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 665 SPP separate power plant producing 300 MW electric power; or as CPDP producing 300 MW and supplies heat of 196 MW to the MSF units. The desalters output is kg/s (14.4 MIGD) and supplied with heat equal to 258-kJ/kg of desalted water. Steam is extracted to the desalting unit(s) from cross-pipe between the IP and LP (intermediate and low pressures) turbine cylinders. If the extracted steam expands in the LP turbine to the condenser, instead of extracting for desalting, it would produce more work. This work is considered as turbine work loss due to steam extraction, and as work equivalent to the heat added to the MSF units. In the reference plant, LP turbine has the following data, and calculated work output: Mass M, kg/s Enthalpy, kj/kg Mass in LP cylinder M 8 = H 8 = 2940 Steam extracted to feed heater at point 9 M 9 = 7.39 H 9 = Steam extracted to feed heater at point 10 M 10 = H 10 =2608 Steam flow to condenser at point 11 M 11 = H 11 = 2333 W(LP turbine) = mihi - Σmehe = (2940) (2769) (2608) (2333) = 74,660 kw So, each kilogram of steam expanded in the LP turbine gives 74660/ = 560 kj work. Then, the lost work due to steam extraction of kg/s to desalters is We = = 43,234 kw = MW. This work is equivalent to 196 MW heat supplied to desalters, and gives specific equivalent work of kJ/kg (15.87-kWh/m 3 ) and kJ/kg specific fuel energy input (counts only for the desalters heat supply). When 4 kwh/m 3 specific energy for pumping is added to that counted for heat, the total specific work is = kwh/m 3, and the specific fuel energy is kJ/kg if η pp = The CO 2 produced per m 3 of desalted water is kg CO 2 /m 3. So, the rate of fuel energy used to produce kg/s is MW. The reference fuel energy rate of (Q f of case 1) would produce kg/s (9.91 MIGD) of desalted water by this arrangement. The results of case 3 are given in Table 5a. Case 3 has saving more than 50% than cases 1 and 2 in specific fuel consumption, but its consumed fuel is still high compared that of the efficient reverse osmosis desalting system. The CPDP are designed and built to produce almost constant water rate, while the produced power changes according to load, from 30% to full turbine rated capacity. Table 6, [8] shows the ratios of installed, minimum, and maximum water to power demands in some Gulf countries. The pace of water demand is higher than that of power. Water plants are needed in most Gulf area, that produces water only, or its water product rate has no relation to power production, as given in the next cases.

12 666 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt Table 6: Minimum, maximum, and installed water to power ratio in some Gulf countries, [8] Country Min. W/ratio (m 3 /d)/mw Max. W/P ratio (m 3 /d)/mw Installed W/P ratio (m 3 /d)/mw Kuwait Bahrain Abu Dhabi Case 4 All water plant using BPST to operate mechanical vapor compression MVC desalting plant and exhausts its steam to multi effect desalting MED system Steam from a boiler can be supplied to BPST to drive the compressor of MVC desalting system, Figure 5, either directly or through its electric power output. The steam exhausted from the turbine at low pressure LP = 0.35 bar (saturation temperature = 70 C) to supply heat to multi effect desalting MED system, Figure 6. This case 4 is shown in Figure 7, and similar to a desalting system installed on barge, [9] using the same type of MVC and MED desalters type, but at different conditions. The steam conditions at the turbine are: Position Temperature, Pressure, Enthalpy, Steam condition C bar kj/kg Turbine inlet Super-heated Turbine exit Quality x = The condensate returns from MED to the boiler through a de-aerator. The steam enters the BPST at enthalpy h s = 3530 kj/kg, expands to enthalpy h d = 2520 kj/kg at the exit. If this steam expands in condensing turbine, its enthalpy at the exit h c = 2333 kj/kg (pressure = 6.3 kpa). Then, the fraction of the BPST power output (used by the MVC unit) to that of condensing turbine is: (h s h d )/(h s - h c ) = ( )/( ) = , and the other fraction is to charged to the MED system. Note that h s, h d, and h c are the steam enthalpy at the steam turbine inlet, at extraction to the desalting unit, and at condenser inlet. Now if 100 MW energy fuel is supplied to the boiler, the condensing steam cycle has typical efficiency of 0.36, then MW fuel energy is to be charged to the MVC unit, and MW is to be charged to the MED system. The work used to operate the MVC system is: W(MVC) = = MW

13 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 667 The boiler heat output Q b = η b Q f = = Ms (h s - h e ), Ms = 27.8 kg/s The data for the MVC desalting units are given by: Compressor total work input, MW Compressor specific work, kj/kg (10 kwh/m 3 ) 36 Pump work, kj/kg (kwh/m 3 ) 7.2 (2) Compressor work, MW Pumping work, MW MVC distillate output D kg/s Qf charged to compressor + pumps), MW Qf /D, kj/s 120 The MW work supplied to the MVC unit(s) produces kg/s based on specific work of compressor 36 kj/kg (10 kwh/m 3 ), and 7.2 kj/kg (2 kwh/m 3 ) for pumping. The MED desalting systems have the following data: n (number of effects) 6 Q d (to desalting unit), MW Performance ratio PR 5.5 D output, kg/s (MIGD) 146(2.8) Steam Enthalpy h d in kj/kg 2520 Q f (for heat), MW Condensate enthalpy h e out in kj/kg 293 Pump work kwh/m 3 2 S (steam flow rate), kg/s 27.8 Q f (heat + pumps) Equivalent we to heat kwh/m Q f /D, kj/kg 127 We counted for heat + pump kwh/m For the MED system, Q d = 27.8( ) = 61,911 kj/s, and PR = 5.5 = 2330D/61911, and D = 146 kg/s. The specific work is: 2 kwh/m 3 (7.2 kj/kg) for pumping, 38.6 (= 5635/146) kj/kg for heat input, and 45.8 kj/kg (to count for heat and pumping energy added). The total pumping energy of the MED unit = MW, and its fuel energy is 2.92 MW. This gives specific fuel for the MED = kj/kg. The total fuel energy for both MVC and MED = 100 MW = MW. The total MVC and MEB desalted water output is kg/s, and Q f /D for the output of the two systems = kj/kg. The CO 2 added to the environment is 9.44 kg CO 2 per m 3 of desalted water. When the reference fuel input is used, the desalted water becomes 855 kg/s (16.25 MIGD). The results of case 4 are given in Table 5a.

14 668 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt Case 5: Back pressure steam turbine operating seawater reverse osmosis SWRO and multi-effect boiling MED desalting systems The more energy efficient seawater reverse osmosis SWRO system, Figure 8, can substitute the MVC used in case 4, and the result is case 5 as shown in Figure 9. The SWRO is assumed to consume 6 kwh/m 3 (21.6 kj/kg). The MW output of BPST produces 1, kg/s when operates the SWRO system. The MED produces the same 146 kg/s as in case 4, and use 2.92 MED fuel energy for pumping. The total fuel energy for both SWRO and MED MW, their total desalted water output is 1, kg/s, and Q f /D for the two systems is 66.3 kj/kg. The CO 2 added to the environment is kg CO 2 per m 3 of desalted water. When the reference fuel input is used, the desalted water becomes 1563 kg/s (29.7 MIGD). The results of case 5 are given in Table 5b. Table 5b: The results of cases 5, 6, 7, and 8 Cases 5(BPST) 6(CST) 7(CST) 8(GT) SWRO MED Total MVC SWRO SWRO D, kg/s Q d (to desalter), MW Qf (for main energy), MW Main energy W(equiv), MW W(pump) W (by main pump), MW Qf (for main pump), MW Qf (total)/d, kj/kg ,4 We, (kj/kg), kwh/m (6) (12.7) (6.63) 43.2 (12) 21.6 (6) 21.6 (6) D (for Qf= ) kg/s and (MIGD) 1563 (29.7) (16.41) (32.82) (33.74) Kg CO 2 /m

15 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 669 Case 6: Steam power plant operating MVC desalting system The same as case 4, but the backpressure turbine is substituted by condensing turbine and all the steam turbine-generator output is used to operate MVC. The MVC consumed energy is 10 kwh/m 3 for compressor, and 2-kWh/m 3 for pumps, a total mechanical energy consumption of 12 kwh/m 3, (43.2-kJ/kg). For the 100 MW fuel input to the boiler of η pp = 0.36, the total power output is 36 MW, and desalted water output = kg/s. The specific fuel input is 120-kJ/kg. The CO 2 added to the environment is 9.35 kg CO 2 / m 3. The reference fuel energy input of gives distillate output = kg/s (16.41 MIGD). The results of case 6 are given in Table 5b. Case 7: Steam power plant operating SWRO desalting system The same steam power plant used in case 6, is used to drive SWRO desalting system (substitutes the MVC system in case 6). The generator output 36 MW can desalt kg/s, and the Q f /D = 60 kj/kg. The CO 2 added to the environment is kg CO 2 /m 3. When the reference fuel rate input of MW is used, the corrected desalted water becomes kg/s (32.82 MIGD). The results of case 7 are given in Table 5b. REFERENCE GAS TURBINE GT POWER PLANT A gas turbine GT power output can be used to operate mechanically driven desalting systems such as SWRO or MVC, while its heat rejected can generate steam. This steam can be used to operate thermal driven desalting system such as MSF, MED, or TVC desalting systems as given in the following cases. Before considering these cases, a short discussion on the GT is presented. The Gas turbines can be used as: 1. Simple power cycle with single gas turbine produces power only, with cycle efficiency up to 40%. 2. Combined heat and power CHP plant (usually called cogeneration power desalting plant CPDP when the process heat is used by desalting units). It consists of simple cycle gas turbine combined with heat recovery steam generator (or sometimes called waste heat boiler WHB). The HRSG recovers the heat content of the hot gases leaving the GT to useful thermal energy in the form of steam. The utilization factor defined heat and power outputs divided by the fuel energy input can reach up to 70-80%. 3. Combined gas/steam turbine cycle with high-pressure steam generated in the HRSG is used to operate steam bottoming cycle, and thus produces more power

16 670 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt output. The gas-steam combined cycle is the most efficient power plants with efficiency up to 60%. The unit power output of the available GT ranges from MW. The GT was used by utilities for peaking capacity. The improvements in GT by raising the unit capacity, turbine inlet temperature, and reliability favor its use as base-load power generator. Large GT generates electricity, with real low NOx emissions (in the single-digit parts per million range), either with catalytic exhaust cleanup or lean pre-mixed combustion. Because of their relatively high efficiency and reliance on natural gas as the primary fuel, gas turbines emit substantially less (CO2) per kwh generated than any other fossil technology in general commercial use. The GT is used extensively in industry to drive pumps, compressors, and other large mechanical equipment, and many facilities use turbines to generate electricity for use on-site. When used to generate power on-site, GT are often used in CHP mode with heat used for winter and summer air conditioning. A well-known gas turbine named LM6000 with data, [10], given in Table 4, is used here to show how a commercial GT can be used to operate different types of desalting system. The data of LM6000 GT are given in Table 6 for simple GT cycle, and when it is combined with HRSG. Case 8 Gas turbine operating seawater reverse osmosis desalting system SWRO In case 8, the typical GT LM6000 is used to operate SWRO desalting system, Figure 10. The LM6000 GT power output is used to drive SWRO desalting system, which consumes 6 kwh/m 3. The turbine has 40 MW net power output, 37% efficiency, and thus its fuel input rate is Qf = MW at the ISO conditions. So, the SWRO desalted water output is 1852 kg/s, and the specific fuel energy is kj/kg. The amount of desalted water for the reference MW fuel input is 1,775.2 kg/s (33.74 MIGD), and the emitted CO 2 is 4.55 kg CO2/m 3. The results of case 8 are given in Table 5b.

17 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 671 Table 6: Data of LM6000 gas turbines Cost and performance characteristics LM6000 Electrical capacity Total installed cost/kw Electric heat rate (kj/kwh), HHV Electrical Efficiency (%) 0.37 Fuel Input (MW) Required fuel gas pressure, bar 30 CHP Characteristics Exhaust flow (kg/s) GT exit temperature, C HRSG exhaust temperature, C Steam turbine output MW Steam output, kg/s Steam output (kw equivalent) Total CHP Efficiency, % HHV 0.74 Power to heat ratio 1.00 Net heat rate (kj/kwh) Effective electrical efficiency 0.69 Case 9: Gas turbine GT power plant operating MVC desalting system In this case 9, the 40 MW power output of LM6000 is used to drive a MVC desalting system, which consumes 12 kwh/m 3 by both the compressor and pumps. The rate of fuel input is MW at the ISO conditions. So, the MVC desalted water output is 926 kg/s, and the specific fuel energy is kj/kg. The amount of desalted water for the reference MW fuel input is kg/s (16.87 MIGD). The CO 2 added to the environment is kg CO 2 /m 3. The results of case 9 are given in Table 5c.

18 672 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt Table 5c: Results of cases 9, 10, 11, and 12 Cases 9(GT) 10(GT+HRSG) 11(CC) 12(CC) MVC SWRO MSF Total SWRO MVC D, kg/s Q d (to desalter), MW 40.1 Qf(for main energy), MW W(pump) kwh/m Qf(by heat + pump), MW Qf(total)/D, kj/kg , We, (kj/kg), kwh/m 3 (12) D (for Q f = ) in kg/s (16.87) (34.616) 2,5142 (47.775) 1398 (26.58) and (MIGD) Kg CO 2 /m Case 10: Gas turbine GT and heat recovery steam generator HRSG operating SWRO and MSF In this case 10, the GT power output is used to operate SWRO desalting plant. The GT exhaust gases are used by unfired HRSG to produce steam. The steam operates an MSF desalting unit, as shown in Figure 11. This arrangement is used in many Gulf countries as given in Table 3A in the appendix. The HRSG steam output of kg/s can operate an MSF unit, which consumes 258- kj heat/kg desalted water. The GT power output is 40 MW. The thermal energy recovered from the hot gases is 40.1 MW obtained from the HRSG and supplied to the MSF units. This heat produces kg/s desalted water by MSF unit. The required pumping energy for the MSF unit (4-kWh/m 3 ) is 2238 kw, and this work is deducted from the 40 MW power output of the GT. So, the net power available to drive the SWRO plant is MW, which can produce kg/s. The total desalted water output is kg/s, and the specific fuel energy is kJ/kg. When the desalted water output is corrected for the reference rate of fuel input of MW, the output becomes kg/s ( MIGD). The CO 2 added to the environment is kg CO 2 /m 3 of desalted water. The results of case 10 are given in Table 5c.

19 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 673 Case 11 Combined gas/steam power cycle operating SWRO desalting system In this arrangement, a bottoming steam cycle, including HRSG, is added to the GT to form a combined cycle. The combined cycle power output operates reverse osmosis RO desalting system. Single or dual pressure HRSG is used to produce steam in combined cycles as shown in Figures 12, and 13 respectively. The main required condition by the generated steam is to have 0.88 or more steam dryness fraction (quality) at the steam turbine exit, beside reasonable HRSG and steam cycle efficiencies. Raising the steam pressure in single pressure HRSG increases the steam cycle efficiency, but lowers the HRSG efficiency. The use of double pressure HRSG gives enough thermal energy to the bottom steam turbine to produce power output of MW. So, the total power output becomes MW. When this power output operates SWRO desalting plant, the desalted water produced is 2, kg/s. The only fuel added is that to GT ( MW). Consequently, the specific fuel input is MJ/m 3, and the CO 2 added to the environment is 3.21 kg CO 2 /m 3. When the desalted water output was corrected for the reference MW, the output is 2,5142 kg/s (47.75 MIGD). The results of case 11 are given in Table 5c. Case 12 Combined gas/steam power cycle operating MVC desalting system In this arrangement, the same combined cycle of case 11 operates MVC desalting system. The total power output is MW, and the desalted water output is kg/s, based on 12-kWh/m 3 compressor and pumping energy. The fuel added to the GT is MW, and consequently, the specific fuel input is MJ/m 3, and the CO 2 added to the environment is kg CO 2 /m 3. When the desalted water output was corrected for the reference MW, the output is 1,2567 kg/s ( MIGD). The results of case 12 are given in Table 5c. Case 13 Combined gas/steam power cycle with backpressure steam turbine operating SWRO and MEB desalting systems This case is similar to case 12 but the steam turbine is a BPST exhausting steam at pressure of 30 kpa (saturation temperature of 70 C) to low temperature multi effect boiling MED system. The characteristics of the MED is similar to case 4 MEB unit, namely, 6 effects, 5.5 gain ratio, 2 kwh/m 3 pumping energy, brine top temperature = 60 C, and steam supply equal to 16 kg/s. The steam turbine power output is MW, compared to MW of condensing turbine case 12, due steam exit at P = 30 kpa and not at condenser pressure of 10 kpa. The MED distillate output is 88 kg/s. The pumping power of the MED unit is kw. So, the net steam turbine output after supplying the pumping energy to the MED system is MW. The SWRO output due the power input of 40 MW by GT and MW by BPST is kg/s. The total desalted water is kg/s.

20 674 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt The specific fuel energy is MJ/m 3. For the reference fuel input of MW, the desalted output is corrected to kg/s (44.99 MIGD). It is noticed here that the decrease of steam power output due to use of backpressure steam turbine is MW can be considered as the real work loss due to the use of MED desalting plant, and specific equivalent work due the heat added is kj/kg (6.93 kwh/m 3 ), and when adding the 2 kwh/m 3, the total equivalent work of MED product is 8.93 kwh/m 3. The CO 2 added to environment is kg/m 3. The results of case 13 are given in Table 5d. Table 5d: Results of cases 13 and 14 Cases Case 13 Case 14 SWRO MED Total SWRO MSF Total D, kg/s , ,155.7 S, kg/s Q d (to desalter), MW Qf (for main energy), MW W(pump), MW Qf (by heat + pump), MW Qf(total)/D, kj/kg We, (kj/kg), kwh/m 3 D (for Qf= ) kg/s and (MIGD) 2473 (44.9) 2428 (46.14) Kg CO 2 /m Case 14 Combined gas/steam power cycle with backpressure steam turbine operating SWRO and MSF desalting systems This case is similar to Taweela plant, where combined gas/steam turbines cycle, using backpressure steam turbine exhausting its steam to multi-stage flash MSF desalting units. The plant has: three gas turbines power output equal to 228 MW at ISO conditions and 185 MW at corrected 46 C ambient temperature, 1190 C turbine inlet temperature TIT, compressor pressure ratio of 16 with 17 stages, and the turbine has 4 stages. The GT can operate with distilled oil or natural gas. Three HRSG producing steam at 70 bar and 522 C, and steam mass flow rate of 115 kg/s each. Two backpressure steam turbines are used, and each has power output of 111 MW, throttling conditions of 520 C and 66.5 bar, and exhaust pressure of steam supply to the MSF unit is 3.5 bar.

21 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt 675 Four MSF desalting units are used, and each has: 12.5-MIGD (657.7) capacity and 290kJ/kg specific consumed heat, (7.7 gain ratio), rated top brine temperature of 110 C, feed water salinity of 45,100 mg/l, and reported extremely low specific pumping work of 3.3 kwh/m 3. The plant power output from gas turbines is 555 MW with fuel input The MSF units of 50 MIGD (2630 kg/s) capacity consumes MW pumping energy. The backpressure steam turbine BPST power output is 222 MW. If the MSF pumping work is deducted from the BPST, its network = MW. The net total power output available from gas turbines and BPST is MW. This power output is not intended to operate SWRO. However, it is capable to produce 34, kg/s from SWRO desalting system. Then, the total desalted water that can be obtained from this plant by MSF and SWRO desalting systems is 37, kg/s by fuel input equal to MW. This gives kj/kg, and amount of CO 2 produced is kg/m 3. When the reference rate of fuel energy MW is applied, it produces 2428 kg/s (46.14 MIGD). The results of case 14 are given in Table 5d. CONCLUSION General comparison between all the cases considered, Table 7, indicates that: 1- It is in-efficient to operate thermal operated desalting systems such as MSF, TVC, and MED by steam directly supplied from fuel fired boilers such as cases 1, and 2. This practice should be avoided. 2- The SWRO is the most energy efficient desalting system, and all the efforts should be done to solve its pretreatment problems and its full applications. 3- The MVC is the most efficient distillation system, and more effort should be done towards increasing its unit capacity by developing compressors of large sizes, and decreasing its mechanical energy consumptions by raising the heat transfer coefficients in their evaporators and pre-heaters. 4- Using high efficiency power production system such as combined cycle can decrease the fuel consumed by desalting systems associated with power production cycle such as cases 11, 12, 13, and All conservation measures in utilization of water should be applied including water pricing.

22 676 Tenth International Water Technology Conference, IWTC , Alexandria, Egypt Table 7: General comparison between the 14 studied cases Cases Qf/D Kg CO 2 /m 3 1 Fuel fired boiler operating TVC Fuel fired boiler operating MSF Steam extracted from ST to MSF BPST power operating MVC, exhaust to MEB BPST power operating SWRO, exhaust o MEB Steam cycle operating MVC Steam cycle operating SWRO Gas turbine GT operating SWRO GT operating MVC GT, HRSG, and MSF Combined GT/ST operating SWRO Combined GT/ST operating MVC Combined GT/ST with BPST, and MEB Combined GT/ST with BPST, and MEB Nomenclature B = brine blow-down flow rate (kg s -1 ) BH = brine heater BPST = Back pressure type steam turbine C = specific heat (kj kg -1 C -1 ) CHP = combined heat and power plant CPDP = cogeneration power desalting plant CST = condensed type steam turbine D = distillate (product water) flow rate (kg s -1 ) Dr = vapor compressed by thermal compressor ECST = extraction-condensing steam turbine F = feed (make-up) water flow rate (kg s -1 ) GR = gain ratio, distillate/steam D/S. GT = gas turbine HHV = fuel high heating value HIS = heat input section HP = high pressure steam turbine cylinder, or high pressure HRSG = heat recovery steam generator HRS = heat recovery section HRJ = heat rejection section HTE = horizontal tube evaporators IP = intermediate pressure steam turbine cylinder, or intermediate pressure h = heat transfer coefficient (kw m -2 K -1 ), or enthalpy in kj/kg l = liters l/c = liters per capita L = latent heat (kj kg -1 ), or length in m